Impact of Pressure Variations on Coal Devolatilization Products. 2

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Energy & Fuels 2004, 18, 520-530

Impact of Pressure Variations on Coal Devolatilization Products. 2. Detailed Product Distributions from 1.0 MPa Nicholas Manton, Joseph Cor, Guido Mul, Donald Eckstrom, and Ripudaman Malhotra* SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025-3493

Stephen Niksa Niksa Energy Associates, 1745 Terrace Drive, Belmont, California 94002-1756 Received July 22, 2003. Revised Manuscript Received December 9, 2003

This paper reports detailed product distributions for the devolatilization of five coals during transient heating at a rate of 7000 K/s and a pressure of 1.0 MPa. Major noncondensable gases were resolved into C1-C4 hydrocarbons, oils, CO, CO2, H2, and H2O. Fuel-N release was monitored in terms of the tar-N and char-N contents. Elemental compositions are reported for the tars and chars. All major products were monitored in individual tests; therefore, closures on the balances on total mass, carbon, hydrogen, and nitrogen clearly indicate the quality of these data sets. All the measurements were resolved in fine time increments from the onset of primary devolatilization through the attainment of ultimate yields. The novel heating configuration in the tests essentially eliminated secondary volatiles pyrolysis chemistry, so the product distributions are especially well-suited to validating the proposed devolatilization mechanisms. FLASHCHAIN, which is one of the network depolymerization models, interprets the most important aspects of the pressure effect on devolatilization for all but one coal, based solely on the proximate and ultimate analyses of the coals and the operating conditions in the tests.

Introduction Across the globe, developers of coal-fired power generators face imperatives to increase conversion efficiencies to better compete with other fuels, especially where CO2 emissions are being reduced. A multitude of process concepts are under development, as surveyed recently by Beer.1 All have one feature in common: primary conversion of the coal feed at elevated pressure. In any of these processes, the partitioning of the coal feed into volatiles and char during devolatilization is crucial, because volatiles are subsequently converted to ultimate products on much shorter time scales than char. Devolatilization also governs the stabilities of flames on fuel injectors and also affects temperature profiles and all the major emissions. Since the mid-1970s, the database on coal devolatilization has grown to represent more than 100 coals from all the major coal producing regions worldwide. For example, one recent survey2 describes a database that was compiled from the literature in English on the devolatilization of 99 different coal samples over an enormous pressure range. However, the vast majority of published work reports only the total weight loss, and fewer than half of it includes tar yields. Only five studies reported the complete distribution of devolatilization * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Beer, J. M. Prog. Energy Combust. Sci. 2000, 26 (4-6), 301. (2) Niksa, S.; Liu, G.-S.; Hurt, R. H. Prog. Energy Combust. Sci. 2003, 29 (5), 425.

products from elevated pressures,3-6 including this one, and only two of these imposed typical suspension firing conditions in the laboratory tests. This study was undertaken to characterize the impact of pressure variations on devolatilization behavior. It provides accurate product distributions for the devolatilization of five coals in entrained flows at 1.0 MPa, as a means to validate devolatilization models for engineering design applications stringently. Here, the product distributions are used to evaluate the predictions from FLASHCHAIN,7 which is one of the network depolymerization mechanisms. Our companion paper8 reports the product distributions from the same five coals at 0.1 MPa and, therefore, provides an essential reference condition for characterizing pressure effects. Here, in Part 2, the product distributions are reported for five coals that represent ranks from subbituminous through high volatile (hv) bituminous at nominal heating rates of 7000 K/s. Major noncondensable gases are resolved into C1-C4 hydrocarbons, oils, CO, CO2, H2, and H2O. Fuel-N release is monitored, in terms of the (3) Suuberg, E. M.; Unger, P. E.; Lily, W. D. Fuel 1985, 64, 966. (4) Bautista, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986, 25, 536. (5) Griffin, T. P.; Howard, J. B.; Peters, W. A Energy Fuels 1993, 7 (2), 297. (6) Matsuoka, K.; Ma, Z.-X.; Akiho, H.; Zhang, Z.-G.; Tomita, A.; Fletcher, T. H.; Wo´jtowicz, M. A.; Niksa, S. Energy Fuels 2003, 17 (4), 984-990. (7) Niksa, S. Combust. Flame 1995, 100, 384. (8) Liu, Y.-L.; Malhotra, R.; Niksa, S. Energy Fuels 2004, 18 (2), 520-530.

10.1021/ef034034e CCC: $27.50 © 2004 American Chemical Society Published on Web 02/27/2004

Effect of Pressure of Coal Devolatilization. 2

Energy & Fuels, Vol. 18, No. 2, 2004 521

Figure 1. Schematic of the Pressurized Radiant Coal Flow Reactor (p-RCFR).

tar-N and char-N levels. Elemental compositions are reported for the tars and chars. All major products were monitored in individual tests; therefore, closures on the balances on total mass, carbon, hydrogen, and nitrogen clearly indicate the quality of these datasets. Experimental Section The original version of the radiant coal-flow reactor developed by Chen and Niksa9 was substantially modified for operation at pressures up to 3.0 MPa, as described by Cor et al.10 The most important differences are that the coal suspension flows upward, to inhibit recirculation currents induced by buoyancy at elevated pressures, and the flow is marginally turbulent. A schematic for the pressurized radiant coal-flow reactor (p-RCFR) is depicted in Figure 1. The facility consists of two vessels that are pressurized to 1.0 MPa during operation. The coal is fed into the system by a positive displacement feeder that is housed in the vessel on the right-hand side in Figure 1. An argon entrainment stream then carries the suspension through the bottom of a U-tube and into the radiant furnace that is housed inside the vessel on the lefthand side in Figure 1. Below the furnace entrance, a second argon flow is introduced into an annulus around the entrainment flow. This sheath flow keeps coal particles from colliding with the walls of the flow tube. The radiant section consists of a quartz tube on the axis of a graphite cylinder, which is inductively heated to 1850-1900 K. Near-blackbody thermal emission from the graphite imposes heat fluxes up to 60 W/cm2 on the suspension, which heats the particles at rates faster than 7000 K/s as they traverse the furnace at 1.0 MPa. The suspension is kept optically thin, to ensure that the radiant (9) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254. (10) Cor, J.; Manton, N.; Mul, G.; Eckstrom, D. J.; Olson, W.; Malhotra, R.; Niksa, S. Energy Fuels 2000, 14 (3), 692-700.

Figure 2. Effect of particle loading on mass-averaged particle and gas temperatures in the p-RCFR; the inlet gas velocity is 36 cm/s, and the pressure is 1.0 MPa. heat flux to individual particles at any axial position is uniform, and the macroscopic behavior can be interpreted in terms of single-particle phenomena. Also, because the argon is transparent to the radiation, its only means of heating is by convection from the tube walls and particles. Dilute suspensions have little interfacial surface area for heat transfer, so the entrainment gas remains relatively cool and quenches chemistry among primary products as they are expelled. Because of the higher gas density, the temperature difference (∆T) between the particles and entrainment flow becomes even greater at elevated pressure. This effect is evident in Figure 2, which shows the thermal histories of the suspension and entrainment flow at several different suspension loadings, based on a one-dimensional heat-transfer analysis in FLUENT.10 The maximum predicted ∆T value decreases for

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progressively higher suspension loadings and reaches a value of 550-650 K. At atmospheric pressure, the maximum predicted ∆T value is 400 K,9 yet the product distributions displayed only minimal evidence of secondary volatiles pyrolysis.8 The cooler entrainment flow at 1.0 MPa provides an even more effective quench. Nominal particle residence times are set by the inlet gas flow rate and the distance between the inlet plane and an argon quench nozzle downstream of the furnace, which is fixed. However, reported values were not measured, but, instead, were simply estimated for plug flow. We know that these values are too long, because of several features that accelerate the flow, including growing boundary layers on the flow tube walls and gas expansion that is due to heating. They should only be used to track the relative differences among different tests. Because the thermal histories of particles and gas at 1.0 MPa are relatively insensitive to the coal loading (cf. Figure 2), the coal feed rate was fixed at a nominal value of 0.5 g/min, while the gas flow rate was changed to vary the residence times. Accordingly, the coal loading diminished in runs with progressively shorter residence times. Products are segregated into bulk solid particles, aerosol, and noncondensable gases with virtual impaction in an aerodynamic classifier (centripeter), as sketched in Figure 1. Beyond the inlet port of the centripeter, the process stream is split between opposing nozzles. Eighty-five percent of the flow passes into the annulus. The aerosol particles are typically a few micrometers in diameter; therefore, they are convected into the annulus and ultimately onto filters. By virtue of their inertia, char particles penetrate the impaction surface and fall into the basket. All tar and char is collected so that total yields are determined gravimetrically. Yields of char and aerosol are based on the weight gain of their respective collection elements and the suspension feedrate, which is assigned from two calibration runs in cold flow both before and after the test. Noncondensable gases are quantified by a nondispersive infrared (NDIR) analyzer, Fourier transform infrared (FTIR) spectroscopy, and on-line gas chromatography. Mass and elemental balances are closed in independent runs, so subsequent interpretations are not subjected to inordinate scatter in the data. At the end of the run, the char is collected from the basket. Some char is deposited in the quench nozzle, and it is also recovered and weighed. Tars are recovered on a three-stage assembly of glass-fiber filters that are dried overnight at 335 K under vacuum. Pure tar samples for subsequent chemical analyses are prepared by extraction with tetrahydrofuran (THF) in an ultrasonic bath, followed by filtration through a 0.2-µm Teflon membrane. The tar solution is concentrated in a rotary evaporator before the remaining solvent is evaporated, following the procedure of Lafleur et al.11 Several tens of milligrams of pure tar are recovered from runs with significant aerosol yields. The aerosols that were entrained with the 15% flow through the char basket were trapped on a preweighed glass wool plug, and its weight gain was determined and added to the weight of tar aerosols collected in the centripeter. The aerosols recovered in the centripeter were examined for soot, according to solubility in THF and filtration. Insolubles were determined to be significant; however, elemental analysis confirmed that the insolubles were coal fines, not soot agglomerates. We conclude that secondary pyrolysis exerts negligible or minor influence on the product distributions. Carbon, hydrogen, and nitrogen contents of condensed products are determined by elemental analysis at commercial testing laboratories. A stream of noncondensable gases is withdrawn through a sidearm on the aerodynamic classifier into a gas analysis train. (11) Lafleur, A. L.; Monchamp, P. A.; Plummer, E. F.; Kruzel, E. L. Anal. Lett. 1986, 19 (21&22), 2103.

Manton et al. Less than 15% of the flow is extracted, so flow patterns within the impactor are hardly perturbed, although aerosol yields must be adjusted for the amount withdrawn into this line. The concentrations of CO, CO2, HCN, NH3, and NO were monitored on-line with FTIR spectroscopy through a 6.5 m multipass gas cell at 335 K. A sidearm in the annulus of the classifier draws gases and aerosol into a tar/soot filter and then into a heated line that fills the multipass gas sample cell in the FTIR spectrometer. Preliminary runs established that water condensation on the transfer lines and all species concentrations stabilize within 40 s of sampling. Hence, five FTIR scans are acquired per test, beginning at 45 s of operation into the test and continuing throughout a total run time of 202 s. The main gas sampling line also includes a sidearm into a NDIR water detector. The only correction to the H2O yields reported here is that the moisture level of the coal, evident as the constant H2O yield at reaction times that were too short to produce any chemically formed H2O, was subtracted from the measured H2O yields to resolve chemically formed H2O only. Gas samples are also extracted for subsequent chromatography into multiport sampling valves at 335 K through a port in the main gas sampling line. C1-C4 hydrocarbons (CH4, C2H2 + C2H4, C2H6, C3H6, C3H8, C4s) are chromatographed on a HayeSep D column into a flame ionization detector (FID). Yields of oils, which are defined as hydrocarbons with carbon numbers of 5 and higher remaining in the gas phase under the conditions in the impactor, are based on the integrated total hydrocarbon signal from the FID reduced by the amounts of the C1-C4 hydrocarbons (determined for the same sample injection). The yield of H2 is determined with chromatography on a HayeSep DB column into a thermal conductivity detector. All data were obtained with the furnace hot zone at 1850 K under a pressure of 1.03 MPa with 99.9999% argon entrainment and sheath flows and 99.99% argon quench streams.

Coal Properties The coal samples are from the collection of coals supplied by the Center of Coal utilization in Japan (CCUJ), which distributed them to many investigators. Coal samples were ground in a disk grinder dosed with liquid nitrogen to displace O2 and dissipate heat. They were classified into the 75-106 µm size grade by sieving on a Ro-Tap for 30 min. The fines were removed by wet sedimentation, and the coal was dried in a vacuum oven for at least 12 h at 16 kPa (25 in. Hg vacuum) and 385 K, and stored in a desiccator with Drierite. A sample of the coal was then sent to a commercial testing laboratory for proximate and ultimate analyses. The values from these measurements are given in Table 1 and they differ slightly from the values reported by CCUJ. Accurate coal analysis is essential for tight closures of the C/H/N balances formulated from the measured product distributions. Chars and tars from the devolatilization tests were analyzed by commercial testing laboratories, and weight loss, product yields, and elemental yields were converted to a dry, ash-free (daf) basis using the ash levels in Table 1. Experimental Results Mass and Elemental Balances. Mass balances are based on the coal feedrate calibrations plus independent determinations of the yields of char, tar, oils, CH4, C2H2 + C2H4, C2H6, C3H8, and C3H6, aggregate C4s, CO, CO2, H2, H2O, HCN, and NH3 for individual runs. C/H/N balances also involve determinations of the elemental compositions of char and tar, the yields of HCN and NH3, plus an ash level. Unfortunately, the nitrogen


Energy & Fuels, Vol. 18, No. 2, 2004 523 Table 1. Coal Properties

proximate analysis, dry wt % volatile matter ash ultimate analysis, daf wt % carbon hydrogen oxygena nitrogen sulfur a

SS001AUS Newlands

SS002AUS Ebenezer

SS003AUS Blair Athol

SS004CHN Tatong

SS005JPN Taiheiyo

28.5 15.2

40.6 11.9

22.1 7.7

28.3 8.5

37.1 13.0

82.3 4.7 10.9 1.7 0.5

77.8 6.1 14.0 1.5 0.6

80.4 4.4 13.0 1.9 0.3

82.6 4.6 11.1 0.9 0.8

76.3 6.0 16.3 1.1 0.3

By difference.

Table 2. Mass and Elemental Closures for Individual Tests with SS003AUS reaction time (ms)

Σmass

Σf Ci

Σf H i

Σf N i

55.0 81.0 120.0 149.0 175.0 248.0 300.0 55.0

99.4 98.9 106.4 101.6 106.6 101.2 103.0 99.4

99.2 96.7 102.9 96.9 99.1 95.8 98.8 99.2

115.0 104.0 114.1 106.7 99.1 114.5 112.1 115.0

104.7 98.4 84.8 91.1 68.0 88.4 88.2 104.7

balances are incomplete, because the HCN and NH3 levels in the products were below the detection limits of the analytical system, because of the dilute suspension loadings and high gas densities. The char composition was converted to the daf basis with the ash content of the coal, assuming that no products were derived from inorganic precursors. Also, the elemental compositions of oils were not monitored. They were assigned as toluenes, based on characterization work at atmospheric pressure.12 Mass and elemental balances are reported for individual tests with SS003AUS in Table 2. Closures for the balances on mass and carbon with SS003AUS are within (5% at all residence times, except for the minor breaches in the mass balance at two intermediate times. The hydrogen balance is systematically oversubscribed by up to 15%. There is a systematic deficit in the nitrogen balance that grows for longer residence times, as expected because HCN and NH3 were omitted. It has already been established that these nitrogen species are released during the later stages of primary devolatilization from these coals,8 so the trend in Table 2 seems to be legitimate. Mass and carbon balances generally satisfy the standard of (5% achieved with the a-RCFR in testing at atmospheric pressure. The only exceptions were the mass balances for SS004CHN and SS005JPN, which were slightly deficient and oversubscribed by 15%, respectively. All carbon balances were closed to within (5%, except that for SS002AUS was deficient by 10%. Hydrogen balances usually did not satisfy the standard. Those for SS001AUS and SS004CHN were oversubscribed by up to 20%, and that for SS005JPN was badly scattered. None of the nitrogen balances were closed, because of the omission of HCN and NH3; however, only those for SS001AUS and SS003AUS exhibited the expected trend toward better closures at progressively longer residence times without excessive scatter or biases. (12) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 627.

Figure 3. (Top) Cumulative transient yields from SS002AUS in the furnace, at 1900 K, of (b) C1-C3 hydrocarbons plus oils, (+) H2O, (]) CO and CO2, and (2) tar. The mass balances on the separate scale at the top of the panel (data indicated by solid squares, 9) also involve char yields. (Bottom) Cumulative transient carbon fractions of (4) C1-C3 hydrocarbons, (]) CO and CO2, (b) oils, and (2) tar. The carbon balances on the separate scale at the top of the panel (data indicated by solid squares, 9) also involve char yields.

Closures of the mass and carbon balances for SS002AUS are shown in Figure 3, along with the major features in the product distributions. These figures show the product distribution on a cumulative basis, according to the mass distribution in the upper panel and according to the carbon distribution in the lower panel. The estimated residence times are longer than actual values by as much as a factor of 2, as noted previously. The

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lines drawn through the data points are spline fits to aid the reader’s eye. The data in these figures depict the entire course of transient devolatilization, from the onset through asymptotic ultimate values for all the major products. Tar is the most abundant product lump with SS002AUS, but noncondensable gases are released with the tar from the beginning. Tar release surges midway through devolatilization with this coal. The total yields of all hydrocarbon gases amount to only half the yields of all the oxygenated gases with SS002AUS. Because devolatilization preferentially releases heteroatoms, the carbon conversion to volatile products comprises only ∼80% of the mass loss. The relative contributions from the major product lumps closely parallels the mass loss distribution, because chemically formed H2O and H2 are the only products that do not contribute to the cumulative carbon distributions. Tar Release. Transient weight loss and tar yields are shown in Figure 4. Despite the similar elemental compositions of these coals, the substantially greater hydrogen contents of SS002AUS and SS005JPN are reflected in substantially greater ultimate yields and tar yields: 28.6 daf wt % total yield and 15.3% tar yield from SS001AUS versus 50.9% and 24.4% from SS002AUS, 29.4% and 10.9% from SS003AUS, 46.9 daf wt % and 23.1% from SS004CHN, and 58.2% and 23.0% from SS005JPN. The notable exception to this tendency is the behavior of SS004CHN. Whereas the total and tar yields from the four other coals are substantially lower than their respective yields at atmospheric pressure,8 as expected, the weight loss and tar yields from SS004CHN are the same within experimental uncertainty. To our knowledge, this is the first observation of pressure-independent total yields and tar yields for the devolatilization of bituminous coal in the literature. Increasing the pressure from 0.1 to 1.0 MPa decreased the ultimate weight loss by 7% for SS002AUS, 25% for SS005JPN, 33% for SS001AUS, and 40% for SS003AUS. Only the value for SS002AUS is typical2 for bituminous coals. The pressure increase reduced the tar yields by 20% for SS002AUS and 33% for SS001AUS and SS003AUS. The tar yields for SS004CHN and SS005JPN actually increased from just under 20 daf wt % to 23% for both coals. However, elevating the pressure also eliminated the extraordinarily high yield of oils from SS005JPN at 0.1 MPa,8 reducing it from 16.2 daf wt % to 2.6%. On the basis of the sum of the yields of tars plus oils, the pressure elevation reduced the yield of aromatic liquids from SS005JPN by more than 30%. This is probably the correct basis to apply, because the atmospheric tars from both SS005JPN and SS004CHN contained unusually large amounts of oxygen,8 which is probably responsible for their high cracking propensity. Even on this basis, however, the liquids yields (and total weight loss) from SS004CHN are independent of pressure. All coals except SS005JPN expel significant amounts of gases from the start, which is consistent with their relatively large oxygen contents and the early release of CO2 and H2O. SS005JPN releases only tar during the onset of devolatilization. After tar evolution, additional amounts of CO, H2, and light hydrocarbons are released. Ultimately, tar yields comprise just under half of the

Manton et al.

Figure 4. (b) Transient weight loss and (O) tar yields versus estimated residence times from the furnace at 1900 K for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

weight loss from SS001AUS, SS002AUS, SS004CHN, and SS005JPN, but only one-third of the ultimate yield for SS003AUS. Hydrocarbon Gas Yields. Distributions of the ClC3 hydrocarbon gases and oils are shown in Figure 5. Yields of C4s were monitored and observed to be negligible for all coals. The evolution of the hydrocarbons over time parallels the tar release for all five coals. With all coals except SS002AUS, the yields of C2s and CH4 continue to grow after tar release has ended. Methane is the most abundant hydrocarbon for each sample, contrasting with atmospheric tests, where C2 species were the most abundant. Methane yields are double the C2 yields from SS001AUS, SS003AUS, and SS004CHN. For all coals, the hydrocarbon gas yields were resolved almost exclusively into methane, acety-


Figure 5. Transient yields of (3) oils, (1) C2s, (O) C3s, and (b) CH4 versus estimated residence times from the furnace at 1900 K for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

lene, and ethylene. Ethane and propylene were both found in small quantities, up to 0.4 wt %. Propane and C4 species were not detected. The total yields of C1-C3 hydrocarbons were similar, at ∼2 daf wt % with SS001AUS, SS002AUS, and SS003AUS, and larger with SS004CHN (3%) and SS005JPN (7.7%). Although oils were the most abundant hydrocarbon gas from all coals in the tests at atmospheric pressure,8 the oils yields were approximately equal to the CH4 yields with SS001AUS, SS003AUS, and SS004CHN. Only SS002AUS generated more oils than CH4, by 50%. Oxygenated Gas Yields. Distributions of CO, CO2, and H2O are shown in Figure 6. The H2O yields do not include any contribution from moisture in the coal samples; either the coals were completely dried or the flat baseline moisture level at early reaction times was subtracted away. CO2 and H2O are among the first


Figure 6. Transient yields of (O) CO, (b) CO2, and (1) H2O versus estimated residence times in the furnace at 1900 K for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

noncondensable products of primary devolatilization from any of these coals. CO2 and chemically formed H2O are released with minor amounts of CO on the same time scale from all coals. For all five coals, chemically formed H2O is the most abundant oxygenated gas, and H2O yields reached asymptotic values before the end of the tar release. These asymptotic yields are ∼60% greater than the ultimate yields for pyrolysis at 0.1 MPa.8 CO production continued after the end of the tar release, as for atmospheric devolatilization, but only with SS003AUS and SS004 CHN. Ultimate CO yields were between one-half to two-thirds of the H2O yields. After the tar release has been completed with SS003AUS, the H2O and CO2 yields remain at their asymptotic levels while the CO yield surges. Such a surge has been

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observed with many coals,13 although, with SS001AUS, the H2O yield grows after the tar release ceases, whereas the CO yield is steady. The oxygenated gases account for most of the weight loss after 80 ms, as noted previously. Yields of all three oxygenated gases from SS001AUS and SS002AUS are approximately half the values from the other three coals. From the reported data sets in the literature on rapid primary devolatilization,13 CO2 and H2O are known to be among the first noncondensable products of primary devolatilization, with CO yields accelerating in the latter stages. Neither feature was apparent in these results. Other inconsistencies with the CO and CO2 measurements were observed: the ultimate yields of these species were less than the values for these coals at 0.1 MPa, and the yields did not increase monotonically as the residence time increased. This is surprising because oxygen functional groups that would be shuttled away as parts of tar molecules during atmospheric pyrolysis should be released as noncondensable oxygenated gases during pressurized pyrolysis. These inconsistencies were probably due to the detectability limit of the FTIR being reached. Even with an increased number density of coal particles, the dilution effect of the carrier gas at 1.0 MPa is much greater than that of the atmospheric tests. H2 Yields. Yields of H2 are shown in Figure 7. This species yield hardly develops throughout primary devolatilization until the end of tar release. With SS001AUS and SS003AUS, H2 is a repository for 17% of coal-H; with SS002AUS and SS004CHN, it represents 11%, and with SS005JPN, it represents 47% (although the latter data set is subject to inordinate scatter). Similar to the yield of HCN, the yield of H2 is not asymptotic, because it would continue to increase until the char-H was eliminated at much hotter temperatures than that in these tests. Hydrogen-to-carbon (H/C) ratios of chars are presented in the next subsection. Time-Resolved Char and Tar Compositions. The evolution of elemental compositions in condensed-phase products is presented in Figure 8. The H/C values of the chars at the shortest residence time agree with the corresponding whole coal values, within experimental uncertainty. The values for all coals then fall continuously for progressively longer reaction times. Whereas the ultimate value of 0.5 for SS001AUS is comparable to the typical value of 0.4 for atmospheric devolatilization, all other values are significantly lower. Ultimate values for SS002AUS and SS004CHN are 0.25, and those from SS003AUS and SS005JPN were only 0.12, which is much lower than the corresponding value for atmospheric devolatilization. The lower values probably reflect longer reaction times in the p-RCFR tests. Tars from all five coals are substantially enriched in hydrogen over the respective whole coal values. The enrichment is up to 20% with SS002AUS, up to 25% with SS005JPN, up to 45% with SS001AUS, and over 50% with SS003AUS and SS004JPN. The degrees of enrichment with SS001AUS, SS002AUS, and SS005JPN are consistent with previous data for comparable coal types from the a-RCFR, including these particular samples.8 However, the enhancements with the other two coals are much higher than those for atmospheric pyrolysis. There are also weak indications that the H/C (13) Niksa, S. Energy Fuels 1996, 10, 173-187.

Manton et al.

Figure 7. Time-resolved H2 yields versus estimated residence times in the furnace at 1900 K for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

values of early tars from SS001AUS, SS003AUS, and SS005JPN are enriched even further in hydrogen, although this aspect is not clearly resolved within the scatter in the data. There are no reasons to indicate that these significant enhancements do not express a real difference between atmospheric and pressurized pyrolysis. The ultimate H/C ratios of the tars from the coals are 1-1.25, whereas the H/C ratios of the whole coals were in the range of 0.65-1.05 and some coals had appreciably less hydrogen than others. All these tar values are significantly higher than those that are reported for other testing configurations in which secondary volatiles pyrolysis is unregulated, corroborating the elimination of secondary tar cracking in the p-RCFR. Although we did not directly measure tar-O and charO, estimates for tar-O, based on the remainder from the



Figure 8. Atomic H/C ratios of (b) char and (1) tar versus estimated residence times in the furnace at 1900 K for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

C/H/N determinations, should be reasonable, because tar contains no ash and its sulfur content should be at least as low as the coal levels. Accordingly, the tar-O level from all coals except SS002AUS is in the range of 10%-13%, and it is 16% with SS002AUS. These levels are only half the values reported for atmospheric devolatilization with these same coals,8 which reflects the elimination of oxygen functional groups from tar precursors at elevated pressure before they are released as tar compounds. The tendency for diminishing tar-O levels for progressively higher extents of primary devolatilization reported for atmospheric devolatilization is only apparent with SS003AUS and SS004CHN at 1.0 MPa. Nitrogen Species. The nitrogen species distributions during primary devolatilization are shown in Figure 9. At 1.0 MPa, the concentrations of HCN and NH3 were below the FTIR limit of detection, so the only reported nitrogen species are char-N and tar-N. Consequently, nitrogen balance closures seem to be poor when taken at face value; instead, we estimate HCN yields as the deficits in the nitrogen balances.

Figure 9. Cumulative transient coal-N fractions versus estimated residence times in the furnace at 1900 K as (O) tar and (b) char for (a) SS001AUS, (b) SS002AUS, (c) SS003AUS, (d) SS004CHN, and (e) SS005JPN.

Tars are virtually the only shuttles for coal-N during primary devolatilization under rapid heating conditions. However, for pressurized devolatilization, relatively little coal-N is released with tar, simply because tar production is suppressed. The magnitudes of the tar-N levels from all five coals are consistent with expectations insofar as these tar-N fractions are very similar to the fractional tar yields, with that of SS005JPN being the largest, followed by SS002AUS, SS004CHN and SS001AUS, and SS003AUS. Because of scatter and biases in the nitrogen species distributions, HCN yields could only be estimated reasonably well for SS002AUS and SS003AUS. The respective coal-N fractions attributed to HCN are 0.2 and 0.4. Both values compare well with the values for atmospheric devolatilization of these coals.8 Most of the HCN at the longest reaction times may be directly

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Manton et al.

Table 3. Evaluation of Ultimate Product Distributions from the p-RCFR SS001AUS predicted

measured

SS002AUS predicted

measured

SS003AUS predicted

weight loss tar yield CO2 content H2O content CO content

31.8 18.6 2.1 4.6 1.3

29.8 16.1 1.1 3.2 1.0

44.5 24.9 2.4 5.1 1.8

50.9 26.6 1.8 5.8 3.5

1.0 MPa 31.1 17.6 2.9 5.8 1.7

weight loss tar yield CO2 content H2O content CO content

39.8 27.7 1.9 4.2 1.2

43.7 29.2 1.1 5.1 1.4

52.8 35.6 2.1 4.5 1.5

54.8 36.1 2.4 5.4 3.8

0.1 MPa 39.0 26.4 2.8 5.4 1.6

expelled from the char, because nitrogen functional groups are the same in char and tar, and char particles are significantly hotter than the tars in the entrainment stream of this experiment. In this respect, the HCN yield at the longest reaction time is not asymptotic. This species yield would increase continuously if the chars were heated further until the nitrogen is exhausted at much higher operating temperatures, such as the H2 yields. Modeling Results Overview of the FLASHCHAIN Simulations. FLASHCHAIN describes the devolatilization behavior of any coal under any operating conditions.7 The proximate and ultimate analyses of the coal are the only required sample-specific input. Indeed, the simulations reported here are predictions based on the coal properties in Table 1 and conditions in the p-RCFR. Thermal histories for p-RCFR tests were assigned with twodimensional CFD simulations. The FLASHCHAIN predictions were based on a much simpler energy balance for individual particles that was tuned to match the CFD predictions. These simulations imposed a nominal heating rate of 7000 K/s to 1100 K with no isothermal reaction period and no decomposition during quenching. None of the model parameters were adjusted in any way to improve the agreement between the model predictions and measured values. Each simulation of a complete test required 6% coal-Hsand correctly predicts that the tar yields from SS002AUS and SS005JPN are up to 70% higher than those from the other coals. Among the oxygenated gases, the predicted H2O yields at 1.0 MPa are predicted and observed to be the most abundant product for all coals. The predicted H2O yields are within 2 wt % of the measured values at both pressures for all coals except SS003AUS and SS005JPN. The predicted yields of CO2 agree well with the measurements for all coals. The CO yields at 1.0 MPa are predicted within useful quantitative tolerances only for SS001AUS, just like in the evaluation at 0.1 MPa. The discrepancies are rooted in uncertainties in the thermal histories assigned for the tests, because most CO is released during the final stages of primary devolatilization. Consequently, uncertainties in the final temperatures and reaction periods will substantially affect the predicted CO yields. Moreover, because CO is the last oxygenated species released during devolatilization, the errors in the predictions for all the other oxygenated compounds ac-


cumulate in the predicted CO yields, through an oxygen balance. The yields of the oxygenated gases should increase as the pressure is increased, based on the other product distributions in the literature2 and the production mechanisms in FLASHCHAIN. However, among these five coals, this tendency is evident only with SS005JPN. It is very difficult to explain how CO2 and CO yields could diminish with increasing pressure for the four other coals. The p-RCFR data sets also include tar and char elemental compositions, which were discussed previously (cf. Figure 8). The predicted tar compositions are generally consistent with the measured values, except that the predicted degree of hydrogen enrichment is greater at 0.1 MPa than at 1 MPa, which conflicts with the data. Also, the predicted char compositions generally show higher carbon contents than the measured values, although the measured values only close the material balance if more oxygen is included than that which satisfied the oxygen balances. Discussion Increasing the test pressure from 0.1 MPa to 1.0 MPa diminished the ultimate weight loss with four of the coals, as expected, but not with SS004CHN. The reduction in weight loss was only 7% with SS002AUS, but it was much more substantial with the three other coals. The reductions in the liquids yields varied over a range of 20%-35% for all coals except SS004CHN, which is within the expected range. Methane, C2H4, and C2H2 are the major C1-C3 hydrocarbons, and CH4 is the most abundant aliphatic hydrocarbon from all five coals. The yields of oils are comparable to the CH4 yields with all coals except SS001AUS, for which it is lower, and SS003AUS, for which it is higher. Yields of the oxygenated gases from all coals were dominated by chemically formed H2O, which is consistent with the nominal rank and oxygen contents. H2 was released only during the latest stages of primary devolatilization. For the short reaction times and moderate temperatures imposed in the p-RCFR, H2 shuttles away ∼15% of the coal-H with all coals except SS005JPN, for which H2 constitutes almost half the coal-H. The impact of pressure on tar characteristics was characterized well over a decade ago in tests with heated wire mesh reactors in the United States3,14-16 but has not received any attention since that time. The molecular weight distributions (MWDs) of tar shift to progressively smaller values for progressively higher pressures, suggesting that a vaporization mechanism is pertinent to any plausible rationale for the impact of pressure on yields. The data sets in this paper expand this characterization, by tentatively indicating that the degree of hydrogen enrichment of tar becomes greater for progressively higher pressures. However, this tendency was evident with only two of the five coals, and more samples need to be tested to establish the scope of the new observation. (14) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J. 1989, 35, 776. (15) Unger, P. E.; Suuberg, E. M. Fuel 1984, 63, 606. (16) Solomon, P. R.; Serio, M. A.; Deshpande, G. V.; Droo, E. Energy Fuels 1990, 4, 42.


Estimated tar-O levels varied over a range of 10%16%, which are only one-half to two-thirds of the values reported for atmospheric devolatilization with these same coals. This difference probably reflects the elimination of oxygen functional groups from intermediate fragments of coal molecules at elevated pressure, before they were released as tar compounds. The H/C ratios of chars decrease continuously throughout devolatilization, as they do for chars prepared at atmospheric pressure. The ultimate values are very sensitive to the severity of the imposed thermal history, especially to reaction time, because char H/C ratios decrease as H2 is eliminated on time scales that are considerably longer than those for primary devolatilization. We have no reason to expect this process to be affected by pressure variations. All the tendencies due to elevated pressures (except for greater hydrogen enrichment of tar at higher pressures) can be interpreted with the phenomenology that is depicted in Figure 10. This so-called “flash distillation analogy”17 invokes an analogy between coal devolatilization and the steam distillation of petroleum. When steam is bubbled through a barrel of crude oil, the lightest fractions pass into the vapor and are transported away with bubbles breaking through the surface of the petroleum. However, the material with high molecular weight remains in the liquid phase and condenses into coke if the temperature exceeds a certain threshold value. According to FLASHCHAIN, coal devolatilization follows this same sequence of steps after depolymerization chemistry has disintegrated the original three-dimensional macromolecular structure of a coal into a mixture of fragments that has a broad MWD. The role of the steam is played by the noncondensable gases that are produced whenever aliphatic components are partially converted to refractory char links. Tar is generated when the depolymerization fragments become small enough to vaporize into the escaping noncondensable gases. (Fragments that vaporize at processing temperatures are still heavy enough to condense into viscous liquids at room temperature.) Char forms by cross-linking among heavier fragments in the condensed phase, whose further depolymerization is suppressed whenever labile connections are converted to refractory char links. Noncondensable gases are produced as a byproduct of charring. All this chemistry occurs in the condensed phase, so no redeposition from the gas phase is involved. Under all practical conditions, the collective mole fraction of all tar components is relatively small and is certainly much smaller than the mole fraction of noncondensable gases. Therefore, the mechanism for the transport of gases also governs the release of tar. The flash distillation analogy does not include any finiterate transport mechanisms. Instead, the escape rate of gases is set equal to their rate of production from the chemical reaction mechanism, under the assumption that a bulk convective flow of gases can be established by a nominally infinitesimal pressure gradient across the particle; hence, internal and ambient pressures are equal and all transport resistances are deemed to be negligible. (17) Niksa, S. AIChE J. 1988, 34 (5), 790-802.

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Manton et al.

Figure 10. The flash distillation analogy.

According to the flash distillation analogy, the phase equilibrium shifts to retain a larger portion of the lighter fragments in the condensed phase as the pressure is increased. These fragments would constitute the heavy end of the tar MWD at low pressures but remain in the coal at elevated pressures. Consequently, tar prepared at higher pressures becomes lighter and the tar yield diminishes. The fragments retained in the char also contain precursors to noncondensable gases that are eventually released; thus, gas yields increase as the pressure is elevated, but not by enough to compensate for the retention of tar precursors. Finite-rate transport mechanisms are not needed to explain the pressure effect. In fact, the scaling for negligible transport resistances in FLASHCHAIN is consistent with the lack of a particle size effect for devolatilization of pulverized coal, which presents problems for older theories that were never reconciled. Conclusions We have developed a reactor system for conducting pyrolysis experiments at elevated pressures. The system allows for determining the yields of all gaseous, tar, and solid product yields to cover the entire pyrolysis process from its onset to fully relaxed ultimate yields. The

evaluations of the FLASHCHAIN predictions demonstrated that the expected trends were apparent in all datasets except those for SS004CHN. Considering that the behavior of SS004CHN conflicts with tendencies for higher pressure that were established with a database of 99 coals, and that the database was recently used to fine-tune the FLASHCHAIN predictions for applications at elevated pressures,2 it is not surprising that the predictions for SS004CHN were erroneous. The predictions for the other four coals were generally as accurate as those for 0.1 MPa, except for the overpredicted weight loss with SS005JPN. Yields of the major noncondensibles were also within useful quantitative tolerances, except for the CO yields, which are subject to the substantial uncertainties in the thermal histories during the later stages of the test period.

Acknowledgment. This work was sponsored by the Center for Coal Utilization, Japan (CCUJ) under the BRAIN-C program, which is under the direction of Dr. M. Harada and Mr. T. Ando. Coal samples were provided by the Coal Research Laboratory of Idemitsu Kosan Co., Ltd. EF034034E

Impact of Pressure Variations on Coal Devolatilization Products. 2

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