Al/Si Based Sorbents Capturing Alkali Containing Vapors And Affecting Particulate Matter Formation During Sewage Sludge Combustion

Co-firing biomass or sole biomass firing in pulverized coal-fired power plants boilers is currently a major development direction of biomass firing power generation technology.Biomass is high in the contents of alkali metals.A great fraction of alkalis releases during combustion to form alkali-containing gases.During the cooling of the flue gas,the alkali vapors transform into submicron particles,which leads to ash deposition and corrosion of heat exchange surfaces and emission of submicron particles.While using Al/Si based sorbents can mitigate these problems through capturing alkali vapors in gas phase,the performances of the sorbents capturing alkalis is the basis for guiding its reasonable and effective application.Co-combustion with coal in pulverized coal-fired boiler is one of the main disposal methods for sewage sludge.Sewage sludge has high ash content and high contents of alkalis and other volatile elements in the ash.The combustion of the sludge also produces a considerable amount of submicron particles.However,the effectiveness and performances of Al/Si based sorbents to control the formation of particulate matter during sewage sludge combustion are unknown yet.Therefore,the present work,based on reaction experiments in a drop tube furnace,investigated the performances of the Al/Si based sorbents including kaolin and coal fly ash capturing alkalis through the reactions with alkali vapor under suspension reaction conditions,and explored the effects of reaction temperature,sorbent properties and dosage,and alkali vapor species on the alkali capturing performance.Aimed at controlling the formation of particulate matter during sewage sludge combustion,the effect of Al/Si based sorbents on the formation characteristics of particulate matter as well as transformations of inorganic constituents during combustion were also studied.The experiments of kaolin reacting with alkali vapors show that the sorbent properties and reaction temperature significantly affecting the alkali capturing performance.At the temperatures below 1100?C,the K capturing reaction is kinetics-controlled.The performance of K capture increases with the temperature,and is hardly influenced by sorbent properties.At higher temperatures,commercial kaolin with larger particle size and lower crystallinity have the K capturing performance decreasing with the temperature increasing because the capturing reaction is diffusion-controlled along with the sorbent deactivation caused by melting and sintering of reaction products.Reagent kaolin with lower melting degree of reaction products has the K capturing performance changing little with the temperature.The effects of reaction temperature on the performances of commercial kaolin reacting with Na OH and KOH are similar.However,at low temperatures（<1100?C）,due to the low eutectic temperature of the products of sorbents capturing Na,sorbents are easy to sinter and melt,resulting in poor Na capturing performance.It was found that the optimal dose of commercial kaolin capturing KOH is equivalent to the K/（Al+Si）molar ratio of 0.5,and the same is true for capturing Na OH.Sorbents capturing alkali vapors significantly reduce the concentration and size of submicron particles formed,and the effect is enhanced with the increase of sorbent dosage.The alkali capturing performance of commercial kaolin reacting with KCl and K₂SO₄ are much worse than that of kaolin reacting with KOH,which may be related to the alkali capturing reaction mechanism as well as the properties of alkali-containing species.The experiments of coal fly ash capturing KOH show that the K capturing performance of coal fly ash is much worse than that of kaolin because of lower content of the effective component and poor reactivity.The K capturing performance of fly ash increases slowly with the increase of the temperature.The optimal dose of fine coal fly ash is equivalent to the K/（Al+Si）molar ratio of 0.5.With the increase of fly ash particle size,the K capturing performance decreases continuously and greatly.The main reason is that the reaction between larger sorbent particles and alkali vapors is preferential to be diffusion-controlled and the product melting and sintering on the surface of sorbent particles cause the sorbent deactivation that further raises the diffusion resistance.It was observed from the experiments of sewage sludge combustion that the submicron particles generated from sludge combustion are mainly composed of volatile elements Na,K,and S and refractory element Si.The experiments on the sorbents affecting particulate matter formation show that the addition of kaolin and coal fly ash has a similar effect on the formation of PM₁₀ during sludge combustion.But the effects of sorbents and its extent depend on the characteristics of particulate matter formation from the sludge combustion such as the main elemental composition of the particulate matter formed.Sorbents capturing alkali vapors reduce the yield of PM_0.1.However,if the particulate matters are mainly formed by alkali sulfates,the sorbents may not necessarily depress the formation of larger submicron particles due to the poor alkali capturing performance of sorbents reacting with sulfates.When the sorbents have performed strong alkali capture,the product melting can increase the surface viscosity of sorbent particles,which enhances the adhering capture of fine particles generated from sewage sludge combustion and is likely to reduce the concentration of the PM₁₀in the flue.