The control of the firing atmosphere of ceramic products is restricted by the kiln structure and equipment configuration, such as the size of the fan air volume, duct diameter, placement of exhaust ports, hot air outlets, and moist air outlets, all of which can affect the control of the firing atmosphere. However, the most crucial factors are maintaining a stable pressure system and operating the burner reasonably.Stable pressure system: Pressure changes can affect the flow state of gases, so fluctuations in the kiln pressure system will cause fluctuations in the atmosphere. In order to control the atmosphere, it is necessary to stabilize the pressure system, and the key to a stable pressure system lies in controlling the zero-pressure surface. In the kiln preheating zone, the pressure is relatively low compared to the external environment due to the need to remove moisture and combustion-generated smoke, resulting in negative pressure inside the kiln; in the cooling zone, cold air is introduced to cool the products, resulting in relatively higher pressure compared to the external environment, leading to positive pressure inside the kiln; between positive and negative pressure, there is a zero-pressure surface, and the firing zone is located between the preheating zone and the cooling zone, so the movement of the zero-pressure surface will cause changes in the firing zone atmosphere.
When the zero-pressure surface is located at the front of the firing zone, between the firing zone and the preheating zone, the pressure in the firing zone is in a slightly positive state, and the atmosphere is reducing; when the zero-pressure surface is at the rear of the firing zone, the firing zone is in a slightly negative pressure state, and the atmosphere is oxidizing.Reasonable operation of the burner:
Whether the fuel is fully burned will affect the kiln atmosphere, especially the atmosphere of the firing zone. Therefore, reasonable operation of the burner and controlling the degree of fuel combustion are important means of controlling the kiln atmosphere. When the fuel is completely burned, all combustible components in the fuel can be completely oxidized in sufficient air, and there are no free C, CO, H2, CH4, and other combustible components in the combustion products, ensuring the realization of an oxidizing atmosphere. When the fuel is incompletely burned, there are some free C, CO, H2, CH4, and others in the combustion products, causing the kiln atmosphere to be reducing.
To ensure complete combustion of the fuel, attention should be paid to the following three points: ① ensuring thorough and uniform mixing of the fuel with air; ② ensuring sufficient air supply and maintaining a certain excess air volume; ③ ensuring that the combustion process takes place at a relatively high temperature.Many people are clear about the theoretical points of stable atmosphere for ceramic products (such as ceramic tableware, ceramic tea sets, etc.), but in practical operations, the kiln atmosphere is often changed unconsciously in order to solve certain firing problems. These changes are often easily overlooked. The following are common problems:Changing the excess air coefficient to increase the firing temperatureSome companies constantly accelerate the firing speed and shorten the firing period in pursuit of maximizing the production of single-kiln porcelain. The most commonly used method by operators is to increase the fuel supply, but after the fuel supply is increased, the adjustment of the supply of secondary air and the adjustment of the total damper of the secondary air fan are often not made in time, causing the firing atmosphere to change from an oxidizing atmosphere to a reducing atmosphere.Changing the atmosphere of the preheating zone to address defectsIn order to reduce the temperature of the rear section of the preheating zone, some operators decrease the opening of the exhaust damper, which affects the kiln pressure balance and gas flow rate, weakening the oxidizing atmosphere in the preheating zone. Poor control can easily cause poor combustion in the front kiln, leading to fluctuations in the atmosphere.Changing the cold air volume to address defects in the cooling zoneThis operation not only affects the changes in the overall kiln pressure system but also causes changes in the atmosphere.
For example, increasing the cold air volume can easily move the zero-pressure surface towards the preheating zone, and conversely, the zero-pressure surface will move towards the cooling zone, both of which can change the atmosphere. In order to stabilize the pressure, it is necessary to adjust the opening of the hot air damper correspondingly to balance the gas inflow and outflow of the entire kiln and stabilize the zero-pressure surface.