The contribution to energy production of the aerobic bioconversion of organic waste by an organic Rankine cycle in an integrated anaerobic-aerobic facility

An integrated anaerobic digestion (AD) and aerobic bioconversion facility, equipped with an organic Rankine cycle (ORC), was analyzed for the management and recovery of energy from organic waste (OW). The ORC was fuelled mainly by the exhaust heat ejected at about 340 K by the aerobic treatment of OW eventually supplemented with the heat generable by the combustion of a given fraction of the biogas generated by AD. For an integrated facility processing 25,000 tonnes/year of OW, the net electrical output of the internal combustion engine fuelled by the biogas was about 1090 kW. If fuelled only with the exhaust heat from the aerobic treatment, the ORC can generate up to 18 kW, leading to a global system efficiency increase of about 2%. For a compression ratio ≤2, exploitation of the biogas generated by AD for increasing the amount of heat and temperature at the ORC inlet up to about 350 K leads to further energetic benefits. A preliminary economic analysis indicates an operational expenditure of about 38 €/MWh for the proposed solution.