Corrosion Characteristic Study On Metallic Heating-surface During Biomass And Coal Co-Combustion

Energy and environment are the two pressing issues recently. With fossil fuel deplenished and environment deteriorated, developing new and clean energy becomes the focus of the attention. With the characteristics of CO2zero emission, abundant resources, renewable and so on, more and more attention has been paid to biomass. However because of the high content of alkali metal, alkaline-earth metal and chlorine elements, which can cause serious problems such as deposition, slag and corrosion, the use of biomass was seriously restricted. Co-firing of biomass and coal can effectively relieve the corrosion problem compared to the pure biomass combustion. In this paper cornstalk, cotton stalk and poplar were co-combusted with lean coal and bituminous coal. The corrosion regulars that blended ash caused to12CrMoVG,15CrMoG and T91was experimental studied. The corrosion mechanism was analyzed and the grey relation analysis was carried out to the corrosion factors. Finally the regression model was established to predict corrosion quantity according to temperature, HC1concentration and the blend proportion.Firstly, experiments were carried out in pipe furnace to investigate the influence regular of temperature, blend proportion, HC1concentration and water vapor. The result showed that the corrosion curve conforms to parabolic regular under any conditions, and with the increase of temperature, corrosion dramatically accelerated, corrosion type also changed from the general corrosion to the complicated corrosion behavior including intergranular corrosion, decarburization corrosion and point corrosion; the effect of blend proportion to the corrosion curve doesn’t conform to the regular of the weighted average. The influence of blend proportion mainly depends on the ash melting temperature difference, sulfur corrosion deriving from coal and chlorine corrosion deriving from biomass; with HC1concentration increasing, the corrosion becomes more and more serious. This is mainly due to the active oxidation of chlorine; the existence of water vapor can relieve corrosion slightly, it is because the presence of water vapor can form loose porous metal oxide film, which will weaken the active oxidation corrosion. Secondly, the microstructure of corroded metal and EDS detection result of corrosion layer were analyzed to explore the co-firing corrosion mechanism. Results showed that the protective metal oxides were formed on the surface by metal elements (mainly is Fe, Cr) and O2, CO2. With the temperature increasing, new phases (CrxCy, FexC etc.)were separated out between the metal grain, leading to some elements dilution in grain boundary area, which caused the formation of corrosion battery between dilution area and grain area, resulting in intergranular corrosion; the low temperature molten salt (KCl-FeCl2, melting temperature were lower than600℃) acted as liquid electrolyte which accelerated the electrochemical corrosion; the oxidation and reduction reaction (REDOX reaction)will take place between metal carbide and corrosive gas, which transferred carbon element to oxidation film, reduced the carbon content in metal, changed the metal structure, resulting in decarburization corrosion. This REDOX reaction makes corrosion stretching along the longitudinal direction; SO2released from coal combustion will react with corrosive material KC1from biomass combustion to release chlorine compounds resulting in active oxidation corrosion.Finally, the correlations of corrosion factors were analyzed with grey relation theory to confirm the main factors. The regression forecasting model was established, found that predict relational formula was a polynomial: y=54.198+0.761x,-115.75x2-0.271x3-0.981x,2+62.38x22+0.052x32This model has high prediction accuracy. It is an easy means to forecast the corrosion of coal blended with biomass combustion, which has instructive significance to corrosion prediction of power station boiler.