Multi-objective optimization of the layout and heating scheme for microwave pyrolysis of a tar-rich coal seam based on RSM, NSGA-II and TOPSIS methods

Microwave heating, as a new type of heating method, has great potential in improving pyrolysis efficiency and achieving targeted regulation of products. A multi-objective optimization was conducted in this study to optimize conversion completed time and the maximum temperature of microwave pyrolysis of a tar-rich coal seam under different parameters by combining RSM, NSGA-II and EWM-TOPSIS methods. Firstly, response surface method was utilized to establish the relationship between two objective functions (conversion completion time, the maximum temperature when the conversion completed) and three independent variables (horizontal spacing, vertical spacing, and the power ratio between the central and lateral heating ports), and the influence degree of each factor was explored through variance and sensitivity analysis. Then, the Pareto frontier was identified through the NSGA-II optimization algorithm to obtain the solution set with the optimal comprehensive evaluation. Finally, the ideal solution in the Pareto frontier was determined through the EWM-TOPSIS evaluation method. The optimal result of microwave layout and heating scheme obtained is a horizontal spacing of 3.61 m, a vertical spacing of 5.31 m, a power ratio of 1.12, the corresponding conversion completion time is 612.34 days, and the maximum temperature is 595.11 °C. Compared to the benchmark conditions, the conversion completion time and the maximum temperature of the optimal solution are reduced by 10.21% and 10.98% respectively. This indicates that the optimized scheme can effectively control the maximum temperature and improve the pyrolysis efficiency. And this optimization framework provides a referable and computationally efficient strategy for optimizing other problems involving multiple conflicting objectives.