Modeling of Chemical Reactions in Gas-Solid Two-Phase Flow and Its Applications to Pulverized Coal Combustion

要約のみTohoku University青木秀之課

Prediction of polycyclic aromatic hydrocarbons formation using flamelet approach with additional transport equations

It is known that the formation of minor species such as polycyclic aromatic hydrocarbons (PAHs) cannot be well captured by the standard flamelet/progress-variable (FPV) model. In this study, the extended method in which additional transport equations for PAHs were solved (FPV-TE model) was verified in the numerical simulations of a laminar counter-flow diffusion flame. The numerical results obtained from FPV-TE model were in better agreement with the solutions of the detailed chemistry than that in the standard FPV model in terms of the mass fractions of PAHs

Application of flamelet/progress-variable approach to the large eddy simulation of a turbulent jet flame of pulverized coals

In this study, the flamelet/progress-variable (FPV) approach was applied to a large eddy simulation of a pulverized coal jet flame. The FPV approach considers the characteristics of the pulverized coal flame, e.g., non-adiabatic system and several types of fuel streams, via additional representative variables. First, the applicability of the FPV approach to a turbulent flame with pulverized coals was confirmed through a comparison of the numerical solutions and experimental data. In this study, the pure pilot case was also investigated to clarify the effects of pulverized coals on the flame. The flame structure changes significantly upon the injection of pulverized coals, and the flame index suggests the coexistence of premixed and diffusion combustion modes even in the downstream region. In particular, the combustion mode fluctuates with time in the middle region of the flame. The fuel gas released from the pulverized coals should increase in this region; therefore, the release and combustion behavior of the volatile matter must be involved in the combustion mode variation. The evaluation of the combustion modes of fuel gas in the coal flame is useful for the design and optimization of pulverized coal combustors with next-generation technologies

Analysis of flame structure using detailed chemistry and applicability of flamelet/progress variable model in the laminar counter-flow diffusion flames of pulverized coals

Pulverized coal is still found in many practical devices even though it is recognized as ‘‘dirty fuel” because of its CO2 and pollutant emissions. To overcome this problem, advanced coal utilization technologies have been developed using numerical simulations. In this study, the structures of the laminar counter-flow diffusion flames of pulverized coals were investigated by performing simulations based on detailed chemistry. The high-temperature region became narrower as the coal/air ratio increased, because of the departure from the stoichiometric mixture and local quenching by the heat transfer between the gas and solid phases. Further, the applicability of the flamelet/progress-variable (FPV) model was investigated through a priori and a posteriori tests. The a priori test confirmed that the FPV model is capable of reproducing the numerical solutions obtained using the detailed chemistry, including the mass fractions of minor species. In the a posteriori test, there was a slight difference between the FPV model and detailed chemistry results due to overestimation of the progress of the chemical reactions. Given the sufficiently high accuracy of the FPV model in various numerical conditions, it can be concluded that the extended FPV model has potential for use in turbulent coal combustion simulations

Effects of infinitely fast chemistry on combustion behavior of coaxial diffusion flame predicted by large eddy simulation

Large eddy simulations (LES) based on turbulent combustion models aid the design and optimization of combustors. Of the various combustion models available, the eddy break up (EBU) model is widely used because it assumes an infinitely fast chemistry. However, omitting the actual chemical kinetics can cause unexpected behavior, and the characteristics of the combustion models need to be elucidated. Here, the effects of an infinitely fast chemistry on the combustion behavior of a coaxial diffusion flame as predicted by an LES were analyzed. Although the EBU model captured the overall behavior of the chemical species as well as the flow field, the gas temperature and mass fractions of the combustion products in the mixing region of the fuel and oxidizer streams were overestimated. In contrast, the flamelet/progress variable (FPV) model yielded results that were in better agreement with the experimental data, because while the EBU model assumes an infinitely fast chemistry, the look-up tables used in the FPV model are based on the actual chemical kinetics. As these models can be used for the CFD simulations of coal and spray combustion, the results of this study should be useful for efficiently simulating practical combustion systems

Effect of natural gas injection point on combustion and gasification efficiency of pulverized coal under blast furnace condition

The reduction of CO2 emission from the ironmaking process is an important issue from the view of environmental problems typified by global warming in recent years. Low RAR (reducing agent rate) operation of the blast furnace is one of effective methods for reducing CO2 emission. Injection of HRA (hydrogenous reducing agents) from the tuyere (where is the lower part of a blast furnace) is also effective measure. In this study, the influence of HRA injection point on combustion and gasification efficiency of pulverized coal (PC) in the case of simultaneous injection of HRA and PC from double-channel lance was examined by small scale combustion furnace and three-dimensional numerical simulation for permeability in the blast furnace. Combustion experimental conditions were in three cases, case 1: injected HRA from the outer side and PC from the inner side of double-channel lance, case 2: injected HRA from inner side and PC from outer side of double-channel lance and case 3: injected HRA and PC premixed. As a result, the combustion and gasification efficiency was increased in the order of case 1, case 2 and case 3. The rate of combustion and gasification of PC was investigated in case1. Not only the oxidation reaction was also accelerated CO2 and H2O gasification reaction in the case of simultaneous injection HRA and PC. A three-dimensional numerical simulation of the experimental furnace was conducted, we confirmed the increase of combustion temperature, the acceleration of oxygen consumption and gasification reaction as with the experimental results in the case of simultaneous injection HRA and PC

Accurate numerical integration of β-PDF for the flamelet approach

When turbulent combustion simulations are performed using the flamelet approach, which indirectly considers the detailed chemical reaction mechanism, the probability density function (PDF) gets applied to statistical distributions for turbulent fluctuations in the mixture fraction that are modeled using the Reynolds-averaged Navier-Stokes or the large eddy simulation. In this case, the so-called “presumed PDF”-i.e., one in which the PDF is assumed and the flamelet table is constructed in advance-is generally employed, and the β-function is widely used for the PDF. This study investigated numerical integration for the equation with the β-function and examined the appropriate method. We succeeded in establishing the optimum value of the parameter for the integration interval and determining the appropriate numerical integration method