METHOD FOR CRACKING AMMONIA

Abstract

The present invention refers to a method for cracking ammonia, producing hydrogen and generating electrical power including electrolysis of water in feed ammonia, evaporation, pre-heating and cracking of ammonia, using ammonia synthesis catalysts at low temperatures. A method for cracking ammonia including a) electrolysis of water in feed ammonia, wherein feed ammonia includes make-up ammonia; b) evaporation; c) cracking; wherein cracking of ammonia takes place between 300-700° C., using ammonia synthesis catalysts.

Claims

1. A method for cracking ammonia, comprising: a) electrolysis of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; b) evaporation; c) cracking; wherein cracking of ammonia takes place between 300-700° C., using ammonia synthesis catalysts.

2. A method for producing hydrogen from make-up ammonia comprising: a) electrolysis of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; b) evaporation; c) cracking; and d) cooling of the gas phase resulting from cracking, wherein cracking of ammonia takes place between 300-700° C., using ammonia synthesis catalysts and wherein at least one of the (i) hydrogen produced by electrolysis and (ii) hydrogen resulting from ammonia cracking, contribute to the hydrogen final yield.

3. The method according to claim 1, wherein ammonia is pre-heated before the cracking stage.

4. The method according to claim 1, wherein said make-up ammonia comprises from approximately 0.2 up to approximately 2% of water.

5. The method according claim 1, wherein ammonia synthesis catalysts are Fe-based.

6. The method according to claim 1, wherein catalysts used are Co, Ru or Ni based.

7. The method according to claim 1, wherein a solution comprising liquid ammonia and non-evaporated water is purged, from evaporator and recycled back into feed ammonia.

8. The method according claim 1, wherein the electrolysis is a high-pressure electrolysis.

9. The method according to claim 1, wherein hydrogen resulting from ammonia cracking is recycled with an external compressor.

10. The method according to claim 1, wherein water is added to the cooling step e) and a scrubber is used for removing traces of unconverted ammonia.

11. The method according to claim 10 wherein unconverted ammonia is condensed and recycled to the electrolyzer when it contains water or to the evaporator when it is water free.

12. A method for removal of water from ammonia, comprising the following steps: a) electrolysis of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; and b) evaporation.

13. A method comprising using ammonia obtainable by the method according to claim 12 in ammonia cracking and/or hydrogen production wherein catalysts are used.

14. A method for producing electricity, feeding a gas turbine with at least one of hydrogen, nitrogen and ammonia obtained by: a) electrolysis of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; b) evaporation; c) cracking; and d) cooling of the gas phase resulting from cracking, wherein cracking of ammonia takes place between 300-700° C., using ammonia synthesis catalysts and wherein at least one of the (i) hydrogen produced by electrolysis and (ii) hydrogen resulting from ammonia cracking, contribute to the hydrogen final yield and wherein waste heat is recovered into ammonia evaporator.

15. The method according to claim 14, wherein part of the ammonia is cracked and part of the ammonia is bypassed to feed a gas turbine.

16. The method according to claim 14, comprising using the method for producing electricity by feeding a gas turbine with at least one of hydrogen, nitrogen and ammonia.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0023] FIG. 1 shows the pre-treatment of ammonia before decomposition, where water is electrolyzed (2).

[0024] FIG. 2 shows the methods of the present invention. In the ammonia cracker, the following reaction takes place: 2NH.sub.3=N.sub.2+3H.sub.2.

[0025] This reaction is rarely or never carried out to completion, meaning that after the ammonia cracker there is still a significant amount of unconverted ammonia. This unconverted ammonia is cooled and it can be recovered in a scrubber by absorption of ammonia in water, or condensed by cooling (7) and the liquid ammonia is recycled back to the electrolyzer (if it contains water) or the evaporator (if it is water free). If we choose to use water in the cooling stage for removing traces of ammonia, we are ensuring that the product gases, H.sub.2 and N.sub.2, are ammonia free. Using the shown configuration, hydrogen may be partially or entirely produced, e.g., during the day, from electrolysis and, e.g., during the night, partially or entirely by cracking ammonia. Furthermore, hydrogen purification can be achieved by PSA or other suitable technology.

[0026] FIG. 3 shows the integration with a gas turbine for achieving maximum overall efficiency in production of electrical power. The product gases hydrogen and nitrogen are used as gas turbine fuel and ammonia could be also present, either unconverted ammonia from cracking (6) or bypassed (9) before cracking. Gaseous ammonia may be added to the cracking process.

[0027] Reference numbers used are the following: [0028] (1) Make-up ammonia (0.2-0.5% water) [0029] (2) Water electrolyzer [0030] (3) Evaporation [0031] (4) Purge H.sub.2O/NH.sub.3 [0032] (5) Pre-heating [0033] (6) Cracking or Decomposition [0034] (7) Cooling [0035] (8) Recycled condensed unconverted ammonia (with or without water) [0036] (9) Bypassed ammonia [0037] (10) Gas turbine [0038] (11) Waste heat

Definitions

[0039] Ammonia Cracking or Decomposition is process of dissociation of gaseous anhydrous Ammonia (NH.sub.3) into a mixture of Hydrogen (H.sub.2) and Nitrogen (N.sub.2) according to the reaction: 2NH.sub.3=N.sub.2+3H.sub.2. This reaction is endothermic. The process is commonly performed at high temperatures between 1560-1740° F. (850-950° C.) in the presence of nickel as catalyst. Due to the higher temperatures required, lifetime of the catalysts will be reduced due to the thermal sintering of the catalyst. The resulting gas mixture is composed of hydrogen and nitrogen in the proportion 3:1 (75% of H.sub.2 and 25% of N.sub.2) with very little amount (20-100 ppm) of residual undissociated ammonia with dew point −60° F. to −20° F. (−51° C. to −29° C.). When performed under the conditions of the present invention, catalysts are preferably Fe-based and the process is performed at lower temperatures, between approximately 300-700° C.

[0040] Ammonia cracker means any suitable reactor where ammonia cracking (6) can take place, comprising fired reactors, preferably SMR.

[0041] Ammonia synthesis catalysts mean, within the context of the present invention, any catalysts suitable for synthesizing ammonia and also suitable for cracking ammonia. These catalysts are preferably iron (Fe) based, but may also comprise other catalysts suitable for the same purpose and operating at similar conditions.

[0042] Ammonia slip means unconverted ammonia (not dissociated in the cracking process) passing through the ammonia cracker.

[0043] Electrolysis of water means decomposition of water into oxygen and hydrogen gas due to the passage of an electric current.

[0044] Feed ammonia or Ammonia Feedstock means a solution comprising make-up ammonia and additional water. Feed ammonia is the solution fed or supplied to electrolyzer (2).

[0045] High-pressure electrolysis (HPE) is the electrolysis of water by decomposition of water (H.sub.2O) into oxygen (O.sub.2) and hydrogen gas (H.sub.2) due to the passing of an electric current through the water at elevated pressure, typically above 10 bar.

[0046] Make-up ammonia or Traded Ammonia comprises ammonia (NH.sub.3) and water (H.sub.2O), preferably between 0.2 to 0.5% of water content. It is usually supplied as a liquid but may also be a solution comprising different physical states. The effect of water comprised in ammonia feedstock in the ammonia decomposition process is primarily that due to poisoning the process, which usually has to take place at a high temperatures. This will increase process cost for ammonia decomposition as well as cost of construction materials in the plant. According to National Bureau of Standards ammonia shall conform to the following properties: minimum purity of 99.98% (wt), maximum 0,0005% (wt) oil and maximum 0.02% (wt) moisture.

[0047] Nitridation means the formation of nitrogen compounds through the action of ammonia.

[0048] PSA means pressure swing adsorption.

[0049] A residual amount of water due to any slip from the electrolysis unit will build up in the evaporator and needs to be purged (4). This purge contains water and ammonia can be recycled back into ammonia feedstock (1) and thus the electrolysis unit which means that the overall water content in the evaporated ammonia is close to zero.

DESCRIPTION OF THE INVENTION

[0050] In order to be suitable as fuel for power production, ammonia needs to be, at least partially, cracked to a gas mixture comprising gaseous hydrogen, nitrogen and ammonia. When using ammonia as an energy carrier for hydrogen production and having only ammonia as the energy source, it is economically essential to maximize the hydrogen production per ton of ammonia consumed. This can be obtained by cracking or decomposing ammonia at the lowest possible temperature since this will result in a lower amount of waste heat to be recovered. In this case, waste heat recovered as steam will have only little value as hydrogen is a product of interest.

[0051] In the ammonia cracking process, gaseous ammonia is dissociated into a mixture of hydrogen and nitrogen in the reversible reaction:

2NH.sub.3 (g)⇄N.sub.2 (g)+3H.sub.2 (g)

[0052] The reaction is endothermic, requiring heat for maintaining the ammonia cracking reaction.

[0053] It has been known that ammonia synthesis catalysts can be used for decomposition or cracking of ammonia. However, it is also well known that water or other oxygen containing compounds will be poisoning to the ammonia synthesis catalysts, such as Fe-based catalysts. Since water is the main compound in traded liquid ammonia or make-up ammonia (1), the poisoning of these synthesis catalysts is considered to be a problem affecting the catalyst performance and therefore how effective and efficient the decomposition process will be. This is at least one of the reasons why other, more expensive, catalysts are commonly used in ammonia cracking.

[0054] The most common method used to remove water from ammonia before decomposition is distillation. The method of this invention to remove water from ammonia by electrolysis is more efficient and effective than typical distillation because the energy spent for electrolysis of water into oxygen and hydrogen will not be lost, since the hydrogen thereby produced will end up contributing to the product gas. Whereas in distillation all the energy spent for separating water from ammonia will be simply lost.

[0055] For a conventional ammonia cracker using a catalyst that can tolerate water, the 0.2-0.5% water content in the make-up ammonia (1) will not harm the catalyst. These catalysts operate at high temperatures, above 600° C. and up to approximately 950° C. By removing water from the make-up ammonia (1), it will be possible to use alternative catalysts, similar to those used for ammonia synthesis at lower temperatures, typically in the range of 300-700° C., preferably between 350-550° C. Such ammonia synthesis catalysts are preferably iron (Fe) based, or other catalysts suitable for the same purpose and are typically purchased at much lower cost than the high temperature catalysts that tolerate water and other oxygen containing compounds, commonly used in ammonia cracking.

[0056] By operating the ammonia cracker at lower temperatures, lower grade materials of construction can be used and therefore capital expenditure is also lower. Lower temperature also creates less waste heat to recover and thus will make the process cheaper and more efficient.

[0057] Make-up ammonia comprises approximately 0.2 to 0.5% water and said water is electrolyzed (2) together with an optional supplementary amount of water added to said make-up ammonia before entering the electrolyzer (2). Feed ammonia is the liquid composition supplied to electrolyzer (2), comprising make-up ammonia and said optional supplementary amount of water, depending on the desired amount of hydrogen to be obtained by electrolysis. The effect of water addition is to increase the hydrogen produced by electrolysis, adjusting the hydrogen production from renewable energy in the electrolyzer. Also, hydrogen protects catalysts and materials from nitriding.

[0058] In a preferred embodiment, water is added to make-up ammonia (1) originating feed ammonia which will be electrolyzed (2) to hydrogen and oxygen and adjust the hydrogen content in the evaporated ammonia feeding the first ammonia cracker (FIG. 2), when having more than one cracker (6).

[0059] In a preferred embodiment of the present invention, the cracked ammonia gas is feedstock to a gas turbine for electricity production, an overall efficiency can be much higher if a significant amount of gaseous ammonia is bypassed the cracker, as shown in FIG. 3. The overall efficiency will be higher since less energy is required to crack ammonia and less waste heat will be generated from the cracker process. In this case it is foreseen not to have sufficient waste heat to evaporate the total ammonia feedstock, then waste heat can be recovered from the gas turbine flue gas without decreasing the gas turbine combined cycle efficiency as the required temperature level for ammonia evaporation is lower than approximately 100° C. and thus available. A significant amount of gaseous ammonia bypassed may be up to approximately 98% of the available ammonia and then we would require heat from the gas turbine exhaust. Depending on the gas turbine technology, if we bypass too much we will not have enough heat to evaporate all the ammonia. This is why the heat from the gas turbine is used. The higher is the ammonia content, the higher is overall efficiency.

Preferred Embodiments

[0060] 1. Method for cracking ammonia, comprising: [0061] a) electrolysis (2) of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; [0062] b) evaporation (3); [0063] d) cracking (6); and

[0064] wherein cracking (6) of ammonia takes place between 300-700° C., most preferably between 350-550° C., using ammonia synthesis catalysts.

[0065] 2. Method for producing hydrogen from make-up ammonia (1) comprising: [0066] a) electrolysis (2) of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; [0067] b) evaporation (3); [0068] d) cracking (6); and [0069] e) cooling (7) of the gas phase resulting from cracking (6),

[0070] wherein cracking (6) of ammonia takes place between 300-700° C., most preferably between 350-550° C., using ammonia synthesis catalysts and wherein at least one of the (i) hydrogen produced by electrolysis (2) and (ii) hydrogen resulting from ammonia cracking (6), contribute to the hydrogen final yield.

[0071] 3. Method according to embodiments 1 and 2 wherein ammonia is pre-heated (5) before the cracking (6) stage.

[0072] 4. Method according to embodiments 1 to 3, wherein said make-up ammonia (1) comprises from approximately 0.2 up to approximately 2% of water.

[0073] 5. Method according to embodiments 1-4, wherein ammonia synthesis catalysts are Fe-based catalysts.

[0074] 6. Method according to embodiments 1-4, wherein catalysts used are Co, Ru or Ni based.

[0075] 7. Method according to embodiments 1 to 5, wherein a solution comprising liquid ammonia and non-evaporated water is purged (4) from evaporator (3) and recycled back into feed ammonia.

[0076] 8. Method according to embodiments 1-6 wherein the electrolysis is a high-pressure electrolysis.

[0077] 9. Method according to embodiments 1-7 wherein hydrogen resulting from ammonia cracking is recycled with an external compressor.

[0078] 10. Method according to embodiments 1 to 9, wherein water is added to the cooling (7) step e) and a scrubber is used for removing traces of unconverted ammonia.

[0079] 11. Method according to embodiment 10 wherein unconverted ammonia is condensed (8) and recycled to the electrolyzer (2) when it contains water or to the evaporator (3) when it is water free.

[0080] 12. Method for removal of water from ammonia, comprising the following steps: [0081] a) electrolysis (2) of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; and [0082] b) evaporation (3).

[0083] 13. Use of ammonia obtainable by the method according to embodiment 12 in ammonia cracking and/or hydrogen production wherein catalysts such as Fe-based catalysts are used.

[0084] 14. Use of ammonia obtainable by the method according to embodiment 12 in ammonia cracking and/or hydrogen production wherein Co, Ru or Ni-based catalysts may be used.

[0085] 15. Method for producing electricity, feeding a gas turbine (10) with at least one of hydrogen, nitrogen and ammonia obtained by: [0086] a) electrolysis (2) of water in feed ammonia, wherein feed ammonia comprises make-up ammonia; [0087] b) evaporation (3); [0088] c) cracking (6); and [0089] d) cooling (7) of the gas phase resulting from cracking (6),

[0090] wherein cracking (6) of ammonia takes place between 300-700° C., most preferably between 350-550° C., using ammonia synthesis catalysts and wherein at least one of the (i) hydrogen produced by electrolysis (2) and (ii) hydrogen resulting from ammonia cracking (6), contribute to the hydrogen final yield and wherein waste heat is recovered (11) into ammonia evaporator (3).

[0091] 16. Method according to embodiment 15 wherein ammonia is pre-heated (5) before the cracking (6) stage.

[0092] 17. Method according to embodiments 15 and 16 wherein part of the ammonia is cracked (6) and part of the ammonia is bypassed (9) to feed a gas turbine (10).

[0093] 18. Method according to embodiments 15 to 17 wherein hydrogen is periodically and/or alternatively produced, e.g. during the day partly or entirely from electrolysis and during the night partly or entirely by cracking ammonia.

[0094] 19. Use of the method according to embodiments 15 to 18 for producing electricity by feeding a gas turbine (10) with at least one of hydrogen (2,7), nitrogen (7) and ammonia (7, 9).

[0095] Use of the method according to embodiments 15 to 18 for producing hydrogen and oxygen (2), evaporated ammonia (3) and gaseous hydrogen, nitrogen and ammonia from make-up ammonia.