w5s6 gaseous fuels

© IFPEN / IFP School 2014

Sustainable Mobility Technical and environmental challenges for the automotive sector Week 5 – Session 6 – Gaseous fuels Ludivine Pidol

W5 – S6 –Gaseous fuels p. 1 © IFPEN / IFP School 2014

Context One of the key drivers for the transport sector is the reduction of CO2 emissions. CO2 emissions can be decreased thanks to adapted human behavior such as car sharing or intelligent driving. A better engine efficiency also allows CO2 emissions to be decreased. As we have already seen, renewable fuels such as biofuels are also a good solution in decreasing CO2 emissions and the last way to reduce CO2 emissions is the use of low carbon content fuels with fossil origins. These alternative fuels are, for example, compressed natural gas (CNG), liquefied petroleum gas (LPG) or hydrogen H2. CNG, LPG Compressed natural gas CNG is mainly composed of methane CH4. LPG Liquefied petroleum gas, also called autogas, is mainly composed of propane (C3H8) and butane (C4H10). CNG and LPG are used in Spark ignition engines.

One of the main interests of these gaseous fuels is their quite good well to wheel balance. This figure, from the Joint Research Center, presents the well to wheel balance for many types of fuels. The Greenhouse gas emissions balance is on the Y axis and the X axis represents the energy balance. Today, Well To Wheel Greenhouse gas emissions for CNG lie between gasoline and diesel. Beyond 2020, greater engine efficiency gains are predicted for CNG vehicles and Well To Wheel Greenhouse gas emissions will be close to those of diesel. The origin of the natural gas and the supply pathway are critical for the Well To Tank Greenhouse gas balance. LPG provides a small Well To Wheel Greenhouse gas emissions saving compared to gasoline and diesel.


The decrease of CO2 emissions for these gaseous fuels is due to their higher Hydrogen Carbon ratio, as shown in this table. These gaseous fuels also display a high knock resistance, as shown by their high octane number. Lastly, relevant properties of these fuels are related to the fact that they are gaseous. Their storage limits the autonomy of the vehicle. Liquid hydrocarbon fuels remain the most efficient in terms of energy content, far in front of gaseous fuels.

So, CNG and LPG are interesting products as they lead to low emission levels, especially for CO2. The resource, natural gas, is widely available. Interesting taxation is proposed in some countries. For example, in Europe, the price of LPG is forty to fifty % lower than the price of gasoline. Also, in Europe, with CNG, you can drive twice as many kilometers for the same price as gasoline. Nevertheless, the use of such gaseous fuels requires adapted engine calibration and the storage and distribution network remain the main challenges for the development of these fuels.


Today, LPG is not used very much for transportation; it is dedicated to domestic usage and chemistry. Concerning CNG, the natural gas vehicle sales are directly linked to the number of refueling stations and the fuel price. This map shows the number of natural gas vehicles worldwide. In 2013, there were 17.7 Million natural gas vehicles worldwide, including 16.3 million passenger cars. The development of natural gas vehicles is really relevant as in 2004, only 4 million vehicles were available.

Hydrogen Hydrogen is used for fuel cell vehicles. Hydrogen is converted into electricity for the motor using a fuel cell. A fuel cell is a device that converts the chemical energy from a fuel (hydrogen in this case) into electricity through a chemical reaction with oxygen.

There are many types of fuel cells, but they all consist of an anode, a cathode and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The only emission is water. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. The fuel cell mainly used in transport applications is the Proton Exchange Membrane Fuel Cell.


Hydrogen can be produced from several resources, such as natural gas (mainly methane CH4), coal or biomass (for example, wood). In order to produce hydrogen, two main routes exist. The first one is thermal production. It consists of a chemical transformation process. It generally involves a decarbonisation of a hydrocarbon or organic feed stock and a splitting of water. The second route is electrolysis which uses electricity to split the water molecule. The most widespread process to produce hydrogen, is steam reforming of natural gas. The catalysed combination of methane and water at high temperatures produces a mixture of carbon monoxide and hydrogen. This mixture is known as syngas. Many potential hydrogen production routes exist. The well to tank balance for Greenhouse gas emissions is critically dependent on the pathway. Indeed, hydrogen produced from wood has a very low well to tank balance compared to hydrogen coming from natural gas and the worst case is hydrogen coming from coal without CO2 storage. Despite technical advantages such as no exhaust emissions and interesting energy conversion efficiency, fuel cell electric vehicles remain prototypes and demonstration cars. The present cost of fuel cell vehicles greatly exceeds that of conventional vehicles, in large part due to the expense of producing fuel cells. As for the other gaseous fuels, storage and the refueling network are the key points for future development. Hydrogen is a highly explosive gas that will have to be treated with care when using it to fill your vehicle. It requires high tankage weights and high storage vessel pressures. Large investment in infrastructure would be required to fuel vehicles. The tank to wheel balance of hydrogen is very interesting. But, concerning the well to tank balance which means the production of hydrogen, the greenhouse gas emissions level strongly depends on the hydrogen production route.

w5s6 gaseous fuels

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