Method and system for producing a fuel for driving a gas turbine

Ammonia is a potential carrier of hydrogen fuel. However, it is not suitable to directly combust in a typical gas turbine burner, mainly due to low flammability, but also due to very high NOx emissions due to the fuel-bound nitrogen.

As of today, ammonia has been used in research kW-scale gas turbines. There is also prior art, WO 2017/160154, in the area of industrial gas turbines operating with ammonia. In detail, WO 2017/160154 discloses a process for generating power using a gas turbine, comprising the steps of: (i) vaporising and pre-heating liquid ammonia to produce pre-heated ammonia gas; (ii) introducing the pre-heated ammonia gas into an am- monia-ammonia cracking device suitable for converting ammonia gas into a mixture of hydrogen and nitrogen; (iii) converting the pre-heated ammonia gas into a mixture of hydrogen and nitrogen in the device; (iv) cooling the mixture of hydrogen and nitrogen to give a cooled hydrogen and nitrogen mixture; (v) introducing the cooled hydrogen and nitrogen mixture into a gas turbine; and (vi) combusting the cooled hydrogen and nitrogen mixture in the gas turbine to generate power. In the process described in WO 2017/160154, in the first place the liquid ammonia is preheated and vaporized to form gaseous ammonia. The gaseous ammonia is then fed to the cracker unit comprising a catalyst. In this case the heat exchanging device and the ammonia cracking device are two different units. Yet, the cracking reaction is endothermic and heat supply is necessary also during cracking of ammonia. In WO 2017/160154, the heat required for the cracking reaction is derived from redirecting 10-20% of the hydrogen/nitrogen fuel stream to the gas turbine.

The object of the invention is, therefore, to provide an improved method for producing a fuel for a gas turbine. The obj ect of the invention is achieved by the independent claims 1 and 6 . The dependent claims describe advantageous developments and modi fications of the invention .

In accordance with the invention there is provided a method for producing a fuel for driving a gas turbine , including the steps of :

- pressuri zing liquid ammonia,

- the pressuri zed ammonia is fed by a feeding line into an ammonia cracking device and partially cracked using exhaust gas heat from the gas turbine ,

- a mixture of hydrogen, nitrogen and uncracked ammonia is fed out of the ammonia cracking device to a cooling unit and cooled down to ambient temperature ,

- in a gas-liquid-separator, the gaseous mixture of hydrogen, nitrogen and uncracked ammonia is separated from the uncracked, liquid ammonia by means of centri fugal and/or gravitational forces , the gaseous mixture of hydrogen, nitrogen and uncracked ammonia is fed to at least one burner of the gas turbine ,

- the uncracked, liquid ammonia from the gas-liquid- separator is mixed with the pressuri zed ammonia in the feeding line .

In accordance with the invention there is provided a system for producing a fuel for driving a gas turbine , comprising :

- a feeding line for liquid ammonia in which a pump is integrated,

- an ammonia cracking device arranged on the feeding line for the production of a mixture of hydrogen, nitrogen and uncracked ammonia and an exhaust gas line connecting an exit of the gas turbine with the ammonia cracking device to trans fer exhaust gas heat from the gas turbine to the ammonia cracking device ,

- a cooling unit for cooling the mixture of hydrogen, nitrogen and uncracked ammonia down to ambient temperature , - a gas-liquid-separator for the separation of the gaseous hydrogen, nitrogen and uncracked ammonia from the uncracked, liquid ammonia by means of centri fugal and/or gravitational forces ,

- a fuel line for feeding the gaseous hydrogen, nitrogen and uncracked ammonia to at least one burner of the gas turbine , and

- a recirculation line connecting the gas-liquid- separator with the feeding line for the transportation of the uncracked, liquid ammonia to the pressurized ammonia .

Advantages relating to the described method may as well pertain to the system and vice versa .

The method overcomes the drawbacks of the known prior art by suggesting an ef ficient way to keep the reaction rate of the cracking process high by continuously suppling heat for from the exhaust gas of the gas turbine . To achieve this , the ammonia is not pre-heated, but the heating step is performed in the ammonia cracking device . In this case , proximity is needed between the gas turbine and the ammonia cracking device , i . e . , both are part of one system for generating power using a gas turbine . In particular, the case is excluded, in which the fuel is produced at a di f ferent site than the gas turbine , and transported to the gas turbine , which is for instance of fshore .

Another essential di f ference between the present method and the prior art is that the ammonia is only partially cracked, so that a mixture containing hydrogen, nitrogen and noncracked ammonia is leaving the ammonia cracking device and fed to the burners of the gas turbine .

Moreover, the mixture containing hydrogen, nitrogen and noncracked ammonia is cooled down to ambient temperature , so that the ammonia condenses , but all hydrogen and nitrogen remain fully gaseous . Ammonia content in the gas phase will be determined by the vapor pressure of the ammonia, which depends on the mixture ' s temperature . This gives the opportunity to control and know the composition of mixture . At typical gas turbine operating conditions , the ammonia content can be about 30-40% , which gives the resulting fuel mixture a laminar flame speed comparable to the one of natural gas .

The uncracked, liqui fied ammonia is eventually recirculated to the incoming ammonia stream . The advantage of this process step is , that the ammonia coming from the gas-liquid- separator is still at high pressure . The proposed system will maximi ze the utili zation of the catalyst , as it allows for high ammonia concentration through the entire catalyst .

By cracking ammonia to a hydrogen-nitrogen mixture prior to inj ection into the burner, a fuel is obtained which has a suitable flame speed, as well as fuel bound nitrogen is lowered . A further advantage is also the elevated heating value of the fuel .

In a preferred embodiment , the ammonia is pressuri zed from a lower pressure to a higher pressure which is in the range between 15 bar and 50 bar, depending on the type and si ze of the turbine . This is done by a pump, integrated in the feeding line , which pumps up ammonia to 15-50 bar . Alternatively, the same pressure can be achieved by heating the ammonia storage container .

In another preferred embodiment , by means of the exhaust gas of the gas turbine , in the ammonia cracking devices the ammonia is heated up from a lower temperature to a higher temperature , which is in the range between 400 ° C and 1000 ° C . In this case the ammonia cracking device works also as heat exchanger and the heat needed for the endothermic cracking reaction is provided by the gas turbine exhaust gas . The exhaust gas of the gas turbine has a temperature of 450 ° C to 650 ° C, but depending on the type of turbine and operating conditions this temperature might be higher . In any case the exhaust gas temperature is high enough to heat up the ammonia to the required temperature level so that the cracking process takes place.

Preferably, the ammonia cracking device is a catalytic ammo- nia-ammonia cracking device containing a catalyst. Alternative embodiments are also possible, e.g. The ammonia cracking device is preferably a reforming reactor.

A solid catalyst is preferably employed in the ammonia cracking device in order to ensure an efficient cracking process. In this case the ammonia cracking device contains a solid catalyst selected from nickel catalysts, iron catalysts, manganese catalysts, platinum catalysts, palladium catalysts, lanthanum catalysts, molybdenum catalysts, ruthenium catalysts, cobalt catalysts, lithium catalysts, and zirconium catalysts, or mixtures thereof.

One embodiment of the invention is now described, by way of example only, with reference to the accompanying drawing. The only figure shows a system 2 for producing a fuel to be burned in a gas turbine 4. Liquid ammonia is fed to the system 2 through a feeding line 6, in which a pump 8 is integrated. The ammonia is pressurized from a lower pressure pi which is e.g. 8 bar to higher pressure P2 at a level between 15 bar and 50 bar, e.g. p ₂ =50 bar. In the feeding line 6 the ammonia has a lower temperature of T ₂ which is 15°C. Alternatively, the pump 8 is omitted, and the ammonia storage is heated to achieve pressure p ₂ .

The pressurized ammonia is then fed into an ammonia cracking device 10, which is also functioning as heat exchanger. Exhaust gas heat from the gas turbine 4 is also transferred to the ammonia cracking device 10 by means of a exhaust gas line 12, so that in the ammonia cracking device 10 the ammonia is heated up to a higher temperature T ₂ , in particular to a range between to 400 °C and 1000 °C. For example, T ₂ =500°C. A suitable solid catalyst material is present inside the ammonia cracking device 10. At the typical gas turbine exhaust temperatures, a substantial fraction (but not all) of the ammonia is cracked to hydrogen and nitrogen. The catalyst speeds up this process.

After leaving the ammonia cracking device 10, the mixture of ammonia, hydrogen and nitrogen is cooled down to ambient temperature, e.g. down to T!=15°C, in the cooling unit 14. This temperature can be different from the temperature in the feeding line 6 though. As the pressure remains high at P2, e.g. 50 bar, part of the ammonia in the mixture will condensate, while hydrogen and nitrogen remain fully gaseous. The partial pressure from gas phase ammonia depends on the temperature Tx of the mixture. At Ti=15 °C, ammonia vapor pressure is about 7 bar, while the remaining pressure is exerted by the cracked products.

Next the gaseous cracking products are separated from the uncracked ammonia by means of centrifugal and/or gravitational forces in a gas-liquid-separator 16. The uncracked liquid ammonia is recirculated to the incoming ammonia stream via a recirculation line 18, while the gaseous mixture (hydrogen, nitrogen and ammonia) is fed to the burners 22 of the gas turbine 4 along a fuel line 20. The fuel mixture fed to the burners 22 has a higher heating value than the incoming ammonia, this energy comes from the exhaust heat. It has also a significantly higher reactivity owing to the presence of hydrogen .