The influencing factors of airflow combustion in coal powder burners

The influencing factors of airflow combustion in coal powder burners
In the pulverized coal boiler, the pulverized coal is sent to the boiler through the burner through the primary air. In order to achieve the ignition temperature and combustion of the coal powder airflow entering the furnace, it is necessary to heat the coal powder airflow. The heat required to heat the coal powder airflow to the ignition temperature is called hot air. The heating of coal powder airflow requires the heating of coal powder and primary air heat, the heat required for the evaporation of water in the pulverization system, and the heat required for the heating, evaporation, and overheating of water in coal powder.To quickly ignite the coal powder airflow, on the one hand, it is necessary to minimize the fire, and on the other hand, it is necessary to quickly raise the temperature of the fire. There are two aspects to the heat source of fly ash flow: convective heat transfer and high-temperature flame radiation heat transfer inside the furnace. Convective heat transfer is the main factor between the two. The calculation results indicate that: ① the normal ignition time exceeds 0.01 seconds; ② The residence time of coal powder in the furnace is 12 seconds; ③ The heating rate of coal powder with a particle size less than 100 microns in the furnace exceeds 10000 ℃/s.Influencing factors:1. Fuel properties. Water vapor and ash content affect the ignition of fuel, while the amount of volatile matter has a significant impact on the ignition and combustion of coal. Coal with low volatile content has a higher ignition temperature. After the coal powder enters the furnace, it requires more ignition heat to heat up to the ignition temperature. In other words, before combustion, the coal powder airflow must be heated to a higher temperature, resulting in a longer combustion time and a longer distance between the combustion point and the burner nozzle. Therefore, when burning low volatile coal such as anthracite and lean coal, the fire zone temperature should be raised to allow the high-temperature flue gas to reflux as much as possible. Coal with high volatility is more prone to ignition, and attention should be paid not to ignite too early to avoid slagging or combustion in the burner.Coal has a high moisture content and requires more heat during combustion. At the same time, due to the consumption of some combustion heat on heating water and vaporization overheating, the temperature of the flue gas in the furnace is reduced, which leads to a decrease in the temperature of the flue gas entrained by the coal powder airflow and the radiation heat of the flame on the coal powder airflow, which is not conducive to combustion. The low temperature of flue gas in the furnace is not conducive to combustion chemical reactions and affects the rapid and full combustion of coal powder. The slow ignition speed of gray coal is not conducive to stable ignition. During combustion, the ash shell hinders the combustion of the coke core and is not easy to burn.2. Primary air temperature. Using high-temperature preheated air as the primary air to transport coal powder reduces the ignition heat required for the coal powder airflow to reach the ignition temperature by increasing the initial temperature of the coal powder airflow, thereby shortening the ignition time. Therefore, hot air powder delivery is widely used in the combustion of 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. Increasing the air flow once will correspondingly increase the ignition area, which is not conducive to ignition. Reducing the air flow once will greatly reduce heat and accelerate combustion. However, if the primary air volume is too small, in the early stage of ignition and combustion, volatile combustion will be affected by insufficient oxygen, which hinders the further development of ignition. Usually, the size of the primary air volume is represented by the primary air rate, which refers to the percentage of the primary air volume to the corresponding total air volume at the furnace outlet. A single wind speed also affects the ignition process. If the wind speed is too high, it will delay the ignition, prolong the ignition distance, and affect the entire combustion process; If the wind speed is too low, it can cause blockage in the primary air duct. Due to early ignition, the burner may be burned out.4. Set up a fire protection belt. In the practice of setting up a flame retardant belt, in order to stabilize the ignition of volatile anthracite, in addition to using hot air to deliver powder and grinding the coal powder finer, the water-cooled wall of refractory coatings such as ore is also used to cover the burner area, forming a so-called flame retardant belt, also known as a combustion belt.5. The characteristics of the burner. The time required for the burner to propagate from the jet boundary to the entire cross-section of the jet is closely related to the size of the burner. The larger the outlet cross-section of the burner, the further away from the nozzle at the end of combustion, and the further the ignition extension. On this basis, it is reasonable to replace high-power burners with burners with lower thermal power to shorten the ignition time of the entire coal powder airflow, because small-sized burners not only increase the surface area of coal powder airflow ignition, but also shorten the time required for ignition to extend to the entire jet cross-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 coal powder. If the boiler load is low to a certain extent, it will endanger the stability of the fire and even cause extinguishing. Therefore, the load regulation range of coal powder boilers is often limited by flame stability. Generally speaking, the minimum load that can ensure fire stability without adding oil is called the minimum stable combustion load. For slag discharge coal powder boilers, the minimum stable combustion capacity is 30% to 70% of the rated load.7. Secondary air introduction method. The secondary air should gradually enter the combustion type coal powder airflow, and the amount of feed in each batch should not be too much to affect ignition. The intake 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 secondary air should be sent in batches according to the needs of the combustion area to ensure that combustion does not lack oxygen and does not reduce flame temperature, so that combustion can be complete.The time for the secondary air to mix with the primary air should be appropriate. If mixed before ignition, it is equivalent to increasing the air volume and delaying ignition; If the secondary air is mixed too late, the coal powder airflow after combustion will lack oxygen, so the secondary air should be mixed in a timely manner after ignition. Mixing secondary air with primary air at once is also not conducive to combustion. As the temperature of the secondary air is much lower than the flame temperature, a large amount of low-temperature secondary air mixing can lower the flame temperature, slow down the combustion speed, and even cause flameout.