Natural Gas Quality

Hannu Jääskeläinen

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• Introduction
• Quality from Well to Vehicle
• Natural Gas Quality Challenges
• Standards

Introduction

Natural gas, consisting primarily of methane, has a number of advantages compared to liquid fuels such as gasoline and diesel fuel. Its high H/C ratio gives it the potential for lower GHG emissions, it is very knock resistant and offers the potential to operate at higher compression ratio and potentially achieve higher thermal efficiencies. Last but not least, natural gas reserves are more plentiful than those of crude oil, suggesting that natural gas may power the world’s transportation systems well into the future.

However, to fully utilize the benefits of natural gas in vehicle applications, a number of fuel quality parameters need to be considered [4079][4096][4563]:

1. Hydrocarbons

   While natural gas is composed mainly of methane, it can have varying concentrations of other hydrocarbons up to C14. Light hydrocarbons are often considered as those with 1 to 6 carbon atoms while those with more than 6 are considered as heavy hydrocarbons. Common light hydrocarbons found in natural gas include ethane and propane. Heavy hydrocarbons are usually removed during processing to avoid hydrocarbon condensation or drop-out. Compressor oils can be introduced into natural gas during compression and typically have 12 to 15 carbon atoms but can include lighter or heavier hydrocarbons. The presence of hydrocarbons heavier than methane can affect the knock resistance of the fuel and affect air-fuel ratio control while some heavy hydrocarbons such as compressor oils can also foul fuel system components such as regulators or injectors and lead to operational difficulties.

2. Diluents

   Diluents are components that have no energy content and thus reduce the heating value of the gas. Common diluents found in natural gas include N₂ and CO₂ but He, CO and O₂ can also be present.

3. Hydrogen

   While hydrogen contains energy, its energy content is relatively low compared to methane (10.8 MJ/m³ compared to 35.8 MJ/m³) and it can thus be considered as a diluent as well. Also, if present in sufficient quantities, hydrogen can lead to embrittlement of steel tanks and their premature failure.

4. Water Vapor

   While water vapor is also a diluent, there are additional concerns. Water vapor can precipitate out as liquid at low temperature and/or at high pressures and collect in tanks and overload dehydration equipment. When combined with trace levels of other common species found in natural gas such as H₂S and CO₂, the resulting acid can lead to corrosion of pipelines and high pressure storage tanks. Ice formation or formation of hydrates that can also lead to operation problems with valves and regulators or restrict gas flow.

5. Sulfur Compounds

   Sulfur can be found in natural gas in several forms including: H₂S, mercaptans, sulfides, thiophenes, elemental sulfur (S₈) and iron sulfide. Some of the sulfur occurs naturally but common odorants such as mercaptans, sulfides and thiophenes that are added to natural gas for safety also contain sulfur. Excessive sulfur compounds can foul equipment or lead to corrosion problems. Sulfur can also deactivate exhaust aftertreatment catalysts in NGVs.

6. Siloxanes

   Biogas can contain compounds such as siloxanes not normally found in fossil-sourced natural gas. Siloxanes can be very harmful to exhaust catalysts and switching type exhaust gas oxygen sensors used in many NGVs [4096].

7. Heating Value

   The heating value (HV) of natural gas is strongly tied to the Wobbe index discussed below. The maximum higher heating value of natural gas expressed on a volumetric basis, i.e., MJ/m³ or Btu/scf, is normally limited to an upper value. Increased concentrations of non-methane hydrocarbons would increase the heating value of natural gas and may need to be accompanied by increased concentrations of diluents to ensure the maximum heating value is not exceeded.

8. Specific Gravity

   The specific gravity (SG)—the ratio of the density of a natural gas blend to the density of air—is typically around 0.6 to 0.7. Pure methane has a specific gravity of 0.554 while for ethane and propane the values are 1.04 and 1.52 respectively. Increased concentrations of non-methane components with a higher molecular mass than methane increase the specific gravity of natural gas relative to pure methane.

9. Wobbe Index

   The Wobbe index is defined as:

   WI = HV/√SG(1)

   In the United States, the higher heating value in Btu/scf is commonly used while in many other parts of the world, MJ/m³ is used. WI can also be calculated using the lower heating value. The units of WI are thus the same as HV but in many cases, the units are left out.

   Wobbe index is a measure of rate of thermal input through a fixed orifice or nozzle in a stationary burner. It is the primary measure used to assess the interchangeability of gas supplies for end-use applications. By keeping Wobbe index constant, periodic adjustments to gas burning appliances can be minimized or avoided. For gas engines, Wobbe index is related to engine power and air-fuel ratio as illustrated in Figure 1 [4079]. For higher WI fuels, the amount of oxygen required to combust a give volume (or number of moles) of fuel increases.

10. Methane Number

    Methane number (MN) is analogous to the octane number for gasoline and reflects the anti-knock behavior of natural gas. It is based on work at AVL in the 1970s where pure methane has MN=100 and pure hydrogen has MN=0. Hydrocarbons heavier than methane have MN < 100 and they tend to degrade the anti-knock characteristics of natural gas. However, it should be noted that ethane and propane still have anti-knock characteristics superior to that of gasoline. Methane number is typically calculated from the analysis of fuel composition.

    To some extent, specifying a maximum heating value or a maximum value for higher order hydrocarbons such a those with 4 or more carbon atoms can address the knock characteristics of natural gas as both approaches would limit the amount of higher order hydrocarbons which are most susceptible to knock [4082]. The combination of compression ratio, EGR, ignition timing, and valve timing employed on some modern light-duty SI natural gas vehicles have been shown to be capable of accommodating fuels with methane number as low as 60 (and possibly lower) without showing any indications of knock [4080]. A methane number of 60 was the lowest value noted in a 2015 survey of US natural gas data from 2013 [4080]. However, some high BMEP stationary engine makers state that a minimum 80 MN is required [4090]. Further discussion can be found in the literature [4080][4096][4083][4084].

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