| Biomass energy as a clean and renewable one has a great potential for exploration.The promotion and application of biomass power generation technology will become an important support for achieving the "dual carbon" goal.Developing high parameter biomass power generation technology is a necessary way to improve power generation efficiency.However,high temperature corrosion problem of the heating surface is the main limiting factor for improving steam parameters.With the diversification of biomass fuels,the content of alkali metals and S,Cl in the fuels is gradually increasing,which exacerbates the degree of high temperature corrosion,seriously affecting the safe and economic operation of the unit.Therefore,it is of great significance to develop new high temperature corrosion resistant material and corrosion protection technology for the safe and efficient operation of biomass power plants based on the high temperature corrosion environment.This article analyzes the high temperature corrosion of the heating surface of typical biomass boilers,designs and prepares the composition of Ni-based alloy coating,and studies the high temperature characteristics of Ni-based alloy coating.The influence of process parameters on the microstructure and element diffusion of Ni-based alloy coating was studied.The corrosion behavior of Ni-based alloy coating in different proportions and compositions of mixed salts was studied.The influence of Fe and Si on the high temperature oxidation and corrosion resistance of Ni-based alloy coating was systematically explored.The combustion characteristics of typical biomass-fired boilers were analyzed by detecting the composition of fuel and ash.It was found that biomass fuels with high content of alkali metals and high chlorine were the main reasons for slagging and corrosion on heating surface.The analysis results of TP347H superheater tube and 15CrMo water-cooled wall tube operating for 180 days in biomass-fired boiler shows that surface corrosion products with cracks can not provide protection to tube substrate.The corrosion medium penetrates into the substrate through the cracks,causing severe intergranular corrosion and thinning the heating surface tube.Ni-Cr-Mo alloy coating was prepared by laser cladding technology.The effects of laser power and scanning speed on the microstructure,microhardness,and element diffusion of the coating were studied.It was found that with the increase of scanning speed and the decrease of laser power,the grains of coating were refined,the microhardness increased,and the dilution rate significantly decreased.The optimal cladding process parameters were obtained based on comparative experiments,where laser power is 2400W and scanning speed is 100mm/s.The high temperature oxidation and corrosion resistance of Ni-Cr-Mo alloy coating at different temperatures were studied.The high temperature oxidation results indicate that the dense and continuous Cr2O3 film formed on the coating surface at 600-800℃ hinders the internal diffusion of O2,making coating have excellent oxidation resistance.Corrosion products such as Cr2O3 and NiO were generated on coating surface after corrosion in a mixed salt of KCl and K2SO4 at 550℃,which effectively hindered the penetration of corrosion media.When the temperature rises above 600℃ the "activation oxidation" process dominant by KCl causes cracking of coating,and K2SO4 gradually penetrates inward to generate NiS and Ni3S2.The coating corrosion degree increases with the increase of K2SO4 proportion at 600-650℃.Intergranular corrosion occurred in coating after corrosion in a mixed salt of NaCl and KCl at 550℃,and the depth of the Cr-depletion zone reached 110 μm.Some Cr2O3 on coating surface is dissolved at 650℃.The ratio of NaCl to KCl has little effect on the corrosion resistance of coating at 600-650℃.A large number of cracks and localized peeling were observed on TP347H surface corrosion products after corrosion in all mixed salts.The corrosion resistance of Ni-Cr-Mo alloy coating is significantly higher than that of TP347H.The influence mechanism of Fe element dilution rate(the addition Fe content of 0wt.%,3wt.%,6wt.%,10wt.%,respectively)on the microstructure,high temperature oxidation,and high temperature corrosion resistance of Ni-Cr-Mo alloy coating was studied.With the increase of Fe content,the grain size of coating coarsens and Mo is significantly enriched in the network eutectic structure.The Fe2O3 and NiFe2O4 generated during high temperature oxidation introduce voids and cracks at the interface between the oxide film and coating,providing channels for the internal diffusion of corrosive medium.After corrosion in a mixed salt of NaCl and KCl at 650℃ for 168 h,the mass loss of coating F3 is the smallest.Once Fe content exceeds 6 wt.%,excessive Fe2O3 is generated during the corrosion process,which results in an increase of cracks,decreasing the density of corrosion layer,and lowering the corrosion resistance of coating.The high temperature corrosion performance in NaCl and KCl mixed salts of pre-oxidized and non pre-oxidized Ni-Cr-Mo coating was studied at 550℃ and 650 ℃.It was found that continuous and dense Cr2O3 was formed on pre-oxidized coating surface,which improved the corrosion resistance of the cladding layer.However,the non pre-oxidized coating exhibited severe corrosion and surface corrosion products was localized peeling.The effect of Si content(the addition Si content of 0wt.%,1wt.%,2wt.%,3wt.%,respectively)on the microstructure,high temperature oxidation,and high temperature corrosion resistance of Ni-Cr-Mo alloy coating was studied.The results show that the grain size of outer oxide layer decreased and the inner oxide layer became denser as the Si content increased.The formed SiO2 layer blocked the internal diffusion of O2 and the external diffusion of Ni and Cr,reducing the depth of Cr-depletion zone.Coating with addition Si content of 3wt.%has the best oxidation resistance at 600-800℃.Coatings formed SiO2 oxide film to hinder the penetration of external corrosion media,effectively inhibiting the selective dissolution of Cr element;On the other hand,the outward diffusion of Cr was suppressed by promoting the enrichment of Mo element at grain boundaries,improving the density of Cr2O3 layer during the corrosion process at 600-700℃.The newly developed Ni-Cr-Mo-Si corrosion resistant alloy material has been applied in heating surface of medium temperature and sub-high pressure biomass boilers.Ni-Cr-Mo-Si alloy coating has excellent corrosion resistance after operation for 8600 h. |
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