(125g) Development of Oxygen Carrier Suitable for Chemical-Looping Combustion of Di-Methyl Ether

Chemical-looping combustion (CLC) has been taken as one of the innovative technologies for CO₂ capture in the next 10-20 years by the International Energy Agency (IEA). It decomposes the conventional combustion into two reactions: reduction of metal oxide by fuel in the fuel reactor, and oxidation of the resulting metal by air in the air reactor. Since direct contact between the air and the fuel is avoided, CO₂ can be separated through condensing method with no extra energy penalty or special separator device.

Most of present studies on CLC processes focus on the use of methane, hydrogen, syngas (H₂, CO) or coal as fuel. With ever growth concerns on energy security and environmental pollution, dimethyl ether (DME) is considered as a promising alternative fuel in the future. It is expected as a candidate for use in a diesel engine or gas turbine to produce power generation. However, the conventional combustion of DME still results in high exergy destruction and large energy penalty for CO₂ separation. The energy penalty for CO₂ separation with conventional MEA absorption method is approximately 4 MJ/kg-CO₂, which decreases the thermal efficiency of a DME-fired combined cycle by 7–8 percentage points.

In our previous study, we have proposed a power generation system with CLC of DME. Due to less exergy destruction and no energy penalty for CO₂ separation, thermal efficiency of the power generation system would be expected to be 8.5 percentage points higher than that of the DME-fired combined cycle with CO₂ capture in the similar condition. It is also found that the DME-fueled CLC process requires a lower reduction temperature below 673 K. Low grade heat, such as mid-temperature waste heat or solar heat, can be added to the fuel reactor, and upgraded to that of the chemical energy associated with the solid fuel of the reduced oxygen carrier. Then, it is recovered in the air reactor at higher temperature as high grade energy. The whole CLC process upgrade the low grade heat to a higher level, which means that more available work can be obtained and higher system efficiency can be achieved.

Due to the lower reduction temperature, the temperature difference between the fuel- and the air-reactor is larger. The oxygen carriers cyclically used between the two reactors suffers stronger thermal stress. Thus, it is important to develop oxygen carriers with high reactivity at low reduction temperature and high redox stability for the DME-fueled CLC process.

In this paper, a new oxygen carrier (CoO+1% PtO₂)/CoAl₂O₄ suitable for the DME-fueled chemical-looping combustion process was synthesized, and the reaction kinetics, the carbon deposition behavior and the regenerability were identified by means of the thermo-gravimetrical analyzer.

Three kinds of oxygen carriers, with Fe₂O₃, NiO and CoO as solid reactants and Al₂O₃ as binder, respectively, have been prepared by dissolution method. The reductions at 673 K showed that, compared with Fe₂O₃/Al₂O₃ and NiO/NiAl₂O₃, CoO/CoAl₂O₄ has higher reactivity and may be an appropriate looping material for the CLC of DME. To improve the reactivity, different additives PtO₂ and Rh₂O₃ with weight fraction of 1.0% were added into CoO/CoAl₂O₄. The reaction rate of (CoO+1% PtO₂)/CoAl₂O₄ with DME was higher than that of (CoO+1% Rh₂O₃)/CoAl₂O₄, and compared with that of CoO/CoAl₂O₄, it was dramatically improved by about 4 times due to the reduction of the activation energy. Oxygen carriers with different PtO₂ weight fraction of 0.2%, 0.5%, 1.0% and 5.0% were prepared to study the effect of the additive content on the reaction behaviors, and the results showed that the optimum PtO₂ content was 1.0%. Due to the lower reduction temperature, carbon deposition was easily to take place for the CLC process of DME. To avoid carbon deposition, DME was saturated with a mole ratio of H₂O/DME as 0.5/1, 0.7/1 and 1/1, respectively. It was found that the carbon deposition rate decreased with the increase of the mole ratio of H₂O/DME, and the reaction rate of the oxygen carriers with DME slightly decreased with the increase of H₂O/DME. To satisfy the operation of the DME-fueled CLC for both good reactivity and avoidance of carbon deposition, the mole ratio of H₂O/DEM should be around 1.0. Finally, the redox stability of the oxygen carrier (CoO+1% PtO₂)/CoAl₂O₄ was tested by cyclically utilization in 30 cycles, and it showed relatively good redox stability and physical strength during the cyclical utilization. The results of this paper would be expected to open the industrial application of DME-fueled chemical looping combustion in power generation system.