Biomass

Biomass and related waste contribute several percent of the nation's current annual fuel resource mix. While commonly used as feedstock for production of biogasses, as discussed previously, biomass is commonly used as a solid fuel, often in the proximity of its production. Wood waste is the primary component of biomass fuel, though peat and other agriculture by-products are also common components.

• Wood contributes more than three-quarters of the total biomass energy. Where there is ample wood in supply, home heating and some large central steam generation (utility, industrial, and institutional) plants use wood or wood waste as a primary fuel source. Wood waste provides about half of the energy consumed by the forest products industry, as the conversion of logs to lumber results in about 50% waste in the form of bark, shavings, and sawdust. Fresh timber typically contains 30 to 50% moisture or more, mostly in the cell structure of the wood. This may be reduced by half after a year of air drying. When moisture content is high, it may be necessary to mix the wood with low-moisture fuel to ensure that there is enough energy density, on an LHV basis, entering the boiler to support proper combustion. Dry wood has an energy density range of 8,500 to 9,100 Btu/lbm (19,800 to 21,200 kJ/kg). This is reduced to about 4,375 Btu/lbm (10,170 kJ/kg) at 50% moisture and 1,750 Btu/lbm (4,070 kJ/kg) at 80% moisture content. In addition to biogasses, wood can be transformed into several other fuel products. Wood charcoal, for example, is made by heating wood to a high temperature in the absence of air. Weight and volume are reduced by 75 and 50%, respectively, producing a product that has a higher energy density and is easier to transport.

• Peat is another major biomass type fuel source. Discussed above as an early stage in the metamorphosis of vegetable matter into coal, peat is a complex mixture of carbon, hydrogen, and oxygen in a ratio similar to that of cellulose and lignin. It is generally low in sulfur, nitrogen, and ash from the parent vegetation. Dry, ash-free ultimate analysis ranges from 50 to 65% carbon, 5.5 to 7% hydrogen, 30 to 40% oxygen, 1 to 2% nitrogen and less than 1% sulfur. While used largely in the United States for soil improvement, mulch, filler for fertilizers, and litter for domestic animals, it is also a viable fuel source. Its energy density, depending on the source and method of processing, may range from 3,700 to 8,000 Btu/lbm (8,600 to 18,600 kJ/kg), with an average of about 6,000 Btu/lbm (14,950 kJ/kg) with standard air-dried processed material.

• There are a number of other biomass type fuels used for generation of industrial steam and power. Aside from their value as fuel, the burning of wastes minimizes disposal requirements. These benefits, however, are often limited by environmental regulations, which vary by region. Typically, these by-products are cellu-losic in character and offer energy densities of 7,000 to 9,500 Btu/lbm (16,300 to 22,100 kJ/kg), depending on the level of resinous material present. These may include bagasse (which is a fibrous by-product of sugarcane processing), corncobs, rice straw or hulls, wheat straw, and pine bark. Cattle manure is another useful by-product type fuel. Other related materials are discussed below under refuse-derived fuels.

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