Energy Recovery from Low Temperature Heat Produced During Aerobic Biological Treatment

The possibility of recovering the heat ejected with the exhaust air arising from the aerobic conversion of organic waste for feeding a micro organic Rankine cycle (ORC) was investigated. This heat was upgraded by the combustion of a given amount of solid recovered fuel (SRF). The exhaust air rate ejected by the aerobic process ranged from 40 to 95 kg/day per each tonne/day of waste processed and the temperature ranged, respectively, from 340 K to 330 K. Calculations refer to a typical aerobic treatment facility able to process 20,000 tonnes of organic waste per year. Maximum efficiency in the utilization of the heat produced by the combustion of SRF, ranging from 14% to 22%, was achieved for ORC operating at a compression ratio from 1.5 to 2.5 and exhaust air temperatures from about 340 to 350 K. Operating the ORC with compression ratios higher than 3.5 and exhaust air temperatures of about 510 K, the power output ranged from about 9 to 12 kW. In these conditions, for the size of the facility investigated, the efficiency of the utilization of the heat generated by the combustion of SRF was from 4% to 7% higher than the ORC thermodynamic efficiency.