Influencing Factors Of Airflow Combustion In Pulverized Coal Burner
Influencing Factors Of Airflow Combustion In Pulverized Coal Burner
In the pulverized coal burner, the pulverized coal is sent to the boiler through the burner through the primary air. In order to make the pulverized coal flow into the furnace reach the ignition temperature and burn, it is necessary to heat the pulverized coal flow. The heat required to heat the pulverized coal flow to the ignition temperature is called hot air. The heating of pulverized coal flow requires the heating of pulverized coal and primary air, the heat required for the evaporation of water in the pulverizing system, and the heat required for the heating, evaporation, and overheating of water in pulverized coal.
To make the pulverized coal flow ignite quickly, on the one hand, the fire should be reduced as much as possible, on the other hand, the fire should be heated up as soon as possible. There are two aspects of fly ash flow heat source, one is convective heat transfer, and the other is high-temperature flame radiation heat transfer in the furnace. Convection heat transfer is the main one. The calculation results show that: ① the normal ignition time exceeds 0.01 seconds; ② The residence time of pulverized coal in the boiler is 12 seconds; ③ The heating rate of pulverized coal with particle size less than 100 microns in the furnace exceeds 10000 ℃/s.
Influencing factors:
1. Fuel properties. Water vapor and ash affect the ignition of the fuel, and the amount of volatile matter has a great impact on the ignition and combustion of coal. The ignition temperature of coal with low volatile matter is high. After the pulverized coal enters the furnace, it needs more ignition heat to be heated to the ignition temperature, that is, before combustion, the pulverized coal flow must be heated to a higher temperature, the combustion time is longer, and the distance between the combustion point and the burner nozzle is longer. Therefore, when burning anthracite, lean coal, and other low volatile coal, the temperature of the fire area should be increased to make the high-temperature flue gas reflux as much as possible. Coal with high volatility is easier to catch fire. At this time, attention should be paid not to catch fire too early, so as not to cause slagging or combustion of the burner.
Coal has high moisture content and requires more heat during combustion. At the same time, part of the combustion heat is consumed in heating water and vaporization overheating, which reduces the flue gas temperature in the furnace, which makes the flue gas temperature entrained by the pulverized coal flow and the radiant heat of the flame on the pulverized coal flow reduce, which is not conducive to combustion. The low temperature of flue gas in the furnace is not conducive to the combustion chemical reaction and affects the rapid and full combustion of pulverized coal. The ignition speed of ash coal is slow, which is not conducive to the stability of ignition. During combustion, the ash shell hinders the combustion of the coke core and is not easy to burn.
2. Primary air temperature. High temperature preheated air is used as primary air to transport pulverized coal. Because the initial temperature of pulverized coal flow is increased, the ignition heat required for pulverized coal flow to reach ignition temperature is reduced, so the ignition time is shortened. Therefore, hot air pulverized coal feeding is widely used to burn anthracite, low-quality bituminous coal, and some lean coal.
3. Primary air volume and wind speed. The principle of primary air volume is to meet the combustion of volatile matter. Increase the primary air volume to increase the fire area accordingly, which is not conducive to fire. Reducing the primary air volume will greatly reduce the heat and accelerate combustion. However, if the primary air volume is too small, at the initial stage of ignition and combustion, volatile combustion will be affected by insufficient oxygen, which hinders the continuous development of ignition. Usually, the size of the primary air volume is expressed by the primary air rate, which refers to the percentage of the primary air volume in the corresponding total air volume at the furnace outlet. The primary wind speed also affects the ignition process. If the primary wind speed is too high, the ignition will be delayed, the ignition distance will be extended, and the whole combustion process will be affected; The primary airspeed is too low, which will cause the primary air duct to be blocked. Due to the early ignition, the burner may be burned.
4. Set fire protection belt. In the practice of setting the fire-proof zone, in order to stabilize the ignition of volatile anthracite, in addition to using hot air to send powder and grinding the pulverized coal finer, the burner area is also covered with water-cooled walls of refractory coatings such as ore, forming the so-called fire-proof zone, also known as the combustion zone.
5. Characteristics of the burner. The time required for the burner to propagate from the jet boundary to the whole section of the jet is closely related to the size of the burner. The larger the outlet section of the burner is, the farther away it is from the nozzle at the end of combustion, and the longer the ignition is. On this basis, it is reasonable to replace the high-power burner with a burner with small thermal power to shorten the ignition time of the whole pulverized coal flow, because a small-size burner will not only increase the surface area of pulverized coal flow ignition but also shorten the time required for ignition to expand to the whole jet section.
6. Boiler operating load. When the boiler load decreases, the average flue gas temperature in the furnace decreases, and the flue gas temperature in the burner area decreases, which is not conducive to the ignition of pulverized coal. If the boiler load is low to a certain extent, it will endanger the stability of the fire and even cause fire extinguishing. Therefore, the load regulation range of pulverized coal boilers is often limited by the flame stability. Generally speaking, the minimum load that can ensure the stability of fire without oil is called the minimum stable combustion load. For the slag discharging pulverized coal boiler, the minimum stable combustion capacity is 30% – 70% of the rated load.
7. Secondary air introduction mode. The secondary air should gradually enter the combustion type pulverized coal flow. Each batch of feed should not be too much, which will not affect the ignition. The inlet air should be strongly mixed with the gas powder mixture. In addition, the introduction of primary air does not affect the mixing of primary air, and the secondary air should be sent in batches according to the needs of the combustion area so that the combustion is not anoxic and the flame temperature will not be reduced so that the combustion can be complete.
The time of mixing the secondary air with the primary air should be appropriate. If mixing before ignition, it is equivalent to increasing primary air volume and delaying ignition; If the secondary air is mixed too late, the pulverized coal flow after combustion will lack oxygen, so the secondary air should be mixed in time after ignition. Mixing the secondary air into the primary air at once is not conducive to combustion. Because the temperature of the secondary air is far lower than the flame temperature, a large number of low-temperature secondary air mixing will reduce the flame temperature, slow down the combustion speed, and even lead to flameout.