Power plants – Combustion products used as motive fluid
Patent
1994-04-07
1995-11-28
Casaregola, Louis J.
Power plants
Combustion products used as motive fluid
60 39281, 60746, F02C 928
Patent
active
054697000
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a fuel control system for a gas turbine engine, particularly an engine whose combustion system has been designed with a view to reducing emission levels of pollutants in its exhaust gases.
There is currently worldwide awareness of the need to limit the pollutants emitted from fossil fuelled engines. This concern is being expressed in terms of increasingly strict legislation in the leading industrial countries.
Pollutants emitted from gas turbine engines are directly related to the temperature at which the air and fuel reactants are burnt in their combustion systems. The principle pollutants are emissions of nitrous oxides (NOx), which begin forming at high temperatures and increase exponentially with increasing temperature, carbon monoxide (CO), and unburned hydrocarbons (UHC). Both the latter two pollutants are produced due to incomplete or weak combustion, typically at low combustion temperatures.
Current technology adopted to meet stricter NOx emission standards includes water or steam injection into appropriately modified conventional types of gas turbine engine combustors. However, this expedient does not necessarily enable the engines to meet all CO limits. A further difficulty with water or steam injection is that it may not be practical in remote or arid locations to provide an adequate supply of suitable water.
It has therefore been necessary to design and develop combustors using alternative combustion techniques which enable combustion temperatures to be kept within certain limits known to substantially avoid pollutant production. The two available techniques are "rich burn rapid quench" and "premix lean burn" and both accomplish the objective by avoiding the high temperatures generated at near-stoichiometric conditions. The terms "rich" and "lean" convey the magnitude of the air/fuel mixture strength with respect to stoichiometric combustion conditions.
One design of combustor employing the premix lean burn technique is shown in copending International patent application number PCT/GB91/01658, having the International publication number WO92/07221, which is hereby incorporated by reference. In this design, combustion occurs in one, two or three successive stages according to the ambient temperature and the power levels being produced. At each stage, controlled amounts of fuel and air are injected, mixed together and burnt in particular ways as disclosed in the patent application and as also described later in this specification. This strategy enables much reduced emissions of NOx, CO and UHC compared to conventional combustors.
One of the problems inherent in employing "lean" fuel/air mixtures is that they necessitate operating the combustion system in close proximity to the reactants' weak extinction limit. Therefore, to provide low emissions over a wide operating range demands that the air/fuel ratio is modulated in some way to sustain combustion, particularly at part-load conditions, where the mixture would otherwise be too weak to burn. The use of staged combustion provides a means of subdividing the expansion of the combustion reactants into a number of smaller, easier to manage, processes. Thus, air/fuel ratios in each stage can be individually set, thereby effectively achieving the desirable modulation of the air/fuel ratio for the overall process without recourse to potentially unreliable variable geometry hardware features for changing airflows through the combustor. Of course, to meet the pollutant emission targets, the combustion temperature within each of these stages must be controlled within the correct narrow temperature bands.
It is an object of the present invention to provide an improved form of fuel control system for regulating fuel flow to a plurality of combustion stages in combustors employing premix lean burn staged combustion techniques.
It is a further object of the invention to provide an improved method of regulating fuel flow to such a staged combustor.
Another object of the invention is to limit the production of NO.sub.x, CO and UHC emiss
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Owen, D. A. et al., "Low Emissions Combustor Design Options for an Aero Derived Industrial Gas Turbine," Canadian Gas Association Symposium on Industrial Application of Gas Turbines, Banff, Alberta, Oct. 16-18, 1991.
Corbett Nicholas C.
Lines Norman P.
Casaregola Louis J.
Rolls-Royce plc
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