•Influence of pyrolysis modeling on CFD entrained coal gasification modeling.•Suitable empirical pyrolysis models calibrated using network models.•Multiple particle heating rates used for the ...calibration.•Very good prediction of flame position considering detailed pyrolysis kinetics.
CFD modeling results for entrained flow coal gasification using advanced submodels for coal conversion are presented and compared to detailed experimental data. The focus of this investigation is on the accurate modeling of the pyrolysis process. An iterative procedure is proposed and validated to bridge the gap between detailed pyrolysis models such as CPD, FLASHCHAIN or FG-DVC and empirical models based on single- or multiple-step kinetic expressions, which are usually used in CFD. Multiple particle heating rates from the CFD solution are taken to perform detailed pyrolysis calculations and these results are used to find optimal kinetic parameters for the empirical models using an automated procedure. It is shown that the heating rate strongly influences the devolatilization process (rate and yield). CFD simulations are performed for the BYU entrained flow gasifier. Due to the high heating rate in entrained flow gasification, the volatile yield can differ significantly from the proximate analysis value. Accurate pyrolysis modeling is shown to be important to capture coal flame ignition, flame location, species distribution and outlet composition. Since the final volatile yield determines the split in carbon conversion between pyrolysis and the subsequent fast conversion in the gas phase and the heterogeneous char conversion, which is a comparatively slow process under gasification conditions, it also directly influences the overall carbon conversion. Overall, the application of the new comprehensive CFD model including the fitted kinetic rates is shown to give similar results to the full coupling of the CFD and pyrolysis software. The comparison between the simulations and the experiments shows very good agreement for three out of four coals. The fourth coal (lignite with high O/C ratio) is well outside the range for which the detailed models were developed, but reasonable agreement is still obtained.
The ignition of non-premixed n-dodecane-air flames is studied in a highly transient counterflow configuration under diesel engine operating conditions. The one-dimensional transient counterflow is ...configured such that ignition is initially physically inhibited through high strain rates. In a short relaxation period the inflow velocities (thereby the strain rate) are lowered and ignition can occur, a behaviour similarly observed in spray flames. The non-premixed flame, which is igniting in this unsteady flow environment, is computed with direct chemistry and tabulated chemistry based on the unsteady flamelet progress variable approach (UFPV). The flamelet look-up table is constructed from igniting unsteady flamelets considering also the parameter range between stable and unstable branches of the characteristic S-shaped curve which is obtained via a continuation method. The mixture fraction, the progress variable, and the local stoichiometric scalar dissipation rate serve as control variables for the three-dimensional table. Particular attention is put on the inclusion of thermal expansion effects and the treatment of the progress variable source term to capture the onset of ignition. Besides varying strain effects, the igniting flame exhibits low temperature chemistry and multi-stage ignition behaviour which is particular challenging for the UFPV modeling. By direct comparison to the reference solution the predictive capabilities of the UFPV approach are analyzed and assessed in detail. Furthermore, the influence of table resolution on computational efficiency is discussed. The systematic analysis and validation of the UFPV approach in the transient counterflow configuration fills a specific gap in the literature and underlines the fidelity of the approach for flames subject to complex transient processes. Furthermore, the 1D configuration may serve as an environment for adjusting and optimizing UFPV tabulation strategies prior to their application to more complex 3D CFD simulations such as spray LES.
•Detailed Kinetics for Coal Combustion and Database Generation.•Machine Learning for Predictive Coal Combustion.•HDMR Reduced-Order Models.•Reduced Computational Cost for CFD Simulations.
Because of ...the complex multiscale turbulence-chemistry-particle (TCP) interactions in solid fuel-supplied combustion systems, developing predictive models remains a formidable challenge even with the improved accuracy of the large eddy simulation (LES) approach. There are three main types of LES-based coal combustion model: a) those for coal particle dynamics, b) those for subgrid TCP interactions, and c) those for solid fuel kinetics. The third type is the focus of this work, as several recent studies have shown that the accuracy of kinetic models used to describe the solid-gas phase kinetic conversion process is of primary importance when it comes to predictability. Therefore, the implementation of detailed solid fuel kinetics is desired when simulating coal combustion However, it is far from being feasible to directly couple detailed kinetics in large-scale LESs of power plants. To overcome the challenge, a reduced-order model based on Machine Learning (ML) is developed in this work to accurately represent the solid-gas phase conversion process at an acceptable computational cost. The ML-based model is trained with databases from the simulations of single-particle combustion with detailed kinetics over a wide range of operating conditions extracted from a novel gas-assisted coal combustion chamber, and then validated by the test databases and unsteady particle trajectories from the LES of the gas-assisted coal combustion chamber. The ML-based model can accurately predict different phases of coal particle combustion at a reduced computational cost. The results indicate that the use of ML-based approaches is promising for implementing detailed solid fuel kinetics in the context of LES.
Rapid Hydrogen Shift Reactions in Acyl Peroxy Radicals Knap, Hasse C; Jørgensen, Solvejg
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
02/2017, Letnik:
121, Številka:
7
Journal Article
Recenzirano
We have used quantum mechanical chemical calculations (CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ) to investigate the hydrogen shift (H-shift) reactions in acyl peroxy and hydroperoxy acyl peroxy ...radicals. We have focused on the H-shift reactions from a hydroperoxy group (OOH) (1,X-OOH H-shift with X = 6, 7, 8, or 9) in the hydroperoxy acyl peroxy radicals, this H-shift is a reversible reaction and it scrambles between two peroxides, hydroperoxy acyl peroxy and peroxy peroxoic acid radicals. The forward reaction rate constants of the 1,X-OOH H-shift reactions are estimated to be above 103 s–1 with transition state theory corrected with Eckart quantum tunnelling correction. The ratio between the forward and reverse reaction rate constant of the 1,X-OOH H-shift reactions is around ∼105. Therefore, the equilibrium is pushed toward the production of peroxy peroxoic acid radicals. These very fast 1,X-OOH H-shift reactions are much faster than the reactions with NO and HO2 under most atmospheric conditions and must be included in the atmospheric models when hydroperoxy acyl peroxy radicals are oxidized. Finally, we have observed that H-shift reactions in a pentane acyl peroxy radical (C5-AOO) is fast (>1 s–1); this can have a significant influence on the possible formation of large acyl peroxy nitrate molecules.
The effect of differential diffusion of two passive scalars having Schmidt numbers of unity and 0.25, respectively, is investigated using direct numerical simulation of a temporally evolving jet. The ...objective of the research is twofold: (i) to compare the turbulent/non-turbulent (T/NT) interface position using the scalar criterion between the unity- and low-Schmidt-number scalar; and (ii) to determine the impact of the T/NT interface on differential diffusion. For the latter, the T/NT interface is detected using the vorticity criterion. To quantify the effect of differential diffusion, a normalised differential diffusion parameter is analysed, clearly showing the dominance of differential diffusion at the T/NT interface. A transport equation for the scalar differences is then evaluated, which shows that differential diffusion originates at the interface. Further, the separation between the passive scalars, arising due to differential diffusion, is studied using conventional and conditional statistics with respect to the interface distance. Since differential diffusion is known to be present at large and small scales, the connection between them is analysed using the scalar dissipation rate. Moreover, the physical mechanism responsible for the departure of the two scalars is analysed using the scalar gradient alignment, the ratio of the diffusive fluxes and by a transport equation for the scalar gradients.
•The laminar burning velocity was measured with the Heat Flux method and the closed combustion vessel.•Liquid biofuels, namely ethanol and iso-butanol, and the reference fuel iso-octane were ...investigated.•The two methods show maximum deviations between 2.3 cm/s and 6.7 cm/s.•The numerical mechanisms reveal prediction performances between 0.2 cm/s and 5.2 cm/s.
Premixed flame characteristics play a crucial role in most advanced combustion applications, such as gas turbines, aircraft combustors or internal combustion engines. The laminar burning velocity is one key parameter to determine the stabilisation and propagation of premixed flames. This study examined the influence of high pressures on the laminar burning velocity of biofuels and the correlation between laminar burning velocity and pressure. The investigated biofuels were pure ethanol as the state-of-the-art surrogate, pure iso-butanol as a possible alternative and several blends of ethanol/iso-octane. Due to the usage of two methods to measure the laminar burning velocity – the Heat Flux burner and the closed combustion vessel – a comparison was added for atmospheric conditions. The measured laminar burning velocities were conducted for equivalence ratios from 0.7 to 1.3, a pressure range from 1 bar(a) to 15 bar(a) and 373 K. The results were further compared to existing numerical mechanisms and literature data.
The numerical mechanisms reveal prediction performances in the range of 0.2 cm/s (0.4%) and 5.2 cm/s (16.2%) for dedicated mechanisms and equivalence ratios between 0.9 and 1.1. The two methods show deviations of the laminar burning velocities between 2.3 cm/s (4.9%) and 6.7 cm/s (11.4%).
A detailed Large Eddy Simulation (LES) of pulverised coal combustion in a large-scale laboratory furnace is presented. To achieve a detailed representation of the flow, mixing and particle ...dispersion, a massively parallel LES was performed. Different phenomenological network models were applied and compared to each other in order to obtain the most adequate devolatilization kinetic data for the LES. An iterative procedure allowed to optimise the devolatilization kinetic data for the studied coal and operating conditions. The particle combustion history is studied by analysing particle instantaneous properties giving a perspective on coal combustion that currently is not available by other means than LES. Predicted major species and temperature were compared with measurements and a good agreement was obtained. The finely resolved near burner region revealed that the flame is stabilised very close to the burner. Furthermore, two distinct zones of CO2 production were found - one in the internal recirculation zone (IRZ) due to gaseous combustion, and one downstream of the vortex breakdown, due to intense char combustion. It was found that particle properties are inhomogeneous within the IRZ, whereas in the external recirculation zone (ERZ) and downstream of the vortex breakdown they were found to be homogeneous.
PDF
