Catalytic Reforming of Biogas for Syngas Production
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.
Chemical engineering, Environmental engineering, Biogas, Synthesis gas, Catalytic reforming, Methane, Carbon dioxide, Aluminum oxide, Methyl chloride, Chlorides--Environmental aspects
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