Asabe Standard X593
The American Society of Agricultural and Biological Engineers (ASABE) is an educational and scientific organization dedicated to the advancement of engineering applicable to agricultural, food, and biological systems. Founded in 1907 and headquartered in St Joseph, Michigan, ASABE comprises 9,000 members in more than 100 countries. ASABE membership is open to all (engineers as well as non-engineers) who are interested in the knowledge and application of engineering in agricultural, food, and biological systems.
ASABE Standard X593 is the standard introduced by the American Society of Agricultural and Biological Engineers (ASABE) entitled “Terminology and Definitions for Biomass Production, Harvesting and Collection, Storage, Processing, Conversion and Utilization.”
The purpose of the standard is to provide uniform terminology and definitions in the general area of biomass production and utilization. This standard includes many terminologies that are used in biomass feedstock production, harvesting, collecting, handling, storage, pre-processing and conversion, bioenergy, biopower, and bioproducts. The terminologies were reviewed by many experts from all of the different fields of biomass and bioenergy before being accepted as part of the standard.
See also: Biomass.
Ash
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.
Ash Analysis
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.
Ash Composition
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.