Burning of suspended coal-water slurry droplet with oil as combustion additive

Burning of suspended coal-water slurry droplet with oil as combustion additive

COMBUSTION AND FLAME 66: 87-89 (1986) 87 Burning of Suspended Coal-Water Slurry Droplet with Oil as Combustion Additive S. C. YAO and P. M A N W A...

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COMBUSTION

AND FLAME

66: 87-89 (1986)

87

Burning of Suspended Coal-Water Slurry Droplet with Oil as Combustion Additive S. C. YAO and P. M A N W A N I Department of Mechanical Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213

Coal-water slurries have been regarded as a potential substitute for heavy fuel oil. Various demonstrations of coal-water slurry combustion have been performed; however, a fundamental understanding of how the combustion process of a slurry fuel is enhanced is still not adequate. The combustion of coal-water mixture droplets suspended on microthermocouples has been investigated by Yao and Liu [1] and later by Matthews and Street [2]. It was found that droplets of lignite coal (which is a noncaking coal) burn effectively; however, droplets of bituminous coal (which is a caking coal) are relatively difficult to burn. During the heat-up of bituminous coal-water slurry droplets may turn to " p o p c o r n " and show significant agglomeration. The incomplete combustion of coal-water slurry droplets in furnaces has been reported [3], and this is a drawback of this process. The objective of the present study is to explore the possibility of enhancing the combustion of coal-water slurry droplets with the use of a combustible emulsified oil. EXPERIMENTAL APPARATUS Hot gas is provided from a blast burner with separate inlets for propane fuel and air. Both flows are adjustable and metered to control gas temperature, velocity, and oxygen concentration. The hot gas stream of combustion product and excess air flows through a section of stainless steel pipe, Copyright © 1986 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

which is insulated on the outside to minimize heat loss, thus allowing mixing to produce a more uniform gas temperature. The gas flows out of a converging nozzle at the top of the pipe: this produces a uniform and higher velocity profile. Since there is heat loss from the mixing pipe, the gas temperature at the exit is measured with a thermocouple. The velocity and oxygen concentration are calculated from input flow conditions by assuming complete combustion. In the present experiment, single droplets of coal-water slurry, with or without oil additives, are suspended on a fine quartz fiber. The observation of evaporation and combustion is recorded by 16 mm color movies with the use of a close-up microlens. The hot gas system is situated on a rotatable platform. When the combustion experiment starts, the platform is rotated rapidly to a lock position, which allows the hot gas stream instantaneously (within 25 ms) to surround the suspended droplet. The coal-water slurries with oil additive are prepared in the laboratory. The selection of the oil is restricted to those immiscible with water. Therefore, an ultrasonic cleaning device is used to produce an emulsion mixture. RESULTS AND DISCUSSION In most of the experiments, the hot gas typically had a temperature of 800°C and a velocity of 1 m/

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88 s with oxygen content about 10%. Two kinds of coal were studied. They are the Pittsburgh Seam #8 bituminous coal and a North Dakota Lignite from the Beulah Mine. The typical bituminous coal is sieved at 200 mesh with a mean diameter of 74 #m. The diameter of drops tested is in the range of 500-1500/~m. For the oil additives, various hydrocarbon fuels have been tested. The use of kerosene appears to be superior to hexane, ethanol, and pentane in terms of the amount needed to cause significant combustion enhancement. Additionally, the kerosene-water emulsion is the most stable one; it can remain as emulsion for up to 24 h, while other emulsions usually last only for a few hours. For droplets of coal-water slurry with emulsified kerosene as the oil additive, combustion is significantly enhanced. The typical concentration of coal in this study was 50% by weight, and the amount of kerosene used was between 8% and 25 % by weight. The typical mixing procedure is to (1) wet the coal particles with a very small amount of water, (2) prepare a water-kerosene emulsion which may contain some separate kerosene, and (3) mix them together by stirring. Right after the droplet is exposed to the hot gas stream, the droplet splatters luminous particles for 1-2 s. A typical flame is shown in Fig. 1. The majority of the splattering flame is in the form of dark red fine particles, which represent the fine coal particles. It is believed that some of the kerosene is also burned in this flame. The range of the splatter can reach a diameter more than 10 times the droplet diameter. Usually, after the splattering, there is a substantial amount of coal remaining on the suspension fiber. But sometimes the original droplet is completely fragmented so that no significant amount of coal remains on the fiber. After the splattering period, the volatiles escape from the suspended coal and burn as a visible red-yellow gas flame in the surrounding; then, the remaining char burns. In general, the burning rate of this char sphere is much faster than that of a coal-water slurry without oil additive. It is clear that the combustion process is enhanced by the presence of emulsified kerosene additive. Attempts have been made to explore the mechanics of combustion enhancement. During the splattering period the hot gas is removed. When

S . C . YAO and P. MANWANI

Fig. 1. Photographsof droplet splattering and devolatization.

the droplet is exposed to hot gas again it is observed that some more splattering occurs before devolatilization happens. That means the splatter occurs at a very early stage of heating when the coal has not fully agglomerated yet. In some of the experiments, the hot gas was removed right after the splattering and the suspended droplet was inspected under a stereomicroscope. As shown in Fig. 2, there are blow holes on the partially agglomerated particles. This is apparently due to the splattering of some coal particles during the heat up phase. Some agglomeration of coal particlesappears; however, because of the significantly enlarged surface area caused by the blow holes, the combustion of the remaining solid particles is enhanced. The popcorn feature, which is observed for conventional combustion of a coal-water slurry without oil additives, does not exist at all. The schematic of the splattering process is shown in Fig. 3. The true mechanism of the splattering is still not known, but some important feature should be pointed out. The splattering

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89 needs to be identified by more refined studies. Experiments have also been performed with lignite. The same type of splattering and combustion enhancement is observed. For the bituminous coal-water slurries, the weight fraction of the kerosene in slurries affects the extent of combustion enhancement. With kerosene concentration less than 6% of weight, there is little splattering. When the concentration is more than 20%, splattering effects are reduced with increasing concentration. At 15% kerosene by weight significant splattering and combustion enhancement occurs. A more refined study may reveal the optimal concentration for combustion enhancement. The procedure for preparation of the coalwater-oil mixture also affects the combustion phenomena. Since there are three components, two of them may be mixed first before being combined with the third one. It has been stated previously that the procedure of mixing water and kerosene as emulsion and then mixing them with the wet coal provides a slurry with significant combustion enhancement. It has also been found that, after the water-kerosene emulsion was made, the dry coal particles can be added into the emulsion in an ultrasonic environment. The resulting slurry also exhibits significant combustion enhancement. However, if the coal and kerosene are mixed first and water is added in by stirring, on combustion, this type of slurry gives weak splattering and usually results in popcorn-type swelling and serious agglomeration. Not much difference is observed as compared with that of pure coal-water slurry without oil additive. REFERENCES

Fig. 3. Schematic of splattering process.

process is, in general, mild as compared with the well reported violent microexplosion of water-inoil emulsions (for example, reported in [4]). It is noticed that kerosene has a boiling point of 250°C and the water in the slurry contains various impurities so that violent spontaneous nucleation is not likely to occur [5]. The splattering of coal particles does occur before the coal particles agglomerate. The exact mechanism of splattering

1. 2.

Yao, S. C., and Liu, L., Combust. Flame, 51:335-345 (1983). Matthews, K. J., and Street, P. J., Sixth International Symposium on Coal Slurry Combustion, Proceedings, 1984, pp. 109-126.

3. McHale,E. T., Scheffe,R. S., and Rossmeissl. N. P., Combust. Flame 45:121 (1982). 4. Lasheras, J. C., Kennedy, I. M., and Dryer, F. L., 5.

Combustion Science and Tech. 26:161-169 (1981). Skripov, V. P., Metastable Liquids, Halstead Press,

Jerusalem, Israel, 1974.

Received 21 October 1985; revised 1 May 1986