Biomass is any organic matter including forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, and municipal and industrial wastes (Bergman and Zerbe 2004). For the purpose of this encyclopedia, we will focus only on the biomass materials generated from forests and wood-processing wastes.

Annual Biomass Resource Potential
from Forest and Agricultural Resources

These woody materials can be forest residues such as thinnings or residual slash from harvest operations, wood wastes such as bark, sawdust, chips, or wood-processing wastes such as black liquor. It is estimated that nearly 5.6 million tonnes of unused wood residue is generated in U.S. sawmills yearly (Smith et al 1994).

Dry wood has roughly twice the caloric value of wet wood (Zerbe 2006) and with varying physical and chemical composition, it is useful to have an understanding of the different ways in which plant species, moisture and energy content, and ash affect biomass utilization.

This section is divided into:

• Wood Composition
• Properties of wood
• Forest Residues
• Wood-processing residues

Stack of Pine

Understanding the structure and composition of woody biomass is vital to efficient utilization. Wood is one of the most abundant resources in the bio-based industry and yet it is also one of the most complex materials, composed of polymers of lignin and carbohydrates that are physically and chemically bound together. Considerable amounts of energy are required to separate the polymers from each other (Committee on Biobased Industrial Products 2000). Deconstructing the hundreds of components that constitute woody material is the key to producing a substitute for petrochemicals. These components can be classified as:

• Cellulose
• Hemicelluloses
• Lignin
• Mineral Elements

Comprising nearly half of woody plant composition by weight, cellulose is a carbohydrate polymer composed of glucose chains (Miller 1999). Cellulose, comprised of carbon, hydrogen, and oxygen in the form of starches, proteins, and sugars, is the most abundant organic material on earth. It is isolated during the pulping process and processed to yield ethanol, cellophane, and cellulose ethers such as acetate, rayon, and nitrates. Due to the relatively high manufacturing cost compared to that of petrochemical polymers, many of these derivatives have only specialty applications.

The primary source of wood cellulose pulp comes from conifer species (Smith et al 1994) but over the last twenty years, hardwood utilization has increased. Processing biomass feedstocks always requires attention to cellulose due to its proportion of total volume and weight.

Cellulose

Starch being Applied to a Plywood Veneer

Starch is the principal carbohydrate reserve of plants. Corn starch currently is a primary feedstock for starch-based ethanol, plastics, loose-fill packing material, adhesives, and other industrial products. Approximately 600 million bushels of corn were used for the production of industrial products during the marketing year 1995 to 1996; of that total, 395 million bushels were used to produce fuel ethanol (ERS, 1996b). While the supply of corn starch has been sufficient to meet current demands, primarily anhydrous motor fuel grade and industrial ethanol, other supplies of sugar feedstocks are being evaluated to meet anticipated increases in demand for oxygenated fuels and chemicals.

Proteins are the primary means of expressing the genetic information coded in DNA. These polymers are based on building blocks of amino acid monomers whose sequence is predetermined by a genetic template. The sequence diversity of proteins is responsible for the wide array of functions performed by proteins in living organisms (OTA, 1993). A variety of plant proteins might one day be commercially exploited as materials, but current understanding of the structural properties of most plant proteins is limited.

Zein is perhaps the best understood plant protein. It has been given the most interest due to its ability to form fibers and films that are tough, glossy, and resistant to grease and scuffing. The Virginia-Carolina Corporation once produced 5 million pounds of Zein fiber before shifting to comparable synthetic fibers. Today Zein is used as a water-impermeable coating for pharmaceutical tablets, binding for bottle-cap liners, and a shellac substitute. The industrial usefulness and merits of Zein have stimulated continual examination of proteins (Wall and Paulis, 1978).

Fermentable sugars are the largest feedstock available to support a bio-based chemicals industry in the United States. A wide range of fermentable sugars can be found in crops and wastes from agriculture and forestry. Major feedstocks include corn, wheat, sorghum, potato, sugarbeet, and sugarcane; other sources include potato-processing residues, sugarbeet and cane molasses, and apple pomace (Polman 1994). Sugars can be produced directly or derived from polysaccharides such as cellulose and starch and then, via microbial fermentation, used to produce a wide range of commodity and specialty chemicals. Existing commercial fermentation primarily utilizes glucose to produce ethanol, acetic acid, amino acids, antibiotics, and other chemicals. Over the long term, new sources of glucose will be required to meet the demands of a bio-based industry. Growth of a bio-based chemicals industry will depend on production of cellulose-rich crops, including those currently under production such as corn and alfalfa and others that presently are not grown commercially, such as switchgrass and hybrid poplar.

Significant increases in glucose reserves are available from lignocellulosic substances found in most plants, crop residues, and waste paper. Cellulose can be hydrolyzed by acid to glucose, although much of the glucose is destroyed during this process. The second most abundant sugar found in hardwood and agricultural residues is xylose, which is derived from xylan hemicelluloses. Xylose is relatively easily recovered by acid or enzymatic hydrolysis and can be fermented to ethanol by naturally occurring organisms or recombinant microbes. The practical sugar yield from lignocellulosics would increase significantly if commercial fermentations could utilize xylose, a 5-carbon sugar or pentose, as well as glucose, a 6-carbon sugar or hexose. Novel genetically engineered microorganisms will play a key role in the direct conversion of cellulose oligomers and 5- and 6-carbon sugars to ethanol.

Enxymatic hydrolysis using mixtures of enzymes, such as cellulase and hemicellulaces, is used to avoid the destruction of sugars associated with acid treatments (hydrolysis) of lignocellulosic material. These enzymes, when combined with effective pretreatment of lignocellulosics, provide high yields of glucose, xylose, and other fermentable sugars with minimal sugar losses. However, these enzymes are currently too costly to use in large-scale conversion of lignocellulosic materials to fermentation substrates.

Hemicellulose is composed of carbohydrates based on pentose sugars, mainly xylose, as well as hexose sugars, such as glucose and mannose. Hemicellulose comprises 25 to 35 percent of the dry weight of wood residues; they are second only to cellulose in abundance among carbohydrates. While use of hemicellulose is currently limited, quantities of hemicelluloses, pectins, and various other plant polymers are abundant in crop residues and have great potential in the production of chemicals and materials. During the pulping process, hemicellulose is pooled with lignin to become the wood-processing residue, black liquor.

Hemicellulose

Lignin Monomer

Lignin is a phenylpropane polymer that holds together cellulose and hemicellulose components of woody biomass. Lignin constitutes about 15 to 25 percent of the weight of woody biomass. Lignin has not yet been used as a raw material for industrial purposes in large quantities. Concerted attempts by pulp and paper research laboratories to develop new markets for by-product lignins have had only limited success (Bozell and Landucci, 1993). Production of low-molecular-weight compounds from kraft lignin, phenols in particular, has also not yet proved to be commercially competitive. This reflects the chemical complexity of lignin and its resistance to depolymerization. Nevertheless, a recent Department of Energy study concluded that pyrolysis of lignocellulosics could make production of phenolics and anthraquinone from lignin competitive, and the potential also exists to produce benzene, toluene, and xylenes from lignin via pyrolysis (Bozell and Landucci, 1993). Lignocellulose pretreatment will be a key step for realizing the presently untapped potential of abundant lignocellulosic materials found in wood (Malherbe and Cloete 2002). Until these higher value products emerge, lignin will continue to be burned to produce energy.

In addition to oxygen, hydrogen, nitrogen, and carbon, trees are composed of many other mineral elements. While these elements do not produce energy during combustion they do affect the energy content of woody biomass. On average, hardwoods have a higher concentration of these mineral elements than softwoods; but the presence of these minerals is more affected by the site where they are grown than their age, species, or size.

Nitrogen is a component of all fuel systems and during the combustion process it is oxidized into NOx. When emitted from combustion facilities at relatively low levels, NOx may have a useful fertilizing effect on forests, however as emission levels increase, NOx produces adverse health effects and increases the acidification of water and soils. Nitrogen concentrations in wood are typically less than that in coal and oil (BROKEN-LINK Alakangas et al. 1987), thus NOx emissions from wood-based fuels are lower and more environmentally friendly than fossil fuels.

Calcium and potassium are the most abundant mineral elements found in woody biomass but other trace elements such as sulfur, chlorine, and heavy metals are also present. Sulfur emissions from combustion of fuels cause extensive damage to ecosystems and buildings, so fossil fuels are often graded by the amount of sulfur present. As occurs for nitrogen, sulfur is oxidized during combustion to form SOx. This compound can have serious environmental effects and causes the acidification of soils and water. The sulfur content in woody biomass resources is much lower than fossil fuels (Richardson et al. 2002). Equipment to remove sulfur during or resulting from the combustion of fossil fuels is expensive. As emission standards become more stringent, woody biomass is considered a viable alternative fuel for reducing SOx emissions associated with fossil fuel sources.

Most chlorine in trees is found in the foilage as an essential component in chlorophyll. Although only present in trace amounts, its ability to form alkali compounds with potassium and sodium can create serious problems for boiler equipment during combustion as a result of oxidation and corrosion (Riedl and Obernberger 1996). Eliminating foilage from woody biomass feedstocks has been shown to reduce corrosion problems, as has co-firing biomass resources with higher sulfur content fuels such as peat or coal (Orjala et al. 2000).

Heavy metals may vaporize during combustion. The remainder contributes to ash formation. Should levels of heavy metals be high, recycling of ash as fertilizer is restricted by environmental legislation, since the metals may leach into ground water or be absorbed by crops. More modern furnaces are often equipped with filters to reduce heavy metal emmisions but it is best that painted or treated wood waste be avoided as a fuel source considering the higher heavy metal concentrations found in these sources.

The properties of wood vary between species and have a direct impact on moisture content which in turn influences the potential energy that is produced from the material.

Properties that influence woody biomass utilization include:

• Plant Species
• Moisture Content
• Energy Content
• Ash Content

The moisture content of biomass material varies greatly. This has a significant effect on many of the energy conversion processes. As an example, when biogas is obtained from an anaerobic digestion process the percentage of solids present in the digestate affects the gas yields. For dry biomass fuels such as wood, the amount of water present has a considerable effect on the proportion of the total heat content of the material that is able to be recovered during combustion.

For wood, the moisture content depends on a combination of climatic conditions, time of year when harvesting takes place, and the duration and method of storage (Simpson and TenWolde 1999). The following simple formulas can be used to calculate the moisture content of biomass:

moisture content (wet basis) = (total weight of wet wood – oven dry weight)/total weight of wet wood * 100

moisture content (dry basis) = (total weight of wet wood – oven dry weight)/oven dry weight * 100.

Energy yields are often expressed as net caloric values. These values increase as wood moisture content is reduced. Thus, the drier the wood, the greater the amount of energy that can be produced. The percent moisture content of the biomass utilized has a large impact upon the design and selection of technologies ideal for energy production.

Net calorific value of waste wood (Pinus radiata)
as a function of moisture content.

The potential energy value of biomass materials is determined by its chemical composition and is measured as Joules of energy in 1 g of fuel (J/g). For convenience this measure is usually expressed as MJ/kg or GJ/t. However, for practical matters the volume-related energy density is a much more important parameter considering the packaging of energy materials, ie. chips vs. sawdust vs. logs.

While the energy value of solid biomass has relatively low variation, the volume of a single unit of fuel equivalent can easily vary by a factor of ten, depending on the method of harvesting or processing. In the case of chipped forest and agricultural biomass, the application of densification technologies can considerably reduce the volume of space required for storage, thus increasing the energy density of the materials.

The total amount of energy released from fuel is called the heating value. Due to differences in chemical composition, softwoods have higher heating values than hardwoods, branches have higher heating values than stemwood, and bark and foilage have higher heating values than wood in general despite the higher ash and heavy metal content (Nurmi 1993). The approximate heating values of wood and various wood residues are shown in the table below (Corder 1976).

Approximate heating values of wood and various wood residues

The energy density of a fuel is the heating value per unit volume. Energy density has been found to be highest in chips from high-density species such as oaks. The southern scrub oak forest type has been shown to be an important component of southern fuelwood resources for industrial heat generation in South Carolina (Harris et al. 2004).

Energy density is relatively low in biomass chips. The space required for transporting and storing chips is 11 to 15 times greater than the space needed for oil, and 3 to 4 times greater than that required for coal (Young 1980). These differences result in higher utilization costs and the biomass material being utilized closer to the source. A commonly used conversion factor shows that a solid cubic meter of wood will produce roughly 2.5 cubic meters of loose chips (Standish et al. 1985).

Factors that influence the solid content of chips include:

1. Particle shape: Reduced ratio between diagonal length and thickness increases solid content (Edberg et al. 1973).
2. Size distribution: Heterogeneous size distribution increases solid content.
3. Tree species: Brittle, low-density materials have a higher solid content.
4. Branch content: Fresh branches and twigs reduce the solid content value.
5. Loading: Compaction from blowing increases solid content versus free-fall loading systems.
6. Storage: Stored materials tend to contain more fine materials and slightly higher solid content.

Comparisons of fuel sources often examine the energy balance ratio or the output energy versus the input to create energy. The energy balance ratio of chips from logging residues and thinnings has been shown to be high compared with other energy crops such as alfalfa, rapeseed, canary reed, and potato (Richardson et al. 2002). Thus, as long as forest fuel is a by-product of other operations and transportation is kept to a minimum, forest residuals make an excellent energy source.

Ash from biomass comes from the minerals present in the structure of the plant and any soil contamination. Characterization of ash by elemental analysis and fusion temperatures is important when selecting biomass fuels because it provides information on how much ash will be generated requiring disposal and indicates the potential for slagging and fouling of burners and boilers (Riedl and Obernberger 1996). When wood is combusted as the only fuel, ash fusion is not usually a problem since the combustion temperatures are likely to be low. However, when biomass is co-fired with coal, combustion temperatures are considerably higher and may reach a level where slagging could occur (Misra et al. 1993).

Ash management presents both a problem and an opportunity. Removal of ash from the furnace and disposal in landfill areas incurs costs for power plants. However, if ash is recycled in the forest or agricultural ecosystems or used to reclaim mine spoils, depletion of plant nutrients, other than nitrogen, and acidification associated with intensive biomass removal is radically reduced.

Sugar and Ash Composition of Feedstocks

All plant species that store primarily carbohydrates or oils are suitable for producing liquid energy sources. Cellulose, starch, sugar and inulin (Ohta et al. 1993) can be used to produce ethanol, and vegetable oils can be used as fuels. Parts of plants containing lignocellulose can provide energy directly as solid fuels or indirectly after conversion (Wright and Berg 1996). Oak Ridge National Lab and Washington State University have conducted research into the utilization of hybrid poplars for bioenergy while the State University of New York has examined other species for utilization.

Populus spp., Salix spp., Eucalyptus spp., and other fast growing species make excellent choices for energy plantations. Various other hardwoods and some softwoods would make better choices for other biobased products due to various differences in sugar composition. Just like making forest management decisions, deciding which species is best for your objectives will depend greatly upon site and economic conditions (Bouton 2002).

To learn about a specific plant species and its role in bioenergy and biobased product utilization, Phyllis has been created. Sponsored by Shell Global Solutions, Agrotechnology and Food Innovations, and maintained by the Energy Research Center of the Netherlands, this site provides analysis and composition data for over 2000 various biomass and waste materials. Biomass materials are group by untreated and treated wood, grasses, agricultural waste, animal waste, and other organic residues. The system also provides comparisons to fossil fuel sources including peat. Phyllis will provide users with information regarding ash and water content, calorific values, and biochemical composition of the material.

Nearly 20 billion cubic feet of wood is removed on an annual basis from lands in the United States. Of that volume, 16 percent is classified as logging residues (Smith et al. 2004). This material is mainly tree tops and small branches that have been considered uneconomical to harvest. The USDA Forest Service Inventory and Analysis program estimates that 61 million dry tons of residuals are available annually from harvesting and fuel reduction activities. A recovery system, that would follow behind a conventional logging operation, could recover 60 percent or 40 million dry tons of this residue for potential bioenergy and biobased product markets (Stokes 1992). To maintain soil properties and nutrients, not all available forest residuals should be recovered during operations (Perlack et al. 2005).

Current Availablity of Logging Residues and Other Removals

Post-logging operations typically result in two categories of waste:

• Harvesting Wastes
• Bark

The outer part of woody stems and branches is comprised of bark and typically comprises 9 to 15 percent of a log's volume (Chang 1954). There are large differences in the types and amounts of various chemical components that comprise bark, even within a single species which depends on growing conditions, age, and various disturbances (Harkin and Rowe 1971). The volume of bark and coarse materials that are available annually in the United States approaches 83 million dry tonnes. Historically, this bark has been used to produce tannins, dyes, resins, flavorings, and medicinal products. As utilization techniques have improved, more and more chemical extracts have become commercially available for utilization. However, bark can be difficult to utilize due to soil contamination during harvesting (Harkin and Rowe 1971).

Bark Pile

The most common use of bark is in mulching or soil amendment (Mater 1969). When worked directly into the soil, bark’s decomposition rate is considerably slower than wood, thus lasting longer; its lower consumption of nitrogen also results in less nitrogen-availability related stress affecting planted vegetation (Allison 1965).

Bark has also been commonly used as a fuel source. On average, 9 tonnes of completely dry bark is the equivalent of nearly 6 tonnes of coal with an effective heating value of 2.34 to 2.93 kWh per pound of bone dry material (Harkin and Rowe 1971). However, these values dynamically decrease as percent moisture content rises. Bark can be processed to create a higher value fuel by manufacturing briquettes but still is limited due to its higher moisture values (Dingwall 1969).

Discarded bark has found an outlet in building materials such as fiber and particleboard (Murphey 1969) and because bark conducts heat less readily than wood, it is a component of insulation board (Martin 1970).

The chemical utilization of bark is still in its infancy, mainly due to the economic expense of transportation, storage, and volume of the material. Very few pure organic compounds have been isolated on a grand scale, such as Salicin from aspen bark but, with advanced fractionation techniques emerging, more compounds are on the horizon (Pearl 1968). More crude fractions such as tannins have been used in larger quantities in the fields of tanning leather, development of drilling muds, adhesives for plywood and particleboard, and taxol for research into cancer treatments (Hathway 1962, Hergert et.al 1965, MacLean and Gardner 1952, Minore and Weatherly 1996).

Forest residues typically refer to those parts of trees unsuitable for sawlogs – treetops, branches, small-diameter wood, stumps, leaves, dead wood and even poorly-formed whole trees – as well as undergrowth and low-value species. While applications may exist for any individual constituent, the difficulty is determining whether there are sufficiently large quantities to warrant separation and harvesting.

If harvested, these residues are sometimes used for combustion or chipping for production of other usable feedstocks. Please see the harvesting section in this encyclopedia for more information.

Primary processing mill residues are very desirable for energy and other bio-based products because they are often clean, uniform, on-site, and have low moisture contents. Because of these desirable traits, very few mill residues are not currently being utilized by industries. Beyond using the residuals for energy and heat, mills are becoming more adept at incorporating them into products such as OSB, acoustic panels, and particleboard.

Forest Products Industry Processing Residues -2004

Wood-processing Residues:

• Black Liquor
• Chips
• Sawdust

Sawdust for Energy

Sawdust is a lignocellulosic biomass source that is the particulate residue of lumber production. Most sawdust is considered green, not dried, and is relatively uniform in size and shape. Sawdust is also refered to as "wood flour", a designation that indicates the particles can pass through a 20-mesh gauge screen.

Sawdust can be used in gasification, combustion, and pyrolysis processes as well as undergo fiber composite manufacturing to produce bedding, abrasives, insulation, and packaging. Sawdust is considered a secondary wood residue in addition to shavings and cut-offs from sawmill operations. Nearly 14.5 million tonnes of seconday wood residues are generated yearly, which sell for about $13 per dry tonne before delivery (Barber 2003).

Chips are described as virgin wood which has beem comminuted by mechanical chippers. Ideal chips are a uniform size and contain no bark or rot. Chip size is a function of wood density, moisture content and orientation in the chipper. Uniformly shaped chips contribute to improved energy production. Chips are often stored on-site for a few months after chipping and care must be taken to limit exposure to the elements. The traditional markets for wood chips have been the pulp and paper industry and landscaping companies.

Wood Chips

Since a load of chips contains much more open space than a solid block of wood, several factors must be considered to determine the tons of chips needed for a specific project. The greater the diagonal-to-thickness ratio of the chips, the lower the solid content (Edberg et al. 1973). More heterogenous chips contribute to reduced space between chips and increase the solid ratio in chip-loads. Chip-loads from whole-trees or logging residues contain more fine materials than uniform chips from pulpwood, and tend to have higher solid content rates. Chip quality also varies according to species, with fuel chips from brittle low-density material containing more fine particles with a higher solid content. The solid content proportion of a fuel chip-load varies from 38 to 44 percent (Nylinder and Tornmarck 1986). A solid cubic meter of wood would produce approximately 2.5 cubic meters of loose chips.

European nations in Scandinavia and the Baltic regions have long used wood chips as a source of energy. One of the newest wood chip suppliers in Europe is GFC, or the Groupe Cooperation Forestiere based in Paris, France.

Groupe Cooperation Forestiere

GCF is the first French forest wood chips producer for bioenergy and the first French group which obtained the long-term durable forest certification of its members. Their goals are to provide private forest owners with the necessary technical assistance to support long-term and durable management, to improve the forests economic competitiveness, and to act as an essential part for regional planning. Beyond providing professional forest management services, harvesting, and reforestation operations, GFC has assisted their landowners in entering the growing market for wood energy by entering into cooperatives with independant enterprises that have produced guaranteed long-term contracts for wood chips off private lands.

In 2004, GFC had 35 members in their wood energy cooperative. They marketed 5 million cubic meters of wood and produced a profit in excess of 230 million Euros, or nearly US $255 Million.

Black Liquor

White liquor is a solution of caustic chemicals, such as sodium hydroxide, that is added to wood chips in Kraft and sulfite pulping processes to dissolve lignin and release cellulose fibers from wood. The fibers are then washed, and the resulting slurry of water, caustic chemicals and dissolved lignin is called black liquor. Black liquor is recycled in a process that gives off heat energy, carbon dioxide, and recoverable chemicals. These chemicals need only a 10 percent supplement of additional caustic chemicals to become white liquor again (Larson and Raymond 1997).

Nearly 47 million dry tonnes of black liquor is produced annually and is typically recovered as part of the pulping process (BEES 2001). Since the liquor is created and consumed in recovery boilers on-site, it is not readily available for off-site utilization. The steam that is generated during the black liquor recovery process contributes significantly to the energy needs of pulp and paper mills. In addition, some chemicals can be recovered from the liquor and through recycling can reduce the pulping processs chemical needs by almost 90 percent.