Today we take a day off from future outlook, projections, uncertainties, and all that. Today, like good engineers, we look at the facts. And we start with natural gas at ambient temperature and pressure.
Natural gas is primarily methane, with small portions of heavier gases. Methane is a hydrocarbon gas with formula CH4, i.e. it is one carbon atom and 4 hydrogen atoms in the molecule. This formula is important to remember when we get to the environmental comparison to other hydrocarbons. Some properties of methane (ref Wolfram Alpha):
- Molecular weight: 16.0425 g/mol
- Density: 6.67151*10^-4 g/cm3
- Boiling point: -161,48 degrees celsius
- Vapor density: 0.55 relative to air
Since what we need the natural gas for is to produce energy, we are particularly interested in the heat of combustion values (ref Wikipedia):
- Specific heat of combustion for natural gas 54 kJ/g
- Specific heat of combustion for gasoline 47 kJ/g
- Specific heat of combustion for diesel 45 kJ/g
So, clearly natural gas offers more energy per mass than the other common fuels.
Back to the molecule. The basic hydrocarbon molecules are chains of carbon atoms connected together, and hydrogen atoms connected to the free spots along the chain. Then, by simple math, we see that with only one carbon atom in the chain you get the best ratio of hydrogen atoms per carbon atoms in the molecule. This translates directly into less CO2 produced in the combustion, simply because there are fewer carbon atoms available.
The emission reductions compared to oil products will broadly speaking be as follows, but this is dependent not only on basic chemical properties, but also on engine technology, combustion characteristics, etc:
- NOx: 85% reduction
- SOx: ~100% reduction
- Particles: ~100% reduction
- CO2: 15-20% reduction
So what has LNG got to do with this? LNG is just a transportation mode, it is not a separate source of energy, it is simply natural gas made liquid for transportation phase, It is regasified before it is used. The reason why it is made liquid is that methane requires 600 times more volume in gas phase than in liquid phase. Hence it is much more efficient to transport it in liquid phase.
In order to get the methane into liquid phase it needs to be cooled down below its boiling temperature of -161 degrees celsius. The LNG is transported at this temperature and at atmospheric pressure. This means the liquid methane is continously boiling during the transportation, and as long as the vaporized gas is released from the tank the pressure will not increase. This is just like boiling water at 100 degrees celsius; the temperature will not increase above 100 degrees, but there will be less and less water left in the pan.
Finally, we better touch upon the safety related properties of LNG. It is important to understand that LNG can not explode or burn. LNG first needs to evaporate and mix with air in gas phase. So if there is a leak and the vapour cloud mixes with air it will form an ignitable cloud when the mix contains between 5 and 15 % methane. Below 5% mix will not be enough too ignite, and above 15% will too much to ignite. Also, in order to get an explosion, instead of just a fire, the vapour cloud needs to be contained in an enclosed area.
There is much more details to discuss on each of the topics above, but this post was intended to be a quick intro, so I think we stop here.