Encyclopedia of Renewable Energy. James G. Speight

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.

      Ash Content

      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.

      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.

The most commonly applied technique for the determination of the ash content and ash composition of coals and other solid fuels in the laboratory involves heating the fuel slowly in air to constant mass at a temperature of 815°C (1,500°F), and subjecting the resultant ash residue to chemical elemental analysis. The ash residue is normally weighed to provide an estimate of the ash content of the fuel, and then analyzed for the 10 major elements present in coal ashes, i.e., Si, Al, Fe, Ca, Mg, Ti, Na, K, P, and S. The elemental concentrations are conventionally expressed as oxides, in their highest oxidation states. The analysis of the laboratory-prepared ash for its trace element content is also fairly common practice. This is a perfectly reasonable and practical approach for most coals, and many other solid fuels, and has been applied for biomass ash analysis.

Biomass feedstocks and fuels exhibit a wide range of physical and chemical properties (Table A-25), but despite the wide range of possible sources, biomass feedstocks are remarkably uniform in many of their fuel properties, compared with competing feedstocks such as coal or crude oil. For example, there are many types of coal whose gross heating value ranges from 8,600 to 12,900 Btu/lb). However, nearly all kinds of biomass feedstocks destined for combustion fall in the range 6,450 to 8,200 Btu/lb). For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb. The values for most woody materials fall into the range 7,750 to 8,200 Btu/lb.

Table A-25 Properties of biomass and other fuel sources.

	Chemical Characteristics
Ash, %	Sulfur, % w/w	Potassium, % w/w	Ash melting temperature (oC)
Bioenergy Feedstocks	corn stover	5.6
	sweet sorghum	5.5
	sugarcane bagasse	3.2-5.5	0.10-0.15	0.73-0.97
	sugarcane leaves	7.7
	hardwood	0.45	0.009	0.04
	softwood