Catalytic Reforming of Biogas for Syngas Production (2012)

Biogas is a mixture of methane and carbon dioxide produced from the anaerobic microbial digestion of biomass. It is an inexpensive, local source of energy but is usually wasted because the CO2 content dilutes the quality of the fuel. Dry and auto-thermal reforming are catalytic methods that convert both the CH4 and CO2 into H2 and CO, or syngas, a valuable product that can be used to produce liquid fuels, provide H2 for fuel cells, or improve the combustion of biogas. A Rh/Al2O3 catalyst is successful in dry reforming biogas to syngas without deactivation from carbon formation at CH4/CO2 ratios of one or lower. In CH4 rich mixtures, auto-thermal reforming (ATR) is effective because it provides additional oxidant that eliminates carbon formation and combusts a portion of the CH4 in-situ to provide the heat needed for the endothermic reforming reactions. In addition to CH4 and CO2, biogas also contains chlorocarbons that are potential catalyst poisons. Chlorocarbons are unique to biogas and bio-derived fuels due to the natural presence of chlorinated compounds in organic material that are released during decomposition or thermal treatment. Despite their presence in biogas in 10-50ppm concentrations, the effect of chlorocarbons on the dry reforming reaction has not been extensively studied. This work investigated the effect of CH3Cl in particular on the activity and selectivity of CH4 dry and auto-thermal reforming using a Rh/Al2O3 catalyst. It was determined that CH3Cl introduction into the reforming reaction deposits chloride on the alumina catalyst support, which increases the surface acidity, poisons the water-gas shift reactions by replacing basic hydroxyl groups, and poisons the dry reforming reaction by reducing hydrogen mobility and the affinity of CO2 for the alumina support. CH3Cl also likely competes and reacts preferentially over CH4 for dry reforming sites. In CO2 rich environments, the reverse water gas shift reaction is poisoned, resulting in an increase of the H2/CO ratio, while in H2O rich environments, the forward water gas shift reaction is poisoned, resulting in a decrease of the H2/CO ratio. With 50 ppm addition of CH3Cl into a dry reforming reaction, the H2/CO ratio increases by 53% at a relatively low temperature of 350°C and increases by only 3% at 700°C. The poisoning of the water gas shift and dry reforming reactions, and the resulting changes in product selectivity and dry reforming activity, are completely reversible upon removal of CH3Cl from the feed. Therefore, the amount of chlorocarbon expected in a biogas mixture, between 10-50ppm, is not particularly harmful for the 4% Rh/Al2O3 catalyst. The degree of chloride poisoning is directly proportional to CH3Cl concentration and inversely proportional to H2O concentration and temperature. Therefore, O2 or air co-feeding minimizes chloride poisoning because it produces H2O and additional heat from the CH4 combustion reaction, both of which decrease chloride poisoning. Auto-thermal reforming is therefore more effective than dry reforming biogas because it keeps the Rh/Al2O3 catalyst clean of carbon and chloride deposition, thereby maintaining the activity and selectivity of the catalyst for conversion of biogas into syngas.