Effect of calcination temperature of red mud POC and hydrothermal pretreatment of wet sludge on syngas quality

Abstract

CaMn_0.5Fe_0.5O_3-δ-type perovskite oxygen carriers (POCs) were prepared using red mud as the main raw material and configuring certain ratios of calcium and manganese elements followed by calcination under different temperatures. A series of experiments were conducted in a fixed bed reactor to prepare hydrogen-rich syngas from municipal wet sludge chemical looping gasification (CLG). The results showed that the calcination temperature has an important influence on the oxygen storage capacity of the modified red mud perovskite oxygen carrier. With the increase of calcination temperature, the oxygen decoupling capacity of the oxygen carrier continues to improve, but the catalytic performance continues to weaken. Compared with the POCs calcined at 1050 ℃ and 950 ℃, the quality of hydrogen-rich gas prepared by the POC calcined at 850 ℃ was better, with a hydrogen gas volume fraction of 44.7 %, exceeding the theoretical value of converting hydrogen elements contained in dry sludge into hydrogen gas. The lower calcination temperature makes POC have larger specific surface area and better catalytic performance, which is conducive to the catalytic reforming of gas components such as CH₄ and CO. The hydrothermal pretreatment of sludge is beneficial to improve the specific surface area of sludge and thus to improve the quality of syngas. The syngas hydrogen gas volume fraction reached 59.9 % using the POC calcined at 850 ℃ with sludge hydrothermal pretreatment at 250 ℃, and the syngas calorific value reached 13.47 MJ/Nm³ . By balancing the carbon and hydrogen elements transferring after the wet sludge gasification, part of sludge moisture participated in the carbon gasification and methane reforming reactions, thereby increasing the hydrogen content in syngas. The study is of guiding significance for the selection of oxygen carriers and fuel pretreatment in CLG of wet sludge.