Cold oxygen plasma oxidation of coal

Short Communications

Cold oxygen

plasma

I. A. Korobetskii,

oxidation

N. V. Balabanova,

of coal

V. K. Popov, V. I. Butakova*

and N. D. Rus’jnova*

institute of Coal, Siberian Branch of Academy of Sciences of USSR, Rukavishnikova Str., 21, Kemerovo, 650610, USSR * Eastern Carbochemical Institute, 8 March Str., 19, Sverdlovsk, USSR (Received 37 May 1989; revised 14 November 1989)

Cold oxygen plasma oxidation (CoPo) is shown to be a selective procedure for the oxidation of the organic matter of coal. Analysis of oxidation products by infrared spectroscopy showed that CoPo and molecular oxygen give quite different results, and that there are differences in the behaviour of certain CH groups during CoPo. (Keywords: structural analysis; fossil fuel; coal)

Structural investigations of fossil fuels are usually preceded by extraction or dissolution procedures. However, very marked changes often occur, and artefacts may be produced, so that mild and selective procedures are required. We have previously shown1-3 how excited gas molecules may be applied as ‘molecular sound’ reagents. Thus, cold oxygen plasma oxidation (CoPo) at 150°C of different coals gave weight-loss curves, which differed from the results of oxidation with molecular oxygen at 35@4oo”C. For example, minimum weight loss was observed in CoPo of lignite, but the greatest weight loss in CoPo of anthracite. Organic structures with linear structures were less affected during CoPo.

and flowmeter to the vacuum system; its pressure (usually 150 kPa) is measured by a manometer. The needle valve serves to regulate the oxygen flow and to maintain a pressure of l-3 kPa in the line, measured by a vacuum gauge. Oxygen is excited by a radio frequency (r.f.) field and interacts with a coal sample in a glass boat located in the reactor. The r.f. radiation is generated by a 40.68 MHz generator at 2@40 W. The gas flow was 20-30 cm3 min- ‘, and the temperature 50-150°C. A quantity (350 mg) of CO.2 mm particle size Kuznetskii subbituminous coal (USSR classification G6, properties summarized in Table 1) was reacted. Every hour, the sample was weighed and a 3 mg sample was taken for infrared spectroscopic analysis on a KBr disc.

EXPERIMENTAL The CoPo apparatus, built in-house, is shown in Figure 1. Oxygen is supplied from a cylinder through a reducing valve

RESULTS AND DISCUSSION Weight loss curves are shown in Figure 2; i.r. spectra of the coal and its CoPo

products after 2,3 and 7 h are compared in Figure 3. Quite different results were obtained in comparison with oxidation by molecular oxygen4,5 , in which a decrease in aliphatic groups and an increase in COOH and

Table 1

Properties

of Kuznetskii

Moisture content, % a.r. Mineral matter, % a.r. Ultimate C H 0 N s

analysis,

coal 2.4 2.6

% d.a.f. 79.7 6.2 11.2 2.7 0.3

Maceral analysis, Vitrinite Exinite Inertinite

%

Figure 2 Weight loss (2) curves as oxygen plasma indicate sampling

loss (1) and rate of weight a function of time of cold oxidation. The asterisks points for i.r. spectroscopy

94 2 4

5

i7-a 2

1

f-

3

Figure 1 Schematic diagram of cold oxygen plasma oxidation apparatus: 1, oxygen cylinder; 2, reducing valve; 3, flowmeter; 4, manometer; 5, needle valve; 6, reactor; 7, glass sample boat; 8, vacuum gauge; 9, reducing valve; 10, radiofrequency generator; 11, vacuum pump 001~2361/90/050654-02 0 1990 Butterworth & Co. (Publishers)

654

FUEL, 1990, Vol69,

May

Ltd.

Short Communications other CO groups is usually observed. CoPo gave no increase in oxygencontaining groups. Initially, oxidation is rapid, and while there is no change in the intensities of i.r. absorptions from oxygen groups, there is a monotonic reduction in the intensity of absorptions from unsaturated CH groups. In the second stage of oxidation, the rate of weight loss is small, but there is a rapid reduction in intensity of signals from both saturated and unsaturated CH groups. Measurements of absorptivities, corrected for the mass of sample remaining, showed more rapid disappearance of aromatic groups (band at 3040cm-i) than aliphatic structures (bands at 2850 and 2920 cm- ‘). It is concluded that a cold oxygen plasma oxidizes coal in a different way compared with the effect of more usual oxidation methods. REFERENCES

c

4000

3000

LOO0

lOO(J

cm -1

Figure 3 1.r. spectra of: 1, fresh Kuznetskii coal; 2, coal after 2 h CoPo; 3, coal after 3 h CoPo; and 4. residual mineral matter after CoPo of coal

Korobetskii, I. A. and Shpirt, M. J. ‘Genesis and properties of mineral matter of coals’, Nauka Press, S.B., Novosibirsk, 1988 Korobetskii, I. A., Balabanova, N. V. and Zaostrovskii, A. N. Fuel Proc. Technol. 1989, submitted for publication Korobetskii, I. A., Abstract of Lectures on an All-union Conference, Irkutsk, 1982 Kister, J., Guihano, M., Mille, G. and Dou, H. Fuel 1988,67, 1076 Milcherski, E. and Stroek, E. V. Koks, Smola, Gas 1986, 1, 3

FUEL, 1990, Vol 69, May

655