| The Zhundong coalfiled in Xinjiang provience is the largerst integrated coalfield in China.The development and utilization of Zhudong coal has been promoted to the national energy strategy.However,due to the high content of Na,Ca,Mg and Fe in the coal,severe slagging and fouling problems have occurred in the boilers when firing Zhundong coal.Which has been a big challenge needed to be solved.In this thesis,foundamental researches on the ash formation,deposition of Zhundong coal and prevention methods were carried out.Firstly,the modes of occurrence of inorganics in four typical Zhundong coals were indentified.Besides,the dispersion state of Ca and Mg in the coal and the microstructure of Zhundong coal were researched.The results showed that sodium was mainly presented as water solube form in Zhundong coals,accounting for 76-90%of total sodium in the coal,which can be either Na2SO4,such as in Wucaiwan coal and Tianchi coal,or NaCl,such as in Wanxiang coal.Ca and Mg were mainly presented as ammonium acetate solube and hydrochloric acid soluble forms,which were highly dispersed in the coal.Fe was maily in the forms of hydrochloric acid soluble and hydrochloric acid insoluble,and the later form was higher.Secondly,the evolution process of inorganic elements from Zhundong coal to the ash were studied in a drop tube furnace at 1300℃,with the use of a sampling probe that can move to different locations of the furnace.The results showed that,in the early combustion stage,organically bond Ca and Mg were oxided to fine grains on the char surface.These fine grains coalesecensed together and formed larger Ca and Mg rich grains as the carbon was consumed.Some of them reacted with the clays in the coal and formed melted spherical particls.In the burnout stage,the char structure would breakup,which resulted in the formation of large number of fine ashes（<10μm）.Partial Na was transformed as Na2SO4and/or NaCl and into PM1.The remaining Na would be fixed by clays in the coarse particles.Thirdly,the fine ash formation during Zhundong coal combustion was investigated,where the impacts of combustion temperature,modes of occurrence of inorganics and coal types were studied.Meanwhile,the bulk ash formation and ash deposition were also investigated.The results showed that the fine ash formation was greatly influenced by temperature.For Wucaiwan coal,the yield and peak size of PM1 were the largest at 900℃.As the temperature increased,both the yield and peak size of PM1 were significantly decreased,which was artributed to the strong influence of temperature on the transformation of Na and S.The yield of PM1-10 decreased as the combustion temperature increased,and the size became bigger.This was due to that high temperature inhibtted the formation of CaSO4and promted the aggregation of fine ash on the char surface.It was found that the PM1-10 in the Wucaiwan coal was mainly formed by the organically bond Ca and Mg.While water soluble Na contributed significantly to the formation of PM1.The Fe in PM1-10 was maily originated from the hydrochloric aid solube Fe.It was also found that the fine ash formation was varied with coal types.On the same coal mass basis,the amount of PM1-10 produced was positive correlation with the ash content in the coal.While on the ash mass basis,the yield of PM1-10 from Wucaiwan coal was the highest,followed by Wanxiang coal,and those from Tianchi coal and Xiheishan coal were much lower.It was because that both Wucaiwan coal and Wanxiang coal were rich of ammonium acetate solube and hydrochloric acid soluble Ca and Mg.The yield of PM1 from Wangxiang coal was the highest,due to its highest content of Na and Cl in the coal.The ash deposition results showed that,at the same coal input,the deposit mass was positive correlation with the ash content,which was in the decreased order of Xiheishan coal,Tianchi coal,Wanxiang coal and Wucaiwan coal.Their deposition propensity were 15.7%,13.3%,13.0%and 10.1%,respectively.While it should be noted that,the deposits from Xiheishan coal and Tianchi coal were loose and could be easily removed.By contrast,the deposit from Wanxiang coal was rich of NaCl,which could accelerate the ash deposition rate.In addition,serious corrosion was observed at the surface of the deposition probe.While for Wucaiwan coal,there was obvious inner layer and outside layer structure in the deposit.The inner layer was composed of fine ashes rich of CaO,SO3,MgO and Na2O.The outside layer was composed of large particles,such as quartz,CaSO4,Ca-containing alunimosilicates,and some Ca/Mg riched sintered particles.Lastly,the effect of kaolin addition and water washing pretreatment on the ash formation and deposition of Zhundong coal was also investigated.It was found that,apart from the capture of volatile Na,kaolin can also effectively capture refractory Ca,Mg and Fe from Zhundong coal,which resulted in the changes of ash melting points and the fine ash formation properties.For Wucaiwan coal,the ash fusion point decreased firstly with the increase of kaolin dosage,then it increased with the further increase of kaolin dosage.When the kaolin dosage was 4%,where the mole ratio of（Na2O+1/2CaO+2/3MgO）/（Al2O3·2SiO2）was about1,the fusion temperature was the lowest.The fine ash and deposition results showed that,the effect was different when kaolin was added to different Zhundong coals.For Wanxiang coal,the addition of kaolin had little impact on the PM1 formation,and the deposit mass even increased.The reason may be that the high Cl content in Wanxiang coal inhibited the capture of Na by kaolin.For Wucaiwan coal,the ash deposition was mitigated after kaolin addition,and the dosage of 2%was the best.For Tianchi coal,the fine ash yield was little affected and the deposit mass was slighty decreased.While for Xiheishan coal,the yield of PM1-10 was significantly decreased after kaolin addition,and the deposit mass was also decreased.The water washing pretreatment can effectively remove the water soluble Na,S and Cl,and can partitially remove Ca,Mg and Fe from the coal.While the fine ash and deposition results showed that water washing pretreatment was not always effective for each coal.The PM1 yield was greatly reduced for Wanxiang coal and Wucaiwan coal,and the PM1-10 was reduced for Xiheishan coal after water washing.And the ash deposition for these three coals were all mitigated.However,for Tianchi coal,the fine ash formation and ash deposition was little affected by water washing pretreatment.The main reason was that iron in the coal was seldom affted by water washing pretreatment. |
