336 Figure 7 – Changes in the major product gas cocentrations and calorific value in the experiment Table 3 – Average concentrations of the product gas and the calorific value Calorific value H2 O2 N2 CO CH4 CO2 (MJ/Nm3) (%) (%) (%) (%) (%) (%) Field experiment (O2: 40~50%) 5.93 21.8 0.7 38.2 23.4 0.4 15.5 Table 4 summarizes the average values of product gas volume, recovered energy, and gasification efficiency obtained in this field-scale experiment and in previous artificial coal seam UCG model experiments (Hamanaka et al., 2021). The product gas volume is presented per unit mass of reacted coal and reacted carbon. Gasification efficiency was calculated as the ratio of the energy recovered in the product gas to the total calorific value of the reacted coal. As shown in Table 4, in this experiment, the product gas volume was 1.33 m3/kg per unit mass of reacted coal, which corresponds to 2.89 m3/kg when converted on a reacted carbon basis. This value is comparable to that reported in previous UCG model experiments (Hamanaka et al., 2021), suggesting that the essential conditions required for gasification, such as reaction temperature or injection conditions, were sufficiently achieved in this field experiment. Furthermore, while the gasification efficiency in the previous artificial coal seam experiments was approximately 60% (Hamanaka et al., 2021), this field experiment achieved a higher value of 71.56%. In general, during UCG process, not all of the heat generated by exothermic oxidation reactions is utilized for the main endothermic gasification reactions. A part of the generated heat is spread as heat loss to the surrounding coal and rock mass. Such heat loss is considered one of the factors resulting to reduced gasification efficiency. The proportion of heat loss is expected to decrease as the total generated thermal energy increases (such as with higher reacted coal mass or product gas volume). Consequently, gasification efficiency is inferred to improve with increasing reaction scale. In addition, while laboratory-scale experiments can cause significant heat loss to the atmosphere, in underground coal seams, heat loss to the surrounding rock mass may have been smaller compared to atmospheric heat loss, which could also contribute to the higher gasification efficiency observed in the field experiment.
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