Reasons for high outlet temperature in the gas generator and its control

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I. Reasons for high outlet temperature in the gas generator and solutions 1. The upward movement of the flame layer causes the high outlet temperature. The adjustment of the fire layer position is mainly achieved by controlling the amounts of steam supplied from above and below as well as the thickness of the ash layer. If too much steam is supplied from above, it will cause the fire layer to rise, leading to an increase in the temperature at the outlet of the gas generator; therefore, it is necessary to maintain an appropriate balance between the amounts of steam supplied from above and below. Based on our experience, it is appropriate to use a blowing volume of around 30,000 NM3/h, a blowing time of 45 seconds, with the upper blowing time set at 29–30 seconds and the steam volume for upper blowing controlled at 5,300–5,500 kilograms per hour. The lower blowing time should be 68–69 seconds, and the steam volume for lower blowing should be maintained at 8,000–8,100 kilograms per hour ； At the same time, controlling the steam pressure applied from above and below is also necessary for gas generation furnaces. If the thickness of the ash layer increases, in order to maintain the original thickness of the fuel layer, it will inevitably lead to an increase in the carbon layer thickness, resulting in an upward shift of the entire fuel layer and higher outlet temperatures ； Therefore, the carbon layer must be kept stable at 2.6-2.8 meters (clearance). 2. The effect of coke particle size on outlet temperature: The impact of coke particle size on outlet temperature is quite significant. If the coke particles are larger, it reduces the resistance of the bed inside the furnace, increases the velocity of the primary airflow, causes the flame layer to rise, raises the outlet temperature, and thus increases heat loss ； At the same time, the heat storage capacity of the carbon layer decreases, resulting in a reduced gas production volume and lower quality of the gas produced by the gas generator. Operational experience has shown that if the coke particle size is between 20 and 75 mm, under the same operating conditions, the maximum temperature at the outlet of the gas generator per cycle rises to around 750°C ； If the coke particle size is between 20 and 40 mm, and the maximum temperature at the outlet of the gas generator per cycle is below 650°C, it is necessary to carefully control the coke particle size to prevent it from being too large. 3. The formation of large lumps inside the furnace leads to uneven ventilation. An improper ratio of steam supplied from the top and bottom causes the fire layer to move downward. The formation of large lumps in the gas generation furnace results in uneven ventilation during blowing, leading to localized overheating on the furnace surface and an increase in the temperature at the outlet of the gas generation furnace. This situation needs to be analyzed primarily based on the condition of the ash and slag as well as the difficulty in lowering the carbon layer. If there are large and hard pieces in the ash and slag, coupled with difficulties in lowering the carbon layer, then it should be considered that large deposits are forming inside the gas generator, which leads to an increase in the outlet temperature of the gas generator. During operation, adjustments should be made promptly based on the CO2 content in the gas blown in from above and below. Appropriately increase the amount of steam blown from above to eliminate large lumps. At this time, it is absolutely not advisable to reduce the outlet temperature by increasing the amount of steam blown downward, as this will have the opposite effect. 4. Other situations that cause an increase in the outlet temperature of the gas generator: If there is internal leakage at the valve opening of the gas three-way valve, the steam supplied for downward injection will take a shortcut and enter the gas cleaning tank directly, resulting in a reduced amount of steam actually reaching the furnace. This leads to an upward shift of the flame layer and an increase in the outlet temperature of the gas generator. In such cases, the reading on the steam meter for downward injection does not decrease; however, the resistance of the bed material inside the furnace decreases during downward steam injection, and maintenance should be carried out promptly. Additionally, a low carbon addition amount can also increase the temperature at the outlet of the gas generator. II. The significance of controlling the outlet temperature of the gas generator for its fuel consumption: Currently, the outlet temperature of gas generators has dropped from an average of 700°C to below 630°C, which is highly beneficial for reducing coke consumption. 1. Calculations for the blowing phase: Components of the blowing gas: H2O2, CO, CO2, N2, CH4; composition (by volume %): 2.96, 1.01, 8.05, 17.98, 70.02, 0. Average molar heat capacities: 6.99, 7.47, 7.2, 10.7, 7.14, 11.51. The average heat capacity of the blowing gas is given by: CP1 = ∑CiMi / 1/22.4 × (6.99×2.36 + 7.47×1.01 + 7.2×8.05 + 10.7×17.98 + 7.14×70.02 + 11.51×0) / 100 = 0.346 kcal/NM3°C. The total time per cycle is 165 seconds, with the blowing time being 45 seconds; the flow rate of the blowing gas is 30,000 NM3/h. Thus, the amount of blowing gas produced per hour is approximately 45/165×30,000 = 8,182 NM3. If the outlet temperature of the gas generator drops from an average of 700°C to below 630°C, the energy loss during the blowing phase can be reduced by approximately 0.346×8,182×70 = 198,168 kcal. With a calorific value of coke of 6,750 kcal/kg, approximately 30 kilograms more coke are consumed per hour during the blowing phase.

2. Calculations for gas production using top-blowing: Components of the gas produced by top-blowing: H2O2, CO, CO2, N2, CH4; composition (by volume %): 48.03, 0.10, 44.28, 4.13, 3.31, 0.15. Average molar heat capacities: 6.99, 7.47, 7.2, 10.7, 7.14, 11.51. The average heat capacity of the gas produced by top-blowing is given by: CP2 = ∑CiMi / 1/22.4 × (6.99×48.03 + 7.47×0.10 + 7.2×44.28 + 10.7×4.13 + 7.14×3.31 + 11.51×0.15) / 100 = 0.324 kcal/NM3°C. The total time per cycle is 165 seconds, with the time spent on top-blowing and secondary top-blowing combined being 44 seconds; the flow rate of the gas produced is approximately 6,000 NM3/h. Thus, the amount of gas produced per hour through top-blowing is approximately 44/(165–45)×6,000 = 2,200 NM3. If the outlet temperature of the gas generator drops from an average of 700°C to below 630°C, the energy loss associated with this gas is approximately 0.324×2,200×70 = 49,896 kcal. With a calorific value of coke of 6,750 kcal/kg, approximately 7.4 kilograms more coke are consumed per hour during the top-blowing phase. Heat carried away by the unreacted steam blown upward: Assuming an upstream steam flow rate of 5500 kilograms per hour (instantaneous flow), the total upstream steam flow per hour is approximately 44/(165–45)×5500 = 2017 kilograms. Of this amount, 2200/22.4/2×18 = 884 kilograms are used for gas generation and decomposition; thus, the amount of unreacted steam blown upward is 2017–884 = 1133 kilograms. Using a table, the molar average specific heat of water vapor at this temperature is 8.8 kcal/kmol. The heat carried away by this unreacted steam is 1133/18×8.8×70 = 38780 kcal. This results in an additional consumption of about 38780/6750 = 5.7 kilograms of coke. It can be seen that if the exit temperature of the gas generator drops from an average of 700°C to below 630°C, 43 kilograms of coke can be saved per hour. Over a year with 330 days of operation, this amounts to a savings of 340 tons of coke, which is equivalent to more than 300,000 yuan in value. Although some of the heat can be used to preheat the steam being blown downward, due to the low heat capacity of superheated steam and its low heat transfer coefficient, not much heat is actually absorbed; most of it is utilized in the waste heat boiler as a by-product. Clearly, using coke to generate steam is highly uneconomical.