Behavior Of Ash Formation And Characteristics Of Particulate Matter Generation During The Combustion Of Coal Slime And Sewage Sludge

As bulk solid wastes,municipal sewage sludge and coal slime both have high moisture contents.They can be synergistically dewatered,and the resulting mixture can be co-fired in pulverized coal-fired boiler for combustion disposal and energy recovery.However,coal slime and sludge are greatly different in ash composition.The interactions between ash particles and between inorganic mineral components of the two fuels affect the ash formation behavior and particulate matter（PM）generation characteristics during co-combustion.Therefore,based on combustion experiments in a drop tube furnace together with systematic characterization and analysis of combustion solid products,the present work investigated the ash formation behavior and PM generation characteristics during the combustion sewage sludge and coal slime and during their co-combustion.Experimental study on the combustion and devolatilization of two types of pulverized sewage sludge show that,with the increase of the temperature,the sizes of ash particles generated during the combustion of pulverized sludge concentrate to the range of 50μm～200μm.The sizes of ash particles are obviously larger than those of char particles due to the coalescence-swelling effect during ash formation,which results in around 2.3%～2.5%（mass）of Cenospheres ash particles formed at 1300°C.During the combustion at the temperature of above 1100℃,a large quantity of molten char and ash particles are generated,but the degree of melting of the particles depends on fuel ash compositions and temperature.PM₁₀（PM with an aerodynamic diameter of?10μm）generated from the combustion of pulverized sludge presents a trimodal particle size distribution.With the increase of the temperature,the yield of PM₁₀increases while the yield of PM₁（PM with an aerodynamic diameter of?1μm）increases significantly.At the same temperature,the BSS combustion produce less PM₁ and PM₁₀ than ASS combustion.The main reasons are that BSS has higher ash and Si O₂ contents and great capacity to capture alkali metals to depress PM₁ formation and BSS has stronger melting and weaker fragmentation during ash formation to reduce the formation of super-micron particles.Experimental study on the combustion of three kinds of pulverized coal slime and co-combustion of coal slime with sewage sludge（blending ratio of 10%,30%and 50%）the ash particles from the three kinds of coal slime are larger in size,and most of them are large aggregates with a few fragments.These reflect the fragmentation-coalescence mechanism of ash formation during pulverized slime combustion,which is related to the compositions and melting characteristics of coal slime ash.The main minerals of coal slime including kaolinite,quartz,pyrite and aluminosilicate clay minerals,which are converted into mullite,hematite and vitreous substances after combustion.There is a collision-agglomeration interaction between the two kinds of ash particles,and sewage sludge ash can promote ash agglomeration and melting.As a consequence,coal slime co-firing with sludge at higher blending ratio forms more large and molten partially-molten ash particles with the particle size distribution and morphology close to that of ash particles from sewage sludge.The melting characteristics of coal slime ash and the blending ratio affect the extent of the interaction.Therefore,controlling the blending ratio of sewage sludge in engineering practice can reduce slagging tendency during co-combustion in the boiler.Investigation of the characteristics of PM generation during coal slime combustion and co-combustion of coal slime mixed with sewage observed that the PM generated by the combustion of three coal slimes is tri-modal distribution while the specific distribution profiles are different from that of sewage sludge.It presents that coal slimes produce coarser fine mode particles while PM₁₀ generated by coal slimes combustion is lower than that of sewage sludge.The main compositional elements of PM₁ are P+S+Cl and Na+K+Mg,while those of PM_1-10 are Al+Si+Ca+Fe.When coal slimes are co-fired with sewage sludge,the inorganic minerals of coal slimes capturing the alkali released from sewage sludge affect the generation of PM₁.The collision-coalescence of two kinds of ash particles and the melting enhancement from sewage sludge ash change the generation of super-micron particles such as central-modal particles.The extent of the change depends on the blending ratio and the ash compositions of coal slime.When ACS and BCS co-combust with sewage sludge at lower blending ratio（<30%and 10%,respectively）,the generation of PM₁ decreases,while the yield of PM₁ increases when CCS is co-fired with sewage sludge.Therefore,in engineering practice,the PM generation characteristics of coal slime and sewage sludge co-combustion can be evaluated according to the ash composition characteristics of the fuels.Co-combustion of sewage sludge and coal slime even can be employed as a measure to reduce PM formation if the blending ratio is properly determined.