Chemistry of Ash Formation

When coal bums in air, as in the determination of ash in proximate analysis, all the organic material is oxidized or decomposed to give volatile products, and the inorganic material associated with the coal is subjected to the combined effects of thermal decomposition and oxidation. As a result, the quantity and composition of the resulting ash differ considerably from those of the inorganic materials originally associated with the pure coal substance. 

It is therefore impossible to determine accurately the composition of the pure coal substance from the usual ultimate analysis simply by making allowance for the quantity of ash left behind as a residue when the coal is burned. Results obtained in this fashion are, as a consequence, quoted as being on a dry, ash-free basis, and no claim is therefore made that these results do in fact represent the composition of the pure coal substance. If, however, it were possible to calculate accurately the quantity of mineral matter originally present in the coal sample, then by making due allowance for this material, the composition of the pure coal material could be deduced with reasonable precision and certainly with a greater accuracy than could be obtained by adopting the analytical figures calculated to a dry, ash-free basis. 

TABLE 5.2 Reactions of Coal Minerals at High Temperature 

Clays Loose structural OH groups with rearrangements of structure and release of H20 

Carbonates Decompose with loss of C02 residual oxides fix some organic and pyritic S as sulfate 

The clay minerals in coal contain water that is bound within their lattices. Kaolinite contains 13% bound water illite contains 4.5% bound water and montmorillonite contains 5% bound water. In addition, montmorillonite that occurs in mixed-layer clays also contains interlayer or adsorbed water. All the water is lost during the high-temperature ashing, for example, 

Calcium carbonate (calcite, CaCOj) is calcined to lime (CaO) during high-temperature ashing and carbon dioxide is evolved, resulting in a 44% redaction in weight from the original calcite, for example, 

Other metal carbonates behave similarly, that is, the oxides are formed during the ashing procedure. 

---

Although Macquer s explanation is correct, A.-L. Lavoisier still held to the more conservative opinion. In his Elementary Treatise on Chemistry, which was first published in 1789, he explained the formation of potassium carbonate in vegetable ash as follows As the potash is not formed, or at least not liberated, said he, except as the carbon of the plant is converted into carbonic acid by the addition of oxygen, either from the air or from the water, the result is that each molecule of potash, at the moment of its formation, finds itself in contact with a molecule of carbonic acid, and since there is great affinity between these two substances, combination must take place (13). 

The major technical barrier associated with biomass combustion systems is the formation of tenacious deposits on heat transfer surfaces caused by the unique high-temperature chemistry of biomass ash. Many types of biomass used in combustion systems contain alkali metal species sodium, potassium, and calcium. The ash content of woody biomass is quite low and not a problem. The ash content of agricultural residues such as wheat straw, rice straw, com stover, and alfalfa stems can be quite high, on the order of 5 percent or up to 20 percent for rice straw and rice husks. The presence of alkali metals in conjunction with the high silica content of some biomass ashes can lead to molten ash at combustion temperatures.553-658 In some cases, the K, Si, and A1 contents are such that very low melting-point eutectic mixtures can form. The materials can be fluid at combustion temperatures, but form glasslike deposits on colder downstream surfaces such as heat exchanger tubes. 

SEM microphotogaphs and EDAX scans of the cross section and outer surface of the slag deposit, illustrated in Figure 10, indicate the chemistry of the deposit is not uniform. The bulk of the fused material is rich in silica, low in iron, and virtually depleted of potassium. The outermost layers, no more than 2 to 3p thick, are very rich in iron and frequently also rich in calcium. On occasion, the outer surface is covered with small particulate, several microns in diameter, or undissolved cubic or octahedral crystals whose origin is pyrites. Similar formations have been observed in full-scale operation. The evidence indicates deposits form under axial symmetric flow conditions in the furnace by the fluxing action at the heat transfer surface of small particles, <8p in diameter, of decidedly different chemical composition and mineral source. Migration of the fly ash to the surface is by means of eddy diffusion, thermophoresis, or Brownian motion. 

Mineral matter in coal, during combustion, transforms into fly ash, and results in the buildup of ash deposits on heat transfer surfaces in PC-fired boilers. The ash formation process determines the ash character, that is, its particle size distribution and variation in chemistry. There are two models that can be used to represent ash formation from mineral matter (1) coalescence of included mineral grains and (2) fragmentation of excluded mineral grains during combustion. 

Hassett, D. J. Thompson, J. S. 1994. Formation of ettringite in disposed low-rank coal fly ash Implications for impact on groundwater chemistry. Proceedings of the North Dakota Water Quality Symposium, 30-31 March, 1994, North Dakota State University Extension Service, 122. 

Eventually, a polymeric substance of the form -(As GaCH2)w- may be formed. Adduct formation is thought to be responsible for the observed lower decomposition temperature of Ga(CH3)3 in the presence of AsH3 relative to Ga(CH3)3 in a carrier gas (120, 121). Furthermore, with AsH3 and D2, the primary reaction product appears to be CH4 rather than CH3D, as expected on the basis of the free-radical mechanism (equations 16a-f). However, the formation of CH4 may be due to reactions of Ga(CH3)t and CH3 with adsorbed AsH v species (122). Estimates indicate that the adduct is too unstable to play a major role in the growth chemistry (129), but this conclusion is subject to uncertainties in the thermochemical data base. 

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

Ash deposition in biomass combustion systems has been the focus of numerous research efforts.559,659 The basic mechanism for deposit formation in biomass combustion systems starts with the vaporization of alkali metals, usually chlorides, in the combustor. Fly ash particles, which are predominantly silica, impact and stick to boiler tube surfaces. As the flue cools the alkali metal vapors and aerosols quench on the tube surfaces. When the ash chemistry approaches equilibrium on the surface and the deposit becomes molten, the likelihood increases that additional fly ash particles will stick, and deposits grow rapidly. Ash deposits can also accelerate the corrosion or erosion of the heat transfer surfaces. This greatly increases the maintenance requirements of the power plant often causing unscheduled plant interruptions and shutdown. 

---