James G. Speight - Encyclopedia of Renewable Energy

The use of fuel with mineral matter that gives a high alkali oxide ash often results in the occurrence of slagging and fouling problems, especially in gasifiers. As oxides, most ash elements have high melting points, but they tend to form complex compounds (often called eutectic mixtures) that have relatively low melting points. On the other hand, high-calcium-low-iron ash coals tend to exhibit a tendency to produce low-melting range slag, especially if the sodium content of the slag exceeds approximately 4% w/w.

The chemical composition of the 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. In addition, fuel with a high iron content (usually >20% w/w ferric oxide) ash typically exhibits ash-softening temperatures under 1,205°C (2,200°F). Also, volatile alkali compounds lower the fusion temperature of ash. In conventional combustion equipment having furnace gas exit temperatures above 790°C (1,450°F, combustion of agricultural residue causes slagging and deposits on heat transfer surfaces. Specially designed boilers with lower furnace exit temperatures could reduce slagging and fouling from combustion of these fuels. Low-temperature gasification may be another method of using these fuels for efficient energy production while avoiding the slagging and fouling problems encountered in direct combustion.

In some test methods, it is recommended that the color of the ash should be noted as it gives an approximate indication of the fusion point. Generally, highly colored ash has a low fusion point while white ash, provided they are relatively no basic oxides, has a high fusion point.

See also: Biomass Ash, Bottom Ash, Fly Ash.

Ash analyses are used for evaluation of the corrosion, slagging, and fouling potential of ash. Typically, the ash constituents of interest are silica (SiO2) alumina (Al2O3), titania (TiO2), ferric oxide (Fe2O3), lime (CaO), magnesia (MgO), potassium oxide (K2O), sodium oxide (Na2O), and sulfur trioxide (SO3). An indication of ash behavior can be estimated from the relative percentages of each constituent.

The determination of mineral ash in a fuel is usually by heating (burning) an accurately-weighed sample of the coal in an adequately ventilated muffle furnace at temperatures in the range 700 to 750°C (1,290 to 1,380°F) for 4 hours. Typically, the experimental data should be reproducible within ±0.2% of the end result. Other standard test methods may vary and require somewhat higher temperatures for the determination of the ash in coal.

See also: Ash, Ash Content.

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 (SiO 2) (both amorphous and crystalline) and calcium oxide (CaO), both being endemic ingredients in many coal bearing rock strata.

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

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 (Al 2O 3), and iron oxide (ferric oxide, Fe 2O 3). Fly ash is generally highly heterogeneous and consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz, mullite (3Al 2O 3.2SiO 2or 2Al 2O 3.SiO 2), 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.

Ash content (which is a thermal manifestation of the inorganic content of a fuel, such as biomass) is the inorganic oxides that remain after complete combustion of the feedstock. The amount of ash between different types of feedstocks differs widely (0.1% w/w for wood and up to 15% w/w for some agricultural products) and influences the use of the fuel as well as the design of the reactor, particularly the ash removal system. The chemical composition of the ash is also important because it affects the melting behavior of the ash. Ash melting can cause slagging and channel formation in the reactor. Slag can ultimately block the entire reactor.

Generally, the ash-forming inorganic materials in most solid fuels, including biomass, can be divided into two broad fractions: (i) the inherent inorganic material and (ii) the extraneous inorganic material.

The inherent inorganic material exists as part of the organic structure of the fuel, and is most commonly associated with the oxygen-, sulfur-, and nitrogen-containing functional groups. These organic functional groups can provide suitable sites for the inorganic species to be associated chemically in the form of cations or chelates. Biomass materials tend to be relatively rich in oxygen-containing functional groups, and a significant fraction of the inorganic material in some of the lower ash biomass fuels is commonly in this form. It is also possible for inorganic species to be present in fine particulate form within the organic structure of some of the fuels, and to behave essentially as an inherent component of the fuel.