EXPANDER FOR SOEC APPLICATIONS

Abstract

In a method for generating various synthesis gases by electrolysis, comprising feeding steam and compressed air to the cathode and anode, respectively, of the electrolysis unit or of the first of a series of electrolysis units into the first of a series of electrolysis units, the electrolysis units are operated under an elevated gas pressure, and the oxygen-rich gas leaving the anode is subsequently expanded down to approximately ambient pressure using a gas expander. The electrolysis units are preferably solid oxide electrolysis cell (SOEC) stacks.

Claims

1. A method for generating synthesis gas containing hydrogen, carbon monoxide or mixtures of hydrogen, carbon monoxide and carbon dioxide by electrolysis, said method comprising feeding steam and compressed air to the cathode and anode, respectively, of the electrolysis unit or of the first of a series of electrolysis units, wherein the electrolysis unit or units is/are operated under an elevated gas pressure, and the oxygen-rich gas leaving the anode is subsequently expanded down to approximately ambient pressure using a gas expander.

2. Method according to claim 1, wherein the electrolysis units are solid oxide electrolysis cell (SOEC) stacks.

3. Method according to claim 2, wherein the SOEC stacks operate in the so-called thermoneutral mode.

4. Method according to claim 1, wherein the synthesis gas is selected from methanol synthesis gas, methane synthesis gas, ammonia synthesis gas and dimethyl ether (DME) synthesis gas.

5. Method according to claim 2, wherein the synthesis gas is selected from methanol synthesis gas, methane synthesis gas, ammonia synthesis gas and dimethyl ether (DME) synthesis gas.

6. Method according to claim 3, wherein the synthesis gas is selected from methanol synthesis gas, methane synthesis gas, ammonia synthesis gas and dimethyl ether (DME) synthesis gas.

7. Method according to claim 2, wherein the air is compressed in an amount sufficient to achieve 50% (v/v) oxygen at an exit of the SOEC stacks.

8. Method according to claim 2, wherein the steam is mixed with recycled hydrogen and pre-heated in a feed/effluent heat exchanged on a cathode side of the SOEC stacks.

9. Method according to claim 8, wherein, on the cathode side, steam is electrolyzed and oxygen is transported across an electrolyte to an anode side of the SOEC stacks.

10. Method according to claim 8, wherein a stream of hydrogen mixed with steam is passed through the feed/effluent heat exchanger prior to being further cooled down by generated high pressure steam.

11. Method according to claim 10, further comprising splitting the stream into a recycle hydrogen stream and residual steam which is sent to ammonia synthesis.

12. Method according to claim 1, wherein a compressor and the gas expander are connected to different lines.

13. Method according to claim 1, wherein a compressor and the gas expander are connected to a mutual line.

14. Method according to claim 1, wherein the gas is expanded down to a pressure of at most 0.2 barg by the gas expander.

15. Method according to claim 1, further comprising pre-heating the air in a feed/effluent heat exchanger to a first elevated temperature T.sub.1.

16. Method according to claim 15, wherein, following pre-heating, the pre-heated air enters an electrical pre-heater which heats the air to a second elevated temperature T.sub.2, wherein T.sub.2>T.sub.1.

17. Method according to claim 16, wherein the second elevated temperature T.sub.2 is an inlet temperature of the SOEC stacks.

18. Method according to claim 15, wherein, after or as oxygen-enriched air leaves the SOEC stacks, heat is recuperated in the feed/effluent heat exchange, and wherein the oxygen-enriched air subsequently enters the gas expander.

19. Method according to claim 1, wherein the compressed air is compressed to a pressure greater than 20 barg and up to 40 barg.

20. A method for generating synthesis gas containing hydrogen, carbon monoxide or mixtures of hydrogen, carbon monoxide and carbon dioxide by electrolysis, said method comprising feeding steam and compressed air to the cathode and anode, respectively, of the electrolysis unit or of the first of a series of electrolysis units, wherein the electrolysis unit or units is/are operated under an elevated gas pressure, and the oxygen-rich gas leaving the anode is subsequently expanded down to approximately ambient pressure using a gas expander, wherein the compressed air is compressed to a pressure of up to 40 barg, and the oxygen-rich gas leaving the anode is of temperature from 650 to 850 C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] The FIGURE shows an exemplary embodiment of an SOEC plant.

DETAILED DESCRIPTION

[0032] The present invention is described in more detail in the example which follows. In the example, reference is made to the appended drawing illustrating the principle of the invention.

Example

[0033] This example, as shown in the FIGURE, shows an embodiment of the present invention, representing an SOEC plant delivering hydrogen to generate 1 ton of ammonia.

[0034] High pressure steam is imported from the ammonia synthesis and also generated within the SOEC plant. The steam is mixed with recycled hydrogen and pre-heated in a feed/effluent heat exchanger Hex1 on the cathode (fuel) side. It is further pre-heated to the operating temperature of the SOEC, using an electrically heated pre-heater ph1. In this example, the SOEC operates in the so-called thermoneutral mode, so the exit temperature from the stack is equal to the inlet temperature.

[0035] On the cathode side, steam is electrolyzed to hydrogen, and the oxygen is transported across the electrolyte to the anode side. The stream of hydrogen mixed with steam is then passed through the above-mentioned feed/effluent heat exchanger Hex1 prior to being further cooled down by generating additional high pressure steam. Finally, the stream is cooled further, and any non-converted steam is condensed out. At this stage, the stream is split into a recycle hydrogen stream and residual steam which is sent to the ammonia synthesis.

[0036] On the air side, air is compressed in a compressor C to 40 barg in an amount sufficient to achieve 50% (v/v) oxygen at the exit of the SOEC stacks. The air is pre-heated to 765 C. in a feed/effluent heat exchanger Hex2 before it enters an electrical pre-heater ph2 which further increases the temperature to 785 C., which is the inlet temperature of the stacks. The oxygen-enriched air leaves the stack, and heat is recuperated in the feed/effluent heat exchanger Hex2 before it enters the expander Eat a temperature of 424 C. The gas is expanded down to a pressure of 0.2 barg, whereby the temperature drops to 91 C.

[0037] Using an efficiency of 85% for the polytropic efficiency and 5% work loss for the air compressor, and a polytropic efficiency of 78% and 4% work loss for the expander, then the work used and the work recuperated will amount to 311 kW and 356 kW, respectively. It can thus be seen that more power is recuperated (45 kWh per ton of ammonia-equivalent synthesis gas production) than what is spent compressing the dilution air.

[0038] In the FIGURE, the compressor and the expander are connected to different lines. They could, however, be connected to a mutual line, which would lead to a better energy efficiency. It could also reduce pressure fluctuations within the cell.