Adaptation and Characterization of Fossilized Hydrocarbon Gas Turbine Combustor for Operation with Green Fuel(s): Experimental and Numerical Investigation

Abstract

Decarbonizing energy conversion systems necessitates the adaptation of conventional fossil-fuel-based systems, which have been designed and optimized over the past century for operation with fossilized hydrocarbons, to operate with green, non-regular fuels. This research focuses on the design intervention and optimization of a gas turbine combustion chamber initially designed to operate with Aviation Turbine Fuel (C12H23) for operation with bioderived gaseous fuels such as producer gas (PG) and syngas (SG). The study employs both experimental and multi-dimensional numerical approaches to address design considerations and process interventions.

A single-can reverse flow type combustion chamber, used in the Auxiliary Power Unit (APU) of a Rover gas turbine with a designed thermal input of 540 kWth using Aviation Turbine Fuel , was analysed. The gas turbine combustion chamber was tested with two gaseous mixtures generated from the thermo-chemical conversion of biomass: PG, composed of H2 (18.15%), CO (18.45%), CH4 (2.12%), CO2 (11.56%), and N2 (49.72%) by volume basis; and SG, composed of H2 (41.61%), CO (18.96%), CH4 (4.09%), and CO2 (35.34%) by volume basis, with lower calorific values of 5 MJ/kg and 10 MJ/kg, respectively, and stoichiometric air-to-fuel ratios of 1.35 for PG and 3.11 for SG. Initial operation of the original combustion chamber with these gases resulted in a maximum thermal input of 19 kWth for PG and 35 kWth for SG, constrained by the need to maintain the flame position within 50% of the dilution zone and acceptable combustor exit emissions. Zero-dimensional thermodynamic analysis indicated that achieving the full thermal input potential for the combustor required optimization. Based on detailed thermo-physical analysis of the two fuels and the combustion chamber geometry, optimization efforts focused on controlling fuel and airflow rates and modifying the air inlet geometry across the primary, secondary, and dilution zones. Multi-dimensional numerical simulations of the combustion chamber and casing were conducted, involving parametric analysis of total thermal energy input and air-to-fuel ratio distribution across the three zones. The Realizable k-ε model was employed to close the Reynold Average Navier Stoke (RANS) equations, and a steady diffusion non-premixed combustion model was adopted for gas-phase reaction kinetics. Skeletal/reduced mechanisms, developed using the GRI Mech 3.0 mechanism and validated with in-house laminar flame speed experiments, were used for temporal optimization of the simulations.

Experimental investigation outside the Auxiliary Power Unit structure required designing a casing for the combustor consistent with the gas turbine setup. Numerical simulations revealed asymmetry in the fluid flow field within the combustor for PG and SG, attributed to their high fuel flow rates and low airflow rates. This was not observed under Aviation Turbine Fuel conditions. Computational Fluid Dynamics (CFD) studies suggested transitioning from a single tangential entry (original combustor housing) to a double tangential entry casing (new combustor casing) to address the observed asymmetry. Hot flow experiments revealed issues such as temperature reversal and incomplete combustion, indicating improper mixing within the original combustion chamber. Redesigning the cross-section of the air inlets to enhance flow rates in the primary zone improved combustion efficiency and eliminated temperature reversal. The modified combustion chamber supported thermal inputs of 151.5 kWth (ṁf-PG: 121.2 kg/hr) for PG and 262.3 kWth (ṁf-SG: 93.5 kg/hr) for SG, constituting 30% and 50% of the rated thermal input, respectively, due to air supply limitations. At these conditions, the maximum CO concentration at the combustor exit was found to be 50 ppmv, with no temperature reversal, meeting all gas turbine qualification criteria. Numerical simulations at the rated thermal input (540 kWth) confirmed that key parameters such as CO, NOx, and flame tip position were within acceptable limits.

This research demonstrates the feasibility and effectiveness of adapting conventional gas turbine combustion chambers for bioderived gaseous fuels through targeted design optimizations and multi-dimensional numerical analyses. The findings provide a pathway for enhancing the performance of gas turbines running on low-calorific green fuels, contributing significantly to the decarbonization of energy conversion systems.