This paper reports on the CO turndown of axially staged combustor, in the context of industrial gas turbines, using chemical reactor network (CRN) modelling. The study investigates the impact of various operating conditions on the exhaust CO emission of each stage, as well as at the combustor exit. Different combustor residence time combinations are explored to determine their potential to mitigate these effects.

The two-stage combustor modelled in this study consists of two combustion zones followed by a discharge nozzle. The primary combustion stage is modelled using a freely propagating flame (FPF), with the exhaust gas extracted at a specified residence time downstream of the flame. These exhaust gases are then mixed and combusted with fresh gases in the secondary combustion stage, modelled by a perfectly stirred reactor (PSR) operating within a specified residence time. These combined gases then flow into the discharge nozzle, which is modelled by a plug flow reactor (PFR) that cools the gas to varying combustor exit temperatures within another specified residence time. Together, these form a simplified CRN model of a dry-low emissions (DLE) combustion system and allows the investigation of how CO turndown behaves.

In this parametric study, the residence times of the primary combustion stage and secondary combustion stage are varied between 0.5 and 2.5 ms. Then, the discharge nozzle residence time is set as the balance of an overall combustor residence time of 10 ms, indicative of DLE combustion systems. This allows the comparison of CO turndown variation between combustion stages of varying characteristic residence times. Next, the effect of pressure is studied to determine how part-load engine pressure reduction can impact exhaust CO concentrations. An equilibrium analysis is then conducted to observe the impact of pressure on the equilibrium CO concentration, given the accompanying increase in combustor inlet temperature. The effect of nominal stoichiometry is then investigated to determine the magnitude of exhaust CO difference for this system. Finally, the primary and secondary fuel flow rate splits are varied to determine their effect on CO emissions, as they change with engine load. The independent variation of these variables allows the observation of their individual impact on CO turndown.

The conclusions from this parametric study are then extended by generating a model engine power curve. The increase in the combustor inlet temperature and pressure, and fuel and air flow rates with power are modelled. The adiabatic flame temperature of the primary combustion stage is kept constant with increasing power to maintain flame stability, while the secondary combustion stage fuel flow is increased to meet the power demand. A constant characteristic discharge nozzle cooling rate is then applied across the load spectrum to model the decrease in the combustor exit temperature as power is decreased. Thus, the model engine study allows the investigation of the coupled effects of these variables on exhaust CO emissions. This provides a first order optimization of the CO turndown of an engine given its operating conditions and residence time.


Optimisation of CO Turndown for an Axially Staged Gas Turbine Combustor

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.