Ash chemistry

Char burns in the combustion zone at the bottom of the reactor, forming C02 and steam, which flow upward through the bed. Residual ash is removed from the bottom of the reactor at the grate. Operational problems can be avoided for feeds with low-melting-point ash by carefully controlling the temperature in the combustion zone or with fuel blending or additives to alter the ash chemistry on the grate. 

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. 

A variety of techniques have been used to model the deposition processes. Frequently a small furnace will be built to provide controlled combustion and careful monitoring of the conditions. The coal is burned and the ash is allowed to impinge on tubes. The increase in deposit weight is noted as a function of time under different conditions. This approach has been helpful and is useful if variations in ash chemistry are the most significant variables. In some furnaces, the aerodynamics are also quite important and cannot usually be scaled. It is also possible to model the effects of impinging ash-like particles by substituting glasses of known fusion or viscosity properties for the coal ash type materials. 

It is hoped that this volume will serve as a further stimulus to workers in and outside of the field of coal mineral matter and ash chemistry. We all benefit from the work of others to extend our current knowledge and can help make coal not only an abundant fuel, but also a more convenient and desirable resource as well. 

Variations Due to Geologic Factors. Ash chemistry is another important factor affecting elemental variations. Its influence is, however, generally more subtle than the other factors. In Table V the contents of selected elements are compared for Appalachian Province coals and Wasatch Plateau coals. Both are bituminous coals with similar ash contents. However, with a few exceptions, the content of the elements in the Wasatch coals is lower than those of Appalachian coals. The lower content of chalcophile elements in Wasatch coals may be due to a lower pyrite content. 

The lower concentration of lithophile elements (e.g., Li, Zr, Nb, Th, Sc, Y) but higher Si content may reflect a higher quartz content in the detrital component of Wasatch coals this, in turn, may be a reflection of differences in the mineralogy of the source rocks. Volcanic ash, which is prevalent in some of our western coals can have a significant impact on ash chemistry. 

It is common practice to make a distinction between the inorganic constituents of so-called "Eastern" and "Western" coals By definition. Western coals are those for which the CaO+MgO content exceeds the Fe203 content of the ash, while the reverse is true for Eastern coals [ 1 I The inorganic constituents in Eastern coals, which are principally bituminous in rank, are predominantly in the form of discrete mineral particles. Clay minerals (kaolinite, illite) are usually dominant, followed by quartz and pyrite. The range and typical values of the mineral distribution and ash chemistry of Eastern coals are shown in Table I. These data were determined from computer-controlled scanning electron microscopy (CCSEM), Mossbauer spectroscopy, and other measurements on over a hundred coals. 

In summary, traditional methods for prediction of ash deposit characteristics are heavily based upon ash chemistry. These conventional analyses do not provide definitive information concerning the mineral forms present in the coals and the distribution of inorganic species within the coal matrix. Such information can be extremely important in extrapolating previous experience, since the nature in which the inorganic constituents are contained in the coal can be the determining factor in their behavior during the ash deposition process. 

The most uniquely suited standardized analyses for ash deposition include ash chemistry and ash fusion temperature. The total ash content from proximate analysis and ash composition provide the fuel irrformation that goes into the majority of common empirical indices of ash behavior, along with ash fusion temperature. The ash chemistry analysis typically reports the ash elemental composition on an oxide basis. This does not mean that all of the species exist as oxides in the fuel (which they do not). It is a convenient method of checking the consistency of the data. The sum of the oxides shoirldbe about the same as the total ash content. The analysis is fundamentally an elemental analysis with no distinction of the chemical speciation of the inorganic species. 

Wessel, R.A., Verrill, C.L., 1998, Black Liquor Combustion Model for Predicting Ash Chemistry in Kraft recovery," The proceeding of AICHE annual meeting. Horida, USA. 

Slagging and fouling issues also must be considered when adding opportunity fuels to the total mass of fuel entering a boiler or combustion system. The ash chemistries associated with many of the opportimity fuels are fundamentally different from the ash chemistries associated with most coals being burned today. 

Miller, S.F., B.G. Miller, and D.A. Tillman. 2002. The Propensity of Liquid Phases forming During Coal-Opportunity Fuel (Biomass) Cofiring as a Function of Ash Chemistry and Temperature. Proc. 27 International Technical Conference on Coal Utilization and Fuel Systems. Clearwater, FL. March 4-7. 

Petroleum coke is, inherently, a low ash fuel however the ash chemistry of petroleum coke remains significant due to issues of slagging and fouling, and trace metal emissions. Bryers [4, 5] has reported general ash characteristics for petroleum cokes as a fiinction of coking method. 

Tables 4-7 and 4-8 present bulk ash chemistries of various wood fuels obtained during test programs. Table 4-7 focuses upon virgin wood material, including sawdust and clean tree trimmings from the urban forest. Table 4-8 presents representative data from some urban wood wastes plus short rotation woody crops. Wide variations in ash compositions can be observed between the various woody fuel samples.

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