d5.1 development of detailed, reduced and tabulated chemical kinetic schemes for ... ·...

D5.1 Development of detailed, reduced and tabulated chemical kinetic schemes for biomass combustion Document Information Contract Number 689772

Project Website www.hpc4energy

Contractual Deadline M9

Dissemination Level PU

Nature R

Author Angelo Greco (ULANCS)

Contributor(s) Xi Jiang, Carmen Jiménez, Fernando A. Rochinha

Reviewer Daniel Mira (BSC)

Keywords Combustion, biomass, kinetics

Notices: The research leading to these results has received funding from the

European Union’s Horizon 2020 research and innovation programme under grant agreement No “689772” And from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Ensino e Pesquisa (RNP) under the 3º Coordinated Call BR-UE Project (WWW.HPC4E.EU) ã 2016 HPC4E Consortium Partners. All rights reserved.

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Table of Contents

Executive Summary ....................................................................................................... 3 1. Introduction ............................................................................................................. 3 2. Physical modelling .................................................................................................. 4 3. Validation of Cantera for laminar flames ............................................................... 4 4. Analysis of fuel variability ...................................................................................... 8 5. Thermochemical database generation ................................................................... 10 6. Ongoing work ....................................................................................................... 11 7. Conclusions ........................................................................................................... 12 References .................................................................................................................... 13

D5.1 Assessment of chemical kinetics for biomass combustion

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Executive Summary This is the WP5 Deliverable D5.1 associated to the validation and assessment of chemical kinetic mechanisms for biomass-derived gaseous fuels. The open-source code Cantera has been tested and validated against well-established solvers (PREMIX and COSILAB), and then used to evaluate the effects of fuel composition on the chemical structure of laminar flames. The next step was to generate thermochemical databases using the laminar premixed flames already obtained and integrate the source terms by a beta-PDF to take into account the effects of turbulence.

1. Introduction Renewable energy sources are playing an increasingly important role in world energy supply, which is facing an unprecedented dilemma in meeting the growing energy demand with depleting natural resources. Biogas and bio-syngas are the most important gaseous renewable energy related to biomass. Biogas is normally produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and agriculture crops. The renewable gaseous fuels such as biogas and bio-syngas (from gasification of biomass) are not broadly used, despite their advantages, mainly because of their complex chemical compositions [1, 2]. Biogas mainly consists of CH4 and CO2, with the presence of other minor constituents such as N2 and H2, where the percentages of these constituents can vary significantly. The variable fuel composition can cause problems in the combustion of these fuels in practical systems. In fact, the fuel composition changes or the fuel variability may lead to unpredictable combustion performances, combustion instability and hot spots which may deteriorate and damage the combustion hardware. This document will present a preliminary study and methodology used to compare and cross-validate chemical kinetics schemes for biomass-derived gaseous fuels. Laminar premixed flames are investigated first, using the open-source software package Cantera 2.2.1(http://www.cantera.org/docs/sphinx/html/index.html, which is suite of object-oriented software tools for problems involving chemical kinetics, thermodynamics, and/or transport processes). This package is first validated and tested for the implementation of other chemical kinetic schemes, from detailed to reduced chemistry mechanism. Initial results from this study will be presented, followed by the presentation of the methodology to develop the tabulated chemistry. Finally, results and future work will be addressed.


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2. Physical modelling The problem to be solved is a premixed, freely propagating, laminar flat flame in adiabatic conditions. A premixed mixture of fuel and air is entering in the reaction zone, where the combustion is taking place. The governing equations to be solved consist of the continuity, species conservation and energy conservation equations, given as follows.

( ) 0tρ

ρ∂

+∇⋅ =∂

u (1)

∂Yk∂t

+∇⋅ ρuYk( ) =∇⋅ ρDk∇Yk( )+ !ωk

(2) ∂ ρcpT( )

∂t+∇⋅ ρcpuT( ) =∇⋅ λ∇T( )+ !ωT − ρ

∂T∂xi

cp,kYkVk ,ik=1

Ns

∑&

'((

)

*++ (3)

In Eqs. (1)-(3), ρ , pc , T , λ , uare the density, specific heat capacity, temperature of the mixture, thermal conductivity and velocity vector of the fluid respectively, while kY , kD and !ωk represent the mass fraction, transport diffusion coefficient and production

rate of the species k from 1 to Ns (total number of species) respectively. Additionally, ,p kc and ,k iV represent the specific heat and diffusion velocity of the species

respectively. The problem is considered as steady where the equations are solved by seating on the frame of the flame. In addition, the hypotheses of a freely laminar propagation allow considering the problem as one-dimensional. The premixed fuel/air fluid mixture is considered as a perfect gas with pressure assumed as constant and with a non-unity Lewis number (Le≠1). The equations were solved on a one-dimensional domain subdivided in N identical elements of size Δx=L/N. Moreover, the diffusion coefficient of a species k in Eq. (2) is described by a mixture averaged diffusion coefficient:

,

1

/

kkm Ns

j k jj k

YDX D

≠

−=

∑

(4)

In Eq. (4), jX designates the molar fraction of species j. These equations are implemented in Cantera 2.2.1 with the possibility to choose the chemistry mechanism.

3. Validation of Cantera for laminar flames The package Cantera 2.2.1 is validated first by comparison to the commercial package PREMIX from CHEMKIN http://www.reactiondesign.com/products/chemkin/). The comparison is performed with the biogas composition BG4 presented in Table1 at the equivalence ratio ϕ=0.7, initial temperature T=298K and p=1 bar. In addition, the GRI-Mech 3.0 chemistry mechanism is considered for describing the chemistry during the combustion of the gas and it contains 53 species and 325 chemical reactions.


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Table 1. Computational cases for biogas fuel variability study Biogas (Volume %) CH4 CO2 H2 N2 Case BG1 50 50 0 0 Case BG2 75 25 0 0 Case BG3 50 40 0 10 Case BG4 50 49 1 0 Case BG5 100 0 0 0

Fig. 1 shows a good agreement between Cantera and Premix for the biogas mixture BG4. Several fuel mixtures have been verified in order to make sure no discrepancies appear for different fuel compositions, and the two codes obtained the same solution files for all of them. Mixtures with and without hydrogen were particularly tested and compared to make sure Lewis number effects do not introduce any trouble for the solvers. A summary of the results is shown in Fig. 1 only for BG4, but all the other cases gave similar results and have been omitted here. In order to further verify the capabilities of Cantera, detailed mechanisms were compared with skeletal and reduced schemes, as this will be an important aspect in the short future. The sk30 mechanism, which is a skeleton mechanism of the GRI-Mech 3.0 and a reduced mechanism developed by Bibrzycki and Poinsot [6] are now compared in Fig 2 for a methane/air flame.

Fig. 1. Comparison between Cantera and PREMIX for a 1D planar premixed BG4/Air (O2 + 3.76 N2) flame at ϕ=0.7, T=298K and p=1bar. Fig. 2 presents a good agreement between sk30 and the GRI-Mech 3.0, while discrepancies are observed with the reduced mechanism. The CM2 [6] mechanism is composed of only two chemical reactions and these discrepancies were therefore expected. However, an additional test is made in order to verify that the discrepancies observed with this reduced mechanism are not related to errors in Cantera.


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Fig. 2. Comparison between the GRI-Mech 3.0 and the skeletal sk30 using Cantera for a 1D planar premixed CH4/Air (O2 + 3.76 N2) flame at ϕ=0.83, T=298K and p=1bar. In order to further verify the possibility to use reduced schemes and compare them with detailed ones, Fig. 3 shows the comparison of the CM2 in Cantera and COSILAB. It indicates Cantera has been successfully setup to run detailed, skeletal and reduced mechanisms.

Fig. 3. Comparison between Cantera and COSILAB for a 1D planar premixed CH4/Air (O2 + 3.76 N2) flame at ϕ=1.0, T=300K and p=1bar. The validation of Cantera for laminar premixed flame simulation with detailed, skeletal and reduced mechanisms have been successfully accomplished and this section is followed by a preliminary analysis of the impact of biogas composition on the chemical structure of the flames. These studies allow us to compare detailed mechanisms and evaluate their performance for modelling biomass flames. We have compared the San Diego mechanism with the GRI 3.0 in Fig. 4.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Y

x(cm)

Equivalence ratio φ=0.83

GRI30 YCOsk30 YCOGRI30 YCO2sk30 YCO2

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

0 0.2 0.4 0.6 0.8 1 1.2 1.4Te

mpe

ratu

re (K

)x(cm)

GRI30sk30


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Fig. 4. Comparison between San Diego and GRI 3.0 for a 1D planar premixed BG4/Air (O2 + 3.76 N2) flame at ϕ=0.7, T=298K and p=1bar. The creation of flamelet databases also relies on 1D flame configurations. Therefore, the generation of 1D flame in premixed conditions and counter-flow diffusion flames is very important. Fig 5 shows the results of a diffusion flame using methane diluted in nitrogen as fuel mixed with air using different chemical kinetic mechanisms. This type of analysis will allow to explore both premixed and diffusion flames and evaluate the thermochemical structure of the flames before running the CFD applications.

Fig. 5. Comparison between different schemes for a 1D counter-flow diffusion flame configuration.

1D planar premixed Biogas/Air flame with cantera: T=298K p=1bar φ=0.7

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.5 0 0.5 1 1.5

Velo

city

(m/s

)

x(cm)

BG4 0.7 cantera GRI30BG4 0.7 cantera SDN2

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

-1 -0.5 0 0.5 1 1.5

Den

sity

(kg/

m3 )

x(cm)


200

400

600

800

1000

1200

1400

1600

1800

-1 -0.5 0 0.5 1 1.5

Tem

pera

ture

(K)

x(cm)


-1e+08

0

1e+08

2e+08

3e+08

4e+08

5e+08

6e+08

7e+08

-1 -0.5 0 0.5 1 1.5

Hea

t rel

ease

(W/m

3 )

x(cm)


counterflow diffusion flame (0.49% CH4, 0.51% N2)/Air

200 400 600 800

1000 1200 1400 1600 1800 2000 2200

0 0.5 1 1.5 2

Tem

pera

ture

(K)

x(cm)

cantera GRI30cantera sk30cantera CM2

OPPDIFF

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 0.2 0.4 0.6 0.8 1

Y k

z

cantera GRI30 YCH4OPPDIFF YCH4cantera GRI30 YO2OPPDIFF YO2cantera GRI30 YN2OPPDIFF YN2

-0.4-0.3-0.2-0.1

0 0.1 0.2 0.3 0.4 0.5 0.6

0 0.2 0.4 0.6 0.8 1

u(m

/s)

z

cantera sk30cantera gri30

ALYAOPPDIFF

200 400 600 800

1000 1200 1400 1600 1800 2000 2200

0 0.2 0.4 0.6 0.8 1

Tem

pera

ture

(K)

z

cantera GRI30ALYA

OPPDIFF

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Dens

ity(k

g/m

3 )

z

cantera sk30ALYA

OPPDIFF


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4. Analysis of fuel variability The combustion behaviours of the fuel mixtures presented in Table 1 are compared to each other in a configuration identical to the one used for the validation test at the equivalence ratio 0.7 and 1.1. These equivalence ratios were chosen to study the effects of the chemical composition in both lean and rich sides. Fig 6 compared combustion of the fuel mixtures in terms of adiabatic temperature, with close results between BG1, BG3 and BG4 despite their different initial chemical compositions.

Fig. 6. Comparison the adiabatic temperature of the different biogas mixtures presented in Table 1. This observation is also made for the flame speed, as presented in Fig. 7, where no major differences were observed between BG1, BG3 and BG4.The biogas mixtures BG1 and BG4 are the closest to each other in terms of flame behaviour, due to the small difference in their initial chemical composition.

Fig. 7. Comparison of the flame speed for the biogas presented in Table 1


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The small dilution by H2 in BG4 did not lead to major variation compared to BG1 at different equivalent ratios, on the adiabatic temperature, flame speed and mass fraction of the species as shown also in Fig 8 and Fig 9.

Fig. 8. Mass fraction of CH4 at the composition described in Table 1.

Fig. 9. Mass fraction of CO2 at the composition described in Table 1. Besides, BG3 stayed close to the behaviour of BG1 and BG4, despite the higher dilution of N2. Cases BG2 and BG5 showed different combustion behaviours due to the elevated CH4 concentrations in these two cases. The observation may be quantitatively different in other configurations, such as a turbulent flow in a combustor or multi-dimensional laminar/turbulent flames. However, the qualitative trend is not expected to change. The next part is dedicated to the presentation of the methodology developed to generate the tabulated chemistry, which will be used for the simulations of the turbulent flames of biogas mixtures.


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5. Thermochemical database generation The method is based on the flamelet approach, considering that a multidimensional flame is a set of one-dimensional flames called flamelets. This method is only valid if the time scale of the flame is smaller than the turbulence, leading to a flame structure not perturbed by turbulent fluctuations. The method can be decomposed in four main steps:

• Determination of the flammability limits • Calculation of multiple solutions of premixed laminar flames at different

mixture fraction (or equivalence ratio) inside the flammability range obtained previously

• Definition of the reaction progress variable (RPV) varying from 0 to 1 and used to identify the equilibrium state (RPV=1 when full equilibrium is reached)

• The flamelets of premixed solution obtained are sent to an in-house algorithm providing the laminar tabulation

• Finally the turbulent tabulated chemistry is obtained by the integration of the laminar source terms using a beta-PDF to take into account the effects of turbulence on the reaction rates [7].

The tabulation describes the thermochemical structure of the flame and gives a quick access to the thermodynamic properties of a particular fuel instantly without solving again the Eqs. (1)-(3), for all the species. This tabulation is built only once and can be used for different simulation. The solutions obtained for each flamelets (or premixed laminar flame) are used to build a first tabulated chemistry defined in the physical space, i.e. in function of the mixture fraction and the physical space. Fig 10 shows for example the temperature in this physical space (z,x) for BG1, where the mixture fraction z takes all the value contained in the flammability limit defined at the first step. Furthermore, x represents the domain where the laminar premixed flame solution was solved.

Fig. 10. Temperature of BG1 (Table 1) in the physical space (mixture fraction z, position in the domain x)


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The tabulation is also defined in the space made of the mixture fraction and the RPV (Reaction Progress Variable) and called (z, RPV). This representation is the one that will be used for the numerical simulation and submitted to the dedicated filter in order to create the turbulent tabulation. This representation of the tabulation in the (z, RPV) provides directly information about a thermochemical property at a point i designating a mixture fraction zi and the reaction progress RPVi. Fig 11 shows the representation of the temperature shown in Fig 10 in the (z, RPV) space.

Fig. 11. Temperature of BG1 (Table 1) in the (z, RPV) space The quality of this tabulated chemistry is dependent on the choice of the parameter used to define the reaction progress variable (RPV). In fact, an improper choice could lead to numerical convergence problem due to the variation stiffness of certain thermochemical parameter.

6. Ongoing work The ongoing work is now focused on developing RANS and LES simulations of the reference fuel in order to validate the integration of the thermochemical database with the CFD solver Alya (D5.2) and validate the results using a well-established jet flame configuration [8]. Sample results of the RANS simulations currently obtained with the Alya using the CFI Model [6] are shown in Fig. 12 for completeness.


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Fig. 12. Mixture fraction (left) and temperature for DLR Jet A flame from TNF Archive [8] The results are currently compared to the experimental data from Meier et al. [8] in RANS, while the LES simulations are being prepared.

7. Conclusions The open-source package Cantera was validated against commercial packages and used to study in details the flame structure for different fuels using a 1D laminar flames in different configurations. The package has shown is capability to use different chemistry mechanism, from detailed to reduced schemes as well as different configurations, freely propagating laminar flames and opposite diffusion flames. Therefore, the tool is now ready to test and evaluate reduced and detailed mechanisms obtained from future analyses.

The initial simulations performed with Cantera has shown a close results for cases BG1, BG3 and BG4, which indicates that CH4 concentration plays a major part in the combustion behaviours. This first analysis was followed by the development of flamelet generated manifold for turbulent combustion of biogas mixtures.

Turbulent databases were generated first for all the biogas mixtures presented in Table 1 and also for a reference fuel, which will be used to validate the CFD simulations with reference experimental data. This reference fuel [8] (22.1% CH4 + 33.2% H2+44.7% N2) will be used to validate our approach and to gather more data about the biogas using an identical configuration, but with different fuel composition. The thermochemical structure of the reference fuel has been determined first through the generation of the laminar and turbulent thermochemistry database.

Numerical simulations (RANS and LES) will be performed with Alya and compared to experimental results from the Sandia TNF Archive database for a full validation (towards D5.2). This study will be followed by the simulation of the different biogas and their comparison to the reference fuel. Besides, an analysis will be performed on the combustion performances and thermochemical structure of the biogas (Table 1) based on the laminar tabulation obtained for the different fuel mixtures.


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References

[1] Fischer M, Jiang X. An investigation of the chemical kinetics of biogas combustion. Fuel 2015;150:711-20. [2] Barnwal A. Combustion properties of biologically sourced alternative fuels. Toronto: University of Toronto; 2012. [3] Lam SH, Goussis DA. Conventional asymptotics and computational singular perturbation for simplified kinetics modelling. In: Smooke MD. Reduced kinetic mechanisms and asymptotic approximations for methane-air flames, Berlin: Springer Verlag; 1991, p. 227–242. [4] Maas U, Pope SB. Simplifying chemical kinetics: Intrinsic low-dimensional manifolds in composition space. Combust Flame 1992;88:239–64. [5] Oijen J. Flamelet-generated manifolds: development and application to premixed laminar flames. Eindhoven: Technical University of Eindhoven; 2002. [6] Bibrzycki J, Poinsot T. Reduced chemical kinetic mechanisms for methane combustion in O2/N2 and O2/CO2 atmosphere. Working note ECCOMET WN/CFD/10/17, CERFACS; 2010. [7] Gövert, S., Mira, D., Kok, J.B.W., Vázquez, M., and Houzeaux, G., 2015. “Turbulent combustion modelling of a confined premixed jet flame including heat loss effects using tabulated chemistry”. App. Energ., 156, pp. 804 – 815. [8] Meier W, Barlow RS, Chen Y-L, Chen J-Y. Raman/Rayleigh/LIF Measurements in a Turbulent CH4/H2/N2 Jet Diffusion Flame: Experimental Techniques and Turbulence–Chemistry Interaction. Combust. Flame 2000;123:326–343.

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