to have 100% variation around its mean flow rate and follows either a normal or uniform distribution, it is seen from Table I that about 50% [=(3.1 - 2.2)/2.2] more total holding time should be provided for such variation. However, the optimal holding time for the first tank is reduced. Uncertainty in other parameters yields a similar conclusion. Expected optimal total holding time based on the uniform probability distribution is greater than that based on the normal distribution for the same interval of uncertainty of each parameter, because the uniform distribution has a larger variance than the normal distribution for the same interval of uncertainty; it reflects correctly the larger uncertainty involved in the parameter (Jaynes, 1959). literature Cited
Chang, T. M., Wen, C. Y., Ind. Eng. Chem. Fundam. 7, 422 (1968). Chang, T. M., Wen, C. Y., “Sensitivity Analysis in Optimal Process Design for Coal Gasification Processes,” Paper V, Office of Coal Research, December 1967. Chen, M. S. K.. “Stochastic Modeling and Stochastic Optimization of Chernical Engineering Processes,” Ph.D. dissertation, Kansas State University, Manhattan, Kan., 1969. Erickson, L. E., Fan, L. T., J . Water Pollut. Contr. Fed. 40, 345 (1968).
Erickson, L. E., Ho, Y. S., Fan, L. T., J . Water Pollut. Cont. Fed. 40, 717 (1968). Hahn, G. J., Shapiro, S. S., “Statistical Models in Engineering,” pp. 229-55, Wiley, New York, 1967. Jaynes, E. T., “Probability Theory in Science and Engineering,” Socony Mobil Oil Co., Dallas, Tex., 1959. Kalinske, A. A., Shell, G. L., “Use of the Activated Sludge Process a t High BOD5,” Annual W P C F Conference, Chicago, Ill., Sept. 22-27, 1968. McBeath, B. C., Eliassen, R., J . Sanit. Eng. Diu.Amer. SOC. Civil Eng. 92, SA2, 147 (1966). McLellan, J. C., Busch, A. W., Proceedings of 22nd Industrial Waste Conference, pp. 537-52, Purdue University, Lafayette, Ind., 1967. Pasveer, A., Sewage Ind. Wastes 26, 149 (1954). Pasveer, A., Sewage Ind. Wastes 27, 783 (1955). Radanovic, L., Ed., “Sensitivity Methods in Control Theory,” Pergamon Press, New York, 1965. von der Emde, W., “Aspects of the High Rate Activated Sludge Process,” Conference on Biological Waste Treatment, Manhattan College, April 1960. RECEIVED for review January 27, 1969 ACCEPTED June 5, 1970 Work financially supported by the Federal Water Pollution Control Administration (Proj. No. 1 WP-01141-02) and the Kansas Water Resources Research Institute Contribution No. 41 (Proj. NO.A-019).
Kinetics of Thermal Decomposition of Pulverized Coal Particles Stanley Badzioch and Peter G. W. Hawksley BCURA Industrial Laboratories, Leatherhead, Surrey, England An apparatus i s described for measuring the extent of thermal decomposition of sizegraded coal particles in the pulverized-fuel size range a t temperatures up to 1000°C and times ranging from 30 to 110 msec. The particles are heated to the decomposition temperatures at high rates (25,000-50,000° C/sec) comparable with those occurring in pulverized-fuel firing. The yield of volatile products under these r a p i d heating conditions was 1.3 to 1.8 times higher than the change in volatile matter found from the difference between the proximate volatile matter of coal and that of char. Analysis of data obtained for 10 bituminous coals and one semianthracite yielded empirical equations suitable for calculating the progress of devolatilization of pulverized-fuel particles when their temperature history is known.
T h e rate and extent of thermal decomposition of coal particles play an important part in the processes occurring in a pulverized fuel flame where as much as 50% by weight of the feed coal may volatilize and burn in the gas phase. Information on the rate of therm11 decomposition is necessary not only for the prediction of ignition d:.:’anres hut also f(11- an accurate knowledge of the heat . -,eased along h ;I~.,i~ c.
The mechanism and the amount of decomposition depend on the rate of heating but there is very little information in the literature (Yellow, 1965) on the thermal decomposition of coal under the heat transfer conditions found in pulverized fuel (p.f.) firing where the rate of heating of the particles may be as high as 10’”C per sec. Chukhanov et al. (1962) found that rapid rates of heating (10’ to lo6”C per sec) gave rise to much h;;h< I Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 4, 1970 521
yields of volatile products than under slow heating conditions or in proximate analysis. The composition of the volatiles was changed, the yield of gaseous products being reduced and that of tars increased-i.e., the volatile products were richer in carbon. Similar conclusions were reached by Loison and Chauvin (1964) on the basis of experiments in which coal particles were supported on a wire gauze heated electrically to about 1000°C a t a rate of about 1500°C per sec. The dependence of the quality of the products on the rate of heating has been noted by Martin (1964) for the decomposition of cellulose; slow heating of cellulose results in conversion of a large fraction of the original weight to a carbonaceous residue or char while rapid heating leaves little or no char and the volatile products are correspondingly richer in carbon. The earliest published work on the kinetics of the devolatilization of coal particles in suspension is that of Shapatina et al (1950) although the purpose of the work was not to study these kinetics but to explore the possibility of controlling the composition of volatiles by varying the heating conditions. The data are not applicable to p.f. firing conditions because no measurements were made for decomposition times shorter than 0.5 sec and the upper temperature limit (550°C) was so low that the amount of devolatilization was small. The particles were coarse (200-p diam) so that heating rates would have been low relative to p.f. conditions. The most serious shortcoming was that the weight loss was calculated from the proximate volatile matter of the char and was therefore underestimated. This paper describes an apparatus for measuring the kinetics of the thermal decomposition of closely size-graded coal particles under conditions similar to those occurring in the pre-ignition zone of pulverized fuel firing and gives empirical expressions for the rate of decomposition based on experimental data obtained for 10 bituminous coals and a semianthracite. Full details of the experimental data are given elsewhere (Badzioch et al , 1968). Apparatus
The principle of the apparatus (the laminar flow furnace) is shown in Figure 1. Closely size-graded coal particles are carried in a small flow of nitrogen through a watercooled feeder tube into a hot stream of nitrogen flowing downwards through a vertical furnace tube a t a Reynolds number low enough to ensure laminar flow. The furnace tube is held a t the same temperature as the preheated nitrogen. The particles are heated rapidly t o that temperature and decompose as they flow through the furnace. Because the flow is laminar the decomposing particles travel in a narrow stream along the axis of the furnace and can be aspirated into a water-cooled collector tube where the decomposition is quenched. The amount of decomposition depends on the transit time between the feeder and collector and on the temperature of the furnace. The transit time can be varied over the range 30 to 110 msec by changing the distance between the feeder and the collector. The temperature of the furnace and gas stream can be adjusted up to 1000°C. The furnace consists of an impervious recrystallized alumina tube (50-mm i.d. and 950-mm long) surrounded over the decomposition zone by a silicon carbide heating element. Nitrogen is fed into the furnace tube through an expansion chamber to ensure a uniform gas flow. The ~ - i mt,1011 p chamber ii sealed to the furnace tube by a 522
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 4, 1970
/
COAL PARTICLES FROM VIBRATING FEEDER
NITROGEN INLET
ILICON CARBIDE
FLOW
STRAIGHTENER-
FURNACE TUBE
SILICON CARBIDE FURNACE HEATER
CYCLONE
Figure 1. Laminar flow furnace
silicon rubber 0 ring. The nitrogen is heated by a silicon carbide element suspended inside the furnace tube with a second silicon carbide element outside the furnace tube as a guard heater. These two heaters have to be operated a t temperatures of 300 to 500°C above the temperature of the furnace to heat the nitrogen to the furnace temperature. The spiral form of the internal element induces a spiral flow in the nitrogen stream. This is eliminated by placing a flow straightener consisting of a zirconia cylinder, 25-mm long pierced by eight 6-mm diam holes spaced equally on a 32-mm diam circle, below the element. The coal particles are fed from a vibrating feeder (not shown) through a water-cooled brass feeder tube centrally located a t its lower end by the flow straightener. The feeder tube (3.8-mm bore and 9.5-mm 0.d.) is sheathed by two concentric mullite tubes separated by asbestos string spacers to avoid cooling the surrounding hot nitrogen stream. The feeder tube projects 28 mm below the flow straightener. This length of the feeder tube is also sheathed by a mullite tube to prevent cooling of the hot nitrogen. The water-cooled brass collector probe has a tapered inlet decreasing from 22- to 6-mm diam within a distance of 40 mm. The collector is supported by an adjustable clamp so the distance from the feeder can be varied. The particles are removed from the aspirated gas by a 25-mm diam cyclone. Any particles that adhere to the inner walls t l f the collector are removed by periodical blasts of nitrogen admitted uia the purge valve. The flowrates of nitrogen through the furnace. itw'Gr and collector tubes are measured R ! i! .
