Dry Low Emission Combustion System Research Articles

A Stochastic and Bayesian Inference Toolchain for Uncertainty and Risk Quantification of Rare Autoignition Events in Dry Low-Emission Premixers

Abstract Quantification of aleatoric uncertainties due to the inherent variabilities in operating conditions and fuel composition is essential for designing and improving premixers in dry low-emissions (DLE) combustion systems. Advanced stochastic simulation tools require a large number of evaluations in order to perform this type of uncertainty quantification (UQ) analysis. This task is computationally prohibitive using high-fidelity computational fluid dynamic (CFD) approaches such as large eddy simulation (LES). In this paper, we describe a novel and computationally efficient toolchain for stochastic modeling using minimal input from LES, to perform uncertainty and risk quantification of a DLE system. More specially, high-fidelity LES, chemical reactor network (CRN) model, beta mixture model, Bayesian inference and sequential Monte Carlo (SMC) are integrated into the toolchain. The methodology is applied to a practical premixer of low-emission combustion system with dimethyl ether (DME)/methane–air mixtures to simulate auto-ignition events at different engine conditions. First, the benchmark premixer is simulated using a set of LESs for a methane/air mixture at elevated pressure and temperature conditions. A partitioning approach is employed to generate a set of deterministic chemical reactor network (CRN) models from LES results. These CRN models are then solved at the volume-average conditions and validated by LES results. A mixture modeling approach using the expectation-method of moment (E-MM) is carried out to generate a set of beta mixture models and characterize uncertainties for LES-predicted temperature distributions. These beta mixture models and a normal distribution for DME volume fraction are used to simulate a set of stochastic CRN models. The Bayesian inference approach through SMC method is then implemented on the results of temperature distributions from stochastic CRN models to simulate the probability of auto-ignition in the benchmark premixer. The results present a very satisfactory performance for the stochastic toolchain to compute the auto-ignition propensity for a few events with a particular combination of inlet temperature and DME volume fraction. Characterization of these rare events is computationally prohibitive in the conventional deterministic methods such as high-fidelity LES.

Extension of Fuel Flexibility by Combining Intelligent Control Methods for Siemens SGT-400 Dry Low Emission Combustion System

Extension of gas fuel flexibility of a current production SGT-400 industrial gas turbine combustor system is reported in this paper. A SGT-400 engine with hybrid combustion system configuration to meet a customer's specific requirements was string tested. This engine was tested with the gas turbine package driver unit and the gas compressor-driven unit to operate on and switch between three different fuels with temperature-corrected Wobbe index (TCWI) varying between 45 MJ/m3, 38 MJ/m3, and 30 MJ/m3. The alteration of fuel heating value was achieved by injection or withdrawal of N2 into or from the fuel system. The results show that the engine can maintain stable operation on and switching between these three different fuels with fast changeover rate of the heating value greater than 10% per minute without shutdown or change in load condition. High-pressure rig tests were carried out to demonstrate the capabilities of the combustion system at engine operating conditions across a wide range of ambient conditions. Variations of the fuel heating value, with Wobbe index (WI) of 30 MJ/Sm3, 33 MJ/Sm3, 35 MJ/Sm3, and 45 MJ/Sm3 (natural gas, NG) at standard conditions, were achieved by blending NG with CO2 as diluent. Emissions, combustion dynamics, fuel pressure, and flashback monitoring via measurement of burner metal temperatures, were the main parameters used to evaluate the impact of fuel flexibility on combustor performance. Test results show that NOx emissions decrease as the fuel heating value is reduced. Also note that a decreasing fuel heating value leads to a requirement to increase the fuel supply pressure. Effect of fuel heating value on combustion was investigated, and the reduction in adiabatic flame temperature and laminar flame speed was observed for lower heating value fuels. The successful development program has increased the capability of the SGT-400 standard production dry low emissions (DLE) burner configuration to operate with a range of fuels covering a WI corrected to the normal conditions from 30 MJ/N·m3 to 49 MJ/N·m3. The tests results obtained on the Siemens SGT-400 combustion system provide significant experience for industrial gas turbine burner design for fuel flexibility.

Experimental and Numerical Investigation of Fuel–Air Mixing in a Radial Swirler Slot of a Dry Low Emission Gas Turbine Combustor

This paper presents the results of an investigation in which the fuel/air mixing process in a single slot within the radial swirler of a dry low emission (DLE) combustion system is explored using air/air mixing. Experimental studies have been carried out on an atmospheric test facility in which the test domain is a large-scale representation of a swirler slot from a Siemens proprietary DLE combustion system. Hot air with a temperature of 300 °C is supplied to the slot, while the injected fuel gas is simulated using air jets with temperatures of about 25 °C. Temperature has been used as a scalar to measure the mixing of the jets with the cross-flow. The mixture temperatures were measured using thermocouples while Pitot probes were used to obtain local velocity measurements. The experimental data have been used to validate a computational fluid dynamics (CFD) mixing model. Numerical simulations were carried out using CFD software ansys-cfx. Due to the complex three-dimensional flow structure inside the swirler slot, different Reynolds-averaged Navier–Stokes (RANS) turbulence models were tested. The shear stress transport (SST) turbulence model was observed to give best agreement with the experimental data. The momentum flux ratio between the main air flow and the injected fuel jet, and the aerodynamics inside the slot were both identified by this study as major factors in determining the mixing characteristics. It has been shown that mixing in the swirler can be significantly improved by exploiting the aerodynamic characteristics of the flow inside the slot. The validated CFD model provides a tool which will be used in future studies to explore fuel/air mixing at engine conditions.

Investigation of Flow Aerodynamics for Optimal Fuel Placement and Mixing in the Radial Swirler Slot of a Dry Low Emission Gas Turbine Combustion Chamber

This paper is concerned with optimizing the fuel–air mixing processes that take place within the radial swirler slot of a dry low emission (DLE) combustion system. The aerodynamics of the flow within the slot is complex and this, together with the placement of the fuel holes with cross injection, controls the mixing of the fuel and air. Computational fluid dynamics (CFD) with the shear stress transport (SST) (k–ω) turbulence model was used for flow and mixing predictions within the radial swirler slot and for conducting a CFD-based design of experiments (DOE) optimization study, in which different parameters related to the fuel injection holes were varied. The optimization study was comprised of 25 orthogonal design configurations in the Taguchi L25 orthogonal array (OA). The test domain for the CFD, and its experimental validation, was a large-scale representation of a swirler slot from the Siemens proprietary DLE combustion system. The DOE study showed that the number of fuel holes, injection hole diameter, and interhole distance are the most influential parameters for determining optimal fuel mixing. Consequently, the optimized mixing configuration obtained from the above study was experimentally tested on an atmospheric test facility. The mixing patterns from experiments at various axial locations across the slot are in good agreement with the mixing predictions from the optimal CFD model. The optimized fuel injection design improved mixing compared with the baseline design by about 60%.

Effect of Change in Fuel Compositions and Heating Value on Ignition and Performance for Siemens SGT-400 Dry Low Emission Combustion System

The influence of changes in fuel composition and heating value on the performance of an industrial gas turbine combustor was investigated. The combustor tested was a single cannular combustor for Siemens SGT-400 13.4 MW dry low emission engine. Ignition, engine starting, emissions, combustion dynamics, and flash back through burner metal temperature monitoring were among the parameters investigated to evaluate the impact of fuel flexibility on combustor performance. Lean ignition and extinction limits were measured for three fuels with different heat values in term of Wobbe Index (WI): 25, 28.9, and 45 MJ/Sm3 (natural gas). The test results show that the air fuel ratio at lean ignition/extinction limits decreases and the margin between the two limits tends to be smaller as fuel heat value decreases. Engine start tests were also performed with a lower heating value fuel and results were found to be comparable to those for engine starting with natural gas. The combustor was further tested in a high pressure air facility at real engine operating conditions with different fuels covering WIs from 17.5 to 70 MJ/Sm3. The variation in fuel composition and heating value was achieved in a gas mixing plant by blending natural gas with CO2, CO, N2, and H2 (for the fuel with WI lower than natural gas) and C3H8 (for the fuel with WI higher than natural gas). Test results show that a benefit in NOx reduction can be seen for the lower WI fuels without H2 presence in the fuel and there are no adverse impacts on combustor performance except for the requirement of higher fuel supply pressure, however, this can be easily resolved by minor modification through the fuel injection design. Test results for the H2 enriched and higher WI fuels show that NOx, combustion dynamics and flash back have been adversely affected and major change in burner design is required. For the H2 enriched fuel, the effect of CO and H2 on combustor performance was also investigated for the fuels having a fixed WI of 29 MJ/Sm3. It is found that H2 dominates the adverse impact on combustor performance. The chemical kinetic study shows that H2 has significant effect on flame speed change and CO has significant effect on flame temperature change. Although the tests were performed on the Siemens SGT-400 combustion system, the results provide general guidance for the challenge of industrial gas turbine burner design for fuel flexibility.

Extension of Fuel Flexibility in the Siemens Dry Low Emissions SGT-300-1S to Cover a Wobbe Index Range of 15 to 49 MJ/Sm3

The extension of gas fuel flexibility in the Siemens SGT-300 single shaft (SGT-300-1S) is reported. A successful development program has increased the capability of the Siemens Industrial Turbomachinery, Lincoln (SITL) dry low emissions (DLE) burner configuration to a fuel range covering a Wobbe index (WI) from 15 to 49 MJ/Sm3. The WI reported in this paper is at a 15 °C fuel temperature. The standard SGT-300-1S SITL DLE combustion hardware allows for gas and liquid fuels within a specified range typically associated with natural gas and diesel, respectively. The range of the WI associated with natural gas is 37–49 MJ/Sm3. Field operation of the standard production SGT-300-1S has confirmed the reliable operation with an extension to the fuels range to include processed landfill gas (PLG) from 30 to 49 MJ/Sm3. The further extension of the fuel range for the SGT-300-1S SITL DLE combustion system was achieved through high pressure testing of a single combustion system at engine operating conditions and representative fuels. The variations in the fuel heating value were achieved by blending natural gas with diluent CO2 and/or N2. Various diagnostics were used to assess the performance of the combustion system, including the measurement of combustion dynamics, temperature, fuel supply pressure, and the emissions of NOx, CO, and unburned hydrocarbons (UHCs). The results of the testing showed that the standard production burner can operate for a fuel with a WI as low as 23 MJ/Sm3, which corresponds to 35% CO2 (by volume) in the fuel. This range can be extended to 15 MJ/Sm3 (54.5% CO2 in the fuel) with only minor modification to control losses through the burner and to maintain similar fuel injection characteristics. The SITL DLE combustion system is able to cover a WI range of 15 to 49 MJ/Sm3 in two configurations. The results of testing showed a lowering in the WI, by diluting with CO2 and/or N2, so that a benefit in the NOx reduction is observed. This decrease in the WI may lead to an increased requirement of the fuel supply pressure.

The influence of changes in fuel calorific value to combustion performance for Siemens SGT-300 dry low emission combustion system

The effect of changes in fuel calorific value on combustion was investigated on a single standard Siemens SGT-300 7.9MW dry low emission (DLE) combustor in a high pressure rig at actual engine operating conditions. The change in fuel calorific value was achieved by blending CO2 and/or N2 with Natural Gas. The lowest fuel calorific value was tested at a Wobbe Index (WI) of 23MJ/m3 which was associated with 35% CO2 (in volume) added to Natural Gas. The engine loads tested varied from 5% to full load. NOx and CO emissions were measured in addition to the frequency and amplitudes of pressure fluctuations and various temperatures. The impact of pilot split (ratio of pilot fuel to total fuel mass flow rate) and fuel composition on these was investigated. The results showed a noticeable benefit where the reduction in WI provided a reduction in NOx emissions. The amplitude of the pressure fluctuations was not an issue being controlled, where necessary, by pilot split. The test results showed that WI of 25MJ/m3 appeared critical to combustion performance compared to Natural Gas. Review of the literature and calculation of the flame properties attempted to provide explanations for the trends observed during testing.

Ultralow-Emission Combustion and Control System Installation Into Mature Power Plant Gas Turbines

The search for power plant sustainability options continues as regulating agencies exert more stringent industrial gas turbine emission requirements on operators. Purchasing power for resale, decomissioning current capabilities altogether, and repowering by replacing or converting existing equipment to comply with emission standards are economic-driven options contemplated by many mature gas turbine operators. One Las Vegas, NV operator, NV Energy, with four natural gas-fired W501B6 combined cycle units at their Edward W. Clark Generating Station, was in this situation in 2006. The units, originally configured with diffusion flame combustion systems, were permitted at 103 ppm NOx with regulatory mandates to significantly reduce NOx emissions to below 5 ppm by the end of 2009. Studies were conducted by the operator to evaluate the economic viability of using a selective catalytic reduction system, which would have forced significant modifications to the exhaust system and heat recovery steam generator, or convert the turbines to operate with dry low-emission combustion systems. Based on life cycle cost and installation complexity, the ultralow-emission combustion system was selected. This technical paper focuses on a short summary of the end user considerations in downselecting options, the ultralow emissions technology, and key features employed to achieve these low emissions, an overview of the conversion scope and a review and description of the control technology employed. Finally, a technical discussion of the low-emission operational flexibility will be provided including performance results of the converted units.

Biodiesel as an Alternative Fuel in Siemens Dry Low Emissions Combustors: Atmospheric and High Pressure Rig Testing

Atmospheric and high pressure rig tests were conducted to investigate the feasibility of using biodiesel as an alternative fuel to power industrial gas turbines in one of the world’s leading dry low emissions (DLE) combustion systems, the SGT-100. At the same conditions, tests were also carried out for mineral diesel to provide reference information to evaluate biodiesel as an alternative fuel. In atmospheric pressure rig tests, the likelihood of the machine lighting was identified based on the measured probability of the ignition of a single combustor. Lean ignition and extinction limits at various air temperatures were also investigated with different air assist pressures. The ignition test results reveal that reliable ignition can be achieved with biodiesel across a range of air mass flow rates and air fuel ratios (AFRs). In high pressure rig tests, emissions and combustion dynamics were measured for various combustor air inlet pressures, temperatures, combustor wall pressure drops, and flame temperatures. These high pressure rig results show that biodiesel produced less NOx than mineral diesel. The test results indicate that the Siemens DLE combustion system can be adapted to use biodiesel as an alternative fuel without major modification.

Design methods of the ABB Alstom Power gas turbine dry low emission combustion system

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.

95/05064 DLE combustor exceeds performance guarantees

The Dry Low Emissions (DLE) combustion system developed by GE for its high-efficiency LM6000 aeroderivative gas turbine is now a commercial reality. The system is exceeding performance guarantees at its first world application ― the combined-cycle, cogeneration facility in Gent, Belgium

Development of an Aeroderivative Gas Turbine Dry Low Emissions Combustion System

This paper gives the development status of GE’s new aeroderivative premixed combustion system. This system consists of a new fuel staged annular combustor, compressor rear frame, first-stage turbine nozzle, electronic staging controller, and fuel delivery system. Component test results along with a description of the combustion system are presented. This new system will reduce NOx emissions by 90 percent relative to the original aircraft engine combustion system while maintaining low emissions of CO and UHCs. Tests of a LM6000 gas turbine equipped with the new system are planned for early 1994.

Carbon Monoxide Emissions From Gas Turbines as Influenced by Ambient Temperature and Turbine Load

The emissions of carbon monoxide (CO) from gas turbines are typically below 100 ppmvd at 15 percent O2 at design full-load operating conditions. The use of water/ steam to reduce NOx emissions from gas turbines results in an increase in CO emissions from gas turbines. This is particularly true when increased rates of water/ steam injection are used to meet stringent NOx limits. Regulations limiting CO emissions from stationary gas turbines were first initiated in the late 1980s by the Federal Republic of Germany and the state of New Jersey in the United States. Since these regulations are silent on ambient and load corrections, these CO limits could be the limiting factor in the current development of dry low-NOx combustion systems by gas turbine manufacturers. In addition, since manufacturers are usually quite specific regarding the conditions for CO guarantees, a conflict for the gas turbine user, who is responsible for the permit application, is readily apparent. This paper attempts to characterize the CO emissions from gas turbines as a function of ambient temperature and turbine load. An ambient temperature correction equation for CO emissions, based on previous work, is presented. The intent is to provide more extensive information on CO emissions such that better defined CO limits can be adopted. Ultimately, this should help the combustion design engineers in developing improved dry low-emissions combustion systems for the gas turbine industry.