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.

Author dc:creator

• Kohn, McKenzie Primerano

• Chemical engineering
• Environmental engineering
• Biogas
• Synthesis gas
• Catalytic reforming
• Methane
• Carbon dioxide
• Aluminum oxide
• Methyl chloride
• Chlorides--Environmental aspects

Language dc:language

English

DOI dc:identifier

https://doi.org/10.7916/D8F76KPR

OAI identifier oai:identifier

oai:academiccommons.columbia.edu:10.7916/D8F76KPR