1Underground coal gasification (UCG) represents a promising and cleaner approach to harnessing unmined coal reserves, offering a viable alternative to traditional coal extraction methods. While conventional mining recovers only 15-20% of total coal reserves, UCG has the potential to significantly extend the life of global coal resources by accessing otherwise unrecoverable deposits. Traditional mining is resource-intensive, requiring substantial time, labour, and machinery, and poses environmental and safety challenges such as landscape disruption, high operational costs, and risks to personnel. UCG circumvents these issues by gasifying coal in situ through a network of wells, where the coal is ignited and sustained by injected oxidants, producing syngas—a mixture primarily of carbon monoxide and hydrogen. This syngas can be used for industrial heating and power generation or converted into hydrogen, synthetic natural gas, and liquid fuels. Compared to surface mining and gasification, UCG offers reduced capital and operational expenses, fewer greenhouse gas emissions, and can be integrated with carbon capture and storage (CCS) technologies. With increasing global energy demand, oil and gas reserves depletion, and growing concerns over climate change, interest in UCG has risen worldwide. The process holds immense potential for exploiting low-grade, inaccessible coal seams and converting them efficiently into syngas, with wide-ranging energy, fuel, and chemical production applications. This report comprehensively analyses UCG technology, tracing its evolution and key projects globally. It examines site selection criteria, highlighting the importance of coal geology, coal properties, and geological and hydrological conditions. The report also reviews various UCG methods and the chemical and physical processes involved and explores process optimization and the influence of different operational parameters. Furthermore, the environmental and economic implications of UCG are assessed, alongside its advantages and disadvantages. This paper is an in-depth review of the fundamental concepts and technologies driving UCG, offering a detailed insight into this evolving coal conversion method.

It is thought that the world coal reserve is close to 150 years, which only includes recoverable reserves using conventional techniques. Mining is the typical method of extracting coal, but it has been estimated that only 15% to 20% of the total coal resources can be recovered in this manner. If unrecoverable coal is considered in the reserves, the lifetime of this resource would be greatly extended, by perhaps a couple hundred years. Mining involves a large amount of time, resources, and personnel and contains many challenges such as drastic changes in landscapes, high machinery costs, elevated risk to personnel, and post-extraction transport. A new type of coal extraction method, known as underground coal gasification (UCG), that addresses most of the problems of coal mining is being investigated and implemented globally. UCG is a gasification process applied toin situ coal seams. UCG is very similar to aboveground gasification where syngas is produced through the same chemical reactions that occur in surface gasifiers. UCG has a large potential for providing a clean energy source through carbon capture and storage techniques and offers a unique option for CO2 storage. This paper reviews key concepts and technologies of underground coal gasification, providing insights into this developing coal conversion method.

The utilization of vast coal deposits around the globe as an energy source is significantly limited in scope owing to environmental risks. In this context, ionic liquids (ILs) provide a novel solution as eco-friendly pretreatment agents. This work examined the effects of IL pretreatment on the gasification of low-grade coal using a simulation model in Aspen Plus®. Four ILs, 1-butyl-3-methylimidazolium chloride [Bmim][Cl], 1-ethyl-3-methylimidazolium chloride [Emim][Cl], trihexyltetradecylphosphonium chloride [P66614] [Cl], and tributylmethylammonium chloride [N1444] [Cl], were employed in this study. A comparative and sensitivity analysis was conducted on untreated and IL-pretreated coal samples using an equilibrium-based steam-O2 gasification model designed in Aspen Plus®. The model was validated and utilized to vary the oxygen/fuel (O2/F) ratio, steam/fuel (S/F) ratio, and temperature within the specified ranges (700-950°C, 0.1-0.6, and 2-4, respectively). The results showed that all the ILs increased CO and H2, and decreased CO2 and CH4, in the syngas produced by coal gasification. The biggest change was affected by [P66614] [Cl], which increased CO and H2 from 12 mol% and 42 mol % to 16 mol% and 50 mol %, respectively, and reduced CO2 and CH4 from 35 mol% and 3.5 mol% to 29 mol% and 2 mol%, respectively. The lower heating value (LHV) of syngas was increased from 7.37 MJ/Nm3 for untreated coal to 7.61, 7.56, 8.1, and 7.56 MJ/Nm3 for coal treated by [Bmim][Cl], [Emim][Cl], [P66614] [Cl], and [N1444] [Cl], respectively. As opposed to raw coal, [Bmim][Cl] and [P66614] [Cl] generated more CGE, ENE, and EXE. The highest CGE, ENE, and EXE of 19, 13.3, and 38.3% were achieved by [P66614] [Cl] treated coal. Syngas having higher H2/CO ratio and LHV were produced at low temperatures, low O2/F ratio, and high S/F ratio. The results of this investigation demonstrated that ILs composed of ammonium and phosphonium might be useful coal pretreatment agents in thermochemical conversion processes.