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 provides a system and method for combustion of ammonia which reduces or eliminates the emission of nitrogen oxides NO _x . The present invention accordingly provides apparatus and methods as set out in the appended claims.

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 system 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 4 and hydrogen 5 are added to the first combustion chamber 2 where combustion takes place producing heat and an exhaust gas flow 22. For example, the operational pressure within the first combustion chamber 2 may lie in the range 8-30 bar, with a typical operational pressure being in the range 12-25 bar. The exit temperature of exhause gases 22 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 180 through mass flow controllers 8 and 11.

The ammonia is provided at an enhanced equivalence ratio, for example in the range 1.0-1.2. Due to the enhanced equivalance ratio, exhaust gases 22 from the first combustion chamber 2 contain a significant proportion of N¾ ^~ ions.

The exhaust gases 22 from the first combustion chamber are provided to a second combustion chamber 3 along with additional ammonia 4 and hydrogen 5.

Control of the ratio of ammonia to hydrogen supplied to the second combustion chamber 3 is achieved by a controller 180 through mass flow controllers 9 and 12. The gas mixture is optimized to deliver maximum power upon combustion, as the exhaust gas flow 24 is provided to turbine 6 to produce a mechanical output, such as rotation of a shaft. For example, the operational pressure within the second combustion chamber 3 may lie in the range 10-30 bar, with a typical operational pressure being in the range 12-25 bar. The exit temperature of exhaust gases 24 from the second combustion chamber 3 may be in the range 1400-2100 K, typically 1500-1800 K. The combustion may be performed at a relatively constant presssure, for example the combustion presure drop may be less than 5% to maintain cycle efficiency, so there is only a small pressure drop over the combustion chamber. However, due to high combustion temperatures, and the high nitrogen content of the ammonia fuel, the exhaust gas flow 24 from the second combustion chamber 3 will have high levels of nitrogen oxides NO _x .

Exhaust gas 26 leaving the first turbine 6 is hot and is routed to a third combustion chamber 14 operating in a relatively low pressure and relatively low temperature regime. For example, the operational pressure within the third combustion chamber 14 may lie in the range 1-10 bar, with a typical operational pressure being in the range 1-5 bar. The exit temperature of exhaust gases from the third combustion chamber may be in the range 300-1300 K, typically 750-880 K.

Prior to entering this third combustion chamber, the exhaust gas 26 containing nitrogen oxides NO _x , from the second combustion chamber 3 and turbine 6 may be measured with an in situ gas analysis sensor 13.

A third mixture of ammonia 4 and hydrogen 5, this time with added air 20 is injected into the third combustion chamber 14 with an enhanced equivalence ratio of ammonia, typically 1.0 - 1.2, that is, an excess of ammonia over that required to react with the supplied hydrogen and oxygen to produce only 2 and H ₂ O. The mixture is combusted. The enhanced equivalence ratio ensures that the combustion produces significant proportion of N¾ ^~ ions which combine with the nitrogen oxides NO _x in the exhaust gas 26 to produce ₂ and H ₂ O thereby removing the NO _x from the exhaust stream 26 and producing an exhaust stream 28 from the third combustion chamber 14 which has a low content of nitrogen oxides NO _x .

The exact fuel ratio of ammonia 4 to hydrogen 5 supplied to the third combustion chamber 14 is set by controller 180 using mass flow devices 17, 18 and mass flow sensors 15, 16 and optionally a mass flow controller 19 for the air or other oxygen-containing gas, in conjunction with an in situ gas analysis sensor 21 to control the ammonia to hydrogen ratio, and optionally also the proportion of oxygen-containing gas such as air, in the gas mixture supplied to the third combustion chamber 14.

The required equivalence ratio is determined by measurement of the input NO _x proportion by gas sensor 13 and by measurement of the output NO _x emissions measured by in situ gas sensor 21. Controller 180 receives data from sensors 13, 21 and issues appropriate commands to mass flow devices 17, 18 and optionally 19. Controller 180 may be the same controller as the controller associated with mass flow devices 8, 9, 11, 12, or may be a separate controller.

Optionally, and in the illustrated embodiment, a heat exchanger 30 may be used to remove waste heat and recover energy from exhaust gases 28 from the third combustion chamber .

In the illustrated example, this may be achieved by recovering heat in heat exchanger 30 and using this to drive a steam turbine 32, although other mechanisms may be provided to recover energy from the waste heat, as appropriate.

In another embodiment of the present invention, as illustrated in Fig. 2, exhaust gases 22 from the first combustion chamber 2 may be routed through a second turbine 34 to recover waste energy as mechanical rotation.

Fig. 3 shows another embodiment of the present invention. In this embodiment, third combustion chamber 14 has an integrated heat exchanger. This may be similar to a heat recovery steam generator with supplementary firing. This may be used to drive steam turbine 32, although other mechanisms may be provided to recover energy from the waste heat, as appropriate .

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 40 may be provided to recirculate a proportion of the exhaust gas from the third combustion chamber 14 back into the first combustion chamber 2. The recirculated exhaust gas may be combined with the input gas flow, for example by mixing with intake oxygen- containing gas at mixer 36. This has the advantage that unburnt N¾ in the exhaust gas is recycled and combusted. The proportion of the exhaust gas may be varied, for example between 0% and 80%, depending on the proportion of unburnt N¾ in the exhaust gas from the third combustion chamber, and the acceptable proportion of N¾ in exhaust 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 exhaust gases;

(2) - overall efficiency of the system is maximised as all ammonia and hydrogen is converted to energy, nitrogen and water;

(3) - gas mixtures, temperatures and pressures in each of the three combustion chambers may be optimised according to their respective allocated function, providing good overall efficiency of the system;

(4) - the combustion chambers 2, 3, 14 can be located at a different location to the turbine (s) 6, 32, 34 so enabling various possible layouts to suit environmental constraints; and

(5) - N¾ content in the exhaust gas is minimised.

The respective technical features that may contribute to the above advantages are as follows.

Use of three combustion chambers 2, 3, 14 enables combustion under appropriate equivalence ratios to allow the formation of N¾ ^~ ions in the first combustion chamber 2, efficient power generation in the second combustion chamber 3 and effective removal of NO _x in the third combustion chamber. The subsequent combination of N¾ ^~ ions with NO _x in the exhaust gas to form ₂ and ¾0 reduces the ammonia content of the exhaust gas.

Measurement 13 of the NO _x content in the exhaust gas 26 from turbine 6 prior to input into the third combustion chamber, control of the NH ₃ /H ₂ gas mass flows into each combustion chamber and measurement 21 of the N0 _X exhaust gas at the output of the third combustion chamber allow the exact setting of the equivalence ratio according to the NO _x content of each respective exhaust gas 26, 28.

This is necessary because the burn conditions in the first and second combustion chambers 2, 3 will determine the NO _x content of the exhaust gases 26. These conditions can change on a dynamic basis and from system to system.

Use of a heat exchanger 30 minimizes the energy loss associated with the third combustion in the third combustion chamber 14. Recirculation of exhaust gas from the third combusion chamber 14 back to the first combustion chamber 2 acts to minimize N¾ emissions.

The present invention accordingly provides a method for combustion of ammonia, wherein a first combusion chamber receives ammonia and hydrogen in controlled proportions, and an oxygen-containing gas such as air, wherein combustion of the ammonia and hydrogen is carried out so as to produce N¾ ^~ ions, among other combustion products. A second combustion chamber receives the N¾ ^~ ions and other combustion products along with further ammonia and hydrogen in further controlled proportions, and produces nitrogen oxides, among other combustion products. A third combustion chamber receives the combustion products of the second combustion chamber including nitrogen oxides along with further ammonia and hydrogen in further controlled proportions and further oxygen-containing gas such as air, such that the nitrogen oxides are combusted into nitrogen and water. Energy from the combustion in the first combustion chamber 2 may be recovered by operation of a turbine 34 to convert the energy released by combustion in the first combustion chamber into mechanical energy.

Energy from the combustion in the second combustion chamber 3 may be recovered by operation of a turbine 6 to convert the energy released by combustion in the second combustion chamber into mechanical energy.

Energy from the combustion in the third combustion chamber 14 may be recovered by operation of a turbine 32 to convert the energy released by combustion in the third combustion chamber 14 into mechanical energy. Operation of the turbine 32 may be by direct action of exhaust gases from the third combustion chamber 14 on the turbine 32, or by heating of water in a heat exchanger 30 to drive third turbine 32 by steam.

The third combustion chamber 14 may incorporate a heat exhanger for recovery of heat from exhaust gases from the third combustion chamber. The heat exchanger may serve to heat steam for the recovery of energy.

A proportion of exhaust gases from the third combustion chamber 14 may be recirculated into the first combustion chamber 2 in order to provide combustion to ammonia remaining in the exhaust gases 28.

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, within the scope of the appended claims.