An Aspen Plus Model of Biomass Torrefaction

University Turbine Systems Research (UTSR) Fellowship 2009

Ryan Dudgeon

University of Iowa

Electric Power Research Institute

Charlotte, NC

Introduction

Biomass offers much potential as a renewable fuel for displacing coal in large-

scale power plants. Given that the carbon contained in biomass is taken directly from the

atmosphere, the fuel is largely considered to be carbon-neutral. Biomass also contains

less sulfur, nitrogen, ash, and heavy metals than coal. However, there are currently

several problems with biomass that prohibit it from being used on a larger scale for

power production. It has a much lower heating value compared to coal and suffers from

logistical issues related to transportation, handling, and storage. Because of high

moisture contents and low energy densities, the cost of transportation to the plant is high.

In addition, the fibrous nature of biomass often causes handling problems, requiring more

energy to be spent on grinding the material. Open-air storage can also be a hindrance as

the hydrophilic nature of biomass can cause it to absorb more moisture over time.

Torrefaction has the potential to solve these problems by improving the properties of the

fuel. It produces a higher quality product with increased heating value, increased energy

density, and improved grindability properties.

Torrefaction is a mild pyrolysis process occurring at low temperatures in the

range of 250 – 300°C and in the absence of oxygen. When biomass is torrefied, a portion

of the volatile matter is driven off in the form of light gases and other condensable

organic compounds. The resulting solid material contains virtually no moisture, less

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volatile matter, and an increased fixed carbon content. A typical torrefaction process

may cause 30% of the mass to be lost in the form of volatile species, but a higher

percentage of the energy, typically 80-90%, is retained in the solid product, resulting in

energy densification of the biomass. The end product is often called bio-char or bio-coal.

Torrefied biomass has much improved grindability properties and heating values much

more comparable to coal. In addition, torrefaction results in a solid product that is

hydrophobic in nature, making it easier to store for longer periods without absorbing

significant amounts of moisture.

Three important definitions related to the torrefaction process are the solid yield,

energy yield, and reaction time. The solid yield is expressed on a dry-ash-free basis since

this is the organic, reactive portion of the material. The solid yield is defined on a mass

basis as shown below.

daffeed

torr

Sm

m

.

The energy yield is also reported on a dry-ash-free basis and can be based on the lower

heating value (LHV) or higher heating value (HHV). For this work the higher heating

value was used.

daffeed

torr

SEHHV

HHV

.

The purpose of this project was to develop a model in Aspen Plus for simulating

the torrefaction reactor and other unit operations associated with the torrefaction process.

The purpose of the model was to determine optimal torrefaction and drying conditions for

maximizing product output, product higher heating value (HHV), and process efficiency.

Another key target of the model was determining conditions for autothermal operation to

minimize utility fuel consumption. Another objective of the model was to serve as a

predictive tool that could be compared to pilot-scale torrefaction testing being done with

a local torrefaction company. A final goal of this project was to develop a first draft of

specifications of torrefied biomass pellets based on literature review, existing

specifications of wood chips, and collaboration with the torrefaction company.

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Torrefaction Model Development

Modeling was performed with Aspen Plus [2]. The torrefaction reactor is

modeled based on the experimental data of [3]. They reported mass yields of solid,

liquid, and gaseous products for torrefaction temperatures from 230°C to 280°C and

reaction times from 1 hr to 3 hr. Data was reported for five biomass feedstocks including

birch, salix, miscanthus, straw chips, and wood chips. For each feedstock, temperature,

and reaction time, the ultimate analysis of the solid torrefied product was reported as well

as the volumetric concentrations of CH4, C2 hydrocarbons, CO, and CO2 in the

torrefaction gas. For each feedstock, a set of equations were created based on multi-

variable linear regression of the experimental data to predict solid, liquid, and gaseous

yields, solid product composition, and gaseous product composition as a function of

torrefaction temperature and reaction time.

Based on a user-specified torrefaction temperature and reaction time, the

regression equations determine the mass flows to be dedicated to solid, liquid, and

gaseous products based on the respective calculated yields. Regression equations also

define the composition of the solid torrefied product in terms of the ultimate analysis.

The ultimate analysis and solid yield are used to calculate the amounts of carbon,

hydrogen, oxygen, and nitrogen consumed for the torrefied solid product. Regression

equations also define the composition of the product gas. As was done with the solid

yield, the predicted gaseous composition is used to determine the amounts of carbon,

hydrogen, and oxygen consumed for the torrefaction gases. The remaining amounts of

carbon, hydrogen, and oxygen not used for solid and gaseous products were thus

designated as constituents of the liquid products. These components were reacted in two

RStoich blocks to predict condensable organic species.

The torrefied solid product, being represented by a non-conventional component,

must be given material properties. The model automatically fills the ultimate, proximate,

and sulfur analyses of the torrefied biomass. However, the heating value must be

predicted because Aspen Plus uses it to calculate the enthalpy of the stream. Test

simulations showed that relatively small variations in the heating value for the torrefied

biomass affected the torrefaction process immensely in the heat demand required for

torrefaction. The correlation chosen for the model was taken form [4]. It uses the

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ultimate analysis to predict the higher heating value (HHV) on a dry ash-free basis as

defined below.

9.58767.133.3019.134/ OHClbBtuHHV daf

Figure 1: Aspen Plus model of torrefaction reactor.

Simulation Procedure and Process Description

The following torrefaction study considers a plant with a dryer, directly-heated

torrefaction reactor, combustor, and heat exchanger. For the woody biomass (wood chips

and salix), the moisture content of the raw material is assumed to be a typically high

value of 35%. For miscanthus, the wet feed was assumed to have a high moisture content

of 50%. If required, a portion of the raw biomass is diverted to the combustor without

being dried, and the rest enters the dryer. The dryer is maintained at a temperature high

enough to guarantee the drying medium does not become saturated, but low enough that

mild volatilization of the biomass is minimal. A dryer temperature of 105°C was

therefore used. The wet biomass was dried to 15% moisture, a value used as a base case

scenario – the effects of moisture on the torrefaction process are discussed later.

Combustion of the wet biomass and torrefaction gases was achieved using 150% of the

theoretical amount of air required for complete combustion. This value was chosen to

ensure that the flame temperature was kept below about 2500°F. Ash was separated from

the combustion products before the flue gas was directed to the heat exchanger. The

majority of the torrefaction gases were circulated back to the torrefaction reactor for

direct heat exchange. These gases were heated to 50°C above the torrefaction

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temperature and the exact amount recirculated to the reactor was controlled to meet the

heat demand of the torrefier. After passing through the heat exchanger, the flue gas was

directed back to the dryer to serve as the drying medium. Finally, the amount of raw

biomass diverted to the combustor was varied to meet autothermal conditions of the

entire process. The process diagram is depicted in Figure 2.

Figure 2: Process diagram of directly-heated torrefaction simulations.

The biomass fuels considered in this study were miscanthus, wood chips, and

salix. Their ultimate analyses and heating values were taken from [3] and [5]. It is

unclear specifically what woody biomass the wood chips are composed of, but it is

assumed to be a hardwood mix. These fuel properties are listed in Table 2.

Table 2: Fuel properties

Miscanthus Salix Wood

chips

C (wt. % daf) 43.5 47.5 48.5

H (wt. % daf) 6.49 6.4 6.6

N (wt. % daf) 0.9 0.63 0.05

O (wt. % daf) 49.11 45.5 44.9

Ash (wt. % daf) 0.3 4.6 0.3

HHV (Btu/lb daf) 7910 8370 8560

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Results and Discussion

Because torrefaction is likely to be most cost effective using reaction times as low

as possible, the following results will focus on torrefaction at a reaction time of 1 hr.

Figure 12 shows the solid yields of the three biomass fuels. Wood chips displays

the highest solid yields ranging from 89.0% to 90.0%, and salix falls between wood chips

and miscanthus. The solid yields of all three fuels decrease with increasing temperature,

although those of miscanthus decrease at the steepest rate of the three.

Reaction Time = 1 hr

60

65

70

75

80

85

90

95

100

220 230 240 250 260 270 280 290

Temperature (°C)

So

lid

Yie

ld (

da

f)

Miscanthus

Wood Chips

Salix

Figure 3: Solid yields of three biomass fuels at 1 hr reaction time.

The HHV of the three torrefied biomass fuels are shown in Figure 4. Most

noticeable is the relatively horizontal line representing salix, which increases only 40

Btu/lb from 230°C to 280°C. This is caused by a rather flat increase in the carbon

content over this temperature range. The hydrogen and oxygen contents display similar

unresponsiveness to temperature change. These ultimate analysis results were compared

to the experimental data upon which the model is based, and the model was found to

mimic the data almost exactly. Considering salix undergoes typical mass loss, it is

possible that approximately equal amounts (on a weight basis) of carbon, hydrogen, and

oxygen are driven off in the torrefaction gases, leaving the torrefied biomass with a

composition similar to the raw material.

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Reaction Time = 1 hr

8600

8800

9000

9200

9400

9600

220 230 240 250 260 270 280 290

Temperature (°C)

To

rrif

ied

Bio

ma

ss

HH

V (

Btu

/lb

da

f)

Miscanthus

Wood Chips

Salix

Figure 4: HHV of the three torrefied biomass fuels at 1 hr reaction time.

Raw biomass HHV (Btu/lb daf): Miscanthus = 7910

Wood chips = 8560

Salix = 8370

Figure 14 presents the energy yields for the three biomass fuels. Wood chips

displays the highest yield, ranging from 100.1% at 230°C to 97.9% at 280°C. It is

important to note that the energy yield can be greater than 100% as it is based on HHV

and not enthalpy; energy is still conserved in the process. The results conclude that

almost all the HHV of the biomass is retained in the solid product. Miscanthus displays

the lowest energy yield, dropping to as low as 77.9% at 280°C, and salix follows the

same trend as miscanthus except its energy yields are consistently about 5% higher.

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Reaction Time = 1 hr

75

80

85

90

95

100

105

220 230 240 250 260 270 280 290

Temperature (°C)

En

erg

y Y

ield

Miscanthus

Wood Chips

Salix

Figure 5: Energy yields of the three biomass fuels at 1 hr reaction time.

Figure 15 shows the amounts of raw biomass that must be combusted as utility

fuel to maintain the process autothermal. Similar to miscanthus, salix displays a

temperature at which no portion of the wet feed must be burned for utility heat, and this

occurs at 275°C. Over the range plotted, wood chips requires at least 6% of the wet feed

to be diverted to combustion. This is a result of the very high solid and energy yields

obtained by the fuel. Because wood chips has a high solid yield and almost all the HHV

is retained in the solid product, there is little HHV potential contained in the torrefaction

gases, and additional fuel is required to supply the heat needed for drying and

torrefaction. It is interesting to note that wood chips and salix each require about 10%

fuel diversion at 230°C. The values for salix are able to decrease to 0 because the energy

yield of salix decreases more sharply with increasing temperature, whereas the energy

yield of wood chips is only slightly affected by temperature.

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Reaction Time = 1 hr

0

2

4

6

8

10

12

220 230 240 250 260 270 280 290

Temperature (°C)

Ma

ss

Pe

rce

nta

ge

of

Bio

ma

ss

Re

qu

ire

d

for

Co

mb

us

tio

n a

s U

tili

ty F

ue

l

Miscanthus

Wood Chips

Salix

Figure 6: Raw feedstock required for combustion to attain autothermal conditions for the

three biomass fuels at 1 hr reaction time.

From Figures 3 – 6, a few disadvantages are inherent with operating at higher

temperatures. The solid and energy yields decrease with increasing temperature, and in

the case of miscanthus and salix, they decrease rather sharply. The process yields also

decrease with increasing temperature as well as the process efficiency in the case of

miscanthus and salix. However, operating at higher temperature presents advantages

which probably outweigh these drawbacks. Higher temperatures provide the opportunity

to decrease the percentage of raw biomass that must be combusted as utility fuel, and in

the case of miscanthus and salix this percentage can be reduced to zero. The torrefied

biomass HHV continues to increase with higher temperature. Specifying a minimum

torrefied biomass HHV of 9000 Btu/lb would require a minimum operating temperature

of about 250°C for miscanthus and 245°C for wood chips. The HHV of salix remains

above 9000 Btu/lb at all temperatures. At these cutoff temperatures the process

efficiency is about 84% for miscanthus and 91% for wood chips, and the process yields

are about 79% and 86%, respectively. If the temperature is increased to 280°C, the

process efficiency of miscanthus decreases from 84% to 78%, and the process yield

decreases from 79% to 71%. These results suggest that it may be more beneficial to

torrefy miscanthus at mid-temperatures around 250°C. On the other hand, operating at

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280°C causes the process efficiency of wood chips to increase from 91% to 92%, and the

process yield decreases only slightly from 86% to 83%. This suggests that it is worth

taking a 3% decline in process yield to gain increased HHV and overall efficiency. Not

only are increased heating values necessary for co-firing with coal, but they will also help

lower transportation costs to the plant. Operating at higher torrefaction temperature also

has the advantage of tar handling. Because temperature gradients should be expected

within the torrefaction reactor, condensation of the organic species can cause major

maintenance problems. It is easier to avoid condensation of the organic tars by operating

at higher temperature. In the case of salix, increased temperature does not attain

significant increases in HHV. If the temperature for salix is increased from 250°C to

280°C, the process yield decreases from 83% to 78% and the process efficiency decreases

from 88% to 83%. These numbers suggest that it may be beneficial to operate at 280°C

to provide better tar handling, or perhaps an operating temperature between 250°C and

280°C could be used to regain lost percentage points in process efficiency and process

yield.

Lower Reaction Times

The benefit of achieving larger production output initiated investigation into

whether the model was capable of accurately modeling lower reaction times. The

torrefaction models for salix and wood chips were compared to experimental results

reported by [1] and [6] using reaction times less than 60 min, and it was determined that

the model could reasonably be extrapolated to these reaction times.

Figure 18 plots the HHV of the torrefied material for the three biomass fuels at

280°C. Lowering the reaction time to less than 1 hr has no significant effect on the HHV.

As the reaction time is decreased from 60 min to 10 min, the HHV decreases by 0.5%,

0.5%, and 0% respectively for miscanthus, wood chips, and salix. The results suggest

that reaction time has little influence on the torrefaction process and the reaction is

mostly completed by merely heating the biomass to the final torrefaction temperature.

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Temperature = 280°C

8500

8600

8700

8800

8900

9000

9100

9200

9300

0 10 20 30 40 50 60 70

Reaction Time (min)

HH

V (

Btu

/lb

daf)

Miscanthus

Wood Chips

Salix

Figure 9: HHV of the three biomass fuels at 280°C.

Raw biomass HHV (Btu/lb daf): Miscanthus = 7910

Wood chips = 8560

Salix = 8370

Figure 19 displays the process efficiency for the three biomass fuels. Much like

the HHV, the process efficiencies are benefited, yet relatively unaffected, by the reaction

time over the range plotted. Decreasing the reaction time from 60 min to 10 min results

in an increase in process efficiency of 1 percentage point for each of the three fuels.

Temperature = 280°C

50

55

60

65

70

75

80

85

90

95

100

0 10 20 30 40 50 60 70

Reaction Time (min)

Pro

cess E

ffic

ien

cy

Miscanthus

Wood Chips

Salix

Figure 10: Process efficiency of the three biomass fuels at 280°C.

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Recommendations for Future Work

There are many areas of this project which merit further study. The most

significant way in which these results can be expanded upon is verification of the Aspen

Plus model results with the upcoming experimental testing. Additional validation,

refinement, and improvement of the model can then be achieved.

One major source of uncertainty in the current model was the prediction of the

HHV of the torrefied biomass. The model used a correlation developed in the literature

based on the ultimate analysis and meant to apply to general biomass samples. It is

recommended that a new correlation be created from the torrefied biomass samples

resulting from the Integro testing. This correlation should be based on the carbon,

hydrogen, and oxygen content of the ultimate analysis and should be based on a dry-ash-

free basis. The correlation should also be developed using as many samples as possible.

In addition, the model could be made more accurate with reactor and heat

exchanger sizing, which means a particular size plant should be chosen based on desired

torrefied biomass production capacity. Finally, additional model simulation could be

performed on the biomass feedstocks pertinent to the experimental torrefaction testing.

References

[1] Bergman, P.C.A., Boersma, A.R., Zwart, R.W.R., Kiel, J.H.A. (2005). Torrefaction

for biomass co-firing in existing coal-fired power stations. Energy Research

Centre of the Netherlands, ECN-C-05-013.

[2] Aspen Plus V7.0. (2008).

[3] Zanzi, R., Tito Ferro, D., Torres, A., Beaton Soler, P., Bjornbom, E. (2004).

Biomass torrefaction.

[4] Sheng, C. and Azevedo, J.L.T. (2005). Estimating the higher heating value of

biomass fuels from basic analysis data. Biomass and Bioenergy, 28, 5, 499-507.

[5] Phyllis, database for biomass and waste. Energy Research Centre of the Netherlands.

www.ecn.nl/phyllis/

[6] Bergman, P.C.A., Boersma, A.R., Kiel, J.H.A., Prins, M.J., Ptasinski, K.J., Janssen,

F.J.J.G. (2005). Torrefaction for entrained-flow gasification of biomass. Energy

Research Centre of the Netherlands, ECN-C-05-067.