Effects Of Torrefaction On The Behaviors Of PM₁₀ Formation,Ash Fouling And Corrosion From Biomass Combustion

Biomass energy has the advantages of renewable and carbon neutral.Therefore,its utilization can effectively cope with the pressure of CO₂ emission reduction.However,the disadvantages of biomass,such as high water content,poor hydrophobicity and low energy density,hinder its large-scale utilization.Torrefaction pretreatment technology can improve the disadvantages of biomass and has the advantage of easy operation,which makes it a kind of biomass pretreatment technology with development potential.On the other hand,the ash related problems(PM₁₀ generation,fouling,slagging,corrosion,etc.)from the combustion of biomass are common and need to be solved.However,the research on these problems is still relatively scarce.In this paper,the generation of PM₁₀ and ash fouling,corrosion behaviors from the combustion of torrefied biomass are systematically studied.Firstly,the release behavior of Cl and S and the migration and transformation behavior of the main inorganic mineral elements（Na,Mg,K,Ca,Fe）in bagasse,wheat straw and rice straw during the torrefaction process were studied.The contents of different states of Na,Mg,K,Ca,Fe in raw and torrefied biomass were quantitatively analyzed by step extraction,microwave digestion and ICP-OES.The results showed that in the torrefaction process,Cl and S were released obviously,while K and Na were slightly released.The release degree varied with samples.At the same time,Na,Mg,K and Ca migrated from higher reactivity states to lower reactivity states during torrefaction process.Secondly,the formation characteristics of PM₁₀ from torrefied biomass combustion were explored.The PM₁₀ formation characteristics of raw and torrefied biomass were studied at 1300℃,and then the effects of combustion temperature on PM₁₀ formation characteristics of raw and torrefied rice straw were explored at 900℃,1100℃and 1300℃.The results showed that compared with raw biomass,the increase of the content of low reactive alkali metal and the decrease of Cl and S content in torrefied biomass resulted in the decrease of gasification degree of alkali metal compounds during the combustion process,which eventually led to the decrease of the yields of PM₁.At the same time,torrefaction promoted the migration of alkali metals to coarse ash particles in the process of combustion,which led to the increase of the melting degree of coarse ash particles,and then the agglomeration degree between coarse ash particles was enhanced,which eventually led to more conversion of PM_1-10 to PM₁₀₊in combustion process.For different combustion temperatures,the yields of PM₁ from the combustion of raw and torrefied rice straw decreased first and then increased with the increase of temperature,and the amount of PM_1-10 decreased with the increase of temperature.In the temperature range selected in this paper,the yields of PM₁ and PM_1-10from torrefied rice straw combustion were lower than that from raw rice straw combustion in the same temperature.At 1100℃and 1300℃,the reduction of Cl in torrefied rice straw had obvious effect on the reduction of PM₁,but at 900℃,this effect was weaker,and the reduction of PM_1-10 was relatively low.In general,for different biomasses and different combustion temperatures,torrefaction is beneficial to reduce the production of PM₁₀.Finally,the ash fouling and corrosion behaviors from torrefied biomass combustion were studied by using the coupling system of drop tube furnace and deposition furnace to simulate the actual combustion process of the fuel,combined with SEM-EDS and other testing methods.The results showed that,compared with raw biomass,the reduction of gasification degree of alkali metal compounds in the combustion process of torrefied biomass led to the reduction of fine particles with strong adhesion,and the adhesion degree to the subsequent large particles.Because of this,the fouling rate of the inner layer from the combustion of torrefied biomass was reduced.However,the fouling rate of the outer layer from the combustion of torrefied biomass was increased.This was due to the increase of alkali metal in coarse ash particles from the combustion of torrefied biomass.Because of this,the melting and adhesion degree of coarse ash particles were enhanced.When these particles collided with the sampling tube,more kinetic energy was absorbed.This resulted that these particles were easier to remain on the sampling tube.On the other hand,the research on the corrosion behavior from the combustion of raw and torrefied rice straw showed that the water-soluble chloride and sulfate of Na and K in the fouling ash played an important role in the corrosion process.Therefore,compared with raw rice straw,the reduction of water-soluble Na,K,Cl and S in the fouling ash from the combustion of torrefied rice straw led to the reduction of the corrosion degree of ash deposition during combustion.