Combustion Characteristics of Small Size Gas Turbine Combustor Fueled by Biomass Gas Employing Flameless Combustion

One approach to achieving 99% combustion efficiency (C.E.) and 10 ppmV or lower NOx (at 15%O2) in a micro gas turbine (MGT) combustor fueled by biomass gas at a variety of operating conditions is with the use of flameless combustion (FLC). This paper compares experimentally obtained results and CHEMKIN analysis conducted for the developed combustor. As a result, increase the number of stage of FLC combustion enlarges the MGT operation range with low-NOx emissions and high-C.E. The composition of fuel has a small effect on the characteristics of ignition in FLC. In addition, NOx in the engine exhaust is reduced by higher levels of CO2 in the fuel.

Expectations and Recent Experience for Gas Turbine Reliability, Availability, and Maintainability (RAM)

Since the early 1990’s there have been significant changes in the gas turbine, and power generation market place. The ‘F-Class’ Gas Turbines, with higher firing temperatures, single crystal materials, increased compressor pressure ratios and low emission combustion systems that were introduced in the early 1990’s have gained significant field experience. Many of the issues experienced by these new product introductions have been addressed. The actual reliability growth and current performance of these advanced technology machines will be examined. Additionally, the operating profiles anticipated for many of the units installed during this period has been impacted by both changes in the anticipated demand and increases in fuel costs, especially the cost of natural gas. This paper will review how these changes have impacted the Reliability, Availability, and Maintainability performance of gas turbines. Data from the ORAP® System, maintained by Strategic Power Systems, Inc, will be utilized to examine the actual RAM performance over the past 10 to 15 years in relation to goals and expectations. Specifically, this paper will examine the reliability growth of the F-Class turbines since the 1990’s and examine the reliability impact of duty cycle on RAM performance.

Development and Application of an Eight-Step Global Mechanism for CFD and CRN Simulations of Lean-Premixed Combustors

In this paper, the development of an eight-step global chemical kinetic mechanism for methane oxidation with nitric oxide formation in lean-premixed combustion at elevated pressures is described and applied. In particular, the mechanism has been developed for use in computational fluid dynamics (CFD) and chemical reactor network (CRN) simulations of combustion in lean-premixed gas turbine engines. Special attention is focused on the ability of the mechanism to predict NOx and CO exhaust emissions. Applications of the eight-step mechanism are reported in the paper, all for high-pressure, lean-premixed, methane-air (or natural gas-air) combustion. The eight steps of the mechanism are as follows: 1. Oxidation of the methane fuel to CO and H2O. 2. Oxidation of the CO to CO2. 3. Dissociation of the CO2 to CO. 4. Flame NO formation by the Zeldovich and nitrous oxide mechanisms. 5. Flame NO formation by the prompt and NNH mechanisms. 6. Post-flame NO formation by equilibrium H-atom attack on equilibrium N2O. 7. Post-flame NO formation by equilibrium O-atom attack on equilibrium N2O. 8. Post-flame Zeldovich NO formation by equilibrium O-atom attack on N2.

Experimental Investigation of Turbulence Structure in a Three-Nozzle Combustor

Proper mixing of fuel, primary and secondary air is a major issue to optimize engine performance in terms of efficiency and pollutant emissions. The underlying turbulent flow field determines these mixing processes. Most experimental and numerical investigations are performed in single nozzle combustors for reasons of optical accessibility and simplicity. The focus of the present study is to compare the variation of the non-reacting turbulent flow field for the case of single-nozzle and three-nozzle operation. In addition, the influence of secondary air entrainment is investigated. The flow configuration is based on commercial geometries. Using a two component laser Doppler velocimeter (LDV) the mean and fluctuating velocities of all three components, as well as two Reynolds-stress components were measured. The autocorrelation function and spectral distributions of the fluctuating velocity signal clearly revealed coherent fluid motions. These observations, together with high speed-flow visualisations indicate a precessing vortex core (PVC). An additional lower frequency for all three nozzles in operation revealed a pulsation of the recirculation zones. A major result of this investigation is that the size and shape of the internal recirculation zones were significantly influenced by operation of adjacent nozzles. Furthermore the generation of PVCs were augmented in the three-nozzle configuration. The additional secondary air entrainment interacts with the primary flow, changing the size and shape of the recirculation zone and affecting the low frequency pulsation.

The Role of the Recirculating Gases at the Mild Combustion Regime Formation

The present work is concerned with the thermodynamic and chemical kinetics of gas turbine combustor operating in the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime. The objective of the present study is to evaluate analytically the effect of the recirculation rate of combustion products within the FLOXCOM gas turbine combustor on a number of combustion parameters, mainly on the ignition delay time, NOx and CO emission, minimum ignition temperature, rate of pollutant formation and the dilution rate. The study also refers to the mechanism of influence of the recirculation rate on these values. Combustion pressure and inlet air temperature are used as parameters. The gas turbine is fueled with methane. The analysis is mainly based on CHEMKIN simulations where the calculation scheme of the combustion process in the combustor is modeled by a combination of plug reactors and mixers. Due to the unique characteristics of gas turbines, inlet air temperature is directly linked to combustion pressure while assuming conventional adiabatic compression efficiencies. It is shown that free radicals, which are part of the reaction products and exists for only a short period of time within the recirculated gases, decrease ignition delay time. The importance of shortening the ignition delay is further highlighted because of the adverse effect oxygen dilution has on this parameter (dilution of the reactants by the reaction products). It was found that there is an optimal recirculation rate, which corresponds to maximum heat density. In addition, results indicate that CO emission values rise with the recirculation rate, however the NOX values are more complicated. NOX depends on recirculation rate when flame temperatures are kept held constant. The NOX emission increases and the CO emission decreases with compressor pressure ratio. The CO concentration that is evaluated in the combustion process is further reduced during last dilution stage. Finally, basic rules for design optimization of the combustor are drafted. These are based on conventional one-dimensional fluid and thermodynamic relations and on the CHEMKIN simulations.

Numerical Assessment of SCAP: A Passive System for Preventing Thermoacoustic Oscillations in Gas Turbine Annular Combustors

ENEL operates a dozen combined cycle units whose V94.3A gas turbines are equipped with annular combustors. In such lean premixed gas turbines, particular operation conditions could trigger large pressure oscillations due to thermoacoustic instabilities. The ENEL Research unit is studying this phenomenon in order to find out methods which could avoid or mitigate such events. The use of effective numerical analysis techniques allowed us to investigate the realistic time evolution and behaviour of the acoustic fields associated with this phenomenon. KIEN, an in-house low diffusive URANS code capable of simulating 3D reactive flows, has been used in the Very Rough Grid approach. This approach permits the simulation, with a reasonable computational time, of quite long real transients with a computational domain extended over all the resonant volumes involved in the acoustic phenomenon. The V94.3A gas turbine model was set up with a full combustor 3D grid, going from the compressor outlet up to the turbine inlet, including both the annular plenum and the annular combustion chamber. The grid extends over the entire circular angle, including all the 24 premixed burners. Numerical runs were performed with the normal V94.3A combustor configuration, with input parameters set so as no oscillations develop in the standard ambient conditions. Wide pressure oscillations on the contrary are associated with the circumferential acoustic modes of the combustor, which have their onset and grow when winter ambient conditions are assumed. These results also confirmed that the sustaining mechanism is based on the equivalence ratio fluctuation of premix mixture and that plenum plays an important role in such mechanism. Based on these findings, a system for controlling the thermoacoustic oscillation has been conceived (Patent Pending), which acts on the plenum side of the combustor. This system, called SCAP (Segmentation of Combustor Annular Plenum), is based on the subdivision of the plenum annular volume by means of a few meridionally oriented walls. Repetition of KIEN runs with a SCAP configuration, in which a suitable number of segmentation walls were properly arranged in the annular plenum, demonstrated the effectiveness of this solution in preventing the development of wide thermoacoustic oscillations in the combustor.

Investigation of Combustion Oscillations in a Lean Gas Turbine Model Combustor

This contribution is a continuation of ASME-GT2006-90300. While still working at atmospheric pressure, the range of operating conditions was extended to more realistic reduced mass flows to reproduce the engine pressure loss and air preheat up to 700K. The thermoacoustic behaviour of the burner was mapped over that operating range. Two different types of oscillations were observed for flames anchored at the nozzle or lifted from it. Both exhibited a frequency dependence on the Strouhal number for constant reduced mass flows. For a selected operating point with the lifted flame at a preheat temperature of 600K and a reduced mass flow of 0.3kg K0.5/(s bar), the thermoacoustic behaviour of the burner was characterised by phase locked Particle Image Velocimetry as well as phase locked OH- and OH-T- LIF measurements and correlated to the acoustic pressure signal obtained by microphones. The combined data showed pulsating combustion being supported through periodic reignition of the main flame zone by a recirculating volume of hot, OH-rich gas, the cycle time being connected to the observed frequency. The characterization of the preheated operating point was completed with a heat balance investigation quantifying the non-adiabatic combustion conditions of the uncooled combustor.

The Reheat Concept: The Proven Pathway to Ultra-Low Emissions and High Efficiency and Flexibility

Reheat combustion has proven now in over 80 units to be a robust, and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultra-low emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Secondly, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely flame propagation in the first combustor stage and auto-ignition in the second combustor stage. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.

Customer Adapted Optimized Maintenance Plan for Gas Turbines

Maintaining high levels of availability and reliability are essential objectives for many industries, especially those that are subject to high costs due to non-payment as a result of shutdowns of critical systems, e.g. gas turbines. To utilize these systems as effectively as possible, maintenance must be optimized. Though, determining what is optimal is a tough multi-variable task requiring detailed knowledge about components building the system, such as damage mechanisms, TMF/LCF crack initiation and propagation, creep deformation, creep damage, general material deterioration, erosion, oxidation and corrosion. Also, customer specific inputs are essential, e.g. value of the produced entity, fuel prices, unplanned and planned standstill cost. To efficiently funnel this information into a customer adapted, optimized maintenance plan, a probabilistic optimization model is proposed. The model will dynamically calculate the most efficient point in time for a renewal. Further, the model can, as a function of risk willingness, adjust the maintenance plan to any customer’s specific demands. This paper describes (i) the model, (ii) how information is gathered and processed, (iii) how the risk assessment is performed and (iv) how the lifetime prediction is carried out. The optimization itself can be adjusted to aim at: minimizing cost or risk or maximizing availability, performance or reliability. Also, this paper describes how the quantitative selection of critical components included in the optimization is performed.

The Air Injection Power Augmentation Technology Provides Additional Significant Operational Benefits

The Air Injection (AI) Power Augmentation technology (HAI for humid Air injection and DAI for dry air injection) has primary benefits of increasing power of combustion turbine/combined cycle (CT/CC) power plants by 15–30% at a fraction of the new plant cost with coincidental significant heat rate reductions (10–15%) and NOx emissions reductions (for diffusion type combustors up to 60%) (See References 1, 2, 3): Figure 1A is a simplified heat and mass balance for the PG7241 (FA) combustion turbine with HAI. The auxiliary compressor supplies the additional airflow that is mixed with the steam produced by the HRSG and injected upstream of combustors. Figure 1B presents the heat and mass balance for the PG7142 CT based combined cycle power plant with HAI. It is similar to that presented on Figure 1A except that the humid air is created by mixing of steam, extracted from the steam turbine, with the supplementary airflow from the auxiliary compressor. The maximum acceptable injection rates are evaluated with proper margins by a number of factors established by OEMs: the compressor surge limitations, maximum torque, the generator capacities, maximum moisture levels upstream of combustors, etc.