The growing strain on natural resources to meet the increasing global demand for energy and chemicals has been a challenge for several decades. This has motivated the emergence of several alternative technologies that provide effective and sustainable solutions while being economically feasible. Chemical looping is one such technology that utilizes the redox gas-solid reaction chemistries to inherently change the reaction mechanism, thus providing new and efficient pathways to produce the desired product. This gives rise to a platform that has higher degrees of freedom as compared to the traditional catalytic systems, which can be leveraged to create economically and environmentally sustainable processes. Several catalytic applications have been investigated as chemical looping alternatives and are given as follows:
Oxidative coupling of methane (OCM):
OCM refers to the reaction where two CH4 molecules couple to form hydrocarbon products such as ethane/ethylene in the presence of oxygen species. The chemical looping OCM technology uses a catalytic oxygen carrier to provide the oxygen species for CH4 coupling, consequently reducing the oxygen carrier. The lattice oxygen of this reduced oxygen carrier is replenished by air in a separate reactor which feeds the oxidized oxygen carrier back into the first reactor, thus completing the loop. Traditionally, O2 is co-fed with CH4 over a catalyst bed to produce these hydrocarbon products. The use of lattice oxygen as compared to O2 improves the selectivity of the desired products by eliminating the undesired gas-phase combustion reactions. Additionally, the use of an oxygen carrier expands the product slate up to C7 hydrocarbons, which has not been reported for the catalytic O2 co-feed system. However, developing an active oxygen carrier has been challenging due to the tradeoff between product selectivity and CH4 conversion. Thus, parametric tests have been conducted in a fixed bed reactor with the goal of understanding this tradeoff, investigating the reaction mechanism of OCM and identifying key reaction steps. These tests indicated a direct correlation of the lattice oxygen vacancy generated on the surface of the oxygen carrier and the tradeoff between selectivity and conversion. These insights combined with density functional theory calculations aided in a dopant screening strategy to induce the OCM selective oxygen vacancies on the oxygen carrier. Several doped oxygen carrier particles were synthesized and tested through which the optimal formulation was identified. Thus, a rational design strategy for developing a highly active and stable OCM particle was established.
Direct NOx decomposition:
Traditionally, NOx decomposition from flue gas streams is carried out catalytically by selectively reducing it with NH3. The novel chemical looping alternative takes advantage of oxygen vacancies that are available on a specialized metal oxide surface to decompose NOx into N2, thereby oxidizing the metal oxide. This oxidized metal oxide can release the oxygen in the form of O2 upon increasing the reaction temperature. This one-of-a-kind technology eliminates the use of NH3 by directly decomposing NOx into N2 and O2 which are produced in separate streams. Several metal oxides have been experimentally screened for the activity towards direct NOx decomposition and subsequent O2 evolution. The goal has been to reduce the reaction temperature, which ultimately lowers the parasitic energy requirement of the system. Further, the effect of other components of the flue gas, such as CO2 and O2, on NOx decomposition activity was investigated. These components showed a significant loss in NOx decomposition activity in a catalytic system. However, the specialized metal oxide has been tailored to show a minimal loss in activity, thus being superior to its catalytic counterpart. Finally, a preliminary techno-economic analysis evaluated the feasibility of the process which indicated significant savings as compared to traditional systems.
Methane to syngas with enhanced CO2/H2O utilization:
Syngas is an essential intermediate for liquid fuel/chemical production. The lattice oxygen from a metal oxide provides unique gas-solid thermodynamics that improves the syngas production efficiency as compared to traditional reforming systems. This thermodynamic benefit can be capitalized for CH4 co-fed with CO2 and H2O through reactor design. The effect of these changes can be effectively captured through process simulations done in ASPEN Plus. Several configurations with gas-solid contact including cocurrent, counter-current and cross-current have been investigated at different conditions to improve the CO2/H2O utilization. These configurations provide a novel approach to take advantage of the differences in gas-solid thermodynamic equilibrium with the change in the solid phase. A systematic study of these process simulations has aided in the design of the actual chemical looping reactor for syngas production. Further, mechanistic studies on the CO2 utilization has been carried out to understand the effect of oxygen vacancies on CH4 conversion and CO2 utilization.
Ammonia decomposition for its use as a hydrogen carrier:
Hydrogen economy is a novel concept that aims to reduce the carbon intensity of the applications pertaining to energy. One of the major hurdles towards adopting such a system lies in the complexity and the cost of storage, transportation, and handling of H2. Thus, ammonia has been proposed as a hydrogen carrier to mitigate these drawbacks, however efficient conversion of ammonia back to H2 can be challenging. A unique chemical looping system has been proposed that uses metal oxides to convert NH3 into N2 and H2. Due to the peculiar process configuration of this system, the N2 and H2 streams can be partially separated in the reactor system itself, providing opportunities for process intensification.