Evolution of oxygen-containing functional groups on coal char surface during gasification in H2O/CO2

Abstract

Gasification converts coal into syngas (CO and H2) for downstream utilization. The most important reaction in an autothermal gasifier is the char−H2O/CO2 reaction. The evolution of C(O) complexes on the coal char surface directly determines the pathways and rates of CO and H2 generation. This study mainly used a fixed bed reactor to prepare Zhundong coal char with different reaction levels in H2O and CO2, and its surface carboxyl, lactone, phenolic hydroxyl, and hydroxyl functional groups were systematically analyzed by using Boehm titration, FT-IR, and XPS. The results show that the contents of the four oxygen-containing groups in raw coal are in the order of carboxyl＞lactone＞phenol hydroxyl＞hydroxyl. In H2O gasification, the lactone groups were more susceptible to H2O resulting in a significant increase in carboxyl content at the early stage of the reaction. The hydroxyl, as a very active group in the reaction, and the amount of its generation and consumption was very obvious. The phenolic hydroxyl−the least thermally stable group among these four types of groups, was unable to participate in the inter-transformation between oxygen-containing functional groups during the gasification. During CO2 gasification, the hydroxyl was rapidly consumed at the initial stage of the reaction and then shown a slow growth trend, and the newly generated hydroxyl would continue to undergo dehydrogenation to form new functional groups. At the end of the reaction, the transformation of other oxygen-containing functional groups into more stable carbonyl groups leads to a sudden increase in the content of carboxyl and lactone. In this work, Boehm titration was introduced into the determination of oxygen-containing functional groups on the surface of char during gasification in H2O/CO2.