Ammonia crackers. V. Hacker and K. Kordesch Volume 3, Part 2, pp

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1 Ammonia crackers V. Hacker and K. Kordesch Volume 3, Part 2, pp in Handbook of Fuel Cells Fundamentals, Technology and Applications (ISBN: ) Edited by Wolf Vielstich Arnold Lamm Hubert A. Gasteiger John Wiley & Sons, Ltd, Chichester, 2003

2 Chapter 10 Ammonia crackers V. Hacker and K. Kordesch Graz University of Technology, Graz, Austria 1 INTRODUCTION Anhydrous ammonia is a widely used commodity and is available worldwide in liquid form in low pressure tanks. Procedures for safe handling have been developed in every country. Facilities for storage and transport by barges, trucks and pipelines from producer to ultimate consumer are available throughout the world. Therefore liquid anhydrous ammonia is an excellent storage medium for hydrogen. Ammonia and its compounds are mainly used in agricultural as fertilizers, especially in the USA and China. Ammonia is also used in large amounts in the Ostwald process (Wilhelm Ostwald) for the synthesis of nitric acid, the Solvay process for the synthesis of sodium carbonate, the synthesis of numerous organic compounds used as dyes, drugs, in plastics and in various metallurgical processes. Important aspects of ammonia as a fuel include it s low toxicity, low flammability and its assured purity which results from the method of manufacturing. The major contaminant is water, which has no adverse effect on the fuel cell operation. From a technical point of view, ammonia offers, in comparison to hydrogen, significant advantages in cost and convenience as a fuel due to its higher density and its easier storage and distribution. The heating value of liquid ammonia is similar to that of methanol. Ammonia contains 1.7 times as much hydrogen as liquid hydrogen per volume. Ammonia dissociates into its constituents hydrogen and nitrogen according to the reaction: 2NH 3 N 2 + 3H 2, H = kj mol 1 (1) The cracking process is thermally efficient and simple. [1 5] A fine purification step after ammonia cracking is usually unnecessary and further co-reactants such as water are not required. The nitrogen generated can be released to the atmosphere without significant local environmental impact. Therefore, an important reason for using ammonia as fuel is that dissociation of ammonia offers by far the simplest approach to produce hydrogen on-site or on-board. It allows a single feed stream, simplicity of start-up and low overall equipment weight. [6 9] The properties of ammonia are shown in Table 1. 2 AMMONIA PRODUCTION At the end of the 19th century great efforts in research on ammonia production were a response to the social requirements of fixed nitrogen for nitrogen-containing fertilizer due to the fast increase in the world population. Many famous scientists were involved in the work to overcome this challenge, which was finally solved by Carl Bosch and Fritz Haber. The Haber Bosch process was one of the most important factors to increase crop yields two to three times over that which existed prior to the introduction of fertilizer so perhaps as much as half of the biomass in our foods today is derived from the use of anhydrous ammonia which is produced directly from natural gas. [10] There are literally dozens of chemical plants that produce anhydrous ammonia. The worldwide ammonia production capacity in 1998 was approximately 151 Mt. The typical Handbook of Fuel Cells Fundamentals, Technology and Applications, Edited by Wolf Vielstich, Hubert A. Gasteiger, Arnold Lamm. Volume 3: Fuel Cell Technology and Applications John Wiley & Sons, Ltd. ISBN:

3 122 Part 2: Hydrogen storage and hydrogen generation Table 1. Properties of ammonia. a Molecular mass Molecular volume (at 0 C, kpa) l mol 1 Gas constant (R) kpa m 3 kg 1 K 1 Liquid density (at 0 C, kpa) g cm 3 Gas density (at 0 C, kpa) g l 1 Liquid density (at C, kpa) g cm 3 Gas density (at C, kpa) g l 1 Critical pressure Mpa Critical temperature C Critical density g cm 3 Critical volume cm 3 g 1 Critical compressibility Critical thermal conductivity kj K 1 h 1 m 1 Critical viscosity mpa s Melting point (triple point) C Heat of fusion (at kpa) kj kg 1 Vapor pressure (triple point) kpa Boiling point (at kpa) C Heat of vaporization (at kpa) 1370 kj kg 1 Standard enthalpy of formation (gas at 25 C) kj mol 1 Standard entropy (gas at 25 C, kpa) J mol 1 K 1 Free enthalpy of formation (gas at 25 C, kpa) kj mol 1 Net heating value (LHV) b kj g 1 Gross heating value (HHV) b kj g 1 Electrical conductivity (at 35 C), very pure cm 1 Electrical conductivity (at 35 C), commercial cm 1 Ignition temperature according to DIN C Explosive limits: NH 3 /O 2 mixture at 20 C, kpa 5 79 vol% NH 3 NH 3 /air mixture at 0 C, kpa vol% NH 3 NH 3 /air mixture at 100 C, kpa vol% NH 3 a Reproduced from Schwartz (2002) [10] by permission of The American Physical Society. b LHV, lower heating value; HHV, higher heating value. ammonia-producing plant first converts natural gas (i.e., methane) or liquefied petroleum gases, such as propane and butane, or petroleum naphtha into gaseous hydrogen. The driving force for the production of ammonia is nitrogen fixation. When ammonia is considered as a fuel, the reason for ammonia production changes from nitrogen to hydrogen fixation but the method of production stays the same and is shown in Figure 1. Figure 2 provides an overview of the raw material sources (apart from water and air) for the world ammonia capacity. New plants are based almost exclusively on natural gas and naphtha. The capital cost and the relative energy requirements for a plant depends on the raw material employed. The relative investment for naphtha is 1.15 times, for oil 1.5 times and for coal 2.5 times that of natural gas. Natural gas has also the lowest specific energy requirement. The significant changes in energy prices from 1973 onwards were a strong incentive to obtain better energy efficiency. The overall energy consumption was reduced from around 40 GJ t 1 NH 3 to 29 GJ t 1 NH 3 (the lower heating value (LHV) of ammonia is GJ t 1 )which equals an efficiency of 64% (see Figure 3). 3 AMMONIA DISSOCIATION System studies demonstrate that hydrogen derived from anhydrous liquid ammonia, via a dissociator and following a hydrogen purifier, offers an alternative to conventional methods of obtaining pure hydrogen for small-scale use. Hydrogen from ammonia dissociation would be the preferred option for the smallest plant sizes where it is projected to be competitive with hydrogen via natural gas of comparable purity. [13] The dissociation rate depends on temperature, pressure and the catalysts being used. The theoretical limitation for the lowest working temperature possible is given by the chemical equilibrium for the dissociation reaction. Figure 4 shows that a nearly complete conversion from ammonia to hydrogen and nitrogen at higher temperatures and atmospheric pressure is possible. For an almost

4 Ammonia crackers 123 Natural gas, naphtha Fuel oil, coal Purification by (hydro)desulfurization H 2 Hydrotreating Sulfur Gas generation Steam Air Primary reforming Secondary reforming Steam Primary reforming O 2 Partial O 2 oxidation Steam Steam Purification Partial oxidation H 2 S and COS Shift conversion HT shift conversion LT shift conversion Off-gas (fuel) HT shift conversion Shift conversion (CoMo cat.) HT shift conversion Final purification Off-gas (fuel) Methanation Molecular sieves Pressure swing adsorption N 2 and H 2 S Liquid nitrogen scrubbing Sulfur N 2 Liquid nitrogen scrubbing Sulfur recovery N 2 Purifier Ammonia synthesis Ammonia Figure 1. Options for generating and purifying ammonia synthesis gas. (Reproduced from Appl (1999) [11] by permission of Wiley VCH.) complete decomposition of ammonia this temperature is approximately 430 C at atmospheric pressure. The ammonia synthesis catalyst problem has been more intensively studied than the catalysis of any other industrial reaction. In principle, metals or metal alloys are suitable as ammonia catalysts, especially those from the transitionmetal group. Catalyst systems currently used for ammonia synthesis have emerged from extensive research efforts to select catalysts that are active for the forward reaction of nitrogen hydrogenation, least active for ammonia

5 124 Part 2: Hydrogen storage and hydrogen generation World ammonia production capacity LHV (EJ year 1 ) 3 Other petroleum products Naphtha Natural gas 2.5 Coke oven gas and coal Figure 2. Feedstock distribution of world ammonia production capacity, based on LHV of ammonia. Compare worldwide electricity production: 50.2 EJ a 1 in decomposition and reasonably resistant to poisoning. A typical chemical analysis of the catalyst precursor yields following values: Fe 3 O wt%; K 2 O 0.8 wt%; Al 2 O wt%; CaO 1.7 wt%, MgO 0.5 wt%; SiO wt%. [14] The influence and kinetic data of materials like porcelain or silica glass, metals like iron, tungsten, molybdenum, nickel, etc., especially noble metals and metal oxides, have been investigated for the dissociation of ammonia. The temperatures for a sufficient catalytic acceleration of ammonia cracking in most of the investigations is between 700 and 1100 C, and therefore too high for a mobile ammonia dissociator. Studies with commercially available simple Price (US$ GJ 1 ) US ammonia US natural gas Figure 3. US natural gas and ammonia price trends. Calculated with an average heat content of natural gas of 38 MJ m 3. (Reproduced from Ref. [12].) catalyst materials such as nickel oxide or iron oxide on aluminum and the influence of the addition of noble metals have been carried out with lab scale reactors (see Figure 5). [1] Catalysts based on nickel on aluminum oxide in the form of small pellets with a diameter of approximately 5 mm at a length of 3 mm have been investigated. The catalysts were tested with the addition of different amounts of platinum and with platinum/lanthanum oxide combinations. Parts per volume bar 5 bar 1 bar NH 3 1 bar H 2 5 bar 15 bar N 2 1 bar 5 bar bar Temperature (K) Figure 4. Chemical equilibrium: 2NH 3 N 2 + 3H 2.

6 Ammonia crackers 125 Conversion rate 1 Ni 0.9 Ni + Pt 0.8 Ni + Pd Ni + La 2 O 3 Ni + Ru Temperature (K) Figure 5. Conversion rate as function of reactor temperature using different catalysts. The catalysts were prepared with the addition of an aqueous solution of dihydrogen hexachloroplatinate, the addition of lanthanum nitrate to the activated catalyst (nickel on aluminum oxide), the addition of an aqueous solution of dihydrogen hexachloroplatinate, and then the addition of ruthenium chloride to the inactivated catalyst (nickel oxide on aluminum oxide). Ammonia cracking devices for small transportable applications have been investigated with different system designs. [15 17] Portable ammonia dissociators with small residual amounts in the ppm-range of ammonia for low power applications up to a 200 W electrical output at the added fuel cell are described in a volume range of less than 6 l. Studies with commercially available catalyst materials like nickel oxide or iron oxide on aluminum and the influence of the addition of noble metals were done with lab scale reactors. [18] Dissociation rates of ammonia on noble metal wires were examined and compared at temperatures from 500 to 1900 K. [19] New catalytic investigations have obtained better cracking efficiencies with new catalyst materials based on Zr, Mn, Fe and Al/alloy catalysts. [20 22] At Graz University of Technology in co-operation with Apollo Energy Systems (USA), catalysts have been investigated with and without the addition of noble metals like platinum, rhodium, palladium, lanthanum oxide and ruthenium, including combinations of these additives (Figure 5). The catalyst test reformer unit was a tube (inner diameter = 12 mm, heated length = 130 mm). The unit was insulated and heated electrically. The temperature was measured inside the catalyst with thermocouples. Ammonia was preheated to obtain a constant temperature over the full catalyst length. The temperature of the reactor is shown in Figure 6 as a function of ammonia flow at a conversion rate higher than 99.99%. The pressure drop in the Temperature ( C) Ammonia flow (l h 1 ) Figure 6. Reactor temperature as a function of ammonia flow (conversion > 99.99%, standard liters per hour) with a Ni/Ru catalyst and tube type cracker. reactor was dependant on the ammonia flow being up to 70 hpa. Based on the results of the catalyst research, the design and development of a lab-scale ammonia cracking reactor was done (Figure 7). In laboratory studies the ammonia used is preheated to about the catalyst temperature. Preheating has been found to improve the efficiency of the cracking reaction, and therefore provides higher flow rates. In further applications this preheating will be done by heat exchangers to use the heat released from the cracked hydrogen/nitrogen mixture. In combination with fuel cells the hydrogen containing anode off-gas will be used as fuel for the burner to heat the cracker. The plate type cracker was filled with 320 ml of catalyst, heated by six burner tubes and isolated with 8 cm rock wool. The gases were not preheated. Figure 8 shows the temperature distribution by four vertically positioned thermocouples at an ammonia flow of 300 l h 1. The temperature at the ammonia inlet is reduced from 660 to 480 C, whereas the temperature in the middle zone remains almost constant. 4 CONCLUSIONS By using materials which are more or less commercially available, it is possible to construct a hydrogen generating plant using ammonia as the fuel, thus eliminating the problems of hydrogen storage and transport. At the high cracking efficiency of the developed catalyst, the ammonia cracker could be constructed at modest mass and volume. The system using ammonia feedstock is technically simple, no recycle loops are required. With alkaline fuel cells especially high efficiency systems are possible as small amounts of ammonia in the cracked gas are permitted and can be burned in an anode off-gas heated cracker device.

7 126 Part 2: Hydrogen storage and hydrogen generation Air inlet Anode off gas inlet Ammonia inlet Burner Catalyst Thermocouple H 3 H 2 H 1 (a) Hydrogen/nitrogen outlet (b) Figure 7. Anode off-gas heated tube type cracker prototype (a) with a plate type ammonia cracker (b) heated with six burner tubes. Graz University of Technology with the courtesy of Apollo Energy Systems, USA. 700 Temperature ( C) Th1 Th3 Th5 Th7 Th9 Th10 Th6 Th2 Th9 Th7 Th5 Th3 Th1 Th8 Th :00 330:00 340:00 (a) (min:s) (b) Figure 8. The temperature distribution during start-up period inside plate type cracker (a) and the position of thermocouples inside cracker (b). REFERENCES 1. G. Faleschini, V. Hacker, M. Muhr, K. Kordesch and R. Aronsson, Ammonia for High Density Hydrogen Storage, presented at the Fuel Cell Seminar 2000, Portland, OR, Oct. 30 Nov. 2, pp (2000). 2. V. Hacker, G. Faleschini, K. Kordesch, R. Aronsson et al., Alkaline Fuel Cells for Electric Vehicles, presented at the 3rd International Fuel Cell Conference, Nagoya, Japan, Nov. 30 Dec. 3 (1999). 3. K. Kordesch, Brennstoffbatterien, Springer Verlag, Wien, p. 64 (1984). 4. M. F. Collins, Life Test of a 200 W Ammonia/Air Fuel Cell System, presented at the 25th Power Sources Symposium, pp (1972).

8 Ammonia crackers O. J. Adlhart and P. Terry, Ammonia/Air Fuel Cell System, presented at the Intersociety Energy Conversion Engineering Conference, Washington DC, September (1969). 6. A. J. Appleby and F. R. Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold, New York, p. 238 (1989). 7. R. Metkemeijer and P. Achard, Int. J. Hydrogen Energy, 19(6), 535 (1994). 8. I. W. Kaye and D. P. Bloomfield, Portable Ammonia Powered Fuel Cell, presented at the Conference of Power Sources, Cherry Hill, pp (1998). 9. R. Metkemeijer and P. Achard, J. Power Sources, 271 (1994). 10. R. D. Schwartz, Population, Fossil Fuel, and Food, Newsletter, The American Physical Society, Vol. 31, No. 1, January (2002). 11. M. Appl, Ammonia, Principles and Industrial Practice, Wiley VCH, New York, p. 66 (1999). 12. Compare (Gordon Johnson); Gas prices from US DOE Website: NH 3 prices from March (2002). 13. G. Strickland, Int. J. Hydrogen Energy, 9(9), 759 (1984). 14. J. R. Jenning, Catalytic Ammonia Synthesis, Plenum Press, New York, p. 20 (1991). 15. H. H. Geissler, Compact H 2 Generators for Fuel Cells, presented at the 17th Power Source Conference, pp (1993). 16. DiMartino, Production of Hydrogen from Ammonia, US patent 4,704,267, Nov. 3 (1987). 17. R. Dong, Y. Dong and Z. Xu, Cracking Process for Producing Hydrogen and Special Equipment for Producing Hydrogen, CN 1,134,912, Jun. 11 (1996). 18. P. N. Ross, Jr, Characteristics of an NH 3 /Air Fuel Cell System for Vehicular Applications, in Proc. 16th Intersoc. Eng. Conf., pp (1981). 19. G. Papapolymerou and V. Bontozoglou, J. Mol. Catal. A: Chem., 120, 165 (1997). 20. G. Boffito, Better Materials for Cracking Ammonia, US patent 5,976,723, Nov. 2 (1999). 21. T. Shikada, M. Asanuma and T. Ikariya, Method of Decomposing Ammonia using a Ruthenium Catalyst, US patent 5,055,282, Oct. 8 (1991). 22. E. Rosenblatt and J. Cohn, Dissociation of Ammonia, US patent 2,601,221, Jun. 17 (1952).