Green Alternatives - Compressed Natural Gas

Natural gas is a mixture of hydrocarbons, mainly methane, and is produced in the gas fields or in conjunction with crude oil production. The interest for gas as an alternative fuel stems mainly from its clean burning qualities. Because of the gaseous nature of this fuel, it is stored onboard the vehicle in a compressed gaseous state.

Natural gas is consumed in residential, commercial, industrial and utility markets and is domestically produced.

Gas streams produced from reservoirs contain natural gas, liquids and other materials.

Processing is required to separate the gas from the petroleum liquids and to remove contaminants. First the gas is separated from the liquids such as crude oil, hydrocarbon condensate, water and entrained solids.

The separated gas is further processed to meet specified requirements. For example, natural gas for transmission companies must generally meet certain pipeline quality specifications with respect to water content, hydrocarbon dew point, heating value, and hydrogen sulphide content. As raw gas is odourless a chemical odorant is generally added prior to entering the distribution system.

Tailpipe emissions of methane the principal component of natural gas are not as well controlled by catalytic converters, and the odorant added to CNG can have a very detrimental effect on the catalytic converter. It is therefore very important that tailpipe losses of unburnt fuel and fugitive/evaporative losses are minimised.

Natural gas vehicles have the potential to significantly reduce pollutants such as CO, NMHCs, SOx, particles and odour. The situation with NOx is less clear and the effects of formaldehyde have yet to be determined.

Natural gas has only short-term sustainability with reserves expected to be depleted by the end of this century. Storage volume of CNG takes 3 to 4.5 times more space than diesel, and the extra weight of the fuel tank results in less payload or reduced fuel consumption. It also requires special refuelling stations that would possibly need a new gas infrastructure.

Liquefied Natural Gas

Liquefied natural gas is starting to be used as a heavy vehicle fuel. The low temperature facilities that are required are expensive, and their manufacture installation and operation increases the life-cycle emissions of greenhouse gases.

The lifecycle emissions of LNG are comparable with CNG, except that CO2 will be higher due to the increased energy needed to liquefy the natural gas.

The LNG driving range is limited because its energy content per volume is relatively low. It requires special refuelling stations and handling of cryogenic liquid, making it suitable only for fleet operators