High pressure CO2 reactivity of Highveld coal from various mines

Abstract

Coal gasification is a process that converts coal into a clean-burning synthesis gas (syngas), which can be used for various applications, including electricity generation, chemical production, and as a feedstock for the production of liquid fuels like synthetic natural gas or transportation fuels. Coal gasification, which is the initial stage of the coal to liquid (CTL) process, is carried out at high pressure in a fixed-bed dry bottom (FBDB) gasifier. While there has been a lot of research in the past about the gasification reactivity of Highveld coal chars, little is known about the correlation between coal properties and intrinsic coal reactivities. However, the relationship between coal characteristics and intrinsic coal reactivities remains a relatively unexplored area of study. The aim of this study is to correlate the intrinsic coal reactivity of seven different single source coal samples to their respective coal and coal char characteristics. The coal samples used in this study were sourced from various seams within the Highveld coalfield. The coal chars were produced by undergoing mechanical size reduction to a particle size range of -150 to +75 μm. Subsequently, the chars were subjected to charring conditions of 900 °C and 30 bar pressure in an environment consisting of nitrogen gas (N2). The specific reaction rates were significantly higher with the use of pure CO2 compared to other studies where lower concentrations of CO2 was used. Because of the higher specific reaction rates, the gasification experiments were conducted at a temperature range of 730 to 750 °C. From the correlations of intrinsic coal reactivity of the seven different single source coal samples and their respective coal characteristics it was observed that the best model for prediction is reactivity as a function of the temperature and ultimate analysis of the coal, which has the highest R2 value and smallest standard error of the model. The chemical and mineralogical properties were considered in the correlations, which included proximate and ultimate analysis, ash composition, gross calorific values, alkali index, Si/Al ratio, CaO content, internal surface area of the coal chars, petrographic and char morphology. A correlation between intrinsic coal properties and coal reactivity revealed that the regression model of reactivity as a function of temperature and proximate analysis can be used to accurately predict gasification reactivities of Highveld coal samples. It is however pertinent that the predictions of other coal samples are compared to the measured values to determine the repeatability of the correlation results when investigating coal samples from different geographical locations.