高碱煤中碱金属分阶演化的灰化学行为及其调控研究

The utilization of high-alkali coal in combustion and gasification processes usually gives rise to severe fouling and slagging problems in equipments, which poses great threats to the stable operation of the system and thus restricts the large-scale use of high-alkali coal. However, the progress in fouling control research is currently undergoing serious resistance, due mainly to the lack of systematical knowledge of the alkalis occurrence, transformation, and especially the ash chemical characteristics of high-alkali coal. In addition, the characteristics obtained by only analyzing the dominated components (i.e., Si, Al, etc.) of general coal ash are inappropriate for high-alkali coal. Therefore, an improved theory regarding high-alkali coal ash, which principally relies on the alkalis fates, is proposed in this project; and moreover, some feasible approaches to solve the fouling and slagging issues are considered to be explored accordingly. Primarily, the occurrence of alkalis in several typical coals will be investigated by combining quantitative and coal petrological techniques; and the interrelations between occurrence and transforming behaviors of alkalis would be tentatively probed. By conducting the pyrolysis, combustion and gasification experiments, the alkalis behaviors in different conversion processes could thus be elucidated. Based on that, a model for tracking the fates of alkalis in different conversion stages is prospectively proposed. Eventually, the influence of alkalis behavior on coal ash constituent, microstructure and phase transformation is likely clarified; as well as the fouling and slagging propensity can be assessed. To some extent, the fouling and slagging problems of high-alkali coal is able to be restrained by changing the features of coal ash and modifying the partitioning behavior of alkalis. Benefiting from this project, specific approaches on control of high-alkali coal ash fouling and slagging propensity would be further developed, and it is also expected to provide a useful fundamental theory for reference to related researches.

煤燃烧或气化过程中碱金属极易引起设备灰污结渣，严重制约我国高碱煤资源的大规模利用。当前，相关技术研究进展缓慢，原因是对高碱煤中碱金属的赋存形态、演化机理，尤其是对其灰化学特性缺乏系统科学认识。以往基于宏量组分(Si、Al等)建立的煤灰特性指标具有局限性，不完全适用于高碱煤。因此，本项目提出构筑以碱金属演化主导的高碱煤灰化学理论，并依此探索抑制灰污结渣的可行方法。研究将明确典型不同变质程度高碱煤中碱金属的赋存规律，考察其对碱金属演化的影响；开展高碱煤热转化实验，建立不同反应阶段的碱金属演化预测模型，揭示碱金属“质”的转化和“量”的再分配机制。探究转化过程中碱金属对煤灰组成、聚合结构和物相转变的作用机理，耦合碱金属演化和煤灰化学特性规律，进行碱金属灰污结渣评价；通过改变煤灰体系特性，调控碱金属演化行为，抑制灰污结渣产生。该项目是煤灰化学基础和应用的延伸，可为高碱煤相关研究提供新思路。

本项目针对新疆高碱煤大规模利用中的碱金属灰污结渣问题，以阐明高碱煤在热转化过程中“碱金属赋存和演化对煤灰化学特性的影响机制”和“碱金属对高碱煤灰聚合结构和相转变的作用机理”两个紧密相关的科学问题为中心，系统研究了高碱煤热转化过程中碱金属演化主导的煤灰化学行为规律。.研究深入探索了典型高碱煤基于煤阶分级的活性组分赋存分类方法。系统分析并对比了典型新疆准东煤及若干西北低阶煤中碱及碱土金属的初始形态。发现高碱煤中Na和K主要以水溶性组分为主；Ca、Mg则以酸溶性形式存在。活性碱金属组分主要存在于煤的孔隙中或与煤有机大分子结合。赋存形态的差异导致煤中活性组分的高温演化过程存在差异。通过对高碱煤进行分选，获得了不同密度显微组分中碱及碱土金属的分布规律及其对煤热转化特性的影响。分阶段考察了不同显微组分在热解、气化和灰化过程中易挥发性物质扩散及含碱矿物质沉积机理。揭示了Na、K、Ca、Mg、S和Cl等活性元素的释放是造成设备沉积和腐蚀的主要原因。S和Cl的存在明显促进碱性矿物质的释放和沉积。设备沉积界面烧结煤灰的导热率和体积密度因煤灰的化学组成不同呈现明显差异。根据264个典型煤样的热力学模拟，详细探讨了SiO2-Al2O3-CaO/FeO等低温共熔体形成对结渣特性的影响。发现低温共熔体的生成是引发结渣的关键，提出了两种修订的煤灰结渣特性预测模型。利用添加外在矿物质并结合热力学计算的方式，探索了S，Cl及碱金属释放的抑制及定向调控方法。.通过耦合碱金属演化与煤灰特性规律，初步构筑了高碱煤灰化学理论，形成了调控碱金属演化行为的技术基础，为解决碱金属在热转化过程中引起的灰污结渣难题提供了依据。