Figure 6.14 shows the net emissions of the three GHGs quantified for these studies: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The biomass IGCC system has a much lower GWP than the fossil systems because of the absorption of CO2 during the biomass growth cycle. Sensitivity analyses demonstrated that even moderate amounts of soil carbon sequestration (1900 kg/ha/7‐year rotation) would result in the biomass IGCC system having a zero‐net GHG balance. Sequestration amounts greater than this would result in a negative release of GHGs, and a system that removes carbon from the atmosphere overall.
The base case presented here assumes no net change in soil carbon, since actual gains and losses will be very site specific. The direct‐fired biomass system in which the residue is used at the power plants has a highly negative rate of GHG emissions because the generation of methane associated with biomass decomposition is avoided. Based on current disposal practices, it was assumed that 46% of the residue biomass used in the direct‐fired and cofiring cases would have been sent to a landfill and that the remainder would end up as mulch and other low‐value products. Decomposition studies reported in the literature were used to determine that if the biomass residue had not been used at the power plant, approximately 9% of the carbon would have ended up as CH4 and 61% as CO2. The remaining carbon is resistant to decomposition in the landfill. Had all the residue biomass been decomposed aerobically, the CO2 produced would have been 1.85 kg/kg biomass. If the biomass residue was not used at the power plant, the decomposition pathways described above would have resulted in total GHG emissions of 2.48 kg CO2‐equiv/kg biomass (1.117 kg CO2 + 0.065 kg CH4). The net difference is the reason for the negative GHG emissions associated with the direct‐fired system.
The NGCC plant has the lowest GWP of all fossil systems because of its higher efficiency, despite natural gas losses that increase net CH4 emissions. Natural gas losses during extraction and delivery were assumed to be 1.4% of the gross amount extracted. Because of the potency of methane as a GHG, nearly one‐quarter of the total GWP of this system is due to these losses. Cofiring biomass with coal at 15% by heat input reduces the GWP of the average coal‐fired power plant by 18%. The reduction in GHGs is greater than the rate at which biomass is cofired because of the avoidance of methane emissions associated with decomposition that would have occurred had the biomass not been used at the power plant. Biomass disposal and decomposition emissions for this scenario are the same as those used in the direct‐fired case.
Air Emissions
Figure 6.15 charts the following emissions: particulates, oxides of sulfur (SOx), oxides of nitrogen (NOx), CH4, CO2, and non‐methane hydrocarbons (NMHCs). Methane emissions are high for the natural gas case due to natural gas losses during extraction and delivery. The direct‐fired biomass and coal/biomass cofiring cases have negative methane emissions due to avoided decomposition processes (landfilling and mulching). CO and NMHCs are higher for the biomass case because of upstream diesel combustion during biomass growth and preparation. Cofiring reduces the coal system air emissions by approximately the rate of cofiring, with the exception of particulates, which are generated during biomass chipping and handling.
Resource Consumption
Figure 6.16 shows the total amount of nonrenewable resources consumed by the systems investigated. Limestone is used in significant quantities by the coal‐fired power plants for FGD. The natural gas IGCC plant consumes almost negligible quantities of resources, with the exception of the feedstock itself, including that lost during extraction and delivery.
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