3rd Workshop on Modelling of Chemical Reaction Systems, Heidelberg, 1996
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MODELLING OF HYDROCARBON AND NITROGEN CHEMISTRY IN
TURBULENT COMBUSTOR FLOWS USING DETAILED REACTION
MECHANISMS
H. C. Magel, U. Schnell, K.R.G. Hein
Institute for Process Engineering and Power Plant Technology
University of Stuttgart
Pfaffenwaldring 23, D-70550 Stuttgart, Germany
e-mail: [email protected]
1 ABSTRACT
The description of chemical kinetics in turbulent reactive flows is an important
task to improve combustion models. This paper describes the inclusion of detailed
chemical reaction mechanisms into the framework of a turbulent flame simulation.
Calculations are based on a finite-volume solution procedure including submodels
for turbulent flow, combustion of fuel and radiative heat transfer. The interaction of
chemical reactions and turbulence is modelled using the Eddy Dissipation Concept
(EDC). The basic idea of incorporating the reaction mechanism into the EDC is
described. The oxidation of methane is described with a detailed C1/C2
mechanism.
The proposed model is applied to a 400kW turbulent diffusion methane flame
in a cylindrical furnace. The measured trends in temperature and species
concentrations of CH4, O2, CO and CO2 are adequately reproduced by the
predicted profiles.
To demonstrate the benefits and limits of this approach, the method is applied to
predict gas phase reactions of the DeNOx technology ‘reburning’ using methane as
reductive, which is applied to a pulverized coal flame. The detailed reaction
mechanism of Miller & Bowman [1] is used to describe the nitrogen chemistry.
The nitrogen chemistry is calculated decoupled from the hydrocarbon chemistry in
a post-processor step. The modelling results are compared to experimental data of a
bench scale test facility.
2 INTRODUCTION
Prediction of flames in furnaces is an important tool for the development of
optimized combustion methods. Since chemical kinetics have a major influence on
formation and destruction of pollutants and intermediate species like carbon
monoxide, a more detailed modelling of turbulent combustion for advanced
combustion systems equipped with air or fuel staging is required. The best way to
describe the hydrocarbon and nitrogen chemistry for various operation conditions
in staged combustion is to apply a detailed reaction mechanism. In the following, a
combustion model will be outlined which is able to handle a detailed reaction
mechanism based on the Eddy Dissipation Concept (EDC).
MODELLING OF HYDROCARBON AND NITROGEN CHEMISTRY IN TURBULENT
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3 MATHEMATICAL MODELS
A prediction code for turbulent reacting flows includes submodels describing
flow field, combustion and heat transfer by radiation. In their entirety the different
submodels form a system of strongly coupled partial differential equations. Each of
these equations can be written in the form of a general transport equation:
∂
∂  ∂Φ 
( ρuΦ ) =
Γ
+ SΦ
∂x
∂x ∂x 
(1)
where ρ, u, x, SΦ and Γ are density, velocity, co-ordinates, source term and diffusion coefficient. This equation describes the local change of the Favre-averaged
variable Φ due to convection, diffusion and production under steady state conditions. Depending on Φ, Eq. (1) represents either mass, momentum, species, or
energy conservation.
Fluid Mechanics and Thermal Radiation
The basic equations of numerical fluid mechanics are well described in
numerous preceding papers, e.g. Schnell et al. [2] and only the newly incorporated
features are related here. The Navier-Stokes equations are solved for the primitive
variables with a standard k,ε-turbulence closure. A transport equation (Eq. (1)) is
solved for the mean mass fraction of each balanced species. The mass density is
determined by the ideal gas equation of state. Thermal radiation is modelled using
the Discrete Transfer Method proposed by Lockwood and Shah [3]. The absorption
coefficient of the gas mixture is assumed to have a constant value of 0.5/m. In the
case of pulverized coal combustion the value depends additionally on the local
particle density.
Turbulent Gas Phase Combustion
Chemical reactions are only functions of the local state specified by density,
species concentrations, and enthalpy and can be properly described with a detailed
reaction mechanism. But due to the statistical nature of these variables in turbulent
combustion and the non-linearity of chemical reaction rates, the current
computational limits prohibit a complete description of chemistry and turbulent
flow. Therefore a turbulent combustion model based on simplifications in
describing the turbulence behaviour and the chemical reaction mechanisms is
necessary to calculate the mean reaction rates.
Chemical reaction kinetics have been included explicitly in a number of
turbulent flame studies, in conjunction with a full H2 mechanism and presumedshape joint PDFs (Bockhorn [4]), or with reduced chemistry and Monte Carlo
calculations of the joint PDF (Pope [5], Nau [6]).
In this study the Eddy Dissipation Concept (EDC) is used to model the
influence of turbulence on chemical reactions. With the EDC it is possible to
include a detailed reaction mechanism in turbulent flame calculations. The
presented EDC combined with chemical kinetics is a general concept which allows
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inclusion of radiation, fuel generated by particles, and the simulation of furnaces
with multiple inlets with different fuel composition.
Pulverized Coal Combustion
Modelling of coal combustion is not the main subject of the present work and
therefore only a short description is given here. Coal pyrolysis is modelled using a
simple one-step model. The amount and composition of the volatiles are taken
from experimental data. Char burnout is controlled by the chemical surface
reaction and the oxygen diffusion to the particle. In the modelling approach, char
nitrogen is released as HCN proportional to the char burnout rate. The release of
the volatile nitrogen species during pyrolysis is modelled as HCN proportional to
the release rate of the volatiles. The basic equations of the coal combustion model
are well described in numerous preceding papers, e.g. [7], [8].
Numerical Solution
A conventional finite volume method is used to solve the system of partial
differential equations. For the non-staggered grid arrangement a special procedure,
known as the pressure-weighted interpolation method (PWIM), is needed to
calculate the convective fluxes at volume faces. To reduce numerical diffusion the
monotonized linear-upwind scheme (MLU) is used for the calculation of the
convective fluxes. The equations are solved using a Gauss-Seidel algorithm. In
order to accelerate the convergence rate, the pressure correction equation is solved
using the Strongly Implicit Procedure. The SIMPLEC algorithm is chosen for the
treatment of velocity-pressure coupling. Further details of the numerical solution
procedure have been published previously by Schneider et al. [9].
4 REPRESENTATION OF CHEMICAL KINETICS
The full C1/C2 mechanism of Warnatz and Maas
The oxidation of methane is quite well understood and various detailed reaction
mechanisms are reported in literature. The mechanisms differ with respect to the
considered species and reactions. Peeters [10] shows the effects of some
mechanisms on laminar flame speed, maximum temperature and CO concentration.
But considering the uncertainties and simplifications included in a turbulent flame
calculation the various mechanisms agree reasonably well.
The reaction mechanism used in this work to model the hydrocarbon chemistry
is the full C1/C2 mechanism from Warnatz and Maas [11] by allowing C1 and C2species only. This mechanism includes about 100 elementary reactions and
considers the following 27 species:
CH4, O2, CO2, H2O, CO, H2, H, O, OH, N2, HCO, CH2O, H2O2, HO2, CH3, CH2,
CH, C2H6, C2H5, C2H4, C2H3, C2H2, C2H, HCCO, CH2CO, CH3CO, CH3HCO.
This full mechanism is believed to capture the most important reactions with sufficient accuracy.
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The full nitrogen mechanism of Miller and Bowman
The full mechanism published by Miller and Bowman [1] is used to calculate
the nitrogen chemistry in a post processing step. It includes about 250 elementary
reactions and considers the following species for the nitrogen and methane
chemistry:
HCN, NO, NH3, NH, NH2, N2O, N, HNO, NCO, CN, NNH, C2N2, NO2,
HOCN, HCNO, HNCO, H2CN, CH4, O2, CO2, H2O, CO, H2, H, O, OH, C2H2,
C2H4, C2H6, CH3, CH2, CH, HCO, CH2O, H2O2, HO2, C2H5, C2H3, C2H, HCCO,
CH2CO, CH2OH, CH3O, C3H3, C3H2, CH2(S), C, C4H2, C4H3, HCCOH, N2
5 EDDY DISSIPATION CONCEPT (EDC)
As proposed by Magnussen [12] [13], the EDC is a general concept for treating
the interaction between turbulence and chemistry in flames. The method is based
on a detailed description of the dissipation of turbulent eddies. In the EDC the total
space is subdivided into a reaction space, called the ‘fine structures’ and the
‘surrounding fluid’. All reactions of the gas phase components are assumed to take
place within this reaction space which represents the smallest turbulence scales
where all turbulent energy is dissipated into heat. All reactions in the surrounding
fluid are neglected. Thus in order to be able to treat the reactions within the fine
structures, the volume fraction of the reaction space γ* and the mass transfer rate
M* between the fine structures and the surrounding fluid have to be determined.
Both quantities are derived from the turbulence behaviour of the fluid. Details of
the applied EDC implementation are given in [19].
Well stirred reactor
By treating the reacting fine structures locally as a well stirred reactor which
transfers mass and energy only to the surrounding fluid (Fig. 1), every chemical
kinetic mechanism can be linked with the EDC combustion model.
heat loss
reactants
products
reaction space
M*
‘fine structures’
M*
mi* ; T* ; ρ*
0
mi ;
T0
;
ρ0
surrounding fluid
Figure 1. Well Stirred Reactor (PSR)
The reaction rates of all species are calculated on a mass and enthalpy balance
for the fine structure reactor. The chemical reactions and the mass transport can be
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described by the following algebraic equations for species conservation and total
enthalpy h.
ω i∗ W i
M∗
-------------- ( m i∗ – m i ) = --------------ρ∗
1 – γ∗
M∗
-------------1 – γ∗
I
∑
( m i∗ h i∗ – m i h i )
i=1
ε
with M∗ = 2.43  --- 
ν
0.5
( i = 1, …, I )
q∗
= -----ρ∗

2
and γ∗ = 2.13  νε ⁄ k 
(2)
(3)
0.25 2
From these equations it is possible to calculate the mass fractions and
temperature in the fine structures as a function of known quantities, e.g. mean
temperature and mean mass fractions. The mean reaction rates of all species can
then be calculated by the chemical reaction rate inside the fine structures. One
should keep in mind that the reactions take place only within a fraction γ∗ of the
total space. Since the chemical reaction rates in the fine structures in general are
functions of all the mass fractions and the temperature, a set of non-linear, coupled,
algebraic equations must be solved.
Numerical Solution
The solution of the Perfectly Stirred Reactor (PSR) equations (2) and (3) must
be obtained by an ‘inner’ iteration procedure, which is embedded in the CFDiteration procedure of the general transport equations, see Eq. (1). This means that
for each CFD iteration it is necessary to calculate at least once the Jacobian matrix
of the PSR equation system in each control volume. This is performed with a
modified version of the PSR-code from the CHEMKIN library [17].
A good initial estimate of the flame behaviour is required to avoid numerical
problems. Therefore calculations with detailed chemistry are always started from a
converged solution obtained with a global two step chemistry [14].
A turbulent flame calculation with detailed chemistry needs a high amount of
CPU time consumed by the solution of a non-linear equation system to determine
the source terms of the species. To reduce the complexity of the system, the
nitrogen chemistry is calculated decoupled from the hydrocarbon chemistry in a
post-processor step.
6 TURBULENT FLAME CALCULATION
Results of a turbulent diffusion flame in a cylindrical furnace with a thermal
input of 400 kW are presented. A full description of this furnace and the
experimental conditions can be found in [18]. Fuel and air are fed through two
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COMBUSTOR FLOWS USING DETAILED REACTION MECHANISMS
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coaxial pipes with diameters of 60mm and 100mm. The diameter and length of the
combustion chamber are 0.5m and 1.7m. The composition of the natural gas is
approximately 90% methane and 10% nitrogen. Fig. 2 shows the predicted
temperature distribution of the furnace.
140
0.0
1.0
1.5
800
600
fuel
0
00
air
16
1200
0.5
2.0
axial distance [m]
Figure 2. Calculated temperature distribution in Kelvin
The chemical reactions are quite slow in the mixing region between the two
jets, leading to a slow temperature rise during the consumption of methane. These
phenomena are reflected in the axial profiles of temperature, CH4 and CO2 data and
the radial profiles of CH4 and O2 data (Fig. 3) which are adequately reproduced by
the predicted profiles using the full C1/C2 mechanism of Warnatz and Maas. For
the radial profiles one must keep in mind that there is a steep gradient near the axis
and in this region the profiles are very sensitive to even small shifts in the axial
distance. A computational grid with 65 nodes in axial and 30 nodes in radial
direction was used. Calculations performed with a refined grid (130 x 70 nodes)
gave almost identical results and therefore grid independence is assumed. A
detailed prediction of this case is discussed in [19].
axial profiles
radial profiles
1500
20
1000
10
500
CO2
0
0
0.0
0.5
1.0
1.5
axial distance [m]
2.0
concentration [vol%]
Temp.
CH4
temperature [K]
concentration [vol %]
30
6
4
O2
2
CH4
0
0.0
0.05
0.1
0.15
0.2
0.25
radial distance [m]
Figure 3. Comparison of predictions and measurements along the flame axis
and at an axial distance of 1.31m (radial profile). Symbols: data;
lines: predictions
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7 COAL COMBUSTION WITH FUEL STAGING
To demonstrate the benefits and limits of the method, it is applied to the
prediction of fuel-NO formation and reduction in a pulverized coal flame equipped
with fuel staging using methane as reductive. In coal combustion the major part of
NO emissions arises from the fuel-bound nitrogen. Fuel staging or reburning
(Fig. 4, left) is an effective DeNOx technology to reduce NOx emissions in the
combustion chamber. One part of the fuel is used to establish a fuel-rich zone after
the main combustion zone. In this substoichiometric zone NOx can be reduced e.g.
by hydrocarbon radicals. The delay in supplying the burnout air provides the
residence time for NOx reduction. Due to the very fuel rich conditions in the
reduction zone, the hydrocarbon and nitrogen chemistry is very complex and
difficult to calculate.
The fuel staging technology is experimentally studied in an electrically heated
entrained flow combustion reactor with a wall temperature of 1300 ˚C. The test
facility has a thermal input up to 30 kW. Fuel and air are fed through three coaxial
pipes. The reactor has a length of 2.5 m and an inner diameter of 200 mm. A
detailed description of the test facility can be found in [20].
In the main combustion zone, a pulverized coal flame with an air-to-fuel ratio
of 1.15 is established. This coal flame produces a flue gas with a nitrogen oxide
concentration of about 850 ppm. At an axial distance of 0.9 m from the burner,
methane is injected. Thus a reburn stoichiometry of 0.75 and a residence time in
the reduction zone of about 1.5 s is achieved. The burnout air injected at an axial
distance of 1.63 m shifts the air-to-fuel ratio to 1.15.
axial profiles of O2, CO and CO2
fuel staging
air
λ = 1.15
methane
λ = 0.75
air
λ = 1.15
ash
flue gas
reburn
zone
burnout zone
15
concentration [vol%]
coal
primary
zone
CO2
10
O2
CO2
CO
O2 data
CO2 data
CO data
O2
5
CO
0
0.0
0.5
1.0
1.5
2.0
2.5
axial distance [m]
Figure 4. Scheme and axial profiles for a pulverized coal flame with fuel staging.
Lines: predictions; symbols: data
The oxygen profile along the furnace axis (Fig. 4, right) demonstrates this
behaviour. The main combustion zone (primary zone) will not be discussed here,
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because the coal combustion model is not subject of this work. After this zone the
coal is almost burnt and heterogeneous reactions can be neglected. The main
interest is put on the chemistry in the fuel rich reburn zone. The injected methane
consumes all existing oxygen. Also the existing CO2 is partially destroyed and a
high amount of CO is present. This behaviour is adequately reproduced by the
predictions, see Fig. 4.
The nitrogen chemistry in this fuel rich zone is very complex and strongly
influenced by intermediates of the hydrocarbon chemistry. The reduction of NO is
mainly accomplished by hydrocarbon radicals. The major reaction paths are shown
in Fig. 5. In addition, the complete oxidation path of HCN and NH3 is required.
3CH
2
+H
+NO
HCNO
+H
+NO
CH
HCN
+H
NH
+NO
CN
N
+NO
N2
+H
+O
+H2
C
+H
NCO
+O2
Figure 5. Reaction path diagram of the NO-HCN-N2 conversion mechanism
Fig. 6 shows measurements and predictions of NO and HCN concentrations.
The existing NO of about 850 ppm after the primary zone is reduced fast as soon as
the reburn fuel is present and HCN is formed. This reduction is overpredicted by
the model, forming too much HCN. In the prediction, HCN and NH3 are the main
nitrogen containing species in the reduction zone. They are partially oxidized to
NO after the injection of burnout air. The effluent NO concentration is a little
overpredicted.
NO concentration
concentration [ppmv]
reburn
zone
primary
zone
burnout zone
1000
800
NO
NO data
600
400
200
concentration [ppmv]
primary
zone
1000
HCN and NH3 concentrations
reburn
zone
burnout zone
800
HCN
NH3
HCN data
600
400
200
0
0
0.0
0.5
1.0
1.5
axial distance [m]
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
axial distance [m]
Figure 6. Axial profiles for a pulverized coal flame with fuel staging.
Lines: predictions; symbols: data
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COMBUSTOR FLOWS USING DETAILED REACTION MECHANISMS
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There are several sources of error in the predictions. First of all the mixing of
reburn fuel and primary gas may be incorrectly predicted. The second reason could
be the coupling concept (EDC) between chemistry and turbulence. But the
promising results of the hydrocarbon combustion do not support these ideas. The
third one is the chemical kinetics, which may be inadequate for the prevailing
conditions. This is also found by other groups calculating the reburn process, e.g
[21]. At present the failure can not be attributed to one specific submodel.
8 CONCLUSIONS
A combustion model is investigated which is capable of representing detailed
chemical reaction mechanisms in the framework of a turbulent combustor flow
simulation based on an Eddy Dissipation Concept. The full C1/C2 reaction
mechanism of Warnatz and Maas [11] is used to describe the methane oxidation.
Calculations are based on a conventional finite-volume solution procedure,
including submodels for turbulent flow, combustion and radiative heat transfer.
This way the combustion of methane could be predicted quite well.
For the simulation of a 400 kW turbulent diffusion flame in a cylindrical
furnace the predictions give good agreement with measured temperature and
species concentration data.
The investigation of the DeNOx technology ‘reburning’ applied to a pulverized
coal flame shows that the model is able to predict the complex hydrocarbon
chemistry in the very fuel rich reduction zone. The nitrogen chemistry is calculated
decoupled from the hydrocarbon chemistry in a post-processor step using the
detailed reaction mechanism of Miller & Bowman [1]. For the nitrogen chemistry
the major trends of the measurements are reproduced by the predictions although
big differences exist in some regions. Nevertheless the model is able to describe the
very complex chemistry of the NO reduction reactions occurring in fuel-staged
combustion.
REFERENCES
1. Miller, J.A. and Bowman, C.T., Proc. Energy Comb. Sci. 15: 287-338 (1989)
2. Schnell, U., Schneider, R., Magel, H.C., Risio, B., Lepper, J., Hein, K.R.G., Third
International Conference on Combustion Technologies for a Clean Environment, Lisbon, 1995
3. Lockwood, F.C. and Shah N.G., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1405-1414.
4. Bockhorn, H., Zur Struktur turbulenter Diffusionsflammen, Habilitation Thesis,
University of Darmstadt, 1989
5. Pope, S.B., Progress in Energy and Combustion Science 11: pp. 119-192 (1985).
6. Nau, M., Wölfert, W., Maas, U., Warnatz, J., Tenth Symposium on Turbulent
Shear Flows, 1995, Vol. 2, pp. 19.25-19.30
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COMBUSTOR FLOWS USING DETAILED REACTION MECHANISMS
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7. U. Schnell, Berechnung der Stickoxidemissionen von Kohlenstaubfeuerungen,
VDI Fortschrittsberichte Reihe 6, Nr 250, 1991
8. Magel, H.C., Schnell, U., Hein, K.R.G., Joint Meeting of the British, Spanish and
Swedish section of the Combustion Institute, Funchal, Madeira, 1996
9. Schneider, R., Risio, B., Schnell, U., Hein, K.R.G., Third International Symposium on Coal Combustion, Beijing, 1995
10. Peeters, T., Numerical modeling of turbulent natural-gas diffusion flames, Ph.D.
Thesis, University of Delft, 1995
11. Warnatz, J. and Maas, U., Technische Verbrennung, Springer, New York, 1993,
p. 101-104
12. Magnussen, B.F., Hjertager, B.H., Olsen, J.G., Bhaduri, D., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh,
1978, pp. 1383-1393.
13. Magnussen, B.F., Nineteenth AIAA Aerospace Meeting, St. Louis, 1981.
14. Magel, H.C., Schneider, R., Risio, B., Schnell, U., Hein, K.R.G., Eighth International Symposium on Transport Phenomena in Combustion, San Francisco,
1995.
15. Magnussen, B.F., Eighteenth International Congress on Combustion Engines,
Tianjin, Int. Council on Combustion Engines, 1989.
16. Gran, I.R., Mathematical Modeling and Numerical Simulation of Chemical
Kinetics in Turbulent Combustion, Dr. Ing. Thesis, University of Trondheim,
1994.
17. Glarborg, P., Kee, R.J., Grcar, J.F., Miller, J.A., PSR - A Fortran program for
modeling well-stirred reactors, Technical report SAND86-8209, Sandia
National Laboratories, 1986.
18. Garreton, D., Simonin, O., First Aerodynamics of Steady State Combustion
Chambers and Furnaces Workshop, EDF-DER, Chaton, 1994.
19. Magel H.C., Schnell U., Hein K.R.G., Twentysixth Symposium (International)
on Combustion, The Combustion Institute, Pittsburgh, 1996
20. Greul, U., Rüdiger, H., Spliethoff, H., Hein, K.R.G. 1996, NOx controlled combustion in a bench scale test facility, 21st Int. Techn. Conf. on Coal Utilization
& Fuel Systems, Clearwater, USA
21. Braun-Unkhoff M., Frank P., Stapf D., Leukel W., Modelling of NOx Reduction
by Reburning, 3rd Workshop on Modelling of Chemical Reaction Systems,
1996
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