Ambient air separation and SOEC front-end for ammonia synthesis gas production

Abstract

In a method for generating ammonia synthesis gas by electrolysis, comprising the steps of compressing air and feeding it to an air separation process, in which the content of nitrogen is concentrated while the content of oxygen and CO.sub.2 is diluted, feeding a mixture of steam and the compressed and refined air into the electrolysis unit or into the first of a series of electrolysis units and passing the outlet from one electrolysis unit to the inlet of the next electrolysis unit, either together with air added after each electrolysis unit or only adding air after the last electrolysis unit, the electrolysis units are run in thermoneutral or endothermal mode and the nitrogen part of the synthesis gas is provided by burning the hydrogen produced by steam electrolysis by the refined air in or between the electrolysis units.

Claims

1. A method for generating ammonia synthesis gas by electrolysis, said method comprising the steps of: compressing air and feeding it to an air separation process to form a compressed and refined air, in which the content of nitrogen is concentrated while the content of oxygen and CO.sub.2 is diluted; feeding a mixture of steam and the compressed and refined air into a first of a series of electrolysis units; producing hydrogen by steam electrolysis in the electrolysis units; providing nitrogen and steam by burning the hydrogen with the compressed and refined air in or between the electrolysis units, wherein the burning produces a stream including steam and nitrogen which is fed to the next electrolysis unit; and passing an outlet from one electrolysis unit to an inlet of a next electrolysis unit, either together with compressed and refined air added after each electrolysis unit or only adding compressed and refined air after a last electrolysis unit, wherein the electrolysis units are run in thermoneutral or endothermal mode, wherein the nitrogen part of the synthesis gas is provided by burning the hydrogen produced by steam electrolysis with the compressed and refined air in the electrolysis units.

2. Method according to claim 1, wherein the air separation process comprises a polymer membrane unit or a ceramic membrane.

3. Method according to claim 1, wherein the air separation process comprises a pressure swing adsorption (PSA) unit or a temperature swing adsorption (TSA) unit.

4. Method according to claim 1, wherein the method comprises passing the outlet from one electrolysis unit to the inlet of the next electrolysis unit together with compressed and refined air added after each electrolysis unit.

5. Method according to claim 1, wherein the method comprises passing the outlet from one electrolysis unit to the inlet of the next electrolysis unit and only adding compressed and refined air after the last electrolysis unit.

6. A method for generating ammonia synthesis gas by electrolysis, said method comprising the steps of: compressing air and feeding it to an air separation process to form a compressed and refined air, in which the content of nitrogen is concentrated while the content of oxygen and CO.sub.2 is diluted; feeding a mixture of steam and the compressed and refined air into a first of a series of electrolysis units; and passing an outlet from one electrolysis unit to an inlet of a next electrolysis unit, either together with compressed and refined air added after each electrolysis unit or only adding compressed and refined air after a last electrolysis unit, wherein the electrolysis units are run in thermoneutral or endothermal mode and the nitrogen part of the synthesis gas is provided by burning hydrogen produced by steam electrolysis with the compressed and refined air in or between the electrolysis units, wherein the burning produces steam, which is fed to the next electrolysis unit, wherein the nitrogen part of the synthesis gas is provided by burning the hydrogen produced by steam electrolysis with the compressed and refined air in the electrolysis units.

7. Method according to claim 1, wherein the air separation process dilutes the content of oxygen to 2-15% in the compressed and refined air.

8. Method according to claim 1, wherein the air separation process dilutes the content of oxygen to 2-10% in the compressed and refined air.

9. Method according to claim 1, wherein the air separation process dilutes the content of oxygen to 2-5% in the compressed and refined air.

10. Method according to claim 1, wherein the content of oxygen in the compressed and refined air feed is balanced to match a required amount of steam for electrolysis.

Description

EXAMPLE 1

(1) It has been verified that an air separation membrane installed front-end of the SOEC gives a good synergy. A simple calculation using compressed air on one side of a membrane and non-compressed air as sweep gas on the other side indicates that the oxygen concentration in the feed air can be reduced from 21% to 4.1%. The calculation is based on feed air (21% O.sub.2 and 79% N.sub.2 plus 400 ppm CO.sub.2) at 10 barg (10 kNm.sup.3/h) and a sweep gas (air, i.e. 21% O.sub.2 and 79% N.sub.2 plus 400 ppm CO.sub.2) at 0.5 barg (10 kNm.sup.3/h). The resulting feed air to the SOEC (6.7 kNm.sup.3/h) at 9 barg consists of 4.1% O.sub.2 and 95.9% N.sub.2.

EXAMPLE 2

(2) Since CO.sub.2 permeates approximately 30 times faster than N.sub.2, a membrane will be able to separate off CO.sub.2 effectively. At 30 bar, the calculation would look like this (dependent of the desired O.sub.2 concentration): Based on feed air (21% O.sub.2 and 79% N.sub.2 plus 400 ppm CO.sub.2) at 30 bar and a sweep gas (air, i.e. 21% O.sub.2 and 79% N.sub.2 plus 400 ppm CO.sub.2) at 0.5 bar, the resulting feed air to the SOEC at 30 bar would consist of 3.3% O.sub.2 and 96.6% N.sub.2 plus 20 ppm CO.sub.2.