Process Effluent Recycling in the Supercritical Water Gasification of Dry Biomass

Abstract

The influence of process water recycling during the Supercritical Water Gasification (SCWG) of dry biomasses was investigated. Dry biomass has to be diluted with water to a dry matter content of approximately 10 wt.% to gasify it in the process of supercritical water gasification. The treatment of wastewater in the SCWG process is cost intensive due to organic contaminants; therefore, the recycling of the process effluent is attractive. Salt separation is needed to avoid accumulation of salts in the effluents, since salts enhance corrosion rates and might cause blocking of the flow when the effluent is recycled. The grass Reed Canary Grass and grapevines were gasified. The recycling of the process effluent did not influence the composition of the product gas. In both cases the carbon efficiency decreased by 4% when wastewater was used to dilute the biomass. An increase in organic carbon and potassium in the reactor effluent was observed after gasification of the biomass with recycled process effluent. The addition of potassium hydroxide to the feed as a homogenous catalyst needs to be closely monitored and adjusted according to the potassium content of the reactor effluent. Insufficient salt separation proved to be an issue regarding formation of solid deposits in the reaction system.