Fanxing Li

Clean coal conversion processes – progress and challenges

Although the processing of coal is an ancient problem and has been practiced for centuries, the constraints posed on todays coal conversion processes are unprecedented, and utmost innovations are required for finding the solution to the problem.With a strong demand for an affordable energy supply which is compounded by the urgent need for a CO2 emission control, the clean and efficient utilization of coal presents both a challenge and an opportunity to the current global R&D efforts in this area. This paper provides a historical perspective on the utilization of coal as an energy source as well as describing the progress and challenges and the future prospect of clean coal conversion processes. It provides background on the historical utilization of coal as an energy source, along with particular emphasis on the constraints in current coal conversion technologies. It addresses the energy conversion efficiencies for current coal combustion and gasification processes and for the membrane and looping based novel processes which are currently under development at various stages of testing. The control technologies for pollutants including CO2 in flue gas or syngas are also discussed. The coal conversion process efficiencies under a CO2 constrained environment are illustrated based on data and ASPEN Plus® simulations. The challenges for future R&D efforts in novel coal conversion process development are also presented.

Role of metal oxide support in redox reactions of iron oxide for chemical looping applications: experiments and density functional theory calculations

Aided by an oxygen carrier such as iron oxide, the chemical looping process can convert carbonaceous fuels while effectively capturing CO2. Previous experimental studies indicate that adding TiO2 support to iron oxide can notably improve the reactivity of iron oxide over multiple redox cycles, making it more suitable for chemical looping applications. In this study, wustite (Fe1−xO) was used to represent pure iron(II) oxide and ilmenite (FeTiO3) was used to represent TiO2 supported iron(II) oxide. The underlying mechanisms for the improved iron oxide performance with TiO2 support are investigated through experiments and periodic Density Functional Theory (DFT) calculations. Both experimental and DFT studies indicate that TiO2 support is unlikely to reduce the activation energy for the reduction of iron(II) oxide. The support, however, can significantly lower the energy barrier for O2− migration within the dense solid phase, thereby enhancing the O2− diffusivity. The good agreements between experiments and DFT calculations confirm that the improved reactivity and recyclability of TiO2 supported iron oxide particles are likely to result from the significantly enhanced O2− diffusivity with the presence of support.

Ionic diffusion in the oxidation of iron—effect of support and its implications to chemical looping applications

Addition of TiO2 was found to significantly enhance the ionic diffusivity of O anion within iron and its oxides, thereby changing the dominating ionic transfer mechanism for iron oxidation from “outward Fe cation diffusion” (in pure Fe case) to “inward O anion diffusion” (in Fe with TiO2 support case).

Fe2O3@LaxSr1−xFeO3 Core–Shell Redox Catalyst for Methane Partial Oxidation

Efficient and environmentally friendly conversion of methane into syngas is a topic of practical relevance for the production of hydrogen, chemicals, and synthetic fuels. At present, methane‐derived syngas is produced primarily through the steam methane reforming processes. The efficiencies of such processes are limited owing to the endothermic steam methane reforming reaction and the high steam to methane ratio required by the reforming catalysts. Chemical looping reforming represents an alternative approach for methane conversion. In the chemical looping reforming scheme, a solid oxygen carrier or “redox catalyst” is used to partially oxidize methane to syngas. The reduced redox catalyst is then regenerated with air. The cyclic redox operation reduces the steam usage while simplifying the heat integration scheme. Herein, a new Fe2O3@LaxSr1−xFeO3 (LSF) core–shell redox catalyst is synthesized and investigated. Compared with several other commonly investigated iron‐based redox catalysts, the newly developed core–shell redox catalyst is significantly more active and selective for syngas production from methane. It is also more resistant toward carbon formation and maintains high activity over cyclic redox operations.

A hybrid solar-redox scheme for liquid fuel and hydrogen coproduction

The feasibility of a hybrid solar-redox process, which converts solar energy and methane into separate streams of liquid fuels and hydrogen through the assistance of an oxygen carrier, is investigated via both experiments and simulations. Fixed and fluidized-bed experiments are conducted to evaluate the redox performances of an oxygen carrier composed of iron oxide promoted with a mixed ionic–electronic conductor (MIEC) support. Over 95% conversion in the methane oxidation step and 60% steam to hydrogen conversion in the water-splitting step are observed. Aspen Plus® simulation based on experimental data and a comprehensive set of assumptions estimates the overall process efficiency to be 64.2–65.3% on a higher heating value (HHV) basis. Through the integration of solar energy, methane to fuel conversion efficiency can approach 100%. The proposed process has the potential to produce transportation fuels and hydrogen at high efficiency with reduced carbon footprint.

Perovskite promoted iron oxide for hybrid water-splitting and syngas generation with exceptional conversion

We report a perovskite promoted iron oxide as a highly effective redox catalyst in a hybrid solar-redox scheme for partial oxidation and water-splitting of methane. In contrast to previously reported ferrite materials, which typically exhibit 20% or lower steam to hydrogen conversion, La0.8Sr0.2FeO3−δ (LSF) promoted Fe3O4 is capable of converting more than 67% steam with high redox stability. Both experiments and a defect model indicate that the synergistic effect of reduced LSF and metallic iron phases is attributable to the exceptional steam conversion. To further enhance such a synergistic effect, a layered reverse-flow reactor concept is proposed. Using this concept, over 77% steam to hydrogen conversion is achieved at 930 °C, which is 15% higher than the maximum conversion predicted by the second law for unpromoted iron (oxides). When applied to the hybrid solar-redox scheme for liquid fuels and hydrogen co-generation, significant improvements in the energy conversion efficiency can be achieved with reduced CO2 emissions.

Effects of bubble–liquid two‐phase turbulent hydrodynamics on cell damage in sparged bioreactor

According to recent experimental studies on sparged bioreactors, significant cell damage may occur at the gas inlet region near the sparger. Although shear stress was proposed to be one of the potential causes for cell damage, detailed hydrodynamic studies at the gas inlet region of gas–liquid bioreactors have not been performed to date. In this work, a second‐order moment (SOM) bubble–liquid two‐phase turbulent model based on the two‐fluid continuum approach is used to investigate the gas–liquid hydrodynamics in the bubble column reactor and their potential impacts on cell viability, especially at the gas inlet region. By establishing fluctuation velocity and bubble–liquid two‐phase fluctuation velocities correlation transport equations, the anisotropy of two‐phase stresses and the bubble–liquid interactions are fully considered. Simulation results from the SOM model indicate that shear and normal stresses, turbulent energy dissipation rate, and the turbulent kinetic energy are generally smaller at the gas inlet region when compared with those in the fully developed region. In comparison, a newly proposed correlation expression, stress‐induced turbulent energy production (STEP), is found to correlate well with the unusually high cell death rate at the gas inlet region. Therefore, STEP, which represents turbulent energy transfer to a controlled volume induced by a combination of shear and normal stresses, has the potential to provide better explanation for increased cell death at the sparger region.

Perovskite-structured AMnxB1−xO3 (A = Ca or Ba; B = Fe or Ni) redox catalysts for partial oxidation of methane

Methane is one of the simplest and most abundant organic compounds on earth. Methane reforming offers the versatility to produce value-added fuels and chemicals. Unlike typical reforming processes, which suffer from efficiency losses associated with endothermic reforming reactions and/or oxygen/steam generation, chemical looping reforming (CLR) utilizes the redox properties of an oxygen carrier, a.k.a. a redox catalyst, to partially oxidize methane into syngas with its lattice oxygen. The reduced redox catalyst is subsequently regenerated with air, providing in situ air separation with minimal energy penalty. The performance of the CLR process is highly dependent on the redox catalyst. In the current study, CLR performances and the underlying mechanisms for perovskite-structured redox catalysts with a general formula of AMnxB1−xO3 (A = Ca or Ba; B = Fe or Ni) are reported. CaMnO3 and BaMnO3 perovskites are worth investigating due to their desirable redox properties and relatively low cost. BaMnxB1−xO3 (B = Fe or Ni) based redox catalysts are shown to be more selective and coke-resistant to methane partial oxidation when compared to CaMnxB1−xO3 (B = Fe or Ni) based redox catalysts. While undoped BaMnO3 exhibited high selectivity towards syngas, addition of B-site dopants such as Ni or Fe leads to higher oxygen carrying capacity without significantly impacting the coke resistance of the redox catalysts. In contrast, nickel doped CaMnO3 is significantly more prone to coke formation. As a low cost and environmentally benign option, a BaMnxFe1−xO3 based redox catalyst was tested for CLR operations in a fluidized bed reactor. >95% syngas selectivity was observed with no signs of deactivation over 20 cycles.

Perovskite nanocomposites as effective CO2-splitting agents in a cyclic redox scheme

A methane-to-syngas selectivity of 96% and a CO yield of nearly 100% in CO2 splitting were achieved over perovskite nanocomposites in a cyclic redox scheme. We report iron-containing mixed-oxide nanocomposites as highly effective redox materials for thermochemical CO2 splitting and methane partial oxidation in a cyclic redox scheme, where methane was introduced as an oxygen “sink” to promote the reduction of the redox materials followed by reoxidation through CO2 splitting. Up to 96% syngas selectivity in the methane partial oxidation step and close to complete conversion of CO2 to CO in the CO2-splitting step were achieved at 900° to 980°C with good redox stability. The productivity and production rate of CO in the CO2-splitting step were about seven times higher than those in state-of-the-art solar-thermal CO2-splitting processes, which are carried out at significantly higher temperatures. The proposed approach can potentially be applied for acetic acid synthesis with up to 84% reduction in CO2 emission when compared to state-of-the-art processes.

Rh-promoted mixed oxides for “low-temperature” methane partial oxidation in the absence of gaseous oxidants

Compared to conventional reforming, chemical looping reforming (CLR), which partially oxidizes methane in the absence of gaseous oxidants such as steam or oxygen, offers a simpler and potentially more efficient route for syngas generation. This is achieved by cyclic removal and replenishment of active lattice oxygen in oxygen carrier particles, a.k.a. redox catalysts. With redox catalysts being at the heart of CLR, their activity and selectivity are crucial for the CLR performance. While many redox catalysts have been developed, their activities toward methane partial oxidation (POx), especially at relatively low temperatures, are often limited due to the high activation energy for the migration and removal of lattice oxygen. Moreover, syngas selectivity is often less than ideal due to the non-selective nature of the surfaces for many oxides. To address these limitations, we investigated the effects of promoting the catalytic activity of oxide surfaces for two redox catalysts: CaMnO3 and LaCeO3. Our findings indicate that promoting mixed oxides with a small amount of Rh can lower the onset temperature of methane POx by as much as 300 °C (0.5 wt% Rh loading). Over 93% syngas selectivity and 7.9 mmol of syngas per gram of redox catalyst were obtained for a highly stable, Rh promoted CaMnO3 at 600 °C, making it a promising redox catalyst for methane POx under a cyclic redox scheme.