electrochemical oxidation of coals: voltammetry and mass spectrometry

Short Communications

3 Chan, L. K., Sarofim, A. F. and Beer, J. M. Combust. Flame 1983,52, 7 Tsujimura, M., Furusawa, T. and Kunii, D. .I. Chem. Eng. Jpn 1983, 37 16,524

4 Kunii, D., Wu, K. T. and Furusawa,T. Chem. Eng. Sci. 1980,35,1701 8 Hasatani, M., Yuzawa, S. and Arai, M. Kagaku Kogaku Ronbunshu

5 Furusawa,T.,Tsunoda, M. and Kunii, D. Am. Chem. SOL Symp. Ser. 1982,8,45 1981, l%, 347 9 Kojima, T., Take, K., Kumii, D. and Furusawa, T. .I. Chem. Eng. Jpn

6 Tsujimura, M., Furusawa,T. and Kunii, D. J. Chem. Eng. Jpn 1983, submitted for publication 16, 232

Electrochemical oxidation of spectrometry

Norman Taylor, Christopher Gibson, Richards*

coals: voltammetry and mass

Keith D. Bartle, Derek G. Mills and D. Gareth

Department of Physical Chemistry, University of Leeds, Leeds LS2 9JT, UK (Received 23 May 1984; revised 16 August 7984)

The anodic oxidation of sulphuric acid slurries of lignite, bituminous, and anthracite coals; a high-yield extract of coal; and carbon black, has been investigated voltammetrically and with simultaneous m.s. monitoring of evolved gases. The previously reported evolution of carbon dioxide at very low voltages was shown to be chemically released from minerals. The low rank coals and the asphaltite did show considerable electrochemical activity near to 1.0-I .2 V, mainly from the conversion of leached ferrous ion to the ferric form. Further activity near 1.4-1.5 V arose from the oxidation to CO, of leached organic matter. For well-washed low rank coals, the coal extract and the anthracite, CO, release was only observed at high (> 2.8V) anode voltages, with simultaneous release of oxygen. Electrochemical gasification of coal via anodic oxidation of macromolecules at low voltages did not occur.

(Keywords: coal; electrochemical oxidation: voltammetry: mass spectrometry)

The electrochemical conversion of coal and its components via anodic oxidation has been the subject of some interest in the past. Much of the early worklp3 was aimed at an understanding of coal structure via degradative oxidation, but more recent investigations4-l4 have foc- ussed attention on the energy aspects, and coal fuel cells incorporating either cathodic consumption of oxygen or cathodic production of hydrogen have been described.

The reactions in the latter have been termed electrochemical gasification by Coughlin and Farooque6*7 and according to these workers have advantages over electrolysis of water for electrochemical hydrogen production, notably because of the lower electrical energy requirements (around one sixth). The observation that no hydrocarbon fuel electrode had previously been found to operate near its reversible potential’ led the present authors to re-examine electrochemical gasification, and in a previous preliminary publication’ 5 the roles of ferrous ion and chemically released carbon dioxide in the results of Coughlin and Farooque were demonstrated. In this Paper, more details of these aspects are given and the electrochemical conversion process is described over a wide range of conditions.

EXPERIMENTAL

The glass cell (Figure I) was essentially similar to that used

*Present address: National Coal Board, Coal Research Establishment, Stoke Orchard, Chehenham, Gloucestershire CL52 4RZ, UK

COl6-2361/85/030415-05$3.00 @ 1985 Butterworth & Co. (Publishers) Ltd

by Coughlin and Farooque. The anode (area 60 cm2) and cathode (area 20 cm2) were fabricated from platinum gauze. Pure nitrogen was aspirated through the anodic compartment to sweep product gases to a four-channel mass spectrometer (Vacuum Generators Model 501) monitoring N,, O,, CO2 and argon. A production rate of lo-r2 m3 min-’ gas could be detected.

The coals, ranging in rank from lignite to anthracite and a Turkish asphaltite (Table I) were ground to 200 mesh (75 ,um) for most experiments. A supercritical gas (SCG) extract of coal supplied by the National Coal Board Coal Research Establishment, and a carbon black

______- _____--_. - __..______ ______ ______- -..-_---. :

Figure 1 Apparatus used for the anodic oxidation of coals

FUEL, 1985, Vol 64, March 415


(Vulcan III, Cabot Corporation) were also examined. Current-voltage curves were recorded for slurries con-

taining between 15 and 150 g dme3 coals and related materials in 4 mol dme3 sulphuric acid as electrolyte. The applied voltage was produced by a triangular-wave voltage generator coupled to a potentiostat in the voltage- follower mode. In some experiments, a three-element system was used in which a salt bridge containing 4moldme3 sulphuric acid electrolyte connected the cell to a saturated calomel reference electrode (SCE). The cell current, cell voltage and refer;ence potential were recorded simultaneously by a Servogor 461 three-channel recorder. Potential scan rates were in the range 5&5OOmVs-‘, generally 100 mV s -I. Analysis of inorganic ions leached from the coals was made by standard methods including polarography and atomic absorption spectrophotometry.

RESULTS AND DISCUSSION

The mass spectrometer readily detected chemical release of CO2 (i.e. before any current was passed) resulting from the attack by the acid electrolyte on the carbonate minerals (Figure 2), the rate being strongly dependent on the ash content of the coal. For example, for the Tuncbilek lignite the CO, release could be observed for several days, and a total equivalent to some 10 cm3 at atmospheric pressure was released. For anthracite only x 1 x lo-’ of this quantity was released. There was no detectable CO, release from the coal extract or from the carbon black. The variation in the ratio of hydrogen and COz production observed by Coughlin, Farooque6*’ and

150 I

Tlme/mfns

Figure 2 M.s. monitoring of chemical release of CO2 from a slurry of Tuncbilek lignite in 4 mol dmm3 sulphuric acid without

passage of current

others’-” is, in the present authors’ view, due to the different inorganic origins of these gases.

Electrochemical activity due to leached inorganic material The electrolysis experiments did not continue to the

point where the level of CO2 release had fallen below the detectable limit. This approach contrasts with other work’ O in which chemical release evidently accompanies electrolysis. Electrochemical activity generally arises at an applied cell voltage near 0.5 V and increases markedly

Table 1 Analytical data for coals and related substances electrochemically oxidized

Coals

Bituminous

Lignite (Tuncbilek)

1 2 (Markham (Dew Main) Mill) Anthracite

Supercritical gas Asphaltitea Carbon black extract of (Avgamasya) (Vulcan Ill) bituminous coal

C (wt%, daf) 74.5 82.7 82.6 91.1 79.6 99.4 82.5 H (wt%, daf) 5.4 5.0 5.2 3.3 6.4 0.6 6.9 0 (wt%, daf) 16.0 9.0 9.7 2.5 ND ND 8.5 N (wt%, daf) 2.5 1.85 1.45 1.3 1.8 Nil 1 .l S (wt%, as received) 1.7 1.55 1.6 1.4 7.1 <O.l 0.95 Cl (wt%, as received) <0.02 0.42 0.24 0.30 ND Nil GO.1 Ash (wt%, dry basis) 16.4 3.8 11.8 3.7 37.4 Nil Nil

Moisture (wt%, as received) 11.5 7.6 5.4 1.7 0.4 Nil Nil

Volatile matter (wt%, daf) 46.8 37.7 40.6 ND 45.4 ND 100.0

aAn asphaltic substance, coal-like in appearance, from which bitumen may be extracted”. Classified as an aromatic asphaltic oil altered during

migration. Originating in SE Turkey ND, Not determined

Table 2 Concentrations of inorganic ions in filtrates from coal and asphaltite slurries (37.5 g dmm3 coal in electrolvte)

Juncbilek Markham Main Daw Mill Avgamasya

lignite coal coal asphaltite

Fe’+ (ppm) 2350 380 340 360

Fe3+ (ppm) 900 Mn+ (ppm) 37

Total halides (ppm) 10

416 FUEL, 1985, Vol 64, March


above zz 1.6 V due to oxygen production at the anode (Figure 3). There are distinct current plateaux in the linear-sweep voltammogram in the regions of + 1.0 and + 1.4 V at the anode, where the currents for bituminous and lignite coals are respectively about 10 and lOO-fold that for electrolyte alone.

Experiments with the filtrates of the coal slurries (Figure 3b), which retained activity below 1.6 V, showed conclusively that much of the electrochemical activity, particularly at the lower voltages, resided in the conversion of leached ferrous ion to the ferric form’ 5. This has been confirmed by Baldwin et a1.16, Okada et ~1.~ and others’-“. Mn’+ and Cl- ions were also shown to be present (Table 2). The small release of Fe2 + from the anthracite and the coal extract correlated with the low electrochemical activity, although the carbon black samples showed considerable initial activity above 1.3 V. The latter was thought to be due to a surface oxidation process enhanced by the large surface area of these samples, and possibly involving the polycyclic aromatic compounds known to be adsorbed on carbon black particles’ *.

The release of Fe2 + was initially rapid for the bituminous coals, the lignite and the asphaltite and continued over a period of days. Some relative rates of Fe2 ’ release are given in ref. 15. Correspondingly, electrochemical activity below 1.6 V was very much reduced after prolonged aqueous washing (Figure 4). The lower voltage plateau (1.0-1.2 V) could, however, be restored by adding Fe2 + ions at concentrations similar to those in the original slurries (Figure 4d).

Activity due to organic material

The re-establishment of only the first voltammetric plateau on restoring the major inorganic components to slurries of washed coals (Figure 4cl) suggests a different origin for the electrochemical activity of coal slurries near 1.4 V. The current in this region is greater for the lignite than for bituminous coals, and very small for the anthracite and the coal extract. The second oxidation process evidently involves organic material leached from the coal since it is also exhibited by the filtrates (Figure 3b). The similarity of the current-voltage curves for the lignite slurry and the filtrate, and the absence of activity for the well washed coals, both suggest that the coal macromolecule has only small electrochemical activity below +2.5 V at the anode. There was also a corresponding decrease in the electrochemical activity of the well-washed lignite in the region of both plateaux. For the filtrate alone, the total current efficiency for 0, and CO, production (assuming 2e and 4e per molecule) ranged from 50 to 70x, with increasing importance of 0, production in the range 1.6-2.0 V.

M.s. confirmed production of carbon dioxide at com- parable rates and current efficiencies from the filtrates and corresponding coal slurries by oxidation at anodic voltages > + 1.5 V. Greatest activity is observed for slurries of the unwashed lignite which show considerable rates of CO, release (z10-6m3min-‘) at z +1.6V at the anode, although oxygen is also evolved here. CO2 evolution is directly related to coal rank, with the anthracite showing detectable CO2 production only at anode voltages > +2.8 V, accompanied by large amounts of oxygen. Unwashed lignite samples gave current efficiencies of CO2 and O2 production of 10-15x and 4065% re-

700 -

600 -

d

0' I I I I I

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Anode Voltage/V

Figure 3 Current-voltage curves at 60°C for slurries of: (a), Tuncbilek lignite; (c), carbon black; and (d), Daw Mill coal in 4 mol dme3 sulphuric acid. Curve (b) is that from the filtrate of the lignite slurry

"i.6 08 1.0 1.2 1.4 1.6 1.8 2.0

Anode Voltage/V

Figure 4 Current-voltage curves at 60°C for slurries of Tuncbilek lignite in 4 mol dmm3 sulphuric acid: (a), original coal; (b), after washing the coal with electrolyte; and (c), after further washing. Curve (d) is that from the slurry of the well-washed coal with 1500ppm Fe*+ added

spectively near 2V, the former dropping to l-2% with repeated washing with fresh electrolyte. Here no CO, was detectable much below this voltage. The current efficiency for 0, production in the latter system was almost lOO%, thus indicating participation in the electrochemical activity by the variety of redox systems leached from the unwashed samples (Table 2). Dhooge et al. observed” an increase in COz release rate for San Juan coal after adding Fe’ + to the slurry, and again on subsequent electrolysis. In the present study the results for the coals are in strong contrast; addition of Fe3+ ions with the stringent ex- clusion of CO2 did not enhance CO, release from the lignite samples, although oxidations not yielding gaseous

FUEL, 1985, Vol 64, March 417


product cannot be excluded. The addition of Fe’+ also serves to enhance oxidations down the redox chain. It is not clear at this stage how thermodynamic and kinetic factors are related in possible incomplete oxidation of the coal macromolecule by the redox systems, although the results of Dhooge suggest that such a process occurs,

Experiments were also carried out in which the ions Ni’+, Cr2+, Fe’+, MnZf, Cl- and Br- were introduced separately into the electrolyte/washed coal slurry (Table 3). For Ni2 + and Cr2 +, the eficiences of both CO, and total gas evolution decreased. For halide ions, a marginal increase in CO, evolution efficiency was observed, while that for total gas decreased. These results indicate a reduction in the fraction of current leading to gaseous products. The presence of Fe2 + ions in low concentration (~200 ppm) led to increased CO, evolution efficiency, while that for the total gas decreased; the current was little changed, suggesting Fe2 + as a catalyst for oxidation to gaseous products. However, the increase in the rate of CO, evolution was small, and with increasing Fe’+ concentration the CO2 and total gas evolution efficiencies decreased dramatically while the current increased. These observations indicate that the major involvement of Fe2 + at the concentrations found in slurries of unwashed lignites is as a current consumer, leading to an overall decrease in gas evolution efficiencies. In the presence of Mn2+ ions, the CO2 evolution efficiency increased to % 90%. Since Mn2 + was observed in the lignite washings (Table 2), the operation might be suspected of a composite redox system responsible for CO, production during electrolysis of coal slurries, especially since higher oxidation states of manganese oxidize coal. However, in the presence of Mn2 + and Fe’+ together the CO, evolution efficiency fell to a more normal level, presumably because Fe2 + now reduced any oxidizing manganese species.

Acid-soluble organic matter (fulvic acids) leached from the lignite and bituminous coals by the electrolyte are evidently responsible for the CO2 release. Both humic and fulvic acids are known to liberate CO, on anodic oxidation in acid electrolyte’*3. For well-washed coals and the coal extract, CO2 release is observed only at the highest anode voltages (> + 2.8 V) and may indicate some conversion of macromolecule. The current efficiency for CO, is, however, very low at ~0.1%.

CO2 production from essentially inorganic free systems was further investigated using high surface area carbon blacks and anthracite samples. In the former case there was the only indication that CO2 production without O2 production increased. The anthracite samples showed low CO2 activity up to the highest voltages used (3.5 V). There was, however, some evidence from the current efficiencies for the formation of non-gaseous oxidation products. The processes in this voltage regime are probably similar to those suggested l9 by Senftle et al. as those occurring at anthracite anodes, i.e. hydroxyl radical formation and subsequent formation of humic acid materials. In preliminary experiments reported elsewhere” using humic acid extracts from low-rank coals, some reaction at lower voltages (z 1.7 V) was observed leading to molecular mass reduction via apparent attack at ether linkages but without CO, production.

CONCLUSIONS

1.

2.

3.

4.

5.

Voltammetric and mass spectrometric experiments show that coal slurries in 4 mol dm-3 sulphuric acid exhibit two kinds of anodic activity below 1.6 V across the cell. Currents below 1.3 V are largely attributable to the oxidation of inorganic ions, particularly Fe2 ‘, while currents in the 1.4-1.7 V region apparently originate in the organic material leached from the coals. Both kinds of activity can be suppressed on prolonged washing of the coal with electrolyte, and are present in the filtrates from the slurries. The magnitudes of both currents depend markedly on coal rank, with a lignite showing greater activity than two bituminous coals, and an anthracite only small activity. A supercritical gas extract of coal showed no anodic activity below 1.6 V. Appreciable CO, production was only observed in conjunction with O2 gas production, except for carbon black.

6. Inorganic ions leached from the coal play no signi- ficant role in oxidation to gaseous products. Their main influence is to reduce the fraction of the current leading to gas evolution. Electrochemically generated

Table 3 Effect of added inorganic ions on gas evolution efficiencies= for anodic oxidation of slurries of washed Tuncbilek lignite

Threshold Mean %eff Doped with: voltage IV) currentb fmA) %eff (COs) %eff (0.2) (total gas)

None (unwashed) 2.02 730 None (1 wash) 2.02 200 None (3 washes) 2.02 160 1000 ppm Ni*+ 2.02 300 1000 ppm Na+ 2.02 190 1000 ppm Cr* + 2.02 400 1000 ppm CI- 2.02 370 1000 ppm Cl- + 1000 ppm Br- 2.02 600

100 ppm Fe’+ 2.02 200

200 ppm Fe*’ 2.02 240 1000 ppm Fe*+ 2.02 400 2000 ppm Fe2+ 2.02 800

200 ppm Mn*+ 1.62 500

200 ppm Mn*+ + 200 ppm Fe*+ 2.02 800

15 5

6*2 552 3+1 9*3

10*4

15* 3 10* 2

3il 1*1

90 f 5 7*2

60 92

95

40 * 2 38 + 2 ND 30 f 2 34 f 2

80 f 5 70 f 3 30? 3 23 i 6 30*4 45 f 3

75 97 96

46 f 4 43 f 4

40 f 5 45 + 6

95 f 5 80* 5 33 f 4 24 f 7

120*9 52 f 5

aCalculated from total charge passed and volume of gas measured by m.s., with the assumption of 4e and 2e requirement for each molecule of CO2 and 02, respectively bFor 30 min oxidations

418 FUEL, 1985, Vol 64, March


ferric ions may slowly oxidize coal to gaseous products, but at the concentrations found in typical coal slurries the rate of such a process is negligible.

ACKNOWLEDGEMENTS

The authors thank the Science and Engineering Research Council for the support of this work through a grant (Award No. GR/B/44819) and through a studentship. They are also grateful to Dr C. E. Snape of the National Coal Board Coal Research Establishment for providing some of the coals and the coal extract, along with their analyses, and to Mr Alan Hedley for a number of other analyses.

REFERENCES

1 Lynch, C. S. and Collett, A. R. Fuel 1932, 11,408 2 Eddinger, T. and Demorest, D. J. Fuel 1947,26, 157 3 Belcher, R. J. Sot. Chem. Ind. 1948,67, 265 4 Posner, A. H. Fuel 1955,34,330 5 Vielstitch, W. ‘Fuel Cells’, Wiley-Interscience, NY, 1965

6 7

8

9

10

11

12

13 14 15

16

17

18

19 20

Farooque, M. and Coughlin, R. W. Fuel 1979,58,705 Coughlin, R. W. and Farooque, M. Ind. Eng. Chem. Process Des. Den 1980,19,76 Okada,G., Guruswamy, V. and Bockris, J. O’M. J. Electrochem. Sot. 1981, 128, 2097 Dhooge, P. M., Stilwell, D. E. and Park, S. M. J. Electrochem. Sot. 1982, 129, 1719 Dhooge, P. M. and Park, S. M. J. Electrochem. Sot. 1983, 130, 1029 Dhooge, P. M. and Park, S. M. J. Electrochem. Sot. 1983, 130, 1539 Anthony, K. E. and Linge, H. G. J. Elecrrochem. Sot. 1983,130, 2217 Lalvani, S., Pata, M. and Coughlin, R. W. Fuel 1983,62, 427 Kreysa, G. personal communication, 1983 Taylor, N., Gibson, C., Bartle, K. D. and Mills, D. G. ‘Proc. Int. Conf. Coal Science’, Dusseldorf, 198 1, p. 278 Baldwin, R. P., Jones, K. F.. Joseph, J. T. and Wong, J. L. Fuel 1981,60, 739 Bartle, K. D., Ekinci, E., Frere, B., Mulligan. M. J., Sarac. S. and Snape, C. E. Chem. Geol. 1981, 34, 151 Lee, M. L. and Bartle, K. D. in ‘Particulate Carbon Formation Durine Combustion’ (Eds. D. C. Sieala and G. W. Smith). Plenum, NY, 1981, p. 91

I.

Senftle, F. E., Patton, K. M. and Heard, 1. Fuel 1981.60, 1131 Bartle, K. D., Taylor, N., Gibson, C., Pomfret, A. and Mills, D. G. ‘Proc. Int. Conf. Coal Science’, Pittsburgh, 1983, p. 22

Effect of particle size on lignite devolatilization in a fixed-bed reactor

A. J. Gokhale and R. Mahalingam Department of Chemical Engineering, Washington State University, Pullman, WA 99164-2710, USA (Received 11 June 1984; revised 20 July 1984)

A fixed-bed coal gasification reactor was set up which specifically simulated the devolatilization zone in a gasifier. Samples (100 g) of lignite coal in three size ranges; - 2+ 1 mm, - 3+ 2 mm and - 4+ 3 mm, were devolatilized in the temperature range 350-55UC with a steam-oxygen mixture, at 1 atm. The effect of these operating variables on tar yield and composition, melting point, viscosity, specific gravity, and molecular weight distribution was determined. A first-order reaction model was fitted to the experimentally observed total loss in weight of the lignite.

(Keywords: lignite; devolatilization; fixed-bed reactor; particle size)

The phenomenon of coal devolatilization observed in a fixed-bed gasifier is a complex series of chemical reactions coupled with heat and mass transfer effects. The rates of devolatilization and the nature of tars generated are influenced by several of the operating variables, e.g. coal particle size, coal type, temperature of devolatilization, reactive gas environment, etc. In the present study, the influences of particle size and temperature on rate of devolatilization have been investigated.

EXPERIMENTAL

A schematic representation of the experimental set-up is shown in Figure I. The fixed-bed reactor used for the devolatilization studies was a 4.1 cm i.d. x 72 cm long, 3 16 stainless steel tube provided with a wire mesh grid located

2 cm from the bottom to hold the coal and product char in the reactor. The reactor was placed in a three-zone Lindberg electric furnace. The total flow rate of oxygen- steam gas mixture was 5000,575O and 6600 ml min - ’ at 350,450 and 55o”C, respectively. The steam content of the gas feed was 57 vol%, the steam being generated in a separate steam-tube prior to entering the reactor. The experiments were performed for 5, 15 and 30 min dur- ation, and at 1 atm pressure, to evaluate the devolatilization rates. The 100 g coal bed reached the desired run temperature in Z 30 s.

The proximate and ultimate analyses of the lignite coal studied are given in Table 1. The lignite coal originated from mines located in Carbon County in Utah.

The total weight loss for the coal and the tar yield were measured. The tar was analysed for its physical properties

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electrochemical oxidation of coals: voltammetry and mass spectrometry

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