Comparison of the liquefaction yields of coals with their composition, free radical density and thermal parameters

Fuel Processing Technology, 25 (1990) 215-226

215

Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands

Comparison of the Liquefaction Yields of Coals with Their Composition, Free Radical Density and Thermal Parameters MANJULA M. IBRAHIM and MOHINDAR S. SEEHRA*

Department o[ Physics, West Virginia University, Morgantown WV 26506 (U.S.A.) and ROBERT A. KEOGH

Center for Applied Energy Research, University of Kentucky Lexington, KY 40512 (U.S.A.) {Received July 25th, 1989; accepted in revised form February 6th, 1990)

ABSTRACT

Pyrolysis behaviour of four coals, a vitrinite,and a bituminite sample has been investigated between 25°C and 600°C by in situ electron spin resonance (ESR) spectroscopy, thermogravimetry/differential thermogravimetry ( T G / D T G ) and differentialscanning calorimetry (DSC) and the resultsare compared with the percentage conversion of these samples to pyridine-soluble liquefactionproducts. The three distincttemperature stagesof pyrolysisreported earlierby Seehra et al. in E S R spectroscopy are observed in these samples also. Linear regression analysis is used to ascertain correlation between percentage conversion and compositional and thermal parameters.Among the compositional parameters, the best correlationis obtained with the atomic H / C ratio (correlationcoefficient,r=0.82) and among the thermal parameters, the rate of mass loss measured in D T G givesthe best correlation (r= 0.92 ),followed by the rate of thermally generated free radicalsin stage 3 (r--0.72).The significanceof these resultsis discussed.

INTRODUCTION

In recent years, several studies dealing with the relationshipsbetween the compositional characteristicsof coals and theirliquefactionbehavior have been reported [1,2].In these studies,relationships have been sought between the conversion of coals to gaseous and liquid products and such parameters as volatilematter, atomic H / C and O/C ratios,reactivemaceral content, mineral content and the like.The studies by Yarzab et al. [1] on 104 U.S. coals led them to conclude that these coals can be divided into three groups: Group 1 *To w h o m correspondence about this paper should be addressed.

0378-3820/90/$03.50

© 1990 Elsevier Science Publishers B.V.

216

consisting of high rank, medium sulfur coals with mean conversion of about 53To; Group 2 of medium rank, high sulfur coals with mean conversion of about 70%; and Group 3 of medium to low rank, low sulfur coals with mean conversion of 64%. In the studies of 20 South African coals by Gray et al. [2], a positive linear correlation (correlation coefficient = 0.95) was found between percentage conversion and volatile matter, H / C atomic ratio and reactive maceral content (consisting of vitrine+exinite÷reactive semi-fusinite). These studies constitute important sources of information on the choice of a coal for liquefaction studies. Much of the current research effort in coal liquefaction at the laboratory scale involving various analytical techniques is directed at understanding the mechanism and devising ways to liquefy coals under less severe conditions of temperature and pressure. Thermal decomposition is generally considered to be the initial step in coal liquefaction [3]. Consequently, coal pyrolysis and liquefaction are linked and several studies of coal pyrolysis using thermogravimetry (TG) have been reported in recent years [4-7]. It is also well known that stable free radicals are present in coals and under pyrolysis and liquefaction conditions, new free radicals are generated [8]. In several recent papers from this laboratory [9-11 ], pyrolysis and extraction behavior of about a dozen U.S. coals has been reported as studied by in situ electron spin resonance (ESR) spectroscopy of free radicals. A common feature discovered from these studies is the presence of four distinct temperature stages as coals were pyrolyzed and since then similar temperature stages have been reported in some British coals [ 12 ]. However, to relate the ESR parameters of the thermally generated free radicals to liquefaction characteristics of coals still remains a challenge. In order to provide a better understanding of the mechanism of coal liquefaction, we recently undertook a combined study involving not only liquefaction but also pyrolysis studies of the same coals using in situ ESR spectroscopy, T G / D T G (Thermogravimetry/Derivative Thermogravimetry) and differential scanning calorimetry (DSC). We also obtained the proximate, ultimate and petrographic analysis of these samples. Four coals, and a vitrine sample and a bituminite sample obtained by density gradient centrifugation, are studied in this work. Whereas T G / D T G detect processes which involve gaseous products resulting in a weight loss of samples [4-7], DSC measures processes involving non-volatiles and transformations occurring without a change in mass. On the other hand, ESR measures only those species which have unpaired spins. Results presented here show that the three techniques provide supporting information and correlations exist between the liquefaction yield and thermally generated free radicals and rate of weight loss. Details of these results are given below. E X P E R I M E N T A L DETAILS

The DSC and T G / D T G measurements were carried out in a Mettler system (Model TA3000). About 10 mg of the sample was used and each experiment

217

was done at a heating rate of 10 ° C/min and nitrogen gas flow rate of 300 cma/ rain. Experiments were carried out at temperatures up to 750 °C for TG and up to 600 °C for DSC measurements. A blank experiment, without sample in the sample holder, was done under identical conditions of heating rate and nitrogen flow. The experimental data for the samples are corrected for the background, as determined in the blank experiment. From DSC data specific heat per unit mass is calculated after correcting for changes in mass due to heating. The ESR measurements were carried out at X-band (9 GHz) frequencies by using a reflection-type spectrometer with a TElo2 mode cavity. Details of the experimental procedures to measure the spin density N, the g-values and the line width/xH, for in situ measurements to about 600 ° C, have been reported previously [9-11 ]. The list of the six samples studied in this work, along with their ultimate, proximate and maceral analysis, is given in Table 1. In all ESR TABLE 1

Proximate,ultimate and petrographic analysis of samples used in this work (weight percentage basis) Sample a

A

B

C

D

E

F

70.83 27.13 2.03 1.03

39.37 60.11 0.53 2.01

36.03 46.37 17.60 4.81

25.64 47.31 27.05 0.46

30.68 63.69 5.63 1.56

38.65 56.38 4.97 0.73

84.55 5.23 1.59 6.62 0.0

81.54 5.72 1.86 5.04 0.0

83.66 4.95 1.75 9.01 0.0

87.74 5.13 1.44 4.02 0.0

84.36 5.55 1.77 7.54 0.17

98.50 0.4 1.10 -

83.9 3.1 1.5 1.0 4.2 0.0 5.9 0.0

22.0 0.0 4.7 37.9 10.3 14.7 9.5 0.9

41.0 0.2 7.6 29.4 8.3 0.9 8.7 3.9

60.0 2.7 6.6 12.3 6.2 0.2 10.1 1.9

Proximate analysis Volatile matter Fixed carbon H - T Ash

Total Sulfur

Ultimate analysis (mad Carbon Hydrogen

Nitrogen Oxygen Chlorine

81.59 8.44 1.79 7.15 0.0

Petrographic analysis (dmmD Vitrinite Pseudovitrinite Fusinite Semifusinite Micrinite Macrinite Exinite Resinite

11.00 0.20 88.80 -

aA-Bituminite, Breckinridge maceral, B-Vitrinite maceral, W.KY. # 11 (71077), C - K C E R 5412, Manchester seam, KY, D - K C E R 2145, Peach Orchard seam, KY, E - K C E R 3796, Elswick seam, KY, F - K C E R 4598, Leatherwood seam, KY.

218

studies, samples were vacuum sealed in a glass tube so that the samples were not exposed to air during heating. The liquefaction experiments were conducted in a microautoclave reactor of 50 ml capacity. The microautoclave was charged with 5 g of coal ( - 100 mesh or < 150/~m; dried overnight under a vacuum of ca. 63 m m H g at 90°C), 7.5 g tetralin and a 0.6 m m diameter (1/4 in. ) steel ball for mixing. The reactor was pressurized with 55 bar (800 psi) of H2 at ambient temperature and immersed in a heated sand bath (427 ° C/445 ° C) for the desired residence time (15 min. ). Typically, it required less than two minutes to reach the desired reaction temperature. To ensure thorough mixing of the reactants, a vertical shaking speed was set at 400 cpm. At the end of the experiment, the reactor was immersed in a cold sand bath. Once the reactor had attained nearly ambient temperature (within 2 rain), a gas sample was taken for analysis. The liquid and solid products were quantitatively removed from the reactor using benzene and extrated with this solvent in a Soxhlet extraction apparatus until the benzene was clear (approximately 48 hours). After weighing the dried thimble to obtain the yield of benzene solubles, the thimble was extracted with pyridine. The percentage ash-corrected pyridine-insoluble material (IOM) determined the extent of conversion obtained through the following equation: percentage conversion = 1 0 0 - IOM. EXPERIMENTAL RESULTS

The values of N, at 25 °C, for the samples of Table 1, are given in Table 2. Noted that at room temperature, bituminite (sample A) has the lowest spin TABLE 2 Comparison of liquefactionconversion with several relevant parameters determined in this work Samplea Liquefaction H/C conversion (To) 427°C/445°C (maf)

Volatile ( V + E + S F ) b From ESR matter (To) (To) (maf) Nat25°C (101~/g)

AN N

From TGA { 1 dm~ ~-~Jat peak temp.

(s-I) A B C D E F

69.3/94.9 62.8/62.2 91.6 51.3/55.7 66.8 63.7/72.4

1.24 0.74 0.84 0.71 0.71 0.79

69.70 37.40 35.10 24.70 30.40 37.40

aSee legend of Table 1. bSum of vitrinite, exinite and semifusinite.

99.80 99.50 90.80 69.40 79.1 82.4

0.26 0.68 0.99 2.1 2.3 2.5

4.4 0.9 1.2 0.4 0.3 0.8

-0.027 -0.0032 -0.0066 -0.0031 - 0.0048 -0.0048

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concentration, vitrinite (sample B ) and coal sample C have the next higher N, whereas the remaining three coals (samples D, E and F) have the highest N values. The variation of the spin concentration N with increasing temperatures for the six samples is shown in Fig. 1. In bituminite (sample A), all four stages [9-11 ] are clearly evident. The first stage of increasing N of bituminite is from 25 to 210 ° C, the second stage of decreasing N is from 210 to 350 ° C, the third stage of very sharp increase of N is from 350 to 475 °C and the fourth stage of decreasing N is above 475 ° C. In the other samples, the first three stages are clearly seen; but the fourth stage, where N begins to decrease with increasing temperatures, is not clearly evident in the temperature range of the present study. However, much of the useful information about liquefaction and pyrolysis is available from the first three stages, as will be discussed below. The results of TG (percentage residual weight of the six samples with increasing temperatures) are shown in Fig. 2. Except for sample A, the relative scales for the ordinates for the other samples are the same, so that total weight loss, as well as rate of weight loss, can be estimated from these plots. The rapid weight loss begins to occur only about 400 ° C. Results of derivative thermogravimetry (DTG), also available from the Mettler TA3000, are shown in Fig. 3. DTG measures the mass change/second at each temperature and it thus represents how fast the mass is being lost at each temperature. This quantity varies from sample to sample and we show later that it has a very strong correlation with percentage conversion. Heat capacity Cpof the six samples as a function of temperature is plotted in Fig. 4. In plotting these data, we have corrected for the weight changes due

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222

to heating as observed in our TG experiments of Fig. 2. In all samples a broad peak in Cp centered around 100°C is observed. There are other peaks in Cp at higher temperatures, although results appear to be different for different samples. In Fig. 5, we combine data of all four sets of measurements (viz. TG, DTG, C~ and spin concentration N) for one sample, viz. sample C. We discuss the significance of these results in the following pages. DISCUSSION AND ANALYSIS

In Fig. 6, we have plotted several quantities against percentage conversion (carried out at 427°C for a residence time of 15 minutes). It is noted that in the ESR, TG and DSC measurements, we only explore initial stages of the kinetics of the coal pyrolysis, so that comparison with the percentage conversion reached in 15 minutes of liquefaction experiment is quite appropriate. (It is possible that in some samples, maximum values of the conversion may be higher than that measured here in 15 minutes). Also, we have excluded bituminite from our analysis, since it is not representative of coals (i.e. its H / C ratio is higher t h a n one). However, the data on bituminite will be used as a reference for an "ideal coal" with very high conversion. lOO

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223

In order to determine any correlations between various measured quantities and conversion to liquefaction products, we have chosen one measured quantity at a time and determined its correlation with conversion by applying a linear regression analysis, assuming a linear relationship would exist. The correlation is respectively termed excellent, good, fair and weak if the correlation coefficient r > 0.9, 0.9 > r > 0.8, 0.8 > r > 0.7 and r < 0.7. The implication of these results is then discussed. Let us first consider N, the spin concentration at room temperature (Fig. 1 ). There is only a weak (r--0.46 ) inverse correlation between the N values at room temperature and percentage conversion (Fig. 6a). It is known [8,13 ] that N values for different maceral groups vary roughly as N (inertinites) > N (vitrinites) > N (exinites). Therefore it follows that higher conversion and lower N values in sample A, B and C are simply due to larger amounts of exinites and vitrinites present in these samples. However, the correlation is too weak to have any predictive value. In Fig. 6(b), we have plotted the percentage conversion as a function of reactive macerals (exinite ÷ vitrinite ÷ semifusinite ). Yarzeb et al. [ 1 ] and Gray et al. [2 ] have found a good correlation between the reactive macerals and percentage conversion {r > 0.9). There is some dispute in the literature regarding the behavior of semifusinite during carbonization. Schapiro et al. [14] classified only 1/3 of the semifusinite as reactive. On the other hand, recent observations [2,15,16] suggest that the whole semifusinite maceral may have a significant role in the liquefaction process. Following this suggestion, including semifusinite as a reactive maceral in addition to vitrinite and exinite, we find the correlation coefficient with percentage conversion to be only 0.46 (Fig. 6b). Thus our finding is that reactive maceral content alone is not sufficient to accurately predict the percentage conversion. Among the other room temperature compositional parameters, we next consider the atomic H / C ratio vs. percentage conversion. Here relatively good correlation (r = 0.82 ) with conversion is indicated (Fig. 6c ), in agreement with the findings of Gray et al. [2]. In fact, atomic H / C appears to yield the best correlation with percentage conversion, relative to the other compositional parameters of coals at room temperature. Our next comparison is with parameters obtained from the thermal decomposition of coals. In Fig. 7 {a), we have plotted against percentage conversion the total weight loss measured by TG at 750 ° C and in Fig. 7 (b), the percentage volatile matter determined by the proximate analysis. Not surprisingly, volatile matter and weight loss measured by TG follow nearly similar trends and their correlation coefficients are also similar, being respectively r=0.46 and r = 0.57. These correlations are fairly weak. In Fig. 7(c), we have plotted the peak magnitude of the rate of mass loss normalized by the initial mass, mo, of each sample against percentage conversion. These quantities are calculated from the DTG data of Fig. 5. Here we

224

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Conversion (wt%, maf) Fig. 7. Comparison of the liquefaction yield with the weight loss at 7 0 0 ° C , volatile matter, rate of mass loss, and A N/N. The straight lines are least squares fits to the data with the correlation coefficient, r for each fit given in f i g u r e s .

observe an excellent correlation (r--0.92) with percentage conversion. Recall that DTG measures how fast volatiles are liberated from coals and by inference it also measures the reaction rate of thermal decomposition of coals. The excellent correlation observed here for the first time may be very important in determining the liquefaction potential of a coal. Another quantity that is perhaps related physically to the same phenomenon is the slope of the N vs. temperature curves of Fig. 1 in stage 3. To get an average value of the slope, wc have plotted AN/N in Fig. 7 (d), where A N is the difference between the N values at the minimum near 400 °C and a temperature 100 °C higher and N is the spin concentration value at the minimum. Here r= 0.72, indicating only a fair correlation. Next we examine the heat capacity data of Fig. 4. Melchior and Luther [17] have reported on the heat capacity data of some coals in the temperature range 30-350°C. The heat flow Q to a coal sample may be written as [17]

Q= f mCpdT+ AH~

(1)

where AHu is the heat of reaction and the first term represents the heat ca-

225 1200 A

800

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0

0

100

200

300

400

500

600

T(°C) Fig. 8. Calculated enthalphy of the samples plotted as a funtion of temperature. The base line of Cp = 0 is used in the calculations.

pacity of a coal (including impurities). Since volatiles are liberated from coals upon heating, Cpdoes not represent the true specific heat of a coal. Nevertheless, we examine the results of Fig. 4. Primarily three endotherms are evident. The first endotherm centered around 100 °C and extending up to 200 °C is probably primarily the result of adsorbed water. It is followed by a broad endotherm extending up to about 400 ° C. These two endotherms approximately coincide with the first two stages seen in our N vs. temperature plots (Fig. 1 ). The third endotherm appears in the range between 400 ° C and 500 ° C, although there are variations between the locations and the magnitudes of these endotherms among different samples. In Fig. 5, we have plotted the results of TG, DTG, Cpand N measurements for one sample (viz. sample C) in order to compare the results obtained from the four different techniques. The first three stages of pyrolysis are evident from all the measurements with the largest changes occurring in the third stage centered around 450 ° C. This is the stage where thermal decomposition of coals occurs and observations in this stage are most crucial to liquefaction. Finally in Fig. 8, we have plotted the calculated enthalphy using the data of Fig. 4 and the base line at Cp--0 for integration purposes. However, there appears to be no significant correlation between enthalphy and percentage conversion. CONCLUDING REMARKS

From the results and discussion presented above, the following conclusions are drawn. Among the room temperature compositional parameters, the strongest correlation (r= 0.82) of percentage conversion appears to be with

226

the atomic H / C ratio. This is in agreement with the findings of Gray et al. [2 ] on twenty south African coals, although in the multivariate analysis of Yarzab et al. [1] on 102 U.S. coals, H / C ratio is only one variable among several affecting the percentage conversion. Among the parameters related to thermal decomposition of coals, the rate of mass loss determined in DTG gives the strongest correlation (r = 0.92 ) with percentage conversion. This is a new result of this work which should be verified by studies on additional coals. Since DTG is a rather simple experiment, this correlation, if valid for a larger variety of coals, could prove to be very valuable in predicting the liquefaction yield of a coal. The effect of catalysts on coal decomposition could also be easily investigated by this technique. ACKNOWLEDGEMENTS

This work was supported in part by grants from the U.S. Department of Energy through CFFLS (Consortium for Fossil Fuel Liquefaction Science). Useful discussions with G. Huffman, J. Zondlo and M. Farcasiu are gratefully acknowledged. The authors also acknowledge D. Taulbee for providing the maceral concentrates used in this work.

REFERENCES 1 Yarzab, R.F., Given, P.H., Spackman, W. and Davis, A., 1980. Fuel., 59: 81; and references therein. 2 Gray, D., Barass, G., Jezko, J. and Kershaw, J.R., 1980. Fuel, 59: 146; and references therein. 3 Neavel, R.C., 1976. Fuel, 55: 237. 4 Cumming, J.W., 1984. Fuel, 63: 1436. 5 Sulimma, A., Leonhardt, P., Van Heek, K.H. and Jtintgen, H., 1986. Fuel, 65: 1457. 6 Morgan, P.A., Robertson, S.D. and Unsworth, J.F., 1986. Fuel, 65: 1546. 7 Carangelo, R.M., Solomon, P.R. and Gerson, D.J., 1987. Fuel, 66: 960. 8 Petrakis, L. and Grandy, D.W., 1983. Free Radicals in Coals and Synthetic Fuels. Elsevier, Amsterdam; and references therein. 9 Seehra, M.S., Ghosh, B. and Mullins, S.E., 1986. Fuel, 65: 1315. 10 Seehra, M.S. and Ghosh, B., 1988. J. Anal. Appl. Pyrolysis, 13: 209. 11 Seehra, M.S., Ghosh, B., Zondlo, J.W. and Mintz, E.A., 1988. Fuel Processing Technol., 18: 279. 12 Fowler, T.G., Battle, K.D. and Kandiyoti, R., 1987. Fuel, 66: 1407. 13 Sibernagel, B.G., Gebhard, L.A., Dyrkacz, G.R. and Bloomquist, C.A., 1986. Fuel, 65: 558. 14 Schapiro, N., Gray, R.J. and Eusner, G.R., 1961. Blast Furnace, Coke Oven and Raw Materials Proc. Am. Inst. Mech. Eng., Vol. 20, p. 89. 15 Taylor G.H., Macowsky, M.Th. and Alpern, B., 1967. Fuel, 46: 431. 16 Steyn, J.G.D. and Smith, W.H., 1977. Coal Gold Base Miner., Sept., 25{9): 107. 17 Melchior, E. and Luther, H., 1982. Fuel, 61: 1071.