Chemistry of Sulfur and Nitrogen in Coal - long range program detailing the chemical nature of organic sulfur and nitrogen in coal and coal macerals.

Abstract of Recent Work: Organic Nitrogen in Coal Macerals: SOx and NOx Release During Coal Combustion

Background: The purpose of this research is to advance the fundamental understanding of the effects of variation of coal composition on SOx and NOx formation during coal combustion. Modern coal-burning power stations are responsible for emissions of millions of tons of SOx and NOx per year into the atmosphere. Not only does this contribute to the acid-rain problem but SOx and NOx also take part in upper-atmosphere chemistry to enhance the ozone-layer problem. A better understanding of how organic sulfur nitrogen in coals, of different chemical compositions and structure, influences the generation of SOx and NOx during combustion, can lead to reductions in these emissions.

Objectives: The objective of this study was to examine the mechanism of release of nitrogen compounds, under strictly controlled experimental conditions from coal lithotypes and macerals. Procedures: Samples of a whole coal and its constituent lithotypes and macerals were pyrolyzed in an entrained flow reactor. The resultant chars were combusted with increasing temperatures in a thermogravimetric-mass spectrometer system to study gas evolution profiles, particularly NOx and SOx compounds.

Results:

1. The results indicate that nitrogen is concentrated in the pyrolysis chars. It increased by about 25-30% in the vitrinite-rich samples and by about 65-75% in the inertinite-rich samples.
2. When the chars are combusted in a gas mixture of 20% oxygen and 80% argon, the SO2 comes off early in the temperature range in two peaks, one before and one just after the start of the CO2 peak as shown in Figures 1 -6. Figure 1 shows the combustion profile of the whole coal, Figure 2 shows the profile of hand-picked but unwashed/separated vitrain and Figure 4 is the profile of hand-picked but unwashed/separated fusain. Figures 3, 5, and 6 show the combustion profiles respectively of separated vitrinite, semifusinite, and fusinite.
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Figure 1
Figure 2
Figure 3

Figure 4
Figure 5
Figure 6
3. Because the early SO2 peak is seen only in unwashed samples (Figures 1 and 2), it is believed that the initial peak is due to pyritic sulfur and the second peak is due to organic sulfur. In all cases NO comes off well after the peak of the CO2 evolution. Thus, the nitrogen is being concentrated in the final part of the char to burn.
4. The gas evolution profiles are distinct for different macerals and the same type of maceral will give a similar pattern even if it comes from a different sample.
Conclusion: Overall these results suggest that in the combustion process the oxygen preferentially strips off the sulfur atoms first, then the carbon atoms and finally the nitrogen atoms which have been concentrated in the remaining char.

References:

Wang, Wanxing, Brown, Stephen D., Thomas, K. Mark, and Crelling, John C., 1994, Reactivity and nitrogen release from a rank series of coals during temperature programmed combustion: Fuel, v.73, no.3, p. 341-347.
Hindmarsh, Christopher J., Wanxing Wang, Thomas, K. Mark, and Crelling, John C., 1994, The release of nitrogen during the combustion of macerals, microlothotypes, and their chars: Fuel, v. 73, no. 7, p. 1229-1234.
Palmer, S.R., Kruge, M. A., Hippo, E. J., and Crelling, John C., 1994, Speciation of hetroatoms in coal by sulfur- and nitrogen- selective techniques: Fuel, v. 73, no. 7, p. 1167-1172.
Crelling, John C., Thomas, K. Mark, and Marsh, Harry, 1993, The release of nitrogen and sulphur during the gasification of chars derived from lithotype and maceral concentrates: Fuel, v. 72, no. 3, p. 349-357.
Palmer, S.R., Hippo, E.J., Crelling, J.C., and Kruge, M.A., 1992, Characterization and selective removal of organic sulfur from Illinois Basin coals: Coal Preparation, v. 10, p. 93-106. Hippo, Edwin J. and Crelling, John C., 1991, Desulfurization of single coal macerals: Fuel Processing Technology, v. 27, no. 3, p. 287-306.
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