Experimental Study On Additive Applied To Against Slagging Of Zhundong Coal

As the largest coal reserves area in China,Xinjiang Uygur Autonomous Region has prediction coal reserves of 219 million which accounts for 40%of coal reserves of the whole country,making it one of the major coal production,coal operation and coal electricity-industry bases in China.Zhundong Coal Field in Hui Autonomous Perfecture of Changji has the largest detected coal preserves in Xinjiang,with coal areas of 14 thousand square kilometers,prediction coal reserves of 390 billion tons and detected coal reserves of213.6 billion tons.Zhundong coal has low percentage of ash,pollution elements such as sulfur and medium heating value,making it a kind of high quality coal for power generation.However,Zhundong coal has defects of low ash melting point,serious slagging and contamination,which will lead to serious problems such as water wall slagging,contamination on high temperature convection heating suifaces,serious ash deposotion on medium and low temperature heating suifaces etc.These defects severely influence the security and cost-effectiveness of boiler operation.At present,the boiler parameters of in-service units have been fixed,which are not applicable to the safe burning of Zhundong coal.The goal of 100%Zhundong coal burning cannot be realized even if the burning system are transformed.Therefore,blend Zhundong coal with low-sodium-coal for burnnig is adopted.Recently,since scientific research institutes such as TPRI and many electricity plants have carried out deep optimization on boiler operation mode over the years,the proportion of Zhundong coal in blended-coal burned in existing boilers has increased.However,the proportion of Zhundong coal in blended-coal in the majority of electricity plants only reaches 60%to 70%,with a maximum proportion of about 80%.In the long run,the production and reserves of low-sodium-coal in Zhundong and the surrounding areas is far unable to meet the demand of Zhundong coal blend-burning.Therefore,adding addictives in boilers to reduce boiler slagging and contamination is one of the important supplementary means in resolving Zhundong coal burning problems.The thesis mainly focuses on techniques of Zhundong coal boiler’s adopting addictives to prevent hearth slagging.Firstly,quality characteristics of Zhundong coal and slagging property are analyzed.By conducting Energy Spectrum Analysis and X-ray Analysis on the slag samples produced by buring pure Zhundong coal,it is concluded that severe slagging in the hearth when burning pure Zhundong coal are the results of several elements such as sodium,sulfur,silicon,calcium,aluminum and iron,among which sodium is the main cause.The process of slagging can be divided into three stages,that is,initial slagging stage,aggravated slagging stage,and melted slagging stage.By analysis,the alkali-metal-sulfate-compound with strong-adhesion and fast-adhension-speed which are produced by Na,S and other elements,is the main cause of Zhundong coal initial slagging.The main reason of slagging aggravating and melting is that in the process of burning feldspar minerals（KAl Si₃O₈-Na Al Si₂O₈-Ca Al₂Si₂O₈）are generated,especially albite（Na₂O·Al₂O₃·6Si O₂）which has lower melting temperature than potassium feldspar and rapidly changes with the temperature.The main reason of the generation of melting slag is that in lower temperature,albite can quickly melt the high melting point substances such as quartz,clay and mullite.According to the research results above,the thesis puts forward the technical solution that addictives can be added to change the constituents of coal ash so as to decrease the formation amount of albite and accordingly mitigate the hearth slagging.The concrete measures to change coal ash constituents are decreasing sodium and adjusting the silicon/aluminum ratio.The thesis conducts one-dimensional flame boiler laboratory slagging test to the silicon-aluminum addictives obtained which are in compliance with the requests above.The test result shows that silicon-aluminum addictives have good against-slagging effects when Na₂O in coal ash is lower than 2.6%after adding addictives.When additives added and Na₂O in coal ash is around 3%,the aluminum/silicon ration of ash is between 1.5 to 1.7.Silicon based silicon-aluminum addictives with a alkali/acid ratio between 0.4 to 0.5 have better effects.Addictives with higher percentage of high melting point Al₂O₃ don’t bring the best effects.By analyzing the results of the tests,it is found that,the ratio of amorphous-phase-material without fixed structure in the slag samples decreased;and the ratio of mineral crystals with high melting point increased;therefore,slag samples are were not melting in large amount.This has proved that the reason why addictives take effects is that,they reduce the generation quantity of of alkali metal feldspar minerals.Therefore,the melting ratio of alkali feldspar minerals in ash samples decrease,and consequently the proportion of Albite-dissolve high-melting-point minerals decrease.As a result,there was no amorphous-phase-material without fixed structure generated in large amount.Finally,through the numerical simulation of a boiler burning pure Zhundong Coal,combining with the actual operation of the field,it is concluded that the space between boiler upper layer burner to the SOFA wind region is the easy-slagging-area.Based on this,an additive method is proposed,that is to spray anti-slag additives towards the waterwall surfaces with the help of inside-furnace air at the easy-slagging-area.The thesis adopts a surface-temperature-controllable simulation system of water cooled wall to conduct a practical test,which has verified the effectiveness of the above mentioned spraying-addictives method.In addition,taking efficiency and economy into consideration,the thesis takes part of kaolin and clay minerals in Zhundong and the surrounding areas as the optional addictives,which has better application perspectives and will provide data and technical support for further industrial test and application.