Pressurized Chemical-Looping Combustion of Chinese Bituminous Coal: Cyclic Performance and Characterization of Iron Ore-Based Oxygen Carrier

Abstract

A pressurized chemical-looping combustion combined cycle (PCLC) system is proposed for solid fuels combustion with potential high system efficiency, improving the fuel conversion and lowering the cost for CO2 sequestration. In this study, pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor focusing on cyclic performance. CVRD iron ore particles were exposed alternately for 20 cycles to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C at atmospheric pressure (0.1 MPa) and a typical elevated pressure of 0.5 MPa. With increasing number of redox cycles, more pyrolysis gases are oxidized by the oxygen carrier. At elevated pressure, the char gasification is intensified with negligible gasification intermediate products released. The CO2 fraction increases from 80% to approximate 90% after 10 cycles at atmospheric pressure. At elevated pressures, the average CO2 fraction stabilizes at 95.75%, approximate to the equilibrium value. The carbon conversion at 0.1 MPa and 0.5 MPa is 76.48 and 84.65%, respectively, and maintains approximately the same during the cycles because excessive steam gasification agent used in this study. The oxygen carrier conversion determined from the oxygen mass balance verifies that reduction level increases with the cycle number. The physical and chemical properties of oxygen carrier particles were characterized. X-ray diffraction (XRD) analysis verifies the extent of reduction level increases with cycles. No detectable formation of compound of iron oxide and coal ash was observed. Scanning electron microscope (SEM) analyses show that the iron ore particles become porous and that more pores formed with cycles. Agglomeration of particles was not observed in all experiments at both pressures. Energy-dispersive X-ray spectroscopy (EDX) analysis show an increasing amount of coal ash on the oxygen carrier particles with increasing numbers of cycles. Pore size analyses show that the oxygen carrier particles maintained mesopores for both atmospheric and elevated pressure. The increase of both surface area and pore volume illustrates that the particles become more porous with redox cycles. This study show that pressurized CLC of coal is promising and a low-cost iron ore-based oxygen carrier may be suitable for pressurized CLC of coal.