褐煤O2/CO2燃烧Cr的氧化机理及模型研究

Organic-Cr is the main Cr species in lignite due to its high content of oxygen-containing functional groups. The behavior of organic Cr is quite different with that of inorganic Cr in the high-rank coals, especially under oxy-fuel combustion, the change of combustion atmosphere and the utilization of limestone furnace-injection technique cause a large number of toxic Cr(VI) emission. The oxidation and transformation mechanism of Cr during lignite oxy-fuel combustion is a critical component for Cr(VI) control. However, the homogeneous and heterogeneous reaction mechanisms of Cr in the system of high concentrations of HCl/SO2/H2O together with Ca/Mg/Na/K/Fe coal-minerals are still unclear. Moreover, no relevant investigation has revealed the effect of competitive reactions on promotion and inhibition of Cr(VI) formation during lignite combustion. This proposed project is to investigate the content distributions/species of Cr in Chinese lignites and the kinetics/model of Cr transformation during combustion upon experiments and simulation calculations. Based on the studies of Cr oxidation mechanisms and the kinetics data of Cr reacted with different compounds, a model is developed to simulate and reveal the promotion and inhibition effects on Cr(VI) formation during lignite combustion. Finally, the control methods of Cr(VI) are proposed, which will lay a theoretical foundation for effective control of Cr(VI) during lignite combustion.

随着褐煤作为动力用煤重要性的提高以及人们对重金属污染的重视，富含含氧官能团的褐煤使Cr向有机态富集导致燃烧过程中Cr的转化行为发生重大改变，特别是O2/CO2燃烧反应气氛改变及炉内喷钙技术的使用，造成大量剧毒Cr(VI)排放的危险，因此研究褐煤O2/CO2燃烧Cr的迁移及氧化路径成为控制Cr(VI)的关键问题。但是高浓度HCl/SO2/H2O和Ca/Mg/Na/K/Fe矿物同时存在的复杂反应体系中Cr的均相非均相反应机理，及对生成Cr(VI)的促进和抑制作用的耦合过程还缺乏深入认识。本项目针对上述问题，研究我国褐煤中Cr的赋存分布及燃烧过程中的反应动力学机理。通过实验与模拟相结合的方法，揭示Cr的氧化路径并获得不同因素与Cr反应的动力学数据，建立模型模拟复杂反应体系下Cr(VI)生成的促进与抑制作用的耦合过程，提出Cr(VI)生成控制思路，为有效控制褐煤燃烧Cr(VI)的排放奠定理论基础。

针对低阶煤O2/CO2燃烧过程中重金属铬的迁移转化特性、氧化影响因素、氧化机理、Cr反应动力学及Cr(VI)的控制进行了详细研究。低价煤中Cr主要以有机态存在，燃烧过程中易于气化释放以及氧化为剧毒六价Cr(VI)，特别是O2/CO2燃烧下，HCl、SO2、H2O浓度大大提高、以及炉内喷钙技术的使用对Cr的氧化带来决定性的影响。与空气燃烧相比，O2/CO2气氛燃烧抑制了煤中Cr的释放，同时在一定程度上促进了Cr(III)的氧化。烟气中水蒸气促进了Cr的氧化，生成高价态Cr化合物，同时影响着矿物对Cr的捕集，当富氧燃烧H2O含量>8%或进行烟气湿式循环时，不利于Cr的控制，特别当CaO同时存在时，燃烧后灰中生成的六价铬高达43%。另外，CaO导致燃烧后灰中Cr(VI)和Cr(III)的水溶性也大大增加，严重危害地下水及环境安全。通过动力学模拟计算及实验详细研究并获得了H2O及CaO对Cr的氧化机理及动力学参数，建立了H2O与HCl竞争反应动力学模型。通过以上研究，提出了低阶煤燃烧六价Cr生成控制思路及方法，在石灰石脱硫的基础上掺混铁基及镁基矿物可弥补石灰石脱Cr的缺陷，有望实现多种污染物的联合脱除。本项目的研究对丰富和补充铬的反应动力学数据库有非常重要的意义，推动了重金属研究及控制的进程，同时为低阶煤大量燃烧铬的排放控制奠定了理论基础。