Tri-reforming and combined reforming of methane for producing syngas with desired hydrogen/carbon monoxide ratios.

摘要

This dissertation is an exploratory study of a new process concept for direct production of synthesis gas (CO + H₂) with desired H ₂/CO ratios (1.5–2.0) for methanol synthesis and F-T synthesis, using CO₂ together with steam and unconverted O₂ in flue gas from fossil fuel-fired electric power plants to react with methane or natural gas. This new process is called tri-reforming, referring to simultaneous CO₂-steam-O₂ reforming of methane or natural gas.; This study included (1) The investigation of carbon formation in the tri-reforming process. For comparison, carbon formation in the combined reforming and CO₂ reforming reaction was studied as well. (2) The effect of reaction conditions and feed compositions on equilibrium composition (e.g. H₂/CO ratio) and equilibrium conversions in the tri-reforming process. (3) The role of catalysts in the tri-reforming process, especially the effect of catalysts on CO₂ conversion in the presence of H ₂O and O₂.; It was clearly evidenced from this study that CO in the product stream is probably the major source of carbon over Ni/Al₂O₃ in the equimolar CO₂-CH₄ reforming at 650°C and 1 atm. Addition of either O₂ or H₂O into the CO ₂ reforming reaction system can suppress carbon formation. It was demonstrated that carbon-free operation can be achieved in the tri-reforming process.; A thermodynamic comparison of tri-reforming with feed compositions of (H₂O+CO₂+0.5O₂)/CH₄ (mol ratio) = 1 showed that O₂ improves equilibrium CH₄ conversion, yet greatly decreases equilibrium CO₂ conversion. H₂O in tri-reforming has a significant effect on the H₂/CO ratio in the products, while O₂ has a minor effect.; A kinetic study and catalytic performance tests indicated that the support in a supported catalyst has a significant role in enhancing CO₂ conversion to CO in the presence of H₂O and O₂ in tri-reforming. The Ni/MgO catalyst showed superior performance with close to equilibrium CH₄ and CO₂ conversions at 850°C, 1 atm, and 32,000 ml/(h.gcat.).; The apparent Activation energy for CH₄ conversion over Ni/MgO was estimated to be 219 kJ/mol, which is higher than over Ni/Al₂O ₃ (69.1 kJ/mol) and Ni/MgO/CeZrO (67.4 kJ/mol). This may be attributed to less CH₄ activation over Ni/MgO or to an experimental artifact caused by catalyst deactivation as reaction temperature decreases from 850°C to 750°C. With the decrease of temperature, Ni may be re-oxidized and form NiO-MgO solid solution in the presence of H₂O, CO₂, or O₂. (Abstract shortened by UMI.)