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Energy & Fuels 1998, 12, 830-831
Measuring the Hydrogen Donor Reactivity and Capacity of Coals John W. Larsen* and Shang Li Department of Chemistry, 6 E. Packer Avenue, Lehigh University, Bethlehem, Pennsylvania 18015
Masakatsu Nomura Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka Suita, Osaka 565, Japan Received October 20, 1997 The hydrogen-donating reactivity and capacity of coals are important in conversion processes,1 in coking,2 and in oxidation.3 Both reactivity and capacity can be measured conveniently by oxidation with sulfur in refluxing o-dichlorobenzene, following the reaction by monitoring the product H2S. All coals lose less than their aliphatic hydrogen content, ranging from 16% of total coal hydrogen for Zap lignite to 48% for Pittsburgh No. 8 coal. The rate of hydrogen loss is a measure of reactivity, and the initial rate of hydrogen loss varies by only a factor of 14 across the series of coals and, for all but one coal, decreases by less than 35-fold between 1 and 300 h. A number of methods have been used to dehydrogenate coals. Benzoquinone has been used4 but we rejected its use because too much of it is incorporated, possibly because it dehydrogenates by an ionic mechanism.5,6 Wender used Pd/CaCO3, a vigorous heterogeneous reagent not useful to us because of its vigor and heterogeneity.7 Benzophenone has been used but at 400 °C, and we feared this high temperature would alter the coal.8 More recently, a variety of aromatic molecules have been used as dehydrogenation agents.9,10 They are effective and the results are interesting, but it is difficult to monitor the extent to which they may become incorporated into the coal. We set out to find another mild dehydrogenation agent whose reaction rate at a convenient temperature could easily be measured. Sulfur is an effective well-characterized dehydrogenation agent.11 It has been used to dehydrogenate coals by pyrolyzing coals together with elemental sulfur.12 We found that elemental S in refluxing o-dichlorobenzene is both effective and convenient. To follow H2S production the reaction off gases are swept into a 2 M NaOH solution
Figure 1. % H lost and H removal rate as a function of time for Pittsburgh No. 8 coal.
Figure 2. % H lost and H removal rate as a function of time for Wyodak coal.
(1) Neavel, R. C. Coal Science I; Academic Press: London, 1982; Cp 1. (2) Larsen, J. W.; Lee, D.; Schmidt, T.; Grint, A. Fuel 1986, 65, 595596. (3) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Energy Fuels 1997, 11, 236-244. (4) Peover, M. E. J. Chem. Soc. 1960, 5020-5026. (5) Penn, J. H.; Lin, Z. J. Org. Chem. 1990, 55, 1554-1559. (6) Jackman, J. M. Adv. Org. Chem. 1960, 2, 329-366. (7) Reggel, L.; Wender, I.; Raymound, R. Fuel 1968, 47, 152. (8) Collins, C. J.; Raaen, V. F.; Benjamin, B. M.; Kabalka, G. Fuel 1977, 56, 107. Raaen, V. F.; Roark, W. H. Fuel 1978, 57, 650-651. (9) Kidena, K,; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678. (10) Yokono, T. Takahashi, N.; Sanada, Y. Energy Fuels 1987, 1, 360362. (11) Fu, P. P.; Harvey, R. G. Chem. Rev. 1978, 78, 317-361. (12) Mazumdar, B. K.; Choudhury, S. S.; Chakrabartty, S. K., Lahiri, A. J. Sci. Ind. Res. 1958, 17B, 509-511. Mazumdar, B. K.; Chakrabartty, S. K.; Lahiri, A. Fuel 1962, 41, 129-139.
to form Na2S. Its concentration is then measured by UV at 229 nm.13 The experimental procedure consists of refluxing ∼5 g of coal in 150 mL of o-dichlorobenzene (bp 180 °C) with a dry N2 sweep passing through the refluxing solution, out through the reflux condenser, and then into the base solution. Aliquots are removed by pipet at intervals, and the Na2S concentration is measured. The reaction in refluxing chlorobenzene (bp 131 °C) is too slow to be useful. Figures 1-5 show the hydrogen lost as a function of time for six coals. In each case the data have been fit by (13) J. H.; Chamberlain, N. F.; Reed, J. R. The Analytical Chemistry of Sulfur and Its Compounds; Wiley-Interscience: New York, 1971.
S0887-0624(97)00192-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998
Communications
Energy & Fuels, Vol. 12, No. 4, 1998 831 Table 1. Dehydrogenation Rates (104 × % H Lost/h) for Coals at Different Reaction Periods time (h) 1 2 5 10 20 50 100 200 300
Pgh No. Wyodak Zap Pittstone-M Workwort Goonyella (75)a (73)a (86)a (85)a (88)a 8 (83)a 170 120 78 53 34 17 11 8.7 5.0
36 31 25 20 16 9.6 6.6 5.8 3.4
12 12 11 11 9.3 6.3 4.2 3.7 1.3
100 82 59 44 30 15 8.1 5.8 2.9
85 65 45 32 22 12 7.5 4.2 0.45
22 21 20 17 14 7.3 4.1 6.9 3.5
Table 2. Elemental Analyses for Pittstone-M Coal Samples (daf)a
Figure 3. % H lost and H removal rate as a function of time for Zap lignite. org coal 2.3% H lost 9.0% H lost 37% H lost a
Figure 4. % H lost and H removal rate as a function of time for Workworth coal.
Figure 5. % H lost and H removal rate as a function of time for Goonyella coal.
a polynomial whose first derivative is shown (right-hand axis). This derivative gives the rate of hydrogen loss, one measure of the hydrogen reactivity. Elemental analyses and characterization of the coals are available elsewhere.9,14 The reaction is capable of differentiating the hydrogendonating capacity of coals. This is clear from the large differences between the amount of hydrogen removed from these coals. The amount of aliphatic hydrogen exceeds the amount of hydrogen removed from these coals, so not all of the aliphatic hydrogen is reactive.15 Three of the coals were also studied by Wender using Pd/ (14) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program, Argonne National Lab Report ANL/PCSP/-93/1; available from NTIS, Springfield, VA.
C (%) H (%)
S (%)
H removed coal wt (g) S incorporated start final (mole ratio)
84.0 80.2 77.2 64.4
0.94 4.61 9.09 24.7
4.50 4.89 4.89
5.24 4.63 3.97 2.60
4.19 4.91 6.35
8.9 5.0 1.9
Data from Galbraith Laboratories, Inc.
CaC03.16 Our results for the percent of total hydrogen removed from Zap (16%), Wyodak, (25%) and Pittsburgh No. 8 coals (48%) differ from Wender’s which are 39%, 27%, and 37% respectively. Given the differences in chemistry and especially accessibility, the different results are not surprising. There have been no previous direct measurements of the variation in oxidizability of hydrogen in coals. Those relative reactivities are available from the derivative curves in Figures 1-5. For Pittsburgh No. 8 coal, 48% of all the hydrogen can be removed with only about 30fold variation in reactivity. Table 1 summarizes the rates of hydrogen loss as a function of reaction time. The reactivity range for most of the coals is low with only Workworth coal showing greater than a 100-fold decrease in hydrogen reactivity after 300 h dehydrogenation. The other five coals have an average reactivity decrease of only 19-fold. The behavior of Goonyella coal suggests dehydrogenation results in significant changes in physical and/or chemical structure. The observation of a constant rate of hydrogen loss may be due to a high population of hydrogens of closely similar reactivity or to dehydrogenation rates limited by the diffusion of S. A potential problem is the amount of sulfur incorporated into the coal during the reaction. It varies with extent of reaction and so is best expressed as the ratio of moles of H removed to moles of S incorporated. These data are shown in Table 2. Less sulfur incorporation would be preferable, but these amounts are not sufficient to vitiate the procedure. More information on the dehydrogenation of these and other coals including structural studies will be reported in a full paper. Acknowledgment. We are grateful to the Exxon Education foundation for partial support of this work (J.W.L. and S.L.). Supporting Information Available: Figure showing the hydrogen lost as a function of time for Pittstone-M coal (1 page). Ordering information is given on any current masthead page. EF970192S (15) Sobkowiak, M.; Painter, P. Fuel 1992, 71, 1105-1125. (16) Reggel, L.; Wender, I.; Raymond, R. Fuel 1968, 47, 373-389.
