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Hydrocarbon Engineering,

Economic considerations dictate that fuel combustion for power generation and transportation must be as efficient as possible. Combustion efficiency is determined by how completely a fuel burns, while greenhouse gas (GHG) emissions are measured by mass per unit of energy produced. Stoichiometric combustion converts all the hydrocarbons to carbon dioxide and water, i.e.:

CH4 + 2O2 ---> CO2 + 2H2O

Combustion efficiencies depend on the design of the combustor and on the fuel/oxidant ratios in the process; the optimal fuel/oxidant ratios for a fuel depend on its physical and chemical properties. Natural gas or LNG fuels have lower GHG emissions than oil or coal.1 These and other factors have quadrupled the use of natural gas for energy production since 1985.2 Natural gas is rapidly displacing coal-fired generation today and is expected to do so well into the future. LNG is finding ever-increasing use as a transportation fuel, especially as a low emission replacement for high sulfur fuel oil (HSFO) in marine applications that must meet IMO2020 regulations.

Natural gas and LNG as fuel

Natural gas and LNG fuels are mixtures of light hydrocarbons, typically C1 – C5 alkanes, CO2, and nitrogen. The relative amounts of the gases in the fuel varies, depending on the source and changes that arise from differential boil-off during storage and transportation. Different compositions in natural gas and LNG produce different combustion characteristics. This is, in part, due to the fact that the energy density of the fuel is the sum of the recoverable energy from chemical combustion plus any energy recoverable by returning all reaction products to 25°C or other standard temperatures (this is the higher heating value [HHV] of the fuel). For example, the HHV for methane is 55.5 MJ/kg. Other light alkane gases have different HHVs (ethane, C2 – 51.9 MJ/kg; propane, C3 – 50.35 MJ/kg; butanes, C4 – 49.50 MJ/kg; pentanes, C5 – 49 MJ/kg; etc.). The energy density of natural gas or LNG fuel depends, therefore, on the relative proportions of C1 to C5 hydrocarbons in the fuel.

Internal combustion engines for power generation must be tuned to the knock resistance of the fuel for safe and efficient operation. The methane number (MN) is used as a metric for the knock resistance of natural gas fuels. It is similar to the octane number used to indicate the quality of gasoline in that it measures the ability of the fuel to undergo compression before it ignites. Fuel/air mixtures that ignite at the wrong point in the ignition cycle produce engine knock. While mild engine knock may only increase pollutant emissions, severe knock can physically destroy an engine.3 For this reason, engine manufacturers typically tune engines to the worst-case fuel scenario at the expense of engine performance.3 The MN for natural gas fuels ranges between 60 to 100, while that for LNG ranges between 50 to 95.4 Convenient MN calculators are available (based on the relative concentrations of C1 – C5 in the fuel) from Cummins, Wärtsilä, and DNV-GL.5,6,7 The ability to determine the MN of a fuel in real-time is therefore highly desirable, since it would allow engines to accept a wider variety of natural gas and LNG fuels while still maintaining optimal engine performance.

Written by Roberto Bosco, MKS Instruments, Inc.

This article was originally published in the October 2020 issue of Hydrocarbon Engineering. To read the full article, view the full issue here. The issue also includes articles on process safety management and implementation, water treatment, sulfur, LNG, simulation, and methods to optimise refinery hydrogen production on steam methane reformers.

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