James G. Speight - Encyclopedia of Renewable Energy

Biomass waste-to-energy conversion reduces greenhouse gas emissions in two ways. Heat and electrical energy are generated which reduces the dependence on power plants based on fossil fuels. The greenhouse gas emissions are significantly reduced by preventing methane emissions from landfills. Moreover, waste-to-energy plants are highly efficient in harnessing the untapped sources of energy from wastes.

Biomass energy projects provide major business opportunities, environmental benefits, and rural development. Feedstocks can be obtained from a wide array of sources without jeopardizing the food and feed supply, forests, and biodiversity in the world and include (i) agricultural residues, (ii) animal waste, (iii) forestry residue, (iv) wood waste, (v) industrial waste, (vi) municipal solid waste, and (vii) sewage.

Agricultural residues encompasses all agricultural wastes such as bagasse, straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. Large quantities of crop residues are produced annually worldwide, and are vastly underutilized. Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields, while processing produces fibrous bagasse, both of which are good sources of energy. Harvesting and processing of coconuts produces quantities of shell and fiber that can be utilized. The current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemically processed to produce electricity and heat. Agricultural residues are characterized by seasonal availability and have characteristics that differ from other solid fuels such as wood, charcoal, and char briquette. The main differences are the high content of volatile matter and lower density and burning time.

Animal waste represents a wide range of biomass that can be used as sources of biomass energy. The most common sources are animal and poultry manures. In the past, this waste was recovered and sold as a fertilizer or simply spread onto agricultural land, but the introduction of tighter environmental controls on odor and water pollution means that some form of waste management is now required, which provides further incentives for waste-to-energy conversion. The most attractive method of converting these waste materials to useful form is anaerobic digestion which gives biogas that can be used as a fuel for internal combustion engines, to generate electricity from small gas turbines, burnt directly for cooking, or for space and water heating.

Forestry residues are generated by operations such as thinning of plantations, clearing for logging roads, extracting stem-wood for pulp and timber, and natural attrition. Harvesting may occur as thinning in young stands, or cutting in older stands for timber or pulp that also yields tops and branches usable for biomass energy. Harvesting operations usually remove only 25 to 50% of the volume, leaving the residues available as biomass for energy. Stands damaged by insects, disease, or fire are additional sources of biomass. Forest residues normally have low density and fuel values that keep transport costs high, and so it is economical to reduce the biomass density in the forest itself.

Wood wastes generated by the wood processing industries primarily include sawmilling, plywood, wood panel, furniture, building component, flooring, particle board, molding, jointing, and craft industries. Wood wastes generally are concentrated at the processing factories, e.g., plywood mills and sawmills. The amount of waste generated from wood processing industries varies from one type of industry to another depending on the form of raw material and finished product. Typically, the waste from wood industries such as saw millings and plywood, veneer, and others are sawdust, off-cuts, trims, and shavings. Sawdust arises from cutting, sizing, re-sawing, and edging, while trims and shaving are the consequence of trimming and smoothing of wood. In general, processing of wood in the furniture industries will lead to waste generation of almost half (45 %) of wood. Similarly, when processing wood in a sawmill, the waste will amount to more than half (52 %) of the wood.

Industrial wastes are generated by the food industry which produces a large number of residues and by-products that can be used as biomass energy sources. These waste materials are generated from all sectors of the food industry with everything from meat production to confectionery producing waste that can be utilized as an energy source. Solid wastes include peelings and scraps from fruit and vegetables, food that does not meet quality control standards, pulp and fiber from sugar and starch extraction, filter sludge, and coffee grounds. These wastes are usually disposed of in landfill dumps.

Liquid wastes are generated by washing meat, fruit and vegetables, blanching fruit and vegetables, pre-cooking meats, poultry and fish, cleaning and processing operations, as well as wine making. These wastewaters contain sugars, starches, and other dissolved and solid organic matter. The potential exists for these industrial wastes to be anaerobically digested to produce biogas, or fermented to produce ethanol, and several commercial examples of waste-to-energy conversion already exist. The pulp and paper industry is considered to be one of the highly polluting industries and consumes a large amount of energy and water in various unit operations. The wastewater discharged by this industry is highly heterogeneous as it contains compounds from wood or other raw materials, processed chemicals, as well as compound formed during processing. Black liquor can be judiciously utilized for production of biogas using anaerobic digestion technology.

Municipal solid wastes are produced and collected each year with the vast majority being disposed of in open fields. The biomass resource in municipal solid waste comprises the putrescible materials, paper, and plastic and averages 80% of the total municipal solid waste collected. Municipal solid waste can be converted into energy by direct combustion, or by natural anaerobic digestion in the engineered landfill. At the landfill sites, the gas produced by the natural decomposition of municipal solid waste (approximately 50% methane and 50% carbon dioxide) is collected from the stored material and scrubbed and cleaned before feeding into internal combustion engines or gas turbines to generate heat and power. The organic fraction of municipal solid waste can be anaerobically stabilized in a high-rate digester to obtain biogas for electricity or steam generation.

Sewage is a source of biomass energy that is similar to the other animal wastes. Energy can be extracted from sewage using anaerobic digestion to produce biogas. The sewage sludge that remains can be incinerated or undergo pyrolysis to produce more biogas.

See also: Biogas, Biomass, Municipal Solid Waste, Waste.

Bio-oil (sometimes called bio-crude) is the liquid condensate produced from biomass (forestry residues, crop residues, waste paper, and organic waste) by means of processes such as pyrolysis (thermal and catalytic). Depending on the feedstock, the bio-oil could be compatible with existing refinery technology and can be converted into fuels, such as gasoline, diesel fuel, and jet fuel.

The product can vary from a light tarry material to a free-flowing liquid that is produced by the thermal decomposition (destructive distillation) of biomass at temperatures on the order of 400°C (750°F) ( Table B-25).

Table B-25Examples of the yield of bio-oil by pyrolysis from agricultural residues at different temperatures ( oC).