CNG Vehicle refuelling solutions - About CNG & NVG's
What is Natural Gas?
Natural gas is a naturally occurring Hydrocarbon formed over millions of years through the decomposition of organic matter such as trees and animals.
It is typically found trapped underground in reservoirs in either gas pockets or trapped in a substrate such as sand or oil, and is extracted either as a by-product of oil extraction or specifically as raw gas.
Methane is the main component of natural gas, and usually makes up over 90 percent of "pipeline quality" natural gas. Other hydrocarbons that may be found in small amounts of natural gas include ethane, propane, and butane.
Most natural gas consumed in the UK is currently domestically produced in the North Sea. Gas streams produced from reservoirs contain natural gas, liquids and other materials. Processing is required to separate the gas from the liquids and to remove contaminants. First the gas is separated from free liquids such as crude oil, hydrocarbon condensate, water and entrained solids. The separated gas is then further processed to meet the specified requirements. A dehydration plant controls the water content; a gas processing plant removes certain hydrocarbon components to hydrocarbon dewpoint specifications and a gas sweetening plant removes hydrogen Sulphide and other sulphur compounds when present. Natural Gas is odourless so odourants are added to give it the familiar 'gas' smell.
What is CNG?
CNG stands for Compressed Natural Gas and is the term used for methane or natural gas that has been pressurised to elevated pressures (typically 250 bar). Natural gas remains in a gaseous state at all times during this pressurisation process.
In order to maximise the quantity of stored gas within a given volume (say in a fuel tank) the gas is pressurised to very high pressures and stored in specialist storage cylinders.
The gas is compressed using high pressure reciprocating (piston) compressors.
What are NGV’s?
NGV’s are Natural Gas Vehicles and are standard vehicles that have been modified to run on CNG.
There are three types of NGV’s: Dedicated, Bi-Fuel and Dual Fuel. Dedicated vehicles run on natural gas only. Bi-Fuel vehicles operate on CNG whilst retaining the ability to use petrol as a reserve fuel. The engine can operate on either fuel but not on both simultaneously. The compression ratio of the engine must remain at a level suitable for petrol. Currently this type of engine is used almost exclusively on vehicles below 3,500kgs. Dual Fuel engines are derived from diesel engines. A small amount of diesel is retained as a pilot source of ignition. The primary fuel Natural Gas is mixed with the incoming air. Dual Fuel engines are auto ignited by compression and require no spark plugs.
 CNG forklift truck - Qantas freight depot Australia |
The compressed natural gas is stored on board the vehicle in cylinders installed in the rear, undercarriage, or on top of the vehicle. Natural Gas is fed to the engine through a high pressure regulator (most often located in the engine compartment), where the pressure is reduced. In carburetted engines, the fuel enters the carburettor (through a special fuel/air mixer) at close to atmospheric pressure through a specially designed natural gas mixer where it is properly mixed with air. In fuel injected vehicles the natural gas enters the injectors at relatively low pressure (up to about 6 bar, 90-100 psi). In either case, natural gas then flows into the engine's combustion chamber and is ignited to create the power required to drive the vehicle. Special solenoid valves prevent the gas from entering the engine when it is shut off. |
 Onyx CNG refuse disposal truck - Paris France |
In bi-fuel vehicles, a fuel selector switch controls the flow of either natural gas or petrol. (In some systems the switchover is done automatically when the vehicle is out of natural gas.) A fuel gauge is provided on the dashboard or it is incorporated into the normal fuel gauge so the driver can determine the amount of natural gas remaining in the fuel tanks.
What are CNG refuelling stations?
CNG Refuelling stations are the collective term for the complete installation required to refuel a vehicle using compressed natural gas, and is made up primarily of a gas supply, gas compressor, storage cylinders, priority panel and dispenser.
CNG stations come in many shapes and sizes but all contain these basic components.
Refuelling stations fall into two main groups, Time Fill (or Slow fill) and Fast Fill.
Time Fill stations use the compressor to directly fill the vehicle over a period of time usually whilst the vehicle is parked up overnight. These stations therefore do not require expensive electronic dispensers, but have the limitation that they are not suited to public refuelling or forecourts.
This type of station is suited to large bus depots where all the vehicles come back to a known base every night.
Fast Fill is the more common approach for light vehicles such as cars and taxis, and is suited to the petrol forecourt approach.
For these stations the gas is dispensed from static storage cylinders which hold the gas at high pressures (250 bar). The gas compressors keep the pressure topped up at all times by periodically cutting in and out as and when required to replenish the gas taken out by each vehicle filling.
This set up is ideally suited to public stations or where a fast fill is required and the vehicle has to move on to another location away from the gas station.
Fast fill stations are suited for cars, taxis, vans, refuse collection vehicles and HGV’s but any kind of vehicle can be refuelled this way.
Safety
CNG, which is always a gas, is safer than traditional liquid fuels such as petrol and diesel for a number of reasons. Firstly, in the event of a fuel leak, the gas will dissipate rapidly upwards whereas with liquid fuels they will pool on the ground creating a potential fire hazard.
Secondly, CNG has a much higher ignition temperature than liquid fuels (580 DegC as opposed to 220 DegC for petrol), therefore less potential ignition sources are likely to be present in the event of a gas leak.
Thirdly, CNG has a narrow range of flammability, igniting only when the concentration in air is between 5 and 15% (petrol is between 1 and 8% mix in air).
Natural Gas is also non-toxic or corrosive and will not contaminate ground water.
Natural gas combustion also produces no significant aldehydes or other air toxins, which are a concern in gasoline and some other alternative fuels.
Another safety feature that NGV’s have is that the storage cylinders both on the vehicles and at the refuelling stations are made from very strong material and are protected by pressure relief and other safety devices.
The design of the cylinders is subject to a number of severe tests such as fire, collision, mechanical damage etc.
The refuelling process at the dispensing pump is also very safe as the whole system is “sealed” which prevents any leakage or spillage of product.
Alternative fuels
Alternative fuels for transport have been considered for many years.
It is not new technology, just realisation that crude oil reserves are finite and dependency on oil has other implications, such as security of supply and price issues.
The main alternative fuels are:
- Ultra Low Sulphur Diesel - This is diesel fuel that meets either the Euro4 specification or the fuel spec proposed by the Commonwealth for implementation 4 in 2006. Euro4 fuel specs were published in Directive 98/70/EC of the European 5 Communities in 1998, and set the sulphur level as being lower than 50 ppm. This has already replaced conventional diesel which has a level of >1000 ppm and will replace low sulphur diesel which is now widely available but which has a sulphur content of <500 ppm.
- Biodiesel & Biogas - Biodiesel is a generic name for fuels obtained by transesterification of a vegetable oil. This produces a fuel with very similar combustion properties to pure diesel, but with lower viscosity. Biodiesel often refers to rapeseed oil methylester (RME), the main European Biodiesel. Esterified Soybean oil is the main United States source of such fuel, called Soy Diesel.
- Liquefied Natural Gas - LNG is also Natural Gas but in a liquefied state. Methane liquefies at –161 Deg C and is generally refrigerated to –180 Deg C for liquefaction and requires vacuum-insulated cryogenic tanks to maintain it in liquid form for storage.
- Liquefied Petroleum Gas - LPG consists mainly of propane, propylene, butane and butylene, in various proportions according to its place of origin. The components of LPG are gases at normal temperatures and pressures, but can easily be liquefied for storage. This is achieved by either an increase in pressure to about 8 atmospheres or by a reduction in temperature.
- Hydrogen - Hydrogen is the chemical element with the smallest molecular mass and is not found as a free element on earth. Because of its high reactivity, it is always bonded to other molecules and as a result hydrogen for automotive use has to be man made. Vehicles can burn pure hydrogen in an internal combustion engine, or use it in a fuel cell to drive an electric motor. The fuel cell option is generally considered preferable for the long term because although it requires more complex changes to existing vehicle design, it allows for higher efficiency and hence a longer range on the same amount of fuel.
- Electric - Many electric vehicles are in use such as Forklift trucks, milk floats and some small passenger vehicles etc. Most electric vehicles suffer from a reduced operating range and require frequent recharging and therefore limit this option to localised inner city transport. Hybrid vehicles, such as Electric/Gas will help to increase the range of these vehicles in the future.
Environmental issues
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There are many reasons why someone would want to invest in alternative fuels, the main drivers being; Environmental, Economical, Entrepreneurial and Technology Evolution. Running vehicles on natural gas can reduce or eliminate:
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The reduction in harmful emissions can be significant:
- Carbon Dioxide – 22-24% reduction against petrol, 10% reduction against diesel
- Carbon Monoxide – 76% reduction against petrol, CNG & Diesel both low
- Nitrogen Oxides – 83% reduction against petrol, 80% reduction against diesel
- Hydrocarbon – 88% reduction against petrol, 80% reduction against diesel
- Benzene – 99% reduction against petrol, 97% reduction against diesel
- Lead – 100% reduction against petrol, not applicable for diesel
- Particulates – not applicable for petrol, Eliminated for diesel
- Sulphur – nearly 100% reduction against petrol, nearly 100% reduction against diesel
Natural Gas is slower burning than petrol or diesel resulting in reduced wear and increased engine life.
There is no lead in natural gas therefore the lead fouling of spark plugs is eliminated and life extended by 2 to 3 times.
Large CNG refuelling stations give added justification for expenditure on pipeline infrastructure.
Improved resource utilisation – national support for NGV’s because of economics and the substitution of an indigenous fuel for oil.
Compressed Natural Gas is between 30% and 60% less than conventional fuels
Security of Supply – A country dependent on imported oil can provide some protection in the event of international oil crises by shifting to indigenous natural gas for its transport.
