Ind. Eng. Chem. Process Des. Dev. 1980, 19, 21 1-219
SV = second virial coefficient Literature Cited
211
Lichtenthaler, R. N., Liu, D. D.. Prausnitz, J. M., Macromolecules, 11, 192 (1978). Liu, D. D., Prausnitz, J. M., J . Appi. Polym. Sci., 24(3), 725 (1979). Luft, G.,Lindler, A., Angew. Makromol. Chem., 56, 99 (1976). Maloney, D. P., Prausnitz. J. M., Ind. Eng. Chem. Process Des. Dev., 15, 216 (1976). Patterson, D., Macromolecules. 2, 672 (1969). "Molecular Theory of Solutions", Chapter XVI, NorthHolland Press, Prigogine, I., Amsterdam, 1957. Steiner, R., Horl6. K., Chem. lng. Tech., 44, 1010 (1972). Swelheim, T., De Swaan Arons, J., Diepen, G. A. M., Red. Trav. Chim. Pays-Bas, 64, 261 (1965). Zwolinski, B. J., Wilhoit, R. C., "Handbook of Vapor Pressures and Heat of Vaporization of Hydrocarbons and Related Compounds", Thermodynamics Research Center and the American Petroleum Institute. Texas A&M University, College Station, Texas, 1971
Alder, B. J., Young, D. A., Mark, M. A,, J . Chem. Phys., 56,3013 (1972). Benzler, H., Koch, A. V., Chem. Zng. Tech. 27, 71 (1955). Beret, S.,Prausnitz, J. M., AIChE J., 21, 1123 (1975a). Beret, S.,Prausnitz, J. M., Macromolecules, 8, 878 (1975b). Beret, S.,Muhle, M. E., Villamil, I.A., Chem. Eng. Prog., 73, 44 (1977). Beret, S.,Ph.D. Thesis, Appendix C, University of California, 1975. Bonner, D. C., Bazua, E. H., Prausnitz, J. M., Ind. Eng. Chem. Fundam., 12, 254 (1973). Bonner. D. C., Maloney, D. P., Prausnitz, J. M., Ind. Eng. Chem. Process Des. Dev., 13, 91 (1974). Carnahan, N. F., Starling, K. E., J . Chem. Phys., 51, 639 (1969). Cernia, E. M., Mancini, C., Kobunshi Kagaku, 22, 797 (1965). Cheng, Y. L., Bonner, D. C., J. folym. Sci., Polym. f h y s . Ed.. 15,593 (1977); 16, 319 (1978). Donohue, M. D., Prausniti!, J. M., AIChE J., 24, 849 (1978). Ehrlich, P., J . Polym. S a . , A3, 131 (1965). Flory, P. J.. J . Chem. Phys., 12,425 (1944). Flory, P. J., Discuss. Fariiday SOC.,49, 7 (1970). Gmehling, J., Liu, D. D., Prausnitz, J. M., J. Chem. Eng. Sci., 34, 951 (1979). Henderson, D., J . Chem. Phys., 61, 926 (1974). Harmony, S. C., Bonner, D. C., Heichelheim, H. R.. AICHE J., 23, 758 (1977). Kaul, 6 . K., Ph.D. Thesis, University of California, Berkeley, 1977. 49, 164 (1970). Koningsveld, R., Discuss. Faraday SOC., Lansig, W. D., Kraemer, E. D., J . Am. Chem. SOC.,57, 1369 (1935).
Received for review October 30, 1978 Accepted October 11, 1979
Supplementary Material Available: Calculation of chemical potentials in ethylene-low-density polyethylene mixtures (9 pages). Ordering information is given on any current masthead page.
Electrochemical Gasification of Coal-Simultaneous Production of Hydrogen and Carbon Dioxide by a Single Reaction Involving Coal, Water, and Electrons Robert W. Coughlln' and Mohammad Farooque Department of Chemical Engineering, The University of Connecticut, Storrs, Connecticut 06268
Coals and other forms of solid carbonaceous fossil fuel are oxidized to oxides of carbon at the anode of an electrochemical cell and hydrogen is produced at the cathode. These gases are thereby produced in relatively pure states. The reaction proceeds at very mild temperatures and at operating electrical potentials lower than 1 V, i.e., significantly lower than the thermodynamic potential of water electrolysis. The process may be viewed as driven simultaneously by energy supplied at low temperatures in approximately equal proportions by the coal and by an external electrical source. I t is expected that coal can supply a larger proportion of the energy if the process is operated at higher temperature for which the required electrical potential will be lower.
Introduction Intense scientific and engineering activity is presently focused on the conversion of the solid fossil fuel coal into clean, fluid synthetic fuels and important chemical intermediates. The degree of fluidity of such a fuel is strongly influenced by its hydrogen content, ranging from a 4:l hydrogen-to-carbon atomic ratio for methane (the major component of natural gas) to the hard coals which are predominantly carbon. Accordingly, almost all coal conversion processes whether they involve liquefaction to an oil or conversion to methane gas require a t least some hydrogasification of coal to produce hydrogen as an intermediate. The chemistry of these processes has been discussed extensively by Mills (1972), and the technology and chemistry are thoroughly adumbrated in the encyclopedia by Kirk and Othmer (1966). Focusing only on the carbon in coal, we can represent the hydrogasification of coal by the well-known steamcarbon reaction 0196-4305/80/1119-0211$01.00/0
This reaction is strongly endothermic and its equilibrium is highly unfavorable a t ordinary temperatures, as indicated by the standard enthalpy and Gibbs free energy changes for reaction I: AH" = +31.4 kcal/mol and AGO = +21.8 kcal/mol. In order to conduct reaction I a t temperatures sufficiently high (-800 "C) to assure favorable equilibrium, and also to supply the endothermic heat of reaction, coal is gasified in practice by treating it with a mixture containing both steam and oxygen. In this way a part of the coal is combusted C(s) + Ozk) COz(g) (11) and the heat released by reaction I1 (So = -94.1 kcal/ mol, AGO = -94.3 kcal/mol) provides the thermal energy and assures the high temperatures required by reaction I. The detailed chemistry and technology of coal gasification are complex and also involve, for example, reactions between COz and coal, between CO and HzO, and between
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Ind. Eng. Chem. Process Des. Dev , Vol. 19, No. 2, 1980
O2 and CO; a detailed discussion of such matters including hardware and gasification equipment has been published by Squires (1974), Klingman and Schaaf (1972), and Verma (1978). The complex gaseous product from coal gasification must be cleaned to remove impurities such as tars, ash, carbon dioxide, and sulfur compounds. The hydrogen and carbon monoxide contents of this gas are often further adjusted by means of the water-gas shift reaction HzO(g)
+ COk)
-
CO,(g)
+ H,(g)
(111)
conducted separately to provide whatever CO/H2 ratio may be required for subsequent reactions to produce desired products, e.g., reaction of CO H 2 to produce methanol, methane (substitute natural gas), or higher hydrocarbons (Fischer-Tropsch reaction). Alternatively, the extent of processing by the water-gas shift reaction may be adjusted to produce hydrogen essentially exclusively and this hydrogen can then be used to manufacture ammonia by the Haber process, to directly hydrogenate coal to liquid fuel (Bergius process and its variations), or to remove sulfur from liquid hydrocarbon fuels (“hydrodesulfurization”). We now report a newly developed electrochemical process which converts coal and water into two separate gaseous products, the one comprising essentially gaseous oxides of carbon and the other essentially pure hydrogen. The process chemistry takes place at mild temperatures (even room temperature) and the gaseous products are essentially free of impurities such as ash, tar, and sulfur compounds. This new electrochemical gasification process involves the anodic oxidation of coal a t an electrode for which we postulate the half-cell reaction
n
COAL GASIFICATION C E L L THERMOMETER
-
\
ANODE GAS OUTLET
GAS OUTLET
ELECTROLYTE
PLATINUM ANODE
F R I T T E D GLASS
.
+
C(s)
+ 2H20(1)
-
C02(g) + 4H+
+ 4e-
(IV)
I
COAL S L U R R I
:
ELECTROLYTE VOLUME ANODE
: :
CATHODE STIRRING
B
8 0 ml 6 5 cm2, P L A T I N U M G A U Z E
90 cm2, PLATINUM GAUZE MAGNETIC, 7 / 8 ” MAGNET
COAL GASIFICATION C E L L
II
ANODE COMPARTMENT GAS O U T L E T
CATHODE COMPARTMENT GAS OUTLET
THERMOMETER I
CATHODE WORKING ELECTRODE
CATHODE COMPARTMENT
in combination with a corresponding half-cell reaction a t the cathode 4H+ + 4e-
-
2H2(g)
(V)
The net sum of these half-cell reactions IV and V is just the equation for the predominant reaction in the electrochemical gasification of coal C(S) + 2H20(1) 2H2(g) + CO2k) (VI) This electrochemical gasification process, instead of producing a complex mixture containing H2, CO, C02, and impurities (as in conventional steam-carbon gasification), produces relatively pure streams of carbon oxides in the anode compartment and hydrogen a t the cathode. I t is important to distinguish between conventional water electrolysis (VIIa) 2H+ + 2eH2 (cathode reaction) +
H20
-
-
1/202 + 2H+ + 2e- (anode reaction) H20
-
H2 + 1/202 (net reaction)
(VIIb) (VI11
and the process of electrochemical gasification represented by eq IV, V, and VI. In principle, water electrolysis (eq VII) requires a theoretical thermodynamic electrical energy input ( L I F O = -nFEo) of 56.7 kcal/g-mol of hydrogen and a corresponding theoretical driving potential of about 1.23 V whereas electrochemical gasification (eq IV, V, and VI) requires according to thermodynamic principles only about 9.5 kcal of electrical energy and a reversible potential of only 0.21 V to produce 1 g-mol of hydrogen. The greatly lower electrical energy requirement for the latter process results, of course, from the consumption of coal which can be viewed as supplying the additional electrons required
FRITTED GLASS SEPARATOR
COAL SLURRY MAGNETIC STIRRING
’
ELECTROLYTE VOLUME ANODE
SUPPORTING ELECTROLYTE
: ‘
CATHODE
f
STIRRING
:
475 ml 9 6 5 c m z , PT G A U Z E , 5 2 Mesh/inch 156cm2 MAGNETIC, I POSITION
’/4
MAGNET AT 3
Figure 1. Diagrams of coal gasification cells.
by the process. Alternatively, electrochemical gasification can be viewed as providing the free energy required to drive reaction VI by supplying an additional reagent in the form of electrons at a theoretical thermodynamic potential of 19.1 kcal X 4.18 J/kcal -AFO EO=-= -0.21 v 4 equiv X 96500 C/equiv nF It is important that this voltage is significantly lower than the theoretical standard thermodynamic potential of 1.23 V for the decomposition of water according to reaction VII. Initial Experimental Results First experiments were conducted in the simple cells shown in Figure 1used in conjunction with a recorder and a potentiostat (both Models PAR 371 and 174A were used); the results demonstrate that the electrochemical gasification of coal is feasible and easy to accomplish. We investigated three coals, one lignite, and one char, all of which were obtained from the DOE Pittsburgh Energy Research Center; analyses of these materials are given in Table I. These coals were subjected to anodic oxidation
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
213
Table I. Characteristics of Coals Studied sample Pittsburgh coal
North Dakota lignite
Illinois no. 6 coal
Analysis (As Received) 2.2 6.2 18.9 35.5 36.9 37.6 33.9 55.3 42.6 11.7 5.6 13.6 5.2 5.3 4.9 50.3 77.5 63.2 0.8 1.5 1.2 30.7 8.9 13.5 1.2 3.6 1.3 8.0 2.5 noncaking 13740 8238 11300
moisture volatile matter fixed carbon ash hydrogen carbon nitrogen oxygen sulfur free-swelling index no. heating value, Btu/lb
Montana Rosebud coal
Montana Rosebud char
7.3 36.1 46.7 9.9 4.9 61.5 0.9 22.1 0.8 noncaking 10021
1.6 6.1 60.4 31.9 1.3 63.0 0.4 3.1 0.2 noncaking 9583
F%O, Ti02 CaO MgO Na,O K20 sulfites
Major Elements in Ash 43.5 44.3 21.9 15.8 25.5 16.3 1.3 1.1 1.4 9.2 1.6 1.1 1.6 0.7 0.9 2.1 0.6 6.8
12.3 11.5 12.5 0.3 23.0 8.9 3.8 0.4 25.4
36.7 18.9 4.9 1.0 16.8 4.4 0.5 0.2 13.2
44.3 21.5 6.0 1.0 18.2 4.9 0.6 0.5 2.3
initial deformation temperature softening temperature fluid temp era tur e
Fusibility of Ash, " F 2630 1980 2680 2100 2910 2180
2030 2080 2130
2150 2180 2210
2150 2180 2210
silica A1203
Table 11. Comparison of Oxidation Rates of Different Coal Samplesa
coal
potential of oxidation, V
coal concn, g/cm3
oxidation rate, mA
Montana Rosebud char North Dakota lignite Pittsburgh coal Illinois no. 6 Montana Rosebud coal
0.78 0.83 0.875 0.875 0.78
0.475 0.475 0.475 0.475 0.475
8.20 9.00 5.30 3.00 1.50
* Electrolyte: 3.7 M II,SO,; temperature: 23 'C; electrode area: 6.5 c m 2 ;coal slurry concentration: 0.475 g/cm3;particle size: 250 p m and below; experimental apparatus: cell I. in the apparatus of Figure 1A and significant currents (i.e., as compared to currents of about 0.05 mA measured in the absence of coal at 40 O C and 1V after 15 min of operation in 4.13 M H,SO,) were measured and hydrogen was produced a t the cathode at voltages significantly lower than those necessary to electrolyze water. Typical results are shown in Table 11. These experiments were conducted at 23 "C using about 0.475 g/cm3 of coal and 3.7 M HzS04 solution as electrolyte. It was found in other experiments that the current observed increased as electrode area increased, as loading of coal-to-electrolyte increased, and as temperature increaa,ed. Smaller particle sizes also seemed to give greater currents, but the particle size effect is complex as discussed below. Additional blank measurements were made using the ash remaining after combusting Montana Rosebud char in air. Currents measured at 0.875 V were less than 0.5 mA with this ash even when the temperature was raised almost to 40 "C; this should be compared to 8.2 mA, measured for the parent char at 0.78 V and 23 "C as seen in Table 11. This table also shows that the lignite appears to be two to three times as reactive as the bituminous coals. A short, preliminary account of this work (Coughlin and Farooque, 1979) and additional details
on the influence of various parameters and operating variables will appear elsewhere (Farooque and Coughlin, 1979a). In the apparatus of Figure 1 agitation by a magnetic stirring bar keeps the coal particles suspended in the anolyte. Oxidation of the coal requires contact with the anode; only background current is measured if the anode is surrounded by a porous membrane which prevents the close approach of the coal particles but permits transport of species dissolved in the anolyte. An electrode arrangement such as employed here as anode is sometimes referred to as a three-dimensional electrode and includes variations such as the fluidized bed, electrode (Goodridge, 19771, packed bed electrode (Kreysa et al., 1975), percolating porous electrode (Coeuret, 19761, pumped slurry electrode (Dworak et al., 1978), or stirred slurry electrode (the configuration shown in Figure 1). Such electrodes are well adapted to continuous processes in which the solid reactant (coal) can be continuously added and products removed without interruption of the operation. One very noteworthy aspect of these early experiments is that Montana Rosebud char gave higher current and appeared easier to oxidize anodically than was the case for the parent coal. Based on other experiments we have done with active carbon and with graphite, we attribute this behavior to the higher surface area, smaller particle size, and more graphitic nature of the char as compared to the parent coal. Influence of Particle Size. North Dakota lignite was sieved into three samples of different size range. These were anodically oxidized at 39 "C and 1.0 V. The course of oxidation is plotted as current vs. time in Figure 2. It is seen that at the outset the larger particles gave higher current but that these relationships shifted after about 30-70 min of oxidation. Electrodes. Preliminary experiments were conducted in the Figure 1A cell using Pt gauze. Superficial areas of anode and cathode were 6.5 and 9.0 cm2, respectively.
214
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980 200,-
.
t
30
2E
29
\
30
31
3
32
I/T x IO~(OK)-'
iicrT
*
. E,
&'it
TCS
Figure 2. Effect of particle size on the oxidation rate; (North Dakota lignite; slurry concentration, 0.0625 g/cm3; supporting electrolyte, 4.13 M H2S04;temperature 39 "C; oxidation potential, 1.0 V; experimental cell, cell I).
I
Figure 4. Oxidation rate vs. 1/T; (North Dakota lignite; slurry concentration, 0.0625 g/cm3; supporting electrolyte concentration, 4.13 M H2S04;particle size: 63 pm and below; oxidation potential, 1.0 V; experimental apparatus, cell I).
50
-
do
-
30
-
20
-
10
-
I 0
4
E
l
SLOPE
: :
-AE/R
- 573
e
- 7P"T - 5P"C
0 0 l L
26
1
,
,
27
28
29
II T
10'~ (
-
30 K
I
I
31
32
AC
1.'
Figure 3. Oxidation rate vs. 1 / T ; (North Dakota lignite; slurry concentration, 0.00125 g/cm3; supporting electrolyte, 4.13 M H2S04; particle size, 63 pm and below; oxidation potential, 1.0 V; experimental apparatus, cell I).
Later experiments were conducted in the apparatus shown in Figure 1B using anode and cathode of 96.5 and 156 cm2, respectively. Experiments currently in progress (but not reported here) produce comparable results using graphite anodes and electrolytes of dilute HC1 or H3P04instead of the Pt gauze anodes and dilute H$04 electrolyte employed in the experiments reported herein (Coughlin and Farooque, 1979b). Effect of Temperature. Using North Dakota lignite and potentials of about 1 V, measured activation energies fall in the range of 10-12 kcal/mol "C. Such low activation energy suggests that the rate-controlling step may not be a typical chemical reaction. Experiments planned for the future will investigate the influence of potential and other conditions on activation energy and hopefully produce data which will suggest the kind of process with which the 10-12 kcal/mol activation energy is associated. Some of this information will be published elsewhere (Farooque and Coughlin, 1 9 7 9 ~ ) . Higher Coal Concentrations Produce Larger Current. Arrhenius plots of log current vs. 1 / T shown in Figures 3 and 4 indicate (1)that a limiting oxidation rate of about 1.5 mA is reached at about 85 "C when the coal-to-electrolyte loading is about 0.00125 g/cm3 (Figure 3), and (2) that increasing the coal-to-electrolyte loading to 0.0625 g/cm3 produces a much higher current (about 13 mA) at a much lower temperature of 50 "C (Figure 4). It is not yet clear why a fivefold increase in coal-to-electrolyte loading (or coal-to-electrode area because the same electrode area and electrolyte volume was used in these
0.08
0 16
3 9 ~
0 24
SLURKY COhCEtITRP.Tlf",
C 40
P.32 OICC
Figure 5. Effect of coal concentration on the oxidation rate; (North Dakota lignite; particle size, 44 pm and below; supporting electrolyte concentration, 4.13 M H2S04;oxidation potential, 1.00 V; experimental apparatus, cell I).
0
6 10
0
-
0
- 6 35 %
"
0
-
21 I %
"
rn - 29 2 %
"
0
0 156 % 01 Tclol Corbon Consumed
:
.
,
I
1
1 101
I02
OXIDATION R A T E ( m A )
Figure 6. Effect of potential on the oxidation rate as the reaction proceeds; (North Dakota lignite; coal slurry concentration, 0.069 g/cm3; supporting electrolyte, 5.60 M H2S04;particle size: 125-149 pm; temperature 114 " C ; experimental apparatus, cell 11).
experiments) causes about a tenfold increase in oxidation current. Further insight into the interrelationship of oxidation current, coal-to-electrolyte concentration and temperature is provided by the combined graph shown in Figure 5. Influence of Potential on Oxidation Rate. The higher the potential the greater the oxidation current. As the coal is consumed by oxidation at a given potential the current diminishes very slowly. This behavior is shown in Figure 6 where current is plotted vs. potential for various
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
i 20
10 0
i
100 0
OXIDATION RATE ( m a )
Figure 7. Effect of potential on the oxidation rate; (North Dakota lignite; coal slurry concentration, 0.069 g/cm3; electrolyte, 5.6 M H2S0,; temperature, 114 "C; experimental apparatus, cell 11).
extents of coal consumption as a parameter. Figure 7 shows similar information but extended to very low potential for 1.75% carbon consumed. Anodic oxidation of carbon anodes has been investigated previously by Binder et al. (1964). The controlled oxidation of purer carbons, whether by electrochemical or chemical means, results in the formation of surface oxides such as hydroxyl, carbonyl, or carboxylic groups, as reported by Binder et al. (1964) and discussed by Coughlin (1969) and Panzer and Elving (1975). We believe that such oxides also form during the anodic oxidation of coal and, as they increase in concentration on the surface of the coal particles, the potential required for additional oxidation increases. These surface oxides of carbon decompose to gaseous oxides of carbon upon heating and this is consistent with the experimental observation that as temperature increases lower potentials are required to produce the same rate of oxidation. Some experiments were performed in which, after significant consumption of the coal had taken place by anodic oxidation at about 100 "C, and the anodic current at 1 V had decreased to low values, the coal was removed from the electrolyte and heated to about 200 "C to decompose surface oxides. When this coal was returned to the anolyte, high oxidation currents were measured at 1 V once again, and these currents were almost equal to those provided by virgin coal under the same conditions. Further details can be found in Farooque and Coughlin (1979~). It is likely that surface carboxyl groups on coal or smaller carboxylated fragments can react via a Kolbe-type pathway to form aliphatic type hydrocarbons RCOO- + RICOO2C02 + RR' + 2e
-
-
Surface free radicals, R., might also form by RCOCCOS + R. + eSuch reactions may be responsible for the tar-like substance that we find can be extracted from anodically oxidized coals using hydrocarbon solvents. Gases Produced and the Current Efficiency. During oxidation of North Dakota lignite at 114 "C and at potentials near 0.85-1.0 V the gas produced a t the cathode was essentially pure Hz. The current efficiency of hydrogen production may be expressed as current efficiency = H2 experimentally produced/ amount of H2 corresponding to coulombs passed Based on 1 2 experiments in each of which about 100 cm3 of H2 was collected, the mean current efficienty was 1.00
215
with a standard deviation of rt0.022. The gas produced within the anode compartment was almost pure COz with small amounts of CO ranging from about 7% early in the electrolysis (about 1870 C passed, equivalent to about 0.34% of coal consumed) to about 3% later on (about 3740 C passed, equivalent to about 0.68% of coal consumed). The volume ratio of the gases collected at the cathode to those a t the anode ranged from about 3.5 to about 8; the higher ratios were obtained at the beginning of the experiment but then decreased. According to reaction I the gas ratio should be about 2. Under the conditions employed, however, a significant portion of the carbon oxides presumably remained bound to the coal (probably as -COOH, -CHO, and -CH20H groups) and this would account for the gas volume ratios greater than 2. The decrease in these ratios as an experiment progresses may be attributed to the buildup of oxygen on the carbon surface until a steady-state saturation is reached. It is expected that operation a t higher temperature (to be attempted in future experiments) will cause more oxides of carbon to be liberated from the coal with an attendant lowering of the ratio of cathode-to-anode gas production rate. Some electrolyte-soluble (probably oxygenated) organic compounds were also produced as indicated by total organic carbon assays of the filtered electrolyte after extended anodic oxidation of coal. A qualitative but sensitive mass spectrometric analysis was made of the gases produced at both anode and cathode. It is noteworthy that no lines were observed for molecular weights corresponding to SOz or HzS even though the parent lignite contains significant sulfur. Consideration of Energy Efficiency Thermodynamic efficiency will depend on the amounts of coal and electricity consumed for a given amount of hydrogen produced; this will in turn depend on operating potential, temperature, and extent of production of other products. A quantitative estimate of such efficiency will be given elsewhere (Coughlin and Farooque, 1979~).Such efficiencies based on the heating value of hydrogen, and which also account for the inefficiencies inherent in generating electricity from coal, are smaller by about half for electrochemical coal gasification at potentials of about 1 V as compared to those cited for conventional gasification processes (Dravo, 1976). The efficiencies for electrochemical gasification become much more favorable, however, as the operating potential is lowered, as might be accomplished by higher temperature operation. Thermodynamic efficiency based on the heating value of the gaseous product(s) is an inadequate basis of comparison in view of the fact it does not account for the energy required to clean the product from a coal gasifier and shift it toward pure hydrogen. A fairer comparison might be an economic estimate of the cost of hydrogen produced by the different processes. A cursory and preliminary attempt at such an economic analysis is given below. Scale-up Estimates. This approximate extrapolation is based on the following data taken from Figure 5 for a coal concentration of 0.36 g/cm3 and a potential of 1 V: (a) 32.5 mA (39 "C) is equivalent to about 0.0036 g of C/h; (b) 50 mA (78 "C) is equivalent to about 0.0056 g of C h; basis = 100 tons coal (as C) per hour (equivalent to 10 of C/h). Assume a cell volume of 6.5 cm3 (i.e., two flat electrodes each of 6.5 cm2 area and separated by a distance of 1 cm). Scale up this volume to lo8 g of carbon/h at 39 "C lo8 X 6.5 cm3 = 1.8 X 10" cm3 0.0036
d
216
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
i.e., a volume of 1.8 X lo5 m3 or a cube about 56.36 m on an edge may be sufficient to gasify 100 tons/h at 39 " C ,
at 78 "C
los X 6.5 cm3 - 1.17 X 10" cm3 0.0056
Le., a volume of 1.17 X lo5 or a cube 48.81 m on an edge may be sufficient to gasify 100 tons/h at 78 "C. The anodic rate of electrochemical coal gasification, i, may be expressed by
i = io exp(ayF/RT) (where 77 is the overpotential, cy, is the transfer coefficient, T i s the temperature, F is the Faraday constant, and io is a function of coal concentration). This equation predicts that increasing coal concentration and temperature would increase the coal gasification rates and thereby lower the estimation of volume requirement. In addition, more effective utilization of electrode area (beyond that of the Pt gauze electrodes we used) should reduce the required electrolysis volumes still further. Of course there may be limitations imposed by electrode area and separatormembrane area. Additional experiments are now in progress to investigate the upper limits of currents that could be obtained by raising temperature and coal loading, as well as possible limitations that may be imposed by cell design parameters, e.g. electrode areas. It should be emphasized that the estimates above are entirely speculative and are given here merely to indicate the amount of additional development work necessary to achieve an understanding of under what circumstances the novel process we report here might be practical. One salient feature that should be borne in mind is that clean hydrogen is directly produced by this new gasification process rather than synthesis gas contaminated by sulfur compounds and other impurities. More extensive but still very rough economic considerations of the potential of the process for practical hydrogen production are given in the last section. Comparison of Electrochemical Gasification of Coal w i t h Conventional Water Electrolysis There are several ways to view the process under investigation. From one standpoint coal is gasified by reaction with water, but using externally supplied electrons as a reagent to make the process thermodynamically feasible at low temperatures and thereby avoiding the need to supply large amounts of thermal energy. From another viewpoint, the process causes the electrolysis of water to hydrogen but uses the electrons in coal to lower the potential of operation by a method that therefore produces C 0 2 and H 2 instead of O2 and H 2 as in ordinary water electrolysis; by using the energy of coal, less energy has to be supplied as electricity compared to ordinary water electrolysis. Yet another interpretation is that the coal depolarizes the anode by reacting with oxygen produced there during water electrolysis, and the consumption of coal lowers the energy barrier and the electrical energy consumption. It is instructive to compare the energy required by the present process ("coal-consuming water electrolysis") with ordinary water electrolysis. The energy consumed by conventional water electrolysis conducted at a potential of E2 to produce NH2mol of H 2 is 2NH,FE2,whereas the energy required by the present process under investigation operating at a potential of E is
E l 0t i dt
+ N,(-hH,)
Figure 8. Flow diagram of a continuous electrochemical coal gasification process.
Upon substituting the stoichiometric relationships NH2= S,,? d t l 2 F and N , = ' I 2 N Hthis 2 becomes The relative energy usage (REU) is accordingly
REU
(
ordinary electrolysis coal assisted electrolysis 2NH2FE2
ZFNHp+ Inserting gives
IMBl
1/ZNH2
)= = &/(E
lm~l/4F)
lMBl
= 94 100 X 4.18 J/mol and the value of F
REU = E,/(E
+ 1.02)
Practical values of E2 are about 2 V, whereas values of E observed in the present work have ranged from about 0.8 to about 1.0 V at room temperature. This means that, per unit of hydrogen produced, the total energy consumption is about the same (REU = 1)for ordinary water electrolysis and for coal-assisted water electrolysis conducted in the experiments near room temperature reported here. In the case of electrochemical gasification to hydrogen, however, about half the required energy comes directly from coal and half from electricity. We expect that the total energy requirement for coal-assisted water electrolysis can be lowered further by conducting it at higher temperatures, thereby permitting operation at lower potentials ( E )than 0.8-1.0 V. Process a n d Pollution Considerations We have observed no sulfur compounds in the gases produced in our experiments and we have been able to re-use the dilute H2S04electrolyte from experiment to experiment with no apparent consumption of the acid. It is reasonable to expect, however, that a practical continuous process will consume some H2S04as it reacts with the basic ash and mineral impurities of the coal. Reaction of acid with FeS mineral impurities in coal (pyrite, marcasite) may be expected to produce H2S. Anodic oxidation of sulfur chemically bound to coal molecules may be expected to produce sulfuric acid. A conceptual flow diagram of a continuous electrochemical coal gasification process is shown in Figure 8. This flow diagram is entirely preliminary and experience teaches that any practical process would be far more complex. In particular, the ease of separating ash and unreacted carbon values by simple differential settling as shown is open to question, Solutions
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980 217 SOL10 POLYMER ELECTROLYTE
Table 111. Cost of Hydrogen in 1980 Dollars from Several Processes (All Costs Include 20% ROIC) process
/ CATHODE
cost in 1980 dollars/KSCF
methane reforming partial oxidation of residuum coal gasification
1.97 2.62
electrolysis (current technology) electrolysis (G.E. SPE technology) electrolysis (G.E. SPE technology)
PURE HYDROGEN
2.58-3.72 (range of 8 separate cases) 6.25 (based on electricity @ $0.027/kWh) 3.75 (based on electricity @ $0.027 / kWh ) 1.65 (based on off-peak electricity @ $O.Ol/kWh)
must also be found for problems of maintaining high coal concentration in the reactor while simultaneously and continuously purging ash (with minimal loss of unreacted carbon values). The purpose of this report, however, is not to present solutions to the many engineering problems inherent in any great departure from more conventional coal conversion. Rather we view this as a report of first experimental investigation into a novel method of coal gasification.
Economic Considerations Some very rough estimates of the cost of hydrogen produced by electrochemical gasification of coal can be made by considering the analysis of production economics for hydrogen during the period 1980-2000 made by Exxon Research & Engineering Company (Corneil et al., 1977) under contract with Brookhaven National Laboratory. This study arrives at the costs of hydrogen in 1980 dollars shown in Table 111. These figures, in $/lo00 ft3 of hydrogen, include a 20% return on invested capital. It is evident from this table that water electrolysis by methods of current technology involves the greatest costs with the result that conventional electrolysis cannot be expected to compete with the other processes. As shown in the table, even the new electrolysis technology using solid polymer electrolytes (SPE) under development by the General Electric Co. will not be economically competitive with coal gasification if electricity costs are taken at $0.027/kWh. If, however, off-peak electric power a t
OXYGEN
+4e+02
4 H * + 4c -2H2
WATER
Figure 9. Schematic diagram of SPE electrolysis cell (from Nuttal, 1978).
$O.Ol/kWh could be used, then water electrolysis using SPE technology could become competitive with the other processes. Because the electrochemical process for gasification of coal under consideration here is closely related to water electrolysis technology, it is of interest to examine the details of electrolysis costs; such information, taken from the Exxon report, is shown in slightly modified form in Table IV. It is evident from this table that the SPE technology has its major effect on the economics by permitting greatly lowered total capital investment which would be only about 30% of that required for a conventional electrolysis plant. This also brings about savings in maintenance, plant overhead, insurance, taxes, depreciation and ROI, each of which are computed as percentages of capital investment. There are also slight savings of electricity cost with SPE because of lowered cell resistance. Because of the economic promise of SPE it is of interest to briefly consider this technology which employs a solid sulfonated fluoropolymer Du Pont’s Nafion) as the sole electrolyte. Figure 9 is a schematic diagram of an S P E electrolysis cell. The basis of the technology is that the design and the use of the S P E permits much greater
Table IV. Comparison of Hydrogen Costs by Electrolysis, 1980$ (Basis: 33 x l o 9 SCF/year; 7920 h/year Operation) new technology (SPE)
current technology investments
$/kW
lo6 $
onsites offsites total % contingency in investments working capital, lo6 $
650
255
167 65.6 39 15.4 206 81.0 30 30 1 8 . 3 (normal power) or 6.9 (off peak)
40
100
lo6 $
$/kW
750 295 15 15 18.9 (normal power) or 7.0 (off peak)
operating costs
normal power, lo6 $/year
off peak power, lo6 $/year
normal power, lo6 $/year
off peak power, l o 6 $/year
electricity @ $0.027 or $O.Ol/kWh water and chemicals labor and supervision maintenance ( 4 %of onsites) plant overhead (2.6%of onsites) insurance, property taxes ( 1 . 5 %of total plant) depreciation (10%of onsites, 4% of offsites) interest on working capital (10%) return on investment (20%of total) total cost including return oxygen credit ($24/ton) net cost including return hydrogen cost, $/KSCF (including return)
111.02 1.25 1.20 10.20 6.63 4.43 27.50 1.89 59.00 223.12 16.70 206.41 6.25
41.12 1.25 1.20 10.20 6.63 4.43 27.50 0.70 59.00 152.03 16.70 135.33 4.10
108.30 0.74 0.60 2.62 1.71 1.22 7.33 1.83 16.20 140.55 16.70 133.85 3.75
40.11 0.74 0.60 2.62 1.71 1.22 7.33 0.69 16.20 71.22 16.70 54.52 1.65
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Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
Table V. SPE System Costs ($/kW)* electrolysis module power conversion and switch gear ancillary equipment installation (not including land or buildings) total
8.15 42.76 18.08 8.26 77.25
1985 cost projection by G.E. Table VI. Costs and Savings Offered by Coal Gasification During SPE Electrolysis Additional Costs consumption of acid t o neutralize ash (for acid electrolytes): $0.76 millioniyear modified cells to permit flow of coal slurry: $6.92 million additional investment coal preparation, handling and storage: $9 million additional investment ash separation and handling: $ 2 million additional investment loss of oxygen credits: $16.70 millionlyear cost of coal: $8.28 million/year Savings reduced electricity cost (by about half) reduced capital investment in power conversion and switch gear, in bus bars and circuit components: $25.9 million lower investment reduced working capital due t o greatly lowered electricity cost (i.e. lower by about half) Table VII. Hydrogen Costs-Coal Assisted Electrolysis-SPE Technology (Basis: 33 X SCF/year; 7920 h/year Operation) investments
lo9
millions of $
onsites 57.6 offsites 14.4 total 72.0 working capital, 9.2 (normal power) 3.4 (off peak) $ million
operating costs, $ millionlyear electricity @ $0.027 or $O.Ol/kWh (off peak) cost of coal at $30/ton water and chemicals labor and supervision maintenance (4% of onsites) plant overhead (2.6% of onsites) insurance, property tax (1.5% of total plant) depreciation (10% of onsites, 4 % of offsites) interest on working capital (10%) return on investment (20% of total) oxygen credit net cost (including return) hydrogen cost, $/KSCF (including return) savings over no-cod case, $/KSCF (%)
normal power, lo6 $/year
off peak power,
lo6
$/year
54.15
20.06
8.28
8.28
1.50 0.60 2.30
1.50 0.60 2.30
1.50
1.50
1.08
1.08
5.82
5.82
0.92
0.34
14.40
14.40
none 90.55
none 55.88
2.74
1.69
1.01 (27%)
none
current densities which require small cells and therefore greatly lowered capital investment. Some savings in electricity costs are also realized due to somewhat lowered cell voltages. A breakdown of projected 1985 SPE system
Table VIII. Hydrogen Costs-Coal-Assisted Electrolysis-Conventional Technology (Basis: 33 x lo9 SCF/year; 7920 h/year Operation) investments
$ million
onsites 240 offsites 2 total 280 working capital, 9.5 (normal power) 3.5 (off peak) $ millions offpeak normal operating costs, power, power, $ millioniyear l o 6 $/year I O 6 $/year electricity @ $0.027 55.51 20.56 or $O.Ol/kWh 8.28 8.28 coal at $30/ton water and chemicals 2.01 2.01 0.60 0.60 labor and supervision 9.60 9.60 maintenance (4% of onsites) plant overhead 6.24 6.24 (2.6% of onsites) 4.20 4.20 insurance, property tax (1.5% of total plant) depreciation (10% of 25.60 25.60 onsites, 4% of offsites) interest on working 0.95 0.35 capital (10%) oxygen credit none none return on investment 56.00 56.00 168.99 133.44 (20% of total) tdtal net cost ‘ (including return) hydrogen cost, 5.12 4.04 $/KSCF (incl. return) savings over no-coal 1.13 (18%) 0.06 (1.4%) case, $/KSCF ( % )
costs recently published by Nuttall, 1977, 1978) is given in Table V. Introduction of coal into water electrolysis processes will entail both savings and additional costs as shown in Table VI. Cost of coal would be 0.276 X lo6 tonsjyear @I $30/ton = 8.28 X lo6 $/year. Assuming that the coal is 10% ash and that half of the ash is acid-consuming NazO we compute a requirement of 0.0218 X lo6 tons of H2S0,/year X $35/ton = 0.76 X lo6 $/year for acid. Capital investment for equipment for dumping, unloading, storing, conveying, crushing, and pulverizing the coal is estimated at $9 million; ash handling, settling, and recycling equipment is estimated at about $2 million (Stone & Webster Engineering Co., 1979). Assume that the cost of the electrolysis cells would double for an additional on site investment of 8.15177.25 X 65.6 = $6.92 million. Savings would also entail lowered electricity costs by half, reduced capital investment in electrical equipment by about half = ‘ I z X (42.76 + 18.08)/77.25 X 65.6 = $25.9 million, and reduced working capital by about half due to lowered purchases of electricity. The savings and additional costs discussed above and outlined in Table VI are incorporated into cost analyses for coal-assisted electrolysis for SPE technology in Table VI1 and for conventional electrolysis technology in Table VIII. The changes suggested by Table VI were incorporated in each case. These analyses show that coal-assisted electrolysis has its most favorable effect when electricity costs are high because cheaper coal energy is substituted for more costly electrical energy. The greatest effect (a 27 % reduction in hydrogen cost) is evident for application with SPE technology at “normal” power costs of $0.027/kWh; the corresponding cost reduction for conventional electrolysis is 18%. For off-peak power at
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
Full- t i m e pressure e l e c l r o l y s i s 5 - 15 Mill power
N a t u r a l oas
U I
001
1
01
I
IO
DAILY HYDROGEN REQUIREMENT,
-
I
100
106 SCFl DAY
Figure 10. Comparison o f hydrogen prices for various rates o f dem a n d ( f r o m D a r r o w et al., 1977). R e p r i n t e d w i t h permission f r o m Znt. J.Hydrogen Energy, 2, 175-187 (1977) by K. Darrow, N. B i e d erman, a n d N. Konopka, copyright 1977, Pergamon Press, Ltd.
$O.Ol/kWh it is seen that direct incorporation of coal into the electrolysis process seems to offer no particular advantage. It should be noted that there are tremendous scale effects that should be considered when comparing the cost of hydrogen by different processes. Figure 10 taken from (Darrow et al., 1975') illustrates these scale effects. It is seen that production of hydrogen by steam-reforming becomes prohibitively expensive for small plants whereas electrolysis processes do not suffer such adverse scale effects for small plants. It should be evident then, that the approximate economic analysis set forth above is very conservative in the savings it shows for coal-assisted water electrolysis because the analysis has been done for very large plants. For smaller sized plants it may be expected that electrolysis will compete better with the other processes in general, and coal-assisted water electrolysis should then provide even greater proportionate savings because of the expected higher unit electricity costs at the lower rates of comsumption in smaller plants. It must be emphasized that the foregoing economic development is very approximate and may not be reliable. In particular, the capital costs are based on a still nonexistent technology for processing coal slurries in electrolytic cells; our estimates and extrapolation of such costs have necessarily been founded on judgment as much as on available facts. It has been assumed, moreover, that the carbon of the coal is completely consumed-an uncertain premise regardless of how reasonable it might appear to use the most reactive fraction of the coal to produce hydrogen by the electrolytic reaction and burn the unreacted portion to produce electric power. The hydrogen costs
219
suggested by this analysis can, therefore, be relied upon as no more than order-of-magnitude estimates. On the other hand, it might also be recognized that these economic estimates, flawed as they may be, nevertheless provide some basis for judging the relative economic sensitivities of the different processing schemes. Major economic impacts of capital costs and power costs do seem to emerge from the effort. The possible importance of innovative engineering of electrolytic cell design seems evident as a means of reducing capital investment. The interaction of coal cost and electric power cost and their importance for the price of the final product are also emphasized by these economic case studies. The relative components of product hydrogen cost seem to show great potential incentives for increasing the substitution of coal for electricity by operating a t lower cell potential. The direction of this interplay is not unexpected but the economic analysis suggests a strong cost-saving incentive. Acknowledgment We are grateful for the financial support of this research by the University of Connecticut Research Foundation and the U.S. Department of Energy. Valuable experimental assistance was provided by Larry Veneziano. L i t e r a t u r e Cited Binder, H., Kohling, A., Richter, K., Sandstede, G., Nectrochim. Acta. 9, 255 (1964). Coeuret, F., Electrochim. Acta, 21, 165 (1976). Corneil, H. G..Heinzblmann. F. J.. Nicholson, E. W. S.,"Production Economics for Hydrogen, Ammonia and Methanol During the 1960-2000 Period", Exxon Research and Engineering Co., Linden, N.J., 07036, BNL-50663, Apr 1977. Coughlin, R. W., Farooque, M., Nature (London), 279, 301 (1979a). Coughlin, R. W., Farooque, M., J . Appl. Nectrochem., to be submitted, 1979b. Coughlin, R. W., Farooque, M., work in progress, 1979c. Coughlin. R . W., Ind. Eng. Chem. Prod. Res. Dev., 8, 12 (1969). Darrow, K., Biederman, N., Konopka, A,, Int. J . Hydrogen Energy. 2, 175-187 (1977). Dravo Corporation, "Handbook of Gasifiers". FE-1772-11, prepared by Dravo Corp. under DOE Contract E(49-18) 1772, published Feb 1976. Dworak, R.. Feess, H,,Wendt, H., presented at the A.1.Ch.E. Meeting, Atlanta, Feb 28-Mar 5, 1978. Farooque, M., Coughlin, R. W., Fuel, accepted for publication, 1979. Goodridge, Electrochim. Acta, 22, 929-933 (1977). Kirk, E. R., Othmer, D. F., "Encyclopedia of Chemical Technology" Vol. 4, Interscience, New York, 1966. Klingman, G. E., Schaaf, R. P., Hydrocarbon Process., 97-101 (Apr 1972). Kreysa, G., Pionteck, S.,Heitz, E., J . Appl. Electrochem., 5, 305-312 (1975). Mills, G. A,, CHEMTECH. 416 (July 1972); Am. Chem. SOC.Div. FuelChem. Prepr., 163rd National Meeting of the American Chemical Society, Boston, Apr 1972. Nuttall, L. J., Inf. J . Hydrogen Energy, 2, 395-403 (1977). Nuttall, L. J., "Water Electrolysis Using SPE", presented at the Hydrogen for Energy Distribution Symposium, IGT, Chicago, July 1978. Panzer, R. E.. Elving, P. J., Electrochim. Acta, 20, 635 (1975). Squires, A. M., Science, 184, 340-46 (1974). Stone & Webster Engineering Co., Boston Mass., private communication, 1979. Verma, A., CHEMTECH, 372-61 (June 1978).
Receiued for recieu: D e c e m b e r 18, 1978 Accepted N o v e m b e r 21, 1979
