A new method for identifying the modes of particulate matter from pulverized coal combustion

Abstract

Knowledge of the modality of the size distribution of particulate matter (PM) from pulverized coal combustion is of great significance from the viewpoint of exposure and risk assessment. Mass or number size distributions are usually used in modality analyses, but sometimes the central particle mode fails to be detected due to overlapping. This work provides a new method for identifying particle modes using mass fraction size distributions of the aluminum (Al). Five Chinese pulverized coals of different ranks were burnt in a laboratory drop tube furnace at 1673 K. The produced PM was size segregated by a low pressure impactor and subjected to elemental composition analysis by X-ray fluorescence (XRF). Particle mass size distributions, mass fraction size distributions of the Al and sulfur (S) were obtained for all the particle samples. The mass size distributions of four coal ash samples all show three distinct particle modes, with a central mode at approximately 2 µm, while the mass size distribution of the LPS coal ash sample only indicates two particle modes. However, the mass fraction size distributions of the Al for all ash samples, including the LPS coal ash sample, generally show three particle modes. The obscurity of the central mode in the mass size distribution of the LPS coal ash sample is expected to be a consequence of the merging of it into the coarse mode. The formation of the central mode is attributed to the more pronounced heterogeneous condensation or adsorption of vaporized species on fine residual ash particles, whose origins are still unclear at present. This is further confirmed by examination of the mass fraction size distributions of the S. These results show that mass fraction size distributions of the Al seem to be more effective in identifying particle modes and their size boundaries than particle mass size distributions.