Fine Particles from Coal Combustion

When coal is combusted in a power station, the resulting ash material is produced in a wide range of particle sizes and chemistries.  The finest ash particles are typically labelled as PM10 and PM2.5, i.e., particles less than 10 and 2.5 µm in size, respectively.  PM10 and PM2.5 are known to be hazardous to human health, causing premature mortality, increased rates of cardiovascular disease, respiratory disease, and lung cancer.  The emission of such fine particles from a flue gas stack (and other processes) is highly regulated and environmental controls are often required. Combustion of coal is known to be a key contributor to atmospheric PM10 and PM2.5.

With increasingly stringent legislation, a greater understanding of the formation of fine particles from coal combustion is needed to predict PM10 and PM2.5 and assist with coal placement.  This project aimed to accurately measure the fine particles produced during combustion of coal at conditions equivalent to an industrial boiler and develop a draft model to predict the formation of fine particles based on mineral type and associations within the given coal.

Four Australian coals were combusted in a drop tube furnace, which has an equivalent heating rate to industrial boilers. The ash produced was collected in two cyclones in series, a cascade impactor and a 0.1 µm filter to ensure as far as possible complete collection of the ash was achieved. The cascade impactor provided a means of collecting and separating fine particles (between 0.3-2 µm) into more discrete size ranges for further analysis.

Particle size distributions of the experimental fly ash showed that PM10 comprised a significant proportion of the ash, even though the coals studied would be considered refractory in nature with very little alkali content. Alkalis are extremely volatile at boiler temperatures and considered key contributors to fine particle formation in coal combustion. PM10 ranged from 20 to 48 vol.% of the ash, or between 33 and 66 grams of PM10 for every kilogram of coal combusted. The more hazardous and harder to collect PM2.5 ranged from 9.75 to 25 grams for every kilogram of coal combusted.

With poorer burnout, the amount of PM10 and PM2.5 increases dramatically due to the presence of agglomerates of nano-sized carbon particles. As burnout increases the last carbon particles to remain are relatively large swollen unreactive char particles, collected in the first cyclone, and the very unreactive agglomerates of nano-size carbon, commonly referred to as soot, collected in the cascade impactor and filter.

Silicon was found to be enriched within the ultra-fine particles, around 0.02 µm, while aluminium enriched 1-2 µm particles and titanium enriched 0.1-1 µm particles. Enrichment of fine particles by calcium, iron, magnesium, phosphorous and sulfur was coal dependent. Enrichment by potassium and sodium was not observed but the coals studied had very low alkali contents. Minerals in the feed coal were identified as the source of each of these enrichments but due to the limited number of coals studied further work is required to confirm the observed deportment.

A model to predict the particle size distribution of the fly ash was developed using SEM-TIMA analysis of each coal as the input. TIMA analysis is an automated Scanning Electron Microscope technique that provides the mineral types, particle size and associations within the feed coal. The model is an excel spreadsheet with controlling macro code and predicts the resulting ash from the following mechanisms:

• Mineral fragmentation;
• Char swelling and fragmentation to form multiple char particles;
• Vaporisation and condensation of volatile elements; and
• Mineral coalescence in each resulting particle.

Modelling suggests vaporisation-condensation dominates the formation of PM2.5, though the other mechanisms can contribute, depending on the coal. While vaporisation-condensation also impacts PM10 because PM2.5 is included in PM10 values, for most coals char swelling had the greatest impact. The mineral type, size and associations within the feed coal directly affect the size of ash particles that form. Therefore, each coal will produce a different ash particle size distribution.