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1038M.C. Heller et al. / Renewable Energy 29 (2004) 1023–1042effect that scale with the cofiring rate, although our model demonstrates that theextent of these reductions can be dependent on the source and type of biomass aswell as the modeling procedure adopted.Willow biomass is grown specifically for electricity generation andthus willowproduction is considered to be within the power generation system boundary. As aresult, electricity generation with willow biomass is nearly GHG neutral (40–50 kgCO 2 eq./MWh elec ). However, the original growth of residue biomass is not consideredwithin the power generation system boundary and a CO 2 absorption creditis not taken for the growth of this biomass. Utilizing biomass residues for electricitygeneration avoids other means of disposal (in this case, landfill) and in turnavoids the GHG emissions that would have occurred as a result of that disposal.While not all of the carbon in the residue biomass decomposes to gaseous productsin our landfill model, methane generation through anaerobic decomposition contributesstrongly to the net greenhouse effect due to methane’s relatively highequivalent global warming potential (GWP CH4 ¼ 23 kg CO 2 eq./kg CH 4 [8]).Decomposition of 100 kg of residue biomass in the ‘‘specific residue’’ landfill scenarioreleases 127.2 kg total CO 2 eq., whereas complete aerobic decomposition (orcomplete combustion) releases 181.9 kg CO 2 . Thus, in this situation, landfilled biomasshas a lower global warming potential than combustion, but there is a significantloss of potential for power generation. Under the ‘‘specific’’ landfill scenario,100 kg of biomass generates 0.03 kWh electricity through landfill gas capture butthe same 100 kg of biomass represents 132 kWh of generation potential in a cofiringoperation. Comparison of the different residue scenarios indicates that smallchanges in biomass characteristics, transportation distances, and disposal managementcan influence the resulting greenhouse gas benefit. Under the ‘‘generic’’ landfillscenario, 100 kg of biomass leads to 166.2 kg total CO 2 eq. A previous reportthat considered a combination of residue disposal through landfill and utilizationas mulch foundthat disposal of 100 kg of biomass releases 248.2 kg total CO 2 eq.[13]. Clearly, such allocation procedures must be standardized if they are to beusedin CO 2 emission regulation and/or trading.Cofiring biomass reduces emissions of SO 2 , Hg, and(in most cases) NO x . Theextent to which biomass cofiring will reduce NO x remains case-specific, as it isdependent on not only biomass composition but also boiler configuration andoperating conditions. Dedicated biomass generation, which in new power plantswill most likely use gasification technology, also provides significant reductions inSO 2 ,NO x , andHg emissions relative to the current coal-dominatedelectricity mix.Recent forecasts by the Energy Information Administration at the US Departmentof Energy predict little to no addition of biomass-powered electricity generation,or indeed any renewable energy sources, when tightened three-pollutant(NO x , SO 2 , Hg) regulations are adopted [4]. Power generators are expectedtoinsteadchoose the less expensive option of installing emission control equipmentwhile maintaining coal as the primary fuel source. However, cases that considerstringent CO 2 reductions or aggressive RPS predict significant increases in the useof biomass, both in cofiring operations and dedicated biomass power plants. WhenaCO 2 cap at 7% below the 1990 level is assumed, biomass cofiring is predicted to
M.C. Heller et al. / Renewable Energy 29 (2004) 1023–10421039increase to 50 billion kWh by 2020, more than 700% above the Annual EnergyOutlook reference case. Adopting the goal of 20% RPS by 2020 is forecasted toincrease total biomass-fueledgeneration to 526 billion kWh by 2020, with 85% ofthis from dedicated power plants. Under this 20% RPS scenario, biomass composes10% of the total electricity generation [4].Establishing energy crops, of course, requires arable land, and there is concernover the availability of this limited resource. According to our model, supplyingenough willow to support a 10% cofire of all willow at the Dunkirk facility wouldrequire an estimated2925 ha of plantation. This is 2.5% of the open landwith suitablesoils and slopes for willow biomass production within an 83 km radius aroundthe Dunkirk plant (note that Dunkirk is on the shore of Lake Erie, so much of thearea aroundthe plant is unavailable) [34]. Operating a 100 MW gasifier at 37%efficiency and80% capacity wouldrequire 26,865 ha of willow plantation, or only1.3% of the total area within an 80 km radius. In an expanded biomass energymarket, however, willow energy crops will be one of many biomass sources includingurban, agricultural, andforestry residues andother energy crops such asswitchgrass. A recent EIA report estimates that 3.9–5.8 million ha of energy cropswill be needed to meet the 20% RPS by 2020 projection [6]. The report also pointsout that it is possible to grow biomass energy crops on Conservation Reserve Program(CRP) land, and that this projected energy crop acreage represents 24–37%of the current allowable CRP land. In addition, acreage devoted to farms and rancheshas been declining steadily since the 1950s [35]. Thus, landuse for biomassenergy crops is not expectedto conflict with landrequirements for foodandfeedcrop production.5. ConclusionsLife cycle analysis demonstrates that electricity generation with willow energycrops, either by cofiring with coal or in dedicated biomass power plants, leads tosignificant reductions in many of the environmental impacts of coal-based electricityproduction. Consumption of non-renewable resources (coal) is reduced, asare net greenhouse gas emissions andcriteria air pollutants including SO 2 , Hg, andlikely, NO x . Cofiring biomass at 10% increases the net energy ratio of producingelectricity by 8.9%. Similarly, the net energy ratio for dedicated biomass gasificationis estimated to be 13, indicating that 13 units of electricity are produced forevery unit of fossil energy consumedacross the entire system life cycle. For comparison,the net energy ratio of the current US electricity gridis 0.26.This study suggests that the environmental impacts from producing electricitywith willow biomass energy crops are similar to using woody residues and that thepollution preventedis comparable to other renewable energy sources (solar andwind). Additional data and experience are needed to determine whether the smalldifferences reported here are indeed significant. It should be noted that choice ofmodeling parameters and allocation procedures can have significant effects onresults. Life cycle assessment basedon ISO 14040 guidelines offers a validmeans of
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