Ind. Eng. Chem. Fundam. 1986, 25, 536-544
536
Time-Resolved Pyrolysis Product Distributions of Softening Coals Jerry R. Bautista Bell Laboratories, Murray Hi//, New Jersey 07974
Wllllam B. Russel' and Dudley A. Savllle Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08540
Time-resolved product distributions of four softening coals pyrolyzed in an electrically heated grid are presented. The data collected over a broad range of temperatures and pressures satisfy a mass balance despite the small sample size (ca. 15 mg). Gaseous product compositions and the weights of tar evolved were determined during heat-up at 1000 K/s and for extended isothermal periods (0-30 s) at various temperatures (450-950 "C). The pressures used ranged from 0.01 to 25 atm of either helium or hydrogen. Quenching with a liquid-nitrogen-cooled helium stream allowed precise time resolution of the pyrolysis reaction over the entire operating range without loss of product.
1. Introduction The pyrolysis of coal, or its thermal degradation in an inert atmosphere, results in the formation of solid, liquid, and gaseous products. The distribution and character of these products reflect the chemical and physical properties of the parent coal as well as the kinetic and transport processes leading to their evolution. We sought to elucidate this complex process through the quantitative collection and characterization of time-resolved product distributions and by mathematical modeling of the relevant kinetic and mass-transfer processes indicated by the data. The experimental methods, data, and interpretation are presented in this paper. Motivation for this study arises from the rapid rates of pyrolysis a t temperatures of industrial interest. The conversion of coal to useful chemicals through gasification and liquefaction, or to heat through combustion, requires reaction at elevated temperatures. Under such conditions, rapid pyrolysis generally precedes conversion processes, necessitating an understanding of primary devolatilization on the time scale and under conditions appropriate to industrial applications (Gavalas, 1982; Howard, 1981; Gorbaty et al., 1982; Meyers, 1982; Speight, 1983). However, the characteristic times for transport of heat, mass, and momentum, as well as chemical reaction, are comparable, so time-resolved product distributions are required to understand the effect of coal type or rank. Furthermore, we have attempted to decouple these phenomena by choosing experimental conditions that tend to affect one more than the other, Le., pressure to probe mass transfer or yield during heat-up to probe reaction kinetics. The dearth of such data in the otherwise extensive coal literature for high heating rate, short residence time pyrolysis attests to the difficulty of a quantitative time-resolved collection of the products. Although a number of different pyrolysis reactors have been devised, two basic designs predominate: the entrained-flow and heated-grid reactors. The entrained-flow reactor introduced by Badzioch and Hawksley (1970), refined somewhat by Kobayashi et al. (1976), Ubhayakar et al. (1976), and Naskala et al. (1977) and recently used extensively by Solomon et al. (1981, 1982, 1984, 1986), provides time-temperature histories appropriate to the study of primary devolatilization. However, heating rates 0196-4313/86/1025-0536$01.50/0
remain uncertain and operation is largely limited to atmospheric pressures. Although on-line qualitative spectroscopic analysis of the products is relatively straightforward (Solomon et al., 1981, 1982, 1984, 1986; Juntgen, 19791, the effects of secondary reactions due to extended residence in the heated zone may obscure the primary pyrolysis product distribution. In addition, quantitative product collection and characterization remains elusive as products tend to deposit on the cooler surfaces downstream of the reaction zone. Variations of the heated-grid reactor, introduced by Loison and Chauvin (19641, also provide reaction conditions appropriate to high heating rate, short residence time pyrolysis studies. The careful control of voltages and currents applied to the mesh in more recent systems allows a well-defined time-temperature history (Hamilton et al., 1979; Niksa, 1981)-an improvement over earlier systems with appreciable overshoot of the steady-state temperature at the end of the heating period (Anthony et al., 1976; Suuberg, 1977). The problem of significant weight loss during cooling was eliminated by Niksa (1981) through rapid cooling by cold nitrogen sprayed directly onto the heated support. Unfortunately, pyrolysis products were vented upon quench, due to the low delivery pressure of the liquid-nitrogen Dewar. Subsequent improvements to the reactor by Bautista (1984) achieve more rapid quenching (ca. -6000 K/s) over the full operating range on the reactor (1torr-100 atm) without product loss with a spray of liquid-nitrogen-cooled helium. In addition to precise control of the time-temperature history, the product collection and analysis train presented here yield time-resolved product distributions with an average closure of 98% of the mass balance over the full operating range. 2. Experimental Section 2.1. Coal Preparation. Four different coals were py-
rolyzed in the study: Pittsburgh Seam HVA bituminous (PSOC-1099 from the Pennsylvania State University coal data base), Beth-Elkhorn, Cambria, and Van Cleaning (from Homer Research Lab of Bethlehem Steel Corp.). Coal characterization data appear in Table I. The coal was received in sealed containers under nitrogen and ground under similar conditions in a glovebox with liquid nitrogen added periodically to the disk grinder 0 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986 537 Table I. Coal Analyses" HVA bituminous 97 pm 90 moisture 70 carbon % hydrogen % nitrogen % chlorine % sulfur % ash % oxygenc
0.81 74.91 5.12 1.49 0.13 1.43 9.21 6.90
180 pm Beth-Elk Ultimate Analysis 0.93 n.a.* 77.27 85.1 5.28 5.45 1.58 1.52 0.14 1.22 0.56 7.63 3.60 5.95 7.37
% moisture 90 ash 9'0 volatile matter 90 fixed carbon
0.81 9.21 32.00 57.98
Proximate Analysis 0.93 n.a. 7.63 3.60 33.27 36.00 58.17 60.39
Cambria
Van Cleaning
n.a. 86.80 5.50 1.50
ma. 90.70 4.85 1.40
0.78 6.89 5.31
0.87 6.29 2.02
n.a. 6.89 35.00 58.11
n.a. 6.29 18.70 75.01
475 433 509 41
443 390 480 24982
Gieseler Plastomer max fluidity, "C bonding, "C resolidification, "C max fluidity (ddpm)
462 411 496 666
Analysis on Pittsburgh Seam HVA bituminous coal was performed by Galbraith Laboratories, Knoxville, TN. Analyses on Beth-Elkhorn, Cambria, and Van Cleaning coals were on a dry basis and performed by Bethlehem Steel Homer Research Labs, Bethlehem, PA. *Not available. By difference.
to avoid heating (Solomon and Mains, 1977). The ground coal was then sieved at ambient conditions with a Ro-Tap machine and standard Tyler mesh sieves. After drying in a vacuum oven at 100 " C for 24 h to establish a constant moisture content, the coals were stored in desiccators under zero-grade nitrogen. The inevitable slight concentration of ash in the smaller particle fractions did not appear to affect experimental results, as indicated by their insensitivity to particle size. 2.2. Experimental Apparatus. The apparatus consisted of the reactor, a synchronous heating controller, a rapid-quench delivery and control system, a downstream product collection system, and a product analysis train. The coal was heated electrically within a stainless steel mesh support measuring 1.5 cm X 6 cm (finished dimensions) prepared from 325-mesh 316 stainless steel screen, folded, and then spot-welded to close the longer side. The temperature of the support was measured by a Type-K thermocouple (chromel-alumel) spot-welded directly to the center of the upper surface of each support; the 50-pm thermocouple leads were attached to a terminal block within the reactor and connected through thermocouple extension wire to the differential amplifier (Tektronix 5A21N) input of a storage oscilloscope (Tektronix 5111). Thermoelectric voltages were isolated from heating voltages by allowing the thermocouple to contact the mesh at only a single point. The uniformity of the support temperature during heating, as well as the thermocouple accuracy, was tested by painting strips of melting point standards along the central two-thirds of the support surface. Differences between the measured temperature and those indicated by the standards were less than 35 "C. A piezoelectric pressure transducer (National Semiconductor LX06002G), optimized for the 0-2 psi range, measured reactor pressure as a function of time. The transducer signal was first amplified and then filtered through an active low-pass filter before being displayed on the oscilloscope along with the temperature traces. The reactor, a stainless steel pressure vessel originally built by Niksa (1981), had a working pressure of 140 atm and an internal volume of approximately 25 cm3. In addition to the modification of the quench system, the internal volume was increased to 250 cm3 and operated in
SAMPLE ELEMENT
AND THERMOCOUPLE
THERMOCOUPLE /-TERMINAL
PRESSURE TAP SEAL
h
\
/
Figure 1. Placement of the support and thermocouple within the reactor.
the bomb mode rather than the purge mode to avoid recirculating flows which affect the product distribution (Bautista, 1984). The coal support and clamp assembly were bolted inside the reactor between two copper electrodes through which the heating current and voltage passed. A four-hole quench manifold in the reactor floor delivered the liquid-nitrogen-cooled, pressurized-helium quench stream. Figure 1 illustrates the internal reactor design and placement of the thermocouple. The typical time-temperature history imposed on the coal consisted of heat-up a t a constant rate, followed by an isothermal period and a rapid quench. The support represents the controlling thermal mass with losses small on the time scale of heat-up, and since the temperature dependence of the electrical resistance compensated that of the heat capacity of the stainless steel mesh, the rate of heating varied as the square of the applied current. Thus, application of a constant current, with a power supply capable of delivering 60 A at 15 V dc (Kepco OPS-15-50 M), produced a constant heating rate preceding the switch to the constant-voltage mode at the onset of the isothermal period. A custom-designed synchronous programmer (TECO Model T-500-1 by Theall Engineering) controlled the duration of the heating period and the onset and duration of the quench and switched the power supply
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Ind. Eng. Chern. Fundarn., Vol. 25, No. 4, 1986
SLP"."
P O S T O h SA-L ILL\/E
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LIQUID NITROGEN SUPPLY DEWAR
A
CHECK VALJE
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PRESSURE GAUCF 'I-TEP SCLENOIZ
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R E L E F /A.,E
Figure 2. Flow schematic diagram.
to the constant-voltage mode for the isothermal period. Switching times of less than 40 ps eliminated any temperature overshoot. Several experimental runs were combined to obtain product distribution data as a function of the independent variable (time, temperature, pressure, etc.), so a well-defined, accurate, and reproducible timetemperature history was needed. Previous work (Niksa, 1981) has demonstrated reproducibility and the absence of overshoot after the linear heating period. Careful control and measurement provided a broad range of reliable operation for the reactor: heating rates of 100-10 000 K/s, isothermal temperatures of 100-1000 OC, isothermal heating periods of C-100 s, quench rates of -6OOO K/s, and reaction pressures of 0.001-100 atm (either He or Hz). For operation in the bomb mode, the reactor inlet and outlet remained sealed during reaction. After quenching, the gaseous products were slowly purged into heated, evacuated collection vessels of known volume (Figure 2) while leaving the liquid (tar) and solid (char) within the reactor (Bautista, 1984). Quantitative collection of the tar fraction was most difficult and accounted for any failure to close a mass balance. Most of the tar evolved condensed onto preweighed foil liners covering both reactor floor and roof. Three filters (a Nuclepore 3-pm membrane filter sandwiched between two quartz fiber filters) at the exit of the reactor efficiently collected entrained tar droplets from the purge stream. A thin sheet of glass wool (approximately 30 mg), resting on a wire support approximately 0.5 cm above the coal support, collected tar from the zone of vigorous free convection. The tar was collected from unlined surfaces by swabbing with small sections (750 "C) CO is one of the primary products of the decomposition of phenolic, ether, and carbonyl functionalities (Attar and Hendrickson, 1982), the phenolic and ether groups being the more abundant (Gavalas, 1982; Speight, 1983). Phenol decomposition at elevated temperatures proceeds through a keto-enol shift to either benzene and water or carbon monoxide and 1,4-cyclopentadiene. The molar generation ratio of CO/H20 observed at 850 "C, 1.7/1.0 (Gavalas, 1982), indicates the CO pathway is preferred. Therefore, increased CO yields at elevated pyrolysis temperatures are expected (Mrazikova et al., 1986). Characterization of the tar fraction provides further evidence of the phenolic origin of CO. 'H NMR of flash pyrolysis tars (Collin et al., 1980) revealed a decreasing phenolic content with increasing pyrolysis temperatures above 595 "C, which implies CO yields increasing with temperature. The decomposition of aliphatic, aromatic, and heterocyclic ethers also evolves CO. The decomposition kinetics of these functional groups differ greatly from one another and are further complicated by the presence of substituent groups. Fortunately, only the decomposition of aromatic ethers, which contain almost all of the non-phenolic oxygen (Bartle et al., 19791, needs to be considered. This decomposition proceeds through free radical mechanisms limited by the formation of the radical itself. Activation energies for these reactions are rather high-on the order of the bond dissociation energy, 50-80 kcal/mol (Weast, 1979), necessitating elevated temperatures. Clearly, many reaction paths lead to the formation of CO. Nonetheless, we can draw several conclusions consistent with the data of Figure 5. The formation of CO is basically a high activation energy process since the mechanisms outlined above require relatively high temperatures (>750 "C) to rupture bonds and thereby to produce an appreciable number of free radicals. Therefore, the yield of CO increases dramatically with temperature, primarily through phenol decomposition with assistance from the decomposition of phenols (Schlosberg et al., 1981) and aldehydes (Cronauer et al., 1979) formed by aromatic ethers at lower temperatures. With regard to the light hydrocarbon products, we note that the saturated and unsaturated species exhibit different trends with temperature. The yields of ethene and propene increase monotonically with temperature while ethane and propane reach constant asymptotes at approximately 950 "C (Figures 6 and 7). The free radical reaction paths giving rise to these products rationalize the difference.
Table 11. a-0 Carbon Bond Strengths bond dissociation energy: bond kcal/mol CGH,CH~-C~H~ 6.7 f 2 C,H&Hz-C2H, 69 f 2 CGH~CHZ-CH~ C6H5-CH3
heat of formation,* kcal/mol
72 93
"Weast (1979). bDean (1979).
Many free radical pathways exist for the production of gaseous hydrocarbons, particularly at high pyrolysis temperatures: the scission of aliphatic groups from the aromatic or hydroaromatic ring periphery (Snape et al., 1985; Nelson and Huttinger, 1986), the breakage of aliphatic bridges between rings, and, to a lesser degree, rupture of hydroaromatic rings. At low temperatures (
