煤粉氧/水蒸气燃烧中的矿物质迁移和细颗粒物生成机制

Understanding the formation mechanism of the fine particulate matter (PM) during coal combustion is essential to reduce its formation and emission in the coal-fired power stations. The emerging coal oxy-steam (O2/H2O) combustion technology, having low emission characteristics of both CO2 and pollutants, has been deemed as a promising oxy-fuel combustion technology for reducing CO2 emission. Focusing on the fine PM in the background of the coal oxy-steam combustion, the proposal project purposefully uses the mineral-loaded model carbon and some typical steam coals as the experimental material, and designs a series of combustion experiments on a well-controlled drop-tube furnace. Systemic samplings are carried out to collect the char samples at different burning stages along the furnace axis. The generated PM are sampled and collected via a dilution probe and low pressure impactors (LPIs). Then, raw materials, char and PM samples are characterized via N2 adsorption method, computer-controlled scanning electron microscopy (CCSEM), inductively coupled plasma mass spectrometry (ICP-MS) and other advanced methods to obtain their physical and chemical properties. Further, an improved model describing the vaporization behavior of the refractory minerals (e.g., SiO2) is developed and based on it, in-depth investigations on the partitioning of the refractory minerals into fine PM are performed. Based upon above analysis, the project aims to solve some key scientific problems existed in the oxy-steam combustion of coal, i.e., the physicochemical structure properties of coal/char and its evolution characteristics, the transformation and portioning behavior of those key PM-forming mineral matter, the migrating progress of the volatile mineral matter into fine PM and its controlling factors, the vaporization behavior of the typical refractory minerals in the fine PM formation. In the end, the mineral matter partitioning behavior and fine PM formation mechanism during the oxy-steam combustion of coal is revealed. The successful implement of this project will be beneficial to develop the emission reduction technologies for CO2 and pollutants during coal utilization.

掌握燃煤细颗粒物的生成机制是实现电厂颗粒物排放控制的重要前提。新兴的煤粉氧/水蒸气（O2/H2O）燃烧技术同时具备CO2和污染物减排优势，被认为是一种很有发展潜力的CO2减排技术。本项目以煤粉氧/水蒸气燃烧中的细颗粒物为研究对象，选用矿物质负载模型碳和典型煤种作原料开展沉降炉燃烧实验，借助沿程取样和撞击式颗粒物采样器（LPI）实现焦、灰和颗粒物收集。利用比表面积与孔径测定仪、计算机控制扫描电镜（CCSEM）、ICP-MS等进行样品物化性质表征，建立并借助矿物质气化模型深入机理分析。项目旨在揭示煤粉O2/H2O燃烧过程中的煤/焦物化结构演变规律；煤中典型矿物质的转化和迁移特性；易挥发矿物迁徙形成细颗粒物的机制；难挥发矿物的气化行为与细颗粒物形成特性。本项目将获知煤粉氧/水蒸气燃烧中的矿物质迁移和细颗粒物生成机制，为发展燃煤细颗粒物和CO2减排技术提供理论支撑。

本项目针对煤粉氧/水蒸气燃烧条件下的细颗粒物生成问题，从燃烧气氛条件、煤/焦物化结构演变、矿物质自身特性、矿物质间交互作用、矿物质气化行为以及气溶胶动力学等方面，深入阐释了煤粉氧/水蒸气燃烧中的矿物质迁移和细颗粒物生成机制。按计划完成了预期研究内容，包括：氧/水蒸气燃烧气氛下煤/焦结构演化特性及其对矿物质迁移行为的影响，氧/水蒸气燃烧条件下易挥发矿物质形成细颗粒物的机制研究，氧/水蒸气燃烧条件下难挥发矿物的转化行为及细颗粒物形成机制。通过以上研究获知了煤在O2/H2O气氛条件下焦燃烧不同阶段时的孔隙结构参数，揭示了煤粉氧/水蒸气燃烧气氛中有机质（焦炭）的物化结构演变规律；掌握了氧/水蒸气燃烧气氛中煤中典型矿物质的转化行为与迁移特性。基于焦炭破碎逾渗理论模型，考虑了碳与O2、CO2、H2O的反应以及CO与H2的氧化反应，建立了考虑燃烧气氛、初温与焦炭表面燃烧温度关系的子模型，构建了不同燃烧环境下的耦合矿物聚合与焦破碎机理的细颗粒物生成模型。改进后的模型能够较好地预测焦炭颗粒在不同气氛下的颗粒表面燃烧温度。温度的变化会影响矿物的聚合过程，不同反应程度下较强的熔融聚合特性会使得产生的颗粒物质量分布峰值粒径更大。不同气氛下的颗粒物粒径分布曲线也会表现出较为显著的差异。增加水蒸气浓度，CO、H2在火焰锋面处与O2进行氧化反应放出热量，焦炭表面燃烧温度升高，生成大粒径的颗粒物生成量随之增加，能有效促进煤中细矿物颗粒向大粒径颗粒物的转化，减少由煤中细矿物颗粒直接转化导致的细颗粒物的生成。易挥发矿物是形成细颗粒物的重要来源，添加少量(0.6%)纳米Al2O3和TiO2，可使PM0.1-2.5(即中间模态PM)和PM2.5的质量产率分别降低24.38%~26.06%和12.85%~19.59%。在中间模态颗粒物中，除了Na和Cl外，还观察到高含量的Al和Si元素，而正是由于Al和S含量的增加导致了中间模态颗粒物的大量形成。本项目揭示了煤粉氧/水蒸气燃烧中的矿物质迁移和细颗粒物生成机制，为发展燃煤细颗粒物和CO2减排技术提供了理论支撑。