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Energy & Fuels 2004, 18, 508-519
Impact of Pressure Variations on Coal Devolatilization Products. 1. Detailed Product Distributions from 0.1 MPa Yan-Lai Liu 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 104 K/s and a pressure of 0.1 MPa. Major noncondensable gases were resolved into C1-C4 hydrocarbons, oils, CO, CO2, H2, and H2O. Fuel-N release was monitored in terms of HCN, NH3, and NO levels and the tar-N and char-N levels. Elemental compositions were reported for the tars and chars. Because all major products were monitored in individual tests, 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; therefore, the product distributions are especially well-suited to validating the proposed devolatilization mechanisms. FLASHCHAIN, which is one of the network depolymerization models, predicted the most important aspects of devolatilization within useful quantitative tolerances for all but one coal, based solely on the proximate and ultimate analyses of the coals and the operating conditions in the tests.
Introduction Devolatilization is the first stage in any coal utilization technology, and it is responsible for releasing almost all the coal-O and coal-S as gases, most of the coal-H, up to half the coal-N, and up to 40% of the coalC. Devolatilization products are responsible for the ignition of autothermal processes, and their rapid combustion provides the heat release near burners and fuel injectors that stabilizes large-scale coal flames. In gasification systems, devolatilization products are partially oxidized and reformed into significant contributions to the synthesis gas product. Detailed product distributions for rapid coal devolatilization are used to guide the development of numerous coal-based technologies, especially now, when the emphasis has shifted toward closer control of the chemical composition of the process streams in gasifiers. 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 survey1 describes a database 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 a handful of studies * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Niksa, S.; Liu, G.-S.; Hurt, R. H. Prog. Energy Combust. Sci. 2003, 29 (5), 425.
have reported the complete distribution of devolatilization products,2-8 and only one8 of these imposed typical suspension firing conditions in the laboratory tests. This study was undertaken for two reasons. First, it provides accurate product distributions for the devolatilization of five coals in entrained flows at a pressure of 0.1 MPa, as a means to evaluate devolatilization models stringently. The reported product distributions are used to evaluate the predictions from FLASHCHAIN,9 which is one of the network depolymerization mechanisms. Second, the study provides an essential reference condition for characterizing the impact of pressure variations on various aspects of devolatilization behavior. This paper describes the reference behavior at 0.1 MPa, and our companion paper reports the product distributions from the same five coals at 1.0 MPa, to characterize the pressure effect and evaluate a current mechanistic interpretation. Here, in Part 1, the product distributions are reported for five coals, representing ranks from subbituminous (2) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Proc. Combust. Symp. 1979, 17, 117. (3) Solomon, P. R.; Hamblen, D. G.; Serio, M. A.; Yu, Z.-Z.; Charpenay, S. Fuel 1993, 72, 469. (4) Bautista, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986, 25, 536. (5) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J. 1989, 35, 776. (6) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 627. (7) Griffin, T. P.; Howard, J. B.; Peters, W. A Energy Fuels 1993, 7 (2), 297. (8) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254. (9) Niksa, S. Combust. Flame 1995, 100, 384.
10.1021/ef034033m CCC: $27.50 © 2004 American Chemical Society Published on Web 02/27/2004
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Figure 1. Schematic of the Atmospheric-Pressure, Radiant Coal Flow Reactor (a-RCFR) at SRI.
through high-volatile (hv) bituminous at nominal heating rates of 104 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 HCN, NH3, and NO levels and 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. Experimental Section These tests used the apparatus described by Chen and Niksa.8 The only difference was to rely more heavily on Fourier transfer infrared spectroscopy (FTIR) in the product analyses. This brief description is supported by additional details8,10 and design specifications11 in other publications. A schematic of the atmospheric-pressure, radiant coal-flow reactor (a-RCFR) is depicted in Figure 1. At the top, a feeder dumps pulverized particles into an argon entrainment stream, forming an optically thin suspension that flows downward into a radiant furnace section. The radiant section consists of a quartz tube on the axis of a graphite cylinder that is inductively heated to 1850 K. Near-blackbody thermal emission from the graphite imposes heat fluxes up to 60 W/cm2 on the suspension, which heats the particles at a rate faster than 104 K/s as they traverse the furnace. The suspension is kept optically thin, to ensure that the radiant 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. 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; reported values were measured under actual conditions. Gas velocities and coal feed rates are simultaneously adjusted, to vary the residence time (10) Chen, J. C.; Niksa, S. Rev. Sci. Instrum. 1992, 63 (3), 2073. (11) Chen, J. C., Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Palo Alto, CA, 1991.
but fix the suspension loading at 300 particles/cm3 at the tube inlet. For all tests reported here, residence times were varied at a fixed furnace temperature of 1850 K. Thermal histories in such a series of runs have almost the same heating rate; however, the suspension achieves different temperatures at the outlet in each case. Higher temperatures are achieved as residence times are extended; however, the outlet temperatures are always well below the furnace temperature. Calculated thermal histories are reported elsewhere.8,10 Products are segregated into bulk solid particles, aerosol, and noncondensable gases with virtual impaction in an aerodynamic classifier (centripeter), as depicted in Figure 1. Beyond the centripeter’s inlet port, the process stream is split between opposing nozzles. Ninety-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 a wire mesh 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 calibration data recorded in cold flow. Preliminary runs established that fines in the coal feed ultimately deposit on the tar collection filters in the aerodynamic classifier. This segregation would be recorded as weight loss, according to our mass determination procedures. To eliminate this artifact, all the weight-loss values are reduced by the amount of fines determined in preliminary cold-flow runs, adjusted for the loss of volatiles from the fines; i.e., the adjustment for fines is reduced in proportion to the ratio of the weight loss recorded in the high-temperature test to the ultimate weight loss. 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. Aerosol yields are adjusted for the flow split in the virtual impactor, deposition losses in the quench nozzle, and losses in the gas sampling line to the water monitor and FTIR (discussed below). Tars are recovered on a four-stage assembly of glass-fiber filters and a polypropylene liner that was dried overnight at 335 K under vacuum. Pure tar samples for subsequent chemical analyses are prepared by extraction with
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Liu et al. Table 1. Coal Properties
SS001AUS Newlands
SS002AUS Ebenezer
SS003AUS Blair Athol
SS004CHN Tatong
SS005JPN Taiheiyo
28.4 15.2
34.0 11.9
22.1 7.7
31.0 8.5
37.1 13.0
80.2 4.7 12.9 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
proximate analysis, dry wt % volatile matter ash ultimate analysis, daf wt % carbon hydrogen oxygena nitrogen sulfur a
By difference.
tetrahydrofuran (THF) in an ultrasonic bath, followed by filtration through a 0.2-µm Teflon membrane. The tar solution is concentrated in a Kudern-Danish concentrator before the remaining solvent is evaporated, following the procedure of Lafleur et al.12 Several tens of milligrams of pure tar are recovered from runs with significant aerosol yield. The membrane residue is weighed and denoted as the soot yield. Insolubles were observed to comprise 1 s. However, such conditions are much more severe than those in the a-RCFR, so no HCN was observed until the very latest stages of tar release with SS001AUS, SS002AUS, SS003AUS, and SS004CHN. HCN appears for the first time during the second stage of devolatilization because char-N is expelled by ring rupture. Although cyanide yields do not distinguish precursors in char or tar, char particles are significantly hotter than tar in the entrainment stream in this experiment. However, with SS003AUS and SS004CHN, a surge in the HCN yields at longer reaction times paralleled diminishing tar yields, which strongly suggests that secondary pyrolysis was a factor in the tests with these coals. Neither NH3 nor NO was observed with SS001AUS, SS002AUS, or SS003AUS under these rapid heating conditions, and NH3 was a substantial product for SS004CHN and SS005JPN at only the longest residence time. Deficits in the coal-N balances with SS003AUS, SS004CHN, and SS005JPN at intermediate reaction times admit the possibility that an unmonitored species, perhaps HNCO, has a role at intermediate stages. Tighter closures on the nitrogen balances are needed to support this hypothesis in any definitive way. FLASHCHAIN predicted the most important aspects of devolatilization within useful quantitative tolerances for all coals except, perhaps, SS003AUS. These predictions were based solely on the proximate and ultimate analyses of the coals and the operating conditions in the tests. The predictions do identify the significantly higher weight loss and tar yields from SS002AUS and SS005JPN, even though the properties of all the coals are very similar. The predicted weight loss and tar yields were most accurate, followed by the CO2 and H2O yields. The predicted CO yields were unreliable for all but one coal, which probably reflects problems in the reported coal-O levels, because oxygen balances were oversubscribed for three of the five coals. Although FLASHCHAIN correctly predicted that tar-N is the predominant shuttle for coal-N during most of the primary devolatilization, it did not accurately depict the abundance of HCN at the longest residence time in the tests. (15) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013.
Effect of Pressure on Coal Devolatilization. 1
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
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provided by the Coal Research Laboratory of Idemitsu Kosan Co., Ltd. EF034033M
