The kinetic parameters of the Kobayashi model [108] are fitted to the experimentally obtained pyrolysis data [107] of the coal fired. The present fit is based on the values presented by Ubhayakar et al. [234]. Compared to the parameters by Ubhayakar et al., the pre-exponential factors for low temperature and high temperaure reactions, A1 and
0.1 0.2
Zvol[-]
x=0 cm
0.1 0.2 Zchar[-]
0.1 0.2
x=25 cm
0.1 0.2
0.1 0.2
x=50 cm
0.1 0.2
0.1 0.2
x=85 cm
0.1 0.2
0.1 0.2
x=125 cm
0.1 0.2
0.1 0.2
0 200 400 600 800
r [mm]
x=195 cm
0 200 400 600 800
0.1 0.2
r [mm]
Figure 5.10: Volatile (left) and char-off mixture fractions (right) for the simulations on the 10 mm ( ) and 5 mm ( ) grid.
0 0.2 0.4 0.6 0.8
0 0.02 0.04 0.06 0.08
Yield[-]
t [s]
1000◦C 1200◦C 1400◦C
Figure 5.11: Volatile yield vs. residence time for the Saar coal obtained experimentally (circles)[107] and for the fitted Kobayashi model with parameters given in table 5.4.
A2, have been reduced by one order of magnitude. The activation energies, E1 and E2 have been reduced by 30% and increased by 20%, respectively. The parameters α1 and α2 have been remained unchanged. The results of the fitting are presented in table 5.4 along with the original parameters and the ones given by Ubhayakar et al.
The experimental conditions used for the fitting are obtained from Knill et al. [107], considering only 80 µm particles subjected to environments of 1000, 1200 and 1400◦C at mean heating rates of 0.73· 105, 0.95· 105 and 1.17· 105 K/s, respectively. The results obtained with the present fit compared to the experiment are shown in Fig. 5.11.
Table 5.4: Kobyashi model parameters.
Source A1 A2 E1 E2 α1 α2
s−1 s−1 J/kmol J/kmol
original [108] 2.0· 105 1.3· 107 1.05· 108 1.67· 108 0.3 1.0 Ubhayakar [234] 3.7· 105 1.46· 1013 7.37· 107 2.51· 108 0.39 0.8 present work 3.7· 104 1.46· 1012 5.16· 107 3.07· 108 0.39 0.8
Chapter 6
Highly Resolved Flamelet LES of a Semi-Industrial Coal Furnace [201]
This chapter including all text, figures and tables is published in the Proceedings of the Combustion Institute ‘Rieth, M., Proch, F., Clements, A.G., Raba¸cal, M., Kempf, A.M.
(2017). Highly resolved flamelet LES of a semi-industrial scale coal furnace. Proceedings of the Combustion Institute, 36(3), 3371-3379.’ [201] and is reprinted with permission from Elsevier. The author of this thesis contributed the code extension and development (including the incorporation of the combustion model and the devolatilization model and updates/optimization of other coal models), running of simulations, post-processing and paper writing. The author A.G. Clements contributed optimization of the radiation rou-tines and the author F. Proch the basic CFD code. The authors A.G. Clements, M.
Raba¸cal and A.M. Kempf contributed discussions, proof-reading and corrections. The author A.M. Kempf developed the original versions of the PsiPhi code.
Abstract
A highly resolved large eddy simulation (LES) of the semi-industrial IFRF coal furnace [250, 251] employing the steady flamelet model is presented. The flamelet table is based on mixture fractions of volatile and char-off gases as well as on enthalpy and scalar dissi-pation rate. Turbulence-chemistry interaction is treated with an assumed pdf approach, with the variance obtained from a transport equation. Radiation is computed by the dis-crete ordinates method and the grey weighted sum of grey gases model. The simulation is conducted with the massively parallel ‘PsiPhi’ code on up to 1.7 billion cells and with 40 million particles. Results are processed and compared against the comprehensive set of experiments to i) validate the new flamelet model and the simulation method and to ii) gain further insight into the combustion process that is not available from the experiment.
The simulation results show that the flamelet LES approach can successfully describe the flow field and combustion inside the furnace; major species and velocities are found in good agreement with the experiment.
The results are further analyzed with a focus on the processes of particle heating, de-volatilization, char combustion and flame stabilization in a highly turbulent environment.
Additionally, the relative importance of scalar dissipation rate is highlighted, showing a large separation of mixing scales between volatile and char-off gas combustion due to the long residence time and generally much lower scalar dissipation rates than typical for lab-scale experiments.
6.1 Introduction
World’s energy demand relies and will rely on burning fuels in the foreseeable future, with coal remaining attractive due to its abundance. The move towards more efficient and less polluting plants drives research efforts into fields like oxy-coal or biomass combustion [27].
Experiments are invaluable for the design and validation of efficient combustion sys-tems, but are increasingly supported by simulations. One of the most promising tools for accurately predicting pulverized coal combustion (PCC) is large eddy simulation (LES).
It offers superior predictions of flow and scalar fields, accurately considering gas phase combustion and turbulent transport. This has become the weakest link in (RANS) coal simulations, since advanced coal sub-models have become available. The first applica-tion of LES to coal combusapplica-tion has been reported by Kurose & Makino [114] followed by several studies on lab-scale jet flames [53, 140, 156, 157, 221, 252, 264], lab- and pilot-scale furnaces and test facilities (≤ 1.0 MWth) [26, 30, 43, 57, 189, 245] and semi-industrial furnaces [153] like the 2.5 MWth swirled IFRF experiment [250, 251] studied in this work. Although we find that LES is the most promising tool for predictive PCC simulations, there are several recent Reynolds-averaged/transported probability density function (RANS/PDF) based studies that were capable of reproducing PCC experiments very well [20, 161, 208, 223, 268], but usually in simulations with non-swirled flow fields.
Most of the reported LES studies rely on early turbulent combustion models (eddy break-up and eddy dissipation), but there is a need to advance the gas combustion models in PCC LES to the state-of-the-art as in pure gas flames to capture turbulence-chemistry interaction phenomena, which are important for flame stabilization and pollutant forma-tion. The flamelet model [162] is particularly promising for PCC applications due to its numerical efficiency and wide success in the field of turbulent combustion LES [85, 174].
After the first application of the flamelet model to volatile combustion by Williams et al. [257], Vascellari et al. [238] and Xu et al. [263] investigated flamelet methods for re-solved single coal particle simulations. Recently, Watanabe & Yamamoto [248] presented a flamelet model that can be applied to turbulent PCC simulations taking volatile and char combustion into account. Their testing in a two-dimensional simulation of a coal jet showed a good agreement compared to data obtained with finite rate chemistry.
This work applies the flamelet model to the LES of the semi-industrial IFRF coal furnace studied experimentally by Weber et al. [250, 251], introduces efficient subgrid modeling and validates the model under realistic conditions and against a comprehensive set of measurements.