Supercritical water gasification of heavy metal contaminated plants with focus on separation of heavy metal contaminants

Soil and groundwater contaminated with heavy metals pose a threat to animals and humans. The use of plants by phytoremediation can help to free the soil and groundwater from the pollutants, but these plants will then become contaminated themselves. The aim of the present work is the energetic utilization of contaminated biomass and the separation of contaminants e.g. heavy metals. This is achieved by gasification of the contaminated biomass under conditions of supercritical water in a continuous laboratory plant. The separation of heavy metal contaminants like zinc, manganese or iron by process-integrated salt separation is necessary to avoid secondary contamination. Reed Canary Grass, Napier Grass and grapevine plants were gasified. Plant type had no effect on gasification efficiency or heavy metal removal. A mean carbon gasification efficiency of 58.3 % was achieved. In all cases, the reactor effluent was free of heavy metal contamination. The general ability to separate salts and heavy metals by salt separation was demonstrated (up to 35 % of heavy metal species detected in the salt brine). The separation is strongly dependent on the preheating temperature (increase in separation of salts when increasing preheating temperature from 460 to 550 °C) and needs to be further improved to avoid the formation of solid deposits in the reaction system. Excessive solid deposition, consisting of coke, salts and corrosion products, limited the experimental duration.