Very rapid coal pyrolysis

Very rapid coal pyrolysis Peter R. Solomon, James R. Markham

Michael

A. Serio,

Robert

M. Carangelo

Advanced Fuel Research, Inc., 87 Church St, East Hartford, (Received 6 December 7984; revised 20 May 7985)

CT 06708,

and USA

Considerable controversy exists concerning the rate of coal pyrolysis. At 800°C the reported rates derived using a single first-order process to define weight loss vary from < 1 s- ’ to 2 100 s- ‘. In an attempt to resolve this controversy, a new pyrolysis experiment has been designed which allows direct measurement of coal temperature and reaction time. It uses a small-diameter electrically heated tube into which coal and helium are injected. The reaction distance is varied by moving the electrode positions. Temperature is measured inside and outside the tube with a thermocouple. Temperatures of the solids are determined at the tube exit using FT-i.r. emission and transmission spectroscopy. The transit time for a pulse of coal is measured using phototransistors at the top and bottom of the tube. Particle velocities are also determined using FT-i.r. transmission. Measurements have been made on sieved size fractions of a North Dakota lignite, Rosebud subbituminous coal and Illinois No. 6 bituminous coal in tubes up to 240cm long at asymptotic tube temperatures of 600, 700, 800 and 935°C. At 800°C primary pyrolysis was completed in a period of 14 ms, during which the maximum coal temperature was increasing from 600 to 740°C. The results are in good agreement with a previously developed functional-group model of coal pyrolysis which employs a distribution of activation energies to describe the evolution of individual pyrolysis species.The results are also

in reasonable agreement with predictions of a single first-order model for weight loss which uses a rate constant k=4.28 x 1Or4 exp(-228500/RT) Jmol-’ s-l. This rate, which is over 1000 s-l inconsistent with the low rates. Reasons for discrepancies in reported rates are discussed. O
IT-i.r.)

Considerable controversy exists concerning the rate of coal pyrolysis. For example, at 800°C rates reported in the literature (derived assuming a single first-order process to define weight loss or tar evolution) vary from < 1 s-l (refs. l-4) to z 100 s-l (refs. 5-9) with intermediate values”-12. One possible explanation is that the rate-limiting step is a chemical kinetic rate and the observed variation is due to differences in the coal type studied. However, this is unlikely, because experiments in which the coal type alone was varied (i.e. all other conditions were held constant) typically show no more than a five-fold variation in rate’. An alternative explanation is that the temperature of the coal particle in some experiments may have been different from that assumed or calculated. It is unlikely that under inert conditions the particle temperatures are underestimated, because experimental measurements and calculations have not indicated significant exothermic heats for coal pyrolysis. However, the assumptions which have been made about the particle heat capacity, emissivity and heat transfer coefficient suggest that particle temperatures have been overestimated. If particle temperatures have been overestimated, the observed rates give lower bounds for the chemical kinetic rate. At 800°C the highest lower bound is % 100 s-l. With such a high chemical rate, any experiment which attempts to obtain isothermal rate data at this temperature must heat the coal in a time short compared with the pyrolysis time, i.e. of the order of 10’ K s-l. At higher temperatures, the heating rate to obtain isothermal data must be even faster. For most experiments at temperatures of > 800°C calculations suggest that such high heating rates are not 00162361/86/020182~13$3.00 I? 1986 Butterworth & Co. (Publishers)

182

at 800°C is

Ltd.

FUEL, 1986, Vol 65, February

achieved with pulverized coal, so pyrolysis typically occurs during heat-up, even assuming zero heats of reaction. Under these circumstances it is necessary to know the coal particle temperature to derive chemical kinetic rates. Coal particle temperatures during rapid pyrolysis have not generally been directly measured. In an attempt to resolve this controversy, a new pyrolysis experiment was designed to provide high heating rates and a geometry which simplified the prediction of particle temperatures13. It used a small-diameter electrically heated tube into which coal and helium carrier gas were injected. The reaction distance was varied by moving the electrode positions. The particle temperatures and the particle residence times were calculated from the measured tube wall temperatures and the coal and gas flow rates, respectively. Even allowingfor the uncertainty in these estimated values, the rates measuredfor both a North Dakota lignite and an Illinois No. 6 bituminous coal at asymptotic tube temperatures of 700,800 and 900°C agreed with the highest reported rates and were inconsistent with the low rates.

These heated tube experiments, therefore, lent support to the high-rate advocates but suffered in conclusiveness, as did the other experiments in this temperature range, in not having a direct measure of particle temperature 2- ’ * or reaction time2’5~7-9,“,‘2. For the experiments reported in this paper, the tube reactor was modified to eliminate these drawbacks. The transit time for the coal was measured using phototransistors at the top and bottom of the tube. Confirmatory measurements of the particle velocity were made using FT-i.r. transmission spectroscopy. Temperature measurements were made

Very rapid coal pyrolysis:

inside and outside the tube with a thermocouple. The temperatures of the solid particles were calculated to be within 50°C of the thermocouple temperatures. In addition, the temperatures of the solids were directly determined at the tube exit using FT-i.r. emission and transmission spectroscopy14*“. Comparison of the FTi.r. measurements with thermocouple measurements made at the same point confirmed the calculated small difference between the thermocouple and particle temperatures. Measurements were made of the amount and composition of the tar, char and gases evolved as a function of the measured reaction time and temperature. The data were used to determine the chemical kinetic rate for primary pyrolysis, during which the initial rapid weight loss, the evolution of tar and lighter hydrocarbons and the disappearance of the aliphatic (or hydroaromatic hydrogen) in the coal/char all happen simultaneously. This paper describes the experimental apparatus and measurement technique and reports the results for pyrolysis of three coals at temperatures between 600 and 935°C. The results are compared with the predictions of a pyrolysis model and with the literature. Reasons for the discrepancies are discussed. EXPERIMENTAL dpparatus

The reactor is illustrated in Figure 1. It consists of a 5 mm i.d. Inconel 702 tube which is heated electrically. Coal entrained in cold carrier gas is injected at the top of the tube. The coal is fed using an entrainment system, described previously7s”. The average reactor pressure ranges from 3.4 to 13.8 kPa above atmospheric, depending on the heated length, flow rate and temperature. The coal-gas mixture enters the heated section of tube and heats rapidly. The heat transfer rate is large because of the small tube diameter, the high thermal conductivity and Secondary gas

,5mm id Inconel tude

Optical sensor

P. R. Solomon

et al.

low heat capacity of the helium carrier gas and the fact that the particles collide with the hot walls of the tube. After a variable residence time, the reacting stream is either ejected from the tube for temperature and velocity measurements or quenched in a water-cooled section of tube for mass balance measurements. The product collection train consists of a cyclone to separate the char, followed by a collection bag to collect the gas, tar and soot. The gas from the bag is analysed by FT-i.r. and the solids and liquids are collected on the bag surface and in a filter. The temperature of the electrically heated tube and reactants is governed by the following constraints. At constant current in the absence of gas and particle flow, the tube reaches an equilibrium temperature such that the external power loss by radiation and convection is equal to the electrical power input. With gas and coal flowing in the tube, the top end of the tube is cooler than the equilibrium temperature, since heat is used to raise the temperature of the incoming gas and coal. The heat absorbed by the coal and gas, and hence the average coal particle and gas temperatures, can be calculated from the measured tube wall temperature. When the reactants reach the equilibrium temperature, the outside of the tube attains a constant temperature. In analysing the results of the tube experiments, it is assumed that for fixed conditions, the temperature is a function of the distance from the top electrode, independent of the electrode position. This is true, provided that the tube is uniform in cross-section, resistivity and emissivity along its length. The assumption has been validated by comparing thermocouple measurements for different positions of the top electrode. The temperatures measured at a fixed distance from the top electrode are within 2&3o”C, independent of the position of the electrode. There is a slight variation in pressure with the electrode position due to the change in heated length ( < 7 kPa). Coals usd

Samples were sieved size fractions. The ultimate analyses are presented in Table 1 and the run conditions in Table 2. The North Dakota lignite and the Illinois No. 6 bituminous coal were used in previous pyrolysis studies7-9,’ ‘. In the previous studies, the rates of evolution of their pyrolysis species were similar to those measured for other coals, so their behaviour does not appear to be unusual. Assessment of temperatures

Power SUPPlY

Thermocouple measurements. The temperature of the gas-coal mixture and the outside tube temperature were Varkble hot zone

I

Table 1

Analyses

of coals used Wt”:, (daf)

Zap, North FT-ir tempemture dlagnostlcs section / optlcal sensor Water cooled sectlon To pump

Figure 1 Schematic

of heated-tube

reactor

C H N S 0 (diff.) Ash”

66.5 4.8 1.1 1.1 26.5 7.1

Dakota

lignite

Montana 12.1 4.9 1.2 1.2 20.3 10.0

Rosebud

Illinois No. 6 13.9 5.1 1.4 4.2 15.4 11.0

“db

FUEL, 1986, Vol 65, February

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Very rapid coal pyrolysis: Table 2

Pyrolysis

P. R. Solomon

run conditions

Particle

size

Case

Coal

(pm)

Asymptotic temperature

a b

Rosebud Zap lignite Zap lignite Zap lignite Zap lignite Rosebud Illinois No. 6

53-74 44-74 44-74 4414 44-74 53-14 44-14

800 800 700 935 600 800 800

C

d F g

et al.

tube Ambient (“C) gas He He He He He He He

measured with a thermocouple. The results for two sets of measurements at different gas feed rates (cases a and b) are shown in Figure 2. The measurements included: thermocouple measurements inside the heated section of the tube and inside a water-cooled tube attached to the hot tube to quench the reaction (closed circles); thermocouple measurements outside the tube (open squares), corrected for heat losses (see below); thermocouple measurements just below the tube end (plus sign); and the FT-i.r. temperature measurements (triangles) discussed later. The positions of the measurements are illustrated in Figure 2~. The thermocouple measurements inside the tube were made with a 0.8 mm diameter type-K thermocouple with an Inconel sheath. A ceramic insulator was located z 25 mm from the tip to keep the thermocouple near the centre of the tube. Heat transfer calculations (see below) suggest that inside the tube during heat-up, the thermocouple reads lO-20°C higher than the surrounding gas, due to radiation from the wall. Of course, since the thermocouple is near the centreline where the gas temperature is lowest, it is possible for the thermocouple to be lower than the average gas temperature during heat-up. When coal is flowing in the tube it is possible that fractional interactions of the particles with the thermocouple could raise the apparent temperature. However, on the basis of the work of Brewster and Seader16, the coal loadings were too small for this effect to be significant. The external wall temperatures was measured with a 0.05 mm bead-diameter unsheathed type-K thermocouple pressed against the tube. This thermocouple reading was slightly lower than the actual tube temperature, due to heat transfer from the thermocouple to the surroundings. However, the maximum effect of this error was determined by comparing the asymptotic values of the external wall temperature and the internal gas temperature, which come to thermal equilibrium for sufficient distances. The knowledge of this temperature difference and the apparent wall temperature was used to determine the error at each measured wall temperature, which decreased as the tube became cooler. The heat losses from the thermocouple were assumed to be primarily due to radiation. In the 800°C experiment, the corrections ranged from + 35°C at 765°C to + 10°C at 490°C. The data points in Figure 2 have been corrected accordingly. The temperature difference between the outside and the inside of the tube wall was calculated to be negligible. The measurements at or just below the tube exit were made with a 0.13 mm bead-diameter unsheathed type-K thermocouple. Below the tube, the thermocouple could read slightly lower than the gas temperature, due to radiation losses. Measurements along the axis oi the

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Coal feed (g min-‘)

Gas feed (dm3 min _ *)

Cold gas velocity (m s-r)

Asymptotic hot gas velocity (m SK’)

3 3 3 3 3 3 0.8

5.6 28.3 31.2 25.2 13.3 28.3 26.8

4.6 23.3 25.7 20.7 10.9 23.3 22.0

16.6 83.9 83.9 83.9 32.0 83.9 79.2

tube showed no more than a 20°C variation from inside to 1 cm below the tube. Heat transfer calculations. The temperatures of the gas, the thermocouple bead and the coal particles were calculated, given the tube wall temperature as a boundary condition. The set of heat transfer equations used is summarized in the Appendix. The particle temperature calculations (see Equation (1)) assumed the temperatureand composition-dependent heat capacity for coal derived by Merrick’ ‘, which agrees with the measurements of Lee’ 8, and a variable heat of reaction. Also assumed was an average spectral emittance of 0.5 for the 6Opm particles of coals, in agreement with recent FT-i.r. measurements’4V’5. These measurements show that for coal particles of this size, the spectral emittance is substantially less than 1 over significant regions of the spectrum. The value of 0.5 is an average assuming a furnace temperature of 800°C. According to Merrick’s model, the heat capacity depends on temperature and the average atomic weight of the coal or char. The heat capacity of the raw coal can be calculated from the elemental composition and ash content before pyrolysis. It is approximately = 1.05 kJ kg-’ K-’ for each of the three coals at room temperature, rising to z 2.5 kJ kg- ’ K - 1 at 500°C before pyrolysis. After pyrolysis starts to occur, a kinetic model is required to keep track of the instantaneous char composition so as to predict the heat capacity. A simple version of the functional-group mode14*7-9,’ ’ was used to determine the char composition. The kinetic model was also used to keep track of the particle mass and the heat of reaction. The particle density was assumed to decrease in direct proportion to the mass loss, while the diameter was assumed to be constant. The heat of reaction was calculated from the difference in the heats of formation of the gases plus char and the raw coal at the reaction temperature. The heats of formation of the raw coal and the char were estimated by the difference in the heat of combustion predicted by the formula of Mott and Spooner” and that of the elements. The heat of the pyrolysis reaction was calculated as described elsewhere”; the method is similar to that proposed by Merrick”. Of the effects considered, the variation of particle heat capacity with temperature is the most significant. The variation of the heat capacity with char composition was relatively modest under the conditions of the tube experiments reported here. The heat-of-reaction term was also not significant, since it was usually at least an order of magnitude smaller than the radiation, convection or wall

Very rapid coal pyrolysis:

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Figure 2 Measuredand calculatedtemperaturesin the tube reactor alth coal and helium. 0, Outside tube temperature corrected for heat loss; 0, thermocouple temperature inside the tube; X, thermocouple temperature at centre of FT-i.r. sample volume; A, FT4.r. temperature of coal particles. -, Calculated coal particle temperature; P-P) calculated thermocouple temperature. (a) slow flow,case a; (b) fast flow, case b

P. R. Solomon

et al.

contact terms. For example, the total heat of reaction predicted for an 8WC, 200 cm experiment with the Zap North Dakota lignite (case b) was only = - 210 kJ kg-‘. The loss in particle mass due to pyrolysis can also have a significant effect on the predicted particle temperature if it occurs while the particle is still heating up. This results in direct coupling of the heat transfer and kinetics. The loss of particle mass also creates a gaseous flux away from the particle which is detrimental to convective heat transfer (and radiation if the gases are opaque). The maximum effect of this correction was estimated (using the correlation of Hoffman and Ross”) to be -7°C for the present experimental conditions. This effect was neglected. As the final reactor temperature increases and the pyrolysis occurs over a wider temperature range, the effect of mass loss on total heat capacity and heat transfer becomes more important with regard to prediction of particle temperature. The calculations assumed convective heat transfer between the tube wall and gas and between the gas and coal particle or thermocouple bead, and radiative heat transfer between the wall and the coal particle or thermocouple bead. The heat transfer coefficient between the wall and the gas, which was determined from the Sieder-Tate equationz2 , was validated by equating the heat transfer to the gas inside the tube with the electric power input minus the power radiated and convected outside the tube (determined from the tube temperature and emissivity). The convective heat transfer between the gas and particle was described by the Ranz and Marshall equation23, which accounts for relative motion between the gas and particles. This relative motion, a result of retardation of particles by the wall, caused a significant enhancement in the heat transfer rate compared with entrained or free-fall pyrolysis experiments. Because of the collisions with the wall and other particles, the particles were assumed to move across the cross-section of the tube. Consequently, each particle experienced the ‘average’ gas temperature. The particles were assumed to be spatially isothermal during heat-up. The temperature difference AT from the centre to the surface of the particle was estimated by a standard methodz4 and is given by Equation (2). Using a value of 1.2 x 10m3 cm2 s-’ for the thermal diffusivity c(at 700 K 25, AT would be 30K for a heating rate of 25 000 K s- ‘. At higher temperatures, c1increases sharply and AT would be even lower. Consequently, the assumption of uniform particle temperature would appear to be valid. To match the measured particle temperatures in the early part of the tube, it was necessary to include a term to account for heat transfer due to collisions of relatively cold particles with the hot walls. This phenomenon, known as wall-contact heat transfer, has been described by Boothroyd26. A heat transfer coefficient was defined by analogy with a conventional convective coefficient, i.e. Q,, =h,,,c(G,ll - T,,,,). The value of &,, used for these calculations was 0.42 Jcme2 s-l K- ‘, giving a maximum increase in the calculated particle temperature of 50°C during pyrolysis at 800°C. The predicted particle temperatures are the solid lines in Figure 2. These are the temperatures used for the functional-group model of pyrolysis and the kinetic analysis. To predict the thermocouple temperature, the reactor cross-section was divided into five coaxial tubular

FUEL, 1986, Vol 65, February

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Very rapid coal pyrolysis:

P. R. Solomon

et al.

shells and the radial conduction equation was solved to predict the temperature of the centre cylinder (Equation (5)). The thermocouple temperature was calculated using the centre gas temperature (for the convective term) and the wall temperature (for the radiative term) as indicated by Equation (4). The predicted centreline thermocouple temperatures (dashed lines in Figure 2) are in good agreement with the measured thermocouple temperatures (solid circles). For the conditions of these experiments the heat transfer between the solids and the gas was so fast that in most cases the thermocouple temperature was predicted to be within 20°C of the coal particle temperature; in the worst case, the thermocouple was 50°C higher than the particle. The thermocouple measurement thus provided a reasonably accurate determination of the particle’s temperature history. Coal particle temperatures by FT-ix. Measurements of

coal particle temperatures were made directly using FTi.r. emission and transmission spectroscopy. As described in previous publications 14,15,the transmittance measurement was used to determine the total emitting surface of the coal particles so that a normalized emission, emission/(l-transmittance), could be compared in both shape and amplitude with theoretical black body. Compared with other techniques for measuring temperature, the FT-i.r. normalized emission measurement has the following advantages: 1. a complete spectrum is obtained, not just two or three colours; 2. measurements are made in the infrared region where the emission is strongest and where measurements of emission and optical properties are relevant to practical conversion processes; 3. measurements are possible with mixed phase (particles, soot and gas); 4. for grey bodies, the measurement of normalized emission allows the use of the spectral shape to obtain temperature and the amplitude to obtain emissivity; 5. it is fast. In the work described below, the coal flow rate was monitored to ensure that both emission and transmittance measurements were made under the same conditions. The coal flow rate was monitored by observing the feed gas flow, which varied inversely with coal flow, due to increased pressure in the small-diameter transfer line between the feeder and the heated tube. The blackbody intensity was calibrated using an enclosed cavity at a known temperature with a small opening at the FT-i.r. focus. The spectra were obtained for measurements made at 0.3 (case a) and 0.75 cm (cases b, c and f) below the bottom of the hot tube (see Figure 2a). For these experiments, the FT-i.r. measurement was able to provide a direct measurement of the coal particle temperature. For the small coal particles used (z 60 pm) the FT-i.r. measurement was an average particle temperature, not the surface temperature. The technique was validated by making measurements under conditions where the particles and gas left the tube at the asymptotic temperature. Equilibrium between the tube, the gas and the particle wasconfirmed on the basis of the observation that all measurements of temperature (thermocouple and FT-i.r.) reached asymptotic values. Two examples are presented in Figure 3a and b. As shown in Figure 2a, the FT-i.r. sample volume crossed the coal stream. The boundaries of the stream were determined by light scattering. There was little

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FUEL, 1986, Vol 65, February

6.0

4.0 0.80

8B (1030

2.0

.95BB(970K)

0 6500

5500

4500

3500

2500

1500

5co

8.0 f

I

co2

+o

6.0

0.90 8B (994

K

4.0

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5500

4500

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2500

1500

5 IO

C

r 6.0

4.0

2.j_ _(

6500

5500

o.go7

4500

3500

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(cm”)

Figure 3 Normalized emission spectra during pyrolysis at an asymptotic tube temperature of 800°C and comparison with theoretical greybody curves. (a) Char previously formed at 1300°C under conditions of case a, 115cm reaction distance (temperature profile in Figure 4a; average thermocouple temperature 994 K). (b) Rosebud coal, case b at 115 cm (average thermocouple temperature 994 K). (c) Rosebud Coal, at 1Ocm (average thermocouple temperature 785 K)

spreading and the width of the stream (indicated on the diagram) was close to the 5 mm tube i.d. The FT-i.r. beam was z 2 mm in diameter, focused in the centre of the coal stream, so the FT-i.r. temperature was the average across the coal stream diameter. To validate the FT-i.r. measurements, thermocouple measurements were also made radially across the stream at the level of the FT-i.r. beam; they are presented in Figure 4a (open circles) for the conditions in Figures 3a and b. As can be seen, the edges of the stream (where contact was made with the cold ambient gas) were cooler than the centre. Thermocouple

Very rapid coal pyrolysis: Dry

Rosebud

800°C

The simplest case to understand

115cm

G e 600 E a e g 500

Coal stream -4

400 \

i-

3ooL ’ -32

I

I

-16

0

Axial

600

position

-

a

I

I

16

or mdial

32

(mm)

Outside

InsIde

tube

tube

i

I

z

Coal

/--

stream

I -16 Axial

or radial

et al.

is that for the pre-

viously formed char, Figure 3~1.The best lit for the shape of

800

j

P. R. Solomon

~4

I

I

0

16 position

,

b

32

(mm)

Thermocouple temperature measurements in the coal stream at the tube exit. 0, Radially at 3 mm below the exit; 0, radially at the tube exit; 0, axially from zero at the optical focus up into the tube. (a) Case a, 115 cm; (b) case b, 10cm. The zero position for the radial measurements is the centre of the coal stream; for the axial measurements it is the FT-i.r. focus and the positive direction is towards the tube end Figure 4

measurements were also made just at the tube exit (closed circles) and there was some cooling at the edges due to the cooling effect of the bottom electrode. The cooler edges of the stream made the average temperature z 50°C lower than the centre temperature. The temperature measured axially at the centre of the stream (squares) dropped by z 20°C from inside the tube to the FT-i.r. focus. For these conditions the average temperature across the FT-i.r. sample volume was therefore z 70°C lower than the temperature inside the tube.

the normalized emission given is by a grey-body temperature of 1000 K. This is in excellent agreement with the averaged thermocouple measurement of 994 K. The amplitude then requires an emissivity E= 0.87. Curves at 1030 K with ~=0.80 and at 970 K with e=0.95 bracket the best-lit curve. If only the emission amplitude at 1500cm-’ were to be used to determine temperature, an uncertainty in the particle emissivity between 0.80 and 0.95 would result in only a +3OC error at 1000°C. To determine temperatures, a value of c = 0.90 is assumed for coal and char. This value is a compromise between the char value of 0.87 determined above and a value of 0.93 expected for large coal particles. The value of 0.93 is based on the measured reflectivity for coal of 0.07 and the assumption that for large particles, the light which is not reflected is absorbed. The normalized emission spectrum in Figure 3b is compared with that of a grey body (E= 0.90) at the average thermocouple temperature of 994 K. The grey-body curve fits, excluding the region around 15OOcm-’ where there is interference from water. While the previously formed char shows a grey-body curve shape close to the average temperature, the newly formed char produced at 800°C does not have a grey-body curve shape. The theoretical and experimental curves match only in the range 160@ 1000 cm-’ and >4500 cm-‘. These and other measurements14,1 5 show that the particle’s emissivity is size- and temperature-dependent. For the size ofcoal particles used here, only specific bands (corresponding to the absorbing bands in coal) have a spectral emittance near 0.9. The result can be understood by considering that a particle’s emittance is related to its absorbance. Particles will emit only where they have sufficiently strong absorbance bands. As discussed by Hottel and Sarofim2’, for large particles (where diffraction can be neglected) the absorption can be calculated from geometrical optics, considering all possible rays through the particles. The absorption within the particle can be calculated using absorbance values measured by the KBr pellet method28. For 60 pm particles, only the band between 1600 and 1COOcm- ’ is strong enough to absorb completely over the particle diameter. For char, there is a broad band absorbance in the KBr spectra which increases at high wavenumbers. This absorbance appears to depend on the extent of pyrolysis and is similar to the broad absorbing feature observed for coals above 95:< carbon (which has been attributed to electron absorption bands) 29, This absorbance is accompanied by an increase in the particle’s spectral emittance at high wavenumbers. The spectra change with particle temperature and with changes in composition (e.g. with ring condensation, which makes the char more graphitic). The emission spectrum approaches that of a grey body like Figure 3a only after sufficient time at elevated temperature (after primary pyrolysis is complete). The char of Figure 3a is a good grey body (t:= 0.87). The char of Figure 3b, which reached only 800°C has regions where E< 0.9. For the sample of Figure 3c which reached 500°C only the region between 1000 and 16OOcm-’ has sufficient absorbance to attenuate the incident light fully over the particle thickness. It is in this region that the normalized emission can be compared with that of a black body for coals and chars.

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The FT-i.r. spectrum measured at 10 cm under the slow flow conditions of Figure 2a is presented in Figure 3c. The thermocouple measurements for this case are presented in Figure 4b. In this case the edges were hotter than the centre because the reactants were being heated by the walls. A 785 K grey-body curve (&=0.9) which tits the 1600-lOOOcm_’ region matches the average thermocouple temperature of Figure 4b. The average temperatures determined by FT-i.r. for the slow flow (case a) are presented as the triangles in Figure 2a. FT-i.r. measurements were made for the high-velocity case b of Figure 2b, but these were z5so”C lower than expected at long distances. An extensive investigation of this problem showed that at the high velocities, the coal particles were breaking up (presumably on hitting the walls) to produce large quantities of lines. The emissivity of these line particles was ~0.9 even in the 160& lOOOcm_’ region. Because of this problem, these points have not been plotted in Figure 2b. The lower-velocity case a did not produce a significant amount of fines. Summary of temperature determinations. The temperature measurements made in this work by thermocouple inside and outside the tube and by FT-i.r. outside the tube provide sufficient information to determine the coal particles’ temperature inside the tube to within +25”C. The wall temperature profile provides an upper bound to the particle temperature, assuming only modest exothermic reaction heats, in accordance with the calculations. The FT-ir. measurements provide a lower bound to the particle temperature inside the tube, because of the cooling effect of the ambient gas on the particle outside the tube. The FT-i.r. temperature is expected to be 2s 70°C lower than the temperature inside the tube. The calculated temperature inside the tube, which lies between these two bounds, is expected to be quite accurate. because of several factors: there is good agreement between the measured and calculated thermocouple readings; the calculated difference between the thermocouple and particle temperatures is within 50°C; after correcting for the cold edges of the stream and the 20°C drop between the inside and outside of the tube, the FT-i.r. particle temperatures are in excellent agreement with the calculated temperatures. Particle residence time

The heated-tube reactor was modified for particle velocity measurements. The passage of a pulse of coal through the system was measured for each electrode position by recording signals on a dual-channel oscilloscope from phototransistors mounted on a glass section at the top of the reactor tube and at the open bottom end of the tube. Photographs of the oscilloscope traces allowed an assessment of the mean particle residence time and the extent of axial dispersion. Measurements were originally made with coal pulses obtained by jarring the coal feeder system. A second technique was developed where short, well-defined pulses could be introduced by using an electrically activated solenoid to inject the contents of a tube containing the coal charge and gas at 70 kPa above atmospheric. While the original method injected the coal at the same velocity as in the experiments for temperature and mass balance measurements, the new technique provided narrower pulses. Both methods gave comparable transit time data.

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The photographs allowed an assessment of the particle residence time and of the particle dispersion, which affects the kinetic analysis. Some representative traces from cold and hot tests are shown in Figure 5. Figure Sa indicates a transit time of z 60 ms for the coal in the cold tube. The average transit time was determined from a number of measurements as z 56 ms. The particles therefore travelled at ~80% of the gas velocity of 28 m s-l in the cold experiments. This compares well with the value of 70% observed by Kim and Seader3* for 329 pm glass beads flowing through a small-diameter metal tube under similar conditions. The particles moved slower than the average gas velocity because of frictional interactions with the wall. Figure Sb-d shows the transit time when the tube was heated over increasing distances (75,100 and 115 cm). The transit time was reduced to = 32 ms when 115 cm was heated (Figure 5d). The extent of axial dispersion was small and was typically almost symmetrical, which would lead to a slight (z lo”/,) underestimation of the rate constant. It was neglected in the analysis of result for this paper. Data are presented in Figure 6 for the mean particle transit times for hot experiments, at a slightly higher gas

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120

160

d

0’

I

40

80

120

160

0’

I

I

40

I

I

80

I

I

120

160

Time (ms)

Figure 5 Phototransistor signals indicating coal particle transit times for four electrode positions: (a) 125 cm cold; (b) 50 cm cold, 75 cm hot; (c) 25 cm cold, 100 cm hot; (d) 10 cm cold, 115 cm hot. Conditions similar to case b but 20% higher helium flow

a Particles

Gas

20

40

60 80 Distance (cm

100

I 120

I 140

1

Figure 6 Measured and calculated particle residence times and calculated gas residence time in the heated-tube reactor (conditions as for Figure 5). The calculation assumes that the particles are at 80% of the gas velocity before and after primary pyrolysis and at 40% between 1 and 75% weight loss

Very rapid coal pyrolysis: P. R. Solomon

velocity than for case b (see Table 2). The hot data were adjusted so that the transit time in the cold part of the tube was not included. This was done by subtracting the heated length from the distance between the detectors (125 cm) and using the cold data to determine the transit time which should be subtracted from the observed transit time for the hot experiment. There is scatter of as much as + 25% in the data. The reason is not known, but it may be due to variations in the coal injection conditions. The uncertainty is reduced by taking an average over many tests. The worst assumption would lead to no more than a -25% variation in the reported kinetic rates which, considering the range of two orders of magnitude in reported rates, is not significant. The particle residence time data definitely indicate that the particles were accelerating in the hot experiments at nearly the same rate as the gas, except for a slow-down in the region where pyrolysis began. The close coupling with the gas velocity is reasonable in light of the small value of the characteristic drag time, 2 1.5 ms (which indicates the relaxation time for a particle for a step change in gas velocity), for the size fraction used in the hot experiments (260 pm)“. The data were fitted by a model which assumed the particles to be moving at 80”/, of the average gas velocity until primary pyrolysis was 1% complete, at 4q/, of the gas velocity between 1 and 75% pyrolysis and again 80% beyond this point. The reason for the slowdown during pyrolysis is not yet clear, but it was probably associated with evolution of gas from the coal or the swelling which has been observed for the lignite at such extremely high heating rates. Additional confirmation of the particle velocities was obtained by comparing the particle feed rate with the density of particles leaving the tube as determined by FTi.r. transmittance r. At short wavelengths, where diffraction effects can be neglected, (l-5) is the fraction of the FT-i.r. sample area blocked by particles: (1 - r) = 4mB/xoL

where m = the particle feed rate (g s-i), B = the percentage area blocked per cm by particles uniformly dispersed at unit density (cm* g-l), u= the particle velocity (cm s-l) and L is the path length (cm). The value of m was measured gravimetrically, L is the tube diameter and B was determined by measuring the transmission through a liquid cell containing measured weights of coal particles suspended in known volumes of oil. Therefore, the particle velocity can be calculated from measured values of (1 -z). Figure 7compares the measured particle velocity with the calculated gas velocity at various points in the tube as the gas is heated, for case a. The particles travelled at z SW/,of the gas velocity for cold gas and an average of 70% for hot gas, in agreement with the phototransistor measurements. Heating rate

As can be seen by comparing the temperaturedistance relation in Figure 2b with time-distance relation in Figure 6, the lignite in case b heated at > 25 000 K s- ‘. From the rapid kinetic rates which were determined under these conditions, this appears to be in the region of the most rapid heating which has been validated in a pyrolysis experiment. An interesting consequence of this rapid heating is shown in Figure 8. The lignite can be seen to have melted, bubbled and swollen. Figure 8a, b and c shows respectively bubbles, and a highly porous char like

et al

that expected from a bituminous coal. This behaviour is not observed at lower heating rates and it is believed that the high heating rate may have reduced the crosslinking reactions responsible for lack of fluidity. RESULTS AND DISCUSSION The results for cases u-d are shown in Figures 9-12 for isothermal tube temperatures of 700, 800 and 935°C. Figures 9a-12a present the yields of char, tar and gas (the sum of measured individual gas species) as a function of the reaction distance. The mass balance was between 96.5 and 101%. The measured and calculated particle temperatures and times as functions of distance are shown in Figures 9b12b. The calculated particle temperatures were validated by thermocouple and FT-i.r. measurements at 800 and 935°C. The calculated particle times were determined at 800°C. Confidence can be placed in the calculated temperature at 700°C because the calculations used the same heat transfer coefficients which gave the validated calculations at 800 and 930°C. The time calculations at 700 and 935°C employed the same relative velocity between gas and particles as was measured at 800°C. For all three temperatures, pyrolysis occurred during particle heat-up, even though the heating rate was > 25 000 K s- ‘. The solid lines in Figures 9a-12a were generated using a previously presented pyrolysis theory’ 9,11 in which the evolution of individual species is calculated using a distributed activation energy, of the form introduced by Anthony et al.‘, for each species. All cases a-g for the three coals were fitted with the same kinetic rates. At 800°C under fast flow conditions, results for the Rosebud subbituminous coal (casef) and Illinois bituminous coal (case g) were almost identical to those for the lignite (case b). The chemical kinetic rates for tar and hydrocarbon gas evolution are shown in Figure 13. As a further comparison, the reciprocals of the times required to achieve

‘“rh E e

r

0

2

4

6

Calculated

8

velocity

10

12

14

1

of gas (ms’)

Figure 7 Comparison of gas velocity with particle velocity determined from FT-i.r. transmittance T, for case a. n . gas at room temperature; 0, gas at elevated temperature

FUEL, 1986, Vol 65, February

189

Very rapid coal p yrolysis: P. R. Solomon

et al.

63% of the ultimate tar yield for each of the cases are plotted (solid circles) in Figure 13 as a function of the reciprocal of the average absolute temperature (from the heat transfer model) durmg this period. A single pyrolysis experiment at 600°C (case e) gave a rate constant of 6.3 s- ‘. These data fall close to the line defining the tar rate. The theory is in excellent agreement with the data. At all three temperatures the observed weight loss is a result

0

50

100

150

Reaction

200 distance

300

250

3

(cm)

b - 400

Reactlon

distance

(cm)

Figure 9 Pyrolysis results for Montana Rosebud, case a. (a) Experifunctional-group mental yield data: 0, char; n , gas; x, tar. ~ model; ---, single first-order rate model. (b) T;me and particle calculations; A, 0, experimental temperature versus distance: p, data

a

100

200

300

Reactlon

400 distance

500

600

7

(cm)

1000

100

b 800

G P z! E 400 F 8

200

e 6000 E 0

Figure 8 Figure 8 Photomicrographs of lignite chars produced at very high heating rates: (a) SOOT, 75 cm reaction distance; (b) 935°C 35 cm; (c) 9C@C, 100 cm

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FUEL, 1986, Vol 65, February

-

I

100

I

400 200 300 Reaction dtstance

I

500 (cm)

600

700

Figure 10 Pyrolysis results for a North Dakota lignite, case b. (a) As Figure Pa. (b) As Figure 9b (calculations only)

Very rapid coal pyrolysis:

s ;

40-

Reactlon

distance

(cm)

10001

1100

b -80

800 -

-60

;; E

-40

i! i-_

-20

0

100

200

300 400 React Ion d Istance

500 (cm)

I 600

70:

et al.

P. R. Solomon

of rapid evolution of tar and slower evolution of the gases. The increase in total weight loss with increasing final temperature is the result of gas evolution due to loss of tighly bound functional groups. For comparison with previous data, the weight loss was modelled assuming a single first-order process using a rate constant k =4.28 x 1Or4 exp( - 228 5OO/RT) s-l, with R in J mol-’ K-’ (dashed lines in Figures 9a-12~). This is one-half of k,, (see Figure 13) and represents a compromise between the rapid evolution of tar and the slower gas evolution. The single first-order rate does not fit the data so well. It gives a steeper weight loss than is observed and the yield does not increase with temperature. Given these limitations inherent in a single first-order model, the theory is in good agreement with the data. The results of the heated-tube reactor experiments provide chemical kinetic rates derived under wellcharacterized conditions. The particle temperatures which have been calculated, determined from thermocouple measurements and measured directly by FT-i.r. (corrected for cooling at the edge) all agree within + 25°C. The transit times measured by phototransistors and the exit velocities by FT-i.r. agree to within le/,. Using the well-characterized time-temperature histories, the chemical kinetic rate for primary pyrolysis tar yield for three coals has been calculated as k = 8.57 x 1014 exp( - 228 5OO/RT). The rate is > 1000 s- ’ at 800°C. The lowest value which could be obtained at 800°C (by assuming that the coal particles are instantaneously at the

Figure 11

Pyrolysis results for a North Dakota lignite, case c. (a) As Figure 9~. (b) As Figure 96 (calculations only)

I800 II

I

1200

I

Temperature (“C) 800 600 500

I

400

’

lo3

a

(5)-Coal

8O-c

d

“F’L,

IO2

IO'

7

0

100

200

300

Reactlon

400 distance

500

600

1

2 0 G

7

(1) L,gn,te single mte

7 \

0’ ; 10-l

(cm) ,100

10001

fl

(1) Bituminous

In :

Tar (4)

5

cc IO'>

(I ) Lignite distributed

1o-3

kmmc gas=8.35x10’4exp

lo-‘ C

I

I

I

0.6

Reaction

distance

(cm)

(-247, 500/k)\

I

0.8

Reciprocal

Figure 12 Pyrolysis results for a North Dakota lignite, case d. (a) As Figure 9a. (b) As Figure 9b (calculations only)

I

n

I

1.0

absolute

I

I

I

1.2

temperature

I

I

1.4 (IO”

Km’

I

1.6

1

Figure 13 Comparison of kinetic rates for weight loss (or tar loss) from several investigations; numbers in parentheses indicate the relevant references. Upper line for ref. 10 for 53-106 pm particles; lower line for 85@1OOO~mparticles. Rates for tar and aliphatic gas are those used in the functional-group model calculations

FUEL,

1986,

Vol 65, February

191

Very rapid coal pyrolysis:

P. R. Solomon

et al.

asymptotic tube temperature) is 100 s-l. Even this lower bound is higher than the high rates originally measured by Badzioch and Hawskley’. While this rate is higher than any previous masurement, it is in agreement with expectations for a chemical process. The activation energy of 230 kJ mol-’ is what is expected from thermochemical kinetics for ethylene bridges between aromatic rings and agrees with pyrolysis rates for model compounds and polymers where these bonds are the weak links31. The rate is also in agreement with data obtained recently in this laboratory for the same coals at much lower temperature (2450°C) at a heating rate of 30 K s-l. What are the reasons for the discrepancies between these rates and rates reported by many other investigators ‘-4,g,1o,32-34? Consider first the discrepancies between rates originally reported by this laboratory for the Zap ligniteay9,’ l and the Illinois No. 6 bituminous coa17. The rate reported here for tar is five times that reported in refs. 7-9 and 50 times that reported in ref. 11. The tar rate from ref. 11 is plotted in Figure 13. The rate for aliphatic gases is five times that previously reported7-9,11. These differences are due mainly to inaccuracies in determining particle temperatures. For example, low-temperature data obtained at a constant heating rate of 30 K min-’ with the heated grid suggested that primary pyrolysis reached a maximum rate at = 5oo”C9. At this temperature a 50-fold higher kinetic rate is equivalent to = 60°C lower temperature (see Figure 13). Recent results have been obtained with an apparatus which achieves improved heat transfer to the coal by flowing preheated helium past the sample located in a furnace whose walls are at the temperature of the helium. At a heating rate of 30 K min-‘, primary pyrolysis occurs at ~450°C. The temperature of the coal particles in the heated grid is based on the thermocouple measurement. Because of significant changes in the physical properties and mass of the coal particle during pyrolysis, the effect of evolving gases and tars on the heat transfer and the convective cooling effect of the cold ambient gas, the particle temperature could be substantially different from that indicated by the thermocouple bead. A temperature error of 50°C is not unexpected for the heated-grid apparatus. The high-temperature data were obtained in an entrained-flow reactor at reactor temperatures between 700 and 1300”C7-Q~“. At these temperatures, a 50-fold higher rate corresponds to a 150-300°C lower particle temperature. Since pyrolysis occurred during particle heat-up, the particle temperature was calculated7-9*’ ‘. A factor of two in the heating rate would account for the 15&3oo”C error in temperature. The temperature calibfixed heat capacity of rations assumed a 1.25 kJ kg-’ K-’ and an emissivity of 0.7. These assumptions would lead to overestimation of the particle heating rate. In addition, the effect of mixing of the cold particle-gas stream with the hot furnace gas was estimated and could lead to errors. The same problem of knowing the particle temperature during heat-up exists in deriving chemical kinetic rates from the data of Kobayashi et d2, which yields a rate of the order of 1 s-l at 800°C and only 100 s-l at 1800°C. At the highest temperature, the overprediction of temperature would have to be more than 1ooo”C to explain the discrepancy. There was no direct measurement or confirmation of the particle temperature in these experiments. Instead a calculation was performed based on an esti-

192

FUEL, 1986, Vol 65, February

mated parameter 8, defined as the ratio of the momentum shape factor to the energy shape factor (used in an integral analysis of the temperature and velocity fields). The particle temperature was calculated assuming 8 = 3, although a value closer to unity would be more likely. At 1827°C there is a 700°C difference between the calculated particle temperatures during pyrolysis for 8= 1 and B= 3. In addition the calculations assumed a smaller value of particle heat capacity than is now believed17*“; a higher value for the absorption of radiation than recent data would indicte for small coal particles14,’ 5 and zero heat of pyrolysis. These assumptions would also lead to overestimation of the particle temperature. Another reason for the discrepancies shown in Figure 13 is interpretation of the rates. The data and analysis of Anthony et al.’ illustrate this problem. They presented two kinetic interpretations for their data: a single firstorder process and a set of parallel processes with a Gaussian distribution of activation energies. Both interpretations fit the data using Arrhenius expressions for kinetic rates. The single first-order process, which uses an activation energy of 46 k3 moi-‘, requires two parameters, while the distributed-rate model, which uses a mean activation energy of 235 kJ mol-‘, requires a third parameter to describe the spread in rates. Niksa et ~1.~ used a single first-order model with a low activation energy of 72 kJ mol-‘. Suuberg et aL6 fitted individual species rather than weight loss. They chose to fit the data with more chemically reasonable rates which were obtained by assuming a preexponental factor of 10’ 3 s- ‘. The problem is that a variety of interpretations may provide good fits to the data over limited ranges of temperature and heating rate. It is only by performing experiments at sufficiently diMerent particle temperatures, with good knowledge of those temperatures, that the ambiguities can be eliminated. To achieve high particle temperatures before pyrolysis is over, very good heat transfer to the particles is required. The data presented in the present investigation, which achieved particle temperatures of 750°C during pyrolysis, indicate that the distributed activation energy model was the better choice to produce k % 100 s- ’ at 800°C. The low rates measured by most grid experiments’,3s4 suggest that pyrolysis is occurring at a lower temperature than assumed by the investigators. As discussed above, the coal particles in the heated grid may be at a different temperature from the thermocouple for a number of reasons. There are other data which do not agree with the present rates and where two-colour temperature measurements were made32-34. These measurements suggest a high solids temperature, but they may indicate the temperature of a hot cloud of soot surrounding the particle or hot spots on the particle surface rather than the temperature in the region of the particle where pyrolysis is occurring. It is also true that the assumption of constant grey-body emissivity used in interpretation of two-colour data should be examined carefully in light of recent data14,“: see Figure 3 and related discussion. ACKNOWLEDGEMENTS This work was supported by the Morgantown Energy Technology Center of the US Department of Energy. The authors are grateful to Warrack Willson of the University of North Dakota Energy Research Center for supplying the Zap North Dakota lignite.

Very rapid coal pyrolysis: P. R. Solomon

REFERENCES I

7

7

8 9 IO

Anthony, D. B., Howard, J. B., Hottel, H. C. and Meissner, H. P. in ‘15th Symposium (Int.) on Combustion’, Combustion Institute, Pittsburgh, 1975, p. 1303 Kobayashi, H., Howard, J. B. and Sarofim, A. F. in ‘16th Symposium (Int.) on Combustion’, Combustion Institute, Pittsburgh, 1977, p. 411 Niksa, S., Heyd, L. E., Russel, W. B. and Saville, D. A. in ‘20th Symposium (Int.) on Combustion’, Combustion Institute. Pittsburgh, I985 (to be published) Solomon. P. R. and Colket. M. B. in ‘17th Symposium (Int.) on Combustion’, Combustion Institute, Pittsburgh, 1979, p. 131 Badzioch, S. and Hawksley, P. G. W. Ind. Eng. Chem. Process Design Dee. 1970. 9, 521 Suuberg. E. M., Peters, W. A. and Howard, J. B. in ‘17th Symposium (hit.) on Combustion’, Combustion Institute, Pittsburgh, 1979. p. 131 Solomon, P. R. and Hamblen, D. G. ‘Measurement and Theory of Coal Pvrolvsis Kinetics in an Entrained Flow Reactor’, EPRI Final Report>or Project RP 1654-8, 1983 Solomon, P. R. ‘Measurements and Theory of Coal Pyrolysis’, US Department of Energy, Report DOE/FE/05122-1668, 1984 Solomon, P. R. and Hamblen, D. G. Prog. Energy Cornbust. Sci. 1984, 9, 323 Freihaut, J. D. ‘A Numerical and Experimental Investigation of Rapid Coal Pyrolysis’, Ph.D. Thesis, Pennsylvania State University, 1980 Solomon, P. R., Hamblen, D. G., Carangelo, R. M. and Krause, J. L. in ‘19th Symposium (En.) on Combustion’, Combustion Institute, Pittsburgh, 1981, p. 1139 Maloney, D. J. and Jenkins, R. G. in ‘20th Symposium (Int.) on Combustion’, Combustion Institute, Pittsburgh, 1985 (to be published) Solomon, P. R., Hamblen, D. G., Carangelo, R. M., Markham, J. R. and DiTaranto, M. B. Am. Chem. Sot. Div. Fuel Chem.

15

Solomon, P. R., Hamblen, D. G., Carangelo, R. M., Markham, J. R. and Chaffee, M. R. Am. Chem. Sot. Dir. Fuel Chem. Preprints

16 17 18 19 20

Brewster, B. S. and Seader, J. D. AIChE J. 1984, 30, 676 Merrick, D. Fuel 1983, 62, 540 Lee, A. L. Am. Chem. Sot. Die. Fuel Chem. Preprints 1968,12(3), 19 Mott, R. A. and Spooner, C. E. Fuel 1940, 19, 226 Solomon, P. R., Serio, M. A., Hamblen, D. G., Best, P. E., Markham, J. R. and Heninger, S. G. ‘Coal Gasification Reactions with On-line In-situ FT-IR Analysis’, 14th Quarterly Report for US Dept. of Energy, Contract No. DE-AC21-81FE05122, 1985 Hoffman, T. W. and Ross, L. L. Int. J. Hear Mass Trangfer 1972,

1985, 30(l),

21

23 24 25 26 27 28 29 30 31 32

33

34

Best, P. E., Carangelo, R. M. and Solomon, P. R. Am. Gem. Sot. Dir. Fuel Chem. Preprints 1984,29(6), 249

EQUATIONS

1

15, 599

22

Preprints 1984, 29(2), 83

APPENDIX:

et al.

Perry, R. H. and Green, D. ‘Chemical Engineers’ Handbook’, 6th Ed., McGraw-Hill, New York, 1984 Ranz, W. E. and Marshall, W. R. Chem. Eng. Prog. 1952,48,141 Carslaw, H. S. and Jaeger, J. C. ‘Conduction of Heat in Solids’, 2nd Ed., Clarendon, Press, Oxford, 1959 Badzioch, S. BCURA Mon. Bull. 1967. 31. 193 Boothroyd, R. G. ‘Flowing Gas-Solids Suspensions’, Chapman and Hall, London, 1971 Hottel, H. C. and Sarofim. A. F. ‘Radiative Transfer’, McGrawHill, New York, 1967 Solomon, P. R. and Carangelo, R. M. Fuel 1982,61, 663 Van Krevelen, D. W. ‘Coal’, Elsevier, Amsterdam, 1961 Kim, J. M. and Seader, J. D. AIChE J. 1983, 29, 353 Solomon, P. R. and King, H. H. Fuel 1984,63, 1302 Ballantyne, A., Chou, H. P., Orozvo, N. and Stickler, D. ‘Volatile Production during Rapid Coal Heating’, Paper to US Dept. of Energy Direct Utilization AR & TD Contractors Review Meeting, Pittsburgh, 1983 Seeker, W. R., Samuelsen, G. S., Heap, M. P. and Trolinger, J. D. in ‘18th Symposium (Int.) on Combustion’, Combustion Institute, 1981, p. 1213 Witte. A. B. and Gat. N. ‘Effect of Rapid Heating on Coal Nitrogen and Sulfur Releases’, Paper of US Dept of Energy Direct Utilization AR & TD Contractors Review Meeting, Pittsburgh, 1983

FOR HEAT TRANSFER

duerage particle temperature

dTJdt=6h,(x)l[p,c,(~)d,l(T,-

gas-particle convection T,) wall-particle radiation + 6aE,F(L)l[p,c,(y)d,](T, - Z4) heat of reaction -(dW/dt)%/[Wc,(y)] wall-particle contact + %&c,,(Y )&I( T, - T,) k(x) = Nu,(x)k,(x)l& Nu,(x)= 2.0+0.60[Re,(x)]0.5[c,~L,(X)/k&x)]o.33 &(x)=p,(x)d,(l/,K)fi,(x) d W/dt = rate of mass loss F(L) =view factor for radiation L=distance into heated zone x=(%+X)/2 Y=T, Particle temperature gradient

ATd: =(dT,/dt)/[24c((y)] a(y) = thermal diffusivity

Y=T,

doerage gas temperature

wall-gas convection &k,,dwllX- T,l gas-particle convection - %(Y )&/[&I - Eck,d,l[ T,- r,l h,(x) = Nu,(x)k,(x)ld, Nu,(x)= l.86{Re,(x)[c,~L,(x)/k,(X)](d,/L))0.33[~~(x)/~~(z)]o.14 Re,(x) = p,(x)& V,/&(x) J%=Pl(l +P) p,z $@&,/p,)(V K) z= T, 4’= CT,+ n/2 g

dT$dt =4h,(x)/b,(l-

Thermocouple temperature

gas convection-wall radiation Tb= q + wlhb(x)(T: - T4) hb(x) = 0.37k, (x)/d,JRe,,(x)]‘.” &b= 0.9 x=(Tb+ T,)/2 Reb(x)=pl(x)dbI:/k(x)

FUEL,

1986, Vol 65, February

193

Very rapid coal pyrolysis: P. R. Solomon et al. Radial gas temperature d~/dt=ki(x)l[pi(x)c,,(x)]l/Ar’[(~+1+ T-1 -2TJ + (Ar/ri)(7; + I- T)] - 6k(y)EJ(pi(lEckpid& T - T,) ri= iAr i=l to n Ar=&/(2n+l) X= Ti n = number of shells = 5 Y = (T+ u/2 Physical properties for helium carrier gas and coal

wall-gas conduction gas-particle convection

k(T)=4.927 x 10-6T0.75 p&T)= 151.6 x 10-7T’.s/(T+97.78) p,(T)= 1.78 x lO-4(273/T)

Nomenclature a CP d EC C f& j it,

R Re r T t

194

average atomic weight of char (g mol- ‘) specific heat capacity at constant pressure (Jg-‘K-l) diameter (cm) fraction of volume occupied by particles solid angle factor for radiation convection coefficient (J cm-’ s-l K-r) heat of reaction (J g-l) thermal conductivity of gas (J cm-’ s-r K-‘) distance into hot zone (cm) mass flow rate (g s-r) Nusselt number = hd/k gas constant=8.31 J mol-’ K-’ Reynolds number = @ Vd)/p radius (cm) temperature (K) time (s)

FUEL, 1986, Vol 65, February

V W a

E p P 0

velocity (cm s- ‘) coal mass (g) thermal diffusivity (cm’ s-l) emissivity viscosity (10-l Pa s) density (g cm- 3, Stefan-Boltzmann constant=5.67 x lo-” W cm-’ Km4

Subscripts thermocouple b C coal gas or average gas g i” gas cylindrical shell i initial 0 wall W WC wall contact 1 gas core