Method for cracking ammonia

Abstract

A process for cracking ammonia to form hydrogen is described, the process comprising the steps of: (i) passing the ammonia through one or more catalyst-containing tubes in a furnace to crack the ammonia and form hydrogen wherein the one or more tubes are heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides, the invention relates to a method for producing ammonium nitrate from flue gas, comprising the steps of (i) cooling the flue gas to a temperature below 170 DEG C, where yH2O is mole% of steam in the flue gas, P * H2O is the equilibrium vapor pressure of water in an aqueous ammonium nitrate solution, and p is the minimum operating pressure of the flue gas, and (ii) cooling the flue gas to a temperature below 170 DEG C. # imgabs0 #

Description

[0001] The invention relates to a process for cracking ammonia, in particular in a furnace heated by burning fuel gas.

[0002] Ammonia can be cracked to form hydrogen. This reaction has been used for many years to provide hydrogen to activate catalysts in ammonia plants, but it is gaining increasing attention as a means of providing hydrogen for power generation or other uses. The reaction can be pictured as follows:

[0003]

[0004] The ammonia cracking reaction is endothermic and can be efficiently effected by passing ammonia over a suitable catalyst in externally heated catalyst-containing reaction tubes arranged in a furnace. Such furnaces are known, for example, for steam reforming of natural gas or naphtha feedstocks. The furnace typically comprises a radiant section containing reaction tubes, where a fuel is combusted with air to provide heat for the ammonia cracking reaction, and a downstream convection section, where the flue gases formed by the combustion are cooled, typically by indirect heat exchange with one or more feeds to the process in a preheating coil.

[0005] The combustion of ammonia-containing fuel gas and air in the furnace will form nitrogen oxides (NO x ), especially nitric oxide (NO) and nitrogen dioxide (NO2).

[0006] If ammonia leaks from the preheat coil or is not completely burned when ammonia is used as a fuel component or is present for selective catalytic reduction, NH3 and NO in the flue gas will x The presence of and H2O can lead to the formation of ammonium nitrate or ammonium nitrite, both of which are explosion risks, especially ammonium nitrite. Ammonium nitrite is particularly dangerous because it is extremely unstable and therefore presents a risk of explosion. Although NO is the main gas formed during the combustion of the hydrogen / ammonia blend, NO2 is also present and further formation of NO2 is driven by the oxidation of NO, which is favored when the temperature in the convection section of the furnace is reduced. The reaction to form ammonium nitrate can be described as follows:

[0007]

[0008]

[0009] Selective catalytic reduction (SCR) can be used in the convection section of the furnace (also known as the flue gas duct) to break down NOx in the flue gas, however, the ammonia cracking process needs to be managed to further minimize the risk of ammonium nitrite and ammonium nitrate formation. Eliminating the conditions that form ammonium nitrate in this system prevents the formation of ammonium nitrite. We have recognized that this can be provided by managing the steam partial pressure in the flue gas.

[0010] Thus, the present invention provides a method for cracking ammonia to form hydrogen, said method comprising the steps of: (i) passing ammonia through one or more tubes containing a catalyst in a furnace to crack said ammonia and form hydrogen, wherein said one or more tubes are heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides which are capable of reacting with ammonia in said flue gas to form ammonium nitrate, and (ii) cooling said flue gas to below 170° C., characterized in that the amount of steam in said flue gas is maintained according to the following formula to prevent the formation of solid ammonium nitrate:

[0011]

[0012] in,

[0013] is the mole % of steam in the flue gas

[0014] is the equilibrium vapor pressure of water in aqueous ammonium nitrate solution, and

[0015] P is the minimum operating pressure of the flue gas.

[0016] Therefore, in the process according to the invention, the amount of steam should be greater than the equilibrium vapor pressure of water in the aqueous ammonium nitrate solution, thereby causing the ammonium nitrate to dissolve rather than be deposited in solid form within the furnace, for example in the convection section or flue gas duct, or downstream of the furnace.

[0017] The steam content in the flue gas can be adjusted or controlled by increasing the hydrogen content in the fuel gas and / or adding steam to the furnace via steam injection.To calculate the minimum 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 can be determined.

[0018] The vapor pressure of water in NH4NO3 solution has been measured and published over a range of temperatures. For example, the vapor pressure at 10°C to 40°C can be found in Kirk-Othmer Encylopedia of Chemical Technology, Vol. 2, 2003, Weston, C., Papcun, J, Dery, M, in the section on ammonium compounds. Crystallization curves are provided in Othmer et al., entitled "Correlating vapor pressures and heats of solution for the ammonjum nitrate--water system: An enthalpy-concentration diagram", published in AIChEJournal, Vol. 6, No. 2, 1960, pp. 210-214, and a complete set of data is published in a paper entitled "NH4NO2 Formation in Cooler Condenser" by Voorwinden, M., published at the ANNA conference held in St. Louis, USA in October 2004.

[0019] The present invention is concerned with a range up to 170°C, above which solid ammonium nitrate is not formed. The following water vapor pressures are used in the present invention.

[0020]

[0021] Confirm NO x The equilibrium vapor pressure of ammonia in the NH3-HNO3 mixture is also useful. This can be achieved as follows: Based on equations 2 and 3 above, the equilibrium constant can be defined as:

[0022]

[0023]

[0024] The equilibrium constant K2 can be derived from the measured value in the paper entitled "The Entropies of Nitric Acid and its Mono- and Tri-hydrates" by Forsythe et al. (published in J. Am. Chem. Soc. Vol. 64, 1942. pp. 48-61), resulting in the following equation (temperature is measured in Kelvin):

[0025]

[0026] K2 can then be derived using Equation 6 for the relevant operating temperature, and if the partial pressures of NO, NO2, and H2O are known, the partial pressure of nitric acid can be derived using Equation 4.

[0027] The equilibrium constant K3 can be derived from the measured value 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), resulting in the following equation:

[0028]

[0029] Once K3 and pHNO3 are calculated, the equilibrium vapor pressure of ammonia can be calculated using Equation 5.

[0030] The temperature of the flue gas from which the ammonium nitrate solid may be formed is 170° C. or less, for example in the range of 10° C. to 170° C. The pressure of the flue gas may be in the range of 0.8 bar to 1.2 bar.

[0031] Ammonia cracking furnaces are known and comprise a furnace box providing a radiant section into which fuel gas and air are fed and in which combustion using one or more burners produces radiant heat for heating one or more reaction tubes containing an ammonia cracking catalyst. There may be dozens or hundreds of tubes in the radiant section. The catalyst may be any ammonia cracking catalyst. Nickel catalysts and ruthenium catalysts may be used. The preferred catalyst is a nickel catalyst. The catalyst may comprise from 3 to 30 weight % nickel, preferably from 8 to 20 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 granular unit which may include one or more through holes, or may be a washcoat on a structured metal or ceramic catalyst. A particularly preferred catalyst is KATALCO available from Johnson Matthey PLC. RTM 27-2, which comprises 12% nickel expressed as NiO on cylindrical particles formed from a high surface area alumina support. The temperature of the ammonia feed at the tube inlet may be in the range of 400°C to 950°C. The temperature of the cracked gas leaving the tube will affect the equilibrium position and may be in the range of 500°C to 950°C. When a nickel catalyst is used, the temperature leaving the tube is preferably ≥ 700°C. The inlet pressure of the tube will be set by the flowsheet design and may be in the range of 1 bar to 100 bar absolute, preferably 10 bar to 90 bar absolute. For example, when a pressure swing absorption unit is used as a hydrogen separation stage, the inlet pressure may advantageously be in the range of 31 bar to 51 bar absolute.

[0032] The fuel gas is combusted to generate heat for the endothermic cracking reaction. The fuel gas contains ammonia so that the combustion produces flue gas containing NO and / or NO2 and steam. The fuel gas may contain 1 volume % to 100 volume % ammonia, that is, the fuel gas may be composed of ammonia, or may contain a lower amount of ammonia, such as 1 volume % to 50 volume %, or 1 volume % to 30 volume % ammonia. The ammonia-containing fuel gas may contain a portion of cracked ammonia, that is, the fuel gas may contain nitrogen, hydrogen and ammonia or be composed of nitrogen, hydrogen and ammonia. It is desirable that hydrogen is present in the fuel gas to support combustion. Hydrocarbon gases such as natural gas may also be used to supplement fuel and provide the energy required for the ammonia cracking reaction. Water vapor may also be present in the fuel gas.

[0033] The combustion of the fuel gas produces flue gases which are conveyed from the radiant section of the furnace to the convection section or flue gas duct of the furnace where they are cooled by indirect heat exchange. One or more heat exchange stages may be provided in the convection section or flue gas duct.

[0034] In addition, it is desirable to include a selective catalytic reduction (SCR) unit in the flue gas duct. The SCR unit is known and generally includes a honeycomb or plate-loaded catalyst that provides a low pressure drop. The SCR catalyst is made of various porous ceramic materials, such as alumina, titania, zirconium oxide, ceria or mixtures thereof, and the active catalytic component is generally an oxide, zeolite or various precious metals of a base metal (such as vanadium, molybdenum and tungsten), such as Pt and / or Pd. Base metal catalysts (such as vanadium and tungsten) lack high heat resistance, but are relatively cheap. Zeolite catalysts are likely to operate at much higher temperatures than base metal catalysts. Zeolite urea SCRs exchanged with iron and copper can be used. The amount of platinum group metals is generally 5 wt % or less. A reducing agent such as ammonia or urea solution is added to the SCR catalyst to convert NOx in the flue gas into nitrogen and water. For example, SCR is performed with anhydrous ammonia according to the following equation:

[0035] 2NO+2NH3+1 / 2O2→2N2+3H2O (8)

[0036] NO2+2NH3+1 / 2O2→3 / 2N2+3H2O (9)

[0037] NO+NO2+2NH3→2N2+3H2O (10)

[0038] The SCR catalyst may operate at a temperature ranging from 225°C to 450°C.

[0039] During combustion, up to 1500 ppmv of NO may be present in the radiant section. x Levels, when passing through the SCR unit, drop to <50ppmv. Due to the excess air, and possible air ingress, there will be oxygen downstream of the combustion.

[0040] Desirably, the SCR unit is 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. Flue gas at this point is particularly susceptible to the formation of solid ammonium nitrate when the temperature drops below 170°C. Therefore, management of the steam content may therefore include adding steam to the convection section of the furnace upstream or downstream of the SCR unit.

[0041] Calculations show that under normal operating conditions, the minimum amount of steam in the flue gas to inhibit the formation of solid ammonium nitrate is ideally about 19.9 mole % or higher. Possible failure or ammonia slip of the SCR unit may increase the minimum amount of steam that is desired to be present, up to about 54.9 mole % or higher, depending on the circumstances.

[0042] The present invention will now be further described with reference to the accompanying drawings, in which:

[0043] Figure 1 An ammonia cracking furnace which can be used in the process of the present invention is described.

[0044] It will be appreciated by those skilled in the art that the drawings are illustrative and that other items of equipment may be required in a commercial plant, such as feedstock barrels, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, liquid level controllers, collection tanks, storage tanks, etc. The provision of such ancillary equipment does not form part of the present invention and is consistent with conventional chemical engineering practice.

[0045] exist Figure 1 In FIG. 1 , an ammonia cracking furnace 10 is depicted as comprising a radiant section 12 and a convection section 14 having a flue gas duct. Ammonia is fed via line 16 to a plurality of reaction tubes 18 containing a nickel catalyst disposed within the radiant section 12. The tubes 18 are heated in the radiant section 12 by combustion of a fuel gas fed via line 20 to a plurality of burners 22. The fuel gas is combusted with air fed to the burners 22 via an air supply line (not shown). The fuel gas contains ammonia, and thus the combustion gas contains nitrogen oxides NO x The ammonia is cracked in the reaction tube 18 to form a gas mixture containing nitrogen and hydrogen and unreacted ammonia, which is collected from the tube 18 via line 24 for further processing to recover the hydrogen.

[0046] Contains NO xThe combustion gases flow from the radiant section 12 to the flue gas duct of the convection section 14, where they are cooled in the first heat recovery unit 26 (including a steam generator) to produce partially cooled flue gas. The partially cooled flue gas then travels in the flue gas duct to a downstream selective catalytic reduction (SCR) unit 26 containing an SCR catalyst, which is fed with ammonia from a reductant storage unit 30 via a pipeline 32. Ammonia 32 reacts with nitrogen oxides in the partially cooled flue gas to form nitrogen and steam. The reaction is incomplete, and trace amounts of NOx and NH3 remain in the flue gas leaving 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 the second heat recovery unit 34 and is recovered from the flue gas duct of the convection section 14 via a pipeline 36.

[0047] To prevent the formation of ammonium nitrate solids in the second heat recovery unit, one or both of the following measures are taken. A stream of hydrogen is added to the fuel gas 20 fed to the burner 22 via line 38. The hydrogen is combusted with air to produce additional steam in the flue gas leaving the radiant section 12 of the furnace 10. Alternatively or additionally, steam is added to the convection section 14 via line 40, preferably at or near the inlet of the convection section flue gas duct. The steam may be generated in part by the first heat recovery unit 26.

[0048] The present invention will be further described with reference to the following calculated examples, all of which are based on the use of Figure 1 The method for operating the ammonia cracking furnace described in . The fuel gas contains 23.6 volume % ammonia, 58.6 volume % nitrogen, 17.6 volume % hydrogen and 0.2 volume % water vapor. The ammonia feed gas contains 0.14 volume % water. The following reaction conditions are set:

[0049] Ammonia feed inlet temperature 550℃

[0050] Cracking gas outlet temperature 700℃

[0051] Ammonia content in cracked gas ≤ 1.2 mol%

[0052] Flue gas duct inlet temperature 780℃

[0053] SCR inlet temperature 370℃

[0054] SCR outlet temperature 380℃

[0055] The NOx level during combustion of the ammonia fuel gas is 1500 ppmv and drops to 50 ppmv when passing through the SCR.

[0056] Example 1: Normal operation

[0057] In normal operation, if there is residual NOx and low levels of slipped NH3, there is a risk of ammonium nitrate formation downstream of the SCR unit 28. Current emission limits require NO x The level is below 50 ppmv and the NH3 level is below 5 ppmv. These levels are used to determine the required steam partial pressure. The operating pressure of the flue gas is 0.8 bar to 1.1 bar. In addition, because NO x The degree of oxidation also has an impact on the steam demand, so oxidation ratios of 0.1, 0.5, and 0.9 (defined as ). Although NO2 does not constitute NO x However, in order to allow for the greatest possible safety margin in the calculations, an oxidation ratio of 0.9 is assumed. Therefore, assuming an outlet pressure of 0.8 bar at the pipeline and an oxidation ratio of 0.9, during normal operation, ammonium nitrate formation is prevented by having a steam content in the flue gas of 19.9 mol% or more.

[0058] Example 2: SCR Failure

[0059] In this case, the SCR failure did not reduce the 1500 ppmv NOx level produced in the radiant section. In the event of an SCR failure and continued ammonia feed into the pipeline through the SCR vessel, NO x The level will be maintained at 1500 ppmv and the NH3 level will be about 1000 ppmv. Then, to avoid the formation of solid ammonium nitrate, the required steam content in the flue gas will be higher, in the range of 32.4 mol% (oxidation ratio of 0.1) to 36.8 mol% (oxidation ratio of 0.9), assuming an outlet pressure of 0.8 bar from the duct.

[0060] Example 3: Ammonia Leak

[0061] In this case, the leakage of ammonia into the flue gas duct (normal operation of the SCR) was investigated.

[0062] In this case, the SCR operates as required to maintain NO x Levels are <50 ppmv. However, leaks within the duct coils introduce ammonia into the flue gas. The worst case scenario is that all the ammonia leaks into the duct, which would result in an NH3 level of 33 mol%. Then, to prevent solid ammonium nitrate formation, the steam content in the flue gas downstream of the ammonia leak would need to be at or above 36.8 mol% (assuming an outlet pressure of 0.8 bar absolute at the duct and an oxidation ratio in the range of 0.1 to 0.9).

[0063] Example 4: Ammonia leak and SCR failure

[0064] In this case, ammonia leakage into the flue gas duct (SCR inoperative) was investigated. In this operating case, the SCR is inoperative and NO x The level remains at 1500ppmv. However, leaks in the duct coils have introduced ammonia into the flue gas. The worst case scenario is that all the ammonia leaks into the duct, which will result in an NH3 level of 33 mol%. 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 ≤ 1 mol%, a steam content in the flue gas (before the ammonia leak) of 37.2 mol% will be sufficient to prevent the risk of solid ammonium nitrate formation. The steam content downstream of the ammonia leak should be equal to or higher than 36.8 mol%.

[0065] Assuming a minimum pressure of 0.8 bar absolute in the duct and using an oxidation ratio of 0.9, the following steam content in the flue gas is calculated to prevent the risk of solid ammonium nitrate formation:

[0066]

Claims

1. A method for cracking ammonia to form hydrogen, the method comprising the following steps: (i) passing ammonia through one or more tubes containing a catalyst in a furnace to crack the ammonia and form hydrogen, wherein the one or more tubes are heated by combustion of a fuel gas mixture to form a flue gas containing nitrogen oxides capable of reacting with ammonia in the flue gas to form ammonium nitrate, and (ii) cooling the flue gas to below 170° C., characterized in that the amount of steam in the flue gas is maintained according to the following formula to prevent the formation of solid ammonium nitrate: in, is the mole % of steam in the flue gas is the equilibrium vapor pressure of water in aqueous ammonium nitrate solution, and P is the minimum operating pressure of the flue gas.

2. The method 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. The method according to claim 1 or claim 2, wherein the ammonia cracking catalyst is a nickel catalyst or a ruthenium catalyst, preferably a nickel catalyst.

4. The process according to any one of claims 1 to 3, wherein the ammonia is fed into the catalyst-containing tubes at a temperature in the range of 400°C to 950°C.

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

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

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

8. The method according to any one of claims 2 to 7, wherein a selective catalytic reduction unit is installed in the convection section to reduce the nitrogen oxide content of the flue gas, preferably wherein the selective catalytic reduction unit is installed 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. The method according to claim 8, wherein the selective catalytic reduction section is located upstream of a second heat recovery unit, and the second heat recovery unit cools the flue gas to below 170°C after the flue gas passes through the selective catalytic reduction unit.

10. A method according to any one of claims 1 to 9, wherein hydrogen is added to the fuel gas mixture to maintain the amount of steam in the flue gas to prevent solid ammonium nitrate formation.

11. A method according to any one of claims 1 to 10, wherein the steam is added to the flue gas to prevent the formation of solid ammonium nitrate.

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

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