Hydrothermal carbonization of biowaste as an alternative treatment path to current waste management practices in Germany

Abstract

Instead of an exclusive composting of the organic fraction of municipal solid waste (OFMSW), anaerobic digestion (AD) has emerged as a widely used upstream process in Germany to additionally harvest energy from this waste stream. However, the energy potential is not fully exploited as only easily biodegradable material undergoes AD. Additionally, a high potential of CH4 and N2O emissions during the composting and spreading of compost may result in an overall high global warming potential (GWP) from these treatment paths. In this study, nine alternative or additional treatment options to AD followed by composting were investigated. For all treatment paths, the hydrothermal carbonization (HTC) reflected a central component and was connected to a downstream energy exploitation step by means of gasification or co-combustion. All treatment paths were dynamically modeled with regards to their energetic performance and evaluated against their exergetic efficiency. The datasets on energy consumption and production formed the basis for determining the global warming potential (GWP) according to a life cycle assessment approach. The functional unit was the production and export of exergy. The assessment showed that the incorporation of an HTC step was able to increase the net exergy exploitation by up to 93% in comparison to a treatment path consisting of AD and composting. Simultaneously, the GWP was reduced by 30%. Here, preferential treatment paths combined AD with HTC.