Combustion and heat transfer characteristics of fuel in a decomposition furnace and their influencing factors
1. Combustion characteristics of fuel in a decomposition furnace
In the decomposition furnace, the heat transfer characteristic of fuel combustion is different from that of fuel combustion heat transfer in rotary kiln. The combustion of fuel in rotary kiln is a process of diffusion control, and its flame must adapt to the calcining system in rotary kiln, that is, it requires a certain flame shape, length and temperature. Generally, the flame shapes we refer to are "brush head", "mallet" and "spherical" flames formed when abnormal conditions occur. "Licking material" flames are the manifestation of this characteristic. When the pulverized coal particle injection decomposition furnace, it is under the influence of vortex and spouting, makes coal particles floating in the stream, after preheating, decomposition, volatile matter of coal absorbing heat release nitrogen, hydrogen, oxygen, etc., combustion emit light and heat, forming a a small Mars, countless formed countless rapid combustion of pulverized coal is small Mars, these small Mars is essentially a small flame, they are under the effect of air suspension decomposition furnace is full of the whole, thus formed throughout the decomposition furnace combustion zone. But on the whole there is no visible flame with a definite outline. Therefore, the combustion of pulverized coal in the decomposition furnace is not a flameless combustion in the general sense, but a combustion reaction consisting of numerous small sparks that fill the whole furnace. Some people also call the combustion in the decomposition furnace flame combustion, this is because the powder particles in the decomposition furnace is scattered in the hot gas flow of high temperature combustion, when the powder particles are heated to a certain temperature, solid particles will also emit light, heat radiation and show the flame state.
2. Heat transfer characteristics in the decomposition furnace
The main heat transfer in the decomposition furnace is convection, which accounts for about 99%, followed by radiation heat transfer. The pulverized coal and raw material into the furnace are suspended in the hot airflow inside the furnace. The fuel combustion heats the airflow to high temperature, and the high temperature airflow simultaneously transfers heat to the material by convection. The heat transfer rate is very high because the gas-solid phase is in full contact. We know that the radiation heat transfer rate varies with the fourth power of temperature changes, the faster the temperature, the higher heat transfer rate, the radiation heat transfer ability is stronger, in the decomposition furnace, air temperature is only about 900 ℃, its radiation ability is far less than rotary kiln burning zone radiation ability, but in a furnace, a large number of CaCO3 decomposition, sent out a lot of CO2 gas (per 100 kg CaCO3 decomposition released 44 kg of CO2 gas) at the same time, and contains a lot of powder particles in gas, this will increase the thermal radiation heat transfer in the furnace capacity, although the radiation heat transfer accounts for only 1% of the total heat transfer in, But it is very advantageous to the uniform temperature distribution of the whole furnace. Due to the decomposition furnace fuel and material is a suspended state mixed together, fuel combustion heat release immediately absorbed by material and when fuel combustion, heat fast, decomposition is fast, when the fuel combustion is slow, the heat release is slow, due to the endothermic decomposition reaction inhibition of the improvement of the combustion temperature, resulting in the decomposition furnace basic constant airflow temperature in normal circumstances in at about 900 ℃.
3. Factors influencing the heat transfer characteristics of fuel combustion in a decomposition furnace
There are many factors affecting the combustion and heat transfer of fuel in the decomposition furnace, such as temperature, tertiary air temperature, pulverized coal fineness, moisture, coal quality, dispersed suspension degree, residence time, excess air coefficient, etc., but the most important is the temperature in the decomposition furnace, tertiary air temperature and coal quality.
1) Temperature in the decomposition furnace
In a traditional rotary kiln, the air temperature of calcining belt is generally 1600 ~ 1700℃, and the temperature of material is 1350 ~ 1450℃. The temperature increase or decrease of 10℃ ~ 20℃ has little influence on the combustion of pulverized coal in the rotary kiln, but has great influence on the decomposition furnace. In decomposing furnace, the decomposition of raw meal in close to equilibrium under the condition of furnace temperature between 860 ℃ ~ 920 ℃, if the temperature has a little change at this moment, such as the rise and fall, will break the original state of equilibrium, thus cause decomposition reaction speed significantly higher or lower, because of CaCO3 decomposition reaction is sensitive to temperature changes, temperature rise and fall will increase or decrease of reaction rate constant.
2) Three wind temperatures
Three air temperature of high and low for pulverized coal combustion in the decomposition furnace plays an important role, especially in high ash content, low content of volatile matter of coal, more significant, three blast temperature is low, slow to pulverized coal combustion, the incomplete combustion can occur, no combustion of pulverized coal particles with the air flow state of decomposing furnace is left after burning, cause after decomposition furnace system overtemperature, skinning, serious when cause blockage. If the three air temperature is too high (sometimes even more than 1000℃), coal combustion speed is too fast, fast heat release, heat concentration, for high volatile coal in the decomposition furnace due to local high temperature and formation of crust, blockage and even burn decomposition furnace.
3) coal
The influence of coal quality on the temperature in the decomposition furnace is more significant: when the pulverized coal has fine fineness, small water content, good dispersion and suspension degree, and the excess air is appropriate, the pulverized coal is completely burned to meet the requirements of the decomposition furnace, then higher combustion efficiency can be obtained. On the contrary, it will affect its performance play, seriously cause crust, blockage and other process accidents. In addition, high volatile content of coal in rotary kiln is easy to form short flame rapid burning, fixed carbon high coal easy shape long flame low temperature. When the coal with high volatile content is injected into the decomposition furnace and mixed with three high temperature winds, it will burn quickly and easily form local high temperature at the bottom of the decomposition furnace and produce crust. For example, the volatilization of coal used in Dongyuan 5000T/D line is 31.7%, and the CDC decomposition furnace often crust, which affects the normal operation; The dragon and the Three lions are even worse. Each shift should be cleaned at least twice, and in serious cases, each shift should be cleaned five or six times.
On 2018-08-08 11:45:46
