Patent Publication Number: US-10767855-B2

Title: Method and equipment for combustion of ammonia

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method and equipment for combustion of ammonia. 
     Description of the Prior Art 
     Ammonia may be used as an energy storage material. Ammonia may be synthesized 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 NOx which should be removed from the exhaust gas in order to reach emission targets. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, in a method and system for the combustion of ammonia, a first combustion chamber receives ammonia and hydrogen in controlled proportions, as well as an oxygen-containing gas, such as air. Combustion of the ammonia and hydrogen in the first combustion chamber produces nitrogen oxides, among other combustion products. The nitrogen oxide content of the combustion products of the first combustion chamber. Ammonia and hydrogen and oxygen-containing gas are introduced into a second combustion chamber in controlled amounts dependent on the measured nitrogen oxide content of the combustion products of the first combustion chamber. The proportions of ammonia and hydrogen and oxygen-containing gas are controlled so that an excess of ammonia is introduced into the second combustion chamber, over that required to react with the supplied hydrogen, so as to produce only nitrogen and water when combustion takes place in the second combustion chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a first embodiment of the method according to the invention, as implemented by a system in accordance with the first embodiment of the method. 
         FIG. 2  is a flowchart of a second embodiment of the method according to the invention, as implemented by a system in accordance with the second embodiment of the method. 
         FIG. 3  is a flowchart of a third embodiment of the method according to the invention, as implemented by a system in accordance with the third embodiment of the method. 
         FIG. 4  is a flowchart of a fourth embodiment of the method according to the invention, as implemented by a system in accordance with the fourth embodiment of the method. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 exhaust 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 NOR. 
     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 exhaust 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 N 2  and H 2 O. The mixture is combusted. The enhanced ratio ensures that the combustion produces significant proportion of NH 2     −    ions which combine with the nitrogen oxides NO x  to produce N 2  and H 2 O 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 NH 3  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 NH 3  in the exhaust gas from the second combustion chamber, and the acceptable proportion of NH 3  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)—NH 3  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 NH 2     −    ions. The subsequent combination with NO x  in the discharge gas to form N 2  and H 2 O 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 combustion 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 exchanger 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.