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CN-117355482-B - Ammonia cracking for hydrogen production

CN117355482BCN 117355482 BCN117355482 BCN 117355482BCN-117355482-B

Abstract

A process for synthesizing hydrogen by catalytic cracking of ammonia, the process comprising the steps of subjecting an ammonia-containing stream (10) to a catalytic cracking step (11) in the presence of heat to produce combustion gases and a thermally cracked stream (14) containing nitrogen, hydrogen and potentially residual ammonia and optionally water, and subjecting the thermally cracked stream to a hydrogen recovery step to produce a high purity hydrogen stream (22).

Inventors

  • S. Panza
  • D. Diadaizio

Assignees

  • 卡萨尔公司

Dates

Publication Date
20260505
Application Date
20220519
Priority Date
20210521

Claims (18)

  1. 1. A process for the catalytic synthesis of hydrogen comprising the steps of: a) Subjecting the ammonia stream (7) to a heating stage to produce an ammonia-containing stream (10); b) Subjecting the ammonia-containing stream (10) to a catalytic ammonia cracking step in the presence of heat to produce combustion gases (60) and a thermal cracking stream (14) containing nitrogen, hydrogen and residual ammonia; c) Optionally mixing the thermal cracking stream with water (74) to produce a thermal cracking stream (75) with water added; d) Feeding the thermal cracking stream (14) or the thermal cracking stream (75) with added water to a cooling stage to produce a cooled stream (79); e) Subjecting the cooled stream (79) to a flash separation step to produce an ammonia-lean stream (81) and an ammonia stream (86) or aqueous ammonia solution (82), and further subjecting the ammonia-lean stream (81) to the steps of: e1 A hydrogen recovery step to produce a high purity hydrogen stream (22) and a tail gas (23), Or (b) E2 A washing step (20) in the presence of water (17) to produce a purified gas stream (51), and further subjecting the purified gas stream (51) to a hydrogen recovery step to produce a high purity hydrogen stream (22) and a tail gas (23); f) Recirculating at least a portion of the tail gas (23) as fuel to provide heat for the catalytic cracking step; g) The high purity hydrogen stream (22) is withdrawn.
  2. 2. The method of claim 1, wherein the heat for the ammonia catalytic cracking step is provided by combustion of the fuel gas (12) in the presence of preheated air (28).
  3. 3. The method of claim 1, further comprising the steps of subjecting the ammonia-retaining fuel gas (12) to a cracking step (100) in the presence of electrical heating to produce a gas mixture (101) retaining hydrogen and nitrogen and optionally unconverted ammonia, and further subjecting the gas mixture (101) to combustion in the presence of preheated air (28) to provide heat in the catalytic cracking step.
  4. 4. The method of claim 1, further comprising the step of: h) Subjecting the aqueous ammonia solution (82) to a distillation step to separate an ammonia stream (86) from the aqueous solution (84); i) Recirculating at least a portion of the ammonia stream (86) as fuel to provide heat for the catalytic cracking step; j) Optionally recycling a portion of the ammonia stream (86) to step a) to undergo the heating stage in the presence of the ammonia stream (7); k) Heat is recovered from the combustion gas (60) by indirectly contacting a portion (87) of the aqueous solution (84) with the combustion gas (60) and feeding the portion of the aqueous solution to the distillation step after heat recovery to provide distillation heat.
  5. 5. The method of claim 4, further comprising the step of mixing a second portion (88) of the aqueous solution (84) obtained from the distilling step with a thermal cracking stream (14) optionally added with a water make-up stream.
  6. 6. The method of claim 1, wherein the hydrogen recovery step is performed by a pressure swing adsorption unit, or a cryogenic separation unit, or a membrane purification unit.
  7. 7. The method of claim 1, wherein the concentration of the high purity hydrogen stream (22) is greater than 99% wt.
  8. 8. The method of claim 1, wherein the concentration of the high purity hydrogen stream (22) is greater than 99.9% wt.
  9. 9. A process according to claim 1, wherein the temperature of the thermal cleavage stream (14) leaving the catalytic cleavage step is between 400 and 950 ℃.
  10. 10. A process according to claim 1, wherein the temperature of the thermal cracking stream (14) leaving the catalytic cracking step is between 550 and 650 ℃.
  11. 11. The process of claim 1 wherein the catalytic cracking step is carried out at a pressure of from 5 to 65 barg.
  12. 12. The process of claim 1, wherein the catalytic cracking step is carried out at a pressure comprised between 15 and 30 barg.
  13. 13. The method of claim 1, wherein the combustion gas (60) is subjected to a nitrogen oxide (NOx) reduction step.
  14. 14. Apparatus for producing hydrogen according to the method of claim 1, comprising at least: an ammonia cracking furnace (11) comprising a plurality of externally heated catalytic tubes, an input line arranged to feed an ammonia-containing stream (10) into said tubes, and an output line arranged to collect a thermal cracking stream (14) from said tubes; optionally a line configured to feed water to an output line arranged to collect the thermal cracking stream (14); a flash separator unit (80) in communication with the output line, the flash separator unit (80) configured to separate an ammonia lean stream (81) and an ammonia stream or aqueous ammonia solution (82); a hydrogen recovery unit (19), the hydrogen recovery unit (19) being in fluid communication with the flash separator unit (80) and configured to recover a high purity hydrogen stream (22) and a tail gas (23); -a line arranged to recycle at least a portion of the off-gas (23) separated from the hydrogen recovery unit (19) to the furnace (11) for use as additional fuel; a line arranged to withdraw a high purity hydrogen stream (22) from said hydrogen recovery unit (19).
  15. 15. The apparatus of claim 14, further comprising: a distillation unit (83) configured to separate water and ammonia in the aqueous ammonia solution (82); -a line connecting the flash separator unit (80) to the distillation unit (83) and configured to convey the aqueous ammonia solution (82) to the distillation unit (83); -a gas production line connecting the distillation unit (83) to the furnace (11); -a heat exchanger section configured to recover heat from combustion gases in the furnace (11) by means of a water flow; -a line connecting the distillation unit (83) to the heat exchanger section and configured to convey the water stream for heat integration purposes between the furnace (11) and the distillation unit (83).
  16. 16. The apparatus of claim 14, wherein the furnace (11) further comprises means for removing nitrogen oxides NOx.
  17. 17. The apparatus of claim 14, wherein the furnace (11) further comprises an SCR unit for removing nitrogen oxides NOx.
  18. 18. The apparatus of claim 14, wherein the hydrogen recovery unit (19) is one of a pressure swing adsorption unit, a cryogenic separation unit, a membrane separation unit.

Description

Ammonia cracking for hydrogen production Technical Field The present invention is in the field of hydrogen production and in particular relates to a method and apparatus for producing hydrogen from an ammonia cracking unit. Background Excessive use of fossil fuels in the power industry and transportation has a detrimental impact on human health and welfare and the environment. There is an urgent need to provide some environmentally friendly and sustainable alternative fossil fuels. Hydrogen and ammonia are carbon-free carriers that are considered ideal substitutes for fossil fuels. On a small scale, hydrogen can be produced by various domestic sources (e.g., solar, wind) and electrolysis. In contrast, on an industrial scale, hydrogen is obtained by reforming fossil fuels, mainly by reforming of natural gas (steam reforming) or water-gas shift of coal-derived synthesis gas. The hydrogen produced by steam reforming requires a multi-step process, starting with natural gas purification, high temperature reforming, high temperature Water Gas Shift (WGS) and purification. Unfortunately, due to the reforming process, a large amount of CO 2 is emitted to the atmosphere. In the art, it is desirable to find an industrial scale hydrogen synthesis process that can produce clean hydrogen without venting any carbon dioxide to the atmosphere. This process should also be economically competitive with conventional processes. Green ammonia synthesized from renewable energy sources is a carbon-free storage carrier for hydrogen gas, with many potential energy applications including the production of green hydrogen gas. Hydrogen can be obtained from ammonia by a thermal decomposition process known as catalytic cracking. In the catalytic cracking process, ammonia is decomposed or cracked back into H 2 and N 2 in the presence of heat and a catalyst (Ni or Ru or Pt) according to the following endothermic equilibrium: 2NH3↔3H2+N2 At temperatures as low as 425 ℃, ammonia can be thermodynamically converted to hydrogen. In practice, however, the conversion depends on the type of catalyst used. Typically, ni is active at higher temperatures (500-750 ℃) than Ru (400 ℃) but the latter catalysts are more expensive. The heat required for the thermocatalytic conversion of ammonia is typically provided by electrical heating in an electrical heating furnace or in a reformer by combustion of fuel. Unfortunately, the ammonia cracking techniques described above suffer from several drawbacks. First, ammonia cracking technology is mature and is commercially available primarily for small scale applications (i.e., having a hydrogen production rate of less than 100 kg H 2/h). The main difficulty in expanding this technology is to design a cracking unit that is sufficiently compact but capable of decomposing ammonia at a rate consistent with consumption. Furthermore, a typical problem observed in projects using adiabatic cracking units is the relatively low conversion of ammonia (i.e., high ammonia slip). In contrast, cracking plants utilizing oxygen converting autothermal reformers require the installation of expensive air separation units ASUs. Furthermore, for high hydrogen production (> 1000 m 3/h), natural gas reforming remains the most cost-effective option. Accordingly, in view of the above-mentioned considerations, it is highly desirable to provide a cost-effective hydrogen synthesis process and apparatus suitable for large-scale hydrogen production. In addition, the improved hydrogen synthesis process should be environmentally friendly and therefore not result in carbon dioxide emissions to the atmosphere. Disclosure of Invention The present invention aims to overcome the above-mentioned drawbacks of the prior art. In particular, the problem addressed by the present invention is how to reduce carbon dioxide emissions and equipment costs, and how to provide methods and equipment suitable for large scale production. The invention relates to a method in which a high-purity hydrogen stream is obtained by cracking ammonia. A first aspect of the invention is a carbon-free hydrogen production process for the catalytic synthesis of hydrogen. The process comprises the steps of subjecting an ammonia stream, optionally with water added, to a preheating step to produce an ammonia-containing stream, subjecting the ammonia-containing stream to a catalytic ammonia cracking step in the presence of heat to produce a thermally cracked stream containing nitrogen, hydrogen, and potentially residual ammonia and water. The process further comprises the steps of subjecting the thermal cracking stream to a hydrogen recovery step to produce a high purity hydrogen stream and a tail gas, or subjecting the thermal cracking stream to a scrubbing step in the presence of water to produce a purified gas stream, and further subjecting the purified gas stream to a hydrogen recovery step to produce a high purity hydrogen stream and a tail gas. In addition, the method includes