Study On The Effect Of Silicon-based Additives On The Thermal Transformation Of Zhundong Coal Fly Ash Using Differential Scanning Calorimetry

Zhundong coal is a kind of major steam coal resource that newly developed in China,which has advantages of low ash content,high calorific value and flammability.However,its characteristics of low ash melting point and high alkali metal content are prone to causing fouling and slagging on the heating surface of utility boilers,so it is of great value to carry out the research on the prevention and control of the slagging and fouling of Zhundong coal boilers.Blended coal combustion is a common measure to prevent slagging and fouling in boilers,but its effect is not ideal due to the large difference of combustion characteristics between Zhundong coal and the other coal.The usage of additives is an important way to slow down the slagging and fouling of Zhundong coal.In order to effectively use silicon-based additives to prevent and control these problems during the combustion of Zhundong coal,it is necessary to study the thermal combination mechanism between silicon-based additives and fly ash.In this paper,the thermal transformation characteristics of two kinds of Zhundong coal fly ashes were contrastively studied by the method of combining differential scanning calorimetry(DSC)which analyses the change of heat flux during the process of heating and X-ray diffraction(XRD)which studies the phase composition of heating products.The thermal transformation characteristics of Zhundong coal fly ashes mixed with white carbon black(industrial silica)with different mesh or mixing ratio were studied as well based on the above analysis in order to study the reaction mechanism between silica additives and fly ash.In addition,the other silicon-based additives rice husk carbon ashes which have practical application prospect were analyzed to compare with the test results of white carbon black.The results show that the remarkable thermal transformation of aluminum silicates minerals occurred in the high temperature stage after 810℃.The original amorphous aluminum silicates in the two Zhundong coal fly ashes reacted with calcium-containing and sodium-containing materials and formed a large number of new aluminum silicates minerals with the ultimate products of anorthite(CaAl2Si2O8)or anorthite sodian intermediate and the intermediate products of gehlenite(Ca2Al2SiO7)and calcium aluminum silicate(CaAl2SiO6).The formation of these minerals is an important stage to reduce the melting point of Zhundong coal fly ash,it is also a significant reaction to the deposition of fly ash in the flue area at a high temperature.The test results of Zhundong blended coal fly ash No.1 with high residual carbon mixed with silica show that when the mixing ratio of silica to ash is 1:9,neither 100-mesh silica nor 2000-mesh silica could react with the substances in fly ash.When the ratio of silica to fly ash is increased to 1:1,both 100-mesh and 2000-mesh silica have obvious endothermic combined reaction with fly ash samples.The test results of Zhundong blended coal fly ash No.2 with higher calcium content mixed with silica show that as the mixing ratio of silica to ash is 1:9,the 100-mesh silica could hardly combine with fly ash,while the 2000-mesh silica increased the temperature at the bottom of the endothermic valley on the spectrum of fly ash by 45℃,which significantly inhibited the ash fusion.When the mixing ratio of silica to ash increased to 1:1,a large amount of 100-mesh silica didn’t react with fly ash.The 2000-mesh silica and fly ash could more sufficiently combined,but it had no more obvious effect on slowing down the melting of fly ash.The above results show that mesh and mixing ratio of silica and the characteristics of ashes are all important factors on influencing the results of the interaction between silica additives and fly ash.The DSC test results of the fly ash No.2 mixed with rice husk carbon ashes show that the rice husk ashes preheated at 500℃ and 700℃ can all react with the substances in the fly ash with the mixing ratio of 1:1.But the two kinds of rice husk ashes are respectively a little less effective than 100-mesh and 2000-mesh silica additives on slowing down the formation of molten substances in fly ash.