ENV60 LNG II Metrological support for LNG custody transfer ...€¦ · ENG60 LNG II ENV60 – LNG II Metrological support for LNG custody transfer and transport fuel applications

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

ENG60 LNG II

• 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 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.

ENG60 LNG II

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

ENG60 LNG II

P = 20 bar, phi = 0.4


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

ENG60 LNG II

P = 40 bar, phi = 1.2


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

ENG60 LNG II

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


99.34 ± 0.20

0.3028 ± 0.0012

0.2986 ± 0.0012


99.57 ± 0.20

0.2024 ± 0.0008

0.1976 ± 0.0008


99.49 ± 0.20

0.2093 ± 0.0008

0.2012 ± 0.0008


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








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

ENG60 LNG II

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

ENG60 LNG II

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.

ENG60 LNG II

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 gascalculation 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).

ENG60 LNG II

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).

Documents

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