高碱金属煤燃烧过程中渣体脱落特性研究

The Zhundong coalfield is the largest integrated coalfield, however, the high alkali content in the coal will induce the serious slagging problem in furnace. The slags of high alkali coal consist of four layers from the tube to the outside: the gaseous inorganic salt condensation layer, the solid particles contact on surface, the solid particles combined each other, the particles fused in whole or in part. The interaction force of particles in each layers are different. The formed slags effected by the internal and external conditions, such as gravity, soot blowing and so on, will usually shedding, however, the shedding modes are different, and the mechanisms of shedding are still ambiguous. This project proposed that use the nano CT technology to obtain the space arranged structure and the combination mode of particles in slags, and according to the results to elaborate the slagging mechanism. After that, the slags growing and shedding experiments will be carried out in full-scale and lab-scale furnace to establish the judgment criterion of slags sheeding modes, study the flowing and the dripping characteristic of fused slags, and the macroscopic fracture characteristics of the solid particles layers in slags. The different sheeding modes of slags will also be studied. For the fused slag layers, the properties of the fused slags was studied to obtain the viscosity, density and the surface tension at high temperature. And then the CFD calculations on the fused slags will be carried out to establish the flowing and dripping calculating modes. Based on the micro space arranged structure of the solid particles layer in slags and the stress distribution of slags, the DEM calculations will be carried out, and couple with the stress-strain behavior of the slags to build the fracture criterion. This project will provide the references on the high alkali coal slagging controlling and the soot blowing system optimization.

准东煤田是我国最大整装煤田，然而其较高的碱金属含量会诱发燃烧设备发生严重的结渣问题。高碱金属煤渣体由内到外依次为无机盐凝结层、表面接触固体颗粒层、相互结合固体颗粒层、及全部熔融或部分熔融层，各部分间相互作用力不同。当渣体形成后受重力、吹灰等内外部条件影响会发生脱落，但脱落形式差别较大，脱落机理也不清晰。本项目首次提出采用纳米CT技术获得渣体内部颗粒的空间排列结构和颗粒间的结合方式，并以此为基础阐述结渣机理；开展渣体生长与脱落实验，建立渣体脱落形式的评判准则、研究熔渣流动与滴落特性和渣体固体颗粒层宏观断裂特性；以所获得的熔渣相关属性为基础，开展CFD仿真计算，建立熔渣流动与滴落的计算模型；以渣体固体颗粒层的微观空间排列结构和渣体的应力分布为基础，开展DEM仿真计算，并耦合渣体固体颗粒层的应力-应变关系，建立渣体固体颗粒层的断裂准则。本项目的研究对治理高碱金属煤结渣、优化吹灰系统具有参考意义。

高碱金属煤燃烧过程中产生的渣体脱落的过程与热态下渣的力学特性紧密相关。本项目通过研究高碱金属煤的结渣机理，熔渣高温下的理化特性，通过Nano-CT对渣体进行扫描与三维重构，研究渣体高温条件下的抗剪特性，并获得抗剪切过程中的力链分布，获得如下结果：.（1）无论煤粉炉还是流化床锅炉在燃烧高碱煤过程中均会产生分层结渣现象，这是由于结渣初期，换热器表面温度较低，与烟气间温差较大，在较强热泳力驱动条件下，大量气相碱金属化合物在换热器表面凝结沉积并凝固；随着温度逐渐上升，渣体表面温度也逐渐升高，热泳力作用下降，产生结渣的主要原因变为熔融颗粒碰撞并凝固；渣体温度继续升高至与烟气温度相同时，渣体表面呈熔融态，可捕捉其他固相颗粒并发生化学反应，进一步使结渣情况恶化。在结渣过程中，还原性气氛也对结渣起促进作用，主要机理在于还原性气氛可将煤灰中Fe3+化合物转化为低熔点Fe2+化合物，促进结渣过程。项目分析了CO、NH3等还原性气体对Fe的还原特性和对煤灰的熔融特性。.（2）获取了准东地区高碱煤燃用后的煤灰，开展高温直剪特性实验，结果表明800℃时剪切面的内摩擦为62.122°，明显小于200℃时的内摩擦角，而黏聚力404.390kPa，明显大于200℃时的黏聚力。这说明温度升高使灰颗粒间的滑动摩擦和镶嵌作用所产生的摩擦阻力降低，使得颗粒间胶结作用增强。在剪切过程中，在初始状态时，法向压应力越大，颗粒间的接触力越大，在法向压应力达到200 kPa时，局部颗粒间接触力最大达到800 N;在剪切结束时，局部颗粒间接触力最大达到1800 N；剪切过程使得颗粒相互挤压，出现了倾斜的力链加强区域，且初始压应力越高，剪切结束时的力链强度越高。.（3）对渣体进行Nano-CT进行扫描，并进行三维重构，结果发现，渣体中以小颗粒为主，说明对于燃用准东煤的结渣问题，主要由密度较小的颗粒引起；渣体不同区域微观形貌大多不同，推测实际锅炉结渣状况可能与燃用煤种和锅炉运行状态有关。