Chlorine,Nitrogen And Sulfur Release During Biomass Torrefaction And Combustion

Coal-fired power plant has supported China’s economic growth and development,but it also has brought serious environment pollution.However,the large-scale utilization of biomass energy is of great significance in either protection of non-renewable energy,carbon dioxide emission reduction or pollutant control.B iomass direct combustion power generation is currently the primary form of biomass utilization,but the problem encountered of slagging and corrosion caused by alkali and chlorine has seriously affected the safety and economical efficiency of boiler operat ion.,safe and stable operation,and chlorine is contributing to the core of the problem,high chlorine content in biomass can make the alkali metal（mainly potassium）at low temperature in the form of gas release,and the release of HCl in combustion wil l have corrosion on boiler heating surface.chlorine concentration often dictates the amount of alkali metal vaporization during combustion more strongly than the alkali concentration in the fuel,and in most cases,the chlorine appears to play a shuttle r ole,facilitating the transport of alkali from the fuel to the cooler combustor surfaces,where the alkalis often form sulfates.Besides,chlorine may cause additional corrosion by formation of HCl,which can promote scale failure.Besides,raw biomass also has many shortcomings,such as its high moisture content and associated low heating value,poor grindability,biodegradability（i.e.,propensity to rot）,etc.But as a pretreatment of mild pyrolysis at the absence of oxygen,torrefaction could alleviate such notable shortcomings of raw biomass.Moreover,it could also decrease the chlorine content in torrefied biomass as most of chlorine was released in torrefaction and increased the heating value of the fuel.As a result,torrefied biomass becomes more coal-like and can be ground and injected in a boiler together with coal,which makes biomass a suitable fuel for power plants.However,some key problems of chlorine release during biomass torrefaction and combustion have not been solved:（1）How was chlorine released during biomass combustion in a fixed bed furnace?（2）How was chlorine release during biomass torrefaction（including aerobic torrefaction and nitrogen torrefaction）?（3）How was chlorine released when biomass cofired with coal?Therefore,several experiments and simulation were conducted in this paper,such as:（1）experiment and assessment on the effects on HCl release during biomass combustion in a fixed bed furnace;（2）evolution of chlorine-bearing gases during biomass torrefaction at different temperatures in nitrogen;（3）HCl and other emissions from combustion of raw and torrefied biomass,and from co-firing of coals with raw and torrefied biomass;（4）evolution of chlorine-bearing gases during biomass aerobic torrefaction.Based on the above studies,this paper proposes several strategies for clear understanding chlorine release during biomass pyrolysis and combustion,which lays the theoretical foundation for the development of biomass utilization and pollutant control technology.1 Release of chlorine during combustion of raw biomass in boilers is detrimental as it contributes to slagging,fouling and corrosion.Combustion of torrefied biomass can alleviate such issues,as it contains less chlorine than its raw biomass precursor.This work assessed the effect of the torrefaction intensity on the chlorine content of residual biomass and of the released gaseous species.For this purpose,corn straw was subjected to torrefaction（mild pyrolysis）at furnace temperatures,in the range of 250-400℃ in an inert nitrogen atmosphere.The major chlorinated species in the evolving pyrolysis gas（“torgas”）were identified as HCl and CH₃ Cl,and were quantified.Finally,emissions of other released gases,such as NO_x,HCN,NH₃,CO,CO₂ and C₂H₆,were also monitored and reported herein.2 Release of chlorine during combustion of raw biomass in boilers is detrimental as it contributes to slagging,fouling and corrosion.Combustion of torrefied biomass can alleviate such issues,as it contains less chlorine than its raw biomass precursor.This work assessed the effect of the torrefaction intensity on the chlorine content of residual biomass and of the released gaseous species.For this purpose,corn straw was subjected to torrefaction（mild pyrolysis）at furnace temperatures,in the range of 250-400 ℃ in an inert nitrogen atmosphere.The major chlorinated species in the evolving pyrolysis gas（“torgas”）were identified as HCl and CH₃ Cl,and were quantified.Finally,emissions of other released gases,such as NO_x,HCN,NH₃,CO,CO₂ and C₂H₆,were also monitored and reported herein.3 The use of nitrogen for torrefaction will enhance the cost,while the flue gas produced by combustion of coal or biomass,which can also provide the anaerobic environment for torrefaction,will increase the economical efficiency.Meanwhile,the heat of the effluent gas can preheat the biomass and improve the overall utilization of energy.Therefore,HCl,CH₃ Cl,SO₂,NO_x and HCN were detected in the process of biomass aerobic torrefaction.The results revealed that HCl,NO and HCN increased with the increase of temperature,and CH₃Cl was affected by temperature and oxygen.At low oxygen concentration（3% O₂）,CH₃Cl first increased and then decreased,while at high oxygen concentration（7% O₂）and high temperature,CH₃Cl dramaticly decreased.Under the condition of low temperature（275-300 ℃）,chlorine was released mainly as CH₃Cl in the process of aerobic torrefaction,but the gap between HCl and CH₃Cl was smaller than nitrogen torrefaction.But when the temperature was more than 325 ℃,CH₃Cl decreased with the increase of oxygen concentration,resulting in HCl gradually replacing CH₃Cl to become major product containing chlorine.When the temperature went up to 375 ℃ and oxygen concentration was 7%,the chlorine conversion to CH₃Cl dropped to below 12%,while the concversion to HCl was more than 50%,the chlorine percent of which was remained in torrefied biomass was less than 10%,suggesting that the existence of the oxygen would like to help chlorine convert to HCl during torrefaction.4 Elevated emissions of hydrogen chloride（HCl）from combustion of biomass in utility boilers is a major issue as it can cause corrosion and,in combination with the high alkali content often encountered in these fuels,it can also deposit molten alkali chloride salts on the boiler’s water tubes.Such deposition can impede heat transfer and cause further corrosion.This work torrefied and then burned herbaceous biomass（corn straw）as well as crop-derived biomass（olive residue and corn-based Distillers Dried Grains with Solubles,DDGS）,all pulverized in the size range of 75-150 μm.It monitored the HCl emissions from torrefaction of biomass and,subsequently,the comparative HCl emissions from combustion of both raw and torrefied biomass.Results showed that during torrefaction most of the chlorine of biomass was released in the gas phase,predominately as HCl.Consequentially,combustion of torrefied biomass,which contained less chlorine than raw biomass,generated significantly lower HCl emissions than raw biomass,particularly so for biomass of low alkali content.This observation complements previous findings in this laboratory that torrefied biomass also generated lower SO₂ emissions than raw biomass,albeit by a smaller factor.Both of these findings enhance the appeal of torrefied biomass as a substitute fuel in utility boilers.5 Acid gas emissions of sulfur dioxide,nitrogen oxide,carbon dioxide,and hydrogen chloride from co-firing biomass with either a high-sulfur bituminous coal or a lowsulfur sub-bituminous coal were conducted.Neat coals,neat biomass,either raw or torrefied,and 50-50 wt% blends thereof were burned in a laboratory-scale electrically-heated drop-tube furnace（DTF）.Pulverized fuel particles were introduced into the DTF,operated at a gas temperature of 1350 K,and combustion emissions were measured in the furnace effluent.Coal particles were in the range of 75-90 μm and biomass particles in the range of 90-150 μm.Results revealed that blending of both coals with raw and torrefied biomass drastically reduced the coal’s SO₂ and NO_x emission yields to values that were below those predicted by linear interpolation of the corresponding emission yields of the two neat fuels.The SO₂ emission yields from torrefied biomass were lower than those of the corresponding raw biomass due to their lower sulfur content.Similarly to the yields,the SO₂ emission factors（based on the energy content of each sample）from the ble nds with torrefied biomass were also lower than the blends with raw biomass.NO_x emissions from neat torrefied were mildly higher than those from raw biomass,as the latter have higher nitrogen content per unit mass.There is no clear reduction trend for NO_x emissions of the blend samples based on their nitrogen content.HCl emission from torrefied corn straw was lower than that from the raw sample as the chlorine content is lower in the former.The HCl emission yields from the blends of corn straw with coal were much higher than those from neat coal combustion.Finally,the HCl emission yield from blends of the high-sulfur coal with corn straw were higher than those from the blends the same biomass with the low-sulfur coal.