Antonio Asti

The ORegen™ Waste Heat Recovery Cycle: Reducing the CO2 Footprint by Means of Overall Cycle Efficiency Improvement

The growing concern for the role of man-made CO2 emissions with respect to global warming combined with the large increase in energy demand spurred by developing nations and a growing global population that is foreseen over the next 15 years have recently turned attention to potential CO2 -neutral energy supply solutions. Waste heat recovery cycles applied to fossil fueled plants offer a local zero-emission solution to producing additional electric energy, thereby increasing the overall plant efficiency with a considerable reduction in the emission of CO2 per unit of energy produced. GE Oil & Gas with GE Global Research Europe has developed a new and attractive solution for recovering waste heat energy from a variety of thermal sources ranging from reciprocating combustion engines to gas turbines. This new recovery cycle is called ORegen™. The ORegen™ recovery cycle is a rankine cycle, with superheating, that recovers waste heat and converts it into electric energy by means of a double closed loop system. The ORegen™ system represents one of the very few viable solutions for recovering heat from sources (such as mechanical drive gas turbines) whose load may vary dramatically over time or where the equipment is located at a site where water is not readily available. For the temperature range of interest, a thorough comparison between many working fluids was performed, leading to the conclusion that the substance that delivers the highest efficiency is Cyclopentane. A high-efficiency Rankine cycle based on such a working fluid places a particularly high demand on the expansion ratio, which influences some of the basic architectural choices of the expander machine. This article introduces the ORegen™ recovery cycle and describes the process used in GE Oil & Gas to design the family of double supersonic stage turboexpanders, covering the power range of 2–17MW. Examples of the application of the ORegen™ cycle to gas turbine are also provided to demonstrate attractive opportunities to increase the overall plant efficiency.Copyright

Development of a Simplified Back-Up Liquid Fuel System for a Heavy Duty Industrial Gas Turbine

Heavy duty gas turbines for power generation and mechanical drive applications are typically fired on natural gas as a primary fuel, providing heat and power with high efficiency and low exhaust emissions. However, fuel gas is not always available when power is needed, and distillate oil is often employed as an easily stored and handled back-up fuel. The present paper describes the development and initial component validation testing of a new, simplified liquid fuel injection system that will provide a back-up liquid fuel option for dry, low NOx combustion systems used in heavy duty industrial gas turbines. This new liquid fuel system offers reduced initial cost and operating cost, lower NOx emissions, and reduced water consumption relative to current technology. Spray test, sub-component ignition test, and full-scale combustion chamber test results will be discussed.Copyright

Application of a 1-D Predicting Tool to the Analysis of Pressure Oscillations in an Industrial Gas Turbine Combustor: GE10 Machine

The development of current industrial gas turbines is strictly constrained by legislative requirements for low polluting emissions. Lean Premixed combustion technology has become through the years the necessary standard to meet such requirements. Premixed technology introduces a new range of problems: combustion instabilities in many operating conditions. Specifically, lean premixed flames pose the threat of pressure oscillations. This phenomenon is the effect of the strong interaction between combustion heat-release and fluid dynamics aspects. The prediction of acoustic oscillations and combustion instabilities is generally difficult because of the complexity of real combustor geometries. As a result, the design phase is usually performed as a trial-and-error task: a specific design is constructed, tested and modified, in a process that continues until acceptable results are found. A specific tool was developed by GE Energy to help predicting the acoustic behaviour of newly designed partially-premixed combustors, avoiding the traditional trial-and-error process: the tool allows the designer to analyze the problem of combustion instabilities since the early design phase, limiting subsequent testing efforts. A mono-dimensional tool based on the 1-D acoustic model was developed by GE Energy and was applied to the single-can combustor of the GE10 machine (a gas turbine in the 10MW class). All the main geometrical features of the GE10 machine, including fuel line geometry, were considered and modeled in a one-dimensional scheme, in order to build an equivalent model for the linear tool analysis. The main frequencies, measured during tests on the GE10 machine, were compared to the numerical results of the tool, showing good agreement between numerical and experimental results and confirming the predictive capability. This good agreement demonstrates that the model can be used for predicting the effects of design changes, with a reduced need of tests.© 2007 ASME

NOx emissions reduction in an innovative industrial gas turbine combustor (GE10 machine): A numerical study of the benefits of a new pilot-system on flame structure and emissions

One of the driving requirements in gas turbine design is emissions reduction. In the mature markets (especially the North America), permits to install new gas turbines are granted provided emissions meet more and more restrictive requirements, in a wide range of ambient temperatures and loads. To meet such requirements, design techniques have to take advantage also of the most recent CFD tools. As a successful example of this, this paper reports the results of a reactive 3D numerical study of a single-can combustor for the GE10 machine, recently updated by GE-Energy. This work aims to evaluate the benefits on the flame shape and on NOx emissions of a new pilot-system located on the upper part of the liner. The former GE10 combustor is equipped with fuel-injecting-holes realizing purely diffusive pilot-flames. To reduce NOx emissions from the current 25 ppmvd@15%O2 to less than 15 ppmvd@15%O2 (in the ambient temperature range from −28.9°C to +37.8°C and in the load range from 50% and 100%), the new version of the combustor is equipped with 4 swirler-burners realizing lean-premixed pilot flames; these flames in turn are stabilized by a minimal amount of lean-diffusive sub-pilot-fuel. The overall goal of this new configuration is the reduction of the fraction of fuel burnt in diffusive flames, lowering peak temperatures and therefore NOx emissions. To analyse the new flame structure and to check the emissions reduction, a reactive RANS study was performed using STAR-CD™ package. A user-defined combustion model was used, while to estimate NOx emissions a specific scheme was also developed. Three different ambient temperatures (ISO, −28.9°C and 37.8°C) were simulated. Results were then compared with experimental measurements (taken both from the engine and from the rig), resulting in reasonable agreement. Finally, an additional simulation with an advanced combustion model, based on the laminar flamelet approach, was performed. The model is based on the G-Equation scheme but was modified to study partially premixed flames. A geometric procedure to solve G-Equation was implemented as add-on in STAR-CD™.Copyright

Numerical heat transfer analysis of an innovative gas turbine combustor: Coupled study of radiation and cooling in the upper part of the liner

A numerical study of a single can combustor for the GE10 heavy-duty gas turbine, which is being developed at GE-Energy (Oil & Gas), is performed using the STAR-CD CFD package. The topic of the present study is the analysis of the cooling system of the combustor liner’s upper part, named “cap”. The study was developed in three steps, using two different computational models. As first model, the flow field and the temperature distribution inside the chamber were determined by meshing the inner part of the liner. As second model, the impingement cooling system of the cold side of the cap was meshed to evaluate heat transfer distribution. For the reactive calculations, a closure of the BML (Bray-Moss-Libby) approach based on Kolmogorov-Petrovskii-Piskunov theorem was used. The model was implemented in the STAR-CD code using its user coding features. Then the radiative thermal load on the liner walls was evaluated by means of the STAR-CD-native Discrete Transfer model. The selection of the radiative properties of the flame was performed using a correlation procedure involving the total emissivity of the gas, the mean beam length and the gas temperature. The estimated heat flux on the cap was finally used as boundary condition for the calculation of the cooling system, consisting of 68 staggered impingement jet lines on the cold side of the cap. The resulting temperature distribution shows a good agreement with the experimental values measured by thermocouples. The results confirm the validity of the implemented procedure, and point out the importance of a full CFD computation as an additional tool to support classic correlation design procedures.Copyright