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ENG60 LNG II
ENV60 – LNG II
Metrological support for LNG custody
transfer and transport fuel applications
Report for the validation of the calculation of the
Methane Number and correlation to the LNG
composition
Task 3.4, deliverable 3.4.3: Report on the validation of a calculation for the MN from
the LNG composition and the correlation of the MN to the LNG composition (VSL, NPL,
PTB, TUBS (REG))
ENG60 LNG II
TABLE OF CONTENTS 1. Introduction ......................................................................................... 3
2. Literature ............................................................................................ 4
2.1 Definition Methane Number ................................................................. 4
2.2 Algorithms ........................................................................................ 4
AVL ..................................................................................................... 4
MWM ................................................................................................... 5
EON-GasCalculation .............................................................................. 5
CARB .................................................................................................. 5
Other .................................................................................................. 6
2.3 Drawbacks ........................................................................................ 6
3. Algorithm............................................................................................. 7
4. Results ................................................................................................ 9
4.1 RCM measurements at PTB ................................................................. 9
Investigation of Reference Mixtures ........................................................ 9
Investigation of LNG-Mixtures .............................................................. 11
4.2 Gas mixture analysis at NPL and VSL .................................................. 18
4.3 Results of the Engine tests from TUBS ................................................ 19
5. Validation .......................................................................................... 23
Algorithm ............................................................................................. 23
Results ................................................................................................. 23
6. Conclusion ......................................................................................... 24
Literature ................................................................................................ 25
ENG60 LNG II
1. INTRODUCTION The methane number (MN) of a fuel gas is a parameter that describes its
properties with respect to the knocking tendency when fired in combustion
engines. Originally, the MN is defined as the volume fraction of methane in a
mixture of methane and hydrogen, expressed as a percentage. A fuel gas with a
certain MN shows the same knocking behaviour as a methane/hydrogen mixture
with a volume fraction of methane equal to the MN. The MN depends, among
others, on the test engine and its operating conditions. The defined test engine
and operating conditions originate from a publication by the Anstalt für
Verbrennungsmotoren Prof. List (AVL) in 1971.1
The paper by AVL also describes a correlation between the MN and the natural
gas composition, including a method for calculating the MN from the gas
composition. This calculation method is commonly referred to as the AVL
algorithm. Since the AVL publication, a number of improvements and
modifications to the algorithm have been proposed, as well as completely
different calculations methods. All methods however have their own
disadvantages and limitations. The calculation of the MN of a fuel gas from the
composition according to these methods can lead to largely different results.2,3
These differences depend also on the composition of the fuel gas. Therefore, a
standardized calculation method for the MN is needed to qualify gas fuels with
respect to the knocking behaviour of the fuel in combustion engines.
Modern engines used, in the automotive industry among others, have a different
combustion behaviour compared to the standard test engine used in the AVL
study which further limits its applicability. It should be noted too that the current
methods for calculating the MN were developed for the high natural gas qualities
seen in pipelines with little impurities of higher (>C4) hydrocarbons. Newer types
of natural gas, including a number of rich LNG qualities which comprise pentanes
and even higher hydrocarbons, may require corrections or modifications to the
models for calculating the MN. Also, when natural gas with higher amount
fractions of CO2, such as in biogas, will be introduced, a modified algorithm
might be required.4
Within work package 3 of the EMRP project ENG60 LNG II an improved
algorithm, based on the original AVL study, which may form the basis of a
ENG60 LNG II
standardized method for the calculation of the MN from the composition of
natural gas, including LNG has been developed. This new algorithm is
accompanied by the calculation of the uncertainty in the MN, based on the input
parameters. This report provides an overview of the literature on the existing
algorithms, a brief description of the new algorithm used, the results from the
tests performed with this algorithm, the validation of the tests and some
concluding remarks on the improved MN.
2. LITERATURE
2.1 DEFINITION METHANE NUMBER
The methane number (MN) of a fuel gas is defined as the volume fraction
methane in hydrogen, having the same knocking tendency as the fuel gas
concerned in a defined test engine under defined operating conditions. The test
engine and the defined operating conditions were developed by the Anstalt für
Verbrennungsmotoren Prof. List (AVL) in the period of 1964‐1969 and published
in 1971.1
A key issue with the methane number is that it is not a thermodynamic property
of the gas, so it cannot be represented by an equation‐of‐state or calculated from
it.5 It is not possible to sufficiently characterize the knocking behaviour of a fuel
in various engines and under different operating conditions by a single index.
2.2 ALGORITHMS
AVL
In the early 1970’s, results from research work on the knock‐rating of gaseous
fuels was published by AVL. Since then, the algorithm developed in this study
has been the de facto standard method to calculate the MN. It is based on a
detailed data set summarized in triangular‐shaped ternary mixture diagrams
(Error! Reference source not found.). In order to make use of the ternary
diagrams the initial gas mixture has to be divided into sub‐mixtures. Before
dividing it, however, a reduced mixture is calculated. It is important to note that
the composition of the initial gas mixture has to be specified in percent volume.
For all existing pipeline quality natural gases the AVL method works reliably and
ENG60 LNG II
sufficiently accurate if nitrogen is excluded from the gas composition.
Figure 1 :Exemplary ternary diagram for a reduced methane/propane/butane mixture
MWM
This method is described in prEN16726:2014 and is based on the original work of
the AVL algorithm. But there are a few key differences: specific weighing factors
for the pentanes and higher are used, the sub‐mixtures are chosen based on
specific selection rules, the MN of the sub‐mixtures is not based on the triangular
shaped diagrams but on a polynomial equation and a correction for the
nitrogen‐carbon dioxide content is always applied. As opposed to the AVL method
where it is only applied when the volume fraction of carbon dioxide and nitrogen
exceeds 2% and 9%, respectively.
EON-GASCALCULATION
The calculation tool called EON‐GasCalculation produces end results similar to the
AVL programme, but with the advantage that corrections for the nitrogen content
can be easily made.
CARB
The Gas Research Institute (GRI) has applied the ASTM octane rating method to
measure the Motor Octane Number (MON).6 Two mathematical relations were
developed to calculate the MON: a linear coefficient relation and a
hydrogen/carbon ratio relation. Both MON are converted to a MN, as described in
ISO technical report ISO/TR 22302.7
The same methodology is also described in ISO 15403‐1: 20068 and it forms the
basis of the calculations done by the Californian Air Resources Board (CARB).
ENG60 LNG II
Although, in the CARB method the MON is calculated with the same formula
using the H/C ratio, but the equation for conversion to MN is different.
OTHER
An additional number of calculation methods can be found in the internet, such
as an online MN calculator from Cummins or from DNV GL.9,10 However, no
information about these methods of calculation is freely available.
2.3 DRAWBACKS
Methane numbers for the same composition can differ significantly, depending on
the calculation method used.2,11 It is therefore essential to mention the used
method.
For Liquefied Natural Gas (LNG), in general, the AVL method gives unrealistic
high values in case of relatively high nitrogen fractions, while this component
does not add that much to the knocking resistance of a fuel gas. On the other
hand, hydrocarbons heavier than the butanes are not taken into account,
although these components have a very strong negative effect on the methane
number.11
The two calculation methods used by the GRI (linear and H/C)7 show consistency
for MNs between 85 and 100, but below this range differences up to 35 points in
MN can occur. In particular, differences occur in case of high fractions of
nitrogen, carbon dioxide, or higher hydrocarbons.
In LNG examples without high fractions of N2, CO2 and high CxHy comparable
results were obtained for all methods, except the CARB method. The CARB
method will lead to a higher MN for any MN>30.
From a metrological point of view, a serious drawback of the existing algorithms
is that none of them provides information about the uncertainties in their results.
ENG60 LNG II
3. ALGORITHM The new algorithm to calculate the methane number from the composition of a
LNG mixture is based on the experimental data gathered by AVL and the
associated evaluation method. The algorithm is programmed in a user friendly
MS Excel spreadsheet.
It uses the ternary diagrams that summarize the results of the engine tests as an
extensive data set for modeling the correlation between the composition of a
natural gas mixture and its resulting methane number. The developed algorithm
follows for the most part the steps described in the AVL report. One major
difference is that the process of deriving the methane numbers for the two
sub‐mixtures by reading the respective ternary diagram is replaced by an
automated procedure. This procedure mimics the way the aforementioned
ternary diagrams are likely to be used by a reader.
In order to increase the repeatability of the calculation of the MN, a routine is
used to minimize the difference in MN for the sub‐mixtures ΔMN by varying the
methane concentration of both mixtures. This approach is very similar to the
procedure used for the MWM method. The reason for the modification is, that in
the original AVL method the composition of the sub‐mixtures only has to be
changed if ΔMN ≥ 5 depicting an insufficient condition in terms of repeatability.
Being published in 1971, the AVL study lacks the availability of digitized data of
any kind. Therefore the relevant ternary diagrams have been digitized using the
freely available Engauge software12 and the iso‐MN curves have been modeled
using polynomial fit functions up to the second order (third order fits may be
implemented in a future iteration of the algorithm). The ternary diagrams
required for the algorithm are two diagrams for the simplified mixtures
(methane/ethane/butane and methane/propane/butane, respectively) plus the
diagram used for the inert gas correction (methane/nitrogen/carbon dioxide).13 A
copy of the respective diagrams can be found in the appendix. The working
principle of the algorithm can be summarized as follows:
• The LNG mixture is reduced following the AVL method and normalized to
100 % volume.1,13
• The reduced mixture is divided into two sub‐mixtures
(methane/ethane/butane and methane/propane/butane) which are then
normalized to 100 % volume.
• The intersection of the two lines representing the methane and ethane (or
propane) concentration of the respective sub‐mixture is calculated giving
the pair of values XMN /YMN
• The intersection points of the ethane (or propane) line with the iso‐MN
curves is calculated
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• The nearest neighbors with respect to XMN are determined.
• The MN corresponding to the intersection point XMN/YMN is calculated by
linear interpolation between the MNs of the neighboring iso‐MN curves
yielding the MNi value of the respective sub‐mixture.
• The difference in methane number ΔMN between both sub‐mixtures is
determined using a numerical procedure.
• The resulting methane number of the LNG mixture MN’ is calculated from
the two MNi of the sub‐mixtures according to the AVL method (Equation 1)
• MN’’ is determined using the methane/carbon dioxide/nitrogen ternary
diagram assuming a methane/nitrogen binary sub‐mixture (methane
concentration is chosen so that both components add up to 100 %) by
linearly interpolating between MNs of neighboring iso‐MN curves.
• The final methane number is calculated according to Equation 2
Equation 1: 𝑀𝑁′ =1
100∙ ∑ 𝑎𝑖 ∙ 𝑀𝑁𝑖
𝑛𝑖=1
Equation 2: 𝑀𝑁 = 𝑀𝑁′ + (𝑀𝑁 − 100)
For a more detailed description of the algorithm used, please read Description of
Algorithm for Calculating Methane Number from LNG composition WP3, D3.1.2.14
ENG60 LNG II
4. RESULTS The mixtures used in the experiments can be found in Table 1
Table 1 : Gas compositions used in experiments expressed as amount- of- substance fractions (cmol/mol) and their expected MNs
Component CH4 C2H6 C3H8 n-C4H10 i-C4H10 n-C5H12 i-C5H12 n-C6H14 N2 CO2 H2 MN
Mix 1 78.80 14.0 3.40 0.90 1.10 0.15 0.15 1.5 59
Mix 2 (Emirates) 84.52 12.9 1.50 0.21 0.22 0.03 0.02 0.6 69
Mix 3 (Norway) 91.80 5.70 1.30 0.15 0.17 0.04 0.04 0.8 77
Mix 4 (Libya) 81.69 13.38 3.67 0.27 0.28 0.01 0.01 0.69 65
Mix 5 (Oman) 87.89 7.27 2.92 0.71 0.65 0.10 0.11 0.35 66
Mix 6 (AVL 6) 95.01 2.62 0.73 0.20 0.15 0.06 0.09 0.22 0.54 0.38 82
Mix 7 97.876 1.00 0.50 0.21 0.18 0.016 0.018 0.2 90
Mix 8 (Alaska) 99.68 0.09 0.03 0.01 0.01 0.005 0.005 0.17 98
calibration MN50 50 50 50
calibration MN60 60 40 60
calibration MN70 70 30 70
calibration MN80 80 20 80
calibration MN90 90 10 90
calibration MN100 100 100
Pentane test 1 99.4 0.3 0.3 84
Pentane test 2 99.6 0.2 0.2 89
Pentane test 3 99.8 0.1 0.1 94
4.1 RCM MEASUREMENTS AT PTB
INVESTIGATION OF REFERENCE MIXTURES
Ignition delay times of different reference mixtures in the range of MN60 to
MN100 were measured at various temperatures and pressures (20 and 40 bar) at
a stoichiometry of 𝜙=0.4. Therefore, the reference mixtures were premixed with
oxygen and diluent gases (nitrogen/argon). An overview about the obtained
results is given in Figure 2. In the latter, the data points are fitted linear to
highlight the given trend in the ignition delay times. Please note, that the linear
fit doesn’t reflect the real situation in the RCM because it doesn’t consider the
heat loss over the walls. It is obvious that with increasing temperature the
ignition delay time becomes shorter for each reference mixture. However, a clear
trend in the behavior of the ignition delay times between the different MN
mixtures is not observable.
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Figure 2 : Measured ignition delay times (IDT) for the reference mixtures at 20 bar (left) and 40 bar (right). The data points are fitted with a linear fit routine not considering the experimental heat loss.
Since a clear scientific picture could not be drawn from the obtained data, exemplarily kinetic simulations for the 40 bar case were performed. Modeling was carried out using the Flame Master software15 with a reduced version of the
n-heptane kinetic mechanism from Lawrence Livermore National Laboratory (LLNL)16, including 1083 reactions and 207 chemical species. The results are
shown in Fig. 4 for the fuel-lean case with an equivalence ratio of 𝜙=0.4 (left)
and for the fuel-rich case of 𝜙=1.2 (right). In the simulations heat loss, like it
occurs in the Rapid Compression Machine (RCM) experiments, was not considered, but this fact should not change the general features that can be seen
from the simulated results and that can partially explain the observed
experimental behaviour.
Figure 3 Chemical kinetic simulations of the reference mixtures’ ignition delay times at
40 bar with fuel-lean (𝜙=0.4) [left] and fuel-rich (𝜙=1.2) [right] conditions. The blue
rectangle indicates the optimal working range of the Rapid Compression Machine (RCM).The light patterned region shows the range of the experimental obtained values.
ENG60 LNG II
Figure 4 shows the simulated IDTs for the different reference mixtures in a wide temperature range for the fuel-lean case (left panel of Fig. 4)) and the fuel-rich case (right panel). The blue rectangles indicate the optimal working range of the
Rapid Compression Machine whereas the light patterned region describes the range of the results obtained in the experiments. Outside of the blue box, i.e.
outside of the optimal working range of the RCM, uncertainties of the experimental values become larger than ±15%. It should be noted that only the temperature is the controllable experimental variable (x-axis) and that the
Ignition delay time is the obtained result (y-axis). From Fig. 4 it becomes clear that in the optimal working range of the RCM the reference mixtures show similar
ignition delay times, which is consistent with the experimental observations shown in Fig. 3. As already mentioned above, heat loss was not considered in the
simulations, shifting the modelled results to lower temperatures in comparison to the experimental range (which strengthen our argumentation that IDTs are very similar for the different reference mixtures). It should be pointed out, that
ignition of MN0 and MN100 (and within the other reference mixtures) belongs to aninterplay of different chemical reactions depending on the amount of hydrogen
and methane in the mixture. However, a clear statement would require a detailed investigation by kinetic modelling including sensitivity and rate of production analyses.
INVESTIGATION OF LNG-MIXTURES
To study the knocking propensity of different LNG mixtures, a set of different mixtures, defined in Table 1 was investigated. The LNG mixtures were premixed
with oxygen and constant amount of diluent gases (nitrogen/argon) allowing
measurements in a similar temperature range at an equivalence ratio of
𝜙=0.4/𝝀=2.5. The measurements were extended to an equivalence ratio
𝜙=1.2/𝝀=0.83 covering also a fuel-rich situation and thus a deeper insight in the
ignition behaviour of LNG mixtures. Measurements were performed each at a minimum of 3 different temperatures within a temperature range from ~870-
1100 K. Ignition delay times were determined for two different pressures (20 bar and 40 bar). Each test series (LNG-Mix) was repeated three times to minimize
the uncertainty of the experimental data and averaged to extract the value for the ignition delay time. Figure 4 gives an overview of the measured ignition delay times in comparison with the reference mixtures. Due to the fact, that the
diluents contain always the same ratio of nitrogen and argon (and the mixture’s heat capacity belongs then only on its composition), some mixtures showed
ignition delay also at temperatures outside of the temperature ranges shown in Fig. 5. These measurements were not included in the figure to allow for a proper
comparison between the different mixtures. A full dataset including all averaged measured IDTs can be found in Table 2.
ENG60 LNG II
Figure 4 Determined ignition delay times for the LNG-mixtures at two different pressures and with two different stoichiometries. The bars’ colour code indicates similar temperatures
As already shown for the reference mixtures, the expected trend that with lower
temperature the ignition delay times of each mixture increases is observable.
However, a clear correlation between the mixtures’ determined methane number
(see Table 1) and the measured ignition delay times cannot be found. This is
consistent with the findings of the ignition behaviour of the reference mixtures as
discussed above. Nevertheless, it can be observed that (in the fuel-lean case)
the ignition delay times of the LNG-mixtures are shorter than for the reference
mixtures for temperatures between 964K and 1009 K (20 bar case) and 921 K
and 979 K (40 bar case), respectively. This behaviour, which is even more
pronounced at higher pressures, indicates strong influence of the other chemical
components, i.e., the higher hydrocarbons, on the ignition and thus on the
knocking resistance of the different LNG-Mixtures. However, from a chemical
point of view, ignition delay times belong strongly to the present radical pool and
the importance of radical forming and consuming reactions. A more detailed
ENG60 LNG II
analysis of a possible correlation between the MN and the ignition delay time
thus require a deeper insight into the combustion chemistry of LNG and
reference mixtures. It is proposed to develop a chemical kinetic model that can
accurately predict the combustion and ignition behaviour of LNG with different
MNs and that can help to understand the observations made during this project.
ENG60 LNG II
Table 2: Overview of all measured averaged ignition delay times for the different LNG-mixtures.
P = 40 bar, phi = 0.4
1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms)
Mix 1 1.0245 976 7.68 1.0427 959 11.32 1.0559 947 17
Mix 2 1.0224 978 3.057 1.0593 944 16.706 1.1098 901 234.3
Mix 3 1.02459 976 5.59 1.06345 940 19.84 1.09051 917 52.98
Mix 4 1.03734 964 7 1.06157 942 14.6 1.11607 896 88.6
Mix 5 1.0416 960 8.89 1.0857 921 24 1.1325 883 225.35
Mix 6 1.0309 970 10.5 1.0752 930 21.7 1.1235 890 205
Mix 7 1.033 968 18.7 1.0775 928 46.4 1.1337 882 240.9
MN0 1.0204 980 5 1.057 946 30 1.1135 898 137.2
MN60 1.02041 980 13.1 1.06045 943 50.5
185.2
MN70 1.02987 971 10.4 1.06724 937 45.99
MN80 1.02669 974 15.2 1.06724 937 50.1
0.999 1.1111 900
MN90 1.0214 979 26.29
0.9871
MN100 1.0683 936 23.66 1.0964 912 45.43 1.1325 883 89.04
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P = 20 bar, phi = 0.4
1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms)
Mix 1 0.95147 1051 4.29
1.01937 981 30.11 1.03627 965 56.24
Mix 2
0.98 1020 2.889 1.00806 992 8.696 1.03413 967 20.22
Mix 3 0.96061 1041 5.97 0.98232 1018 11 1.00604 994 19.36 1.03093 970 49.92
Mix 4
0.98912 1011 7.54 1.01626 984 14.54 1.02284 978 27.34
Mix 5
0.97466 1026 4.97 0.99701 1003 9.35 1.02459 976 18.4
Mix 6 0.95969 1042 16.5 0.98328 1017 22.8 1.00908 991 36.3 1.03413 967 92.9
Mix 7 0.96432 1037 29.4 0.98912 1011 40.28 1.0142 986 88.3
MN0
0.97943 1021 5.06 0.99305 1007 9.6 1.01833 982 46.7
MN60 0.95694 1045 11.9 0.98135 1019 24.2 0.99108 1009 30.8
MN70 0.95238 1050 6.8 0.97656 1024 19.5 1.002 998 34.1 1.03306 968 93.9
MN80 0.95877 1043 16 0.97943 1021 24 1.002 998 39.1 1.02987 971 114.3
MN90
0.9871 1013 11.25 1.007 993 17.56 1.0214 979 26.29
MN100 0.97847 1022 69.28 1 1000 98.43
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P = 40 bar, phi = 1.2
1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms)
Mix 1 1.0845 922 5.604 1.121 892 17.304 1.1337 882 22.61
Mix 2 1.1025 907 14.45 1.1286 886 49.92
1.1655 858 161.48
Mix 3 1.10334 906 15.28 1.11565 896 34.62 1.14155 876 100.63
Mix 4
1.11359 898 12.77 1.12909 886 24.2 1.16279 860 106.95
Mix 5 1.0928 915 10.63 1.1173 895 18.5
1.1614 861 58.66
Mix 6 1.08 926 18.3 1.1135 898 33.5
1.1587 863 112.2
Mix 7 1.0893 918 28.53 1.1173 895 72
1.156 865 244
MN0
SIMULATIONS
SIMULATIONS
SIMULATIONS
SIMULATIONS
MN60
SIMULATIONS
SIMULATIONS
SIMULATIONS
SIMULATIONS
MN70
SIMULATIONS
SIMULATIONS
SIMULATIONS
SIMULATIONS
MN80
SIMULATIONS
SIMULATIONS
SIMULATIONS
SIMULATIONS
MN90
SIMULATIONS
SIMULATIONS
SIMULATIONS
SIMULATIONS
MN100 SIMULATIONS SIMULATIONS SIMULATIONS SIMULATIONS
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P = 20 bar, phi = 1.2
1/T (1/K) Temperature (K)
IDT
(ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms) 1/T (1/K) Temperature (K) IDT (ms)
Mix 1
1.0638 940 9.811 1.0952 913 17.11 1.1198 893 41.57
Mix 2
1.0582 945 17.21 1.0834 923 41.81 1.1049 905 148
Mix 3
1.05932 944 24.38 1.08302 923 48.4 1.09529 913 101.16
Mix 4
1.04384 958 29 1.06724 937 49.75 1.10619 904 97.69
Mix 5
1.04932 953 10.51 1.07411 931 24.16 1.09649 912 46.36
Mix 6
1.0395 962 46.7 1.0626 941 87.1
Mix 7 1.0162 984 43.9 1.0395 962 75.6 1.0638 940 127.5
ENG60 LNG II
4.2 GAS MIXTURE ANALYSIS AT NPL AND VSL As described in section 2.1.2 of the validation protocol for the calculation of the
Methane number from the LNG composition, WP3, D3.2.117, the gas mixtures
used in the methane number engine tests were supposed to be analyzed after
the tests had been performed. Two samples of each of the gas mixtures
described in Table 1 were send to NPL or VSL to be certified against traceable
primary gas standards. The results from the analyses of these mixtures are
shown in Table 3 and Table 4. However, due to particles in the mixtures it was
shown to be problematic to connect pressure regulators to the cylinders without
any leaking during the sampling to the gas chromatograph. A number of the
samples were analysed by the NMIs against primary gas standards, but others
were unable to be analysed by the NMIs’ sensitive instrumentation due to
particulate contamination within the samples. For these samples,if available, the
results from the certificates of analysis from the gas suppliers were instead used.
For mixture 3, 6, 7 and 8 the nominal mix with a higher uncertainty was used.
Table 3 : Overview of component analysis for the different LNG-mixtures.
Component (% mol/mol):
CH4 C2H6 C3H8 n-C4H10 i-C4H10 n-C5H12 i-C5H12 N2
Mix 1 79.12 ±
0.40 13.75 ±
0.28 3.34 ± 0.07
0.877 ± 0.018
1.110 ± 0.022
0.1493 ± 0.0030
0.1495 ± 0.0030
1.497 ± 0.030
Mix 2 84.63 ±
0.42 12.80 ±
0.26 1.494 ± 0.030
0.209 ± 0.004
0.219 ± 0.004
0.0313 ± 0.0006
0.0191 ± 0.0004
0.596 ± 0.012
Mix 3 91.80 ±
0.69 5.70 ± 0.17
1.300 ± 0.039
0.150 ± 0.005
0.170 ± 0.005
0.0400 ± 0.0012
0.0400 ± 0.0012
0.800 ± 0.024
Mix 4 81.80 ±
0.41 13.28 ±
0.27 3.66 ± 0.07
0.269 ± 0.005
0.281 ± 0.005
0.0101 ± 0.0002
0.0099 ± 0.0002
0.692 ± 0.014
Mix 5 87.94 ±
0.44 7.21 ± 0.14
2.93 ± 0.06
0.707 ± 0.014
0.661 ± 0.013
0.0994 ± 0.0020
0.1106 ± 0.0022
0.348 ± 0.007
Mix 6 95.25 ±
0.71 2.00 ± 0.06
1.000 ± 0.030
0.300 ± 0.009
0.300 ± 0.009
0.0220 ± 0.0007
0.0250 ± 0.0008
1.100 ± 0.033
Mix 7 97.88 ±
0.73 1.000 ± 0.030
0.500 ± 0.015
0.210 ± 0.007
0.180 ± 0.006
0.0160 ± 0.0005
0.0180 ± 0.0006
0.200 ± 0.006
Mix 8 99.68 ±
0.75 0.0900 ± 0.0027
0.0300 ± 0.0009
0.0100 ± 0.0003
0.0100 ± 0.0003
0.0050 ± 0.0002
0.0050 ± 0.0002
0.170 ± 0.005
Table 4 : Results from analysis of pentane test samples by VSL
Component (% mol/mol):
CH4 n-C5H12 i-C5H12
[Mix 9] Pentane test 1 sample 1
99.38 ± 0.20
0.3104 ± 0.0012
0.3062 ± 0.0012
[Mix 9] Pentane test 1 sample 2
99.34 ± 0.20
0.3028 ± 0.0012
0.2986 ± 0.0012
[Mix 10] Pentane test 2 sample 1
99.57 ± 0.20
0.2024 ± 0.0008
0.1976 ± 0.0008
[Mix 10] Pentane test 2 sample 2
99.49 ± 0.20
0.2093 ± 0.0008
0.2012 ± 0.0008
[Mix 11] Pentane test 3 sample 1
99.68 ± 0.20
0.1000 ± 0.0004
0.1001 ± 0.0004
[Mix 11] Pentane test 3 sample 2 Not analysed due to particles in the sample
ENG60 LNG II
Table 5 : Results from analysis of calibration MN samples by VSL
Component (% mol/mol)
CH4 H2
calibration MN50 sample 1 Not analysed due to particles in the
sample
calibration MN50 sample 2 Not analysed due to particles in the
sample
calibration MN60 sample 1 59.56 ± 0.12 40.34 ± 0.20
calibration MN60 sample 2 59.52 ± 0.12 40.34 ± 0.20
calibration MN70 sample 1 69.76 ± 0.14 30.02 ± 0.15
calibration MN70 sample 2 69.76 ± 0.14 30.06 ± 0.15
calibration MN80 sample 1 79.14 ± 0.16 20.70 ± 0.10
calibration MN80 sample 2 79.14 ± 0.16 20.69 ± 0.10
calibration MN90 sample 1 89.82 ± 0.18 10.08 ± 0.05
calibration MN90 sample 2 89.80 ± 0.18 10.07 ± 0.05
4.3 RESULTS OF THE ENGINE TESTS FROM TUBS The methane number can only be measured with the equipment used by AVL and
under the boundary conditions used by them. This equipment unfortunately is
not available any more. If another engine and other boundary conditions are
used the result is called the service methane number (SMN). It has different
absolute values but shows the same trends as the MN. SMNs of gas mixtures 1-8
(natural gas type mixtures) and the three pentane test mixtures (mixtures 1-11)
were measured in a series of engine tests at TUBS. In these tests, the intake
manifold pressure was used as a measure for the knocking propensity of the
gases. Four different centers of combustion were used in the tests: 8°CA
(=Crank Angle), 10°CA, 12°CA & 14°CA, which yielded different service methane
numbers for the mixtures.
The charts in figure 5 and 6 show plots of SMN against the MN calculated using
the AVL method for the 11 mixtures tested.
ENG60 LNG II
Figure 5 : Results of the experimental service MN on a 200 cc single cylinder Spark Ignition engine
Figure 6: Results of the experimental service MN on a 600 cc single cylinder Spark Ignition engine.
ENG60 LNG II
The methane number of each mixtures was also calculated using the novel MN
algorithm developed in this project. The results (‘NPL MN’) are plotted in Figure 7
and Figure 8 below.
Figure 7: Methane Numbers and Service Methane Numbers for the mixtures investigated
40
50
60
70
80
90
100
110
0 1 2 3 4 5 6 7 8 9 10 11 12
Meth
ane n
um
ber
Mixture
DGC NPL MN SMN 8oC SMN 10oC SMN 12oC SMN 14oC
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Figure 8: Methane Numbers and Service Methane Numbers for the mixtures investigated
The above results show, a strong correlation between the MN results obtained
from the NPL and AVL algorithms.
Regarding the results from the SMN engine tests, mixtures 1-7 show a trend of
SMN decreasing with increasing crank angle This trend is reversed for mixtures
10 & 11, whereas for mixtures 8 & 9, no significant trend appears to exist. There
is a general pattern that the SMNs are lower than the AVL and NPL MN at low
MNs and higher than the AVL and NPL MN at higher MNs.
0
20
40
60
80
100
120
1 2 3 4 5 6 7 8 9 10 11
Meth
ane N
um
ber
Mixture
DGC EN16726 NPL MN SMN 8oC SMN 10oC SMN 12oC SMN 14oC
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5. VALIDATION
ALGORITHM Since the newly developed algorithm is based on the experimental data gathered
by AVL,1 it has been initially validated by comparing it with other popular
methods based on the AVL data namely the MWM and the Danish Gas Company
(DGC) methods. A detailed version of this comparison together with descriptions
of the different methods can be found in the document describing the developed
algorithm that has been submitted together with the algorithm.14
In summary, it was found that the MN values calculated with the new algorithm
agree well with the values derived by the MWM and the DGC methods. Figure 9a
shows calculated MN using the different methods for of a set of exemplary LNG
mixtures. Further analysis indicates that the results of the new algorithm
correlate approximately linearly with the other two methods (Figure 9 b)
Figure 9. Calculated methane numbers for a set of LNG mixtures with the DGC method (center) and the MWM method (right) plotted against the corresponding values
calculated with the new algorithm fit.
RESULTS The final uncertainty budget has validated with the input from D3.3.1, D3.3.2
and D3.4.1. Resulting values for the expanded uncertainty of the methane
number strongly depend on the composition of the LNG mixture, and are
typically between 0.3% and 0.8% relatively .
ENG60 LNG II
6. CONCLUSION A new algorithm, based on existing algorithms such as the AVL and MWM, has
been developed for LNG type of chemical composition. The new algorithm pays
special attention to the influence of higher hydrocarbons such as butanes and
pentanes as these components have a high impact on the methane number while
the concentration of these components in an amount of stored LNG can increase
over time (ageing effect). The algorithm is programmed in a user-friendly MS
Excel spreadsheet and an expanded uncertainty on the result is calculated to be
typically between 0.3% and 0.8% relative on the MN. This is the first algorithm
to provide a MN accompanied by an uncertainty.
Experimental results on two types of four-stroke single cylinder engines show a
good correlation between the calculated MN and the measured service MN.
However, the correlated line is slightly “tilted”, showing a lower SMN at the low
MN range and a higher SMN in the high range. More experiments are needed to
understand this behaviour and/or to modify the algorithm(s).
Ignition delay time measurements using a Rapid Compression Machine show
good correlation of the results with the hydrogen in methane mixtures. However,
the results with the more complex natural gas mixtures do not show the
expected correlation. The complex chemistry of the LNG related combustion
process has to be better understood. New experiments, with less complex
mixtures, such as e.g. propane in methane and butane in methane, have to be
performed to describe an improved kinetic model for LNG combustion.
The algorithm, the experimental results and the content of the report will be
communicated to relevant standardization committees to be used in the creation
of a harmonized method for the calculation of the MN.
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LITERATURE 1. Leiker, M., Christoph, K. & Al., E. Evaluation of Anti Knocking property of
Gaseous Fuels by Means of Methane Number and its Practical Application to
Gas Engines. ASME Publ. (1971).
2. prEN 16726:2014 Gas Infrastructure- Quality of gas- Groep H.
3. POSITION, E. Methane number as Paramete for Gas Quality Specifications.
(2012).
4. Bottino, G. et al. EMRP METROLOGY FOR LNG FEEDBACK.ppt. (2014).
5. Gas Infrastructure Europe, G. Position paper on Impact of including
Methane Number in the European Standard for Natural Gas. (2012).
6. Standard Test Method for Motor Octane Number of Spark-Ignition Engine
Fuel- ASTM D2700.
7. ISO\TR 22302 Natural gas- calculation of Methane Number. (2014).
8. ISO 15403-1:2006: Natural Gas- Natural Gas for use as a compressed fuel
for vehicles part 1: Designation of the quality. (2006).
9. MN calculator. Available at: http://www.cumminswestport.com/fuel-
quality-calculator.
10. DNV GL methane number calculator. Available at:
https://www.dnvgl.com/oilgas/natural-gas/fitness-for-purpose-of-lng-pki-
methane-number-calculator.html.
11. Agentschap NL, ministerie van Economische Zaken, Landbouw en
Innovatie: nota Methaan getal, vergelijking van methodieken. (2011).
12. http://digitizer.sourceforge.net/. Available at:
http://digitizer.sourceforge.net/.
13. DIN 51624:2007-11, Automotive fuels- Compressed natural gas-
Requirements and test methods. (2007).
14. Gieseking, B. & Brown, A. S. Description of Algorithm for calculating
Methane Number for LNG Composition.
15. Pitsch, H. A C++ computer program for 0-D and 1 D laminar flame
calculations. (1998).
16. Mehl, M. et al. Kinetic modeling of gasoline surrogate components and mixtures under engine conditions. Proc. Combust. Inst. 33, 193–200
(2011).
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17. Brown, A. (NPL), Eilts, P. (TUBS), Moshammer, K. (PTB), Murugan, A. (PTB) & Nieuwenkamp, G. (VSL). Validation Protocol for the Calculation of
the Methane Number from the LNG Composition. (2016).