Study On The Mechanism Of Alkali Metal Sodium On Coal Pyrolysis-derived Soot

Coal derived soot can affect the temperature field distribution and pollutant formation during coal pyrolysis/combustion;and soot in the atmosphere have bad impact on the environment and health,so its generation mechanism has always been concerned.Research on the effects of ion-exchangeable Na(INa)and physically adsorbed Na(ANa)in coal on the generation characteristics and physicochemical properties of coal-derived soot can reveal the mechanism of alkali metal Na affecting coal-derived soot formation.Therefore,in this thesis,the emissions of organic carbon(OC)and elemental carbon(EC)from coal-fired boilers were analyzed in the field test of pulverized coal boilers and grate fired boilers.The pyrolysis experiments of raw coal,acid washed coal,Na loaded coal and synthetic pulverized coal prepared from synthetic char supported with biphenyl or pyrene were carried out in a drop-tube reactor.The influence mechanism of Na on the formation of soot during coal pyrolysis was studied.In this thesis,ELPI was used to measure the concentration distribution of PM _2.5in coal-fired boilers,and PM_2.5 sampler was used to collect PM_2.5 samples.After that,the chemical composition of PM_2.5 was analyzed by XRF and carbon analyzer,and the morphology of PM_2.5 was analyzed by SEM.The study found that the content of soot and tar in the PM_2.5 generated/emitted by grate fired boilers with poor combustion effect is much higher than that of the pulverized coal boilers,and they agglomerate with metal mineral particles to form soot aggregate(chain or cluster).Due to the existence of soot aggregates,the PSDs of grate fired boilers'PM_2.5 are also different from the bimodal distribution(ultrafine particles and residual ash particles)of pulverized coal boilers.The EC content in PM _2.5 increases after passing through dust removal equipment,indicating that the dust collector has a poor effect on removing soot,which increases the harm of soot emitted by coal-fired boilers.In view of this,this thesis carried out the research on coal derived soot,and focused on the role of alkali metal Na in soot formation.ELPI was also used to measure the concentration distribution of coal pyrolysis aerosols,hydrochloric acid washing method was used to remove metal minerals in raw coal,the organic structure of coal and aerosol was analyzed by FTIR,and the yields of soot and tar were measured by ICP-OES test and dichloromethane extraction.The analysis of the soot generation characteristics of 6 raw coals and corresponding acid washed coals shows that the organic structure of coal and the metal elements in coal both affect the formation of soot.The absorption bands area of aliphatic hydrogen and aromatic hydrogen in the infrared spectrum of acid washed coal is positively correlated with soot yield,but aromatics contribute more to soot.Since oxygen-containing substances can inhibit the sooting of tar,the absorption band area of oxygen-containing structure is inversely related to the tar conversion rate.During pyrolysis,metals in coal can catalyze the cracking of aliphatic and aromatic substances in tar,and can also hinder the polymerization between aromatic compounds.The pyrolysis experiment of Yimin acid coal loaded with INa and ANa respectively showed that:INa remaining in char during pyrolysis can catalyze the cracking of free radical fragments,reduce the yield of primary tar,and significantly reduce soot yield.ANa has little effect on the tar release process,but it is easier to vaporize,and the soot yield first increases slightly and then decreases as ANa content increases.Small molecule aromatic compounds and high-aliphatic substances that count considerable amount in Yimin lignite volatile matter begin to transform into soot at 1175°C;and the oxygen-containing substances have the strongest inhibitory effect on soot at 1250°C.Therefore,there are"turning points"in these two places in the curve of soot yield changing with temperature.Oxidation of oxygen-containing substances exceeds the surface growth with residence time extension,which reduce the yield of soot.Gasified Na also affects particle size distribution of soot,but its mechanism is affected by the negatively charged radicals in pyrolysis gas.As the amount of gasified Na increases,the collisional aggregation between basic soot particles increases first and then decreases.The surface and overall chemical properties of soot samples were analyzed by XPS and FTIR.It was found that gasification of Na(INa or ANa)increases the reactivity of volatile matter and decreases the temperature of two turning points.At low temperature(1100°C)or low Na concentration,the main role of gasified Na is to promote the condensation of aromatic compounds,which is conducive to soot formation;However,when temperature is higher than 1000°C and the concentration of Na is high enough,the main functions of gasification Na are to catalyze tar cracking,inhibit aromatic compounds polymerization and promote the reaction of oxygen-containing substances with soot and its precursors,which inhibits the formation of soot.Part of INa could maintain the organic binding state into soot,promote the formation of ether and sulfone structures,ANa has no such effect,and the Na Cl used to load ANa could form gaseous Cl,inhibiting ether and sulfone formation.The microscopic and nanoscopic structure of soot were analyzed by HRTEM and Raman spectrometer,and the results showed that:The basic soot particles grow up through the collision of initial soot particles and surface growth reaction,and the particle size mainly obeys the normal distribution in the range of 10?40 nm;the soot aggregates are formed by the collision and aggregation of basic soot particles,and the stronger the collision aggregation effect,the higher the fractal dimension of aggregates.The length and curvature of the graphite-like crystallites in basic soot particles follows lognormal distribution,and they change inversely with the experimental parameters change;the amorphous carbon inside basic soot particles produced by coal is mainly aliphatic,and their content in the particle core is higher.With the increase of Na concentration,the influence of Na extends from the core to the shell in the order of soot formation.The Na⁺—?structure promote the polymerization of aromatic compounds and make the inner core smaller;while Na in the shell region increases microcrystalline defects,so the average length of crystallites increases and the average curvature of crystallites decreases.In this thesis,the model compounds(biphenyl and pyrene)were loaded on synthetic coke to prepare modeled coal powder for pyrolysis experiment of model compounds.Biphenyl and pyrene,two“small”aromatic hydrocarbons,do not produce soot at 1100°C,and due to the absence of aliphatic structure,this two aromatic hydrocarbons hardly produce tar during pyrolysis.The effects of Na content,temperature and residence time on aromatic substances and oxygen-containing functional groups in soot samples produced by model compounds are the same as those of coal;however,aliphatic substances in soot produced by synthetic pulverized coal come from aromatic ring opening reaction,and their variation law with experimental parameters is different from that of coal.The reaction paths of PAHs polymerization and PAHs oxidation by·OH before and after Na participation are constructed by quantum chemistry calculation software Gaussian 09.The results show that Na⁺—?structure increases the surface electrostatic potential of naphthalene molecules,increases the reaction barrier of polymerization reaction between naphthalene molecules and naphthalene free radicals,and hinders the reaction;both Na⁺complex and C—O—Na structure can reduce the H transfer energy barrier in the process of·OH oxidation of pyrene radicals,and promote the reaction.