Energy & Fuels 1989,3, 692-703
692
aggregation of iron atoms occurred more easily over bituminous coals than brown coals even at the same loading. This is because brown coals have more surface functional groups that can interact with iron species.
Conclusions The use of Mossbauer and EXAFS techniques was demonstrated to be quite effective in revealing the type of iron species formed during pyrolysis of Fe(N03)3-loaded brown coal in an inert atmosphere. The main species observed at low temperatures was highly dispersed FeOOH. At higher temperatures, reduced species, like y-Fe, a-Fe, and Fe3C, became predominant. The change of chemical forms of iron species during heat treatment was affected not only by temperature but also (30)Yamashita,
H.;Tomita, A. Unpublished data.
by iron loading. The aggregation and reduction of iron species were rather slow in the low-loading samples.
Acknowledgment. The partial financial support of a Granbin-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture, Japan (62603014), is acknowledged. Coal Corporation of Victoria, Australia, kindly supplied the raw brown coal. The X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee (87-139). We thank Dr. M. Nomura of KEK-PF for his advice in measuring EXAFS spectra. Thanks are due to Dr. A. Kunii of the Faculty of Science and the staff of the Cyclotron and Radioisotope Center at Tohoku University for Mossbauer spectroscopy measurements. Registry No. Fe, 7439-89-6; Fe(N03)3,10421-48-4.
Chemical Characteristics of Tars Produced in a Novel Low-Severity, Entrained-Flow Reactor J. D. Freihaut,* W. M. Proscia, and D. J. Seery United Technologies Research Center, East Hartford, Connecticut 06108 Received M a y 19, 1989. Revised Manuscript Received July 28, 1989 Understanding tar formation, desorption, and gas-phase secondary reactions during rapid coal devolatilization is critical to the formulation of a comprehensive understanding of rapid coal devolatilization. To overcome some of the limitations of traditional entrained-flow systems, a novel flow reactor and separation system was established to investigate the tar devolatilization properties of a range of coals. Results indicate a wide range of coal ranks devolatilize in a phenomenologically similar sequence, with tar formation and evolution dominating the initial phases of particle hydrocarbon mass loss under the heat-transfer conditions characteristic of this system. The average structural characteristics of the tars change significantly during the temperature-resolved evolution process, with hydrogen-rich, lower molecular weight species evolving before the relatively hydrogen-poor, high-molecular-weight species in the latter stages. The lower the rank index of the parent coal, the more dissimilar the primary tars are relative to the parent coal at any given extent of tar evolution. The integrated mass of tars evolved from medium- and low-volatile bituminous coals appears to approach the parent coal elemental and functional group infrared absorbance characteristics as an asymptotic limit. But for a given coal, the earlier in the tar evolution process, the more dissimilar, relative to the parent coal, are the evolved tars with respect to these characteristics. Generalizations concerning the “similarity” of primary tars to the parent coal structure have limited applicability in the formulation of general models of tar formation and evolution. Such generalizations apply to particular coal ranks and describe the integrated sum of a process that, upon deconvolution, reveals systematic differences between chemical characteristics of tars and the parent coal.
Introduction and Approach The devolatilization of a high-volatile bituminous coal follows the sequence indicated in Figure 1. The formation and evolution of heavy molecular weight hydrocarbons, tars, account for more than half the total mass loss of such ~ o a l s . ~ As - ~observed * ~ ~ ~ under ~ ~ a wide range of heating (1)Suuberg, E. M.;Peters, W. A.; Howard, J. B. Seuenteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979;pp 117-130. (2)Arendt, P.; van Heek, K. H. Fuel 1981,60,779-789. (3)Solomon, P. R.;Colket, M. B. Seventeenth Symposium (International) on Combustion;The Combustion Institute: Pittsburgh, PA, 1979; pp 131-143. (4)Freihaut, J. D.; Zabielski, M. F.; Seery, D. J. Nineteenth Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1982;pp 1159-1167.
0887-0624/89/2503-0692$01.50/0
conditions, the formation of detached tar precursors (DTP) from attached complexes (ATP) and the subsequent evo(5)Doolan, K. R.;Mackie, J. C.; Tyler, R. J. Fuel 1987,66,572-578. (6)Serio, M. A.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Res. 1987,26,1831-1838. (7)Tyler, R.J. Fuel 1980,59,218-226. (8)Teo, K.C.; Watkinson, A. P. Fuel 1986,65,949-959. (9)Solomon, P. R.;et al. Nineteenth Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1982, 1139-1149. (10)Squire, K.R.;et al. Fuel 1986,65,833-843. (11)Calkins, W. H.;Tyler, R. J. Fuel 1986,63,1119-1124. (12)Klose, W.; Lent, M. Fuel 1984,64, 198-199. (13) Oh, M. S.; Howard, J. B.; Peters, W. A. Modeling Volatile8 Transport in Softening Coal Pyrolysis. Presented at the AIChE National Meeting, San Francisco, CA, Nov 1984. (14)Suuberg, E.M.In Chemistry of Coal Conuersion;Schlosberg, R. H., Ed; Plenum: New York, 1985;pp 67-100.
0 1989 American Chemical Society
Chemical Characteristics of Tars
Energy & Fuels, Vol. 3, No. 6,1989 693
kdes = kvap
+
conv
+
ned
Figure 1. Coal devolatilization/pyrolysis.
lution of tars dominates the initial mass loss of bituminous coal p a r t i c l e ~ . ~ ' ~In ~ 'addition, ~ J ~ ~ gas-phase, "secondary" reactions of tars can account for major fractions of the light gas yields, depending on the heating conditions. The and chemical chardistribution of the light gases5-7J1*22 acteristics of the collected tars are dependent on both the transient particle temperature and ambient gas temperature. A number of investigators have observed that a range of coal types follows the same phenomenological sequence, although the tar yields and characteristics vary significantly with coal rank characteristic^.^*^*^*^*^^-^^ Some investigators report the chemical structural characteristics of primary tars are " ~ i m i l a r "to ~ ~those * ~ ~present in the parent coal; that is, "primary" tar "monomers" reflect the coal " p 0 1 y m e r " , ~ *with ~ J ~ ambient ~~~ pressure apparently having little effect on the structure of evolved primary tars relative to the parent coal. "Primary" tars are those collected in conditions in which intraparticle or extraparticle "secondary" reactions are thought to be minimized. Orning and Griefer27 noted that "under conditions of molecular distillation", the vacuum pyrolysis of coal gave (15) Oh, M. S. Softening Coal Pyrolysis. Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1985. (16) Unger, P. E.; Suuberg, E. M. Fuel 1984, 63,606-611. (17) Zielinski, E. Fuel 1967, 46, 329-340. (18) Hertzberg, M.; Ng, Daniel. In Fundamentals of the PhysicalChemistry of Pulverized Coal Combustion; Lahaye, J., Prado, G., Eds.; NATO AS1 Series, Series E 137, 1987; pp 104-125. (19) Fong, W. S.; Peters, W. A.; Howard, J. B. Fuel 1986,65,251-254. (20) Fitzgerald, D. Trans. Faraday SOC.1956,52,362-369. (21) Chermin, H. A. G.; Van Krevelen, D. W. Fuel 1959,36,85-104. (22) Nelson, P. F.; Tyler, R. J. Twenty-First Symposium (Znternatioml) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986, 427-435. (23) Wei-Chun Xu; Akira, Tomita. Fuel 1987,66,6264331. (24) Wei-Chun Xu; Akira, Tomita. Fuel 1987, 66, 632-636. (25) Granger, A. F.; Ladner, W. R. Fuel 1970,49,17-25. (26) Phenoc, Tran. X.; Maloney, D. J. Presented at the Twenty-Second Symposium (International) on Combustion, Pittsburgh, PA, 1988. (27) Orning, A. A.; Griefer, B. Fuel 1956,35, 381-383. (28) Brown, J. K.; et al. J . Inst. Fuel 1958, 31, 259-273. (29) Solomon, P. R;Hamblen, D. G. Bog. Energy Combust. Sci. 1983, 9,323-363. (30) Collin, P. J.; et al. In Coal Liquefaction Products, I; Shultz, H. D., Ed.; John Wiley & Sons: New York, 1983; pp 86-123. (31) Calkins, W. H.; Hazaman, E.; Zeldes, H. Fuel 1984,63,1113-1118. (32) Friedel, R. A.; Pelipetz, M. G. J . Opt. SOC.Am. 1953, 43, 1051-1052. (33) Given, P. H. In Coal Science, III; Gobartz, M. L., Larsen, J. W., Wender, I.; Eds.; Academic Press: New York, 1984; pp 63-252. (34) Solomon, P. R.; Hamblen, D. G. In Chemistry and Physics of Coal Utilization; Cooper, B., Petrakis, L. Eds.; AIP Conference Proceedings 70; American Institute of Physics: New York: 1981; pp 121-140.
a condensible solid that "resembles" the molecular configuration of coal, as justified by infrared absorption spectra. In a more comprehensive investigation, Brown et al.28indicated vacuum pyrolysis of coal resulted in a kind of "depolymerization" followed by separation of the smaller units from the larger. The room-temperature condensible volatiles "resembled" the parent coal in chemical structure. In attempts to formulate a general model of coal devolatilization, some of these investigators have noted that, within the limits of experimental resolution, the chemical kinetic parameters that describe tar evolution from a wide range of coals do not vary with coal type.39g*m* From this latter perspective, the monomer-polymer relationship between primary tar and parent coal structural characteristics and the chemical kinetic parameters describing tar formation and evolution are invariants in coal devolatilization. These become the underlying hypotheses of general models of tar devolatilization. On the other hand, some investigators, while indicating the common phenomenologicalsequence of disperse-phase devolatilization, note that tar yields and chemical characteristics vary significantly with coal type.7J1*25*30 In addition, secondary gas-phase pyrolysis behavior varies according to chemical characteristics of the primary tars.5*7*11 Implicit in these structural observations with respect to primary tars is the implication that the underlying mechanisms of tar formation, evolution, and secondary reaction vary with coal rank characteristics, which, in turn, implies a variance in associated kinetic parameters. In addition, the primary tar structural differences directly imply significant differences in homogeneous decomposition mechanisms, making it unlikely the same secondary decomposition kinetic parameters are applicable to all primary coal tars. Others have noted the appreciable role transport parameters can exert in determining devolatilization phenomena. The formation of a glass-like "melt" as an intermediate phase during devolatilization of a bituminous ~ o a l ~and ~ the - ~observed ~ J ~changes ~ ~ in yields and molecular weight characteristics of heavy hydrocarbons with ambient pressure15J6have led to detailed examinations of intraphase and interphase mass-transport phenomena contributions in mass-loss kinetics. Ignoring the details of product distributions and chemical characteristics of tars with changes in pyrolysis conditions, others have indicated that weight-loss kinetic behavior is dominated by heat-transfer con~iderations,'~J~ implying independence of coal type. As apparent from the diverse perspectives, a conceptual understanding of dis-
694 Energy & Fuels, Vol. 3, No. 6, 1989
Freihaut et al. Total flow Q Collection probe Acceleration THORIAAKONIA
Fractionization
SIGHT WINDOW
ISOTHERMAL ZONE
cyclone system
To Impactor train system
Figure 3. UTRC-EFR aerosol-char separation apparatus. Water cooled probe separator To EXHAUST
’I\
TO EXHAUST COOLING AND REJECTION
i
TO PHASE SEPARATION FILTERS AND GAS ANALYSIS SYSTEMS
WATER COOLED (WATER QUENCHED FOR CHAR AND TAR COLLECTION)
(ADJUSTABLE POSITION)
aerosol transfer line
Filter
Andersen impactor train (8 stages) 5 stage cyclone
Rotameter
Figure 2. UTRC entrained-flowreactor. perse-phase coal devolatilization remains elusive, and consequently, kinetic models remain difficult to extrapolate to a wide range of conditions. From any of the above perspectives, heavy hydrocarbon (tar) yields and characteristics emerge as the intrinsic chemical tracers to aid in deconvoluting the rapid devolatilization process. Among the considerable experimental difficulties in investigating rapid devolatilization via tar evolution are rapidly quenching a heterogeneous process whose overall tar evolution rate appears to adjust itself to heating conditions, isolating char particles, tar aerosols, and light gas species that are coupled in product streams, minimizing gas phase reactions of thermally labile, heavy molecular weight organic compounds, and subsequent analysis of large, fragile organic species that have considerably different functional forms and molecular size parameters, depending on heating conditions and parent coal characteristics. Batch reactors and single particle systems are often criticized for generating too small a tar sample in conditions unrepresentative of rapid heating. Continuous reactor systems, such as entrained-flow reactors, provide acceptable heating conditions, but due to substantial residence time in hot entrainment gases and the large entrainment flow/sample mass, it is difficult to either time-resolve mass loss or isolate and capture primary tars for subsequent analyses. This communication reports the utilization of a novel entrained-flow reactor and product isolation system. In contrast to the operation of most entrained-flow reactors, no attempt is made to match the entrainment gas and wall temperatures. Entrainment gas temperatures are purposely kept below wall temperatures to minimize extraparticle gas-phase reactions of tars. To obtain char-free tar samples, an aerosol-phase separation system is employed to isolate entrained tar species from the char mass. Both the char particle and aerosol arms of the product separation system contain serially staged particulate sep-
-Exhaust
to hood or
Figure 4. UTRC-EFR sample collection system. 25 20
Flux w / cm2
15 10
5 n ”
0
5 10 15 2 0 25 3 0 3 5 4 0 45 5 0 Position, cm
Figure 5. UTRC-EFR reactor total flux profiles. aration systems followed by porous metal final filters that collect aerodynamically isolated, “pure” tar species. The phase-separation and gas analysis system handles the entire reactor mass flow. After particulate and aerosol separation, the gas stream is analyzed on-line by use of a Fourier transform infrared spectrometer (FT-IR) coupled to a multipass cell having an equivalent path length of 43.5 m.
Experimental Reactor and Product Separation System Figures 2-4 display the essential components of the entrained-flow reactor and product separation and gas analysis systems. Figures 5-7 display the total power density, radiative power density, and gas temperature profiles as a function of reactor position. The flux rate profiies are measured by specially designed, calibrated probes. Incident, center-line radiative flux rates range from several watts per square centimeter at wall temperatures of 700 “C to approximately 25 W/cm2 at wall temperatures of
Energy & Fuels, Vol. 3, No. 6,1989 695
Chemical Characteristics of Tars 25 2o 15
Flux
7
t
800 -
600 -
Wicm2
400 -
5
0 0
200 -
5 10 1 5 20 2 5 30 3 5 40 4 5 50 Position, cm
Figure 6. UTRC-EFR reactor radiative flux profiles. 0
I
I
I
0.2
0.4
0.6
12001------
n
1000 -
Tw I'Cl 1241
C
8
TIME [SEC]
Figure 8. UTRC-EFR gas and particle temperature profiles for a 939 "C wall temperature (25-pm particle).
800-
1200
c A
S C
600-
1000 400-
800 200 -
T
600
! 10 30 C
0
5
1 5 20 2 5
THERMOCOUPLE
3 5 4 0 4 5 5 0 5 5 60
POSITION,
400
CM
Figure 7. UTRC-EFR axial gas temperatures for given wall temperatures with argon carrier gas. 1240 "C. Gas temperature profiles vary as shown. Gas temperature profiles were obtained by making a series of measurements with a set of decreasing thermocouple bead sizes with the asymptotic temperatures approached defined as the "true" local gas temperature. The reactor creates a heat-transfer field in which entrained particles are heated by radiation to temperatures slightly above the local, axial, entrained gas temperature within the reactor (Figures 8 and 9). The radiation flux induces an inverse diameter dependent heating rate on particles in an attempt to drive the particles to equilibrium with the radiating walls. The carrier gas imposes an inverse diameter squared component on the particle heating rate, which in effect insures that small particle temperatures are not very different from the local gas temperature, assuming the devolatilization process is weakly endothermic or thermally neutral. Particles temperatures are calculated by using the measured radiative fluxes and gas temperatures, a particle emissivity of 0.7t2and the Merrick model%for the heat capacity of coal. (35)Merrick, D.Fuel 1983,62,540-546. (36)Rodgers, P. A,; Creagh, A. L.; Prange, M. M.; Prausnitz, J. M. Ind. Eng. Chem. Res. 1987,26,2312-2318. (37)Campbell, J. H. Fuel 1978,57,217-224. (38)Carangelo, R. N.; Solomon, P. R.; Gerson, D. J. Fuel 1987,66, 960-967. (39)Bidlingmeyer, B. A.; Warren, Jr., F. V. LC-GC 1988,6,780-786. (40)Suuberg, E. M.; Scelza, S. T., Fuel 1982,61,198-199. (41)Anthony, D. B.; Howard, J. B.;Hottel, H. C.; Neissner, H. P., Fifteenth Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; pp 1303-1317. (42)Tyler, R. J., Fuel 1979,58, 680-686. (43)Bennett, H. Industrial Waxes, I; Chemical Publishing Co., Inc.: New York: 1975;Chapter 3,pp 146-164. (44)Tsai, Ching-Yi; Scaroni, A. W., Twentieth Symposium (International) on Combustion:The Combustion Institute: Pittsburgh, PA, 1984, pp 1455-1462. (45)Calkins,W. H.; Hagamon, E.; Zelda, H. Fuel 1984,63,1113-1118. (46)Calkins, W. H.; et al. Fuel 1984,63,1226-1229.
200
0.2
0.4
0.6
0.8
TIME [SEC]
Figure 9. UTRC-EFR gas and particle temperature profiles for a 1241 "C wall temperature (25-pm particle). Estimated particle heating rates are of the order of 2000-5000 "C/s in these conditions. Such heating rates are greater than that reproducibly obtainable in a heated-grid apparatus operated in the same laboratory, but less than that expected in conventional entrained-flow reactors wherein heating rates approaching 100OOO "C/sec are indicated. Operating an entrained-flow reactor in this manner minimizes gas-phase pyrolysis reactions of the initial tar species and allows collection of large quantities of physically isolated tar species, thereby allowing a greater degree of temperature and coal rank deconvolution of the tar evolution process. Normally, 60-75% of the total reactor flow (-20.6 L/min of argon) is drawn through the aerosol separation (impactor train) system; that is, F of Figures 3 and 4 is on the order of 0.25-0.40. The temperature and path length of the cooled aerosol transfer line are varied according to the nature of the tars being collected, which, in turn, varies with the parent coal characteristics and (47)Meuzelaar, H.; et al. Presented at the First Symposium on Advances in Coal Spectroscopy; Snowbird, UT, 1989. (48)Stenberg, V.I.; et al. In Coal Science, II; Gobarty, N. L., Larson, 3. W., Wender, I., Eds.; Academic Press: New York, 1983;pp 125-171. (49)Green, T.;Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Myers, R. A., Ed.; Academic Press: New York, 1982,pp 199-282. (50)Davidson, R. N. in Coal Science,I; Gobarty, N. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982;pp 84-160. (51)Freihaut, J. D.;Proscia, W. M. Energy Fuels 1989,3,625-635. (52)Fletcher, T.;Baxter, L. L.; Ottesen, D. K. International Conference on Coal Science; Moulin, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier: New York, 1987;pp 945-950.
696 Energy & Fuels, Vol. 3, No. 6, 1989
Freihaut et al.
Table I. Nomenclature for Coal Devolatilization/Pyrolysis Schematic symbol definition ATP attached tar precursor DTP detached tar precursor LG light gas species T tar species k global rate process subscript definition char 2 char species formed by high-temperature secondary reactions after tar evolution chem chemical pyrolysis process conv convective transport det detachment process gaseous state specific chemical component 1 liquid state m mass transport ned nonequilibrium desorption physical bond-breaking process PhY vaporization VaP 1 low-temperature devolatilization/pyrolysis-formed light gases (H20, COz, CO, CHI, CzH6and higher alkanes, C2H4) 2 high temperature pyrolysis light gases (CO, HZ,HCN, C&, C,HA superscript definition , low-temperature secondary reactions of tars /I high-temperature secondary reactions of tars
Table 11. Composition of PSOC Coals A. Elemental Composition of PSOC Coals on a Dry, Ash-Free Basis (High Temperature) PSOC seam no. rank %C % H %N %0 % S location 20-30-pm Particles 71.42 5.17 1.35 21.01 1.06 lower Wilcox, 1443D lignite
reactor heating conditions. The aerosol-phase separator is designed to "pull off" all particles or aerosols that are less than 2 pm. A qualitative check of the performance by scanning electron microscope examination of the deposits indicated that designed behavior is followed, provided proper flow rates are maintained throughout an experiment. Inertia carries larger particles into the char separation (cyclone train) system. Both separation trains contain porous metal filters as final stages. Performance of the system is monitored by measuring the flow through the separation arms and pressure drop acrms the filter housings and determining the tar mass deposited on each of the filter systems. Sample Selection. The feed system utilized is capable of sustained delivery of optically thin streams of particles at constant mass delivery rate, provided the feeder is loaded with a narrow particle size range initially. Coal sample feed rates of 1-3 g/h were employed in this investigation. Particle sizes as low as 10 pm and as large as 300 pm have been utilized. Since the feeder operates on aerodynamic principles, aerodynamically separated samples were employed (see acknowledgements). The elemental compositions of the parent coals are given in Table 11. Sample Measurements. Mass low is determined by ash tracer techniques. It is observed that experiment-specific determinations of the parent coal ash need to be made. This is accomplished by operating the system under cold-flow conditions before and after a particular hot-wall devolatilization test and determining the high-temperature (ASTM) ash content of the particles collected in the first-stage cyclone. This value becomes the initial, experiment-specific feed ash value. It is observed that lower density particles are fed into the reactor initially despite the rather extensive efforts to match the aerodynamic characteristics of the feeder with those of the size-separated samples provided. Consequently, sustained operation over a sequence of devolatilization conditions leads to significant variations in the average density (mineral content) of the feed, which, in turn, leads to inconsistent determinations in particle mass loss via ash tracer. Elemental characteristics of evolved tars and chars are determined by use of a Perkin-Elmer 240 instrument. IR absorbance characteristics are determined by an alkali-metal halide technique using the same FT-IR used to determine the composition of the pyrolysis gases. The particulate-free gas streams from the two separation systems are recombined and continously passed through a multipass cell. The 43.5-m path length is needed to measure the low levels of IR-active light gases generated. Gas concentrations are low because the initial stages of devolatilization, the focus of this investigation, generate small amounts of such
20-30~ coal feeds
TX 1507D lignite 1520D sub C 14451) 1451D 1493D 1508D
HVC bit HVA bit HVA bit MVB
1516D LVB
72.49 4.32 73.67 5.90 76.72 83.98 78.79 89.71
5.44 5.48 5.16 4.29
88.88 4.71
1.09 20.22 1.88 Beulah, ND 1.11 18.22 1.10 Smith Roland, WY 1.28 15.65 0.91 Blue, NM 1.67 7.41 1.46 Pitt. No. 8 1.48 10.15 4.42 Ill. No. 6 1.10 4.27 0.63 Pocahontas No. 3 1.49 4.30 1.25 Lower Kittaning
63-75-pm Particles 1451D HVA Bit 84.70 5.40 1.71 7.26 0.92 Pitt. #8
B. Volatile Matter (VM) (Moisture-Free Basis) of PSOC Coals PSOC no. rank 70 VM particle size, um lignite 1443D 40.6 20-30 lignite 41.6 1507D 20-30 1520D sub C 42.2 20-30 1445D 42.2 HVC 20-30 1451D HVA 20-30 34.40 1493D 1508D 1516D
% of tar
HVA MVB LVB
37.10 38.73 16.0 17.8
63-75 20-30 20-30 20-30
9: Aerosol train filler 10: Cyclone lraln !Mer
- 2 0
2 4 6 8 Collection stage
1 0 1 2
Figure 10. Tar distribution within the aerosol collection system. gases which are entrained in a relatively large volume of carrier gas. For example, a 1% yield of a particular gas a t a feed rate of 2 g/h into 20.6 L/min of argon would generate a concentration of 10 ppm of the gas. Most IR-active light gases have sufficient oscillator strengths to generate usable calibration curves down to the 1ppm level a t a path length of 43 m and resolution of 0.5 cm-'. Tar Sampling. The wide ranges of polarity and size properties of the molecular species that compose the tar yields lead one to consider the possibility that certain fractions of tars may be preferentially condensed or captured a t certain stages of the separation system. Preferential entrapment of fractions of the collective tar species would then lead to an unrepresentative deposition of hydrocarbons on the final filters. T o ascertain the mass fraction "representativeness" of the final filter tar samples, a detailed mass fraction analysis was performed of the tar mass collected at the various points of tar deposition in the separation system. Figure 10 displays the percent of total aerosol mass deposited a t each of the indicated deposition points of the separation system. Results are shown for coal samples of three different rank characteristics and, therefore, widely varying tar properties-HVA bituminous (PSOC 1451D), subbituminous C (PSOC 1520D), and low-volatile bituminous (PSOC 1516D). The results indicate the major fraction of the captured tar mass is found on the final filter stages of the separation-collection system, irrespective of the appreciably different chemical and volatility (to be published) characteristics of the rank-varying tars. The
-
Energy & Fuels, Vol. 3, No. 6, 1989 697
Chemical Characteristics of Tars 9 d
PSOC IISID, 20-3Op Feed
Peak Cas Temp B B O T
Y+ ?d
wm
u
z
* -
U m U 0
$? U 9
:-
O
I
Tar: Impactor l h i n Filter Sample
*+---l-----l---~--l-----
9000
3600
3200
2800
2900 ~ ~ ~ - 1 6 0 0 ~ ~ 8 0 0 ~ 1 9 0 0 WAVENUMBER
Figure 11. IR absorbance spectra of aerosol and cyclone filters and aerosol train stage deposits for PSOC 1451 devolatilization.
ratio of impactor train to cyclone train filter tar masses mirrors the mass ratio of the flow separation at the phase separator stage, indicating the tar aerosol closely follows the gas flow through the collection system. As a result, only about 15% of the train-deposited tar mass occurs in the prefilter stages. Figure 11displays the mid-Et absorbance spectra of the cyclone and impactor train filters as well as the species deposited on aerosol impactor plate 1for a given set of conditions. Within the limited sensitivity of the infrared absorbance technique, the average mass-normalized functional group characteristics of the filter tars are the same, but aerosol impactor plate 1 shows the presence of detectable amounts of mineral matter. The mineral matter could be due to coal particle fines produced during the feed or devolatilization process or small quantities of fly ash convected away from the particles during the tar evolution process. Aside from the presence of small but detectable mineral absorbance bands in stages 1-5, the absorbance spectra of samples from stages 1-8 indicate the deposited tar samples in each stage are structurally similar; that is they have absorbance bands of the Same shape and mass-normalized intensity throughout the aerosol train. Such consistency is observed for each of the coals investigated, although the chemical nature and deposited amounts of the tars vary substantially with rank characteristics of the parent coal and operating conditions. In summary, both mass distribution and chemical characterization of the species deposited within the sampling system components indicate the tars collected on the final filters are representative of the collective, captured tar mass. The prefilter impactor plates serve to separate the small amount of fines, that is, mineral matter/coal particles, from the entrained tar aerosol. Relatively small amounts of tars are deposited in these stages before the final filter stages. Size-Exclusion Chromatography (SEC). Size-exclusion chromatography (SEC) was used to determine relative molecular-weight distributions of the tar samples. Tar samples were prepared by dissolving several milligrams of tar in 3-4 mL of tetrahydrofuran and then filtering through a 0.5-fim filter to remove insoluble material. SEC was performed on 500- and 100 8, p-styragel (Waters) columns in series using tetrahydrofuran (THF) as the mobile phase at a flow rate of 1.0 mL/min. UV detection a t 330 nm was used, and a uniform response with molecular weight was assumed. SEC separates molecules according to their effective size in solution. Separation according to molecular weight occurs only to the extent that solute molecule size is related to weight.
0.55
v Fraction voiatiies (DAF)
v
2 0 - 3 0 microns
o,25
v
u
, ,
0.051
v
6 3 - 7 5 microns I
, 1
900 1100 1300 Reactor wall temperature, C
700
Figure 12. Mass fraction volatile yield vs reactor wall temperature ("C) for vortec-separated PSOC 1451D (600 ms).
Molecules of similar molecular weight but different structure, and therefore size, will elute at different retention volumes. The retention behavior is also complicated by absorption interactions of certain condensed ring aromatic compounds with the styrene-divinylbenzene column packing. A correlation technique using SEC measurements combined with additional characterization data to account for differences in the size-molecular weight relationship of samples has recently been presented by Rodgers, ets al. The molecular weights determined by using this technique showed the largest deviation from FIMS measurements for coal liquifaction products, most likely due to the heteroatom content of the samples. We are presently working on including heteroaromaticity into this type of correlation for use with coal tars. However, for the present study, the SEC columns were calibrated by using acenaphthene (MW = 154), polyisoprene (MW = loOO), and polystyrene standards (MW = 1800, 3600, 8500). The reported molecular weights are therefore actually weights for polystyrene of equivalent size. The molecular weights presented here are useful for relatiue comparisons only. A more comprehensive examination of the molecular-weight characteristics of the tar samples will be published elsewhere.
Results HVA Bituminous Coal. Relationship of Tar Production to Light Gas Yields. For the PSOC 1451D coal, a n Appalachian high-volatile bituminous coal, t h e ash
698 Energy & Fuels, Vol. 3, No. 6, 1989
Freihaut et al. Table IV. Relative Molecular Weight Characteristics of HVA Bituminous Tars (PSOC 1451D, 20-30-pm Feed Particles)' reactor wall max gas residence temp, "C temp, "C time, ms M,, M, 668 507 580 438 641 704 569 545 452 665 825 660 515 481 719 939 796 450 491 743 1058 895 410 518 801 1163 969 355 509 803 1241 1053 335 506 807
1.o
0.9 0.8
Relative yield
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 200
400
600
800
'Argon carrier gas. Tars = species collected on final filter of impactor train.
1000
Peak gas temperature, C Figure 13. Selected light gas yields vs peak gas temperature ("C) for PSOC 1451D (20-30-pm particles; 600 ms).
1.2 I
Table 111. Elemental Composition of Tars-Temperature Effects'
temp, "C reactor wall maxgas 668 704 825 939 1058 1163 1241
% C % H % N % (S + 0) A. HVA Bituminous Coal (PSOC 1451D) 20-30-pm Particles 507 84.05 6.07 1.64 8.24 569 84.07 5.94 1.67 8.32 660 84.37 5.86 1.68 8.09 84.46 5.93 1.76 7.83 796 84.62 5.55 1.69 8.14 895 85.22 5.40 1.73 7.65 969 85.55 5.27 1.74 7.44 1053 86.00 5.08 1.73 7.19
668 825
507 660
1241
1053
63-75-pm Particles 83.97 6.22 1.64 84.47 5.83 1.72 84.16 5.79 1.74 85.50 5.29 1.76
H/C Oa4 0.2
0.87 0.85 0.83 0.84 0.78 0.76 0.74 0.71
8.14 7.98 8.29 7.43
0.89 0.83 0.83 0.74
B. Subbituminous Coal (PSOC 1520D) 507 78.21 8.43 0.63 12.73 78.53 8.19 0.68 569 12.57 660 78.15 7.83 0.79 13.23 77.78 7.63 0.81 13.77 895 78.50 7.14 0.92 13.43 1053 78.92 6.72 0.98 13.32
1.29 1.25 1.20 1.17 1.09 1.02
C. Low-Volatile Bituminous (PSOC 1516D) 569 89.38 5.60 1.39 3.61 660 88.78 5.25 1.29 4.69 796 89.60 5.24 1.44 3.70 89.75 5.23 1.46 3.54 1058 895 90.12 5.08 1.48 3.29
0.75 0.71 0.70 0.70 0.68
668 704 825 1058 1241 704 825 939
a Tars = species collected on final filter of impactor train. Argon carrier gas. Average particle residence time varies as indicated in Table IV.
tracer volatile yields for two particle sizes are shown in Figure 12. As indicated, despite the factor of 3 difference in particle size, the reactor temperature sensitivity of the mass loss is very similar between the size cuts. Temperature calculations using the measured reactor heat-transfer characteristics indicate this should indeed be the case (see above). Figure 13 shows the relative gas yields, normalized with respect to maximum yields of each gas and plotted with respect to the peak reactor gas temperature, since gas-phase reactions of tars are thought to account for substantial fractions of "coal" pyrolysis gases. It is noted that significant increases in acetylene and hydrogen cyanide gases are not observed until peak gas temperatures of 700 "C are achieved. At lower gas temperatures, light gas yields are dominated by CHI, C2H4,CO, and H,O, but these gases account for only 10-15% of the total 0.2-0.25 particle mass fraction loss observed. For this coal, tar
-
I\
0 300 5 0 0 7 0 0 900 1100
Peak reactor gas temp., C Figure 14. -CH,- stretch absorbance (2920 cm-') vs gas temperature ("C)(20-30-pm particles): circle = PSOC 1451D; square = PSOC 1520D.
yields dominate the mass loss at these and lower gas/ particle temperatures. Appreciable tar yields, 0.10-0.15 mass fraction, are observed at gas/particle peak temperatures of 500 "C (wall temperatures of 660 "C). Hydrogen and Heteroatom Concentrations. Examination of the elemental composition and relative molecular weights of the tars as a function of characteristic temperature, reactor wall, or peak gas temperature, indicates that the tar evolution process resembles a distillation process (Tables IIIA, IV). The hydrogen-rich and somewhat heteroatom-poor (relative to the parent coal), lower molecular weight species devolatilize initially. As implied by the insensitivity of the total mass loss with particle size and as indicated by the temperature calculations, the elemental composition of the tars evolved from the two different particle sizes and for a particular stage of the evolution process, should be similar. As indicated in Table IIIA, the elemental composition of the tars produced from either the 20-30-pm feed or 63-75-pm feed are quite similar. IR Absorbance Characteristics. A distillation-like tar evolution process would also imply changing functionality characteristics of the samples with characteristic temperature. Figure 14 displays the relative -CH2- concentration (2920 cm-l) of the tars for increasing reactor temperature. Other functional group changes are also systematic. For example, low-temperature lignite and subbituminous tars have significant concentrations of carboxylic and carbonyl functional groups. Tars that have undergone extensive high-temperature (700 "C+)homogeneous decomposition reactions develop intense aromatic hydrogen bands while the aliphatic and oxygen-containing functional forms decrease (to be published). The polymethylene results shown here provide the most apparent tracer, indicating changes in tar characteristics with conditions. Tar Yields. Investigation of the tar evolution process on a heated-grid apparatus indicate potential tar yields
Chemical Characteristics of Tars
Energy & Fuels, Vol. 3, No. 6,1989 699
,..I
" 0.35 ' A
5
0.30
v
v
( - 1
L45ID
s
F 0.25
0.8
R
A
c
0.20
I
R
T
0.15
0.6
: 0.10 _ I
0 1520D
0.54 R 0.05 I
I
I
I
I
I
70
75
80
85
90
95
70
75
Figure 15. Tar yields vs % C in heated-grid and entrained-flow reactor. Conditions: 1 atm pressure; 63-75-pm particles in heated-grid reactor; 20-30-pm in EFR.
1.
PSOC 1520D
1.8
% H, DAF
O
A L
i
1.41 -
l
90
L
A
0
85
Figure 17. % (S + 0) in tar/% (S + 0) in coal vs coal rank (20-30-pm particle parent feed; 660 "C peak gas temperature).
2.2
;
80
% C ( 0 A F ) OF P A R E N T COAL
% C ( 0 A F ) OF P A R E N T COAL
1
!
70
7.0
'
PSOC 14450 "OC
. I
75
I
I
II
80
85
90
%C ( O A F ) C O A L
Figure 16. % H(tar)/ % H ( 4 ) vs coal rank (20-30-pm particles; peak reactor gas temperature 660 "C).
from zero-hold-time experiments are 0.25-0.30 of the parent coal mass on an ash-free basis for this Long hold time yields increase to 0.30-0.35 of the parent coal mass. The heated grid apparatus generates heating rates on the order of 1000 OC/s to peak temperatures of 1000 "C. Tar yields on the entrained-flow reactor can be determined by using a combination of ash and nitrogen balance considerations (to be published). Tar yields determined in this manner are comparable to those observed in the heated-grid apparatus for the same range of peak temperatures (see Figure 15). In summary, the results indicate the hydrogen level and in particular the polymethylene concentration of the tars systematically decrease with peak reactor temperature as the total tar yield increases. As the hydrogen level gradually decreases, the relative molecular-weight moments of the evolved tars increase to the point where the tar yield is maximized. Lower temperature tars contain less total oxygen and sulfur mass fractions than the parent coal on a dry, ash-free basis. Coal Rank Effects on Tar Characteristics. Hydrogen and Heteroatom Concentrations. It is desirable to determine the change in the nature of the tars produced with changes in the chemical characteristics of the parent coal, while keeping the heating conditions constant. The heating conditions that maximized the tar yields for the high volatile bituminous coal (825 "C wall, 660 "C gas temperature) while minimizing high-temperature gas-phase
1493D PSOC 1 4 5 1 0
4.0
400 600 800 1000 1200 Reactor peak gas temperature, C
Figure 18. % H in tars vs peak reactor gas temperature (2030-pm particles).
secondary reactions of all tars were selected. Lower heating rate data2p37,38 indicate that lower and higher rank coals may require a somewhat different peak gas temperature to maximize tar yield. However, no data indicates peak temperatures greater than 700 OC are needed to maximize tar yields from coal ranks ranging from lignite to lowvolatile bituminous. Figures 16 and 17 compare the hydrogen and sulfur + oxygen levels of the tar species to those observed in the parent coals. As the rank characteristics of the parent coal increase, the elemental composition of the tars becomes more like that of the parent coal. The ratios asymptote toward unity. Conversely, the lower the rank of coal, the more unlike in elemental compositions are the primary tars relative to the parent coal. Figure 18 displays the tar % H (daf) determined for all the coals investigated. The % H of some low-rank, low-temperature tar components produced in these conditions approaches 9% (daf). Table IIIB,C contains detailed elemental composition of subbituminous and low-volatile tars as a function of reactor conditions. IR Absorbance Characteristics. The most striking difference in the IR absorbance spectra of the low-rank coal tars relative to the higher ranks resides in the polymethylene absorbance region (2910-2940 cm-'). The lowrank coal tars generally display much greater absorbance levels in this region per unit mass of sample than the high-rank coal tars. Assuming the absorption coefficients of this band do not vary significantly with coal rank, such behavior implies increasing concentrations of -CH2- levels with decreasing coal rank. The -CH2- absorbance band correlates with the % H level in the tars (Figures 18-20).
Freihaut et al.
700 Energy & Fuels, Vol. 3, No. 6, 1989 1
S
W=Weight Average N=Number Average
l.oj
0 1 R 0 . 8 7 B I A I
m
700
0.64
l
E
.
i2 o 4 1
0 /
0 . 2 1
C
Model Compound Calibration
.
.
--A---A
100
M
1
I
I
I
I
70
75
80
85
90
95
% C(DAF) OF PARENT COAL 70
I
I
I
75
80
a5
%C ( D A F )
90
COAL
Figure 21. Relative number- and weight-average molecular weights of tars vs coal rank.
Figure 19. 2920-cm-’ absorbance of tars vs coal rank (20-30-pm particles; peak gas temperature 660 “C).
t\ 2
c
Absorbance 2920lcm
-
0
Subbituminous
\ocky
Mt. bituminous Central, Appalachian bituminous Medium, low volatile bituminous
o.6
O0.2 a4
1
Lignite
1 -
increasing coal rank
Figure 22. Primary tar-parent coal: structural comparisons. 5
6
7 0 %H (DAF) tars
9
Figure 20. Absorbance (2920 cm-’) vs % H (daf)in tars (2030-pm particles; peak gas temperature 660 “C).
Relative Molecular Weight Characteristics. The difference in structural characteristics of the low-temperature tars evolved from the lignite and subbituminous coals, relative to the bituminous coal tars, is again emphasized by comparing the molecular weight moments obtained by the size-exclusion chromatography technique. The data in Figure 21 indicate the tars evolved from the low-rank coals at a particular temperature have apparently greater molecular weight moments than the corresponding high-rank coal tars. In the most elementary sense, the polystyrene-calibrated SEC technique indicates only that aliphatic, hydrogen-rich low-rank coal tars are structurally “larger” than the more aromatic bituminous coal tars with respect to a length to weight ratio parameter. These results likely reflect variation in conformational aspects of the tars as a function of rank rather than weight size or polarity difference^.^^ The long-chain, low-polarity aliphatic tars characteristic of the low-rank coals are not efficiently retained by the size-exclusion column and, consequently, elute in the short retention times characteristic of heavy polystyrene standards. The molecular-“weight”moments appear to be more characteristic of relative molecular geometry than actual mass size when comparing tars from a range of coal ranks. To verify this structure-retention hypothesis, a series of long alkane and stearate molecules were injected in the SEC system. As expected, relatively small molecules by mass species but large by length to weight ratio gave apparently large equivalent polystyrene masses. Tar Yields. As noted above, tar yields in the entrained flow reactor can be determined by heated-grid investigations of the same coals or estimated directly from coupled
ash and nitrogen balance considerations (to be published). Figure 15 indicates transient tar yields determined for four of the coals by the heated-grid and entrained-flow reactor systems. Summary of Coal Rank Variation Results. The general observations with respect to the variations in low-temperature coal tars with rank characteristics of the primary coal are indicated in Figure 22; the lower the rank of the parent coal, the more unlike the parent coal are the low-temperature, primary tars. The dissimilarity appears primarily related to the aliphatic to aromatic hydrogen distribution and the oxygen-sulfur heteroatom content of the tars relative to the parent coal. Significant polymethylene concentrations variations are observed with variation in coal type. The so-called “dissimilarity” index of Figure 22 can obviously be quantified once a particular structural aspect is selected. For example, if % H (daf) is chosen, the dissimilarity index would be defined as % H,/ 9a Hmdand would have the form given in Figure 16. To obtain the same functional form for S + 0 comparisons, the dissimilarity index could be defined as (S + O)c,,d/(S + O)w Other structural indices could be defined: polymethylene absorbance ratios, carboxylic absorbance ratios, etc. Relationship of Results to Previous Investigations. HV Bituminous Tars. The “similarity”or “resemblance” reported between the “primary” tars and the corresponding parent coal s t r u c t ~ r e s appears ~ ~ ~ ~to~ be ’ ~based ~ ~ ~on~a~ qualitative, rather than quantitative, application of “similar” and “resemblance” to rather limited data. In addition, devolatilization conditions in which collected tars are considered to be “primary” are vastly different.s1 Orning and Griefer2’ collected tars evolved in vacuum conditions from a Pittsburgh seam coal heated on an electric plate to 510 “C. They compared the infrared absorbance of a thin film of the evolved tar to that of a thin
Chemical Characteristics of Tars section of vitrain (“anthraxylon”) of a coal sample from the same seam. The vitrain-rich absorbance spectrum was generated on a different grating instrument by a different investigator. Nevertheless, for the same tar film and vitrain section thicknesses, the transmission spectra of the two samples were “essentially identical” with respect to “shape”, “shoulders”, and “relative intensities” of absorption bands. However, the spectra are reported relative to the same thickness (20 pm). Since the different sample types undoubtedly had different mass densities, one concludes that band intensities reported on a mass-normalized basis would be very different. Indeed, the hydrogen and heteroatom mass contents of the tars are reported as 7.5% and 7.0%,whereas the hydrogen and heteroatom content of the vitrain are reported as 5.8% and 12.6%,respectively. Such large differences in elemental composition and associated IR-active functional groups should produce striking differences in a mass-normalized transmission or absorbance spectra, particularly in the strongly absorbing aliphatic H (2600-3000 cm-’) stretching and moderateintensity aromatic H bending (760-920 cm-’) regions. In short, the lack of difference in the transmission spectra of the two samples of such different elemental composition must be due to differences in techniques and assumptions employed in generating the coal spectra in one laboratory and the tar spectra in another and/or indicative of the quantitative insensitivity of the grating instruments utilized. Brown et a1.28also collected tar in a vacuum from a prime coking coal in a similar apparatus to that of Orning and Griefer, but at a peak temperature of 400 “C. Similar to Orning and Griefer, they also observed the volatile solids to have a much greater hydrogen composition (7.0% H) than the parent coal (5.0% H). Contrary to the results of Orning and Griefer, however, the absorbance bands of the tar solids do not show the same absorbance intensities as those observed for the parent coal. The tar samples display more intense aliphatic stretch and bending (1300-1400 cm-’) bands, aromatic hydrogen-bending bands, and carbonyl bands (1650-1725 cm-’), but lower broad band oxregion. Again, two ygen structures in the 1100-1300-~m-~ different sampling techniques were employed to obtain the spectra-thin film of tars vs alkali-metal halide pellets for the parent coal. Again no mass normalization factors are reported. Consequently, quantitative band intensity comparisons between tar and coal are not possible, but qualitative differences are apparent, as expected from the significant differences in elemental composition. Such instrumental and technique considerations, the awareness of the intrinsic broad-band nature of the infrared absorbance spectra of complex hydrocarbon mixtures, and the resulting limited sensitivity of the technique to detect small but important changes in structure from one sample to the next lead one to the conclusion that the term “similar” is more qualitative than quantitative. Consequently, in expressing reservations concerning the realism of his “average” coal molecule structure, Given33 notes “...My reason for believing that a molecule could be representative of the whole coal was chiefly the fairly close similarity of the infrared spectra of coals and their extracts. I now regard this as poor evidence. The “close similarity” of spectra means in fact merely that the bands are centered on more or less the same frequencies. These bands are quite broad, and the shapes, including widths and intensities, are not necessarily the same for coal and extract. In general terms, the same functional groups are present, but this does not mean much when one is talking about a complex mixture.”
Energy & Fuels, Vol. 3, No. 6, 1989 701 The elemental composition of the tars isolated in this atmospheric pressure investigation display the same type of temperature and rank variation in hydrogen content as that observed in other atmospheric pressure systems wherein secondary reactions of tars are minimized to the point of maximum tar evolution.‘J1~22~30 The relatively high polymethylene content of the low-temperature tars from a wide range of coal ranks and the dominance of such structures in low-rank tars before the onset of secondary reactions also agrees with previous i n v e s t i g a t i ~ n s . ~ ~ ~ ~ ~ J ~ Intraparticle or gas-phase reactions of primary tars are rapid at particle or gas temperatures of 700 “C and above, common operating temperatures of conventional entrained-flow reactors. Such homogeneous pyrolysis reactions are coincidental with the evolution of the heavier tar species from the devolatilizing particle itself. However, as Collin et note in a review of coal devolatilization work performed in Australia, the elemental composition of tars produced in some heated-grid devolatilization i n v e s t i g a t i o n ~of~ high-volatile ~~~ bituminous coals have considerably lower hydrogen concentrations than that observed in other studies. Perhaps more importantly, the tar elemental compositions were generally observed to be practically independent of the extent of tar evolution or reported final particle temperatures between 400 and 900 “C. Analogous to the hot plate work reported a b o ~ e ,tars ~ ~from , ~ ~these early heated-grid studies are reported to have IR absorbance bands “similar” to those of the parent coal. By use of the computerized advantage of FT-IR instrumentation, typical mass normalized spectra as a function of peak grid temperature are reported as a function of isothermal hold temperatures for tars evolved from a bituminous coalNin a heated-grid system. Relative to the spectra of either the parent coal or the tars evolved at the maximum tar yield (830 “C), the lowest hold temperature (450 “C) tar spectra indicates the presence of appreciable quantities of polymethylene functionality, in agreement with the observations of this investigation and others.11~28~30*31 However, no temperature dependence is apparent in the corresponding hydrogen elemental composition data for the tars from bituminous ~ 0 a l s . ~Again 1~ the authors appear to use the term “similar” in a qualitative sense with respect to either the elemental composition or infrared absorbance characteristics of the bituminous coals. No distinction is made between the collective tar mass at the point of maximum yield and the differential components evolved at intermediate extents of reactions. In comparing the tar properties, one must also note the rather different conditions in which the tars are formed. The heated-grid experiment^^^ noted above employed vacuum conditions whereas this investigation and the fluid-bed i n v e s t i g a t i ~ n s ” Jcited ~ ~ ~ were ~ * ~ ~performed in atmospheric pressure conditions. The vacuum conditions may promote the evolution of heavier, hydrogen-poor tar species at lower temperatures due to a reduction in interphase mass-transfer resistance. Ambient pressure is known to reduce total tar yields relative to vacuum cond i t i o n ~ . ~However, * ~ * ~ ~ the low temperature tars (300-600 “C) are always H rich relative to the parent coal and also show a systematic decrease in H concentration with increase in extent of devolatilization. As noted above, Orning and GriefeF and Brown et a1.%have demonstrated that vacuum heating bituminous coals to temperatures below that needed for maximum tar yield (600-650 “C) produce tars H rich relative to the parent coal. Low-Rank Coal Tars. Irrespective of heating conditions and extent of tar evolution, low-rank coal tars are
702 Energy & Fuels, Vol. 3, No. 6, 1989
Freihaut et al.
consistently observed to be H rich relative to the parent vary with these parameters. Lower rank coal tars generally coal. For example, using multiple runs with a heated-grid have greater concentrations of long-chain aliphatic species apparatus in 1atm of helium, Suuberglo observed a North than middle- and high-rank coals presumably due to the Dakota lignite (4.06% H) produced tars having 7.7% lower degree of coalification of the tar precursor structures H(daf). These experiments were conducted at thermoin the parent coal. Lignites for example have been utilized couple heating rates of approximately 1000 "C/s. At as a source of industrial waxes.& Coal samples from seams generally associated with middle- and high-rank coals of heating rates approaching 10000 "C/s in a fluidized-bed the Appalachian province may also display large concensystem, Australian brown and subbituminous coals7~22~30~42 trations of long-chain aliphatic species. The cannel coal evolve tars having hydrogen concentrations around 85% samples provide the most easily recognized example.% The (daf) for reactor temperatures of 450 "C to approximately maceral composition of a coal has been demonstrated to 7% at the temperature (625 "C) of maximum tar yield. have a significant influence on the rapid devolatilization These values compare to 4.9 %H in the parent brown coal mass of a coal and, in light of the above results, the and 6.4 %H in the subbituminous coal. Similar results tar evolution and secondary r e a ~ t i o n s . 4 Variations ~>~~ in were observed by Calkins using an identical reactor system the distribution of bonding interactions of the tar prebut a Texas lignite coal." In a slow heating rate expericursors within the maceral and intramaceral macromoment using a wire-basket approach, Campbell37has oblecular matrix of the parent coal sample are undoubtedly served the integrated sum of tars evolved from a Wyodak associated with variations in the chemical mix of tar presubbituminous coal during pyrolysis to 610 "C at 0.05 "C/s cursors found in the parent coal. Thermally "detaching" have a hydrogen composition of 9.6% compared to that the tar precursors from the "whole" coal matrix involves of the parent coal of 5.9%. disruption of bond strengths ranging from physical (van General Summary of Results. The entrained-flowder Waals, hydrogen bonding, donor-acceptor complexing) reactor investigation indicates that the phenomenology of to chemical covalent bond strengths. A distribution by size coal devolatilization and pyrolysis is similar for a wide and polarity indices of detached tar precursors is created range of coal ranks. The phenomenology is summarized at each stage of the intraparticle tar formation process. in Figure 1 (see Table I). Heavy hydrocarbons (ATP) are It is tempting to associate the aliphatic-rich material that initially detached (DTP) by physical (kphy)and chemical evolves early in the tar evolution process with physically (kchem)processes within the coal particle at relatively low associated waxes and resins present in weakly coalified temperatures (300-450 "C). Further heating results in the low-rank coals (liptinites) or wax- and resin-containing extraparticle evolution (k,) of some detached heavy hymacerals (exinites) in the middle- and high-rank coals.33 drocarbons (tars), the onset of light gas production Some evidence indicating this may be the case has been (450-650 "C) by intraparticle pyrolysis reactions, and the pre~ented,4~9~~ although more detailed investigations are detachment of more tar precursors. The low-temperature warranted. In addition, high-volatile bituminous coals light gases consist mainly of CH4 and higher alkanes, CO, appear to contain significant fractions of aromatic fractions COz, HzO, and some CzH4. The higher alkanes probably that are physically associated with the parent coal maconsist of olefin/paraffin pairs" as is evidenced by the trixM and whose intraparticle detachment process is best broad-band absorption observed in the IR absorbance described as a melting proce~s.'~Whether a significant spectra of the low-temperature light gases. The absolute fraction of these species can evolve from a particle without quantities of these species vary with coal type as do the pyrolytic modifications that lower the size and polarity absolute yields of "primary" tars and their chemical indices, thereby raising their vapor-pressure properties, characteristics. Gas and particle temperatures greater than remains to be demonstrated. Significant fractions of 700 "C preferentially produce C2H2,HCN, CO, and C2H4 middleand high-rank tar precursors are also formed by and undoubtedly Hz, which is not measured in this system. thermal degradation of covalent bond structures within On a mass basis, the tar species dominate the initial the coal m a t r i ~ . ~ ~ - ~ O particle weight loss for subbituminous and higher ranks and can represent substantial fractions of low-rank coal Concluding Remarks mass loss. For any given extent of tar evolution, the lower the rank Irrespective of variations in the mix of intraparticle of coal, the more dissimilar the evolved tars are to the physical and pyrolytic degradation processes and the parent coal. Primary low-rank coal tars consist mainly of chemical characteristics of the evolved tars, the temperalong-chain aliphatic species with significant levels of asture-resolved transient tar evolution process indicates a sociated carboxylic and carbonyl groups. As the rank index common tar evolution phenomenology is followed by a of the feed coal is increased from lignites, the primary tars wide range of coal types. The distillation-liketar evolution structural characteristics approach that of the parent coal process implies heat- and mass-transport factors play a with respect to aliphatic and aromatic hydrogen ratio, significant role in the temperature-resolved, heavy hypercent hydrogen, and percent heteroatom content. Pridrocarbon evolution process. mary tars from bituminous coals have structural characThe chemical characteristics of the parent coal deterteristics more reflective of, but never identical with, the mines the chemical nature of the primary tars and poparent coal. Corresponding observations can be made with tential yields. Heat- and mass-transport parameters of the respect to tars evolved for a given coal but with respect devolatilization system determine the extent of evolution, to extent of tar evolution. That is, the tars evolved initially the timetemperature-resolved evolution pattern, and the in the evolution process are more unlike the parent coal degree of secondary reaction modification of the primary than the integrated sum of tars at the point of maximum tars. Generalizations based on the "similarity" to the tar yield. The earlier in the tar evolution process, the parent coal of the integrated sum of the evolved tar mass greater the aliphatic hydrogen concentration of the tars. from middle-rank coals are noted to be more qualitative Relationship of Tar Evolution Phenomenology to than quantitative and of limited applicability to the deCoal Structure. The chemical nature and yields of tars velopment of a rank-dependent, devolatilization model. varies significantly with coal rank and geologic origin beThe relative importance of interphase and intraphase cause the mix of tar precursors found in the parent coal mass transport in evolution rates may be ascertained in-
Energy & Fuels 1989,3, 703-706 directly by investigating the effects of particle size on tar yields and characteristics. Within the particle size range employed in this investigation for a HVA bituminous coal, intraphase transport considerations appear to have a second-order effect on tar yields and properties relative to interphase considerations. Of course, changes in maceral comp~sition~ with ~ ~ 'particle size must be considered. A more extensive investigation is being conducted to affirm this phenomenology and to test its applicability to the range of coals investigated in this investigation. The questions of similarity in kinetic parameters among a range of coal types and whether chemical or transport phenomena dominate a stage of tar-evolution processes require a multireactor approach in which the phases of tar
703
formation and evolution can be deconvoluted.
Acknowledgment. We are grateful to the U.S.Department of Energy, Pittsburgh Energy Technology Center, for coal samples provided in this investigation. In addition some of the results presented were obtained by using funding provided by Grant DE-AC22-84PC70768. The technical expertise of Gerald Wagner and Dave Santos is present throughout the work performed in this investigation. Brian Knight was responsible for the implementation and practical design of the phase-separation system for this particular reactor. Registry NO.CH4,74-82-8;CzH4,74-85-1;C2H2,74-86-2;CO, 630-08-0; HCN, 74-90-8.
Effect of Acid Treatment Atmosphere on the Thermoplasticity of a Low-Volatile Bituminous Coal Chun W. Lee* and Robert G. Jenkins? Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received July 7,1989
A strongly caking low-volatile bituminous coal was treated with HC1 and HF under air and N2 atmospheres. The thermoplastic properties of the raw and treated coals were examined in N2 (1atm) by use of a microdilatometer. It was found that coals treated with the acids in air exhibited the lowest maximum swelling parameter (V8,vol '%) and weight loss during the dilatometry measurements. The above measured values were significantly restored for the coal acid treated under NP. The results indicate that acid treatment of this coal in air promotes low-temperature air oxidation of the coal, consequently reducing its thermoplastic properties. It is shown that the acids may be important in catalyzing low-temperature coal oxidation. The significance of the acid treatment atmosphere and its implication for the reaction behavior of acid-treated coals are also discussed.
Introduction An enormous body of literature has clearly indicated that the presence of mineral matter can have profound influences on almost all aspects of coal utilization and experimentation. I t is beyond the scope of this paper to review the literature, but it suffices to say that mineral matter has been implicated in the catalysis of pyrolysis, gasification, liquefaction, and carbonization reactions. Treatment of coals with hydrochloric (HC1) and hydrofluoric (HF) acids h the method most commonly employed for the removal of mineral matter from coals for studying the effect of mineral matter on the reaction chemistry of coal aqd coal chars. This paper examines the thermoplastic properties of a highly caking bituminous coal demineralized under air and N2 atmospheres. Extraction of coal with concentrated hydrofluoric acid removes major minerals in coal quantitatively. Additional extraction with hydrochloric acid before and after hydrofluoric acid extraction is used to obviate the formation of insoluble calcium fluoride. Treatment of coals with HCl and HF can have a profound influence on the reaction behavior of the treated coals. Significant reduction in the
dilatometric properties of caking Polish coals resulted from demineralization using hydrochloric and hydrofluoric acids has been reported.' Coal demineralization is usually carried our in air. It should be noted that the possibility of oxidation during acid treatment was ruled out by Bishop and Ward.2 Their conclusion was drawn based on a finding that the carbon and hydrogen contents for a low rank vitrain treated with HC1 and HF under inert and air atmospheres were essentially the same. On the basis of this conclusion Wachowska et al.' also suggested that it is unlikely that the reduction observed in thermoplasticity of acid-treated coals is caused by oxidation. However, enhanced air oxidation of coal caused by acid demineralization was reported by Kister et al.3 They observed, by IT-IFt and UV fluorescence,that both air oxidation and demineralization of a French subbituminous coal resulted in the loss of aliphatic groups attached to polycyclic aromatics with three or four rings and the appearance of oxygenated groups, mainly carbonyls and carboxylic acids. The loss of aliphatic carbon during air oxidation and acid demineralization of coal was also observed by high-resolution solid-state 13C NMR.4
'Current address: College of Engineering, The University of Cincinnati, Cincinnati, OH 45221.
(1) Wachowska, H.; Pawlak, W.; Andrzejak, A. Fuel 1983,62,85-88. (2) Bishop, M.; Ward, D. L. Fuel 1958, 37, 191-200. (3) Kister, J.; Guilino, M.; Mille, G.; Dou,H. Fuel 1988,67, 10761082.
0887-0624/89/ 2503-0703$01.50/0 0 1989 American Chemical Society
