(2c) Development and Performance of Cobalt Oxide as Oxygen Carrier In Dimethyl Ether (DME) Fueled Chemical-Looping Combustion

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. Although it is a relatively clean fuel, direct combustion of DME still causes CO₂ diluted by N₂, which complicates the separation of CO₂. Recovering CO₂ from a DME fueled power cycle will cause the thermal efficiency decreased by about 7.7 percentage points. Thus, we have to face a serious issue how to more efficiently and compatibly utilize DME fuel in the power generation systems with CO₂ capture of super-lower energy penalty. 

In our previous study, a thermal power cycle combined with DME fueled chemical-looping combustion (CLC) was proposed, and no extra energy penalty is required for CO₂ separation of this system. Besides, compared with natural gas, DME has high oxygen content. Its C-O bond energy is smaller than that of the C-H and the distortion of C-O bonds in the DME molecule weakens the boning strength. As a result, the C-O bond breaks easier than the C-H bond and the reaction between DME and looping material may be feasible at relatively low temperatures. Thus the lower grade heat such as waste heat or solar thermal energy can be supplied to the endothermic reaction in the fuel reactor, which would further increase the amount of heat produced in the high-temperature air reactor, improving the thermal efficiency of the CLC power generation system. 

To identify suitable looping material candidates for DME fueled chemical-looping combustion, we have conducted experiments by using a thermogravimetrical analyzer. Seven oxygen carriers, with Fe₂O₃, NiO and CoO as solid reactants and Al₂O₃, MgAl₂O₄ and YSZ as binders, respectively, have been prepared by dissolution method. The reductions at 400^oC showed that, compared with Fe and Ni based oxygen carrier, the Co based one have higher reactivity and may be an appropriate looping material for chemical-looping combustion fueled by C-O bonded fuels like DME. Kinetic behaviors of CoO/CoAl₂O₄ and CoO/YSZ were studied at the reduction temperature of 400^oC, 450^oC and 500^oC, and the results suggested that suitable reduction temperature is around 450^oC. Scanning electron microscopy (SEM) was used to characterize the morphological features of the fresh, reduced and oxidized oxygen carriers, and it is observed that porous structure were formed after a reaction cycle, which facilitated the diffusion of gaseous reactant into the core of oxygen carrier. The energy-dispersive X-ray (EDX) identified that serious carbon deposition occurred during the reduction process, and its rate became lower with the growth of reduction temperature. To restrain carbon deposition, DME was saturated with a mole ratio of H₂O/DME as 1/1, 2/1, 3/1, and 4/1, respectively. It is found that when the ratio of H₂O/DME is above 3/1, carbon deposition could be completely avoided. The findings of this paper may broad the CLC field in the utilization of alternative fuel for CO₂ capture.