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Journal ArticleDOI

A 5-step reduced mechanism for combustion of CO/H2/H2O/CH4/CO2 mixtures with low hydrogen/methane and high H2O content

01 Jan 2013-Combustion and Flame (Elsevier)-Vol. 160, Iss: 1, pp 56-75

AbstractZMN and NS acknowledges the funding through the Low Carbon Energy University Alliance Programme supported by Tsinghua University, China. ZMN also likes to acknowledge the educational grant through the A.G. Leventis Foundation.

Summary (3 min read)

1. Introduction

  • Recent developments in gas-turbine power generation include the use of low calorific value fuels.
  • Generally, these mechanisms were developed systematically by introducing steady-state and/or partial equilibrium assumptions respectively for some species and reactions involved in a skeletal mechanism.
  • The hydrogen content is low in the BFG as noted earlier and one may like to mix it with small amounts of H2, CH4 and H2O or other gases containing high fractions of these species in order to enhance the BFG combustion characteristics.
  • To the best of their knowledge this is the first attempt to obtain a reduced mechanism for a multi-component fuel mixture with a good accuracy over a wide range of thermodynamic and thermo-chemical conditions.

2. Development of skeletal mechanism: Sensitivity analysis

  • The chemical kinetics of CO/H2 mixture oxidation has been investigated by numerous studies in the past and a sustained interest on the combustion of Syngas in gas turbines for power generation has led to publication of a dedicated volume on this topic in the Combustion Science and Technology journal in 2008.
  • Flame speed sensitivity analyses are conducted using the GRI [19], at high (20%) and zero water vapour content in the fuel mixture in order to (1) identify the most important reactions in each case and (2) to obtain a suitable skeletal mechanism for CO/H2/H2O mixtures.
  • CO2 has large sensitivities for both dry 9 and wet mixtures and OH + CO = H+ CO2 remains as the most important reaction with sensitivity nearly five times larger than for the HO2 reaction for CO consumption.
  • Thus, the effect of small CH4 amounts in the fuel mixture is adequately captured by the extra 9 reactions noted above, something which was neglected while developing reduced mechanism in a previous study [14].

3. Development of reduced chemistry

  • By removing certain intermediate species from the detailed mechanism, the computational effort is reduced as the number of ODEs that must be solved is decreased.
  • Intermediate species can be systematically identified and removed from the ODE system via two major sequential steps.
  • Second, further reduction of the skeletal mechanism results in a reduced mechanism.
  • For fast development of reduced chemistry, the interactive Computer Assisted Reduction Mechanism (CARM) algorithm [40, 43] was used for the automatic generation of reduced chemistry with the ability to produce source codes needed for computing the chemical sources.

4. Reduced mechanism

  • The same would apply in cases where Ar is the inert.
  • Also, for fine tuning of the reduced chemistry, the activation energy of reaction 2 in Table 3 was increased by 27.5%, a procedure similar to the correction factor employed by Boivin et al. [14] to correctly predict the ignition delay times.
  • The steady-state relationships can be written as dCA dt = ψA(ss, ss )− gA(ss, ss)CA = 0, where ψA(ss, ss) and gA(ss, ss) are functions of species both in steady-state, denoted by ss, and non steady-state, denoted by ss.
  • Since the current QSS species are not strongly coupled, the point iteration scheme is found to be sufficient for the present case.

5. Validation

  • Both the skeletal and reduced mechanisms are validated over wide range of conditions shown in Table 4, by comparing laminar flame speeds, ignition delay times and the flame structure with experimental results and/or the computational results obtained using the GRI Mech 3.0 [19].
  • The flame speeds are calculated using the PREMIX [45] code of the CHEMKIN package [46] including the thermal diffusion and multi-component formulation for the species’ diffusivities.
  • In the cases where no experimental data are available, the skeletal and the reduced mechanisms are validated against the predictions of the GRI Mech 3.0 [19] and so readers are cautioned while interpreting this particular comparison.
  • In calculating the ignition delay times with the reduced mechanism, the correction factor used in the study of Boivin et 18 al. [14] is employed.
  • This correction factor was originally developed in [47] from an analysis of the autoignition eigenvalue under lean conditions.

5.1. Premixed flames

  • Comparisons of computed flame speeds, sL, against available experimental data for the mixtures listed in Table 4 are presented in Figs. 1-10.
  • The above comparisons show that overall both the skeletal and the reduced mechanism give good agreement with the experimental data and the computations with the GRI Mech 3.0 [19].
  • The skeletal mechanism of [14] as implemented in this study, under-predicts the flame speeds for all equivalence ratios and the level of under-prediction increases with the H2O content in the fuel mixture.
  • Again there is a good agreement with the full GRI Mech 3.0 [19] and it is somewhat improved in the high pressure case, compared to the predictions of the methane-containing fuel mixture in Fig. 11.

5.2. Autoignition

  • Figure 21 compares the computed ignition delay times (with the correction factor in Eq. 3 applied) with the experimental results of Kalitan et al. [55] for CO/H2 mixtures over a range of conditions listed in Table 4.
  • Overall, the agreement is very good for both low and high pressures and for the entire range of temperatures considered.
  • Figure 22 compares ignition delay times computed for a CO2-diluted mixture to the measured values in [28] at different pressures.
  • The reduced mechanism shows good agreement with the experimental data for the entire temperature range.
  • As noted in [28] using sensitivity analysis, the most important reactions at the conditions tested were the chain-branching reactions and the three body recombination reaction H + O2 + CO2 = HO2 + CO2.

6. Speed up times

  • Table 5 shows the time in seconds taken for each run for each of the conditions shown in Table 4.
  • The flame speeds were calculated using the PREMIX code [45] with thermal diffusion and a multi-component formulation for the species’ diffusivities, in a 2.5 cm domain with adaptive grid.
  • It is clear that both the skeletal and reduced mechanisms reduced the computational time significantly compared to the GRI Mech 3.0 [19], while maintaining the same level of accuracy.
  • In particular for case 3 the skeletal mechanism is about 50 times faster and the reduced mechanism about 300 times faster.

7. Conclusions

  • A 5-step reduced chemical kinetic mechanism involving 9 species for accurate prediction of the combustion characteristics of multi-species fuel mixtures of CO/H2/H2O/CH4/CO2, having low hydrogen/methane and high water vapour content is derived.
  • These two mechanisms are tested for their ability to predict laminar flame speeds, flame structure and ignition delay times over a wide range of pressure, temperature and fuel mixture composition.
  • It is also worth to note that these conditions are relevant for stationary gas-turbines for power generation.
  • Furthermore, it is found that use of the reduced mechanism decreases the computational time significantly compared to the GRI Mech 3.0, while maintaining a a very good degree of accuracy.
  • ZMN also likes to acknowledge the educational grant through the A.G. Leventis Foundation.

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A5-stepreducedmechanismforcombustionof
CO/H
2
/H
2
O/CH
4
/CO
2
mixtures with low
hydrogen/methane and high H
2
Ocontent
Z. M. Nicolaou
a
, J. Y. Chen
b
, N. Swaminathan
a
a
Cambridge University, Department of Engineering, Trumpington Street, Cambridge
CB2 1PZ, UK
b
University of California at Berkeley, Department of Mechanical Engineering, 6163
Etcheverry Hall, Mailstop 1740, USA
Abstract
In this study a 5-step reduced chemical kinetic mechanism involving 9 species
is developed for combustion of Blast Furnace Gas (BFG), a multi-component
fuel containing CO/H
2
/CH
4
/CO
2
, typically with low hydrogen, methane and
high water fractions, for conditions relevant for stationary gas-turbine com-
bustion. This reduced mechanism is obtained from a 49-reacti on skeletal
mechanism wh i ch is a modified subse t of GRI Mech 3.0. Th ese skeletal and
reduced mechanisms are validated for laminar flame speeds, ignition delay
times and flame structure with available experimental data, and using com-
putational results with a comprehensive set of elementary reactions. Overall,
both the skeletal and reduced mechanisms show a very good agreement over
a wide range of pressure, reactant temperature and fuel mixture composition.
Email addresses: zn209@cam.ac.uk (Z. M. Nicolaou ) , jychen@me.berkeley.edu
(J. Y. Chen), ns341@cam.ac.uk (N. Swaminathan)
Preprint submitted to Combustion and Flame September 13, 2012
Published in Combust. Flame, 2013, 160(1), p. 56-75
DOI: 10.1016/j.combustflame.2012.09.010

1. Introduction
Recent developments in gas-turbine power genera t i on include the use of
low calorific value fuels. These fuels may be Synthetic Gas which is commonly
known as Syngas, Coke Oven Gas (COG) and Blast Furnace Gas (BFG)
or suitable combinations of these gases [1]. The constituents of and their
relative proportions in these gases vary considerably. The Syngas obtained
by coal gasification is mostly composed o f hydrogen an d carbon monoxide
with varying levels of carbon dioxide, water and other trace species [2, 3].
The rel at ive proportions of the predominant gases vary wi d ely depending on
the gasification pro cess and the ratio of hydrogen to carbon monoxide mole
fractions, f
H
2
= X
H
2
/X
CO
,istypicallylargerthan0.1anditcanbeashigh
as 3 [1, 2, 4, 5, 6, 7, 8]. The industrial COG includes considerable amount
of CH
4
in addition to these species with f
H
2
as high as 11 and f
CH
4
5
[9], whereas BFG has f
H
2
,CH
4
ranging from 0 to 0.15 [1, 5, 9]. In terms of
calorific values, BFG has the lowest value of about 2.95 MJ/m
3
Ncompared
to 40 MJ/m
3
Nforthestandardnaturalgasusedingasturbines[1].
Robust and accurate models for combustion chemistry and its i nteraction
with tu r bu l en ce are required for the design and development of gas turbines
intend to operate with the above fuels. The combustion chemistry is of par-
ticular interest to this study and the turbulence-chemistry interaction will be
addressed in future. The wide variation in fuel mixture composition noted
above oers a consi der ab l e challenge to construct a reliable, robust and com-
putationally ecient chemical kinetic scheme. The computationa l eciency
is specifically of high importance from the view point of turbulent combus-
tion calculation. Reduced mechanisms oer a convenient way to achieve this
2

objective by reducing number of species involved in the combustion kinetics
and yet maintaining an acceptable level of accuracy for important attributes
such as laminar burning veloci ty, flame structure, ignition delays, extinction
limits etc. There have been many developments of such reduced mecha-
nisms for the most commonly used single component fuels [10, 11, 12, 13].
Generally, these mechanisms were developed sy st emat i cal ly by introducing
steady-state a n d/o r partial equilibrium assumptions respectively for some
species and reactions invol ved in a skeletal mechanism. Sensitivity analyses
were typically used to obtain a skeletal mechanism from a full comprehensive
set of elementary reactio n s. As noted earlier, these strategies have been used
in many past studies to obtain reduced kinetic mechanisms for single compo-
nent fuels and there has not been an attemp t to obtain a reduced mechanism
for a multi-species fuel mixture such as the BFG, to the best of the authors’
knowledge. Thus, this study makes an attempt in that regard.
The range of validi ty of a reduced mechanism strongly depends on the
fuel composition and operating con d i t i o n used to d evelop it. The hydrogen
content is low in the BFG as noted ear li er and one may like to mix it with
small amounts of H
2
,CH
4
and H
2
Oorothergasescontaininghighfractions
of these species in order to enhance the BFG combustion characteristics.
The need of a reduced mechanism for such multi-species fuels then becomes
imperative. Most of the attempt s in the past to get redu c ed mechanisms
for a multi-species fuel mixture were for syngas and were validated only for
relatively high f
H
2
values and very low water vapour content [14, 15]. More
importantly, the eect of CH
4
was not considered since it was generally taken
that the CH
4
content in such fuels was too low to aect the combustion
3

characteristics which might not be entirely correct. F or example, it is later
shown in this work that small amounts of CH
4
in a CO/CH
4
/H
2
O-air mixture
directly aect the flame speed response to water content in the fuel mixture.
In this stud y, an accurate reduced ki n et ic mechanism is developed fr om
a49-reactionskeletalmechanismwhichisshowntobesuitableformulti-
component fuel mixtures containing CO, H
2
,H
2
O, CO
2
and CH
4
,withlow
f
H
2
and f
CH
4
typical for the BFG mixture. This reduced mechanism is
then validated for laminar flame sp eed and its structure, and ignition de-
lay times for pressure and temperature conditions relevant to ground-based
heavy weight gas-turbines with typical overall pressure ratios of ab out 20 or
small [3, 5, 16] and combustor inlet temperature not exceeding 1000 K [3, 16].
The redu ced mechanism is also assessed for its suitability for high H
2
Ocon-
tent in the fuel mixture. To the best of our knowledge this is the first attempt
to obtain a reduced mechanism for a multi-component fuel mixture with a
good accuracy over a wide range of thermodynamic and thermo-chemical
conditions.
The rest of the paper is organi sed as follows. The skeletal mechanism re-
quired for the development of the reduced chemistry is discussed in section 2.
The techniques used to develop the reduced mechanism is discussed in sec-
tion 3 and the reduced mechanism is presented in section 4. The validation
results are discussed section 5, its computational advantage is demonstrated
in section 6 and the conclusions are summarised in the final section.
4

2. Development of skeletal mechanism: Sensitivity analysis
The chemical kinetics of CO/H
2
mixture oxidation has been investigated
by numerous studies in th e p as t a n d a s u st ai n ed interest on the combustion
of Syngas in gas turbines for power generation has led to publication of a
dedicated volume on this topic in the Combustion Science and Technology
journal in 2008. The reviews by Chaos and Dryer [3] and Sung and Law [4]
clearly identified the important reactions for CO oxidation are CO + OH =
CO
2
+ H and CO + HO
2
=CO
2
+ OH, wi t h the seco n d reactions becoming
important at e l evated pressures. Comprehensive kinetic mechanisms for dry
and moist C O oxidation have been proposed in the p a st [17, 18] and has been
updated in a number of later studies as has been noted by Sung and Law [4].
The interested readers are referred to [4] for furth er det ai l .
Out of these many ava i l a b l e comprehensive m echanisms, a set of 22-
reactions suggested in [15] as a guideline along with the GRI Mech 3.0 is
used to obtain a skeletal mechanism in this study. This choice is mainly for
the following two reasons. (i) The stiness of the reduced mechanism, sig-
nified by the non-linear coupled equat i o n s for stead y -st a te species, strong l y
depends on the skeletal mechanism used. Wang and Rogg [15] produced
anon-stiandworkingmechanismformoistCOusingtheir22reactions.
(ii) The interest i n this study also i n cl u d es the eects of CH
4
on moist CO
and thus the GRI Mech 3.0 [19] is used, since this mechanism is widely val-
idated using experimental data for methane [19], H
2
O-diluted and oxygen
enriched methane [20], moist H
2
/CO mixtures at elevated temperature [21]
and 323 K [22]. This mechanism was also observed to give reasonable results
for flame speeds and ignition delay times for multi-species fuel mixtures over
5

Figures (28)
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Frequently Asked Questions (2)
Q1. What are the contributions in "A 5-step reduced mechanism for combustion of co/h2/h2o/ch4/co2 mixtures with low hydrogen/methane and high h2o content" ?

In this study a 5-step reduced chemical kinetic mechanism involving 9 species is developed for combustion of Blast Furnace Gas ( BFG ), a multi-component fuel containing CO/H2/CH4/CO2, typically with low hydrogen, methane and high water fractions, for conditions relevant for stationary gas-turbine combustion. 

The computational results are compared to experimental measurements of the flame speeds available in the literature for a wide range of pressure, 1-20 atm., temperature, 298- 700 K and thermo-chemical conditions. The authors thank the reviewers for suggesting many validation data which helped to show the robustness of the mechanisms over wide range of conditions for flame speeds and autoignition delay times.