An Experimental Facility for the Study of Coal Pyrolysis at 10

692

Energy & Fuels 2000, 14, 692-700

An Experimental Facility for the Study of Coal Pyrolysis at 10 Atmospheres Joseph Cor, Nicholas Manton, Guido Mul, Donald Eckstrom, William Olson, 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 November 9, 1999

This paper introduces an experimental facility to study coal pyrolysis at 10 atm (1.0 MPa). The system processes a stream of coal entrained in argon gas through an inductively heated furnace at 1850 K. Due to the different absorptive properties of the coal and gas, the coal is heated to temperatures on the order of 1100 K, while the gas remains relatively cool, quenching the products of primary pyrolysis. The extent of devolatilization is controlled by varying the residence time of the coal within the furnace. In this way, the products of primary coal devolatilization can be resolved in time from the onset of devolatilization to the attainment of ultimate yields. Char and tar yields are measured at each residence time, while major noncondensable gases are resolved into C1-C3 hydrocarbons, oils, carbon oxides, water, hydrogen, and nitrogen species. To validate the accuracy of the test method, the complete product distribution from individual runs are analyzed to formulate mass and elemental C/H/N balances. The performance of the test facility is illustrated with test results for a high-volatile bituminous coal.

Introduction Economic forces and environmental regulations are driving the manufacturers and operators of coal-fired electrical generation facilities toward combustion systems that operate at higher efficiencies. One technique for increasing efficiency is combustion and/or gasification at pressures of 1.0 MPa or above.1 At the same time, there is an increasing reliance on computer simulations to model coal behavior in utility boilers and gasifiers, and these models represent coal pyrolysis as an essential first step. Estimating heat release from the combustion requires only the knowledge of the elemental composition, but modeling the combustion behavior, particularly with respect to pollutant formation, requires knowledge of detailed product distributions. Moreover, strategies to manage pollutant chemistry must distinguish mechanisms for the formation of primary products from those for the subsequent secondary transformations in hot gases. For all these reasons, measurements of product and elemental distributions from coal pyrolysis under high-pressure conditions are essential. Primary devolatilization products are the volatiles released by chemistry among species and functional groups in the coal. Secondary products are formed from subsequent transformations of the primary products after they pass through the interface between solid and * Author to whom correspondence should be addressed. (1) Fundamentals of Coal Combustion: For Clean and Efficient Use (Coal Science and Technology, Vol. 20); Smoot, L. D., Ed.; Elsevier Science Ltd.: New York, 1993.

gas. Chen and Niksa2 have previously described a radiant coal flow reactor (RCFR), which uniquely heats a coal suspension from a near-blackbody enclosure, not by a preheated gas stream. The suspension’s entrainment gas is transparent to the radiation and thus remains much cooler than the suspension, especially if the system pressure is elevated. Pristine products that have been quenched as soon as they were expelled can be recovered from the flow. The extent of primary or secondary pyrolysis can be varied by changing the velocity of the entrainment gas through the furnace. In this manner, the stages of coal pyrolysis can be studied through the complete expulsion of primary products or even through the complete transformation of primary products into secondary products. Once they have passed through the radiant furnace, the char particles are separated from the aerosols and gaseous products by means of a virtual impactor (centripeter). Aerosols are trapped on filters and the soluble fraction is identified as tar, and gaseous products are analyzed by gas chromatography, a nondispersive infrared (NDIR) analyzer, and Fourier transform infrared (FTIR) spectroscopy to quantitatively determine oils, light hydrocarbons, hydrogen, water, CO, and CO2. A careful mass balance is carried out in each test to determine the percentage of initial coal mass converted to each product. Furthermore, the elemental compositions of chars and tars are determined so that elemental balances on carbon, hydrogen, and nitrogen can be (2) Chen, J. C.; Niksa, S. Energy Fuel 1992, 6, 254.

10.1021/ef9902348 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/26/2000

Coal Pyrolysis at 10 Atmospheres

Energy & Fuels, Vol. 14, No. 3, 2000 693

formulated. The results of these tests form a rigid basis for comparison against simulation models. The RCFR was originally designed for and used at 0.1 MPa.2 The new system, designated the pressurized, radiant coal flow reactor (p-RCFR), retains all of the essential elements of the RCFR but can operate continuously at pressures to 2.0 MPa, although at present it has only been used at 1.0 MPa. To mitigate the effects of buoyancy at high pressure, the flow direction of the entrainment gas and particles was reversed in the p-RCFR. The high pressure has also required that the radiant furnace, which cannot hold a large pressure differential, be housed inside an outer housing, which is also kept at 1.0 MPa, with cooling gas flowing through it to maintain it at near-room temperature. In addition, the centripeter section of the p-RCFR is now built of stainless steel parts to withstand the high pressure. The separation of the char particles, the trapping of oils, the detection of gaseous species, and the elemental analysis are carried out in much the same manner as in the earlier RCFR tests. The following sections present a detailed description of the p-RCFR and the experimental procedures used with this system. Then the primary devolatilization results at 1.0 MPa for a bituminous coal, designated SS001AUS, are presented. Experimental Overview Pressurized, Radiant Coal Flow Reactor (pRCFR). The p-RCFR has many features in common with the first-generation RCFR described by Chen and Niksa3 for atmospheric pyrolysis testing. Both reactors have the basic configuration shown schematically in Figure 1. Coal particles, entrained in argon gas, flow through a transparent quartz tube. An outer sheath gas flow surrounds the entrainment flow and helps prevent particles from colliding with the tube wall. To counteract the effects of buoyancy at 1.0 MPa, the gas and particle flows move upward in the p-RCFR. Also, the diameter of the quartz tube was changed from 20 mm in the RCFR to 14 mm in the p-RCFR, to reduce the total argon mass flowrate and thereby increase the concentrations of the gas-phase devolatilization products in the effluent. The quartz tube is located on the central axis of a 10cm (i.d.) graphite cylinder encased in zirconium oxide and alumina insulation and wrapped by five turns of a water-cooled copper induction coil. This graphite and insulation is mounted within water-cooled copper plates and a quartz housing. The induction coil routinely heats the graphite to 1850 K. Axial temperature profiles of the graphite are uniform to within 98% of the mean temperature, except for the outermost 5 mm at both ends, over which the temperature falls to about 75% of the mean value. The graphite length is 6 cm, although the overall hot zone length, which accounts for radiation leakage out of the ends of the entrainment tube, is 12 cm. To ensure attainment of complete primary pyrolysis in the p-RCFR, the graphite length was made 1 cm longer than in the RCFR. Furnace temperatures are monitored to within an uncertainty of 5 K with a twocolor IR pyrometer aimed through a hole in the insula(3) Chen, J. C.; Niksa, S. Rev. Sci. Instrum. 1992, 63 (3), 2073.

Figure 1. Schematic of a Radiant Coal Flow Reactor.

tion around the graphite. Nominal particle residence times are set by the inlet gas flow rate and the distance between the inlet plane and an argon quench nozzle, which is fixed. Values reported here are assigned from the nominal gas velocity into the furnace and the length of the hot zone. Such estimates are significantly longer than actual values due to the combined effects of growing boundary layers and density reductions. Residence times were varied at a fixed furnace temperature of 1850 K. Thermal histories in such a series of runs have similar heating rates, but the suspension achieves different temperatures at the outlet in each case. Higher particle temperatures are achieved as residence times are extended, but particle temperatures at the outlet are always well below the furnace temperature. The near-blackbody radiation from the graphite rapidly heats the particles as they traverse the furnace. As the coal particles are heated, they expel their primary devolatilization products. The coal suspension is kept optically thin to ensure that the radiant heat flux to individual particles at any axial position is uniform. In this way, macroscopic behavior can be interpreted in terms of single-particle phenomena. Also, since the entrainment gas is transparent to the radiation, its only means of heating is by convection from the tube wall and particles, so the entrainment gas remains relatively cool and quenches chemistry among devolatilization products as they are expelled. Typical particle and entrainment gas temperature profiles, simulated with FLUENT, are shown in Figure 2. This figure shows the temperature of a gas flow at 1.0 MPa with no particles in the suspension (N ) 0/cm3) and with particle loadings of 100 and 400 particles per cm3. Even in the N ) 400/ cm3 case, the peak gas temperature is more than 500 K lower than the peak particle temperature. This figure

694

Energy & Fuels, Vol. 14, No. 3, 2000

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.

also demonstrates that the coal particle loading has a relatively small effect on the peak gas and particle temperatures at elevated pressures in the reactor. Care must be taken during the run to control the differential pressure between the inner quartz entrainment tube and the oven. At the temperatures of this experiment, a sufficiently large pressure differential can either burst or crush the tube. Therefore, the differential pressure is monitored, and regulated within 2.6 to 4.0 kPa (20-30 Torr) with fine flow adjustments. The coal particles are fed into the entrainment stream by a coal feeder inside the pressurized housing, shown schematically on the right-hand side of Figure 3. The feeder dumps pulverized particles into an entrainment stream, forming an optically thin suspension that flows downward into a U-tube. The other end of the U-tube delivers the upward, two-phase flow into the radiant furnace section within the pressure vessel (left-hand side of Figure 3). Coldflow visualizations of the coal-gas suspension revealed that the high Reynolds number of this flow at 1.0 MPa promotes turbulence, causing the coal suspension to collide with the tube wall. Moreover, gravitational deceleration can reverse the particle flow, causing particles to fall downward near the walls, which also promotes collision with the tube wall. These problems were eliminated by constricting the flow to accelerate it as soon as it enters the quench nozzle section and by installing flow straighteners for both the entrainment and sheath streams at the entrance of the quartz tube. After these modifications, the visualizations showed that the most stable flow regime occurs when the sheath and entrainment flow velocities are equal. At this condition, deposits that form on the tube wall involve only a small fraction of the coal feed, whose weight is factored into the assignment of yields, as explained below. A quench nozzle mounted at the entrainment tube outlet blasts argon into the process stream, rapidly quenching all chemistry and nucleating tar into an aerosol. After the quench nozzle, the process stream passes through a tube with a transpiring argon flow to prevent aerosol deposition. Above the transpiring tube, the stream flows into another pressurized housing where virtual impaction in an aerodynamic classifier segregates the products into bulk solid particles, aerosol, and

Cor et al.

noncondensable gases. Eighty-five to ninety-five percent of the flow into the centripeter passes into its lower annulus. By virtue of their greater inertia, char particles penetrate the impaction surface; they then deflect off a cone on the top of the centripeter into a collection vessel. Since the aerosol particles are typically a few micrometers in diameter, they stay with the flow and are convected onto filters on the bottom of the annulus. Noncondensable gases pass through the filters. A small amount of the gas flow downstream of the centripeter filters is sampled for analysis. Its pressure is reduced to 120 kPa (910 Torr) before seven or more sampling loops are filled for gas chromatographic (GC) determination of light hydrocarbon and hydrogen concentrations. Downstream of the sampling loops, the pressure is decreased to 83 kPa (633 Torr) before the flow passes through NDIR analyzer and an FTIR spectrometer. Experimental Procedure At 1.0 MPa, the thermal histories of particles and gas are insensitive to the coal loading, because of the relatively high thermal capacitance of the pressurized gas (see Figure 2). Consequently, 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. Under this procedure, the coal loading diminishes in runs with progressively shorter residence times. A set of flow rate specifications, shown in Table 1, have been formulated to impose nominal residence times varying from 55 to 350 ms at 1.0 MPa, which covers devolatilization from its onset through the release of ultimate, asymptotic yields. Enough variation exists in the coal feed rates in a sequence of runs that a calibration of the feeder must be performed before and after each experimental run. These calibrations are performed by feeding the coal through the system at pressure, but with the furnace turned off, for the 200-s duration of a typical run both before and after the actual test. The feed rate of the coal during the experiment is assigned as the average of the two calibration values. After the devolatilization test, the radiant furnace section is turned off and the tar and char samples are recovered from the centripeter for weighing. About 1-2% of the coal feed inevitably settles in the U-tube between the feeder and the furnace. This material is swept onto clean filters in the centripeter and is factored into the weight loss assignments. This settling of coal is more severe at low entrainment velocities, which sets an upper limit to the achievable residence time with this system. Tars are recovered on a four-stage assembly of glass-fiber filters, which are dried overnight at 385 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-mm Teflon membrane. The membrane residue is weighed and denoted as the soot yield. The tar solution is concentrated in a KudernaDanish concentrator or in a rotary evaporator before the remaining solvent is evaporated, following the procedure of Lafleur et al.4 Several tens of milligrams of pure tar are recovered from runs with significant aerosol yields. Carbon, hydrogen, and nitrogen contents of condensed products are determined by elemental analysis at Galbraith Laboratories, a commercial testing laboratory. As described in the previous section, a stream of noncondensable gases is withdrawn through a sidearm on the main flow from the centripeter after the tar collection filters, and a (4) Lafleur, A. L.; Monchamp, P. A.; Plummer, E. F.; Kruzel, E. L. Anal. Lett. 1986, 19 (21&22), 2103.



Figure 3. Schematic of the p-RCFR experimental facility. Table 1. P-RCFR Flow Rate Specifications reaction time (ms)

entrainment gas inlet velocity (cm/s)

55 83 96 113 138 170 220 350

220 144 125 106 87 71 55 36

portion of the stream is extracted through a port into a multiposition sampling valve at 335 K. Downstream of this port, the sample stream is sent through an NDIR water detector. Then concentrations of CO and CO2 are monitored on-line with FTIR spectroscopy through a multipass gas cell at 335 K having a total path length of 6.5 m. The line that fills the multipath gas sample cell in the FTIR is also heated. Preliminary runs established that all species concentrations stabilize within 40 s of sampling. Hence, at least seven FTIR scans are acquired per test, beginning at 45 s of operation in the test and continuing throughout a total run time of 202 s. Seven of the multiport sampling loops are typically filled during a normal run for subsequent gas chromatograph analysis. C1-C3 hydrocarbons (CH4, C2H2 + C2H4, C2H6, C3H6, C3H8) are chromatographed on a HayeSep D column into a flame ionization detector (FID). Yields of oils, defined as hydrocarbons with carbon numbers of 5 and higher that

remain in the gas phase under the conditions in the impactor, are assigned by difference from the total yield of all noncondensable hydrocarbons and the yields of the C1-C3 species as determined through a separate open column and a second FID. The yield of H2 is determined with chromatography on a HayeSep DB column into a thermal conductivity detector (TCD).

Particle Thermal Histories The analysis of particle thermal histories was developed by Chen5 and Chen and Niksa3 and has been refined for two-dimensional geometry using FLUENT. The analysis determines the temperatures and velocities of individual particles and the entrainment gas as functions of radial and axial location within the quartz tube. These thermal histories pertain to the entire suspension, provided that the loading is below 1200 particles/cm3, at which point its radiant absorption becomes significant and it is no longer optically thin in some directions. The radiant flux from the furnace onto the flow axis is defined by an analysis that does not consider the suspension, because optically thin suspensions absorb insignificant amounts of the total radiation (5) Chen, J. C. Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Stanford, CA, 1991.

696


Cor et al. Table 2. Elemental Analysis for SS001AUS Coal, daf wt % asha

C

H

Ob

N

S

15.2

80.2

4.7

12.9

1.7

0.5

a

Weight percent (dry). b Assigned by difference.

The simulation results in the bottom plot in Figure 4 also show how the high-pressure, upward-flow configuration affects time-temperature histories. In this figure, the inlet entrainment gas velocities are 15 cm/s at 0.1 MPa and 36 cm/s at 1.0 MPa. However, in upward flows, the particle slip velocity causes particles to lag behind the mean gas flow, increasing the residence time of the coal particles. Also, since the entrainment gas remains much cooler at 1.0 MPa, the flow is accelerated much more slowly. The combined effect of these factors nearly quadruples the required reaction time for devolatilization at 1.0 MPa. Initially, the particle heating rates are similar (7100 K/s at 0.1 MPa and 9250 K/s at 1.0 Mpa) but due to the higher convective loss rate to the cooler entrainment gas, the heating rate at 1.0 MPa falls to 3500 K/s during the later stages of pyrolysis. Coal Properties

Figure 4. Simulated thermal histories of coal and entrainment gas for tests at 0.1 and 1.0 MPa versus (top) position and (bottom) calculated residence time. Particle loading is 300/ cm3.

flux across the furnace. This flux, along with other heat fluxes within and between particles and gas, defines the thermal histories of the suspension and gas stream. A more detailed discussion of the radiation flux analysis is provided by Chen and Niksa.3 Since this facility is intended for coal devolatilization tests and devolatilization rates are nominally independent of pressure, operating conditions that impose the same heating rates and ultimate sample temperatures at 1.0 and 0.1 MPa will achieve the same extent of devolatilization. As seen in Figure 4, FLUENT simulations show that nearly the same particle temperature profiles can be imposed at 1.0 MPa as at 0.1 MPa. Both simulations are for operating conditions that are severe enough to achieve ultimate primary devolatilization during the available residence time in a 5-cm furnace at atmospheric pressure with a particle loading of 300 particles/cm3. In these figures, the leading edge of the graphite furnace element is at 3 cm and the trailing edge is at 8 cm. Coal suspensions begin heating upstream of the hot zone inlet because radiation escapes through the transparent flow tube. They heat up at similar rates as they move through the furnace, and they ultimately attain the same temperature before reaching the furnace outlet. In fact, the only significant difference in pressurized case, at least in the spatial coordinates (top plot in Figure 4), is that the pressurized entrainment gas remains much cooler and will therefore provide a much more effective quench for primary devolatilization products.

The coal in this experiment was ground in a disk grinder dosed with liquid nitrogen to displace O2 and dissipate heat. It was then classified into the 75- to 106µm size grade by sieving on a Ro-Tap machine for 30 min, dried in a vacuum oven for at least 12 h at 16 kPa (25 in. Hg vac) and 385 K, and stored in a desiccator with Drierite. A sample of the coal was then sent to Galbraith Laboratories, a commercial testing laboratory, where the properties shown in Table 2 were determined. 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 also analyzed by Galbraith Laboratories, and weight loss, product yields, and elemental yields were converted to the dry, ash-free (daf) basis by using the ash level in Table 2. Experimental Results This section presents the main features in the measured product distributions for the devolatilization of an Australian hv bituminous coal (SS001AUS) at 1.0 MPa. All data were obtained with the furnace hot zone at 1850 K, with 99.999% argon entrainment and sheath flows and 99.99% argon quench streams. Gas flow rates were regulated to vary reaction times. As gas flows were adjusted, the coal feed rate was fixed at a nominal value of 0.5 g/min, so coal loadings decreased as gas flow rates were increased to reduce residence times. Cumulative Mass Conversion and Carbon Fractions versus Time. Mass and elemental balances for individual tests at various reaction times for the coal are reported in Table 3; the mass and carbon balances also appear in Figure 5. Mass balances are based on the calibrated coal feed rate plus independent determinations of the yields of char, tar, oils, CH4, C2H2 + C2H4, C2H6, C3H6 + C3H8, CO, CO2, H2, and H2O for individual runs. C/H/N balances also involve determinations of the elemental compositions of char and tar, plus an ash level. We converted the char composition to the dry-ashfree (daf) basis with the coal’s ash content, assuming



Table 3. Mass and Elemental Closures for Individual Pressurized Tests with SS001AUS Coal reaction time (ms)

Σmass

ΣfiC

ΣfiH

ΣfiN

54.6 83.1 95.8 113.3a 138.4 169.8 219.6

104.7 100.7 100.1 106.8 106.6 100.2 101.0

98.5 98.4 97.5 87.1 101.1 96.4 94.8

97.6 106.1 113.7 99.1 113.2 108.8 121.9

99.7 98.1 97.9 79.6 100.6 90.1 88.5

a Ultimate analysis for tar not available due to insufficient sample size.

Figure 5. (Top) Cumulative transient yields from SS001AUS in the furnace at 1850 K of C1-C3 hydrocarbons plus oils (b); plus H2O (+); plus CO and CO2 (]); and plus tar (2). The mass balances on the separate scale (9) also involve char yields. (Bottom) Cumulative transient carbon fractions of C1-C3 hydrocarbons (4); plus CO and CO2 (]); plus oils (b); and plus tar (2). The carbon balances on the separate scale (9) also involve char yields.

that no products were derived from inorganic precursors. Also, the elemental compositions of oils were assigned as that of benzene. Since no aliphatic hydrocarbons heavier than C3 are ever observed, oils are probably mixtures of several single-ring aromatic compounds, including benzene, toluene, xylene, phenol, cresol, and xylenol, as has been reported by other investigators.6 Due to the high mass flow rates of argon required in these experiments, the gaseous nitrogen (6) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 627.

species HCN and NH3 existed at concentrations below the sensitivity of our FTIR measurements. Hence, the elemental nitrogen balances are incomplete. The balances for SS001AUS in Table 3 exhibit mass closures comparable to previous data sets for tests at atmospheric pressure in the RCFR. The range of mass closures from 100.2% to 106.8% is only slightly broader than for atmospheric tests, and all but two of the closures satisfy the previous standard of (5% on mass balances. The carbon balances for SS001AUS close to within (5% in all but two cases, with the largest discrepancy being where the tar sample was not available for analysis. The hydrogen balance is on the order of what has been observed in previous devolatilization tests for coal at 0.1 MPa. In general, however, the values exceed 100%. Insofar as hydrogen is always distributed over char, tar, hydrocarbon gases, and water, it is difficult to pinpoint a single source of error that could be responsible for discrepancy in the hydrogen balance. The nitrogen balances for SS001AUS are also scattered, although the range is smaller than found in many comparable 0.1-MPa devolatilization studies. Since neither HCN nor NH3 are monitored in these tests, the N-balances should all be below 100%, which they are in all but one case. The greatest imbalance appears at the longest residence times, as expected because HCN is released only during the latest stages of primary devolatilization. Closures of the mass and carbon balances are also illustrated in Figure 5. This figure shows 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 earlier. The lines drawn through the data points in this and subsequent figures are spline fits to aid the reader’s eye. Product Conversions. Even though elevated pressures suppress tar release, tar remains the most abundant product category. The ultimate tar yield represents over half the total weight loss. Yields of noncondensable hydrocarbon gases include substantial amounts of C1C3 compounds. Oils yields are appreciable. The total yields of all hydrocarbon gases are much lower than the yields of all the oxygenated gases. Since devolatilization preferentially releases heteroatoms, the carbon conversion to volatile products is always less than the mass loss. With this coal, carbon conversion comprises only about 70% of the mass loss. The relative contributions from the major product categories closely parallel the mass loss distribution, since chemically formed H2O is the only product that does not contribute to the cumulative carbon distributions. The aerosols recovered in the centripeter were examined for soot, according to solubility in THF and filtration. Small amounts of insolubles were present, but elemental analysis showed that they were fine char particles, not soot, so their amounts were added to the char yields. Tar Release. Transient weight losses and tar yields appear in Figure 6. Initially, tar comprises most of the mass loss, as expected for hv bituminous coals. Ultimately, tar yields comprise just about one-half (15.5%) of the total weight loss for this coal (30%). The tar yield

698


Figure 6. Transient weight loss (O) and tar yields (b) from SS001AUS versus estimated residence times in the furnace at 1850 K.

Figure 7. Transient yields of oils (3), C2’s (1), C3’s (O) and CH4 (b) from SS001AUS versus estimated residence times in the furnace at 1850 K.

for this coal was 25 daf wt % at 0.1 Mpa but has been suppressed by the elevated pressure. Hydrocarbon Gas Yields. Distributions of the C1C3 hydrocarbon gases and oils appear in Figure 7. The most striking feature in the light gas yields is the way that their evolution in time parallels tar release. The oil yields also follow this tendency. The yields of C2’s and especially CH4 continue to grow after tar evolution has ended. CH4 is the most abundant aliphatic hydrocarbon in this coal at pressure, whereas C2’s predominate in its atmospheric pyrolysis. The C1-C3 hydrocarbons consist primarily of methane, acetylene, and ethylene, as expected. The C3 yields are not appreciable. The yield of oils from SS001AUS is less than 1 daf wt %, which is even lower than the CH4 yield. Oxygenated Gas Yields. Distributions of CO, CO2, and H2O appear in Figure 8. H2O yields do not include contributions of more than 1.5% from moisture in the coal samples, consistent with our coal drying procedures. During devolatilization, the water yields reach their asymptotic values before the end of tar release. Chemically formed water is the most abundant oxygenated gas throughout the primary devolatilization of SS001AUS. The ultimate H2O yields in Figure 8 were roughly 60% greater than the ultimate H2O yield for atmospheric pyrolysis of this coal. From many reported

Cor et al.

Figure 8. Transient yields of CO (O), CO2 (b), and H2O (1) from SS001AUS versus estimated residence times in the furnace at 1850 K.

Figure 9. Time-resolved H2 yields from SS001AUS versus estimated residence times in the furnace at 1850 K.

data sets on rapid coal devolatilization,7 we know that CO2 and H2O are among the first noncondensable products of primary devolatilization and that CO yields surge during the latest stages. However, neither of these features is apparent in this data set. H2 Yields. Yields of H2 appear in Figure 9. This species yield hardly develops throughout primary devolatilization until the end of tar evolution. With SS001AUS, H2 is the shuttle for almost one-quarter of the coal’s hydrogen. The yield of H2 from this coal is not asymptotic, because it would continue to increase until the char-hydrogen was eliminated at much higher temperatures than used in these tests. Hydrogen-to-carbon (H/ C) ratios of chars are presented below. Cumulative Gas Yields. At 1.0 MPa, the ultimate yield of primary pyrolysis gaseous products for SS001AUS is 14.9%. This compares with an ultimate yield of 12.1% of gaseous products for SS001AUS at 0.1 MPa. Therefore, while tar production for this coal is suppressed at elevated pressures (15.5% at 1.0 MPa versus 29.2% at 0.1 MPa), a portion of this apparent suppression can be accounted for as the decomposition of tar precursors into additional gaseous products. Time-Resolved Char and Tar Compositions. The evolution of the elemental compositions of condensed(7) Niksa, S. Energy Fuels 1996, 10, 173.


Figure 10. Atomic hydrogen-to-carbon ratios of char (9) and tar (2) from SS001AUS versus estimated residence times in the furnace at 1850 K. The ratio for the whole coal appears as the dashed horizontal line.

phase products is shown in Figure 10. The H/C ratios of the chars at the shortest reaction time agree with the corresponding whole coal values, within experimental uncertainty. The values then fall continuously as reaction times become progressively longer. The ultimate value for SS001AUS is 0.51. Typically, ultimate values of H/C for chars prepared in the atmospheric pressure RCFR are less than 0.4 at the end of primary devolatilization. Tars are substantially enriched in hydrogen compared to the respective whole coal values. The enrichment is 35-45% for the ultimate tar samples from SS001AUS. This degree of enrichment is consistent with previous data for comparable coal types from the RCFR for atmospheric pyrolysis, including this particular coal. There are also weak indications that the H/C values of early tars are even more enriched in hydrogen, although this aspect is not clearly resolved within the scatter in the data. Although we did not directly measure the oxygen contents of tars and chars, the estimated weight percentages of oxygen in the tars based on the remainder from the C/H/N concentrations should be reasonable, because tar contains no ash and its sulfur content should be at least as low as the level in the coal. According to this estimation scheme, tars contained 9-10% oxygen throughout devolatilization. These levels are only half the values reported for atmospheric pyrolysis with this coal. Nitrogen Gases and Fuel-Nitrogen Balances. The nitrogen species distributions during primary devolatilization are shown in Figure 11. Neither HCN nor NH3 was detectable, so the nitrogen balance closures are too low at the longest residence times, as discussed earlier. No ammonia was observed with this coal during atmospheric pyrolysis, so none was expected in these tests either. The scatter in the data precludes an estimate for the HCN yields. Tars are virtually the only shuttles for fuel nitrogen during the first stages of primary devolatilization under rapid heating conditions. But for pressurized devolatilization, relatively little coal-N is released with tar, simply because tar production is suppressed. The magnitudes of the tar-N levels are consistent with expectations insofar as these tar-N fractions are very similar to the fractional tar yields.


Figure 11. Cumulative transient coal-nitrogen fractions from SS001AUS versus estimated residence times in the furnace at 1850 K as tar (O) and char (b). HCN and NH3 levels were not measurable.

Summary of Primary Devolatilization Data. Increasing the test pressure from 0.1 to 1.0 MPa diminishes total volatiles yields, especially the tar yields, as expected. SS001AUS has ultimate total volatiles yields of 30 daf wt %, of which 15% is tar. Methane, acetylene, and ethylene are the major C1-C3 hydrocarbons, and CH4 is the most abundant aliphatic hydrocarbon. The yields of oils are only half the aggregate yield of C1-C3 hydrocarbons. Yields of the oxygenated gases were dominated by chemically formed H2O, 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 less than onefourth of the coal’s hydrogen with SS001AUS. The relatively very high H/C ratios in tars from the p-RCFR can be attributed to the minor role of secondary chemistry in this test facility. As expected, tar H/C ratios decrease throughout primary devolatilization because aliphatic peripheral groups and bridge remnants are progressively eliminated from aromatic nuclei before they are released from the coal matrix as tar. Discussion Pressurized coal pyrolysis under rapid heating conditions has been characterized previously with two types of laboratory reactor systems, wire grid microreactors and drop tube furnaces. Electrically heated wire grids are easily modified for pressurized service and were therefore employed earlier and more frequently than drop tubes. The earliest pressurized wire grids8,9 were plagued by poor control of the thermal history and inferior thermometry. Notwithstanding, complete product distributions from pressurized wire grids were first reported more than 20 years ago.9 Newer applications featured improved control and monitoring of the thermal histories, which appears to have reduced the experimental uncertainties associated with the reported (8) Anthony, D. B.; Howard, J. B.; Hottel, H. C.; Meissner, H. P. Fifteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; p 1303. (9) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Seventeenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1978; p 117.

700


pyrolysis product distributions.10-14 The best systems added a gas flow to sweep volatiles away from the heated mesh to inhibit unregulated secondary pyrolysis of the volatiles.15,16 Modern pressurized wire grids are very well suited to monitoring volatiles yields and pyrolysis product distributions, provided that the heating rates of interest are below 1000 K/s and reaction times are extended to achieve ultimate yields. But they have not yet been used to resolve the process dynamics. Whereas gas quenching is routinely used to resolve reaction times into increments of 100 ms or less in atmospheric wire grid systems, it has not yet been used in a pressurized system. So pressurized wire grids have accurately determined ultimate pyrolysis yields at elevated pressures, but have not been used to characterize the reaction dynamics during heating periods. Because of the complex engineering required to develop pressurized drop tube reactors, there are far fewer reports on pressurized pyrolysis under entrained flow conditions than in wire grids. We know of 10 research institutions worldwide that published reports in English during the past two decades on the devolatilization behavior in pressurized coal flow reactors, including a handful that generated data during the past decade.17-20 Collectively, this work characterized swelling behavior, changes in the composition and structure of char and tar samples, fuel-nitrogen evolution, and the distinctive behavior of Australian brown coals. All drop tube reactors, pressurized or not, are plagued by two generic disadvantages. First, heating rates of the particles are governed by two-phase convective mixing phenomena at the injector. Consequently, temperatures through the particle jet vary both radially and axially. Such nonuniform thermal fields cannot be diagnosed directly. They can only be analyzed with complex heat transfer models subject to very large uncertainties because two-phase fluid mechanics determines the heat transfer rate. Under many practical situations, the (10) van Heek, K. H. German Chem. Engr. 1984, 5, 319. (11) Lowenthal, G.; Wanzl, W.; van Heek, K. H. Fuel 1986, 65 (3), 346. (12) Gibbins-Matham, J.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (13) Gibbins, J.; Kandiyoti, R. Energy Fuels 1989, 3, 670. (14) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73 (4), 591. (15) Guell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (16) Cai, H. Ph.D. Thesis, Imperial College of Science, Technology, and Medicine, University of London, 1995. (17) Tomita, A. Analysis of product distribution in coal pyrolysis under high pressure, Report No. 97TM5021; Institute for Chemical Reaction Science, Tohoku University, Sendai, Japan. (18) Yeasmin, H.; Matthews, J. F.; Ouyang, S. Fuel 1999, 78, 11. (19) Hamalainen, J. P.; Aho, M. J. Fuel 1996, 75 (12), 1377. (20) Lee, C.-W.; Jenkins, R. G.; Schobert, H. H. Energy Fuels 1991, 5, 547.

Cor et al.

thermal time scale to heat the entrainment gas is actually much longer than the time scale for particle heating, so heating rates are often much slower than expected. The second generic disadvantage is that primary products undergo uncontrolled secondary volatiles pyrolysis in the hot process stream, which is necessarily much hotter than the coal suspension. Compounding these generic disadvantages, adapting drop tube designs for pressurized service is a substantial undertaking, and even under routine operation, pressurized drop tubes are expensive, labor-intensive, and hazardous to operate. We believe that the p-RCFR system circumvents many of these limitations. Even though the system processes coal suspensions at technologically relevant loadings, the induction furnace is compact and no huge pressure vessels are involved. Thermal histories of the coal can be assigned with conventional heat transfer mechanisms for individual particles, because no twodimensional convective mixing phenomena are involved. Pristine primary devolatilization products are recovered because the entrainment flow remains much cooler than the coal particles throughout the entire test. This intrinsic quenching mechanism is even more effective at elevated pressure than in its previous application at atmospheric pressure, where it was used to recover tars with the lowest aromaticities and highest H/C ratios ever reported for rapid coal devolatilization.2 The process dynamics are easily resolved with gas quenching, and all the major devolatilization products are routinely monitored in every run. Conclusions These data represent a significant advance in our capability to monitor coal decomposition products for pressurized pyrolysis within useful tolerances. SRI’s p-RCFR can now be operated continuously with the furnace at 1850 K for test pressures to 1.0 MPa. The battery of analytical methods to monitor all major pyrolysis products was successfully incorporated into the p-RCFR’s inverted flow field. We now routinely monitor char, tar, soot, and the major noncondensables from tests at 1.0 MPa. Mass and carbon balances nearly meet the standards achieved in SRI’s previous tests of atmospheric pyrolysis and are more accurate than any ever previously reported for pressurized pyrolysis tests. Acknowledgment. This work was sponsored by the Center for Coal Utilization, Japan (CCUJ) under the BRAIN-C program, under the direction of Dr. M. Harada and Dr. T. Ando. Coal samples were provided by Idemitsu Coal Research Labs. EF9902348

An Experimental Facility for the Study of Coal Pyrolysis at 10

Recommend Documents