低负荷燃煤锅炉烟气再循环方式下SO3与Hg的生成及飞灰富集机理研究

Flue gas recirculation can give rise to enrichment of SO3 and Hg in flue gas for low-load coal-fired boiler, causing the potential low-temperature corrosion in cold parts of the boiler system, which affects the steady operation of the system. At present, for low-load coal-fired boiler, the effects of flue gas recirculation on the formation of SO3 and Hg and their retention in fly ash are still not clear. In this work, the influences of flue gas composition on the formation of SO3, Hg species and Hg speciation are studied, through simulating the low oxygen combustion condition and flue gas composition of low-load coal-fired boiler with flue gas recirculation. Also, the chemical kinetic models of SO3 formation and Hg oxidation are established. Moreover, the effects of the changes in flue gas composition and physicochemical properties of fly ash, caused by flue gas recirculation, on the retention of SO3 and Hg in fly ash and heterogeneous catalytic oxidation of SO2 and Hg are studied. By the analysis of gas composition upstream and downstream of fly ash filtration, and the identification of key functional groups and intermediates at the surface of fly ash, the relationships between the retention of SO3 and Hg and above-mentioned groups are established, and the impacts of key group and species on the retention of SO3 and Hg in fly ash and heterogeneous catalytic oxidation of SO2 and Hg are clarified. The results of this work provide fundamental data and scientific basis for the emission and control of SO3 and Hg emitted from low-load coal-fired boiler with flue gas recirculation.

低负荷燃煤锅炉烟气循环引起烟气中SO3与Hg的富集，对系统造成潜在的低温腐蚀，影响系统稳定运行。目前，烟气循环对低负荷燃煤锅炉SO3与Hg的生成及飞灰富集的影响机理尚不明确。项目拟通过模拟低负荷燃煤锅炉烟气循环方式下的“低氧”燃烧条件与氛围，研究烟气成分对SO3与Hg的生成及Hg的形态分布的影响规律，建立SO3生成与Hg氧化的化学动力学模型。同时，研究烟气循环引起烟气成分及飞灰物化特性变化，对飞灰富集SO3与Hg、飞灰催化氧化SO2与Hg的影响规律；并结合飞灰过滤前后气体成分、飞灰表面关键基团及中间产物分析，建立SO3、Hg在飞灰表面的富集量与以上基团产物的相关性关系；阐明关键基团及物种对飞灰富集SO3、Hg及飞灰催化氧化SO2与Hg的影响机理。项目研究为低负荷燃煤锅炉烟气循环方式下SO3与Hg的排放及过程控制可提供基础数据与科学依据，具有重要理论意义和应用价值。

随着锅炉大气排放标准日益严格，烟气循环脱硝技术在部分低负荷燃煤工业锅炉上得到应用。但低负荷燃煤锅炉烟气循环引起烟气氛围与飞灰物化特性发生变化，尤其是对SO3与Hg的生成及飞灰去除产生重要影响。因而，针对该烟气循环方式下SO3与Hg的生成特性及飞灰吸附研究具有重要意义。项目搭建煤粉沉降炉，研究了燃烧烟气成分对SO3与Hg生成及Hg形态分布的影响规律。结合固定床汞吸附装置，研究了烟气循环引起飞灰物化特性变化的规律，并对其飞灰Hg吸附的影响进行了研究；结合飞灰表征分析，阐明了关键基团对飞灰Hg吸附的影响机理。研究结果表明，烟气循环方式下，烟气循环比例、煤种、关键烟气组分如SO2、NO等对SO3与Hg生成、Hg形态分布，以及飞灰吸附去除Hg有重要影响。随着烟气循环比例的增加，烟气中SO3浓度与SO3生成率均呈下降趋势。相比非烟气循环方式，烟气循环比例在20-60%，褐煤与烟煤SO3浓度分别下降了18.4-59.3%、66.6-92.5%，SO3生成率分别降低了0.22-0.88%、0.61-0.99%；净烟气循环方式下烟气中总汞浓度略有下降，烟气中Hg2+/Hg（总）比例均有降低。20-60%烟气循环比例下，褐煤与烟煤条件下总汞浓度分别下降21.1-36.8%、13.2-15.2%，烟气中Hg2+/Hg（总）比例分别降低了1.9-5.4%、3.5-3.7%。SO2对烟气中总汞含量有明显的促进作用，其他组分影响并不明显。随着NO浓度增加，飞灰Hg吸附量显著增加，NO浓度由0提升至300ppm时，褐煤、烟煤飞灰吸附量分别增加6.4倍、4.0倍。随着SO2浓度增加，飞灰Hg吸附量显著降低，并抑制NO促进作用。随烟气循环比例提高，C=O官能团含量增多，将增强了飞灰汞吸附能力。项目研究为燃煤锅炉烟气循环方式下SO3与Hg的排放及控制可提供基础数据与科学依据，具有重要理论意义和应用价值。