| The coal resource of China is abundant, widely distributed and low prices, therefore coal has been widely used as high quality fuel in China’s power production. In the practical coal-burning process in power plant, there will be heavy fouling and slagging on the heating tube surface of the boiler backpass, that is because the presence of alkali substance in the coal. Fouling and slagging on the heat exchange tube can lead to the deterioration of the heat transfer conditions, pipe wall corrosion, and may also burst pipes because of over-temperature and cause unplanned shutdown. Those problems bring a serious threat to the stable operation of the boiler. Zhun Dong coalfield was recently discovered in Xinjiang area. ZD coal is of huge reserves and high quality, but its features of rich calcium and sodium have led to serious fouling problems in the actual combustion process. Those disadvantages greatly limit the mass extensive use of ZD coal. Therefore, studying the deposition characteristics of the coal fly ash is of great significance, because it can ensure stable operation of power generation facilities and the widespread use of Zhun Dong coal.Three kinds of typical coal ash were selected with significantly different properties. Diao Bing Shan coal ash was obtained from external heat exchanger of Diao Bing Shan power plant CFB boiler. Huo Lin He lignite coal fly ash was from Tong Liao power plant. Wu Cai Wan coal was from Zhun Dong area. We focused on the effect of flue gas velocity, the tube surface temperature and ash particle size distribution on the coal ash deposition characteristics, and summarized the basic multi-stages laws of ash deposition formation process. Considering the relationship between the content of alkali metal and the combustion temperature in Zhun Dong coal, we carried out some experiments of the influence of different combustion temperature on coal ash deposition features, and got valuable knowledge and conclusions.Firstly, we completed the visualization of thermal state fly ash deposition test rig, established the basic methods of fouling image analysis, and laid a good equipment basis for the real-time observation of the fouling formation process. Because the environment of the boiler heating surface is of high-temperature and heavily polluted, it is very difficult to observe the mechanism of deposition of ash particles in situation. Obviously, the visualization of reforming test bench is necessary for discovering laws. In this paper, the position of sampling probe and furnace side tube had a reasonable match, making a no-hinder vision pathway. With the help of self-designed high-temperature observation window and auto-focus camera-integrated video sampling system, details of the ash particles deposition process was recorded comprehensively. We used the Image Pro Plus software to measure the thickness of ash deposition layer for the first time, and Image Pro Plus was originally applied on biological analysis. On that basis, we obtain a "deposit layer growth rate curve", which could give a characterization of fly ash deposition.Secondly, the deposition characteristic experiments of DBS and TL coal ash were carried out at different flue gas flow rate and probe surface temperature. We also studied the influence of coal ash particle sizes on the deposition characteristics of DBS ash. It was found that increasing flue gas flow rate would dampen the ash particle deposition, and increase the surface temperature of the probe would accelerate the growth rate of fouling layer. The reason of those phenomenon may be the strong erosion effect of high-speed flue, and ash particles softening on hot surfaces, which enhanced adhesion. In particle size influence experiments, we found that the ash particles of different size had different chemical composition and content in the same ash sample, leading to the differences of deposition characteristics of coarse and fine ash. Fine ash was preferential and more likely to deposit, forming a rough initial deposited layer greatly improved the environment of subsequent deposition of ash particles. The above effect is called “first deposition surface roughness”, and enhances the ash deposition trend.Finally, the deposition characteristic experiments of Wu Cai Wan Zhun Dong coal ash were carried out at different flue gas flow rate, probe surface temperature and combustion temperature. The results show that Zhun Dong coal ash on the hightemperature surface of the probe has a faster deposition layer growth rate, and decreasing the simulated flue gas velocity is conducive to the growth of the deposition layer development. Those conclusions are consistent with the DBS and TL lignite coal ash experimental results, and the analysis of its causes are also similar. The size and composition content of the ash obtained in different combustion temperature were significantly different. Particle size of 800℃ burning ash is much larger than the case of 500℃ and 650℃.The particle size distribution of two kinds of low-temperature ash are similar,and 650℃ burning ash particles are the smallest. The alkali metal content in the ash changed with the combustion temperature. with burning ash temperature increased, Na element content decreased, while the Ca element content increased, changes in Mg, Al, K content is not monotonous. The reason speculated is that Na will evaporate with the form of compounds at high temperatures, and Ca CO3 would decompose to releases CO2 at high temperatures, which causes a change in the relative content of each element. Fouling characteristics test showed that the high temperature ash deposition trend significantly stronger than the two kinds of low temperature ash. Coal ash samples under 500℃ and 650℃ had low fouling tendency, and their deposition characteristic curve are close.Image analysis also found that ash deposits borders remained uneven in the growth process, which was significantly different with the smooth edges of DBS and TL ash deposits layer. The reason speculated is that alkali metal content in Zhun Dong coal ash is very high. Alkali metal would react with other elements to form the low-temperature eutectic, resulting in a lower ash melting point and sintering temperature. Considering the small size ash particles easy to soften and heat tolerance of large ash particles is strong. Therefore area of fine ash relatively concentrated that melts, leading to the ash layer collapses and make deposition outline sunken. However, area of a relatively high share of the coarse ash can maintain the status, and become convex peak. The real-time rough surface also provides great convenience for the subsequent ash particles deposition, so the above effect is called " second deposition surface roughness". |
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