The (large) gas family – Part 1

Gas, methane, autogas, propane, natural gas, biogas, butane, biomethane…

This week we are continuing with the topic of ‘what exactly is gas’. We have already discussed some fundamental topics such as what is gmobility, what is biomethane, how gmobility is used in transport and others.

Nevertheless, it is now time to talk about the gas family since apart from methane (gmobility), there are other gases used in transport too.

So, you might have heard the terms ‘gas, methane, autogas, propane, natural gas, butane…’ and probably some others too. Probably you are wondering – if all these are gases, aren’t they all the same?

Or are they somewhat (or completely) different?

It certainly is easy to speak about gas for mobility but… what do we actually mean by ‘gas’? Let’s explain what gas is when referring to transport.

From one side we have natural gas, mainly composed by methane (CH4 for those of you who were great at chemistry in school). It sold at the gas station as CNG (Compressed Natural Gas) for light-duty vehicles or, for long-haul truck applications, as LNG (Liquefied Natural Gas).

But at many gas stations, you can also find LPG (aka Autogas)… What is it exactly?

LPG stands for Liquified Petroleum Gas, and it is mainly composed by butane (C4H10) and propane (C3H8). A key characteristic of LPG is that it in gaseous form at ambient pressure, however, can also be stored under a liquid form in a pressurized tank.

The level of pressure inside the tank depends on the ambient temperature and from the butane/propane ratio, but normally sweeps from 2-3 bars under cold conditions going up to 10-12 bar during summer time.

It is well known also for domestic use (heating and cooking) and butane is what you have in your lighter (great to start your BBQ and avoid using your cigarette!).

LPG is normally produced from the oil refining process (so it is an oil-derived product), but it can also be produced while extracting natural gas from wells, where it is then separated.

LPG and CNG…are they friends or foes?

Actually, this is the wrong question!

As CNG, also LPG is considered a low carbon fuel: what does it mean?

Gasoline, diesel, LPG and CNG are hydrocarbon fuels, simply meaning that they are composed of hydrogen (H) and carbon (C). This last one transforms into CO2 during the combustion process. We can rank all of them according to the ratio between H and C. The higher the ratio, the lower the emissions of CO2 generated.

Here’s how they stack up in terms of hydrogen (H) to carbon (C) ratios:

  • gasoline and diesel: both around 1,86
  • LPG: 2,58 (with a 50/50 blend between butane and propane)
  • CNG is at 4,00.

As we said, the higher this ratio, the lower CO2 emitted. Therefore, this translates into a CO2 emissions reduction by 10% from LPG and by 23% from CNG compared to conventional fuels (petrol and diesel).

What about the renewable dimension – does ‘bioLPG’ exist?

While biomethane is easily produced from biogas as a natural product from many fermentation processes, production of LPG is very limited today (<0.1% over the total LPG consumption) and is mainly obtained as a co-product from the biodiesel production.

In the future, however, the development of advanced chemical processes on lignocellulose and/or waste could open a way for more significant production of bioLPG. We will have to wait and see how that one develops!

Euro/litre for LPG…Euro/kg for CNG…pricing cannot be more complicated!  

Yet again, another reason why you might get lost…

At the fuel station, LPG is sold as €/l and CNG as €/kg… so how can you know which of the two is cheaper???

To compare prices we must refer to the same content energy – makes sense, right? So, 1 litre of LPG contains 24,2 MJ energy, 1 litre of gasoline – 31,3 and 1kg of CNG – 47,5 MJ on average.

This is why when you are at the gas station you have to compare the fuel price taking into account that the litre of LPG contains about 23% less energy compared to gasoline and, at the same time, CNG contains about 50% more.

Do we need to have our calculator ready before even entering the station?  

In the EU, a measure has been recently approved to solve this issue – in addition to the fuel price indication, complementary information will be displayed, directly providing the value in €/100km.

But, is this the right indication you need? Well, yes and no. In fact, behind this indication, there is the assumption of typical fuel consumption (l/100km or kg/100km) but your car is probably a bit different than the typical model used…

Nevertheless, the good news is that even with the information about energy content above, you can get a pretty close picture of how much the difference actually is…so, now, you are ready to start!  


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