Energy & Fuels 1993, 7,495-504
495
Methane Steam Reforming over Fe Electrodes in a Solid Electrolyte Cell Haytham Alqahtany, Douglas Eng,* and Michael Stoukidest Department of Chemical Engineering, Tufts University, Medford, Massachusetts 02155 Received January 4, 1993. Revised Manuscript Received April 26, 1993
The production of synthesis gas, CO + H2, from methane steam reforming was studied in a yttriastabilized zirconia reactor at 850-950 “C and atmospheric total pressure. The reactor also served as an oxygen-ion-conducting solid electrolyte cell with reduced Fe and Pt used as anodic and cathodic electrodes, respectively. The effect of two oxygen sources-gaseous oxygen and ionically transported oxygen-on methane steam reforming was studied. It was found that 0% transported through the electrolyte promoted CO formation more favorably than gaseous oxygen. When 0% + H2O was supplied, up to 87% yield to CO was obtained. Ionically transported oxygen and gaseous oxygen, however, did not significantly differ in H2 formation. The HdCO ratio could be influenced by the amount of 0% pumped through the electrolyte. Carbon formation occurred with both oxidants, however, less with 0%.Iron was compared to nickel and found to be comparable in catalytic activity. These results could be used in the development of an in situ methane steam reforming solid oxide fuel cell (SOFC). It is suggested that operation of the reactor cell as a fuel cell may be at least as good as existing industrial methods and still coproduce electrical energy.
Introduction
Internal reforming of natural gas within a solid oxide fuel cell (SOFC) is expected to simplify the overall system Methane is the major component of natural gas, an design and produce electrical power as well. A SOFC is inexpensive abundant natural resource used in energy a fuel cell not with liquid but solid electrolyte. Solid production. Methane utilization has recently become one electrolytes are solid-state materials with electrical conof the most investigated areas in catalysis and energy ductivity partly or wholly due to ionic displacement. research. One such path of utilization is the production Oxygen-ion-conducting solid electrolytes are solutions of of synthesis gas from methane via steam reforming. oxides of divalent or trivalent cations (Y2O3, YbzO3, or Synthesis gas can then be used as feedstock to produce CaO) in oxides of tetravalent metals such as ZrOz, ThOz, other hydrocarbons. and CeO2.’ Yttria-stabilized zirconia (YzO3 in ZrOz abSteam reforming is the catalytic conversion of hydrobreviated as YSZ) is a frequently used solid electrolyte carbons with steam to produce gases with high hydrogen which exhibits considerable 02-conductivity. YSZ can and carbon monoxide content. The conversion is highly conduct oxygen ions over a wide temperature range (250+33.9 kcal). The most frequently endothermic ( A G ~ ~= K 1000°C).8 Hence,YSZcan beutilizedforsteamreforming. used industrial catalyst is nickel although all transition In a SOFC, steam reforming of methane to synthesis gas metals of group VI11are known to be active to some extent.’ could be the reaction a t the anode. A t the cathode, exposed Recently, several investigators have reexamined the partial to air, atomic oxygen can be converted to ionic oxygen via oxidation of methane to CO and Hz without the addition of steam and promising results have been rep~rted.~B oa&,-,rbd + 2e 0%.At the anode, the reverse half-cell reaction occurs promoting the reaction of oxygen with Steam reforming is the most common method for promethane to produce CO + H2. Current is generated by ducing hydrogen and hydrogen/ carbon oxide mixtures in this ionic transfer. the manufacture of basic chemicals (e.g., ammonia or methanol), in oil refining, and in other industrial appliOxygen-ion-conductingsolid electrolytes have other uses cations.4 in addition to materials for SOFCs. They can be used in Steam reforming may also be important in compact fuel oxygen sensors, solid electrolyte potentiometry, and eleccells with natural gas or liquid hydrocarbon feedstock.6*6 trochemical oxygen pumps (EOPS).~JO If the spontaneously The reforming process produces hydrogen for the fuel cell, generated current in a fuel cell is either very low or in an and excess gas is used for higher hydrocarbon synthesis. undesirable direction, an external power source can be Units with capacities over 20 MW have been constructed.e used to direct and control the current. Electrochemical oxygen pumps have been used to study several industrially To whom correspondence should be addressed. important catalytic oxidation reactions including NO t Current address: Chemical Procees Engineering Research Institute, decompositionll and CO, CH30H, CzH4, and CH4 oxidaP.O. Box 1517, 64006 University City, Thessaloniki, Greece.
-
_ _ _ _ _ _ ~ ~ ~
(1) Grenoble, D. J. Catal. 1978,51, 203. (2) Hickman, D. A.; Schmidt, L. D. Science 1993,259, 343. (3) Hickman, D.A.; Schmidt, L. D. J. Catal. 1992,138, 267. (4) Twigg, M.V., Ed.Catalyst Handbook, 2nd ed.; Wolfe: London, 1989. (5) Gilles, E. Chem. Eng. h o g . 1988, 76 (lo), 88. (6) Mint, G.; Olesen, 0. Hydrogen Production and Marketing; ACS Symposium Series 116; American Chemical Society: Washington, DC, 1980.
(7) Subbarao, E. C.Solid Electrolytes and their Applications; Plenum: New York, 1980. (8) Etsell, T. H.;Flengas, S. N. Chem. Rev. 1970, 70,339. (9) Vayenas, C.G. Solid State Ionics 1988, 28-30, 1521. (10) Stoukides. M.Ind. Em. Chem. Res. 1988.27.1745. (11) Pancharatnam, S.;Huggins, R. A.; Mason, D. J . Electrochem. SOC.1976, 122, 869.
0887-0624/93/2507-0495$04.00/0 0 1993 American Chemical Society
496 Energy & Fuels,
Alqahtany et al.
VoZ. 7, No. 4,1993
t i ~ n . ' ~InJ ~many of these reactions, oxygen pumping led to an increase in catalytic activity and hence oxygen consumption. Studies have shown that oxygen supplied via pumping can be more active or selective than gasphase o ~ y g e n . ' ~Consequently, J~ product selectivity and yield were influenced by electrochemical oxygen pumping. In this study, electrochemical oxygen pumping is utilized for the production of synthesis gas from methane oxidation via steam reforming. Nickel is the predominantly used catalyst for steam reforming and, as an electrode, is frequently used as nickel-zirconia cermets.16J7 In this study, however, iron electrodes are used. Recent work has indicated that iron catalysts are most effective as reduced Fe.18J9 It has been suggested that the stability of the reduced Fe along with severe carbon deposition20-22 make iron a commerciallyunfavorable catalyst compared to supported Ni.23This study will reevaluate the prospects of iron electrodes for chemical cogeneration. The solid electrolyte cell in this study is utilized as an oxygen pump to investigate the effect of ionic oxygen on methane steam reforming to synthesis gas. Implicitly, oxygen pumping can shed light on actual chemical cogeneration in a fuel cell where current generation is high. The effect of pumping oxygen ions will be compared with the effect of admixing oxygen gas with feed methane. Steam is added to reduce carbon formation as in the steam reforming process. Three different reactions are compared: methane steam reforming without oxygen added, methane steam reforming with pumped oxygen, and methane steam reforming with oxygen admixed in the gas phase. With a comparison of the effects of pumped and gaseous oxygen, it is possible to contribute to existing steam reforming technology and future chemical cogeneration technology. Experimental Section Apparatus. The apparatus, including analytical devices, is schematically shown in Figure 1. Oxygen and methane gases (diluted in helium) and helium diluent gas were supplied by Middlesex Welding. Reactant gases flowed through rotameters (MathesonFM 10504T) and were mixed. The mixed gases flowed into the reactor cell and could be admixed with water vapor using a saturator. Reactor gases could either bypass or pass through the saturator depending on whether steam was intended to admix with methane, oxygen, and helium diluent. The saturator consisted of a stainless steel tube 12in. long and 2.5 in. in diameter. The gases passed through stainless steel tubing a t the bottom of the saturator where a 2-in. packed glass bed was located. The saturator was placed on a water bath where the temperature could be controlled. (12)Vayenas, C.;Bebelis, S.; Neophytides, S. J.Phys. Chem. 1988,92, 5083. (13)Vayenas, C.; Bebelis, S.; Ladas, S. Nature 1990,343,625. (14)Seimanides, S.; Stoukides, M. J. Electrochem. SOC.1986, 133, 1535.
(15)Otsuka, K.;Suga, K.; Yamanaka, I. CataZ. Lett. 1988, 1,423. (16)Lee, A. L.;Zabransky, R. F.; Huber, W.J. Znd. Eng. Chem. Res.
1990,29, 766.
(17) Middleton, P. H.; Seiersten, M. E.; Steele., B. C. H.Proceedings of the 1st International Symposium on Solid Oxide Fuel Cells;
Electrochem Soc: Pennington, NJ, 1989;p 90. (18)Alqahtany,H.Ph.D. Dissertation,TuftsUniversity,Medford,MA, 1993. (19)Miineter,P.; Grabke, H. Ber. Bunsen-Ces. Phys. Chem. 1980,84, 1068. (20)Baker, R.Harris, P.; Thomas, R.; Waite, R.J. Catal. 1973,30,86. (21) Labo, L.; Trimm, D. J. CataZ. 1975,29, 15. (22) Albright, L.; Grynes, B.; Corcoran, W. PyroZysis Theory and Practice; Academic Press: New York, 1983. (23) Rostrup-Nielsen,J. R. CataZysis Science and Technology; Anderson, J. R., Boudart, M., Ms.; Springer-Verlag: New York, 19W,Vol 5.
gas
chromatograph
O2 CH4 He
Figure 1. Experimental apparatus. Feed gas Effluent gas
l t
Iron electrode
11
Air
L I I I 1I I
/Gas
mixture
'te
Platinum electrode
Figure 2. Solid electrolyte reactor cell. The gases were analyzed using on-line gas chromatography (Perkin-Elmer Sigma 300 HWD)with a molecular sieve column and a porapak N column. Both columns were 6 ft long. The molecular sieve detected 0 2 , N2, H2, CHI, and CO. The porapak N column detected C02, C2H4, C2H6, and H2O. The rates of formation of carbon and hydrogen were calculated from the steady-state atomic carbon and atomic hydrogen balances, respectively. An EG&G (Princeton Applied Research) Model 363 potentiostat-galvanostat supplied current to the solid electrolyte reactor cell. Digital multimeters (Fluke 8600A) were used to monitor potential, current, and cell resistance. A Beckman hydrogen analyzer was used to monitor the hydrogen content of the effluent gas. Reactor. The experiments were run in a solid electrolyte cell reactor as shown in Figure 2. The reactor consisted of an 8% yttria-stabilized zirconia (YSZ) tube (Zircoa Products) with a 16-mm i.d. and a 1.8-mm wall. YSZ is a frequently used 02--conducting solid electrolyte. The YSZ tube was inserted into a quartz tube (Figure 2). The quartz tube had a 22-mm i.d. with a 3-mm wall thickness. A water-cooled stainless steel cap was tightened to the top of the zirconia tube sealing two separate gas compartments. One compartment was formed in the annulus between the two tubes and the other compartment was inside the YSZ tube. CatalystPreparation.The electrodeexposed to air was made of platinum obtained from Engelhard (EMS-SCA3786). Platinum paste was applied to the outside of the YSZ tube and covered a superficial area of 10 cm2(4.5 cm long). The electrode inside the YSZ tube, which was exposed to the reactants, was iron. Iron powder was obtained from Sigma Chemical (C-3518) as reduced pentacarbonyl iron (99%). The iron was mixed with ethylene glycol and painted inside the zirconia tube covering a superficial surfacearea of 10cm2(4.5 cm long). Both electrodeswere calcined a t 950 OC for 7 h. Wires were attached, and a stream of hydrogen gas was passed over the iron electrode overnight at 950 "C. The mass of the iron catalyst was 2.5 g. Severalsamples were examined using SEM. The effect of HzO/ CHI on the morphology and structure of the iron catalyst surface was examined. Figure 3, parts a and b, and c and d, shows the
Energy & Fuels, Vol. 7, No. 4, 1993 497
Methane Steam Reforming over F e Electrodes
3
1
Figure 3. Scanning electron microscope photographs showing the effect of the H20/CH4ratio on the iron electrode: (a) for a feed H20/CH4 ratio of 1.0; magnification, X500; (b) for a feed H20/CH4 ratio of 1.0; magnification, X6000; (c) for a feed H20/CH4 ratio of 0.5; magnification, X500; (d) for a feed H20/CH4 ratio of 0.5; magnification, X6000. effect of passing H20/CH4 feed ratios of 1.0and 0.5, respectively. The catalytic surface area of reduced iron was estimated by using the SEM pictures for the reduced catalyst** and the loading amount of metallic iron. From the SEMs, an average particle
size of 5pm was considered. With this particle size and assuming that the total catalytic surface area consisted of 5-pm-diameter iron spheres, the 2.5 g of metallic iron was calculated to have a surface area of about 1900 cm2.
498 Energy & Fuels, Vol. 7,No. 4, 1993 trpe a b C
d
e
Table I: Compositions of Reactants feed composition admixed gaseous C& and 02 gaseous C& and pumped 0% ions admixed gaseous CHI, HtO, and 0 2 admixed gaseous CHI and H2O and pumped 0% ions admixed gaseous CHI and H2O
It was found that reduced Fe electrodes had higher catalytic activities and CO selectivities than oxidized Fe.18 For all but transient experiments,the iron electrodewas oxidized to remove carbon fragments,and subsequentlyit was reduced in a stream of hydrogen gas overnight at 950 O C before each experiment was conducted. Thus,for eachexperiment, a similar1y“clean”reduced Fe surface was initially present. Results Methane Oxidationin the Absence of Steam. Table I shows the five different reactant mixtures employed in this study. The different cases were (a) methane oxidation via admixed oxygen in the gas phase (CH4 + 0 2 1 , (b) methane oxidation via pumped oxygen ions (CH4 + OS), (c) methane steam reforming with admixed oxygen (CH4 + HzO + 02), (d) methane steam reforming with pumped oxygen ions (CHI + H2O + Os),and (e) methane steam reforming without oxygen (CH4 + HzO). Initially, simple experiments were conducted comparing the first four cases in Table I. These results are shown in Table 11. Before each experiment, the surface of the iron electrode was always oxidized to remove (hydro)carbon fragments and then reduced in a stream of hydrogen gas overnight at 950 “C. To directly compare gaseous and pumped oxygen, the ionic oxygen was converted to an equivalent amount of gas-phase oxygen. The flux of 0% is equivalent to i/4F where i is the imposed current and F is the Faraday constant. This 0% flux was equivalent to an inlet feed oxygen stream which is shown in Table 11.The total flow rate was kept constant a t 2.5 mL/s. The temperature was 950 “C, and the total pressure was 1.0 atm. Experiments aand b had the same feed methane content without steam. In experiment a, 1.3% gaseous oxygen was fed into the reactor. In experiment b, an equivalent amount of oxygen was pumped through the electrolyte. The ionic oxygen was more selective to CO with a higher yield for case b where ionic oxygen was fed. The CO selectivity is defined as (rate of CO formed)/(rate of methane consumption). The CO yield is defined as (rate of CO formed)/(rate of methane fed). For experiments c and d, the methane feed was admixed with 4.4% steam. The steam contents significantly improved the selectivity and yield to CO. Again, the pumped oxygen in experiment d yielded more CO than the gaseous oxygen in experiment c. In Table 11, a greater factor than the addition of ionic oxygen was the addition of steam, which approximately tripled the CO yield. Ionic oxygen is more selective than gaseous oxygen not only during methane oxidation but also during methane steam reforming. Effect of Gaseous and Ionic Oxygen during Methane Steam Reforming. Work was elaborated to investigate the effect of ionic 02-upon CO formation during methane steam reforming. The Fe electrode was reduced before each experiment. Figures 4-7 and Table I11 summarize the results with methane steam reforming. Table I11 tabulates the experiments for Figures 4-7. In this table, it can be seen that the H2/CO ratio is reduced
Alqahtany et al.
by supplying current. This technique could be useful to alter the HdCO ratio in industry. Figure 4 shows the effect of varying inlet pCH4 with or without ionic oxygen at 950 “C and 1.0atm totalpressure. The inlet P H 2 0 was constant at 0.044 atm, near saturation at room temperature. The total flow rate was constant at 2.5 mL/s. The flux of 0% is equivalent to i/4F where i is the imposed current and F is the Faraday constant. The current imposed was i = 600 mA. This 0% flux was equivalent to that of inlet feed oxygen at 0.0152 atm. Major producta formed during the experiment were CO, H2, C02, and carbon. No C2+ was observed. In Figure 4A, the rate of hydrogen formation is shown increasing with PCH4. Apparently, the addition of 0% to methane steam reforming did not appreciably affect hydrogen formation. In Figure 4b, however, the rate of carbon monoxide formation was affected when PCH4 exceeded 0.044 atm or when the feed H20/CH4 ratio exceeded 1.0. Carbon monoxide formation increased w i t h P c ~ with 4 and without pumped Os,but the presence of 0%for pCH4 > 0.044 atm nearly doubled the rate of CO formation. The rates of C02 formation were relatively small (CO/CO2 > 20) and decreased with increasing PCH4. The addition of 0% did not appreciably affect COz formation. Carbon formation was nonexistent for PCH4 < 0.044 atm or for a feed HzO/ CH4 ratio > 1.0. In the case of methane steam reforming without oxygen, when PCH4 exceeded 0.044atm, the rate of carbon formation increased significantly to become comparable to the rates of CO formation (i.e., 1O-emol/s). Carbon formation significantly diminished upon addition of Os, which increased CO formation. Figure 4, parta C and D, shows the effect of PCH4 on the CO selectivity and yield, respectively. In the range of experiments, the CO selectivities for 0% always exceeded those corresponding values for no oxygen. The presence of 0%made a significant difference in CO yield only when PCH4 exceeded 0.044 atm or when the H20/CHd ratio exceeded 1.0. The CO selectivity and yield plots both show maxima. Figure 5 shows the effect of inlet methane partial pressure comparing gaseous and ionic oxygen at 950 “C and 1 atm total pressure. The feed stream was saturated with water to 0.044 atm. The total feed flow rate was 2.5 mL/s. The current pumped across the cell was 300 mA, which was equivalent to 0.0076 atm. For comparison, an equivalent amount of gas-phase 0 2 was admixed in the feed. In Figure 5A, the rate of hydrogen formation is shown increasing with pCH4. There was no apparent difference between gaseous 02 and ionic 0%.In Figure 5B, although both rates of CO formation are increasing withpCH4, there was about twice as much CO produced when 0% was pumped than was produced when 02was admixed. Figure 5, parta C and D, shows the CO selectivity and yield, respectively. The maximum selectivities were 83% for steam reforming with 0% and 40% for steam reforming with 0 2 . Both CO selectivity curves reached a maxima. The maximum CO yield with 0% was 51 % . Figure 5E shows the rates of COZformation, which were similar and decreasing with increasing The rates of C02 formation (lo-’) were easily an order of magnitude below those for CO and H2 (lW-lV). In parts C and D of Figure 5,it was seen that the CO selectivities and yields no longer increased when PCH4 exceeded 0.05 atm. The reason was that carbon formation was becoming an increasing problem as shown in Figure 5F. The presence
Methane Steam Reforming over Fe Electrodes
Energy & Fuels, Vol. 7, No. 4, 1993 499
Table 11: Comparison of Gas-Phase Oxygen and Ionic Oxygen in the Absence and Presence of Steam T = 950 “C, PM = 1atm, Q = 2.5 mL/s, catalyst mass = 2.5 g % co
% c02
% H2
in
out
in
out
in
out
out
out
% Yco
4.7 4.7 4.7 4.7
0.75 0.70 0.79 0.70
0.0
0.25 0.0 2.18 2.30
1.3 1.3 1.3 1.3
0.95 1.20 3.0 3.3
0.70 0.70 0.91 0.70
7.65 8.0 10.04 10.01
20.2 25.5 63.8 70.2
% H20
%CHI a b C
0.0 4.4 4.4
%0
2
% sco 24.0 30.0 75.0 83.0
d a = no H2O + gaseous 0 2 , b = no H2O + pumped Os,c = HzO + gaseous 0 2 , d = HzO + pumped 0%. For experiments (b) and (d), a current of 500 mA was passed through the cell. This current was equivalent to 1.3% 02. a
Table III: Methane Steam Reforming Results
T, %CHI %CHI %H2O %HzO
OC
in
out
in
out
950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 950 850 850 850 900 900
1.457 2.602 4.010 5.791 8.001 1.868 2.807 4.01 5.79 8.00 4.265 7.913 1.761 2.800 4.333 7.913 6.035 6.035 6.035 6.035 6.035 6.035 6.035 6.035 6.035 4.70 4.70 4.70 4.70 4.70 4.70 4.70 4.70 4.70
0.853 1.457 0.978 0.248 0.417 0.910 1.565 1.503 0.50 0.333 1.663 1.125 0.924 1.673 2.279 1.555 0.9997 1.497 1.584 1.545 1.497 1.418 1.291 1.408 0.880 3.277 3.522 3.345 2.815 3.410 2.974 1.867 3.384 2.093
4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40
3.560 2.862 1.632 1.653 1.807 4.40 4.40 4.40 2.417 2.354 2.876 1.837 4.40 4.40 4.275 3.233 1.693 1.613 2.016 2.015 2.226 2.308 2.25 2.907 2.621 3.04 4.40 4.206 2.791 4.40 4.00 2.511 4.40 3.280
900 950 950 950
%02
i,
in
mA
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.76 0.76 0.76 0.76 0.0 0.0 0.37 0.0 0.0 0.89 0.0 1.604 0.0 0.0 0.77 0.0 0.0 0.77 0.0 0.0 0.77 0.0
0 0 0 0 0 300 300 600 600 600 300 300 0 0 0 0 0 100 0 200 300 0 400 0 700 0 0 300 0 0 300 0 0 300
rco, mol/s
%02
in
0.0 0.0 0.0 0.0 0.0 0.76 0.76 1.52 1.52 1.52 0.76 0.76 0.0 0.0 0.0 0.0 0.0 0.253 0.0 0.507 0.76 0.0 1.01 0.0 1.77 0.0 0.0 0.76 0.0 0.0 0.76 0.0 0.0 0.76
3.76 X 8.16 X 2.51 X 2.77 X 2.62 X 4.27 X 9.95 x 2.06 X 4.70 X 5.00 x 2.21 x 3.95 x 2.33 X 5.77 x 8.37 X 2.57 X 2.64 X 3.24 X 3.08 X 3.38 X 3.65 X 3.45 x 4.00 x 3.50 X 5.10 X 7.46 X 5.28 X 1.00 x 1.00 x 6.68 X 1.55 X 1.85 X 8.37 X 2.60 X
of 0% serves as a deterrent for carbon formation up to When the feed H20/CH4 ratio was under 1.0, there was a tendency to form carbon regardless of the type of oxygen source. Figure 6 again compares gaseous 02 and ionic 02-.This figure shows the effect of inlet Po2 a t 950 OC and 1.0 atm. Inlet P C Hand ~ p H 2 0 were fixed at 0.06 and 0.044 atm, respectively. The total flow rate was fixed at Q = 2.5 mL/s. Pumped oxygen was converted via i/4F to an equivalent gas-phase inlet composition to allow direct comparison of gaseous and ionic oxygen. In Figure 6A, the rate of hydrogen formation was generally unaffected by Po2 and by the type of oxygen feed ( 0 2 or 02-). In Figure 6B, CO increased with increasing oxygen. In addition, the rates of CO formation for 0% always exceeded those corresponding rates for 0 2 in the investigated range of experiments. The CO selectivities and yields are shown in Figures 6C and 6D. In Figure 6C, the CO selectivity in presence of 02-was always about 8-1676 higher in the experimental range than the corresponding selectivity in the presence of admixed 0 2 . CO selectivity increased reaching a maximum of 99% at
PCH4 = 0.05 atm.
w02,
molh
2.42X le7 3.54 x 10-8 1.61 X 10-8 2.00 x 10-8 1.65 X le7 5.30 X 10-7 2.75 X 10-8 5.03 X 10-8 1.98 X 10-8 8.10 X 10-8 4.50 X 10-8 1.13 X le7 6.22 X 10-7 4.88 X 4.23 X 10-8 8.90 X 10-8 6.43 X 10-8 6.43 X 10-8 6.43 X 10-8 4.78 X 10-8 6.43 X 10-8 2.59 X 10-8 1.40 X 10-8 6.54 X 10-8 1.70 X lb7 3.23 X lV7 5.22 X 10-8 3.80 X 10-8 3.23 X le7 4.46 X 10-8 2.10 x 10-8 4.12 X lk7 3.67 X 10-8 6.00 X 10-7
m2,
mol/s
2.10X 10-7 3.91 X lP7 8.78 X 10-8 1.41 X 10-8 1.82 X le7 1.96 X le7 2.54 X 5.13 X lk7 1.28 X 10-8 1.78 X 6.88 X le7 1.65 X 1.71 X lP7 2.30 X l V 7 4.33 x 10-8 1.42 X 10-8 1.03 X 10-8 1.21 x 10-8 1.15 X 10-8 1.16 X 10-8 1.15 X 1.16 X 10” 1.19 x lk7 1.10 x 1.23 X 4.29 X 2.40 X 2.96 X 5.49 x 2.62 X 3.93 x 10-7 10-8 7.72 X 2.68 X 10-8 6.47 X 10-7
rc, mol/s
10-8 1.OX 10-9 10-8 1.0 x 10-8 10-8 4.29 X 10-8 106 2.88 X 10-8 106 5.11 X 10-8 10-8 1.0x 10-9 10-8 1.0x 10-9 10-8 1.0 x 10-9 1 P 5.11 X le7 106 2.76 X 1 P 10-8 1.0 x 1o-g 106 2.88 X 10-8 10-8 1.0 x 1o-e 10-8 8.45 X 10-8 10-8 8.40 X 106 3.84 x 10-8 106 2.44 X 10-8 106 1.33 X 10-8 106 1.41 X 10-8 106 1.16 X 10-8 1 P 9.26 X 106 1.01 x 10-8 106 7.10 X le7 106 5.75 x 10-7 106 1.0 x 10-0 10-8 3.81 X lk7 10-8 1.49 X lP7 10-8 1.0x 10-9 10-8 5.97 x 10-7 10-8 2.00 x 10-7 10-8 1.0x 10-9 10-8 1.00 x 10-8 10-8 2.63 X 10-8 1.0x 10-9
%SCO %Yco 60.9 69.7 81.0 48.9 33.8 43.6 78.4 80.4 86.9 63.8 83.1 56.9 27.2 50.1 39.9 39.5 51.3 69.8 67.7 73.6 78.7 73.1 82.5 74.0 96.8 51.3 43.0 72.2 52.0 50.7 87.8 63.9 62.2 97.6
25.2 30.7 61.2 46.8 30.0 22.4 34.7 50.3 79.4 61.1 50.7 48.8 12.9 20.1 18.9 31.8 42.8 52.5 50.0 54.8 59.2 55.9 64.8 56.7 82.7 15.5 11.0 20.8 20.8 13.9 32.3 38.5 17.9 54.1
Hd
CO
5.57 4.79 3.50
5.09 6.95 4.59 2.55 2.48 2.72 3.56 3.11 4.18 7.34 3.99 5.17 5.52 3.90 3.73 3.73 3.43 3.15 3.36 3.00 3.14 2.40 5.75 4.54 2.96 5.49 3.92 2.53 4.17 3.20 2.50
Po2 = 0.0178 atm. Maximum CO yield with 0%was 83%, noticeably higher than that with gaseous 0 2 . In Figure 6E, the rate of carbon dioxide formation increased significantly in the case of gaseous oxygen but was small (1O-e mol/s) and nonincreasing when 0% was pumped. Carbon formation decreased with increasing oxygen for both types of oxygen feeds but was always significant (10-8 mol/@ when 02 was admixed. Upon pumping Os, carbon formation, as shown in Figure 6F, sharply decreased to immeasurable values mol/s or smaller) particularly when Po2 exceeded 0.0101 atm (or i = 400 mA). Figure 7 shows the effect of temperature on the rates of CO and H2 formation. Three different temperatures-850, 900,and 950 OC-were used. Specific conditions were PCH4 = 0.043 atm, p H 2 0 = 0.044atm, and Po2 = 0.008 atm (or 300 mA) for Q = 2.5 mL/s. The cases of 02-+ H2O and only H20 similarly increased the CO rate, as shown in Figure 7A, unlike that of H2O + 0 2 which did not appreciably increase CO. The calculated activation energies are shown in Table IV. The activation energy for methane steam reforming with pumped oxygen (21.3kc&
Alqahtany et al.
500 Energy & Fuels, Vol. 7,No. 4, 1993 ’
2.0 10.5
I
/
I
j
I
I
I
I
I
100
I
1
I
0.08
0.10
I
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I
Al RATE OF HYDRCGEN FORMATION
-->
80
-
% e L
P
so-
u) 0
I
I
1
0.02
0
’
1
’
1
0.06
0.04
’
0.08
‘
I
0.10
0
0.02
0.04
pCH4 ( a t m ) 6.010-61
’
’
0.06
pCH4 ( a t m )
’
’
0
’
’
I
100
B) RATE OF CARBON MONOXIDE
t
1
I
D) COYIELD 80
L
0.02
0
0.06
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0.08
0.10
0
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Figure 4. Effect of pumping 0% on methane steam reforming. Inlet P C His~varied, inlet P H = ~0.044 atm, Q = 2.5 mL/s, T = 950 and Pm = 1.0 atm. The flux of 0%is equivalentto i/4Fwhere i 5 600mA and Fie the Faraday constant. This 0%flux was equivalent to an inlet Po2 = 0.0152 atm. Plots show the (A) Rate of HSformation, (B)rate of CO formation, (C)selectivity to CO, and (D)yield to co. OC,
~~
mol) was significantly higher than that for the reaction with gaseous oxygen (9.3 kcal/mol). This effect verifies the observed rate increases with 02-pumping. It may be possible to dramatically increase CO yield at slightly higher temperatures. Also in Figure 7B, the rate of hydrogen formation showed more similar behavior between H2O and H2O + 0%whereas H20 + 02 produced less Hz. Pumping 0% appeared to have a positive effect on CO selectivity and yields as evidenced by Figures 4-6. Compared to gaseous 0 2 admixed with H2O or HzO alone, 0% H2O was more selective and active to CO formation. Hydrogen formation, however, appeared to be similar for both 0% and 0 2 . Another observation in Figures 4-6 was that the carbon formation was reduced more easily by 0%feed than by 0 2 . In Figures 5F and 6F, there was a clear difference between pumped and gaseous oxygen. The critical O2-/CH4 ratio at which carbon deposition became significant was 0.25 for both figures. The critical HzO/CHd ratio was 0.75-1. The amount of oxygen atoms added per feed methane molecule was O/CH4 = 1.25-1.50. Consider the possible reactions between carbon, steam, and oxygen according to the following:
+
+ + co + + +
C(s) + HzO C(s)
H2
(1)
1/202
C(S) C(ads)
CO
0”
(2)
CO2
0 2
-
CO(ads)
(3) 2e
(4)
CO(ads) + 0” C02 + 2e (5) The Gibbs free energies at 950 “C for the first three
reactions are -17.5, -57.6, and -99.3 kcal, respectively. The charge-transfer reactions 4 and 5 occur at the triple contact point between the catalyst, the electrolyte, and the carbon deposits. That pumped and admixed oxygen produced very similar hydrogen formation but unsimilar CO formation rates suggests that ionic oxygen may involve a mechanism that predominantly affects CO but not H2 production. Ionic oxygen, however, tended to only enhance CO and not C02 formation. It may be that reaction 5 contributes much less than reaction 4 in 0% consumption. The admixing of 0 2 and the pumping of 0%resulted in similar rates of H2 formation (Figures 5B and 6B). The independence of hydrogen formation may be explained by H2 formingfrom C& decomposition,areaction in which oxygen is not important. In addition, hydrogen may have been rapidly oxidized to water-which was already present-because the hydrogen to water reaction may have been near equilibrium as is further discussed below. Most interesting is the fact that 0” only enhanced CO formation when carbon deposits were significant. In this case, pumped oxygen may activate carbon more easily than admixed oxygen. Admixed oxygen may not diffuse easily to an iron surface that is physically blocked by carbon. As opposed to O s , admixed oxygen may combust directly with carbon, a noncatalytic process. Role of Carbon in Synthesis Gas Formation. Observations suggest that as carbon built up, 0” converted carbon to CO more efficiently than 02.Comparisons of carbon and CO formation in Figures 5 and 6 seem to confirm this effect. To study the effect of carbon on CO formation in more detail, it was of interest to see how carbon is removed from
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Figure 5. Comparison of 02 and 0% varying inlet P c H ~Inlet . P H= ~0.044 atm, Q = 2.5 mL/s, T = 960 OC, and Ptot= 1 atm. The flux of 0%is equivalent to i/4F where i = 300 mA and F is the Faraday constant. This 0%flux was equivalent to an inlet Pa = 0.0076 atm. An equivalent amount of gas-phase 02 was passed in the feed for comparison. Plots show the (A) rate of H2 formation, (B)rate of CO formation, (C) selectivity to CO, (D)yield to CO, (E) rate of CO, formation, and (F)rate of carbon formation.
the surface by pumped 0%.To this end, transient experiments were performed with carbon initially deposited on the electrode surface. Figure 8 shows the results of pumping 0%or passing 0 2 a t 950 "c,and at p H 2 0 = 0.044 atm and Po2 = 0.012 atm. Methane was not co-fed. To deposit carbon on the Fe surface, at time t = 0, a mixture of one part steam to two parts methane was run for more than 4 hover the electrode. Methane flow was then stopped a t t = 4 h, and only steam with or without oxygen was passed through the reactor. Every 2 h, three readings of products were taken for (a) only steam, (b) steam and pumped oxygen, and (c) steam and gaseous oxygen. In Figure 8, parts A and B, CO and H2 formation rates decreased with time. Figure 8A shows a clear difference between the CO formation for steam + 0% and the other two types of oxygen feeds. Pumped oxygen produced the highest CO formation rates and, in fact, produced negligible C02. In Figure 8B,however, the difference among the transient hydrogen formation rates was less pronounced.
The carbon deposited in the reactor during the transient experiments in Figure 8 was most likely pyrolytic carbon. Occurring a t elevated temperatures, this form of coking physically forms dense shales which accumulate over the catalyst and often conform to the shape of the reactor.% It appears that reactions 3 and 4 are of importance but not reaction 5. Discussion Thermodynamics. In addition to the methane steam reforming reaction CH, + H 2 0 CO + 3H2 (6)
-
there are several other reactions that could positively or negatively contribute to the overall conversion to CO and (24) RostrupNieleen,J. R.;Tettrap, P.B.In Symposium on Science of Catalysis and its Application in Industry; FPDIL: Sindri, 1970.
(25) Allen, D.;Gerhard, E.;Linkins, J. Znd. Eng. Chem. Process Des. Dev. 1975, 14, 3.
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CO,
+ H,
(7)
In addition to (6) and (7), the reactions of methane pyrolysis, which has been experimentally observed,ll can be considered as a source for carbon formation:
-
CH4 C + 2H, (8) Finally, the addition of gaseous or electrochemical oxygen in the system suggests that several oxidation reactions should be also considered. These reactions are
-
+ 20, CH, + '/,O,
CH,
+ 2H,O
(9)
CO + 2H,
(10)
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-
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co + 1/,0, CO, CH, + 3/20,CO + 2H,O
-
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(12) (13)
C + H,O+CO + H, (14) Due to the high experimental temperatures, it was of interest to determine if and how many of the above reactions could have reached thermodynamicequilibrium at the conditions of the reactor exit. If such an equilibrium were established for any of these reactions, it would be meaningless to study kinetics at the given conditions. To this end, a computer program was prepared in order to determine the outlet composition assuming that thermodynamic equilibrium was attained for reactions 6-14. The reactor was assumed to operate isothermally, and the inlet molar flow rates of methane, oxygen, water, and diluent helium were known. The total pressure Pt was considered atmospheric. A system of the following 9 equations was then solved simultaneously: hydrogen balance: (4nCH4
+ 2nH20)h = (2yH20
oxygen balance:
+
+ 4YcH4)outnt (15)
Energy & Fuels, Vol. 7, No. 4, 1993 603
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Figure 7. Effect of temperature on the rates of CO and Hz formation. T = 850,900, and 950 O C . PCW = 0.043 atm, PHZO = 0,044atm, and Po2 = 0.008 atm (or 300 mA), Q = 2.5 d / s . Plots show the (A) rate of CO formation and (B)rate of HZ formation. Table IV: Apparent Activation Energies Based on Data at 860-!KiO OC
feed CHI + HzO CH4 + HzO + 02 C& + Ha0 + 0"
0
E& (kcal/molCO formed) 15.1 9.5 21.3
(20)
0.5 p 0.5)] (23) K4 = (YHZO)J[(YHZYOZ)( t where ni and yi, represent the rate of inlet molal flow rate of component i and the outlet mole fraction of component i, respectively. The total outlet molal flow rate is denoted as nt, and the rate of carbon deposition is denoted as nc. The equilibrium constants for reactions 6-8 and 11 are denoted as K1, Kz,Ks, and K4, respectively.
Figure 8. Transient effect of carbon. T = 950 "C,Pbt = 1 atm, P m = 0.044 atm, and Po2 = 0.012atm. Oxygen was pumped in an amount equivalent to Po2 = 0.012atm. Plots show the (A) rate of CO formation and (B) rate of HZformation.
It is not necessary to set reactions 6-14 a t equilibrium because if only four of them are, then the remaining reactions can be produced as linear combinations of the selected four and, therefore, are also a t equilibrium.18 Furthermore, if carbon deposition does not take place, eq 22 would be replaced by an atomic carbon mass balance. Nevertheless, most of the data were in the regime where carbon formation did take place to a small or large extent and, therefore, eq 22 was not disregarded. To numerically solve the system of equations, a NEQNF/DNEQNF (single/double precision version) program located on the IMSL Math/Library (VAX system) was used. The program utilized the Levenberg-Marquardt algorithm and a fiiite-difference Jacobian approximation to solve a system of nonlinear equations. Parts A and B of Figure 9 compare experimental results with the predictions of the thermodynamic model. In Figure 9A, the predicted exit concentrations of HzO and CH4 were plotted vs pCH4 for T = 950 "Cand PHZO = 0.044 atm. No oxygen was fed. The experimentally attained values are also shown. Similarly, Figure 9B shows the predicted model values as well as the experimentally attained methane and steam exit concentrations when = 0.044 atm, P C H = ~0.06 atm, varying inlet Po2 for PHZO and T = 950 OC. In both figures, methane and steam actual conversions were below their thermodynamically predicted conversions. In general, it was found18 that the water gas shift reaction 7 was very close to equilibrium followed by the reaction between HZand 02 (reaction 11). The methane-steam reaction 6, on the other hand, was far from its equilibrium. It has been suggested that experimental COz measurementa are the most difficult to consistently reproduce and correlate to thermodynamically predicted values because
Alqahtany et al.
504 Energy & Fuels, Vol. 7, No. 4, 1993
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