Experimental Study On Characteristics Of Ash Deposition During Co-Combustion Of Biomass And Coal

With the depletion of fossil fuel and the deterioration of environment from that, biomass as a kind of clean and renewable energy has been attached increasing importance to the countries all over the world. Power generation technology of biomass and coal co-combustion in large power plants both make use of the substantial investment and infrastructure in existing coal-fired power plants, and achieve the effective use of biomass energy. It's a bright future. However, large amount of alkali metals and chlorine in biomass fuels may lead to ash melting temperature reduction and serious fouling and slagging in the surface of superheater during co-combustion of biomass and coal, which may endanger the safe operation of boiler and restrict the large-scale use of biomass fuels. Therefore, the study on characteristics of ash deposition during co-combustion of biomass and coal, which is of great significance to guarantee the safe operation of boiler and promote the large-scale use of biomass fuels.In this paper, the ash melting characteristics, ash physical phase, and ash chemical compositions were analyzed to study ash deposition characteristics during co-combustion of biomass and coal.First of all. the ash melting temperature was measured using YX-HRD Ash Melting Analyzer during co-combustion of various biomass and coal. The impact of mixing ratio, alkali metals and alkaline earth metals on the ash melting temperature were discussed. The ash melting point decreased gradually with the increase of mixing ratio of biomass. And also fouling and slagging tendency increased gradually. Since alkali metals oxides have a low melting point, alkaline-earth oxides and SiO2 are apt to form low-melting eutectics, the ash melting temperature decreased with the increase of alkali metals and alkaline earth metals contents, and SiO2/Al2O3 ratio in the ash during co-combustion.Secondly, tube-type furnace was used to study the release characteristics of K and Cl during co-combustion of biomass and coal. The release characteristics of K and Cl are quite different between biomass and coal. The element of K in biomass is easy to release. The releasing amounts of K and Cl are proportional to the total contents of K and Cl in the fuels. The element of K is released in the form of the KCl(g), and Cl element exists as species of KCl(g),HCl(g) and Cl2(g), HCl(g) is the main form. The releasing amounts of K and Cl during co-combustion biomass and coal are more than coal combustion. Relatively, co-combustion helps to reduce the release of K and increase release of Cl. The release characteristics of K and Cl are affected by many factors during co-combustion. The influence factors of mixing ratio, combustion temperature and fuel compositions were discussed. The results show that the releasing amounts of K and Cl both increase with the increase of mixing ratio of biomass. However, the releasing amounts of K and Cl with the mixing ratio of biomass are no-linear relationship. The combustion temperature at 1100℃, the releasing amounts of K less than at 900℃, whereas Cl increases. The releasing amounts of alkali metals depent on fuel compositions. The increase of the CaO contents in mixing fuels can promotes the release of K.The phase compositions of ash from co-combustion of biomass and coal was analyzed in XRD. And MDI jade5.0 software was used to analyze the XRD spectrum. The results show that chemistry reaction happened between the ash of biomass and coal. Potassium aluminosilicate is formed during the reaction of KCl(g) with aluminosilicates. And also KCl(g) can be sulfated by SO2 in the flue gases. CaO and MgO in mixing fuels also can react with aluminosilicates to form CaAl2Si2O8 and Mg3Al2Si3O12. These reactions are primary cause to result in the non-linear changes of the releasing amounts of K and Cl.Finally, the experimental study on characteristics of ash deposition during co-combustion of biomass and coal was made in a drop-tube furnace. Microstructure and phase compositions of ash deposits were analyzed by SEM/EDS, and then discussed the mechanism of ash deposition. The results show that with the increase of the biomass propotions in mixing fuels, there are more obvious conglobation and fusion, and more amounts of K and Cl in ash deposits. These indicate that there would be a more severe fouling and slagging tendency., KC1(S,S2),K2SO4(S,S2)å'ŒK2Si4O(liq,s) are main materials in ash deposits burning wheat-straw and maize-straw. In the fuels combustion process, K and Cl behavior can be divided into two steps:volatilization, and capture and condensation. K is likely to be released as KCl(g). During the gas cooling, part of KCl(g) is condensed in the ash particles surface, and part of KCl(g) is react with SO2 to form K2SO4(g) or K2SO4(s,s2).In addition, KCl(g) may condense directly during the gas cooling. The ash deposits on the probe can be divided into inner and outer layers. KCl(g) and aerosols are condensed in the probe surface to form inner layer which has a great viscosity. The outer layer is formed through the inner layer continuous capturing the fly particles. Because the viscosity of K2SO4 is less than KCl and potassium aluminosilicate has a relatively higher melting point, the formation of potassium aluminosilicate and K2SO4 both can improve the fouling and slagging in the surface of superheater tube.