Laboratory Studies on Cavity Formation and Product Compositions During Gasification of Subbituminous Coal in Underground Coal Gasification (UCG) reactor

摘要

Underground coal gasification (UCG) is a process of 'in-situ' conversion of coal into gaseous products with usable heating value through combustion with oxygen/air and gasification with steam. From the economic and environmental point of view, UCG is a highly promising clean energy technology which has a potential to meet the increasing energy demands in several countries. UCG process involves principally four steps: drilling, linking, ignition, and gasification. Injection and production wells apart are drilled from the surface into the coal seam. In the following step, a link is created between two wells, which allow flow of gases. It is necessary to ignite the coal initially to raise the coal temperature using combustible gas and oxygen mixture with an ignition source. After this, mixture of air/oxygen and steam are injected into seam to gasify the coal and the syngas is recovered from production well. As the coal is being combusted/gasified a void space is created, which is referred as cavity. The formation of cavity and the composition of product gas depend on different phenomena like gas-solid reactions of char after devolatilization and diffusion of gas in the char layer, spalling of char/coal etc. However, these phenomena are also functions of operating parameters such as, injected oxidant flow rate, steam-to-oxygen ratio, operating pressure, combustion time, total operation time, coal particles size and distance between wells etc. Since, in-situ UCG is a complex process, laboratory-scale experiments on coal blocks can provide significant insight into UCG process. In this investigation, an approximately 15 cm diameter by 60 cm cylindrical block of sub-bituminous coal enveloped with refractory concrete is imbedded in an ex-situ reactor. The experimental system is equipped with a gas feeding system, product gas collection system, temperature and pressure monitor system and a gas chromatograph. So far, the first three steps (i.e. drilling, linking and ignition) have been carried out successfully. One of the objectives of ex-situ UCG experiments is to comprehend the cavity formation due to the variation of different operating parameters using coal block made up of different size of coal particles such as, fine coal powder, one centimetre coal particles and bigger coal chunks. However, the main objective of this research is to develop a model to describe cavity growth during UCG process in coal seams and to investigate the effect of various physical properties and operating parameters on performance of the process. Validation of the model will be performed by utilizing the data obtained from the ex-situ UCG operations.