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Methane Number 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 sin- gle 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 DOI: 10.7569/JNGE.2017.692506 J. Natural Gas Eng., Vol. 2, No. 2, December 2017 135 Gary Palmer: Methane Number 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 qual- ity standard such as establishing a defined Methane Number based on gas compo- sition. 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 liqui- fied 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 bil- lions 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 propri- etary method to calculate Methane Number. DOI: 10.7569/JNGE.2017.692506 136 J. Natural Gas Eng., Vol. 2, No. 2, December 2017 Gary Palmer: Methane Number 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. DOI: 10.7569/JNGE.2017.692506 J. Natural Gas Eng., Vol. 2, No. 2, December 2017 137 Gary Palmer: Methane Number 1.5.2 Conflicts 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.
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