Status of underground coal gasification as a clean energy: a critical review

Underground Coal Gasification (UCG) could remain a potential clean energy act in this era of global energy transition. The syngas produced from coal can be converted into valuable chemicals and fuels, including hydrogen, ammonia, and methanol. The oxidation of coal can be achieved through injecting oxygen, air, steam, or carbon dioxide (CO2), which produces a fuel gas. UCG process has a dual advantage in the sense that it not only emits reduced CO2, but it also becomes easier to capture the released CO2 from a UCG unit. Furthermore, the UCG technique could become energy-efficient by utilizing renewable energy sources. However, this process poses environmental risks, including contaminating groundwater, subsidence, and complexities in monitoring. This paper evaluates the current status, factors affecting the UCG process by examining the role of different oxidizing agents in influencing syngas composition and future prospects of UCG as a clean, low-energy-intensity solution. This study also highlights the challenges encountered and recent advancements in monitoring and mitigating the contaminants. From a broader perspective, UCG, when coupled with Carbon Capture and Storage (CCS) and Renewable usage, can become a reliable and potentially energy-non-intensive, sustainable energy solution supporting the shift to low-carbon fuels.