The influence of carbon dioxide on the gasification rate of Highveld coal chars at elevated pressures

Abstract

Gasification is an important process which is applied extensively for coal utilization. In the South African context, coal gasification is the first step in the coal to liquid (CTL) process and is performed in a fixed-bed dry bottom (FBDB) gasifier at high pressure ( 30 bar). Although the gasification reactivity of Highveld coal chars has received considerable attention in the past, only limited information is available on the high pressure reactivity. It is not known how the coal/char reactivity at elevated pressures is related to the standard reactivity test which is performed at atmospheric pressure. In this study, the carbon dioxide (CO2) gasification reactivity of different coal chars was investigated over a wide pressure range (1-30 bar). The samples originated from different seams of the Highveld coalfield. Gasification experiments with char particles in the size range of 425-500 μm were performed in a laboratory-scale high pressure fixed bed reactor within a temperature range of 780-855°C. The CO2 concentration in the reagent gas was varied between 5-30% with the balance N2. These conditions were selected to conduct the experiments in the chemical reaction controlled regime. It was observed that the reaction rate was solely a function of temperature and CO2 partial pressure, and the effect of total system pressure was insignificant. The reaction rate increased with increasing CO2 partial pressure throughout the range of partial pressures investigated (0.05-9.0 bar). The reactivity was well described by both the Langmuir Hinshelwood and the empirical power law kinetic models. Overall activation energies for the Seam 2, 4 and 5 coal chars were 261 ± 20 kJ/mol, 260 ± 26 kJ/mol and 250 ± 30 kJ/mol respectively. Char structural changes are known to occur during gasification, however up to date the effect of CO2 partial pressure on the extent of pore development has not been quantified or explained. To investigate the effect of CO2 partial pressure on pore growth, CO2 and N2 adsorption measurements were performed on chars and partially converted chars. The analyses showed that at fixed conversions an increase in CO2 partial pressure generally resulted in an increase in both the char micro- and mesopore surface areas. These observations suggest that the degree of pore development is a function of CO2 partial pressure. This is significant and has fundamental implications for the reaction of CO2 with coal chars. It is required to be incorporated into rate equations for improved kinetic modelling at high pressures. The structural Random Pore Model (RPM) accurately described the experimental data up to 20% conversion. The structural parameter (ψ) increased with increasing CO2 partial pressure suggesting an increased char pore growth at higher CO2 partial pressures which corresponds to the surface analysis results.