The Structure of Submicron Ash from Combustion of Pulverized South African and Colombian Coals

Abstract

The formation of submicron ash particles during the utility-scale pulverized combustion of South African Klein Kopie and Colombian El Dorado coals was studied by measuring the ash particle number and mass size distributions in the size range 0.01–1 pm upstream of the electrostatic precipitator (ESP). Ash morphology, composition and microstructure were studied by high resolution scanning and transmission electron microscopes (SEM and TEM).

From 0.75 to 1.5 percent of the ash on a mass basis was in the particles smaller than 0.5 pm, i.e. had volatilized during combustion. Number mean sizes as determined by the differential electrical mobility method (DMA) varied from about 0.09 pm up to about 0.15 pm. Particles above 0.2 pm were spherical in shape, being formed due to transformations of the non-volatilized coal ash fractions. Volatilized ash species formed high number concentrations of submicron agglomerates having characteristic lengths up to 1 pm. They dominated the submicron number size distribution mode. The agglomerates were formed from a few to up to about one hundred partially fused primary particles. The size of the primary particles varied from 20 to 60 nanometers.

The amorphous primary particles from the combustion of Klein Kopie coal were not exactly spherical. They seemed to have formed from particles less than 10 nm in diameter. These tiny particles were almost completely fused together to form primary particles that appeared spherical in the lower magnification TEM and SEM micrographs. Similarly, the primary particles from the combustion of El Dorado coal seemed to have formed from the almost completely fused 20 nm spherical particles. Areas with clear crystalline fringes were observed within the amorphous primary particles formed during the combustion of El Dorado coal, suggesting sufficiently high temperatures were reached for the crystallization of some of the condensed ash components.

We propose new mechanisms for the formation of submicron agglomerated ash particles in pulverized coal-fired boiler flames.