AN ARRANGEMENT FOR PREPARATION OF LIQUID FUEL FOR COMBUSTION AND A METHOD OF PREPARING LIQUID FUEL FOR COMBUSTION

TECHNICAL FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a preparation of liquid fuel for combustion, particularly for ignition in burners in gas turbines .

In such a gas turbine known in the state of the art, for ex ^¬ ample as disclosed in EP 1 953 454 Al, a gas duct or gas flow path is routed through a combustion section/system located between a compressor and a turbine section. The combustion section may include an annular array of combustors or a sin ^¬ gle annular combustor. High pressure air from the compressor flows through the combustion section where it is mixed with fuel and burned. As mentioned above, the combustors comprise (at least) one burner, and (at least) one igniter for ignit ^¬ ing the air/fuel mixture especially during start up of the gas turbine.

Combustion gases exit the combustion section to power the turbine section which drives the compressor. In single-shaft arrangements a high-pressure and a low-pressure turbine of the turbine section are mechanically connected and together drive an output power shaft. In twin-shaft arrangements a low-pressure turbine (power turbine) is mechanically inde ^¬ pendent, i.e. only drives the output power shaft, and a high- pressure turbine, or so called compressor turbine, drives the compressor. This combination acts as a gas generator for the low-pressure turbine. The combustion gases exit the turbine section through an exhaust duct.

In recent years, increasingly stringent emissions standards have made a lean, premixed combustion more desirable in power generation and industrial applications than ever before, since this combustion mode provides low NOx and CO emissions without water addition. Lean, premixed combustion of natural gas or liquid fuel avoids the problems associated with diffu ^¬ sion combustion and water addition.

As a result, lean, premixed combustion has become a founda ^¬ tion for modern Dry Low Emissions (DLE) gas turbine combus ^¬ tion systems. The DLE combustor, for example as disclosed in EP 0 747 636 Al , being developed require precise control of the combustion process. The pre-mixers and combustor must generate and receive, respectively, the correct mixture of air and fuel, and control the combustion temperature to limit emissions to acceptable levels. Starting such gas turbines on liquid fuel, e.g. kerosene or diesel, is associated with poor reliability. In starting phases gas turbines operate at low speed while supplying liq ^¬ uid fuel at low pressure and at low temperature. Low pressure as well as low temperature makes it harder for the liquid fuel to ignite and less reliable for the gas turbine to start .

The pressure of the liquid fuel supplied is used to break the liquid fuel in small droplets - using a mechanical device, i.e. an atomizer, - prior to the ignition. Breaking liquid fuel in droplets increases a surface area of the liquid fuel, and a small amount of fuel vapour forms around the droplets. This is ignited by the ignition spark. While supplying liquid fuel in the starting phase at low pressure the amount of droplets and the surface area of the liquid fuel are decreased. This makes it harder for the liq ^¬ uid fuel to ignite and less reliable for the gas turbine to start. Further more - liquid fuel provided in the starting phase for ignition at lower temperature needs more energy to be added to the liquid fuel for the liquid fuel to ignite and gas turbine to start. As a solution for these ignition difficulties by starting a gas turbine on liquid fuel the liquid fuel could be supplied at higher pressure to increase the amount of small droplets and the surface area of the liquid fuel to ignite. As the fuel flow needed to start the gas turbine is fixed for each ambient condition, the increased pressure would have to be e.g. supplied via separate starting nozzles that are switched off when the gas turbine is running. Other solutions for these ignition difficulties are based on extra gas systems providing pressurized gas in the starting phase, for example by using gas bottles, which makes it eas ^¬ ier for the gas turbine to start. But it is seen to be diffi ^¬ cult to ensure a sufficient reliability without using gas systems.

SUMMARY OF THE INVENTION

It is a first objective of the present invention to provide a method and an arrangement for preparing liquid fuel for com ^¬ bustion to make it easier for the liquid fuel to ignite in the burners .

It is a second objective of the invention to provide an ar- rangement and a method by which the above-mentioned shortcom ^¬ ings can be mitigated, and especially an easier and a more reliable ignition of liquid fuel in the burners of a gas tur ^¬ bine . These objectives are according to the invention achieved by providing an arrangement for preparation of a liquid fuel for combustion, particularly for ignition. This arrangement comprises a main fuel feed for feeding liquid fuel from a fuel supply to a burner and a reservoir for liquid fuel. The res- ervoir is arranged in parallel with said main fuel feed. The arrangement further comprises a means for heating liquid fuel in said reservoir. Said reservoir is connected to said main fuel feed to be filled at a first predetermined condition with liquid fuel fed in said main fuel feed and/or said reservoir, and is connected to said main fuel feed to be evacuated at a second predetermined condition of heated liquid fuel heated in said reservoir into said main fuel feed.

These objects are according to the invention also achieved by providing a method of preparing a liquid fuel for combustion, particularly for ignition. This method comprises the steps of:

- filling at first predetermined condition a reservoir with liquid fuel of a main fuel stream fed in a main fuel feed wherein said reservoir is arranged in parallel to said main fuel feed,

- heating liquid fuel in said reservoir,

- evacuating at a second predetermined condition said reser ^¬ voir of heated liquid fuel heated in said reservoir into said main fuel feed/main fuel feed.

In other words - the invention provides a source of heated liquid fuel, particularly heated high pressurized liquid fuel, for example kerosene, in a reservoir. The reservoir is arranged in parallel with a main fuel feed of a combustion system and can be filled by said main fuel feed and/or evacu ^¬ ated into said main fuel feed. By evacuating heated liquid fuel in said main fuel feed the temperature of the liquid fuel for the burners to ignite will be increased - and the liquid fuel will be ignite more easily, i.e. the gas turbine will start more reliably. The reservoir contains a heating means, which heat the liquid fuel filled in the reservoir, particularly while circulating in the reservoir.

By arranging the reservoir in parallel with the main supply, the reservoir, i.e. the heating means could be removed for cleaning or replacement without interrupting a gas turbine operation . While using the invention for preparing liquid fuel for starting a gas turbine the first predetermined condition could define or correlate with (time when) the gas turbine is being prepared to start. The second predetermined condition could be a date when the gas turbine is started. This second condition/date could correlate - but not necessarily - with the point when the liquid fuel to be heated in the reservoir achieves a specific/suitable temperature or a heating pe ^¬ riod/duration expires.

The heating operation can be stopped at a predetermined or suitable temperature. This temperature, preferably measured by a thermocouple and/or controlled by a control system, should preferably be high enough to significantly increase an amount of vaporisation of a fuel when it is atomised in a burner. Such a suitable temperature could be about 50 °C up to 150 °C or more.

The heated liquid fuel can - particularly by use of at least one valve - be directed to burners of the combustion system - via the main fuel feed - when the gas turbine is started. Af ^¬ ter successful ignition of the fuel in the burners of the combustion system, the fuel flow/feed can be switched back - particularly by use of at least one valve - to normal un- heated fuel.

The reservoir can be sized to contain enough liquid fuel for one, two or several starts of a gas turbine as required, with a signal to indicate when the temperature has reached a pre- determined or desired level. The signal could be generated by a sensor - or based on signal generated by a sensor, particu ^¬ larly of a thermocouple, arranged in the reservoir - and/or could be evaluated and/or processed in a control system. According to a preferred embodiment the reservoir is sized to contain liquid fuel for two starts of the gas turbine, so that - if the gas turbine is stopped after a first start - enough (heated) liquid fuel is available in the reservoir for a second start.

It has been realized that the invention provides - in a easy and efficient way - a source of heated high pressure liquid fuel, so that - when the fuel is expanded to lower pressure and broken into droplets - it vaporises more easily, allowing more reliable ignition. In other words - the invention pro ^¬ vides in an easy and efficient way a recirculating circuit for heating pressurized liquid fuel in order for it to evapo ^¬ rate more easily when expanded to a lower pressure during start of a gas turbine.

Usually pressurized liquid fuel is supplied at a pressure several times e.g. 3-10 that of the compressor delivery pres ^¬ sure in order to provide the fine droplets required for DLE combustion. During start the supply pressure is reduced to only a few bars over the compressor delivery pressure, which in itself is then typically not significantly above atmos- pheric or ambient pressure.

According to a preferred embodiment of the invention a flow circuit could be realized in said reservoir or with said res ^¬ ervoir as part of the flow circuit. Said reservoir could com- prise a pump to pump the liquid fuel along a pipe, a duct from the reservoir towards the burners, with a return pipe to an opposite side of the reservoir - defining said flow circuit . During the heating operation/period the liquid fuel circu ^¬ lates around said flow circuit to ensure that both - fuel and pipework - reach a required temperature. The continuous flow of fuel around the circuit to be heated also means that the fuel is not in contact with the heating means for an extended period of time. These reduce the risk of coking/overheating of liquid fuel on/by the heating means. According to a further preferred embodiment of the invention said reservoir or other elements of the flow circuit, as for example pipes, ducts, comprises at least one thermocouple po ^¬ sitioned at a probing point in said reservoir or said other elements, i.e. in said flow circuit. Said at least one ther ^¬ mocouple provides a signal to a control system, indicating that the temperature of the fuel and/or the pipework has reached a required level for liquid fuel starting. Therefore a control system could be provided to evaluate and/or to process said signal generated by said thermo sen ^¬ sor .

According to a preferred embodiment of the invention the con- nection of the reservoir with the (main) fuel feed can be realized by valves, particularly by two-way valves, arranged at an inlet end ("fill in end") and/or an outlet end ("evacuat ^¬ ing end") of said reservoir or flow circuit providing different switch settings, i.e. phases of operation.

A valve positioned in the flow circuit, particularly at the outlet end of the reservoir or flow circuit, could be

switched to direct the liquid fuel (in the reservoir) either (1) back to the reservoir, in a heating phases, or (2) into the pipes to the burners, in a starting phase. A valve posi ^¬ tioned in the flow circuit, particularly at the inlet end of the reservoir or flow circuit, could be switched to direct the liquid fuel of the main fuel feed either (1) (directly) into the burners, both for normal operation and in a heating phase, or (2) towards the reservoir, in a starting phase.

When the valves are in the starting positions, i.e. in posi ^¬ tion of/for the starting phase, a pressure of the un- heated/cold fuel of the main supply then "drives" the heated fuel out of the reservoir into the burners.

According to a preferred embodiment of the invention the ar ^¬ rangement/method could operate in two phases, a heating phase and a starting phase. In the heating phase - a gas turbine is preparing to start on liquid fuel - the reservoir is filled with (unheated) liquid fuel by/out of the main fuel feed. Valves - preferable con- necting the reservoir with the main fuel feed at an inlet end and an outlet end of the reservoir or flow circuit - , par ^¬ ticularly two-way valves, are switched so that the liquid fuel is isolated in the reservoir/in a heating circuit real ^¬ ized in the reservoir - or with the reservoir as part of the flow circuit. A (small) fuel pump could circulate the liquid fuel in the reservoir, which could dump back into an inlet end of the reservoir. The heating means/the heater is

switched on for heating the isolated liquid fuel in the res ^¬ ervoir. A thermo sensor, for example a thermocouple, indi- cates when the temperature of the liquid fuel to be heated in the reservoir is high enough for the gas turbine starting.

When this temperature is reached, the heating means/the heater is turned off. If the temperature drops too low before the gas turbine is started, the heater is switched on again.

In the starting phase - the gas turbine is started - the valves, i.e. the two-way valves, are switched so that liquid fuel of the main fuel feed (main fuel flow) enters the inlet end of the reservoir (or flow circuit) and pushes (evacuates) the heated liquid fuel into the supply, i.e. back into the main fuel feed, to the burner. The evacuation of the reservoir could be supported by the fuel pump of the reservoir if needed .

An advantage of the proposed arrangement by arranging the reservoir in parallel with the main fuel feed, the reservoir, i.e. the heating means, can be removed for cleaning or re ^¬ placement without interrupting the gas turbine operation.

According to a preferred embodiment of the invention the gas turbine is a Dry Low Emission gas turbine, that means the gas turbine is operating with a Dry Low Emission combustion. While supplying heated liquid fuel, particularly heated pres ^¬ surized liquid fuel, only for starting - during operation the flame of the combustion system will provide enough tempera- ture as well as more/higher pressure is available during op ^¬ eration - the rate of heating does not need to be high. Fur ^¬ ther to that - while supplying heated liquid fuel from a res ^¬ ervoir in which a predetermined amount of liquid fuel is heated - instead of heating a continuous flow of liquid fuel flowing upstream to the burners - the rate of heating does not need to be high.

Because the rate of heating does not need to be high, the temperature of the heating means and/or the sizes of the heating means themselves do not need to be high/large which reduce the risk of coking/overheating of liquid fuel by the heating means. Small heaters - preferably depending on a size of the gas turbine - could be used by the invention. Further advantages as well as advantageous features of the invention appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a spe ^¬ cific description of embodiments of the invention cited as examples .

In the drawings :

Figure 1 is a schematic cross-sectional view through a part of a known gas turbine engine with burners, to which an arrangement for preparation of a fuel for combustion according to the present invention may be applied;

Figure 2 is an enlarged schematic view of a recirculating circuit for heating pressurized liquid fuel operat- ing in a heating phase according to the present invention;

Figure 3 is an enlarged schematic view of a recirculating circuit for heating pressurized liquid fuel operat ^¬ ing in a starting phase according to the present invention .

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention is directed to an arrangement for preparation of a liquid fuel for combustion in a gas turbine as used in for instance a power plant as schematically illus- trated in Figure 1.

The gas turbine engine as shown in Figure 1 has an air inlet 1 at one end followed by a compressor 2 for compressing the air from said inlet. Combustors 3 having a can-like shell are distributed around a turbine shaft 4. Liquid Fuel, that means in this case heating oil, supplied by a fuel supply 13 is in ^¬ troduced into the respective burners 9 at 5 and is there mixed with a part of said air from the air inlet 1 for the combustion .

Hot gases resulting from the combustion drive turbine blades 6 of the turbine part of the gas turbine engine and are guided by guide vanes 7. The combustion chamber 8 has a burner aperture 10 through which the burner 9 is fitted. The burner 9 comprises a burner space where the fuel and air is mixed before being released into the combustion chamber 8 via the burner aperture 10. A supplying means is adapted to supply liquid fuel through an internal passage inside the burner body member for said com ^¬ bustion. This liquid fuel is atomized and sprayed into said burner space of the burner 9 through a pilot fuel injector nozzle. An ignition means (not shown) is located close to said nozzle for igniting the fuel inside the burner space for commencing combustion.

One or more liquid fuel nozzles are located in said burner space 9 for supplying to the space atomized liquid fuel for evaporation and later combustion.

The general function and operational modes of an arrangement for preparation of the liquid fuel for combustion for in- stance in a gas turbine will now be explained with reference to Figures 2 and 3.

The present invention is based on the use of a recirculating circuit 10 as shown in Figures 2 and 3 for heating pressur- ized liquid fuel in order for it to evaporate more easily when expanded to a lower pressure and broken into droplet' s during start of the gas turbine allowing a more reliable ig ^¬ nition . The recirculation circuit 10 is arranged in parallel with the main liquid fuel feed 11 - downstream a fuel supply 12 and upstream the burners of the gas turbine supplied by a fuel supply 12 via the main fuel feed 11. The recirculation cir ^¬ cuit 10 comprises a reservoir 20 with one or more heaters 24 for heating liquid fuel inside the reservoir 20, a pipe 21 from the reservoir 20 towards the burners and a return pipe

22 to the opposite side of the reservoir 20 - all together completing a flowing circuit, i.e. the recirculating circuit 10 - so that liquid fuel, pressurized liquid fuel - supplied by the main fuel feed 11 into the reservoir 20 - could circu ^¬ late around this circuit 10.

The recirculation circuit 10 further comprises a small pump

23 arranged in the return pipe 22 for pumping the liquid fuel along the pipe 21 from the reservoir 20 towards the burners and to circulate in the recirculating circuit 10. A thermocouple 40 is arranged in the recirculating circuit 10, i.e. the thermocouple 40 is - as it can be seen in Fig ^¬ ures 2 and 3 - located in the pipe 21. The thermocouple 40 is generating signals - according to a temperature of the liquid fuel in the reservoir 20, i.e. circulating in the recirculating circuit 10. The signals are transmitted to a control sys ^¬ tem for controlling the temperature of the liquid fuel, i.e. a temperature in the recirculating circuit 10, as well as the operational modes of the recirculating circuit 10.

The recirculating circuit 10 is connected to the main fuel feed 11 by a first 30 and a second two-way valve 31 located at an inlet end 25 and an outlet end 26 of the recirculating circuit 10. The inlet end 25 - with the first valve 30 - is located upstream of the outlet end 26 - with the second valve 31 - in relation to the main fuel feed 11 so that liquid fuel flowing in the main fuel feed 11 could flow into the reser ^¬ voir 20 at the inlet end 30. The reservoir 20 could be evacu ^¬ ated via the outlet end 26, i.e. the second valve 31, back to the main fuel feed 11 again.

The reservoir 20 is sized for enough liquid fuel for two starts of a gas turbine as required. The valves 30, 31 could be switched - in control of the con ^¬ trol system - in different switch settings relating to dif ^¬ ferent operation modes as shown in Figures 2 and 3 and as de ^¬ scribed as follows. Figure 2 is illustrating the recirculating circuit 10 in a heating phase.

The gas turbine is preparing to start on liquid fuel. The reservoir 20 is filled with liquid fuel at a normal starting pressure. The heaters 24 are switched on. The two-way valves 30, 31 are switched so that the liquid fuel is isolated in the recirculating circuit 10. The small fuel pump 23 circu ^¬ lates the fuel, which dumps back into the inlet end 25 of the reservoir. The thermocouple 40 indicates when the temperature of the liquid fuel is high enough for the gas turbine start ^¬ ing . When this temperature, for example about 80 °C, is reached, the heaters 24 are turned off. If the temperature drops too low before the gas turbine is started, the heaters 24 are switched on again. Figure 3 is illustrating the recirculating circuit 10 in a starting phase.

The gas turbine is started. The two-way valves 30, 31 are switched so that the main fuel flow enters the inlet end 25 of the recirculating circuit 10 and the reservoir 20, and pushes the heated liquid fuel into the main fuel feed 11 - via the outlet end 26 of the recirculating circuit 10 - to the burner. After successful ignition of the burners, the valves 30, 31 could be switched back in the heating positions, i.e. in po ^¬ sition of/for the heating phase (Figure 2), or normal posi ^¬ tion, i.e. in position of/for the normal operating phase (Figure 2), so that the fuel supply 12 fed normal unheated fuel directly to the combustion system 8.

As looking at the switch settings of the valves 30, 31 as shown in Figures 2 and 3: the valve 31 could be switched to direct the liquid fuel either (1) back to the reservoir 20, in the heating phases, or (2) into the main fuel feed 11 to the burners, in a starting phase; the valve 30 could be switched to direct the liquid fuel of the main fuel feed 11 either (1) (directly) into the burners, both for normal op ^¬ eration and in the heating phase, or (2) towards the reser- voir 20, in the starting phase.

When the valves 30, 31 are in the starting positions, i.e. in position of/for the starting phase (Figure 3), a pressure of the liquid fuel of the main fuel supply 12 then "drives" the heated liquid fuel out of the reservoir 20, i.e. the recircu ^¬ lating circuit 10, into the burners, that means that heated liquid fuel can be directed into the burners when the gas turbine is started.

The heating operation will be stopped at a suitable tempera ^¬ ture - measured by the thermocouple 40 and controlled by the control system. This temperature will be high enough to sig- nificantly increase an amount of vaporisation of a fuel when it is atomised in the burner.

The recirculation circuit 10 is arranged in parallel with the main fuel feed 11, so that the heaters 24 can be removed for cleaning or replacement without interrupting the gas turbine operation .

Because the rate of heating does not need to be high, the temperature of the heaters 24 themselves does not need to be high. Also the continuous flow of liquid fuel around the re ^¬ circulation circuit 10 means that the liquid fuel is not in contact with the heaters 24 for an extended period of time. Both these factors reduce the risk of coking/overheating of liquid fuel by the heaters 24.

More generally, in a preferred embodiment, the invention may be directed to a method of preparing a liquid fuel for com ^¬ bustion according to any preceding method claim, used for preparing liquid fuel for starting a gas turbine, particu- larly starting a DLE gas turbine.

In a further preferred embodiment, the invention may be di ^¬ rected to a method of preparing a liquid fuel for combustion according to any preceding method claim, wherein said heated liquid fuel is directed to a burner 9 of a combustion system 3 of a gas turbine when the gas turbine is started.