Study On Adhesion And Control Mechanism Of Zhundong High-Calcium Coal Ash

There are abundant coal reserves in Zhundong region of Xinjiang Province.The Zhundong coal mined there has the characteristics of high calorific value,high moisture volatile content and low ash.Zhundong coal is a good power coal,which is of great significance to alleviate the domestic energy shortage.The operation practice shows that serious ash deposition occurs in the furnace of the boiler burning Zhundong coal,which leads to the blockage of large-scale use of Zhundong coal and a great waste of resources.In order to promote the sustainable development of energy in China,it is urgent to break the technical bottleneck of 100%burning Zhundong coal.At present,many researches focus on the mechanism of ash deposition caused by the release and condensation of alkali metals and alkaline earth metals in the coal,but there are few reports on the correlation of ash adhesion strength,which also affects ash deposition.Therefore,it is necessary to deeply reveal the adhesion mechanism and control method of Zhundong coal ash,supplement the research on coal ash deposition mechanism,and provide theoretical basis for the safe utilization of Zhundong coal.This paper innovatively adopts the research method combining experiment and molecular dynamics simulation to explore the ash adhesion mechanism under different parameters from the macro and micro scales for solving the ash adhesion deposition of Zhundong coal.The main conclusions are as follows.A self-designed high temperature ash adhesion strength tester was used to analyze Wucaiwan coal,a Zhundong high-calcium coal.And it was found that Wucaiwan coal ash showed adhesion strength to metal surface at 800℃,and the adhesion strength increased with the increase of temperature.From 800℃to 1000℃,the adhesion strength of Wucaiwan coal ash gradually increased from 1.858k Pa to 21.725k Pa.Above 1000℃,the adhesion strength increased rapidly,and reached 184.933k Pa at 1200℃.Comparing with Hongliu coal,it was found that the early appearance and rapid increase of Wucaiwan coal ash adhesion strength were important reasons for the serious ash deposition in the furnace.The main reason for the increase of adhesion strength was the formation of liquid substances caused by the melting of minerals in Wucaiwan coal ash.The melting of sulfates,especially Na₂SO₄ and Ca SO₄,was the main reason for the appearance and growth of adhesion strength of Wucaiwan coal ash below 1000℃.When the temperature exceeded 1000℃,the melting of feldspar minerals led to the continuous increase of ash adhesion strength.Meanwhile,the content of calcium oxide had a significant impact on the ash adhesion strength of Wucaiwan coal,because calcium oxide promoted the low temperature co-melting of minerals such as gehlenite.Therefore,the high calcium content is an important reason for the serious deposition of Zhundong high-calcium coal ash.The adsorption and diffusion characteristics of main molten minerals on the iron oxide surface were studied by molecular dynamics to reveal the microscopic ash adhesion mechanism of Zhundong coal.The main effect of Na₂SO₄ and Ca SO₄ onα-Fe₂O₃（110）surface was chemical adsorption,which was an important reason for the appearance of ash adhesion strength.The peaks of adsorption and binding capacity of Na₂SO₄ and Ca SO₄ to iron oxide surface appeared at 900℃and 1100℃respectively within the calculated temperature range.The adsorption capacity of Ca SO₄ to iron oxide surface was stronger than that of Na₂SO₄.The main effect of Ca₂Al₂Si O₇ and Ca₂Mg Si₂O₇ onα-Fe₂O₃（110）surface was physical adsorption,and the binding capacity peaked at 1200℃and 1100℃,respectively.The adsorption capacity of Ca₂Al₂Si O₇ and Ca₂Mg Si₂O₇ on iron oxide surface was similar,but stronger than that of Na₂SO₄ and Ca SO₄.Compared with Ca₂Al₂Si O₇ and Ca₂Mg Si₂O₇,Na₂SO₄ and Ca SO₄ had more active molecular motion on the iron oxide surface,and were more likely to form chemical bonds withα-Fe₂O₃（110）surface.Although feldspar minerals had weaker diffusion capacity than sulfates,they had obvious advantages in the content of liquid and adsorption binding capacity.The increase of temperature promoted the conversion of Ca SO₄ to Ca₂Al₂Si O₇ and Ca₂Mg Si₂O₇ with stronger adsorption capacity onα-Fe₂O₃（110）surface.The ash adhesion strength of Zhundong coal at 1000～1200℃increased rapidly under the double action of liquid phase content increase and adsorption capacity increase.The inhibition effect and mechanism of additives on Zhundong coal ash adhesion strength were studied,and it was found that kaolin,vermiculite and struvite as additives had obvious anti-Zhundong coal ash adhesion effect.Kaolin had stable inhibition of ash adhesion strength growth at 800～1200℃,and can effectively reduce the adhesion strength of Zhundong coal ash by 55.9%at 1200℃.Vermiculite had excellent adhesion resistance below 1150℃.Vermiculite was an excellent anti-ash additive for boilers with combustion temperature below 1000℃,such as circulating fluidized bed boilers.At800～1000℃,struvite had no good inhibiting effect on Zhundong coal ash,but struvite can effectively reduce the ash adhesion strength of Zhundong coal at temperatures above1000℃.Kaolin and vermiculite can convert sodium compounds in ash from sodium sulfates to nepheline and albite with high melting point,inhibit the increase of ash adhesion strength caused by sulfate melting,and improve the appearance temperature of ash adhesion strength.At 1200℃,the addition of kaolin and struvite can increase the content of high melting point minerals such as corundum and forsterite in the ash,inhibit the increase of liquid content,block the adhesion of liquid,and reduce the adhesion strength of Zhundong coal ash.The effect and mechanism of Ni and Cr coatings on the adhesion properties of Zhundong coal ash were analyzed by experiments and calculations.In the ash deposition experiment,Ni and Cr coatings reduced the ash mass of the deposited pipe by 49.8%and37.3%,respectively.Compared with austenitic steel surface,nickel and chromium coating can improve the appearance temperature of ash adhesion strength from 800℃to 1000℃,which can alleviate the problem of premature ash adhesion strength.At 1100～1200℃,Ni and Cr coatings can significantly reduce the adhesion strength between coal ash and austenitic steel surface.The wettability of Zhundong coal ash on three kinds of metal surfaces from strong to weak was as follows:austenitic steel sheet,chromium sheet and nickel sheet.According to the results of molecular dynamics calculations,the austenitic steel surface had stronger adsorption capacity to molten minerals than nickel and chromium coating.In particular,the nickel oxide surface formed by nickel coating was more stable than the chromium oxide surface formed by chromium coating,and had lower adsorption capacity of molten minerals,revealing the reason why the adhesion strength between nickel coating and Zhundong coal ash was lower than that of chromium coating.