Comparison of Char Structural Characteristics and Reactivity During Conventional Air and Oxy-Fuel Combustion

Abstract

The capture and sequestration of CO2 generated from large- scale stationary power plants is considered to be one of the leading technologies that could potentially have a significant impact on reducing greenhouse emissions. Among these emerging technologies, the oxy-fuel combustion is a near-zero emission technology that can be adapted to both new and existing pulverized coal-fired power stations. The goal of this work is to make a comparative study on char structural characteristics (including char yield, swelling ratio, BET surface area, pore distribution, morphology) and reactivity during conventional air and oxy-fuel combustion. Specific experimental designs include two series. One is carried out in pure N2 and CO2 (pyrolysis experiments), and another is prepared in N2 + 5%O2 and CO2 + 5%O2. Coal samples included raw coal, low density fraction coal and medium density fraction coal in all experiments. The present study is a further effort to extend our knowledge about physical and chemical structural characteristics and reactivity of char in the presence of high concentration CO2. Combustion and pyrolysis of a density fractionated China coal at drop tube furnace yielded the following conclusions. Compared to oxy-chars obtained under pure CO2 atmosphere, the swelling ratios of char obtained in pure N2 atmosphere are higher. When adding 5%O2, experimental results are completely different with those of the pyrolysis experiment. In comparison with the oxy-chars obtained under CO2 + 5%O2 atmosphere, the swelling ratios of the char obtained in N2 + 5%O2 atmosphere are lower. In the pyrolysis experiment, the BET surfaces Area of the oxy-chars are about 10–20 times as much as chars. When adding 5%O2, the BET surfaces Area of the oxy-chars are about two to four times as much as chars. During pyrolysis experiment, the total pore volumes of the oxy-chars obtained under pure CO2 are larger in comparison with chars obtained in pure N2. The pore distributions of the oxy-chars are mainly mesopore and the chars have macropore mostly. The micropore and mesopore volume of oxy-chars are about ten times as much as chars separately and the macropore volume of oxy-chars are about two times as much as chars. When adding 5%O2, the total pore volumes of the oxy-chars obtained under CO2 + 5%O2 are also larger in comparison with chars obtained in N2 + 5%O2. In the pyrolysis experiment, the reactivity indexes of oxy-chars obtained under pure CO2 are lower in comparison with chars obtained in pure N2. When adding 5%O2, no significant differences can be observed, which may be due to the ordering of carbon microcrystalline structure. In pure CO2 atmosphere, the oxy-chars have a highly order polycrystalline structure, but chars obtained at pure N2 atmosphere have a highly disordered carbonaceous structure.