Catalyst development and process intensification towards syngas production through methane reforming
Creators: name: Mozammel, Tibra
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340601
subject: Catalysis and mechanisms of reactions
classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)
classificationCode: 401807
subject: Nanomaterials

description: Abundance of natural gas in addition to its lower carbon emission compared to other fossil fuels including coal and crude oil makes it a preferred feed-stock for producing liquid fuels and chemicals. There are number of chemical processes that have been developed to convert natural gas/methane into liquid fuels and other fuel additives, collectively referred as Gas-to-Liquid technologies (GTL). Catalytic reforming of methane for producing syngas, a mixture of hydrogen and carbon monoxide, is a well-established but least efficient and dearest part of GTL processes. Majority of the industrial syn gas production technologies are large scale highly energy intensive steam reforming (SMR) processes. Other catalytic reforming processes, including catalytic partial oxidation (CPOx), dry reforming (DRM) and autothermal reforming (ATR) of methane, can produce synthesis gas of different compositions as well as varying energetics. DRM produces CO rich syn gas, SMR produces H2 rich syn gas. CPOx produces the syn gas having the H2:CO ratio 2, desirable for downstream methanol and DME production, but the exothermic nature of process often leads to the formation of hot spots in the catalyst surface that deactivate the catalyst. Moreover, this process requires handling of oxygen and methane mixture, which is a safety concern. Autothermal reforming of methane combines non-catalytic complete oxidation (by flame conventionally) of methane and catalytic reforming of methane by steam and CO2. This process can be miniaturized and carried out at high pressure, which can avoid syngas compression step during the downstream high pressure conversion of syngas into methanol. However, ATR requires flame (or burner) and it carried out at very high temperatures; higher than 1000 ⁰C. Major factor governing to optimize these reforming processes is to develop stable and active catalysts against coking, active metal sintering and support degradation. Developing robust catalysts with high activity has always been challenging. Another factor that needs to be addressed for optimization of reforming processes is efficient heat and mass transfer, which is difficult to achieve in small scale processes.<br><br>The focus of this study is to develop an intensified/efficient reforming process with smaller footprint so this can be used to convert methane onsite in remote areas. In order to integrate the heat and energy transfer in a small-scale process as well as to get the desirable H2/CO ratio, it has been proposed to develop a dual catalyst beds reactor consisting of oxidation catalyst and reforming catalyst. In such processes, methane oxidation occurs at a very short contact time on the first bed, produces heat and CO2 with high space velocities. Then CO2 and steam reforming occurs on the second bed utilizing the heat generated from the first bed. This process is similar to autothermal reforming except the flame, which is replaced by an oxidation catalyst. This process is referred here as “catalytic ATR”. The process parameters such as temperature, pressure, feed composition (O2 and steam) were varied for the in depth understanding of the system. The key advantage that Catalytic ATR has over other reforming process is, tailored syn gas composition in the product by changing temperature and feed ratios.<br><br>For the quest of suitable reforming catalyst, seven catalysts had been synthesized for dry reforming of methane, on mesoporous alumina supports, including mono, bi and trimetallic catalysts consists of transition metals nickel, cobalt and rhodium (Ni, Co and Rh). The bimetallic catalyst NiCo/Al2O3 and (RhNi/Al2O3) were found to be highly active, produced CO rich syn gas (H2:CO ratio ~ 1) and particularly stable for hours. The fresh and spent catalysts were characterized using different characterising technology; BET, XRD, TGA, XPS and TEM, to understand stability and activity of these catalysts. The alloy phase of Ni and Co in the case of NiCo/Al2O3 and the hydrogen spill-over effect of Rh in the case of NiRh/Al2O3 were found to be responsible for the enhanced activity. <br><br>Lower temperature partial and/or complete oxidation catalysts were required for the first bed of the duel catalyst bed of catalytic autothermal reforming process. Hydrotalcites of varying Mg/Al (Magnesium and Aluminium) ratio supported rhodium catalysts were screened for partial oxidation of methane. Rhodium weight percentage was kept as 1%. All these catalysts (Fresh and spent) were also characterized with BET, XRD, TGA, TPO, XPS and TEM. Rhodium catalysts favour partial oxidation of methane due to the dual role of rhodium as an oxidation catalyst and reforming catalyst. Among the rhodium hydrotalcite catalysts, Rh/hydrotalcite (HT-1) was oxidized at around 450 °C and found to be highly active CPOx catalyst for syn gas production.<br><br>1 weight percentage of Pd/CeO2/Al2O3 was synthesized in a noble synthesis process and tested in catalytic autothermal reforming. The result reveals that Pd/CeO2/Al2O3 favoured complete oxidation of methane at even lower temperatures (from 290 ⁰C). For lower oxidation temperature and tendency of complete oxidation, Pd/CeO2/Al2O3 catalyst was chosen as an oxidation catalyst for the catalytic autothermal reforming of methane.<br><br>Finally, the dual catalyst bed consists of total oxidation catalyst and best performing/economic reforming catalysts placed in sequential manner were tested with oxygen and steam as oxidant feed. Syngas produced from this autothermal reforming contains hydrogen to carbon ratio higher than DRM and CPOX but lower than conventional SRM and ATR processes. Pd/CeO2/Al2O3 showed methane oxidation as low as 290 °C and produced significant heat at higher temperature; enough to initiate the subsequent reforming processes by the reforming catalyst.