ASTM D8221-23

Standard Practice for Determining the Calculated Methane Number (MN<inf>C</inf>) of Gaseous Fuels Used in Internal Combustion Engines

SIGNIFICANCE AND USE
5.1 The methane number (MN) is a measure of the resistance of the gaseous fuel to autoignition (knock) when used in an internal combustion engine. The relative merits of gaseous fuels from different sources and having different compositions can be compared readily on the basis of their methane numbers. Therefore, the calculated methane number (MNC) is used as a parameter for determining the suitability of a gaseous fuel for internal combustion engines in both mobile and stationary applications.
SCOPE
1.1 This practice covers the method to determine the calculated methane number (MNC) of a gaseous fuel used in internal combustion engines. The basis for the method is a dynamic link library (DLL) suitable for running on computers with Microsoft Windows operating systems.  
1.2 This practice pertains to commercially available natural gas products that have been processed and are suitable for use in internal combustion engines. These fuels can be from traditional geological or renewable sources and include pipeline gas, compressed natural gas (CNG), liquefied natural gas, liquefied petroleum gas, and renewable natural gas as defined in Section 3.  
1.3 The calculation method within this practice is based on the MWM Method as defined in EN 16726, Annex A.2 The calculation method is an optimization algorithm that uses varying sequences of ternary and binary gas component tables generated from the composition of a gaseous fuel sample.3 Both the source code and a Microsoft Excel-based calculator are available for this method.  
1.4 This calculation method applies to gaseous fuels comprising of hydrocarbons from methane to hexane and greater (C6+); carbon monoxide; hydrogen; hydrogen sulfide; nitrogen; and carbon dioxide. The calculation method addresses pentanes (C5) and higher hydrocarbons and limits the individual volume fraction of C5 and C6+ to 3 % each and a combined total of 5 %. (See EN 16726, Annex A.) The calculation method is performed on a dry, oxygen-free basis.  
1.5 Units—The values stated in SI units are to be regarded as standard. Other units of measurement included in this standard are provided for information only and are not considered standard.  
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 75.160.30 - Gaseous fuels
• Technical Committee: D03 - Gaseous Fuels
• Drafting Committee: D03.03 - Determination of Heating Value and Relative Density of Gaseous Fuels

Relations

• Refers: ASTM D4150-19 - Standard Terminology Relating to Gaseous Fuels
• Effective Date: 15-Dec-2019
• Refers: ASTM D4150-08(2016) - Standard Terminology Relating to Gaseous Fuels
• Effective Date: 01-Jul-2016
• Refers: ASTM D4150-08 - Standard Terminology Relating to Gaseous Fuels
• Effective Date: 01-Dec-2008
• Refers: ASTM E29-08 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Oct-2008
• Refers: ASTM E29-06b - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Nov-2006
• Refers: ASTM E29-06a - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Sep-2006
• Refers: ASTM E29-06 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-May-2006
• Refers: ASTM E29-04 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Dec-2004
• Refers: ASTM D4150-03 - Standard Terminology Relating to Gaseous Fuels
• Effective Date: 10-Aug-2003
• Refers: ASTM E29-02e1 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 10-May-2002
• Refers: ASTM D4150-00 - Standard Terminology Relating to Gaseous Fuels
• Effective Date: 10-Jun-2000
• Refers: ASTM E29-93a(1999) - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 10-May-1999

Overview

ASTM D8221-23, "Standard Practice for Determining the Calculated Methane Number (MNC) of Gaseous Fuels Used in Internal Combustion Engines," establishes a standardized method for calculating the methane number (MN) of various gaseous fuels. The methane number is a key indicator of a gaseous fuel’s resistance to autoignition, or “knock,” when used in internal combustion engines. This standard provides an essential tool for evaluating and comparing the knocking resistance of fuels from different sources and compositions, enabling informed decisions in selecting the most suitable gaseous fuels for mobile and stationary engine applications.

Key Topics

• Methane Number (MN & MNC):

  • The MN reflects a fuel's tendency to resist knock, which is vital for efficient and safe engine performance.
  • The calculated methane number (MNC) is an analytical estimate, derived from gas composition data.

• Applicable Fuels:

  • Covers a broad range of commercially available natural gas products, including:
    • Pipeline gas, compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), and renewable natural gas (RNG).
  • Fuels may be of traditional geological or renewable origin.

• Calculation Method:

  • Based on the MWM Method as outlined in EN 16726, utilizing an optimization algorithm with gas component tables.
  • Requires input of fuel composition in volume percent at reference conditions (15.55 °C, 101.325 kPa).
  • Supports conversion from mass or mole fractions in accordance with ISO 14912.

• Software Tools:

  • The method is implemented as a dynamic link library (DLL) for Microsoft Windows platforms.
  • Includes a Microsoft Excel-based calculator and a VBA macro for practical application and data processing.

• Component Limitations:

  • The calculation addresses hydrocarbons from methane (CH₄) to hexane+ (C₆+), as well as non-hydrocarbon components like hydrogen, hydrogen sulfide, nitrogen, and carbon dioxide.
  • There are specified limits for certain higher hydrocarbons to ensure calculation accuracy.

Applications

ASTM D8221-23 is widely used across the energy, automotive, and gas utility sectors for:

• Engine Fuel Suitability Evaluations:

  • Determining if a specific gaseous fuel is fit for use in internal combustion engines, such as those found in heavy-duty vehicles, power generation, and industrial facilities.
  • Assists in maximizing engine efficiency and minimizing knock-related damage.

• Comparative Analysis:

  • Comparing the performance and suitability of fuels from various geographic and technological sources, including biomethane and synthetic natural gases.

• Quality Control and Compliance:

  • Supports quality assurance and regulatory compliance by providing consistent and reliable methods to report fuel knock resistance.
  • Useful for suppliers, engine manufacturers, fleet operators, and testing laboratories.

• Research and Development:

  • Provides a consistent metric for R&D initiatives aimed at developing new gaseous fuels or optimizing engine designs for multifuel capability.

Related Standards

Several standards are referenced or complementary to ASTM D8221-23, ensuring comprehensive fuel assessment and compatibility:

• EN 16726: Gas infrastructure - Quality of gas - Calculation of methane number.
• ISO 14912: Gas analysis - Conversion of gas mixture composition data.
• ASTM D4150: Terminology Relating to Gaseous Fuels.
• ASTM E29: Practice for Using Significant Digits in Test Data.
• ASTM D1835, D1945, D1946, D2163, D2650, D3588: Methods for analysis and specification of natural gas and LPG.
• GPA Standards (2145, 2166, 2172, 2261): Industry-recognized methods for gas sampling and analysis.
• ANSI Z132.1-1969 & ISO 14532: Provide foundational measurement and vocabulary standards.

Keywords: methane number, MN, calculated methane number, MNC, gaseous fuels, natural gas, CNG, LNG, LPG, RNG, engine fuels, autoignition resistance, fuel quality, knock resistance, ASTM D8221-23, gas engine performance, standard practice, fuel suitability.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D8221 − 23
Standard Practice for
Determining the Calculated Methane Number (MN ) of
C
Gaseous Fuels Used in Internal Combustion Engines
This standard is issued under the fixed designation D8221; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.5 Units—The values stated in SI units are to be regarded
as standard. Other units of measurement included in this
1.1 This practice covers the method to determine the calcu-
standard are provided for information only and are not consid-
lated methane number (MN ) of a gaseous fuel used in internal
C
ered standard.
combustion engines. The basis for the method is a dynamic
1.6 This standard does not purport to address all of the
link library (DLL) suitable for running on computers with
safety concerns, if any, associated with its use. It is the
Microsoft Windows operating systems.
responsibility of the user of this standard to establish appro-
1.2 This practice pertains to commercially available natural
priate safety, health, and environmental practices and deter-
gas products that have been processed and are suitable for use
mine the applicability of regulatory limitations prior to use.
in internal combustion engines. These fuels can be from
1.7 This international standard was developed in accor-
traditional geological or renewable sources and include pipe-
dance with internationally recognized principles on standard-
line gas, compressed natural gas (CNG), liquefied natural gas,
ization established in the Decision on Principles for the
liquefied petroleum gas, and renewable natural gas as defined
Development of International Standards, Guides and Recom-
in Section 3.
mendations issued by the World Trade Organization Technical
1.3 The calculation method within this practice is based on
Barriers to Trade (TBT) Committee.
the MWM Method as defined in EN 16726, Annex A. The
calculation method is an optimization algorithm that uses 2. Referenced Documents
varying sequences of ternary and binary gas component tables 4
2.1 ASTM Standards:
generated from the composition of a gaseous fuel sample.
D4150 Terminology Relating to Gaseous Fuels
Both the source code and a Microsoft Excel-based calculator
E29 Practice for Using Significant Digits in Test Data to
are available for this method.
Determine Conformance with Specifications
1.4 This calculation method applies to gaseous fuels com-
2.2 CEN Standard:
prising of hydrocarbons from methane to hexane and greater
EN 16726 Gas infrastructure—Quality of gas—Group H,
(C6+); carbon monoxide; hydrogen; hydrogen sulfide; nitro-
Annex A—Calculation of methane number of gaseous
gen; and carbon dioxide. The calculation method addresses
fuels for engines
pentanes (C5) and higher hydrocarbons and limits the indi-
2.3 ISO Standard:
vidual volume fraction of C5 and C6+ to 3 % each and a
ISO 14912 Gas analysis—Conversion of gas mixture com-
combined total of 5 %. (See EN 16726, Annex A.) The
position data
calculation method is performed on a dry, oxygen-free basis. 7
2.4 ASTM Adjuncts:
ASTM_D8221_mzdll_ver2.32.0.dll
ASTM_D8221_MNc_Method_ver1.32.1.xlsb
This practice is under the jurisdiction of ASTM Committee D03 on Gaseous
NOTE 1—The adjuncts listed above are the Microsoft Office 32-bit
Fuels and is the direct responsibility of Subcommittee D03.03 on Determination of
Heating Value and Relative Density of Gaseous Fuels.
Current edition approved May 1, 2023. Published June 2023. Originally
ɛ1
approved in 2018. Last previous edition approved in 2018 as D8221 – 18a . DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/D8221-23. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
European Standards (ENs) are documents that have been ratified by one of the Standards volume information, refer to the standard’s Document Summary page on
three European Standardization Organizations (ESOs), CEN, CENELEC or ETSI; the ASTM website.
recognized as competent in the area of voluntary technical standardization as for the Available from European Committee for Standardization (CEN), Avenue
EU Regulation 1025/2012. EN16726 was developed by the Technical Committee Marnix 17, B-1000, Brussels, Belgium, http://www.cen.eu.
CEN/TC 234. Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
Leiker, M., Christoph, K., Rankl, M., Cartellieri, W., and Pfeifer, U., “Evalu- 4th Floor, New York, NY 10036, http://www.ansi.org.
ation of Antiknocking Property of Gaseous Fuels by Means of Methane Number and Available from ASTM International Headquarters. Order Adjunct No.
its Practical Application to Gas Engines,” ASME, 72-DGP-4, 1972. ADJD822119-EA. Original adjunct produced in 2018.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8221 − 23
version.
5. Significance and Use
5.1 The methane number (MN) is a measure of the resis-
ASTM_D8221_mzdll_ver2.64.0.dll
tance of the gaseous fuel to autoignition (knock) when used in
ASTM_D8221_MNc_Method_ver1.64.1.xlsb
an internal combustion engine. The relative merits of gaseous
fuels from different sources and having different compositions
NOTE 2—The adjuncts listed above are the Microsoft Office 64-bit
can be compared readily on the basis of their methane
version.
numbers. Therefore, the calculated methane number (MN ) is
C
used as a parameter for determining the suitability of a gaseous
3. Terminology
fuel for internal combustion engines in both mobile and
3.1 Definitions:
stationary applications.
3.1.1 For definitions of general terms used in D03 Gaseous
Fuels Standards, refer to Terminology D4150.
CALCULATION METHOD
3.1.2 methane number (MN), n—an experimental determi-
6. Scope
nation of a gaseous fuel’s resistance to knock based on a
Cooperative Fuel Research (CFR) Motor Octane Number
6.1 This calculation method applies to gaseous fuels as
(MON) test engine and indicated by the volume of methane in
defined by the Scope, and defined in Section 3, and listed in
a blend with hydrogen.
Procedure 8.1, Table 2.
3.1.2.1 Discussion—Methane has a value of MN = 100 and
hydrogen has a value of MN = 0. 7. Significance and Use
3.1.3 methane number, calculated (MN ), n—a calculation 7.1 The methane number of a gaseous fuel can be calculated
C
of a rating index, indicating the resistance to knock of a based on its composition by various analytical methods which
gaseous fuel when compared to a reference methane/hydrogen can give different results. The results of the method described
mixture. in this practice match measured methane numbers (see X1.1
and X2.1) with reasonable certainty for the fuels as defined in
3.1.3.1 Discussion—Multiple methods have been developed
this practice.
in the past for providing this analytical estimate based on gas
composition. A MN is determined using volumetric fuel
C 7.2 The method requires input of composition in the form of
composition. Sometimes MN is described as “methane index
C
volume fractions at reference conditions of 15.55 °C and
(MI).”
101.325 kPa and expressed as a percentage. Compositions
available as either mole fraction or mass fraction are to be
3.2 Definitions of Terms Specific to This Standard:
converted to volume fraction using the methods cited in ISO
3.2.1 commercially available, adj—a natural gas product
14912. For general use, it may be assumed that mole fraction
from more than one non-governmental entity that may be
and volume fraction are equivalent.
distributed to more than one user.
3.2.1.1 Discussion—Examples of commercially available
7.3 The final result, the calculated methane number, is
natural gas products are pipeline gas from a utility, CNG from
expressed as an integer per Practice E29.
a fueling station, and so forth.
7.4 While this calculation method will provide results for
3.2.1.2 Discussion—Examples of natural gas that is not
gaseous fuel compositions outside the scope of this practice,
commercially available include well-head gas, landfill gas,
the results may have a higher degree of uncertainty.
methane digester gas, and so forth.
7.5 Errors—This calculation method may not apply to
3.3 Abbreviations:
certain gas compositions based on the component inputs
3.3.1 CNG—Compressed Natural Gas
violating input limit requirements or due to the method’s
internal calculations and regressions being unable to develop a
3.3.2 DLL—Dynamic Link Library
calculated methane number value. Error messages are provided
3.3.3 LNG—Liquefied Natural Gas
to the user and are listed in Table 1 and described in 7.5.1 to
3.3.4 LPG—Liquefied Petroleum Gas
7.5.4.
3.3.5 RNG—Renewable Natural Gas
TABLE 1 Error Messages
4. Summary of Calculation Method
Error Message Comment
4.1 The application of analytical methods to groups of
“The input sample total must be from No MN value displayed
C
ternary and binary mixtures of gas components with known 99 % to 101 %, inclusive; please
correct!”
methane numbers that reflect the makeup of the gaseous fuel
mixture is the basis for this calculated methane number
“The MNc calculation is not possible due No MN value displayed
C
method. to internal program errors!”
4.2 This calculation method originates with the data of a “A valid MNc value cannot be calculated!” No MN value displayed
C
research program performed by AVL Deutschland GmbH for
“A MNc value with reasonable certainty is No MN value displayed
C
FVV and was updated and revised by MWM GmbH and then
not possible due to high C5+ content!”
finally published for global use in EN 16726, Annex A.
D8221 − 23
TABLE 2 Components
Excel 32-bit or Excel 64-bit versions of Microsoft Office. An
Component Formula Component Formula Excel program (see A1.1) using the VBA macro (see A1.2) has
Methane CH Hexane C H been developed allowing for either individual or multiple gas
4 6 14
Ethylene C H Heptane C H
2 4 7 16
samples. It also provides conversions for mass or mole
Ethane C H Octane C H
2 6 8 18
fractions to volume fraction per ISO 14912 as well as a
Propylene C H Nonane C H
3 6 9 20
Propane C H Decane C H normalization routine. A graphical user interface could also be
3 8 10 22
Butadiene C H Hydrogen H 9
4 6 2
developed by a user to use the dynamic link library file.
Butylene C H Hydrogen Sulfide H S
4 8 2
i-Butane C H Carbon Monoxide CO
4 10
n-Butane C H Nitrogen N
4 10 2 8. Procedure
i-Pentane C H Oxygen O
5 12 2
n-Pentane C H Carbon Dioxide CO
5 12 2
8.1 Determine the volume composition of the gaseous fuel
Neopentane C H Water Vapor H O
5 12 2
in accordance with any ASTM or GPA method (see X2.2.1 and
X2.2.2) that yields the complete composition in component
amounts of 0.1 % or more, in terms of the components listed in
Table 2. Not all components listed in Table 2 must be input for
7.5.1 “The input sample total must be from 99 % to 101 %,
a MN value to be calculated. Several of the components are
inclusive; please correct!”—The calculated methane number
C
not typically included in a standard gas chromatograph analysis
program requires a gas sample composition that totals 99 % to
of natural gas, for example, oxygen, water, hydrogen sulfide,
101 %, inclusive. Please correct the gas component inputs
carbon monoxide, and hydrogen. However, these components
manually or normalize the gas sample composition.
can be quantified by other techniques if their concentrations are
7.5.2 “The MN calculation is not possible due to internal
C
program errors!”—A calculated methane number is not pos- considered significant.
sible for the given gas sample because the internal program
8.2 Conversion from a mass-based composition or a molar-
iteration routines failed. During the program’s internal optimi-
based composition and/or at different pressure or temperature
zation processes, the differences between calculated methane
conditions should be done in accordance with ISO 14912. All
numbers for various ternary mixtures used within the program
significant inputs should be in the form of volume fractions
were outside allowable internal program limits and a reason-
expressed as percentages at reference conditions of
able MN value could not be generated. An example of a gas
C
101.325 kPa, 15.55 °C except H S, which is in ppm(v). All
sample causing this error would be a high CH composition
measured hexane and higher hydrocarbons are combined and
with the rest of the sample containing H and CO components
input as hexane in the calculation regardless of if their
(for example, 98 % CH , 1.8 % H , and 0.2 % CO).
4 2
individual component amounts are less than 0.1 %. If the
7.5.3 “A valid MN value cannot be calculated!”—A cal-
C
measurement reported a value for hexane-plus hydrocarbons,
culated methane number is not possible for the given gas
then that value should be input as hexane. If the measurement
sample because internal program data is not available to
reported a value for heptane-plus hydrocarbons, then that value
support the calculation method or the internal program pro-
should be input as heptane. If nitrogen and oxygen measure-
cesses were unable to complete the calculation. An example of
ments are reported as a single nitrogen-oxygen value, then that
a gas sample causing this error would be a N free composition
value may be input as nitrogen with little impact to the MN
C
with low CH content and high CO content (for example,
4 2
value. Oxygen and water vapor component amounts may be
39 % CH and 61 % CO ).
4 2
input as part of the overall gaseous fuel composition, but they
7.5.4 “A MN value with reasonable certainty is not pos-
C
are later removed by the calculation method, which is per-
sible due to high C5+ content!”—The calculation method
formed on a dry, oxygen-free basis.
limits the volume fraction of C5 and C6+ to 3 % each and a
total of 5 % for a given gas sample and will not provide a
8.3 An Excel program, augmenting the VBA macro de-
calculated methane number if these limits are exceeded due to scribed in A1.2, is executed after numerical values of the
the high uncertainty in the result (for example, 94 % CH , 3 %
gaseous fuel composition (Table 2) are input. A calculated
C H , and 3 % C H ). methane number (MN ) is provided, and possible warning or
5 12 6 14 C
error messages may also be provided as described in 7.5, Table
7.6 The basis for this method is a dynamic link library
1. See A1.1.
(DLL) suitable for running on computers with 32-bit or 64-bit
Microsoft Windows operating systems.
8.4 Alternatively, an Excel VBA macro has been developed
allowing a user to develop their own Excel program. See A1.2.
7.7 An Excel Virtual Basic for Applications (VBA) macro is
available for a user to tailor their calculations for a specific
9. Report
application (see A1.2). The DLL files currently support either
9.1 A calculated methane number (MN ) should be reported
C
8 as “per Practice D8221.”
The sole source of the calculation method is a dynamic link library suitable for
running on computers and was created by ASTM based on the FORTRAN code
developed by MWM GmbH. ASTM will update the DLL file associated with this
practice as revisions are published. If you are aware of any revisions that should be
made to the calculation method and the DLL file, please provide this information to Support of this calculation tool for use on current and future computer systems
ASTM International Headquarters. Your comments will receive careful consider- is important. An updated interface for the calculation is being planned for broader
ation at a meeting of the responsible technical committee, which you may attend. software compatibility.
D8221 − 23
10. Precision and Bias 10.3 Bias—No general statement is made on bias of this
calculation method since no agreed upon reference values are
10.1 The determination of the MN described by this
C
available.
practice is exact and is reported as an integer value per Practice
E29.
11. Keywords
10.2 Precision—The precision of the MN method de-
C
11.1 CNG; gaseous fuels; LNG; LPG; methane number;
scribed by this practice is dependent on the precision of the
MN ; motor vehicle fuel; natural gas; RNG
original gaseous fuel components that are entered into the
C
method.
ANNEX
(Mandatory Information)
A1. CALCULATED METHANE NUMBER (MN ) METHOD
C
A1.1 Microsoft Excel Program opportunity to normalize the data will be offered. Alternatively,
there is a “Normalize” button that when clicked will normalize
A1.1.1 Save the Excel file “ASTM_D8221_MNc_Method_
the sample input. The normalized values overwrite the input
ver1.32.1.xlsb” and the dynamic link library file “ASTM_
values in cells “C6” to “C29.”
D8221_mzdll_ver2.32.0.dll” to the same directory/folder in a
A1.1.3.7 The status or error message associated with the
local drive for use with Excel 32-bit software. Alternatively,
MN value is displayed in cell “I20” (see Table 1).
C
save the Excel file “ASTM_D8221_MNc_Method_
A1.1.3.8 The “Clear Input” button resets the input cells
ver1.64.1.xlsb” and the Dynamic Link Library file “ASTM_
“C6” to “C29.”
D8221_mzdll_ver2.64.0.dll” to the same directory/folder in a
A1.1.3.9 The “Clear Output” button resets the output cell
local drive for use with Excel 64-bit software.
“I16” an
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: D8221 − 18a D8221 − 23
Standard Practice for
Determining the Calculated Methane Number (MN ) of
C
Gaseous Fuels Used in Internal Combustion Engines
This standard is issued under the fixed designation D8221; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—The Excel filenames in 2.4, A1.1.1, and A1.1.4.5 were updated editorially in August 2019.
1. Scope
1.1 This practice covers the method to determine the calculated methane number (MN ) of a gaseous fuel used in internal
C
combustion engines. The basis for the method is a dynamic link library (DLL) suitable for running on computers with Microsoft
Windows operating systems.
1.2 This practice pertains to commercially available natural gas products that have been processed and are suitable for use in
internal combustion engines. These fuels can be from traditional geological or renewable sources and include pipeline gas,
compressed natural gas (CNG), liquefied natural gas (LNG), gas, liquefied petroleum gas (LPG), gas, and renewable natural gas
(RNG) as defined in Section 3.
1.3 The calculation method within this practice is based on the MWM Method as defined in EN 16726, Annex A. The calculation
method is an optimization algorithm that uses varying sequences of ternary and binary gas component tables generated from the
composition of a gaseous fuel sample. Both the source code and a Microsoft Excel-based calculator are available for this method.
1.4 This calculation method applies to gaseous fuels comprising of hydrocarbons from methane to hexane and greater (C6+);
carbon monoxide; hydrogen; hydrogen sulfide; nitrogen; and carbon dioxide. The calculation method addresses pentanes (C5) and
higher hydrocarbons and limits the individual volume fraction of C5 and C6+ to 3 % each and a combined total of 5 %. (See EN
16726, Annex A.) The calculation method is performed on a dry, oxygen-free basis.
1.5 Units—The values stated in SI units are to be regarded as standard. Other units of measurement included in this standard are
provided for information only and are not considered standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
This practice is under the jurisdiction of ASTM Committee D03 on Gaseous Fuels and is the direct responsibility of Subcommittee D03.03 on Determination of Heating
Value and Relative Density of Gaseous Fuels.
Current edition approved Dec. 1, 2018May 1, 2023. Published April 2019June 2023. Originally approved in 2018. Last previous edition approved in 2018 as
ɛ1
D8221D8221 – 18a –18. DOI: 10.1520/D8221-18AE01. DOI: 10.1520/D8221-23.
European Standards (ENs) are documents that have been ratified by one of the three European Standardization Organizations (ESOs), CEN, CENELEC or ETSI;
recognized as competent in the area of voluntary technical standardization as for the EU Regulation 1025/2012. EN16726 was developed by the Technical Committee CEN/TC
234.
Leiker, M., Christoph, K., Rankl, M., Cartellieri, W., and Pfeifer, U., “Evaluation of Antiknocking Property of Gaseous Fuels by Means of Methane Number and its
Practical Application to Gas Engines,” ASME, 72-DGP-4, 1972.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8221 − 23
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D4150 Terminology Relating to Gaseous Fuels
E29 Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
2.2 CEN Standard:
EN 16726 Gas infrastructure—Quality of gas—Group H, Annex A—Calculation of methane number of gaseous fuels for
engines
2.3 ISO Standard:
ISO 14912 Gas analysis—Conversion of gas mixture composition data
2.4 ASTM Adjuncts:
ASTM_D8221_mzdll_ver2.32.0.dll
ASTM_D8221_MNc_Method_ver1.32.1.xlsb
NOTE 1—The adjuncts listed above are the Microsoft Office 32-bit version.
ASTM_D8221_mzdll_ver2.64.0.dll
ASTM_D8221_MNc_Method_ver1.64.1.xlsb
NOTE 2—The adjuncts listed above are the Microsoft Office 64-bit version.
3. Terminology
3.1 Definitions:
3.1.1 compressed natural gas (CNG), n—natural gas that has been compressed after processing for storage or transportation
purposes.
3.1.1.1 Discussion—
CNG is primarily used as a fuel for vehicles, typically compressed up to 24 821 kPa in the gaseous state.
3.1.1 For definitions of general terms used in D03 Gaseous Fuels Standards, refer to Terminology D4150.
3.1.2 liquefied natural gas (LNG), n—natural gas that has been liquefied after processing for storage or transportation purposes.
3.1.2.1 Discussion—
Liquefied natural gas is revaporized and introduced into pipelines for transmission and distribution as natural gas and may be used
as a fuel for internal combustion engines.
3.1.3 liquefied petroleum gas (LPG), n—a gaseous fuel that has been liquefied after processing for storage or transportation
purposes composed primarily of propane, propene (propylene), butane, and mixtures of these gases.
3.1.3.1 Discussion—
This special mixture of gases was specifically developed for use as a fuel in spark ignited internal combustion engines. Limits are
applied to propene and butane, which are used to adjust the vaporization pressure. When used in vehicles, LPG is specified as HD-5
propane or HD-10 propane.
3.1.2 methane number (MN), n—an experimental determination of a gaseous fuel’s resistance to knock based on a Cooperative
Fuel Research (CFR) Motor Octane Number (MON) test engine and indicated by the volume of methane in a blend with hydrogen.
3.1.2.1 Discussion—
Methane has a value of MN = 100 and hydrogen has a value of MN = 0.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from European Committee for Standardization (CEN), Avenue Marnix 17, B-1000, Brussels, Belgium, http://www.cen.eu.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from ASTM International Headquarters. Order Adjunct No. ADJD822119-EA. Original adjunct produced in 2018.
D8221 − 23
3.1.3 methane number, calculated (MN ), n—a calculation of a rating index, indicating the resistance to knock of a gaseous fuel
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when compared to a reference methane/hydrogen mixture.
3.1.3.1 Discussion—
Multiple methods have been developed in the past for providing this analytical estimate based on gas composition. A MN is
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determined using volumetric fuel composition. Sometimes MN is described as “methane index (MI).”
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3.1.6 natural gas, n—a naturally occurring mixture of hydrocarbon and nonhydrocarbon gases found in porous geological
formations (reservoirs) beneath the earth’s surface, often in association with petroleum. The principal component of natural gas
is methane.
3.1.7 pipeline gas, n—gas that is refined to high quality levels and transported in natural gas pipeline systems.
3.1.8 renewable natural gas (RNG), n—a pipeline-quality gas that is all or in part from renewable sources, and is fully
interchangeable with geological natural gas.
3.1.8.1 Discussion—
RNG can be biogas or biomethane (the gaseous product of the decomposition of organic matter) that has been processed to purity
standards and thus can be used as a fuel for internal combustion engines.
3.1.8.2 Discussion—
Like geological natural gas, RNG can be used for transportation purposes in the form of compressed natural gas (CNG) or liquefied
natural gas (LNG).
3.2 Definitions of Terms Specific to This Standard:
3.2.1 commercially available, adj—a natural gas product from more than one non-governmental entity that may be distributed to
more than one user.
3.2.1.1 Discussion—
Examples of commercially available natural gas products are pipeline gas from a utility, CNG from a fueling station, and so forth.
3.2.1.2 Discussion—
Examples of natural gas that is not commercially available include well-head gas, landfill gas, methane digester gas, and so forth.
3.3 Abbreviations:
3.3.1 CNG—Compressed Natural Gas
3.3.2 DLL—Dynamic Link Library
3.3.3 LNG—Liquefied Natural Gas
3.3.4 LPG—Liquefied Petroleum Gas
3.3.5 RNG—Renewable Natural Gas
4. Summary of Calculation Method
4.1 The application of analytical methods to groups of ternary and binary mixtures of gas components with known methane
numbers that reflect the makeup of the gaseous fuel mixture is the basis for this calculated methane number method.
4.2 This calculation method originates with the data of a research program performed by AVL Deutschland GmbH for FVV and
was updated and revised by MWM GmbH and then finally published for global use in EN 16726, Annex A.
5. Significance and Use
5.1 The methane number (MN) is a measure of the resistance of the gaseous fuel to autoignition (knock) when used in an internal
combustion engine. The relative merits of gaseous fuels from different sources and having different compositions can be compared
readily on the basis of their methane numbers. Therefore, the calculated methane number (MN ) is used as a parameter for
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determining the suitability of a gaseous fuel for internal combustion engines in both mobile and stationary applications.
D8221 − 23
CALCULATION METHOD
6. Scope
6.1 This calculation method applies to gaseous fuels as defined by the Scope, and defined in Section 3, and listed in Procedure
8.1, Table 2.
7. Significance and Use
7.1 The methane number of a gaseous fuel can be calculated based on its composition by various analytical methods which can
give different results. The results of the method described in this practice match measured methane numbers (see X1.1 and X2.1)
with reasonable certainty for the fuels as defined in this practice.
7.2 The method requires input of composition in the form of volume fractions at reference conditions of 15.55 °C and 101.325 kPa
and expressed as a percentage. Compositions available as either mole fraction or mass fraction are to be converted to volume
fraction using the methods cited in ISO 14912. For general use, it may be assumed that mole fraction and volume fraction are
equivalent.
7.3 The final result, the calculated methane number, is expressed as an integer per Practice E29.
7.4 While this calculation method will provide results for gaseous fuel compositions outside the scope of this practice, the results
may have a higher degree of uncertainty.
7.5 Errors—This calculation method may not apply to certain gas compositions based on the component inputs violating input
limit requirements or due to the method’s internal calculations and regressions being unable to develop a calculated methane
number value. Error messages are provided to the user and are listed in Table 1 and described in 7.5.1 to 7.5.4.
7.5.1 “The input sample total must be from 99 % to 101 %, inclusive; please correct!”—The calculated methane number program
requires a gas sample composition that totals 99 % to 101 %, inclusive. Please correct the gas component inputs manually or
normalize the gas sample composition.
7.5.2 “The MN calculation is not possible due to internal program errors!”—A calculated methane number is not possible for
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the given gas sample because the internal program iteration routines failed. During the program’s internal optimization processes,
the differences between calculated methane numbers for various ternary mixtures used within the program were outside allowable
internal program limits and a reasonable MN value could not be generated. An example of a gas sample causing this error would
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be a high CH composition with the rest of the sample containing H and CO components (for example, 98 % CH , 1.8 % H , and
4 2 4 2
0.2 % CO).
7.5.3 “A valid MN value cannot be calculated!”—A calculated methane number is not possible for the given gas sample because
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internal program data is not available to support the calculation method or the internal program processes were unable to complete
the calculation. An example of a gas sample causing this error would be a N free composition with low CH content and high
2 4
CO content (for example, 39 % CH and 61 % CO ).
2 4 2
TABLE 1 Error Messages
Error Message Comment
“The input sample total must be from No MN value displayed
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99 % to 101 %, inclusive; please
correct!”
“The MNc calculation is not possible due No MN value displayed
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to internal program errors!”
“A valid MNc value cannot be calculated!” No MN value displayed
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“A MNc value with reasonable certainty is No MN value displayed
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not possible due to high C5+ content!”
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TABLE 2 Components
Component Formula Component Formula
Methane CH Hexane C H
4 6 14
Ethylene C H Heptane C H
2 4 7 16
Ethane C H Octane C H
2 6 8 18
Propylene C H Nonane C H
3 6 9 20
Propane C H Decane C H
3 8 10 22
Butadiene C H Hydrogen H
4 6 2
Butylene C H Hydrogen Sulfide H S
4 8 2
i-Butane C H Carbon Monoxide CO
4 10
n-Butane C H Nitrogen N
4 10 2
i-Pentane C H Oxygen O
5 12 2
n-Pentane C H Carbon Dioxide CO
5 12 2
Neopentane C H Water Vapor H O
5 12 2
7.5.4 “A MN value with reasonable certainty is not possible due to high C5+ content!”—The calculation method limits the
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volume fraction of C5 and C6+ to 3 % each and a total of 5 % for a given gas sample and will not provide a calculated methane
number if these limits are exceeded due to the high uncertainty in the result (for example, 94 % CH , 3 % C H , and 3 % C H ).
4 5 12 6 14
7.6 The basis for this method is a dynamic link library (DLL) suitable for running on computers with 32-bit or 64-bit Microsoft
Windows operating systems.
7.7 An Excel Virtual Basic for Applications (VBA) macro is available for a user to tailor their calculations for a specific
application (see A1.2). The DLL files currently support either Excel 32-bit or Excel 64-bit versions of Microsoft Office. An Excel
program (see A1.1) using the VBA macro (see A1.2) has been developed allowing for either individual or multiple gas samples.
It also provides conversions for mass or mole fractions to volume fraction per ISO 14912 as well as a normalization routine. A
graphical user interface (GUI) could also be developed by a user to use the dynamic link library file.
8. Procedure
8.1 Determine the volume composition of the gaseous fuel in accordance with any ASTM or GPA method (see X2.2.1 and X2.2.2)
that yields the complete composition in component amounts of 0.1 % or more, in terms of the components listed in Table 2. Not
all components listed in Table 2 must be input for a MN value to be calculated. Several of the components are not typically
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included in a standard gas chromatograph analysis of natural gas, for example, oxygen, water, hydrogen sulfide, carbon monoxide,
and hydrogen. However, these components can be quantified by other techniques if their concentrations are considered significant.
8.2 Conversion from a mass-based composition or a molar-based composition and/or at different pressure or temperature
conditions should be done in accordance with ISO 14912. All significant inputs should be in the form of volume fractions expressed
as percentages at reference conditions of 101.325 kPa, 15.55 °C except H S, which is in ppmv.ppm(v). All measured hexane and
higher hydrocarbons are combined and input as hexane in the calculation regardless of if their individual component amounts are
less than 0.1 %. If the measurement reported a value for hexane-plus hydrocarbons, then that value should be input as hexane. If
the measurement reported a value for heptane-plus hydrocarbons, then that value should be input as heptane. If nitrogen and
oxygen measurements are reported as a single nitrogen-oxygen value, then that value may be input as nitrogen with little impact
to the MN value. Oxygen and water vapor component amounts may be input as part of the overall gaseous fuel composition, but
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they are later removed by the calculation method, which is performed on a dry, oxygen-free basis.
8.3 An Excel program, augmenting the VBA macro described in A1.2, is executed after numerical values of the gaseous fuel
composition (Table 2) are input. A calculated methane number (MN ) is provided, and possible warning or error messages may
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also be provided as described in 7.5, Table 1. See A1.1.
8.4 Alternatively, an Excel VBA macro has been developed allowing a user to develop their own Excel program. See A1.2.
The sole source of the calculation method is a dynamic link library suitable for running on computers and was created by ASTM based on the FORTRAN code developed
by MWM GmbH. ASTM will update the DLL file associated with this practice as revisions are published. If you are aware of any revisions that should be made to the
calculation method and the DLL file, please provide this information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting
of the responsible technical committee, which you may attend.
Support of this calculation tool for use on current and future computer systems is important. An updated interface for the calculation is being planned for broader software
compatibility.
D8221 − 23
9. Report
9.1 A calculated methane number (MN ) should be reported as “per Practice D8221.”
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10. Precision and Bias
10.1 The determination of calculated methane number (MNthe MN ) described by this practice is exact and is reported as an
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integer value per Practice E29.
10.2 Precision—The precision of the calculated methane number (MNMN ) method described by this practice is dependent on
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the precision of the original gaseous fuel components that are entered into the method.
10.3 Bias—No general statement is made on bias of this calculation method since no agreed upo
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