(383d) Simultaneous Removal of Biomass-Generated Syngas Contaminants Using Biochar-Based Catalysts

In the quest for low-cost and efficient technologies to convert biomass into fuels, chemicals, and power, gasification offers great potential. However, to effectively utilize syngas, the main product of biomass gasification, it must be cleaned of its contaminants. Contaminants such as ammonia, tars, and H₂S cause various problems in downstream applications of the gas. Simultaneous removal of these contaminants using a single catalytic bed and same conditions will significantly reduce costs. The primary objective of this work was to investigate the use of carbon-based catalysts, biochar and activated carbon derived from biochar, for individual and simultaneous removal of tars, ammonia, and H₂S.

Biochar is a low surface area waste product generated during biomass gasification. Biochar has shown good potential for tar removal. To improve upon the low surface area of biochar, activated carbon was synthesized using the biochar derived from switchgrass gasification. Activated carbon was also modified using dilute acid to give an acidic surface to help improve removal of toluene (model tar) and NH₃. The three catalysts (biochar, activated carbon, and modified activated carbon) were evaluated for their contaminant removal efficiency, first separately and individually with each contaminant and then simultaneously with all of the contaminants. Evaluation was carried out at temperature of 700 °C and 800 °C and a gas residence time of 0.001 kg/(m³/hr) in a fixed-bed catalytic reactor fed with syngas.

Several key findings, to date, are following. First, both the activated carbons and biochar were effective in tar removal (up to 92%). Second, as compared to biochar, activated carbon catalysts resulted in higher toluene conversion due to its higher surface area (~900 m²/g compared to <10 m²/g of biochar), larger pore diameter (19 A° compared to 15.5 A° of biochar) and larger pore volume (0.44 cc/g compared to 0.085 cc/g of biochar). Chemisorption experiments showed that CO₂ was weakly adsorbed (physiosorption) onto activated carbon surface. This indicates that the sorbent can easily be regenerated. The adsorptive capacity was as high as 1.8 mmol/g at 60 kPa indicating a high sorption capacity.