In my previous LNG project, we need to supply gas with specified minimum methane number and specified higher heating value.
What is methane number?
It is the measure of resistance of fuel gases to engine knock, known as detonation. Detonation is produced by the auto-ignition of fuel/air mixture ahead of the propagating flame. Detonation must be immediately stopped to avoid increase of wear and tear and engine damage.
If MN is too low, the detonation can reduce engine efficiency and increases the quantity of harmful engine emissions.
MN Calculation
I searched if there is formula for the calculation, but I cannot find it. MN is not gas thermodynamic property, so no equation of state can be used to calculate it. I found there are several online methane number calculators. I tested three of them, Cummin, Wartsilla, and DNV MN calculator, respectively by using LNG Tangguh composition. These are the results.
Those three calculators give very slightly different results.
MN is determined by knock testing unit. The references used are:
- Pure methane is assigned as knock resistance reference fuel with MN of 100
- Pure hydrogen is assigned as knock sensitive reference fuel with MN of 0
There are several empirical models to measure MN of different artificial natural gas. Those models showed different results. The difference can be as high as 5 points.
Minimum Methane Number
ISO 15403-1 (Natural Gas for use as a compressed fuel for vehicles – Part 1: Designation of the quality) does not mention required minimum MN of gas. However DIN 51624:02 does mention.
Requirement for minimum MN is different from one country to another.
- Minimum MN in the USA is 75. But common practice is 80(90)
- Minimum MN in China is 90
- Minimum MN by Euromot (The European Association of Internal Combustion Engine Manufacturers) is 80
- Minimum MN refer to Wartsila engine is 80
Concern
A methane number of minimum 80 will endanger the security of natural gas market, as the supplier of this gas/LNG is limited. For example, in Indonesia, only Tangguh which has MN of min 80. In addition, limiting MN to min 80 will require expensive gas treatment to increase MN. GIIGNL recommend that if methane number should be included in regulation, it should be maximum 65 to ensure security of LNG supply and access to the market.
Reference:
You cannot arbitrarily limit the minimum methane number to 65. You cannot change the combustion characteristics of every spark ignited engine to allow for such low quality gas. If the LNG producers want to participate in the vehicle fuel market, they must remove the heavy hydrocarbons and make a compatible fuel (>80MN).