Coal combustion theory

1. Coal Classification
Due to physical and chemical reactions, there are differences of metamorphic grade of coal. Owing to different coal producing area and different years of geological formation, the combustion performance of coal is extermely different.

Solid combustion for cement kiln is bituminous coal, anthracite, lean coal, lignite, petroleum coke, and other industrial waste. In regard to fuel coal, the classification in china is as follows:

volatile flux

In American, depending on the volatile flux of coal, the coal is classified as followed.(ASTM D338 standard): ≥69 <78> 22 ≤31 

In German, industrial coal's classification criteria are:

2. Coal powder's combustion processes
Fuel combustion flame formed in cement kiln belongs to turbulent diffusion flame. Its combustion process can be divided: Fuel and air mixture,fuel and air which are heated to the ignition temperature, volatile initial fire burning, coke combustion and burnout.

The combustion of volatile species has relation with coal type,the heating rate, the structure of the burner,thus，improving burner structure, accelerate the heating rate of the coal volatiles and precipitation, can improve the performance of the burner.
After voilating media burn, the burning lefted has relation with the surrounding air and carbon particles.

Therefore, the diffusion rate determines the rate of fuel combustion, it is mainly affected by carbon particles of porous, fuel particle size, O2 partial pressure, ambient temperature and other factors.

Due to its high temperature environment in the kiln, the combustion is controlled by the particle boundary diffusion rate, generally called diffusion controlled process.

3. The main factors that influence coal combustion
3.1 Coal powder burner
Burner structure, design parameters, operating parameters, etc. have a significant impact on coal combustion.Thus each burner manufacturers improve the structure and design parameters of the burner by Cold test, thermal test, the theoretical model to let the performance of burners get better.

The speed of the primary air is low(40~70m/s.The combustion of pulverized coal is slow, whose function is completing coal transportation.The primary air has little inflence on mixing coal and air, absorbing secondary air, adjusting flame shape and temperature distribution. Thus it couldn't meet production needs.

At the late of 1970s,France Pillard Company firstly develope three-channel burner for pulverized coal combustion.Them German KHD company, Demark F.L.Smidth company, Japanese UBE company develope various types of multi-channel fuel burner to use fuel burning, low-quality fuel and protect environment on the basis of constantly optimization in the modern precalciner system. Therefore, the main aim like multi-channel burner's research and development is put forward.

Reducing the amount of primary air, increasing the use of secondary air to improve the thermal efficiency of the system; Increasing the mixing of pulverized coal and combustion air to improve the combustion rate. Enhancing the burner's thrust to strengthen the entrainment of secondary air and increase the flame temperature.

Increase the adjustment means for each channel wind and wind speed, thus easy to flexibly control the flame shape and temperature field according to the need. It is beneficial to use low volatile and low active fuel. Improving production efficiency of kiln system, achieving the aim like high-quality, high yield, low consumption, and reducing NOx generation amount.
As can be seen from the history of the burner, coal powder combustion technology has made rapid progress by the continuously optimizing the structure of the burner. Therefore burner pulverized coal combustion is an important factor of influence.

3.2. Coal performance
3.2.1 The calorific value of coal
Calorific value of coal is an important indicator to measure the performance of coal-fired.Generally speakinghigh calorific value coal combustion represents good properties of coal.Currently Cement large companies tend to use high calorific value coal.Because that high calorific value coal, not only reduce the amount of coal, but improve coal fast heating。 Cement kiln achieve high yield and low consumption, the overall economic efficiency is better than using low quality coal

3.2.2Volatiling coal
Volatile coal is the organic compound composed of carbon with hydrogen, oxygen, sulfur etc,t was first precipitated gasification and combustion before pulverized coal combustion. With the improvement of volatiles, the flame becomes longer, which exhibits the characteristics of a gas flame. Burning capacity is proportional to the volatile content of coal.Therefore, when using low volatile fuel, in order to obtain the same fuel burn time, it save pulverized coal combustion to control the particle size.
Coal volatile has relationship with the ignition temperature is shown as the chart.

3.3.3Coal fineness
When the rotary kiln is burned as a ribbon, secondary air temperature is greater than 900 ℃, the flame temperature is higher than 1800 ℃,Pulverized coal combustion rate is mainly affected by the diffusion rate.Pulverized coal burn time is proportional to the square of the particle size in the reflection process the diffusion rate is controlled. As long as the diameter of reactant particles is reduced, the reaction rate can be accelerated.So kiln burner, coal fineness is the main factor affecting the rate of pulverized coal combustion. The coal quality on the burn rate was less affected. Therefore, anthracite or bituminous coal will reduce coal fineness to ensure the complete combustion of pulverized coal and the firing temperature of firing zone. According to the practical experience of rotary kiln burner,Kiln pulverized coal's fineness is half of Volatile content of combustible base

3.3.4 Coal powder moisture content
When pulverized coal containing water is sprayed into the rotary kiln,heat is transferred to the surface of coal particles,water is vaporized, while pulverized coal is cooled at ambient temperature environment. Therefore, prior to the combustion, pulverized coal need a higher temperature,coal began to burn after Combustibles are separated out.At high temperatures, coal moisture evaporates quickly.It will produce high vapor pressure in the coal particles, thus particles will burst.High moisture will cause slow ignition of pulverized coal and produce long flame.

3.3.5The combustion performance of coal
Coal combustion is an extremely complex process.There is no theory can explain the dynamics of coal powder combustion. Therefore, in the actual work, identify and study specific coal combustion performance. by analogy, experience, experimentation, etc. Although analogy, empirical method is common, but the results are sometimes inaccurate.So now the experimental method is adopted more.Experiment is divided as the dynamic and static simulation experiment. Since the new fuel, large type changes fuel, dynamic simulation test is used more , but the dynamic simulation test method is more complex an heavy workload, thussome static test method is used in practice. Using thermal gravimetric analysis, differential thermal to do reasearch of coal ignition point, combustion rate, combustion completion temperature. By contrast test the performance of different types of coal, pulverized coalignition point and appropriate fineness can be determined, in order to guide the decomposition furnace, kiln burners design. The right is analysis results of anthracite, bituminous coal thermal gravimetric.