Gas turbine fuel flexibility: pressurized swirl flame stability, thermoacoustics, and emissions

Power generation gas turbine manufacturers and operators are tasked increasingly with expanding operational flexibility due to volatility in global gaseous fuel supplies and increased renewable power generation capacity. Natural gas containing high levels of higher hydrocarbons (e.g. ethane and propane) is typical of liquefied natural gas and shale gas, two natural gas sources impacting gas turbine operations, particularly looking forward in the United Kingdom. In addition, hydrogen-blending into existing natural gas infrastructure represents a potential energy storage opportunity from excess renewable power generation, with associated combustion impacts not fully appreciated. This thesis aims to address the specific operational problems associated with the use of variable gaseous fuel compositions in gas turbine combustion through a combination of experimental and numerical techniques, with a focus on natural gas blends containing increased levels of higher hydrocarbons and hydrogen. Parametric experimental combustion studies of the selected fuel blends are conducted in a new fully premixed generic swirl burner at elevated ambient conditions of temperature and pressure to provide representative geometry and flow characteristics typical of a can-type industrial gas turbine combustor. New non-intrusive diagnostic facilities have been designed and installed at Cardiff University’s Gas Turbine Research Centre specifically for the characterization of the influence of fuel composition, burner geometry, and operating parameters on flame stability, flame structure, thermoacoustic response, and environmental emissions. Experimental measurements are supported through the use of numerical chemical kinetics and acoustic modelling. Results from this thesis provide an experimental validation database for chemical kinetic reactor network and CFD modelling efforts. In addition, it informs gas turbine manufacturers on potential burner design modifications for future fuel flexibility and provide enhanced empirical tools to power generation gas turbine operators for increased operational stability, reduced environmental impact, and increased utilization

Development and commissioning of a chemiluminescence imaging system for an optically-accessible high-pressure generic swirl burner

A chemiluminescence imaging system has been commissioned at Cardiff University’s Gas Turbine Research Centre. OH* and CH* chemiluminescence measurements were initially made on swirl-stabilised methane flames to find the optimal settings of intensifier gate timing, gain, and UV lens f-stop. Measurements with gate widths down to 100 μs have been achieved on methane flames with thermal powers up to 100 kW, pressures up to 3 bara, and global equivalence ratios of 0.6 to 1.2. OH* and CH* chemiluminescence intensities are found to vary with each parameter and yield sufficient spatial information to confirm visual evidence of stable flame operation as well as both lean and rich stability limits. Additionally, the OH*/CH* chemiluminescence intensity ratio is used for evaluation of the local equivalence ratio within the flame. Further OH* and CH* chemiluminescence measurements were made on BOS gas (65% CO, 34% N2, 1% H2) flames at comparable conditions to the CH4 flames to investigate the change in intensities, with marked variation identified between the two fuel blends. In addition to developing measurement capability, image processing and deconvolution techniques have also been integrated into the chemiluminescence system, extending the fundamental combustion research capabilities of the Gas Turbine Research Centre

Lean methane flame stability in a premixed generic swirl burner: Isothermal flow and atmospheric combustion characterization

Gas turbine combustors operating in lean premixed mode are known to be susceptible to flame blowoff due to competing influences of increasing chemical timescales and decreasing flow time scales under these conditions. In this study, combustion stability and the onset of flame blowoff in particular, are characterized in a new swirl burner operated with fully premixed methane (CH4) and air at thermal power of 55 kW, atmospheric combustor inlet pressure, and ambient (∼290 K) combustor inlet temperature. The onset of flame blowoff was shown repeatedly to exhibit high amplitude, low frequency combustion instabilities as a result of periodic flame extinction and reignition events. In addition to detailed isothermal characterization of the burner velocity field using particle image velocimetry, a combination of dynamic pressure sensing and optical combustion diagnostics, including OH∗ chemiluminescence and OH planar laser induced fluorescence, give indication of the combustion rig acoustic response and changes in flame acoustic response, heat release, and flame anchoring location related to the onset and occurrence of blowoff. This analysis shows that the onset of this instability was preceded by a marked reduction in dominant frequency and amplitude until frequency collapse and high amplitudes were observed throughout the burner inlet mixing plenum, burner pilot, combustion chamber, and exhaust ducting. Acoustic and optical signal analysis show potential viability for use in practical applications for precursor indications of lean blowoff. The flame anchoring location within the combustion chamber was shown to detach from the burner exit nozzle and stabilize within the outer and central recirculation zones near the lean blowoff limit, providing evidence of changes to both chemical and flow time scales. Chemical kinetic modelling is used in support of the empirical studies, in particular highlighting the relationship between maximum heat release rate and OH∗ chemiluminescence intensity

Catalytic influence of water vapor on lean blowoff and NOx reduction for pressurised swirling syngas flames

It has become increasingly cost-effective for the steel industry to invest in the capture of heavily carbonaceous BOF (Basic Oxygen Furnace) or converter gas, and use it to support the intensive energy demands of the integrated facility, or for surplus energy conversion in power plants. As industry strives for greater efficiency via ever more complex technologies, increased attention is being paid to investigate the complex behavior of by-product syngases. Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O on heavily carbonaceous syngas mixtures. Direct formation of CO2 from CO is slow due to its high activation energy, and the presence of disassociated radical hydrogen facilitates chain branching species (such as OH), changing the dominant path for oxidation. The observed catalytic effect is non-monotonic, with the reduction in flame temperature eventually prevailing, and overall reaction rate quenched. The potential benefits of changes in water loading are explored in terms of delayed lean blowoff, and primary emission reduction in a premixed turbulent swirling flame, scaled for practical relevance at conditions of elevated temperature (423 K) and pressure (0.1-0.3 MPa). Chemical kinetic models are used initially to characterize the influence that H2O has on the burning characteristics of the fuel blend employed, modelling laminar flame speed and extinction strain rate across an experimental range with H2O vapor fraction increased to eventually diminish the catalytic effect. These modelled predictions are used as a foundation to investigate the experimental flame. OH* chemiluminescence and OH planar laser induced fluorescence (PLIF) are employed as optical diagnostic techniques to analyze changes in heat release structure resulting from the experimental variation in water loading. A comparison is made with a CH4/air flame and changes in lean blow off stability limits are quantified, measuring the incremental increase in air flow and again compared against chemical models. The compound benefit of CO and NOx reduction is quantified also, with production first decreasing due to the thermal effect of H2O addition from a reduction in flame temperature, coupled with the potential for further reduction from the change in lean stability limit. Power law correlations have been derived for change in pressure, and equivalent water loading. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed for laminar flames, are compared against the challenging environment of turbulent, swirl-stabilized flames at elevated temperature and pressure, characteristic of piratical systems

Ammonia-methane combustion in tangential swirl burners for gas turbine power generation

Ammonia has been proposed as a potential energy storage medium in the transition towards a low-carbon economy. This paper details experimental results and numerical calculations obtained to progress towards optimisation of fuel injection and fluidic stabilisation in swirl burners with ammonia as the primary fuel. A generic tangential swirl burner has been employed to determine flame stability and emissions produced at different equivalence ratios using ammonia–methane blends. Experiments were performed under atmospheric and medium pressurised conditions using gas analysis and chemiluminescence to quantify emission concentrations and OH production zones respectively. Numerical calculations using GASEQ and CHEMKIN-PRO were performed to complement, compare with and extend experimental findings, hence improving understanding concerning the evolution of species when fuelling on ammonia blends. It is concluded that a fully premixed injection strategy is not appropriate for optimised ammonia combustion and that high flame instabilities can be produced at medium swirl numbers, hence necessitating lower swirl and a different injection strategy for optimised power generation utilising ammonia fuel blends

Additive manufacture and the gas turbine combustor: challenges and opportunities to enable low-carbon fuel flexibility

Advances in gas turbine (GT) combustion are enabled by metal additive manufacturing (AM) using selective laser melting (SLM) and other methods. In future low-carbon energy systems, AM will be critical for GTs operating on fuels such as hydrogen, ammonia, and biofuels. This paper evaluates the impact of AM on GT combustors, focusing on design freedom for novel geometries, reduced product development timelines, multiple component integration, and high-temperature materials suitable for harsh environments. Current AM challenges and research needs for GT combustors are discussed with industry input. These challenges are shown to be priority R&D areas across the GT value chain. Recent academic advances show the positive influence of widening access to SLM platforms and AM facilitates research using materials and geometries relevant to the GT community. Micro GTs are well-suited to SLM platforms, enabling novel geometries incorporating multiple functional parts including heat exchangers and porous media using advanced metal alloys. For industrial GTs, AM reduces new combustor product development time, as rapid prototyping and testing complements numerical methods. This review provides compelling evidence for continued AM R&D for GT combustion applications to meet future decarbonization goals

Development and commissioning of a chemiluminescence imaging system for an optically-accessible high-pressure generic swirl burner

A chemiluminescence imaging system has been commissioned at Cardiff University’s Gas Turbine Research Centre. OH* and CH* chemiluminescence measurements were initially made on swirl-stabilised methane flames to find the optimal settings of intensifier gate timing, gain, and UV lens f-stop. Measurements with gate widths down to 100 μs have been achieved on methane flames with thermal powers up to 100 kW, pressures up to 3 bara, and global equivalence ratios of 0.6 to 1.2. OH* and CH* chemiluminescence intensities are found to vary with each parameter and yield sufficient spatial information to confirm visual evidence of stable flame operation as well as both lean and rich stability limits. Additionally, the OH*/CH* chemiluminescence intensity ratio is used for evaluation of the local equivalence ratio within the flame. Further OH* and CH* chemiluminescence measurements were made on BOS gas (65% CO, 34% N2, 1% H2) flames at comparable conditions to the CH4 flames to investigate the change in intensities, with marked variation identified between the two fuel blends. In addition to developing measurement capability, image processing and deconvolution techniques have also been integrated into the chemiluminescence system, extending the fundamental combustion research capabilities of the Gas Turbine Research Centre