(1) Construct a pressure system based on the calorific value of the fuel used. High-calorific-value fuels, such as natural gas, heavy oil, light diesel oil, and coke oven gas, should use a pressure system with small pressure differential and dispersed ignition. Small pressure differential and dispersed combustion refers to a small absolute difference between the positive and negative pressures inside the kiln, a large coal feeding area, or a large and dispersed burner arrangement. This method can make the temperature inside the kiln uniform and reduce heat consumption. In this case, the zero pressure level should be in front of the ignition zone. High-calorific-value fuels should not be burned in a small area per unit time, otherwise it will lead to local overburning. If high-calorific-value fuels are baked with a large pressure differential, the air flow inside the kiln will be very fast, and some combustible components of the fuel will not be fully burned and will be discharged outside the kiln, changing the atmosphere inside the kiln and wasting fuel. At the same time, due to the large pressure differential, the kiln car structure, kiln walls, and kiln roof are easily damaged, the gas stratification in the preheating zone is severe, the temperature difference is large, the product quality is uneven, and the yield is low.
(2) Construct a pressure system based on the calorific value of the internal fuel. Generally, low-internal-fuel billets operate under both positive and negative pressure, with a relatively small pressure difference. In this case, the zero-pressure level should be before the ignition zone. However, high-heat or superheated materials, especially coal gangue, require a large pressure difference during roasting. Besides the heat required for normal firing, billets with high internal fuel or internal combustion have excess heat that needs to be rapidly dissipated, thus necessitating a lower pressure. In this case, the zero-pressure level should be after the ignition zone. In the pressure system, the negative pressure position, negative pressure value, and zero-pressure position of the preheating zone significantly affect the sintering process. Once the billet stack configuration is determined, the negative pressure value of the preheating zone becomes an indicator of the overall tunnel kiln ventilation. Low negative pressure and insufficient draft often result in rapid firing, slow bottom firing, and slow cooling after firing, severely impacting production. Excessive negative pressure increases the excess air coefficient in the ignition zone, leading to weak ignition, under-firing of the top section, and increased coal consumption. Therefore, careful pressure control during operation is crucial.
Some simplified tunnel kilns, in an effort to reduce power consumption or for other purposes, have omitted the under-car balancing fan, which is highly unreasonable. All tunnel kilns without an under-car balancing fan are prone to problems in the kiln car and sand seal. The function of the under-car balancing fan is to blow cold air from the outlet end to the inlet end of the tunnel kiln, creating a positive pressure state under the kiln car, with the intake air drawn from the preheating zone pit. Its main function is to ensure that the pressure curve formed in the under-car tunnel is consistent with or nearly consistent with the pressure curve in the kiln tunnel above the car, minimizing gas exchange between the upper and lower parts of the kiln.
