Agricultural and forestry residues provide the largest proportion of biomass used for the production of biomas bioenergy. Some estimates suggest that globally available biomass role in the form of recoverable residues represents about 40 Ejyr -1, enough to meet 10% of the total present energy use of 406 Ejyr -1 . However, realizing this potential is limited by factors such as ease and cost of recovery and environmental concerns relating to sustainable land use practices.
In developed countries, sawmill and pulp mill residues comprise the largest proportion of residues in the existing bioenergy mix. Industries make use of recovered operational waste such as sawdust and off cuts in timber mills and black liquor in pulp and paper production to supply process energy needs. Additionally, techniques such as densification can produce a usable fuel from sawmill sawdust and bark as well as shavings from furniture manufacturing.
Substantial opportunities exist in the forestry sector. Forest management residues including branches, tree tops, and small diameter stems from thinning and harvest of commercial plantations are a potentially large source of biomass, which is underutilized. Full utilization of thinnings, which may represent approximately 25% of the biomass produced, permits energy recovery from a resource that may otherwise decay on the forest floor. In both cases, the carbon in the biomass is returned to atmosphere, but where bioenergy is produced, biofuels greenhouse mitigation benefit is obtained from displacement of fossil fuel emissions. Technological advancements are being made in this area, making use of bundling technology for the densification of harvest residues. This process, largely being developed in Finland, involves recovery of the residues and bundling at source to produce an easily transportable and storable fuel.
Used vegetable oils and animal fats can be reprocessed into biodiesel process. Biodiesel has similar physical properties to conventional diesel to which it can be blended or alternatively used in its pure form. Biodiesel is commercially available at service stations in countries such as the United States, Brazil, France, Germany, and Austria. In 1996, the International Energy Agency (IEA) reported that 21 countries produced some 591,000 ton of biodiesel. Direct substitution of diesel oil would result in the mitigation of over 500,000 ton of CO2 emissions. As approximately 20% of global CO2 emissions can be attributed to the transportation sector, further development of biofuels, including biodiesel benefits, can contribute significantly to reduction in GHG emissions from this sector.
Wet waste streams such as domestic, agricultural, and industrial sludge and animal manure emit methane (CH4), as a product of anaerobic decomposition of organic matter. CH4 has a greenhouse warming potential 21 times higher than that of CO2 and therefore, avoidance of CH4 emissions produces significant greenhouse benefits. Anaerobic digestion is a waste management and energy recovery process highly suited to wet waste streams. The bacterial fermentation of organic material produces biogas (65% CH4 , 35% CO2 ), which is captured and utilized to produce heat and power. Although anaerobic digestion of sewage, industrial sludge and wastewater are fully commercialized, systems involving the processing of animal manure and organic wastes are still being developed. Technology can generate approximately 200 kilowatt-hours (kWh) of electricity from 1 ton of organic waste. If this was used to replace electricity from coalbed methane production, direct GHG emissions could be reduced by approximately 220 kgCkWh_ 1.
The energy contribution of animal sludge and municipal solid energy from waste biomass globally was estimated at 5300 to 6300 MW in 1995, 95% of which can be attributed to Asia. Globally, anaerobic digestion of organic wastes is predicted to increase to 8915 to 20130 MW in 2010. This projection is largely attributed to the developed world increasing its contribution.
Energy hydrogen production from solid municipal waste offers a potentially sustainable approach to a growing environmental problem. Such sources include urban wood waste (i.e., furniture, crates, pallets) and the biomass portion of garbage (i.e., paper, food, green waste.). The biogas emitted when waste decomposes in landfill can be extracted for use in the generation of electricity. Maximizing the potential of such waste streams is hampered by logistical issues relating to recovery, negative public perception of ‘‘incineration,’’ and opposition from environmental groups to the establishment of a market for waste.
