Performance of potassium-modified Fe2O3/Al2O3 oxygen Carrier in coal-direct chemical looping hydrogen generation

Abstract

Coal-direct chemical looping hydrogen generation (CLHG) is an innovative technology developed to produce hydrogen with in situ CO2 capture. Most oxygen carrier (OC) candidates for other chemical looping processes are not suitable for the technology. In this work, potassium modified Fe2O3/Al2O3 OCs were prepared by co-precipitation method, and screening tests were conducted to selected appropriate OC and char particle size. Experiments on the technology were carried out in a fixed bed reactor to evaluate the redox and cyclic performance of the composite OCs using Shenmu char as fuel. The OC residues in the process were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and N2 adsorption analyzer. Moreover, the variations of potassium content in OC residues were also discussed. The results show that the appropriate particle ranges were 425 μm–600 μm for OC and less than 75 μm for char. During the first cycle, the carbon conversion could reach about more than 95% within 45 min and the total H2 concentration under an N2-free basis in steam oxidation period was about 99.3%. In the multi-cycle tests, the carbon conversion and hydrogen yield remained almost unchanged during the first three cycles and decreased thereafter. The selectivity toward CO2 was less than 80% during the first three cycles, increased in the next five cycles and reached about 99.3% in the 8th cycle. The OC exhibited a good anti-sintering ability and stable pore structure in the eight cycles. The decrease of potassium content in OCs was the reason for the decline of carbon conversion and hydrogen yield, and the supplement of K2CO3 could recover the reactivity of OCs.