Methane Number Algorithm with Defined Uncertainty

Bjoern Gieseking

4th International Workshop Metrology For LNG

National Physical Laboratory

16.06.2016

LNG: UK Perspective

Department of Energy & Climate Change

1: DUKES, 2015 2:Updated energy and emissions projections 2014

• 27% of imported natural gas is LNG (2014)

• 92% of LNG from Qatar (2014)

• 4 LNG terminals

• Natural gas important share in energy mix

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• Increasing share of natural gas in primary energy mix

• Increased share of LNG in global trade

1&2: BP, Energy Outlook 2016 edition

LNG: Global Perspective

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Methane Number (MN)

• MN is natural gas counterpart to octane number

• Describes knocking resistance of gas mixture to engine knocking

• Used by engine manufacturers to describe quality of natural gas

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1 www.vka.rwth-aachen.de

• Based on detailed set of experiments carried out between 1964 and 1970 by AVL

• By definition derived from specific test engine

• Results summarised in ternary diagrams

MN Algorithm: Working principle

LNG composition

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Sub-mixtures

MN’ (ΔMN)

Inert gascorrection

MN

MinΔMN

• Calculate reduced mixture and normalise to 100% volume

• Divide into two sub-mixtures and normalise again

• Calculate MNi of sub-mixtures using ternary diagrams

• Minimise ΔMN between sub-mixtures using automated routine

• Derive MN by applying inert gas correction

B. Gieseking, A. S. Brown, submitted, 2016

MN Algorithm: Working principle

• Convert ternary diagram into Cartesian coordinates

• Calculate intersection point of C1

and C2 representing methane and ethane (propane) volume fractions

• Calculate intersection points of ofC2 with iso-methane curves

• Identify nearest neighbouring iso-methane curves

• Calculate MNi by linear inter-polation

Uncertainty of MN Algorithm

LNG

compositionMN

Uncertainty

algorithm

Pcompr.- SMN

transfer function

Uncertainty of MN Algorithm

LNG

compositionMN

Uncertainty

algorithm

Pcompr.- SMN

transfer function

Uncertainty not included in

calculation tools or

LNG legislation so far!

Uncertainty of MN Algorithm

• Assumption: Values of iso-MN curves and intersection point XMN/YMN precise

• Uncertainty resulting from digitising ternary diagrams dominates (line thickness)

• Defining symmetric uncertainty in x for intersection points with iso-MN curves

• Resulting uncertainty strongly depends on LNG composition

• Values vary between 0.2 and 0.7 MN or 0.3 and 0.8% (k=2)

MN Algorithm Results

• Novel algorithm shows good agreement with other popular methods for set of exemplar LNG mixtures

• MWM methods yields MN values that are in average smaller by one

MN Algorithm Results

NPL algorithm correlates approximatelylinearly with DGC and MWM methodsover a broad range between 60 and 99MN

Slope Intersect

DGC(NPL) 1.01 ± 0.01 -0.12 ± 0.37

MWM(NPL) 1.03 ± 0.02 -3.07 ± 1.78

Results of generalised least squares fits:

MN Algorithm Results

• Novel algorithm correctly models influence of nitrogen (N1-N5) and pentanes (HC1 – HC5)

• Observed differences between methods might stem from different approaches for Cn>4 and inert gas corrections

Increased N2 content Increased pentane content

Outlook

SMN = f(pcompr.)

Exp. results

SMN

SMN=f(MN)

Engine calibration

MN

AlgorithmCorrelation

Summary

• Novel algorithm shows good agreement with other popular calculation tools & correlates linearly

• For first time: Uncertainty of algorithm included

• u/c values strongly depend on LNG composition ranging from 0.2 to 0.7 MN or 0.3 to 0.8% (k=2)

• Differences between compared methods presumably due to different approaches for inert gas and Cn>4 correction routines

• Recommendation: Incorporation of u/c associated with MN calculation(calc. tools & legislation for LNG)

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1: GIIGNL, The LNG Industry, 2013

Thank you for your attention!

Composition of LNG mixtures I

Volume fraction [%]

CH4 C2H6 C3H8 n-C4H10 i-C4H10 n-C5H12 i-C5H12 N2

Mix 1 78.8 14.00 3.40 0.90 1.10 0.15 0.15 1.50

Emirates 84.52 12.90 1.50 0.21 0.22 0.03 0.02 0.60

Norway 91.80 5.70 1.30 0.15 0.17 0.04 0.04 0.80

Libya 81.69 13.38 3.67 0.27 0.28 0.01 0.01 0.69

Oman 87.89 7.27 2.92 0.71 0.65 0.10 0.11 0.35

Mix 6 95.253 2.00 1.00 0.30 0.30 0.022 0.025 1.10

Mix 7 97.876 1.00 0.5 0.21 0.18 0.016 0.018 0.20

Alaska 99.68 0.09 0.03 0.01 0.01 0.005 0.005 0.17

Mix 9 99.54 0.10 0.10 0.08 0.08 ----- ----- 0.10

Composition of LNG mixtures II

Volume fraction [%]

CH4 C2H6 C3H8 n-C4H10 i-C4H10 n-C5H12 i-C5H12 Cn>5 N2 CO2

N1 92.85 5.00 2.00 ----- ----- ----- ----- ----- 0.15 -----

N2 92.40 5.00 2.00 ----- ----- ----- ----- ----- 0.60 -----

N3 91.50 5.00 2.00 ----- ----- ----- ----- ----- 1.50 -----

N4 90.00 5.00 2.00 ----- ----- ----- ----- ----- 3.00 -----

N5 87.00 5.00 2.00 ----- ----- ----- ----- ----- 6.00 -----

HC1 92.27 5.00 2.00 0.20 0.20 0.015 0.015 ----- 0.30 -----

HC2 92.18 5.00 2.00 0.20 0.20 0.06 0.06 ----- 0.30 -----

HC3 92.00 5.00 2.00 0.20 0.20 0.15 0.15 ----- 0.30 -----

HC4 91.70 5.00 2.00 0.20 0.20 0.30 0.30 ----- 0.30 -----

HC5 91.10 5.00 2.00 0.20 0.20 0.60 0.60 ----- 0.30 -----

Engine Knocking

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1: www.troublecodes.net/pcodes/p0324/