Experimental Investigation On Ash Eposition And Ash Sintering Behavior During Co-combustion Of Coal And Biomass

China now is facing severe fossil energy crisis and environmental pollution,and one suitable solution is to utilize biomass which is renewable.Plenty of operation experience and investigations have demonstrated that co-firing biomass fuels in coal-fired plant is a low-cost,low-risk and promising technology.However,ash-related problems restrict the application of biomass co-firing.The investigation on ash deposition during coal and biomass co-firing,therefore,is of great value.This study,on one hand,conducts lab-scale experiments on sintering characteristics of the mixture of ashes prepared from coal and biomass.The shrinkage rate of ash block is obtained using a method combining sintering probe and digital image processing technique.The effects of biomass fuel properties and blend ratio are evaluated by several analysis methods,such as XRD and SEM-EDS.On the other hand,the pilot tests are performed in a 3 00KW down-fired furnace to study the ash deposition during the co-combustion of coal and biomass fuels.With an ash deposition probe,a CCD monitoring system,and a temperature collector,on-line measurements of deposit thickness and heat flux are carried out.The microstructure and mineral composition of the ash deposits are analyzed to investigate the differences in ash deposition behavior during typical biomass fuels co-firing with coals which have different ash melting points.The research contents are as follows.Firstly,the sintering and melting characteristics of ash mixtures prepared form Zhundong lignite and six different biomasses are studied in a horizontal-chamber furnace.The result turns out that blending corn stalk ash and rice husk ash promote the formation of alkali minerals and Ca-Si-Al-O eutectics,which have low melting points.Thus,the sintering behavior of Zhundong ash is promoted.Nevertheless,ashes from wood,pumpkin seed shell,pine nut shell,and walnut shell have opposite effects,and the inhibitory effect of wood ash is most notable.Wood ash is rich in calcium-minerals and tends to react with aluminium silicates in coal ash to form gehlenite with a very high melting point.Secondly,the formation of bubbles during the sinter of the ash mixture of Zhundong coal and corn stalk is investigated by a polarizing microscope and digital image processing technique.The variations of bubble number,bubble size,and porosity with time are measured.The result shows that 50%corn stalk ash changes the mineral composition of sintered ash remarkably.The release of gaseous matter is promoted and the formation of bubbles is advanced.Thirdly,corn stalk ash and wood ash are blended into coal ashes(Shanxi and Shenhua)with different melting points to study the sintering and melting behaviors.The result turns out that corn stalk ash promotes the sintering degree of both two coal ashes,and the effect is enhanced with higher blend ratio.It is because that the interactions between biomass and coal ashes inhibit the formation of minerals with high melting points.Blending high ratio of wood ash increases the sintering degree of Shanxi ash(high melting point)as a result of an inhibition on the formation of mullite which has a pretty high melting point.However,the sintering and melting behaviors of Shenhua ash(low melting point)are inhibited with high ratio of wood ash added.The formation of gehlenite accounts for this phenomenon.Fourthly,the co-combustion of Shanxi Coal and wood is carried out in the pilot-scale furnace to study the influences of the biomass blend ratio and furnace temperature on ash deposition.The result shows that co-firing wood exacerbates the slagging,even though the ash content in wood is rather low.In addition,the deposit thickness increases with rising blend ratio.The high content of calcium in wood plays a significant role.The formation of mullite is inhibited on one hand,on the other hand,calcium-sulfate minerals are formed and contribute to the formation of a melt.Frequent shedding of deposits occurs when co-firing 10%wood at a low furnace temperature.Higher furnace temperatures inhibit the shedding and promote the slagging.Fifthly,in order to demonstrate the role of calcium in wood promoting the slagging when co-fired with high-fusion-temperature coal,CaO additive is added to Daotong Coal and combusted in the pilot-scale furnace.During the test,the concentration of SO2 in the flue gas is continuously measured.The result shows that ash deposition is remarkably increased with CaO additive,and SO2 concentration decreased.It indicates that CaO reacts with SO2 and contribute to calcium-sulfate-bonded deposits.Finally,wood and corn stalk are co-fired with Shenhua Coal,respectively.The result shows that co-firing corn stalk significantly increases the slagging tendency and results in severe heat transfer deterioration.The mean size of fly ash particle decreases with rising blend ratio.Co-firing wood has an inhibition effect.When 10%wood is co-fired,no shaped deposit is produced and the size of fly ash particle increases a lot.The mineral composition of fly ash differs little among the cases,indicating that co-firing biomass influences the ash deposition mainly through the volatile compounds.