About CNG (Compressed Natural Gas) and LNG (Liquefied Natural Gas)
Natural gas is one of the cleanest, safest, and most useful forms of energy. It represents about 25 percent of all primary energy used in the United States. The natural gas used for NGVs is the same natural gas that is piped to millions of homes for cooking and heating. Information on the superior safety characteristics of natural gas can be found here. Natural gas is a very simple fuel. 
Around 90% of natural gas is methane (CH4),which is just one carbon atom with four hydrogen atoms attached, with the remainder comprising of propane, butane and other components. Composition varies according to the source of the natural gas.
The only simpler fuel available is hydrogen, which is technically an energy 'carrier' rather than an energy source itself. As yet there is no economic method of creating and distributing large quantities of hydrogen, so, until this occurs, natural gas will remain the clean fuel of choice for some time. Being rich in Hydrogen, natural gas is often used as a feedstock, which is one of the reasons why natural gas vehicles are often referred to as the 'pathway to the hydrogen economy'. Ultimately, an investment in natural gas infrastructure is an investment in hydrogen infrastructure.
Natural gas, although a fossil fuel, is considered to be an alternative energy source because it is a preferable alternative to oil. You may say, "Why is natural gas considered to be an alternative fuel? Isn’t gas a fossil fuel?" You are right – natural gas is a fossil fuel but it is different from gasoline, petroleum and coal because it does not contain the same harmful compounds found in other fossil fuels. Unlike gasoline, petroleum and coal, natural gas has a negligible sulfur dioxide content, does not contain lead, has a low nitrogen dioxide content, a low particulate content, and a low carbon monoxide content. As well, natural gas does not require carcinogenic (cancer-causing) additives to boost octane levels because natural gas is naturally high in octane. In addition, natural gas is still abundantly available which means that it is practical to rely on its continued supply for hundreds of years into the future.
CNG - is natural gas under pressure which remains clear, odorless, and non-corrosive. Although vehicles can use natural gas as either a liquid or a gas, most vehicles use the gaseous form compressed to pressures above 3,600 pounds per square inch.
LNG - is natural gas in its liquid form. When natural gas is cooled to minus 259 degrees Fahrenheit (-161 degrees Celsius), it becomes a clear, colorless, odorless liquid. LNG is neither corrosive nor toxic. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is primarily methane with only small amounts of other hydrocarbons. LNG is an odorless, non-toxic and non-corrosive liquid, and if spilled, LNG would not result in a slick. Absent an ignition source, LNG evaporates quickly and disperses, leaving no residue. There is no environmental cleanup needed for LNG spills on water.
Benefits from using CNG (Compressed Natural Gas)
1. Making America less dependent on foreign oil is a national priority
2. Urban smog reduction
3. It’s safe and lighter-than-air, CNG is nontoxic and disperses quickly. It has a higher ignition temperature than gasoline and diesel fuel, which reduces the chances of accidental ignition.
4. Cost effective. Current prices are between $0.99 and $1.74 per gasoline gallon equivalent. Natural gas as a vehicle fuel actually costs less today than conventional gasoline
5. CNG burns cleaner and produces lower levels of harmful pollutants than gasoline or diesel vehicles
6. CNG vehicles reduce emissions of carbon monoxide and reactive hydrocarbons
7. CNG contains no particulates such as those associated with diesel fuel, and also reduces emissions of carbon dioxide - the principal "greenhouse" gas
8. Vehicle emissions are lower with natural gas than with gasoline because ignition temperatures are higher and combustion is more efficient
9. Engine maintenance cost can be reduced by extending time between oil changes because the particulate materials that are produced during the combustion cycle of gasoline engines and cause the engine oil to get dirty are not present in the CNG engine
10. Fewer environmental hazards combared to other fuels
11. Readily available. The United States has a huge natural gas resource base. For example, 22% of all new transit buses on order nationwide now are natural gas-powered.
12. It’s the most practical. Natural gas is the most efficient feedstock for the production of hydrogen. It’s part of an infrastructure that’s convertible to hydrogen. CNG stations can easily be converted to hydrogen delivery due to their common properties.
13. It’s possible to produce hydrogen-natural gas-blended fuel. A hydrogen/cng blend makes good economic and environmental sense.
14. All 50 states and all major metropolitan areas have access to natural gas and are not dependent on other countries
15. Natural gas is also piped directly to the fueling station, reducing the need to transport the fuel, as well as, eliminating the need of ordering and delivery schedules
16. It’s Cheaper Than Hydrogen: The projected cost of hydrogen currently is in excess of $8.00 per gallon.
Petroleum Displacement
Making America less dependent on foreign oil is a national priority. In his 2006 State-of-the-Union address, President Bush stated that we are “addicted to oil,” and his Administration is dedicated to reducing that addiction. Congress, too, is strongly on record in support of reducing petroleum use, and has passed a number of pieces of legislation to incentivize Americans to move to non-petroleum fuels. While in 2005, the U.S. imported over 65 percent of the oil it used, 97 percent of the natural gas used in the U.S. was produced in North America (85 percent from the U.S. and 12 percent from Canada). Every gallon equivalent of natural gas used in vehicles is one less gallon of petroleum that has to be imported.
Typical dedicated NGV's can reduce exhaust emissions of:
• Benzene emissions by 97% compared to diesel and 99% compared to gasoline
• Nitrogen oxides (NOx) more than 50% compared to diesel and 60%-90% compared to gasoline
• Carbon dioxide (CO2) by 10% compared to diesel and 30%-40% compared to gasoline
• Carbon monoxide (CO) by more than 90% compared to gasoline and diesel
• Non-methane hydrocarbons (NMOG) by 50-75% compared to gasoline
• Lead and sulfur emissions by 100% compared to both diesel and gasoline
• Smoke and particulate matter (PM10) up to 90% compared to diesel
Greenhouse Gases
Per unit of energy, natural gas contains less carbon than any other fossil fuel, and thus produces lower carbon dioxide (CO2) emissions per vehicle mile traveled. While NGVs do emit methane, another principle greenhouse gas, any slight increase in methane emissions would be more than offset by a substantial reduction in CO2 emissions compared to other fuels.
Emissions
Exposure to the levels of suspended fine particulate matter found in many U.S. cities has been shown to increase the risk of respiratory illness and other health problems. Much of the particulate matter in urban areas is due to transportation. Natural gas produces only tiny amounts of particulate matter. Natural gas is abundant, low-cost, and domestically produced.
Natural gas vehicles (NGV's) also produce far less urban emissions than diesel vehicles. For example, even when the stringent 2007 EPA heavy-duty engine emission standards become applicable, NGV's will be producing only one-sixth the NOx of comparable diesel engines. NGV's have a deserving reputation for being one of the cleanest transport fuels available, not just with respect vehicle emissions, but also for fuel production. Studies around the world consistently show that natural gas engines produce fewer harmful emissions than petrol, diesel or LPG, in spite of the considerable improvements made to these fuels in recent years. Even when measured against other 'clean' fuels or methods, such as gasoline-electric hybrids, NGVs emissions are frequently lower.
The world's cleanest internal combustion production line car is an NGV - the Honda Civic GX, available in the US. It has a dedicated engine which is reported in high polluting areas to produce exhaust emissions that are cleaner than the air going into the engine! The Civic can drive from the West Coast of the US to the East Coast and emit less non-methane hydrocarbons than if you were to spill one teaspoon of petrol!
Apart from producing fewer emissions, natural gas also poses fewer environmental hazards than other fuels. In the event of an accident, natural gas dissipates into the atmosphere rather than spilling on to the ground - a major benefit for our waterways and wildlife.
Emissions, what are they? The main emissions currently regulated throughout the world are:
Carbon monoxide (CO) - Carbon monoxide is a gas that in high concentrations can lead to asphyxiation.
Particulate matter (PM) - Particulates are any materials that are trapped by a gauze filter during emission testing. This means not only is it a measure of soot particles but also liquid aerosols that have been trapped. Particles from any source that enter the lung can often cause lasting damage. In addition, particles that are highly reactive are believed to cause further damage. Obviously the best thing is have the fewest particles possible to reduce any risk as much as possible. PM emissions have been the key focus of recent international emission standards.
Nitrogen oxides (NOx) - These are one of the constituents of photochemical smog/haze. In many areas it is the amount of NOx that is released that governs air quality. This is because there is already a large amount of HC in the air ready to react with the NOx. NOx has also been the key focus of recent international emission standards.
Hydrocarbons (HC), sometimes excluding Methane (NMHC) - As mentioned above HCs react with NOx to form smog/haze. As methane is a 'non-reactive' hydrocarbon, it is not involved in this smog forming process. When NGVs are discussed, the methane emission is usually excluded from the air quality measuring process.
Greenhouse' gas emissions - As CO2 is usually the more predominant greenhouse gas, greenhouse emissions factors are usually quantified in CO2 equivalents. As methane is also a greenhouse gas, any unburnt methane emissions are usually included in NGV greenhouse emissions calculations. Due to the increasing efficiency of natural gas engines, this problem is progressively being minimised.
About Natural Gas Vehicles
• There are over 150,000 NGVs on U.S. roads today and over 5 million worldwide.
• There are over 1,500 NGV fueling stations in the U.S. - over half are available for public use.
• Natural gas costs, on average, one-third less than conventional gasoline at the pump.
• Over 50 different manufacturers produce 150 models of light, medium and heavy-duty vehicles and engines.
• Roughly 22 percent of all new transit bus orders are for natural gas.
• Natural gas is sold in GGEs or gasoline gallon equivalents. A GGE has the same energy content (124,800 BTUs) as a gallon of gasoline
Were does Natural Gas come from? Most natural gas consumed in the United States is domestically produced in 2 ways.
1. Gas streams produced from fossil fuel reservoirs contain natural gas, liquids, and other materials. Processing is required to separate the gas from petroleum liquids and to remove contaminants.
2. Biogas is a mixture of methane and other gases produced from the decomposition of organic materials. It is produced naturally in landfills, and from the processing of animal waste, sewage, crop waste, and cellulosic and non-cellulosic crops. An energy-wise and greenhouse gas-wise alternative is to capture the biogas from these renewable waste sources, convert that biogas to biomethane, and use the biomethane to displace petroleum or other fossil fuels in transportation or other energy applications.
A dehydration plant controls water content; a gas processing plant removes certain hydrocarbon components to hydrocarbon dewpoint specifications; and a gas sweetening plant removes hydrogen sulfide and other sulfur compounds (when present).
