(583dj) Modeling and Prediction of Coal Devolatilization At High Temperatures With Different Heating Rates

Coal devolatilization is acknowledged as the first step and also of primary importance in coal thermal conversion techniques, such as coal coking, gasification, combustion and so on. Thus, the heating history and release of volatile matters in coal particles have a significant impact on the overall reactor performance. However, the heating rate and the ultimate temperature are much different in coal utilization processes. The heating rate is in the range of 10^-1~10¹ K/s in a coking plant, 10²~10⁴ K/s in the processes of gasification or combustion and 10⁵~10⁷ K/s in a plasma pyrolysis process. Therefore, establishing the fundamental knowledge of coal pyrolysis under various conditions (such as various kinds of coal, a wide temperature and heating rate range) is very important to understand the common pyrolysis mechanism in industrial coal conversion processes.

In this work, a comprehensive heating and devolatilization mechanism model with special consideration of the particle-scale physics (such as the heat transport inside particle) was established to study coal devolatilization behavior under various conditions, especially under some extreme conditions (such as high temperatures greater than 1800 K and milliseconds of reaction time). The chemical percolation devolatilization (CPD) model was improved based on the experimental data of coal pyrolysis, which was further applied to describe the physical and chemical transformations of various kinds of coal to explore the dominant role of coal rank in coal devolatilization process. The predictions of the improved CPD model agreed well with the experimental data under various conditions.

The effects of surrounding gas temperature, gas composition and particle size on particle heating history and coal devolatilization behavior were simulated based on this mechanism model. The results revealed that the heat transport resistance inside particle is of great importance for large particle heating in pure hydrogen atmosphere, which would impede the thermal energy transportation from surrounding gas to the particle. The heating rate and devolatilization time were then related to temperature of surrounding gas, thermal conductivity of heating gas, and diameter of particles. It was summarized that the coal devolatilization time has excellent linearity with the particle heating rate in the double logarithm coordinate, independent of the operating conditions and coal ranks.

According to the proposed single-particle heat transfer model, it was recommended that the particle residence time in high temperature zone should be larger than the corresponding coal devolatilization time under the same conditions in the practical application. Since the proposed model works well in understanding the heating history and devolatilization behavior of a single particle with different heating rates, this model can provide more scientific foundation to help design the industrial reactor, eventually to guarantee the overall coal conversion performance.