Methane Number

Gary Palmer Independent Consultant, Calgary, Canada

Received October 24, 2017; Accepted October 25, 2017

Abstract: The Methane Number is a measure of the resistance of natural gas to detonation when it is burned as a motor fuel in an engine. Pure Methane is assigned a Methane Number of 100 and pure Hydrogen is assigned a Methane Number of zero. A natural gas having a Methane Number of 80 for example, would have the detonation properties of a mixture consisting of 80 vol% Methane and 20 vol% Hydrogen. The Methane Number concept is similar to the Octane Number for gasoline. Unlike gasoline however, there is not yet a universal standard for testing natural gas Methane Number as there is in the motor test for gasoline. Also, there is no universally accepted method for calculating the Methane Number based on the composition of natural gas.

Keywords: Methane number, gas combustion

1 Methane Number The definition of Methane Number is simple if the gas consists only of the two components Methane and Hydrogen. But for hydrocarbon gas mixtures consisting not only of methane but also of Ethane and heavier, the definition is much more difficult and is the subject of much debate and controversy. There are various proposed methods of calculating Methane Number based on gas composition. Some are methods proposed by standards associations in Europe and some are proprietary methods proposed by engine manufacturers.

1.1 Standards Associations The following standards have been proposed to establish a universal method to relate gas composition to Methane Number but so far there has been no agreement on which method to use.

*Corresponding author: [email protected] DOI: 10.7569/JNGE.2017.692506

134 J. Natural Gas Eng., Vol. 2, No. 2, December 2017 Gary Palmer: Methane Number

1. The Gas Research Institute in the United States proposes using the European standard ISO-15403-2006 Part 1 as a method to define Methane Number based on gas composition for motor fuels. There is no official status for the method in the United States. 2. Some European countries propose standard ISO-TR22304 as a method for calculating Methane Number and suggests having two grades of natural gas to be used as motor fuel: • Grade X (Regular Grade): regular grade has a minimum Methane Number of 65. It is intended mainly to be used as burner fuel or as fuel for internal combustion engines, having a very low compression ratio. • Grade Y (Premium Grade): premium grade natural gas would have a minimum Methane Number of 80. It would be for general use as motor fuel for all engines. The standard listed above has no official status. 3. Standards associations in Germany propose the following two standards to be used to define Methane Number: • DIN standard uses the AVL method to calculate the Methane Number. The AVL method is believed to be part of DIN standard 51624. • DIN research report on internal combustion engines bulletin 3, 1971. The method has no official status.

1.2 Engine Manufacturers Almost every engine manufacturer has developed its own proprietary method for calculating Methane Number for natural gas. But there is no widely accepted single method that they all agree on. The situation is complicated by the competing claims of various engine manufacturers that it is the design of their engines that defines what fuel is acceptable, not an arbitrary Methane Number. Such things as compression ratio, ignition timing, or simple derating of the horse power output provide the necessary flexibility in making the engine more tolerant of fuels with low Methane Number. Manufacturers resist establishing an arbitrary Methane Number to define what is acceptable as motor fuel. They also resist being forced to accept an arbitrary formula for calculating Methane Number. The following manufacturers of heavy industrial engines and others all have their own proprietary formulas for calculating the Methane Number of natural gas to be used as motor fuel:

• Waukesha • Caterpillar • Wartsila • MWM • Cummins

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None of these individual calculation methods have been mutually agreed upon and none have official status.

1.3 Suppliers and Users of Natural Gas as Motor Fuel A controversy is raging, especially in Europe, about the lack of official standards for natural gas which is used as motor fuel. Users of this type of natural gas claim that poor quality fuel is wrecking their engines. They are agitating for the adop- tion of something similar to ISO standard TR22304 which specifies that motor fuel grade Y should have a Methane Number of at least 80. European suppliers of natural gas that is to be used in internal combustion engines are fighting this proposal stating that ordinary pipeline quality gas is completely acceptable as engine fuel. They resist any attempt to determine a quality standard such as establishing a defined Methane Number based on gas composition. They insist that the free market reacts to pressure and will establish its own standards. The suppliers propose that the present system of loose none mandatory standards is best, where fuel quality is a matter to be decided between the buyer and the seller and state that the normal processing of raw natural gas to produce acceptable pipeline quality gas should also be sufficient for quality motor fuel.

1.4 CNG and LNG CNG and LNG typically begin with either a pipeline quality gas or a field gas that approaches pipeline quality standards. Then the gas is either compressed or liquified to make it transportable to market. However, the final product of compression or liquification is still basically pipeline quality gas. Methane Number, depending on composition, is typically in the range of 65 to 75 and most of the commercial products fall short of the goal of a Methane Number greater than 80. The process of manufacturing CNG is relatively simple and uncomplicated, requiring only compression and sometime dehydration to produce a commercial product. In most cases, when the CNG must meet an arbitrarily imposed Methane Number standard, complex refrigeration and fraction equipment must be added to the simple compressor station process. CNG producers therefore do not wel- come the imposition of Methane Number standards for their CNG product. Major suppliers of LNG propose a similar argument. They have invested billions of dollars to convert ordinary pipeline quality gas to LNG. But meeting Methane Number standards would require the investment of billions of dollars more to make major modification to their processing schemes. They quote the following export statistics from all parts of the world as shown in Table 1 below. These figures apply to huge processing and liquification facilities located on tidewater in seaports around the world. Table 1 uses the MWM proprietary method to calculate Methane Number.

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Table 1 Percent of LNG Meeting Methane Number [2]. Methane number Pass Fail No 65 100% 0% No 70 97% 3% No 75 34% 66% No 80 12% 88%

1.5 Natural Gas Characterization 1.5.1 Wobbe Number The Wobbe Number is commonly used in Europe to compare the performance of different fuel gases in appliances with burners. If the Wobbe Numbers are identical for two different gases, then the gases can be used interchangeably in combustion equipment. The Wobbe Number is sometimes used to specify gas heating value in gas purchasing contracts and is usually expressed in metric units MJ/Sm3.

HigherHeatingValueofGas HHV Wobbe Number = = SpecificGravity SG

3 Higher Heating Value of Gas = ∑(HHV)i · (mol fraction)i MJ/Sm 3 Where (HHV)i is the higher heating value of component i in MJ/Sm Calculating Wobbe Number (Imperial Units):

Higher Heating Value of Gas Mixture = ∑(HHV)i · (mol fraction)i Btu/scf

Where (HHV)i is the higher heating value of component i in Btu/scf MoleWt Specific Gravity Gas Mixture = .2885 HHV of Mixture Imperial Unit Wobbe Number = Btus/ cf SpecificGravity

 Btu   MJ   35. 315 Metric Unit Wobbe Number =        SCF  948Btu  Sm3  Higher heating values of pure hydrocarbons is provided in Table 2. Some of the Methane Number formulas incorporate the Wobbe Number in the calculations. European tabulated values of HHV are very different from GPA values.

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1.5.2 C onflicts Between Pipeline Gas Specifications and Methane Number Wobbe Number is directly related to the heating value of the fuel. The richer the gas the higher the Wobbe Number and the lower the Methane Number. Contrarily, increasing the Methane Number will decrease the heating value of the fuel. Since pipeline quality gas and natural gas motor fuel are destined for different uses there is a strong possibility that different standards to be met by the two gases will be in conflict. Many pipeline gases when compressed to CNG or liquidized to LNG, because of their low Methane Number, do not make ideal fuels for engines. Quality standards for pipeline gas are negotiated as individual contracts between the pipeline companies and the producers or between the pipeline companies and the consumers. There are no officially recognized universal standards for pipeline quality gas. Specifications for the gas will be part of the contract between the par- ties involved and may include standards of purity, composition, hydrocarbon dew point, water content, and heating value. Pipeline contracts usually specify the low- est heating value (LHV) whereas the Wobbe Number deals with the higher heating value (HHV), the difference being the latent heat of condensation of the stoichio- metric water vapour formed as a product of combustion. This lower heating value (LHV) specified in pipeline contracts typically varies between 950 Btu/SCF and 1150 Btu/SCF. This would mean in most cases that the Methane Number is less

Table 2 Higher Heating Value (HHV) of Pure Hydrocarbons [3]. Component HHV (Btu/scf) Metric MJ/Sm3

H2 324.2 12.08

H2S 637.1 23.74

C1 1010.0 37.63

C2 1769.6 65.93

C3 2516.1 93.75

iC4 3251.9 121.16

nC4 3262.3 121.55

iC5 4000.9 149.07

nC5 4008.9 149.37

C6 4755.9 177.20

C7 5502.5 205.02

C8 6248.9 232.83

C9 6996.5 260.68

C10 7742.9 288.49

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138 J. Natural Gas Eng., Vol. 2, No. 2, December 2017 Gary Palmer: Methane Number than 80 and before a typical pipeline gas could be used as a motor fuel it should be first processed to increase the Methane Number to 80 or higher. Pipeline gases with excessive inerts such as nitrogen or carbon dioxide will probably also contain extra Ethane and will be heavier to boost the Btu content and to offset the loss of heating value due to dilution by inerts. This practice will tend to drag down the Methane Number. For most pipeline gases, if they are to approach 99% Methane, it will be necessary to remove essentially all of the light gases such as Hydrogen, helium, Nitrogen, and also to remove essentially all of the Ethane and heavier components. It is a two-step process, first to remove the light gases, then to remove the heavy ends. Simple compression to CNG or liquification to LNG will not, in most cases, produce an ideal motor fuel, and further processing is necessary to raise the Methane Number.

1.6 Calculation of Methane Number from Composition There are numerous formulas for calculating the Methane Number. But so far none of them have been recognized officially. Many of the calculation methods are proprietary and are not in the public domain. But several technical papers have been presented by standards associations and technical societies. The suggested methods are as follows.

1. Linear Correlation of Methane and Wobbe Numbers The Methane Number and Wobbe Numbers of various LNG compositions from suppliers around the world were calculated using the MWM method for Methane Number and the results were plotted showing the relationship between Wobbe Number and Methane Number. Plotting of the data from 22 different sources was very consistent and showed very little scatter of the data points when Methane Number was plotted against Wobbe Number on an arithmetic scale (see Figure 1). The plot suggests intersecting straight lines with a break in the slope occurring at a Wobbe Number of about 54.9 MJ/Sm3 and a Methane Number of about 77. Above a Wobbe Number of about 54.9 MJ/Sm3 the Methane Number is relatively insensitive to changes in Wobbe Number. In this range for an increase of 1.0 in Wobbe Number the Methane Number decreases by only 5.2 points. Below a Wobbe Number of 54.9 MJ/Sm3 the Methane Number is more sensitive to changes in Wobbe. For an increase of 1.0 in Wobbe, the Methane Number decreases by 14.6 points above a Wobbe of 54.9. This suggests that it is relatively easy to approach a Methane Number of 80 by removing the Ethane and heavier components in the mixture but it will become increasingly difficult to raise the Methane Number above 80. Pure Methane would have a Methane Number of 100. Use of the linear correlation provides a quick and simple way to estimate the Methane Number. The Methane Number calculated is the result of the MWM

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J. Natural Gas Eng., Vol. 2, No. 2, December 2017 139 Gary Palmer: Methane Number method upon which the line was based. The data is plotted between Wobbe Numbers from 53.5 to 57. The accuracy of the chart depends on the validity of the MWM method for calculating the Methane Number. Also, European values of higher heating values do not agree with North American values which will invali- date this method.

2. The “Anstalt fur Verbrennungskraftmachinen List (AVL)” Method The AVL method to calculate Methane Number is base on empirical measure- ments taken on different gases in Germany in 1970. The method is described in the German report “Research Combustion Engine Bulletin 120 Part 3, 1970“ which is not readily available. Possibly the details of the AVL method are proprietary and confidential and not revealed in the report. Many software programs such as MWM, Gascale, DGC, and Gasyle have incor- porated the AVL procedure in their calculation of Methane Number, none of which are readily available.

56.5

56 ) 3 55.5

54.5 Wobbe index (mj/m

53.5

53 60 65 70 75 80 85 90 95 100 Methane number

Figure 1 Methane Number vs. Wobbe Index of LNG from Different Suppliers [1].

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3. The Hydrogen/Carbon Ratio Method A method of calculating Methane Number has been suggested in Europe which describes a method of quantitively relating Methane Number to Hydrogen/ Carbon ratio in the natural gas motor fuel. The development of the method involved using Iso-Octane dosed with Tetra Ethyl Lead as the reference fuel and then measuring the knock resistance of various natural gas fuels in a motor Octane test similar to that used for gasoline. It was then possible to relate motor Octane Numbers for natural gas to the composition of the gas based on the Hydrogen/Carbon ratio for the sum of components. If the fuel composition is known, the Hydrogen and carbon atoms in the mixture are counted, and the ratio of Hydrogen atoms to carbon atoms is calculated to calculate the motor Octane Number [1]. Octane Number = -406.14 + 508.04 x H/C – 173.55 × (H/C)2 + 20.17 × (H/C)3 Having the motor Octane Number, the Methane Number is calculated as follows, Methane Number = 1.624 × Octane Number –119.1 To produce a calculated value of Methane Number of 80 requires a motor Octane Number of 122.6 and a Hydrogen/carbon ratio of 3.758. The accuracy of the method is not known.

4. The ISO/TR 22302-2014 Method This European standard proposes a method to calculate Methane Number based on gas composition. It proposes that natural gas should be marked in two grades.

• Grades X: Methane Number no less than 65 to be used as burner fuel. • Grades Y: Methane Number no less than 80 to be used as motor fuel.

2 Conclusions Suppliers of natural gas, whether in the form of standard pipeline gas, compressed natural gas (CNG), or liquified natural gas (LNG) all strongly oppose the setting of international standards for Methane Number. Contrarily, the operators of natural gas powered vehicles and stationary engines strongly recommend a Methane Number of 80 to protect their engines. Manufacturers of engines are hesitant to admit their engines require better fuel and insist that modifications can be made so that engines can tolerate poor quality fuel. ISO specification TR 22302-2014 suggests a compromise that would satisfy both sides of the debate. Instead of having a single Methane Number for all users of natural gas, including pipeline gas, compressed natural gas (CNG), and Liquified natural gas (LNG), the standard recommends having two grades of natural gas. One is for use in burners and a second class with a higher Methane Number is to be used as motor fuel.

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• Grade X: Methane Number not less than 65 • Grade Y: Methane Number not less than 80

Where Grade X and Y correspond to Regular and Premium grades of gasoline. According to international LNG producers’ surveys, 100% of their product meet the Methane Number of 65 without further processing. Because ordinary LNG has a Methane Number of 65 and normally has the same composition as pipeline gas, pipeline gas can also normally meet a Methane Number standard of 65. It is only the small segment of the natural gas market that is used for motor fuel that would be required to meet a Methane Number standard of 80. The most logical marketing plan for CNG and LNG that is used as motor fuel would be to provide relatively small processing centers to process pipeline gas to make motor fuel having a Methane Number of no less than 80. The product could be sold directly to local users and could also be transported by truck to more dis- tant users.

References

1. Proposed Amendments to the California Alternative Fuels for Motor Vehicle Regulations, California Environmental Protection Agency, Air Resources Board, December 21, 2001, https://www.arb.ca.gov/regact/cng-lpg/isor.PDF 2. International Group of Natural Gas Exporters. Position paper on the impact of including methane number in natural gas regulation. http://www.giignl.org/publications/ position-paper-impact-including-methane-number-natural-gas-regulation 3. GPSA Engineering Data Book, 12th edition, Figure 23-2.

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