title: The development of coal particle gasification model with application leading to underground coal gasification
Creators: name: Sutardi, Tata
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: underground coal gasification, coal particle model, kinetic parameter study, thermochemical behaviour.

description: The development of Underground Coal Gasification (UCG) modelling has been carried out for many years and this study is an attempt to improve the mechanisms of thermochemical processes in an existing model. The understanding of the thermochemical behaviour of coal gasification reactions is important because it influences gas production simultaneously with a coal mass reduction. A coal particle model was developed to investigate the thermochemical processes of gasification for underground coal applications. The chemical reactions were defined with an Eddy Break Up (EBU) model for controlling the reaction mechanisms and the study was particularly focused on identification and roles of the important kinetic parameters. At initial validation, coal particle oxidation based on the combustion experimental results with drop tube furnace, is used for comparison. With regards to the results, the best agreement of coal oxidation is achieved with the pre-exponent factor (A) of 0.002 and 85500, for the reactions, R2 (C + O2 = CO2) and R3 (C + 0.5O2 = CO), respectively. The gasification reactions are subsequently applied for the thermochemical process investigation and the kinetic parameters for this application are also identified. A kinetic parameter study was also conducted to identify the difference between bituminous and lignite coals through the comparison parameter of ignition delay time. With seven reaction mechanisms applied to represent coal combustion, this study identified that the ignition delay time difference was significantly affected by the devolatilization reaction. This reaction is important for predicting the ignition delay time of coal particle combustion. For the simulation case, two types of coal, named PSOC 1451 and PSOC 1443, were examined numerically and the results are compared with the experimental data. Existing kinetic parameters for the devolatilization reaction R1 (Coal  Coalvolatile + char) underestimate the ignition delay time which is largely influenced by the value of the pre-exponent factor (A) of R1. Results giving the best agreement with the experiment are obtained with A= 3.12 x 105 and 9.36 x 107 for PSOC 1451 and PSOC 1443, respectively. The UCG application could be friendlier to the environment, since the cavity formed potential to be used as CO2 storage and the process itself has a promising role on utilising CO2. For initial investigation, several gasification simulations were conducted by involving the CO2 at the drop tube furnace as a reactor, and syngas production was investigated. The results showed that the syngas production at the reactor environment’s condition with higher CO2 has better products of H2, CO and CH4. When investigating the syngas quality, several gasification simulations with the addition of steam (H2O) into the reactor were carried out and the results showed that the more concentration of H2 was obtained at the higher steam condition. However, the study with combining the CO2 and H2O in the reactor’s environment was also carried out with the results showing a promising indicator in producing the better syngas quality. This investigation through the simulation performance also identified the gas formation behaviour in the gasification reactions. The production of H2 and CO is controlled significantly by the level of oxygen concentration via the char reactions. However, their production rates are strongly dependent upon the reaction zones of gasification. For example, CO is produced in both oxidation and reduction reaction zones, while H2 production dominates the reduction zone. Spatio-temporal distributions of the gas species along with the coal particle temperature provide additional information for further development of UCG modelling. With these results, the model indicates a capability to provide good guidelines with the associated thermochemical processes that can help to develop robust coal gasification technology and lead to improved syngas quality. The effects of particle size have been identified through the model simulation and experiments. In the results of the simulation, the particle size has a greater effect on the heterogeneous reactions. In the case of CO formation, the smaller particle size has greater products in the unit of mole fraction over the area of generation. However, in the experimental results the effect of particle size variation causes the varieties of coal in the packed bed porosity. The smaller particle size causes less porosity, and therefore a lower rate of gas productions. This is because the porosity contributes to providing access to oxygen to react with coal. The effects of temperature variation has also been investigated through the model simulation and experimental procedures. The results through the simulation suggest that the temperature encourages better reactions and therefore more gas products are obtained at the higher temperature either in the results of model simulation or experimental procedures. Finally, an experiment was also conducted to identify the effect of gas flowrate variations. The air flowrate needs to be injected in order to keep coal reactions occurring simultaneously, because the coal stock moves downstream during the gasification. The results show that a higher flowrate resulted in a greater area of coal surface reactions and also a higher concentration of gas products. It indicates that the greater flowrate need to be presented as more pressure is needed to maintain the reactions occurring at the coal stock which lies further upstream.