Historically, fossil fuels have dominated the energy generation domain and it is projected that they will continue to remain a dominant source of energy for the foreseeable future. Although essential, the usage of fossil fuels has deemed to be deleterious for both human health and the surrounding environment as the release of anthropogenic CO2 because of burning fossil fuels has been asserted as one of the leading causes of global warming and climate change. It is important to understand that fossil fuels are not only used for energy generation, but they also contribute significantly towards the production of various industrially important chemicals and derivatives. Syngas and hydrogen, two of the petrochemical industry’s most sought-after commodities are typically generated using fossil fuels, wherein both gaseous and solid fossil fuels can be employed for syngas and hydrogen generation. Naturally occurring carbonaceous sources (fossil fuels) such as natural gas, shale gas, coal, etc. are reformed/gasified in the presence of steam/CO2/molecular O2 to generate syngas, which is processed further for generating hydrogen. These processes however collectively suffer from certain limitations including high endothermic heat requirement, placement of amine-based units for capturing CO2 generated across various operations in the process, catalyst deactivation due to coking, and the requirement of a highly energy and cost-intensive cryogenic air separation unit. Consequently, new technologies are being explored that can facilitate syngas/hydrogen generation in an efficient and clean manner such that the parasitic energy and cost of syngas generated per unit carbon is reduced. Chemical looping is one such technology platform that has the potential to replace the conventional syngas generating processes. The chemical looping process utilizes metal oxide-based oxygen carriers that react with carbonaceous feedstocks at elevated temperatures to carry out their partial oxidation for syngas generation through the transfer of lattice oxygen. The lattice oxygen lost in this step is made available to the reduced oxygen carriers using suitable oxidizing agents such as steam, CO2, or air, where the selection of the oxidizing agent is contingent to the desired process.
The work presented here aims to use the chemical looping technological platform for generating syngas using calcium-iron based oxygen carriers. When mixed in stochiometric proportions, the oxides of Ca and Fe tend to form a unique brownmillerite phase, which has exceptional thermodynamic selectivity for syngas production coupled with the occurrence of sufficient oxygen vacancies that promote rapid rates of ionic diffusion for enhanced reaction kinetics. These oxygen carriers also exhibit another unique trait as they tend to re-oxidize fully in the presence of CO2 or steam while generating CO and H2, respectively. The effect of adding MgO as an inert support on the reactivity of Ca2Fe2O5 has been ascertained while also investigating the regeneration performance in the presence of CO2. A detailed set of density functional theory calculations has been presented that investigates the reaction mechanism pathway and compares the activation energy barrier for syngas formation in Ca2Fe2O5 and pure Fe2O3. Extensive thermodynamic simulations have been conducted to carry out a systematic modeling study for various reactors such as reducer, oxidizer, and combustor associated with the methane chemical looping reforming system. Both isothermal and adiabatic modes of operation have been investigated under a wide range of operating pressures ranging from 1 to 30 bar. Comparison has been drawn between the various proposed chemical looping schemes and the industrial standard of autothermal reforming for generating syngas. The use of pure Ca2Fe2O5 for generating syngas using the chemical looping route is limited by its high endothermic heat requirement. To overcome this limitation, modification of the oxygen carrier has been envisioned through the addition of transition metal oxides (NiO, CuO, and Co3O4). The effect of adding varying concentrations of these metal oxides to Ca2Fe2O5 on process parameters such as a methane conversion, syngas selectivity, and endothermic heat requirement has been assessed through computational simulations carried out using thermochemical software. As already established, Ca2Fe2O5 possesses the capability to generate high quality syngas and get re-oxidized in CO2 to generate CO. Apart from this, it possesses another trait wherein any sulfur (typically found in solid fuels such as coal) entering the reactor can be selectively captured and removed using CaO present in the carrier. Using this principle, a process scheme has been devised for producing acetic acid from petcoke where syngas is generated in one reactor for methanol synthesis and CO along with H2S is produced in others , wherein methanol and CO further react with one another catalytically to produce acetic acid. The effect of solid circulation rates on syngas quality, parasitic energy requirement, and the sulfur capture capability has been studied using process simulations. Also, a comparison has been made for acetic acid generation using the proposed process and syngas generated through the conventional petcoke gasification route. One of the challenges of operating the chemical looping system for syngas generation from methane in adiabatic mode is the drop in temperature along the length of the reducer reactor in the absence of external heat source. As a result, methane conversion is high near the reactor top, but it decreases substantially as the temperature drops. To overcome this limitation, the Ca2Fe2O5 carrier is doped with Ni, where this doping is found to create oxygen vacancies that lower the energy barrier for methane adsorption and enable its rapid conversion to syngas. A detailed density functional theory- combined experimental approach has been utilized to study the effect of Ni doping on methane reactivity as well as the tendency of the carrier to be regenerated fully in CO2.
The work done as a part of this dissertation provides a holistic view of syngas generation using the Ca-Fe oxygen carriers, such that it provides a detailed account of the various aspects of technology development to propel the proposed chemical looping scheme towards commercialization pathway as a cost and energy efficient alternative to the state-of-the-art technologies already in practice. The work here covers a wide range of aspects for chemical looping process development that includes design and development of the oxygen carrier, understanding the reaction thermodynamics, and extensive process modeling studies for obtaining the optimum operating conditions, investigating the reaction kinetics and syngas generation performance through thermogravimetric and fixed bed analysis, and lastly, conducting density functional theory calculations to gain a mechanistic insight into the underlying reaction chemistry.