The present invention relates to a method for the prepara tion of a gaseous fuel based on ammonia and hydrogen.

More particularly, the gaseous fuel is by the invention prepared by cracking a part of an ammonia feed in a solid oxide fuel cell to hydrogen and nitrogen when operating the fuel cell at a reduced fuel utilization and mixing the hy- drogen and nitrogen containing off-gas from the fuel cell with the rest of the ammonia feed to the gases fuel. In that way a useful fuel for gas engines is prepared.

There is presently a rapidly growing interest in using re- newable ammonia as a fuel to produce power in combustion engines, gas turbines or fuel cells.

Combustion engines cannot operate on pure ammonia due to its poor combustion characteristics but require a cofeed of another fuel with better properties.

Research has therefore focused on co-combusting hydrogen together with ammonia. The hydrogen can conveniently be produced by cracking part of the ammonia in a dedicated cracking unit upstream the combustion engine. Such cracking units are, however, expensive and use up to 13 % of the lower heating value in the processed ammonia.

Solid Oxide Fuel Cell (SOFC) can also be used to convert ammonia into heat and power in a stand-alone plant with very high efficiency. SOFC plants are unfortunately much more capital expensive compared to gas engines. SOFC stacks do, however, operate as very efficient ammonia crackers utilizing waste heat from the fuel cell operation to drive the ammonia cracking while at the same time producing power. As already mentioned hereinbefore, combustion engines can not operate on pure ammonia. This can be solved by cracking part of the ammonia to hydrogen and nitrogen in a dedicated ammonia cracking unit. This requires, however a substantial investment and entails a loss in efficiency. Alternatively, a different fuel like f.inst. DME can be used as a combus tion property enhancer, but this will require a dual fuel system including storage facilities and the alternative fuel will also most likely be more expensive than ammonia if produced from renewable sources

Power generation using ammonia as fuel for a SOFC is well known and it has also been proposed to use a gas engine to convert the unreacted anode off gas. The focus has, how ever, always been on maximizing the electric output and ef- ficiency from the SOFC.

The present invention is based on to use the capability of SOFCs to crack ammonia into hydrogen and nitrogen in con junction with a gas engine so that enough of the ammonia is converted in the SOFC, when keeping the fuel utilization of the SOFC at a low range. Thereby a minimum amount of hydro gen in the fuel mix to the gas engine is provided by non- converted off gas from the SOFC plant. Thus, the invention provides a method for the preparation of a gaseous fuel comprising the steps of:

(a) providing a gaseous stream of ammonia; (b) splitting the gaseous stream of ammonia into a first and second substream;

(c) introducing the first substream into a solid oxide fuel cell; (d) cracking the gaseous ammonia in the first substream to hydrogen and nitrogen by means of heat created by electro chemical reactions performed in the solid oxide fuel cell; (e) withdrawing an off-gas from the solid oxide fuel cell containing the hydrogen and nitrogen formed in step (d); (f) mixing the off-gas with the second substream of the gaseous ammonia to provide the gaseous fuel, wherein the solid oxide fuel cell is operated at a fuel utilization of between 35 and 70%.

When using ammonia as a fuel for SOFC, two reactions are occurring in the anode chamber:

NH ₃ = 0.5 N ₂ +1.5 H _{2 DH} NH3 Crack ₌ 247.87 kJ/mol) (1)

1.5 H ₂ + 0.75 0 ₂ = 1.5 H ₂ 0 DH ^H20K = 56.69 kJ/mol) (2)

The fuel utilization i.e. the amount of ammonia and hydro gen converted in the SOFC is defined as:

1.5 mole NH^ ^ut + mole H

1.5 mole NH + mole H

As an example, minimum fuel utilizations are obtained at operating volts of 0.7, 0.8 and 0.9 V to be 33.5 %, 40.4 % and 50.9 % respectively

This invention provides the following advantages: • The investment in a dedicated cracking unit is avoided

• There is no parasitic loss of ammonia to drive the crack ing reaction. On the contrary power is produced while con verting the ammonia to hydrogen and nitrogen · The SOFC unit can operate at low fuel utilization (ammo nia and hydrogen conversion) which makes it cheaper in in vestment in SOFC stacks

• Due to the low fuel utilization in the SOFC less waste heat must be removed by excess air flow on the cathode side, which translates to savings on the air compressor and cathode feed/effluent exchange.

In an embodiment, the gaseous stream of ammonia is provided from a pressurized and liquid ammonia source, which is heated and expanded in an expander to form the gaseous stream of ammonia.

In an embodiment a part of the off-gas from the solid oxide fuel cell is recycled to the anode of the fuel cell.

When expanding pressurized ammonia in the expander, elec trical energy can be generated in the expander. The elec trical energy can be utilized for driving a number of de vices in the method according to the invention.

Thus, in an embodiment electrical energy is generated in the expander.

In an embodiment the electrical energy is utilized for op- erating a blower for blowing air into the cathode chamber of the solid oxide fuel cell. In an embodiment the electrical energy is utilized for op erating a compressor for recycling a part of the off-gas from the solid oxide fuel cell to the anode chamber of the fuel cell.

The present invention can advantageously be used for the preparation of gas fuel for engines installed on maritime vessels and for revamping gas fuel systems for existing en gines. Additionally, the invention is useful for operating combined heat and power plants.

The invention is further described in more detail by refer ence to Fig.l, which is a flow sheet showing a method ac cording to a specific embodiment of the invention.

The feedstock is pressurized ammonia either taken from a pressurized tank or pumped in the form of liquid, refriger ated ammonia. The ammonia is vaporized in heat exchanger El by sensible heat in exhaust gas from a gas engine and is further heated by the same exhaust gas in heat exchanger E2 before it is expanded to near atmospheric pressure in a turboexpander .

This expander is used to generate electric power and uses the exergy invested in condensing the ammonia in the ammo nia plant. Part of the ammonia is subsequently sent to the gas engine via an optional cooler, E9. The rest of the am monia is then mixed with gas recirculated from the anode in the SOFC and then heated to the operating temperature of the SOFC in the feed/effluent heat exchanger E3. The ammo nia and recirculated anode off gas is converted to elec- tricity and heat in the anode chamber of the SOFC. The am monia will be effectively cracked in the anode chamber driven by the waste heat from oxidation the resulting hy drogen.

The conversion of the hydrogen in the SOFC is deliberately kept low as the main purpose is supply the hydrogen to the gas engine. After exiting the SOFC the anode off gas is cooled down in the anode feed/effluent exchanger E3 and then split into two streams, 2081 and 1070. Then minor stream, 2081 is fur ther cooled in the anode recycle feed/effluent heat ex changer, E200, and then in the cooler, E100, before enter- ing the anode recycle gas blower.

The anode recycle stream is provided to supply a hydrogen rich gas back to E3 and the SOFC in order to avoid material degradation due to nitriding by pure ammonia according to:

3 Ni + NH ₃ = Ni ₃ N + 1.5 ¾

The rest of the anode off gas, stream 1070, is cooled in exchanger E7 and the water formed by oxidation of hydrogen in the SOFC is separated out in a separator before being mixed with the ammonia bypassing the SOFC in stream 1016.

A relevant amount of air is added to the fuel stream and the mixture is combusted in a gas engine providing (elec tric) power and a hot exhaust gas. The exhaust gas is used to preheat the ammonia in heat exchanger E2 and is then further cooled in cooler E5 and finally used to evaporate the ammonia in El.

In the cathode chamber of the SOFC, air is provided by an air blower and sent to the cathode feed/effluent heat ex changer, E4, which brings the temperature up to the operat ing temperature of the SOFC. The depleted air leaving the SOFC cathode is used for preheat in E4 and finally cooled down in the cooler E6.

The heat released in E, E6, E100 and E7 can be used as dis trict heating.

The present invention focuses on minimizing the investment of the combined SOFC + Gas Engine system, as the present capital investment for SOFC are very high. The operating regime is thus changed to have a low fuel utilization in the SOFC - enough to provide a minimum of hydrogen to the gas engine and at the same time provide enough temperature increase across the SOFC to enable operation of the heat exchanger E3 and E4 without unduly large heat exchange sur faces. In fact, the low fuel utilization provides other benefits because a lower amount of waste heat that have to be removed by the air stream which reduces the size and in- vestment in the air blower and exchanger E4 compared with

SOFC with high fuel utilization. Furthermore, the parasitic power used by the air blower is reduced. Compared to a sys tem with an ammonia cracker providing the hydrogen for the gas engine the investment in the cracker is avoided as well as the parasitic use of fuel to drive the reaction. On the contrary electric power generated with high efficiency is provided by the SOFC in parallel with its function as ammo nia cracker.

Examples

With reference to Fig. 1 a detailed calculation has been performed to illustrate how to perform the invention and to compare the result with operation of a SOFC + Gas Engine plant designed to maximize the electrical efficiency by having a high fuel utilization in the SOFC. The results are shown in Table 1 for an inlet flow of ammonia corresponding to a lower heating value of 10 MW and 15 vol % in ammonia in the feed to the gas engine.

Table 1 The relative number of stacks have been roughly estimated by using the following relationship:

Stack area = (Average Nernst potential - Operating Volt age)/Area specific Resistance Due to the average higher operating temperature of the ded icated SOFC this is somewhat overestimating the stack area for this option but on the other hand the lower operating temperature as per the invention will prolong the lifetime of the stacks.

The SOFC + gas engine as per the invention, obviously has a lower electrical efficiency but it should recalled that in- vestment for a gas engine is probably around 500 $/kW but at present it is probably 4 - 10 times higher for the SOFC system and the stack replacements are also expensive.