Basic Research On Combustion Characteristics And Ash Behaviors Of High-alkali Zhundong Coal

Xinjiang Zhundong is rich in coal reserves. Zhundong coal is characterized by high sodium and calcium contents, heavy deposition and fouling occur on the heating surface, which will result in serious danger and uneconomical operation of Zhundong coal-fired boilers. Beginning from basic problems, this thesis selected five kinds of typical Zhundong coal to do experimental studies on their coal qualities and combustion characteristics. Results show that Zhundong coal is generally high in moisture, low in ash, medium to high in calorific value, low in nitrogen and sulfur contents, etc; the ash melting point is higher, both of its coal rank and iginition point are between lignite and bituminous coal. With the increase of mining depth, there are less moisture, less ash content in Zhundong coal. Its calorific value increases and sulfur content decreases. The coal quality gets closer to bituminous coal and the slagging tendency mitigates. Ignition point of Zhundong coal slightly decreases while its combustion rate increases, the combustion characteristic gets better.Deep Tianchi Energy coal was selected as research object, and phase changes in high and low temperature ashes were comparatively analyzed. Mineral conversion in heating process was thoroughly and meticulously studied through thermogravimetric experiment of low-temperature ash. Contents of Na and Ca in coal ashes made at different temperatures were measured respectively to observe their release regularities, and precipitating types of Na, Ca were simulated by Factsage. Results show that the original minerals of Zhundong coal mainly include calcium sulfate hydrate, calcite, quartz, kaolinite and pyrite. During the process of combustion, complex chemical reactions occur among these minerals, and they transform to brownmillerite, calcium silicate, et al. Experiment results show that Na in coal begins to precipitate during400-600℃, Ca partially precipitates before600℃and precipitates fastest after1000℃.Combining with simulation results, it’s speculated that Na precipitates in the form of NaCl(g) before1000℃. After1000℃， Na precipitates in the form of NaOH(g) and Na2SO4(g). Eutectics are formed when Ca interacts with iron and silicon containing minerals after1000℃. The eutectics precipitate at molten state, which leads to sharp decrease of Ca.Fouling and slagging characteristics of Zhundong coal in actual industrial production were studied in a power plant burning Zhundong coal. The results show that obvious changes occur with the flow of flue gas. Slag in burner blog is hard and close to vitreous, shining with metallic lustre. Its microstructure presents to be a porous molten block whose surface is dense and smooth. With the flow of flue gas, the temperature of heating surface decreases, fouling condition is alleviated, the size of ash particles decreases, and agglomerate phenomena gradually disappears. Na in flue gas condensates in the vertical flue area, resulting in a large amount of soda-lime feldspar appears in the ash. Slag grows with time, the quantity and thickness have been increased significantly. Main minerals of the initial deposition layer of slag sample are quartz, albite, calciclase and hematite. With the growth of slag, the content of quartz decreases significantly, while the content of alkali metal decreases gradually. The change of SOFA throttle opening will affect the temperature and atmosphere in burning area of the furnace. High temperature and reducing atmosphere can lead to heavy fouling and slagging. With the increase of the blending ratio of ZhunDong coal, temperature and Na content of the furnace increases, resulting in the significant increase of the amount of slag.