Talking about the low pollution burning method of steel drum painting fuel gas drying furnace (1)

Talking about the low pollution burning method of steel drum painting fuel gas drying furnace (1)

At present, there are more and more steel drum cleaning and painting and drying furnaces using fuel and gas in China. Since the volatile substances in the paint can be burned by recycling and then entering the burner for burning, not only energy is saved, but also pollution is reduced, so the fuel gas drying oven is a major advancement in the environmental protection of steel drum production. However, since most enterprises are still unfamiliar in operation technology and have many problems in understanding, many companies in the barrel industry often have serious pollution and waste problems, resulting in considerable environmental pollution and product pollution. In order to let the technical personnel in the industry understand the fuel gas drying furnace and reduce the environmental pollution, some necessary introductions are made to the common combustion operation problems and low pollution combustion methods of the steel drum fuel gas drying furnace.

First, low oxygen combustion

Both NOx and SOx are products of combustion. Their production is harmful to the human body, pollutes the environment, and brings many disadvantages to the operation of the oven. Therefore, we should try our best to reduce the generation of NOx and SOx. At present, under the advanced technical conditions, the excess air coefficient can be as low as α≤1.03. Low-oxygen combustion reduces the amount of flue gas, which increases oven efficiency.

1. Use low-oxygen combustion to reduce the amount of SO2 produced.

Due to the high sulfur content in the fuel, the low temperature hot surface is seriously corroded and ash blocked, thus starting the research work of low oxygen combustion. In the early 1960s, low-oxygen combustion was considered 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 produced.

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

After combustion of sulfur in the fuel, sulfur dioxide is formed, and part of the SO2 is further oxidized to form sulfur trioxide, SO3, which combines with water in the flue gas to form sulfuric acid H2SO4. When sulfuric acid vapor encounters a low-temperature metal surface, it will condense the surface to form a small amount of sulfuric acid droplets. If these sulfuric acid droplets are generated on the metal surface of the heated surface, the heated surface will be subjected to low temperature corrosion; if it is condensed on the surface of the fly ash, large particles containing acid will be formed to cause acid dust. The factors affecting the amount of sulfur trioxide produced are mainly 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, the conversion rate of SO3, in addition to the sulfur content of the fuel, is mainly related to the concentration of oxygen in the flue gas. Reducing the concentration of residual oxygen can reduce the conversion rate of SO3. Therefore, low-oxygen combustion can effectively control the hazard caused by sulfur combustion.

However, if certain technical measures are not taken during low-oxygen combustion, the incomplete combustion loss will increase, and the dust concentration in the exhaust gas will increase. The relationship between the concentration of exhaust smoke and the residual oxygen concentration in the flue of the economizer outlet: as the excess air ratio decreases, the dust concentration increases. When the residual oxygen concentration is less than 1%~2%, the dust concentration increases sharply. At this time, the chimney emits black smoke, the flame in the furnace becomes dark, the flame extends out of the furnace outlet, and combustion instability may occur. Therefore, the combustion is low. When oxy-combustion, the combustion equipment should be more perfect, so that it can completely burn under the condition of the theoretical air volume.

In order to control the soot within the appropriate range, usually the full-load working condition, the residual oxygen concentration is 0.8%~1%; when the load is 50%, the low-temperature corrosion is also considered light, and it is unnecessary to use too low excess air coefficient. Generally, the residual oxygen concentration is about 2%.

During low-oxygen operation, the concentration of SO3 in the flue gas is lowered, and the dew point of the flue gas is lowered, so that the phenomenon of white smoke is reduced. Generally, when the 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 the amount of NOx produced.

Generally, the more excess air coefficient in the furnace, the higher the oxygen concentration in the flue gas, and under the same conditions, the NOx emission concentration will increase. The operation of low excess air ratio is to reduce the 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 NOx formation condition is destroyed.

Some studies have shown that when the oxygen concentration is lowered, the NOx concentration is lowered, especially when the oxygen concentration is less than 1%, the NOx concentration will drastically decrease. Reducing the excess air ratio will increase the flame temperature, with the result that the amount of NOx produced increases. When the excess air ratio is 1.0, the flame temperature is the highest and the NOx production is also the largest. However, this is the case of premixed combustion flames. Most combustion and drying ovens are classified as diffusion combustion. Due to the different mixing conditions, changing the excess air coefficient will produce different results. In general, the combustion equipment is not perfect. The amount of air varies from place to place. In some places, the excess air coefficient 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 that the temperature is lowered to reduce the amount of NOx generated. Therefore, since nitrogen in the air and nitrogen in the fuel do not have oxygen, NOx can be reduced.

However, since a part of the air is insufficient to be reproduced, when the low excess air ratio is operated, the soot concentration is increased. Excessively reducing the excess air ratio, although reducing the NOx discharge concentration, will cause a sharp increase in soot concentration.

3. Organizational issues that should be considered for hypoxic combustion

To organize low-oxygen combustion, 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 the air. To this end, the atomized oil flow and air flow conditions must be carefully studied, especially to solve the carbon burnout problem. Since the concentration of oxygen in the late stage of combustion is low and the temperature is low, it is technically difficult to burn off the carbon before leaving the furnace. Solving this problem is the key to low-oxygen combustion. It requires uniform distribution of air and burners, and uniform distribution of fuel and air between burners. When the load of the oven changes, in order to ensure the ratio between the fuel and the air, higher requirements are also imposed on the automatic adjusting device. In summary, to organize hypoxic combustion, the following points should be considered:

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

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

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

Second, two-stage combustion method

Experiments have shown that when the excess air ratio is less than 1, the amount of NOx produced is significantly reduced. Excessive air ratios of less than 1, that is, excessive combustion of the fuel, have a significant effect on controlling 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 amount of air, the combustion is performed under an excessively rich condition, and the excess air is sent to the furnace through a special discharge port to be mixed with the smoke generated by the excessive combustion of the fuel. Complete the entire combustion process. Since the air is supplied to the combustion twice, it is called two-stage combustion, and the air sent by the main burner is called a section of air, and the air sent by the dedicated nozzle is called two-stage air, which is called a section of combustion and two sections of combustion.

Before the second stage air is fed, a part of the air can only be burned for part of the fuel due to insufficient air, so 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. presence. The reaction of nitrogen and atomic oxygen is carried out because the activation energy is large, and it is not necessary until the fuel is substantially burned. Therefore, the amount of NOx generated must be small. When the two-stage air is fed in, the temperature of the flue gas has been lowered due to the cooling effect in the furnace. Although the oxygen is excessive, the NOx generation rate is slow due to the low temperature, and the NOx generation can be effectively controlled. When the temperature is high, it is difficult to reduce the temperature to reduce NOx. Therefore, the reaction activation energy of nitrogen and oxygen is generally higher than that of carbon and oxygen, so that the combustion is performed under conditions of insufficient air. To achieve better control results.

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 restriction of the secondary air injection port, or the flame becomes a long flame compared with the original, and it is restricted by the size and shape of the combustion chamber, the secondary combustion is sometimes not used.

With secondary combustion, the introduction position of the air is different, and the concentration of NOx finally obtained is also different. The closer the introduction position of the second-stage air is to the top of the first-stage burner, the larger the concentration of NOx is discharged.

Third, flue gas recirculation combustion method

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

Exhaust circulation rate = {flue gas circulation amount (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 combustion air and burned, if the flue gas recirculation rate is increased, the rate of decline in NOx formation also increases, but there is a limit value. Moreover, in actual work, it is subject to factors such as flame stability and vibration of the oven body. Therefore, when determining exhaust gas recirculation, various working conditions must be considered, and the recycling rate is usually 20% to 30%.

When using the flue gas recirculation method, recirculation pumps and piping equipment are required in the plant. In some cases, a cooler may be required and an exhaust gas recirculation rate control mechanism is required.