The Determination and Use of Specific Surface Values For Coals

47 The Determination and Use of Specific Surface Values For Coals D. H. T. SPENCER and R. L. BOND

Downloaded by IOWA STATE UNIV on October 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch047

The

British

Coal Utilisation

Research Association, Leatherhead,

Surrey,

England

Several methods used during the last 30 years in attempts to measure the specific surfaces of coals include gas and vapor adsorption and heats of immersion. The specific surface value obtained for a given coal can vary according to both the type of method used and the particular variant of the method used—e.g., vapor adsorption at different temperatures. The consequent controversies that have been evoked by such contradictory results have led to considerable confusion. Growing evidence suggests that the concept of "specific surface," when applied to sorbents containing pores with diameters of a few tenths A. or less, has no physical meaning.

j y j u c h effort has been expended during the last 30 years on investigating the sorptive properties of coals (12, 13). M a n y different sorbates, under various conditions of temperature and pressure, have been used; rare gases, nitrogen, oxygen, carbon dioxide, carbon monoxide, nitrous oxide, sulfur dioxide, hydrocarbons, alcohols, etc. have been sorbed from the gas or vapor phase, and a wide range of liquids of different molecular sizes and properties has been used i n experiments on heats of immersion. Following directly from these measurements, specific surface values have often been deduced in attempts to elucidate coal structure and to explain the behavior of coals and their chars i n various processes such as gasification or carbonization. Disaccord between the values obtained with the different methods or sorbates used has given rise to much controversy, which some workers believe has now been resolved. Coal Structure The main area of doubt was generally thought to be the uncertainty regarding the extent to which the molecules of a particular sorbate could pene724 In Coal Science; Given, P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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Specific Surface Values for Coals

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trate the pore structure a n d regarding the role of specific interactions between the sorbate molecules and the coal. However, we believe there is a more important fundamental area of doubt—namely, the meaning that can be ascribed to computed values for the specific surfaces of coals. W e have reached these views by considering the dual structural character of coals. C o a l is microporous, w i t h certain partial molecular sieve properties. ( A microporous solid herein refers to that w h i c h contains pores w i t h diameters of a few tens of A . or less.) Micropores can be considered as entities capable of sorbing foreign molecules, a n d it is known that additivity of their sorption potential fields enhances the sorption owing to dispersion interactions. A s the pores become progressively narrower, the vapor adsorption isotherm ( Figure 1 ) in the initial region u p to point Β becomes progressively steeper (toward the

Figure I .

Type I and Type II BET sorption

isotherms

sorption uptake ordinate). T h e differential heat of adsorption shows a corre sponding rise. T h e magnitude of the effect, and the pore-diameter below w h i c h it becomes apparent are all different for different sorbates. Such enhancement, which may act across several molecular diameters of sorbed molecules by induction ( 3 8 ) , can lead to the micropores' becoming completely filled by "condensed" sorbate i n the region of apparent monolayer formation. Although such a drastically distorted isotherm ( T y p e I) can still be represented mathe matically by the L a n g m u i r and B E T equations, values derived for specific surface b y using these equations (or from the point Β value) lack meaning. References to the literature i n which these problems are discussed are indicated in Table I. Coals are "macromolecular,"—i.e., low rank coals, at least, appear to be able to absorb certain molecules such as methanol and hydrocarbons. In l o w and medium rank coals the ultimate units are linked by chemical and physical forces, a n d i n high rank coals physical forces predominate. T h e presence of hydroaromatic structures in low rank coals must lead to rather distorted frame works. Although it is not difficult to visualize that spaces exist i n which foreign

In Coal Science; Given, P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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COAL SCIENCE Table I.

Some References to Literature on the Pore-Narrowing Effect and the Sorptive Behavior of Microporous Solids

Material Type Silicas* Dehydrated zeolites A phosphomolybdate Carbons

Reference 20, 22, 23, 24, 26, 28, 29, 30, 39, and see 3 5, 6, 7,11,16,31 18 2, 4, 8,14,15,19, 23, 25, 31,32, 34, 36, 37, 38 a

The general comments of Gregg (17) are noteworthy. The studies of Kiselev and his co-workers particularly exemplify the pore-narrowing effect and the role of specific (/. 21. 27) sorbate'adsorbent interactions, such as hydrogen bonding, which are not affected by pore narrowing. β


6

molecules may be accommodated and that the entrances, at least, to these spaces must be of molecular diameter, doubt must be expressed as to the meaning of the term specific surface when applied to such a structure. This argument, which has been mentioned briefly by Dryden (13), is in line with the recent views of D u b i n i n and his co-workers (16) on dehydrated zeolites and other microporous solids such as active carbons. Micropore

Filling

by

Methanol

W e should like to give one example illustrating our contentions. Previous work (34, 37) has indicated that micropore filling by methanol at 2 7 3 ° K . occurred with a polymer carbon (treated with hydrogen) and the polymer carbon steam-activated to high burn-off; the activation caused a shift of the knee of the T y p e I isotherm to higher relative pressures. This is in accord w i t h some widening of the pores that must have occurred, and it indicates that the pore-narrowing effect must have been more intense in the unactivated material. W e obtained Type I methanol isotherms at 2 9 8 ° K . for a high rank coal ( 9 ) , its 6 0 0 ° C . char ( 9 ) , and the same char steam-activated to high burnoff, with a similar shift in the knee of the isotherm between char and activate. In contrast, although a 6 0 0 ° C . low rank coal char (9) and activates gave T y p e I isotherms up to at least a relative pressure of 0.95, that of the raw coal (9) was T y p e II, apparently owing to absorption. Summary O u r current views, with some elaboration, are summarized below. W e do not consider it meaningful with coals, or with microporous solids in gen eral, to deduce specific surface values from sorption data, nor even to apply the concept of "specific surface" to these materials. W e believe that "sorption uptake" (moles per unit weight or volume of a given adsorbent) under defined conditions is the correct parameter that should be used to describe the sorptive properties of such materials. Thus, whenever the sorption uptake by an active carbon, for a particular sorbate, is required in a practical application such as solvent recovery, purification, or gas sorption, what should be determined in the laboratory is the uptake under the conditions for which the value is to be used; it is not possible to predetermine unequivocally the value for one



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sorbate from the behavior of other sorbates—except inasmuch as it might be useful to find that in the absence of molecular sieve or absorption behavior, the maximum uptakes (expressed as volume of bulk l i q u i d ) of different vapors by a given microporous structure appear to be constant. In particular, "specific surface" values, determined under one set of conditions, should not be used to calculate sorption uptakes for different practical conditions. Notwithstanding the above comments, we do not deny that adsorption/ desorption isotherms can give revealing information on the pore structures of microporous solids and on the changes these undergo upon various treatments. Further, the heat of immersion "molecular probe" technique should still be regarded as a valuable tool for revealing partial molecular sieve behavior in a given carbon, but we feel it should not be used to estimate specific surface values, of coals for example or even of wide-pore carbonaceous materials since adequate reference materials are not available. Thus, for example, while we ourselves (JO) have used heats of immersion values for a series of carbons to show the relative gross effect that progressively increasing burn-off has on the accessibility of the pore structures to penetrating molecules, we do not n o w consider it valid to translate the values into specific surface values. Since with very fast gas-carbon reactions, where reaction occurs at the external surface and in large pores that effectively extend the external surface, the use of any computed specific surface value that includes a contribution owing to micropores or fine pores must be incorrect, we suggest that greater attention should be given to characterizing directly the structure of such large pores. A m o n g the methods that might be considered for this application are x-ray stereo-microradiography with electron staining ( 3 5 ) , mercury injection porosimetry, and the silver impregnation procedure (33) for obtaining replicas of the pores whose surface area could then be computed. In studies of slower reactions in which the role of fine pores and micropores may become important, the use of a specific surface value involving a contribution owing to the latter must also be inadequate, and here it is more realistic to include an investigation of the penetration behavior, at the same elevated temperature, of a nonreactive counterpart of the reacting gas. Acknowledgment Acknowledgment is made to the British C o a l Utilisation Research Asso ciation for permission to publish this communication.

Literature Cited (1) Aristov, B. G., Kiselev, Α. V., Zh. Fiz. Khim. 37, 2520 (1963). (2) Ανgul', Ν. Ν. Kiselev, Α. V. Kovalyova. Ν. V,. Khrapova, Ε. V., Proc. Intern. Congr.Surface Activity, 2nd, London, 1957, 2, 218 (1957). (3) Babkin, I. Yu., Kiselev, Α. V., Zh. Fiz. Khim. 37, 228 (1963). (4) Barrer, R. M., Proc. Roy. Soc. (London) A161, 476 (1937). (5) Barrer, R. M., Proc. Symp. Colston Res. Soc. 10, 6, 53 (1958). (6) Barrer, R. M., Bultitude, F. W., Sutherland, J. W., Trans. Faraday Soc. 53, 1111 (1957). (7) Barrer, R. M., Sutherland, J. W., Proc. Roy. Soc. (London) A237, 439 (1956). (8) Beebe, R. Α., Millard, B., Cynarski, J., J. Am. Chem. Soc. 75, 839 (1953).



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(9) Bond, R. L., Spencer, D H. T., "Industrial Carbon and Graphite," p. 231, Society of Chemical Industry, London, 1958. (10) Bond, R. L., Spencer, D. H. T., Proc. Conf. Carbon, 3rd, Buffalo, 1957, 357 (1959). (11) Breck, D. W., Eversole, W. G., Milton, R. M., Reed, T. B., Thomas, T. L., J. Am. Chem. Soc. 78, 5963 (1956). (12) Chiche, P., J. Chim. Phys. 60, 792 (1963). (13) Dryden, I. G. C., "Chemistry of Coal Utilization," Suppl. Vol., H. H. Lowry, ed., p. 232, Wiley & Sons, New York, 1963. (14) Dubinin, M. M., Chem. Rev. 60, 235 (1960). (15) Dubinin, M. M., "Industrial Carbon and Graphite," p. 219, Society of Chemical Industry, London, 1958. (16) Dubinin, M . M., Zhukovskaya, E. G., Murdmaa, K. O., Zh. Fiz. Khim. 37, 426 (1963). (17) Gregg, S. J., "Powders in Industry," pp. 79, 93, Society of Chemical Industry, London, 1961. (18) Gregg, S. J., Stock, R., Trans. Faraday Soc. 53, 1355 (1957). (19) Kipling, J. J., Wilson, R. B., Trans. Faraday Soc. 56, 562 (1960). (20) Kiselev, Α. V., Gas Chromatog., Proc. Symp., 4th, Hamburg, 1962, XXXIV (1962). (21) Kiselev, Α. V., Rev. Gén. Caout. 41, 377 (1964). (22) Kiselev, Α. V., Proc. Intern. Congr. Surface Activity, 2nd, London, 1957, 2, 179 (1957). (23) Ibid., p. 189. (24) Kiselev, Α. V., "Surface Chemical Compounds and Their Role in Adsorption Phenomena," Moscow University Press, 1957; transl. by U.S. At. Energy Comm., AEC-tr-3750, p. 93. (25) Kiselev, Α. V., Proc. Symp. Colston Res. Soc. 10, 51 (1958). (26) Kiselev, Α. V., Ibid., p. 195. (27) Kiselev, Α. V., Quart. Rev. Chem. Soc. (London) 15, 99 (1961). (28) Kiselev, Α. V., Uspekhi Khim. 25, 705 (1956). (29) Kiselev, Α. V., El'tekov, Yu. Α., Proc. Intern. Congr. Surface Activity, 2nd, Lon don, 1957, 2, 228 (1957). (30) Kiselev, Α. V., Kulichenko, V. V., Isirikyan, Α. Α., Ibid., p. 199. (31) Lamond, T. G., Marsh, H., Carbon 1, 281 (1964). (32) Lamond, T. G., Marsh, H., Carbon 1, 293 (1964). (33) Lang, F. M., Magnier, P., private communication. (34) Millard, B., Beebe, R. Α., Cynarski, J., J. Phys. Chem. 58, 468 (1954). (35) Nelson, J. B., Proc. Conf. Carbon, 5th, Univ. Park, Penna., 1961, 1, 438 (1962). (36) Pierce, C., J. Phys. Chem. 63, 1076 (1959). (37) Pierce, C., Smith, R. N., J. Phys. Chem. 54, 354 (1950). (38) Pierce, C., Wiley, J. W., Smith, R. N., J. Phys. Chem. 53, 669 (1949). (39) Sing, K. S. W., Swallow, D., J. Appl. Chem. 10, 171 (1960). RECEIVED March 15, 1966.

Discussion R. B . Anderson: W e certainly agree with M r . B o n d that "surface area" i n solids with pores of molecular size has little meaning; however, adsorption studies have provided much insight into the physical structure of coal. A w o r d of caution is i n order on the method of estimating "surface areas" using adsorption of gases at high temperatures. Near the boiling point of the a d sorbate, physical adsorption may be regarded as enhanced condensation; this process is relatively nonspecific, a n d the success of the B E T method probably results from this factor. A t high temperatures the surface is only sparsely occupied b y adsorbate, and the number of molecules adsorbed probably varies significantly w i t h the



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number and k i n d of high energy sites on the surface. Adsorption w i l l occur only on these sites of high energy. F o r this reason "surface areas" estimated from high temperature adsorption may be incorrect by factors possibly as large as 3 to 5. It must be remembered, i n interpreting adsorption isotherms on coal, that coal expands sizably when temperature is increased, and coal swells appreciably during adsorption. These factors are relatively unimportant for porous inorganic solids. O n heating from — 1 9 5 ° C . to room temperature the volume of coal expands about 1 0 % , and at least more or less consistent with this, pore openings at — 1 9 5 ° C . seem to be 4 A . while at room temperature they are about 5 A . Further, on adsorption of water or methanol the volume of coal increases 1 0 - 2 0 % , and with some exceptional polar "adsorbates" coal eventually dissolves. P h i l i p L . W a l k e r , Jr.: I was interested in M r . B o n d s suggestion that we should no longer report the surface area of coals since the figure has little meaning. I w o u l d agree that there is some question as to what areas should be assigned to adsorbate molecules i n the case of adsorption in the very fine pore structure of coals. However, D r . L a m o n d has shown recently that surface areas reported for solids containing pores primarily less than 10A. in diameter, calculated from the B E T equation or the P E equation, agree closely ( 5 ) . Therefore, I do not think that we need be concerned that the areas we report are too high because of the confusion of capillary condensation and multilayer adsorption, which is a problem in activated carbons. K . A . K i n i : The paper by Bond and Spencer shows the skepticism w i t h which they view the problem of the surface area of coal. M o r e than two decades have passed since the question was first raised, and we are still debating it. I wonder whether this debate needs to continue in view of the work on CO2 adsorption by coals by K i n i ( 3 ) , Walker and Geller ( 9 ) , Anderson and co-workers ( 2 ) , and the recent work using krypton and xenon under pressure by K i n i (4). Surely, the data in these papers represent reasonably acceptable values of the total surface areas of coals. The meaning of "surface area" i n pores of molecular dimensions is admittedly a difficult question, but I feel that the difficulties are exaggerated by the authors. George R. K a p o : A n interesting technique has been developed for determining the surface areas of carbon blacks by fatty acid soap adsorption from aqueous solutions ( 7 ) . The technique used is relatively simple and is based on a break i n the potentiometric titration curve of the carbon black w i t h a standard soap solution. It was confirmed that the break in the curve corresponded to a monolayer of soap covering the available surface area. Generally the soap adsorption gave a much lower surface area than the B E T area presumably owing to the 10' difference in diffusivities and to pore size restriction to the large soap molecules. This work was done on carbon blacks to provide a more meaningful criterion for ability of a black to act as a rubber reinforcement agent. It is suggested that this technique be applied to coal and coke to provide a realistic estimate of the available surface area (compared with the B E T area)


COAL SCIENCE


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for l i q u i d phase reaction involving large molecules. F o r example, semicokes and coke breeze ( w h i c h have different B E T areas) have been found to have the same capacity for absorbing large molecules such as thermobitumens ( 6 ) . Also, Naucke has reported a method for determining the surface area of cokes b y phenol adsorption from solution ( 8 ) . R. L . B o n d : W e are grateful to the contributors for their comments. It is clear that various techniques for estimating apparent specific surfaces of coals provide reproducible values a n d that the sorption data provide useful com parative information. However, w e still feel that the only reliable data are the "sorption uptakes" of a given sorbate by a given adsorbent, and these should not be converted into specific surface values, i.e., b y using values for apparent monolayer capacities. W e hope to support our contentions more fully elsewhere, and the com ments of the various contributors w i l l be given full consideration, but we w o u l d like to note here with respect to D r . Kini's remarks that B E T specific surface values for low rank coal, using CO2 at 195°K. (3) or xenon at 2 7 3 ° K . (4) as sorbate, can agree with the corresponding values obtained using methanol at 3 0 3 ° K . (2, 4) which are spurious. Literature Cited (1) Anderson, R. B., Hall, W. K., Lecky, J. Α., Stein, K. C., J. Phys. Chem. 60, 1548 (1956). (2) Anderson, R. B., Hofer, L. J. E., Bayer, J., Fuel 41, 559 (1962). (3) Kini, Κ. Α., Rept. Intern. Congr. Ind. Chem., 27th, Brussels 2, 110 (1954). (4) Kini, Κ. Α., Fuel 43, 173 (1964). (5) Lamond, T. G., Marsh, H., Carbon 1, 281 (1964). (6) Lowry, Η. H., "Chemistry of Coal Utilization," Suppl. Vol., p. 182, John Wiley & (7) Maron, S. H., Bobalek, E. G., Fok, S. M., J. Colloid Sci. 11, 21 (1956). (8) Naucke, W., Brennstoff-Chem. 44, 302 (1963). (9) Walker, P. L., Jr., Geller, I., Nature 178, 1001 (1956).


General

Discussion

731


General Discussion Peter H . G i v e n : There are two general matters related to the theme of this session on which I should be interested to hear discussion. In some coal combustion systems the ignition process occurs at relatively low temperatures. N o doubt the character of this process depends on the particle size and mode of ignition (by radiation, flow of hot air, etc.) and hence on the radiation intensity or air temperature and flow rate. However, if one compares the ignition of a series of whole coals of different rank under the same physical conditions, the property of the coal that determines the change of ignition characteristics with rank is no doubt the nature and quantity or rate of release of volatile matter in the crucial temperature range ( 3 5 0 ° 4 5 0 ° C . , approximately); this idea was implied i n D r . Essenhigh's paper i n this session ( D r . Essenhigh's verbal presentation with J . B. H o w a r d on " C o a l Combustion Phenomena and the Two-Component Hypothesis of Coal Constitution"). If this idea is accepted, then the ignition characteristics of a coal may well depend on its pétrographie composition. T h e volatile matter released by exinites is different in composition from that released by vitrinites, the yield of volatile matter is much greater ( 2 - 3 times with many bituminous coals), and the rate of release at 3 5 0 ° - 4 5 0 ° C . is probably higher. O n the other hand, the inert macérais yield little volatile matter at a l l , and virtually none at low temperatures. It may well happen therefore that two whole coals of apparently similar rank but different pétrographie makeup could have different ignition characteristics. This seems to be a worthwhile topic for research. W e have heard several papers on the reactions of coal or coal volatiles under conditions of very high energy input (in plasmas, laser irradiation, etc.). Potentially useful yields of valuable chemical raw materials result. T h e work therefore appears to be a new variety of cook-book preparative chemistry i n which new and highly active species are involved as intermediates, and it is not easy for the ordinary chemist to visualize what goes on and what these intermediates are. W o u l d any of the authors care to comment? Marie-Therese M a c k o w s k y : I believe that during combustion in wet or dry boilers, especially i n wet boilers in which one has a temperature i n the fire chamber around 1 7 0 0 ° C , the differences between macérais are not as important as transport phenomena. I am involved with coal petrology and coal microscopy, but I feel that there are some problems which can't be solved by coal petrology. R. H . Essenhigh: D r . Given has accurately summarized and interpreted our views—namely, that ignition appears to be established in the first pulse of volatiles generated i n the temperature range ( 3 0 0 ° - 4 5 0 ° C . ) . Given that condition, coals of nominally similar rank (by total carbon or V M %) but having different ignition characteristics because of different "inflammabilities" are most likely to have these differing characteristics because of different pétrographie makeup. W e would agree with D r . Given that this could be a most worthwhile and fruitful topic for research.


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Experimental results obtained very shortly after this Conference demonstrated a need to modify our views to some extent. W e expected coals to break u p at their normal decomposition temperature, even i n flames under conditions of fast heating. In the plug flow reactor we have been operating, the rates of heating are 1 0 C . / s e c , and the new results show that this is so fast that the particles can reach temperatures of 100Q°C. before significant pyrolysis sets i n ; we would expect the previous comments to hold true only under conditions of slow heating, say not exceeding 1 ( P C . / s e c . However, we would still expect inflammability characteristics to depend on pétrographie makeup. Downloaded by IOWA STATE UNIV on October 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch047

,o

In reply to D r . Mackowsky's comments we would concede that other factors may well dominate i n boilers at 1 7 0 0 ° C ; we would submit that the influence of coal constitution may perhaps still be felt. In the first place the combustion of coal u p to 1500° or 1600°C. at least is no longer regarded as being determined completely by transport processes. It is now clear that the role of boundary layer diffusion has been substantially overestimated. That being so, the nature of the reactant and its reactivity become important. T h e results referred to above indicate that there can be a significant residue of volatiles left i n the solid coke (identified as the carbonization p o l e ) . T h e temperatures involved ( 1 5 0 0 ° C . ) are within 200° of those cited by D r . Mackowsky, a n d the influence times (approaching one second) are also comparable. W e must therefore accept the probability that the material burning out is a carbon-hydrogen complex—even i n wet bottom boilers—and is therefore likely to have a higher reactivity than pure carbon. A s far as the rank or pétrographie constituents are concerned, these however, should not affect the situation if it is true that (a) a l l pyrolyzing coals pass through the carbonization pole, and (b) they stop pyrolyzing at the pole (even i n boilers) provided that the coal is initially of lower equivalent rank than the carbonization pole. This, of course, says nothing of ignition. Broadly speaking, rank does affect ignition; otherwise there would be no difficulty i n burning anthracites and no need for designing different boilers for anthracites. The significant point, nevertheless, is the strong possibility of finding volatiles still in the residue, even i n boiler combustion chambers operating at 1700°C. Andrew G . Sharkey, Jr.: Positive, negative, and neutral species have been found by probe techniques i n flames. Direct mass spectrometric techniques should lead to identification of many of the primary species obtained by heating coal to extreme temperatures. Robert A . F r i e d e l : T o D r . Sharkey's remarks on mass spectroscopy I would like to a d d a few pertinent comments on other types of spectroscopy which would be applicable to the study of species obtained from coal subjected to plasma, laser irradiation, and similar high energy processes. H i g h speed emission spectroscopy has been used to study free radicals and positive, negative, and multiple ions produced in explosions and flames. M a n y excited states would exist for many different species from coal subjected to high energy. Complex spectra would result. T h e combination of electronic-vibration-rotation transitions observable in emission spectroscopy


General Discussion

733


would further complicate the spectra. However, the spectra may not be any more complex than coal itself. H i g h speed infrared spectroscopy is also available and has been used to study explosions and shock-tube experiments. Specific information on vibration and rotation states can be obtained by this method. Again the spectral information would be complex, but at least the electronic transitions are hereby eliminated. Microwave spectroscopy is another possibility for studying high energy processes. T h e technique is most suitable for investigating rotational ground and excited states of small molecules. Practically a l l vibration transitions are eliminated by operating in this region of the spectrum. T h e technique closely allied to microwave spectroscopy—electron paramagnetic resonance—is certainly a feasible method for studying free radicals in situ in high energy processes. Studies of radiation processes in situ are being carried out. F o r example, the free radicals produced from organic vapors bombarded by electrons and gamma rays can be measured during the radiation process. It should be possible to use similar techniques for investigating organic substances subjected to high energy. In addition to direct measurements on high energy processes it should also be possible to isolate species from a plasma—for example, by freezing them on cold surfaces or in matrices. Observations by various spectral methods, including absorption, scattering ( R a m a n ) , and emission (luminescence), could then be used at leisure to study ions, radicals, and other species. For studying metals in high energy processes, the flame emission technique should be applicable. A more sensitive technique for investigating metal ions is the atomic absorption method, in which the high energy flame or plasma absorbs the energy from a source emitting the spectrum of the metal being studied. H . R. L i n d e n : H i g h temperature pyrolysis of coal with high energy sources seems to follow readily predictable paths similar to hydrocarbon pyrolysis. The effects of pressure, gas atmosphere, reaction time, and the "volatile matter" content of the coal bear the same relationship to yields of methane, ethane, ethylene, acetylene, and hydrogen as for simple hydrocarbons. Effective reaction temperature, although not directly measurable, could be estimated by means of a suitable chemical thermometer, such as the Ci»Hi»-C.»H4-H-.» system which approaches equilibrium very rapidly. As D r . Given also noted, equating the "volatile matter" to the reactive portion of the coal is an oversimplification but adequate for empirical purposes; the C H ratio of the coal would probably be more suitable. D r . Sharkey: Attempts were made to use C H ratio following laser irradiation as an indication of the reaction temperature, as D r . L i n d e n suggests. Collecting the irradiated coal was difficult, and special techniques are now being devised. Use of a spectrum-line reversal technique is another possibility. R. L . B o n d : In studying high temperature systems the problems associated with minerals in coal should also be investigated, and I would make a plea that as much work should be done on the minerals at high temperatures as on the carbon/hydrogen-containing particles.



734 COAL SCIENCE


The Determination and Use of Specific Surface Values For Coals

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