Study On Effect Of Additive On Characteristics Of Ash Deposition And Corrosion During Biomass Combustion

Energy and environment are the intensively concerned issues all over the world. Developing the biomass energy becomes one of the current international hot trends. Due to the large amount of alkali metals and chlorine in biomass fuels, which can cause serious problems such as deposition and corrosion of heating surface in the process of boiler combustion, the large-scale use of biomass energy is restricted. Adding the additive can effectively lessen the problems of deposition and corrosion caused by biomass burning. In this paper choosing Al2O3, SiO2, CaO, kaolin and brown coal as the additives, the influence regular that the additive caused to deposition and corrosion in the process of biomass burning were experimentally studied. Meanwhile by means of FactSage software simulating the process of biomass combustion thermodynamic equilibrium, the influences that different additives caused to the distribution patterns of potassium and chlorine were studied.The experiments of ash deposition during the process of biomass combustion were done in the drop-tube furnace. Adding different additives in the corn stalk and collecting deposited ash on the probe after combustion, the morphology, composition and phase of the deposited ash were analyzed and studied by SEM, EDS and XRD testing methods. The result showed that the deposited ash during the process of corn stalk combustion was the alkali metal chloride KCl as the main material with the small particle size, compact structure and strong depository. The more content of KCl, the ash melting point was lower and the ash deposition was more serious. Al2O3could effectively reduce the KC1content of deposited ash. Al2O3could convert KC1to potassium aluminosilicate compounds with SiO2, while adding Si only could lead to more serious sintering. The mechanism of Kaolin to decrease the content of KC1was the same as Al2O3, but adding Kaolin was more effective. The content of alkali metals in deposition wasn’t decreased by adding CaO, and so was the content of deposited ash, while CaO could convert some low melting compounds into high melting compounds to prevent the ash sintering and make the ash loose.The influence laws of different additives on ash corrosion during the corn stalk combustion were studied in a tube furnace. The result showed that the corrosion curve under different additives accorded with parabola law. The corrosion of boiler superheater was mainly ash corrosion, and the contents of KCl in ash were main factors about corrosion. The ash corrosion was greatly lessened by adding Al2O3, which has a better effect, while dramatically accelerated by adding CaO. The micro-analyses were made for the ash simples after corrosion and metal polishing by SEM, EDS and XRD testing methods. The result showed that Fe2O3and Fe3O4were the major components of metal oxide film, the more contents of which, the higher the degree of corrosion, and the pitting corrosion on metal surface was more damage than the uniform corrosion. Whether closing to the metal substrate, the Cl ion always being in metal oxide layer was easier to penetrate the oxide layer to react with metal substrate, so that reducing the content of Cl in ash by adding additive was the most effective way to lessen corrosion.The process of corn stalk combustion was simulated by the Equilib model of FactSage software. Under the effects of different additives, the main distribution forms of K. and Cl under thermodynamic equilibrium, transformation regular with the reaction temperature change, as well as the change regular of contents of KCl and HCl in gaseous phase were discussed and studied, flic result showed that K and Cl in gaseous phase mainly took the form of KCl, and the content of KCl would increase with the increase of reaction temperature. The increasing concentration of gaseous KCl could result in the more KCl to condense on the heating surface when the flue gas flowed through the heating surface, and the content of KCl in deposition was increasing. Therefore, the method of adding additives to reduce the concentration of gaseous KC1could reduce the deposition of KCl, then effectively improved the conditions of deposition and corrosion in furnace. The content of gaseous KCl was decreased by adding Al, and the Cl element separated out in the form of HCl. After adding Al, K element in solid phase was converted from potassium silicate into potassium aluminosilicate. Adding Si could reduce the content of gaseous KCl, but largely increase the content of molten K2Si4O9-When adding Ca element, the gaseous HC1was completely converted, but the content of gaseous KC1was increased slightly. Under high-temperature environment, Ca was tend to combine with Si to form calcium silicate to prevent the appearance of molten potassium silicate. Adding Si and Al at the same time, the inhibitory effect on gaseous KCl was obvious. Especially when the molar ratio of silicon aluminum was1and0.5, the content of KCl would reduce to zero with the increase of the amount of additive, but it wouldn’t when the molar ratio of silicon aluminum was greater than1. When adding Ca, Si and Al at the same time, Ca would combine with Si and Al to form calcium silicate or calcium aluminum silicate, thereby affected Ca on HCl transformation, also affected Si and Al on KCl transformation. Adding S would lead to the production of SOX, but SO2could react with KC1to decrease the content of gaseous KCl. Adding S, Si and Al at the same time, with the increase of reaction temperature, K2SO4was converted into KAlSi2O6and released large amounts of SO2. When the reaction temperature was below950℃and adding S, Ca, Si and Al at the same time, Ca would combine with SO2to form CaSO4, and Si, Al reacted with KCl to generate potassium aluminosilicate, thus the contents of KCl and SOX were better controlled.