PROCESS FOR CRACKING AMMONIA

Abstract

A process for cracking ammonia to form hydrogen is described comprising the steps of (i) passing 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 capable of reacting with ammonia in the flue gas to form ammonium nitrate, and (ii) cooling the flue gas to below 170 C., characterised by maintaining an amount of steam in the flue gas according to the following equation to prevent solid ammonium nitrate formation: (I) where, y.sub.H2O is the mol % of steam in the flue gas, P*.sub.H2O is the equilibrium vapor pressure of water in an aqueous solution of ammonium nitrate, and p is the minimum operating pressure of the flue gas.

Claims

1. A process for cracking ammonia to form hydrogen comprising the steps of (i) passing 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 capable of reacting with ammonia in the flue gas to form ammonium nitrate, and (ii) cooling the flue gas to below 170 C., wherein maintaining an amount of steam in the flue gas according to the following equation to prevent solid ammonium nitrate formation: $y_{H_{2}O}>\frac{p_{H_{2}O}^{*}}{P}$ where, y.sub.H.sub.2.sub.O is the mol % of steam in the flue gas, p*.sub.H.sub.2.sub.O is the equilibrium vapor pressure of water in an aqueous solution of ammonium nitrate, and P is the minimum operating pressure of the flue gas.

2. The process according to 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 gas is cooled to below 170 C.

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

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

5. The process according to claim 1, wherein the ammonia is fed to the catalyst containing tubes at a pressure in the range of 1 to 100 bar abs, preferably 10 to 90 bar abs.

6. The process according to claim 1, wherein the fuel gas mixture contains 1 to 100% or 1 to 50% by volume of ammonia.

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

8. The process according to claim 2, wherein a selective catalytic reduction unit is installed in the convection section to reduce the nitrogen oxides content of the flue gas, preferably wherein a selective catalytic reduction unit is installed in the convection section downstream of a first heat recovery unit that cools the flue gas to an inlet temperature for the selective catalytic reduction unit.

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

10. The process according to claim 1, 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. The process according to claim 1, wherein the steam is added to the flue gas to prevent solid ammonium nitrate formation.

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

Description

[0030] The invention will now be further illustrated by reference to the FIGURE in which;

[0031] FIG. 1 is a depiction of an ammonia cracking furnace useful in the process of the present invention.

[0032] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0033] In FIG. 1, an ammonia cracking furnace 10 is depicted comprising a radiant section 12 and a convection section 14 comprising a flue gas duct. Ammonia is fed via line 16 to a plurality of nickel catalyst-containing reaction tubes 18 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 air supply lines (not shown). The fuel gas comprises ammonia and the combustion gases therefore contain nitrogen oxides, NO.sub.x. Ammonia is cracked in the reaction tubes 18 to form a gas mixture containing nitrogen and hydrogen and unreacted ammonia, which is collected from the tubes 18 via line 24 for further processing to recover the hydrogen.

[0034] The combustion gases containing NO.sub.x flow from the radiant section 12 to the convection section 14 flue gas duct and are cooled there in a first heat recovery unit 26, which includes steam generation, to generate a partially cooled flue gas. The partially cooled flue gas then passes within the flue gas duct to a downstream selective catalytic reduction (SCR) unit 26 containing a SCR catalyst that is fed 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 steam. The reaction is incomplete and trace amounts of NOx and NH.sub.3 remain in the flue gas leaving the SCR unit 28. The flue gas leaving the SCR unit 28 is further cooled within the flue gas duct in a second heat recovery unit 34 to below 170 C. and recovered from the flue gas duct of the convection section 14 via line 36.

[0035] In order to prevent ammonium nitrate solids formation in the second heat recovery unit one or both of the following measures are adopted. A hydrogen stream is added via line 38 to the fuel gas 20 fed to the burners 22. Combustion of the hydrogen with air thereby generates additional steam in the flue gas leaving the radiant section 12 of the furnace 10. Alternatively or on addition, steam addition is made 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 in part generated by the first heat recovery unit 26.

[0036] The invention will be further described by reference to the following calculated examples all of which were based on a process operated using the ammonia cracking furnace depicted in FIG. 1. The fuel gas contained 23.6% volume ammonia, 58.6% vol nitrogen, 17.6% vol hydrogen and 0.2% vol water vapour. The ammonia feed gas contained 0.14% vol water. The following reaction conditions were set: [0037] Ammonia feed inlet temperature 550 C. [0038] Cracked Gas Outlet temperature 700 C. [0039] Ammonia content in the cracked gas 1.2 mole % [0040] Flue gas duct inlet temperature 780 C. [0041] SCR inlet temperature 370 C. [0042] SCR outlet temperature 380 C. [0043] NOx levels of 1500 ppmv during combustion of the ammonia fuel gas, reducing to 50 ppmv when passing through the SCR.

Example 1: Normal Operation

[0044] In normal operation, the risk of ammonium nitrate formation downstream of the SCR unit 28 would occur where residual NO.sub.x and a low level of slipped NH.sub.3 were present. Current emission limits require NO.sub.x levels below 50 ppmv and NH.sub.3 levels below 5 ppmv. These levels were used to determine the steam partial pressures required. The operating pressure of the flue gas was 0.8 to 1.1 bara. In addition, because the degree of oxidation of NO.sub.x also may impact on the steam requirements, an oxidation ratio

[00005]$\left(\mathrm{defined}\mathrm{as}\frac{p_{{\mathrm{NO}}_2}}{P_{{\mathrm{NO}}_x}}\right)$

was investigated at 0.1, 0.5 and 0.9. While NO.sub.2 does not form a majority of the NO.sub.x, to give the largest safety margin within the calculation, an oxidation ratio of 0.9 was assumed. Accordingly, assuming an exit pressure from the duct of 0.8 bar, and an oxidation ratio of 0.9, ammonium nitrate formation was prevented during normal operation by having a steam content in the flue gas of 19.9 mol %, or higher.

Example 2: SCR Malfunction

[0045] In this scenario a SCR malfunction has not reduced the NO.sub.x level from the 1500 ppmv produced in the radiant section. In the case of the SCR malfunctioning, and ammonia continuing to be fed into the duct through the SCR vessel, NO.sub.x levels would remain at 1500 ppmv and NH.sub.3 levels would be around 1000 ppmv. Then, to avoid solid ammonium nitrate formation, the required steam content in the flue gas would be higher, ranging from 32.4 mol % (with an oxidation ratio of 0.1) to 36.8 mol % (with an oxidation ratio of 0.9), assuming an exit pressure from the duct of 0.8 bar a.

Example 3: Ammonia Leak

[0046] In this scenario an ammonia leak into the flue gas duct (with SCR functioning) was investigated. In this case, the SCR was functioning as required to maintain NO.sub.x levels <50 ppmv. However, a leak within the duct coils introduced ammonia into the flue gas. The worst case, in which all ammonia leaks into the duct, would result in NH.sub.3 levels reaching 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 exit pressure from the duct of 0.8 bar abs. with an oxidation ratio in the range of 0.1 to 0.9).

Example 4: Ammonia Leak and SCR Malfunction

[0047] In this scenario an ammonia leak into the flue gas duct (with SCR not functioning) was investigated. In this operating case, the SCR in non-functioning and NO.sub.x levels remain at 1500 ppmv. However, a leak within the duct coils has introduced ammonia into the flue gas. The worst case, in which all ammonia leaks into the duct, would result in NH.sub.3 levels reaching 33 mol %. The steam content required to prevent solid ammonium nitrate formation varies depending on the extent of the leak. For small leaks resulting in ammonia levels 1 mol %, having a steam content in the flue gas (prior to ammonia leak) of 37.2 mol % would be sufficient to prevent the risk of solid ammonium nitrate formation. The steam content, downstream of the ammonia leak, should be at or above 36.8 mol %.

[0048] Assuming the lowest pressure in the duct is 0.8 bar absolute and using an oxidation ratio of 0.9, the following steam content within the flue gas to prevent the risk of solid ammonium nitrate formation was calculated:

TABLE-US-00002 NO.sub.x Steam Operating NH.sub.3 content content Regime content [ppmv] [mol %] Normal 5 ppmv 50 19.9% operation (with SCR) SCR 1000 ppmv 1500 36.8% malfunction NH.sub.3 leak 1 mol % 50 36.8% [SCR operating] (37.2% upstream of leak) 5 mol % 50 36.8% (38.7% upstream of leak) 10 mol % 50 36.8% (40.9% upstream of leak) 33 mol % 50 36.8% (54.9% upstream of leak) NH.sub.3 leak 1 mol % 1500 36.8% [SCR malfunctioning] (37.2% upstream of leak) 5 mol % 1500 36.8% (38.7% upstream of leak) 10 mol % 1500 36.8% (40.9% upstream of leak) 33 mol % 1500 36.8% (54.9% upstream of leak)