Study On Emission Behavior Of Particulate Matter During Co-combustion Of Biomass And Coal

Co-firing biomass with coal is a promising short-term option for reducing the net CO₂emissions from existing coal-fired power plants,due to its low cost,simplicity,ability toprovide a rapid solution and easily adaptive to the vast existing infrastructure.However,using biomass as a co-firing feedstock in traditional coal-fired plants is associated withsome facility security and environment problems,such as aggravation of ash deposition andcorrosion,environment hazard for more fine particulate matter emission,mainly because ofthe higher amounts of alkali metals and chlorine in biomass compared to coal.Therefore,it's very important to study on the formation and emission behavior of particulate matter(PM₁₀) from co-combustion of coal with biomass comprehensively.The test in this thesis was mainly carded out on the drop tube furnace and verticalthermal balance furnace.During the test,it analyzed the main factors for the formation andemission characteristics of PM₁₀ from co-firing of biomass with coal at different testconditions.Meanwhile,the content of bio-chemical components in different biomass fuelsand its effect on PM₁₀ formation were also investigated.Furthermore,the transfer behaviorof alkali presented as different forms in biomass was discussed by both test analysis andsimulated calculation.Consequently,the role of alkali in biomass on PM₁₀ emission fromco-firing of coal with biomass was also involved in this paper.The key findings of thisdissertation are as follows:Firstly,the influences of test conditions on PM₁₀ emission behavior were investigated.The test results indicate that:except for the emitted concentration,there is no big differencefor the mass distribution of PM₁₀ from co-firing of different biomass and coal blends.It isvery similar with that from combustion of single biomass or coal sample,being a bimodalparticle-size distribution.During co-firing of biomass with coal,most alkali,S and C1 arerich in submicron particles (PM₁),mainly existing as alkali chlorides and sulphates.Withincreasing oxygen content,the emission concentration of fine (PM₁) and coarse (PM_1-10)both increases,while the ratio of PM₁ to PM₁₀ decreases.With increasing combustiontemperature,both PM₁ and PM_1-10 increase,especially PM₁.When the biomass blend ratio is small,the concentration of PM₁ and PM_1-10 from co-firing coal with biomass is lowerthan that of simulated results from combustion of single fuel sample.It means that thecontent of PM₁ and PM_1-10 from co-firing decreases compared to single ones.However,asthe addition of biomass increases further,the whole emission of PM₁ and PM_1-10 isenhanced and tends to be some differences among the different blend fuel samples.Withbiomass addition increase,the content of main ash formation elements in PM₁₀ raises,especially for the alkali metal in PM₁.Moreover,the variation trend of alkali and S is close.When the co-firing fuels are different,the effect of biomass blend ratio on elementcomposition in PM₁₀ is also different.Secondly,the content of biochemical components in biomass was measured and theeffects of different cellulose and lignin contents on both the pyrolysis and combustionbehavior of biomass and emission characteristics of PM₁₀ from co-firing were discussed.The test results indicate that:the content of cellulose is higher than that of lignin.Inbiomass,the cellulose content is between 55～78%,while that for lignin is around 19～34%.During pyrolysis and combustion of biomass,with the increment of cellulose content,theweight loss of sample increases obviously.The main decomposing reaction for celluloseoccurs between 300-400℃,which is against to the rapid vaporization of most volatilematters in sample resulting in the remarkable mass loss.Due to part of lignin consists ofbenzene rings,the lignin is harder to decompose than the cellulose and the main reactiontemperature is also much higher and broader.In the co-firing test,the samples withdifferent content of biochemical components have a similar bimodal PM₁₀ mass distribution.But with the increment of lignin content in sample,the PM₁ peak shifts to big particle sizeand the PM₁₀ emission of simulated biomass samples (composed by the mixture of celluloseand lignin chemicals with different blend ratio) increases linear.However,for actualbiomass fuel and simulated sample,the influence of cellulose and lignin content on PM₁₀emission behavior during co-firing is not completely the same,which is attributed to thedifferences of their combustion characteristics,char structures and inorganic matters.Finally,the transfer behavior of alkali metal with different exist forms during biomasscombustion and its effect on the PM₁₀ emission from co-firing were investigated.By usingchemical fractionation method to treat biomass fuel,the test results indicate that:the alkalisalts in biomass that are ionically bound are more reactive.With increasing of combustion temperature,the vaporization amount of alkali metal increases.Depending on the amountof available C1 and S,most of alkali may react with them to form chlorides or sulphates.And they end up predominantly in the fine particle fraction in the fly ash duringcombustion.When biomass sample is pretreated by using chemical fractionation method,the content of volatile alkali in fuels decreases clearly and large amount of available C1 andS is also leached.So,during combustion the vaporization amount of alkali reduces and theformation of fine particle fraction in the fly ash is limited.Meanwhile,after chemicalfractionation,the content of organic matter in sample raises.It promotes the combustionreaction.With the temperature getting higher,more inorganic matters appears,such asaiuminosilicate,and their fusion and aggregation are also enhanced.And all of these mayaccelerate the combination or"capture"of A1,Si to the alkali metal in gaseous.Moreover,most of these alkali-silicate and alkali-aluminosilicate may typically end up in the coarseash fraction either in the bottom ash or in the fly ash.From thermo-equilibrium calculationof combustion process,it showed that after chemical fractionation both the vaporizationamount and efficiency of alkali in the sample reduce and in addition the vaporizationtemperature of alkali metal also increases obviously.In the co-firing of coal withwater-washed biomass test,the mass distribution and concentration of PM₁₀ decreasesremarkably,especially for PM₁.The water leached method for pre-treat biomass fuel candecrease the PM₁₀ emission and alkali concentration in PM₁₀ during co-firing of coal withbiomass to a large extent.The alkali concentration in PM₁₀ from co-firing of water leachedsamples can decrease more than 95% compared to that from co-firing of raw fuels.In thewater leached process,even though the deprivation efficiency of K (which is ionicallybound) is larger than that of Na,however,the effect of removal of Na per unit deprivationefficiency on PM₁₀ reduction during co-firing is much more remarkable than that of K.Itmeans that the ionically bound Na in biomass plays a more important role on PM₁₀formation and emission during co-firing of coal with biomass.