Ash Formation And Deposition During Co-combustion Of Torrefied Biomass And Coal

Torrefaction is able to effectively improve the physical and chemical properties of biomass,benefiting the utilization of biomass combustion or co-combustion for power on a large scale.Compared to the biomass-coal combustion,torrefied biomass-coal combustion has better technical,environmental and economic advantages.It is one of the most important choices to cope with the environmental pollution and CO₂ emission,and has been concerned worldwide.After torrefaction,the changes of properties,especially the inorganic elemental characteristics of biomass can influence the ash formation and deposition during biomass combustion and co-combustion.The ash-related issues during combustion are the key scientific issues to be solved.However,the present investigations mainly focus on the raw biomass-coal co-combustion,yet are lack of comprehensive analysis related to ash formation and deposition during biomass combustion and co-combustion.Aiming at the existing gaps,this work fouced on the effects of torrefaction on the transformation of the key ash-forming elements in the biomass,and revealed the rules of ash formation and deposition during co-combustion of torreffied biomass and coal.Firstly,aiming at the shortage of understanding on the transformation of the key ash-forming elements in biomass during torrefaction,this work investigated the effect of torrefaction on the release of Cl,S and the transformation of alkali and alkaline earth metals(AAEM),Fe in three biomasses(rice husk,corn stalk and saw dust),by means or instruments of micro-wave digestion,chemical fractionation,ion chromatography(IC)and inductively coupled plasma-mass spectrometry(ICP-MS).The results showed that about30.2-58.6%of S and 5.0-8.1%of Cl were released from the biomasses after torrefaction.Na and K were released about 2.8-7.9%and 5.0-8.1%respectively,mainly in the form of water-soluble species.Torrefaction evidently reduced the contents of water-soluble and ion-exchangeable AAEM which are reactive and easy-vaporized during combustion in the biomasses,while had negligible effect on the content and occurrence modes of Fe.Secondly,considering the fine ash particles have significant effects on ash fouling and slagging,the devices such as drop-tube furnace,low pressure impactor(LPI),scanning electron microscope equipped with energy dispersive spectrometer(SEM-EDS)were applied to investigate the formation of particulate matters during combustion of biomasses and co-combustion with high Ca coal(C1)and high Si,Al coal(C2).The results showed that the main interactions between biomass and coal were capture of AAEM by particles rich in Si,Al and competitive reactions between alkalis and Mg?Ca?Fe.For co-combustion of C1 coal and rice husk,C1coal and saw dust,C2 coal and corn stalk,the interactions resulted in promoted transformation of PM₁ to PM₁₊.For co-combustion of C1coal and corn stalk,the interactions resulted in more formation of PM₁.Viewed from the emission of particulate matters,Si,Al-rich coal is properer to be co-fired with biomass rich in K and Cl(e.g.corn stalk)than the Ca-rich coal.After torrefaction,the alkalis,Cl,S together with the reactive-forming Ca and Mg in the biomasses were reduced.It resulted in less vaporization of AAEM,Cl and S during combustion and further decreased the formation of PM₁.Meanwhile,the less vaporization of AAEM promoted the agglomeration of ash particles,resulting in transformation of the finer particles to the coarser ones.For co-combustion of C1 coal and rice husk,C1 coal and corn stalk,torrefaction increased the formation of PM_1-10 consequently,while for co-combustion of C1 coal and saw dust,C2 coal and corn stalk,torrefaction increased the formation of PM₁₀₊.Thirdly,via the combined system of drop-tube furnace and fouling experimental systems,SEM-EDS,IC etc.,the fouling characteristics of ash during biomass combustion and co-combustion at simulated convection zone were investigated.According to the fouling mechanisms,the deposits were divided into inside deposits which were difficult to be removed and outside deposits which were easy to shed,and investigated respectively.The results showed that co-combustion effectively inhibited the formation of inside deposits,mainly resulting from the interactions which reduced the vaporization of alkalis or alkaline earth metals.Besides,the considerable amount of S introduced from coal promoted the sulfation of chlorides,resulting in less condensation of AAEM-compounds on the tubes or coarse particles.However,for co-combustion of C2 coal and corn stalk,because the formation of sticky alkaline aluminosilicates was promoted,which further resulted in more outside deposits.Therefore,compared to the C2 coal rich in Si and Al,co-firing Ca-rich coal with corn stalk rich in K and Cl was more effective for controlling ash fouling.Torrefaction reduced the alkalis,Cl,S and reactive-forming Ca and Mg in the biomasses,which further decreased the condensation of vaporized chlorides and sulfates on the tubes or coarse particles.As a result,the fouling propensities of biomass combustion and co-combustion were decreased.Lastly,based on the drop-tube furnace and mechanism model of inertia impaction,the growth of slagging during biomass combustion and co-combustion at simulated radiation zone were investigated via laser particle size analyzer,SEM-EDS and computer controlled scanning electron microscope(CCSEM).The results showed that the interactions between biomasses and coal reduced the content of minerals with high adhesion such as multiple silicates,aluminosilicates,Si-rich particles and so on in the bulk ash.It weakened the ash agglomeration and adhering deposition,further reduced the slagging deposits.Because the coarse particles rich in Si and Al from C2 coal captured the vaporized K from corn stalk,the formation of sticky alkaline aluminosilicates were increased,aggravating the slagging propensity of the bulk ash.Compared to the coal rich in Si,Al and had larger particle size of bulk ash,the Ca-rich coal with smaller particle size of bulk ash was more favorable to co-fired with biomass for controlling ash slagging.After torrefaction,the reduction of Cl,S and reactive-forming AAEM in the biomasses resulted in more retention of AAEM in the coarse particles.It promoted the formation of multiple silicates,aluminosilicates,Si-rich particles etc.which were prone to slag,consequently,the slagging propensities during co-combustion were increase by torrefaction.Furthermore,the inertia impaction model was firstly applied for prediction of slagging propensity during torrefied biomass combustion and co-combustion.The results showed that for growing phase of slagging,the calculated results based on inertia impaction model presented to be much more accurate than the results from slagging index based on the chemical composition of the fuels.While combined the Urbain viscosity model and the value of critical viscosity was set as 10⁵Pa·s,the calculated values and experimental values had the optimal correlation.