Experimental Study On The Pore Evolution Of Ash Sintering Process And The Thermal Conductivity Of Fly Ash Aggregate

Due to the traditional energy structure,fossil fuel depletion and global warming are the long-term challenges for China.Co-combustion of coal and biomass is an effective way to make use of biomass energy for alleviating the shortage of fossil energy.However,the problem of ash sintering in biomass combustion restricts its development.The pore structure in the ash sintering process is the main reason to affect heat transfer and reduce efficiency.Therefore,the pore evolution in the ash sintering process of coal and biomass was studied.The previous methods to study the pore structure of ash sintering have their defects in the accuracy,intuition,or research scope.In this paper,X-ray computed microtomography technology was applied for the first time to study the pore structure evolution in the sintering process of coal and biomass blended ash,and the pore parameters were calculated.Additionally,the genesis of pores was explored by mineral analysis.Besides,fly ash,the main solid waste of power plants,is harmful to the environment and human health.The preparation of sintered fly ash aggregate is an important way of resource utilization for fly ash.Fly ash aggregate can be used as thermal insulation materials in the tank foundation of the tower solar power plant.Based on XCT technology,the pore structure was analyzed while the thermal conductivity was predicted.In addition,the influence of molten salt leakage on the thermal conductivity of foundation materials was also researched,which was of great significance to the structural design of storage tanks and the safety of power plants.The specific research contents are as follows.Firstly,in the sintering experiment system with a CCD monitoring system,the sintering and melting characteristics of coal and biomass ash were preliminarily studied to guide the selection of subsequent experimental temperature.The results showed that co-firing corn straw ash promoted the sintering and melting of coal ash.Although the fusion characteristic temperature of walnut shell ash was low,the blending of walnut shell ash increased the fusion temperature of coal ash.What's more,the sample had an obvious expansion phenomenon.Secondly,based on XCT technology,the pore structure evolution of Zhundong coal and corn straw ash during the sintering process was studied.The porosity,pore number,and pore diameter distribution were calculated and counted.The mechanism of pore formation was explored.The condensed matter in the pores was mainly composed of Na Cl,Ca SO₄,and KCl,which indicated that Cl made alkali metals,especially K,easier to convert into the gas phase while S tended to react with Ca to form calcium sulfate.Thirdly,the law of pore evolution in the sintering process of coal and walnut shell ash was obtained.After adding a small amount of walnut shell ash,the pore evolution process changed greatly.The porosity and pore volume increased greatly.Besides,walnut shell ash inhibited the sintering and melting of coal ash,and the sintered samples became loose and porous with low sintering strength,which is conducive to the actual power plant soot blowing and slag removal.To verify the effect of Ca on the walnut shell co-firing process,the effect of Ca O additive on pore structure evolution of coal ash sintering was explored.The results showed that the addition of 5%Ca O promoted the sintering of coal ash while the addition of 10%Ca O inhibited the sintering process.Fourth,based on XCT technology,the thermal physical properties of stacked sintered ultra-light fly ash aggregate were explored.The effective thermal conductivity of accumulated fly ash aggregate changed from 0.097 to 0.113 W/(m K).The accumulated fly ash aggregate has excellent thermal insulation performance,low permeability,and enough compressive strength.Its properties are related to particle size distribution and porosity.In addition,the properties of the foundation material after the molten salt leakage were studied.It is found that the molten salt leakage will increase the thermal conductivity of the foundation material to 4?5 times,and endanger the safety of the storage tank.Fifthly,based on the principle of measuring thermal conductivity by hot-wire method,the effective thermal conductivity of a large amount of aggregate accumulated in the molten salt tank foundation was measured by using a self-made hot wire experimental platform.The thermal conductivity of bulk fly ash aggregate is linear with bulk density and porosity.At the same time,the accuracy and reliability of the XCT simulation method for predicting effective thermal conductivity were verified by using the law.