超低排放燃煤机组硒的迁移转化及飞灰对其富集特性

Selenium transformation in ultra-low-emission coal-fired power units and its enrichment characteristics in fly ash

采用微波消解法和氢化物发生-原子荧光光谱法考察了9台超低排放在役机组硒迁移转化规律,探究了循环流化床(CFB)和煤粉炉(PC)机组飞灰特性差异对硒吸附能力的影响。燃烧后煤中硒几乎全部呈现挥发态,底渣中残留量极低。与浓度归一化和质量分布法相比较,相对富集系数法可以客观地评价燃煤副产物中硒的富集能力,两类机组中硒均主要富集于飞灰中。CFB较低炉膛温度和添加CaO可以降低入炉煤中硒释放比例并增强飞灰对硒的吸附能力,故其底渣和飞灰中硒的富集程度均高于PC,导致脱硫石膏中硒富集程度低于PC。飞灰对硒的吸附量随比表面积或孔容积增大而增大,但随粒径或孔径增大而减小。CFB飞灰中未燃尽碳含量高、形状不规则、表面粗糙且存在较多蜂窝状孔隙,导致其对硒的富集程度高于PC飞灰。

Microwave digestion and hydride generation-atomic fluorescence spectrometry were used to investigate the migration and transformation of selenium(Se)in nine ultra-low emission units in service.The characteristics of fly ash from circulating fluidized bed(CFB)and pulverized coal(PC)units are compared for understanding the Se adsorption ability in fly ash.After combustion,Se in coal is essentially volatilized in gas but only a small amount is remained in bottom slag.Compared to concentration normalization method and mass distribution method,relative enrichment factor method is more suitable to evaluate the Se enrichment ability for feed coal and combustion byproducts,which indicates that Se is enriched in fly ash from both CFB and PC units.Low combustion temperature and CaO additive in CFB boiler can not only reduce Se release ratio from coal but also enhance the Se adsorption ability in fly ash.As a result,Se enrichment in bottom slag and fly ash from CFB is more than that from PC,while Se in gypsum from CFB unit is less than that of PC.Moreover,Se adsorption content is heavily depended on the physical properties of fly ash.The amount of selenium adsorbed by fly ash increases with the increase of specific surface area or pore volume,but decreases with the increase of particle size or pore size.The fly ash from CFB unit has high unburned carbon content,irregular shape,rough surface and more honeycomb pores,which makes Se enriched in fly ash from CFB more than that of PC.