Environmental sustainability of integrating the organic Rankin cycle with anaerobic digestion and combined heat and power generation

Given the growing scarcity of primary energy resources, increasing the efficiency of energy conversion is one of the key challenges for optimising energy use. For this reason, low-grade or waste heat from various processes is becoming increasingly more attractive as an energy source. This study considers anaerobic digestion (AD) coupled with a combined heat and power plant (CHP) as a source of low-grade heat for electricity generation utilising an organic Rankine cycle (ORC) system. The aim is to evaluate the environmental sustainability of such a system relative to the AD-CHP system without heat recovery. Ten real AD-CHP plants using cereal silage and animal slurry as feedstocks are considered for these purposes and their impacts have been estimated through life cycle assessment. The results suggest that systems with the ORC have lower impacts than those without it, but the average reductions are relatively small (1.6–5.8%). However, for the smaller plants fed mainly with animal slurry, climate change increases significantly (up to 27 times). The reduction in impacts is greater for the bigger plants where the surplus heat available for the ORC is higher. The impacts from the ORC plant are insignificant, with its electrical efficiency and lifespan showing little effect on the results. Small slurry-fed plants without the ORC have lower environmental impacts than the bigger silage-fed plants fitted with an ORC system for nine out of 13 categories considered; climate change is up to 32 times lower. They are only slightly worse than the bigger plants with the ORC for ozone depletion and human toxicity due to the economies of scale. Therefore, while there are clear benefits of fitting an ORC system to an AD-CHP plant, greater benefits can still be achieved by utilising waste feedstocks, such as animal slurry, instead of fitting an ORC to a plant utilising cereal silage.