Biomass Attributes

8/7/2019 Biomass Attributes

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By Daniel Mahr, P.E.

Energy Associates, P.C.

Montville, NJ, USA

WHY BIOMASS?

The power industry is confronting challenges with seemingly conflicting goals. Large, solid-fuel power

plants provide the reliability and flexibility utilities require for baseload, cycling, and on-demand

situations. They provide the economy of scale needed to minimize the cost of production. Consumers,

including industry, rely on affordable, dependable electrical energy. Its an important part of our

economy and our daily lifestyle. Reducing emission levels and conserving our finite resources are key

components for achieving a sustainable environment.

Historically, biomass powered societys early development, and its ability to power our needs today is

being reassessed, as a means to recycle carbon emissions. Biomass is a resource that can be substituted

for coal, in varying degrees for existing pulverized coal (PC) plants. New, large power plants are being

designed to utilize biomass as the primary fuel, most notably in circulating fluidized bed combustion

(CFB) boilers.

Biomass is available now. New products and sources are being developed, as the market unfolds.

While biomass-fired plants have been a part of the scene for some time, they have typically been

relatively small, 25 to 50 MW, and often address specialized, local conditions. Biomass units on the scale

of 300 MW and larger present a number of different and important challenges. The plants ability to

effectively utilize biomass fuel products is more important for this technology scale-up.

Biomass fuel procurement decisions have a greater impact on a solid-fuel power plants design and its

fuel processing, handling, and storage features. Compared to other solid-fuel-fired plants, some

components will appear quite familiar, but there are important differences. Other system components

and processes will be completely foreign. A successful design must consider the fuel first and provide

the flexibility needed to handle the range of properties and characteristics that will be experienced. A

number of examples and lessons learned from smaller biomass plants provide some of the guidance

needed for scaling up to 300 MW and larger biomass units.

CO-FIRING IN A PC BOILER


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The European Union expects all member states to supply 20% of their energy requirements by 2020

from sustainable forms of energy. These include wind, landfill gas, and biomasswith small amounts

from solar, geothermal, and wave power.

In the United Kingdom, perhaps the largest co-firing project has recently been installed by Drax Power at

its 4,000 MW, coal-fired Drax Power Station, located in Selby, North Yorkshire. This is the largest coal-

fired station in the U.K. and provides enough power to meet 7% of the U.K.s electricity needs. Drax

Power began experimenting with a system that blends biomass with coal, using the existing emergency

coal reclaim hopper. Based upon the success of this project, Drax Power recently completed an 80

million ($150 million) direct biomass injection system. Together, the 100 MW blending and the 400 MW

direct inject provide 500 MW of biomass generating capacity. Figure No. 1 views the handling and

storage area of the biomass co-firing system during construction.

An important aspect of Drax Powers project is the U.K. governments long-term subsidies for biomass.

The uncertainty of the level of the current subsidy has dozens of companies delaying their further

investment in biomass. The amount and duration of the subsidy will affect how much biomass is co-fired

at the Drax Power Station.


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CO-FIRING IN A CFB BOILER

One of the attributes of circulating fluidized bed combustion technology is its ability to utilize a variety

of solid fuels. Very often, a CFB boiler will be using a high-ash, high-sulfur fuel. The relatively low

combustion temperatures, reduced fuel preparation requirements, and inherent control of emissions

within the boiler itself make it well suited to low-quality coal and alternative solid-fuel products.

The low-ash and low-sulfur characteristics of biomass fuel make it a convenient fuel foil for low-quality

coal. It nicely offsets the high-ash, high-sulfur components of low-quality coal.

ENELs Sulcis plant in Sardinia, Italy, currently has two units, a 240 MWe pulverized coal boiler, which

has a FGD system, and a new 350 MWe, Alstom Power fluidized bed boiler. The fluidized bed boiler was

retrofitted to the plant and two 240 MWe pulverized coal-fired boilers were removed.

The plant now uses a blend of South African, Colombian, and Sardinian coals. The Sardinian coal is from

a local mine/preparation plant. It has moderate ash content, relatively high sulfur levels, and high

moisture content.

While the CFB boiler was designed for coal, ENEL added a biomass system. Figure No. 2 views Sulciss

biomass handling, processing, and yard storage area. Wood chips are received by truck from local


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sources. They are stockpiled outdoors and reclaimed by payloaders. Wood chips are screened and

stored in the yard and boiler bins. A completely independent biomass handling and feed system was

constructed. The biomass is fed to the boiler via two of the three cyclone sealpots at the back wall of the

furnace. Biomass is a maximum of 15% of the fuel input by heating value.

The worlds largest fluidized bed boiler is the 460 MW, once-through supercritical unit at the Lagisza

plant. It began commercial operation in June 2009, replacing two of the seven existing units. The

fluidized bed unit is designed to co-fire up to 10% biomass by weight. The biomass feeding equipment

was considered in the design, so it could be added at a later date.

The Shaw Group is currently constructing the $1.8 billion, 585 MWe, Virginia City Hybrid Energy Center

near St. Paul, Va., for Dominion Virginia Power. The plant will have two Foster Wheeler CFB boilers that

will utilize waste coal, gob, and use up to 20 percent biomass. Construction began on June 30, 2008,and passed the halfway point in December 2009. Figure No. 3 is an artists rendering of the plant.

DIRECT FIRING IN A CFB BOILER


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Technology developments for the direct firing of biomass benefited from the Public Utility Regulatory

Policies Act (PURPA) that was passed as part of the National Energy Act in 1978. It promoted the

conservation of energy, efficient use of facilities and resources, and equitable rates for customers. It

encouraged a greater use of renewable energy by creating an expanded market by adding non-utility

producers.

A variety of boiler designs were used under PURPAs regulations including bubbling fluidized bed,

circulating fluidized bed, fixed grate stokers, sloping grate, traveling grate stokers, and water-cooled

vibrating grates. These technologies typically are all available up to a 50 MWe capacity for a single unit.

As unit size ramps up, the circulating fluidized bed boiler becomes the technology of choice. Like

pulverized coal technology, CFB technology scales up more easily for coal/biomass than stokers or

grates. Major boiler manufacturers like Foster Wheeler, Alstom Power, and Metso Power are offering

300 MWe units for biomass and looking to the next larger size generation.

For new generation capacity, there are good reasons to consider a CFB boiler. This technology is proven

and has been used since the 1980s. There are numerous plants that utilize agricultural, forest, mill, and

urban biomass products. With forethought for boiler and plant design, a variety of solid fuels can be

used according to availability, market, regulatory, or other circumstance.

Drax Power has been investigating the addition of three 300 MWe biomass-fired plants in the U.K. One

would be located adjacent to its existing 4,000 MWe coal-fired plant and another in the Port of

Immingham. Sites for the third plant are being evaluated. A variety of biomass products are being

investigated including wood chips, wood pellets, miscanthus briquettes, straw pellets, bagasse

briquettes, and logs. Much of the fuel will be initially imported while indigenous sources are developed.

The current uncertainty of sufficient long-term subsidies in the U.K. for biomass, however, has these

three plants and dozens of companies slowing or delaying their investment in biomass.

UTILIZATION ATTRIBUTES

Fuel properties and characteristics are important to boiler design and operation. Different boilers have

unique design and fuel requirements, i.e. whats injected into a pulverized coal-fired (PC) boiler is an

inappropriate fuel product for a stoker boiler. Heating value, percent volatiles, total ash and moisture

content, ash constituents, and particle size are all key parameters considered by the boiler designer.

There are a wide number of choices in selecting the component materials that are generically known as

biomass. Some biomass products have unique utilization issues. Wheat straw, for instance, has very

high levels of chlorine and its ash chemistry is dominated by silica from the phytolith inorganic

structures with significant potassium present. The chemical fraction behavior of biomass materials is

quite different from that of typical coals.


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For co-firing applications, the properties of biomass, coal, and possibly other solid fuels can be blended

as a designer fuel. The objective is to best meet boiler, combustion, emission, and economic

requirements.

CFB boilers are different from other combustion technologies. They have relatively low combustion

temperatures and long combustion residence time, and the injection of limestone into the furnace

makes the CFB technology unique in its ability to utilize a wide range of fuels while controlling

emissions. In a CFB boiler, fuel is generally combusted in a bed or matrix of material that is expanded at

a pressure and velocity that are:

1. above the particles saltation velocity but

2. below the particles transport velocity,

to sustain fuel particles in the fluidized state. The particles mass, volume, and shape are important to

the efficiency of the process.

As a practical matter, the fuel feed to a CFB boiler will cover a range of particle sizes, from sand-size

grains to small lumps. For any given fluidizing velocity, the smaller particles will transport much easier

than larger particles, but they will also fully combust more quickly. CFB boilers use cyclones to separate

the smallest (fly ash) particles from larger particles that may be only partially combusted. The larger

particles are discharged to the bottom of the cyclones sealpot or loop-seal and reintroduced to the

combustion bed. The combustion gases and fly ash are discharged through the top of the cyclone to the

superheater and economizer. The design and operation of the cyclones themselves are dependent upon

particle size and flue gas velocity.

Particle size curves have been developed by boiler manufacturers to identify fuel requirements. These

curves are dependent upon the particles fluidizing and combustion characteristics. Particle mass, size,

shape, and volatile content are key parameters for the curve. Some boiler manufacturers impose

distinct fuel requirements to meet contract performance guarantees and warranties, depending upon

the design fuel and contract terms.

Controlling fuel quality and cost by processing and blending biomass products on-site is a standard

industry practice. Biomass handling systems typically include screens and hogs. Not all biomass products

are completely suitable and the amount of some, like straw or miscanthus pellets, may be limited due to

their chlorine and potassium content, which are deposition and corrosion concerns. When different,

less-suitable products are used, accurate control of the blending process is important for combustion.

For a pulverized coal-fired boiler, it is well known that for coal to be combusted much like a gaseous

fuel, it must be pulverized to a fineness where 70% typically passes a 200 mesh size. Combustion

residence time is counted in seconds. The co-firing of biomass fuel in a PC boiler imposes similar


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requirements, although the particles are larger, perhaps crushed by an air-swept hammermill to less

than 1/4 inch.

The requirements for a biomass-fired stoker boiler is seen in Figure No. 4, as published in Steam, Its

Generation and Use, 40th Edition, page 15-8, by Babcock & Wilcox. B&W adds a note of caution that this

distribution is rarely achieved, a processing problem.


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Figure No. 5 illustrates the acceptable size range of biomass, as used by one CFB boiler manufacturer.

The curve conforms to Austrian standard No. M 7133, Chipped wood for energetic purposes

Requirements and test specifications.

Figure No. 6 illustrates the fuel sizing curve used by one boiler manufacturer for coal. With a top size of

perhaps 3/8 inch, it is very different from the curves for biomass that have a top size of something more

like 4 to 6 inches. Figure No. 6 is considered to be conservative for commercial commitments and drawn

for a low-volatile coal. For coals that are relatively high in volatiles, the upper curve might be adjusted

by the user at the plant, so that larger particles could be used.

Comparing the curves in Figures No. 4, 5, and 6, it can be seen that Figure No. 5 has a much wider,

forgiving range (distance between the upper and lower curves and slopes of the two curves themselves)of particle sizes. This has a lot to do with the high volatile levels and low specific gravity of biomass

products. Both parameters are important for combustion and fluidization. Fluidization parameters are

not an issue for a stoker boiler and the higher specific gravity and lower volatility of coal make their

combustion characteristics different for these applications and technologies.


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Some boiler manufacturers do not define particle sizing per se, only the maximum and minimum sizes

and maximum percent within the size distribution. In one case for wood chips, the maximum size is 30 x

30 x 5 mm and the minimum size, those particles smaller than 3.15 mm, is limited to 10% of the fuel

matrix. Both the minimum and maximum values are more restrictive than Figure No. 5, i.e. fewer fines

and a smaller top size are allowed. This makes the sizing issue more important than what would be

supposed by Figure No.5. This can be a key consideration in matching fuel processing techniques to

boiler requirements.

As seen, controlling particle size and fuel quality to meet contract terms and boiler guarantees are

important issues during the plant design and development stage of a project. Typically the boiler

manufacturer is not responsible for supplying the fuel nor does he furnish the fuel preparation system. It

can be an issue of contention for the parties to the contractfuel supplier, fuel handling system

designer/vendor, EPC contractor, boiler supplier, owner, and engineer.

HANDLING ATTRIBUTES

One of the limitations of biomass products such as wood chips is the relatively low, volumetric heating

value or energy density. This is a property that is not normally of concern for other solid fuels. Both the

lower mass heating value in MJ/kg (Btu/lb.) and low bulk density in kg/m3 (lb./ft.3) conspire to

significantly reduce its volumetric heating value. As a result, many more railcars, much larger stockpiles,

and wider belt conveyors are necessary to deliver and store the energy equivalent of other solid fuels.

Wood chips require six times the volume as bituminous coal on a MJ/m3 (Btu/ft.3) basis.

Fuel degradation and spontaneous combustion are more important concerns for biomass fuel products.

This is a moisture-dependent issue.

Dry biomass can be stored for long periods. Farmers store their grain harvests in silos to protect it for

markets. Their hay was once stored in barns as the winter diet for horses, cows, and other farm animals.

The grains discovered in Egyptian pyramids survived for thousands of years. So dry biomass can be

stored for long time periods.

At moisture levels between 20 and 60 percent, degradation and spontaneous combustion become a

concern. There is a high-temperature process due to oxidation of the cellulosic materials and a low-

temperature process involving the growth and respiration of microorganisms, such as aerobic mold-

fungi and bacteria. Biological heating, under the influence of water content or air humidity can increase

product temperature to a sufficiently high value to trigger oxidation of the cellulose material to initiate a

fire.

Above a 60 percent moisture level, wet biomass does not pose a spontaneous combustion problem. As

any camper can attest, its hard to start or sustain a campfire with wet wood. At high moisture levels,


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too much energy is needed to increase the water temperature to 100 oC (212 oF) and then evaporate it.

This high energy demand drops the temperature of the biomass below the level needed to sustain

combustion. Biomass will, however, continue to degrade due to biological heating.

The large surface area of particles like wood chips and their irregular shape that traps small air pockets

provide a near-ideal environment for the breakdown of fibers, increasing surface temperatures, and the

potential for spontaneous combustion. Smaller chip sizes increase surface area and the probability of

biological heating.

Wood chips, chopped straw, and other agricultural products have poor flow characteristics. During

storage in stockpiles and bins, they compress in volume and particles can become entwined, matted,

gaining strength as a mass, rather than behaving as unique individual particles. Instead of having a

sloping angle of repose, the sides of a partially reclaim stockpile can be vertical. Storage bins are often

designed with negative wall angles; the bins have walls that diverge. The bins are wider at the bottom

than they are at the top, which is very different from the hoppers typically used for storing coal or other

solid bulk products. Chute angles and the choice of liner materials should consider these poor flow

properties.

Different biomass products can be blended for availability, economic, combustion, and other reasons.

Tests have found that the blends of different biomass products can result in poorer flow conditions than

if any product is used alone. Particle size, shape, and compressibility have a lot to do with this

phenomenon.

PRE-TREATMENT

A biomass pre-treatment industry is developing to address some of the following issues:

1. The wide variety of biomass products often does not match with the narrow fuel specifications for the

boiler.

2. To lower the relatively high costs of transportation, handling, and storage.

3. To reduce plant investment, maintenance, and labor costs by using a homogenous, consistent fuel for

the combustion process.

Pellets are formed in a process that dries, mills, conditions with binders, extrudes, and then cools the

product. A variety of biomass feedstocks can be used including bagasse, corn cobs and screenings,

peanut shells, sawdust, switchgrass, and wheat middlings. The process grinds oversized material and

compacts the fine biomass feedstock into a homogenous, high-energy-density fuel. The pellets are


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cylindrical shapes, with dimensions of 6-8 mm diameter and 15-25 mm length. Pellets are a convenient

biomass product for co-firing in a coal-fired plant.

The properties and quality of biomass pellets can vary widely. While pellets from different

manufacturers may appear to be similar, fuel properties are inherited from the parent or source

feedstock material and quality is dependent upon the manufacturing process. While pellets

manufactured from sawdust might be directly suitable as a fuel, pellets manufactured from switchgrass

might be limited to no more than 10% of the blended biomass fuel because of their high alkali, chlorine,

and silicon content, which can increase erosion and corrosion problems. Likewise, some manufacturers

may use a binder such as starch while other processes will depend upon the biomasss cellulosic lignin,

which with heat and moisture acts as a binder to form a dense pellet. The amount of handling and

exposure can also affect the quality of the delivered product. Receiving a shipment of wood pellets that

has degraded into mostly sawdust can be very disconcerting when you are expecting a cargo of free

flowing, hard, clean, relatively dust-free pellets.

Biomass briquettes have much in common with pellets. The primary difference is that briquettes are

larger than pellets. They may be 30 to 100 mm (14 inches) in diameter. They can be a composite

product formed from a blend of biomass and coal.

The properties of biomass can be improved through a thermo-chemical process known as torrefaction.

Biomass is heated at a temperature of 200-300 oC (390-570 oF), in a reducing environment, typically for

an hour. During this time, the biomass partially decomposes giving off volatiles. The remaining product

or char alters material properties as follows:

1. increased energy density,

2. improved hydrophobic nature,

3. improved grindability,

4. increased uniformity, and

5, improved durability.

Torrefaction can help to mitigate certain constraints related to co-firing of biomass with coal, such as

size reduction of biomass (which is problematic for untreated biomass due to its fibrous structure),

storage (dry matter losses, fungi growth), or feeding. A unique advantage is that different types of

feedstocks can be used and the torrefaction process will produce a uniform product. It reduces some of

the quality control issues that might otherwise be encountered with biomass products.

Torrefied biomass can be further processed into pellets, which improves physical properties. Torrefied

pellets can be stored in outdoor stockpiles and handled much like coal. They have perhaps half of the


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energy density of coal, which is a big improvement in comparison to untreated biomass products. The

ability to handle and store torrefied biomass much like coal can significantly reduce the capital cost for

an existing coal-fired plant to get into the biomass business with a co-firing project. Integro Earthfuels

in the U.S. and Topell B.V. in the Netherlands are two companies that are moving from the pilot stage to

their first commercial biomass torrefaction and pelletizing plants.

LESSONS LEARNED

In a non-regulated, competitive market environment, plants are less inclined to share information that

could be valuable to a competitor. Both the good and bad experiences are of value to those seeking a

path that avoids the pitfalls.

The National Renewable Energy Laboratory of the U.S. Department of Energy examined the experience

of twenty biomass plants in 2000. They identified several key issues that impacted these plants: fuel

cost, location, fuel handling, reliability and dependability, partnerships, and subsidies.

Fuel cost was the highest priority at most plants. Since there is normally a direct correlation between

fuel cost and fuel quality, fuel quality trade-offs played an important role in plant design and operation.

The location affects a biomass power plant in a couple of ways. First, there can be local circumstances

that become permit and community requirements. These can increase operating cost due to the need to

curb traffic, to restrict operations that are noise and odor sources, and to pay high tax or labor rates.

Second, the distance to biomass resources is important because the typically low energy value of

biomass in comparison to coal, oil, and gas can quickly raise fuel cost as the transport distance increases.

Most plants in the NREL survey experienced a significant learning curve. They spent a lot of time and

money the first couple of years solving problems such as fuel stockpile heating and odors, excessive

equipment wear, handling hang-ups and bottlenecks, tramp metal problems, and wide fluctuations in

fuel moisture content. A plant must meet environmental standards operating with variable conditions

and without excessively corroding heat transfer surfaces or slagging boilers beyond the point of prudent

operation.

Many biomass plants significantly changed fuels over the years. This is not an unusual case for the utility

industry. Many PC boilers, which were originally designed to use bituminous coal, switched to low-sulfur

PRB coal and, perhaps once fitted with scrubbers, have now switched to a high-sulfur Illinois coal.

For biomass, the differences can be significant and designing for fuel flexibility is a good strategy. The

Colmac plant found it economical to modify its permit to allow the use of petroleum coke. At times,

waste fossil fuels can be more economical than biomass products and the properties of one can offset

those of another.


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Plants with the best long-term operating records placed a high priority on plant reliability and

dependability. This must be stressed during both plant design and operation. Staying atop of

maintenance programs and maintaining a clean, neat workplace are essential elements for long-term

reliability.

For those plants with close ties to a limited number of customers and/or suppliers, the relationship or

partnership with these firms is important. For instance, a sawmill may provide the fuel as a primary

supplier and be a consumer of the generated power. The mill and power plant have a mutual interest in

the success of one another. In instances where the interest of the partners diverges, both parties suffer.

Many of these plants were constructed with the aid of PURPAs regulations or state regulations where

the legislature obligates utilities to either generate or purchase power at rates that are higher than what

is otherwise available from other sources, i.e. a coal or nuclear power plant. Subsidy programs, however,

do not last and competition will affect the long-term financial future of the plant.

WHY NOT BIOMASS?

The conflicting goals confronting the power industry make a sustainable, dependable fuel like biomass a

candidate option to consider. It allows large, existing solid-fuel power plants to diversify their fuel base

by co-firing biomass with coal or other solid fuels. Because existing plants can be employed and biomass

can be a secondary fuel, it is an opportune technology that allows utilities to test the technology and

build a network of suppliers in a relatively low-cost and low-risk program.

Consumers and industry rely on affordable, dependable electrical energy. Reducing emission levels and

conserving our finite resources are key components for achieving a sustainable environment. Utilities

build plant fleets to handle baseload, cycling, and peak demands. For a variety of reasons, there is no

one technology that best meets all operating conditions. Biomass is a fuel that can deliver on many

counts now and new pre-treatment technologies are at hand to make it a fuel that is more familiar,

convenient, and economical. For utilities that are planning new power plants, biomass is a strategic fuel

for expanding the plant fleet and one to add to the checklist of options.

This paper is from the Proceedings of ASME 2010 Power Conference, POWER2010, held July 13-15 in

Chicago.

REFERENCES


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Drax Power Ltd., http://www.draxgroup.plc.uk/media/press_releases/?id=69889, Press releases, Drax

signs contract with Spencer for supply of biomass co-firing infrastructure April 7, 2009. Drax Power Ltd.,

Drax Facts & Figures.

Hotta, Arto, Features and Operation Performance of Lagisza 460MWe CFB Boiler, 20th International

Conference On Fluidized Bed Combustion, May 18, 2009.

Lord Jenkin of Roding, The U.K.s Energy Future, Transmission & Distribution World, January 2010.

Maciejewska, Anna et. al., Co-Firing of Biomass with Coal: Constraints and Role of Biomass Pre-

Treatment, DG JRC Institute for Energy, 2006.

Mahr, Daniel et. al., Biomass plant relies on variety of local fuels, Power, April 1992.

Paul, Dilo et.al., Drivers for Biomass Co-Firing in the U.S. Economics of Legislation, 26th Coal Utilization

& Fuel Systems, March 5-8, 2001.

Plasynski, Sean I. et. al., Biomass Cofiring In Full-Sized Coal-Fired Boilers, 24th Coal Utilization & Fuel

Systems, March 8-11, 1999.

Virginia City Hybrid Energy Center Newsletter, Volume 5, October 2009.

Webster, Ben, Drax suspends plan to replace coal with greener fuel, TIMESOnline, Feb., 19, 2010.

Wiltsee, G., Lessons Learned from Existing Biomass Power Plants, National Renewable Energy

Laboratory, USDOE, SR-570-26946, February 2000.

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