In chemical-looping combustion (CLC), an oxygen carrier provides lattice oxygen for complete combustion of a fuel for heat and power production. The reduced metal oxide is then oxidized in a separate reactor. The combustion products CO2 and H2O are obtained in pure form, without any nitrogen in the gas. As no gas separation work is needed, this could be a breakthrough technology for carbon capture (CCS). Normally, the fuel-and air-reactor are designed utilizing interconnected fluidized beds. The same underlying reversible redox reactions of CLC can be used for other fuel conversion technologies. These include fluidized bed processes for gas, solid and liquid fuels for heat, power, syngas or hydrogen production. Some of these concepts were suggested as far back as the 1950's, while others have just recently been proposed. Chalmers University of Technology has been involved in CLC research for over 18 years, and this paper will provide a review of some recent developments with respect to CLC with gaseous, liquid and solid fuels. Further, the paper will provide an overview some related technologies where Chalmers is conducting research: i) Chemical-looping gasification (CLG), ii) Chemical-looping reforming (CLR) and iii) Chemical-looping tar reforming (CLTR). In these processes, a pure syngas/hydrogen can be produced effectively, which could be utilized for chemical or fuel production. CHEMICAL-LOOPING COMBUSTION (CLC) Background-As is evident from Fig. 1, the gas stream from the fuel reactor contains a concentrated stream of CO2 and H2O, while the nitrogen from the combustion air is obtained in the air reactor outlet. Possible C28E5−TR1132 (900...950°C) C28E1S2 (975°C) C28 (940°C) C14 (930...950°C) Ni−based (900°C)

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The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with inherent separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air reactor to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When solid fuel is used in CLC, the char must be gasified by, e.g., steam to form H2 and CO, that can be subsequently oxidized to H2O and CO2 by the oxygen carrier. In the case of CLOU, the oxygen carrier releases oxygen gas in the fuel reactor. This enables a high rate of conversion of char from solid fuels, because it eliminates the need for the gasification step needed in normal CLC with solid fuels. In this work, the rate of oxygen release and oxidation of an oxygen carrier consisting of CuO supported by MgAl2O4 (40/60 wt %) for the CLOU process is investigated. The oxygen carrier was produced by freeze-granulation, calcined at 950 °C, and sieved to a size range of 125–180 μm. The reaction rates were obtained in a laboratory-scale fluidized-bed reactor in the temperature range of 850–900 °C, under alternating reducing and oxidizing conditions. The rate of oxygen release was obtained using devolatilized wood char as the fuel in N2 fluidization. Care was taken to obtain reliable measurements not affected by the availability of the fuel and temperature increase in the bed during combustion of the fuel with the released oxygen from the carrier. The Avrami–Erofeev mechanism was used to model the rates of oxygen release and the values of k o and E app were estimated to be 2.5 × 105 s–1 and 139.3 kJ mol–1, respectively. The rates of Cu2O oxidation were investigated in a flow of 5% O2 at the inlet of the reactor. However, it was observed that the oxidation rate is limited by the oxygen supply, indicating rapid conversion of the oxygen carrier. From the obtained reaction rates, the minimum total amount of the investigated oxygen carrier needed in the air and the fuel reactor is estimated to be between 69–139 kg MWth –1.

Chemical‐looping combustion (CLC) is an emerging technology that can be used to meet the growing demands for electrical energy production without CO2 emissions. In CLC with solid fuels, the gasification of the carbonaceous fuel by steam is envisaged to be performed directly in the fuel reactor. This requires high steam‐gasification rates for the effective use of the solid fuel. Recently, it has been observed that the choice of oxygen carrier can have a profound effect on the char‐conversion rates in the fuel reactor. More specifically, the char‐conversion rate with a Brazilian manganese ore was a factor of five higher than that with ilmenite. In this work, the reaction mechanism of the char gasification was investigated in the presence of this manganese ore with the aim to explain the high rates observed. Steam‐gasification experiments with petroleum coke were performed by using a batch fluidized‐bed reactor with manganese ore as the bed material. In addition, partial gasification experiments of petroleum coke were conducted, and detailed energy‐dispersive X‐ray spectroscopy (EDX) analyses were performed on the surface and interior of the fuel and manganese‐ore particles. The effect of the possible gas‐phase release of oxygen from the manganese ore was also investigated. It was found that the release of gas‐phase oxygen by the oxygen carrier does not explain the high gasification rates observed. Instead, the transfer of a catalytically active material, potassium, from the bed material to the solid fuel was observed, which in turn catalysed the steam–carbon‐gasification reaction. As the catalytically active compound is included in this naturally occurring bed material, it may offer cost‐efficient, catalytic gasification in a CLC process.

The removal of condensable hydrocarbons or tars from raw gas derived from biomass gasification presents an obstacle in the widespread application of biomass gasification. Hot catalytic tar cleaning as a secondary tar removal strategy is discussed as a tar cleaning technology. This can be realized in a dual-fluidized-bed reactor system, in which a catalytically active bed material is continuously regenerated. Such a process is termed chemical looping reforming (CLR). In such a process, it has been suggested that oxygen carrier particles employed for chemical looping combustion may be used, with the oxygen transfer from the particles to the gas promoting tar decomposition. Experiments were conducted in a small-scale, batch-wise fluidized-bed reactor with the aim of investigating a variety of bed materials for this process. The purpose of the present work is thus to conduct a screening study of a variety of bed materials based on the transition metals Fe, Mn, Ni, and Cu. The experiments were conducted in a batch fluidized bed, where the particles are exposed to reformer and regenerator conditions alternatingly. The conversion of ethylene from a synthetic gasification gas mixture was used as an indicator for the suitability of the materials for tar conversion. It was found that the natural material bauxite and the synthetic bed materials NiO/α-Al2O3, CuO/MgAl2O4, and La0.8Sr0.2FeO3/γ-Al2O3 exhibit high ethylene conversion rates and, thus, possess promising properties for their application in CLR.