Patent Description:
As is known, ammonia is a promising fuel that may be used to run gas turbine engines and provide a strong contribution to decarbonization. Combustion of ammonia is in fact completely carbon free, on the one side, and, on the other side, ammonia is an energy dense fuel suitable for efficient and durable storage of large amounts of chemical energy. Required volume may be remarkably low, for example compared to hydrogen, because ammonia is rather easy to liquefy and can be kept in the liquid state under relatively low pressure even at environmental temperature. Thus, not only storage, but also transportation and distribution may be made efficient, safe and cost-effective.

However, combustion of ammonia in ordinary operating conditions of gas turbine engines is known to produce significant amounts of nitrogen oxides (NOx). Emissions of NOx are critical and need to be minimized as well as emissions of carbon monoxide. Also blends of ammonia and carbon based fuels such as natural gas or methane may not meet current strict requirements for NOx emissions if directly supplied to gas turbine engines. The benefit of carbon free or at least low-carbon combustion may be thus cancelled by a corresponding increase in NOx emissions.

Moreover, reactivity of ammonia is relatively low and represents another hurdle that makes use of ammonia in gas turbine engines difficult.

In other words, gas turbine engines as normally operated are not compatible with feed of ammonia as the sole fuel or as a component of a mixture of different fuels.

<CIT> discloses a method for controlling a sequential combustor of a gas turbine engine, comprising a first combustor stage and a second combustor stage. The method comprising supplying the first combustor stage with a first air/fuel mixture (M1) and supplying the second combustor stage with a second air/fuel mixture (M2). The first air/fuel mixture (M1) contains ammonia and is stoichiometric or sub-stoichiometric.

Other examples of known methods are disclosed in <CIT>, in <CIT>, in <CIT> and in <CIT>.

It is an aim of the present invention to provide a method for controlling a sequential combustor of a gas turbine engine and a gas turbine power plant that allow the above limitations to be overcome or at least reduced.

According to the present invention, there is provided a method for controlling a sequential combustor of a gas turbine engine, the sequential combustor comprising a first combustor stage and a second stage;.

The supply of an excess of ammonia with respect to air oxygen prevents or at least strongly reduces the formation of nitrogen dioxides, which is instead a major problem in conventional engines operated on lean air/fuel mixtures containing ammonia. Moreover, residual ammonia is thermally decomposed into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>, which are therefore suitable for resuming the combustion without increasing the production of NOx.

The solution thus allows to exploit advantages of ammonia as a fuel for gas turbine engines and its potential contribution to decarbonization, without incurring in adverse effects such as high NOx emission levels. Advantages of ammonia to other carbon-free fuels mainly reside in ease of storage and transportation in amounts sufficient to meet the need of the market.

Heat in the first combustor stage causes the residual ammonia which has not taken part to first stage combustion to be thermally decomposed into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>, which are available to resume combustion in the presence of additional oxygen. Injection of dilution air at a location where molecular ammonia has been decomposed and is no longer present, or is present in a negligible amount, determines the conditions for resuming the combustion without causing critical production of NOx. Together with control of the amount of the first fuel supplied to the first combustor stage combustion, injection of dilution air allows also to control the temperature of the gas at the inlet of the second combustor stage.

According to an aspect of the invention, a fuel content of the first air/fuel mixture is entirely ammonia.

Completely carbon-free combustion is thus ensured, without the drawbacks associated with supplying other fuels which are not as easy and safe to store and distribute as ammonia (e.g. direct supply of molecular hydrogen).

According to an aspect of the invention, the method comprises decomposing additional ammonia outside the sequential combustor.

According to an aspect of the invention, the method comprises supplying the decomposed additional ammonia to the second combustor stage in the second air/fuel mixture.

Decomposition of additional ammonia outside the sequential combustor and feed of such decomposed additional ammonia to the second combustor stage create the conditions for running the gas turbine engine on ammonia alone, if so desired, without incurring in excessive production of NOx.

According to an aspect of the invention, the second air/fuel mixture contains at least one of hydrogen, methane, ethane, propane, butane pentane and any other gaseous fuel.

A conventional fuel, such as natural gas or syngas, may be used in the second combustor stage of the sequential combustor for hybrid operation of the gas turbine engine.

According to an aspect of the invention, the second air/fuel mixture (M2) is over-stoichiometric (lean).

If hydrocarbons are present in the second fuel (e.g. natural gas or syngas), the excess of oxygen allows to keep production of carbon monoxide very low. Instead, in case products of ammonia decomposition are burned, the absence of molecular ammonia prevents critical production of NOx even in a lean air/fuel mixture.

According to an aspect of the invention, the method comprises controlling a temperature of a first stage flame, an equivalence ratio of the first stage and a fuel massflow of the first stage so that the second air/fuel mixture is auto-ignited in the second combustor stage.

According to the present invention, there is also provided a gas turbine power plant comprising:.

According to an aspect of the invention, a content of the first air/fuel mixture is entirely ammonia.

According to an aspect of the invention, the second fuel source comprises a reactor coupled to the first fuel source to receive the first fuel and configured to decompose ammonia in the first fuel and to feed the decomposed ammonia to the second combustor stage as the second fuel.

According to an aspect of the invention, the second fuel contains at least one of hydrogen, methane, ethane, propane, butane, pentane or any other gaseous fuel.

According to an aspect of the invention, the second air/fuel mixture is over-stoichiometric.

According to an aspect of the invention, the control system is configured to control at least one of a temperature of a first stage flame, an equivalence ratio of the first stage and a fuel massflow of the first stage so that the second air/fuel mixture is auto-ignited in the second combustor stage.

The present invention will now be described with reference to the accompanying drawings, which show a number of non-limitative embodiments thereof, in which:.

In the following, when dealing with mixtures of fuel and air, reference will be made to the air-to-fuel ratio λ. Therefore, the terms "sub-stoichiometric" and "over-stoichiometric" will be in any case understood as meaning an excess of fuel and an excess of air in the mixture, respectively, compared to the stoichiometric ratio λST.

With reference to <FIG>, number <NUM> defines a gas-turbine plant as a whole comprising a gas turbine engine <NUM>, a first fuel supply line <NUM>, second fuel supply line <NUM> a control system <NUM>.

The control system <NUM> is configured to determine set-points of the gas turbine engine <NUM> for desired operating conditions and to use actuators of the gas turbine engine <NUM>, so that set-points may be achieved.

The gas turbine engine <NUM>, in turn, comprises a compressor <NUM>, a combustor assembly <NUM> and an expansion section or turbine <NUM>. Moreover, sensors <NUM> are arranged and configured to sense operating quantities of the gas turbine engine <NUM> and to send measurement signals to the control system <NUM>.

The compressor <NUM> feeds the combustor assembly <NUM> with a flow of air drawn from outside.

The combustor assembly <NUM> comprises a plurality of sequential combustors <NUM>, which cause the combustion of mixtures of air and fuel. An air plenum <NUM>, coupled to the sequential combustors <NUM>, is supplied with compressed air by the compressor <NUM> and defines a compressed air source for the sequential combustors <NUM>.

The turbine <NUM> receives and expands a flow of burnt hot gas from the combustor assembly <NUM> to extract mechanical work, which is transferred to an external user, typically an electric generator, which is not shown here.

The first supply line <NUM> supplies a first fuel F1, which contains molecular ammonia and in one embodiment is sole ammonia.

The second supply line <NUM> supplies a second fuel F2, which contains one of hydrogen and methane or a mixture thereof. For example, the second fuel F2 may be natural gas or syngas and contain one or more of ethane, propane, butane, pentane or any other gaseous fuel.

<FIG> schematically shows one of the sequential combustors <NUM>, being it understood that the other sequential combustors <NUM> have basically an identical structure.

The sequential combustor <NUM> comprises a first combustor stage <NUM> and a second combustor stage <NUM>, sequentially arranged along a gas flow path <NUM>. A mixer <NUM> is arranged along the gas flow path <NUM> between the first combustor stage <NUM> and the second combustor stage <NUM> and is configured to add a controlled amount of dilution air A into gas flowing through the gas flow path <NUM>.

The first combustor stage <NUM> is fluidly coupled to the first fuel supply line <NUM> to receive an amount of the first fuel F1, which is adjustable by a first stage fuel actuator <NUM>, e.g. a fuel valve system. The first combustor stage <NUM> also receives an airflow from the plenum <NUM>, which is in turn fed by the compressor <NUM>. The first fuel F1 and the airflow from the plenum <NUM> form a first air/fuel mixture M1 which is burned in the first combustor stage <NUM>. The amount of first fuel F1 is controlled by the control system <NUM> through the first stage fuel actuator <NUM> so that the first air/fuel mixture M1 is stoichiometric or sub-stoichiometric (rich) for the first fuel F1. Under both stoichiometric or sub-stoichiometric conditions, only part of the ammonia in the first fuel F1 is burned in the first combustor stage <NUM> and rate of production of NOx can be kept low, within acceptable limits. The remaining portion, which is not burned because there is no sufficient oxygen available, is thermally decomposed (or cracked) into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>. Therefore no or negligible molecular ammonia NH<NUM> is any longer present in the mixture that leaves the first combustor stage <NUM>. In particular, at the location where the mixer <NUM> adds dilution air A into gas flowing through the gas flow path <NUM> ammonia has been burned or decomposed. Injection of air causes the temperature of gas to drop. However, oxidation of decomposed ammonia resumes downstream of the mixer <NUM> and the temperature starts rising soon after, until the mixture of dilution air A and exhaust from the first combustor stage <NUM> reaches the second combustor stage <NUM>.

The second combustor stage <NUM> is fluidly coupled to the second fuel supply line <NUM> to receive an amount of the second fuel F2, which is adjustable by a second stage fuel actuator <NUM>, e.g. a fuel valve system. The second fuel F2 from the second fuel supply line <NUM> is added to the mixture of dilution air A and exhaust from the first combustor stage <NUM> flowing through the gas flow path <NUM> to form a second air/fuel mixture M2. Thus, a fuel content of the second air/fuel mixture M2 may include hydrogen and nitrogen from first combustor stage <NUM> and the second fuel F2 from the second fuel supply line <NUM>. No or negligible amount of undecomposed ammonia is present in the second air/fuel mixture M2.

The amount of second fuel F2 is controlled by the control system <NUM> through the second stage fuel actuator <NUM> so that the second air/fuel mixture M2 is over-stoichiometric (lean).

The control system <NUM> also controls at least one of a temperature of a first stage flame in the first combustor stage <NUM>, an equivalence ratio of the first stage and a fuel massflow of the first stage and the feed of the dilution air A through the mixer <NUM> so that the second air/fuel mixture M2 is auto-ignited in the second combustor stage <NUM> and a flame front position of the second combustor stage <NUM> is adjusted as desired. As already mentioned, in fact, the dilution air A determines both temperature drop of the gas flowing from the first combustor stage and temperature rise due to reactivation of combustion of decomposed ammonia.

According to another embodiment of the invention, shown in <FIG>, a gas-turbine plant <NUM> comprises the gas turbine engine <NUM>, the (first and here sole) fuel supply line <NUM>, which supplies the first fuel F1 containing molecular ammonia and, in one embodiment, only ammonia, and the control system <NUM> as already described. Additionally, the gas-turbine plant <NUM> comprises a reactor or ammonia cracker <NUM> coupled to the fuel source <NUM> and configured to decompose ammonia into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>.

The gas turbine engine <NUM> comprises the compressor <NUM>, the combustor assembly <NUM> and the expansion section or turbine <NUM>. The combustor assembly <NUM> comprises the sequential combustors <NUM>, each one with a respective first combustor stage <NUM> and a respective second combustor stage <NUM>.

The first combustor stage <NUM> is coupled to the fuel supply line <NUM> to receive a first fraction of an overall supply of the first fuel F1. The supply of the first fuel F1 is controlled by the control system <NUM> to have a stoichiometric or sub-stoichiometric first air/fuel mixture M1 in the first combustor stage <NUM>.

The ammonia cracker <NUM> is coupled to the fuel source <NUM> to receive a second fraction of the first fuel F1 from the supply line <NUM> and to the second combustor stage <NUM> to supply the second fuel F2. Specifically, the ammonia cracker <NUM> decomposes the molecular ammonia contained in the received flow of the first fuel F into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>. The ammonia cracker <NUM> supplies the decomposed ammonia to the second combustor stage <NUM> as the second fuel F2 to form the second air/fuel mixture M2 with exhaust from the first combustor stage <NUM> and dilution air A provided by the mixer <NUM>.

The supply of the second fuel F2 to the second combustor stage <NUM> is controlled by the control system <NUM> through second stage fuel actuator <NUM> so that the second air/fuel mixture M2 is over-stoichiometric.

Again, the control system <NUM> also controls at least one of a temperature of a first stage flame in the first combustor stage <NUM>, an equivalence ratio of the first stage and a fuel massflow of the first stage and the feed of the dilution air A through the mixer <NUM> so that the second air/fuel mixture M2 is auto-ignited in the second combustor stage <NUM> and a flame front position of the second combustor stage <NUM> is adjusted as desired.

According to another embodiment, shown in <FIG>, a gas-turbine plant <NUM> comprises the gas turbine engine <NUM>, the first fuel supply line <NUM>, which supplies the first fuel F1 containing molecular ammonia, the second supply line <NUM>, which supplies a fuel flow containing hydrogen, methane ethane, propane, butane, pentane or any other gaseous fuel, the control system <NUM> and, in addition, a reactor or ammonia cracker <NUM> coupled to the fuel source <NUM> and configured to decompose ammonia into molecular nitrogen N<NUM> and molecular hydrogen H<NUM>.

The second combustor stage <NUM> may receive the second fuel F2 either from the second supply line <NUM> or from the ammonia cracker <NUM>, or even from both. The control system <NUM> controls the supply of the second fuel F2 through second stage fuel actuators <NUM>, <NUM> so that the second air/fuel mixture M2 is over-stoichiometric, is auto-ignited in the second combustor stage <NUM> and a flame front position of the second combustor stage <NUM> is adjusted as desired.

Claim 1:
A method for controlling a sequential combustor (<NUM>) of a gas turbine engine, the sequential combustor (<NUM>) comprising a first combustor stage (<NUM>) and a second combustor stage (<NUM>);
the method comprising supplying the first combustor stage (<NUM>) with a first air/fuel mixture (M1) and supplying the second combustor stage (<NUM>) with a second air/fuel mixture (M2);
the first air/fuel mixture (M1) contains ammonia and is stoichiometric or sub-stoichiometric;
characterized by supplying dilution air into the sequential combustor (<NUM>) between the first combustor stage (<NUM>) and the second combustor stage (<NUM>), at a location where unburnt ammonia from the first combustor stage (<NUM>) is decomposed.