High Temperature Corrosion Mechanism And Protection On Heating Surface Of Straw Fuel Boiler

It is of great significance to utilize biomass resource in a large scale for substitutingthe fossil fuel and solving the environmental problems.Crop straw is one of the mostimportant parts of biomass resource in China.Direct combustion of straw is suit forutilizing straw in a large scale.However,during direct combustion of straw,its high contentof chlorine and alkali metal causes many problems,such as deposition of the heatingsurface and high temperature corrosion. All these problems must be faced when peoplewant to develop the straw fuel boiler energy utility efficiency with the developedtemperature and pressure of main steam.Therefore, it's necessary to research themechanism and protection technique of the deposition corrosion of the heating surface.Firstly, this paper analyzes the quantity and distribution of straw resource. It givespriority to the straw of maize, rice and wheat, in which the maize straw is the largest innumber and distributes mainly in the north of China. It is followed by rice and wheatstraws which concentrate mainly in the south.This paper also discusses the inspectionstandard and analysis method of biomass fuel, and compares fuel properties of the strawand coal in China. The analysis discovers that CET/TS and ASTM are not suitable forinspection analysis of agriculture resources in our country, so inspection standard andanalysis method of biomass fuel urgently needs to be developed in our country.Thecharacteristics of the biomass straw in our country lie in high chlorine,high potassium,highsulphur and low sulphur. So its combustion products are mainly alkali chlorine.However,coals in our country contain high sulphur, high sulphur,high potassium and low chlorine.Itscombustion products are mainly alkali sulphur.According to the characteristics of the biomass straw,this paper develops a numericalmodel of travelling grate boiler combustion of the straw fuel, based on the straw fuelanalysis and primary air as the input conditions. The model of integrated travelling grateboiler combustion comprises bed model, furnace model and their coupling model. The simulation analysis focuses on its thermodynamic state and temperature field in the furnaceof boiler. Compare of composion test results of deposition corrosion of the heating surfacein pure straw burning boiler, this paper establishes corrosion sedimentary environment ofthe straw fuel boiler, the corrosive compound is made of KCl,NaCl,K₂SO4and Na2SO4,mass ratio of KCl: NaCl: K₂SO4: Na2SO4is 63:13:5:1, and corrosion reaction temperatureis 500℃, 550℃, 600℃and 650℃. In order to explore depositon corrosion process andcorrosion mechanism of heating surface in straw boiler,the experiments of hightemperature corrosion are conducted to simulate deposition condition in biomass boilerheating surface for four kinds of materials: 1Cr19Ni11Nb, 10Cr9Mo1VNb,12Cr1MoVGand 20G. According to the biomass straw boiler heating surface deposion corrosionenvironment and physical and chemical properties of elements,Ni,Cr,Mo,Al and rare-earchmetal elements fine powders are used as corrosion pretection matericals.The Ni-Crprotection coatings are prepared with arc spraying to research its corrosion resistance indeposition condition in biomass boiler heating surface.The results show that corrosion resistance decreases as temperature rises andcorrosion resistance of four kinds of materials is 1Cr19Ni11Nb＞10Cr9Mo1VNb＞12Cr1MoVG＞20G.According to thermal analysis and kinetic theory,it sets the model ofdynamics analysis for corrosion process.The thermal analysisα-t curves are plotted byusing the mass gain curves.The D4 corrosion reaction type is plotted by using G(α)-t linearfitting curves and fitting error. The corrosion mechanism function isG (α)=1-2/3α- (1-α)^2/3.Under simulation test of deposition corrosion of heatingsurface in straw boiler conditons, these kinds of 1Cr19Ni11Nb,10Cr9Mo1VNb,20G and12Cr1MoVG materials activation energy E and pre-exponential factor A in the reactionhave been calculated,which are 3632.88J/mol,1421.92J/mol,635.30J/mol,1027.70J/moland0.026181558s-1,0.019152922s-1,0.017641521s-1,0.018077269s-1.The metal elements areanalyzed after the experiment using SEM-EDS method to detect the appearance,theelement contents and the compositions of the corrosion products. The results show that thecorrosion products are mainly Fe oxide,Cr oxide and Ni oxide.The results of arc spraying protection coatings corrosion test show that two kinds ofNi-Cr coatings corrosion resistance decreases as temperature rises.The anti-corrosionabilities of two coatings are better than body metal. And the anti-corrosion abilities of 1#coating are better than 2# coating. The D4 corrosion reaction type are plotted by usingthermal analysis and kinetic theory. The function of corrosion mechanism is G (α)=1- 2/3α- (1-α)^2/3.The kinds of materials activation energy E andpre-exponential factor A in the reaction have been calculated,which are 4383.19J/mol,4284.97J/mol and 0.022393154s^-1,0.020931502s^-1.The activation energy value shows twokinds of protection coatings corrosion ability and mass gain curves are in agreement. Theelements of protection coatings are analyzed after the experiment by using Opticalmicroscope and SEM-EDS method to detect the appearance,the element contents and thecompositions of the corrosion products.The results show that the anti-corrosion resistanceof high Ni and Cr metals is better than that of common metal materials. Although thereaction activity of Cr and Cl is large,Cr still has some of the anti-corrosion resistance.And Ni's corrosion resistance is more excellent than Cr in the chlorine corrosionenvironment.In conclusion, according to the characteristics of the biomass straw fuel,the corrosionmechanism and protection technology of deposition of the straw boiler heating surfaceneed take chloride and sulfide corrosion of alkali into consideration, given priority tochlorinated corrosion incorporated with sulfide corrosion.