METHANE AS VEHICLE FUEL.

A fuel in expansion

The methane gas extracted from Lake Kivu could also be an excellent fuel for road vehicles. Methane as a fuel is known in France under the name GNV : vehicle natural gas. It has to be compressed at 200 bars in special reservoirs. It is stored and used in the form of a gas. The amount of gas in 1 m3 for one bar of pressure and at 20°C corresponds to about a litre of deisel fuel. The calorific energy of the gas (is) 8.9 to 12.8 KwH/m3 depending on the amount of other gases, particularly CO2, mixed with the methane.

Because of the high pressure for storing a reasonable reservoir, size limits methane-powered vehicles to short distance usage. Methane offers an attractive alternative to deisel and petrol for buses, taxis, vans for artisans, delivery, civil service etc...

Environmental attractiveness

The combustion of natural gas gives off less toxic pollutants. No sulphur, lead or benzene. No measurable particles. No smell or black smoke from the exhaust.

Possibility of reducing global emission of CO2 because this carburant contains the least carbon in its formula.

Chemical equation of the combustion of methane in air : CH4 +2(02 + 3.76 N2) = 2 H2O + CO2 + 2 53.76 N2)+ Energy

Chemical equation of the combustion of deisel in air : C12 H26 + 37/2(02+3.76N2) = 12 CO2 + 13 H2O + 37/2 (3.76 N2) + Energy

Gas powered vehicles

Methane gas is very high octane (120 - 130) which allows it to function with high output in spark ignition engines. To compare, the octane level of ordinary petrol in France is 95. The octane level measures the resistance to spontaneous ignition when petrol is compressed and heated. The higher the level, the more utilisation is possible in high output engines.

Light vehicles which are powered by natural gas have petrol engines equipped with gas injection. So that they can work with both carburants the volumetric ratio is adapted to the petrol. But this does not allow for the exploitation of the gas at its octane level - consumption could be less with an engine specifically for gas.

Heavy vehicles are equipped with deisel engines transformed into spark ignition engines : lowering of the volumetric ratio, specific systems for starting and gas injection (the engine no longer works on a deisel cycle but on the spark ignition cycle). In this case the potential of the gas is reached but double carburation is impossible.

Gas storage in vehicles

The reservoirs for French urban buses are grouped by 7 (9 as an option) for a total volume of 882 litres (single tank volume 126 litres) at 200 bars, this giving the equivalent of 210m3 of gas at atmospheric pressure. This volume is equivalent to 210 litres of deisel fuel and gives the bus an autonomy of 300 to 400 km. These reservoirs in carbon fibre and epoxy resin (or aluminium) have a maximum service pressure of 288 bars and a minimum working pressure of 20 bars. Test pressure is 600 bars.

Light vehicles have a reservoir mounted behind the front seats. Some have the reservoirs mounted under the floor, thus retaining useful space (see van at top of page).

Distribution of methane carburant

A natural gas service station is more complex than its traditional cousin since the gas needs to be compressed.

There are two possible types of service station :

Simple compression (200 bars). Filling is thus rather slow (several hours). Parking places have to be created for the purpose of recharging.

Supercharging, followed by storing at high pressure (250 bars) allows for rapid refilling on dedicated lanes. This solution is obviously more expensive than the previous one;
A high capacity station (500 vehicles) costs around 500K€. A medium capacity station (50 vehicles) costs 80 K€.

The 'individual' compressor

This small scale apparatus permits the filling of a private car's tank in a few hours (about 5 hours). It is designed for small fleets of vehicles. In France it costs about 5000 €.

Safety

Because of its composition, natural gas is non toxic and lighter than air (0.55 density) and thus rises very quickly (0.8 m/s) in the atmosphere, this avoiding all accumulation at ground level in case of a leak. Do not confuse it with GPL (liquified petroleum gas : a mixture of butane and propane) which has a density of 2.15 and which spreads at ground level if there is a leak.

Risk of fire or explosion

In order to have combustion with natural gas there must be :

• A mixture of air and gas (between 5% and 15% of gas)
• A spontaneous ignition temperature of 540°C
• A confined space

The mixture of air and natural gas has a very small range of ignition and the temperature needed for spontaneous ignition is very high. Inflammation is highly unlikely with a vehicle.

Tests show that methane gas is one of the safest fuels.

Risk of asphyxiation/anoxia

Natural gas is not toxic

• 80/95% methane
• no toxic ingredient
• no carbon monoxide

If there is a leak, the natural gas can take the place of the air and thus deprive a person in an enclosed space of oxygen.

Because of the speed with which natural gas rises this risk is minimal in properly-ventilated areas and even less in vehicle users in the open.