Fly ash

The fine ash from combustion processes that becomes dispersed in the air. To the casual observer it might appear as smoke , and indeed it is often found mixed with smoke. Fly ash arises because fuels contain a small fraction of incombustible matter. In a fuel like coal , ash has a rock-like siliceous composition, but the high temperatures at which it is formed often means that metals such as iron are incorporated into the ash particles, which take on the appearance of small, colored, glassy spheres. Petroleum produces less ash, but it is often associated with a range of oxides such as vanadium (in the case of fuel oils) and, more noticeably, hollow spheres of carbon . In traditional furnaces, much ash remained on the grate, but modern furnaces produce such fine ash that it is carried away in the hot exhaust gas.

Early power stations dispersed so much fine fly ash throughout the nearby environment that they were soon forced to adopt early pollution abatement techniques. They adopted "cyclones" in which centrifugal force removes the particles by causing the waste gas stream to flow on a curved or vortical course. This technique is effective down to particle sizes of about 10 μm, but smaller particles are not removed well by cyclone collectors and here electrostatic precipitation process often proves more successful, coping with a size range 30—0.1 μm. In electrostatic precipitators the particles are given a negative charge and then attracted to a positive electrode where they are collected and removed. Cloth or paper filters and spraying water through the exhaust gases can be useful in removing fly ash.

Fly ash is a nuisance at high concentrations because it accumulates as grit on the surfaces of buildings, clothes, cars, and outdoor furnishings. It is a highly visible and very annoying aspect of industrial air pollution . The deposition of fly ash increases cleaning costs incurred by people who live near poorly controlled combustion sources. Fly ash also has health impacts because the finer particles can penetrate into the human lung. If the deposits are especially heavy, fly ash can also inhibit plant growth.

Each year millions of tons of fly ash are produced from coal-powered furnaces, most of which are dumped in waste tips. Care needs to be taken that toxic metals and alkalis are not leached from these disposal sites into watercourses. Fly ash may be used as a low grade cement in road building because it contains a large amount of calcium oxide but generally, the demand is rather low.

[Peter Brimblecombe Ph.D. ]

RESOURCES

BOOKS

Sellers, B. H. Pollution of Our Atmosphere. Bristol: Adam Hilger, 1984.