Use and Potential
Biomass waste products arising from traditional agricultural and forest production, and processing contributes significantly to the energy mix (Hall, 1991; Pingoud et al., 1999; Hall et al., 2000). Resources include rice husks, corn cobs, cereal straw, bagasse, wood process residues (bark, sawdust, off-cuts), black liquor, nut shells, animal manures, and municipal solid wastes. A wide range of energy conversion routes exist to produce electricity, heat, or fuels (landfill gas, biogas, producer gas, ethanol, methanol, briquetted pellets, charcoal, etc.). Currently, a significant proportion of biomass waste at food and fiber processing plants is either burned to waste or dumped into landfills for disposal. Where this dumping entails a disposal cost, use of the resource for energy production may be economically feasible and reduce the possibility of local air and water pollution.
Monitoring and Verification
The resource is widely distributed, highly variable, and difficult to assess with any degree of accuracy. It tends to have a low energy density and high moisture content. In countries with extensive forest industries-such as Sweden, Finland, and Austria-energy from woody biomass by-products can be monitored relatively easily and can supply up to 30 percent of the country's primary energy. In developing countries with dispersed rural populations, use of biomass wastes for cooking and heating can only be estimated.
Agricultural and forest production wastes are traditionally left in the field after harvest to decompose; hence, these wastes return organic matter and nutrients to the soil and carbon to the atmosphere. Sustainable production methods would need to be carefully evaluated for each site and soil type if this biomass were to be removed from the site along with the traditional food and fiber products. Conversely, removal of these waste by-products (straw, logging slash, etc.) may have benefits-such as ease of cultivation and replanting, disease control, and avoiding methane production during natural decomposition. Because of the importance of organic residues to the maintenance of soil quality, it has been suggested that only about 50 percent of agricultural residues can be removed from fields without affecting future crop productivity (Sampson et al., 1993).
Waste-to-energy plants that produce biofuels or generate heat and electricity may create local emission problems relating to heavy metals, dioxins, and so forth, depending on their design and operation, as well as the nature of the biomass. For example, considerable debate continues about the benefits of incineration of municipal solid waste and sewage sludge (e.g., Aumonier, 1996) versus disposal to properly designed landfills after increased diversion of the organic component to composting facilities or anaerobic digestors (e.g., Finnveden and Ekvall, 1998). If GHG emissions reduction is the primary objective, incineration with energy recovery may be preferred. When other economic and environmental factors are also considered, there may be no general solution.
Collection and transport of biomass by-products such as cereal straw and forest residues to a central processing plant are energy-intensive. Full life-cycle analyses need to be undertaken and energy ratios evaluated.
Many commercial waste-to-energy plants are already being operated successfully. Bagasse is used for on-site co-generation and electricity exports in Australia, South Africa, and Hawaii. Cereal straw is used for district heating in Denmark, Germany, and the United Kingdom. Wood processing residues are used in the United States, Australasia, and northern Europe. Landfill gas plants are widely distributed in most developed countries. Community-scale biogas plants are gaining in popularity in Denmark, India, and China. Several power plants that burn chicken litter are operating in the United Kingdom. Development of further plants depends on local energy prices and government strategies for waste avoidance and carbon trading.
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