High-temperature solar pyrolysis of coal

Solar Energy Vol. 31, No. 2, pp. 13%143. 1983

0038-092X/83/080137~)7503.00/0 Pergamon Press Ltd.

Printed in Great Britain.

HIGH-TEMPERATURE SOLAR PYROLYSIS OF COALt W I L L A R D H . BEATTIE

University of California, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, U.S.A. and RENI~ BERJOAN and JEAN-PIERRE COUTURES C.N.R.S. Laboratoire des Ultra-refractaires, Odeillo, France

(Received 13 October 1982; accepted 16 October 1982) A~tract---Subbituminouscoal from Western United States was pyrolyzed by directly exposing 50 mg powdered samples to concentrated solar radiation. It was found that exposure to flux levels > 200 W/cm2 for 12.5 s devolatilized 51 per cent of the coal. At flux levels between 100 and 200 W/cm2 devolatilization was slightly less. Gas yield was a maximum of 31 mmol/g coal at a flux of lOOW/cm 2 and decreased slightly with increasing flux. Gas yields were more than twice as great as those obtained by a laser technique developed to simulate solar pyrolysis. In experiments with spectral cut-off filters there was no effect due to changing the wavelength distribution of sunlight. l. INTRODUCTION The utilization of coal through conversion to gaseous and liquid fuels embodies a number of different technical approaches. The energy necessary to drive endothermic coal gasification reactions can be supplied by partial coal combustion, by preheating the reactant gas, or from an external energy source, such as the sun. Pyrolysis is the initial step that occurs in most coal gasification processes. In this process, at temperatures between 600 and 1000°C, coal is decomposed into fuel gases, tars, and a residue of char containing carbon and mineral matter. Subsequent gasification reactions between the char and reactive gases such as CO2 or H20 may form additional fuel gases, or the gasification process may be carried out in the presence of high pressure H 2, which reacts with the pyrolysis products. The use of high-temperature solar energy to drive the endothermic reactions associated with coal gasification has been suggested by several investigators[I-5]. Such a process would give not only a highly useful and transportable end product, but would also result in the storage of a significant fraction of the solar energy in the bonds of the fuel molecules. Thus, solar energy may be transformed into a form that is both storable and transportable, and which can be used in existing equipment without modification. Depending on the nature of the solar gasifier, it has been calculated that a net savings in coal up to one-third of the supply can be realized. Economic analyses of solar-driven coal gasification [2-4] predict that the cost of "sunfuels" ['Work performed under the auspices of the U.S. DOE, and jointly funded 19ythe Solar Test Facility Users Association, Albuquerque, New Mexico and Los Alamos National Laboratory.

will be very comparable to costs for fuels generated by other advanced combustion-driven gasification concepts. Bituminous and subbituminous coal deposits in Western U.S. comprise a significant portion of the nation's reserves, and they are located in a region of abundant sunshine. This coal has a high volatility, which is advantageous for pyrolysis. In a previous work, Beattie and Sullivan produced an effective technique utilizing a continuous-wave carbon dioxide laser as the source of radiant energy for simulating concentrated sunlight in the direct irradiation of milligram-size samples of powdered coa115-8]. The laser technique has enabled us to perform parametric studies of coal gasification reactions. This technique employs a smaller, more versatile apparatus, and offers more rapid turnaround than is possible on a solar furnace. However, the laser radiation differs from solar radiation in three respects: the laser emits a small diameter beam that is limited to irradiation of small areas, the beam is unidirectional rather than a focused spot from a wide angle, and the radiation is a single frequency rather than the broad distribution characteristic of solar radiation. The primary purpose of this work was to evaluate the laser technique by comparing it with similar experiments in an actual solar furnace. A secondary purpose of this work was to investigate the possible role of photochemical reactions in the solar pyrolysis of coal. The use of photochemistry for the generation of readily storable fuels is under active consideration[9, 10]. However, the photochemistry attendant with coal gasification has not been previously investigated. Fluorescence and photochemical reactions have been observed in low-rank coals[11], and one might expect the degree of photo137

138

W . H . BEATTIE et al.

chemical reactivity of the volatile pyrolysis products of coal to be greater than that for solid coal.

EDGE

OF

FOCUSED

/ QUARTZ

DOME

2. EXPERIMENTAL

A single lump of subbituminous coal from a mine in Northeast Arizona was ground, mixed and sieved under an inert N2 gas atmosphere, and stored in closed containers. A sieved fraction of - 5 0 + 100 mesh size was used for th.is as well as most of the previous study[8], and its properties are given in Table 1. Table 1. Properties of coal sample Particle

diameters

Average p a r t i c l e

density

~'LON IG

GAS INLE

TO SPE

0 . 1 5 - 0 . 3 0 mm 1.34 g,/cm 3

C o m p o s i l l u n (wt%) Moisture a

9,40

Volatility b (exciuding moisture) Ash c

27.55 7.20

Nonvolatile

carbon d

55.85 100.00

aDetermined by d r y i n g ] hour a t llO°C i n f l o w i n g N2 (ASTM p r o c e d u r e ) , bDetermaned by heating to 950°C for 7 min in flowing N 2 (ASTM procedure). CResJdue atter combustion in air; not necessarily equal to original mineral matter in the coal. dDetermJned

by difference.

Experiments were carried out on a vertical solar furnace located at the C.N.R.S. Laboratorie des Ultra-refractairies in Odeillo, France. This furnace, which is described in several reports[12, 13], consists of a heliostat located on the south side of the laboratory building directing light vertically upward to a 2 m dia., 0.85 m focal length parabolic mirror in an overhanging section of the building. The experimental apparatus was supported on a track below the parabolic mirror allowing the entire assembly to be positioned at the focal point or withdrawn to the side. Large sliding doors located below the experimental area were closed to block the radiation during shutdown. All experiments were conducted in the apparatus shown in Fig. I. The 0.8 cm dia. of the coal sample closely corresponds to the diameter of the focused beam, and the angle subtended by the quartz dome is equal to the 123 ° core focal angle of the solar furnace. Sample temperatures were measured pyrometrically from below the sample in order to avoid interference from the reflected concentrated solar radiation. Because the sample dish temperature would probably be lower than that of the coal, tempc,'atures were measured using a surrogate sample consisting of a 0.05 cm thick graphite disk that replaced the sample dish and sample. The flux falling on the sample was varied by moving the supporting track and entire apparatus vertically so that the sample would be positioned above or below the focal plane of the furnace. The

Fig, I. Apparatus for solar pyrolysis of coal. The coal is held in a stainless steel or molybdenum sample dish 0.8 cm i.d. and 0.1 cm deep. The dish rests on a stainless steel support disk with a 0.8 cm hole in the center. The volume of the quartz dome and connecting tubes to the valves is 399 ml. flux falling on the sample was measured by putting an aperture at the sample position and a flowing water calorimeter below it, as shown in Fig. 2. EDGE

OF

QUARTZ DOME

/

.ON

Fig. 2. Pyrolysis apparatus equipped with a flowing water calorimeter. The sample dish and support disk are replaced by a molybdenum disk covered with zirconia felt and having a 0.8 cm aperture in the identical position as the sample. The flux passing through the aperture is absorbed at the grooved black painted surface (Aquadag paint) of the copper block, and is determined from the differential temperature of the flowing water. During irradiation the apparatus was purged with N 2. The variation of solar flux with aperture or sample height is shown in Fig. 3. In most experiments the sample was placed below the focal plane. Chemical analysis of the product gas was done on a portable quadrupole mass spectrometer connected

139

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SAMPLE INLET

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0 300

; 4

, 0

500

600

700

WAVELENGTH (nm)

Fig. 5. Transmittance of cut-off filters: (1) Plexiglass, (2) Yellow Kodagraph sheeting, and (3) Orange Kodagraph sheeting. irradiated for 12.5s by manually uncovering and recovering the quartz dome with an aluminum plate. The time required to remove or replace the aluminum plate was estimated to be approximately 0.3 s. The apparatus was allowed to cool to its initial temperature with the sliding doors of the furnace in the closed position, and the final equilibrium pressure was recorded. Gas yields were calculated from the pressure increase, and a gas sample was expanded into the mass spectrometer mixing tank and stirred; then a portion of the gas was transferred to the 0.3 1 sample tank for a mass spectrometer scan. Finally, the char was weighed and the per cent devolatilization was determined from the mass loss. Flux measurements were made at positions identical to the sample positions and corrected for changes in insolation with time of day. 3. RESULTS During the first one to two seconds of pyrolysis, a plume of smoke plunged upward from the coal sample, depositing soot-like particles in a 2-3 cm dia. spot on the dome surface directly over the sample. At fluxes of 100-200 W/cm 2 the remainder of the quartz dome remained clean; with increasing flux, the plume velocity increased and the area surrounding the spot was somewhat darkened; at fluxes of 600-900 W/cm: almost all surfaces inside the reactor were partially darkened, but with greatest density in the spot over the sample. During pyrolysis the coal swelled and sintered into a highly irregular porous lump of char 0.44).8 cm high. In Fig. 6, the per cent devolatilization of the coal is given as a function of solar flux. As a result of the violence of the pyrolysis reaction, it was sometimes difficult to quantitatively recover the char for weighing. Probable errors were greater for high than for low per cent devolatilization, and this prejudice is reflected in the line drawn through the points. At fluxes above

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Fig. 6. The per cent devolatilization of coal as a function of solar flux for 12.5 s exposures. A buffer gas of N 2, O, or H 2, O, at 630 torr was used. The solid line corresponds to runs in N 2 and the dashed line runs in H 2. approximately 200 W / c m 2, devolatilization reaches a level value of approximately 51 per cent. Below 200 W/cm 2, devolatilization decreases with decreasing flux, and in H2 buffer gas it falls off faster than in N 2 buffer gas. (However, there is insufficient data to be certain of the latter statement.) During the first part of the experimental program, reproducible gas yields could not be obtained. Subsequently, it was found that small horizontal displacements of the sample with respect to the focal point of the furnace seriously degraded the gas yield while making only small decreases in the flux falling on the sample. This is illustrated in Fig. 7. In experiments employing filters, and where positioning errors could be eliminated, the flux levels were reduced approximately in proportion to the integrated filter transmittances, but the yields were degraded only slightly if at all. The effect was to shift the Fig. 7 lines for a given aperture height to the left. Similar effects were observed in H2 buffer gas. Temperature measurements with the optical pyrometer were made early in the program before positioning errors were eliminated. Recorded tem35

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400 600 FLUX (W/crn 2)

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Fig. 7. Variation of gas yield with solar flux incident upon sample as samples are moved off-center, for the four given aperture heights. The upper line gives gas yields for wellcentered samples. Measurements were made without use of filters.

peratures of the graphite disk fell between 1800 and 2500°C at flux levels between 200 and 450 W/cm 2. These temperatures were always within 30ff'C of the calculated equilibrium blackbody temperatures of the irradiated disk. Although no assessment of the accuracy of the temperature measurements could be made, it appears reasonable to assume that equilibrium coal temperatures were close to the equilibrium blackbody temperature. In Fig. 8 the gas yield is given as a function of solar flux in a series of carefully positioned experiments. The gas yield reaches a maximum of 31 mmol/g at a flux of 100 W / c m 2, and falls off slowly with increasing flux. An unexpected result of these experiments was that the gas yields obtained under solar irradiation were more than twice as great as those previously obtained under laser irradiation. {~

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400

600

800

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FLUX (W/cm 2)

Fig. 8. Variation of pyrolysis gas yield with solar flux using no filter, O, or the Plexiglass filter, Fg, and with laser flux, O. The solar irradiated samples were carefully centered with respect to the focal point, located 2-14.5mm below the focal point. Data for the laser irradiated samples is taken from Ref. 8, Fig. 15.

The compositions of pyrolysis gases were determined from the corrected ratios of mass spectral peak heights using the buffer gas as an internal standard. In N2 buffer gas the product gases determined were H2, It20 , CH4, C O 2 and combined C2H 4 plus C2H 6 and in 1't2 buffer gas the same products were determined excepting H2 and adding CO. The H20 in the product gas was always less than its vapor pressure. Because of the unknown loss of H20 during evacuation of the apparatus, all gas compositions were calculated on a dry basis. The gas analyses for some well centered runs are given in Fig. 9. The most abundant product was H2, comprising 68-78 tool per cent of the gas at flux levels between 100 and 9 0 0 W / c m L The H2 yield did not change much with flux, but a small variation would be masked by the scatter of the points. The average H 2 yield and average deviation is 23.7 + 3.8 mmol per gram of coal. The second most abundant product, CO, was approximately 14 mol per cent in H2 buffer gas at

High temperature solar pyrolysis of coal I

35

I

assumed to be the carbon deposited on the reactor walls as soot-like particles. The hydrogen in the pyrolysis gas agrees, within the reproducibility of the measurement, with the combined hydrogen in the coal, and we conclude that the combined hydrogen in the coal was completely devolatilized. Three-quarters of the combined oxygen is found in the pyrolysis gas, but since both measurements are subject to significant errors, it is possible that all of the combined oxygen could have been in the pyrolysis gas.

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Fig. 9. Yields of the indicated gases as a function of the flux at the sample surface under the conditions: no filters and N 2 buffer gas, (3, plexiglassfilter and N2 buffer gas, [7, no filter and H2 buffer gas,/k. The symbol ZCH represents the sum of CH4, C2H4and C2H 6. The runs in N2 buffer gas were the same as given in Fig. 8. The dashed line through the H z data points was drawn with the same slope as the line in Fig. 8. Note the scale change in the ordinate. high flux levels. At high flux levels in N 2 buffer gas, the combined yield of the hydrocarbons CH4, C2H4 and C2H6 was approximately 10 mol per cent. In H2 buffer gas this yield was approximately twice as great as it was in the inert buffer gas. In either N 2or H z buffer gas the average yield of CO 2 was approximately 3 mol per cent, and the yield increased slightly with flux. Figure 8 shows that the total gas yield decreases slightly with increasing flux. At moderate flux levels of 100-200 W/cm 2most of the combined hydrogen in the coal devolatilized into H2 gas, but at high flux levels a greater fraction of the combined hydrogen was liberated in the form of hydrocarbons. This trend corresponds to the slopes of the H 2 and ZCH lines in Fig. 9. The elemental fractions in the pyrolysis gas are compared with those in the original coal in Table 2. The carbon in the pyrolysis gas and the solid residue is less than that in the original coal. The remainder is

Table 2. Compositions of coal and pyrolysis products

Complete devolatilization of the coal occurred at fluxes >~200 W/cm 2 for 12.5 s exposure times. Under such high fluxes, coal temperatures in excess of 1800°C are indicated by both measurements and calculations. These temperatures are far above the nominal 1000°C maximum temperature ordinarily assumed to be sufficient for complete coal pyrolysis. The average devolatilization of 51 per cent also far exceeds the 37 per cent ASTM volatility that was determined at 950°C. At fluxes < 200 W/cm 2 the devolatization decreased with decreasing flux under either solar or laser irradiation. Volatile fractions measured under laser irradiation at several fluxes are given in Table 3, and these values agree with those under solar irradiation (Fig. 6). Table 3. Devolatilization of coal under laser irradiation" Laser Flux (W/cm 2)

Devolatilization in 12.5-s (wt%)

Buffer Gas

58 73 166 166 166

29 37 50 41 45

Ar Ar Ar CO

aData

from Ref,

64.34 11.5. 41.8 d 11.0

N

4.75 5.5 ± 0.8

1.53 --

Coal

12.78 b 9.1

dc = 49~ residue

i n H2 b u f f e r - 7.2~

ash.

gas.

and Fig.

16.

H and 0 in moisture

, gas + vapor + char.

(1)

cooling ,

(2)

tar.

tadisat heating

is

CAverage values at 450 W/cm 2 and in N 2 buffer gas. The sum of H2, CH4, C2H4, C2H6, CO, and CO 2 is lncluded. The CO fraction w a s a s s u m e d t o b e t h e s a m e i n N2 b u f f e r gas as measured

IV and VI,

9.4

boxygen determined by difference, taking ash = 7.2~, and ignoring the fraction sulfur (< I~).

that

Table

radiant heating

0a

Vapor

aonly combined R and 0 included; excluded.

8,

H2

Gas yields under solar irradiation far exceed those under laser irradiation. The reason for the high yield cannot be found in any photochemical processes, because the results of the experiments with filtered light indicate that changes in the frequency distribution have no detectable effect upon gas yield. An explanation of the difference that is based upon the different geometries of laser and solar irradiation is compatible with the data. For this purpose the following model of the pyrolysis mechanism is proposed.

Component Wt~

Ha Coal Pyrolysis gas c Solid residue Remainder

141

Vapor-

,

particulate solids + gas. (3)

In reaction (1), the coal is initially heated at a rate of 10a-105°C/s. As the coal is heated, competing reactions of bond scission and polymerization occur[14].

142

W. H. BEATTIEet al.

The coal first liquifies, then vapors force their way through cracks and pores in the particle with explosive-like force[15]. The low vapor pressure fraction and mineral matter remain in the solid, and polymerization reactions cement the particles into an irregular mass. The observation of extremely irregular expanded masses of char is evidence for the existence of a cementing liquid phase. The gases and vapors that escape are the high vapor pressure fractions consisting of the degraded fragments of coal molecules. Upon mixing with the colder buffer gas, the heavier molecules condense into particles of tarry liquid phase, reaction (2). If, instead of cooling, the heavier fraction should pass through a region of high radiant flux, it would absorb energy and very rapidly approach its equilibrium blackbody temperature. Bond scission and polymerization would occur and the net reaction would be disproportionation into small molecules of gases high in hydrogen content and carbonaceous solid particles as given in reaction (3). This secondary pyrolysis reaction can account for the deposit of soot-like particles. When coal is pyrolyzed under laser irradiation, only a small fraction of the vapor passes through the laser beam with attendant secondary pyrolysis, and the major fraction cools in the volume outside of the laser beam. Under solar irradiation with the coal sample located at or below the focal point, the entire volume of vapor is irradiated, and is subject to secondary pyrolysis. The gas yield is thereby maximized, the tar yield minimized. The slight decrease in gas yield with increasing solar flux, shown in Fig. 8, is also explainable. The solar flux incident upon the coal was regulated by adjusting the distance between the focal point and the coal sample. At an aperture height of - 1 5 m m the flux was 100 W/cm 2(Fig. 3) and the gas yield was maximum. At this relatively low flux the velocity of the ejected vapor may be expected to be relatively low, and further decreased by passage through ~ 15 mm of buffer gas before passing through the high flux region around the focal point. With a relatively long residence time in the high flux region, secondary pyrolysis of the vapor and gas yield would be maximized. At higher flux levels, the sample was located closer to the focal point, causing the velocity of the ejected vapor through the focal region to be increased and the residence time of the vapor in the high flux region to be decreased; consequently secondary pyrolysis and gas yield would be somewhat decreased. The degradation of gas yield when the sample is off-center is also explainable by this mechanism. The off-center flux is not significantly decreased from that at center if the sample is below the focal point. However, the secondary pyrolysis would be considerably decreased because some of the vapors could pass around one side of the high flux focal region. 5. CONCLUSIONS When samples of subbituminous coal were pyrolyzed in an inert buffer gas at 1 atm pressure by direct

absorption of concentrated solar radiation at flux levels > 200 W/cm 2, approximately 51 wt per cent of the coal was volatilized, and virtually no tar was produced. Maximum gas yield was 31 mmols per gram of coal. On a molar basis the principle gas was H2, with lesser amounts of CO, CH4, C02, C2H4 and other hydrocarbons in that order. The gas yield was not affected by changes in the spectral distribution of sunlight, and we therefore believe that there are no photochemical reactions of interest in this process. The gas yield varied with the position of the sample, relative to the focal point of the furnace. In our apparatus, gas yield was greatest when the coal sample was approximately 15 mm below the focal point of the furnace, and decreased if the sample was either raised or moved laterally off-center. A reaction mechanism involving two steps of pyrolysis is proposed. The first step is devolatization of the coal, and the second step is gas-phase pyrolysis of the vapor. The difference in yields between laser and solar irradiation may be explained using this model. The gas yields from direct solar pyrolysis at flux levels of 100 W/cm 2 are significant and may be high enough to sustain an economically viable solar coal gasification process. Future engineering scale solar coal gasification work should be undertaken in order to investigate the economic advantages of this direct solar heating technique.

Acknowh, dgements--The authors wish to thank Margaret Salopek of Los Alamos National Laboratory for her technical assistance.

REFERENCES

1. D. W. Gregg, R. W. Taylor, J. H. Campbell, J. R. Taylor and A. Cotton, Solar gasification of coal, activated carbon, coke, and coal and biomass mixtures. Solar Energy 25, 353-364 (1980). 2. D. W. Gregg, W. R. Aiman, H. H. Otsuki and C. B. Thorsness, Solar coal gasification. Solar Energy 24, 313-321 (1980). 3. W. R. Aiman, C. B. Thorsness and D. W. Gregg, Solar Coal Gasification: Plant Design and Economics. Lawrence Livermore Laboratory, Livermore, California. UCRL Preprint 84610 (1981). 4. S. Lakshmanan, F. K. Manasse and V. K. Mathur, Production of fuels from high temperature solar thermal systems-~conomic analysis. Fundamentals and Applications of Solar Energy (Edited by I. H. Farag and S. S. Melsheimer),AICHE Syrup. Series No. 198, Vol. 76, p. 156. New York (1980). 5. W. H. Beattie, Flash pyrolysis of coal through solar heating. Proc. of 1981 Ann. Meet. Am. Section Int. Solar Energy Soc., p. 179. Newark, Delaware. 6. W. H. Beattie and J. A. Sullivan, Laser simulation of solar coal gasification. Proc. Solar Fuels Workshop, Albuquerque, New Mexico, SERI 902-3 (1979). 7. W. H. Beattie and J. A. Sullivan, Flash pyrolysis and gasification of coal through laser heating. Proc. 15th Intersoc. Energy Conver. Engng Conf., p. 637. Seattle, Washington (1980). 8. W. H. Beattie, Laser Simulation of Solar Pyrolysis and Gasification Using Static Coal Samples. LA-8617, Los Alamos National Laboratory (1981).

High temperature solar pyrolysis of coal 9. R. W. Harrington, Nonbiological photochemical energy conversion, can it compete? J. Solid State Chem. 22, 9-15 (1977). 10. J. R. Bolton, Solar fuels. Science 202, 705-711 (1978). 11. F. T. C. Ting, Petrographic techniques in coal analysis. Analytical Methods for Coal and Coal Products (Edited by C. Karr, Jr.), Vol. I, Chap. 1. Academic Press, New York (1978). 12. R. W. Taylor, R. Berjoan and J. P. Coutures, Solar Gasification of Carbonaceous Materials. UCRL-53063, Lawrence Livermore Laboratory, NTIS (1980).

143

13. S. H. Bomar, Jr., A proposed vertical-axis solar facility. Proc. Solar Fuels Workshop, Solar Thermal Test Facility Users Association, Albuquerque, New Mexico, SERI 9090-2, NTIS (1979). 14. D. B. Anthony and J. B. Howard, Coal devolatilization and hydrogasification. A . L C h . E . J . 22(4), 626-656 (1976). 15. W. J. McLean, D. R. Hardesty and J. H. Pohl, Direct observations of devolatilizing pulverized coal particles in a combustion environment. Proc. 18th Syrup. (Int.) on Combustion, Combustion Institute (1980).