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The chemical composition of ash is an important factor in fouling and slagging problems and in the viscosity of ash in wet bottom and cyclone furnaces. The potential for the mineral constituents to react with each other as well as undergo significant mineralogical changes is high. The use of biomass feedstocks with mineral matter that gives a high alkali oxide ash often results in the occurrence of slagging and fouling problems. As oxides, most ash elements have high melting points, but they tend to form complex compounds (often called eutectic mixtures) which have relatively low melting points. On the other hand, high-calcium-low-iron ash coals exhibit a tendency to produce low-melting range slags, especially if the sodium content of the slag exceeds approximately 4% w/w.

One form of ash is fly ash, one of the residues generated during combustion. Fly ash is generally captured from the chimneys of power plants, and is one of two types of ash that are jointly known as ash; the other form of coal ash is bottom ash, which is removed from the bottom of coal furnaces.

Depending upon the source and makeup of the feedstock to the combustor, the components of fly ash vary considerably (Table A-24), but all fly ash includes substantial amounts of silicon dioxide (SiO₂) (both amorphous and crystalline) and calcium oxide (CaO), both being endemic ingredients in many coal bearing rock strata.

Table A-24 Common constituents of fly ash (% w/w).

Coal:	Bituminous	Subbituminous	Lignite
SiO2	20-60	40-60	15-45
Al2O3	5-35	20-30	20-25
Fe2O3	10-40	4-10	4-15
CaO	1-12	5-30	15-40

Toxic constituents depend upon the specific coal bed makeup, but may include one or more of the following elements or substances (in alphabetical order and not in order of occurrence) in quantities from trace amounts to several percent: arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium. Organic constituents of ash include dioxins and polynuclear aromatic compounds.

Fly ash solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags (Baghouse). Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 micron to 100 µm. They consist mostly of silica (SiO2), which is present in two forms: amorphous (rounded and smooth) and crystalline (sharp, pointed, and hazardous), aluminum oxide (Al₂O₃), and iron oxide (ferric oxide, Fe₂O₃). Fly ash is generally highly heterogeneous and consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz, mullite (3Al₂O₃.2SiO₂ or 2Al₂O₃.SiO₂), and various iron oxides.

In the past, fly ash was generally released into the atmosphere, but pollution control equipment mandated in recent decades now requires that it be captured prior to release. In the United States, fly ash is generally stored or placed in landfills or is often used to supplement Portland cement in concrete production as well as in the synthesis of geopolymers and zeolites.