Coal Science

39 The Dry Oxidation of Subbituminous Coal E. J. JENSEN, N. MELNYK, J. C. WOOD, and N. BERKOWITZ

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 21, 2015 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch039

Research Council of Alberta,

Edmonton,

Canada

Detailed results of a study on the formation of humic acids during dry oxidation of a typical subbituminous coal are reported. The reaction was carried out in a closed circuit system at temperatures between 180°-300°C.

and oxygen concen-

trations in the range 10-90% and was allowed to proceed for approximately 100 hours in most test runs. Particular attention was directed to the development of acidity and alkali solubility in the reactant mass. Data thus obtained are used to outline a general reaction path and to delineate some of the principal chemical changes which appear to occur during the oxidation process. Of special interest is the observation that significant formation rates are confined to a strictly limited temperature range and that even here, alkali solubility and acidity may be consequences of two essentially unrelated reactions.

gy

a convention w h i c h apparently originated from observations b y C . Vauquelin (14), A . Klaproth (6), and R. Jameson ( 5 ) , alkali soluble organic constituents of soil and coal are designated as "humic acids." C o l lectively, these substances make u p an ill-defined series of dark colored, weakly acidic solids w h i c h form whenever plant components (such as lignin) are exposed to fungal oxidases, or when coal is allowed to weather or otherwise oxidize. Some evidence now exists that humic acids isolated from these varied sources do indeed contain several common peripheral structures (2, 11, 16), but b y any more discriminating test, the term possesses little chemical significance or fixed meaning. Prevailing uncertainties are perhaps most clearly illustrated b y the properties of humic acids obtained from a nominally single source—e.g., oxidized coal. Such humic acids w i l l resemble each other i n color and typically have equivalent weights around 250. However, depending upon factors which are 621 In Coal Science; Given, Peter H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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COAL SCIENCE

still not fully understood, their molecular weights can range from less than 600 to more than 10,000. Their acidities can assume values between ca. 7 and 12 meq. per gram. Their - O H / - C O O H ratios can vary from 0.8 to 2.5. W h i l e a rather close structural relationship between them and the parent coal possibly may be taken for granted, the nature of this relationship and the manner i n w h i c h humic acids form from coal remain equally unknown. A l l that can be asserted w i t h confidence is that humic acids are, i n themselves, reaction intermediates w h i c h can be easily further degraded by oxidation. In these circumstances—and i n v i e w of increasing industrial interest i n coal-based humic acids as chemical source materials (4)—we thought it pertinent to reinvestigate the mechanism of humic acid formation and, as a first step, to direct particular attention to the development of acidity and alkali solubility during progressive uncatalyzed oxidation of a subbituminous coal (Table 1) w i t h dry oxygen. The choice of this particular system is, pnma facie, arbitrary since conversion of coal into humic acids can, i n principle, be accomplished b y several methods. ( A m o n g those commonly used are reactions Table I.

Composition of Coal

Proximate Analysis

Ultimate Analysis

Equilibrium Moisture Basis

Dry Basis

23.0

0.6

Moisture % Ash% Volatile Matter % Fixed Carbon %

0

7.3

9.4

28.6

36.9

41.1

53.1

100.0

100.0

% daf Carbon

72.1

Hydrogen

4.3

Sulphur

0.4

Nitrogen Oxygen (bydiff.)

1.4 21.8 100.0

• As charged to reactor

of static or fluidized coal beds w i t h air or oxygen; " w e t " reactions with H2O2, ΚΜηθ4, or HNO3, the latter yielding so-called nitro humic acids; reac tions with aqueous alkali and air or oxygen under pressure.) It is, however, worth recalling that oxidizing coal with liquid-phase reagents, while usually much faster than dry oxidation, is characteristically less subject to control, and its greater severity w i l l also result i n reduced yields of humic acids (and i n correspondingly greater proportions of secondary degradation p r o d u c t s — notably water soluble acids—in the reactant mixture). These unnecessary complications we wished to avoid. Experimental

Procedures

In order to permit meaningful measurements w h i c h could be related to particular reaction conditions, the oxidations were carried out in the apparatus shown in Figures 1 and 2. This gas-tight, closed circuit installation allowed pressure and temperature control, gas monitoring, and periodic withdrawals of small coal samples while reaction proceeded. The total free volume of the apparatus amounted to some 2200 cc.

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


39.

JENSEN ET AL

1) 2) 3) 4)

623

Dry Oxidation

Fluldlzer 7) C 0 absorber Heater 8) Connectors to 0 analyxer Coal sampler 9) Manometers Dust absorber 10) Pressure regulator Gat sampling valve 11) Pump unit (6 Uters/mln.) 6) H 0 Total volume of apparatus 2200 cc. 2

2

2

Figure 1.

Coal oxidation apparatus (not to scale)

F o r each r u n , the fluidized bed reactor was charged with 15.0 grams of freshly ground (-t35 + 150 mesh) coal w h i c h had been dried i n vacuo at 5 0 ° C . Operating pressures were always kept to a few millimeters above atmospheric, but temperatures i n the different test runs varied between ca. 180° and 3 0 0 ° C . (Below 1 8 0 ° C , little or no reaction occurs even when reaction periods are extended to several hundred hours; above 3 0 0 ° C , excessive burnoff, as evidenced b y very high H2O and CO2 yields, is encountered.) Oxygen concentrations i n the various runs ranged from 10 to 9 0 % a n d were closely controlled b y admitting oxygen at rates which always just balanced the rates of oxygen consumption. T h e latter were measured b y a calibrated cylinder which formed one-half of the constant-head oxygen supply bottle (Figure 2 ) , and instantaneous oxygen concentrations were continuously monitored on a Beckman F 3 oxygen analyzer. D u r i n g the first 30 minutes or so


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COAL SCIENCE


f 1) 2) 3) 4) 5) 6) 7) 8)

4 liter O 2 - réservoir 280 cm measuring column Adjustable mercury-filter disc air trapfor(2) O 2 - trap Manometer H 0 absorber Mercury bead capillary valve Auxllllory valves 2

Figure 2.

Oxygen supply apparatus (not to scale)

after start-up and attainment of reaction temperature, oxygen consumption rates would occasionally exceed the capacity of the supply valve; when this happened, an auxiliary oxygen source was switched into the line. In no case, however, d i d oxygen concentrations in the system deviate from the chosen value by more than a few percent. The principal off-gases (H2O and CO2) were routinely collected in tared scrubbers and quantitatively determined by weighing. However, as each run progressed, 1-cc. gas samples were also withdrawn from time to time and analyzed by gas chromatography. Particular attention was paid to oxygen (to check the performance of the Beckman analyzer) and to carbon monoxide. A t


39.

JENSEN ET AL

Dry

Oxidation

625

regular intervals 0.1-0.2-gram coal samples were withdrawn for infrared spectroscopy, elemental analysis, and functional group determinations. Measurements of alkali solubles in these coal samples—conventionally accepted as indices of humic acid concentrations—were initially performed by using Kreulen's method (7). However, even when the most stringent precau tions were taken to exclude air, this method yielded markedly time-dependent results (presumably owing to oxidation of the coal by the relatively strong alkali solution), and a more satisfactory colorimetric technique (by J. F . Fryer) was therefore employed. This entailed extracting the coal sample with 0.1 λ aqueous sodium hydroxide for 16-20 hours i n an inert atmosphere and subse quent photoelectric scanning of the extract solutions. Actual humic acid con centrations were then obtained from specially constructed reference curves which related optical density (at an appropriate wavelength) to humic acid contents. The inherent error in this determination is estimated at less than 10%. Total acidities and concentrations of carboxylic acid groups were measured by what are now well-established procedures ( I , 8).


7

Results The resultant data, exemplified by Figures 3 to 15 and by the material balances summarized in Table II, outline a definite and rather characteristic pattern. As might have been anticipated, carbon and hydrogen contents of the oxidizing coal decrease progressively as reaction proceeds and tend towards limiting values which depend on the severity and duration of the reaction. (Under the conditions of the present study, these limits lie in the neighbor hood of 6 1 % and 1.5%, respectively). Similarly, total acidity and carboxyl contents (with the difference be tween these two parameters almost entirely accounted for by the concentra tion of phenolic-OH) rise rapidly towards asymptotic limits from which, how ever, they w i l l again decline if reaction conditions are severe—i.e., if reaction temperatures and oxygen concentrations are high. Concentrations of — C O O H typically remained below ca. 5 meq. per gram, while the total acidity attained values which, depending on conditions, could range from 9 to 12 meq. per gram. (In the parent coal, which contained only traces of alkali soluble matter, — C O O H and total acidity amounted to ca. 0.6 and 6 meq. per gram, respectively. ) O n the other hand, concentrations of humic acids—i.e., of alkali solubles in the reacting coal mass, start in the range 0 - 5 % (consistent with minimal concentration in the parent coal) and show a very pronounced, if somewhat diffuse, general dependence on reaction conditions. For example, at 1 8 0 ° C . and oxygen concentrations below about 2 0 % , conversion to humic acids is negligible, and significant reaction rates at this temperature are only observed when [ O ] > 5 0 % . However, as temperatures rise, reaction rates at all oxygen levels increase until, at 2 5 0 ° C , even relatively low oxygen concentrations w i l l generate large humic acid yields within a few hours. At still higher tempera tures, oxygen concentrations in excess of 20% cause very rapid conversion and, thereafter, destruction of humic acids by secondary degradation. Graphs of humic acid concentration vs. time w i l l therefore then pass through a more or less clearly discernible maximum.


COAL SCIENCE


626

Significantly, very rapid conversion of coal into humic acids is always accompanied by a sharp increase in oxygen consumption rates, formation of large quantities of water and carbon dioxide, and a marked decline in the amounts of recoverable coal—i.e., by all the symptoms of extensive combustion. However, similar (if less rapid) carbon losses to burn-off are also observed under much less severe reaction conditions, and such losses will steadily accumulate even when the net rate of humic acid formation has fallen to zero—i.e., when concentrations of humic acids in the oxidizing coal mass have become independent of reaction time. Steady evolution of carbon dioxide and water is therefore a general feature of the reaction. Except at relatively low temperatures and oxygen levels, where conversion to humic acid is slow and where stripping losses can be more or less effectively balanced by adding oxygen to the reacting coal mass, this manifests itself also in increasing weight %HA

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100 ο

90

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time in hours

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Legend and Explanation for Figures

3-15

Experimental conditions are indicated under each graph. The following abbreviations and symbols are used: • C = carbon content (% ) of solid reactant O H = hydrogen content ( % ) of solid reactant Φ TA = total acidity in meq./gram θ C O O H = carboxyl content in meq./gram β HA = Tiumic acid"—i.e., alkali soluble material in w/w % of reactant Θ O2 = cumulative oxygen consumption in liters (STP) Ο CO = carbon monoxide content in volume % of reactor gas The abscissae and ordinates are self-explanatory, but note that the C O ordinate should be read on the 0-100% ( H A ) scale.


39.

JENSEN ET A l . Ί


627

Dry Oxidation ί

~4Ô

6 ·

\\mt in h e u r t

180 C

2 I % 0 «

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Figure 4 %C%H

time in bourt

I80*C

5 0 *

0,

Figure S


COAL SCIENCE

628

%C%H

%HA 100

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time In keurs 180· C

70%

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Figure 6

40

60

time in hours

225*C

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Figure 7


39.

629

Dry Oxidation

JtN&N ET AL %HA 100

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Coal Science

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