Energy efficient thermochemical conversion of very wet biomass to biofuels by integration of steam drying, steam electrolysis and gasification

A novel system concept is presented for the thermochemical conversion of very wet biomasses such as sewage sludge and manure. The system integrates steam drying, solid oxide electrolysis cells (SOEC) and gasification for the production of synthetic natural gas (SNG). The system is analyzed by thermodynamic modelling and the analysis shows that the system can handle mechanically dried biomasses with a water content of 70 wt% and an ash content of up to 50 wt% (dry basis). A high tolerable ash content is an advantage because very wet biomasses, such as sewage sludge and manure, have a high ash content. The analysis shows that the total efficiency of the novel system is 69–70% depending on the biomass ash content, while the biomass to SNG energy ratio is 165%, which is near the theoretical maximum because electrolytic hydrogen is supplied to the synthesis gas. It is proposed to combine the novel system with an anaerobic digester for conversion of biomasses with high nitrogen content, such as sewage sludge. The organic nitrogen in the sewage sludge will be mineralized in the digester instead of ending up as N₂ in the SNG product. - Highlights: • A novel system concept is analyzed by thermodynamic modelling. • Waste heat from SNG synthesis is used to dry biomass in a steam dryer. • The biomass to SNG energy ratio achieved is 165%. • The biomass + net electricity to SNG energy efficiency achieved is 69–70%. • Alternative systems capable of handling high N fuels are presented.