Gas turbine inlet temperature suppression during under frequency events and related method

This invention relates to gas turbine operation, particularly during under-frequency events when demand exceeds supply.

When electricity demand exceeds supply into a grid, the grid frequency will dip below the target of either 50 or 60 Hz. For gas turbine based power plants, the power capability of the gas turbine typically decreases as the frequency decreases. Accordingly, in order to provide grid stability during large frequency dips, the output of the gas turbine power plant may need to be increased, at least temporarily. The output power of a gas turbine used to generate electricity can be increased by, for example, additional compressor mass flow or additional fuel flow. An increase in compressor mass flow, however, may be limited due to compressor surge margin or other limitations, while an increase in fuel flow may be limited due to parts life considerations, resulting from operation beyond normal operating temperatures.

Accordingly, there is a need for a mechanism by which gas turbine output power may be augmented during short term under-frequency events without requiring the turbine to be fired at higher than normal operating temperatures, or to minimize the increase in firing temperature to meet the requirements.

In accordance with an exemplary but non-limiting implementation of the technology disclosed herein, advantage is taken of the known phenomenon that a decrease in compressor inlet temperature typically increases gas turbine output capability. Thus, there is provided a method of augmenting power output in a gas turbine electrical power-generating plant (or other mechanical drive application) comprising a multistage compressor, a combustor and a multistage turbine component, during events when grid frequency drops below a predetermined target frequency, the method comprising: a) providing a supply of liquid or liquified air, or blend of gases (subsequently referred to as “Air” for simplicity), arranged to permit selective addition of liquified air to an ambient air inlet to the compressor; and b) flowing controlled amounts of the liquified air into the ambient air inlet during the events.

In another aspect, the invention relates to a gas turbine electric power generating plant comprising: a multi-stage compressor having an ambient air inlet; a multi-stage turbine component; a combustor arranged to receive compressed air from the compressor and to supply gaseous combustion products to the multi-stage turbine; and a source of liquified air arranged to supply liquified air to the ambient air inlet of the compressor.

The invention will now be described in connection with the drawing identified below.

With reference to the Figure, a gas turbine plant 10 includes a multistage compressor 12 that supplies air to a combustor 14 which, in turn, supplies hot combustion gases to a multi-stage gas turbine 16. As illustrated, the compressor 12 and turbine 16 operate on a common rotor shaft 18 which may also be connected to a generator (not shown) downstream of the turbine 16. Other turbine arrangements, may also benefit from this invention. The gas turbine arrangement per se, is not the subject of this invention, and need not be described in any further detail.

In accordance with an exemplary but non-limiting embodiment of the invention, substantially open-ended inlet plenum 20 to the compressor 12 is arranged to supply cooled intake air to the compressor inlet 22. A storage tank 24 is arranged to supply liquified air via conduit 26 to an injection manifold 28 comprised of plural nozzles 30. Liquified air is air that has been cooled to very low temperatures by means of compression and heat removal. It has a density of about 870 kg/M³, which may vary depending on the elemental composition of the air. The liquified air injection is controlled by a control valve 32 in the conduit 24, upstream of the manifold 28 and nozzles 30.

Placement of the manifold 28 and associated nozzles 30 may be varied within the plenum 20. For example, by locating the nozzles closer to the plenum inlet 34, a more uniform temperature may be achieved as the air flows toward the compressor inlet 22, but some undesirable temperature increase along the path may result. Cooler but less uniform temperature may be achieved by placing the manifold 28 and associated nozzles 30 closer to the compressor inlet 22. Thus, the exact placement of the plenum 28 and nozzles 30 will depend on specific applications, but is well within the ability of the ordinarily skilled worker in the art.

The above-described inlet arrangement allows short term output power augmentation by injecting liquified air into the inlet system of the compressor 12 to thereby decrease the temperature of the ambient air entering the compressor 12. The decrease in temperature with increased flow of liquid Air, increases the power output of the gas turbine 16, at least temporarily, to a level that would otherwise require the turbine 16 to be fired beyond its normal operating temperatures.

The control scheme may be one that is based on power output requirements at various frequency levels, or on maintenance of a particular compressor inlet temperature. It may also be advantageous to control the supply of liquified air via valve 32 to the minimum amount required to maintain a required output at predetermined frequency levels, i.e., on an intermittent or modulated basis that conserves the expensive liquified air, with longer replenishment intervals.

It will be appreciated that the power augmentation achieved by additional compressor mass flow as described herein may improve compressor surge margin and parts life.