煤粉再燃过程化学结构和矿物质协同影响非均相还原反应的机理研究

Coal reburning is referred to as an extremely potential and prospective low-NOx combustion technology applied in routine coal-fired utility boiler. Induced by variations of coal reburning conditions, the physical and chemical property of coal itself, such as char structure and forms of mineral matter in coal will have a remarkable impact on coal reburning, especially heterogeneous reduction reaction between char and nitric oxide (C-NO reaction). The main objectives of the present work are to study co-influence of coal char chemical structure and mineral matter on heterogeneous reduction reaction during coal reburning based on microscopic view. By means of systematical basis tests and advanced characterization methods, the evolutions of coal char chemical structure characterized by Raman spectroscopy and their independent effects on heterogeneous reduction reaction will be investigated. The quantitative correlations between the contribution to the total NO reduction by the reacted char and carbon structure will be revealed in detail. The transformations of alkali and alkaline earth metallic species,as well as iron-bearing mineral matter during coal reburning and their independent influence on heterogeneous reduction reaction will be also studied based on tests and XRD, SEM-EDX analysis. Finally, the mineral-char interaction during coal reburning and their co-influence on heterogeneous reduction reaction will be stated. The results will provide theoretical basis for quantitative characterization of NOx emissions and control and prediction of heterogeneous reduction reactivity during coal reburning.

煤粉再燃技术被誉为极具潜力和应用前景的常规燃煤电厂的低NOx燃烧技术。在再燃条件的外因诱导下，煤粉自身的物理化学特性(焦结构、矿物质的存在形态)将显著影响煤粉再燃过程，特别是煤焦的非均相还原反应(C-NO反应)。本申请项目的目标是从微观层面研究煤粉再燃过程中煤焦化学结构和矿物质对非均相还原反应的协同作用。通过系统的基础试验并结合先进的表征手段，①基于拉曼光谱研究煤粉再燃过程中化学结构的演变及其对非均相还原反应的影响，揭示非均相还原反应性与煤焦的碳结构之间的关联；②研究煤粉再燃过程中煤中碱金属、碱土金属和含铁矿物质的转化及其对非均相还原反应的影响；③研究煤粉再燃过程中煤焦结构和矿物质的相互作用及其对非均相还原反应的协同影响。研究结果将为煤粉再燃过程NOx排放和控制的定量描述以及非均相还原反应性的预测提供理论依据。

煤粉再燃技术是极富应用前景的低NOx燃烧技术，国内外学者在燃料的孔隙结构、燃料粒径和烟气成分等的影响方面已取得了很大的进展，但是针对煤和煤焦的化学结构对再燃脱硝的影响还报道很少。为了揭示这种影响，本项目利用拉曼光谱、傅里叶变换红外光谱（FT-IR）和X射线衍射（XRD）表征煤焦的微观结构，利用热重分析仪和管式炉进行了煤焦再燃脱硝实验，探索了煤焦化学结构随热解温度（600~1100℃）变化对再燃脱硝的影响。同时，研究了煤粉再燃过程中煤中矿物质成分转化对脱硝的影响。研究结果表明，当煤中矿物质含量较高时，其FT-IR吸收峰会掩盖低频区的有机官能团。在研究的热解温度范围内，煤焦的拉曼光谱峰面积比值ID1/IG在900℃之前随着热解温度升高逐渐增大，在900℃以后逐渐减小，拉曼光谱峰面积之和则持续缩小。随着热解温度升高，D1峰峰位总体上先降低再升高。在热解温度低于900℃时，煤的大分子结构发生缩合和解聚，尚未发生石墨化，解聚产生了孤立的sp2碳原子，导致ID1/IG逐渐增大；在900℃之后，无序的sp2碳原子向有序的晶体sp2碳原子转变，逐渐石墨化，因此ID1/IG逐渐减小；煤焦结构的有序度先降低后升高。无烟煤焦的ID1/IG小于烟煤焦和褐煤焦的值，表明无烟煤焦的结构有序度更高。煤焦的拉曼光谱体现了无定形碳和微晶碳的结构信息，没有出现矿物质的特征峰，表明碳结构并没有受到矿物质的干扰，因此采用拉曼光谱在这方面优于XRD及FT-IR。煤焦的再燃脱硝实验结果表明，更高的反应温度和适当的氧气浓度（2%）有利于煤焦再燃脱硝。煤焦脱硝率和脱硝反应活化能分别随着热解温度降低和增大，脱硝反应活化能与拉曼光谱峰面积之和存在指数关系，高温热解时拉曼光谱峰面积之和缩小导致活性结构的减少，相应引起煤焦的脱硝反应性降低。煤焦由于酸洗脱除矿物质后，其脱硝率显著低于未酸洗煤焦。在高钙烟煤再燃过程中，含钙矿物质在高温时转化为较稳定的钙铝黄长石，导致矿物质的催化作用减弱。煤粉再燃过程中煤焦的碳结构和矿物质表现出对脱硝反应有协同影响作用。本项目相关研究成果为煤粉炉再燃过程NOx排放和控制以及煤粉再燃非均相还原反应性的预测提供理论基础，进一步丰富了煤粉燃烧理论。