生物质锅炉中温受热面碱蒸气冷凝与飞灰黏附多重结渣模拟

Simulation of alkali gaseous condensation and fly ash viscous capture-induced dual slagging on mid-temperature heating surface of biomass boiler

生物质作为CO_(2)零排放的清洁可再生能源,在部分替代化石燃料用于燃烧发电的领域具有良好的应用价值。但生物质燃料普遍含有大量碱金属成分,在燃烧过程中易于以气相形式析出凝结在受热面形成有黏性的初始沉积层,强化壁面对烟气中飞灰颗粒的捕获,导致锅炉受热面结渣情况进一步恶化。针对以上问题,利用Factsage软件计算生物质锅炉中温过热器初始沉积层黏附特性,结合临界速度模型,建立综合考虑KCl冷凝-飞灰捕捉的受热面结渣综合模型;利用ANSYS FLUENT软件编写用户自定义函数(UDF),对生物质锅炉中温过热器受热面沾污行为进行模拟研究,并与现场采样结果对比验证模型准确性。结果表明,中温过热器区域的沉积速率与取样结果相吻合;壁面温度升高对飞灰碰撞效率影响较小,但抑制KCl冷凝导致总沉积量降低;烟气入口速度变化影响KCl冷凝和飞灰碰撞效率,大颗粒(50、80μm)碰撞效率随烟气速度增大而增大,而10μm小颗粒更易被粘性壁面捕获,导致沉积效率更高;相同烟气入口速度下,粒径50μm和80μm飞灰为主要沉积;总体上,碱蒸气冷凝在中温过热器区域结渣过程贡献显著,KCl直接冷凝在壁面的沉积质量占总沉积质量的5.41%,黏性沉积表面捕获飞灰颗粒沉积质量占19.24%。模型适用于生物质锅炉以中温过热器为代表的中温受热面(约700℃)积灰沾污预测。

As a renewable energy source with zero CO_(2)emission,biomass has a good application potential in partially replacing fossil fuels for power generation.However,biomass fuels generally contain high content of alkali metal components that easily precipitate and condense to form a viscous initial deposition layer on the heating surface during combustion.Capture of fly ash by viscous surface is thus enhanced,resulting in further severe slagging in the biomass-fired boiler.Aiming at these problems,the adhesion characteristics of the initial deposition layer in the mid-temperature superheater of biomass boiler has been calculated using FactSage software.Combined with the critical velocity model,an integrated model of slagging considering KCl condensation and fly ash viscous capture is established.The user-defined function(UDF)is employed with ANSYS FLUENT software to simulate the fouling behavior of the heating surface of the midtemperature superheater of biomass boiler,and field sampling results are used for model validation.The results show that the deposition rate in the mid-temperature superheater is consistent with the sampling results.The increase of wall temperature has little effect on the ash impaction efficiency,but the inhibition of KCl condensation leads to fewer deposition.The change of flue gas inlet velocity affects the condensation and impaction efficiency.The impaction efficiency of large particles(50,80μm)increases with the increase of flue gas velocity,while the small particles of 10μm are more easily captured by the viscous wall,resulting in higher deposition efficiency.Under a same gas inlet velocity,the fly ash particles with diameter of 50μm and 80μm dominate the deposition.Overall,the contribution to slagging of mid-temperature superheater by alkali gaseous condensation is significant.The mass of KCl direct condensation accounts for 5.41% of the total deposition mass,while the mass of fly ash viscous capture accounts for 19.24%.The proposed model is suitable for predicting ash deposition and fouling on mid-temperature heating surface(about 700℃)represented by a mid-temperature superheater of a biomass boiler.