The quantification of the actual reduction in green house gases sourcess emissions resulting from the substitution of fossil fuels with energy from waste biomass requires a complete lifecycle assessment (LCA). A systematic framework for estimating the net Green House Gases emissions from bioenergy systems and comparing them against the fossil fuel reference system that it would replace has been developed. The major considerations of the life cycle assessment approach to quantifying the greenhouse impacts of bioenergy are as follows:
Carbon stock dynamics. Changes in the carbon stock of plants, plant debris, and soils result from biomass growth and harvest. This necessitates time-dependent analysis, as such changes might extend over long periods of time until a new equilibrium is reached.
Trade-offs and synergies. The trade-off between competing land uses, namely biomass resources production, food production, and carbon sequestration, should be considered in determining the benefits of bioenergy. A timber plantation, thinned to maximize value of wood products, where thinning residues are used for bioenergy, is an example of synergy.
Leakage. The use of biomass fuels does not always avoid the use of fossil fuels to the full extent suggested by the amount of bioenergy actually used and therefore the reduction in fossil fuel use that can be attributed to the project will be reduced. This is commonly referred to as leakage.
Permanence. Irreversible Green House Gases effect mitigation offered by the system is required to permanently offset the CO 2 emissions. The greenhouse mitigation benefit from substituting bioenergy for fossil fuel effects on environment use is irreversible. In contrast, the mitigation benefit of reforestation will be lost if the forest biomass is reduced by harvest or natural disturbances.
Emissions factors. The quantity of Green House Gases produced per unit of fossil fuel energy consumed. This factor influences the net benefit of a bioenergy project: if bioenergy displaces natural gas supply demand, the benefit is lower than if it displaces coal, as coal has a higher emissions factor than natural gas.
By-products. The emissions and offsets associated with both products and by-products of a biomass production system must be considered.
Efficiency. In determining the quantity of fossil fuel energy displaced by a quantity of biomass feedstock productions, it is important to know the energy output per unit of that feedstock.
Upstream and downstream Green House Gases emissions. Auxiliary (energy inputs) emissions resulting from the bioenergy and biomass reference systems should be considered, such as from fossil fuels used in production/extraction, processing, and transport of the feedstock.
Other greenhouse gases. The emissions of other Greenhouse Gases associated with biomass and fossil fuel chains, such as CH4 and N2O, expressed in CO2 equivalents, should be included in the calculation of net greenhouse mitigation benefits.
LCA assessments have been applied to a variety of fuel systems to generate an understanding of their contribution to atmospheric CO 2 concentrations. For example, full fuel cycle CO 2 emissions per GWh for coal have been calculated as 1142 t compared to 66 to 107 t for biomass.
