Tokyo Gas has been working to develop combustion simulation technology since 1980s. The development of our combustion simulations started from considering only flow and heat transfer, and through the implementation of turbulence combustion models and detailed reaction mechanisms, our technology has been evolved. Recently, in order to evaluate combustion emissions and reveal the complex phenomena, increase in the scale of simulation is inevitable. Examples of our simulations from past to present and expectations for large-scale simulation in the future are addressed in this paper.
The mitigation of greenhouse gas emission from thermal power plants is strongly required towards 2050. CCUS (Carbon dioxide Capture, Utilization and Storage) is one of the promising technologies for drastic reduction of greenhouse gas from thermal power plant. However, capturing carbon dioxide from flue gas requires huge energy and it causes the reduction of thermal efficiency. Recently, the new concept of carbon dioxide capturing thermal power generation system with keeping high efficiency, named “oxy-fuel IGCC system” was proposed and has been developed. This system injected CO2 into a coal gasifier to purify CO2 concentration in flue gas. This paper firstly introduces the outline of coal gasification and chemical reaction model used for numerical simulation. A coal gasifier is one of the most important utilities in oxy-fuel IGCC system as well as the conventional IGCC system. The results of numerical simulation in a coal gasifier under oxy-fuel IGCC system are also introduced. Gaseous temperature in the combustor decreases when N2 is just replaced with CO2, while gaseous temperature recovers with increasing O2 concentration. The CO concentration increases with replacing N2 with CO2, while the H2 concentration decreases. This is due to the fact that rich CO2 concentration promotes the reverse reaction of the water gas shift reaction. Although gaseous temperature drastically decreases by replacing N2 with CO2, the performance of coal gasifier, such as carbon conversion efficiency does not decrease very much and it increases with increasing O2 concentration. Therefore, the advantage of oxy-fuel IGCC system was clearly showed by the numerical simulation. Finally, the expectations of “K-computer” and advanced numerical simulation technology are mentioned.
As a result of technical innovation and progress of computers and software, the combustion simulation has become an indispensable method as a tool for further development of a combustor. Since lead time and cost of combustion tests are large, there is a demand of transposing a combustion test to simulation. Advancement of combustion simulation as a tool which can be easily used for design is expected to grow further, and the same expectation also exists for the large scale combustion simulation technology. In this report, examples of combustion simulations for a low NOx combustor, a liquid fuel combustor and supercritical CO2 combustor of gas turbine are described.
A methodology to reproduce international variations in diesel spray ignition is proposed. Through two-dimensional gas chromatography and hydrogen nuclear magnetic resonance spectroscopy, a total of 22 species are defined as palette species to model commercial light oils. A surrogate fuel model is formulated with the palette species in such a way that the surrogate fuel model takes over combustion-related properties such as distillation curve, reactivity and soot propensity from the real fuel. The spray combustion simulation coupled with a reaction model showed that the proposed methodology well reproduces the magnitude relationship in international variations of the spray ignition delay times. Moreover, sensitivities of the spray ignition delay time to elemental uncertainties such as the reactivity of the surrogate fuel model, the distillation curves, etc., are systematically analyzed, and the uncertainty of a simulated ignition event is thus quantified. For relatively low temperature condition of 925 K studied in this paper, the combined uncertainty is estimated to be [-7%, 20%] for RANS while [-9%, 28%] for LES, respectively. The uncertainty budget shows that the reactivity of the surrogate fuel model most affects the simulated spray ignition event.
Reaction mechanism and elementary processes become more important in catalysis as well as combustion. In this article, we will introduce how to construct and analyze the reaction mechanism of catalytic reactions based on density functional theory (DFT), using examples from our recent study for steam methane reforming with nickel catalysts. Important energies on surface, such as adsorption, activation, and reaction energies, can be theoretically calculated using a slab model with periodic boundary conditions. Gibbs free energies and rate coefficients of elementary reactions is also calculated by phonon analysis based on DFT. Microkinetic simulations with constructed reaction mechanism enable us to compare directly with experimental results, for example, conversion, selectivity, and temperature profile in a fixed bed reactor filled with catalyst pellets. Detailed reaction mechanism analyses clarify the main reaction pathway and rate limiting step on surface. Finally, our latest study for the reactivity at the defects on surface will be shown as an example of future prospects for kinetic modeling in catalysis.
An experimental study has been performed by observing the internal and atomization behavior, and by measuring the droplet temperature during spheroid-type evaporation of water-in-oil (W/O) emulsion on a heated surface. The base fuel was n-dodecane. In this study, the effects of surfactant concentration were mainly examined. By decreasing the surfactant concentration from 0.8 to 0.2 % in volume, stability of the emulsion was reduced. When the surfactant concentration was decreased, number of puffing microexplosion much decreased. Aggregation and coalescence of dispersed water droplets were accelerated in the emulsion droplet, and the water volume contained in the fuel droplet increased. Occurrence frequency of disruptive microexplosion increased even at lower surface temperatures. Moreover, it became earlier at lower droplet temperatures.