煤灰流动性研究方法进展

Advances in research methods of coal ash fluidity

随着煤转化工业对转化率和生产效率要求的进一步提高,煤的热转化过程更趋向于在高温高压转化器中进行。在高温高压的液态排渣燃烧炉和气化炉中,煤中矿物质完全熔融成熔渣形式再排出。对于采用液态排渣和水冷壁的气流床气化炉,要求煤灰熔融温度低于操作温度,熔渣黏度范围为2.5~25.0 Pa·s,且在操作温度范围内黏度随温度的波动较小,因此气化过程中煤灰的熔融性和黏温特性是影响熔渣流动的关键因素。笔者论述了传统灰熔融评价方法的发展过程,各国标准方法的原理都是通过被压实样品在升温过程中的形变来判断得出熔融温度,但仅靠熔融温度无法提供实现现代大型气化过程精细化控制所需信息,而对煤灰熔融过程的全阶段测试有助于更准确地指导实际生产。对比各国研究者对熔融过程的定性和定量研究表明,熔融温度中的变形温度并非煤灰开始熔融的温度,针对煤灰沉积、烧结等问题,熔融全过程测试提供的开始收缩温度和热力学计算预测的液相最初形成温度有助于更准确地预测煤灰可能产生沉积或烧结的温度。黏温特性的测试目前仍依靠高温旋转黏度测试法,该法耗时较长且流程繁琐,因此研究者更趋向于用更简便和省时的方法实现对适用样品的黏温特性的快速筛选。除了试验方法,模拟计算方法在煤灰流动性研究中的应用越来越普遍,通过热力学计算和分子模拟方法,能够获得试验难以测得的矿物质组成及熔体的微观结构变化,且分子模拟中非平衡分子动力学方法可更准确模拟复杂流体的剪切稀化过程,从而获得更接近试验值的黏度计算结果。采用非平衡方法提高了计算结果的准确度,但也增加了计算的复杂程度及所耗费的机时,且目前煤灰体系的计算模型选择不多,因此采用分子模拟方法应综合考虑体系的复杂度与计算结果的准确性。随着熔融过程研究的进一步深入和模拟计算方法的普遍应用,试验结果呈现的宏观性质变化机理将更易于通过微观结构变化来阐明,反过来也将有助于优化现有的模拟计算方法和参数。

As the requirements for conversion rate and production efficiency in coal conversion industry further improving,the thermal conversion process of coal tends to be performed in converter with high temperature and high pressure. In the high-temperature and highpressure combustion furnaces and gasifiers with slag tapping,the minerals in coal are completely melted into slag and then discharged. For an entrained-flow gasifier with slag tapping and a water-cooled wall,the melting temperature of coal ash is required to be lower than the operating temperature,and the viscosity of the molten slag ranges from 2.5 to 25.0 Pa·s. Moreover,the fluctuation of viscosity with temperature should be small in the operating temperature range. The melting temperature and the viscosity-temperature characteristics of coal ash during gasification are the key factors affecting the flowing of slag. The development of traditional testing methods for ash melting temperature was reviewed and analyzed in this paper. The principle of standard methods in various countries is to determine the melting temperature by the deformation of the compacted sample during the heating process. However,the melting temperature alone cannot provide enough information to achieve fine control of modern large-scale gasification processes,and the full-stage testing of the coal ash melting process helps to guide the actual production more accurately. A few qualitative and quantitative studies of the melting process by researchers in various countries show that the deformation temperature in the melting temperature is not the real temperature at which the coal ash starts to melt. In view of the deposition and sintering problems of coal ash,the start temperature of shrinking and the initial formation temperature of the liquid phase that predicted by thermodynamic calculation helps to predict the temperature at which coal ash may deposit or sinter more accurately. At present,the test of viscosity-temperature characteristics still relies on the high-temperature rotational viscosimeter,which is time-consuming and complex. Therefore,a simpler and time-saving method is used to quickly screen the viscosity-temperature characteristics of suitable feedstocks. In addition to the experimental methods,the application of simulation calculation methods is more and more widely used in the study of coal ash fluidity. Through thermodynamic calculations and molecular simulation methods,it is possible to obtain mineral composition and microstructure changes of the melt that are difficult to measure in experiments. The non-equilibrium method in molecular dynamics can simulate the shear thinning process of complex fluids more accurately,thereby the obtaining viscosity calculation results are closer to the experimental values. Not only does the use of non-equilibrium methods improve the accuracy of the calculation results,but also increases the complexity of the calculation and the time required for the calculation. At present,there are not many choices of calculation models for the coal ash system. Therefore,the complexity of the system and the accuracy of the calculation results should be considered comprehensively when using the simulation calculation methods. With the further study of the melting process and the general application of simulation calculation methods,the mechanism of the change for macroscopic property presented in experiment will be easier to be clarified through the microstructure changes,which in turn will help optimize existing simulation calculation methods and parameters.