Design methods of the ABB Alstom Power gas turbine dry low emission combustion system
Authors: Alkabie H 
Published: October 2002

This paper describes the conceptual design and aerothermal approach of the ABB ALSTOM POWER UK Limited (ABB ALSTOM POWER) dry low emission (DLE) combustion system directed towards all current and future power ranges. A set of parameters was employed as a common criterion to design the company's combustion system across the product range. The parameters made it possible to scale the combustion system to operate at any required firing temperature without the penalty of achieving the low emission targets. The logic and sensible air mass mapping and testing management based on a basic knowledge of the aerothermodynamics of the combustion system and its integral parts produced reliable operation and engine results.

The combustion system aerothermodynamic design considers the constraints of maintaining combustor specifications while introducing a reliable ultra-low NO x concept with minimal structural change to engine design. Furthermore, the ignition loop, piloting and stability margins were issues continuously assessed through a defined application programme to examine their impact on engine operability at various load conditions.

A main radial swirler injector is utilized for both gas and liquid to produce a fuel lean mixture for combustion at full-load operating conditions, while a pilot fuel injector guarantees a local fuel-rich zone to sustain stability at low power conditions. The carefully planned switch-over between the pilot and main swirler provides smooth operation and low emission across the load range conditions.

The company combustion system relies upon the principle of aerodynamic variable mixing with a single vortex generator burner. The partial premixing process takes place at the radial inflow swirler slots before the fuel—air mixture is introduced into a prechamber and thereafter into the combustion chamber. For vane passage injection the liquid liquid fuel atomization should be at an optimum and the vane passage and prechamber length set to provide the vaporization period. The impingement cooling scheme aided by a thin layer of thermal barrier coating (TBC) achieved an excellent combustor wall temperature at all operating conditions and across the DLE engines product range. The scheme made it possible to direct a large amount of air towards the swirler to achieve the lean premix combustion.

Substantial CO emission reduction at part load was achieved by variable guide vanes (VGV) modulation for the single-shaft engines. This action inhibits NO formation without slowing the CO oxidation process to a frozen condition. The variable mixing regime approach and accurate air management has demonstrated an ultra-low NO x emission of 12–15 and 32–35 ppmv (corrected to 15% O2) at the full-load condition while operating on gas and distillate-2 fuel respectively. Moreover, the system exhibited low CO and dynamics oscillation capability for both gas and liquid fuel operation.

At the present time, the one-dimensional analysis has been proved to be a reliable tool for combustion system designers to produce a preliminary design configuration. The particular choice for a commercial computational fluid dynamics (CFD) code to satisfy various criteria of the swirling combusting flow is proving to be difficult due to constraints regarding slover and/or methods limitation.

The results obtained from the high-pressure air facilities, engine beds and field trials represent a true technological achievement through collective engineering design efforts towards emission and pollution control for small- and medium-range gas turbine engines.