METHOD FOR IMPROVING EFFICIENCY OF AN AMMONIA SYNTHESIS GAS PLANT

Abstract

Method for improving efficiency of an existing ammonia synthesis gas plant or a new ammonia synthesis gas plant by establishing a combination of secondary steam reforming using oxygen from electrolysis of water for the production of ammonia synthesis gas.

Claims

1. A method of improving efficiency of an ammonia synthesis gas plant, the ammonia synthesis gas plant comprises a fired primary steam reformer and a secondary steam reformer operated with an oxygen containing atmosphere, a water gas shift unit, a carbon dioxide removal unit, a methanation step and an ammonia synthesis gas compressor, the method comprises the steps of (a) establishing an electrolysis unit and preparing a separate hydrogen gas containing stream and a separate oxygen gas containing stream by electrolysis of water; (b) establishing a gas pipe for transporting the separate hydrogen gas containing stream from the electrolysis unit to the synthesis gas compressor and/or to the methanation step; and (c) establishing a gas pipe for transporting at least part of the separate oxygen gas stream from the electrolysis unit to a burner in the secondary reformer.

2. The method according to claim 1, wherein the electrolysis unit is powered by renewable energy.

3. The method according to claim 1, wherein the oxygen containing atmosphere, is air enriched with oxygen from the separate oxygen gas stream.

4. The method according to claim 1, wherein the electrolysis unit is pressurized.

5. Improved ammonia synthesis gas plant comprising a fired primary steam reformer and a secondary steam reformer operated with an oxygen containing atmosphere, a water gas shift unit, a carbon dioxide removal unit, a methanation reactor and an ammonia synthesis gas compressor, wherein the ammonia synthesis gas plant further comprises an electrolysis unit providing a separate hydrogen containing stream and a separate oxygen gas containing stream by electrolysis of water and a gas pipe for transporting the separate hydrogen gas containing stream from the electrolysis unit to the synthesis gas compressor and/or to the methanation reactor and a gas pipe for transporting at least part of the separate oxygen gas stream from the electrolysis unit upstream or into a burner in the secondary reformer.

Description

[0012] The present invention is based on establishing a combination of the fired primary steam reforming process and the secondary reforming process using air or oxygen enriched air in the operation of the secondary reformer burner and a new implemented step of electrolysis of water for the production of ammonia synthesis gas.

[0013] Thus, this invention provides method of improving efficiency of an ammonia synthesis gas plant, the ammonia synthesis gas plant comprises a fired primary steam reformer and a secondary steam reformer operated with an oxygen containing atmosphere, a water gas shift unit, a carbon dioxide removal unit, a methanation step and an ammonia synthesis gas compressor, the method comprises the steps of

[0014] (a) establishing an electrolysis unit and preparing a separate hydrogen gas containing stream and a separate oxygen gas containing stream by electrolysis of water;

[0015] (b) establishing a gas pipe for transporting the separate hydrogen gas containing stream from the electrolysis unit to the synthesis gas compressor and/or to the methanation step; and

[0016] (c) establishing a gas pipe for transporting at least part of the separate oxygen gas stream from the electrolysis unit to a burner in the secondary reformer.

[0017] The method of the invention can be used to improve efficiency of an existing ammonia synthesis gas plant operated with primary and secondary reforming or in a new plant with primary and secondary reforming. The improvement of an existing or a new ammonia synthesis gas plant by the method of the invention aims to increase the production capacity of the plant and/or to save fuel in the fired primary steam reformer at a fixed capacity, as oxygen from water electrolysis provides heat for the reforming reaction in the secondary reformer. Thereby, the duty of the primary reformer is decreased, when the oxygen content in the oxygen containing atmosphere in the secondary reformer is increased with the oxygen prepared in the water electrolysis. As a result, the hydrocarbon slip in the gas from the primary reformer increases and the gas exit temperature decreases, which again results in lower fuel consumption for firing the primary reformer. Due to the lower fuel consumption, the reformer tube wall temperature is reduced, resulting in a significantly longer tube life time.

[0018] Another advantage is that the overall hydrocarbon slip outlet the secondary reformer can be the same as in conventional plants without electrolysis or can be reduced to obtain improved synthesis gas composition because of reduced content of inserts resulting in reduced purge from the ammonia loop and thus a more efficient utilization of the feed stock.

[0019] The method according to the invention provides further advantage of less emission of CO.sub.2 from the primary flue gas stack.

[0020] Still an advantage is that the CO.sub.2 partial pressure is increased at inlet to the carbon dioxide removal unit, which improves the carbon dioxide removal efficiency by reducing the required energy consumption.

[0021] Compared to prior art methods using electrolysis of water for hydrogen production and air separation for nitrogen production, the oxygen product from electrolysis of water is advantageously used for partial oxidation in secondary reformer resulting in a reduced size of the primary reformer in a new plant or reduced load in an existing plant, which is a costly and an energy intensive unit and process.

[0022] Still an advantage of the invention is that energy for operating the electrolysis unit can be renewable energy generated by windmills, solar cells, hydraulic energy or other renewables.

[0023] Thus, in a preferred embodiment of the invention, the electrolysis unit is powered by renewable energy.

[0024] Preferably, the electrolysis of water is performed at elevated pressure according to process air compressor discharge pressure, which delivers the prepared stream of oxygen at elevated pressure to the burner of the secondary reformer and the hydrogen stream to the synthesis gas compressor and/or to the methanation step.

[0025] Thus, in a preferred embodiment of the invention, the electrolysis unit is pressurized.

[0026] The synergy in combining water electrolysis with secondary reforming technology for ammonia synthesis gas production, results in overall savings of hydrocarbon feedstock and fuel for the reforming process.

[0027] In Table 1 below, key figures of ammonia synthesis gas preparation are given for a 2200 MTPD ammonia plant for comparison of conventional syngas technologies and conventional syngas technology combined with water electrolysis.

TABLE-US-00001 TABLE 1 Natural gas Natural gas Power Primary T.sub.out Technology feed con- fuel con- for elec- CO.sub.2 in reformer Primary for syngas sumption, sumption, trolysis, flue gas, duty, Reformer, preparation Nm.sup.3/h Nm.sup.3/h MW Nm.sup.3/h Gcal/h C. Conventional 57,408 19,273 0 21,899 108.82 807 Conventional 57,108 14,072 54 16,438 82.34 748 with water electrolysis (25% oxygen in air)