Investigation On Control Of Ash Fouling And Slagging And On Removal Of Fly Ash During Thermochemical Conversion Of Biomass

A large amount of biomass ashes can be generated during the thermochemical conversion of biomass.These fly ashes are prone to melt and volatilize at high temperatures,which easily causes great harm for the heat transfer surface of boilers and equipments,such as fouling,slagging,and corrosion,etc.This poses a great threat to the safe and steady operation of biomass thermochemical conversion system.So it is of important and practical significance to conduct an investigation on the ash fouling and slagging problems,as well as removal of fly ashes.Aiming at preventing and controlling the harm of fouling,slagging,and corrosion,this paper proceeded a fundamental study on the key scientific issues on ash-induced problems from the point of problems of existing studies,including particle characteristics of fly ashes from biomass conversion,pyrolysis behavior of biomass,optimal condition for reducing fly ash generation,sintering and melting properties of various biomass ash,ash fouling and slagging properties,thermal migration and transformation behavior of alkali/alkaline metals,trapping and removal of fly ash in cyclone separator at high temperatures,etc.And a series of research results have been obtained in this study.Firstly,this study focused on the particle characteristics of fly ash from thermochemical conversion of corncob,peanut shell,rice husk and rice straw.In order to provide a basis for preventing ash fouling and slagging problems,and for optimizing effective ash removal measures,particle characteristics including particle size distribution,chemical composition,crystal structure,element occurrence condition of fly ashes from combustion and from gasification process were comparatively studied by laser particle size analyzer(LPSA),X-ray fluorescence(XRF),X-ray diffraction(XRD).Based on these results,the effects of ashing temperature on mineralogical properties of fly ash was analyzed.Moreover,muffle furnace burning test was used to study the effects of temperature on ash contents.The results showed that as the ashing temperature elevated,the particle size of fly ash obviously decreased,and mineral crystal phases with simple structure was integrated into complex phases,resulting in formation of low melting eutectic compounds.Under the same ashing conditions,the content of K,Na and other alkali metals in corncob ash(CCA)was the highest among the studied ash samples,the content of alkali metals in rice straw ash(RSA)and peanut shell ash(PSA)were relatively lower,and that of rice husk ash(RHA)was the lowest.The emission of potassium during biomass gasification is mainly in form of KCl.The fouling and slagging severity of ash from gasification was proved to be more severer than that from combustion.Then,the pyrolysis behaviors of different biomass during thermochemical conversion were investigated by the technique of thermogravimetry coupled with mass spectrometry(TG-MS).In this section,the effects of heating rates,particle granularity and carrier gas flow on biomass pyrolysis and thermal dynamics were studied.The optimum condition of reducing the generation of fly ash was explored on premise of ensuring thermal conversion efficiency.This part can provide scientific basis for pyrolysis parameters optimization and for radically reducing ash generation.The rank of volatilization release characteristic index is corncob>rice straw>rice husk>peanut shell.The dynamic results obtained by Coats-Redfern method corresponded well with TG results.The maximum weight loss rate increased with the increase of heating rate and carrier gas flow.The total weight loss for biomass samples with d ? 450?m increased with the increase of particle size.Under experimental conditions in this study,it is found that the pyrolysis of biomass particles with size in 154?280 ?m at the heating rate of 20 ?/min and in the gas flow of 30?60 mL/min was proved to be the most appropriate for syngas production,and under this condition,the content of ash impurities was relatively least.Afterwards,the sintering,melting and bonding regularities of biomass ash from different thermochemical conversion processes were studied by muffle furnace burning testash fusion test,thermogravimetric and differential thermal analysis(TG-DTA).The influence of various ashing temperature,ashing process,pyrolysis atmosphere on ash fusion characteristics was comparatively analyzed.The results indicated that ash fusion points and ash content increased at elevated ashing temperatures and time.The sintered structure of biomass ash was emerged at above 600 ?.When the ashing temperature was 815 ? the surface of ash started to melt with the increase of ashing time.At the same temperature,the ash color changed from black to light gray,finally turned to pale red.The weight loss in the fusion process of biomass ash from gasification showed a step-by-step mechanism,and the total weight loss and maximum weight loss rate increased with the increase of oxygen content.Under aerobic atmosphere,as for combustion ashes,the weight loss in their fusion processes showed a stepwise mechanism while there was no stepwise laws under inert atmosphere.Based on the research described above,the microstructure evolutions of biomass ash together with the thermal migration and transformation behaviors of alkali/alkaline metals in ash were investigated with scanning electron microscopy(SEM)and energy dispersive X-ray(EDX).Meanwhile,based on those experimental results of particle characteristics of fly ash,ash sintering and melting properties,microstructure evolutions of biomass ash,the fouling and slagging mechanism of biomass ash was studied in depth.And some new approaches and measures for effectively solving ash fouling and slagging problems were put forward in this study.Furthermore,the fouling and slagging tendencies of different kinds of biomass ashes were analyzed and predicted by using multiple distinguishing methods,and the influence of ashing temperature on ash fouling and slagging as well as on the thermal transformation of alkali/alkaline metals were both acquired.With the increase of ashing temperature,there was and obvious phenomenon of ash fouling and slagging.Alkali metal materials gradually dissolved out,and most of them was in molten state,which further aggravated ash fouling and slagging.When the gasification ash happened to melt,chlorides were released from the interior of ash particles,then moved towards the surface of ash particles.And this was the main reason for resulting in the internal porous characteristics of gasification ash with external protuberance structure.Finally,considering the complicated motion of strongly turbulent flow under high temperature in cyclone separator of biomass gasification station,together with the difficulty of measuring fly ash concentration,an optimal mathematic model for strong swirl turbulence,renormalization group coupled with reynolds stress model(RNG-RSM),was innovatively put forward for numerical simulation of high temperature gas phase flow field.Besides,as for the particle-phase flow field,the multiphase particle-in-cell was used.The crude syngas under high temperature released from gasifier was regarded as the object of this study.The air-fly ash coupling migration law inside the cyclone separator at high temperature were studied on numerical simulation by FLUENT software,using proposed gas-particle coupling mathematic model,based on the principle of computational fluid mechanics.Through the simulation study,the distribution laws of three dimensional velocities and fly ash concentration in strong swirl turbulence field under high temperature were both obtained,and the relations between inlet velocity and separation efficiency was also predicted accurately.The results indicated that as the inlet velocity elevated,firstly the separation efficiency increased,and then decreased.The simulation results were in good agreement with the experimental data.And in order to reduce ash fouling and slagging for follow-up equipments after cyclone separator,the optimization measures and suggestions for capture and removal of fly ash particles in cyclone separator under high temperature were put forward according to the simulation results,which had a key practical value for engineering application.