Lang Qin

Shale gas-to-syngas chemical looping process for stable shale gas conversion to high purity syngas with a H2 : CO ratio of 2 : 1

The shale gas-to-syngas (STS) chemical looping process was conceived by Fan and associates in 2013 for the production of high-purity syngas from shale gas. The STS process producing syngas does not require the use of molecular oxygen from air separation and steam. This paper describes the rationale for the process concept with experimental data that substantiates the process validity. Specifically, the STS process consists of a co-current gas (shale gas)–solid (metal oxides) moving bed contact mode reducer operation with metal oxides for shale gas conversion to syngas. The reduced metal oxides from the reducer operation are regenerated via an oxidation operation with air. Various active metal oxides or metal oxide composites can be utilized. However, it is through the combination of desired metal oxides and co-current moving bed reducer that high syngas purity and a desirable H2 : CO molar ratio of ∼2 : 1 can be achieved. In this study, active iron–titanium composite metal oxide (ITCMO) materials are used as the oxygen carrier for the demonstration of the STS process. The desirable thermodynamic property of ITCMO is a key factor for the generation of high quality syngas. The co-current moving bed provides a desirable gas–solid contacting pattern that minimizes carbon deposition and maximizes the syngas yield. The syngas produced by the STS process can achieve a H2 : CO molar ratio of ∼2 : 1 with little CO2, CH4 and steam, which is required for downstream processes to produce liquid fuels and chemicals. The experimental results for reaction kinetics including oxygen carrier recyclability and pressure effects are obtained by thermogravimetric analysis (TGA), and syngas generation using a fixed bed, a bench-scale moving bed, and a sub-pilot scale moving bed reactor demonstrations are achieved in this study. The bench and sub-pilot demonstrations confirm that the syngas produced by the STS process is close to thermodynamic equilibrium with the reduced ITCMO. Furthermore, simulation studies are conducted to compare the efficiency of the STS process with a conventional autothermal natural gas reforming process.

Evolution of nanoscale morphology in single and binary metal oxide microparticles during reduction and oxidation processes

Metal oxide composites are enabling materials for many energy conversion systems such as chemical looping and photocatalysis. Their synthesis into electronic materials and operation in chemical looping technologies are based on reduction and oxidation reactions that involve exchanges of ions and electrons. These processes result in the creation and diffusion of defects that determine the nanoscale crystal phases and morphologies within these materials and their subsequent bulk chemical and electrical behavior. In this study, samples of metal oxide composites undergoing cycles of reduction and oxidation are examined at the nano- and micro-scale; the interfacial characteristics of dissimilar metals and metal oxides within the composites are examined. Specifically, structural transformations during redox processes involving pure Fe, FeNi alloy, and CuNi alloy microparticles are investigated. In Fe and FeNi systems, nanowires and nanopores are observed to simultaneously form on the microparticle surface during oxidation, while no such structuring is observed in the CuNi system. Additionally, uniform FeNi microparticles are transformed into particles with a NiO-rich core and a Fe2O3-rich shell during oxidation, due to differences in the oxidation and ion diffusion rates of Ni and Fe. In all material systems, the oxidized form of the microparticles exhibited porous cores due to ion transport described by the Kirkendall effect. A fundamental understanding of these phenomena will help direct the fabrication of electronic oxide materials and the development of metal oxide-based oxygen carriers for chemical looping applications.

Nanostructure formation mechanism and ion diffusion in iron–titanium composite materials with chemical looping redox reactions

Iron oxide composites are enabling materials in energy conversion systems including chemical looping and photocatalysis. Extensive earlier experimental findings indicate that inert oxides such as titanium oxide can greatly improve the reactivity of iron oxide over multiple redox cycles. Knowledge on the evolution of the nanoscale morphology of the Fe–Ti materials during the oxidation and reduction is thus of considerable importance. It is also of interest to the fundamental understanding of the ion diffusion mechanism in the reaction processes. In this study, Fe–Ti composite microparticles undergoing cycles of oxidation and reduction are examined at the nano-scale, and the interfacial characteristics of the iron titanium oxides within the composites are probed. Nanobelts are observed to simultaneously form on the microparticle surface during the oxidation at 700 °C, while microblades are found at 900 °C. Additionally, unlike pure iron microparticles that become dense on surface due to sintering effect, Fe–Ti microparticles are transformed into porous particles after redox cycles. The atomistic thermodynamics methods and density functional theory calculations are carried out to investigate the ionic diffusion and vacancy formation during the oxidation and reduction process. A number of surface configurations are considered, and the Ti–Ti–O– terminated surface is computed to the most stable surface structure at experimental conditions. It was found that in oxidation processes, surface Ti atoms are more favorable for oxygen adsorption and dissociation than Fe atoms. The energy barrier of Fe ion diffusion towards the surface, on the other hand, is lower than Ti ion diffusion, which contributes to the Fe2O3-dominant nanobelt formation. The volume change due to high temperature associated with the solid state transformation at the Fe2O3/FeTiO3 interface produces compressive stresses, which stimulate Fe2O3 nanobelt growth to accompany the interface reaction. Also, as the vacancy formation energy of FeTiO3 is lower than Fe2O3 in the reduction process, it indicates that it is easier for a FeTiO3 surface to form vacancy defects, thereby enhancing the porous surface structure formation and O2 diffusivity. The good agreements between experiments and DFT calculations further substantiate nanostructure formation mechanism in redox reactions of iron titanium composite materials.

Oxygen vacancy promoted methane partial oxidation over iron oxide oxygen carriers in the chemical looping process

We perform ab initio DFT+U calculations and experimental studies of the partial oxidation of methane to syngas on iron oxide oxygen carriers to elucidate the role of oxygen vacancies in oxygen carrier reactivity. In particular, we explore the effect of oxygen vacancy concentration on sequential processes of methane dehydrogenation, and oxidation with lattice oxygen. We find that when CH4 adsorbs onto Fe atop sites without neighboring oxygen vacancies, it dehydrogenates with CHx radicals remaining on the same site and evolves into CO2via the complete oxidation pathway. In the presence of oxygen vacancies, on the other hand, the formed methyl (CH3) prefers to migrate onto the vacancy site while the H from CH4 dehydrogenation remains on the original Fe atop site, and evolves into CO via the partial oxidation pathway. The oxygen vacancies created in the oxidation process can be healed by lattice oxygen diffusion from the subsurface to the surface vacancy sites, and it is found that the outward diffusion of lattice oxygen atoms is more favorable than the horizontal diffusion on the same layer. Based on the proposed mechanism and energy profile, we identify the rate-limiting steps of the partial oxidation and complete oxidation pathways. Also, we find that increasing the oxygen vacancy concentration not only lowers the barriers of CH4 dehydrogenation but also the cleavage energy of Fe-C bonds. However, the barrier of the rate-limiting step cannot further decrease when the oxygen vacancy concentration reaches 2.5%. The fundamental insight into the oxygen vacancy effect on CH4 oxidation with iron oxide oxygen carriers can help guide the design and development of more efficient oxygen carriers and CLPO processes.

Chemically and physically robust, commercially-viable iron-based composite oxygen carriers sustainable over 3000 redox cycles at high temperatures for chemical looping applications

High attrition of oxygen carriers from ionic diffusion induced morphological changes is one of the most serious obstacles inhibiting commercialization of chemical looping processes. By confining active oxygen carriers in an Al-based skeleton, a low-cost oxygen carrier achieving high chemical and physical stability over 3000 TGA redox cycles with exceptional attrition resistance and reactivity in a high pressure and temperature, chemical looping moving bed pilot plant, is enabled for commercial implementation.