How to Decompose Ammonia

Abstract

1. A process for decomposing ammonia to form hydrogen, comprising: (i) passing ammonia through one or more catalyst-containing tubes in a furnace to decompose the ammonia to form hydrogen, the one or more tubes being heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides that can react with the ammonia in the flue gas to form ammonium nitrate; and (ii) cooling the flue gas to less than 170° C., wherein y is a hydrogen atom or a nitrogen atom that is capable of reacting with the ammonia in the flue gas to form ammonium nitrate. H2O is the mole percent of water vapor in the flue gas, and P * H2O is the equilibrium vapor pressure of water in an aqueous solution of ammonium nitrate, and p is the minimum working pressure of the flue gas. [0010] JPEG2025512105000016.jpg10128

Description

[Technical field]

[0001] The present invention relates to a process for decomposing ammonia, and in particular to a process for decomposing ammonia in a furnace heated by the combustion of a fuel gas. [Background technology]

[0002] Ammonia can be decomposed to form hydrogen. This reaction has been used for many years to provide hydrogen to activate catalysts in ammonia plants, but is becoming increasingly interesting as a means to provide hydrogen for power generation or other uses. The reaction can be illustrated as follows:

[0003]

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[0004] The ammonia decomposition reaction is endothermic and can be usefully accomplished by passing ammonia over a suitable catalyst in externally heated catalyst-containing reactor tubes located within a furnace. For example, such furnaces are known for steam reforming of natural gas or naphtha feedstocks. The furnace generally includes a radiant section containing reactor tubes in which fuel is burned with air to provide heat for the ammonia decomposition reaction, and a downstream convection section in which flue gases formed by combustion are cooled by indirect heat exchange with one or more feeds for the process, usually in a preheat coil.

[0005] When fuel gas containing ammonia is burned with air in a furnace, nitrogen oxides (NO x ), in particular nitric oxide (NO) and nitrogen dioxide (NO2), are formed.

[0006] If ammonia leaks from the preheat coil, or if it is incompletely combusted when used as a component of fuel or present for selective catalytic reduction, NH3 and NO2 may be present in the flue gas. xAs a result of the presence of H2O, ammonium nitrate or ammonium nitrite may be formed, both of which, but especially ammonium nitrite, pose an explosion hazard. Ammonium nitrite is particularly dangerous as it is highly unstable and an explosion hazard. NO is the main gas formed during the combustion of hydrogen / ammonia blends, but NO2 is also present and further formation of NO2 is driven by the oxidation of NO, which is advantageous as the temperature in the convection section of the furnace is reduced. The reaction to form ammonium nitrate can be represented as follows:

[0007]

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[0008] Selective catalytic reduction (SCR) can be used in the convection section of the furnace, also called the flue gas duct, to destroy NOx in the flue gas, but the ammonia decomposition process must be managed to further minimize the risk of ammonium nitrite and ammonium nitrate formation. Eliminating the conditions under which ammonium nitrate forms in this system protects against the formation of ammonium nitrite. The inventors have determined that this can be provided by managing the water vapor partial pressure in the flue gas. Summary of the Invention

[0009] Accordingly, the present invention provides a process for decomposing ammonia to form hydrogen, comprising the steps of: (i) passing ammonia through one or more catalyst-containing tubes in a furnace to decompose the ammonia to form hydrogen, the one or more tubes being heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides that can react with the ammonia in the flue gas to form ammonium nitrate; and (ii) cooling the flue gas to less than 170° C., to prevent the formation of solid ammonium nitrate,

[0010]

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[0011]

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[0012]

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[0013] Thus, in the process according to the invention, the amount of water vapor must be greater than the equilibrium vapor pressure of water in the aqueous ammonium nitrate solution, so that the ammonium nitrate dissolves rather than precipitates as a solid either inside the furnace, e.g., in the convection section or flue gas duct, or downstream of the furnace.

[0014] The water vapor content in the flue gas can be adjusted or controlled by increasing the hydrogen content in the fuel gas and / or by adding water vapor into the furnace via water vapor injection. To calculate the minimum water vapor partial pressure to prevent the formation of solid ammonium nitrate, the equilibrium vapor pressure of ammonia and the vapor pressure of water in an aqueous solution of ammonium nitrate may be determined.

[0015] The vapor pressure of water in NH4NO3 solutions over a range of temperatures has been measured and published. For example, the vapor pressures at 10-40°C can be found in the chapter on Ammonium Compounds by Weston, C., Papcun, J, and Dery, M. in the Kirk-Othmer Encylopedia of Chemical Technology, Vol 2. 2003. The crystallization curves are provided in a paper by Othmer et al. entitled "Correlating vapor pressures and heats of solution for the ammonium nitrate-water system: An enthalpy-concentration diagram", published in the AIChE Journal, Vol 6, Issue 2. 1960. Pp 210-214, and the complete data set is published in a paper by Voorwinden, M. entitled "NH4NO2 Formation in Cooler Condenser", presented at the ANNA meeting in St. Louis, USA, October 2004.

[0016] The range of interest for this invention is up to 170° C. above which solid ammonium nitrate is not formed. The following water vapor pressures were used in this invention:

[0017] [Table 1]

[0018] NO x It is also useful to determine the equilibrium vapor pressure of ammonia in the -NH3-HNO3 mixture. This can be accomplished as follows: From equations 2 and 3 above, the equilibrium constant can be defined as follows:

[0019]

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[0020] The equilibrium constant K2 can be derived from measurements by Forsythe et al. in the paper entitled "The Entropies of Nitric Acid and its Mono- and Tri-hydrates", published in J. Am. Chem. Soc. Vol. 64, 1942, Pp 48-61, which gives the following formula (temperature measured in Kelvin):

[0021]

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[0022] Equation 6 can then be used to derive K2 for the relevant operating temperature, and equation 4 can be used to derive the partial pressure of nitric acid if the partial pressures of NO, NO2, and H2O are known.

[0023] The equilibrium constant K3 can be derived from measurements in the paper entitled "Vapor Pressure of Ammonium Nitrate" by Brandner, JD et al., published in J. Chem. Engineering Data, 7 (1962), pp. 227-228, which gives the following formula:

[0024]

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[0025] Once K3 and pHNO3 have been calculated, the equilibrium vapor pressure of ammonia can be calculated using Equation 5.

[0026] The temperature of the flue gas at which ammonium nitrate solids may be formed is below 170° C., for example in the range of 10 to 170° C. The pressure of the flue gas may be in the range of 0.8 to 1.2 bar.

[0027] Ammonia decomposition furnaces are known and include a furnace box providing a radiant section, where fuel gas and air are supplied and where radiant heat is generated by combustion using one or more burners to heat one or more reaction tubes containing an ammonia decomposition catalyst. There may be dozens or hundreds of tubes in the radiant section. The catalyst may be any ammonia decomposition catalyst. Nickel and ruthenium catalysts may be used. A preferred catalyst is a nickel catalyst. The catalyst may comprise 3-30% by weight nickel, preferably 8-20% by weight nickel (expressed as NiO), on a suitable refractory support, such as alumina or a metal aluminate. The catalyst may be in the form of a pelletized unit, which may contain one or more through holes, or may be provided as a washcoat on a structured metal or ceramic catalyst. A particularly preferred catalyst is KATALCO® 27-2 available from Johnson Matthey PLC, which comprises 12% nickel (expressed as NiO) on a cylindrical pellet formed from a high surface area alumina support. The temperature of the ammonia feed at the inlet of the tubes may range from 400 to 950 °C. The temperature of the cracked gas leaving the tubes affects the equilibrium position and may range from 500 to 950 °C. If a nickel catalyst is used, the temperature leaving the tubes is preferably 700 °C or higher. The inlet pressure of the tubes is set by the flowsheet design and may range from 1 to 100 bar abs, preferably from 10 to 90 bar abs. For example, if a pressure swing absorption unit is used as the hydrogen separation stage, the inlet pressure may typically range from 31 to 51 bar abs.

[0028] The fuel gas is combusted to generate heat for the endothermic decomposition reaction. The fuel gas contains ammonia such that combustion produces flue gas containing NO and / or NO2 and water vapor. The fuel gas may contain 1-100% ammonia by volume, i.e. the fuel gas may consist of ammonia, or may contain lesser amounts, for example in the range of 1-50% or 1-30% by volume of ammonia. The ammonia-containing fuel gas may contain a portion of the decomposed ammonia gas, i.e. the fuel gas may contain or consist of nitrogen, hydrogen, and ammonia. It is desirable for hydrogen to be present in the fuel gas to support combustion. Hydrocarbon gases such as natural gas may also be used to supplement the fuel and provide the energy required for the ammonia decomposition reaction. Water vapor may also be present in the fuel gas.

[0029] Combustion of the fuel gas produces flue gas which is conveyed from the radiant section of the furnace to the convection section of the furnace, or to a flue gas duct, where it is cooled by indirect heat exchange. There may be one or more stages of heat exchange within the convection section or flue gas duct.

[0030] Additionally, it is desirable to include a selective catalytic reduction (SCR) unit in the flue gas duct. SCR units are known and generally include a honeycomb or plate supported catalyst with low pressure drop. SCR catalysts are made from various porous ceramic materials such as alumina, titania, zirconia, ceria, or mixtures thereof, and the active catalyst component is usually an oxide of either a base metal (such as vanadium, molybdenum, and tungsten), a zeolite, or various precious metals such as Pt and / or Pd. Base metal catalysts such as vanadium and tungsten lack high thermal durability but are less expensive. Zeolite catalysts have the potential to operate at substantially higher temperatures than base metal catalysts. Iron and copper exchanged zeolite urea SCR can be used. The amount of platinum group metal is typically 5% by weight or less. A reducing agent such as ammonia or urea solution is added to the SCR catalyst to convert NOx in the flue gas to nitrogen and water. For example, SCR proceeds with anhydrous ammonia according to the following equation: 2NO+2NH3+1 / 2O2→2N2+3H2O (8) NO2+2NH3+1 / 2O2→3 / 2N2+3H2O (9) NO+NO2+2NH3→2N2+3H2O (10)

[0031] The SCR catalyst may operate at temperatures ranging from 225 to 450°C.

[0032] NO levels up to 1500 ppmv during combustion in the radiant section x may be present and is reduced to less than 50 ppmv when passing through the SCR unit. Oxygen is present downstream of the combustion due to excess air and the possibility of air ingress.

[0033] The SCR unit is preferably downstream of the first heat recovery section. If desired, a further heat recovery section may be provided downstream of the SCR unit to cool the flue gas to below 30° C. The flue gas at this point is particularly susceptible to the formation of solid ammonium nitrate if the temperature drops below 170° C. Thus, management of the water vapor content may include the addition of water vapor to the convection section of the furnace upstream or downstream of the SCR unit.

[0034] Calculations have shown that the minimum amount of water vapor in the flue gas to inhibit the formation of solid ammonium nitrate under normal operating conditions is desirably about 19.9 mole percent or greater. A possible SCR unit malfunction or ammonia leak increases the minimum amount of steam that is desirably present, which may be up to about 54.9 mole percent or more, depending on the circumstances. [Brief description of the drawings]

[0035] The present invention will now be further described with reference to the drawings. [Figure 1] FIG. 1 is a diagram of an ammonia cracking furnace useful in the process of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Those skilled in the art will understand that the drawings are schematic and that in a commercial plant additional items of equipment may be required, such as raw material drums, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, etc. The provision of such ancillary equipment does not form a part of this invention, but is in accordance with conventional chemical engineering practice.

[0037] In Figure 1, an ammonia cracking furnace 10 is shown that includes a radiant section 12 and a convection section 14 that includes a flue gas duct. Ammonia is supplied via line 16 to a number of nickel catalyst-containing reaction tubes 18 located within the radiant section 12. The tubes 18 are heated in the radiant section 12 by combustion of a fuel gas that is supplied via line 20 to a number of burners 22. The fuel gas is combusted with air that is supplied to the burners 22 via an air supply line (not shown). The fuel gas contains ammonia, and therefore the combustion gases produce nitrogen oxides NO. x The ammonia is decomposed in reaction tube 18 to form a gas mixture containing nitrogen and hydrogen and unreacted ammonia, which is collected from tube 18 via line 24 for further processing to recover the hydrogen.

[0038] NO xThe combustion gases containing NOx and NH3 flow from the radiant section 12 to the flue gas duct of the convection section 14 where they are cooled in a first heat recovery unit 26 including steam generation to generate partially cooled flue gas. The partially cooled flue gas then passes through the flue gas duct to a downstream selective catalytic reduction (SCR) unit 26 containing an SCR catalyst supplied with ammonia from a reductant storage unit 30 via line 32. The ammonia 32 reacts with nitrogen oxides in the partially cooled flue gas to form nitrogen and water vapor. The reaction is incomplete leaving traces of NOx and NH3 in the flue gas exiting the SCR unit 28. The flue gas leaving the SCR unit 28 is further cooled to below 170° C. in the flue gas duct in a second heat recovery unit 34 and is withdrawn from the flue gas duct of the convection section 14 via line 36.

[0039] To prevent the formation of ammonium nitrate solids in the second heat recovery unit, one or both of the following measures are employed: A hydrogen stream is added to the fuel gas 20 supplied to the burner 22 via line 38. Combustion of hydrogen with air generates additional water vapor in the flue gas leaving the radiant section 12 of the furnace 10. Alternatively or additionally, water vapor is added to the convection section 14 via line 40, preferably at or near the inlet of the flue gas duct of the convection section. The water vapor may be partially generated by the first heat recovery unit 26.

[0040] The invention will be further illustrated by reference to the following calculated examples, all of which were based on a process operating using the ammonia cracking furnace shown in Figure 1. The fuel gas contained 23.6% by volume ammonia, 58.6% by volume nitrogen, 17.6% by volume hydrogen, and 0.2% by volume water vapor. The ammonia feed gas contained 0.14% by volume water. The following reaction conditions were set: Ammonia supply inlet temperature 550℃ Cracked gas outlet temperature: 700℃ Ammonia content in cracked gas: ≦1.2 mol% Flue gas duct inlet temperature 780℃ SCR inlet temperature 370℃ SCR outlet temperature 380℃ NOx levels during combustion of ammonia fuel gas are 1500 ppmv and are reduced to 50 ppmv when passing through the SCR.

[0041] Example 1: Normal operation During normal operation, residual NO x And if low levels of slipped NH3 are present, there is a risk of ammonium nitrate formation downstream of the SCR unit 28. Current emission limits are below 50 ppmv NO x The required levels were 0.8-1.1 bara and NH3 levels below 5 ppmv. These levels were used to determine the required water vapor partial pressure. The flue gas operating pressure was 0.8-1.1 bara. In addition, NO x The degree of oxidation can also affect the steam requirements, so oxidation ratios of 0.1, 0.5, and 0.9 (

[0042]

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[0043] Example 2: SCR Malfunction In this scenario, the SCR malfunction did not reduce the NOx levels from the 1500 ppmv generated in the radiant section. If the SCR malfunctions and ammonia continues to be fed into the duct through the SCR vessel, the NO xThe level would remain at 1500 ppmv and the NH3 level would be around 1000 ppmv. The required vapour content in the flue gas would then be higher, assuming an outlet pressure from the duct of 0.8 bar absolute, and would range from 32.4 mol% (oxidation ratio is 0.1) to 36.8 mol% (oxidation ratio is 0.9), in order to avoid the formation of solid ammonium nitrate.

[0044] Example 3: Ammonia Leakage In this scenario, ammonia leakage into a flue gas duct (with SCR in operation) was examined.

[0045] In this case, the SCR will detect less than 50 ppmv of NO x The ammonia leaks functioned as necessary to maintain the levels. However, ammonia was introduced into the flue gases through a leak in the duct coil. In the worst case scenario, where all the ammonia leaks into the duct, the NH3 level would reach 33 mole %. The steam content in the flue gas downstream of the ammonia leak would then need to be greater than or equal to 36.8 mole % to prevent the formation of solid ammonium nitrate (assuming an outlet pressure from the duct of 0.8 bar absolute and an oxidation ratio in the range 0.1-0.9).

[0046] Example 4: Ammonia Leak and SCR Malfunction In this scenario, we looked at ammonia leakage into the flue gas duct (without SCR functioning). In this case, the SCR is not functioning and NO xThe level remains at 1500 ppmv. However, ammonia was introduced into the flue gas through a leak in the duct coil. In the worst case scenario, where all the ammonia leaks into the duct, the NH3 level would reach 33 mole %. The steam content required to prevent the formation of solid ammonium nitrate varies depending on the extent of the leak. For small leaks resulting in ammonia levels of 1 mole % or less, a water vapor content of 37.2 mole % in the flue gas (before the ammonia leak) would be sufficient to prevent the risk of solid ammonium nitrate formation. The water vapor content downstream of the ammonia leak must be greater than or equal to 36.8 mole %.

[0047] Assuming a minimum pressure in the duct of 0.8 bar absolute and using an oxidation ratio of 0.9, the following vapour contents in the flue gases were calculated to prevent the risk of solid ammonium nitrate formation:

[0048] [Table 2]

Claims

1. 1. A process for decomposing ammonia to form hydrogen, comprising: (i) passing ammonia through one or more catalyst-containing tubes in a furnace to decompose the ammonia to form hydrogen, the one or more tubes being heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides that can react with ammonia in the flue gas to form ammonium nitrate; and (ii) cooling the flue gas to less than 170° C. to prevent the formation of solid ammonium nitrate, wherein the reaction is carried out according to the formula: [0010] (In the formula, [0025] is the mole percent of water vapor in the flue gas; [0030] is the equilibrium vapor pressure of water in an aqueous solution of ammonium nitrate, P is the minimum working pressure of the flue gas.

2. 10. The process of claim 1, wherein the furnace comprises a radiant section in which the catalyst containing tubes are heated and a convection section downstream of the radiant section in which the flue gases are cooled to below 170°C.

3. 3. The process according to claim 1 or claim 2, wherein the ammonia decomposition catalyst is a nickel catalyst or a ruthenium catalyst, preferably a nickel catalyst.

4. The process of any one of claims 1 to 3, wherein the ammonia is supplied to the catalyst containing tube at a temperature in the range of from 400 to 950°C.

5. 5. The process according to any one of claims 1 to 4, wherein the ammonia is supplied to the catalyst containing tube at a pressure in the range of from 1 to 100 bar absolute, preferably from 10 to 90 bar absolute.

6. 6. The process of any one of claims 1 to 5, wherein the fuel gas mixture contains 1 to 100% or 1 to 50% by volume of ammonia.

7. 7. The process of any one of claims 1 to 6, wherein the fuel gas comprises nitrogen, hydrogen and 1 to 50% by volume of ammonia.

8. 8. The process according to any one of claims 2 to 7, wherein a selective catalytic reduction unit is provided in the convection section to reduce the nitrogen oxide content of the flue gas, preferably a selective catalytic reduction unit is provided in the convection section downstream of a first heat recovery unit which cools the flue gas to an inlet temperature of the selective catalytic reduction unit.

9. 9. The process of claim 8, wherein the selective catalytic reduction section is located upstream of a second heat recovery unit that cools the flue gas to less than 170°C after the flue gas has passed through the selective catalytic reduction unit.

10. 10. The process of any one of claims 1 to 9, wherein hydrogen is added to the fuel gas mixture to maintain the amount of water vapor in the flue gas to prevent the formation of solid ammonium nitrate.

11. The process of any one of claims 1 to 10, wherein the water vapor is added to the flue gas to prevent the formation of solid ammonium nitrate.

12. 12. The process of claim 11, wherein steam is added to the flue gas upstream of the selective catalytic reduction unit.

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