A Study on Coal Properties and Combustion Characteristics of Blended Coals in Northwestern China

The utilization of blended coals has become popular in northwestern China. In this paper, investigations on coal properties and combustion characteristics of blended coals have been carried out to provide guidance for blended-coal-fired plants. Experimental results show that proximate, ultimate, and calorific value analyses of blended coals can be calculated by mass-weighted average of individual coals. However, grindability, ash composition, ash fusion temperatures, and some characteristics related to combustion performance of blended coals cannot be predicted accurately from those of individual coals and the mixture ratio. Blending a coal of high fusion temperatures with a coal of low fusion temperatures can significantly increase the ash fusion temperatures of blended coals. Thermogravimetric experimental results indicate that the intense combustion stage of blended coals is quite different from those of individual coals. Thermogravimetric curves of blended coals lie between those of individual coals but show nonadditive behavior. Furthermore, combustion characteristics of blended coals improve significantly with increased oxygen concentration, and combustion performance of low-reactivity coal is less sensitive to changes in oxygen concentration. Kinetic analysis indicates that the apparent activation energy of coal combustion is not constant at different conversion fractions. The combustion process of blended coals is charactered by segmentation and the apparent activation energy in low-temperature zone is higher than that in high-temperature zone. The mean activation energy of blended coals shows nonadditivity performance in both Kissinger-Akahira-Sunose (KAS) and Coats-Redfern methods due to the interaction between individual coals. The KAS method is more adequate than the Coats-Redfern method for analyzing the kinetics of blended coal combustion because the Coats-Redfern method requires prior assumption of reaction mechanism and its analysis results sometimes differ clearly from experimental results.