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How to Generate Gas Using the \"Heavier Below, Lighter Above\" Principle Author/Source: Zhengda Thermal Energy Date: 2006-8-28 The process of producing synthetic ammonia feed gas from coal is essentially a matter of effective utilization of fuel heat, and this is also one of the key factors determining the coal consumption in gas production. Reducing heat loss and improving heat utilization have always been the main goals pursued in gas production. The forms of heat loss include: heat loss carried away by unreacted steam in the water gas, sensible heat loss from the blast air and dry gas, latent heat loss from the exhaust blast air, heat loss caused by gases carried away with the gas and ash residues, and heat loss due to heat dissipation through the wall layers. Among them, the heat of the unreacted steam, along with the sensible heat losses from the blowing gas and dry coal gas, account for about 60% of the total heat. An important measure to reduce these heat losses is to adopt a process control method that involves \"more pressure at the bottom and less at the top\". The so-called \"heavy downward, light upward\" process refers to a process control method in which, during the intermittent gas generation in the fixed layer, the proportion of time spent on the downward flow is increased while the proportion of time for the upward flow is reduced. In traditional cycle time allocation, the percentage of time allocated to downward flow is generally between 36.5% and 37.5%. Reduce the percentage of top-blown and diffused air, and correspondingly increase the percentage of bottom-blown air to 45.5–46.5%. With two different processes, there are apparent differences in surface characteristics, as reflected in the changes in the temperature of the gas in the downward air ducts. In the first process, the temperature of the gas in the upper gas channel is generally set at around 300 degrees, while the temperature of the gas in the lower gas channel is usually around 260 degrees. The actual difference lies in the thermal efficiency; the fact that the latter is much higher than the former is most clearly reflected in the coal consumption per ton of ammonia produced. Studies conducted on various manufacturers show that by adopting a \"heavy bottom, light top\" process, the coal consumption per ton of ammonia can be reduced by 100–200 KG, all else being equal. The most efficient and economical way to utilize the heat generated during the blowing process is to store it within the furnace as much as possible, rather than recovering it outside the furnace. The heat released during the blowing process can be utilized most effectively and economically by storing as much of it as possible within the furnace, rather than recovering it at the furnace itself. The heat released during blowing, the heat required for gas production, and the heat lost throughout the gas production process all contribute to the total heat loss, which includes losses due to steam carried away during decomposition, heat dissipation through the furnace walls, and other such factors. Therefore, in a properly operating gas furnace, regardless of its thermal efficiency, this heat balance always exists; however, different levels of thermal efficiency lead to varying degrees of heat loss. Under certain conditions, the heat loss is primarily related to the sensible heat losses of the blowing air and dry gas. When Q_in is constant, reducing the sensible heat losses from the blast air and dry gas dampers requires a corresponding reduction in the heat generated during blasting, thereby decreasing fuel consumption. When Q_out is constant, reducing the heat losses carried away by the blast air and dry gas means that, according to the heat balance equation, more heat must be absorbed during gas production. In this way, without increasing the coal consumption for blasting, the steam decomposition rate can be increased, leading to an increase in the output of semi-water gas and a reduction in the coal consumption per ton of ammonia produced. Therefore, reducing sensible heat loss is an effective way to improve heat utilization and reduce the amount of coal required for gas production. To minimize the dispersion of combustion gases, it is crucial to lower the temperature of the gas exiting the furnace, especially during the blowing phase of the upward-flowing gas. Due to the high volume and fast flow rate of air entering the furnace, a portion of the heat generated by combustion is carried away from the gasification zone, causing the heat storage capacity in the carbonization and drying zones to reach saturation and leading to an increase in the amount of heat carried away. During upward blowing, the upward-flowing gas, which includes steam with a large specific volume that has not yet decomposed, carries more heat upward, ultimately resulting in the heat storage capacity in the carbonization and drying zones reaching saturation, an increase in the temperature of the gas exiting the furnace, and significant sensible heat losses. Therefore, while maintaining stable furnace conditions and the temperature of the gasification zone, it is necessary to shorten the time during which the gasifying agent moves upward as much as possible, and extend the time for downward flow. This allows the heat accumulated in the furnace to be reused, thereby improving the heat storage capacity of the gasification zone and reducing heat loss. During the process adjustment, the upper blowing time is first shortened, while the lower blowing time is correspondingly increased; as a result, the temperature in the gasification zone rises. When this temperature exceeds the ash melting point, scorching and caking can occur. Therefore, it is necessary to reduce the heat input by further shortening the blowing time, in order to maintain thermal balance and achieve the process control goal of \"more force on the lower side and less on the upper side\".