Sources for CH4 Emissions in the Algae Pathway, AD Process, and Biogas Clean-up
532 WordsFeb 17, 20182 Pages
There are two potential sources for CH4 emissions in the algae pathway, the AD process and biogas clean-up. Based on a literature review (Liebertrau, et al., 2010; Flesch, et al., 2011), we assume a total fugitive CH4 loss of 2% for this pathway. Biogas flaring efficiency was observed to be less than usually expected, averaging 81% CH4 combustion in the flare with a range of 48% to 99%. This range is usually assumed to be 90% to 95%. However, the N2O emission from the N-containing sludge was ignored because chemical fertilizers replaced by the biogas residue are associated with the same issue.
Nitrous Oxide Emissions from Soil Application of Digestate Solids
The nitrogen in the digestate might be converted to nitrous oxide (N2O), a GHG even more potent than CH4, after application to soil. The International Panel on Climate Change (IPCC) task force on greenhouse inventories emission factor for direct N2O emissions from organic fertilizers is 0.01 kg N2O–N per kilogram of applied N where each kilogram of N2O–N equates to 44/14 g of N2O (IPCC, 2006). Direct N2O emissions from ﬁeld application of digestate were estimated in this manner and added as a GHG burden. Emissions from transporting digestate to the ﬁeld assumed the digestate was dewatered by centrifuge to 30 wt% solids.
Integrating all factors over 100 years, Bruun et al. (2006) computed that the scenario with digestate substituting for a portion of the mineral fertilizer had additional emissions of 0.013 to 0.017 kg