Methods for decomposing 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 more particularly 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 produce hydrogen. This reaction has long been used in ammonia plants to supply hydrogen and activate catalysts, but it is gaining increasing interest as a means of supplying hydrogen for power generation or other applications. The reaction can be described as follows:

[0003]

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

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

[0006] If ammonia leaks from the preheating coil, or if ammonia undergoes incomplete combustion while being used as a fuel component or present for selective catalytic reduction, NH3 and NO will be released into the flue gas. xAs a result of the presence of H2O, ammonium nitrate or ammonium nitrite may be formed, both of which, especially ammonium nitrite, pose an explosive risk. Ammonium nitrite is particularly dangerous because it is highly unstable and explosive. NO is the gas mainly formed during the combustion of the hydrogen / ammonia blend, but NO2 is also present, and the further formation of NO2 is driven by the oxidation of NO, which is advantageous because it lowers the temperature in the convection part of the furnace. The reaction that forms ammonium nitrate can be expressed as follows:

[0007]

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[0008] Selective catalytic reduction (SCR) can be used in the convection section of a furnace, also known as the flue gas duct, to decompose NOx in flue gas. However, the ammonia decomposition process needs to be controlled to further minimize the risk of ammonium nitrite and ammonium nitrate formation. Protection from ammonium nitrite formation is achieved by eliminating the conditions under which ammonium nitrate is formed in this system. The inventors have shown that this can be achieved by controlling the partial pressure of water vapor in the flue gas. [Overview of the project]

[0009] Accordingly, the present invention relates to a process for decomposing ammonia to form hydrogen, comprising: (i) passing ammonia through one or more catalyst-containing tubes in a furnace to decompose ammonia to form hydrogen, wherein one or more tubes are heated by the 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 below 170°C, wherein to prevent the formation of solid ammonium nitrate, the following formula is used:

[0010]

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[0011]

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[0012]

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[0013] Therefore, in the process according to the present invention, the amount of water vapor must be greater than the equilibrium vapor pressure of water in the aqueous solution of ammonium nitrate, and as a result, the ammonium nitrate dissolves rather than precipitates as a solid inside the furnace, for example, 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 to the furnace via water vapor injection. To calculate the minimum water vapor partial pressure required 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 an NH4NO3 solution within a certain temperature range has been measured and published. For example, the vapor pressure at 10 - 40°C can be found in the chapter of Ammonium Compounds by Weston, C., Papcun, J, Dery, M. in Kirk - Othmer Encylopedia of Chemical Technology, Vol 2, 2003. The crystallization curve is provided in a paper titled "Correlating vapor pressures and heats of solution for the ammonium nitrate - water system: An enthalpy - concentration diagram" by Othmer et al., which was published in AIChE Journal, Vol6, Issue 2, 1960, Pp 210 - 214. The complete dataset was published in a paper titled "NH4NO2 Formation in Cooler Condenser" by Voorwinden, M., which was presented at the ANNA Society in St. Louis, USA in October 2004.

[0016] The range targeted by the present invention is up to 170°C, beyond which solid ammonium nitrate is not formed. The following vapor pressures were used in the present 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 achieved as follows. From the above equations 2 and 3, the equilibrium constant can be defined as follows.

[0019]

Equation

[0020] The equilibrium constant K2 can be derived from measurements in the paper "The Entropies of Nitric Acid and its Mono- and Tri-hydrates" by Forsythe et al., published in J.Am.Chem.Soc.Vol.64.1942.Pp48-61, and the following equation (temperature measured in Kelvin) is given.

[0021]

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[0022] Next, K2 can 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.

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

[0024]

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

[0026] The temperature of the flue gas in which solid ammonium nitrate can form is 170°C or less, 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 radiating section, to which fuel gas and air are supplied, and where combustion using one or more burners generates radiant heat for heating one or more reaction tubes containing an ammonia decomposition catalyst. Dozens or hundreds of tubes may be present in the radiating section. The catalyst may be any ammonia decomposition catalyst. Nickel catalysts and ruthenium catalysts may be used. A preferred catalyst is a nickel catalyst. The catalyst may contain 3 to 30% by weight of nickel, preferably 8 to 20% by weight of nickel (expressed as NiO) on a suitable refractory carrier, such as alumina or a metallic aluminate. The catalyst may be in the form of pelletized units, 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 contains 12% nickel (expressed as NiO) on cylindrical pellets formed from a high-surface-area alumina carrier. The temperature of the ammonia feed at the tube inlet may be in the range of 400 to 950°C. The temperature of the decomposed gas exiting the tube affects the equilibrium position and may be in the range of 500 to 950°C. When a nickel catalyst is used, the exit temperature is preferably 700°C or higher. The tube inlet pressure is set by the flow sheet design and may be in the range of 1 to 100 absolute bar (bar abs), preferably 10 to 90 absolute bar. For example, when a pressure fluctuation absorption unit is used as the hydrogen separation step, the inlet pressure may typically be in the range of 31 to 51 absolute bar (bar abs).

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

[0029] Flue gas is generated by the combustion of fuel gas, and the flue gas is transported from the radiating section of the furnace to the convection section or flue gas duct, where it is cooled by indirect heat exchange. One or more heat exchange stages may be provided within the convection section or flue gas duct.

[0030] Furthermore, it is desirable to include a selective catalytic reduction (SCR) unit in the flue gas duct. SCR units are well known and generally contain a honeycomb or plate-supported catalyst with minimal 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 one of the following: base metals (such as vanadium, molybdenum, and tungsten), zeolite, or various noble metals such as Pt and / or Pd. Base metal catalysts such as vanadium and tungsten lack high thermal endurance but are less expensive. Zeolite catalysts have the potential to act at substantially higher temperatures than base metal catalysts. Iron and copper exchange zeolite urea SCR can be used. The amount of platinum group metals 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 using anhydrous ammonia according to the following formula: 2NO + 2NH3 + 1 / 2O2 → 2N2 + 3H2O (8) NO2+2NH3+1 / 2O2→3 / 2N2+3H2O (9) NO + NO2 + 2NH3 → 2N2 + 3H2O (10)

[0031] SCR catalysts can operate at temperatures ranging from 225 to 450°C.

[0032] NO levels of up to 1500 ppmv are present during combustion in the radiating region. x Oxygen may be present, but decreases to less than 50 ppmv when passing through the SCR unit. Due to the possibility of excess air and air intrusion, oxygen is present downstream of combustion.

[0033] The SCR unit is preferably located downstream of the first heat recovery unit. If desired, an additional heat recovery unit may be provided downstream of the SCR unit to cool the flue gas to below 30°C. At this point, the flue gas is particularly susceptible to the formation of solid ammonium nitrate when its temperature drops below 170°C. Therefore, controlling the water vapor content may include adding water vapor to the furnace convection section upstream or downstream of the SCR unit.

[0034] Calculations indicate that the minimum amount of water vapor in the flue gas to inhibit the formation of solid ammonium nitrate under normal operating conditions is preferably about 19.9 mol% or more. Possible SCR unit malfunction or ammonia leakage may increase the minimum amount of vapor that is desirable to be present, which may be up to or above about 54.9 mol%, depending on the circumstances. [Brief explanation of the drawing]

[0035] Next, the present invention will be further described with reference to the figures. [Figure 1] This is a diagram of an ammonia decomposition furnace useful in the process of the present invention. [Modes for carrying out the invention]

[0036] Those skilled in the art will understand that the drawings are schematic and that commercial plants may require additional equipment such as raw material drums, pumps, vacuum pumps, compressors, gas recirculation compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, and storage tanks. The provision of such ancillary equipment does not form part of the present invention and follows conventional chemical engineering practices.

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

[0038] NO xThe combustion gas containing the 32 flows from the radiating section 12 into the flue gas duct of the convection section 14, where it is cooled in the first heat recovery unit 26, which includes steam generation, to produce partially cooled flue gas. The partially cooled flue gas then passes through the flue gas duct to the downstream selective catalytic reduction (SCR) unit 26, which contains an SCR catalyst supplied from the reducing agent storage unit 30 along with ammonia 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, and trace amounts of NOx and NH3 remain in the flue gas exiting the SCR unit 28. The flue gas exiting the SCR unit 28 is further cooled to below 170°C in the flue gas duct of the second heat recovery unit 34 and recovered 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 steam in the flue gas exiting the radiating section 12 of the furnace 10. Alternatively, or in addition to this, steam 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 steam may be partially generated by the first heat recovery unit 26.

[0040] The present invention will be further illustrated by the following calculated examples, all of which were based on processes operating using the ammonia decomposition furnace shown in Figure 1. The fuel gas contained 23.6 vol% ammonia, 58.6 vol% nitrogen, 17.6 vol% hydrogen, and 0.2 vol% water vapor. The ammonia feed gas contained 0.14 vol% water. The following reaction conditions were set: Ammonia supply inlet temperature: 550°C Decomposition gas outlet temperature: 700℃ Ammonia content in decomposition gas ≤ 1.2 mol% Flue gas duct inlet temperature: 780℃ SCR inlet temperature 370℃ SCR outlet temperature is 380 °C The NOx level during the combustion of ammonia fuel gas is 1500 ppmv and decreases to 50 ppmv when passing through SCR.

[0041] Example 1: Normal operation In normal operation, when there is residual NO x and low levels of slipped NH3 present, there is a risk of ammonium nitrate formation downstream of the SCR unit 28. Current emission limits require NO levels of less than 50 ppmv x and NH3 levels of less than 5 ppmv. Using these levels, the required water vapor partial pressure was determined. The operating pressure of the flue gas was 0.8 - 1.1 bara. In addition, since the oxidation degree of NO x can also affect the water vapor requirements, oxidation ratios of 0.1, 0.5, and 0.9 (

[0042] [Number] as defined) were investigated. Although NO2 does not form the majority of NO x for the sake of giving the largest safety margin within the calculation, the oxidation ratio was assumed to be 0.9. Therefore, assuming an outlet pressure from the duct of 0.8 bar and an oxidation ratio of 0.9, ammonium nitrate formation during normal operation was prevented by setting the vapor content in the flue gas to 19.9 mol% or more.

[0043] Example 2: SCR malfunction In this scenario, due to a malfunction of the SCR, the NOx level did not decrease from 1500 ppmv generated in the radiation section. When the SCR malfunctions and ammonia continues to be supplied into the duct through the SCR vessel, NO xThe level remains at 1500 ppmv, and the NH3 level will be approximately 1000 ppmv. Next, to avoid the formation of solid ammonium nitrate, the required vapor content in the flue gas will be higher, assuming an outlet pressure of 0.8 absolute bar from the duct, ranging from 32.4 mol% (oxidation ratio of 0.1) to 36.8 mol% (oxidation ratio of 0.9).

[0044] Example 3: Ammonia Leakage In this scenario, we investigated ammonia leakage into a flue gas duct (where the SCR is functioning).

[0045] In this case, SCR is less than 50 ppmv of NO. x It was functioning as needed to maintain the level. However, ammonia was introduced into the flue gas due to leakage within the duct coil. In the worst-case scenario where all the ammonia leaks into the duct, the NH3 level would reach 33 mol%. Then, to prevent the formation of solid ammonium nitrate, the vapor content in the flue gas downstream of the ammonia leak needs to be 36.8 mol% or higher (assuming an outlet pressure of 0.8 absolute bar from the duct and an oxidation ratio in the range of 0.1 to 0.9).

[0046] Example 4: Ammonia leakage and SCR malfunction In this scenario, we investigated ammonia leakage into a flue gas duct (where the SCR is not functioning). In this case, the SCR is not functioning, NO xThe level remains at 1500 ppmv. However, ammonia has been introduced into the flue gas due to leakage within the duct coil. In the worst-case scenario, where all the ammonia leaks into the duct, the NH3 level would reach 33 mol%. The vapor 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 less than 1 mol%, a water vapor content of 37.2 mol% in the flue gas (before the ammonia leak) should be sufficient to prevent the risk of solid ammonium nitrate formation. The water vapor content downstream of the ammonia leak must be 36.8 mol% or higher.

[0047] Assuming a minimum pressure of 0.8 absolute bar in the duct and using an oxidation ratio of 0.9, the following vapor content in the flue gas was calculated to prevent the risk of solid ammonium nitrate formation.

[0048] [Table 2] This disclosure includes the following aspects. [Aspect 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 ammonia to form hydrogen, wherein the one or more tubes are heated by the 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 below 170°C, wherein to prevent the formation of solid ammonium nitrate, the following formula is used:

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Claims

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 ammonia to form hydrogen, wherein the one or more tubes are heated by the 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, wherein to prevent the formation of solid ammonium nitrate, the following formula is used: [Math 1] (In the formula, 【Number 2】 This is the mole percent of water vapor in the flue gas, [Math 3] This is the equilibrium vapor pressure of water in an aqueous solution of ammonium nitrate, A process characterized by maintaining the amount of water vapor in the flue gas according to the minimum operating pressure of the flue gas (where P is the minimum operating pressure of the flue gas).

2. The process according to claim 1, wherein the furnace comprises a radiating section in which the catalyst-containing tube is heated, and a convection section downstream of the radiating section in which the flue gas is cooled to less than 170°C.

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

4. The process according to claim 1, wherein the ammonia is supplied to the catalyst-containing tube at a temperature in the range of 400 to 950°C.

5. The process according to claim 1, wherein the ammonia is supplied to the catalyst-containing tube at an absolute pressure in the range of 1 to 100 bar.

6. The process according to claim 1, wherein the fuel gas mixture contains 1 to 100% by volume of ammonia.

7. The process according to claim 1, wherein the fuel gas comprises nitrogen, hydrogen, and 1 to 50 volume percent of ammonia.

8. The process according to claim 2, wherein a selective contact reduction unit is installed in the convection section to reduce the nitrogen oxide content of the flue gas.

9. The process according to claim 2, wherein a selective catalytic reduction unit is installed in the convection section downstream of the first heat recovery unit, and the first heat recovery unit cools the flue gas to the inlet temperature of the selective catalytic reduction unit.

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

11. The process according to any one of claims 1 to 7, wherein hydrogen is added to the fuel gas mixture to maintain the amount of water vapor in the flue gas and prevent the formation of solid ammonium nitrate.

12. The process according to any one of claims 1 to 7, wherein the water vapor is added to the flue gas to prevent the formation of solid ammonium nitrate.

13. The process according to claim 12, wherein the water vapor is added to the flue gas upstream of the selective catalytic reduction unit.

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