Electrical energy production from the integrated aerobic-anaerobic treatment of organic waste by ORC

The energetic performance of an ORC system fueled by the heat generated from the integrated aerobic/anaerobic treatment of organic waste was analyzed. The temperature and heat content of the exhaust air arising from the aerobic treatment were increased by the combustion of the biogas produced by the anaerobic digestion of a fraction of the same waste. On the basis of the amount of excess air exploited in the process, for each tonne of organic waste treated, it was possible to produce from 30 to 90 kg of exhaust air per day with a mean temperature ranging from 330 to 340 K. By processing from 0.5% to 16% of the whole organic waste in an anaerobic digestion section instead of the aerobic one, it was possible to increase the exhaust air temperature from 340 to 510 K, leading to an increase in the ORC size from about 0.05 to about 1 W/tonne/year. The best energetic utilization of the biogas was achieved for ORC compression ratios from 1.5 to 2 and for maximum air temperatures from 335 to 340 K. In these conditions, by using a micro-ORC system (i.e. <15 kW), it was possible to convert about 20% of the energy content of the biogas into electrical energy.