Combustion and Self-Desulfurization Characterization of Blended Low-Rank Coals for Improved Resource Utilization in Fluidized Bed Boilers

The co-combustion of low-rank coals through fluidized bed boiler (CFB) is an effective approach to enhance the level of resource utilization. To date, there has been a lack of investigation concerning the co-combustion kinetics and self-desulfurization characteristics of coal slime, coal gangue, and raw coal. In this study, we adopted multiple model-free and model-fitting methods to comparatively analyze co-combustion kinetics of blended coals on the basis of thermogravimetric data. Then, the sulfur balance and self-desulfurization of blended coals in the co-combustion were intensively investigated using a tube furnace set-up. The results reveal that in the presence of coal gangue in blended coals, the average activation energy (E_a) falls within the range of 65.7 kJ/mol to 100.4 kJ/mol, as determined by four model-free methods. Conversely, in the absence of coal gangue, only the Flynn-Wall-Ozawa (FWO) and Friedman (FM) methods are deemed appropriate for calculating the average E_a, yielding a value of 77.3 kJ/mol. The first order reaction model is confirmed to be reliable for analyzing the co-combustion kinetics of low-rank blended coals. Irrespective of the specific composition of the blended coal, a significant linear correlation exists between the E_a and the natural logarithm of the pre-exponential factor (lnA) within an extensive range of parameters. Moreover, the addition of coal gangue to the blended coal substantially enhances the self-desulfurization level, resulting in an increase from 25.7% to 60.7% at 1073 K. The self-desulfurization efficiency exhibits a good linear relationship with both the mass ratio of coal gangue to coal slime and the molar ratio of calcium to sulfur. In a practical implementation, the optimal addition ratio of coal gangue is a trade-off between the self-desulfurization efficiency and the ignition capacity.