Can thermal energy recovery from digestate make renewable gas from household waste more cost effective? A case study for the Republic of Ireland

Abstract This article assesses the impact of the energy recovery from the digestate on the economics of biomethane produced from the organic fraction of municipal solid waste. Six waste-to-energy routes are investigated and assumed to be deployed in the regional context of the Republic of Ireland. Anaerobic digestion without energy recovery, and landspreading of dewatered digestate, presents the lowest levelised cost for the biomethane produced, from 86.8 to 108.9 €/MWh, and highest net present value, from 70.6 to 116.4 M€. However, if the digestate is dried, synthetic natural gas production from the digestate through steam gasification, maximising the amount of renewable gas produced, presents the lowest levelised cost, from 93.4 to 113.8 €/MWh, and highest net present value, from 65.5 to 111.8 M€ and highest net present value. Thanks to the largest substitution of natural gas, this process presents also the largest CO2 emission saving, from 12.1 to 20 kilotonnes of CO2 per annum. Transportation costs of the residues generated, because of the proximity of farm lands in which the digestate is landspread, and because of the small amount of ashes generated, are negligible when compared to CapEx, OpEx and energy expenditures. CapEx and OpEx are the most sensitive parameters, and the more the energy demand of the process is not covered the more the expenditures for energy supplies become relevant. Although all the alternatives presented costs greater than natural gas price for household consumers, 70.3 €/MWh, additional revenues for waste management services would make the renewable gas produced profitable.