PROCESS FOR CRACKING AMMONIA
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 H2O is the mol % of steam in the flue gas, P* 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.
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
>
p
H
2
O
*
P
where,
y H 2 O is the mol % of steam in the flue gas,
p* H 2 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.