Exploring the mechanism of alkali metal K-catalyzed biomass char gasification using in-situ DRIFTS and molecular simulation

Abstract

Although alkali metal potassium (K)-catalyzed char gasification has been widely studied, the detailed mechanism of this process is still not fully understood. To study this catalytic mechanism in more detail, high-temperature in-situ DRIFTS experiments were carried out for the gasification of cellulose char with K and cellulose char without K under H2O and O2 atmospheres, transient gasification experiment of char with K under H2O atmosphere, and char with K gasification under isotope water atmospheres. Peaks at 1945 cm−1 and 2020 cm−1 were observed. Combined with the theoretical spectra obtained by quantum chemical simulations and the prediction of catalytic gasification intermediates by molecular dynamics simulations, these two peaks were confirmed to represent char-polyynes species. Based on this result, a new mechanism is proposed: K interacts with carboxyl groups and aromatic ketones to promote desorption, thereby exposing active sites. Then, K reacts with the edge of the activated char to form numerous highly active char-polyynes species. The polyynes edge of this char-polyynes phase provides ideal adsorption–desorption sites, enabling further reaction with the gasifier. Thus, gas desorption is promoted. The adsorption and desorption peaks of catalytic gasification under the H2O atmosphere were located at 1400 cm−1 and 1300 cm−1, and these peaks represented stable adsorption states and unstable desorption precursors, respectively. Based on this, adsorption–desorption theory was analyzed more comprehensively, and the adsorption–desorption process was divided into three stages: initial adsorption, intermediate conversion, and desorption. The comprehensive new mechanism proposed in this paper provides a better understanding of the nature of char catalytic gasification at a deeper level.