Primer on Wood Biomass for Energy

Richard Bergman, Research Chemical Engineer

John Zerbe, Wood Technologist

USDA Forest Service, State and Private Forestry Technology Marketing Unit
Forest Products Laboratory, Madison, Wisconsin

Introduction weight of the wood, and sometimes ash may be used as a fer-
tilizer.

This paper explains and describes the concepts of wood en- Economic
ergy on a residential, commercial, and industrial scale in the
United States so that the Forest Service can help meet the de- Low Fuel Cost
mands of communities involved in the forest-products indus-
try. In addition, terminology associated with this field is The principle economic advantage of wood-burning systems is
explained so individuals can develop a basic understanding of that wood fuel is usually less expensive than competing fossil
and familiarity with technical terms common to bioenergy. fuels.
Definitions specific to wood energy are given at the end of this
report. However, the price of wood for use as fuel can be extremely
variable. Sometimes when surplus supplies of wood residues
Advantages of Wood Biomass are available at nearby forest-products manufacturing plants or
municipal solid-waste handling facilities, the cost can be very
Environmental low or even negative. But today, most manufacturing wood-
plant residues are being used internally as fuel or sold exter-
Renewable nally as a higher valued product. Transportation for delivering
from the supply site to the wood combustion or wood-
Wood fuel has several environmental advantages compared processing unit is the primary expense of wood fuel.
with fossil fuels. Wood can be continually replenished, which
leads to a sustainable and dependable supply. However, proper At other times, mostly dependent on location of the wood-
forest management must be practiced to ensure that growing power facility, the cost of wood fuel can be quite high because
conditions are not degraded during biomass production. large volumes are needed to have a dependable and consistent
supply of wood fuel (~1,360 green kg (~1.5 green tons) per
Low Carbon Emission hour per megawatt of power generated for a 27% overall
power plant efficiency). However, wood power plants can find
Wood combustion produces little net (~5%) carbon dioxide and do maintain a fairly low price and consistent fuel supply
(CO2), the major greenhouse gas, because the CO2 generated when adequate quantities are available. Staff foresters allow
during combustion of wood equals CO2 consumed during the plant personnel to focus on plant operation while foresters fo-
lifecycle of the tree. Transporting wood using petroleum gen- cus on wood-fuel procurement.
erates some excess CO2.
Typically, the average cost of fuel wood for small-scale com-
Minimal Metals and Sulfur bustors is similar to the reported prices of pulpwood for a
given location. Pulpwood is one of the lower valued forest
Wood fuel contains minimal heavy metals and extremely low products, ranking between industrial boiler fuel and the lowest
levels of sulfur; therefore, combusting wood fuel will not cre- quality saw logs, pallet logs, and stud wood. According to re-
ate acid rain pollution through sulfur emissions. However, gional data from the first quarter of 2007 (International Wood-
burning wood in the forest does emit significant amounts of fiber Report, May 2007, Vol. 13, No. 5), weighted average
nitrous oxide, a greenhouse gas, if either by wildfire or broad- price in dollars per green short ton delivered to mill was $29
cast burning for stand improvement. for softwood and $30 for hardwood for roundwood pulpwood
across all U.S. regions. The Southern Pine pulpwood stump-
Minimal Ash age price average reported across the U.S. South in the 1st
quarter 2007 Timber Mart-South (Vol. 12, No. 1) was $7.89
Particulate emissions from wood are controllable through per green short ton, whereas the southern hardwood pulpwood
standard emission control devices such as bag houses, cyclone stumpage price average was $6.51 per green short ton. These
separators, fly-ash injectors, and electronic precipitators. Bot- numbers are derived from average prices of the previous year
tom ash is minimal. Usually, wood ash is less than 1% of the ($1 for pine and $2 for hardwood). Considering small-scale

Revised January 2008

operations, likely delivered costs of chips would be upwards cause of the different types and capacities of equipment and
of $33/1000 green kg ($30/ton), resulting in estimated average whether equipment is new, used, or in-place and can be con-
fuel costs of $3.34/GJ ($3.53/million Btu) assuming roughly verted to burn wood.
$16.5 to $22 per 1,000 green kg ($15 to $20/ton) transporta-
tion costs. Scales of Operation
In comparison, the 2007 price of residential No. 2 distillate oil
Micro Scale
was $2.22 per gallon, excluding taxes ($15.17/GJ) ($16.01
million Btu), and the average price of residential natural gas
Space Heat
was $12.53 per 1000 ft3 ($11.55/GJ ($12.18/million Btu)).
Numerous wood-burning facilities use less than 1 MW
However, the prices of oil and natural gas were less at com-
(3.4 million Btu/h) of electrical energy and 1 MW of thermal
mercial and industrial plants and significantly less at utility
energy and are used for residential or small institutions
plants. In January 2005, coal prices at utility plants were $1.45
(schools) in Vermont. Steam turbines that generate electricity
per million Btu ($1.37/GJ). One dollar per million Btu equals
can be rated based on the thermal (th) energy inputted or elec-
$20.2 per ton of coal used at electrical utilities. According to a
trical (e) energy outputted at full power (I kW = 3,413 Btu/h;
published report by the Energy Information Administration,
1 MW = 3,413,000 Btu/h or 3.413 million Btu/h).
(May 2007 Receipts, Average Cost and Quality of Fossil Fu-
els), coal prices have increased in the last two years mostly For residential use of wood for fuel, common types of fur-
because of high demand by China and India. On May 2007, naces use split lengths of firewood to heat air in a plenum.
the price for utility coal was $1.79 per million Btu ($1.69/GJ), This hot air is then circulated through a duct system to various
an increase of roughly 20%. At the wood-burning McNeil points in the building. In an even simpler arrangement, heat is
Power Station (Burlington, Vermont), the price per green ton accumulated from burning logs in a fireplace and fan-blown to
was almost $30 ($33/1,000 green kg) or $60 per dry ton, or the surrounding space.
$3.53 per million Btu ($3.34/GJ) in 2006.
Split fuel wood can be fed to the fire from a magazine, and
Least Costly Option
some automated controls of the burning and heat distribution
rate can be applied. However, a greater degree of automation
Because the market for wood biomass energy may be uncer-
can be obtained through use of wood chips or wood pellets as
tain or uncommon in a particular area, potential wood biomass
fuel in specialized combustion units. Charcoal is another pos-
users may want to do a brief, informal feasibility study before
sibility for use to attain better control.
undertaking a rigorous economic analysis.
In 2006 in the United States, about 0.64 exajoules (0.61 quad)
A full life-cycle cost analysis can be used to compare the costs of energy from wood were used in the residential and com-
of a biomass combustion system (BCS) with a fossil fuel sys- mercial sectors. This is equivalent to about 32,600 million ov-
tem. When incorporating initial costs, analysis should be de- endried kg (35.9 million ovendried tons) of wood. It is
termined on an annual basis over the entire expected life of the reasonable to assume that wood from small-diameter trees
project, typically 20 years for a BCS. It is necessary to con- could provide additional fuel for this market, up to a 5% in-
sider the full lifetime costs of a project, because initial costs of crease or 1,630 million ovendried kg (1.8 million tons at thin-
a BCS are generally greater (approximately 50% to 200%) ning prescriptions of 10 green tons/acre).
than a fossil-fuel system. The reasons for the high initial costs
are the fuel-handling and storage systems required. Therefore, Electricity
comparing only initial costs of energy systems would suggest
the benefit of purchasing a fossil fuel system. At the micro-scale level, small gasifiers coupled to internal
combustion engines and generators can produce up to 100 kW,
A full life-cycle analysis considers annual costs for an ex- (341,000 Btu/h) of electricity for decentralized use. In the fu-
tended period of operation, and because of relatively high fos- ture, improvements should lead to more efficient arrangements
sil fuel costs, the BCS might be the least-costly option. In with turbine generators or fuel cells as the producer (wood)
general, to find the equivalent price of wood compared with gas cleaning technology improves.
oil or gas on a cost per GJ (× 106 Btu), add approximately
50% to the wood price to account for higher capital and oper- Cogeneration
ating and maintenance costs (O&M) of burning wood. Be-
cause of technology advances, however, O&M costs are Micro-scale cogeneration is sometimes used for village power
becoming less of a factor. applications in developing countries. In the Philippines, two
units were installed that provided electrical energy to a coco-
In general, wood combustion system costs are $50,000 to nut processing plant and thermal energy for copra processing.
$150,000 for a 0.6 MW (2 million Btu/h) system, $100,000 to In the future, micro-scale cogeneration should be capable of
$350,000 for a 0.6 to 1.5 MW (2 to 5 million Btu/h) system, operating at electrical power levels as low as 2 kWe (6,830
and $250,000 to $500,000 for a 1.5 to 3 MW (5 to 10 million Btu/h) and could be used in domestic household applications
Btu/h) system. Cost of installation is extremely variable be- for combined heat and power.

2 Revised January 2008

One small generating plant might use 454 ovendried kg (0.5 total of about 31.2 MW (106 million Btu/h). This is the
ton) of wood fuel per day or 0.164 million ovendried kg (180 equivalent of about 136,000 ovendried kg (150 tons) of wood
tons) per year. In the short term, 20 of these plants might con- per day or 50 million ovendried kg (55,000 tons) per year.
sume 3.27 million ovendried kg (3,600 tons) of wood from Here, a 5% increase that might be supplied from forest resi-
small-diameter trees per year. dues amounts to 2.5 million ovendried kg (2,750 tons).

Small Scale Electricity

Space or Process Heat Forest-products manufacturing plants also have medium-scale

generating facilities. Sometimes separate companies located
Many U.S. schools use wood combustion to produce space close to the manufacturing plant site will buy plant residues
heat in the range of 1 to 5 MW (3.41 to 17.1 million Btu/h). for fuel to generate and sell electricity back to the manufactur-
Types of fuel used are whole tree and mill chips, pellets, and ing facility and the grid. With such arrangements, the forest-
briquettes. The typical heating medium is hot water instead of products company does not finance the cost of the generating
steam. Low- and high-pressure steam systems may require ad- plant. California has medium-sized power-generating plants in
ditional operator attention and maintenance that could make Mount Lassen, Rio Bravo, and Hayfork.
wood heat uneconomical.
Cogeneration
Known capacity at educational institutions in the Midwest and
several other states is a total of about 120 MW (410 million Medium-scale cogeneration plants would be suitable for pro-
Btu/h). This is the equivalent of about 0.548 million ovendried ducing electricity and processing steam for dry kilns at a lum-
kg (600 tons) of wood per day or 200 million ovendried kg ber manufacturing plant. A new sawmill installation at
(220,000 tons) per year. Here, a potential 5% increase sup- Vilppula in central Finland demonstrates this application. The
plied from forest residues amounts to 10 million ovendried kg new unit has a thermal capacity of 13.5 MW (46.1 million
(11,000 tons). Btu/h) and produces heat for a district-heating network in ad-
dition to providing energy for a 9.0 MWth heat boiler. This
Electricity plant with an electrical output of 2.9 MWe (9.9 million Btu/h)
also produced over 70% of the electricity needed internally.
Small-scale electrical generation with wood fuel is in place in
many locations in the United States; often these facilities are Total power-generating capacity in the 5- to 15-MW (17.1 to
at forest-products manufacturing plants. Frequently, excess 51.2 million Btu/h) range from wood in the United States is
capacity or generation of electricity during times of low-load about 1,160 MW (3,960 million Btu/h). Based on high heating
demand results in power that can be sold back to the local value, this is the equivalent of about 5 million kg (5,500 tons)
power grid. of ovendried wood per day or 1,830 million ovendried kg
(2 million tons) per year. Here, a 5% increase that might be
Cogeneration supplied from forest residues amounts to 90.9 million oven-
dried kg (100,000 tons).
A few Vermont schools use boilers with close-coupled gasifi-
ers at the 1- to 3-MWth (3.41- to 10.2-million Btu/h) level to Large Scale
generate hot water for space heating. If configured to produce
both heat and electricity, these units could produce between Space and/or Process Heat
500 kWe and 1.5 MWe (1.71 and 5.12 million Btu/h).
Large-scale plants using wood fuel are common in forest-
Total power-generating capacity in the 1- to 5-MW range from products manufacturing plants. At Fort James Corporation in
wood in the United States is about 310 MW (1,075 million Green Bay, Wisconsin, a combustor boiler produces 27.8 MW
Btu/h). This is the equivalent of about 1.36 million ovendried (95 million Btu/h) of electricity using fuel from the paper mill
kg (1,500 tons) of wood per day or 500 million ovendried kg and deinking sludge. Other combustors made by the same
(550,000 tons) per year. Here, a 5% increase that might be company operate on fuels such as medium-density fiberboard
supplied from forest residues amounts to 25 million ovendried waste, sander dust, board trim, and hog fuel.
kg (27,500 tons).
An educational institution in Moscow, Idaho, operates a
Medium Scale hogged wood-fuel burning facility with a capacity of about
25.8 MW (88 million Btu/h). Another institution in Rolla,
Space or Process Heat Missouri, has a facility with a capacity of about 39.6 MW
(135 million Btu/h) that burns coal and wood. If these two fa-
A few educational facilities in the United States (e.g., Massa- cilities operated totally on wood at maximum capacity, there
chusetts, Minnesota, Mississippi) use wood for space heating would be a demand for 104 million ovendried kg (115,000
in this category. Various types of combustors, boilers, and fu- tons) of wood per year. A 5% increase in demand would
els are used. Known capacity at educational institutions is a amount to 5.21 million ovendried kg (5,800 tons).

3 Revised January 2008

Electricity IGCC plants and a few smaller biomass (mainly wood) IGCC
plants are operational. Nuon Power in Buggenum, Nether-
Biomass-fueled utility power plants are located in California, lands, uses a hard coal and biomass blend feedstock in a 253-
New England, and other areas of the United States. The aver- MW (863-million-Btu/h) plant. At Cascina, Italy (near Pisa),
age size of these plants is 20 MWe (68.3 million Btu/h). Larger Bioelettrica SpA has a 16 MWe (55 million Btu/h) and 41
plants can exceed 50 MWe (171 million Btu/h). MWth (140 million B/h) IGCC plant. These plants use agricul-
tural residues, sawdust, short-rotation coppice, and wood chips
In Vermont, two power plants (Burlington and Ryegate) use as feedstock.
whole-tree chips as their primary fuel source, although mill
chips and pellets are combusted as well. Large utility systems The Värnamo, Sweden, wood-using IGCC plant produced
are designed with fuel storage and handling systems and com- 6 MWe (20 million Btu/h) and 9 MWth (30 million Btu/h),
bustion systems that can use virtually any wood fuel. The most which was channeled into the district heating system of the
viable source is whole-tree chips that cost $22.05 to $33.07 city during the heating season. The Värnamo plant is the
per 1,000 green kg ($20 to $30/ton) in September 2004. A rule world's first biomass-fueled IGCC plant and was developed by
of thumb is that harvesting (cutting and skidding) costs are Sydkraft AB and Foster Wheeler International. The plant was
about $7.72 to $11.02 per 1,000 green kg ($7 to $ 10/green shut down in 2000 and reopened for research in December
ton), stumpage costs about $1.10 per 1,000 green kg ($1/ton), 2003.
and chipping about $4.41 per 1,000 green kg ($4/ton). Truck-
ing is in addition to these amounts. The installed capacity of power plants burning timber residues
in the United States was about 7,497 MW (25,600 million
Transportation is the highest variable cost because of the dis- Btu/h) as of 2002. Some of these plants are operable, but are
tances that chips travel to the plant (i.e., the closer the chip not currently operating. If the 7,497-MW (25,600 million
source, the less expensive the chips). Typically, the majority Btu/h) installed capacity is converted to wood requirements
of wood chips are transported within an 80.4-km (50-mile) based on 5.5 MW per 1,000 ovendried kg (17 million Btu/
radius of the plant. Therefore, location of a new plant requires ton), the requirement would be 32.7 million ovendried kg
much foresight to ensure the plant would have a continuous (36,000 tons) per day or 11,900 million ovendried kg (13.2
chip supply available for the years of plant operation. Provid- million ovendried tons) per year. This number is not adjusted
ing the necessary tonnage requires appropriate estimations. to account for efficiency, (electrical power generation is
Total tonnage on a 0.40-hectare (1-acre) area could vary from roughly 25% efficient). However, because some plants are not
55,100 to 110,200 green kg (50 to 100 tons), depending on operating and all plants do not operate at full capacity for 24
species, stocking, and past harvest practices. Harvested ton- hours day in and day out, the calculated wood requirement
nage could be a third to half those amounts or more. based on total capacity without adjustment for efficiency
should be reasonable. If 5% of the market could be served by
Chip texture is the main quality-control issue for plants, and increased harvests of small-diameter timber through added ca-
consistent, uniform size is the main reason that mill chips are pacity, greater use of existing capacity, or substitution of wood
used in small-scale wood systems. In general, mill chips are from harvests of small-diameter material for existing wood
high quality and cost $11.02 to $16.53 per 1,000 green kg fuel supplies, this would amount to 600 million ovendried kg
($10 to $15 per ton) more than whole-tree chips. Maximum (660,000 tons) per year.
daily wood consumption and energy production of the steam
turbines for the two plants are 1.66 million green kg (1,825 Thermal and Electric Power
tons) and 50 MWe (171 million Btu/h) for the Burlington plant
and 0.636 million green kg (700 tons) and 20 MWe (70 million Residential
Btu/h) for the Ryegate plant. Both Burlington and Ryegate are
operating at approximately 25% efficiency. Housing represents the largest share of wood-fuel use in the
United States. A large volume of wood is burned in fireplaces
Cogeneration for ambience, and many houses have wood-heating and wood-
pellet furnaces. Some heating units burn wood chips, and
With a backpressure steam turbine, combined generation of wood sawdust fuel has been successfully used. At a Vermont
thermal energy and power results in relatively low power out- Public Housing Authority project, an efficient wood burner
put, compared with thermal load output. Recent economic provided heat at only $26 per apartment per month for the en-
studies of large units have not been favorable. Instead of using tire apartment complex for 9 years.
steam turbines, gas turbines have a greater overall efficiency.
Demonstration of integrated gasification combined cycle
(IGCC) power plants (also called gasification combined-cycle Commercial
plants or bottoming-cycle gasification plants) is under way.
Gas turbines are extremely sensitive to particulates that easily Public institutions, including schools, hospitals, prisons, and
erode turbine blades, so with solid fuels that tend toward more municipality-owned district heating projects, are prime possi-
contamination in the gases they produce, progress in solid fuel bilities for using biomass energy. Many schools in Michigan,
use has been slow. Nonetheless, several large-capacity coal Minnesota, Vermont, Wisconsin, Arkansas, Georgia, Ken-

4 Revised January 2008

tucky, Missouri, Tennessee, Pennsylvania, and Maine heat was projected to burn 280,000 tons of wood waste each year,
with wood. feeding 25 MW (85 million Btu/h) of power into the Minne-
sota power grid. The heat from the plant that incorporates a
A number of colleges have central heating systems, and at unique combination of renewable energy, CHP, and district
least 10 of them use wood. Fredericton, New Brunswick, has a heating technologies meets 80% of the annual energy re-
wood-energy heating system for the university and town. At quirement in downtown St. Paul.
the university campus, buildings including laboratories, a
greenhouse, and a large hospital with high steam requirements In Nederland, Colorado, a town 20 miles west of Boulder, a
are heated with wood. At the State House in Montpelier, Ver- community biomass project set out to prove the viability of
mont, a wood-fired steam plant serves the campus of state using forest waste to provide heat and power. The community
government buildings. A hospital in Michigan, a prison in center with 20,000 ft2 of conditioned space was used to con-
New York, and a forestry laboratory and greenhouse in Nova duct pilot studies. The system used a 100-horsepower boiler
Scotia all heat with wood. The Oujé-Bougoumou community (3.3 million Btu/h or 3,450 lb of steam/hour). The total cost of
in northern Quebec uses sawmill waste, including sawdust, for the project, including purchase of a used boiler, was $443,000,
central heating of all buildings. In 2002, a wood combustion and the saving in fuel cost was estimated at $8,150 per year.
system was installed at Mount Wachusett Community College
in Gardner, Massachusetts. The 8-million-Btu/h (2.4-MW) Beyond saving on fuel costs, the environmental and social
wood-fired hydronic heating plant, which uses wood chips for benefits include reduction of air emissions (as compared with
fuel in a direct-combustion process, replaces the college's prescribed burns), use of a rapidly renewing fuel source, im-
costly electric 11.3-million-Btu/h (3.3-MW) heating system. provement of forest health, reduction of losses from wildfires,
The system will use 1,000 tons of wood chips during one heat- economic development, and public relations value.
ing season to heat the 427,387 ft2 of space at the college's
Gardner campus. Electricity savings are estimated to be Industrial
3,382,518 kW (12,180,000 MJ or 1.55 billion Btu) annually.
Brick and lime kilns are effective users of wood and wood
In central or district heating for municipalities, using wood charcoal in large quantities in foreign countries such as Brazil.
may reduce coal consumption. Chilled water for central cool- Such applications also exist in North America, with potential
ing in summer can also be produced. Charlottetown, Prince for greater use of wood in brick kilns. The potential also ap-
Edward Island, Canada, has two wood-fired district heating pears logical for expanded use of fuel derived from wood in
systems. lime kilns in the kraft pulp industry.
Several conference centers and other privately owned build- The potential seems even greater in the cement industry,
ings use wood heating and cooling effectively. A good exam- where the primary raw material for cement manufacture is cal-
ple of a modern wood-burning system is the demonstration cium carbonate or limestone. Depending on the type of proc-
plant at the Lied Conference Center (Nebraska City, Ne- ess, cement manufacturers can require large amounts of fuel to
braska). The plant consists of a bin and an auguring and me- heat materials to 1,500°C (2,700°F). It takes about 180 kg
tering system for wood-chip fuel, two fire-tube boilers, and a (400 lb) of coal to make about 900 kg (1 ton) of cement. Ce-
computerized control system. The boilers are rated at 1.2 MW ment production results in emitting high levels of CO2 into the
(4 million Btu/h or 115 boiler horsepower or 4,000 lb of atmosphere from the calcining process, the conversion of cal-
steam/hour) and 2.3 MW (8 million Btu/h or 230 boiler horse- cium carbonate (limestone) to calcium oxide (lime) through a
power or 8,000 lb of steam/hour). At an installed cost of about burning process. As a result of the high CO2 emission levels,
$375,000, the plant in winter provides steam to generate hot cement plants are recognized as being major generators of this
water for space heating, bathrooms, a laundry, and a large greenhouse gas. High amounts of sulfur in coal used in cement
swimming pool. Water is chilled through a refrigeration cycle manufacture also result in lower cement yields from lime-
in which water vapor from an evaporator is absorbed by a lith- stone. The calcium sulfate produced in removing sulfur with
ium bromide (LiBr) solution. The diluted LiBr solution gives limestone becomes a disposal problem.
up its refrigerant water again when energy in the form of heat
is added to vaporize the water. Thus, water performs the func- Utility
tion of other refrigerants such as freon.
Utilities are firing more wood fuel in response to the Public
In New Hampshire, a resort hotel produces space heat, hot wa- Utilities Regulatory Power act of 1978 (PURPA) and Renew-
ter, and process heat for manufacturing from wood fuel. At able Fuel Standards by States in the last few years. Companies
least one motel in Vermont is heated with wood. are also choosing to co-fire biomass with coal to save fuel
costs and earn emissions credits. As a result of such regulatory
Municipality requirements and consumer demand, an increasing number of
power marketers are starting to offer environmentally friendly
St. Paul, Minnesota, is now drawing on wood waste to heat electricity from wood and other sources.
and cool most of its downtown buildings while also generating
electricity. The new combined heat and power (CHP) plant

5 Revised January 2008

Besides co-firing are direct fired and gasification systems. were also used as a source of direct heating energy. Some-
Most of today's biomass power plants are direct-fired systems times gasifiers were oxygen-blown; oxygen instead of air re-
that are similar to most fossil-fuel fired power plants. sults in a medium-Btu energy gas.

Whereas steam generation technology is very dependable and Today, a new generation of low energy, gas-producing gasifi-
proven, its efficiency is limited. Biomass power boilers are ers with better systems for cleaning and control are being de-
typically in the 20 to 50 MW range, compared with coal-fired veloped. Not only are these new gasifiers more reliable for
plants in the 100 to 1500 MW range. The small-capacity conventional applications, such as driving internal combustion
plants tend to be lower in efficiency. Because of economic engines, but they also may find suitability for use with Stirling
trade-offs, efficiency-enhancing equipment cannot pay for it- engines, micro-turbines, and fuel cells.
self in small plants. When wood plants replace coal, they re-
duce sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon In July 2007, the state of Georgia awarded a development
dioxide (CO2) to the air. Wood gasifiers can be more efficient company a construction permit to build a 100-million-gallon-
than direct burning, and usually the gas may require cleaning per-year cellulosic ethanol plant. This is the first plant to use
to remove problem chemical compounds. synthesis gas from wood to produce transportation fuel.

Circulating Fluidized Bed Units

Wood System Design
Air-blown circulating fluidized bed appliances for use with
The most important factors in performance of biomass- biomass that provide hot-fuel gas for lime kilns and boilers
combustion systems are solid engineering design and effective have been in use since the 1980s. Size and moisture content of
controls, regardless of the type of combustion system used. the fuel can vary in this type of combustion bed. Circulating
Institutional and commercial heating systems primarily use fluidized beds are now being demonstrated with coal and natu-
direct-burn and two-stage combustors. ral gas-fired utility boilers, and development of circulating flu-
idized bed gasifiers for use with gas turbines is under way.
In a direct-burn system, the combustor is a single-combustion
chamber with a large volume that allows combustion gases to Combined Cycle Gas Turbines
rise directly to the opening of the heat exchange passages at
the top of the boiler. Relative simplicity and low costs are fea- The new low energy-producing gasifiers gain improved per-
tures of direct-burn systems. The firebox may also be sur- formance in power generation through the use of integrated
rounded by a water jacket containing a large volume of water gasification combined cycles (IGCC) in turbine operation. In
for use as hydronic heating for residences or small wood- these systems, gas undergoes combustion in the turbine, and
product operations. the heat recovered from gas-turbine exhaust (flue gas) can be
used to generate power and heat in a steam turbine. The envi-
For the two-chamber systems, a separate refractory-lined ronment is the primary beneficiary of the combined-cycle
combustor, the primary chamber, sits next to the boiler con- technology because more energy can be produced per pound
nected by a short opening that is also refractory-lined (a blast of CO2 emitted than in simple-cycle technology.
tube). The primary chamber houses the grates, the fuel, and
the air-fed components, just like the direct-burn system. Hot A circulating fluidized bed gasifier is proposed for use with
gases from the combustor pass through the blast tube or di- gas turbines at the Vienna University of Technology (TU Vi-
rectly into the combustion chamber of the boiler, the secon- enna, Austria).
dary chamber. The two-chamber system can burn both high
and low moisture biomass fuels. A variation of the Fuel Cells
two-chamber system is the close-coupled gasifier that restricts
the combustion of air so that wood gases produced are not al- Woody biomass gasification is promising to generate a prod-
lowed to bum in the primary chamber but in the secondary uct suitable for use with the rapidly developing fuel cell tech-
chamber. nology. A major advantage of wood for producing fuel-cell
fuels is low sulfur content, because fuel cells are very sensitive
New and Existing Technology to this contaminant.

Additional advantages are high volatility and reactivity. Thus,

Gasification biomass gasifier fuel for fuel cells could lead to lower operat-
ing temperatures and pressures than would be possible with
Low Energy Gas coal gasifiers.
Gasification of wood and charcoal flourished around the world
during World War II. Gas with low energy content could be
produced to run internal combustion engines for over-the-road Cofiring
transportation as well as marine transport. Even some military
tanks were operated with gasifiers. These gasifiers were Cofiring often refers to the practice of introducing biomass as
downdraft and air-blown, but updraft and side-draft gasifiers a supplementary energy source in coal plants. It is a near-term,

6 Revised January 2008

low-cost option for using woody residue to produce electricity lons of ethanol produced from cane sugar to the United States
costing approximately $2 per kW of total capacity for capital in 2006 (66% of total ethanol imported into the United States).
investments on a power plant burning 10% wood ($200 kW In Germany, sources being considered are beet sugar, starches
per biomass kW). For example, a 500-MWe coal plant burning from grains such as wheat and rye, and potatoes. Germany is
10% wood would pay $1 U.S. million for retrofitting their sys- also considering lignocellulosic sources that include fast-
tem to handle woody biomass. According to the U.S. Depart- growing poplars, willows, miscanthus, and Jerusalem arti-
ment of Energy Biomass Power Program (May 1999 Biomass chokes. In the United States, wood residues could be an eco-
Cofiring: A Renewable Alternative for Utilities and Their nomical and environmentally desirable raw material. For
Customers), seven utilities burning at least 7% wood reduced ethanol from wood to be economically viable, availability of
their NOx emissions by 15% compared to burning 100% coal. the raw material, efficient manufacturing, well-managed prod-
Extensive demonstrations and trials have shown that effective uct marketing, and federal and state subsidies are important
substitutions of biomass energy can be made up to 15% of the factors.
total energy input.
Ethanol burns cleaner than gasoline and diesel, and its octane
Investments are expected to be $100 to $700 per kW of bio- rating is greater than that of regular gasoline. Ethanol has a
mass capacity, with the average ranging from $180 to $200 lower energy density than regular gasoline, but because of its
per kW. Cofiring results in a net reduction in emissions of sul- higher octane rating, can be burned more efficiently in high-
fur dioxide, nitrogen oxides, and non-renewable carbon diox- compression engines than gasoline. Other aspects of using
ide. ethanol and preventing some previous problems, such as
eliminating coatings on interiors of fuel lines and facilitating
Cogeneration cold weather starting, are readily attainable.

Cogeneration is the simultaneous production of heat and elec- As a Fuel Additive - In some cities and surrounding areas,
tricity, commonly called combined heat and power (CHP), known as non-attainment areas, ethanol may be used as an
from a single fuel. Traditionally, a steam turbine is used to oxygenate in gasoline during summer months. Its use is man-
produce electricity; although a wood gasification/internal dated in some cases, where other agents, mainly methyl terti-
combustion engine combination can also be a cogeneration ary butyl ether (MTBE), are banned. Production of ethanol
unit. Several factors affect the economic feasibility of a CHP from corn in the Midwest has increased dramatically in the
unit, including wood waste disposal problems, high electricity last several years, partially because of the MTBE ban in 21
costs, and year-round steam use. states. Legislation has also been introduced to ban MTBE na-
tionwide, but this is not proposed to take effect until 2012.
Two common mistakes when installing a CHP system are buy- Probably most states would already have banned it by then. As
ing a steam boiler that is designed for less than 100 lb- of January 2007, present total existing biomass ethanol capac-
force/in2 (689 kPa) or over-sizing the system. Buying a steam ity is 5635.6 million gallons per year with total under new
boiler that is designed for less than 100 lb-force/in2 (psig) re- construction or expansion plans increasing to 6.123 billion
sults in a quality of steam that is not adequate for turbine op- gallons per year.
eration. Over-sizing the system results in additional capital
and operating costs, not better quality steam. Methanol

More electricity and heat are generated for a lesser amount of Methanol is another potential liquid fuel that can be manufac-
fuel by a CHP unit than by a separate heat and power (SHP) tured from wood. Methanol is known as wood alcohol, as it
unit. Common challenges for all wood-fired systems are en- was most commonly made from wood during the 1920s.
suring adequate fuel procurement and solving the complex However, methanol was a byproduct of charcoal manufacture
fuel-handling and storage issues. through destructive distillation. When it began to be synthe-
sized from natural gas, methanol from wood could no longer
compete. Today, some methanol is made from wood and coal
Liquefaction through gasification, forming synthesis gas (syngas), and con-
verting syngas to methanol, much in the way natural gas is re-
Ethanol formed to syngas and converted to methanol. However,
making methanol from wood is more complex than making it
As a Motor Fuel - Although different types of liquid fuels, from natural gas.
including gasoline and diesel, could be made from wood,
ethanol is most commonly produced from biomass. Biomass Methanol has a lower energy density than ethanol, and metha-
ethanol is mostly produced through fermentation with poten- nol is a toxic substance. However, methanol can be made from
tial production through gasification. In the United States, wood at higher yields than ethanol. Making methanol from
ethanol is made mostly from corn grain with an annual pro- wood uses all wood components, including lignin and bark;
duction of 4.86 billion gallons in 2006 in an Energy Informa- but ethanol is only made from cellulose and hemicelluloses
tion Agency report (December 2007 Petroleum Supply with currently available hydrolysis and fermentation technolo-
Monthly, Appendix D). Also, Brazil exported 434 million gal- gies.

7 Revised January 2008

Bio-oil charcoal is commonly used for cooking and manufacturing
steel.
Pyrolysis oil (bio-oil) is a liquid fuel with medium heating
value that can be used to generate electricity and heat at indus- Prices
trial locations such as saw mills, pulp and paper mills, wood
processors, agricultural facilities, and recycling facilities. Be- Figure 1 shows the cost of five fuel types based on representa-
cause it is derived from biomass, pyrolysis oil is deemed to be tive average residential unit costs that DOE is required to
greenhouse-gas neutral. It has virtually no sulfur, low nitrous maintain. These costs for the five fuels were published in the
oxide emissions and very low particulates (significantly lower Federal Register on March 16, 2007 to take effect on April 20,
than diesel) when combusted. Pyrolysis oil can be used di- 2007. The reported costs are shown with assumed costs for
rectly at the point of production. Pyrolysis oil is also trans- wood fuel types including pellets and chips. No allowance has
portable, opening potential for small power-generation plants been made for conversion efficiency. Because market prices
to service installations such as hospitals, schools, universities, for fuels vary, this comparison should be considered as a gen-
hotels, and other commercial and industrial facilities. eral guideline only.

Efficiency is an important determination of how well a fuel is

The produced oil is acidic with a pH of 1.5 to 3.8 and has an utilized through existing technology. In Table 1, note that for
elevated water content 8% to 20% by weight. This leads to wood, the greater the moisture content, the lower the overall
corrosion problems, especially at higher temperatures. The efficiency.
oxygen content is 40% to 50%, mostly from the water content.
The lower heating value is approximately 16–21 MJ/kg Table 1. Overall weighted-average efficiency of wood and
(6,900–9,000 Btu/lb). The pyrolysis oil is not auto-igniting in other competing fuels
a diesel engine. The cetane number is only – 10. The viscosity
increases to a maximum in 12 months because of
polymerization. The pyrolysis oil is not stable reacting with air
and degasing. Pyrolysis oil cannot be blended with diesel. Fuel Power plants (%) Other uses (%)
Coal 35 45–60
Gas 45 80–95
A new plant in Guelph, Ontario, Canada, used approximately
Wood 22–25 65–80
40 m3 (1,400 ft3) of wood waste to produce an intermediate Nuclear 34–37 NA
grade bio-oil. The first run was at the equivalent daily rate of Oil 38 80
50 tonnes (55 tons) of feedstock processed and the second at a Propane NA 80
rate of 100 tonnes (110 tons).

Pellets and Briquettes

As wood is refined into other forms, its value as a fuel in- Residential Fuel Costs for Various Fuels
creases. Benefits of refining include facilitation of handling,
transportation, and storage; improved durability; burning with $35.00
increased efficiency; lower variability; and higher energy den- $30.00
($/million Btu)

sity. $25.00
Cost

$20.00
$15.00
Manufacture of pellets and briquettes provides most of these $10.00
advantages, with the exception of higher energy density. $5.00
These fuels are dry and better energy carriers than wet wood. $0.00
Also, in the case of fireplace log briquettes that are usually
il

ts
ne

s
ty

ga

ip
an

lle
ci

se

ch
g
tri

pe
op

al

made with the addition of petroleum-derived wax, they have a

tin

ro
ec

ur

d
Pr

ea

Ke

oo
El

at

oo
H

higher energy density than wood. Pellets are easily manufac-

N

W
W
2
o.

tured and provide an excellent fuel for automated controlled

N

burning in pellet stoves and pellet boilers.

Charcoal
Throughout history, charcoal manufacture has been used to Figure 1. Representative average costs per million Btu of
five fuels as published in the Federal Register by the U.S.
improve fuel characteristics of wood. It is a simple, but cum-
Department of Energy on March 16, 2007. These fuels are
bersome, process that characteristically requires much atten- compared with wood pellets selling at $150 per ton and
tion to details to prevent air pollution. Charcoal manufacture wood chips selling at a minimum of $30 per green ton.
in the United States is limited primarily to briquettes for resi-
dential and recreational use and, to a lesser degree, to manu-
facture activated carbon for industry. In some countries,

8 Revised January 2008

Glossary Cogeneration—Combined heat and power (CHP).

Ash—The noncombustible components of fuel. Combined heat and power (CHP)—The simultaneous pro-
duction of heat and mechanical work or electricity from a sin-
Ash fusion temperature—The temperature at which ash gle fuel.
melts.
Combustion air—Air that is used for the burning of a fuel.
Biogas—A gas produced from biomass, usually combustible.
Combustion efficiency—The efficiency of converting avail-
Biomass—Organic matter available on a renewable basis. able chemical energy in the fuel to heat. It measures only the
Biomass includes forest and mill residues, agricultural crops completeness of fuel combustion that occurs in the combus-
and wastes, wood and wood wastes, animal wastes, livestock tion chamber.
operation residues, aquatic plants, fast-growing trees and
plants, and municipal and industrial wastes. Combustor—The primary combustion unit, usually located
next to the boiler or heat exchanger.
Bottom ash—Ash that collects under the grates of a combus-
tion furnace. Cyclone separator—A flue-gas particulate-removal device
that creates a vortex to separate solid particles from the hot gas
Boiler horsepower (BHP)—The equivalent of heat required stream.
to change 15.6 kg (34.5 lb) per hour of water at 212°F (100°C)
to steam at 212°F (100°C). One BHP equals 9.81 kW (33,479 Densified biomass fuel—Biomass material that has been
Btu/h). dried and compressed to increase its density (e.g., pellets).

Bridging—Wood fuel in a storage bin, hopper, or conveying District energy system—A system using central energy
system that supports itself although the fuel beneath has plants to meet the heating or cooling needs or both of residen-
moved. Bridging is one of the most common problems associ- tial, institutional, commercial, and industrial buildings.
ated with wood-handling systems.
Excess air—The amount of combustion air supplied to the fire
British thermal unit (Btu)—A standard unit of energy equal that exceeds the theoretical air requirement to give complete
to the heat required to increase the temperature of 1 lb (0.45 combustion.
kg) of water 1°F (0.56°C).
Flue gas—All gases and products of combustion exhausted
Carbon cycle—The process of transporting and transforming through the flue or chimney.
carbon throughout the natural life cycle of a tree from the re-
moval of CO2 from the atmosphere to the accumulation of Fly ash—Ash transported through the combustion chamber by
carbon in the tree as it grows, and the release of CO2 back into the exhaust gases and generally deposited in the boiler heat
the atmosphere when the tree naturally decays or is burned. exchanger.
Carbon sequestration—The provision of long-term carbon
storage in the terrestrial biosphere, underground, or oceans, so Fuel cell—A cell similar to a battery that uses an electro-
that the buildup of carbon dioxide (principal greenhouse gas) chemical reverse electrolysis process to directly convert the
concentration in the atmosphere reduces or slows. chemical energy of a fuel (gas, propane) into electricity, heat,
and water.
Char—Carbon-rich combustible solids that result from pyro-
lysis of wood in the early stages of combustion. Char can be Gasifier—Any device that changes solid biomass into a gase-
converted to combustible gases under certain conditions or ous fuel.
burned directly on the grate.
Hog fuel—Fuel generated by grinding wood and wood waste
Clinker—A slag-like material formed in the combustion for use in a combustor.
process when the temperature of combustion exceeds the ash
fusion temperature of the fuel. Kilowatt—A standard unit for expressing the rate of electrical
power and useful heat output. The symbols e and th stand for
Chipper—A large device that reduces logs, whole trees, slab electrical and thermal, respectively.
wood, or lumber to chips of more or less uniform size. Sta-
tionary chippers are used in sawmills, whereas trailers Live-bottom trailer—A self-unloading tractor-trailer with a
mounted whole-tree chippers are used in the woods. hydraulically operated moving floor used to remove biomass
fuel.
Cofiring—Utilization of bioenergy feedstocks as a supple-
mentary energy source in high-efficiency boilers. Metering bin—A bin in the fuel-feed stream that allows a
precise feed rate of the fuel to the fire.

9 Revised January 2008

Mill chips—Wood chips produced in a sawmill.

On/off fuel feed—A fuel-feed system that transports fuel to

the grates on an intermittent basis in response to boiler water
temperature and load variations.

Over-fire air—Combustion air supplied above the grates and

fuel bed.

Particulates—Minute, solid, airborne particles that result

from biomass combustion.

Pyrolysis—A process of reduction at oxygen-starved condi-

tions, involving the physical and chemical decomposition of
solid organic matter by the action of heat into liquids, gases,
and a carbon char residue.

Residence time—The length of time the fuel remains in a

combustion zone.

Seasonal efficiency—The ratio between the total useful en-

ergy delivered to the thermal load over the full operating sea-
son and the total potential energy within the fuel burned over
the period.

Steady-state efficiency—Ratio of output-to-input energy

when combustion system is operating under design conditions.

Turndown ratio—A ratio found by dividing the maximum

energy output by the minimum output at which efficient,
smoke-free combustion can be sustained.

Under-fire air—Combustion air added under the grates.

Whole-tree chips—Wood chips produced in the woods by

feeding whole trees or tree stems into a mobile chipper that
discharges directly into a tractor-trailer.

Wood gasification—The process of heating wood in an oxy-

gen-starved chamber until volatile pyrolysis gases (e.g., CO,
H2, O2) are released from the wood. The gases emitted are
low- or medium-energy-content gases that can be combusted
or used to produce chemicals in various ways.

10 Revised January 2008