Ammonia may be used as an energy storage material. Ammonia may be synthesised and stored for later combustion. Combustion of ammonia in a gas turbine may allow chemically- stored energy to be released into mechanical energy. However, combustion of ammonia produces nitrogen oxides NO _x which should be removed from the exhaust gas in order to reach emission targets.

The present invention accordingly provides apparatus and methods as set out in the appended claims. Such apparatus and methods reduce or eliminate the emission of nitrogen oxides NO _x during ammonia combustion.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of non-limiting examples only, in conjunction with the appended drawings, wherein:

Figs. 1-4 schematically illustrate respective embodiments of the present invention.

In a certain embodiment of the invention, illustrated in Fig. 1, an ammonia combustion includes a compressor 1 which compresses air, or other oxygen-containing gas, and passes it into a relatively high-pressure and high-temperature first combustion chamber 2. A first mixture of ammonia 3 and hydrogen 4 are added to the first combustion chamber 2 where combustion takes place producing heat and an exhaust gas flow. For example, the operational pressure within the first combustion chamber 2 may lie in the range 10-30 bar, with a typical operational pressure being in the range 12-25 bar. The exit temperature of exhause gases 102 from the first combustion chamber may be in the range 1400-2100 K, typically 1500-1800 K. Control of the ratio of ammonia to hydrogen supplied to the first combustion chamber 2 is achieved by a controller 18 through mass flow controllers 5 and 6 coupled with an in situ gas analysis sensor 7. The gas mixture is optimized to deliver maximum power upon combustion. However, due to high combustion temperatures, and the high nitrogen content of the ammonia fuel, the exhaust gas flow 102 from the combustion chamber 2 will have high levels of nitrogen oxides NO _x .

The exhaust gas 102 is provided to a first turbine 8 where work is transferred to a shaft or similar to provide a mechanical output. Exhaust gas leaving the first turbine 8 is hot and is routed to a second combustion chamber 13 operating in a relatively low pressure and relatively low temperature regime. For example, the operational pressure within the second combustion chamber 13 may lie in the range 1-10 bar, with a typical operational pressure being in the range 1-5 bar. The exit temperature of exhause gases from the second combustion chamber may be in the range 300-1300 K, typically 750-880 K.

Prior to entering this second combustion chamber, the exhaust gas containing nitrogen oxides NO _x is measured with an in situ gas analysis sensor 9. A second mixture of ammonia 3, hydrogen 4 and air is injected into the second combustion chamber 13 with an enhanced equivalence ratio, typically 1.0 - 1.2, that is, an excess of ammonia over that required to react with the supplied hydrogen to produce only 2 and ¾0. The mixture is combusted. The enhanced ratio ensures that the combustion produces significant proportion of N¾ ^~ ions which combine with the nitrogen oxides NO _x to produce 2 and ¾0 thereby removing the NO _x from the exhaust stream 102. The exact equivalence ratio of ammonia to hydrogen in the second mixture is set by controller 18 using mass flow controllers 10, 11 and optionally an air mass flow controller 19 in conjunction with the in situ gas analysis sensor 12 to control the ammonia to hydrogen ratio, and optionally also the proportion of oxygen-containing gas such as air, in the second gas mixture supplied to the second combustion chamber 13. The required equivalence ratio is determined by measurement of the input NO _x proportion by gas sensor 9 and by measurement of the output NO _x emissions measured by in situ gas sensor 14. Controller 18 receives data from sensors 12, 9, 14 and issues appropriate commands to mass flow devices 11, 12 and optionally 19. Controller 18 may be the same controller as the controller associated with sensor 7 and mass flow devices 5, 6, or may be a separate controller.

A heat exchanger 15 may be used to remove waste heat and recover energy from discharge gases from the second combustion chamber. In the illustrated example, this is achieved by recovering heat in heat exchanger 15 and using this to drive steam turbine 16, although other mechanisms may be provided to recover energy from the waste heat, as appropriate.

For example, as illustrated in Fig. 2, discharge gases from the second combustion chamber 13 may be routed to a second turbine 22 to recover waste energy as mechanical rotation.

Fig. 3 shows another embodiment of the present invention. In this embodiment, second combustion chamber 24 has an integrated heat exchanger. This may be similar to a heat recovery steam generator with supplementary firing. A heat recovery steam generator (HRSG) is a heat exchanger designed to recover the exhaust 'waste' heat from power generation plant prime movers, such as gas turbines or large reciprocating engines, thus improving overall energy efficiencies. Supplementary (or 'duct' ) firing uses hot gas turbine exhaust gases as the oxygen source, to provide additional energy to generate more steam if and when required. It is an economically attractive way of increasing system output and flexibility. Supplementary firing can provide extra electrical output at lower capital cost and is suitable for peaking. A burner is usually, but not always, located in the exhaust gas stream leading to the HRSG. Extra oxygen (or air) can be added if necessary. At high ambient temperatures, a small duct burner can supplement gas turbine exhaust energy to maintain the designed throttle flow to the steam turbine.

In a further embodiment of the present invention, illustrated in Fig. 4, a recirculation line 20 may be provided to recirculate a portion of the discharge gas from the second combustion chamber 13 back into the first combustion chamber 2. The recirculated discharge gas may be combined with the input gas flow, for example by mixing with intake oxygen- containing gas at mixer 26. This has the advantage that unburnt N¾ in the exhaust gas is recycled and combusted. The proportion may be varied, for example between 0% and 80%, depending on the proportion of unburnt N¾ in the exhaust gas from the second combustion chamber, and the acceptable proportion of N¾ in discharge gases from the system.

The present invention accordingly aims to provide one or more of the following advantages:

(1) - nitrogen oxides NO _x content is reduced or eliminated from the discharge gases; (2) - overall efficiency of the system is maximised as all ammonia and hydrogen is converted to energy, nitrogen and water;

(3) - the first and second combustion chambers 2, 13, 24 can be located at a different location to the turbine (s) 8, 16,

22 so enabling various possible layouts to suit environmental constraints ;

(4) - N¾ content in the discharge gas is minimised. The respective technical features that may contribute to the above advantages are as follows.

(1) Use of a second combustion chamber 13, 24 enables combustion under appropriate equivalence ratios to allow the formation of N¾ ^~ ions. The subsequent combination with NO _x in the discharge gas to form ₂ and ¾0 reduces the ammonia content of the discharge gas.

(2) Measurement 9 of the NO _x content in the exhaust gas 102 from turbine 8 prior to input into the second combustion chamber, control of the NH ₃ /H ₂ gas mass flows into the first combustion chamber and measurement 14 of the NO _x emissions at the output of the second chamber allow the exact setting of the equivalence ratio according to the NO _x content of the exhaust gas and discharge gas. This is necessary because the burn conditions in the first combustion chamber will determine the NO _x content of the exhaust gases 102. These conditions can change on a dynamic basis and from system to system.

(3) Use of a heat exchanger 15, 24 to minimize the energy loss associated with the second combustion in the second combustion chamber 13, 24.

(4) Recirculation of discharge gas from the second combusion chamber back to the first combustion chamber acts to minimize NH ₃ emissions. The present invention accordingly provides methods and systems for combustion of ammonia, as defined in the appended claims . Energy from the combustion in the first combustion chamber 2 may be recovered by operation of a first turbine 8 to convert the energy released by combustion in the first combustion chamber into mechanical energy. Energy from the combustion in the second combustion chamber 13 may be recovered by operation of a second turbine 16, 22 to convert the energy released by combustion in the second combustion chamber into mechanical energy. Operation of the second turbine 22 may be by direct action of exhaust gases from the second combustion chamber 13 on the turbine 22, or by heating of water in a heat exchanger 15 to drive second turbine 16 by steam.

The second combustion chamber 24 may incorporate a heat exhanger for recovery of heat from exhaust gases from the second combustion chamber. The heat exchanger may serve to heat steam for the recovery of heat.

A proportion of discharge gases from the second combustion chamber may be recirculated into the first combustion chamber in order to provide combustion to ammonia remaining in the exhaust gases.

While the present application has been described with reference to a limited number of particular embodiments, numerous modifications and variants will be apparent to those skilled in the art.