Mineral Distribution in Pulverized Blended Zhundong Coal and Its Influence on Ash Deposition Propensity in a Real Modern Boiler Situation.

This study aims to explore the mechanism under which ash deposition propensity is improved by coal blending in a real modern boiler situation. In this paper, Zhundong coal (ZD), from northwestern China, known to have a heavy ash deposition problem in boilers, was blended with Jincheng anthracite (JC), which has a high ash fusion temperature (AFT). The density composition of the coal blend, which reflects the mineral distribution in pulverized coals, was found to change during the intergrinding process. The higher rank coal JC was found to be more concentrated in its lower and highest density fractions. The variances in the chemical composition among density fractions in the pulverized coal blend were found greatly narrowed as compared with the parent coals. AFT results indicate that the ash melting behavior in the coal blend (ZD/JC = 50:50) varies with density, which is confirmed by corresponding slag contents calculated with FactSage 7.1. Particle size distributions of the density fractions in ZD, JC, and the coal blend were determined with a laser particle analyzer. The size distribution of ash particles in each density fraction was estimated according to the char morphology, which is deduced from the ash content and coal particle size distribution. Minerals in the lowest density fractions will form ash with a particle size of around 2 μm. Included minerals in the medium density fractions will form ash particles with size related to its ash content, and excluded minerals will undergo slight fragmentation and have a ash particle size similar to the corresponding coal particles. A comprehensive comparison between ash volume, ash particle size, and softening temperature indicates that ash deposition propensity of the coal blend is improved not only because of an apparent increase in AFT but also because of an apparent decrease in the total volume of ash particles possibly arriving on the deposition surface.