BR-112021019898-B1 - Separation of ambient air and front section of SOEC for ammonia synthesis gas production.

BR112021019898B1BR 112021019898 B1BR112021019898 B1BR 112021019898B1BR-112021019898-B1

Abstract

AMBIENT AIR SEPARATION AND FRONT PART OF SOEC FOR AMMONIA SYNTHESIS GAS PRODUCTION. In a method for generating ammonia synthesis gas by electrolysis, comprising the steps of air compression and feeding to an air separation process, in which the nitrogen content is concentrated while the oxygen and CO2 content is diluted, feeding a mixture of steam and compressed and refined air to the electrolysis unit or to the first of a series of electrolysis units and passing the outlet of one electrolysis unit to the inlet of the next electrolysis unit, either along with the air added after each electrolysis unit or only adding air after the last electrolysis unit, the electrolysis units are run in thermoneutral or endothermic mode and the nitrogen part of the synthesis gas is supplied by burning the hydrogen produced by steam electrolysis by the refined air in or between the electrolysis units.

Inventors

PETER MØLGAARD MORTENSEN
John Bøgild Hansen

Assignees

HALDOR TOPSØE A/S

Dates

Publication Date: 20260310
Application Date: 20200331
Priority Date: 20190405

Claims (3)

1. Method for generating ammonia synthesis gas by electrolysis, said method characterized in that it comprises the steps of: - compressing air and feeding it to an air separation process, in which the nitrogen content is concentrated while the oxygen and CO2 content is diluted; - feeding a mixture of steam and compressed and refined air into the electrolysis unit or the first of a series of electrolysis units; and - passing the outlet of one electrolysis unit to the inlet of the next electrolysis unit, either together with the air added after each electrolysis unit or only adding air after the last electrolysis unit, wherein the electrolysis units are solid oxide electrolytic cell (SOEC) stacks and run in thermoneutral or endothermic mode and the nitrogen part of the synthesis gas is supplied by burning hydrogen produced by steam electrolysis by refined air in or between the electrolysis units.
2. A method according to claim 1, characterized in that the air separation process comprises a polymeric membrane unit or a ceramic membrane.
3. A method according to claim 1, characterized in that the air separation process comprises a pressure swing adsorption (PSA) unit or a temperature swing adsorption (TSA) unit.

Description

[001] The present invention relates to an improved process for generating synthesis gas for the production of ammonia. [002] A typical ammonia production plant first converts a desulfurized hydrocarbon gas, such as natural gas (i.e., methane) or LPG (a liquefied petroleum gas, such as propane and butane) or petroleum naphtha into hydrogen gas by steam reforming. The hydrogen is then combined with nitrogen to produce ammonia via the Haber-Bosch process: 3 H2 + N2 → 2 NH3 [003] Thus, the synthesis of ammonia (NH3) requires a synthesis gas (synthesis gas) comprising hydrogen (H2) and nitrogen (N2) in a suitable molar ratio of about 3:1. [004] Ammonia is one of the most widely produced chemicals, and is synthesized directly using gaseous hydrogen and nitrogen as reactants without precursors or byproducts. In its gaseous state, nitrogen is widely available as N2, and is normally produced by separating it from atmospheric air. Hydrogen (H2) production is still challenging and, for the industrial synthesis of ammonia, it is most often obtained from steam methane reforming (SMR) of natural gas. Furthermore, when air is used for reforming processes, N2 is also introduced, thus making the need for an air separation unit superfluous, but a cleaning process is necessary to remove oxygen-containing species such as O2, CO, CO2, and H2O to prevent catalyst poisoning in the ammonia converter. Carbon dioxide is a product of SMR and can be separated and recovered within the plant. Hydrogen production is therefore a critical process in ammonia synthesis, and sustainable ammonia production is desirable to reduce consumption of a primary source, such as natural gas, and to avoid CO2 emissions from the process. [005] In a prior patent application by the Applicant (PCT/EP2018/076616, now published as document WO 2019/072608 A1), a process is described in which the synthesis gas for ammonia production is prepared by electrolysis using solid oxide electrolytic cell (SOEC) stacks without having to use air separation. This process uses a combination of water electrolysis and air combustion to facilitate high-temperature steam electrolysis, which effectively means that any oxygen in the water and air feed can be separated into a separate stream and an intermediate H2/N2 product in a ratio that is suitable for ammonia production. The present invention can be viewed as an extended embodiment of the PCT/EP2018/076616 process, in which the air fed to the SOEC stacks has undergone an air separation step upstream of the SOEC stacks. Air separation is ideally achieved using a membrane or, alternatively, by pressure swing adsorption (PSA) or temperature swing adsorption (TSA). This way, some of the oxygen is removed from the air, and less oxygen needs to be burned and subsequently separated in the SOEC. This allows for a smaller stack area in the SOEC and improved process integration. [006] The teaching of PCT/EP2018/076616 allows the production of pure ammonia synthesis gas from sustainable resources. Combining this with partial air separation, an improved process integration can be achieved, since the oxygen in the air feed can be balanced to exactly match the steam production required in the SOEC layout combined with the ammonia cycle. Furthermore, upstream air separation reduces the content of other impurities in the air, especially CO2. [007] By performing only partial air separation, the operating costs of the initial air separation step can be kept very low, since a high selectivity for oxygen rejection can be achieved in the separation step when the remaining oxygen content is left at 15%, 10%, or even 5%, or potentially also at 2%. [008] Until now, the standard solution within this field has been to carry out a two-stage front-end retrofit for an ammonia plant that is operated exclusively on fossil fuels. [009] The preparation of ammonia synthesis gas by electrolysis has been described in several patents and patent applications. Thus, a method for the anodic electrochemical synthesis of ammonia gas is described in US 2006/0049063. The method comprises supplying an electrolyte between an anode and a cathode, oxidizing negatively charged nitrogen-containing species and negatively charged hydrogen-containing species present in the electrolyte at the anode to form adsorbed nitrogen species and hydrogen species, respectively, and reacting the adsorbed nitrogen species with the adsorbed hydrogen species to form ammonia. [0010] In US 2012/0241328, ammonia is synthesized using both electrochemical and non-electrochemical reactions. The electrochemical reactions occur in an electrolytic cell with a lithium-ion conducting membrane that divides the electrochemical cell into an anolyte compartment and a catholyte compartment, the latter including a porous cathode closely associated with the lithium-ion conducting membrane. [0011] Document WO 2008/154257 discloses a process for the production of ammonia that includes the production of nitrogen fr