Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 108

Ash is the noncombustible residue remaining after complete combustion of a fuel and is composed primarily of oxides and sulfates, and it should not be confused with mineral matter, which is composed of the unaltered inorganic minerals in the fuel. In very general terms, the inorganic materials in most solid fuels, including biomass, can be divided into two broad fractions: (i) inherent inorganic material and (ii) 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.

The extraneous inorganic material has been added to the fuel through geological processes, or during harvesting, handling, and processing of the fuel. Biomass fuels, for instance, are commonly contaminated with soil and other materials, which have become mixed with the fuel during collection, handling and storage.

Ash is quantitatively and qualitatively different from the mineral matter originally present in the fuel because of the various changes that occur, such as loss of water from silicate minerals, loss of carbon dioxide from carbonate minerals, oxidation of iron pyrite to iron oxide, and fixation of oxides of sulfur by bases such as calcium and magnesium. In fact, incineration conditions determine the extent to which the weight changes take place and it is essential that standardized procedures be closely followed to ensure reproducibility.

Thus, ash is formed as the result of chemical changes that take place in the mineral matter during the ashing process. The quantity of ash can be more than, equal to, or less than the quantity of mineral matter in the fuel, depending on the nature of the mineral matter and the chemical changes that take place in ashing. The various changes that occur include (i) loss of water from silicate minerals, (ii) loss of carbon dioxide from carbonate minerals, (iii) oxidation of iron pyrite to iron oxide, and (iv) fixation of oxides of sulfur by bases such as calcium and magnesium. In fact, incineration conditions determine the extent to which the weight changes take place and it is essential that standardized procedures be closely followed to ensure reproducibility.

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.