Talking about the Low-pollution Combustion Method of Drum Boiler Fuel Gas Dryer (I)

At present, there are more and more domestic steel drum cleaning and painting drying furnaces that use fuel and gas. Because the volatile substances in the paint can be recycled and then burned into the burner, which not only saves energy, but also reduces pollution. Therefore, the fuel gas drying furnace is a major advancement in steel drum production toward environmental protection. However, due to the fact that most enterprises are still unfamiliar with the operation technology and there are many problems in understanding, many companies in the barrel making industry often have serious problems of pollution and waste, causing serious environmental pollution and product pollution. In order to enable the technical personnel in the industry to understand the gas-fired gas drying furnace and reduce environmental pollution, some necessary introductions are made to the common combustion operation problems and low-polluting combustion methods of the steel drum fuel gas drying furnace.

First, low oxygen combustion

NOx and SOx are products after combustion. Their production is harmful to the human body and causes pollution to the environment. It also brings many disadvantages to the operation of the oven. Therefore, we should try to minimize the generation of NOx and SOx. At present, under the advanced technology conditions, the excess air coefficient can be as low as α ≤ 1.03. Low-oxygen combustion reduces the amount of flue gas, which can improve oven efficiency.

1, using low oxygen combustion to reduce the amount of SO2 production

Due to the high content of sulfur in the fuel, causing severe corrosion and clogging of the low-temperature hot surface, the study of low-oxygen combustion began. In the early 60s, low-oxygen combustion has been considered as an effective means of solving low-temperature corrosion. Nowadays, low oxygen combustion technology has been widely used to reduce the amount of NOx and SOx generated.

The low-oxygen combustion method operates with a low excess air ratio to reduce the oxygen concentration and reduce the amount of SOx generated.

After the sulfur in the fuel burns, sulfur dioxide will be generated, and part of the SO2 will be further oxidized to generate sulfur trioxide SO3. The combination of SO3 and the water in the flue gas will generate sulfuric acid H2SO4. When the sulfuric acid vapor meets the surface of the low-temperature metal, sulfuric acid droplets having a small particle size are generated on the coagulation surface. If these sulfuric acid droplets are generated on the metal surface of the heating surface, the heated surface will be subject to low temperature corrosion; if condensed on the surface of fly ash, large particles containing acid will be formed, causing acid dust. The factors affecting the production of sulfur trioxide are mainly related to the concentration of oxygen in the flue gas, in addition to the sulfur content of the fuel. By reducing the concentration of residual oxygen, the conversion rate of SO3 can be related to the concentration of oxygen in the flue gas, in addition to the sulfur content of the fuel. Reducing the concentration of residual oxygen can reduce the conversion rate of SO3. Therefore, low-oxygen combustion can effectively control the hazards caused by sulfur combustion.

However, if a certain technical measure is not taken during low-oxygen combustion, the incomplete combustion loss will increase and the concentration of dust in the exhaust will increase. The relationship between the concentration of flue gas and the remaining concentration of oxygen in the outlet flue of the economizer: As the excess air coefficient decreases, the dust emission concentration increases. When the remaining oxygen concentration is less than 1%~2%, the dust concentration increases sharply. At this time, the chimney smokes black smoke, the flame in the furnace darkens, and the flame stretches out of the hearth exit, and combustion instability may also occur. When oxygen is burned, the combustion equipment should be more perfect so that it can completely burn near the theoretical air volume.

In order to control the smoke in an appropriate range, the oxygen residual concentration is usually 0.8%~1% for full load conditions; when 50% load is taken into account, low temperature corrosion is also lighter, and it is unnecessary to use an excessively low excess air ratio. The remaining oxygen concentration is generally about 2%.

During low-oxygen operation, the concentration of SO3 in the flue gas decreases, the dew point of the flue gas decreases, and the occurrence of white smoke decreases. Generally, when SO3 concentration is less than 0.001% to 0.002%, white smoke can be effectively prevented. At this time, the excess oxygen concentration is about 1.5%.

2. Use low-oxygen combustion to reduce NOx production

Generally, the more the excess air coefficient in the furnace, the higher the concentration of oxygen in the flue gas. Under the same conditions, the NOx emission concentration will increase. The operation of the low excess air ratio is to reduce the amount of air supply as much as possible so that the oxygen in the air is completely combined with the fuel, so that the nitrogen in the air or the nitrogen in the fuel does not get oxygen, and the generation conditions of NOx are destroyed.

Some studies have shown that when the oxygen concentration decreases, the NOx concentration decreases, especially when the oxygen concentration is less than 1%, the NOx concentration will drastically decrease. Lowering the excess air coefficient will increase the flame temperature, and as a result, the NOx production will increase. When the excess air coefficient is 1.0, the flame temperature is the highest and the amount of NOx generated is also the highest. However, this is the case of premixed combustion flames. Most of the combustion drying furnaces are diffusive combustion. Due to different mixing conditions, different results can be obtained by changing the excess air ratio. The general situation is not a perfect combustion equipment. The amount of air is different everywhere. In some places, the excess air factor is just right. In some places, there is too much air. When the excess air ratio is reduced, the air may become insufficient. One of the results is a decrease in temperature, which reduces the amount of NOx generated. Therefore, since nitrogen in the air and nitrogen in the fuel do not have oxygen, the NOx can be reduced.

However, due to the reappearance of a portion of the air shortage, the low excess air coefficient operation causes an increase in soot concentration. Excessive reduction of the excess air ratio can reduce the NOx emission concentration, but it will cause a sharp increase in smoke concentration.

3. Problems to be considered when organizing hypoxic combustion

To organize the low-oxygen combustion, it is necessary not only to ensure good atomization, but also to increase the surface area of ​​the oil; it is also necessary to mix the oil mist with air just right. To this end, careful research must be conducted on the atomized oil flow and air flow conditions, in particular to solve the carbon burnout problem. Since the oxygen concentration is low and the temperature is low during the later stage of combustion, it is technically very difficult to burn carbon before leaving the furnace. Solving this problem is the key to low-oxygen combustion. It requires uniform burner and air distribution and uniform fuel and air distribution among burners. When the load of the oven changes, in order to ensure the ratio between fuel and air, higher requirements are also put on the automatic adjustment equipment. In short, the organization of low oxygen combustion should consider the following points:

(1) Select a burner with good performance to obtain the required aerodynamic working conditions and ensure that the fuel and air mix well and achieve complete combustion.

(2) Select a suitable air distribution system to ensure uniform air distribution for each burner.

(3) Select high quality instruments and automatic adjustment equipment.

Two-stage combustion method

Experiments show that when the excess air coefficient is less than 1, the amount of NOx generated is significantly reduced. Excessive air ratios of less than one, that is, excessive combustion of the fuel, have a significant effect on the control of NOx. This is the theoretical basis of the two-stage combustion method. When the amount of air supplied to the burner is reduced to less than the theoretical air amount, the combustion is performed under excessively rich conditions. The excess air is sent into the furnace through a special discharge port and mixed with the flue gas generated by the over-concentrated combustion of the fuel. Complete the entire combustion process. Because the air is fed twice for combustion, it is called two-stage combustion. The air fed by the main burner is called a section of air, and the air provided by the special nozzle is called two-stage air, which is correspondingly referred to as a combustion and two-stage combustion.

Before the second stage air is fed in, due to lack of air, a section of air can only be used for burning part of the fuel. Therefore, the flame temperature is low; on the other hand, there is a large amount of unburned fuel and a large amount of incomplete combustion products in the flame. exist. The reaction between nitrogen and atomic oxygen can only be performed after the fuel is completely burned due to its large activation energy. Therefore, the amount of NOx generated must be small. When the secondary air is fed in, the temperature of the flue gas has been reduced due to the cooling effect in the furnace. Although the oxygen is excessive, due to the low temperature, the NOx generation rate is slow and the generation of NOx can be effectively controlled. When the temperature is high, it is difficult to reduce NOx by lowering the temperature. Therefore, the reaction activation energy of nitrogen and oxygen is generally used to be higher than the activation energy of carbon and oxygen reaction, so that the combustion is performed under insufficient air conditions. , In order to obtain better control effect.

It should be noted that this method is prone to incomplete combustion and unstable combustion, so special care must be taken to prevent the generation of incomplete combustion products such as coal dust and CO. For existing facilities, because of the limitation of the secondary air injection port, or the fact that the flame becomes longer than the original flame, and because of the size and shape of the combustion chamber, secondary combustion may not be used.

With two-stage combustion, the introduction position of air is different, and the final concentration of NOx is also different. The second-stage air is introduced closer to the top of the first burner, and the NOx emission concentration is greater.

Third, the flue gas recycling combustion method

In order to reduce the generation of NOx in the furnace, a method of recirculating the flue gas is also adopted, in which case the temperature and oxygen concentration of the flame itself can be uniformly reduced.

Exhaust gas circulation rate={flue gas circulation volume (m3/h)/ combustion air volume (m3/h)}×100%

Flue gas recirculation reduces the furnace temperature and oxygen concentration, thereby reducing the amount of NOx produced. The composition of the exhaust gas is N2, CO2, H2O, and O2. When they are mixed with air for combustion and burned, if the flue gas recirculation rate is increased, the rate of decrease in NOx production also increases, but there is a limit value. In practice, it is subject to factors such as flame stability and vibration of the oven body. Therefore, when deciding the exhaust gas recirculation, various working conditions must be considered, and the recirculation rate is usually taken as 20% to 30%.

When using the flue gas recirculation method, a recirculation pump and piping equipment are required in the device. In some cases, a cooler may also be required and an exhaust gas recirculation rate control mechanism may be provided.