Exploring the potential for biomethane production by the hybrid anaerobic digestion and hydrothermal gasification process: A review

For a cleaner and healthier planet, industries must decarbonize and move away from ‘take-make-waste’ linear economies to circular economies, in ways that maximize value and minimize negative environmental impact. Anaerobic digestion (AD) is a well-known technology used to process domestic and industrial wastes to produce biogas (mainly CH4) but with large volumes of high moisture content digestate as by-product The digestate is both difficult and expensive to manage, often requiring more than half the operating cost of treatment plants. Hydrothermal gasification (HTG) can convert the recalcitrant digestate into renewable gases including CH4. So, for high carbon conversion a hybrid AD and HTG technology is an attractive solution. In the hybrid process, AD can be used to treat the wet biomass (an existing practice), and the liquid AD digestate can then be processed by HTG to optimize CH4 yield. This paper reviews the pros and cons of the AD and HTG processes, examines the valorization of co-products, and assesses the potential of the hybrid process with additional information from other AD-thermochemical process hybrids that have widely reported economic feasibilities. [Display omitted] •AD-HTG is an efficient process for sustainable energy production.•Hybrid AD-HTG results in higher power and CH4 generation.•Benefits of recycling HTG products to higher energy density gases were discussed.•Valorization of by-product CO2 to chemicals were discussed.•Current challenges and future directions of AD-HTG process were presented.