Reactivity of Coal, Coal Macerals, and Combustion Byproducts - unique program studying the combustion properties and char reactivity of single coal macerals, coal particles and fly ash carbon.

Abstract of Recent Work: Combustion Reactivity of Coal Macerals

Background: Understanding coal combustion properties aids in the design and maintenance of boilers, helps to maximize burning efficiency, and assists in reducing carbon particle emissions. Coal is, however, an extremely complex and heterogeneous material whose physical and chemical properties are difficult to categorize. Not surprisingly, the present predictive capability of coal combustion behavior is somewhat imperfect due, in part, to this heterogeneity. Macerals are the individual organic subcomponents of coal and, have distinct physical and chemical properties which reflect differences in original plant material, alteration during deposition, and the degree of coalification or rank. The distribution of the different maceral types within a particular coal, along with rank changes, greatly control coal properties and, as a consequence, affect combustion behavior.

Objective: The overall objective of this study was to examine the combustion characteristics of single coal macerals, comparing them not only with each other, but also with the parent whole coals from which they were derived.

Procedures: In a recent set of experiments the combustion properties of a channel sample of the Herrin No. 6 coal, its selected lithotypes, single maceral concentrates, and associated chars were studied. The chars were made in an Entrained Flow Reactor at 1000°C in a nitrogen atmosphere at heating rates comparable to pulverized fuel conditions (104 to 105 °C/second). The chars were then combusted in a TGA in an atmosphere of 80% argon and 20% oxygen and the combustion gases were analyzed in a mass spectrometer.

Results: The results of ultimate analyses (Table 1) show that compared to the whole coal channel sample the separated vitrinite had slightly less carbon and about half the sulfur content, while the DGC fusinite had slightly more carbon and about two-thirds less hydrogen and nitrogen. In the corresponding chars (Table 2), the nitrogen increased by almost one-third in the vitrinite and by about two-thirds in the fusinite.


Table 1
Table 2
The morphology of the chars were strikingly different. The extremes were the uniform open thin- walled cenospheres of the separated vitrinite (Figure 1) and the totally unfused and unchanged forms of the fusinite (Figure 2). The semifusinite showed a mix of thick-walled open cenospheres, thick-walled honeycombed cenospheres, and unfused particles Figure 3). The char from the whole coal showed a proportional mix of all these forms.

Fig. 1
Fig. 2
Fig. 2
The differences in reactivity (rate of weight loss) in temperature programmed experiments (Table 3) are also striking. While the DGC vitrinite is just slightly more reactive than the whole coal which has about 85% vitrinite, the DGC fusinite was only about half as reactive. The DGC semifusinite is intermediate between these two extremes. Thus, there can be a wide variation in reactivity between different coal particles from the same sample.

Table 3
Conclusions: The overall conclusion of this work is that the various macerals behave differently in the combustion process. The macerals have differences in chemical composition, the yield different char morphologies., and the react at significantly rates in the combustion process.

References:

Milligan, Jimmy B., Thomas, K. Mark, and Crelling, John C.,1997, Temperature-programmed combustion studies of coal and maceral group concentrates: Fuel, v.76, no. 13, p. 1249-1255.
Beeley, T. J., Crelling, J. C., Gibbins, J. R., Hurt, R., Man, C. K., and Williamson, J., 1995, Coal char reactivity characterization for burn-out prediction in utility boilers: In, Pajares, J. A., and Tascon, J. M. D., eds. Coal Science - Proceedings of the Eighth International Conference on Coal Science, Elsevier, Amsterdam, v. 1, p. 615-618.
Crelling, John C. and Thomas, K. Mark, 1994, Review of some recent research on the combustion properties of coal macerals: Preprints - Div. of Fuel Chem., Am. Chem. Soc., v. 39, no. 1, p. 209- 213.
Crelling, John C., Hippo, Edwin J., Woerner, Bruce A., and West, David P., 1992, Combustion characteristics of selected whole coals and macerals: Fuel, v. 71, no. 2, p. 151-158.
Crelling, John C., 1988, Skorupska, Nina M., and Marsh, Harry, 1988, Reactivity of coal macerals and lithotypes: Fuel, v. 67, no. 6, p. 781-785.
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