The Release Of Alkali Metal In Biomass Model Compounds And The Effect Of Alkali Metal On Ash Fusibility

Coal and biomass are main fuels of combustion electricity production in our country.The presence of alkali metals,i.e.potassium and sodium,will cause problems in boiler,such as deposition,slagging and corrosion.In biomass,the concentration of potassium is usually high and that of sodium is low.The concentrations of alkali metals in coal are usually low,while in some high sodium coal,e.g.Zhundong coal,the high content of sodium in ash will cause slagging.Therefore,according to the difference of fuel characteristics between coal and biomass,the main content of this thesis is to study the mechanism of potassium release in biomass and the effects of alkali metals in ash on ash fusibility.Experimental and theoretical methods were applied to study the release and transformation of potassium during pyrolysis of KCl doped biomass model compounds and pine wood,as well as the formation mechanism of CH₃Cl during pyrolysis.The results illustrated that during pyrolysis of biomass model compounds and pine wood,parts of K in KCl was transformed into organic K and then released at higher temperatures.The conclusion was supported by the observation of XRD and SEM-EDX.Due to the methoxyl groups on rings in their structures,significant amount of CH₃Cl was observed during pyrolysis of KCl doped pine wood,lignin and pectin.CH₃Cl was mainly formed at low temperatures?200-350??,which was therefore inhibited by high temperature or high heating rate.Intelligent ash melting apparatus was applied to study the effects of adding Na₂O to synthetic ashes on ash fusibility.When the content of Na₂O in ash was below 3%,increasing the content of Na₂O could increase the initial deformation temperature?IDT?,but had little influence on softening temperature?ST?,hemispherical temperature?HT?and flowing temperature?FT?.When the content of Na₂O in ash was between 3-10%,increasing the content of Na₂O lowered all the four fusion temperatures significantly.Thermodynamic database and equilibrium module provided by FactSage 7.0 were adopted to calculate the behaviors of minerals with the increasing Na₂O content in ash and the increasing temperature.Albite and Nepheline were proposed to cause the decline of the ash fusion temperatures.Sodium containing minerals can take place of the calcium minerals in ash which has higher melting temperatures such as Anorthite,meanwhile react with them to form eutectics,thus lowering the ash fusion temperatures.The effects of adding K₂O to synthetic ashes on ash fusibility were studied.For coal ash with relatively low CaO content?0-5%?,adding K₂O in ash in a proper amount?0-5%?reduced IDT,ST and HT,but had little influence on FT.For coal ash with high CaO content?20-25%?,adding K₂O in ash in a proper amount?0-5%?reduced all the four ash fusion temperatures.For biomass ash,the ash fusion temperatures could be reduced by increasing the content of K₂O until it reached 30%.Thermodynamic calculation illustrated that the formation of potassium containing minerals microcline and leucite were related to the decline of ash fusion temperatures.The effects of adding different oxides to real coal ash on ash fusibility were studied.Na₂O,MgO,SO₃,and CaO were added to a real coal ash to study the effects of them on ash fusibility.The results showed that adding Na₂O?0-5.5%?can lower the ash fusion temperatures of real coal,which was the same as the results obtained in synthetic ash.Increasing the content of SO₃ in coal ash?6.5-13%?can increase the ash fusion temperatures of real coal.Adding MgO in coal ash?3-9%?caused a primary decrease and a further increase of the real coal's ash fusion temperatures.Adding CaO in coal ash?13-47%?also caused a primary decrease and a further increase of the coal's ash fusion temperatures.