Production of CH4 from Biomass via Supercritical Water Gasification

Abstract

A common route to generate methane-rich gas from renewable biomass feedstocks (such as agricultural/forestry residues and municipal/industrial wastewater sludges) is anaerobic digestion. However, biological processes are limited in many cases by low reaction rates (days to weeks reaction) and low tolerance to bacteria, low tolerance to temperature and a high footprint (large reactor and land requirement). Supercritical water gasification (SCWG) of biomass/waste can be a promising alternative technology for renewable CH4 production as the process has a much smaller footprint and higher efficiency. Many early studies on biomass SCWG proved that methane-rich gas can be achieved at lower temperatures (<500 °C), compared with a high yield of hydrogen at above 600 °C. Production of methane-rich gas or synthetic natural gas (SNG) via SCWG can be carried out at low temperatures, since the methanation reaction is a highly exothermic reaction which requires less external heat to maintain the desired reaction temperature. Processing at low temperatures can improve cold gas efficiency of the process and utilize waste heat from some high-temperature processes (i.e. iron/steel manufacturing). Moreover, methane-rich syngas has a higher Btu value per unit volume than hydrogen, so it is a promising substitute for natural gas for heat or electricity production, or it can be injected into existing natural gas grids. However, a low temperature process generally requires the presence of a catalyst to maximize its reaction rate. For CH4 production via low-temperature SCWG of biomass, the presence of an active catalyst is critically important for reducing the tar/char yields, lowering the reactor volume and increasing the carbon conversion rate. This chapter overviews the state-of-the-art in production of CH4 from biomass via SCWG. The focus of this chapter is placed on effects of operating conditions (temperature, residence time, pressure, etc.) and types of catalysts on the gasification efficiency and CH4 yield.