CONVERSION OF GREENHOUSE GAS COMPONENTS TO VALUE ADDED FUEL VIA SYNGAS ROUTE

Abstract

Carbon capture and utilization (CCU) has been drawing significant attention in recent years especially as a near to the medium-term strategy to decrease the global carbon footprint swift and meet the climate change commitments of NDCs including the net-zero CO2 emissions. The present global warming is majorly contributed by carbon dioxide (74.4%), methane (17.3%), and the total GHG emissions were estimated as 48.94 billion t CO2e in 2018, wherein 3.35 billion t CO2e was contributed by India. The main causes of this unprecedented growth of GHGs are fossil fuel combustion such as coal, oil and natural gas. Moreover, with increasing global energy demand from expanding economies and population growth, fossil fuels usage becomes unavoidable resulting in increased greenhouse gas (GHG) emissions. As per the Statistical review of world energy 2022, the world's primary energy consumption was estimated at 595 Exajoules in 2021 and is predicted to grow about 50% by 2050. Despite the rapid progress in renewable energy such as solar, wind, tidal, biogas, and geothermal, it may not deliver the present energy needs due to their geographical and technological limitations. On the other hand, fossil fuel-based power plants (coal, gas and oil usage) and high-temperature industrial applications release flue gases containing high amounts of CO2 and CH4, H2O, O2 etc., which can be effectively utilized to produce dimethyl ether (DME), a versatile fuel and chemical alternative via intermediate syngas route by tri reforming of methane to augment the energy needs. Tri-reforming of methane (TRM) can produce synthesis gas with a variable H2/CO ratio of 1-2 suitable for methanol, dimethyl ether, lower olefins and Fischer-Tropsch fuels (diesel, gasoline and naphtha) etc. However, the susceptibility of Ni-based catalysts for coke deposition is limiting its commercial application. Thus, the development of a stable and robust catalyst with high resistance to coke formation is desired. The direct syngas to DME (STD) process over Cu-Zn/Al hybrid catalyst has also drawn wide attention, although DME can be produced from syngas indirectly via intermediate methanol in a two-step process. Significant economic benefits can also be achieved in the direct synthesis of DME by increasing per-pass syngas conversion, decreasing H2 demand with low feed syngas H2/CO ratios, and alleviating thermodynamic limitations. DME evolved as an essential substrate of the 21st century as a substitute for diesel and LPG, producing chemicals like acetic acid, methyl acetate, dimethyl sulfate, and lower olefins. Concerning the above climate and energy issues, efforts have been made to develop a stable and robust catalyst for the effective utilization of industrial flue gases via the methane tri-reforming process for syngas production and its direct conversion to ultra-clean fuel dimethyl ether. This thesis is divided into six chapters consisting of the preparation of catalysts, physicochemical characterization, structure-activity relationship and their systematic activity investigation, including conclusions from the present work and future direction for syngas generation by methane tri-reforming process and its direct conversion to dimethyl ether. Chapter-1: Introduction This chapter summarises the current greenhouse gas emissions scenario, global energy challenges and reports some carbon capture and utilization (CCU) processes. Discusses some of the promising CCU approaches such as syngas generation by various processes, gasification and methane-CO2 reforming and its conversion to many value-added products via GTL technology. It elaborates on the syngas generation by different CO2 reforming of methane processes and its direct conversion to dimethyl ether (DME), the major focus of this work, wherein, specifically the tri reforming focuses on utilizing the major greenhouse gases CH4 and CO2 effectively to produce synthesis gas. Chapter-2: Literature review A detailed literature survey has been carried out and presented in this chapter on state of the art on catalysts developed for syngas generation by methane tri-reforming and its direct conversion to dimethyl ether. Also, it details the emergence of these processes for effectively utilizing industrial flue gases that mitigates GHG emissions to produce an alternate eco-friendly ultra-clean fuel “DME.” Chapter-3: Instrumentation and experimental setup The chapter details the process and analytical instrumentation used for the present study. This describes the experimental reactor setup and process instruments used for catalyst evaluation in both TRM and STD processes. It also explains the details of various analytical instruments such as XRD, XPS, physisorption (BET), chemisorption (TPR and TPD), SEM, TEM, FTIR and TG-DTG used for the structural elucidation and physico-chemical characterization of prepared catalytic materials, as well as GC (RGA) for the analysis of reactant and product components in TRM and STD process Chapter-4: Enhanced CO2 utilization via methane tri-reforming over Ru incorporated Co/MgO-Al2O3 catalyst: Influence of La and Ce promoters The catalytic efficiency of Ru incorporated Co-based catalysts supported on cylindrical extrudates of MgO-Al2O3 mixed oxide with promoters La and Ce for syngas production by methane tri reforming for the effective utilization of CO2 present in varied sources, including industrial waste flue gases has been explored in this chapter. Ru was chosen as one of the active metals due to its characteristic hydrogen spillover phenomena and high coke resistivity. Moreover, Co and Ru coexistence play a pivotal role in enhanced reactants’ conversion and catalyst stability via the bimetallic synergy. The activity study revealed that Co content, Co-Ru synergy and La, Ce-doping influenced the catalytic activity as the prepared catalysts were evaluated at different operating conditions like temperature 600-800 °C, pressure 1 atm and GHSV 4000-10000 ml gcat-1 h-1 under an optimized molar feed ratio of CH4:CO2:H2O:O2 = 1:0.75:0.75:0.1. The variation in feed composition was also tested over the Co-Ru catalyst and obtained a product syngas ratio 1-2. The other key factors that influenced the catalytic performance were Co crystallite size, Co3O4 reducibility, surface basicity and metal-support interface as observed from the stability studies and detailed physicochemical characterization of fresh and spent catalysts. Further, the addition of La facilitated the formation of a large number of surface basic sites than Ce via a better La-(Co-Ru)-support interface, resulting in superior coke resistivity for the La-doped Co-Ru catalyst with high conversions of CH4 and CO2 during 100 h time on stream study. Chapter-5: Insights of precursor phase transition of (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst for one step dimethyl ether synthesis from syngas The structural insights of the active catalytic components of (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst, such as methanol synthesis catalyst (CuO:ZnO:Al2O3) and dehydration catalyst (γ-Al2O3) for direct DME synthesis from syngas were studied in this chapter. The influence of precursor phase transition of methanol synthesis catalyst with varied Cu/Al content from hydrotalcite to zincian malachite on the hybrid catalyst’s activity was investigated. A series of Cu-Zn hybrid catalysts were prepared via a kneading extrusion process by physical mixing of methanol synthesis catalyst prepared via facile forward precipitation route with CuO/Al2O3 wt. ratios 1.5 to 6 and the dehydration component boehmite (γ-AlO(OH)). The activity studies revealed that the catalyst with a lower CuO/Al2O3 ratio (1.5) derived from the phase pure hydrotalcite precursor showed enhanced catalytic activity with a high CO conversion of 60.5 % and DME selectivity of 71.5 %. Whereas the catalyst with a high CuO/Al2O3 ratio (6) showed relatively lower DME selectivity, although it was derived from the phase-rich zincian malachite, which is a preferred active phase for methanol synthesis. The higher yield of DME with hydrotalcite-derived hybrid catalyst was most likely attributed to the well dispersed nanostructured active Cu sites and the higher number of weak and medium strength acidic sites. Moreover, the phase transition of boehmite to γ-Al2O3 may also be helped in increased methanol dehydration rate towards enhanced DME yield with its solid Lewis acidity. Chapter-6: Single-step dimethyl ether synthesis from syngas over layered hydroxy double salts (HDS) derived (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst In this chapter, we have reported the catalytic activity of (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst derived from layered hydroxy double salt (HDS) precursor of methanol synthesis catalyst with additional structural promoters Ga, In and Cr for direct DME synthesis from syngas. Here, boehmite was employed as a precursor for the dehydration catalyst γ-Al2O3 of the hybrid catalyst. The activity studies revealed that the (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst derived from layered (Cu, Zn) hydroxy double salt with NO3- as anion showed relatively better activity. Further, the Ga-incorporated hybrid catalyst showed higher CO conversion 60.9% and DME selectivity 79.1% than other hybrid catalysts. The In modified hybrid catalyst showed inferior activity in terms of CO conversion. The higher activity of Ga-promoted hybrid catalyst might be attributed to the huge number of facile reducible active Cu sites along with a better amount of surface acidity from weak and medium Lewis acidic sites. Chapter-7: Conclusions and future scope This last chapter of the thesis details the essential findings and conclusions from the investigations in the present study. It also presents a way forward for future work to take the present study to a higher technology readiness level.