Flash pyrolysis of Montana Rosebud coal. 2. Experimental data and

Aug 31, 1987 - The flash pyrolysis of Montana Rosebud subbituminous coal was studied in a 7.62-cm-i.d., down-flow entrained reactor with a reaction ...
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Energy & Fuels 1988,2,827-833

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Figure 10. Regression model predictions for sulfur retention in

char at various reaction temperatures and 2.23-MPa pressure. Plotted experimental data in the order of increasing gas residence time are from test no. 3A331, the average of the 3A333 tests,and from test no. 3A335. to temperature. Apparently a low-sulfur char is initially formed that increases in sulfur content with residence time. This implies that a large percentage of the sulfur in the coal is quickly volatilized into the extraparticle environment and is then partially returned to the char, most likely via back-reaction of the hydrogen sulfide as discussed earlier in conjunction with Figure 8. Since hydrogen sulfide forms directly from pyrite or indirectly from the decomposition of various organic sulfur species, the trends shown in the figure may predominantly reflect the kinetics of either the hydrogen sulfide back reaction or organic sulfur compound cracking reactions. The suggested asymptote at approximately 75% sulfur retention probably reflects either an approach to equilibrium or a saturation of available reaction sites for the hydrogen sulfide as the char anneals.

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Conclusions A composite factorial experimental design and response surface methods were successfully applied to study the flash pyrolysis of Montana Rosebud coal. Statistically significant regression models, which proved to be complementary and internally consistent, were used to predict product gas and char trends. Temperature was generally shown to be the most important parameter. This is consistent with the present global view of pyrolysis and demonstrates that this trend prevails over wide ranges of temperature, pressure, and gas residence times that have not been investigated before in combination. At the higher temperatures, total gas yield decreased and char yield and carbon retention correspondingly increased, indicating deposition from the cracking of volatiles. The predicted maximum gas yield was slightly less than the average volatile matter content of the coal, indicating that there was no net enhancement of volatile release due to high heating rates and small particle sizes with this coal at the conditions studied. Pressure effects were small, and the few variables that exhibited residence time dependencies tended to approach asymptotic values at the longer residence times, indicating approach to equilibriums or yield limits. These statistically substantiated experimental trends provide a data base for validating and/or improving the predictive capabilities of comprehensive, semiempirical models, which was outside the scope of this paper. Acknowledgment. This work was sponsored by the Surface Coal Gasification Program of the US. Department of Energy. NO.CO, 630-08-0;COP, 12438-9;Hz, 1333-740;CH,, 74-82-8;HZC=CHZ, 74-85-1;HZS, 13465-07-1;C&, 71-43-2;02, 7782-44-7; Nz, 7727-37-9; S, 7704-34-9.

Flash Pyrolysis of Montana Rosebud Coal. 2. Experimental Data and Response Surface Model Predictions for Polycyclic Aromatic Compounds Larry A. Bissett* and Steven C. Lamey Morgantown Energy Technology Center, United States Department of Energy, Morgantown, West Virginia 26507-0880 Received August 31, 1987. Revised Manuscript Received July 11, 1988 The flash pyrolysis of Montana Rosebud subbituminous coal was studied in a 7.62-cm-id., down-flow entrained reactor with a reaction environment of 75 vol % nitrogen and 25 vol ?& argon. A threevariable, composite factorial experimental design was used to investigate the effects of reaction temperature, pressure, and gas residence time over the ranges 1089-1644 K, 0.79-6.31 MPa, and 2.19-10.00 s, respectively. These conditions promoted the substantial occurrence of secondary volatiles reactions, and as a result, the net recovery of polycyclic aromatic compounds (PAC’s) was small and the extent of alkylation and hydroxylation of the PAC’s was low. Many of the principal compounds were the same as those reported in tars from carbonization processes that operated at temperatures in the lower range of this study; however, these results showed that only three- and four-ring compounds tend to persist at higher temperatures. Quadratic response surface analyses were used to identify additional experimental trends that may aid the development of semiempiricalmodels not considered here.

Introduction This is the second paper in a series dealing with entrained-flow, flash pyrolysis of Montana Rosebud subbituminous coal over wide ranges of reaction temperature, pressure, and residence time. In the first paper,l experi-

mental data and regression model predictions pertaining to product gas and char c h a ” i s t i c s were presented and discussed. The present paper is a similar treatment of the (1) Part 1: Bissett, L.A. Energy Fuels, preceding paper in thia issue.

This article not subject to U.S.Copyright. Published 1988 by the American Chemical Society

Bissett and Lamey

828 Energy & Fuels, Vol. 2, No. 6, 1988

small quantities of polycyclic aromatic compounds (PAC’s) recovered in the same test series. Tars are produced during the devolatilization of coal and can be a major byproduct, depending on the coal used and process conditions and configuration. Knowledge of tar characteristics is important not only from environmental, health, and commercial production aspects, but also from the viewpoint of understanding and eventually modeling devolatilization and secondary reactions. As described by McNeil,2 crude high-temperature tar, which is most applicable to this paper, is typically an extremely complex mixture containing hundreds of compounds and is dark brown to black in appearance and characterized by an unpleasant smell. The main components are cyclic aromatic hydrocarbons ranging from single-ring compounds to polycyclic aromatic hydrocarbons (PAHs) with possibly as many as 20 or more rings. Heterocyclic aromatic compounds, containing oxygen, nitrogen, and sulfur atoms, and paraffinic hydrocarbons may also be present. The aromatic structures are present in both unsubstituted and substituted forms. The main substituent groups are methyl,, ethyl, and hydroxyl. It has been noted2 that compounds containing up to four rings are generally fully condensed and that more complex compounds tend to have branched structures. A considerable body of data exists showing how the yields of tar obtained from rapid coal pyrolysis are affected by various parameter^.^ However, the amount of published data pertaining to the compositions of these tars seems to be considerably less. Fillo et al.4 conducted a systematic study on the effects of process conditions on the chemical composition of tars produced during rapid devolatilization of pulverized lignite and subbituminous coal. The results showed that the tar was primarily a mixture of aromatic compounds and that the number of rings dropped from a range of four or more to a range of one to three as the planned test temperature varied from 873 to 1273 K. The data also indicated that the large components tended to decompose to compounds with three to four rings and that the ring-bound oxygen components decomposed to phenolic compounds. As temperature increased, the extent of alkylation and hydroxylation decreased markedly. Grand’ry6 reported the main components of four high-temperature tars produced at carbonization temperatures ranging from 1073 to 1373 K. These data showed that as temperature increased, alkyl substitution and phenolic content decreased and the tar base fraction remained relatively unchanged. The number of different compounds reported also tended to decrease, and the relative percentage of hard pitch and tar increased with carbonization temperature. Further indications of the general effects of process conditions on the nature of the tars can be obtained from commercial carbonization data and supportive investigations. McNei12pointed out that commercial low-temperature tars, verticle retort tars, and coke oven tars form a progression in which yield decreases, aromaticity increases, and the proportions of phenols, paraffins, and substituted aromatics decrease. In addition to changes resulting from temperature increases, this progression was partly attrib(2) McNeil, D. In Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: New York, 1981; Second Supplementary Volume, Chapter 17. (3) Howard,J. B. In Chemistry of Coal Utilization;Elliott, M. A., Ed.; Wiley: New York, 1981; Second Supplementary Volume, Chapter 12. (4) Fillo, J. P.; Stetter,J. R.; Stamoudis, V. C.; Vance, S. W. DOE/ ET/14746-11(DE83016417);US.Department of Energy: Springfield, VA, 1983. (5) Grand’ry,E.Presented at the Colloquiumon High Temperature Gas Chromatography, Essen, FRG, 1959.

uted to differences in the occurrence of secondary reactions, such as dehydration of phenols and the dealkylation of aromatic hydrocarbons and heterocyclic ring compounds. In laboratory tests to simulate commercial carbonization operations, Walters et al.6 reported the results of carbonizing over 500 coals by a standard procedure at temperatures of 773-1273 K. As summarized by McNeil? these results showed that the acid fraction of the tar decreased with carbonizing temperature and increased with the oxygen content of the coal and that the base fraction depended mostly on the nitrogen content of the coal and was fairly independent of temperature. From reported data, it is evident that tars produced from various coals have many chemical similarities. However, thermal history and the extent of secondary reactions can also result in significant yield and compositional differences. The results reported here on the nature of the PAC’s produced in pyrolysis tests cover a wide range of conditions up to and including those that result in nearly the complete destruction of these compounds and appear to fill a gap in the existing data base in this regard.

Experimental Section The tests were conducted in a 7.62-cm-i.d.,down-flow entrained reactor that turbulently combined highly preheated nitrogen with argon-conveyedcoal and maintained isothermal conditions as the resulting mixture flowed in a laminarlike manner through a 1.22-m-long,alumina reaction tube. The Montana Rosebud subbituminous coal used in the tests was a 200 X 270 mesh (U.S. Standard) fraction with a mean particle diameter of 57 pm. On a weight percent, as-fed basis, the average composition of the coal was 64.1% carbon, 4.4% hydrogen, 17.9% oxygen, 1.1%nitrogen, 1.0%sulfur, 10.4% ash, and 1.0% moisture. The average volatile matter content was 40.6 wt %. Fifteen tests under different experimentalconditions and three replicate tests to determine normal variation were conducted to execute a composite factorial experimentaldesign. Test conditions covered 1089-1644-K reaction temperature, 0.79-6.31-MPa pressure, and 2.19-10.00-s gas residence time. Details of the equipment, test procedures, and experimental design were provided in part 1 of this series.’ PAC’s were recovered in the experimentalsystem where the total product gas flow was indirectly cooled to nominally 300 K and where approximately 5-10% of the product gas flow was passed through a cold trap at 233 K. The condensates were t y p i d y recovered on the day after a teat. The recovered material from the 300 K condenser was mixed with toluene and subjected to a Dean and Stark distillation to remove water. Following this and filtration,the remaining solution was evaporated to recover the toluene-soluble organic material. The cold trap was given successive washings with methanol, toluene, chloroform, and methylene chloride, and the resulting solution was evaporated to recover the dissolved organic material. After weigh back, the PAC’s were chemically characterized with a Hewlett-Packard 5985B gas chromatograph/mass spectrometer set to scan a range of 40400 m u with scan times up to 3 s. The gas chromatograph was equipped with a silicon-coated,fused-silica, capillary column, which was connected directly to the ion source of the quadrapole mass spectrometer.

Results Table I presents the test numbers, the associated test conditions, and the weight fractional yield of PAC’s with respect to coal on a dry, ash-free basis. The yields here and in subsequent tables include the recovery of PAC’s at both locations with the material recovered in the cold trap appropriately proportioned into the total. The test numbers are related to the composite factorial design as explained in part 1 of this series.l The table shows that (6) Walters, J. G.;Ortuglio, C.; Glaenzer, J. Bull.-US., Bur. Mines 1967, No. 643.

Flash Pyrolysis of Montana Rosebud Coal Table I. Weight Fractional Yield of PAC’s yield, pressure, gas residence kg/kg of test no. temp, K MPa time, s mafcoal 1089 2.23 4.68 0.0178 3A133 3A222 3.20 0.0144 1189 1.33 3A224 1.33 6.84 0.0040 1189 3A242 3.76 3.20 0.0125 1189 1189 3A244 3.76 6.84 0.0108 1310 3A313 0.79 4.68 0.0014 1310 3A331 2.23 2.19 0.0095 1310 2.23 4.68 0.0060 3A333 2.23 4.68 3A333-2 1310 0.0037 1310 2.23 4.68 0.0116 3A333-3 1310 2.23 4.68 0.0123 3A333-4 2.23 0.0042 10.00 1310 3A335 0.0024 1310 6.31 3A353 4.68 1458 1.33 0.0001 3A422 3.20 1458 1.33 0.0002 3A424 6.84 3.76 o.Ooo1 3.20 3A442-1 1458 3.76 o.Ooo1 6.84 3A444 1458 1644 2.23 4.68 0.0003 3A533

the substantial occurrence of secondary reactions at these reaction conditions resulted in small yields of PAC’s that varied from nominally 1.8 w t % of the products at the lowest temperature to practically none at the higher temperatures. As shown by Table I, there were relatively large variations in yield in the replicate tests (e.g., test no. 3A333-2) at the center point of the experimental design. This was most likely due to recovery errors associated with collecting very small quantities of material in the relatively large receiver used for the 300 K condenser. This receiver was sized for testa in which large quantities of condensed steam would accumulate. It was impractical during the test series reported here to solvent wash the receiver after each test. At the completion of all pyrolysis tests, which included more than what is reported here, the receiver was disassembled and washed repeatedly with toluene. The amount of material recovered indicated that overall about 25% of the PAC‘s had been retained on the vessel wall. Thus some recovery errors were present and some sample cross-contamination may have occurred. However, the compositional differences of the material recovered in the receiver during the test series and the way the material solidified on the vessel wall suggested that the extent of cross-contamination was probably small. In a later long duration test to produce significant amounts of char at the center point condition, 13 times more coal was fed to the system than the average amount fed per center point test for the series reported here. The recovery errors in the long duration test would presumably be proportionally smaller due to the greater quantity of material that was collected. The weight fractional yield of PAC’s in the long duration test was only 7.5% higher than the average yield for the center point tests reported here. This was considered close agreement in light of the relative size of the recovery equipment. No material was ever recovered in the condenser receiver at test temperatures of 1458 K and higher. Since the cold trap was always solvent washed after each test, the recoveries for those tests were probably subject to smaller errors and there was little opportunity for sample cross-contamination. The 20 most abundant and consistently detected PAC’s in these tests are listed in Table 11. About half of these were also listed as main components in the tars reported by Grand’rys and Ferrand.’ Overall, approximately 60 (7) Ferrand, R. Separation Immediate et Chromatographie; Groupe dAvancement des Methodes Spectroscopiques: Paris,1962.

Energy & Fuels, Vol. 2, No. 6,1988 829 Table 11. Principal PAC’s PAC mol w t naphthalene 128 134 benzothiophene 142 methylnaphthalene 152 acenaphthylene fluorene 166 dibenzofuran 168 178 phenanthrene anthracene 178 dibenzothiophene 184 fluoranthene 202 202 PYene phenylnaphthalene 204 chrysene/ triphenylene 228 252 benzo[k]fluoranthene 252 benz[e]ppene 252 benz[a]pyrene 252 perylene 276 indeno[1,2,3-cd]fluoranthene indeno[1,2,3-cd]pyrene 276 276 benzo[ghi]perylene

mol formula

no. of rings 2 2 2 3 3 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6

different compounds were detected during this test series, ranging from phenol with a molecular weight of 94 to an unknown PAH with a molecular weight of 302. The number of compounds detected generally decreased with temperature. Nearly 50 compounds were detected in many of the lower temperature tests, while only 12 PAC’s were detected in the test at the highest temperature. The range of average molecular weights for the PAC’s recovered in the condenser receiver was 186-212 and for the PAC’s recovered in the cold trap was 133-186. When there was sufficient material recovered, elemental and simulated boiling point analyses were conducted on some of the samples recovered in the condenser receiver. The atomic hydrogen-carbon ratios of these varied from 0.71-0.83, and the boiling point temperature range from initial to end points was 434-819 K. From these limited data, there seemed to be a slight tendency for the hydrogen-carbon ratios to decrease and boiling points to increase with reaction temperature. Further details of all the analyses presented above are available elsewhere.8 Fractionation of some of the samples showed that most of the components were in the neutral fraction. The base fraction was less than 5% by weight of the samples and the acid fraction was less than 2%. As reported by Stamoudis et al.? the acid fractions of tars from several pilot-scale, fixed-bed gasifiers’OJ1and from a liquefaction process12were significantly higher, being in the range of 10-30%. However, like the samples reported here, the base fractions of these other tars were low and most of the components were generally concentrated in the neutral fraction. In contrast though, these other tars contained alkanes and alkenes, and the extent of alkylation and hydroxylation and the amounts of heterocyclic aromatic compounds were higher. Since these other tars were produced at much lower temperatures, the differences with (8)Bissett, L.A.Morgantown Energy Technol. Cent. [Rep.],DOE1 METC (US.Dep. Energy) 1986,DOE/METC-86/2026 (DE86006618). (9)Stamoudis, V. C.; Haugen, D. A.; Heffley, D. C.; Picel, K. C. Proceedings of the 24th Hanford Life Sciences Symposium: Health and Environmental Research on Complex Organic Mixtures; U.S.Department of Energy: Oak Ridge, TN, 1985. (10)Wilzbach, K.E.;Stetter, J. R.; Reilly Jr., C. A.; Willson, W. G. Argonne National Laboratory Report No. ANLISER-1;NTIS: Spring-

field. VA., 1982. ~ ~ (il)Stamoudis, V. C. Argonne National Laboratory Report No. ANLISER-6; NTIS: Springfield, VA, 1986. (12)Griest, W. H.;Guerin, M. R.; Coffin, D. L. Health Effects Investigation of Oil Shale Development; Ann Arbor Science Publishers: Ann Arbor, MI, 1981.

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test no. 3A133 3A222 3A224 3A242 3A244 3A313 3A331 3A333 3A333-2

test no. 3A133 3A222 3A224 3A242 3A244 3A313 3A331 36333 3A333-2

test no. 3A133 3A222 3A224 3A242 3A244 3A313 3A331 3A333 3A333-2

test no. 3A133 3A222 3A224 3A242 36244 3A313 3A331 3A333 3A333-2

Bissett and Lamey

Table 111. Relative Weight Fractional Yields of Principal PAC’s yield, lo3 kg/kg of maf coal yield, lo3 kg/kg of maf coal benzomethylacebenzomethylacenaphthathionaphtha- naphthynaphthathionaphtha- naphthylene phene lene lene fluorene test no. lene phene lene lene fluorene 0.934 0.101 0.887 0.969 0.490 3A333-3 0.620 0.027 0.069 0.854 0.348 0.331 3A333-4 0.258 0.116 0.946 0.381 0.260 0.591 0.768 1.750 0.096 0.427 0.224 3A335 0.482 0.040 0.442 0.163 1.061 0.033 0.283 0.031 0.527 0.357 3A353 0.426 0 0.004 0.110 0.063 0.096 0.279 0.554 0.611 0.318 3A422 0.026 0.110 0.001 0 0.021 0.010 0.528 0 0.035 3A424 0.004 0.813 0.398 0.013 0.077 0 0.002 0.031 0.018 0.206 3A442-1 0 0.499 0 0 0.003 0.006 0.145 0.070 1.165 0.347 0.268 3A444 0 0 0 0.002 0.016 0 0.049 0.532 0.417 0.173 3A533 0 0 0 0 0.016 0.018 0.036 0.547 yield, lo3 kg/kg of maf coal yield, lo3kg/kg of maf coal dibenzodibenzodibenzo- phenananthrathiofluroandibenzo- phenan- anthrathiofluroanfuran threne cene phene thene test no. furan threne cene phene thene 0.212 0.798 1.337 3A333-3 0.490 0.106 0.296 1.398 0.837 0.154 0.740 0.870 0.541 3A333-4 0.246 0.512 0.115 0.113 0.757 0.569 0.526 0.239 3A335 0.140 0.266 0.498 0.261 0.079 0.219 0.155 0.051 0.184 0.171 0.075 0.518 3A353 0.039 0.159 0.097 0.012 0.112 0.610 0.577 0.270 3A422 0.012 0.035 0 0 0 0.079 0.984 0.212 0.983 0.362 3A424 0.016 0.027 0 0 0.015 0.006 0.075 0 0.003 0.020 0.065 0.509 3A442-1 0.002 0.021 0.012 0.003 0.015 0.493 0.258 0.137 0.044 0.393 3A444 0.010 0.023 0.014 0.005 0.017 0.502 0.214 0.225 3A533 0.013 0.095 0.024 0.012 0.036 0.036 0.182 0.129 0.303 0.152 yield, lo3 kg/kg of maf coal yield, lo3 kg/kg of maf coal chryschrysphenylene/tribenzo[k]phenylene/tribenzo[k]naphthaphenyfluoranbenz[e]naphthaphenyfluoranbenz[e]Dvrene lene lene thene DyTene test no. pvrene lene lene thene Dvrene 0 3A333-3 0.747 0.144 0.912 0.772 0.416 0.923 0.880 1.356 0.218 0.456 0.418 3A333-4 0.750 0.142 0.895 0.758 0.408 0.234 0.423 0.538 0.180 0.092 0.268 0 3A335 0.206 0.109 0.178 0.032 0 0 0.377 3A353 0.116 0.030 0.144 0.407 0.687 0.066 0.073 0.049 0.529 0.228 3A422 0 0 0.344 0.575 0 0 0 0.922 0.140 0 3A424 0.002 0.003 0 0 0 0 0 0 0 0.004 0.294 3A442-1 0.013 0 0 0.508 0.109 0.293 0.383 0 0.006 0.190 3A444 0.016 0 0 0.066 0.417 0.353 0 0.382 3A533 0.021 0.004 0.033 0.208 0.176 0.095 0 0 0 0.182 yield, lo3 kg/kg of maf coal yield, lo3 kg/kg of maf coal indenoindeno[1,2,3-cd]- indenobenzo[1,2,3-cd]- indenobenzobenz[a]- perfluoran- [1,2,3-cd][ghilbenz[o]- perfluoran- [1,2,3-cd][ghilpyrene ylene thene pyrene perylene test no. pyrene ylene thene pyrene perylene 0 0.194 0.271 0.241 0.171 3A333-3 0 0.298 0.115 0.376 0.296 3A333-4 0 0.474 0.386 0.401 0.410 0.369 0.293 0.113 0.369 0.291 0 0 0 0 0 3A335 0.129 0.087 0.107 0.101 0.071 0.289 0.193 0.388 0.386 3A353 0.078 0.040 0.054 0.050 0.036 0.438 0 0 0 0.146 0.080 3A422 0 0 0.108 0.153 0.334 3A424 0 0 0 0 0 0 0 0 0 0 0.206 0.318 0.259 0.176 3A442-1 0 0 0 0 0 0.400 3A444 0 0 0 0 0 0.136 0.172 0.135 0.052 0 0 0 0 3A533 0.086 0.068 0.026 0 0 0 0.068

the PAC’s recovered in the tests reported here are consistent with trends identified by others.44 Table I11 gives the weight fractional yields of the principle PAC’s recovered in these tests. These yields and the data reported in the following tables are expressed on a relative basis because the background ion counts of the gas chromatograph/mass spectrometer were necessarily factored out when the compositions of the recovered materials were determined. With the tests arranged in order of increasing temperature level, the table shows that nearly all the principal components were present in the lower temperature tests but that only the intermediate compounds with three and four rings generally remained at the higher temperatures. Apparently the sequence and kinetics of secondary cracking reactions and thermal stability considerationsresulted in the broad spectrum of one-

to six-ring PAC’s produced in the lower temperature testa to be reduced to a smaller intermediate spectrum of threeand four-ring compounds at the higher temperatures. Further insights can be gained by combining the results from the various tests at the same temperature level. This is a reasonable exercise since the studies cited earlier showed major temperature effects and since temperature was generally shown to have the most effect on the product gas and char characteristics in these tests.l The results of doing this are presented in Tables IV and V. For Table IV, the relative weight percentage yield of a compound at a particular temperature was calculated by summing the relative yields of that compound in all the tests conducted at the particular temperature level and then dividing by the total PAC yield for those tests. Thus for example, acenaphthalene was 7.2% of the total PAC yield in the

Flash Pyrolysis of Montana Rosebud Coal Table IV. Relative Weight Percentage Yield of Principal PAC’s to Total PAC Yield Der Reaction Temperature Level re1 yield for temp level PAC 1089 K 1189 K 1310 K 1458K 1644 K naphthalene 5.2 9.3 10.5 1.1 0 0 0 benzothiophene 0.6 0.6 1.7 2.2 5.0 0.5 0 methylnaphthalene 0.7 5.4 11.9 6.5 3.7 acenaphthylene 7.2 2.7 10.5 12.8 2.9 fluorene 3.2 2.1 1.7 8.4 4.8 dibenzofuran 2.2 6.4 7.8 21.9 33.9 phenanthrene 6.8 4.1 4.7 anthracene 3.7 8.7 8.6 dibenzothiophene 0.8 0.9 2.7 4.3 0.8 5.4 7.5 7.3 12.7 fluoranthene 5.6 5.2 7.6 6.4 7.6 5.6 Pyrene 1.2 1.3 1.4 1.2 phenylnaphthalene 2.9 5.2 2.8 0 0 6.0 chrysene) triphenylene 4.1 4.9 5.3 0 0 benzo [k]fluoranthene 0 0 0 2.5 benz [e]p yrene 3.1 1.2 benz [a]pyrene 0 0 0 3.0 1.9 1.1 2.2 perylene 0 0 1.5 1.8 indeno[ 1,2,3-cd]0 1.5 0 fluoranthene 1.4 2.3 2.8 0 0 indeno[ 1,2,3-cd]pyrene 2.0 2.1 0 1.0 0 benzo[ghi] perylene 73.4 82.3 92.6 65.4 64.4 tot. Table V. Relative Weight Percentage Yield of Principal PAC’s to Total PAC Yield per Reaction Temperature Level and Number of Rings re1 yield temp level, K 2 rings 3 rings 4 rings 5 rings 6 rings 23.2 1089 10.8 21.5 6.0 3.9 1189 20.0 12.1 14.7 11.5 6.1 1310 23.9 12.9 18.4 11.8 6.4 1458 1.6 64.1 16.6 0 0 1644 0 70.9 21.7 0 0

eight tests at 1310 K. As temperature increased, Table IV shows that the relative proportions of some compounds pass through a maximum, while others generally increase or decrease. The table also shows that the principal compounds became greater proportions of the total PAC yield as temperature increased, reflecting the smaller number

Energy & Fuels, Vol. 2, No. 6,1988 831

of PAC’s present at the higher temperatures. For Table V, the results in Table IV are further reduced and organized according to the number of rings in the compounds. Thus for example, the two-ring compounds naphthalene, benzothiophene, and methylnaphthalene were collectively 12.1% of the total PAC yield in the four testa at 1189 K. Presentation in this manner clearly shows the previously identified tendency for the PAC yield to be dominated by three- and four-ring compounds at the higher temperatures.

Discussion As previously explained,’ quadratic response surface models were used to analyze the data by least-squares regressions and a (significance) levels were used as statistical tests to determine the adequacies of the overall model fits and the significances of the predicted effects. Due to the uncertainties involved in experimentally determining total PAC yields and compositions, only regression model fits with CI levels of 0.05 or higher (numerically lower) were considered as significant enough for predictive purposes. A lack-of-fit diagnostic was also used to test for possible inadequacy of a model by comparing the error variance estimated from the model to the pure error variance estimated from the replicate data at the center point test condition. Significant lack of fit can indicate an inadequacy of the model form to fit the true nature of the data and/or replicate data under one or more conditions that are not representative of all the experimental points. Application of this diagnostic did not change the significances of the data reported here. The total PAC yield and 13 of the principal PAC yields were adequately represented by quadratic response surface models at a levels higher than 0.05; of these, nine were hgher than 0.02. Except for benzothiophene and chrysene/triphenylene, all the principal compounds with three, four, and five rings had statistically significant regression model fits. Of the five-ring and six-ring groups, benzo[klfluoranthene and indeno[1,2,3-~d]fluoranthene, respectively, were the only two with adequate fits to at least the 0.05a level. All the models with adequate representations had statistically significant predicted temperature effects to at least the 0.03~1 level, with over half of them

Table VI. Regression Statistics for Weight Fractional Yields Drobabilitv fraction variable coeff of determ, R2 inadequate fit no temp effect no pressure effect tot. PAC 0.8747 0.0086 0.0027 0.1606 naphthalene 0.8661 0.0109 0.0772 0.0035 0.0026 fluorene 0.9098 o.oO09 0.0266 0.0016 0.0005 dibenzofuran 0.9207 0.0159 0.0031 0.0090 phenanthrene 0.8729 0.0969 0.0096 0.0242 fluoranthene 0.8322 0.1310 0.0263 0.0499 benzo[klfluoranthene 0.7924 0.1010 0.0040 0.0051 indeno[ 1,2,3-~d]fluoranthene 0.8914 0.2928

tot. PAC 8.062 -4.771 0.421 -1.411 0.079 -1.712 -0.475 -0.610 1.525 1.068

Table VII. Regression Coefficients for Weight Fractional Yields regression coeff, lo3 kg/kg of maf coal benzo[k]naDhthalene fluorene dibenzofuran Dhenanthrene fluoranthene fluoranthene 0.578 26 0.282 58 0.199 13 0.586 15 0.487 50 0.486 21 -0.359 71 -0.130 43 -0.088 58 -0.322 53 -0.301 81 -0.217 33 0.01063 0.005 91 0.042 25 0.059 48 0.062 24 -0,156 89 -0.013 41 -0.006 83 -0.009 90 -0.057 61 -0.031 36 -0.12996 0.024 14 -0.009 89 -0.013 08 -0.025 82 0.034 39 -0.047 07 -0.044 42 -0.133 25 -0.063 43 -0.12665 -0.123 11 -0.008 95 0.042 04 -0.029 49 -0.017 12 -0.067 73 -0.050 57 -0.062 10 -0.011 09 -0.08899 0.21564 -0.015 69 -0.068 03 -0.100 68 -0.017 51 0.020 41 0.017 09 0.071 00 0.089 69 -0.038 53 0.056 70 0.098 24 0.008 66 0.001 79 0.043 05 0.082 51

no gas res effect 0.2241 0.2532 0.3223 0.3569 0.5426 0.6169 0.4315 0.0071

indeno[l,2,3-cd]fluoranthene 0.071 27 -0.080 64 0.003 36 -0.053 88 0.013 48 -0.01363 0.032 74 0.006 83 0.055 15 0.045 18

832 Energy & Fuels, Vol. 2, No. 6,1988

Bissett and Lamey

A

1 2

.

0010-

?

0005-

p x

Reaction temperature (K)

Figure 1. Regression model prediction for the yield of PAC's at 2.23-MPa pressure and 4.68-5gas residence time. Plotted experimental data in the order of increasing temperature are from test no. 3A133, the average of the 3A333 tests, and test no. 3A533.

having a levels higher than 0.005. Regression statistics and coefficients of the total yield and some of the principal PAC's including the only ones with significant predicted pressure or residence time effects are given in Tables VI and VII. The regression equation form is given in part 1.' As explained earlier,' this regression approach was selected as an experimentally consistent way to identify and quantify statistically significant trends in a composite factorial experimental design and, especially with respect to total yield, was never intended to supplant notable, semiempirical models, such as those being developed by Solomon and Serio at al.13-lg However, this approach provides a substantiated experimental data base that, as a minimum, can be used for validating the predictive capabilities of more comprehensive models and, in particular for the components, possibly give some useful insights for improvements. Comparisons are being made by Solomon and Serio et al.20s21 and are not the subject of this paper. Figure 1shows how the predicted total yield of PAC's varies with reaction temperature under the experimental center point pressure and gas residence time conditions. Center point values are used here and in other figures to represent trends with the highest level of confidence. Thus, at 2.23-MPa pressure and 4.68s gas residence time, the total yield of PAC's is predicted to be about 1.8% of the coal at the lowest temperature and to decrease monotonically and reach essentially zero at the highest temperature. In this and other figures, experimental data are also plotted when possible to give some fee1 for the regression fits. Under the center point test condition, plotted data are average results of the four center point tests. The predicted variations in six of the relative yields with reaction temperature are shown in Figure 2. Of the 13 principal PAC's with statistically significant regression (13) Solomon, P. R.; Hamblen, D. G. DOE/FE/05122-1485 (DE84000225);US. Department of Energy: Springfield, VA, 1983. (14) Solomon, P. R.; Serio, M. A.; Hamblen, D. G. Morgantown Energy Technol. Cent. [Rep.],DOEIMETC (U.S.Dep. Energy) 1987, DOE/METC-87/6079 Vol. 2 (DE87006496),548. (16)Solomon, P. R.; Serio, M.A.; Carangelo, R. M.;Markham, J. R. Fuel 1986,65, 182. (16) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.;Serio, M.A.; Deshpande, G. V. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32. (17) Serio, M.A,; Solomon, P. R.; Heninger, S.G. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986, 31(3), 210. (18) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, 1 , 138. (19) Solomon,P. R.; Smoot, L. D.; Serio, M.A.; Hamblen, D. G.; Yang, D. B.; Brewster, B. S. Morgantown Energy Technol. Cent. [Rep.], DOEIMETC ( U S . Dep. Energy) 1987, DOE/METC-87/6079, Vol 1 (DE87006495),43. (20) Solomon, P. R.; Serio, M.A.; Hamblen, D. G. Morgantown Energy Technol. Cent. [Rep.],DOEIMETC ( U S . Dep. Energy) 1986, DOE/METC-86/6043 (DE86006617),182. (21) Serio, M. A.; Solomon, P. R.; Kroo, E.; Deshpande, G. V. US. Dep. Energy [Rep.],DOEIMC 1987, DOE/MC/21004.

1089

1144

1200

1255

1311

1367

1422

1478

1533

1589

1644

Reaction temperalure [K]

Figure 2. Regression model predictions for relative yields at 2.23 MPa pressure and 4.68-5gas residence time: (A) naphthalene (A);(B) fluorene (A);(C)phenanthrene (0);(D) fluoranthene (0); (E) benzo[k]fluoranthene (v);(F)indeno[1,2,3-~d]fluoranthene (X). Plotted experimental data in the order of increasing temperature me from test no. 3A133, the average of the 3A333 tests, and teat no. 3A533.

0

079

148

217

286

355

424

493

562

631

700

Reaclion pressure lMPa1

Figure 3. Regression model predictions for relative yields at 1310 K reaction temperature and 4.68-5 gas residence time: (A) naphthalene ( 0 ) ;(B) fluorene ( 0 ) ;(C) dibenzofuran (A);(D) phenanthrene (A). Plotted experimental data in the order of increasing pressure are from test no. 3A313, the average of the 3A333 tests, and test no. 3A353. model fits, these were selected so a range of two- to six-ring compounds would be presented. The figure shows that the relative yields of all six compounds are predicted to decrease with reaction temperature. In general, the compounds with the higher relative yields at the lower temperatures are predicted to have greater temperature dependencies and their yields decrease more sharply with temperature. Indeno[ 1,2,3-~cl]fluoranthene,which has six rings and the highest molecular weight in the figure, is predicted to disappear at the lowest temperature. It is followed next by the extinction of naphthalene, which has only two rings and the lowest molecular weight. Of these six compounds, only phenanthrene and fluoranthene are predicted to remain at the highest temperature. This graphically illustrates the previously noted tendency for a broad spectrum of compounds to be present at the lower temperatures and for only a smaller, intermediate spectrum to persist at the higher temperatures. Figure 3 shows the predicted variation of relative yields with pressure for the four PAC's with models that had statistically significant predicted pressure effects. Several interesting features are revealed by these trends. For instance, naphthalene yield is predicted to steadily decrease with pressure, whereas the yields of the three-ring compounds shown are predicted to pass through maximums in the 2.2-2.5-MPa pressure range. Of the three-ring

Energy & Fuels, Vol. 2, No. 6, 1988 833

Flash Pyrolysis of Montana Rosebud Coal

1255K 1311K

ature. The predicted increases in yield at the longer residence times probably stem from the questionably low experimental values obtained in tests 3A333 and 3A333-2.

\

1367K

2

3

4

5

6

7

8

9

10

11

Gas residence lime Is)

Figure 4. Regression model predictions for the relative yield of indeno[ 1,2,3-cd]fluorantheneat various reaction temperatures and 2.23-MPa pressure. Plotted experimental data in the order of increasing gas residence time are from test no. 3A331, the average of the 3A333 tests, and test no. 3A335.

compounds, the heaviest one is predicted to have the most pressure dependency, and the other two, which have comparable molecular weights, are predicted to have similar trends with pressure. The pressure at which maximum yield occurs is also predicted to increase slightly as molecular weight increases. These trends, together with the prediction that naphthalene has the highest yield at the lower pressures, may be a reflection of a tendency for higher pressures to favor the net production of heavier ' compounds, at least at the lower temperatures investigated. This same conclusion was also reached from the assessment of the pressure effects on the hydrogen yield in the product gas and on the hydrogen retention in the product char in these tests.' Figure 4 shows the predicted relative yield of indeno[1,2,3-cd]fluoranthene as a function of gas residence time. Since this was the only PAC to have a model with a statistically significant predicted residence time effect, no rate comparisons with other PAC's were possible. As would be expected for cracking or stabilization reactions, the yield is predicted to decrease with time. The times required for the relative yield (X1000) to decrease from 0.2 to 0.1 are predicted by the regression model to be 1.50,1.17, and 0.87 s at reaction temperatures of 1255, 1311, and 1367 K, respectively, thus indicating that the rate is predicted to increase with temperature. These predicted times are nearly linear with respect to reciprocal absolute temper-

Conclusions The reaction conditions of the study promoted the substantial Occurrence of secondary volatiles reactions, and as a result, the net production of PAC's was small and the extent of alkylation and hydroxylation of the PAC's was low. Many of the principal comounds were the same as those reported in tars from simulated and actual commercial carbonization processes that operated at temperatures in the lower range of this study. However, at higher temperatures, this study showed that only a narrower, intermediate spectrum of three- and four-ring compounds tends to remain. Some additional effects of the reaction variables were obtained from the use of a composite factorial design and the associated regression analysis. Temperature was shown to be the most important parameter and caused all component yields to decrease. On the basis of the predicted trends for some of the component yields, there seemed to be a tendency for compounds with the higher relative yields at the lower temperatures to have greater temperature dependencies. A few statistically significant predicted pressure effects suggested that pressure may tend to favor the net production of heavier compounds, at least at the lower temperatures investigated. Although only one PAC yield had a statistically significant predicted residence time effect, it provided a qualitative measure of the refractory nature of these compounds. These trends may aid the continued development of semiempirical models not considered here. Acknowledgment. This work was sponsored by the Surface Coal Gasification Program of the U.S.Department of Energy. Registry No. Naphthalene, 91-20-3; benzothiophene, 1109543-5; methylnaphthalene, 1321-94-4;acenaphthylene, 208-96-8; fluorene, 86-73-7;dibenzofuran, 132-64-9;phenanthrene,85-01-8; anthracene, 120-12-7;dibenzothiophene, 132-65-0;fluoranthene, 206-44-0; pyrene, 129-00-0; phenylnaphthalene, 35465-71-5; chrysene, 218-01-9;triphenylene, 217-59-4;benzofluoranthene, 56832-73-6; benz[e]pyrene, 192-97-2; benz[a]pyrene, 50-32-8; perylene, 198-55-0; indeno[ 1,2,3-~d]fluoranthene,193-43-1; indeno[l,2,3-cd]pyrene, 193-39-5;benz[ghi]perylene, 191-24-2.