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Biomass

Biomass refers to biological material derived from living or recently living organisms, such as plants or plant-derived materials. As an energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel. Conversion of biomass to biofuel can be achieved by different methods, which are broadly classified into: (i) thermal, (ii) chemical, and (iii) biochemical methods. This biomass conversion can result in fuel in gas, liquid, or solid form.

Biomass is a renewable energy source unlike other resources such as crude oil, natural gas, tar sand, coal, and oil shale. Agricultural products specifically grown for biofuel production include crops such as corn, soybeans, rapeseed, wheat, sugar beet, sugar cane, palm oil, Jatropha, as well as wood (Table B-14).

Biomass is generally produced in a sustainable manner from water and carbon dioxide by photosynthesis. There are three main categories of biomass—primary, secondary, and tertiary.

Table B-14 Major categories of biomass feedstocks.

Algae	Prokaryotic algae, Eukaryotic algae, Kelps
Aquatic plants	Algae, Water weed, Water hyacinth, Reed and rushes
Biorenewable wastes	Agricultural wastes, Crop residues, Mill wood wastes, Urban wood wastes, Urban organic wastes
Energy crops	Short rotation woody crops, Herbaceous woody crops, Grasses, Starch crops, Sugar crops, Forage crops, Oilseed crops, Switchgrass, Miscanthus
Food crops	Grains, Oil crops
Forest products	Wood; Logging residues; Trees, shrubs, and wood residues; Sawdust, bark etc.
Landfill	Hazardous waste, Non-hazardous waste, Inert waste, Liquid waste
Lichens	Crustose lichens, Foliose lichens, Fruticose lichen
Mosses	Bryophyta, Polytrichales
Organic waste	Municipal solid waste, Industrial organic wastes, Municipal sewage, and sludge

Biomass is biological organic matter but is more often used to refer to (i) energy crops grown specifically to be used as fuel, such as fast-growing trees or switch grass, (ii) agricultural residues and by-products, such as straw, sugarcane fiber, and rice hulls, and (iii) residues from forestry, construction, and other wood-processing industries.

Many different types of biomass can be grown for the express purpose of energy production (Table B-15).

Table B-15 Sources, processing options, and uses of biomass-derived products*.

Resources	Collection	Conversion	End products
Stage 1	Stage 2	Stage 3	Stage 4
Agricultural crops
Energy crops
Forestry
Herbaceous plants
Oil-bearing plants
Wastes
	Harvesting
	Handling
		Pretreatment
		Biochemical processes
		Chemical processes
		Physical processes
		Thermochemical
			Biodiesel
			Electrical power
			Heat
			Solid fuels
			Transportation fuels
*The sub-categories are listed alphabetically and are not listed in an order of preference for processing. The choice of processing preference is dependent upon the type of biomass and the desired product.

Crops that have been used for energy include sugar cane, corn, sugar beets, grains, elephant grass, kelp (seaweed), and many others. There are two main factors which determine whether a crop is suitable for energy use. Good energy crops have a high yield of dry material per unit of land (dry tonnes/hectare). A high yield reduces land requirements and lowers the cost of producing energy from biomass. Similarly, the amount of energy which can be produced from a biomass crop must be less than the amount of energy required to grow the crop. In some circumstances like the heavily mechanized corn farms in the U.S. Midwest, the amount of ethanol which can be recovered from the corn is barely larger than the fuel required for tractors, fertilizers, and processing.

The components of biomass include triglycerides, sterols, alkaloids, resins, terpenes, terpenoids, and waxes. This includes everything from primary sources of crops and residues harvested/collected directly from the land to secondary sources such as sawmill residuals, to tertiary sources of post-consumer residuals that often end up in landfills. A fourth source, although not usually categorized as such, includes the gases that result from anaerobic digestion of animal manures or organic materials in landfills.

Primary biomass is produced directly by photosynthesis and includes all terrestrial plants now used for food, feed, fiber, and fuel wood. All plants in natural and conservation areas (as well as algae and other aquatic plants growing in ponds, lakes, oceans, or artificial ponds and bioreactors) are also considered primary biomass. However, only a small portion of the primary biomass produced will ever be harvested as feedstock material for the production of bioenergy and by-products.

Biomass feedstocks and fuels exhibit a wide range of physical and chemical properties (Table B-16).

Table B-16 Selected properties of representative biomass materials.

Mass %, dry	Wood	Grain*	Municipal Solid Waste**	Animal Wastes (Manure)
Carbon	50-53	45.0	47.6	35.1
Hydrogen	5.8-7.0	5.8	6.0	5.3
Nitrogen	0-0.3	2.4	1.2	2.5
Sulfur	0-0.1	0	0.3	0.4
Oxygen	38-44	42.5	32.9	38.7
Volatile matter	77-87	70-80	77	76.5
Fixed carbon	13-21	—	11	0
Ash	0.1-2.0	4.0	12	23.5
H/C atom ratio	1.4-1.6	1.5	1.5	1.8
GCV, MJ/kg (dry)	19.8-21.0	16.8	19	13.4
Moisture, %	25-60	16	20	7-35
* Red corn cob (corn stover) contains approximately 25% cellulose, 10% lignin, and 15% moisture. ** Combustible portion; may contain 9% metals and 12% glass/ceramics on an as-received basis.

For example, there are many types of coal and the gross heating value of these types varies from 8,600-12,900 Btu/ lb. However, nearly all kinds of biomass feedstocks destined for combustion range from 6,450 to 8,200 Btu/lb. For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb; the values for most woody materials are 7,750 to 8,200 Btu/lb. Moisture content is probably the most important determinant of heating value. Air-dried biomass typically has approximately 15 to 20% moisture, whereas the moisture content for oven-dried biomass is around 0%. Moisture content is also an important characteristic of coals, varying in the range of 2 to 30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, approximately 10 and 40% of the bulk density of most fossil fuels. Liquid biofuels have comparable bulk densities to fossil fuels.

Combustion offers the most direct route for energy recovery from biomass, and is an effective means of utilizing the total energy content of whole wood and other biomass. Technology for small-scale combustion is certainly well developed. Larger-scale operation for electricity generation at the 5 to 50 MW level should be feasible, although it is not expected to be economically competitive with crude oil- or natural-gas-fired systems. Due to the large land area over which wood must be harvested, the seasonal nature of the supply, and the large volume of material to be transported and stored, the reliable provision of wood to a large plant can present considerable problems.

Biofuels are derived from biomass and have the potential to produce fuels that are more environmentally benign than crude oil-based fuels. In addition, ethanol, a crop-based fuel alcohol, adds oxygen to gasoline thereby helping to improve vehicle performance and reduce air pollution. Biodiesel, an alternative or additive to crude oil diesel, is a nontoxic, renewable resource created from soybean or other oil crops.

Direct biofuels are biofuels that can be used in existing unmodified crude oil engines. Because engine technology changes all the time, direct biofuel can be hard to define; a fuel that works well in one unmodified engine may not work in another. In general, newer engines are more sensitive to fuel than older engines, but new engines are also likely to be designed with some amount of biofuel in mind.