WO2013119281A1

WO2013119281A1 - Ammonia flame cracker system, method and apparatus

Info

Publication number: WO2013119281A1
Application number: PCT/US2012/062570
Authority: WO
Inventors: Shawn Grannell; Donald E. Gillespie
Original assignee: Shawn Grannell; Gillespie Donald E
Priority date: 2012-02-10
Filing date: 2012-10-30
Publication date: 2013-08-15

Abstract

Apparatus, systems, and methods reside in the decomposition of ammonia into a hydrogen-containing gas mixture. Ammonia and air enter a heat exchanger in a substantially ammonia-rich proportion. A portion of the ammonia is combusted, and the remainder is decomposed, thereby forming the hydrogen-containing gas mixture. The incoming ammonia and air are heat exchanged with the outgoing hydrogen- containing gas mixture. The hydrogen may then be directed at further uses.

Description

AMMONIA FLAME CRACKER SYSTEM,

METHOD AND APPARATUS

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Patent Application Serial No. 13/537,125, filed June 29, 2012, and 13/370, 910 filed February 10, 2012, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present disclosure relates to decomposing (or "cracking") ammonia into a hydrogen-containing gas mixture, and more particularly, to a heat exchanged ammonia flame cracker which converts ammonia into a hydrogen-containing gas mixture at a gas temperature that is high enough for rapid decomposition of ammonia without the aid of a catalyst.

BACKGROUND OF THE INVENTION

[00031 Due at least in part to high petroleum prices, environmental concerns, and concerns about future fuel availability, many have sought to replace petroleum-based fuels. Ammonia is one such replacement fuel, storable indefinitely as a liquid at a pressure of about 10 bars at 300 Kelvin. Ammonia can be made from a wide variety of sources of energy, and is thus said to be "source-neutral". Systems that make ammonia from nuclear energy have been disclosed. The sustained manufacture of very large quantities of ammonia, from nuclear energy, does not appear to be a scientific issue. If ammonia is converted into hydrogen, then that hydrogen can be used in any of the applications that currently use any fuel, or that currently use hydrogen for any purpose. Hence the motivation to invent the ammonia flame cracker.

[0004] An ammonia flame cracker converts ammonia into a mixture of hydrogen and substantially inert gas. The hydrogen can be used as the combustion promoter for an engine that is fueled by mostly ammonia, according to U.S. Patent No. 7,574,993, the entire content of which is incorporated herein by reference. The hydrogen can be used for fueling engines, in applications for which battery/electric was previously the only option, such as forklifts which can be safely operated indoors. The hydrogen can be used as a lift gas for a balloon. The hydrogen can be supplied to a hydrogen-SCR, which uses the hydrogen as the reductant for removing NOx from exhaust gas.

[0005] An ammonia flame cracker is especially useful for supplying hydrogen to processes that specifically require hydrogen. In some instances, the cost of transporting hydrogen and other usability concerns, such as hydrogen's well known and long enduring problems concerning tank size and pressure, are sufficient to warrant a preference for purchasing ammonia, and then converting the ammonia into hydrogen, using an ammonia flame cracker, rather than purchasing the hydrogen as hydrogen.

[0006] Some ammonia crackers that are known to the art have difficulties and limitations because of large size and intricate design required for heat transfer, large quantities of sometimes expensive catalyst required to obtain a substantial ammonia decomposition yield, an uncontrolled and often low ammonia decomposition yield, and lack of rapid start capability. U.S. Patent Application Publication No. 2003/0232224 discloses that about 1 -2 kg of catalyst is needed to catalytically decompose ammonia at a thermal rate of 3 kilowatts, or about 0,06 moles of ammonia per second,

[0007] Ammonia crackers that use engine exhaust heat to decompose ammonia, such as the ammonia crackers disclosed in U.S. Patent Nos, 2, 140,254; 4,478, 1 77; and 4,750,453, are large, expensive, and intricate devices which must be placed in the engine exhaust flow. Furthermore, an engine's exhaust gas temperature is not high enough to decompose any of the ammonia without using an ammonia cracker catalyst. Such cracker catalysts are relatively large and expensive when sized for providing enough catalytic sites for catalytically decomposing ammonia at a high rate or at a high ammonia decomposition yield. In some instances, an engine's exhaust gas temperature is not high enough to give acceptable ammonia cracker performance even if a catalyst is used.

[0008] U.S. Patent No. 3,598,538 discloses a heat exchanged, electrically powered ammonia cracker, which can be operated at temperatures approaching 3000°F ( 1649°C), but more typical ly heats the ammonia to 1 700°F (927°C). U.S. Patent No. 2,578, 193 discloses a heat exchanged, electrically powered, catalytic ammonia cracker, which operates at 1200°F (649°C). The ammonia flame crackers disclosed herein obtain energy for decomposing ammonia principally from the combustion of some of the ammonia and not from electricity.

[0009] Ammonia burners, disclosed in U.S. Patent Nos. 5,904,910 and 6,488,905, can decompose ammonia non-catalytically by combustion of some of the ammonia with either pure oxygen, or some combination of oxygen and air. However, neither patent discloses a provision for heat exchanging the burner inputs and outputs, immediately before and after combustion and decomposition of ammonia in the burner. The Applicants have discovered that this preheating of the reactants, via heat exchange with the products, causes non-catalyzed ignition of rich ammonia/air mixtures that would not otherwise be flammable. This exchange of heat is also required for the efficient recovery of hydrogen from ammonia. Ammonia can be fully combusted and decomposed with air at equivalence ratios greater than 3, if the products and reactants are heat exchanged. The inclusion of a heat exchanger thus confers a significant efficiency improvement to the non-catalytic ammonia burner.

[0010] U.S. Patent No. 2,013,809 discloses a catalytic ammonia cracker, which decomposes ammonia at an unspecified temperature. U.S. Patent Nos. 2,601 ,221 and 2,606,875 describe catalytic ammonia combustion and/or decomposition at temperatures of 500°C or higher, but these patents do not further disclose the rapid and substantially non-catalyzed decomposition of ammonia, which occurs at temperatures higher than about 1400°C. U.S. Patent Nos. 1 ,915, 120; 2,013,652; 2, 161 ,746; 2,264,693; 2,578, 193; 3,025, 145; 3,379,507; 3,505,027; 3,804,596; 4,069,071 ; 4, 1 57,270; 4, 179,407; 4,219,528; 4,755,282; 4,788,004; 5,055,282; 5, 139,756; 5,976,723; 6,007,699; 6,299,847; 6,800,386; 6,936,363; and 7,875,089, and U.S. Patent Application Publication Nos. 2002/00281 71 , 2003/0103878, 2003/0232224, 2004/0154223, 2004/0163313, 2005/0037244, and 200601 12636 disclose ammonia crackers which operate at peak temperatures of 1200°C or lower, and thus do not disclose or anticipate the rapid and non-catalyzed decomposition of ammonia, which only occurs at temperatures higher than 1400°C.

[0011] U.S. Patent Application Publication No. 2012/0148925, included herein by reference, discloses ammonia crackers which decompose ammonia at temperatures higher than 1 100°C, but the description indicates, in paragraph 0045, that "...it appears that the ammonia cracking temperature is lower than 1400°C for a broad range of throughput.". The Applicants discovered later that gas temperatures higher than 1400°C are required for rapid and non-catalyzed decomposition of ammonia. Non-catalyzed decomposition of ammonia, at gas temperatures higher than 1400°C, appears to be necessary for achieving a high ammonia decomposition yield at a high throughput.

[0012] U.S. Patent Application Publication No. 2005/0037244 discloses, in paragraph 0036, that "it is to be understood that ammonia oxidation also takes place during hydrogen combustion, which also contributes heat for the autothermic reaction", hence non-catalyzed ammonia combustion may occur at temperatures lower than 1200°C. Oxygen may directly attack ammonia molecules at temperatures lower than 1400°C. However, when the equivalence ratio is greater than 3, there is enough oxygen to combust only a minor portion of the ammonia, and the remaining major portion of the ammonia must decompose without the assistance of direct attack by oxygen. This occurs rapidly, for ammonia molecules that do not contact a catalyst, only when the gas temperature is higher than 1400°C.

[0013] U.S. Patent No. 7,794,579 incorporates a heat exchanged, autothermal, catalytic ammonia reformer which may be operated within a temperature range of 200-2000°C. The invention discussed in the '579 Patent appears to operate in the same manner at 200°C as it does at 2000°C. Also, the ' 579 Patent discloses, in the ninth paragraph from the end of the text, that the invention ". ..combines endothermic heterogeneous ammonia decomposition reaction, into hydrogen and nitrogen on a supported catalyst, with the exothermic homogenous oxidation of ammonia (into nitrogen and water) in the gas phase." Hence, non-catalyzed ammonia combustion may occur in the gas phase, but the inventors, in the '579 Patent, do not anticipate non-catalyzed ammonia decomposition. In other words, the inventors, in the '579 Patent, clearly do not possess the knowledge that non-catalyzed ammonia decomposition may proceed at operationally significant rates in different temperature regimes. Moreover, all disclosed embodiments require the use of a catalyst, and no description is given for the rapid decomposition of non-combusted ammonia molecules that do not contact a catalyst.

[0014] Apparatus for cracking ammonia was disclosed by Lee, Park, and won at the 2008 Ammonia Fuel Conference. The 2008 presentation, delivered on September 29, 2008, is titled "Properties of Laminar Premixed Hydrogen-Added Ammonia/Air Flames'¹. Additional apparatus for cracking ammonia was reported by won, Joo, Lee, and Urn at the 201 1 Ammonia Fuel Conference. The 201 1 presentation, delivered on September 19, 201 1 , is titled "Reforming and Burning of Ammonia in Micro Hydrogen and Power Generation Systems". Neither of these two presentations specifies a temperature range for non-catalyzed ammonia decomposition. In particular, no specification is given for a gas temperature that is higher than 1400°C, which is now known to be required for rapid and non-catalyzed decomposition of ammonia.

[0015] Based on the foregoing, there is a need for a heat exchanged ammonia cracker that rapidly and non-catalytically converts ammonia into hydrogen at a high ammonia decomposition yield, and at a high overall thermal conversion efficiency.

SUMMARY OF THE INVENTION

[0016] This invention relates to the conversion of ammonia into a hydrogen- containing gas mixture. A method of cracking ammonia in accordance with the invention comprises flowing ammonia and air into a heat exchanged ammonia cracker, wherein combustion and decomposition of the ammonia occurs. The decomposition of the ammonia occurs within the gaseous phase, and at a temperature that is high enough for rapid and non-catalyzed decomposition of the ammonia. A mixture of gaseous products, resulting from the reactions, is expelled from the outlet of the ammonia cracker, the mixture including non-combusted hydrogen gas, which may then be used for other purposes.

[0017] An elementary ammonia flame cracker embodiment, constructed in accordance with the invention, includes an inlet for receiving a "reactant mixture" that contains ammonia and oxygen. A mixture of ammonia and air is an example of such a reactant mixture. This incoming reactant mixture is preheated by counterflow heat exchange with an equal mass of an outgoing hydrogen-containing gas mixture. This preheating causes the incoming reactant mixture to ignite and combust non- catalytically within the heat exchanger, upon reaching an ignition temperature. The now-combusting reactant mixture attains a gas temperature that is higher than 1400°C during its transformation into the hydrogen-containing gas mixture, so as to rapidly and non-catalytically decompose any non-combusted ammonia molecules. The outlet of the ammonia flame cracker expels the converted mixture including non-combusted hydrogen gas.

[0018] An alternate ammonia flame cracker embodiment includes separate inlets for receiving ammonia and for receiving air. The ammonia and air are preheated separately while flowing toward a combustor. Upon arrival at the combustor, the ammonia and air are combined into a combined mixture that combusts. The now- combusting mixture attains a gas temperature that is higher than 1400°C during its transformation into a hydrogen-containing gas mixture, so as to rapidly and non- catalytically decompose any non-combusted ammonia molecules. The preheating of the ammonia and air is achieved by counterflow heat exchanging the incoming ammonia and air with the outgoing hydrogen-containing gas mixture. The outlet of the ammonia flame cracker expels the converted mixture including non-combusted hydrogen gas.

[001 ] In a compounded ammonia flame cracker embodiment, a combustor fuel is combusted with air within a first exothermic conduit within a heat exchanger, and ammonia is at least partially decomposed into hydrogen and nitrogen within a second, separate, endothermic conduit within the same heat exchanger. The combustor fuel and air enter the first conduit, and are preheated separately until they arrive at a combustor, in which the combustor fuel and air mix and combust. The ammonia, entering the second conduit, is heated, and the ammonia is at least partially decomposed into hydrogen and nitrogen at a gas temperature that is higher than 1400°C. Some or all of the hydrogen, exiting from the second conduit, may be directed at further uses. The combustor fuel may be ammonia, or it may be substantially hydrogen. If the combustor fuel is ammonia, then the total mass of incoming ammonia is divided into: a portion of ammonia entering the second conduit as ammonia destined for decomposition, and a portion of ammonia entering the first conduit as combustor fuel. If the combustor fuel is substantially hydrogen, then the hydrogen-containing gas mixture, exiting from the second conduit, is divided into: a portion of the hydrogen-containing gas mixture entering the first conduit as combustor fuel, and a portion of the hydrogen-containing gas mixture directed at further uses. The combustor fuel may be a fuel other than ammonia or products of ammonia decomposition. Hence the endothermic decomposition of ammonia may accept an input of heat yielded by combustion of an entirely different fuel such as natural gas.

[0020] The elementary, alternate, and compounded ammonia flame cracker embodiments have particular elements in common. All contain counterflow heat exchangers. All decompose non-combusted ammonia non-catalytically. All preheat the incoming ammonia and air, before decomposing at least a portion of the ammonia at a gas temperature that is higher than 1400°C. All can provide heat for the endothermic decomposition of ammonia, by combustion of ammonia or by combustion of products of ammonia decomposition, although the compounded embodiment may burn other fuels in its combustor. This combustion, of ammonia or of products of ammonia decomposition, is supported by an input of air into the ammonia cracker. All are capable of operation on ammonia and air as the only substantial inputs, although the compounded embodiment may accept other fuel inputs. All can be operated as stand-alone devices, or they may be incorporated into engine systems or other systems that consume hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIGURE 1 is a graph of equivalence ratios, used for the calculation of a theoretical upper performance limit;

[0022] FIGURE 2 is a graph showing the rate of non-catalyzed ammonia decomposition at a low decomposition yield;

[0023] FIGURE 3 is a graph showing the temperature trend for the time required to decompose 1 % of the ammonia;

[0024] FIGURE 4 is a graph showing the rate of non-catalyzed ammonia decomposition at a high decomposition yield;

[0025] FIGURE 5 is a graph showing the temperature trend for the time required to decompose 100% of the ammonia;

[0026] FIGURE 6 is a graph showing the general form of non-catalyzed ammonia decomposition, as a function of time;

[0027] FIGURE 7 is a graph showing the temperatures required for ignition, and for full ammonia decomposition; [0028] FIGURE 8 is a schematic diagram of an elementary ammonia flame cracker embodiment constructed in accordance with the invention;

[0029] FIGURE 9 is a schematic diagram of an alternate ammonia flame cracker embodiment;

[0030] FIGURE 10 is a schematic diagram of a compounded ammonia flame cracker embodiment;

[0031] FIGURE 1 1 is a schematic diagram of a basic system that converts ammonia into hydrogen for further uses, the system incorporating an ammonia flame cracker;

[0032] FIGURE 12 is a graph of the vapor pressure of ammonia, as a function of temperature;

[0033] FIGURE 13 is a schematic diagram of a supercharged engine system, the system incorporating an ammonia flame cracker; and

[0034] FIGURE 14 is a schematic diagram of a system that supplies hydrogen to an SCR, the system incorporating an ammonia flame cracker.

DETAILED DESCRIPTION OF THE INVENTION

[0035] This invention relates to the conversion of ammonia into a hydrogen- containing gas mixture. A method of cracking ammonia in accordance with the invention comprises flowing ammonia and air into a heat exchanged ammonia cracker, wherein combustion and decomposition of the ammonia occurs. The decomposition of the ammonia occurs within the gaseous phase, and at a temperature that is high enough for rapid and non-catalyzed decomposition of the ammonia. A mixture of gaseous products, resulting from the reactions, is expelled from the outlet of the ammonia cracker, the mixture including non-combusted hydrogen gas, which may then be used for other purposes.

[0036] Typically, the oxidizer, which is used for combusting a portion of the ammonia, is "atmospheric air", which refers to the naturally occurring atmospheric mix of mostly nitrogen and oxygen, which is not enriched in oxygen content. The term "air" is used wherever purified oxygen is permitted but not required. Hence the scope of "air" is intended to cover pure oxygen, oxygen with substantially inert impurities, and atmospheric air. In some cases, partially or fully purified oxygen may be usefully incorporated into a startup strategy. The scope of "purified oxygen" is intended to cover oxygen which is partially or fully purified to a concentration that is greater than that of the naturally-occurring atmospheric mixture. Hence, a mixture of purified oxygen and atmospheric air is also purified oxygen.

[0037] The ammonia may contain small quantities of substantially inert impurities such as water or others, which are ignored in the following analysis. Hence the scope of "ammonia" is intended to cover pure ammonia, and ammonia with a minor fraction of impurities, both of which are gaseous unless specified otherwise. The "equivalence ratio" herein refers to the ratio, of ammonia that is available for combustion or decomposition, to the quantity of ammonia that the air can burn. A total ammonia and air reaction, for which a full ammonia decomposition yield is assumed, is described by the following equation:

0.79N₂ + 0.210₂ +0.28ΦΝΗ₃→

0.42H₂O + (0.79 + 0.14Φ)Ν₂ + 0.42(Φ - 1)H₂

Φ = Equivalence Ratio, and Φ > 1

[0038] In an ideal example, the reactants and products are heat exchanged, such that no energy is expended in preheating the reactants up to an appropriate range of reaction temperatures, and the equivalence ratio is chosen, such that when the ammonia combustion and decomposition reaction is run to completion, the heat, that is released by the formation of water, equals the heat, that is absorbed by the decomposition of ammonia, and the mixture undergoes zero temperature change when it reacts. Once started, the reaction continues without an external input of heat, and is thus an autothermal reaction. The theoretical limits of efficiency and yield are unaffected by substantially inert impurities which may be present in the ammonia, or by the use of specifically pure oxygen instead of air.

[0039] FIG. 1 illustrates a graph 100 showing a curve 102 which is used for calculating a theoretical upper performance limit of an ammonia flame cracker that incorporates an ideal counterf!ow heat exchanger. Curve 102 shows equivalence ratios for which the quantity of heat, released by the exothermic formation of water, is just sufficient to decompose all of the ammonia at a constant temperature, for each temperature. The enthalpies of formation of ammonia, water, or any other compound vary with temperature because, over a given temperature range, the total heat capacities of the reactants and products, in any given chemical equation, are not generally the same. Each equivalence ratio on curve 102 is 1.5 times the molar enthalpy of formation of water, divided by the molar enthalpy of formation of ammonia. The enthalpy of formation assigns a value of zero to pure chemical elements in their usual form, for example, diatomic hydrogen.

[0040] The rate of non-catalyzed ammonia decomposition increases exponentially with increasing temperature. Experimental measurements of the non-catalyzed ammonia decomposition rate indicate that about 1% of the ammonia decomposes in 1 second at 1 150°C. It appears that ammonia does not appreciably decompose in less than 1 second at temperatures lower than 1 1 0°C. Hence, the substantial bulk of the reactions, involving ammonia, occur within a range of temperatures that is higher than ] 150°C, which is also higher than 1 100°C. Curve 102 has a minimum of 6.66 at about 1 100°C.

[0041] At temperatures higher than 1 100°C, the total heat capacity of the reactants is very slightly greater than the total heat capacity of the products. The effect, of this difference in heat capacities, on the reaction energy balance of an ideally heat exchanged reaction, can be compensated exactly by using the equivalence ratio of 6.66. The theoretical upper performance limit, for an ammonia flame cracker incorporating an ideal counterflow heat exchanger, thus corresponds to an ammonia- to-hydrogen thermal conversion efficiency of 97.3% on a lower heating value basis, obtained at an equivalence ratio of 6.66.

[0042] FIG. 2 illustrates a graph 200 showing the fraction of pure ammonia (without oxygen) decomposed, as a function of residence time, in a heated, non- catalytic chamber. Curves 202, 204, 206 and 208 correspond to temperatures of 1 156°C, 1215°C, 1272°C, and 1315°C, respectively, and a pressure of 1 bar. Ammonia appears to decompose at an approximately constant rate at a given temperature, from zero decomposition, up until about 0.05 (5%) of the ammonia is decomposed. The rate of ammonia decomposition declines above 5%. The rate of decline can clearly be seen near 10%, and the decline is faster than can be explained by simple exponential decay of the ammonia. - 1 ! -

[0043] FIG. 3 illustrates a graph 300 showing a temperature trend, for the time required to decompose 1% of the ammonia. The data for line 302 are derived from the results shown in graph 200. The Ln( ) function is the base e logarithm, where e = 2.71 8, and the Ln( ) function is defined to be zero when the time is f second. The temperature range shown is about 1 150- I 320°C. The time required to decompose 1 % of the ammonia is about 1 second at 1 150°C. The slope of line 302 indicates that when the fraction of ammonia decomposed is small, the apparent activation energy for the non-catalyzed decomposition of ammonia is about 455 kilojoules per mole of ammonia.

[0044] This activation energy of 455 kilojoules per mole is very nearly equal to the enthalpy change for breaking the first of the three H-N bonds of the ammonia molecule. The enthalpy change for breaking the first H-N bond is about 444 kilojoules per mole of ammonia at 0 Kelvin, and about 462 kilojoules per moie near 1 00-2000 Kelvin. Ammonia molecules undergo a series of intermediates including NH₂, NH and N, wherein the transition from NH₃ to N¾ involves the largest enthalpy change, and is thus the rate-limiting step. The transition, to subsequent intermediates and then finally to hydrogen and nitrogen, appears to be relatively much faster.

[0045] FIG. 4 illustrates a graph 400 containing additional data on the decomposition of pure ammonia, at a higher range of decomposition yields than the range shown in graph 200, Curves 402, 404, 406 and 408 correspond to temperatures of 1361 °C, 1373°C, 1420°C, and 1455°C, respectively, and a pressure of 1 bar. The fraction of ammonia decomposed is approximately proportional to the cube root of time, and the overall reaction rate is exponentially dependent on temperature. The cube-root function is a solution to a differential equation, in which the instantaneous rate of ammonia decomposition is inversely proportional to the square of the fraction of ammonia decomposed.

[0046] FIG. 5 illustrates a graph 500 showing the temperature trend associated with the time required to decompose 100% of the ammonia, that is, for full decomposition of ammonia. The data for line 502 are based on the results shown in graph 400. Again, the Ln( ) function is the base e log, where e ~ 2.718, and the Ln() function is defined to be zero when the time is 1 second. The temperature range shown is about 1360-1460°C. The time required to decompose 100% of the ammonia is about 1 second at 146CTC. The slope of line 502 indicates that when the fraction of ammonia decomposed is large, the apparent activation energy for the non-catalyzed decomposition of ammonia is about 657 kilojou!es per mole of ammonia.

[0047] This activation energy of 657 kilojoules per mole is nearly halfway between the enthalpy change for breaking the first of the three H-N bonds of the ammonia molecule, and the enthalpy change for breaking both the first and second of the three H-N bonds. The enthalpy change for breaking both the first and second H-N bonds is about 830 kilojoules per mole of ammonia at 0 Kelvin. The approximate inverse square dependence, of the ammonia decomposition rate, on the concentration of hydrogen in the mixture, suggests that molecular hydrogen is recombining with two of the intermediates, with NH₂ being the most energetically favored, and NH being the second most energetically favored, for recombination back down to N¾ and NH₂, respectively. It appears that the larger activation energy, of 657 kilojoules per mole, is required to overcome the recombination of ammonia-related intermediates when molecular hydrogen is present. Water may also participate in this recombination, but probably to a much lesser extent,

[0048] FIG. 6 illustrates a graph 600 showing the general curve of ammonia decomposition as a function of time. The data are graphed on an equivalent time axis, wherein a longer time spent at a lower temperature, is equivalent to a shorter time spent at a higher temperature. The time rescaling is normalized to 1455°C, such that at 1455°C, the rescaled time is the actual time. In one example, for the previously determined activation energy of 657 kilojoules per mole, 1 second, spent at 1420°C, is equivalent to about 0,389 seconds, spent at 1455°C. A similar rescaling of time, based on the activation energy of 657 kilojoules per mole, collapses all of the decomposition data from graphs 200 and 400 into a single curve 602. Curve 602 is 0.924, multiplied by the cube root of the time in equivalent seconds spent at 1455°C, which is the best fit to the data for the range shown. Curve 602 approaches 1 ( 100%) in 1.27 seconds.

[0049] Therefore, as demonstrated by the curves described above, the Applicants have discovered the unexpected result that ammonia fully decomposes non- catalyticaily in about 1.27 seconds at 1455°C, or about 3,26 seconds at 1420°C, etc. when the pressure is 1 bar. The frequency of collisions between gas molecules is proportional to pressure. A higher collision frequency, and thus a higher ammonia decomposition rate at a given temperature, or a slightly lower temperature at a given ammonia decomposition rate, is thus achieved by the use of elevated pressures. Residence time is also proportional to pressure for a given mass throughput and set of dimensions. Based upon these observations, the Applicants have additionally discovered that the dimensions of an ammonia flame cracker can be made smaller for a given residence time and throughput, if elevated pressures are used. Near 100% decomposition of the ammonia may be achieved in practice, even at elevated pressures of perhaps 10 bars, because less than one millionth of the ammonia survives intact in an equilibrium mixture at temperatures higher than 1400°C, and at a pressure of 10 bars.

[0050] FIG. 7 illustrates a graph 700 showing the combinations of time and temperature required for ignition 702 and full decomposition 704 of ammonia at a pressure of 1 bar. It is a goal of the apparatus disclosed herein to decompose ammonia, at a high ammonia decomposition yield, and within a reasonable time. Ignition and decomposition times, that are longer than 1 second, are burdensome from the standpoint of design. An ammonia cracker becomes large for a given throughput, if either of these times is longer than 1 second, and heat loss also becomes comparatively large. Therefore, peak gas temperatures higher than 1400°C are required to obtain a full ammonia decomposition yield. How much higher, depends on dimensions and throughput, which in turn determine an ignition time and a holding time at peak temperature. Representative examples are given.

[0051] Ignition occurs in an ammonia/air mixture when enough ammonia decomposes and combusts, at the ignition temperature, during the ignition time, such that the ammonia decomposition rate increases by a factor of e, where e ~ 2.71 8. Over a wide range of temperatures of interest, the ammonia decomposition rate increases by a factor of e for every 40-50°C temperature increase. For equivalence ratios of about 6, a temperature increase of 40-50°C is yielded when about 1 % of the ammonia decomposes and combusts adiabaticaliy. Therefore, about 1% of the ammonia should decompose during the ignition time in order for ignition to occur. Ignition curve 702 is thus derived from an extrapolation of line 302 in graph 300.

[0052] The ignition time is the distance traversed along a 40-50°C rise in the heat exchanger, divided by the gas velocity in an ignition region. In one example, a heat exchanger has a temperature gradient of about 40-50°C per centimeter down the length, and a gas velocity of 10 meters per second in an ignition region. The ignition time is 0.001 seconds, and the mixture ignites at 1460°C, according to the ignition curve 702, In another example, a heat exchanger has a temperature gradient of about 40-50°C per 2 centimeters down the length, and a gas velocity of 2 meters per second in the ignition region. The ignition time is 0.01 seconds, and the mixture ignites at 1340°C, according to the ignition curve 702. In both examples, which are considered typical, the ignition temperature is higher than 1200°C.

[0053] Regarding ignition, the principal assumption is that only ammonia that has decomposed can be burned. However, oxygen may attack molecules ammonia directly at temperatures that are lower than the ignition temperature 702, so it is possible that the ammonia could ignite at combinations of ignition time and temperature that are below curve 702. The ignition curve 702 is thus an upper bound on combinations, of time and temperature, that a rich, premixed ammonia/air mixture could be expected to tolerate without igniting. Separately flowed ammonia and air components are guaranteed to ignite upon mixing, at combinations of preheat temperature and holding time in the combustor that are on or above the ignition curve 702.

[0054] It is this preheating of the ammonia and air, by the heat exchanger, which causes the non-catalyzed ignition of a premixed ammonia/air mixture in ammonia flame cracker 800. This preheating causes the non-catalyzed ignition, upon mixing, of separately flowed ammonia and air in ammonia flame cracker 900. This preheating also causes the non-catalyzed ignition, upon mixing, of separately flowed combustor fuel (possibly ammonia) and air in the combustor 1004 of ammonia flame cracker 1000.

[0055] The Applicants have discovered that a rich, premixed ammonia/air mixture ignites non-catalytically in about 0.1 seconds at a pressure of 1 bar and a temperature of 1200°C, a result which is in close agreement with ignition curve 702. Therefore, fully non-catalyzed ignition of premixed or separately flowed ammonia and air occurs when the preheat temperature is about 1200°C or higher, depending on flow conditions. An ammonia flame cracker can be restarted, following a brief cessation of flow, by simply resuming flow, if the preheat temperature is still high enough for ignition, when flow resumes. [0056] The full decomposition curve 704 defines a region of combinations, of peak temperature and holding time at peak temperature, which are sufficient to fully decompose ammonia. Curve 704 is thus derived from an extrapolation of line 502 in graph 500. In one example, all of the ammonia is decomposed when the ammonia is held at a peak temperature of about 1 770°C for 0.001 seconds. In another example, all of the ammonia is decomposed when the ammonia is held at a peak temperature of about 1660°C for 0.01 seconds. In yet another example, all of the ammonia is decomposed when the ammonia is held at a peak temperature of about 1560°C for 0.1 seconds, according to curve 704. In all three examples, which are considered typical, the temperatures required for full decomposition of the ammonia are higher than 1400°C. A temperature range of about 1500-1800°C appears to be appropriate for fully decomposing ammonia in about 0.001 to 0.1 seconds at a pressure of 1 bar. The rate of ammonia decomposition increases with increasing pressure. A temperature range of about 1400-1700°C appears to be appropriate for fully decomposing ammonia in about 0.001 to 1 seconds at a pressure of 10 bars.

[0057] FIG. 8 illustrates an elementary ammonia flame cracker 800 embodiment including concentric, preferably round tubes 802, 805, and 806, and an igniter 808. The igniter 808 may be a spark gap, a traveling flame, or any of the other various igniters which are known to the art. The igniter 808 is used for igniting a flammable mixture at startup. A starting mixture is burned near a turnaround region 804, and the burned gases are purged through, from an operationally relatively hot end 812, toward an operationally relatively cool end 814, thus warming surfaces within the ammonia flame cracker 800 up to operating temperature during a starting period. In some cases, leftover hydrogen in this burned starting mixture is applied to further uses during the starting period.

[0058] Tubes 802 and 805 are joined near cool end 814, and these tubes form a counterflow heat exchanger, in which one incoming gas mixture is heat exchanged with one outgoing gas mixture. The incoming gases reverse their direction of travel in the turnaround region 804. The turnaround region is the region at or near the innermost limit of gas travel in the heat exchanger. A rich mixture of ammonia and air enters through inlet 801 , and travels through the space between tubes 802 and 805, toward a turnaround region 804. The mixture of ammonia and air is preheated to ignition by heat conduction and/or radiation through the walls of tube 805. The mixture of ammonia and air ignites upon reaching the ignition temperature 702 somewhere within the space between tubes 802 and 805, upstream of the turnaround region 804. The ignited mixture combusts, thereby forming a hydrogen-containing gas mixture.

[Θ059] Gas temperatures that are higher than 1400°C are attained by the combusting reactant mixture, during its transformation into the hydrogen-containing gas mixture, so as to non-catalytically decompose any non-combusted ammonia molecules. At least the surfaces contacting the mixture, in the space between tubes 802 and 805, are selected or treated so as to be non-catalytic, thus preventing the mixture of ammonia and air from igniting before it is preheated to the maximum possible extent. The hydrogen-containing gas mixture travels toward outlet 807, through the space inside tube 805. The outgoing hydrogen-containing gas mixture is cooled by giving up a portion of its sensible heat to the next equal mass of incoming ammonia and air, by heat conduction and/or radiation through the walls of tube 805. Finally, the hydrogen-containing gas mixture leaves the ammonia flame cracker 800, through outlet 807. The small arrows indicate the direction of flow.

[0060] In accordance with a preferred construction, tube 806, joined to tube 802 near cool end 814, forms the outer wall of a vacuum jacket, and tube 802 forms the inner wall of this same vacuum jacket. This vacuum jacket prevents heat loss from the heat exchanger that is formed by tubes 802 and 805. A vacuum space 8 18 is bounded by the inner surface of tube 806 and the outer surface of tube 802. The inner surface of tube 806 may be coated with a metal film, for example gold, silver, copper or aluminum, which has a high average reflectivity for blackbody radiation at 1400°C. The high reflectivity coating on the inner surface of tube 806 reduces radiative heat loss from tube 802, and also serves to keep tube 806 relatively cool. The outer surface of tube 802 may also be constructed of, or coated or covered with possibly reflective materials that have a very low vapor pressure at I 400°C so as to minimize contamination of the vacuum space 818, and to avoid substantial contamination of any reflective coatings. Further insulation on the outside of tube 806 only serves to raise the temperature of tube 806, and has minimal impact on the rate of net heat loss from tube 802. [0061] Ammonia flame cracker 800 is inherently stable in its operation. Headroom is afforded for variations of flow, temperature, pressure, etc, which are compensated by movement of the ignition point, back and forth along the length, within the space between tubes 802 and 805. At a given operating point, the margin of stability is proportional to the distance, from the turnaround region 804, to a point at which the incoming mixture ignites. The mixture of ammonia and air, entering inlet 801 , ignites and combusts when it reaches the ignition temperature 702, and thus does not accept preheating beyond the ignition temperature 702. Operation is thus not subject to thermal runaway. However, if the ignition time and the holding time at peak temperature are about the same, then the full ammonia decomposition temperature 704 is about 320°C higher than the ignition temperature 702. This is shown by the vertical displacement of curve 704 above curve 702.

[0062] The full ammonia decomposition temperature 704 can be made equal to the ignition temperature 702, by making the holding time at peak temperature about 300- 1000 times longer than the ignition time. This is shown by the horizontal displacement of curve 704, to the right of curve 702. Therefore, the preheat temperature, which is limited by the ignition temperature 702, cannot closely approach the full ammonia decomposition temperature 704, and the theoretical limit, corresponding to an equivalence ratio of 6.66, is not closely approached, unless the holding time at peak temperature is made relatively very long, which is burdensome from the standpoint of design. Despite this l imitation, ammonia flame cracker 800 can be usefully incorporated into systems in which the sensible heat and chemical energy, of the hydrogen-containing gas mixture exiting 807, have equal value, such as a barbecue grill or other cooking or heating appliance, or a turbine. The above limitation is addressed by an alternate ammonia flame cracker 900 embodiment, in which the ammonia and air are separately preheated and separately flowed into a combustor. Separate preheating prevents ignition of the ammonia and air until they are combined, and thus allows a higher preheat energy state to be achieved.

[0063] FIG. 9 illustrates an alternate ammonia flame cracker 900 embodiment including concentric tubes 902, 903, 905, and 906, and an igniter 909, which is similar to igniter 808. Again the tubes are preferably round, although other cross sections and geometries are possible, as described earlier. The igniter 909 is used for igniting a flammable starting mixture at startup. The starting mixture is combusted in region 904, and the combusted gases are purged through from an operationally relatively hot end 912 toward an operationally relatively cool end 914, thus warming surfaces within the ammonia flame cracker 900 up to operating temperature. In some cases, leftover hydrogen in this combusted starting mixture is directed at further uses during warm- up.

[0064] The tubes 902 and 905, and 905 and 903 are joined near cool end 914, and these tubes form a counterflow heat exchanger, wherein two separate streams of incoming gas are heat exchanged with one stream of outgoing gas. The construction methods and materials requirements are similar to those of corresponding tubes in ammonia flame cracker 800. Gaseous ammonia enters the ammonia flame cracker 900, through inlet 901 , and travels through the space between tubes 902 and 905 until the ammonia arrives at region 904. Air enters through inlet 908, and travels through the space within tube 903 until the air arrives at region 904.

[0065] Region 904 is a combustor, into which the ammonia and air are separately flowed. Combustor 904 also happens to be a turnaround region. The turnaround region is the region at or near the innermost limit of gas travel in the heat exchanger. Generally, both the combustor 904 and a turnaround region will be substantially near hot end 912, and will therefore be substantially near each other. The ammonia and air form a combined mixture that combusts within or downstream of the combustor 904. Gas temperatures that are higher than 1400°C are attained by the combined mixture, during its transformation into a hydrogen-containing gas mixture, so as to non- catalytically decompose any non-combusted ammonia molecules. The holding time at peak temperature is at least partially determined by the volume of the combustor 904. Finally, the hydrogen-containing gas mixture travels toward outlet 907, through the space between tubes 905 and 903, and leaves the ammonia flame cracker 900, through outlet 907. The smalt arrows indicate the direction of flow.

[0066] The ammonia is preheated while flowing from inlet 901 to combustor 904 and the hydrogen-containing gas mixture is cooled while flowing from combustor 904 to outlet 907, by heat conduction and/or radiation through the walls of tube 905. The air is preheated while flowing from inlet 908 to combustor 904 and the hydrogen- containing gas mixture is cooled while flowing from combustor 904 to outlet 907, by heat conduction and/or radiation through the walls of tube 903. It is through this heat exchange process, that the ammonia and air are preheated before they react in the combustor 904 and become the hydrogen-containing gas mixture. The outgoing hydrogen-containing gas mixture is cooled by giving up a portion of its sensible heat to the next equal mass of incoming ammonia and air.

[0067] No effort is made to prevent or impede preheating of the air or of the ammonia. For all discussion herein pertaining to the alternate ammonia flame cracker 900 embodiment, both the ammonia and the air are preheated, and the preheat temperatures are assumed to be the same. Preferably, ammonia flame cracker 900 should be controlled so as to obtain preheat temperatures of about 1200°C or higher, according to curve 702, and also according to flow conditions.

[0068] The ammonia, entering inlet 901 , is substantially devoid of molecular oxygen. That is, no effort is made to include oxygen or air in the ammonia, although the ammonia may contain small quantities of oxygen-bearing impurities such as water, which do not normally support combustion of ammonia. The air, entering 908, is substantially devoid of ammonia. That is, no effort is made to include ammonia or any other fuel in the air. Neither mixture can ignite until the two are combined in the combustor 904. Therefore, the total preheat energy state of the separate mixtures is not limited by the ignition temperature 702.

[0069] In practice, some of the ammonia may decompose during preheating, before arriving at combustor 904, and the preheat temperatures may be somewhat lower than the full ammonia decomposition temperature 704, If enough ammonia is decomposed during preheating, then such a preheat state can be energetically equivalent to preheating to the full ammonia decomposition temperature 704, and the theoretical limit, corresponding to an equivalence ratio of 6.66, may be approached. In the case that all of the ammonia is decomposed during preheating, the decomposed mixture retains its label as the "ammonia" until it is combined with "air" in the combustor 904.

[0070] The combined mixture undergoes an additional 7°C temperature rise above the preheat temperature, for every 1 % of the ammonia that is decomposed before the ammonia is mixed with the air. The partially or fully decomposed ammonia may combust at a peak gas temperature that is substantially higher than the preheat temperature. The energy, that is associated with this temperature rise, is recovered in the partial decomposition of the next mass of incoming ammonia, A substantial fraction, perhaps about 1 /4 to 1/3 or more of the ammonia, may easily be decomposed, before arriving at combustor 904, thus resulting in a temperature rise of about 200°C above the preheat temperature, when the air reacts with the partially decomposed ammonia. The preheat temperature may be limited by the maximum temperature that can be tolerated by materials, for example, about 1700°C for aluminum oxide. In one example, a 200°C temperature rise, above the 1700°C preheat temperature, produces a peak gas temperature of 1900°C. Peak gas temperatures, possibly exceeding material durability limits, can thereby be achieved by ammonia flame cracker 900 in some instances.

[0071] Means of monitoring one or more temperatures, and means for feedback control of the equivalence ratio, (not shown) may be provided so as to prevent thermal runaway and to maintain a desired set of operating temperatures. A temperature sensor (not shown) may measure a temperature near hot end 912. A target temperature may be chosen, such that no ammonia passes through the outlet 907 when the measured temperature is maintained at the target temperature. The equivalence ratio may be varied in response to this measured temperature, such that if the measured temperature is lower than a target temperature, then the equivalence ratio is decreased, and vice versa.

[0072] Tube 906 is joined to tube 902 near cool end 914. Tube 906 forms the outer wall of a vacuum jacket and tube 902 forms the inner wall of this same vacuum jacket. This vacuum jacket prevents heat loss from the heat exchanger that is formed by tubes 902, 903 and 905. A vacuum space 918 is thus bounded by the inner surface of tube 906 and the outer surface of tube 902. Other description of the vacuum jacket is the same for ammonia flame cracker 900 as it is for ammonia flame cracker 800.

[0073] A heat exchanger geometry, possibly differing from the particular examples shown in 800 and 900, may be chosen so as to improve heat exchange or to impede heat loss. For example, multiple tubes 805, and 903 and 905, may be housed within tubes 802, and 902, of ammonia flame crackers 800, and 900, respectively. In another example, a heat exchanger monolith with hollow channels, each of which is about a millimeter or a few millimeters wide, may be used instead of the single tube 805, or instead of the single tube arrangement comprising tubes 903 and 905. Such a monolith may contain, for example, an N by N array of square channels. Adjacent channels may alternate between connection to reactant in!et 801 and product outlet 807 of ammonia flame cracker 800, in a checkerboard pattern or other pattern. Channels may be similarly divided into 3 groups separately connecting, one group each, to separate inlets 901 and 908, and outlet 907 of ammonia flame cracker 900, in a pattern that is chosen to promote mixing of the ammonia and air in the combustor 904, and also to promote exchange of heat. Any other ammonia flame crackers disclosed herein may be similarly expanded into multiple channels, or use a geometry that differs from the particular examples shown.

[0074] Components may have a layered or graded composition that varies down the length. For example, a heat exchanger monolith may be composed of predominantly aluminum oxide or silicon carbide near hot end 812, or hot end 912, and muilite near cool end 814, or cool end 914. Any other ammonia flame crackers disclosed herein may incorporate similarly graded or layered components.

[0075] Such a layering or grading may include variable doping of a semiconductor heat exchanger monolith, of which silicon carbide is an example of a semiconductor, from which the monolith may be constructed. Variable doping can be used for obtaining a desired variation in electrical resistivity, so as to permit starting by resistive heating. In one example, an electric current is passed through contacts brazed onto the monolith near coo! end 814, or 914, and the current is conducted down the length through a pair of highly conductive channels in the monolith, toward a moderately conductive channel connecting the two highly conductive channels near hot end 812 or 912, the remainder of the monolith being substantially electrically insulating. An electric current is passed through a portion of the heat exchanger monolith, thereby resistively heating a portion of the heat exchanger up to operating temperature during a starting period. If a portion of the monolith is resistively heated to a temperature that is higher than the ignition temperature 702, then igniters 808 and 909 may be omitted.

[0076] FIG. 10 illustrates a compounded ammonia flame cracker 1000 embodiment having separate conduits for ammonia combustion and ammonia decomposition, and system 1001. Separate conduits are used in 1000 and 1001 for obtaining a hydrogen-containing gas mixture that is devoid of both water and residual oxygen, and of reduced nitrogen content. The theoretical upper performance limits concerning energy balance, overall equivalence ratio, and conversion efficiency are all the same, regardless of whether ammonia is combusted and decomposed in separate conduits, or within the same conduit. The theoretical upper performance limit corresponding to an equivalence ratio of 6.66 may be approached by ammonia flame cracker 1000, in the U-shaped configuration shown.

[0077] The gas temperatures, at which ammonia is decomposed in the endothermic conduit 1002, can be no higher than the highest material temperature reached by the ammonia flame cracker 1000. All ammonia flame crackers 800, 900, and 1000 exchange heat from the products to the reactants so as to preheat the reactants to an appropriate range of reaction temperatures, and also to cool the products to a range of temperatures that is appropriate for further uses. However, unlike ammonia flame crackers 800 and 900, in which the ammonia combustion and ammonia decomposition occur autothermally within the same mixture, Ammonia flame cracker 1000 must also exchange the heat of ammonia decomposition.

[0078] Ammonia enters inlet 1008, and the ammonia is preheated in conduit 1002. This ammonia decomposes upon further heating in conduit 1002 at a gas temperature that is higher than 1400°C. The ammonia in conduit 1002 is thereby transformed into the hydrogen-containing gas mixture in the endothermic decomposition conduit 1002. Then the hydrogen-containing gas mixture is cooled by the heat exchanger 1006 before exiting outlet 1010. If the ammonia fully decomposes in conduit 1002, then the mixture exiting outlet 1010 contains about 75% hydrogen and 25% nitrogen by volume.

[0079] A combustor fuel enters inlet 1012 and air enters inlet 1014. The combustor fuel may be ammonia, or it may be a portion of the hydrogen-containing gas mixture exiting from outlet 1010. The combustor fuel and air are preheated separately, and are then combined and combusted in a combustor 1004. The combustor 1004 may include an igniter, (not shown) which may be a spark gap, a traveling flame, or any of the various other igniters which are known to the art. The combustor 1004 may combine the combustor fuel and air in a distributed way, such that the heat release is spread out, thus lowering peak temperatures in the combustor 1004 and facilitating heat transfer from combustor 1004 to endothermic decomposition conduit 1002. The temperature near the combustor 1004 must be higher than 1400 Celsius, such that heat will flow from the combustor 1004 to the endothermic decomposition conduit 1002. Heat is recovered from the exhaust gas after it leaves the combustor 1 004, and before the exhaust gas exits from outlet 1016. The arrows show the direction of flow. The temperature of the heat exchanger 1006 has an approximately uniform gradient with respect to vertical height as drawn, such that hotter items are higher on the vertical axis, and the items at a given vertical height are approximately the same temperature. The heat exchanger 1006 generally transfers heat into the mixtures that are traveling upward in the schematic, and transfers heat out of the mixtures that are traveling downward, and is therefore a coiinterflow heat exchanger.

[0080] At least enough air is supplied to the combustor 1004, such that all of the combustor fuel is combusted, and more than sufficient air may be used for the purposes of balancing the heat capacities between the exothermic conduit containing the combustor 1004 and the endothermic conduit 1002, for reducing NOx emissions by the combustor 1004, for lowering peak temperatures in the combustor 1004, or to ensure the complete burn-up of the combustor fuel entering inlet 1012. The combustor fuel entering inlet 1012 and air entering inlet 1014 may be mixed in stoichiometric proportions in combustor 1004. Heat is recovered from the exhaust gas after it leaves the combustor 1004, and before it exits from outlet 1016. Generally, a heat exchange relationship may exist between any two or all flow channels or other elements contained within the heat exchanger 1006. In particular, heat is transferred from the combustor 1004 to the endothermic ammonia decomposition conduit 1002.

[0081] Ammonia flame cracker 1000 may be incorporated into a fueling system 1001 for a fuel ceil 101 1 that receives a hydrogen-rich gas mixture and expels a hydrogen-depleted gas mixture. The hydrogen-rich gas mixture may contain non- decomposed ammonia, and the fuel cell 101 1 may be selected from any of the fuel cells which are known to the art, possibly one of the ammonia-tolerant fuel ceils. The hydrogen-depleted gas mixture may contain leftover hydrogen not consumed by the fuel cell 1 01 1 , and possibly non-decomposed ammonia. [0082] Ammonia enters inlet 1008, and the ammonia is at least partially decomposed within conduit 1002. The hydrogen-containing gas mixture, exiting from outlet 1010, is received, as the hydrogen rich gas. by the fuel cell 101 1. At least a portion of the hydrogen is consumed by the fuel cell 101 1 , thereby transforming the hydrogen-rich gas mixture into the hydrogen-depleted gas mixture. The hydrogen- depleted gas mixture is expelled by the fuel cell 101 1 , and the hydrogen-depleted gas mixture is used as the combustor fuel, which enters inlet 1012. This hydrogen- depleted gas m ixture, which is retrieved from the fuel cell 101 1 after some of the hydrogen is consumed, is an example of an "anode off-gas". System 1001 is operated toward the advantages that none of the ammonia is wasted, no gas separators are needed, impurities do not accumulate in the fuel cell 101 1, and the ammonia flame cracker 1000 and system 1001 can be operated with ammonia and air as the only substantial inputs.

[0083] Other configurations of ammonia flame cracker 1000 are possible, such as a multichannel square prism monolith with one long dimension and 2 ends, the "conduits" referred to herein being divided into multiple straight channels running parallel to the long dimension. Separate combustor fuel and air enter multiple channel pairs on a first end of the monolith, and the decomposed ammonia exits a separate set of ammonia decomposition channels on the same end. The channel pairs for combustor fuel and air are combined into single channels about halfway down the length, and this combination region is the combustor. Exhaust gas exits through these combined channels, and ammonia enters the ammonia decomposition channels on the second end of the monolith. The monolith is operationally hot in the middle, and operationally cool on the ends. A fuel cell may receive the decomposed ammonia from the ammonia decomposition channels, and expel an anode off-gas into the combustor fuel channels on the first end of the monolith. The monolith may be vacuum-jacketed along the sides, and it may be started by electrical resistive heating or by combustion of flammable mixtures within the monolith.

[0084] Ammonia flame crackers 800, 900 and 1000 can be wanned up to operating temperature by combustion of a starting mixture and then flowing the combusted starting mixture through the ammonia flame crackers, such that the combusted starting mixture thermally contacts the heat exchangers and/or combustors of ammonia flame crackers 800, 900 and 1000, during a starting period. A "starting period" is a time interval, in conjunction with which, starting methods or starting apparatus are used for heating at least a portion of an ammonia flame cracker up to an appropriate operating temperature. The starting mixture generally comprises a fuel component and an oxidizer component. The fuel and oxidizer components of the starting mixture may be flowed as a single mixture into inlet 801 , ignited by igniter 808, and combusted within or upstream of the turnaround region 804 of ammonia flame cracker 800. The fuel and oxidizer components of the starting mixture may be flowed separately into inlets 901 and 908, respectively, ignited by igniter 909, and combusted in the combustor 904 of ammonia flame cracker 900. The fuel and oxidizer components of the starting mixture may be flowed separately into inlets 1012 and 1014, respectively, and combusted in the combustor 1004 of ammonia flame cracker 1000.

[0085] Mixtures of ammonia and purified oxygen are flammable at a wide range of temperatures, and thus can be used as starting mixtures. In that case, the fuel component of the starting mixture is ammonia, and the oxidizer component is either purified oxygen, or a combination of purified oxygen and atmospheric air. Purified oxygen can be used for rendering flammable, a comparatively larger mass of starting mixture that consists mostly of ammonia and atmospheric air. Purified oxygen may be obtained by pressure swing adsorption, or by any other means known to the art for extracting partially or fully purified oxygen from air. Ammonia flame crackers 800, 900 and 1000 may thereby be started, using ammonia, atmospheric air and purified oxygen.

[0086] As the surface temperatures increase during warm-up, it is permitted to do one or more of the following: increase the total flow of the starting mixture, decrease the oxygen fraction of the oxidizer component, or increase the overall richness of the starting mixture. Any of these may be achieved by holding the flow of purified oxygen constant, and increasing the ammonia flow, while possibly also increasing the atmospheric air flow, to the extent permitted by the increasing surface temperatures. In each case, the ammonia flow may optionally be increased during warm-up. Ammonia flame crackers 800, 900 and 1000 can thereby be quickly started even on a comparatively small flow of purified oxygen. The flow of purified oxygen is turned off at or near the end of the starting period.

[0087] An oxygen extractor that provides purified oxygen for starting may be more burdensome or expensive than is an ammonia decomposer that provides hydrogen for starting. Hydrogen can be used for rendering flammable, a comparatively larger mass of starting mixture that consists mostly of ammonia and atmospheric air. In that case, the fuel component of the starting mixture is either hydrogen, or a combination of ammonia and hydrogen, and the oxidizer component is atmospheric air. The ammonia decomposer, that provides hydrogen for starting, may be selected from any of the ammonia decomposers disclosed herein or known to the art. The ammonia decomposer that provides hydrogen for starting could be a smaller ammonia flame cracker, which itself must be started. Hence, in an absurd scenario, an infinite starting series of ever-smaller and fully non-electric ammonia flame crackers is incorporated into a starting strategy. The starting series can be truncated, perhaps to only one ammonia cracker that provides hydrogen for starting, by including, within the starting series, an ammonia cracker that accepts electricity as at least an initial or partial energy input. Ammonia flame crackers 800, 900 and 1000 may thereby be started, using ammonia, atmospheric air and a relatively small quantity of electrical energy as the only inputs.

[0088] An exclusively electrically powered ammonia decomposer may be used for providing hydrogen for starting if it is desired to obtain a maximally flammable starting mixture, which is devoid of water, and is of minimal inert gas content. An exclusively electrically powered and possibly heat exchanged ammonia decomposer may contain a resistively heated electric element, which may be non-catalytic, and which may be immersed in flowing ammonia, without air.

[0089] In one example, a near-stoichiometric starting mixture comprising atmospheric air and 1/4 decomposed ammonia is flammable at a wide range of temperatures. As the surface temperatures increase during warm-up, it is permitted to do one or more of the following: increase the total flow of the starting mixture, increase the ratio of ammonia to hydrogen in the starting mixture, or increase the overali richness of the starting mixture. Any of these may be achieved by holding the hydrogen flow constant, and increasing the ammonia flow, while possibly also increasing the atmospheric air flow, to the extent permitted by the increasing surface temperatures. In each case, the ammonia flow may optionally be increased during warm-up. Ammonia flame crackers 800, 900 and 1000 can thereby be quickly started even on a small flow of hydrogen and a correspondingly small consumption of electric power by the ammonia decomposer that provides the hydrogen for starting. The flow of hydrogen is turned off at or near the end of the starting period. After starting, ammonia flame crackers 800, 900, and 1000 can revert to operation on ammonia and possibly atmospheric air as the only substantial inputs.

[0090] FIG. 1 1 illustrates a basic system 1 100 for operating an ammonia flame cracker 1 1 12. The ammonia flame cracker 1 1 12 may be any of the ammonia flame crackers 800, 900 or 1000. wherein ammonia and air enter the various inlets and hydrogen exits an outlet as described earlier, possibly including starting apparatus and methods also described earlier. A mass flow control module 1 1 10 comprises combinations of pumps, compressors, expanders, regulators, sensors, actuators or valves which are known to the art, and it controls the mass flows of air and ammonia into the ammonia flame cracker 1 1 12.

[0091] The system 1 1 00 incorporates a source of gaseous ammonia 1 102 comprising a tank of ammonia 1 104, ammonia vaporizer 1 108, and a pump and/or pressure regulator 1 106, which regulates the pressure of the ammonia in vaporizer 1 108 and/or entering the mass flow control module 1 1 10. If regulator 1 106 includes a pump, then the source of gaseous ammonia 1 102 may be controlled so as to build a pressure that is higher than the pressure in the ammonia tank 1 104. Engine coolant, engine exhaust, output from the ammonia flame cracker 1 ] 12, or another hot or warm fluid may be circulated through the heating loop 1 109 of the ammonia vaporizer 1 108, thereby heating the ammonia vaporizer 1 108. Ammonia is fully vaporized before entering the mass flow control module 1 1 10. Any liquid ammonia entering the mass flow control module 1 1 10 or ammonia flame cracker 1 1 12 could surge in response to pressure and temperature variations, thus causing the instantaneous flow of ammonia to vary in an unknown or uncontrolled manner. Therefore, the ammonia source 1 102 is controlled so as to supply only gaseous ammonia to the mass flow control module 1 1 10, which supplies the gaseous ammonia to the ammonia flame cracker 1 1 12. [0092] The vaporizer 1 108 may be omitted if ammonia is drawn from the tank 1 104 as a gas. in some cases, the regulator/pump 1 106 may be omitted. The ammonia tank 1 104 may contain liquid anhydrous ammonia, which may be drawn from the tank 1 104 as either a liquid or a gas. The ammonia tank 1 104 may contain aqueous ammonia, which possibly aiso includes an antifreeze component, in which case the ammonia is drawn from the tank 1 104 as a gas, possibly at a pressure of less than 1 bar. The output of the ammonia flame cracker 1 1 1 2 is directed toward one or more further uses 1 1 14 of hydrogen which are disclosed herein or known to the art.

[0093] FIG. 12 illustrates a graph 1200 depicting the vapor pressure of ammonia 1202. The pressure of the ammonia leaving the source of gaseous ammonia 1 102 is limited by the vapor pressure of ammonia at a given temperature. If it is desired to operate the source of gaseous ammonia 1 102 at an output pressure of 200 kPa, for example, then the temperature of the gaseous ammonia, leaving the source of gaseous ammonia 1 102, must be higher than -19°C.

[0094] FIG. 13 illustrates a supercharged engine system 1300. The system 1300 is a more specific case of the system 1 100, wherein the engine 1314 is a specific example of a further use 1 1 14 of the hydrogen. A supercharger 1305 supplies atmospheric air at a possibly supercharged pressure to the mass flow control module 1310, which controls the mass flows of ammonia and air entering the ammonia flame cracker 1312. The ammonia flame cracker 1312 may be any of the examples previously listed for ammonia flame cracker 1 1 12. The mass flow control module 1310 also controls the overall stoichiometry of the hydrogen-containing gas mixture and air entering the engine 1314. The mass flow control module 13 10 is supplied with gaseous ammonia from the gaseous ammonia source 1302, which is similar to the gaseous ammonia source 1 1 02. The ammonia flame cracker 13 12 operates at substantially the same, possibly supercharged pressure of the air, entering the engine 1314. Hence the system 1300 represents one specific case, in which the ammonia flame cracker 1312 is operated at a pressure that is possibly greater than 1 bar, in this case, for the purpose of appropriately matching input and output pressures. The hydrogen-containing gas mixture and air, entering engine 1314, may enter a working chamber through separate intake valves, or the hydrogen-containing gas mixture and air may mix immediately upstream of an intake valve, so as to avoid the possibility of detonating a iarge volume of hydrogen/air mixture. Those skilled in the art will recognize an appropriate choice of additional design parameters for the engine 1314 combusting flammable mixtures of hydrogen, inert gas, and atmospheric air.

[0095] FIG. 14 illustrates a hydrogen SCR (Selective Catalytic Reduction) system 1400. The system 1400 is a more specific case of the system 1 100, wherein the exhaust system 1414 is merely another further use 1 1 14 of the hydrogen. A mass flow control module 1410 controls the mass flows of ammonia and air into the ammonia flame cracker 1412. The ammonia flame cracker 1412 may be any of the examples previously listed for ammonia flame cracker 1 1 12. The mass flow control module 1410 is supplied with gaseous ammonia from the gaseous ammonia source 1402, which is similar to the gaseous ammonia source 1 102.

[0096] NOx (Nitric Oxide, possibly also containing Nitrogen Dioxide) cleanup is achieved by the hydrogen entering the exhaust pipe 141 8 of the exhaust system 1414. The hydrogen reduces the NOx to nitrogen on the SCR catalyst 1420. The engine 1416 may be a diesel engine that incorporates a particulate trap (not shown), which may also remove hydrocarbons and carbon monoxide from the exhaust. The hydrogen SCR system 1400 differs from conventional ammonia/urea SCR systems, in that an excess of hydrogen can be delivered into the exhaust pipe 141 8, so as to completely reduce NOx to nitrogen in the exhaust system 1414, without causing excessive passage of ammonia through the SCR catalyst 1420. If a conventional ammonia/urea SCR system delivers an excess of ammonia to an SCR catalyst so as to reduce all of the NOx to nitrogen, then a significant quantity of ammonia passes through the SCR catalyst. Conventional ammonia/urea SCR systems may operate at a deficit of ammonia so as to avoid passage of ammonia through the SCR catalyst, but in doing so, they fail to fully reduce the NOx to nitrogen. Therefore, system 1400 can substantially eliminate both NOx and ammonia emissions simultaneously, whereas conventional ammonia/urea SCR systems cannot.

[0097] Hydrogen SCR systems, and various SCR catalysts, including hydrogen SCR catalysts, are known to the art. The hydrogen SCR system 1400 differs from conventional hydrogen SCR systems, in that the reductant is stored as ammonia, which is then decomposed non-catalytically as needed. In conventional hydrogen storage systems, the hydrogen is stored as either a cryogenic liquid or as a high pressure gas. Also, hydrogen forms explosive mixtures with air that are easily ignited. Ammonia can be stored indefinitely at ambient temperature as a moderate pressure liquid, and ammonia is not easily ignited in air.

[0098] The present subject matter may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0099] Having described our invention, we claim:

Claims

1. A method of fnitothcnaally and non-caialytically dewjnipoeing or "cracking" ammonia with air, comprising the steps of:

providing a counterflow heat exchanger having one or more inlets in gas flow conwrnmicarion with one or more incoming gas flow channels, one or more outgoing gee flow channel* in gas flow communication with an outlet, and a turnaround region in gaa flow sotmrnmicatjan withmeine ming and outgoing gas flow channels;

flowing ammonia into an inlet of the heat exchanger, flowing air into an inlet of the heat exchanger preheating the incoming ammonia within one or more of the incoming gas flow channels; preheating the incoming air within one or more of the incoming gas flow channels; combusting the preheated ammonia and the preheated air w hin the heat exchanger at a gas temperature that is higher than 1400 Celsius, thereby transforming the ammonia and air into a hydrogen- containing gas mixture;

cooling the hydrogen>coirtaining gas mixture within an outgoing gas flow channel or channels; and flowing the hydrogen^ooteining gas mixture out of the outlet of the heat exchanger; whereby the cooling of the h drogen-eontaiiung gas mixture is achieved substantially in conjunction with the preheating of the next incoming ammonia and air.

2. The memod of claim l.wherem tto

method further comprising the steps of: combining the ammonia and the air into a single reactant mixture; flowing the reactant mixture into an inlet of the heat exchanger; and preheating the incoming reactant mixture to ignition within an farming gas flow channel or channels in gas flow communication with the inlet; and wherein the cooling of the hydrogcB<ont*iniiig gas mixture is achieved substantially m conj -taction whh the preheating of the next incoming reaotant mixture.

3. Hie method of claim 1, wherein the ammonia inlet and the air inlet an separate inlets, comprising the steps of: providing a combustar within the heat exchanger, the combustor being substantially near a turnaround region within the heat exchanger; and combining the preheated ammonia and the preheated air into a combined mixture in the combustor wherein the combined mixture is combusted.

4. The method of claim 1, further including a rtarting method that is used in conjunction win a starting period, the starting method comprising the step of: flowing an electric current through a portion of the heat exchanger, thereby ixsiJtively heating a puroon or me neat exchanger.

3. The method, of claim I, further including a storting method thai ύ used in conjunction with a starting period, the starting method compriiiing the steps of: providing a source of hydrogen;

flowing the hydrogen into on inlet of the heat exchanger; flowing air into an inlet of the heat exchanger; combusting the hydrogen and the air within the heat exchanger, thereby tnufonning the hydrogen and ah* into a combusted starting mixture; and flowing the combusted starting mixture through the heat exchanger so as to contact at least a portion of the heat exchanger, thereby heating a portion of the heat exchanger.

6. The method of claim 1, further including a starting method that ia used in conjunction whh a starting period, the starting method cornprislng the steps of: providing a source f purified oxygon; flowing the purified oxygen into an inlet of the heat exchanger flowing ammonia into an inlet of the heat exchanger; combusting the ammonia and die purified oxygen within the heat exchanger, thereby transforming the ammonia and purified oxygen into a combusted starting mixture; and flowing the combusted starting mixture through the heat exchanger to as to contact at lent a portion of the heat exchanger, thereby heating a portion of the heat exchanger.

7. The method of claim 5, the starting method furttaffcofrroriaiag flowing ammonia into an inlet of the heet exchang r, combusting the ammonia, the hydrogen and the air within the beat exchanger, thereby transforming the ammonia, hydrogen and air into a combusted starting mixture; and flowing the combusted starting mixture through the heat exchanger so as to contact at least a portion of the heat exchanger, thereby heating a portion of the heat exchanger.

8. The method of claim 6, the starting method further comprising the step or:

eubrtuitiaDy iaamuiiig k i* uf ammonia flow in Corytmction with the starting period.

9. The method of claim 7, the Parting method further comprising the step of: substantially increasing a rate of ammonia flow in conjunction with the startmg period.

10. The method of claim I, further including a temperature feedback control method, the feedback control method comprising the steps of: providing a tempaaturc sensor that measures an opexsting temperature, the measured operating temperature preferably being a rear-maximum tonpcrature;

providing a flow control module that controls a ratio, of the anrmonia to tbe air, and controlling the ratio, of the armnonia to the air, according to the measured operating temperature.

11. The method of claim 1,

12. The method of claim I, whtueiu Lh preheated ammonia and the prehMted air are owulsuoLaJ widjlu ibe heei excaanger ta n gos tmpcraturc of about 1500-1900 Celsius.

13. A αοη-catalytic ammonia decomposer or "cracker", comprising; an ammonia decomposition region that accepts an incoming flow of ammonia and eacpo!s an outgoing hydrogen-containing gas mixture, within which nca-ccmbusted ammonia is decomposed at a gas temperature that is higher than 1400 Celsius; a combustor that accepts an incoming flow of combustor fuel and an iiKOming flow of air and expels an exhaust gas, and within which the combustor fuel and air are combusted at a gas temperature that is higher dun 1400 Celsius; and

a heat exchanger that preheats inr/vning ammonia in eenjunctiuu with cooling of an outgoing gas; wherein non-combusted ammonia is decomposed in the ammonia decomposition region in conjunction with combustion of the combustor fuel and air in the combustor.

14. The ammonia cracker of clflim I \ wherein:

the combustor and the ammonia decomposition region are substantially the same region; the combustor fuel is the ammonia;

the hydrogen-containing its mixture is the exhaust ges;

the ammonia and the air are cnmhinnd mrr> a aingla raaftt^nt τ ϊ¾ΐιι^ l-yf f^ prch*«†i"g; the heat exchanger preheats the rcactant raixrare in conjunction with cooling of the hydrogen- containing gas mixture flowing from the ammonia decomposition region; and non-combusted ammonia is decomposed in die ammonia decomposition region in conjunction with combustion of & portion of the ajnmoniaand the air.

15. The ammonia racker of claim 13; wherein the combustor and the ammonia decomposition region an substantially the same region; wherein the combustor fuel is die ammonia wherein the hydrogen ontaining gas mixture is the exhaust gas; wherein, the heat exchanger separately prebeals the ammonia and the air in conjunction with cooling of the h aVogen-containfaig gas mixture flowing from the ammonia decomposition region; wherein the ammonia and the air are separately flowed into die combustor; and wherein non-combusted ammonia is decomposed in the ammonia decomposition region m conjunction wit combustion of a portion of the ammonia and the air.

16. The ammonia cracker of claim 13; wherein the combustor and the ammonia decomposition region are separate;

wherein the heat exchanger preheats the ammonia in conjunction with one or more of the following: cooling of the exhaust gas flowing from the combustor, or oooliag of the hyoYogerMairnaining gas mixture flowing from the ammonia decomposition region; wherein the heat exchanger preheats the air and the combustor fuel in conjunction with one or more of the following: cooling of the exhaust gas flowing from the combustor, or cooling of die hydrogen-containing gas mixture flowing from the ammnnia decomposition region; sod wherein rKXKXimbusted ammonia is decomposed in the ammonia decomposition region m

oftfac combustor fuel oxi the air ro the combustor.

17. The anmonk crackorof claim 16;

wherein die combustor fuel is ammonia or a portion ofthehydrogen-cootauung as mixture.

18. A ammonia crackerof claim 16:

wherein the combustor fuel is a hydrocarbon.

19. The ammonia cracker of claim 13, including a temperature feedback control apparatus, the control apparatus comprising:

a thermal radiation detector or other temperature sensor mat measures an operating temperature, the measured operating temperature preferably being a neax-maximum temperature; and

a flow control module mat controls a ratio, of the ammonia to me air;

wherein the ratio, of the ammonia to the air, is controlled according to the measured operating temperature.

20. The ammonia cracker of claim 13;

wherein the non-combusted ammonia is decomposed at a gas temperature mat is about 1500-1900 Celsius.

21. The ammonia cracker of claim 16, incorporated into a fuel cell system, the system comprising:

a fuel cell that receives a hydrogen-rich gas mixture and expels a hydrogen-depleted gas mixture; an engine that combusts the hydrogen-containing gas mixture with engine air; and

a mass flow control module that controls flows of the ammonia and the air into the ammonia cracker, and flow of engine air into the engine.

23. The ammonia cracker of claim 13, incorporated into an SCR exhaust cleanup system, the system comprising:

a source of gaseous ammonia, wherein the gaseous ammonia is the ammonia that is flowed into the ammonia cracker;

a mass flow control module; and

an exhaust line further comprising an SCR catalyst, onto which a reductant and a NOx-containing exhaust gas are flowed;

wherein the reductant substantially reduces the NOx to nitrogen on the SCR catalyst;

wherein the mass flow control module controls flows of the ammonia and the air into the ammonia cracker; and

wherein the reductant is a portion of the hydrogen-containing gas mixture.

24. The ammonia cracker of claim 13, including a starting apparatus that heats a portion of the heat exchanger in conjunction with a starting period, the starting apparatus comprising:

an ammonia cracker that provides hydrogen that is included in a starting mixture, and which accepts electricity as at least an initial or partial energy input; wherein the starting mixture is combusted within the ammonia cracker; and wherein the combusted starting mixture is flowed contactingly through a portion of the heat exchanger.

26. The ammonia cracker of claim 13, including a starting apparatus that heats a portion of the heat exchanger in conjunction with a starting period, the starting apparatus comprising:

an oxygen extractor that provides purified oxygen that is included in the starting mixture; wherein the starting mixture is combusted within the ammonia cracker; and wherein the combusted starting mixture is flowed contactingly through a portion of the heat exchanger.

28. The ammonia cracker of claim 13, including a vacuum jacket insulator for preventing heat loss from the ammonia cracker, the vacuum jacket comprising: a vacuum space that is bounded by an inner wall and an outer wall, the vacuum space preventing conductive and convective heat loss from the inner wall;

an inner wall that is constructed at least partially of ceramic, for example, aluminum oxide; and

an outer wall, the inner surface of which is constructed of, or coated with, a high-reflectivity material for preventing radiative heat loss from the inner wall.

29. The ammonia cracker of claim 28, wherein the high-reflectivity material comprises one or more of the following:

gold, silver, copper, or aluminum.

30. The ammonia cracker of claim 13, wherein the heat exchanger comprises:

a multichannel ceramic or semiconductor monolith.