PROCESS FOR THE THERMAL DECOMPOSITION OF AMMONIA AND REACTOR FOR CARRYING OUT THE PROCESS

Abstract

The invention relates to a process for the thermal decomposition of ammonia. The process comprises passing ammonia through a conduit which contains an ammonia decomposition catalyst in a part thereof. At least a section of the part of the conduit which contains the catalyst is immersed in molten lead as heat transfer medium, which is at a temperature at which the catalyst is capable of catalyzing the decomposition of ammonia into hydrogen and nitrogen. A reactor for carrying out this process is also disclosed.

Claims

1-25. (canceled)

26. A process for the thermal decomposition of ammonia, wherein the process comprises passing ammonia through a conduit which contains an ammonia decomposition catalyst in a part thereof, at least a section of the part of the conduit which contains the catalyst being immersed in molten lead as heat transfer medium which is at a temperature at which the catalyst is capable of catalyzing the thermal decomposition of ammonia into hydrogen and nitrogen.

27. The process of claim 26, wherein the molten lead is present in a vessel whose outer wall is at least in part in direct contact with a hot gas whose temperature is higher than the temperature at which the catalyst is capable of catalyzing the decomposition of ammonia.

28. The process of claim 27, wherein the hot gas is or comprises a combustion gas generated by the combustion of a gas or gas mixture which is or comprises hydrogen and/or ammonia.

29. The process of claim 28, wherein the gas mixture comprises hydrogen and nitrogen.

30. The process of claim 29, wherein at least a part of the gas mixture is an exhaust gas of an anode part of a hydrogen fuel cell which had been supplied with a gas mixture generated by the thermal decomposition of ammonia in the conduit.

31. The process of claim 26, wherein the exhaust gas is or comprises ammonia.

32. The process of claim 27, wherein the hot gas is passed through a gap between at least a part of the outer wall of the molten lead containing vessel and an inner wall of a thermo-isolated external casing which completely surrounds at least a part of the molten-lead containing vessel.

33. The process of claim 26, wherein the conduit comprises a substantially U-shaped tube.

34. The process of claim 26, wherein a plurality of conduits is used.

35. The process of claim 26, wherein the conduit comprises a part which does not contain decomposition catalyst and through which ammonia to be decomposed is passed to heat it to a temperature which is suitable for contact with the decomposition catalyst.

36. The process of claim 35, wherein at least a portion of the part of the conduit for heating the ammonia is in direct contact with hot gas generated by a combustion of a gas or gas mixture which is or comprises hydrogen and/or ammonia and had previously been in contact with the outer wall of a vessel which contains the molten lead.

37. The process of claim 26, wherein the process further comprises passing the decomposition products in the conduit to the anode part of a hydrogen fuel cell.

38. The process of claim 26, wherein the ammonia decomposition catalyst comprises one or more of Ru, Ni, Rh, Co, Ir, Fe, Pt, Cr, Pd or Cu.

39. The process of claim 26, wherein the ammonia decomposition catalyst comprises one or both of Ru and Ni.

40. A reactor for the thermal composition of ammonia, wherein the reactor is suitable for carrying out the process of claim 26.

41. The reactor of claim 40, wherein the reactor comprises a device for generating a hot gas by combusting a hydrogen and/or ammonia containing gas or gas mixture, a vessel containing lead and at least one conduit containing the ammonia decomposition catalyst in a part thereof, at least a section of the catalyst containing part of the conduit being surrounded by the lead present in the vessel, and a thermo-isolated external casing which completely surrounds at least a part of the lead-containing vessel such that there is a gap between an outer wall of the vessel and an inner wall of the external casing through which gap the hot gas can pass.

42. The reactor of claim 41, wherein the at least one conduit comprises a substantially U-shaped tube.

43. The reactor of claim 41, wherein the at least one conduit comprises a part which does not comprise catalyst and is capable of being heated by hot gas which had previously been in contact with the outer wall of the lead containing vessel to thereby heat ammonia gas entering the conduit to a temperature suitable for contacting the decomposition catalyst.

44. A unit, wherein the unit comprises a hydrogen fuel cell and the reactor of claim 26 connected to each other.

45. A method of increasing the energy efficiency of a reactor for the thermal decomposition of ammonia in the presence of a decomposition catalyst, wherein the energy required for maintaining the decomposition reaction is supplied by a stream of hot gas, the energy being transferred from the hot gas to the ammonia and decomposition catalyst through a mass of molten lead as heat transfer medium which is heated by the hot gas and in turn heats the ammonia and decomposition catalyst to thereby increase the amount of energy contained in the hot gas which is used for heating the ammonia and the decomposition catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention is further described in the detailed description which follows, in reference to the accompanying drawings by way of non-limiting examples of exemplary embodiments of the present invention. In the drawings:

[0036] FIG. 1 schematically represents an ammonia decomposition reactor according to the present invention;

[0037] FIG. 2 is a schematic representation of the bottom part of an embodiment of the reactor according to the present invention;

[0038] FIG. 3 is a schematic representation of the top part of an embodiment of the reactor according to the present invention;

[0039] FIG. 4 shows an arrangement of (six) U-shaped tubes inside the lead containing vessel;

[0040] FIG. 5 shows a heating element for melting the lead in the lead containing vessel; and

[0041] FIG. 6 is a schematic top view of an embodiment of a decomposition reactor according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0042] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

[0043] FIG. 1 schematically represents an ammonia decomposition reactor according to the present invention. The reactor 1 comprises an outer thermo-isolated casing 2 and a vessel 3 inside the casing which contains lead 4 in which a substantially U-shaped tube 5 containing an ammonia decomposition catalyst 6 in a part thereof is immersed. Ammonia is introduced at one end of the tube 5 and decomposition products exit the tube at the other end thereof. The reactor 1 further comprises a burner 7 (e.g., in the form of a torch) at the bottom thereof for the combustion of a hydrogen and/or ammonia containing gas (combined with an oxygen containing gas such as air). The hot combustion gas passes through the gap between the outer casing 2 and the vessel 3 and thereby maintains the molten lead 4 inside the vessel 3 at a temperature which is sufficient for maintaining the catalyzed decomposition reaction of the ammonia inside the tube 5. After having been in direct contact with the vessel 3 the hot gas comes into direct contact with that part of the tube 5 which does not contain catalyst in order to preheat the fresh ammonia introduced into the tube 5 at one end thereof, preferably to or close to a temperature which is suitable for contacting the catalyst 6 (i.e., the decomposition initiation temperature, which depends at least in part on the catalyst). Thereafter the combustion gas exits the reactor 1 through the gas outlet 9. The residual heat in this gas may optionally be used for other purposes, e.g., for evaporating liquid ammonia to be decomposed.

[0044] FIG. 2 is a schematic representation of the bottom part of an embodiment of the reactor according to the present invention. It shows, in addition to the elements discussed with respect to FIG. 1, an inlet 8 for the gas mixture that is to be passed to the burner 7. FIG. 2 further shows a vessel 3 which contains a total of six substantially U-shaped tubes 5, the arrangement of which inside the vessel 3 being shown in more detail in FIG. 4. FIG. 2 also shows a (preferably electric) heating element 10 inside the vessel 3, shown in more detail in FIG. 5. The heating element 10 can be used at the start of the process (at which the lead is usually at about room temperature and thus, a solid) to melt the lead (the melting point of lead is 327° C.). A suitable temperature of the heating element is, for example, about 500° C.

[0045] FIG. 3 is a schematic representation of the top part of an embodiment of the reactor according to the present invention. It shows the U-shaped tubes 5, the outer casing 2 and the outlet 9 for the hot combustion gas after heat transfer to the lead, catalyst and ammonia to be decomposed.

[0046] FIG. 6 is a schematic top view of an embodiment of the decomposition reactor according to the present invention. It shows the outer casing 2, the lead containing vessel 3, the inlets and outlets of the six tubes 5, the top of the heating element 10, the inlet 8 for the gases used for combustion and the outlet 9 for the hot combustion gas after heat transfer.

[0047] In the following an exemplary embodiment of a system which comprises the reactor-heat exchanger according to the present invention will be described in more detail. This embodiment comprises the following elements: [0048] 1. Reactor-heat exchanger. [0049] 2. Fan for supplying air into the burner (torch) of the reactor-heat exchanger. [0050] 3. External torch with a faucet and consumption measurement device for combustion Of hydrogen and/or ammonia containing gas; [0051] 4. Ammonia tank in water bath. [0052] 5. Electrical water heating element for bath of ammonia tank. [0053] 6. Consumption controller of gaseous ammonia. [0054] 7. Device for measuring hydrogen content in decomposition products (e.g., katharometer). [0055] 8. Computer with software for control and data acquisition.

[0056] The first step of launching the reactor-heat exchanger is turning on an electrical heating element for melting lead by setting the temperature of that element, for example to about 500° C. Heating control is carried out according to readings of a pair of thermocouples which sense the temperature at the bottom and at the top of the lead containing vessel. The temperature of the top thermocouple is higher than that of the bottom thermocouple during heating, which causes lead to melt from the top to the bottom, thereby preventing temperature tensions.

[0057] When proceeding towards reactor heating by means of burning hydrogen and/or ammonia containing gas the following sequence is followed: a minimal consumption (e.g., 14 volts) air supply fan is turned on, an ammonia tank is opened, the supply to the decomposition reactor is turned on with consumption of 0.5 nm.sup.3/hour and ignition of the internal burner is carried out by a gas torch through a special opening in the burning chamber. After the ignition, the opening is closed and further heating of the reactor is carried out according to readings of the thermocouples and a sensor of the hydrogen concentration in the decomposition products.

[0058] In order to accelerate heating, it is possible to gradually increase the supply of ammonia up to a consumption rate of 1.5-2 nm.sup.3/hour. Increasing consumption by 0.5 nm.sup.3/hour is possible when the hydrogen concentration in the gas leaving the reactor is higher than 30%. The heating process may be considered to be finished when readings of the thermocouple at the bottom of the lead containing vessel reaches 600° C.

Testing of Reactor

[0059] The reactor was designed as an autonomous power source with a capacity of five kilowatts. Properties and advantages thereof were as follows:

[0060] 1. A rapid decrease in the temperature of combustion products from an adiabatic combustion temperature (~ 1400° C.) to approximately 650° C., which is determined by the kinetic properties of the catalyst. As a result, almost all structural elements operate at temperatures below 650° C.

[0061] 2. When using lower temperature catalysts, the operating temperature can still be reduced.

[0062] 3. Liquid lead provides intensive heat exchange with the surfaces of tubes filled with a catalyst, enabling an almost isothermal mode of operation of a tubular reactor and a degree of decomposition of ammonia close to equilibrium.

[0063] 4. The relatively low operating temperature of the structure contributes to the extension of its life and reduces heat loss to the environment.

[0064] In order to assess the thermal efficiency of the reactor tests were carried out at different ammonia consumption rates. Since the decomposition of ammonia is an endothermic reaction energy is required to maintain the working (decomposition) temperature. In addition, heat loss through thermal insulation is inevitable. At a fixed ammonia flow rate a part of the decomposition products was used as fuel in the combustion chamber of the reactor. In the experiments the minimum consumption of decomposition products which must be directed to the burner to maintain a stationary temperature was determined. To determine the minimum flow rate the following method was used. The decomposition products exiting the reactor were cooled to a temperature of 50° C. and were divided into two streams. One stream was sent to the burner of the reactor combustion chamber while the other stream was disposed of. The gas flow into the combustion chamber was measured using a flowmeter. This flow rate was reduced to the minimum value which still provided a high degree of ammonia decomposition. The test results are shown in the table below, in which in addition to the fraction of decomposition products used for combustion, four temperatures are set forth: T.sub.Pb_bott and T.sub.Pb_up = temperatures of the lead in the lower and upper parts of the reactor, T.sub.comb = temperature of the combustion products, T.sub.H2_N2_out = temperature of the decomposition products, and C.sub.H2 = volumetric concentration of hydrogen in the decomposition products.

[0065] The first line in the table refers to the “idle” mode, in which all decomposition products were combusted to heat the reactor. As the consumption of ammonia increased, the proportion of decomposition products used as fuel decreased. At a maximum productivity of 5 nm.sup.3/h of ammonia the decomposition costs and the heat losses amounted to 33% of the flow rate.

TABLE-US-00001 Consumption of NH.sub.3, nm.sup.3/h Fraction of decomposition products used for combustion, % C.sub.H2, % T.sub.Pb.sub._bott, °C T.sub.Pb_up, °C T.sub.comb, °C T.sub.H2_N2_out, °C 0.25 100 74.7 616 606 332 104 0.5 72 74.72 629 620 336 124 1 43 74.8 630 618 313 162 2 38 74.8 645 630 307 217 3 35 74.75 650 640 340 272 4 34 74.6 660 647 382 320 5 33 74.6 670 655 396 358