335 Figure 6 – Coal ignition system Table 2 – Ignition condition and point during the ignition operation No. LP gas duration Ignition point Injection condition 1st < 15 seconds 19 m Air: 100 L/min, O2: 30 L/min, LP gas: 2 L/min 2nd 20 hours 18.5 m Air: 140 L/min, O2: 50 L/min, LP gas: 2 L/min 3rd 30 minutes 17.85 m Air: 100 L/min, O2: 20 L/min, LP gas: 4 L/min 4th 8.5 hours 17.55 m Air: 140 L/min, O2: 50 L/min, LP gas: 3 L/min 4. RESULTS AND DISCUSSION 4.1 Product gas Figure 7 shows the variations in the major product gas components and calorific value from the start to the end of the experiment. Table 3 lists the average concentrations of the product gas and the calorific value obtained in this field experiment. The calorific value of the product gas was calculated by from multiplying its specific calorific value by the concentrations of each combustible components such as H2, CO, and CH2 (Su, F et al., 2015). In this experiment, the average concentrations of the main gas components were 21.8% for H2, 23.4% for CO, 0.4% for CH4, and 15.5% for CO2, with H2 and CO being the primary combustible components. These results indicate that the main gasification reactions were steam gasification (Equation (1)), the water-gas shift reaction (Equation (2)), and the boudouard reaction (Equation (3)), which primarily contributed to the generation of combustible gases. As shown in Figure 7, significant changes in the concentration of each gas were observed throughout the experiment. For example, during approximately the first 5 hours after ignition, oxygen remained in the product gas at a concentration of about 10%, indicating that the injected oxygen was not fully consumed. As the injection pipe was moved, the oxygen concentration in the product gas gradually decreased, and oxygen was not detected after approximately 12 hours. In contrast to the decrease in oxygen concentration, CO2 increased with the expansion of the combustion zone, followed by an increase in H2 and CO concentrations. The significant increase in H2 and CO observed after approximately 15 hours is considered to have resulted from the stepwise increase in oxygen supply and the transition into a new coal reaction face by moving the tip position of the injection pipe. Periods during which the calorific value of the product gas decreased sharply are attributed to operational adjustments associated with the hydrogen purification unit installed next to the experimental equipment. Specifically, closing the control valve in the product gas line to regulate the gas supply to the purification unit restricted the product gas flow rate, thereby influencing the reactions within the gasification zone. Nevertheless, these sharp changes tended to gradually return to previous levels once operating conditions stabilized. Although variations were observed throughout the experiment, combustible gas was generated continuously, indicating that a sufficiently large and stable gasification reaction zone had been established during the field experiment.
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