Evaluation of coal char gasification kinetics and pore development in high pressure steam and carbon dioxide
Coal is utilised in the coal-to-liquid (CTL) process on a large scale in South Africa and significantly contributes to the country’s energy demand. In this conversion process, the primary step is the high pressure gasification of coal, and only limited studies have related to high pressure gasification kinetics with CO₂, and specifically steam for South African coals. In this study, inertinite-rich coal from the Highveld coalfield was used to evaluate the coal char gasification kinetics and the associated surface area development at high steam and CO₂ partial pressures up to 20 and 30 bar, respectively. A coal char sample of -150+75 µm was prepared through mechanical size reduction of lump coal and charred at 950 °C in a N₂ atmosphere. The coal char gasification experiments were subsequently conducted at isothermal conditions at a temperature of 780 °C for CO₂ and 740 °C for steam in the chemical-controlled regime. The reaction rate was determined by the analysis of the carbon-based products, and the micropore surface area of the raw and reacted chars (conversion of ~10, 20, and 30%) was analysed by CO₂ adsorption with use of the Dubinin-Astakhov (D-A) method. From the reaction rate and the subsequent micropore surface area data, the intrinsic rate (g/m².s) was determined for the quantification of the kinetic parameters described by the Langmuir-Hinshelwood (LH) rate type model. The Random-Pore Model (RPM) was used to model the development of micropore surface area with the extent of carbon conversion. From the obtained results, it was found that the specific reaction rate and micropore surface area were significantly affected by the extent of carbon conversion and more pronounced for steam. With an increase in reactant partial pressure, the intrinsic reaction rate increased with a reaction order of 0.57 (±0.09) and 0.32 (±0.05) for steam and CO₂ respectively at a reactant partial pressure of up to 10 and 20 bar, decreasing to a value of close to zero with a further increase. The LH type model was suitable for describing the effect of partial pressure on the reaction rate. The development of micropore surface area was found to be not affected by the reactant partial pressure for steam gasification in contrast to CO₂ gasification. A mixed model (combination of LH type model and RPM) was used to model the specific rate and it was found that the model can fairly predict the reaction rate and a directly fitted RPM rate type was suitable to describe better the specific rate. The intrinsic reaction rate was found to be only a function of partial pressure for steam gasification, which was described well by a single LH type model with the intrinsic [Ct]k₁ and k₁/k₃ values of 7.4x10⁻⁹ (g/m².s.bar) and 0.13 (1/bar), respectively over the studied conversion range. For CO₂ gasification, intrinsic [Ct]k1 and k1/k3 values were found to be in the range of 4.6-5.7x10⁻⁹ (g/m².s.bar) and 0.11-0.12 (1/bar), respectively.