Influence factors of pulverized coal burner airflow combustion

Influence factors of pulverized coal burner airflow combustion
In the pulverized coal boiler, 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 airflow requires the heating of pulverized coal and primary air, the heat required for evaporation of water in the pulverizing system, and the heat required for 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, and on the other hand, the fire should be warmed up as soon as possible. The heat source of fly ash flow has two aspects, one is convection heat transfer, the other is radiation heat transfer of high temperature flame in the furnace. The convective 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 furnace 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 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 content is high. After the pulverized coal enters the furnace, it needs more ignition heat to be heated to the ignition temperature. That is to say, the pulverized coal gas flow must be heated to a higher temperature before combustion. The combustion time is longer, and the distance between the combustion point and the burner nozzle is longer. Therefore, when burning low volatile coal such as anthracite and lean coal, the temperature of the fire zone should be raised to make the high-temperature flue gas return as much as possible. Coal with high volatility is more likely to ignite. At this time, attention should be paid not to ignite prematurely to avoid slagging or combustion of the burner.
The coal has a high moisture content and requires more heat during combustion. At the same time, because part of the combustion heat is consumed in heating water and vaporization overheating, the flue gas temperature in the furnace is reduced, which makes the flue gas temperature and flame entrained by the pulverized coal flow have a reduction effect on the radiant heat of the pulverized coal flow, which is not conducive to combustion. The low temperature of flue gas in the furnace is not conducive to combustion chemical reaction and affects the rapid and full combustion of pulverized coal. The slow ignition speed of ash coal is not conducive to ignition stability. During combustion, the ash shell hinders the combustion of coke core and is not easy to burn.
2. Primary air temperature. High-temperature preheated air is used as the primary air to transport pulverized coal. As the initial temperature of pulverized coal flow is increased, the ignition heat required for pulverized coal flow to reach the ignition temperature is reduced, so the ignition time is shortened. Therefore, hot air pulverized coal is widely used for burning anthracite, inferior 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 primary air volume will increase the ignition area, which is not conducive to ignition. Reducing the primary air volume will greatly reduce the heat, thus accelerating combustion. However, if the primary air volume is too small, the volatile combustion will be affected by insufficient oxygen at the initial stage of ignition and combustion, thus hindering the continuous development of ignition. Generally, 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 air speed also affects the ignition process. If the primary air speed is too high, it will delay the ignition, extend the ignition distance and affect the whole combustion process; If the primary air speed is too low, the primary air duct will be blocked, and the burner may be burnt out due to early ignition.
4. Set up fire protection belt. In the practice of setting up the heat preservation zone, in order to stabilize the ignition of volatile anthracite, in addition to using hot air to send the powder and grinding the pulverized coal to finer size, the water-cooled wall of refractory coating such as ore is also used to cover the burner area, forming the so-called heat preservation zone, also known as the combustion zone.
5. Characteristics of 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 farther away from the nozzle at the end of combustion, and the farther the ignition extension. On this basis, it is reasonable to replace high-power burners with burners with smaller thermal power to shorten the ignition time of the entire pulverized coal flow, because small-scale burners will not only increase the surface area of pulverized coal flow ignition, but also shorten the time required for ignition to extend to the entire 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 adjustment range of pulverized coal boiler is often limited by the flame stability. Generally speaking, the minimum load that can ensure the fire stability without adding oil is called the minimum stable combustion load. For slag-discharging pulverized coal boiler, its minimum stable combustion capacity is 30%~70% of the rated load.
7. Secondary air introduction mode. The secondary air shall gradually enter into the combustion pulverized coal flow. The amount of feed in each batch shall not be too much to affect the ignition. The feed air shall be strongly mixed with the gas-powder mixture. In addition, the introduction of primary air does not affect the mixing of primary air. The secondary air should be delivered in batches according to the needs of the combustion area, so that the combustion can be complete without oxygen deficiency and flame temperature reduction.
The mixing time of secondary air and primary air should be appropriate. If the mixture is mixed before ignition, it is equivalent to increasing the primary air volume and delaying ignition; If the secondary air is mixed too late, the pulverized coal flow after combustion will burn and lack oxygen, so the secondary air should be mixed in time after ignition. The mixing of secondary air with primary air at once is not conducive to combustion. Because the temperature of secondary air is far lower than the flame temperature, a large amount of low-temperature secondary air mixing will reduce the flame temperature, slow down the combustion speed, and even cause flameout.