煤灰动态熔融特性对飞灰沉积的影响及机理研究

The deposit and slagging produced by coal ash in the syngas cooler are one of the obstacles to the stable operation of the gasifier. The fusibility of coal is commonly used in prediction of deposition type in the coal-fired boiler. However, due to the lack of systematic study on the dynamics of the ash melting process, the relation between coal ash deposition process and its melting characteristics cannot be established. Therefore, the deposition characteristics is hard to predict based on the sintering characteristic or fusibility of the coal ash. The project aims to study the influence and the mechanism of the dynamic fusion behavior on the deposition characteristic at the three stages, the formation of initial deposits, the growth of bond layers, and the solidification of slagging layers, which is the three stages in the deposition formation process. The dynamic melting process of coal ash and the factors affect the nature of sintering phase and melting phase is studied. The classification of coal ash is conducted on these characteristics. The effect of the sintering characteristics on the initial stage of the deposition layer and the formation mechanism is investigated. The influence of the quantity and the viscosity of melting phase on the formation rate of slagging layer was studied. The synergistic effect of liquid phase production and liquid viscosity was used to elucidate the impact mechanism. A comprehensive deposition-slag sequence that can predict deposition at different temperature is to be established based on this research. The model that assemble the type of coal ash dynamic melting characteristics, the ease of formation of the initial deposition layer, and the tendency of slagging will support the operation of entrained flow gasifier.

煤灰在合成气冷却器产生的积灰、结渣问题是实现气化炉长周期稳定运行的障碍之一。燃煤锅炉中常用煤灰的熔融性预测灰渣的沉积类型，但由于缺乏对煤灰动态熔融过程的系统研究，无法建立煤灰沉积过程与其烧结熔融特性关系，因此无法通过煤灰的烧结熔融性准确预测其沉积特性。项目拟针对沉积过程中的三个阶段：初始沉积层形成、沉积层的粘结长大、烧结固化形成结渣层，分别研究煤灰动态熔融特性对沉积过程的影响及机理。具体的研究内容包括：通过对煤灰动态熔融过程的分析确定影响煤灰烧结阶段和熔融阶段特性的本质，并据此分类；研究煤灰烧结阶段特性对初始沉积层性质和形成机理的影响；研究煤灰熔融阶段特性对结渣层形成速率的影响，并通过液相数量和黏度的协同作用阐明其影响机理。在此基础上，以煤灰动态熔融特性类型、是否易形成初始沉积层、结渣倾向高低，建立综合沉积结渣序列，预测不同操作温度下煤灰的沉积特性，为气化炉的运行操作提供科学依据。

以大规模气化技术中合成气冷却器的积灰问题为背景，深入认识了煤灰组分对其烧结熔融特性的影响规律，通过对高硅铝灰、高钙灰、高钾灰的动态熔融特性的考察，明确了煤灰组成对典型灰成分煤的结渣特性的影响规律，高硅铝灰硅铝比较高时结渣性较低；硅铝比低时高于初始熔融温度极易发生严重结渣。硅铝较高的高钙灰的熔融类似于晶体，超过其烧结温度将发生严重结渣；对于硅铝比较低的高钙灰，其熔融过程可类比于多个晶体混合物的熔融，结渣程度较硅铝比高的样品较轻，但是超过变形温度后沉积灰也将严重结渣。高钾灰中大量可溶性钾发生低温共熔引发液相烧结和严重结渣。先进行预处理去除部分可溶性钾，再掺烧煤可改善高钾灰的结渣特性。对比了不同成分特征灰的黏温特性与熔融行为特征，揭示了烧结阶段和主要熔融阶段的固液相转变规律和黏温特性之间的关系，只有同时满足FT与DT温差大且熔融过程第三阶段收缩速率低时，熔渣才可能是非结晶渣，因FT与DT温差大和第三阶段的低收缩速率意味着熔渣的固液相转变速率低且黏度高。明确了熔融最后阶段难熔物溶解与其熔融结渣特性之间的关系，液相温度相同的样品灰不论在熔融行为特征和黏温特性上都表现迥异，只有当样品的液相温度相同，位于同一矿物析出区域且熔融行为特征类似，才会表现出相近的熔融温度和黏温特性。提出了沉积结渣预测方法，基于灰成分、动态熔融特性、固液相转变速率先判断类型再进行沉积结渣性预测可提高准确性。建立了基于固液相转变和动态熔融特性的沉积结渣序列，具有典型灰成分的样品可根据该序列判断是否归属于长渣、低温短渣、高温短渣。“高温短渣”适用于燃煤锅炉，很难在燃烧过程中产生熔融煤灰颗粒，亦不会导致结渣问题，对维持除尘系统和换热系统的长周期运行十分有利。“长渣”则可能有利于采用液相排渣的气化炉，因较宽的熔融范围意味着流动性熔渣不会在较窄的温度波动范围内产生明显的固化以至于影响其流动性。“低温短渣”则既不适用于燃煤锅炉也不适用与液态排渣的气化炉，是需要调节灰成分的原料。