As the volatile content of anthracite is less than 10%, four corner tangentially boiler, W-type flame boiler and CFB boiler in pulverized coal power plant where anthracite are used always faced some problems, such as the difficulty of ignition, low load combustion stability and high carbon content of fly ash, and so on. Nowadays, the most widely used method to realize stable and efficient combustion of anthracite is to improve the combustion temperature, but the problem is high combustion temperature may lead to the generation of a large amount of thermal-NOx. Realizing stable and efficient combustion of anthracite and controlling the formation of NOx are mutual contradictory sides, therefore, to realize the stable, efficient, clean coal combustion becomes a difficult problem in the clean coal combustion field. Based on the preheating strengthening basis theory, the laboratory of CFB first proposed the anthracite combustion system preheated by CFB, which can realize the effective unity of combustion stability of anthracite and control of NOx on the premise of fully guarantee of the stable combustion and hierarchical control of pulverized coal line of NOx formation.
In the anthracite combustion system preheated by CFB, pulverized coal firstly was conveyed into CFB boiler for preheating or internal combustion by wind, and then the preheated high temperature fuel was sent into pulverized coal boiler through the burner. In the process of preheating or internal combustion, pyrolysis, gasification or combustion reactions occured between the pulverized coal and conveying wind. About 5-10% combustion heat was released for the whole preheating of pulverized coal with temperature reaching over 800 ℃. At the same time, the conveying air was transferred into reducing gas composition containing H2, CO in the loop CFB reaction. When the high temperature fuel and the reducing gas contacted air in the burner, combustion reactions would occur as a continuation of the combustion process. Thus, the problem of ignition and combustion stability could be overcome during the combustion of high temperature fuel. In the process of preheating or internal combustion, volatile was precipitated, nitrogen contented in coal was conversed and the pore structure of particles was changed. Preheating before combustion of pulverized coal, that was, cascade combustion process, advantages in the characteristics of stability and high efficiency.
The research team from the laboratory of CFB set up a 30 kW CFB pulverized coal preheating combustion experimental platform, and carried out deep research on the low volatile fuel especially preheating characteristics of anthracite, revealing the particle behavior before and after preheating and gas conversion characteristics. Their work provided physical basis for the theory that anthracite coal preheated by CFB can strengthen the ignition and combustion stability. The preheating combustion test of Yangquan anthracite at the volatile content of 6.7% showed that, the fuel preheated by CFB reached 850 ℃, and volume of fuel particles decreased significantly after preheating with the inner hole surface area and total pore volume increasing significantly. The CO, H2 content were about 7.3% and 8.4%, respectively, calorific value of gas reached 800 kcal/Nm3 (including sensible heat). In the preheating process of pulverized coal, part of nitrogen was precipitated, and conversed to N2, NH3 and HCN in the strong reducing atmosphere, which provided conditions to reduce NOx emissions. The results showed that, through preheated by CFB, anthracite with higher preheat temperature, smaller particle size and larger specific surface area could be combusted more efficiently and stable. At the same time, the conversion of coal nitrogen to nitrogen during the preheating provided control approach for low nitrogen oxide emissions.
On the experimental platform, researches on the combustion characteristics of low volatile especially anthracite were also carried out as well as the formation characteristics of nitrogen oxide. The combustion process and combustion mode for pulverized coal in combustion chamber were deeply studied. Besides, researchers also explored the effect of combustion air distribution and temperature parameters on the combustion of preheated fuel, transformation of coal nitrogen to nitrogen oxides, and transformation way of preheated coal nitrogen of anthracite in CFB, revealing the main reasons for control of nitrogen oxide emissions in the system. Research showed that, when the high temperature fuel went into the combustion chamber, the rising rate of temperature was much higher than that of heating rate of conventional combustion. Preheating in CFB has been part of, or early internal combustion, and the combustion of high temperature fuel in the chamber was the continuing process. As pulverized coal combustion rate was high and stable, the air staging combustion of pulverized coal could control release rate of energy and transformation route of coal nitrogen, thus, pulverized coal combustion chamber had a uniform temperature distribution. Combustion efficiency of anthracite reached 96%, and NO emission value reached 259 mg/Nm3. Compared to anthracite without preheated, emission is higher than 800 mg/Nm3, and the combustion efficiency is about 90%.
The research was funded by the national natural science fund project, with related research results published in some academic journals, such as "FUEL", "JOURNAL OF THERMAL SCIENCES" and "ENERGY AND FUELS".
On the preheating research, combustion characteristics of pulverized coal and emission characteristics of NOx, research team from laboratory of CFB will continue to widen and deepen their researches of preheating combustion of pulverized coal, in order to form a new technology research direction, develop new types of high efficient combustion technology for pulverized coal and make contributions to the development of energy-saving and emission reducing coal combustion technology.
