FI3878806T3 - Method for the preparation of hydrogen or hydrogen-containing fuels by catalytic cracking of ammonia - Google Patents

Claims (9)

1 20000101.4 METHOD FOR THE PREPARATION OF HYDROGEN OR HYDROGEN-CONTAINING FUELS BY CATALYTIC CRACKING OF AMMONIA Description

Field of the invention

The present invention relates to a method for the conversion of ammonia by partial oxidation and cracking into a combustible, hydrogen-containing gas mixture with the main components hydrogen, water vapor and nitrogen.

This converts ammonia, which is hardly usable as a fuel in practice, into an easily combustible gas.

Background to the invention

Due to its energy density or calorific value, ammonia is suitable in principle as a fuel and offers the advantages of being CO2-neutral, providing the hydrogen required for synthesis is produced by electrolysis based on solar energy, wind energy or hydroelectric power.

This means that the ammonia fuel does not require a carbon carrier at all.

The feedstocks nitrogen and green electricity are available worldwide in unlimited quantities.

Furthermore, ammonia can be stored in liquid form in pressure tanks (8 bar) or at -33°C without pressure and can thus be transported cost-effectively over long distances.

The main disadvantage of ammonia for use as an energy source is its poor combustibility, which makes it difficult to use it directly as a fuel in a thermal engine.

To avoid this disadvantage, ammonia can be converted back into hydrogen or a hydrogen-

containing gas mixture before combustion.

The ammonia is cracked catalytically via a separate cracking reactor.

To use the fuel obtained in this way, it is not necessary, for example, to produce pure hydrogen by cracking in order to operate a thermal engine or a burner; it is sufficient if a combustible gas mixture with a sufficiently high hydrogen content is produced.

Additional components such as nitrogen, water vapor or residues of unreacted ammonia do not interfere.

In further use, additional ammonia can be admixed to the hydrogen-containing gas mixture to increase energy.

Summary of the invention

2 20000101.4

The cracking reaction of ammonia to hydrogen and nitrogen is endothermic.

It usually takes place with the support of a cracking catalyst at temperatures above 400°C, usually above 600°C.

Due to the endothermic nature of the reaction, an external energy supply is required to maintain the reaction.

This energy can be generated, for example, by burning some of the hydrogen produced by the ammonia cracking.

Because of the large amounts of energy released during hydrogen combustion compared to ammonia cracking, only a small portion of the hydrogen produced by cracking is required.

The hydrogen not required for this purpose is removed from the method and put to other uses.

The nitrogen that is also formed passes through the process as an inert gas.

Various methods for cracking ammonia and obtaining hydrogen according to the above. principle have previously been described (see for example US2003/0232224, US 2005/0037244 or US 2013/0266506, and EP 3028990). However, these methods have the disadvantage that the energy reguired for ammonia cracking must be supplied to the cracking catalyst via heat exchanger surfaces.

Due to the already high temperaturesat the catalyst, even higher temperatures are reguired with respect to the heat source, which places high demands on the heat exchanger and ultimately leads to an increase in the size of the cracking reactor and higher material costs.

In the method of US 2013/0266506, heat exchange takes place with the combustion exhaust gas of the ammonia combustion engine.

In EP 3059206, the energy reguired for the cracking of the ammonia is provided by admixing air or oxygen with partial oxidation of the ammonia.

The energy reguired for the cracking is thereby released within the gas mixture itself and it is possible to do without heat exchanger surfaces for the energy supply to the catalyst layer.

The disadvantage is the lower hydrogen content in the fuel gas produced, but this can be tolerated if the gas mixture remains combustible in air.

In EP 3059206, a circulation reactor is reguired.

A disadvantage here is the expense of the circulation pump, especially because the pump has to be operated at the high temperature of the cracking catalyst.

Furthermore, it requires the use of moving parts, which involves deterioration in terms of operational safety.

US 2012/040261 discloses a method for the production of hydrogen from ammonia.

A mixture of ammonia and air is fed into a reactor.

The reactor contains an ammonia oxidation catalyst followed by an ammonia cracking catalyst downstream.

A

3 20000101.4 part of the ammonia is oxidized at the oxidation catalyst.

The remaining ammonia is decomposed at the cracking catalyst.

In the present case, another method for ammonia cracking is described (see Fig. 1). Ammonia and air are fed into a catalytic reactor as feedstocks.

The air volume is limited to the extent necessary to provide energy for ammonia cracking by oxidation.

This oxidizes part of the ammonia to nitrogen and water, while the larger part is cracked into hydrogen and nitrogen.

The ammonia conversion takes place on a catalyst consisting of a mixture of two different catalysts.

One part consists of a conventional oxidation catalyst {preferably platinum or palladium on aluminum oxide or other carrier materials), the other part of the mixture consists of a conventional ammonia cracking catalyst (e.g. nickel or iron on suitable carrier materials such as magnesium oxide or aluminum oxide). If necessary, the active components can also be applied together on a suitable carrier material.

In order to obtain the highest possible hydrogen content in the product mixture,

hereinafter also referred to as cracked gas, the air flow is limited as far as possible.

However, a minimum supply of air is required to maintain the cracking reaction.

The required air supply thus results from the fact that a certain exotherm must still be maintained for the overall reaction in order to compensate for the heat losses in the cracking reactor.

There is no need for an external supply of further energy, e.g. by an additional electric heater.

Based on the simplifying assumption that air consists of 20% oxygen and 80% nitrogen, the summary conversion in the cracking reactor is approximately according to the following equation:

8NH3+02+4N2=2H20+8N2+10H2

AHr = 2 (-242 kJ) - 8 (-46 kJ) = -116 kJ

Under these conditions, the reaction remains weakly exothermic when considered at standard conditions, and a gas mixture containing about 50% hydrogen can be obtained.

At the reaction temperature of, say, 600°C, the enthalpy of reaction continues to decrease because of the differences in the specific heats of the starting and final products, but it remains exothermic.

To reduce heat losses, the input stream to the reactor is preheated with the product gas from the reactor via a heat exchanger.

For the example assumed in Fig. 1, the product

4 20000101.4 gas temperature is assumed to be 700°C at the catalyst outlet and the preheated inlet stream temperature is assumed to be 500°C.

The temperature of the product gas decreases to approx. 200°C until it leaves the heat exchanger.

The temperature level of this heat exchanger is significantly below the temperature level with an indirectly heated catalyst bed as described in EP 3028990, where temperatures above 1000°C are reached on the warmer side of the heat exchanger, resulting in correspondingly higher material usage.

Another advantage of the described method compared to that of EP 3028990 is the significantly simpler design of the reactor structure including the heat exchanger.

With an amply-sized design of the heat exchanger according to the present application, the temperature difference can be reduced even further.

In order to bridge the losses caused by this temperature difference and compensate for the insulation losses of the reactor, the proportion of air is to be set somewhat higher than in the above- mentioned equation.

This oxidizes slightly more ammonia and slightly reduces the hydrogen content.

However, the above-mentioned equation can be used to explain the principle relationships.

Temperature control of the reactor is carried out by readjusting the mixing ratio between ammonia and air.

The process is controlled in such a way that the temperature in the cracking reactor does not fall below a minimum value.

This temperature depends on the type of cracking catalyst and is high enough for the catalyst to work reliably and at a high reaction rate.

Usual temperatures for nickel or iron catalysts, for example, are around 700°C, but considerably lower temperatures may be sufficient if ruthenium is used.

The above-mentioned total reaction is separated into two partial reactions, namely oxidation of part of the ammonia as long as oxygen is available and cracking of the remaining ammonia.

The two stages can be described by the following reaction equations.

The first reaction is exothermic and the subsequent reaction endothermic.

8 NH3 + 02 +4 N2 => 2 H20 + 14/3 N2 + 20/3 NH3 2 H20 + 14/3 N2 + 20/3 NH3 => 2 H20 + 8 N2 + 10 H2

The oxidation proceeds at a much higher reaction rate than the ammonia cracking, so in designing the reactor one has to assume the extreme case that the first reaction is already completed before the second reaction starts.

The oxidation reaction causes an adiabatic temperature increase of the gas mixture of approx. 600°C.

At steady-state

20000101.4 operation and an inlet temperature at the reactor of 600°C, for example, a temperature rise to 1200°C can thus be expected, if heat losses are neglected. In the subsequent reaction zone, the temperature would then drop from 1200 to 600°C as a result of the ammonia cracking. 5 In order to avoid damage to the cracking catalyst, the catalyst can be designed for the high temperatures. When using aluminum oxide or magnesium oxide carriers, high thermal stability is provided. Another possibility is to divide the catalyst bed into several sectors and not to admix the air to the ammonia from the beginning but only before the respective sectors. A corresponding setup is shown in Fig.

2. In this way, the adiabatic temperature increases are mitigated and the temperature gradients in the catalyst bed are reduced. Because of the high reaction rate of the oxidation reaction, it is generally sufficient to admix a small amount of less than 10% of oxidation catalyst to the total amount of catalyst, thereby reducing the cost. Further cost reductions are possible if the oxidation catalyst is concentrated in the initial part of the bed. Although the entire cracking process is energy-autonomous, the reactor has to be preheated from the outside by means of an additional electric heater in order to start it up. Preheating must be carried out at least up to a temperature at which the oxidation reaction starts. This is around 200 - 300°C. If the heat exchanger is designed economically, however, it must be at least large enough to preheat the feedstocks to the ignition temperature for the oxidation reaction. In order to achieve a high hydrogen concentration and a high level of efficiency, the heat exchanger must be designed to be as large as possible. By means of the described method, a cracked gas with a hydrogen content of 40- 50% can be obtained, which can be used directly as fuel in a thermal engine. The calorific value is sufficient for this purpose, the other components water vapor and nitrogen do not interfere. If necessary, water vapor can be separated to increase the calorific value. Unreacted residual ammonia is also tolerated by the thermal engine. On the other hand, there is even the possibility of admixing further ammonia to the fuel produced in order to increase the total firing thermal power supplied to the thermal engine. If required, the hydrogen content of the cracked gas can be additionally increased by using pure oxygen instead of air as the oxidant. The hydrogen content can thus be increased to approx. 60% and, with additional separation of the water vapor formed, to

6 20000101.4 approx. 70%. Under these conditions, it is possible to achieve the maximum possible value of 75% as is approximately obtained when pure ammonia is cracked with external energy input.

In order to increase the hydrogen content in the cracked gas, it is also possible to use enriched oxygen, which can be obtained inexpensively from air separation plants.

The production rate of hydrogen can also be increased for a given reactor size by increasing the total pressure in the reactor.

The pivotal factor for hydrogen production is the residence time of the ammonia in the catalyst layer.

The residence time is prolonged with increasing pressure, resulting in higher conversion with increasing pressure compared to reactor operation at atmospheric pressure.

If the cracked gas is to be used in a thermal engine, it is advisable to cool the gas as far as possible via the heat exchanger described.

However, the cracked gas can also be used in industrial combustion processes, where it is burned with air in an open flame.

Examples include the burning of bricks or cement production.

In these cases, the cracked gas can be supplied to the burner directly from the catalyst outlet, e.g. at the temperature of 600 - 700°C.

This obviates the need for the heat exchanger.

In this case, however, the ammonia-air mixture must be preheated at the inlet to the catalyst bed at least to the point where ignition occurs or the oxidation reaction starts.

In order to do entirely without a separate heat exchanger here, the heat required for preheating can be transferred by heat conduction through plates or rods installed longitudinally to the direction of flow of the gas.

Starting in the front part of the catalyst bed with the oxidation zone, the rods project into the incoming fresh gas against the flow.

The feedstocks are preheated by means of heat transfer from the rods to the fresh gas.

In the case of reactor diameters that are not too large, this function is already fulfilled by the reactor wall, via which heat is transported in the direction of the fresh gas supply.

A corresponding setup is shown in Fig.

3. Specifically, a stainless-steel tube with an inner diameter of 25 mm and a wall thickness of 2.5 mm was used here.

The space in front of the catalyst bed was filled with metallic packing to improve heat transfer from the metal tube to the fresh gas.

The length of the preheating section was 10 cm.

With the setup shown, a volume flow of ammonia was cracked in such a way that a heat output of 3 kW was released when the cracked gas was burned.

A catalyst bulk of 150 ml content was used.

For larger outputs, for example, a large number of such tubes can be connected in parallel.

Furthermore, heat conduction against the gas flow can also be carried out via

7 20000101.4 suitable rods or sheets installed in the catalyst bed. In the case of the use of copper rods, which lend themselves because of their good heat resistance, these must be covered with an appropriate protective layer to prevent corrosion by ammonia or enclosed with stainless steel tubes. In the method according to the invention, a conversion of ammonia in the range of 65-100% is obtained. Preferably, a conversion of ammonia in the range of 70-100% is obtained, more preferably 75-100% and most preferably 80-100%. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a method flow diagram for the intended process for ammonia cracking in a reactor with oxidation and cracking catalyst incl. the heat exchanger.

Fig. 2 shows the flow diagram according to Fig. 1, but in which the catalyst bed is divided into separate trays and the air is admixed with the gas stream in a divided manner upstream of each tray.

Fig. 3 shows an exemplary embodiment in the form of a tubular reactor in which a separate heat exchanger is dispensed with and the preheating of the feedstocks is carried out by heat baffles, here in the form of the reactor wall.