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It is widely known that anthropogenic carbon dioxide in the atmosphere has been linked to global warming, and in recent years, it has been proposed that fossil-fuel power plants should “capture” the carbon produced and pump it underground so that it is not added to the atmosphere to contribute to global warming. However, many present post-combustion capture technologies are capital intensive, have high energy penalties, and are difficult, if not feasible or economic to retrofit to many present-day power plants. Chemical looping combustion has been considered a promising technological alternative to direct fuel combustion because of its inherent ability produce electricity from carbonaceous fuels without a significant carbon dioxide separation penalty.In recent years, there has been an interest in solid fuel systems for the chemical looping processes because of the ability to convert fuel directly without a need for external gasification. As a result, The Ohio State University has developed an iron-based Coal Direct Chemical Looping (CDCL) process. The iron-based CDCL process directly converts solid fuels into electricity with in-situ carbon capture by reducing and oxidizing iron oxide based oxygen carrier particles in separate reactors without using a traditional gasifier and an energy-costly air separation unit. Like most chemical looping combustion processes, the system consists of a reducer reactor, which converts the fuel and reduces the oxygen carrier, and a combustor reactor, which combusts the oxygen carrier. Many chemical looping processes use a fluidized bed for the reducer reactor; however, the iron-based CDCL process developed at Ohio State uses a moving bed configuration; in this work, it is tested in a bench-scale apparatus. In the moving bed reducer, the oxygen carrier particles, solid fuels and enhancing gas are introduced from the top, middle and bottom sections, respectively. In order to effectively convert the solid fuels, the reducer is designed with two main stages. In the top stage the iron oxygen carrier converts the volatiles that are driven off the coal, while in the bottom stage the iron oxygen carrier converts the devolatilized coal char. The volatile conversion for methane, carbon monoxide and hydrogen has been successfully tested in previous works. The focus of this work is the char conversion in the countercurrent moving bed. Different operating conditions were tested for char conversion by changing the following parameters: reactor temperature, oxygen carrier flow rate, oxygen-carrier-to-char flow ratio, and enhancing gas flow rates (steam or carbon dioxide). Char conversion was calculated by either comparing the inlet and outlet gas flowrate difference caused by the evolved gases from the char conversion or measuring the gas concentration at the gas outlet using a gas chromatograph. The gas profile at various sections in the reactor can be also retrieved by analyzing the gas composition from different sampling ports along the reactor. Oxygen carrier conversion was calculated by measuring the weight change of the carrier with a carbon analyzer. It is noted that when optimized, the countercurrent moving bed reactor can produce near-complete conversion of solid fuel. The oxygen carrier conversion was measured and determined to be mainly FeO, which is a significantly better conversion compared to fluidized bed reducers. Furthermore, carbon analysis after runs showed that carbon deposition is less than 0.2%, allowing for minimal contamination in the combustor. The data obtained from the unit are analyzed and will be used for process scaleup. |
