Heat Integration of Ethanol and Yeast Manufacture - Anastasovski Aleksandar
Integration of first and second generation of ethanol ... · Integration of first and second...
Transcript of Integration of first and second generation of ethanol ... · Integration of first and second...
2010-05-28
Integration of first and second generation of ethanol production from
wheat
Projektering, KET050
Project mentor: Guido Zacchi, Ola Wallberg
Written by: Emelie Arvinius, Pia-Maria Bondesson, Ilia Komorin, Maria Navasa, Patrik Svensson
Abstract Nordisk Etanol & Biogas AB is planning to build a new ethanol plant that will produce 130 000 m3 of
ethanol per year from wheat. Ethanol is used more and more as fuel for cars. Most of the ethanol is
imported because only 200 000 m3 of ethanol fuel are produced in Sweden, mainly from wheat and
barley.
The goal is to study the integration of a straw based with an existing starch based ethanol plant, where
the stillage is used for biogas production. The lignin from the straw is combusted to obtain electricity
and heating for the process and the surplus electricity is sold. This was done by first designing an
ethanol plant based on the starch production of 130 000 m3 of ethanol/year and then designing a straw
based plant with a capacity of 200 000 tons of straw/year to produce ethanol and biogas. An economic
analysis was performed together with sensitivity analyses on the most uncertain values as cost of raw
material, selling price of products and the design of the simultaneous saccharification and fermentation
of the straw based ethanol process.
In the straw based process, there are many ways to pretreat the lignocellulosic material. It was thought
that steam explosion with dilute acetic acid was the most suitable setup. The acetic acid chosen is also
an intermediate in the biogas process so it will not harm the process downstream as other acids might
do and it is converted to biogas.
The integration of the processes can be done in two places of the processes, either in the fermentation
step or where the streams to the distillation are connected. In this study it was assumed that integration
in the distillation step was the best option.
When integrating the processes, the energy demand is higher than if considering having the two
processes separately. The stripper column is the element that requires most energy. However, the heat
integration study should be improved to lower the energy demand, especially in the distillation step.
The production of ethanol mainly comes from the first generation process while the biogas comes
mainly from the second generation process. When looking at the energy efficiencies, the best result was
obtained when integrating the two processes due to the larger amount of electricity produced.
From an economical point of view, building the first generation plant is a profitable investment due to
the low investment cost and the high output of ethanol. The income for the methane and the dried
destillers grain (DDG) is much lower than for the ethanol which makes the plant sensitive and
dependent on the ethanol selling price and the cost for wheat, which is the main cost in the process.
Building a second generation plant and integrating the process in the distillation step showed to be the
most profitable option. Even though the investment cost would be twice as high as for the first
generation plant, the production of electricity would become a major income (almost 4 times higher
than for DDG which is no longer produced). Methane income would also be more significant. This makes
the process less sensitive to market price fluctuations. This means that the integrated process is more
economically stable and more economically profitable, depending on the spread of costs and incomes.
Contents 1. Background ............................................................................................................................................... 1
1.1 The company: Nordisk Etanol & Biogas AB ......................................................................................... 1
1.2 The project task................................................................................................................................... 1
2. Ethanol/biogas production from wheat ................................................................................................... 2
2.1 Ethanol production with 1st generation, starch based production ..................................................... 2
2.1.1 Wheat ........................................................................................................................................... 2
2.1.2 Pretreatment of wheat ................................................................................................................ 3
2.1.3 Pre-Mixing step ............................................................................................................................ 6
2.1.4 Gelatinization ............................................................................................................................... 6
2.1.5 Liquefaction ................................................................................................................................. 7
2.1.6 Fermentation ............................................................................................................................... 8
2.1.7 Distillation .................................................................................................................................. 10
2.1.8 Alcohol dehydration technologies ............................................................................................. 11
2.2 Ethanol production with 2nd generation, straw based production ................................................... 13
2.2.1 Pretreatment methods .............................................................................................................. 14
2.2.2 Hydrolysis and fermentation ..................................................................................................... 17
2.2.3 Distillation .................................................................................................................................. 17
2.3 Biogas ................................................................................................................................................ 17
2.3.1 Biogas process [18] .................................................................................................................... 17
2.4 Possible integration steps ................................................................................................................. 21
3. Suggested process ................................................................................................................................... 22
3.1 Ethanol production with 1st generation, starch based production ................................................... 22
3.1.1 Pre-Mixing step .......................................................................................................................... 23
3.1.2 Gelatinization ............................................................................................................................. 23
3.1.3 Liquefaction ............................................................................................................................... 24
3.1.4 Fermentation ............................................................................................................................. 24
3.1.5 Distillation .................................................................................................................................. 24
3.2 Ethanol production with 2nd generation, straw based production ................................................... 25
3.2.1 Process overview ....................................................................................................................... 25
3.2.2 Pretreatment.............................................................................................................................. 26
3.2.3 SSF .............................................................................................................................................. 27
3.2.4 Distillation .................................................................................................................................. 28
3.2.5 Solid removal ............................................................................................................................. 28
3.2.6 Heat and electricity generation ................................................................................................. 28
3.3 Biogas ................................................................................................................................................ 29
3.4 Integration of 1st and 2nd generation ethanol production ................................................................ 30
4. Results and analysis ................................................................................................................................ 31
5. Economics and sensitivity analysis .......................................................................................................... 36
5.1 Economy ............................................................................................................................................ 36
Equipment cost ....................................................................................................................................... 36
Chemicals, raw material and products ................................................................................................... 39
Operational plant costs and investment analysis ................................................................................... 42
5.2 Sensitivity analysis ............................................................................................................................ 43
5.2.1 WIS concentration in the SSF ..................................................................................................... 43
5.2.2 Enzymatic yield .......................................................................................................................... 45
5.2.3 Product price .............................................................................................................................. 46
6. Discussion and conclusions ..................................................................................................................... 49
7. References .............................................................................................................................................. 51
Appendix ..................................................................................................................................................... 55
Appendix I ............................................................................................................................................... 55
Appendix II .............................................................................................................................................. 56
Appendix III ............................................................................................................................................. 57
Appendix IV ............................................................................................................................................. 61
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1. Background Ethanol is used more and more as fuel for cars, either as an additive or in FFV-cars (Flexible Fuel Vehicles)
which can run on regular unleaded gasoline or on a gasoline blended fuel like ethanol. Most of the
ethanol is imported because only 200 000 m3 of ethanol fuel are produced in Sweden, mainly from
wheat and barley.
Nordisk Etanol & Biogas AB is planning to build a new ethanol plant that will produce 130 000 m3 of
ethanol per year from wheat in the beginning. This plant will be located in Karlshamn, Blekinge. The
production will increase to reach 260 000 m3/year.
At the start, it is planned that the ethanol will be produced from starch (out of wheat) while biogas will
be produced from the cereal stillage (first generation plant). To get as high energy efficiency as possible,
it is planned later on to use straw for the production of ethanol and biogas (second generation plant).
From the sludge, biofuel and fertilizer products will be produced. The aim of this project is to look for
the best and most suitable method in which the straw based part can be integrated in a starch based
plant.
The interest for producing ethanol from straw is of big interest, because the raw material is available in
large quantities and it does not compete as a food source, like wheat.
The problem with using straw as a sugar source is that the structure is hard to brake in order to release
the cellulose and the hemicelluloses content. Therefore, the pretreatment step requires more effort.
1.1 The company: Nordisk Etanol & Biogas AB Nordisk Etanol & Biogas AB was founded in 2006. Their goal is to become Sweden’s biggest and most
effective production plant for bio based ethanol, with a clear focus on the environment, market
development and profitability. The company will contribute to a faster conversion towards renewable
fuel sources, both nationally and internationally. The company will also help Sweden drastically
decrease its need for importation of fossil fuels and the use of them [1].
The company will use the residue stillage which always occurs during ethanol production to produce
biogas which will be used as fuel for vehicles and it can also be used for producing green electricity [1].
1.2 The project task The goal is to study the integration of a straw based with an existing starch based ethanol plant, where
the stillage will be used for biogas production. The lignin from the straw will be combusted to obtain
electricity and heating for the process and surplus electricity will be sold.
The main task of his project is to design an ethanol plant based on the starch production of 130 000 m3
of ethanol/year. Moreover, different process options should be investigated in order to design a straw
based plant with a capacity of 200 000 tons of straw/year to produce ethanol and biogas. Operation and
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investments costs as well as project valuation and economic analysis should be performed. Finally,
sensitivity analysis of the most uncertain parameters should also be done.
2. Ethanol/biogas production from wheat
2.1 Ethanol production with 1st generation, starch based production A general flowsheet of a first generation ethanol plant is shown in Figure 1. Other starch based ethanol
production plants may differ a little but the basic components are the ones shown in the figure. The
production of ethanol consists of a grain pre-mixing step followed by a gelatinization and a liquefaction
step. Afterwards, the fermentation takes place where ethanol is produced. The ethanol is then purified
to obtain pure ethanol at 99.8%.
Figure 1 General picture of process for the first generation
2.1.1 Wheat
Wheat consists mostly of starch. Starch is composed of two polysaccharides, where one has a linear
chain structure of glucose units called amylose and the other has a branched structure called
amylopectin, see Figure 2. The ratio between these two polysaccharides gives starch its specific
properties.
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Figure 2 Structures of the starch components [2]
High Amylose content in the starch gives a low viscosity at a given temperature due to its unbranched
structure compared to the branched structure in amylopectin. Because of the higher temperature
required to make amylase gelatinize due to the linear structure, it is not economic defendable to use
this kind of starch because of the high energy requirement. On the contrary, starch with high amount of
amylopectin content swells more easily in solution and has a lower gelatinization temperature and is
more advantageous for ethanol production [2].
2.1.2 Pretreatment of wheat
When the wheat arrives to the factory it is inspected for mold infestation, moisture content, weight etc.
The wheat is then stored in silos. Afterwards, the wheat continues to the sieving step were it is cleaned
from stones and other undesirable substances. After the sieving, the wheat is grinded by hammer mills
[3].
The Dry Milling process
The purpose of the milling is to increase the surface area to make the slurry more accessible. By
breaking down the wheat to very fine particles the penetration of water increases in the cooking step [3],
as well as the different enzymes used will break down the starch more effectively in less time. This is
important for the economy of the process [4]. There is a lot of different milling equipment used to grind
cereals but the most popular for distilleries are the hammer mills [3], see Figure 3.
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Wheat is transferred from the silos to the hammer mill before the pre-mixing. The particle size
distribution is controlled by the size of the screen that covers the walls of the inside of the machine
where the hammers are rotating and preventing bigger particles to pass until they reach the proper size.
Figure 3 Hammer mill for milling grain [4]
It is important to check the screens regularly so the size distribution is correct. It can differ as much as 5-
10 % in ethanol yields if the screens are incorrect [3]. As the hammer mill is a crucial step, replacement
of mills or screens should be considered from time to time [3]. If the particles are too big, the enzymes
have difficulties to reach the starch and if the particles are too small, they can cause problems in the
stillage centrifuge [3] as well as they can cause coagulation in the slurry tank. As long as the particles are
in the right range, the yield will not be affected.
The temperature of the flour should not exceed certain temperatures (25-35 °C) because it can pre-
gelatinize which can result in starch that cannot be converted downstream.
The wet- milling process
Several steps are used in the wet milling process. The wet milling process is divided in two sections. The
first section is the millhouse which produces slurry consisting of starch and many by-products. The other
section is the “finishing” part that consists of different steps that process the starch slurry from the
millhouse. It begins with soaking where the water soluble compounds are removed. After this step, the
starch is impure and in the last step, it is washed with pure water countercurrent to produce a pure
product.
The steeping step (soaking)
The millhouse produces starch using a series of steps beginning with soaking. When the wheat is
cleaned, it is stored for 20-40 hours with acid containing about 1600 ppm SO2 at a temperature of 52 ⁰C.
The steeping process swells the kernel, softens the shell and the protein structure, which will make it
easier to separate the starch and the protein from the germ and the hull, see Figure 4.
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Figure 4 Nomenclature for the different parts of the wheat [3]
Germ system
The steeped corn passes through the mills to separate the germ from the rest of the kernel. The germ is
separated from the slurry in hydro cyclones. After two stages of milling and separation, the germ is
washed counter currently in three steps and then is dewatered and dried.
After these steps, the seed contains about 50 % of oil with fiber protein and starch. The whole germ is
processed to recover the oil. This can be done at the plant or the whole germ is sold.
Fiber system
After the removal of the germ, the remaining is slurry of starch, protein, fibers and particles of different
sizes. Screens are used to recover the fibers and the particles. The fine particles of starch and protein
are processed in the next step.
Gluten separation
The slurry which contains the starch and proteins is dewatered so that the viscosity gets higher and
centrifuged so there is a separation between the starch and the proteins. The stream of gluten is also
thickened, dewatered and dried. The product is gluten meal which is sold.
Starch washing
The starch that comes out from this step contains small amount of impurities which must be removed.
This is done by countercurrent washing with water and the use of hydro cyclones. The product is almost
pure starch.
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To make ethanol from starch, it must be broken down to fermentable sugars by the hydrolysis reaction
where water combines with starch.
When dry milled wheat is used, the whole wheat is fermented and there are a lot of other materials
within the wheat. When using the wet mill process, those materials are not present and pure starch is
hydrolysed and fermented. The yeast can be recovered and used many times instead of being wasted in
the stillage.
The advantages of the wet milling process are several: the fermentation starts immediately and fast
because of high yeast concentration. Due to the high concentration, other microorganisms are displaced
so they cannot infect the process that easy. Because of the yeast recirculation, there is not a significant
growth of new yeast cells, which means there is a saving of 4 % of sugars that can be converted to
alcohol instead of yeast growth.
Nowadays, almost all the wet milling plants use maize as the feed but wheat can be used as well.
2.1.3 Pre-Mixing step
In this step, flour is mixed with water and backset from the distillation towers. The thin stillage (backset)
is rich in nutrients from the fermentation and has a low pH which is optimal for the enzymes [5]. The
slurry from the distillation stripper is pumped to a decanter, the water from the decanter is called thin
stillage and is rich in nutrients, and the centrifuged biomass is used for biogas production [5]. This step is
critical for the process. The stillage also reduces the water consumption and lowers the pH to about
between 4.5- 5.5 which is the optimal pH for the fermentation step.
If the dry matter content is too low, it will result in poor economy of the process. On the other hand, if
the dry matter content is too high, the viscosity will be too high and that will cause problems with heat
exchangers, enzyme kinetics and fermentation. Reduced viscosity will reduce the energy needed for all
the process steps. That is something gained with lower viscosity, but the dry substance should be as high
as possible.
In this step, a small quantity of α-amylase is added (0.02 % v/v) to facilitate agitation because of the high
viscosity of the slurry.
2.1.4 Gelatinization
After the pre-mixing step, the slurry is pumped to the gelatinization tank where the structure of the
starch is broken down. Starch gelatinization is a process that breaks down the bonds of the starch
molecule. This allows the hydrogen bonding sites to absorb water and swell. As the water enters the
starch, it decreases the number of crystalline regions and the heat added makes chains separate into an
amorphous form. When increasing the temperature over a specific value, the starch granules start to
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absorb water, swell and burst, releasing the starch into solution [3]. This process is irreversible and is
called gelatinization. The gelatinization temperature for wheat is between 58-65 ° C. At this
temperature, the starch grain swells and opens up so the enzymes in this step, the α-amylase, can
hydrolyze the long chains into shorter sugars. When the sugars are hydrolyzed to shorter chains, the
viscosity of the mash will decrease rapidly and will not increase again in the process, see Figure 5.
Figure 5 Gelatinization cycle with a decrease in viscosity [5]
If the temperature drops when the starch is gelatinized, it will re-crystallize and it cannot then be
degraded by α-amylase and thus, the starch will go through the whole process undegraded.
2.1.5 Liquefaction
As yeast cannot use the starch present in the process for fermenting, the starch needs to be hydrolyzed
first. During liquefaction, starch is converted to shorter carbohydrates by the presence of the enzyme α-
amylase, added to the slurry after the pH is adjusted [6] as explained before. The enzymes used in the
production of ethanol from wheat are the α-amylase and glucoamylase. α-amylase can be used to
hydrolyze the starch randomly to dextrin. The glucoamylase converts dextrin to glucose units by
hydrolyzing the α-1, 4 linkages by the non reducing end of the dextrins [6].
Enzymes are biological catalysts which can improve the fermentation by breaking down proteins into
amino acids and thereby, increase the yield. Moreover, they also reduce fouling downstream caused by
the proteins, which is very good in means that the system does not need to be cleaned that often.
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Liquefaction and gelatinization are generally carried out simultaneously. The viscosity is also reduced
because of the degradation to shorter carbohydrates (dextrins) [7]. The liquefaction is carried out at
about 88-93 °C and at a pH about 6-6.5. This is the optimum temperature for the enzymatic breakdown
of the starch to dextrins (carbohydrates). If the temperature is lowered, the energy demand is
decreased and pressure vessels for high temperature are not necessary. The enzymes catalyze the
hydrolysis reaction at this temperature for about 2 hours till the proper dextrose equivalent is obtained
[8].
Proteases are a group of enzymes that break down proteins. They are very important because they
break down the peptide bonds in proteins to liberate amino acids. They are able to break down almost
any kind of proteins [9]. The proteases can be used for increasing the capacity of the plant. They reduce
the viscosity so more wheat feed can be handled and energy consumption can be decreased in the plant.
The reaction time is proportional to the enzyme concentration. It is an important design factor if high or
low concentrations of enzymes are to be used and thus, the reactors must be designed for the correct
residence time [3].
Rhizozyme which is also an enzyme can be used instead of glucoamylase (0.5 kg/ton starch) or as a
supplement (0.1 kg/ton starch). With this, higher yields can be achieved. The optimal temperature for
Rhizozyme is 30-35°C and a pH around 3.5-4.5.
2.1.6 Fermentation
The most common modes of fermentation are batch fermentation and continuous fermentation. Most
of the processes use batch fermentation where the sugars can be converted to 75-95 % of the
theoretical and the final ethanol concentration going into the distillation tower is 10-16 % (v/v).
The continuous process does not have the drawbacks that are present in batch fermentation. It can be
run for longer periods without the need for shutting them down and for this reason the reactors can be
smaller in size. They can also be fully automated. To increase the productivity even further, two stirred
tank reactors in series can be used to avoid inhibition by ethanol. As the conversion of sugar will not
reach the maximum in the first reactor, it is pumped to the second.
To increase the productivity, the yeast cells are recycled. The recycle of the yeast cells creates a high
biomass concentration which reduces the time for the conversion of sugar to ethanol. This in turn
increases productivity because of the rapid fermentation. This process also has drawbacks because high
ethanol concentrations increase the death rate of the cells, has downtimes and is hard to automate.
With the continuous process with cell recycle, higher productivity is obtained because the high yeast
concentration can be maintained and the sugars can be fermented faster than if no cell recycle was
performed. The biomass leaving the reactor is separated by a centrifuge and pumped back to the
reactor. The cost of this system is higher than without recycling because of the cell separation.
The best yeast at the moment suited for the fermentation step is normal baker’s yeast, Saccharomyces
cerevisiae, under anaerobic conditions. The yeast Saccharomyces cerevisiae is classified as facultative
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anaerobic, which means that it can grow with or without the presence of oxygen. When the oxygen level
is sufficient and the sugar concentration is kept low, almost no ethanol is obtained. The sugars are
instead used for producing new cells. When the oxygen level is reduced or the sugar concentration is
over a certain value, ethanol is produced.
In a factory for producing ethanol, the substrate is fermented in an anaerobic way even if the
environment contains a small amount of oxygen. If oxygen is not supplied, the yeast will not produce
ethanol from sugars. The yeast will stop to grow and will not complete the fermentation. The yeast
requires this small amount of oxygen to synthesize vital molecules for the cell [3]. In order for the yeast
to grow in an optimal way, they need to have nutrients. If the right nutrients are not supplied, the yeast
will not grow fast enough and ferment well so the yield3will decrease.
Bacterial growth is also inhibited due to the lack of substrate. If the bacteria, that produce lactic acid,
are present in the system, they will start to consume the sugars and a decrease in the ethanol yield will
be observed.
In the separate hydrolysis and enzymatic fermentation (SHF) process, the enzymes catalyze the
breakdown of cellulose by the hydrolysis reaction to form glucose in one reactor. The product stream
then goes into a fermentor where the glucose is converted to ethanol due to the action of yeast.
The sequence steps for the simultaneous saccharification and fermentation process are the same as in
SHF except for that the hydrolysis and fermentation are combined in the same vessel.
Simultaneous Saccharification and Fermentation
Simultaneous Saccharification and Fermentation (SSF) has become a popular process on new plants and
is one process option for producing ethanol from starch. Many distillers changed from having a separate
saccharification reactor to adding the saccharification enzyme directly into the fermentor.
SSF has become the most low-cost process from which a high ethanol yield is obtained. The benefits
gained by performing the enzymatic hydrolysis together with the fermentation, instead of having a
separate step after the hydrolysis, is that the sugar concentration in this process will never reach any
high level that inhibits the enzymes used. As soon as the glucose molecules are formed, they are
consumed by the yeast and converted to ethanol.
The drawback of SSF is to find optimal conditions (pH and temperature) for both the yeast and the
enzyme. There are also difficulties to recycle the yeast and the enzymes. The temperature is usually kept
below 37 °C to maintain the yeast in a good growing phase [10]. The heat in the liquefaction slurry that
has to be cooled down to about 37 °C is reused by heat exchange in the process. The ethanol
concentration leaving the fermentor is about 10 % (w/w).
Both methods have their advantages and disadvantages but SSF was chosen as it is the method which
presents a higher ethanol yield and that is what was wanted to achieve [11].
For example, SHF has the possibility to operate the process at optimal temperatures as the two
processes are separated. These temperatures are around 30ºC for the yeast and 50-60 ºC for the
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enzymes. On the contrary, the temperature used in SSF is a compromise as low temperatures are
preferred by the yeast so no damage occurs but the enzymatic hydrolysis rate is slowed down.
Moreover, it is known that end-product inhibition occurs during the enzymatic hydrolysis step when the
concentration of glucose is high. In SHF, in order to avoid end-product inhibition, the solution must be
diluted or the enzymatic hydrolysis must take place longer time. Furthermore, in SHF the yeast cells can
be recycled as the liquid hydrolysate is separated from the solid residue before fermentation so, the
glucose consumption for the cell growth is kept low but the separation of the solid material requires an
extra washing step to avoid sugar losses. This means that the operating volume stream is higher and
thus, the sugar concentration is lower [11].
2.1.7 Distillation
Distillation is required to concentrate the low content of alcohol solution. By heating the liquid, ethanol
can be separated from water and other impurities in order to leave the distillation tower at 93 wt/wt%
of ethanol. There are different types of distillation systems; multi-stage, countercurrent, continuous.
The section above the feed entry is defined as the rectifying section and the part below the feed is called
the stripping section, see Figure 6 [3].
The good things about using distillation towers are a high alcohol recovery and that columns are easy to
scale. But it even has a downside. For example, if there is an azeotrope, the dryness specification cannot
be reached. The energy demand is higher for low feed concentrations of alcohol compared to high feed
concentrations [3].
Nowadays, a modern facility for producing ethanol from wheat can use as many as three separation
columns (stripper, rectifier and an additional rectifier). This system is used to improve the heat
integration. The heat from the first column is condensed and connected to the reboiler that drives the
second distillation tower and the same connection is made on the third column. If there is low grade
steam, it can be used and recovered from other parts of the facility, instead of producing expensive
steam [12].
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Figure 6 A typical distillation system [13]
In the stripper, the liquid going in is separated from all the ash, organic acids and other low boiling
products. The end product from the stripper is a mixture of ethanol and water with volatile compounds.
After the stripping column, the mixture comes into the rectifier column that removes most of the water.
[12].
The stillage from the stripper is centrifuged and the water that comes out is called thin stillage. Some of
the thin stillage can be recycled, where the recycled stream is called backset. The amount of thin stillage
recycled varies and is usually around 10-15 % by volume in the pre-mix step in order not to get
accumulation of substances in the process. The rest of the thin stillage can be used in the biogas
production. The wet cake can be used for biogas or for cattle feed.
2.1.8 Alcohol dehydration technologies
To use ethanol as transportation fuel, the ethanol content must be increased even further. A mixture of
ethanol-water forms an azeotrope at 95.6 wt % ethanol. To get beyond this point, a third component is
needed, the entrainer. Some techniques are explained through the text.
Nowadays, the most common method is to use molecular sieves, an adsorption method, see Figure 7.
They can selectively remove the water from the mixture and produce a very dry product. Molecular
sieves are granular and hard substances which can be spherical or cylindrical. The pores in the sieves are
typically 3 Angstroms (3*10-8 centimeters). The water molecules have a diameter of less than 3
Angstroms and the ethanol molecules have a diameter greater than 3 Angstroms. This means that the
water molecules can be adsorbed in the internal structure of the sieve and dehydration is achieved [13].
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By this technique, you can produce anhydrous ethanol of 99.9 % (wt) [13]. Usually a cyclic process is
used for loading and unloading the adsorbent, (water). To unload the water; vacuum or high
temperature can be used to desorb the water.
Figure 7 The adsorption process for both the loading and unloading cycle [13]
Another method to dehydrate the ethanol is to add some compounds to avoid azeotrope formation.
This can be done by the addition of solvents or salts. Examples of entrainers are benzene and ethylene-
alcohol.
Gas stripping is common in various chemical process industries in liquids containing volatiles substances.
Gas strippers achieve high recovery of ethanol. In gas stripping the gas is an inert gas that will not be
condensed in the condenser.
Another method is liquid-liquid extraction where the broth from the fermentation is placed in a mass
transfer contact with an organic liquid that can extract the ethanol from water. Because the extractant
has to be reused, the extracted ethanol must be removed in the regeneration unit, see Figure 8. This can
be done if a difference in volatility between the two liquids exists.
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Figure 8 Liquid-liquid extraction process [13]
Pervaporation is a membrane based process in which the fermentation broth is brought into contact
with a porous membrane, see Figure 9. A vacuum is applied on the other side. The use of pervaporation
was a very strong competitor to molecular sieve adsorption but today this system is rarely used. For
ethanol dehydration, a hydrophilic membrane is used which selectively transports water. The good thing
about these membranes is that they can remove water from alcohol even when the VLE (Vapor liquid
equilibrium) is unfavorable [13]. The components of the feed diffuse through the membrane and
evaporate into another vapor phase. Because different compounds have different diffusion
characteristics, some compounds will be enriched in the permeate.
Figure 9 The pervaporation process [13]
2.2 Ethanol production with 2nd generation, straw based production Instead of using starch for ethanol production, lignocellulosic material can be used. This can be wood,
waste products or crop residues. Lignocellulosic material contains mainly three different constituents;
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cellulose, hemicellulose and lignin. The production of ethanol from lignocellulosic material consists of
five steps; pretreatment, hydrolysis of cellulose, fermentation of hexoses, separation and effluent
treatment, see Figure 10.
Figure 10 General flowsheet for second generation ethanol production
2.2.1 Pretreatment methods
There are several different technologies available for ethanol production from lignocellulosic materials.
For the moment, the conversion processes, based on the use of cellulosic enzymes seem to be the most
promising ones for large scale application. The efficiency of this technology is limited due to the
chemical structure of lignocellulose biomass which inhibits the enzymatic breakdown of cellulose and
the solubilization of hemicellulose-derived sugars [14]. Cellulose strains are bundled together and tightly
packed in such a way that neither water nor enzyme can penetrate through the structure [15]. For this
reason, the pretreatment is an essential step for ethanol production in purpose of deconstructing the
structure to obtain an adequate yield of released fermentable sugars in the hydrolysis step [16]. In the
pretreatment, the solubilization of lignocelluloses components depends on temperature, pH and
moisture content18. In addition, this is considered to be one of the most expensive processing steps,
estimated to account for up to 33% of the total cost by the National Renewable Energy Laboratory [14].
Further on the goal is to achieve high sugar release pretreatment efficiencies; low production of toxic
compounds and low energy consumption as possible [15].There are four groups of processes that can be
used for pretreatment: physical, physico-chemical, chemical and biological.
Physical pretreatments
The first step in pretreatment of the wheat straw for ethanol production is size reduction through
milling, grinding or chipping. This should improve the efficiency of downstream processing. Both particle
size and moisture content are variables that influence the energy consumption as well as the
effectiveness of the following step in the process. Smaller particle size requires higher energy for
production while higher moisture content of straw will lead to higher specific energy consumption in
downstream steps [15].
Stillage
Distillation
Ethanol
Solid
Liquid
Fermentation Separation
Biogas
Straw Pretreatment
Anaerobic
digestion
15
Physico-chemical
In lignocellulosic materials, such as wheat straw, hemicellulose is the most thermal-chemically sensitive
fraction. Hemicellulose compounds start to solubilize into the water at temperatures higher than 150C,
where xylan is the most easily extracted.
Liquid Hot Water Pretreatment
In the Liquid Hot Water (LHW) pretreatment, the process is kept under pressure to maintain the hot
water in a liquid state. Commonly used are temperatures in the range of 170-230C and pressures over
5 MPa [15]. To improve the efficiency, it is possible to homogenize the straw by mechanical
pretreatment (for example milling) and the slurry of water and straw is agitated. The pressure applied,
the ratio solid/liquid, the processing time as well as the temperature are parameters that have been
tested in different ranges but the conclusion is that a two-step LHW pretreatment gives the maximum
recovery of fermentable sugars. However, economic and energetic aspects have to be considered in
proportion to the gained ethanol. As no acids are used, the materials costs are remarkably reduced
when hydrothermal pretreatment that does not employ any catalysts, as LHW, is used. Two fractions
are obtained after filtration of the pretreated slurry, one cellulose-enriched (water-insoluble solids
fraction) and one rich in hemicellulose-derived oligomeric sugars (liquid fraction or prehydrolyzate) [14].
Steam Explosion
Steam explosion is one of the most cost-effective and widely used pretreatment methods for wheat
straw. For this method, size-reduced (by physical pretreatment) wheat straw is rapidly heated by high
pressure steam for a period of time. Then, the pressure is suddenly reduced which causes an explosive
decompression of the material. Most commonly temperatures in the range of 160-230C are used and
the time period varies from several seconds to few minutes. Addition of chemicals such as H2SO4 or SO2
can improve the rate and extent of hemicelluloses removal and lead to enhanced yield of sugar in
enzymatic hydrolysis at lower temperatures [15]. Here, the economical aspect in more expensive
materials in comparison to the higher ethanol yields should be taken into consideration.
Ammonia Fiber Explosion (AFEX)
In this pretreatment, the wheat straw is exposed to liquid ammonia at high temperature and pressure
for a period of time, followed by a sudden pressure release causing an “explosion” of the structure. For
AFEX, there is no need for size reduction and it does not produce any inhibitors that may affect later in
the process. A comparison between steam and ammonia explosion has been made and the results
showed that the enzymatic hydrolysis was more or less the same with both pretreatments for wheat
straw. The main drawback with pretreatment method is handling large amounts of ammonia under
pressure, which is a high risk. However AFEX pretreatment of wheat straw is rarely reported and more
experimental work is needed [16].
Chemical pretreatments
For the chemical pretreatment of wheat straw, different chemicals such as acids and oxidizing agents
are used. Most commonly used is H2SO4, but depending on which one is used, the effect on the
lignocellulosic material is different. Alkaline, ozonolysis, peroxide and wet oxidation pretreatments are
16
more effective in lignin removal whereas dilute acid pretreatment is more efficient in hemicellulose
solubilization [15].
Acid hydrolysis
When inorganic acids, often H2SO4 are used for pretreatment, the downstream enzymatic hydrolysis
improves. Depending on the dose of acid the method is categorized as dilute- and/or concentrated-acid
hydrolysis. When the latter method is used, the wheat is treated with high concentration of acids at
ambient temperatures, resulting in high yields if sugars (30C, 30 min). The advantage with this method,
except the high yields, is that no enzymes for saccharification are needed. On the other hand,
drawbacks such as high energy consumption, equipment corrosion, obligation of acid recovery and
longer reaction times should be taken into consideration.
In the dilute-acid method, low concentration acids (0,5-1%) and high temperatures (>180C) are used.
The high temperatures make it possible to attain acceptable rates of hemicellulose conversion into sugar
oligomers and monomers. Even though low concentrations are used, the high temperatures increase the
equipment corrosion but the main drawbacks of this method are the formations of many inhibiting
byproducts and the required pH-neutralization for downstream processes [16].
Dilute organic acids, often fumaric or maleic acid, have been investigated in comparison to sulfuric acid
using temperature ranges 150-170C for 30 minutes. The author’s conclusion was that maleic acid
pretreatment of wheat straw was almost as effective as sulfuric acid in respect to enzymatic digestibility
where more xylose and less furfural (a byproduct that inhibits the hydrolysis) is produced. Another
positive aspect is the fact that pH-adjustment is not needed to achieve a pH of 4-5 in the SSF as well as
the use of organic acids might improve the biogas production.
Alkaline
In the alkaline pretreatment dilute bases such as sodium, potassium, calcium or ammonium hydroxides
are used at lower temperatures (50-65C) and longer periods (up to 24h) to achieve an effective method
with high sugar yields. Sodium hydroxide is the most studied alkaline method, but due to the high cost
of alkali utilization, the cheaper option calcium hydroxide (lime) seems to be the most promising one
[15]. The down side with lime is its ability to precipitate and clog up the instrumentation. Another option
is the use of ammonia due to the positive effect it might have later in the biogas production process.
See the biogas section for detailed information.
Biological
In this pretreatment method, microorganisms such as brown, white or soft-rot fungus do a selective
degradation of both lignin and hemicellulose. The suitable fungi for biological pretreatment should have
higher affinity for lignin and degrade it faster than carbohydrate components. For now, the most
effective one is the white-rot fungi but the rate of hydrolysis reaction is very low (~5 weeks) and needs a
great improvement to be applied to a bigger scale. There is interest in further studies as the biological
pretreatment is safe, environmentally friendly and has rather low energy consumption in comparison to
other pretreatment methods [15].
17
2.2.2 Hydrolysis and fermentation
Once the pretreatment of the wheat straw has been performed, the enzymatic hydrolysis and the
fermentation steps take place. The hydrolysis (converts the complex polysaccharides to sugar
monomers) uses the same principles as for the first generation ethanol production, see Section 2.1.6.
The enzymes for the second generation ethanol production are not the same as those used in the first
generation. To hydrolyze cellulose, enzymes as cellulases are used to depolymerize cellulose to first
cellobiose, two glucose units linked together, and then to glucose. The fermentation also uses the same
principles as for the first generation production, see Section 2.1.6.
In the present case, SSF has been chosen to be used as method to perform the calculations. If recycle of
yeast was to be considered, separation of yeast from the lignin would be required as the lignin would
otherwise accumulate. So, new yeast has to be produced each time and this means that the glucose loss
due to yeast cell production is higher in SSF than in SHF [17].
2.2.3 Distillation
The product stream from the SSF contains ethanol, non-converted glucose, pentoses, water and several
other products. Distillation is the method of choice for the recovery of ethanol from the fermentation
broth in second generation ethanol production, see Section 2.1.7 for more information about ethanol
recovery techniques.
2.3 Biogas
2.3.1 Biogas process [18]
After the distillation, the stillage from the first and second generation process using SSF will go to the
biogas production site. Organic matter will be broken down under anaerobic conditions in the presence
of different microorganisms.
The process can be summarized into four stages, see Figure 11:
1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis
Figure 11 The different stages in the biogas production process [19]
18
All these stages may take place in the same reactor. The first step in the process is the hydrolysis, where
the complex organic (cellulose, starch, protein and fat) matter is broken down to simple sugars, amino
acids and fatty acids.
During the acidogenesis step, the products from the hyrdolysis step are broken down even further. The
products from this step are volatile fatty acids (VFA), ammonia, carbon dioxide, hydrogen sulfide and
some other. These products are further broken down in the acetogenesis step to mainly acetic acid,
carbon dioxide and hydrogen. The production of hydrogen can only be maintained if the concentration
remains at a low level. If it is not maintained at a low level, the anaerobic oxidation will stop and so will
the whole process.
The last stage in the biogas production is the methanogenesis step. The bacteria in this step use the
intermediate products from all previous stages to convert them to methane, carbon dioxide and water31.
The Acetotrophic methanogens are the dominating methane producing organisms, which use acetate as
a substrate. In their metabolism, the acetate is converted into two parts, where one carbon is used for
the production of methane and the other for carbon dioxide. As these organisms grow very slowly, this
is often the rate determining step in the biogas process. If the residence time is too short, less than 12
days, the organisms can be washed out because of too short time to reproduce. The residence time
depends on the reactor, if the methanogens are immobilized on a support material shorter residence
times can be used.
Process optimization
To get a good conversion, stirring is needed in order to obtain a good contact between the substrate
and the organisms and get an even temperature distribution in the tank.
For the biogas site to work well there is a need for right different environmental conditions for good
growth of the organisms. Examples are oxygen content and salt concentration. Every microorganism has
its own environmental conditions that need to be satisfied but they can adapt to the system
environment to some extent.
The bioprocess is best when the pH is neutral or between 7.0 and 8.5. To maintain this pH, a good buffer
(alkalinity) is needed and if high protein content is present in the substrate, with much nitrogen, high
alkalinity can be achieved due to the formation of ammonium bicarbonate.
When the microorganisms digest the substrate, the C/N (carbon/nitrogen) ratio has the highest
influence for the biogas production. If this ratio is too low, the process will stop due to inhibition by
ammonia. If the ratio is too high, the organisms’ nitrogen content will be too low. A good C/N ratio is
between 10-30, with an optimum between 15-25.
The temperature is a very important factor. The biogas process contains many kinds of microorganisms
that differ in the temperature range for optimal growth. Many microorganisms follow the growing
curves, see Figure 12. The biogas process is set after these temperatures, approximately around 35-40
ºC and 50-60ºC. At temperatures above 60 ºC, the activity of the methane producers is reduced and
accumulations of fatty acids in the process are obtained.
19
Figure 12 Growing phases for different organisms
When a microorganism uses a substrate, new cells are formed but also different toxic byproducts from
the cells are produced. Those byproducts cannot usually be used by that cell but can be a substrate for
another type of cells. That is what happens in the biogas process where different kinds of
microorganisms use each other’s byproducts.
In the absence of oxygen, fermentation or anaerobic respiration usually occurs. The end products from
this process are mainly acids, alcohols, hydrogen and carbon dioxide. If many electron acceptors are
available, the organisms that use the most energizing component will dominate. This is what happens in
the biogas process. In this process, the microorganisms that produce methane dominate. They use the
carbon dioxide as the final electron acceptor. The biogas process is very sensitive to oxygen. The
methanogenes are very sensitive and do not survive if they come into contact with oxygen and other
bacteria in the process.
During the anaerobic digestion, there is very little heat release from the organisms so the heat must be
supplied from another source because most of the energy released is obtained in the end product.
There are two temperatures used for the biogas digestion, which is around 37 ºC (mesophilic) and
around 55 ºC (thermophilic). Once the temperature is set, it should not vary more than +/- 0.5 ºC to get
the best results. If the temperature is lowered under the optimal temperature, the fermenting organism
will continue to produce acids and alcohols but the methane producing organisms will stop producing
and cannot take care of the fermentation products. The fermentation products will start to accumulate,
the pH will decrease and the process will stop.
The choice of temperature varies. At a higher temperature, the digestion is faster but it gets more
unstable because the optimal temperature for the organism is closer to the maximum temperature the
organism can tolerate. More heat is also needed to be supplied. Also ammonia is released at a higher
temperature that will slow down the organisms.
The process can be performed in different ways depending on which substrate is to be digested. During
continuous digestion, material is pumped in as slurry continuously, see Figure 13. It gives an even
20
amount of biogas production during the whole time. For this to work the slurry should not have a dry
substance of more than 15 % dry solids, otherwise the slurry cannot be pumped. The slurry is usually
pumped between 1-8 sets/day to the biogas process; this is called fed-batch production. The opposite is
batch production, where all the material is digested at the same time as it is maintained in the same
reactor during the whole process, see Figure 13. Afterwards, the material is taken out and new material
can be put in to be digested. In a batch production, when the material is degraded and new material is
put inside a certain amount of the digested material which contains sludge is used as inoculate. The
amount of inoculate varies and depends on the substrate, but it is between 20 and 70 % [20].
The decomposition rate is fast at the beginning of the process and decreases over time. It can be hard to
maintain an even digestion rate in this system if the dry matter content is high. Also the methane
production is greatest at the beginning of the production and declines over time.
Figure 13 Picture of a batch and continuous digestion
Either one or two digester chambers can be used for the different steps. The simplest case is when the
whole process occurs in one chamber. When two digester chambers are used, the process is divided into
two steps. In the first step only hydrolysis and fermentation take place and most of the acid production
is done. The second step is customized for methane production. This process is well suited when easily
digested products as monomer sugars, fatty acids and amino acids are present.
The temperature has a big influence on the residence time. In the mesophilic range, the residence time
is about 15 days or longer but in the thermophilic range it can be around 10 days. The methane
producing bacteria undergo mitosis during a time of 12 days, this means that the residence time of the
substrate cannot be lower than that value or otherwise, the bacteria will be washed out. If the methane
producing bacteria are immobilized on a support material, the residence time can be shorted down to
two days to get an economy in the system if the feed into the process is high. To use a system which has
a residence time for two days, the bottom products from the distillation are separated in a way that only
the liquid fraction will go for biogas production and the solids will be taken away and be sold as animal
feed if having a first generation ethanol production plant. If having a second generation ethanol
production plant, the solids will instead be used for production of heat and electricity.
21
Fertilizer
The degradation of organic material in the bioprocess gives biogas and if the remaining sludge has good
quality, it can be used as fertilizer. The different minerals that are available are concentrated in the end
product. Many factors contribute to the quality of the fertilizer like the type of substrate, pretreatment
method and process conditions.
The liquid fertilizer has a dry substance of 2-7 %. In some biogas plants the fertilizer is divided into a
solid and a liquid part. When the fertilizer is divided into two parts, the liquid part will contain more
nutrients and the solid part will contain more humus formation materials. Under the microbial
breakdown in a biogas process many different minerals are released. Bio fertilizer therefore contains
phosphor, potassium and manganese for the plant to take up. Bio fertilizer that will be used must
contain at least 2 kg/tone of ammonia and 3-4 kg/tone of total nitrogen content. The content of
nitrogen in the digestate can be increased if more proteins are added in the substrate. However, too
much protein is not good either because it will produce too much ammonia that will inhibit the methane
producers in the biogas process.
2.4 Possible integration steps There are different steps where it is possible to integrate the first and second generation ethanol
production, like the fermentation or the distillation step.
A good way to integrate the first and second generation plants is in the fermentation step. This can be
done because the same yeast is used and it operates at the same pH. The integration results in a higher
ethanol concentration from the fermentation and is better than running the both processes separately.
The advantage for getting higher ethanol concentration is that the energy consumption for the
distillation is reduced dramatically because less water needs to be removed. During the pretreatment,
several sugar degradation products that come from hexoses, pentoses and weak organic acids have
been shown to inhibit yeast and enzymes. Small concentrations of acids have a stimulating effect for
producing more ethanol. When mixing the pretreated slurry from the 1st generation plant with the one
from the 2nd generation plant, the inhibitor concentration will be diluted and the fermentation of
glucose from starch and cellulose will be improved [21].
Another process integration alternative that also works is to integrate the first and second generation
processes in the distillation step. The processes are run separately and combined in the distillation step
as the incoming feed to this step has the same physical properties. The first stripper acts to remove
unwanted products with low volatility, as ethanol and water mixture to concentrate the ethanol. In the
second distillation tower were only ethanol and water remains, the ethanol reaches the azeotropic point
which is the same for all integration possibilities.
22
3. Suggested process
3.1 Ethanol production with 1st generation, starch based production
Wheat grain composition used in the process is shown in Table 1 and is based on approximate figures and reactions from literature, see Table A1 Appendix I, it is also assumed that 3% of the fibers are soluble and labeled as fermentable sugars which react as if they were starch. The water content is assumed to be 15%.
Table 1 Dry solid composition of wheat grain composition
DS%
Starch 69
Fermentable sugars 3
Protein 13
Fiber 10
Fat 3
Ash 2
Glucose is not produced until the SSF step. The steps before SSF (see Figure 14) soften and break down the starch/amylopectin to shorter chains to lower the viscosity and so that glucoamylase can penetrate the structure more easily. The operation time for the plant is 8000 hours a year.
The pH in the first generation ethanol process is not taken into account in the calculations and is not adjusted during the process.
The overall reaction for the process is the following, for 72 DS% starch (69 DS% starch + 3 DS%
fermentable sugars). The theoretical yields for the reaction are shown in Table 2.
Table 2 Theoretical yields for the reaction from starch to ethanol. The yields are given in kg/1000 kg wet starch.
Yield kg/1000 kg
wet starch
Glucose 800
Ethanol 409
Carbon dioxide 391
The suggested process for the first generation ethanol production process is shown in Figure 14.
23
E-1
Silo
Wheat transportation
P-2
E-2
Hammer mill
Pre mixing tank Gelatinization/Liquifaction
35 % solids
vvxSSF
CO2 out
Warming stream to the pre-mix tank
Cold water
vvxPump
Stripper Rectifier
E-9
Condensor
Fuel ethanol
Bioreactor Bioreactor
Methane/CO2
Methane
Waste products
Glucoamlylose
Yeast
Water
Alfa-amylose
Centrifuge
H2O out
Upgrading plant
Solids for animal feed
Backset (10 %)
Condenser
Mixer
Water slurry
Slurry for biogas
Figure 14 Flowsheet for suggested 1st
generation ethanol plant
The wheat is kept in silos with an average temperature of 10°C. Before pre-mixing, the wheat grain is
milled in a hammer mill. This is done to gain more surface area for the enzymes to work. The milling is
important for the ethanol yield. [22]
3.1.1 Pre-Mixing step
Milled wheat flour, α-amylase, fresh water and backset water are mixed all together in a vessel and heated up to 55°C under stirring for 60 min, see Table A3 Appendix I. The slurry in the pre-mix vessel is mixed until a 35 wt% of solids is reached, based on the report from Novozyme. The α-amylase in the premix is purchased from Novozyme, the trade name is Termamyl SC/Liquozyme SC, the choice was based on information gathered and based on the specifications for the process. That is the temperature interval and the pH.
0.2 kg enzyme/ton starch is added in the pre mixing to lower the viscosity and make mixing with the enzymes better, even though the main work by the enzyme is done in the liquefaction step where the rest of the enzymes are added.
3.1.2 Gelatinization
After the premixing, the slurry is fed into the gelatinization vessel and heated up to 60°C. And then,
stirring takes place during 30 minutes for further swelling of the starch granules. The slurry contains 35%
of solids. This is an optimal composition that is used in first generation production in the industry and
recommended in many articles, see Table A4 Appendix I. It keeps the viscosity of the slurry acceptable
and result in high ethanol yields.
Keeping the temperature low keeps the starch granules from building coatings so that enzymes would
not be able to penetrate the structure and which leads to lower yields [22].
Heating is done by direct steam or heat exchanging with a hot process stream.
24
3.1.3 Liquefaction
In the liquefaction, the slurry is heated up to 90°C and stirred for 120 minutes. Heating is done by direct
steam or heat exchange. An additional 0.2kg α-amylase/ton starch is added. Retention times and
temperatures do not vary that much in the literature, see Table A5 Appendix I. The choice was based on
literature but also on the assumption that no other pretreatments were taken into account and the
shorter times in premixing and gelatinization.
3.1.4 Fermentation
Before the fermentation, the slurry needs to be cooled down. This is done rapidly to avoid
retrogradation. To save energy, the heated stream is heat exchanged with the cold water feed entering
the pre-mixer warming it up to 55°C.
The incoming stream to the fermentation enters one of the six fermentation vessels, where the slurry
then ferments for 50h. Each fermentor is working batch wise. The feed in and the feed out are adjusted
so the rest of process can run continuously. The fermentation process is chosen to be SSF as it results in
a higher yield, shorter reaction time and less process steps, according to literature, see Table A6
Appendix I.
The temperature is set to 31°C, as 30-33°C is the optimum. A temperature rise of 6-8°C is caused by the
reaction so 31°C is chosen to keep the temperature inside the yeast optimal temperature interval. For
cooling the mash, an external cooling jacket is used. The fermentation works best under yeast growth
which has an optimum at 28°C, so 31°C in the suggested fermentation and an adequate cooling will keep
the fermentation under good conditions.
The total yield for the suggested SSF process is set to be 84.3% of theoretical for starch to ethanol. For
calculations, the yields for starch to glucose and glucose to ethanol were divided into 98% and 86% from
given information and guides from Professor Guido Zacchi and Doctor Ola Wallberg from the
department of Chemical Engineering, Lund University. There have been documented higher total yields
but 84.3% was the best figure that could be found from full scale industrial ethanol plant with SSF, see
Table A2 Appendix I.
In SSF, both the enzyme for breaking up the oligosaccharides into glucose and the yeast to ferment
glucose into ethanol and CO2 are used simultaneously without inhibition of each other. The enzyme
chosen is Novozymes Spirizyme Plus® which is a glucoamylase with a dosage of 0.8 kg/ton starch and the
yeast used is Saccharomyces cerevisiae. The dosage is chosen to be 1.5 kg yeast /ton mash.
Except for the main reaction in the SSF, there are other byproducts formed where glycerol is the most
common and is taken into account. 4% of the glucose is assumed to be converted into glycerol [5]. The
byproduct does not affect any reaction or contributes to contamination and is later on used in the
biogas production. CO2 is of course a major product from the fermentation and is almost 1:1 with the
ethanol. The gas is purged out from the SSF.
3.1.5 Distillation
The slurry from the SSF is fed to the distillation unit, which was set to produce an ethanol stream where
the outgoing concentration was 93 w/w%. The distillation unit consists of two different columns. The
25
first unit is a stripper column consisting of 30 trays. The energy needed in the reboiler in the stripper is
generated in a heat and power plant, where bought pellets are burnt. To diminish the heat needed in
the reboiler in the stripper, the stillage from the stripper is used for heat exchange with the feed going
into the stripper.
The vapor stream from the stripper is condensed in the reboiler for the rectifier and then fed to the
rectifier column. The heat provided is used in the reboiler. The bottom product from the stripper is
centrifuged and some of the liquid is recirculated as backset to the pre-mixing step and the rest is taken
to the biogas plant. The solids with a dry matter content of 50% are used for cattle feed. These solids,
which are the remaining fermentation residues, are also called DDG (distilled dried grains). The DDG can
be used as animal feed after it has been dried. It is an excellent protein and energy source for cattle. It is
rich in residual yeast protein, minerals, vitamins, fibers and fat. It can also be used as fuel [23].
The product from the stripper, assumed only to contain water and ethanol, enters the rectifier in the
45th tray of a total of 50 trays. The distillate stream, having a concentration of 93 w/w%, is taken to the
dehydration step to remove the rest of the water. The dehydration step consists of molecular sieves.
This results in a 99.8 w/w% ethanol stream. 20 % of the ingoing ethanol was assumed to be used for
regeneration in the dehydration step. The bottom stream from the rectifier is taken into water
treatment since it is assumed to not contain more than water.
The calculations were performed with the flow sheeting program Aspen Plus®.
3.2 Ethanol production with 2nd generation, straw based production
3.2.1 Process overview
The straw based ethanol production process has a capacity of 200 000 tons straw/year, as defined by
NEAB. The working hours/year is assumed to be 8 000.
The wheat straw being used in this project was assumed to have a 90% dry matter content. The
composition of the straw is assumed to be as given in Table 3. This assumption is made from different
articles, where the composition of straw, has been analyzed. For cellulose, hemicelluloses, lignin and ash,
an average have been used. The leftover has been called residuals. The different compositions are listed
in Appendix II.
Since the majority of the hemicellulose is xylan and only a small part is arabinan and other sugars, the
hemicellulose is said to only contain xylan. Lignin consists of both acid soluble and acid insoluble lignin,
where the soluble part is small compared to the insoluble part. Therefore, lignin is assumed to contain
only acid insoluble lignin. The residuals are hard to determine. Proteins and fats are a part of the
residuals but also other components can be a part of it. In the project, assumptions are made that 50%
of the residuals are fat and 50% are proteins.
26
Table 3 Composition of wheat straw
Weight (%)
Cellulose 35
Hemicellulose 23
Lignin 23
Ash 5
Residuals 14
The process designed for straw based production is shown schematically in Figure 15. The straw is being
pretreated in a first step and then separated with a filter press where half of the liquid goes directly to
the biogas production and the rest goes with the solid part to the SSF. After the SSF, the products go to
the distillation step, where 93wt% ethanol is obtained in the distillate and the rest product, the stillage,
is fed to the biogas plant. Before the stillage enters the biogas plant, the solids are filtered and taken to
a heat and power plant for heat and electricity generation.
Straw transport
Straw in
HAC-Spray
vvxPretreatment
Steam in (20 bar)
Flash-vesselCondesor
Filter
SSF
CO2 out
Nutrient
Ammonia
Water
Enzyme
Yeast
vvx Pump
Stripper Rectifier
Condensor
P-15
H2O out
Dehydration
Fuel Ethanol
Bioreactor Bioreactor
Methane/CO2
Methane
Waste products
Filter Unit for solid
separation
Lignin for burning
Mixing tank
Upgrading plant
Kondensor
Heat for SSF
Heat
Water +volatiles
Filter unit
Figure 15 Schematic over 2nd
generation process
3.2.2 Pretreatment
As pretreatment method, steam pretreatment, of diluted acetic acid impregnated straw, at 20bar was
chosen. Using sulphuric acid, which is the most commonly used acid for this pretreatment method was
not an option since the sulphur affects the downstream process negatively. The sulphur inhibits the
27
methane production in the biogas plant [18]. Organic acids have the potential to achieve high yields as
using sulphuric acid. Organic acids also have the positive effect that smaller amounts of byproducts in
the pretreatment step are produced since it is not promoted by the organic acids, [24]. Since it is an
organic acid, it can be used in the downstream process as a substrate generating methane in the biogas
plant. Acetic acid is chosen because it is one of the last intermediates during the anaerobic digestion
where organic material is converted into biogas. The acetic acid being used is diluted to 1 weight-%
assuming that is gives the best yields in the pretreatment.
Since no data was found for pretreatment of wheat straw with this method, the parameters and yields
were taken from Linde where the pretreatment was made with sulphuric acid [25]. In Linde’s
experiments the highest yields for both cellulose and hemicelllulose after the pretreatment were
achieved when the material was heated up with steam to 190°C during 10 minutes. The yields of sugar
recovery from the pretreatment are almost the same but since Linde got more than 100 % yield, the
yields being used have been scaled down. The glucose recovery was scaled down and it was assumed
that no HMF was produced. This assumption was based on that smaller amounts of byproducts are
generated when using organic acids. Also less furfural is assumed to be generated compared to Linde’s
results. The assumed yields are listed in Table 4, where the pretreated wheat straw is divided into the
liquid part (hydrolysate) and the water insoluble solid part (WIS).
Table 4 Yields of sugar recovery in pretreatment where a stands for furfural
Yield (% of theoretical)
WIS Hydrolysate Byproduct
Glucose 96.0 4.0
Xylose 23.9 75.9 0.2a
Before feeding the straw into the pretreatment reactor, the straw is treated with the diluted acetic acid.
This can be done by soaking, where the material is impregnated in a batch and then pressed, or with
spraying, where the material gets sprayed with the acid. The soaking leads to an evenly impregnated
material. The spraying method involves lower amounts of chemicals and no pressing step [11]. This
makes spraying a more feasible alternative in big scale and was also chosen in the process. The spraying
is assumed to change the dry content of straw to 50%.
The steam used in the pretreatment plant was set to 20 bars (212°C). The energy losses in the
pretreatment reactor is said to be 10% of the steam required for heating up the material to 190°C.The
pressure is then lowered by a flash to 1 bar and the material is then cooled down to 35°C. The flashed
steam is condensed and taken to the biogas plant. The heat generated is used in SSF to heat up to and
maintain the needed temperature.
3.2.3 SSF
Before the slurry from the pretreatment is fed to the SSF, 50% of the liquid is taken away with a filter
press. The liquid is later mixed with the stillage stream from the stripper and taken to the biogas plant.
28
SSF has been chosen to be used as method to produce ethanol. Process data are based on Linde’s
results [25]. The temperature and pH was set to 35°C and pH 5 just as Linde. The residence time was set
to 55 hours since in Linde’s results the concentration of ethanol is almost as high as possible after that
time. Running for longer time is assumed to result in higher costs for the reactors than the extra amount
of ethanol gained.
In Linde’s experiments, the enzyme dosage was varied. Enzyme dosage of 12 FPU/g WIS was chosen
instead of the highest dosage of 14 FPU/g WIS since the difference between the results was not that big
and the enzyme costs get lower. Instead of Celluclast and Novozyme that Linde used, an enzyme
mixture, Cellic Ctech, which combines both enzymes that depolymerize the cellulose into cellobiose and
converts the cellobiose into glucose units, is used. This has an activity of 96 FPU/ml enzyme mixture.
The nutrients and the amounts of these are the same as in Linde’s study, 0.5 g/l (NH4)2HPO4 and 0.025
g/l MgSO4·7H2O. The yeast being used is normal baker’s yeast, which is cheapest. In this process there is
no yeast cultivation, all yeast that is being used is bought. The amount being added is decided to be 3 g/l
based on Linde’s experiment.
The WIS going into the reactor is set to 15% by diluting it with water, which is higher than the 5 % Linde
uses. Since the WIS is higher in this process, the yields are set lower, based on Linde’s results for various
WIS concentration in the SSF. [26]
The yield for the enzymatic hydrolysis was set to 60% of theoretical after the pretreatment and for the
fermentation 90% of the theoretical, which results in an overall ethanol yield in the SSF to 54%. Linde
achieved an overall yield of 62% after 55 hours.
Ammonia is used to set the pH to 5. In Linde’s experiments, sodium hydroxide was used but the nitrogen
in the ammonia is better for the downstream process. The nitrogen prevents the chance of too high
acidity in the biogas plant as an effect of the fermentation of the sugars in the anaerobic digestion [18].
3.2.4 Distillation
For information about the distillation and the following dehydration system see Section 3.1.5
Distillation. The difference between the first and second generation ethanol plant is that the bottom
stream from the stripper is filtered and the solids are taken to the heat and power plant and no liquid is
used as backset. All the liquid is instead used for biogas.
3.2.5 Solid removal
The solids are removed before the biogas plant because some of them are not digestible. The complex
structure of lignin makes it for example impossible to digest. Also the time to digest the stillage is
shortened when removing the solids. The solids can be used for heat and electricity generation. It is
assumed that the solids are fed to the heat and power plant with a dry content of 50%.
3.2.6 Heat and electricity generation
The lignin and other solids, which are removed before the biogas step, are used for generating heat and
electricity, see Figure 16. The process is based on Sassner’s model [27]. Like in this model, the substrate
is burnt and the flue gases are used to produce 91 bar steam at 470°C. Into the burner, air is provided.
29
The flow rate of air is set to be 25% higher than that required for burning. The steam is expanded in the
turbines from which 20 and 4 bar steam is taken out to cover the energy demand in the process. The 20
bar steam is used in the pretreatment reactor. The 4 bar steam is divided in different streams, some are
used to preheat the sprayed straw to 95°C before it enters the pretreatment reactor and some are used
to produce that amount of heat required in the reboiler in the stripper. The rest of the 4 bar steam is
expanded in a condensate turbine at 0.1 bar to generate electricity. The condensates from the
preheating of the feed and the heating of the reboiler are taken back to the system and are being mixed
with the stream from the last turbine and then reused together with fresh water to produce new 91 bar
steam. The isentropic efficiencies are 85% for the different turbines and the mechanical efficiencies are
97%.
Combustion
Lignin-ash
Boiler
Flue gases
vvx
Warm air
Flue gases out, 150 degrees C
Turbine 1 Turbine 2 Turbine 3
Steam 91 bar
E-7
E-8 Steam 20 bar
E-9
Electricity
vvx
Steam 4 bar
E-11
Water, 10 degrees C
E-12Return condensate, 4 bar E-13
Steam 20 bar
Air in, 15 degrees C
Figure 16 Heat and power plant
In this model, more electricity is produced than being used in the process and is assumed to be sold as a
co-product. An alternative would be not burning more solids than are needed to supply the process
energy demand. Then the rest can be sold as pellets.
3.3 Biogas The biogas process is divided in two phases. In the first part all the steps, except for the methane
producing step, are active. In the second phase the methane is produced. The ingoing material contains
a high amount of easily digestible substrates like sugar monomers, for which the two phase process is
recommended [18]. The process is continuous where the methane is taken out and the rest products
from the anaerobic digestion are also taken out. The residence time is assumed to be 48 hours because
it is assumed that the most of the material has been digested at that time.
30
The sludge being used is assumed to be 10% of the reactor volume. The organisms are reproduced
during the anaerobic digestion so a part of them are taken out with the rest products.
The temperature is set to 37°C, a mesophile process, and the pH to 7. There are wider varieties of
organisms in mesophile sludge than in thermophile sludge and with more kinds of organisms the
process is more stable to changes [18]. Ammonia is used to set the pH to 7.
Since it is living organisms that digest the material, some of the material is used as substrate for the
organism growth. The maximum yield for the organism for each reaction in the biogas process is
assumed to be the same as for yeast in fermentation of glucose to ethanol as no data has been found.
The yield for fat is assumed to be much lower (30%) than the rest substrates. This is assumed because
the fat would need longer time than 48 hours to digest maximal. The reactions that are assumed to
occur in the process are defined as below with the assumed yield in % of theoretical in parenthesis. HAc
stands for acetic acid.
Glucose → 2Ethanol + 2CO2 (95%)
3Xylose → 5Ethanol + 5CO2 (85.5%) [28]
2Ethanol + CO2 → 2HAc + CH4 (95%)
2Glycerol → 2HAc + CO2 + CH4 + 2H2 (95%)
Furfural + 4H2O → 2HAc + CO2 + 2H2 (84%) [28]
Fat + 4CO2 + 2H2 + 8H2O → 9HAc + 4CH4 (30%)
HAc → CH4 + CO2 (95%)
The biogas is then upgraded in a biogas plant, which is not designed here, while the liquid part from the
rest products is taken to water treatment. The solid parts can be used as fertilizers. The raw biogas
obtained in the digestion is not high quality enough if it is planned to sell this gas or use it as a fuel. Thus,
by the use of a biogas upgrading or purification process, contaminants in the raw biogas are adsorbed or
scrubbed increasing the methane concentration.
3.4 Integration of 1st and 2nd generation ethanol production The integration was done in the distillation step. This was chosen since it would be easier to combine
the two processes in this step since the first generation ethanol plant already exists when the second
generation plant is planned to be built. Then it is only the distillation unit and the downstream process
with the biogas plant and the heat and power unit that has to be rebuilt.
Another option is to integrate the two processes in the SSF but then the SSF also has to be redesigned.
The best advantage with integrating in that step is the decreased inhibitor concentration. But since the
inhibitors are already diluted in the process, where half of the liquid is removed before the straw based
slurry is fed to the SSF with fresh water, integration in SSF is not necessary from that point of view.
31
The process flowsheet for the integrated process is shown in Figure 17.
Straw transport
Straw in
HAC-Spray
vvxPretreatment
Steam in (20 bar)
Flash-vesselCondesor
Filter
SSF
CO2 out
Nutrient
Ammonia
Water
Enzyme
Yeast
Pump
Stripper Rectifier
Condensor
H2O out
Dehydration
Fuel Ethanol
Bioreactor Bioreactor
Methane/CO2
Methane
Filter Unit for solid
separation
Lignin for burning
Mixing tank
Upgrading plant
Kondensor
Heat for SSF
Heat
Water +volatiles
Filter unit
E-28
Silo
Wheat transport
Hammer millPremixing/gelatinization Liquefaction
SSF
CO2 out
Cold water
Yeast
Glucoamylose
Amylase
Cooler
Integration vvx
vvx
Methane
Figure 17 Flowsheet for the integrated ethanol and biogas process
In the first generation ethanol plant, heat and electricity is generated by burning pellets, straw or what
else being the best alternative to produce that amount of electricity and heat being needed. In both the
second generation ethanol plant and in the integrated plant, heat and electricity is generated from rest
products from the ethanol plant. If not integrating the two plants, the electricity and heat demand in
both first and second generation can be satisfied by the production of heat and electricity from the heat
and power plant in second generation ethanol production plant.
4. Results and analysis In this project, a simulation has been made for a first generation plant, for a second generation plant
and for a plant working with both processes in order to see if there are advantages when integrating the
first generation with the second generation in the distillation step.
32
The energy demands have been calculated by Aspen in the most cases, except for the pretreatment
reactor in the 2nd generation ethanol process. This has been calculated with the flow of the 20 bar steam
(8300 kg/h) and the enthalpy of vaporization (1890 kJ/kg). According to the calculations performed, it
can be seen in Table 5 that the total primary energy demand in the integrated case is higher compared
to the sum of energy demand when considering two individual plants. However, this difference is not
very big and as it also can be seen, the system which demands more energy is the second generation.
This is due to the fact that high amount of water is needed in second generation ethanol process and
thus, higher flows must be taken into account which leads to high energy consumption when heating is
needed as in the stripper. From the results obtained in Table 5, one would say that there is no point in
having an integrated plant but an economic evaluation is needed before any conclusions can be
withdrawn.
Table 5 Total primary energy demand comparison
Energy demand integrated 29.7 MW
Energy demand 1st 13.7 MW
Energy demand 2nd 15.2 MW
Energy demand 1+2 sep 28.9 MW
Moreover, the step which is most heat demanding is the stripper step. The pretreatment in the second
generation plant is also very energy demanding because it uses vapor at 20 bar, see Table 6.
Table 6 Primary energy demand comparison by process steps
Primary energy demand (MW)
1st generation 2nd generation integrated
Heater in liquefaction step 1.5 1.7
Preheating of sprayed wheat straw 2.8 2.8
Pretreatment 2nd 4.4 4.4
Stripper 12.2 8.0 20.8
Total (MW) 13.7 15.2 29.7
Heat integration has been performed in the whole integrated plant. Stillage preheats the feed into the
stripper and the fresh water needed in the pretreatment of the first generation plant is heat exchanged
with the slurry after the liquefaction step. Also the heat needed to heat up the SSF in the second
33
generation plant is obtained by heat integration, where it is heated up by condensate the flashed steam
from the pretreatment step in the second generation plant. But probably more effective heat
integrations could be done. Lots of streams need to be cooled down with water and the heat generated
by condensate the flashed steam is more than is needed in the SSF. For energy demand and energy heat
release, see Table 7.
Table 7 Energy demand and release in the different process steps. The underlined values are those steps where the energy demand is satisfied with 4 and 20 bar steam. The other values are secondary energy used in the process. All the process steps marked with 1
st is meaning that it is a part of the first generation ethanol process, while 2
nd means that it is a part of the 2
nd
generation ethanol process.
Energy demand and release (MW)
1st generation 2nd generation integrated
Heater in liquefaction step, 1st (74 -
> 90°C)
1.5 1.7
Cooler before the SSF, 1st (40 ->
31°C)
- 0.8 - 0.8
Preheating of sprayed wheat straw,
2nd (10 -> 95°C)
2.8 2.8
Pretreatment, 2nd 4.4 4.4
Condenser, flashed steam 2nd - 5.0 - 5.0
SSF, 2nd 1.5 1.5
Cooler before filter the pretreated
slurry, 2nd (100 -> 35°C)
- 2.2 - 2.2
Stripper 12.2 8.0 20.8
Condenser after stripper - 11.5 - 3.2 - 11.1
Reboiler, rectifier column 9.2 1.7 10.7
Condenser, rectifier column - 10.0 - 1.9 - 11.8
Cooler before biogas plant 0.0 - 3.0 - 3.3
As it can be seen, there is a lot of energy that could be used from the condensed flash steam as well as
the pretreated slurry could be used to preheat some other stream diminishing the amount of cold water
and primary energy (steam) needed. If an efficient plant is desired, the distillation step should be
34
treated separately, looking more into the details in this step in order to optimize the energy demand.
The high energy consumption in this step is due, among other factors, that a lot of heat is required in
the stripper column because the feed does not enter the tower at its boiling point. Also the number of
trays and other process design parameters should be investigated into more detail to optimize the
energy demand in the towers.
When comparing the different steps between the integrated plant and the non-integrated, see
Appendix III for results, the results obtained were those expected. The data obtained from the
pretreatment and SSF in both first and second generation were the same as those obtained from the
integrated plant. This is obvious as it is the same process. Nevertheless, when the integration is done,
that is in the distillation step, values differ quite a lot. Of course the flow downstream increases but it is
mainly because of the high amount of water required which, as said before, turns into higher energy
consumption.
The same amount of ethanol is produced, 15.5 ton/h, when integrating the two different plants
compared to the case where plants are not integrated. The ethanol mainly comes from the first
generation plant, 13 ton/h, compared to the 2.5 ton/h from the second generation plant. These values
are the ethanol flows after dehydration. On the other hand, the biogas production is a bit higher when
integrating than if the two plants are considered. 8.1 ton/h is the biogas flow when integrated, 2.8 ton/h
from first generation and 5.0 ton/h from second. All biogas has 26% in weight in methane and 74% in
carbon dioxide, see Table 8. The higher amount of biogas produced when the two ethanol plants are
integrated is due to a design specification in the simulated process. It should be the same value as
considering the two processes working separately.
Table 8 Ethanol and Biogas production comparison
Ethanol-biogas production
1st generation 2nd generation integrated
Ethanol produced (ton/h) 13.0 2.5 15.5
Biogas produced (ton/h) 2.8 5.0 8.1
Mass fraction of methane 0.26 0.26 0.26
Mass fraction of carbon dioxide 0.74 0.74 0.74
From a mass and energy balances point of view, seems not to be any reason to integrate the first
generation and second ethanol production as the results obtained from the integrated plant, are almost
the same as considering the two plants working separately. However, the economical study will
determine if it is reasonable or not to integrate the two plants once it is shown that it is possible to
perform the integration. Before taking into consideration if the integrated plant should be built, other
35
studies should be performed, specially heat integration improvement and maybe integration in other
steps.
The heat and electricity produced in the process by burning the solids are given in Table 9. When the
two processes are integrated, both solids from the 1st and the 2nd generation ethanol process are used.
When not integrating, only the solids from the 2nd generation ethanol process are used. The column 2nd
+ 1st shows the products when the plant in the second generation is used for heat and electricity
generation for both the first and the second generation ethanol plant.
Table 9 Production of heat and electricity from burning solid residues. The electricity in brackets is the net electricity that can be sold. The first column is generation of heat and electricity from 2
nd generation plant to 2
nd generation. The second column
is generation of heat and electricity from 2nd
generation plant to 2nd
and 1st
generation. And the last column is the generation of heat and electricity from integrated plant.
Total energy production (MW)
2nd generation 2nd + 1st Integrated
20 bar steam 4.4 4.4 4.4
4 bar steam 10.8 24.6 25.2
Electricity 15.4 (10.6) 11.2 (0.4) 26.8 (17.0)
TOTAL (MW) 30.6 40.2 56.4
As can be seen, less electricity is generated when the heat demand for the stripper is supplied in both
the first and the second generation ethanol plant. Moreover, when not integrating the processes, the
solids from the 1st generation ethanol plant are sold as pellets which are not taken into account when
comparing the energy production and the energy efficiency. The pellets are calculated within the
economy analysis.
The energy efficiency for the plant is calculated as the ratio between the heat of combustion (lower
heating value) in the products divided by the heat of combustion in the raw material, see Appendix IV.
The electricity is used as it is and has not been corrected with any factor to have a comparable value
versus the heating value. The efficiencies for the different cases are shown in Figure 18.
36
Figure 18 Energy efficiency for the different cases based on lower heating value. The electricity is not recalculated to heat required for products. Calculated from the Appendix IV.
In the two cases, integrated process and separate plants; the integrated plant presents higher energy efficiency, 54.0% compared to 48.2%. The energy efficiency in the 1st generation plant is high (52.2%) due to the fact that the supplied heat and electricity are taken into account but not as the lower heating values for the pellets but as the energy needed in electricity and heat (6 MW + 13.7 MW) in the calculations. It should in reality be smaller than the case with separated plants.
5. Economics and sensitivity analysis
5.1 Economy The economical analysis was mainly done to investigate if an integration of first and second generation
ethanol production is profitable. Parameters that are taken into consideration are the amount of
ethanol, biogas and electricity that can be produced by building the second generation process. Three
scenarios were considered:
1. First generation ethanol and biogas production alone.
2. Second generation and first generation not integrated but with joint heat and biogas
production.
3. The integrated process.
The economical analysis is based on an estimation of the cost of the equipment and operating the plant
as well as an investment analysis. Calculations of equipment size, energy and raw material usage as well
as amount of products produced were based on figures simulated in Aspen Plus.
Equipment cost The estimation on major equipment in the process was based on calculations using the Ulrich method.
Some parts of the plant are not possible to estimate with Ulrich, that is why price proposals from similar
equipment are taken into account. The Ulrich method is based on 1982 figures, for actual prices the
0%
10%
20%
30%
40%
50%
60%
1stgeneration
2ndgeneration
1st + 2nd Integrated
Ene
rgy
eff
icie
ncy
Electricity
Methane
Ethanol
37
Marshall and Swift Equipment Cost Index is used where I1982=746, I2009,2ndQ=1462.9 [29] and the exchange
rate is 7 SEK/USD (2009 2nd Q) [30].
(
)
∑
The prices are then adjusted by multiplying with factors for contracting and unforeseen cost
(fcont/unfor=1,25), for extra sizing for equipment (fsize=1,15) as well as for help equipment, plant buildings,
surrounding etc. (frest=2,85). CBM is the estimated price for the equipment and is specifically calculated
from Ulrich method. For equipment based on price proposals the prices are only multiplied with the
factors.
By this method the following costs in Table 10 were calculated for building the plant(s) and the amount
of equipment needed (n):
38
Table 10 Equipment cost and amount for 1st, 2nd and integrated plant
Equipment n Price 1st generation
(MSEK)
n Price 1st+2nd generation
(MSEK)
n Price Integrated process
(MSEK)
Silos 3 4.6 5 7.3 5 7.3
Pre mixer 1 10.5 1 10.5 1 10.5
Gelatinization 1 7.1 1 7.1 1 7.1
Liquefaction 2 18.6 1 18.6 1 18.6
SSF (1st) 6 24.7 6 24.7 6 24.7
Acetic Acid pretreatmenta
- - 1
86 1
86
Filter - - 1 5 1 5
SSF (2nd) - - 7 31.4 7 31.4
Stripper 1 12 2 22.1 1 15.8
Distillation tower 1 17 2 32.1 1 19.5
Ethanol purifierb 1 20 2 40 1 20
Lignin separatorc - - 1 49 1 49
Bioreactors 1 3.1 1 9.3 1 9.3
Clarifier 1 5.5 1 5.5 1 5.5
Methane purifierd 1 20 1 20 1 20
Minor (HE, condenser, mills)
- 4.3 - 4.8 1 4.3
Total equipment cost
343.1 373,4 334
Total plant cost, (after multiplying with the factors, except heat and power plant)
604 1536 1368
Heated and power plante
1 200 1 250 1 400
Total investment: 804 1786 1768
a Price for the equipment for pretreatment of wheat straw is based on Andritz [31] price proposal where
impregnation with acetic acid, heat exchanging and flashing is included, which is 85 MSEK.
b Equipment cost for ethanol purification is estimated to 20 MSEK, based on information from Robert,
Jianlong Chemical.
c Price for the lignin separation equipment is from a Master Thesis from LTH found and recalculated by
Professor Guido Zacchi, which is 45 MSEK.
39
d The methane purifier equipment cost estimated to 20 MSEK, based on consultation with Doctor Tobias
Persson (Process manager for Biogas at Malmberg Water AB, Åhus).
e Heat and power plant is an estimation based on (referens per sassner page 69). The boiler used in the
Sassners process had an effect of 67 MW and cost roughly 300 MSEK. The first generation has a boiler
size of 25 MW (200 MSEK), second generation has 56 MW (250 MSEK) and the integrated has 96 MW
(400 MSEK).
Chemicals, raw material and products Prices for chemicals and raw materials can be seen in Table 11 and were taken from different sources
though it was not possible to find all prices summarized in one source. Therefore, prices may fluctuate
but for more accuracy, prices are no older than three years. Annual cost and income could be calculated
for each plant(s) based on flows (kg/h) from Aspen Plus and is seen in Table 12.
Table 11 Prices for chemicals and raw materials
Substance SEK/kg
Acetic acid [32] 5.61
Ammonia [32] 12.52
Amylas LIQUOZYME SC DS [33] 37.60
ENZYME 30.00
Ethanol [34] 6.36
Glucoamylas, SPIRIZYME PLUS [33] 29.88
Methane 13.44
Nutrient [27] 31.00
Wheat [35] 1.00
Wheat Straw [36] 0.69
Yeast [37] 24.00
Pellets from solids [38] 0.8
40
Table 12 Annual cost/income for consumed chemicals or raw materials and the products.
1st
generation 1st
+2nd
generation Integrated
ton/year MSEK/year ton/year MSEK/year ton/year MSEK/year
CONSUMED
Ammonia 910 11.4 1 817 22.8 1 817 22.8
Amylas, LIQUOZYME SC DS
89 0.6 88 0.6 88 0.6
ENZYME - - 18 0.5 18 0.5
Acetic Acid - - 3 200 17.9 3 200 17.9
Glucoamylas, SPIRIZYME PLUS
179 5.4 179 5.4 179 5.4
Nutrient - - 461 14.3 461 14.3
Wheat 354 481 354.5 354 481 354.5 354 481 354.5
Wheat Straw - - 200 000 138,0 200 000 138.0
Yeast 1 327 31.9 3 962 95.1 3 962 95.1
Total Cost - 403.8 - 649.1 - 649.1
PRODUCTS
Ethanol 103 295 657.2 122 872 781.8 121 813 775.0
Methane 5 866 78.9 16 514 222,0 17 074 229.6
DDG 45875 36.7 45875 36,7
NET INCOME 369.0 391.4 355.5
Other costs estimations were done for electricity and water consumption for the processes. The annual
water consumption can be calculated from the Aspen simulation. On the other hand, it is not possible to
calculate the electricity consumption (kWh/year) for the 1st generation plant, while 1st+2nd generation
and the integrated plants are self sufficient with both heat and electricity. An approximation is made
based on Murphys & Powers [39]here the size of the “Murphy” plant is similar to the one investigated
for first generation in Table 13:
41
Table 13 Comparison with investigated plant and Murphys & Power
Murphys & Power Investigated 1st generation Fraction (Invested/Murphys)
Wheat, (ton/year) 403 200 354 500 0.88
Water, (l/year) 603 722 300 460 800 000 0.76
Produced ethanol, (l/year) 150 930 600 103 300 000 0.68
Electricity requirement, (MWh/year)
48 300 n.a
Thermal requirement, (MWh/year)
362 200 109 600 0.5
The requirements and ethanol production for the reference plant are higher at the same time as the
thermal requirement is much lower for the investigated plant. So the electricity consumption for the
investigated plant is set to be 48 000 MWh/year for 1st generation plant. The need of thermal energy for
the 1st generation plant is 109 600 MWh/year, which are supplied by the heat production plant by
burning wood pellets or oil. While 1st+2nd generation and the integrated plant are producing an
electricity surplus that is later on sold and can be seen in Table 14.
Price for pellets is set to 550 SEK/MWh [40], which is the price for producing thermal energy and
electricity in the heat and power plant in first generation plant. The electricity surplus is assumed to be
sold for 1000 SEK/MWh.
Table 14 Income and cost for electricity
Electricity consumption (MWh/year)
Electricity surplus
(MWh/year)
Cost for electricity production
( MSEK)
Income for electricity
surplus (MSEK)
1st generation 48 000 - 157.6 -
1st+2nd generation
89 600 3 200 - 3.2
Integrated 78 400 136 000 - 136.0
The price for water is set to 20.8 SEK/m3[41], which gives the following cost in Table 15.
Table 15 Water cost
m3/year Cost (MSEK/year) Fixed price (MSEK/year)
1st generation 460 800 9.58 0,15
1st+2nd generation 1 364 640 28.38 0,15
Integrated 1 373 500 28.57 0,15
42
Operational plant costs and investment analysis A method based on Table 16 was used for the calculations of operational costs for the plant(s) [42]. The
annual cost is later used for the investment analysis. In all three scenarios the factors are the same.
Table 16 Costs for operational plant cost and costs estimations
Description of cost Factors
Frozen capital
Storage of raw material + chemicals
Delivery once a week
Storage of products Delivery twice a week
Direct variable costs
Chemicals + Raw material Cost of consumed chemicals and raw materials
Energy and water Cost of electricity, thermal energy and water consumption
Operators (Staff) 8000 h/year; Salary 110 SEK/h + 50% in workers taxes; 10 operators
Operator managment 15% of operator costs
Laboratory staff 15% of operator costs
Indirect variable costs
Overhead for personal 70% of operator costs for shift personal
50% of operator costs for day personal
Administration 25% of overhead for personal
Annual plant costs
3% of annual plant costs for license
5% of annual plant costs for selling & distribution
2% of annual plant costs for research and development
5% of annual plant costs for maintains and reparations
15% of maintains and reparations for spare parts
Total annual plant operation cost
All three options have an annual surplus. By doing an investment analysis it is possible to make a
conclusion of which option is most profitable. The annuity method is used to see the real annual profit
by taking the investment cost and interest into account. The economical life time (calculated time to pay
off the investment) is 15 years (N) with an interest of 8% (X). There is no rest value of the investment
after 15 years and the net payment each year is constant. The annuity factor is multiplied with the
investment cost to get the annual capital costs which are taken off from the annual net income, this
gives the real annual profit as in Table 17.
43
Table 17 Annual income, plant cost and profit
Ethanol income (MSEK)
Biogas income (MSEK)
Pellets from solids
income (MSEK)
Electricity income (MSEK)
Operational plant cost
(MSEK)
Investment cost
(MSEK)
Real annual profit
(MSEK)
1st generation
661.4 78.2 36.7 - 632.1 804.2 50.3
1st+2nd generation
788.6 218.1 36.7 3.2 845.4 1785.9 -7.0
Integrated 788.6 226.4 - 136.0 845.7 1768.1 99.9
The result is that building only the 1st generation plant makes the investment profitable.
Depending on the low investment cost and the high ethanol output.
Building the 1st and 2nd generation plants parallel is unprofitable depending on very high
investment cost and no extra gain from the integrated process.
Building an integrated process gives a profitable investment, depending on the high output of
products compared to a relative low investment cost.
So an extra investment cost to build the 2nd generation ethanol production is recommended considering
economical profit. While the 1st generation process will produce satisfying amount of ethanol, the 2nd
generation process will double the biogas income and replace the high energy consumption cost when
1st generation is operated alone, even making a high profit (10% of total income). Compared to the pay-
off time (Investment/net annual income), the 1st generation plant would have 5.1 years and the
integrated plant would have 6.4 years. From an economical point of view the pay-back times are
basically the same. The interesting though is the real annual profit which is twice higher for the
integrated plant, even though the investment is twice as high then for the 1st generation plant it is still
more profitable to integrate. Operational plant costs would of course be higher for the integrated
process, but the higher net income makes worth the extra cost.
The separated 1st and 2nd generation is not a profitable option. Depending on the extra investment made
in building double distillation and ethanol purifiers. While producing and extra income out of pellets
from solids and the same amount of ethanol and methane as in the integrated plant, the need for
energy in the separated process is just too high to make a profit. The pay-of time for the separated plant
is much higher than for the two others, which is 10.2 years.
5.2 Sensitivity analysis Sensitivity analyses have been performed for the WIS concentration and enzymatic yields in the SSF of
the straw based process in the integrated plant. Analyses have also been performed for the price
changes of straw, methane, ethanol, electricity, DDG and yeast.
5.2.1 WIS concentration in the SSF
How much the feed, going into the SSF in straw based process, has to be diluted is unsure because there
are not any available data for full scale production. Increasing the WIS can damage the equipment since
44
there are more solids going into the SSF. Because of less dilution of the feed, the risk of inhibition of the
reactions will increase. Furthermore, the stirring is affected since more power is needed when there are
more solids. When lowering the WIS, not so much energy for stirring is needed but much more water
has to be added which has then to be removed in downstream processing. In our sensitivity analysis, we
varied the WIS content from 5% to 20% and investigated how this affected the energy demand in the
SSF, the distillation unit, the amount of ethanol and biogas obtained and the electricity generation for
the integrated process. The results are shown in Table 18.
Table 18 Results for various WIS concentrations in the SSF in the integrated process. The line marked in grey shaded is the base case
WIS (%)
Energy demand
in SSF (MW)
Energy demand in stripper (MW)
Ethanol 99.8% (ton/h)
Biogas (ton/h)
Electricity to sell (MW)
(Total amount
produced electricity
(MW))
5 6.7 34.7 15.2 8.7 15.7 (25.5)
10 3.3 25.8 15.3 8.4 16.4 (26.2)
15 1.5 20.8 15.3 8.1 17.0 (26.8)
20 0.9 19.1 15.3 7.8 17.3 (27.1)
The same amount of ethanol is assumed to be produced. However the volume of the SSF is changed
with the WIS in the way that lower WIS concentration means bigger volume of the reactors. The biogas
product is larger with a lower WIS. This depends on the fact that the solids leaving the filter before the
biogas plant have a dry content of 50% and with more water in the SSF, less of the components in the
liquid goes with the solids to the burning plant and can instead be used in the biogas production.
However, less electricity is produced.
The energy demand in the SSF increases with decreasing WIS concentration like in the distillation unit,
mostly the stripper. The increase depends on the increase of water. The higher energy demand in the
SSF is supplied by the heat generated by condensing the flashed steam after the pretreatment unit like
before. The stripper is supplied with heat from the 4 bar steam produced from the heat and power plant.
Increasing the energy demand means less produced electricity that can be sold.
As seen in Table 19, an increase in WIS content is profitable due to the lower investment and water cost.
The change in the income is not affected by the lower methane production since more electricity is
produced then. A higher WIS content seems to be recommendable from this analysis but the risks with
higher solid contents in the SSF need to be investigated in lab scale.
45
Table 19 Economical analysis depending on WIS content
WIS Income products (MSEK/year)
Water cost (MSEK/year) Investment cost (MSEK/year)
Real annual profit (MSEK/year)
5 1 158 59 411 34
10 1155 39 360 78
15 1152 28 334 99
20 1145 24 324 103
5.2.2 Enzymatic yield
Like in the former sensitivity analysis, the enzymatic yield is unsure in the SSF in the straw based process
because of non existing data for full scale production. Also data of how the yield is affected with a WIS
of 15% is missing since most lab scale processes have a WIS of less than 10%. In our sensitivity we varied
the enzymatic yield from 50% to 80%. In this sensitivity analysis, the same parameters as in the
sensitivity analysis of WIS content have been checked. The results are shown in Table 20.
Table 20 Results for various enzymatic yields in the SSF in the integrated process. The line marked in grey shaded is the base case
Enzymatic yield (%)
Energy demand in SSF (MW)
Energy demand in stripper (MW)
(Energy demand in distillation unit
(MW))
Ethanol 99.8%
(ton/h)
Biogas (ton/h) Electricity to sell
(MW) (Total
amount produced
electricity (MW))
50 1.6 20.6 15.1 7.8 18.3 (28.1)
60 1.5 20.8 15.5 8.1 17.0 (26.8)
70 1.4 21.0 15.9 8.2 15.7 (25.5)
80 1.3 21.2 16.3 8.5 13.3 (23.1)
The results in Table 20 show that the energy demand does not vary so much with varying the enzymatic
yield. The amounts of ethanol and biogas produced increase with increasing yield while the electricity
produced decreases.
As seen in Table 21 an increase in enzymatic yield is profitable, which was expected after the discussion
above, due to the increased income from the products. This means that the biogas and ethanol pays off
better than electricity. A higher yield means more enzymes are needed. The higher income shows that it
is possible to have more enzymes in the process to gain higher amounts of ethanol and biogas.
46
Table 21 Economic analysis depending on enzymatic yield
Enzymatic yield (%)
INCOME products (MSEK/year)
50 1133
60 1152
70 1164
80 1183
5.2.3 Product price
Both the first generation process and the integrated process are economically profitable. Analyses have
been made below on different price changes for the sources of income and different cost of
expenditures to compare and see when the first and integrated process will be more profitable than the
other.
The major incomes for the processes are summarized as:
o Ethanol (largest income in both processes)
o Methane
o DDG (only first generation process)
o Electricity (only integrated process)
The major costs for the processes are summarized as:
o Wheat (highest cost in both processes)
o Straw (only in the integrated process)
o Yeast
o Electricity demand (only first generation plant)
Methane is an interesting product to examine, it showed that the integrated process is much more
sensitive to methane price fluctuations and the investment is as profitable as the first generation
process when the methane price is between 10-11 SEK/kg (if the methane price is fixed in the first
generation process). A break even is when the price on methane is 9 SEK/kg, see Figure 19 and then
both processes are as profitable. Such price decrease is relatively high (35% decrease) but is possible.
47
Figure 19 Real annual profit compared to price on methane
Comparing the fluctuation on ethanol price in the two processes is not interesting though ethanol is the
highest income in both process and would vary in the same way. More interesting was to examine at
what price the ethanol from the first generation and the integrated process become unprofitable and at
what price the integrated process becomes as profitable as in the first generation. The result can be
seen in Figure 20. To make the integrated process as profitable as in the first generation, the ethanol
price should decrease from 6.36 SEK/L to 5.95-6 SEK/L, which is a very possible scenario that should be
considered. The price of ethanol has to decrease to 5.55 SEK/L and to 5.87 SEK/L to make the integrated
process respectively the first generation process unprofitable. An expected decrease considering the
economical situation in the world, but it also shows that the integrated process is less sensitive to
ethanol price.
Figure 20 Real annual profit compared to price on ethanol
Wheat straw represents one of the major costs in the integrated process, the original price for wheat
straw is 0.69 SEK/kg. If the price of wheat straw would increase a break even between first generation
and the integrated process will occur at the price of 0.9 SEK/kg. The integrated process would be
unprofitable if the price would increase to 1.12 SEK/kg, see Figure 21. Such increase is unlikely at the
time considering that there is a lot of wasted wheat straw, but price can increase if ethanol from wheat
straw production will increase and price conflict will become an issue.
0
20
40
60
80
100
8 9 10 11 12 13 14
Re
al a
nn
ual
pro
fit
(MSE
K)
Methane price (SEK/kg)
1st
Int
1st (fix)
-40
10
60
110
160
5 5,5 6 6,5 7 7,5Re
al a
nn
ual
pro
fit
(MSE
K)
Ethanol price (SEK/L)
1st
Int
Int (fix)
48
Figure 21 Real annual profit compared to price on wheat straw
When using pellets as a fuel in the first generation and when electricity is one of the major costs, the
process gets sensitive to price change. Pellet price (electricity price) has to increase to 0.83 SEK/kWh to
make the first generation unprofitable and decrease to 0.3 SEK/kWh to be as profitable as the
integrated process, see Figure 22. While in the integrated process the electricity is one of the major
incomes. A change in the selling price from 1 SEK/kWh to 0.27 SEK/kWh would make the investment
unprofitable but is very unlikely to happen, while a decrease to 0.67 SEK/kWh will make the investment
as profitable as first generation process, see Figure 23.
Figure 22 Real annual profit compared to electricity selling price
-5
15
35
55
75
95
0,65 0,75 0,85 0,95 1,05 1,15
Re
al a
nn
ual
pro
fit
(MSE
K)
Wheat straw price (SEK/kg)
1st (fix)
Int
0
20
40
60
80
100
0,25 0,45 0,65 0,85 1,05
Re
al a
nn
ual
pri
ce (
MSE
K)
Electricity selling price (SEK/kWh)
Int
1st (fix)
49
Figure 23 Real annual profit compared to electricity price if buying pellets as fuel
To make the process unprofitable, the yeast price must be doubled in the integrated process. However,
the first generation can withstand an increase in the yeast price but not to a large extent. Further
examination is not made though such price change is mostly unreal.
The income from DDG in the first generation is one of the major incomes. After some examination, it is
concluded that the selling price for the DDG, which is 800 SEK/ton, has to increase to almost 2000
SEK/ton to be as profitable as in the integrated process, which is very unlikely. Even if no DDG is sold,
the first generation would still be profitable.
6. Discussion and conclusions To integrate the first generation and second generation ethanol production processes in the best and
economically feasible way is not an easy part because it has never been done before. The second
generation process exists only in the pilot scale stage and has not yet been commercialized.
In the second generation process there are many ways to pretreat the lignocellulosic material. It is
important to choose the right pretreatment method as it is the most costly part of the process. It was
thought that steam explosion with dilute acetic acid was the most suitable setup. The acetic acid chosen
is also an intermediate in the biogas process so it will not harm the process downstream as other acids
might do and it is converted to biogas.
The integration of the processes can be done in two places of the processes, either in the fermentation
step or where the streams to the distillation are connected. To integrate in the SSF might be a good
choice since there have been reports that say it would facilitate the ethanol production due to the yeast
cells at the right inhibitor concentration will increase the ethanol concentration. In our study we
assumed that the inhibitors are already diluted before the SSF in the straw based process so it was not
necessary in this case. Due to these conclusions the integration is done in the distillation step.
When integrating the processes, the energy demand is higher than if considering having the two
processes separately. The stripper column is the element that requires most energy. However, the heat
-5
15
35
55
75
95
115
0,15 0,35 0,55 0,75
Re
al a
nn
ual
pro
fit
(MSE
K)
Electricity buying price (SEK/kWh)
1st
Int (fix)
50
integration study should be improved to lower the energy demand, especially in the distillation step.
The production of ethanol mainly comes from the first generation process while the biogas comes
mainly from the second generation process. When looking at the energy efficiencies, the best result was
obtained when integrating the two processes due to the larger amount of electricity produced.
From an economical point of view, building the first generation plant is a profitable investment due to
the low investment cost and the high output of ethanol. The income for the methane and the dried
destillers grain (DDG) is much lower than for the ethanol which makes the plant sensitive and
dependent on the ethanol selling price and the cost for wheat, which is the main cost in the process.
This results in an economically profitable plant but unstable due to market price fluctuations on ethanol
and wheat.
Building a second generation plant and integrating the process in the distillation step showed to be the
most profitable option. Even though the investment cost would be twice as high as for the first
generation plant, the production of electricity would become a major income (almost 4 times higher
than for DDG which is no longer produced). Methane income would also be more significant. This makes
the process less sensitive to market price fluctuations. This means that the integrated process is more
economically stable and more economically profitable, depending on the spread of costs and incomes.
Even if the factors in the operational plant costs (Table 16) would be higher, depending on more
operators and more costs for maintenance because of new technology, the investment would be as
profitable as the first generation plant.
Building a second generation plant parallel to the first generation plant and integrating in the biogas
showed to be an unprofitable investment. Even if the investment cost is almost the same as for the
integrated process and incomes for methane and ethanol and the operational plant costs are also
almost the same. The main reason that the investment is not profitable is the surplus electricity which is
practically zero. If more electricity could be produced by perhaps more heat integration, the parallel
plant would also be a profitable option.
51
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55
Appendix
Appendix I Table A1 Content of wheat from literature
Novozyme [5] HGCA [2] Article 1 [43] Article 2 [44] Suggested
content
Water % 15 15 12.4 14 15
Starch DS% 67-70 69 68.5 75.6 69
Fermentable sugars DS% 1-2 3 - - 3
Biomass DS%
Protein 12-14 11.5 11 15 13
Fiber 10-13 11 - 7 10
Fat 2-3 2.5 - 3
Ash 2 2 - 2.4 2
Lignin - 1 - - -
Table A2 Yields in 1st
generation SSF from literature
Article 5 [48] HGCA [2] Article 3 [45] Article 4 [46] Suggested values
SSF yield [%] 83.38 84 87-89 82 84.3
Starchglucose - - - - 98
Glucoseethanol - - 90 - 86
Table A3 Literature values for the pre-mixer
Agro [47] Alcohol book [22] Novozyme [5]
Temperature (°C) 58 60 <60
Time (min) ~60 - 0-30
Enzyme (kg/ton
starch)
0.4-0.6 Amylase 0.2 Termamyl 120L Can be added
Table A4 Literature values for gelatinization
Agro [47] Alcohol book [22] Novozyme [5]
Temperature (°C) 73 58-64 58-65
Time (min) 120 6-120 -
56
Table A5 Literature values for liquefaction
Article 3 [45] Agro [47] Alcohol book [22] Novozyme [5]
Temperature 95 89 85 85-95
Time (min) 120 120 60-120 30-120
Enzymes (kg/ton
starch)
0.2 (Termamyl 120L) - 0.4-0.6 (Termamyl
120L)
0.2-0.4 (α-amylase)
Table A6 Literature values for 1st
generation SSF
Article 3 [45] Agro [47] Article 5 [48] Alcohol book [22] Novozyme [5]
Temperature (°C) 35 31.5 - 32 30-32
Time (h) 30 (with di-
ammonium
phosphate)
55 72 10-48 45-55
Yeast (kg/starch) 1.5 kg/ ton
(batch)
- - 0.1 kg/ton (batch) -
Living cells
(million/mL)
- - Start: 20 - Start: 10-15
- - - Top: 150 Top: 200
Enzyme (kg/ton
starch)
0.9 - - - 0.5-0.9
Appendix II Table A7 Composition of wheat straw from different articles
Composition of wheat straw (%w/w)
Article 1[43] Article 2 [44] Article 3 [45] Article 4 [46] Mean value used
Cellulose 36.3 35.9 32.6 35 35
Hemicelluloses 23 23.9 24.2 22.3 23
Lignin 25.5 19.3 26.5 15.6 23
Ash 6.7 4.1 4.6 6.5 5
Residuals 8.5 16.8 12.1 20.6 14
57
Appendix III Table A8 Calculated heat duties for the 1
st generation pretreatment
Pretreatment 1st generation
1st generation 2nd generation integrated
heater (MW) 1.6
1.7
cooler (MW) 0.8
0.8
Table A9 Calculated values for the 1st
generation SSF
SSF 1st generation
1st generation 2nd generation integrated
Heat duty (MW) -3.1
-3.1
Product (ton/h) 109.2
111.5
Mass fraction of EtOH 0.14
0.13
T(ºC) product 31
31
Pressure (bar) 1
1
The product stream consists of water, ethanol, glucose, xylose, furfural, acetic acid, carbon dioxide,
oxygen, glycerol, ammonia and nutrients. The mass fraction given in the table is the mass fraction of
ethanol in the product stream. The mass fractions obtained are too high to be considered. A common
value should be around 7-8% in weight. However, the results obtained will not affect the distillation
results very much.
Table A10 Calculated values for the 1st generation gas separator
Gas separator 1st generation
1st generation 2nd generation integrated
Heat duty(MW)
-0.2
Gas stream (ton/h) 13.0
12.9
Product (ton/h) 96.2
98.6
The main task of this separator is to separate the carbon dioxide from the stream outgoing the SSF. So,
the gas stream consists of 99.8% in weight of carbon dioxide and 0.2% of oxygen. The product stream is
formed by water, ethanol, glucose, xylose, furfural, acetic acid, glycerol, ammonia and nutrients.
58
Table A11 Calculated values for the 2nd
generation pretreatment
Pretreatment 2nd generation
1st generation 2nd generation integrated
Energy demand (MW)+10%
4.4 4.4
Energy released--> 10%
0.4 0.4
steam 20bar
8.3 8.3
Product
53.3 53.3
T(ºC) product
190.0 190.0
The energy demand is the heat duty required for the pretreatment. However, a 10% more of the
required heat duty was taken into account to guarantee that the temperature in the reactor reaches
190ºC. Assuming no heat losses and that the steam provided to the reactor covers the energy demand,
the 10% extra of the energy demand will be in excess, this value is referred to in the table as energy
released.
The product consists of water, glucose, xylose, acetic acid and furfural. The main component is water
with a mass fraction of 0.842, 0.136 of xylose, 0.011 of acetic acid and the same mass fraction for
glucose. The mass fraction for furfural is very small, 229ppm.
Table A12 Calculated values for the 2nd
generation cooling before prefilter
Cooling before prefilter
1st generation 2nd generation integrated
Heat duty (MW)
-2.2 -2.2
T(ºC) in
100 100
T(ºC) out
35 35
Pressure (bar)
1 1
After the pretreatment and before filtering the stream to remove 50% of the liquid, mainly water and
the stream needs to be cooled down with a heat exchanger.
Table A13 Calculated values for the 2nd
generation SSF
SSF 2nd generation
1st generation 2nd generation integrated
Heat duty (MW)
1.5 1.5
Supplied by condensed vapor 100ºC,1 bar(MW)
5.0 5.0
Net heat duty (MW)
-3.4 -3.4
Product (ton/h)
113.1 113.1
Mass fraction of EtOH
0.03 0.03
T(ºC) product
35 35
Pressure (bar)
1 1
59
The heat duty for the SSF is the energy required to preheat the feed stream in the reactor. This energy is
supplied by condensed vapor at 1 bar. As the energy supplied by the vapor is higher than the energy
required to preheat the ingoing feed, heat will be released from the SSF as shown in the table above.
The product stream of the second generation SSF contains water, ethanol, glucose, xylose, furfural,
acetic acid, carbon dioxide, ammonia and nutrients. The main component is water in 90.8%w/w. The
mass fraction of ethanol is the mass fraction in the product stream.
Table A14 Calculated values for the 2nd generation gas separator
Gas separator 2nd Generation
1st generation 2nd generation integrated
Heat duty (MW)
-0.003 -0.003
Gas stream (ton/h)
2.5 2.5
Product (ton/h)
110.6 110.6
The gas separator separates the carbon dioxide from the outgoing stream of the SSF. The gas stream is
the carbon dioxide while the product stream contains water, ethanol, glucose, xylose, furfural, acetic
acid, ammonia and nutrients.
Table A15 Calculated values for the stripper column
Stripper column
1st generation 2nd generation integrated
Number of stages 30.0 30.0 30.0
Pressure (bar) 3.0 3.0 3.0
Pressure drop (bar) 0.01 0.01 0.01
Q demand reboiler (MW) 11.9 8.0 20.8
T(ºC) reboiler 137.0 136.9 136.9
Feed (ton/h) 96.2 110.6 208.5
T(ºC) Feed 88.0 96.6 91.8
Vapor (ton/h) 20.8 6.9 28.5
T(ºC) Vapor 117.8 126.9 121.4
Waste-liquid (ton/h) 75.4 103.8 180.1
T(ºC) Waste-liquid 134.3 135.2 135.0
Vapor condenser (MW) 11.5 3.2 11.1
T(ºC) condenser in 118.0 126.9 121.4
T(ºC) condenser out 110.7 113.8 111.6
P (bar) condenser 3.0 3.0 3.0
Mass fraction of EtOH vapor 0.65 0.38 0.56
60
Table A16 Calculated values for the furfural separator
Furfural separator
1st generation 2nd generation integrated
Heat duty (MW) 0 0.004 0.13
EtOH-H20 (ton/h) 20.8 6.8 28.4
waste-furfural (ton/h) 0 0.004 0.01
The furfural separator is located after the stripper column. It takes the distillate from the stripper and
removes the furfural contained in that stream as it is an undesired byproduct.
Table A17 Calculated values for the rectifier column
Rectifier column
1st generation 2nd generation integrated
Number of stages 50 50 50
Pressure (bar) 0.5 0.5 0.5
Pressure drop (bar) 0.01 0.01 0.01
Reflux ratio 1.53 1.5 1.5
Feed stage 45 45 45
Feed (ton/h) 20.8 6.8 28.4
T(ºC) Feed 110.7 113.8 111.6
Ethanol (ton/h) 14.6 2.8 17.2
T(ºC) Ethanol 61.37 61.4 61.37
Water (ton/h) 6.2 4.1 11.1
T(ºC) Water 99.1 99.3 99.2
Mass fraction of EtOH vapor 0.93 0.93 0.93
Q condenser (MW) 10 1.9 11.8
T(ºC) condenser 61.4 61.4 61.4
Q reboiler (MW) 9.2 1.7 10.7
T (ºC) reboiler 99.1 99.3 99.2
Q left after using vapor condenser (MW) 1.8 1.5 0.4
Table A18 Calculated values for the biogas cooler
Biogas cooler
1st generation 2nd generation integrated
Heat duty (MW) 0.0 -3.0 3.3
Pressure (bar) 3 1 1
61
Table A19 Calculated values for the anaerobic digester 1
Anaerobic digester 1
1st generation 2nd generation integrated
Heat duty (MW) -0.5 -1.0 -1.5
Ammonia (ton/h) 0.5 0.5 0.5
Product (ton/h) 48 100.8 148.8
T(ºC) 37 37 37
Pressure (bar) 1 1 1
Table A20 Calculated values for the anaerobic digester 2
Anaerobic digester 2
1st generation 2nd generation integrated
Heat duty (MW) -0.05 -0.1 -0.1
Biogas (ton/h) 48 100.8 148.8
T(ºC) 37 37 37
Pressure (bar) 1 1 1
Mass fraction of CH4 0.02 0.01 0.01
Table A21 Calculated values for the gas separator biogas
Gas separator biogas
1st generation 2nd generation integrated
Heat duty (MW) -0.05 -0.08 -0.1
CH4-CO2 (ton/h) 2.8 5.0 8.1
non-CH4 (ton/h) 45.1 95.8 140.7
Mass fraction of CH4 0.26 0.26 0.26
Mass fraction of CO2 0.74 0.74 0.74
Appendix IV Table A22 Mass flow for raw material and products
Mass flow (ton/h)
IN 1st generation 2nd generation integrated
Wheat 44.3 44.3
Straw 25.0 25.0
OUT
Ethanol 12.9 2.4 15.2
Methane 0.7 1.3 2.1
62
Table A23 Lower heat of combustion for raw material and products
Stream Heat of combustion (MJ/kg)
Ethanol 27.1
Methane 50
Wheat (8.95 % moisture) 15.2
Wheat straw 16.5
Table A24 Energy content in the raw material and the products based on the lower heat of combustion
Energy content (MW)
IN 1st generation 2nd generation integrated
Wheat 187.1 187.1
Straw 114.6 114.6
OUT
Ethanol 97.2 18.4 114.6
Methane 10.2 18.5 29.6
Table A25 Electricity needed in the ethanol plant
Electricity needed (MW)
1st generation 2nd generation integrated
Electricity 6 4.8 9.8
The electricity value for the first generation is taken from an article [49] without considering the biogas
part, the value from second generation is an average from Sassners’ data [27] where the biogas part is
also not included and the integrated is the sum of the first and second generation values assuming that
1 MW is not needed when integrating.