Synthesis gas production from CO.SUB.2 .and H.SUB.2.O in a co-electrolysis

Abstract

A synthesis gas production process from CO.sub.2 and H.sub.2O with a co-electrolysis, wherein the CO.sub.2 and CH.sub.4 content of the produced gas is reduced on the cathode side.

Claims

1. A synthesis gas production process from CO.sub.2 and H.sub.2O with a co-electrolysis (9), wherein the CO.sub.2 and CH.sub.4 content of the produced gas (18) on the cathode side (17) is reduced, wherein for this purpose, subsequent to the co-electrolysis (9), the gas (18) of the cathode side, containing H.sub.2, CO, unreacted steam and CO.sub.2 as well as CH.sub.4, is additionally fed to a catalytic reactor (20, 24) favoring a reverse water-gas shift reaction R3 and/or steam reforming reaction R4 and/or fed to a coke-filled container (20, 24) favoring at least one of the reactions R5, R6 and R7, with:
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O  R3:
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2  R4:
CO.sub.2+C.fwdarw.2 CO  R5:
H.sub.2O+C.fwdarw.CO+H.sub.2  R6:
2H.sub.2+C.fwdarw.CH.sub.4  R7: wherein, while feeding to the at least one catalytic reactor (20, 24) and/or at least one coke-filled container (20, 24), preheating by means of at least one electric heater (19, 23) takes place.

2. The synthesis gas production process according to claim 1, wherein after the catalytic reactor(s) and/or the coke-filled container(s) (20, 24), a partial gas separation of H.sub.2 or H.sub.2-rich gas from the gas mixture takes place after cooling (6, 26) of the gas (25), wherein the gas (40) separated and enriched with Hz is recycled to the cathode side (17) of the electrolysis stack (9).

3. The synthesis gas production process according to claim 2, wherein as a separation device (39) a membrane separation plant and/or a pressure swing absorption plant is used.

4. The synthesis gas production process according to claim 1, wherein the preheating takes place by means of a preheater (19, 23), which is a component of the catalytic reactor(s) (20,24).

5. The synthesis gas production process according to claim 1, wherein, after feeding to (i) the at least one catalytic reactor (20, 24) favoring at least one of a reverse water-gas shift reaction R3 and a steam reforming reaction R4 and/or (ii) the at least the one coke-filled container (20, 24) favoring at least one of the reactions R5, R6 and R7 take place, a feeding additionally takes place to an all-ceramic stack (35) operated at temperatures higher than the stack (9), wherein the all-ceramic stack (35) is dimensioned smaller than the first electrolysis stack (9) or a second electrolysis stack (46), operated at the same temperature level as the first electrolysis stack (9).

6. The synthesis gas production process according to claim 5, wherein prior to the feeding of the gas (18) to the all-ceramic stack (35), the second electrolysis stack (46) or any temperature increase and/or an electric heater (34), the gas (18) is fed to a gas separation device (43) suitable for higher temperatures, wherein from the all-ceramic stack (35), the second electrolysis stack (46) or the gas (18) to be fed to temperature increase and/or the electric heater (34) a proportion of CO and H.sub.2 from more than 0% to up to 100% is separated as gas (44, 49) and is made available as synthesis gas (30) together with the gas (47, 50) produced from the all-ceramic stack (46) or the second electrolysis stack (46) for further processing.

7. The synthesis gas production process according to claim 6, wherein a ceramic membrane is used as gas separation device (43).

8. A synthesis gas production process from CO.sub.2 and H.sub.2O with a co-electrolysis in at least one electrolysis stack (9), wherein the CO.sub.2 and CH.sub.4 content of the produced gas (18) on the cathode side (17) is reduced, wherein for this purpose, subsequent to the co-electrolysis (9), the gas (18) of the cathode side, containing H.sub.2, CO, unreacted steam and CO.sub.2 as well as CH.sub.4, is additionally fed to a catalytic reactor (20, 24) favoring a reverse water-gas shift reaction R3 and a steam reforming reaction R4 and/or fed to a coke-filled container (20, 24) favoring at least one of the reactions R5, R6 and R7, with:
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O  R3:
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2  R4:
CO.sub.2+C.fwdarw.2CO  R5:
H.sub.2O+C.fwdarw.CO+H.sub.2  R6:
2H.sub.2+C.fwdarw.CH.sub.4  R7: wherein, while feeding to the at least one catalytic reactor (20, 24) and/or at least one coke-filled container (20, 24), preheating by means of at least one electric heater (19, 23) takes place.

9. The synthesis gas production process according to claim 8, wherein upstream of the electrolysis stack (9) and/or upstream of or in the catalytic reactor (20, 24) and/or the coke-filled container (20, 24) a recuperator or an additional temperature increasing means is provided in addition to the electric heater (19, 23).

Description

(1) There is shown in:

(2) FIG. 1 a schematic representation of an embodiment for increasing the temperature and for downstream provision of catalyst beds for carrying out the RWGS reaction after the electrolysis stack

(3) FIG. 2 a schematic representation of an embodiment for increasing the temperature and connecting a small all-ceramic stack;

(4) FIG. 3 a schematic representation of an embodiment with H.sub.2 excess;

(5) FIG. 4 a schematic representation of an embodiment of a H.sub.2/CO separation at high temperatures and downstream provision of a second stack and

(6) FIG. 5 a schematic representation of an embodiment for a separate electrolytic decomposition of H.sub.2 and CO.sub.2.

(7) FIG. 1 shows a schematic representation of the temperature increase and subsequent connection of catalyst beds for carrying out the RWGS reaction after the electrolysis stack.

(8) Steam 1, H.sub.2/CO-containing cathode purge gas 32 and CO.sub.2 2, which is preheated in a heat exchanger 3 to prevent steam condensation, are mixed in a gas mixer 4. The preheating of the CO.sub.2 can be done with an electric heater, with waste heat of co-electrolysis or from another external heat source. Steam 1, cathode purge gas 32, and CO.sub.2 2 are under increased pressure to allow synthesis gas 30 to be discharged at a desired pressure. The cathode purge gas 32 may be a partial flow of the synthesis gas 30 recirculated by means of a blower (not shown).

(9) The steam-CO.sub.2-purge gas mixture 5 is heated to the extent possible in the recuperator 6 against the hot reaction gas 25 to be cooled and can then, if necessary, be further heated in the electric heater 7.

(10) In the electrolysis stack 9, an electrolytic splitting of steam and CO.sub.2 of the supplied gas 8 to H.sub.2, CO and O.sub.2 is carried out by electric energy 10. The oxygen accumulating at the anode 11 is removed from the stack 9 with purge gas 12, which is supplied at the required pressure and preheated in the recuperator 13 as well as in the electric heater 14. The hot purge gas O.sub.2 mixture 15 is cooled in the recuperator 13 against the purge gas 12 to be preheated and then discharged as exhaust gas 16 to the atmosphere.

(11) The approximately 850° C. hot gas 18 accumulating at the cathode 17 contains H.sub.2, CO, unreacted steam and CO.sub.2 as well as formed methane and due to the high temperature and the nickel-containing material is largely in chemical equilibrium.

(12) In order to reduce the CO.sub.2 and CH.sub.4 content in the gas 18, the gas 18 is supplied to the electric energy 10 operated heater 19 for further preheating and subsequently is supplied to the catalytic reactor 20, wherein the reheated gas reacts to chemical equilibrium, mainly through the reactions R3 and R4. Since the reaction final temperature of the gas 21 after the catalytic reactor 20 will be higher than in the gas 18 after the electrolysis stack 9, the CO.sub.2 and CH.sub.4 content in the gas 21 is lower than in the gas 18.

(13) Since the maximum preheating temperature of the gas 22 is limited due to the materials, one or more heater-reactor combinations 23-24 may be necessary until the required reaction end temperature 31 and thus the desired CO.sub.2 and CH.sub.4 concentration are achieved in the gas 25.

(14) After cooling of the reaction gas 25 in the recuperator 6 against the steam-CO.sub.2-purge gas mixture 5 and with the cooling water operated final cooler 26, steam 28 which is unreacted or is formed by the chemical reaction R3 in the electrolysis stack 9 and condensed by the cooling 6 and 26 is discharged from condensation vessel 27 via the condensate removal 29.

(15) The synthesis gas 30 remaining after the separation vessel 27 having the desired H.sub.2:CO molar ratio and the low CO.sub.2 and CH.sub.4 contents is supplied to the subsequent synthesis.

(16) FIGS. 6 and 7 in the appendix show the gas composition following the stack of the co-electrolysis as a function of temperature at a pressure of 1 bar (FIG. 6) and 10 bar (FIG. 7), a H.sub.2:CO molar ratio of 2 and a H.sub.2O/CO.sub.2 decomposition degree of 80%.

(17) The following tables show the gas compositions for selected temperature values at 1 bar (Table 1) and 10 bar (Table 2) and a H.sub.2CO.sub.2 decomposition rate of 80%:

(18) TABLE-US-00001 TABLE 1 Gas compositions I for selected temperature values at 1 bar p [bar(a)] 1 1 t [° C.] 850 950 1000 850 950 1000 kmol/kmol kmol/kmol, anhydrous CO 0.2662 0.2666 0.2670 0.3082 0.3128 0.3150 CO.sub.2 0.0647 0.0524 0.0477 0.0749 0.0615 0.0563 H.sub.2 0.5325 0.5332 0.5329 0.6163 0.6256 0.6287 H.sub.2O 0.1360 0.1477 0.1523 0.0000 0.0000 0.0000 CH.sub.4 0.0005 0.0001 0.0000 0.0006 0.0001 0.0000 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 Total CO.sub.2 + 0.0755 0.0616 0.0563 CH.sub.4

(19) TABLE-US-00002 TABLE 2 Gas compositions I for selected temperature values at 10 bar. p [bar(a)] 10 10 t [° C.] 850 950 1000 850 950 1000 kmol/kmol kmol/kmol, anhydrous CO 0.2420 0.2617 0.2647 0.2897 0.3093 0.3134 CO.sub.2 0.0784 0.0546 0.0486 0.0938 0.0646 0.0575 H.sub.2 0.4840 0.5233 0.5288 0.5795 0.6187 0.6259 H.sub.2O 0.1648 0.1541 0.1552 0.0000 0.0000 0.0000 CH.sub.4 0.0308 0.0062 0.0027 0.0369 0.0074 0.0032 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 Total CO.sub.2 + 0.1307 0.0720 0.0607 CH.sub.4

(20) The higher the temperature, the lower the CO.sub.2 and CH.sub.4 content in the produced gas. The gas compositions at 850° C. correspond to the gas composition in the prior art.

(21) In the following, a further solution, namely by increasing the temperature and providing coke beds downstream of the electrolysis stack, is explained, wherein continued reference may be made to FIG. 1.

(22) In contrast to the previous solution, the reactors 20 and 24 in FIG. 1 are not filled with catalyst but with coke which is consumed by the chemical reactions (R5, R6 and R7) and therefore has to be replaced regularly. The coke can be, e.g., charcoal produced from biomass.

(23) Via the heaters 19 and 23, the gas stream 18 is supplied with that amount of heat that the gas 18 reacts with the coke in the containers 20 and 24 and produces, as a function of the pressure and the gas qualities and reaction end temperatures listed in the following tables (Table 3 and Table 4), respectively at a H.sub.2H/CO.sub.2 decomposition degree in the electrolysis of 80%:

(24) TABLE-US-00003 TABLE 3 Gas Compositions II for selected temperature values at 1 bar p [bar(a)] 1 1 t [° C.] 850 950 1000 850 950 1000 kmol/kmol kmol/kmol, anhydrous CO 0.3229 0.3296 0.3314 0.3272 0.3310 0.3322 CO.sub.2 0.0063 0.0015 0.0008 0.0064 0.0015 0.0008 H.sub.2 0.6455 0.6590 0.6613 0.6541 0.6618 0.6629 H.sub.2O 0.0133 0.0042 0.0025 0.0000 0.0000 0.0000 CH.sub.4 0.0121 0.0057 0.0040 0.0122 0.0057 0.0041 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 Total CO.sub.2 + 0.0186 0.0072 0.0049 CH.sub.4

(25) TABLE-US-00004 TABLE 4 Gas Compositions II for selected temperature values at 10 bar p [bar(a)] 10 10 t [° C.] 850 950 1000 850 950 1000 kmol/kmol kmol/kmol, anhydrous CO 0.2633 0.3015 0.3115 0.2889 0.3126 0.3187 CO.sub.2 0.0421 0.0126 0.0070 0.0462 0.0130 0.0072 H.sub.2 0.5259 0.6027 0.6230 0.5769 0.6249 0.6374 H.sub.2O 0.0884 0.0354 0.0224 0.0000 0.0000 0.0000 CH.sub.4 0.0802 0.0477 0.0360 0.0880 0.0495 0.0368 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 Total CO.sub.2 + 0.1342 0.0625 0.0440 CH.sub.4

(26) At this point, it is noted that the temperature of 850° C. does not correspond here to the prior art, rather heat was supplied to the gas 18 to reach the reaction end temperature after the coke bed of 850° C.

(27) It is not possible, while maintaining a constant H.sub.2:CO molar ratio in the synthesis gas 30 (here=2), to pass the gas 18 after the stack 9 directly, i.e., without further heating, over a coke bed. The endothermic chemical reactions of the gas 18 with the coking carbon (R5 and R6) cool the gas. As the CO.sub.2 and the CH.sub.4 content increases with decreasing temperature, the gas deteriorates compared to the variant without coke bed (=prior art).

(28) The values in the tables show that an improvement in the gas quality can be achieved by using a coke bed after the electrolysis stack. At higher pressures (10 bar) it is necessary at the same time to use higher temperatures, since the higher methane content again deteriorates the gas quality.

(29) FIG. 2 shows a schematic representation of an embodiment for increasing the temperature and connecting a small all-ceramic stack.

(30) Steam 1, cathode purge gas 32 and CO.sub.2 2, which is preheated in a heat exchanger 3 to avoid steam condensation, are mixed in a mixer 4. The preheating of the CO.sub.2 can be done with an electric operated heater, with waste heat of co-electrolysis or from another external heat source. Steam 1, cathode purge gas 32, and CO.sub.2 2 are at increased pressure to allow synthesis gas 30 to be discharged at a desired pressure. The cathode purge gas 32 may be a partial flow of the synthesis gas 30 recirculated by means of a blower (not shown).

(31) The steam-CO.sub.2-purge gas mixture 5 is heated to the extent possible in the recuperator 6 against the hot reaction gas 36 to be cooled and then, if necessary, further heated in the electric heater 7.

(32) In electrolysis stack 9, which is operated at usual temperatures of about 850° C., an electrolytic splitting of steam and CO.sub.2 of the supplied gas 8 to H.sub.2, CO and O.sub.2, takes place by electrical energy 10, wherein the H.sub.2O/CO.sub.2 degree of conversion has not yet reached its maximum allowable value. The oxygen accumulating at the anode 11 is discharged from the stack 9 using a partial flow 33 of the purge gas 12, which is supplied with the required pressure and preheated in the recuperator 13 and in the electric heater 14.

(33) The 850° C. hot gas 18 forming at the cathode 17 contains H.sub.2, CO, unreacted steam and CO.sub.2 as well as formed methane and is largely in chemical equilibrium due to the high temperature and the nickel-containing material.

(34) For a further electrolytic CO.sub.2/H.sub.2O decomposition, the gas mixture 18 is further heated in optionally existing heater 34 operated with electric energy 10 and supplied to the all-ceramic stack 35 also powered by electric energy 10, in which the further electrolytic decomposition of CO.sub.2 and H.sub.2O takes place up to the permissible H.sub.2O/CO.sub.2 degree of decomposition at higher temperatures, for example at 1000° C.

(35) Due to the higher temperatures in the stack 35, the chemical balance of the produced gas mixture 36 is lower in CO.sub.2 and CH.sub.4 contents than in the co-electrolysis of the prior art.

(36) The oxygen resulting on the anode side 11 of the stack 35 is discharged from the stack 9 with the second partial flow 37 of the purge gas 12 and after mixing with the hot purge gas-O.sub.2 mixture from the stack 9 as gas 15 is cooled in the recuperator 13 against the to-be-preheated purge gas 12 and then discharged as exhaust gas 16 to the atmosphere.

(37) After cooling of the reaction gas 36 in the recuperator 6 against the to-be-heated steam-CO.sub.2-purge gas mixture 5 and in the cooler 26 operated with cooling water, steam 28 not converted in the electrolysis stack 35 and condensed in the cooling 6 and 26 is separated in the condensation container 27 and discharged via the condensate separator 29.

(38) The synthesis gas 30 remaining after the separation vessel 27 with the desired H.sub.2:CO molar ratio and the low CO.sub.2 and CH.sub.4 contents is supplied to the subsequent synthesis.

(39) The gas compositions which are established at 1,000° C. and in each case at 1 bar and at 10 bar with the same total H.sub.2O/CO.sub.2 decomposition rate correspond to the compositions of the variant with downstream catalytic reactors and are shown in the corresponding tables in the description associated with FIG. 1.

(40) In FIG. 3 is a schematic representation of an embodiment shown with H.sub.2 excess.

(41) In contrast to the variant of FIG. 1, hydrogen or a hydrogen-enriched gas 40 is separated from the gas 38 after the final cooling 26 and condensate separation 27 in the device 39, is recirculated by means of the compressor 41, and together with the steam 1 and the CO.sub.2 2 is returned to the process.

(42) The device 39 for hydrogen separation may be, for example, a membrane separation plant or a pressure swing absorption plant (PSA).

(43) For precise control of the amount of H.sub.2 to be separated off, for example, part of the gas stream 38 can be guided as a bypass around the separating device 39 (not shown).

(44) The gas 30 remaining after the gas separation unit 39 is the synthesis gas with the desired H.sub.2:CO molar ratio for the subsequent synthesis.

(45) The hydrogen 42 supplied to the process assumes at the same time the function of the cathode purge gas 32.

(46) By H.sub.2-recirculation, the SOC stack is operated with H.sub.2 excess, and there is a shift of the chemical equilibrium of the reactions R3 and R4 to less CO.sub.2 and more CH.sub.4, as the following table shows (H.sub.2O/CO.sub.2 decomposition degree 80%):

(47) TABLE-US-00005 TABLE 5 Gas Compositions III for selected temperature values at 1 and 10 bar Pressure 1 bar 10 bar Temperature 850° C. 950° C. 850° C. 950° C. without with without with without with without with H.sub.2-Loop kmol/kmol anhydrous CO 0.3082 0.3205 0.3128 0.3241 0.2897 0.2914 0.3093 0.3165 CO.sub.2 0.0749 0.0366 0.0615 0.0276 0.0938 0.0574 0.0646 0.0325 H.sub.2 0.6163 0.6412 0.6256 0.6481 0.5795 0.5812 0.6187 0.6325 H.sub.2O 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 CH.sub.4 0.0006 0.0017 0.0001 0.0002 0.0369 0.0701 0.0074 0.0185 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 2.00 2.00 1.99 2.00 2.00 Total CO.sub.2 + CH.sub.4 0.0755 0.0383 0.0616 0.0278 0.1307 0.1275 0.0720 0.0510

(48) The variant at 850° C. corresponds to a co-electrolysis with H.sub.2 excess but without the reheating of the gas 18 from the stack in the heaters 19 and 23 and the post-reaction in the reactors 20 and 24 at higher temperatures.

(49) In each case, the gas qualities with H.sub.2 recirculation (“with”) are compared with the qualities without recirculation (“without”). The gas qualities without recirculation correspond to the qualities with downstream catalytic reactor.

(50) With H.sub.2 excess, an improvement in gas quality over the prior art can be achieved. At higher pressures (10 bar), a higher reaction end temperature is also advantageous for a significant improvement in gas quality.

(51) FIG. 4 shows a schematic representation of an exemplary embodiment of an H.sub.2/CO separation at high temperatures and subsequent connection of a second stack.

(52) Starting from the description of FIG. 1, the gas 18 produced in the first stack 9 with a H.sub.2O and CO.sub.2 decomposition degree of, for example, 80% is first fed to a gas separation device 43, which is suitable for high temperatures. This gas separation device 43 may be, for example, a ceramic membrane.

(53) With the help of this gas separation device, a high proportion of CO and H.sub.2 and possibly no H.sub.2O and CO.sub.2 is separated as gas 44 from the gas 18 and then cooled in the recuperator 6 against a partial flow of the gas mixture 5.

(54) The remaining amount 45 of H.sub.2O and CO.sub.2 is supplied as a feed gas to a second stack 46, in which the gas 45 is further decomposed electrolytically using electric energy 10. Since the H.sub.2 and CO content in the gas 45 is initially low, a high degree of decomposition, e.g., 70%, in relation to the amount of H.sub.2O and CO.sub.2 supplied in the gas 45, can again be achieved in the stack 46, so that the total H.sub.2/CO.sub.2 degree of decomposition based on the amount of steam 1 and CO.sub.22 in both stacks is much higher than in only one stack (e.g. 80%), without the risk that the oxidic electrolyte of the stack is reduced.

(55) The CO and H.sub.2-rich gas 47 leaving stack 46 is cooled in the recuperator 48 against the second partial flow of the gas 5. Both cooled gas streams 49 and 50 are mixed and further cooled in the final cooler 26, where the remaining water 28 contained in the gas mixture is condensed out and separated in the condenser 27 from the gas stream.

(56) The synthesis gas 30 having the desired H.sub.2:CO molar ratio is fed to the subsequent synthesis.

(57) If, in addition, a higher reaction temperature is to be achieved, heaters and catalytic reactors for example can be connected downstream of the stack 46 as described in FIG. 1.

(58) The gas qualities resulting at temperatures of 850° C. and 950° C. and at a pressure of 1 bar and 10 bar and at a H.sub.2O/CO.sub.2 decomposition of 80% in the first and 70% in the second stack are summarized in the following table:

(59) TABLE-US-00006 TABLE 6 Gas Compositions IV for selected temperature values at 1 and 10 bar Pressure 1 bar 10 bar Temperature H.sub.2 + CO- 850° C. 950° C. 850° C. 950° C. Abtrennung + without with without with without with without with 2. Co-SOC kmol/kmol anhydrous CO 0.3082 0.3258 0.3128 0.3275 0.2897 0.3146 0.3093 0.3257 CO.sub.2 0.0749 0.0224 0.0615 0.0176 0.0938 0.0261 0.0646 0.0177 H.sub.2 0.6163 0.6515 0.6256 0.6549 0.5795 0.6308 0.6187 0.6522 H.sub.2O 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 CH.sub.4 0.0006 0.0003 0.0001 0.0000 0.0369 0.0285 0.0074 0.0043 Total 1.0000 1.0000 1.0000 1.0000 1.0000 0.9999 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 2.00 2.00 2.01 2.00 2.00 Total 0.0755 0.0227 0.0616 0.0176 0.1307 0.0546 0.0720 0.0220 CO.sub.2 + CH.sub.4

(60) In each case, the gas qualities with 2nd co-electrolysis (“with”) are compared with the qualities without second co-electrolysis (“without”). The gas qualities without 2nd co-electrolysis correspond to the qualities with downstream catalytic reactor.

(61) With a second co-electrolysis and thus higher H.sub.2O/CO.sub.2 decomposition degree, an improvement in gas quality over the prior art can be achieved.

(62) In FIG. 5 is a schematic representation of an embodiment for a separate electrolytic decomposition of H.sub.2O and CO.sub.2 is shown in which a separate CO.sub.2 and H.sub.2O electrolysis is applied.

(63) The CO.sub.2 2 is decomposed catalytically by means of electric energy 10 after recuperative heating in the heat exchanger 6 against the hot CO-rich gas 18 and further heating in the heater 7 in the stack 9.

(64) The steam 1 is decomposed electrolytically after recuperative heating in the heat exchanger 55 against the hot H.sub.2-rich gas 54 and further heating in the heater 51 in the stack 53 by means of electric energy 10.

(65) After the final cooling of the two cooled gas streams 56 and 57 in the coolers 26 and 58, and the separation of the condensate 28 from the gas stream 59 in the condenser 27, both streams 59 and 60 are mixed to synthesis gas 30 with the desired H.sub.2:CO molar ratio and supplied to the subsequent synthesis.

(66) The following table (Table 6) shows the gas compositions at 850° C. and 950° C. and a pressure of 1 bar and 10 bar each at a H.sub.2O/CO.sub.2 decomposition degree of 80%:

(67) TABLE-US-00007 TABLE 6 Gas compositions V for selected temperature values at 1 and 10 bar Pressure 1 bar 10 bar Temperature H.sub.2 + CO- 850° C. 950° C. 850° C. 950° C. separate CO.sub.2- and without with without with without with without with H.sub.2O-SOC kmol/kmol anhydrous CO 0.3082 0.3077 0.3128 0.3077 0.2897 0.3077 0.3093 0.3077 CO.sub.2 0.0749 0.0770 0.0615 0.0770 0.0938 0.0770 0.646 0.0770 H.sub.2 0.6163 0.6153 0.6256 0.6153 0.5795 0.6153 0.6187 0.6153 H.sub.2O 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 CH.sub.4 0.0006 0.0000 0.0001 0.0000 0.0369 0.0000 0.0074 0.0000 Total 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 H.sub.2:CO 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 Total CO.sub.2 + CH.sub.4 0.0755 0.0770 0.0616 0.0770 0.1307 0.0770 0.0720 0.0770

(68) In this variant, the gas quality is determined solely by the H.sub.2O or CO.sub.2 decomposition degree. A temperature and pressure dependence of the gas composition is not present, since no homogeneous gas reactions (R3 and R4) can proceed. This leads to lower CO.sub.2+CH.sub.4 contents being achieved in comparison to the prior art with comparable H.sub.2O and CO.sub.2 decomposition levels in the electrolysis only at high pressures and temperatures <950° C. The cause is the lack of methane formation.

(69) TABLE-US-00008 LIST OF REFERENCE NUMBERS 1 steam 2 carbon dioxide 3 heat exchanger 4 gas mixer 5 steam-CO.sub.2-purge gas-mixture 6 recuperator 7 electric heater 8 feedgas stack 9 electrolysis stack 10 electric energy 11 anode of stack 12 purge gas 13 recuperator 14 electric heater 15 purge gas-O.sub.2-mixture 16 exhaust gas 17 cathode of the stack 18 gas after stack 19 electric heater 20 catalytic reactor or coke filled container 21 reaction gas after catalytic reactor or coke filled container 22 preheated gas 23 electric heater 24 catalytic reactor or coke filled container 25 reaction gas 26 final cooler 27 condensate separator 28 condensate 29 condensate separator 30 synthesis gas 31 reaction end temperature 32 cathode purge gas 33 partial flow of purge gas 12 34 electric heater 35 all-ceramic electrolysis stack 36 gas after all-ceramic stack 37 partial flow of purge gas 12 38 gas after final cooling 39 device for the separation of H.sub.2 from the gas mixture 38 40 H.sub.2 enriched gas 41 recirculation compressor 42 supplied, recirculated H.sub.2 stream 43 gas separator, suitable for high temperatures 44 H.sub.2- and CO-enriched gas 45 H.sub.2O and CO.sub.2 enriched gas 46 electrolysis stack 47 gas after stack 46 48 recuperator 49 partial stream of cooled reaction gas 50 partial flow cooled reaction gas 51 electric heaters 52 steam in front of stack 53 53 electrolysis stack 54 H.sub.2 rich gas following stack 53 55 recuperator 56 cooled partial flow CO 57 cooled partial flow H.sub.2 58 final cooler 59 H.sub.2 stream after condensate separation 60 CO stream to final cooler