1,720,967 research outputs found
New-concept gas turbine burner simulation in moderate intense Low-Oxygen Combustion Regime
No full textIn a trapped-vortex combustor (TVC) flame stabilization is achieved through intense internal exhaust gases recirculation, which is promoted by the adoption of cavities. Thanks to its peculiar features, a trapped-vortex burner produces low pressure drop and emissions and it is characterized by extended blow-out limits. The strong mixing of fresh reactants with flue gases due to internal recirculation represents the basis for the establishment of a distributed MILD, i.e. "Moderate Intense Low-Oxygen Dilution Combustion" regime, which is characterized by reduced temperature peaks, volumetric distributed reactions, low NOx emissions and no thermo-acoustic instabilities. Aim of the work is to study the possibility to obtain a MILD regime in our available trapped-vortex device, taking the advantage of the combined effect of TVC strong internal exhaust gases recirculation and of oxy-combustion external exhaust recirculation, attaining the benefits of CO2 capture at the same time. To this end a series of computational fluid dynamics simulations were conducted on our TVC device, in order to understand the influence on combustion of the main operating parameters, such as the equivalence ratio, the level of dilution, the injection temperature, the velocity, etc.. A preheating temperature and a range of oxygen concentrations that at the same time complies with a distributed reactions regime and an efficient combustion were identified for the premixed and non-premixed operating modes
Micro gas turbine combustor performances in CO2/O2 oxidizer atmosphere
No full textIn fossil fuel energy power plants the oxy-combustion technique, is one of the possible approaches to the problem of greenhouse gases emissions, through the CO2 capture and subsequent storage. It is realized using recirculated flue gas enriched with oxygen as oxidizer and it is suitable more than other techniques to retrofit existing plants. The commercial gas turbine combustors currently available are however designed and optimized for air combustion. In this work, through a series of CFD simulations, a typical commercial micro turbine burner has been tested in oxy-combustion conditions, in order to verify the performances. Through this study it has been shown how these class of combustors cannot be used in an optimal way in terms of efficiency, pollutant emissions and oxygen consumption. Some possible solutions have been also proposed
Modeling and simulation of an oxygen-blown bubbling fluidized bed gasifier using the Computational Particle- Fluid Dynamics (CPFD) approach
No full textFluidized beds are conventional components of many industrial processes, such as coal gasification for energy generation and syngas production. Numerical simulations help to properly design and understand the complex multiphase flows occurring in these reactors. Two modeling approaches are usually adopted to simulate multiphase flows: the two fluids Eulerian-Eulerian model and the continuous/discrete Eulerian-Lagrangian model. Since fluidized beds account for an extremely large number of particles, tracking each of them could not assure to get results within a reasonable computational time. The Computational Particle-Fluid Dynamics (CPFD) approach, which belongs to the Eulerian-Lagrangian models class, groups together particles with similar key parameters (e.g. composition, size) into computational units (parcels). Parcel collisions are modeled by an isotropic solid stress function, depending on solid volume fraction. In this paper, the bubbling fluidized bed (BFB) upstream gasifier of the EU research infrastructure ZECOMIX (Zero Emissions of Carbon with Mixed technologies) has been simulated using a CPFD approach via Barracuda® software. The effect of different fluidizing agent injection strategies on bed bubbling and mixing, for non-reacting cases, has been studied. The numerical results for a reacting case have been compared to the available experimental data, gathered during the coal gasification campaign. The model has proved to be very useful in the choice of the more efficient injection configuration that assures a more effective contact of the gas with the solid bed and a good bubbling fluidization regime, together with a satisfactory prediction of the outlet gas composition. The numerical approach has turned out to be robust and time-saving and allowed to dramatically reduce the computational cost with respect the classical two fluids Eulerian-Eulerian models. © 2018, Isfahan University of Technology
Approfondimenti sul bruciatore trapped-vortex. Analisi di sensibilità e scalatura del prototipo
Full text linkNel report in oggetto viene approfondito lo studio sul nuovo bruciatore trapped-vortex alimentato a syngas. È importante infatti valutare sia gli effetti della variazione dei principali parametri operativi, nonché indagare su quali sono gli effetti di scala sul prototipo. Altrettanto interessante è studiarne il comportamento in regime dinamico attraverso una Large Eddy Simulation (LES), allo scopo di verificare la stabilità del vortice intrappolato. Nelle pagine seguenti viene innanzitutto ripreso e descritto il principio di funzionamento del prototipo, passando poi alla vera e propria analisi di sensibilità alle grandezze operative del sistema. Successivamente vengono descritti i diversi criteri di scalabilità e riportati i risultati nel caso di un bruciatore di potenza sia raddoppiata che dimezzata. Vengono infine riportati i risultati dell’analisi LES
Specifiche tecniche relative alla realizzazione del bruciatore trapped-vortex ed alle modifiche dell'impianto MICOS
Full text linkNel presente report si riportano le dimensioni geometriche definitive e le specifiche del nuovo bruciatore trapped-vortex alimentato a syngas. Vengono inoltre descritte alcune modifiche da apportare all’impianto MICOS su cui il prototipo verrà testato, per garantire le nuove condizioni operative. Sarà infatti necessario predisporre e regolare in maniera indipendente, due linee per l’aria (primaria + secondaria) ed una per il combustibile. Ciascuna di queste tre linee dovrà dividersi in due parti uguali in modo da alimentare le due serie di fori contrapposte sulle due piastre forate del bruciatore. La temperatura dell’aria deve essere di circa 700K. Sarà pertanto necessario dotare l’impianto di un sistema di preriscaldamento di tutta l’aria addotta, in un range 400-800K, allo scopo di avere un certo margine di manovra. Nella sperimentazione si indagherà, tra le altre cose, sull’effetto prodotto dall’utilizzo di diversi rapporti aria/syngas e sull’effetto delle velocità in ingresso sulla fluidodinamica, sul campo termico e sulle reazioni chimiche. Le piastre forate destra e sinistra, dovranno pertanto essere sostituibili al fine di variare il numero e/o le dimensioni dei fori. Le pareti laterali del combustore saranno realizzate in quarzo in modo da permettere l’accesso ottico, al fine di effettuare misure che prevedono l’uso di apparecchiature laser. L’alimentazione del combustibile sarà effettuata mediante bombole
CO2 methanation in a shell and tube reactor CFD simulations: high temperatures mitigation analysis
No full textCO2 methanation is gaining increasing interest in the last years as a way of storage of the energy surplus produced by renewable energy sources. Shell and tube reactors are among the most widespread type of methanation reactors. One-dimensional pseudo-homogenous/heterogeneous models and CFD (Computational Fluid Dynamics) models using some simplifications, as imposed heat exchange coefficients and constant coolant temperatures, have been applied up to now to study the problem. In this article those simplifications have been removed and the CO2 methanation in a cooled multi-tubular catalyst reactor has been investigated through 3D CFD simulations, using a commercial software and a porous model. Nitrogen and oil cooling have been studied and the results have been compared. The goal is to identify suitable solutions that allow the use of gas cooling, while maintaining low temperature levels and high conversions. The oil cooling case has been also analyzed for comparison and in the perspective of a future system upgrade. After a validation of the model using the available experimental data, a sensitivity analysis has been conducted for what concerns coolant and reactants temperatures and flow rates, catalyst load and operating pressure in the tubes. Strategies for the high temperatures mitigation in the reactive zones have been evaluated and discussed. In particular the proposed technique of catalyst uneven distribution has revealed to be very effective. The model has proved to be very useful for a detailed analysis of the phenomenon and for obtaining essential data that resulted difficult to measure by experimentation
Gas Turbine Combustion Technologies for Hydrogen Blends
Full text linkThe article reviews gas turbine combustion technologies focusing on their current ability to operate with hydrogen enriched natural gas up to 100% (Formula presented.). The aim is to provide a picture of the most promising fuel-flexible and clean combustion technologies, the object of current research and development. The use of hydrogen in the gas turbine power generation sector is initially motivated, highlighting both its decarbonisation and electric grid stability objectives; moreover, the state-of-the-art of hydrogen-blend gas turbines and their 2024 and 2030 targets are reported in terms of some key performance indicators. Then, the changes in combustion characteristics due to the hydrogen enrichment of natural gas blends are briefly described, from their enhanced reactivity to their pollutant emissions. Finally, gas turbine combustion strategies, both already commercially available (mostly based on aerodynamic flame stabilisation, self-ignition, and staging) or still under development (like the micro-mixing and the exhaust gas recirculation concepts), are described
Sorption enhanced steam methane reforming in a bubbling fluidized bed reactor: Simulation and analysis by the CPFD method
Full text linkThis work reports the modelling and simulation results of a bubbling fluidized bed reactor using the Computational Particle Fluid Dynamics (CPFD) method of the Barracuda® software. The reactor under investigation is the carbonator installed in the ENEA ZECOMIX research infrastructure, where Steam Methane Reforming (SMR) happens simultaneous with CO2 capture via solid sorbents. In this intensified process, namely Sorption Enhanced Steam Methane Reforming (SE-SMR), steam methane reforming is coupled with high temperature CO2 sorption and calcium looping (CaL) process, in order to increase the H2 yield, beyond thermodynamic limits. Currently, the reactor is operated in batch mode and is used also for sorbent regeneration, by switching the fluidizing gas flow from steam/methane to oxy-burner combustion products. With the aim of studying the process when it is operated as a closed loop, in this paper the reactor is continuously fed by a fresh sorbent flow and a riser/calciner reactor for sorbent regeneration, to be connected with the carbonator, has been sized. The continuous circulation of solid material between the two reactors ensures the maintenance of different operating temperatures and therefore greater operational optimization. The numerical analysis presented in this paper will serve as a valid support for the experimental activities. For this purpose, a sensitivity study on the SE-SMR process has been conducted, by varying the main operating conditions (e.g. sorbent conversion, sorbent/catalyst ratio, fluidizing gas flow), to evaluate the hydrogen purity yield. Two different kinetic mechanisms have been compared for the gas phase reactions. A post-processing routine has been written, in order to analyze bubbles sizes and velocities inside the fluidized environment. The effect of sorbent and catalyst particles segregation has been also investigated. The same modelling approach has been used for the sizing of the fast riser calciner reactor
Computational particle fluid dynamics 3D simulation of the sorption-enhanced steam methane reforming process in a dual fluidized bed of bifunctional sorbent-catalyst particles
No full textSorption enhanced steam methane reforming (SE-SMR) is an efficient hydrogen production technology that combines steam methane reforming with CO2 capture. Reactions are shifted over their thermodynamic limits and hydrogen yield is enhanced, in more favorable thermodynamic conditions. Bifunctional sorbent-catalyst particles have been developed to assure stability over multiple cycles and to make the endothermic methane reforming and the exothermic CO2 capture completely integrated. Moreover, handling one granular material simplifies system management, reduces bed inventory and avoids the problem of segregation in case of two separated materials. The continuous and simultaneous operation of the process in a dual fluidized bed reactor (DFB), which combines a reformer and a calciner, for sorbent regeneration through high temperature calcination, increases productivity and allows a greater optimization. Aim of this work is the 3D simulation of the SE-SMR process in a DFB reactor, by the Computational Particle Fluid Dynamics (CPFD) method of the Barracuda VR® software. The simulation results in terms of solid flow, pressure balance and particles segregation are reported. A post-processing routine has been written to characterize bubbles in the two fluidized beds. The effects of bed inventory, superficial velocity and steam to methane ratio on hydrogen purity and methane conversion are discussed in the paper
Effects of hydrogen blending and exhaust gas recirculation on NOx emissions in laminar and turbulent CH4/Air flames at 25 bar
No full textIn this work, the effect of Exhaust Gas Recirculation (EGR) on NOx emissions in a CH4/H2/air combustion at 25 bar, at an equivalence ratio Φ = 0.7, is analyzed in laminar and turbulent partially premixed flames. Numerical simulations of twenty premixed and partially premixed counterflow flames with H2 ranging from 0 to 100%, with and without EGR, are carried out. Analysis of NO formation mechanisms shows that the thermal path is the main responsible for the NOx production in each flame, with a contribution of nearly 80%. In laminar flames an increase of H2 leads to an increase in NOx emissions. The addition of the exhaust gas decreases the flame temperature and therefore NOx: each laminar flame shows a NOx reduction of about 60% with the presence of exhaust gas. Four turbulent slot jet flames with a CH4/H2/Air/EGR premixed central core and Air/EGR as coflow are studied in a two-dimensional framework using the Reynolds Averaged Navier–Stokes (RANS) approach and the Large Eddy Simulation (LES) methodology, to take into account some flame dynamics, despite the 2D context. Accurate molecular transport properties are considered and, a reduced specifically designed chemical mechanism for methane/hydrogen-air combustion at Φ=0.7, consisting of 48 transported species and 465 elementary reactions is adopted. The four turbulent flames were simulated with 0 and 50% hydrogen concentration, with and without EGR. The presence of hydrogen reduces CO2 emissions, but at the same time increases NO concentration, the thermal path being the main NO formation mechanism. The use of exhaust gas recirculation leads to NO reduction as in laminar flames. The results obtained in this work show that at high pressure, the hydrogen enrichment of natural gas in the EGR mode leads to lower NOx as well as CO2 emissions
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