Woody biomass is converted into useful forms of energy (solid, liquid, or gaseous fuels) as well as useful products (polymers, bio-plastics, char, pellets, and acids) at a biorefinery. A biorefinery is a facility that uses biomass conversion technologies to convert biomass into fuels, power, and value-added chemicals. Each biorefinery process yields different amounts and types of co-products and by-products. Coproducts describe the useful and marketable by-products, other than energy, that are produced simultaneously during biomass conversion. Many of today’s co-products may have traditionally been defined as waste or by-products. Biorefinery process technologies include thermochemical (gasification, pyrolysis), biochemical (fermentation), or chemical (chemical synthesis) pathways.
Each route is currently being developed by many different entities. The main challenge is to determine or discover processing technologies that can collect and convert currently under-utilized woody biomass into products with higher value.
Wood is one of the most abundant feedstock resources in the world available for the production of bioenergy and biofuels. It is also a very complex material. Woody plant species are so diverse that they grow in virtually every part of the world and can be harvested to produce solid, liquid or gaseous energy. Wood, in general, is composed of polymers, which are large organic molecules of lignin and carbohydrates (cellulose and hemicellulose) that are physically and chemically bound together. Different species types have varying amounts of cellulose, hemicellulose and lignin. To understand more about the make-up of various wood species, please see The USDA Forest Service’s Forest Products Lab handbook titled Wood Handbook: Wood as an Engineering Material. All of these components plus the presence of mineral elements affect energy value and utilization potential.
All tree and woody shrub species that store carbohydrates or oils are suitable for producing liquid and gaseous energy sources. Cellulose, starch, sugar and inulin (Ohta K, Hamada S, Nakamura T. 1993) can be used to produce ethanol, while plant oils can be used to produce bio-oil. Parts of plants containing lignocellulose can provide energy directly as solid fuels or indirectly after conversion (Wright and Berg 1996).
To learn about a specific plant species and its role in bioenergy and biobased product utilization, you can access Phyllis. Phyllis is an online database sponsored by Shell Global Solutions, Agrotechnology and Food Innovations and is 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 grouped 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 provides users with information regarding ash and water content, calorific values, and biochemical composition of the material.
Composition of Wood
Wood is composed primarily of cellulose, hemicellulose, lignin, and mineral elements.
- Cellulose: Cellulose is the structural component of a green plant’s cell walls. It is comprised of carbon, hydrogen, and oxygen in the form of starches, proteins, and sugars and is the most abundant organic material on earth. Cellulose makes up approximately 50% of woody biomass’ dry weight. Cellulose from wood is isolated during the pulping process and processed to yield ethanol, cellophane, cardboard, paper, and cellulose ethers such as acetate, rayon, and nitrates.
- Hemicellulose: Hemicellulose is another component of a green plant’s cell walls. Unlike cellulose, which contains only one sugar (glucose), hemicellulose can include a number of sugar monomers (xylose, hexose, mannose, and galactose, for example). Hemicellulose makes up 25 to 35% of the dry weight of woody biomass.
- Lignin: Lignin is a polymer (or glue-like substance) that holds cellulose and hemicellulose together. It makes up approximately 15 to 25 percent of the dry weight of woody biomass and is what can make the biochemical conversion of woody biomass difficult. Lignin has not yet been used as a raw material for industrial purposes in large quantities.
- Mineral Elements:Woody biomass is composed of many elements. Principal elements include carbon, oxygen, and hydrogen. Woody biomass is also composed of mineral elements such as nitrogen, sulfur, chlorine and heavy metals and while these elements do not produce energy during combustion, they do affect the energy content of different types of woody biomass. On average, hardwoods have a higher concentration of mineral elements than softwoods. However, the presence of mineral elements in woody biomass is more dependent on the site where the particular species of tree is grown rather than the particular species itself.
- Nitrogen is a component of all fuel systems. During the combustion process, it is oxidized into nitrogen oxide (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.
- 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 with nitrogen, sulfur is oxidized during combustion to form sulfur oxide (SOx). This compound can have serious environmental effects and causes the acidification of soils and water.
- Most chlorine in trees is found in the foliage as an essential component in chlorophyll. Although only present in trace amounts, its ability to form alkali compounds with potassium and sodium, resulting in oxidation and corrosion, can create serious problems for boiler equipment during combustion2. Eliminating foliage from woody biomass feedstocks can reduce corrosion problems, as can co-firing biomass resources with higher sulfur content fuels such as peat or coal3.
- Heavy metals tend to 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 groundwater or be absorbed by crops.
The moisture content of biomass material varies greatly and plays a large role in determining the most suitable energy conversion process. Wet conversion processes such as fermentation are often more suited to biomass with a higher moisture content (e.g. corn, sugarcane, barley straw). Dry conversion processes such as pyrolysis, gasification, and combustion are more suited to biomass with a lower moisture content (e.g. wheat straw, pine, switchgrass, etc). Generally, wet conversion processes are used when the moisture content of the biomass requires excessive energy for drying, compared to the energy content of the end product.
Energy yields are often expressed as net caloric values. These values increase as wood moisture content is reduced.
The moisture content in wood depends on a combination of climatic conditions, time of year when harvesting takes place, and the duration and method of storage. These simple formulas can calculate the moisture content of woody 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
- Ohta K, Hamada S, Nakamura T. 1993. Production of high concentrations of ethanol from inuling by simultaneous saccharification and fermentation using Aspergillus niger and Saccharromyces cerevisiae. Appl Environ Microbiol 59(3):729-733.
- Wright LL, Berg S. 1996. Industry/Government Collaborations on Short-Rotation Woody Crops for Energy, Fiber, and Wood Products. In: BIOENERGY ’96: The Seventh National Bioenergy Conference; Partnerships to Develop and Apply Biomass Technologies. Nashville, TN.