Incremental life cycle analysis: Using uncertainty analysis to frame greenhouse gas balances from bioenergy systems for emission trading

There are many scales that can be employed to calculate net greenhouse gas emissions from bioenergy systems, ranging from single point source (stack gas) measurement, to full, multi-layered life cycle analyses considering all of the inputs and outputs throughout the economy. At an appropriate scale within these extremes, a method can be selected to support verification activities related to project-based trading of greenhouse gas emissions. The boundaries of the analysis must be carefully selected in order to meet the twin goals of the verification activity: (1) to meet scientific standards for emission balance quantification, and (2) to meet cost-effectiveness criteria of the emission trading community. A methodology is proposed for verifying greenhouse gas reductions arising from substitution of bioenergy fuel for fossil fuel in electric generation. The method utilizes an incremental progression through the fuel life cycle, pausing at each level to evaluate the potential benefits and uncertainties of a next-level expansion. A case study is analyzed for greenhouse gas benefits of switchgrass substitution for coal combustion in an existing electric generation facility. Using the incremental technique to set boundaries on the analysis, replacement of coal with switchgrass is found to provide a reduction of net greenhouse gas emissions at a rate of CO2-equivalent per million Joules (g/MJ) of switchgrass input. At full production, co-firing 5 percent switchgrass, the emission reduction would total more than 305,500 metric tons CO2-eq annually.