Natural Gas Vehicles
A close relative of natural gas is called "biogas", produced by the anaerobic (oxygen free) decomposition of organic materials, which is approximately 60% methane, 30% carbon dioxide and 10% nitrogen and water vapor. As the most plentiful and technologically inexpense alternative source of methane, natural gas reserves could be supplemented by extracting biomethane from biogas locked in landfills, plant wastes, swamps ("swamp gas"), animal waste, sewage and other biomass sources.
Here are some data about natural gas in the USA from the CIA World Factbook as of October 2008:
According to the Pickens Plan website in 2008, US natural gas reserves are twice those of US petroleum (but it doesn't state whether these are proved or estimated), accounting for "our country's second largest energy resource" (presumably meaning wind is the first, but this is unstated). With numerous sources being discovered and new technologies coming on line, such as those to extract methane from biogas (with anaerobic digesters), methane hydrates, and coal gasification, both reserves and production could increase dramatically.
Of the 7 million NGVs worldwide in 2008, only 150,000 were in the US. While US manufacturers offer NVGs in Asia and Europe, and they have built some bi-fuel cars for the US market, incredibly the only production NGV auto available in the US in 2008 was the Honda Civic GX.
Conversion kits are available to make a gasoline engine run on CNG. In the US a conversion costs between $8000 and $18000 with an EPA certified kit, but this cost can offset by federal and state tax credits of up to $32000. A big part of the conversion cost is to offset the EPA certification, and in some states such as California a state certification, which the OEM must pay.
There are fewer CNG fueling stations than gasoline stations but operators of NGV fleets, such as urban bus lines, delivery truck and taxi cab companies often have their own CNG fuel pumps. There are also appliances for CNG fueling at home which connect to the existing natural gas supply and compresses it as CNG into the NGV tank.
Natural gas reserves are measured in volume, trillions of cubic feet or meters, but it is also measured in terms of energy content, typically in British Thermal Units (BTUs) per cubic foot or therms. A therm is equivalent to 100,000 BTUs, the heat energy in about 100 (actually 96.7) cubic feet at standard temperature and pressure (STP). (In the US the National Institute of Standards and Technology (NIST) defines STP as 68°F and 14.696 psi which is one standard atmosphere (atm).) So according to this, there are about 1000 BTUs per cubic foot of natural gas at STP, but these figures are used by gas companies based on the Higher Heating Value (HHV). CNG engines are rated using the Lower Heating Value (LHV), which subtracts the energy of water vapor produced by combusting hydrogen, typically 10% of the energy measure at the HHV. Hence, CNG engine ratings are based on 900 BTUs per cubic feet.
At vehicle fueling scales CNG is measured and sold in gasoline gallon equivalents (GGEs), but the equivalency depends on calculations of the energy content in both CNG and gasoline which vary, and whether those calculations use HHV or LHV. Measurements of the energy in CNG, hence GGEs, are also inconsistent because CNG varies in quality and the pressure at which it is compressed from region to region and over time. With more impurities the energy content is lower and the GGE is higher. With more heavy hydrocarbons like butane present, the energy is higher and the GGE is lower. With higher pressure the energy is higher and the GGE is lower. With lower pressure the energy is lower and the GGE is higher.
Figures for gasoline BTU/gal. range from 114,100 (for regular unleaded), to 115,000 to 124,800. Figures for GGEs in cu.ft. range from 121.5 to 124.8 to 127.77. These differences can be confusing but keep in mind that some sources like gas suppliers use HHV while engine ratings use LHV. Using data from the US Dept. of Energy (DOE), we get the following figures:
| Heating Value | Gasoline | CNG | GGE |
|---|---|---|---|
| HHV | 124,800 BTU/gal. | 1000 BTU/cu. ft. | 124.80 cu. ft. |
| LHV | 115,000 BTU/gal. | 900 BTU/cu. ft. | 127.77 cu. ft. |
While CNG therms are calculated at a standard temperature and pressure (STP), commercial CNG is distributed at much higher pressures which can vary at the fuel station pump and into your NGV. Older NGV tank pressures were 2400, 2600, 2800 and 3000 psi, but the 2008 standard is 3600 psi. The GGE of 127.77 cu. ft. is based on the CNG heat energy at one atmosphere, or 14.969 psi. Compressed to 3600 psi that same 127.77 cu. ft. is reduced about 245 times to about .52 cu. ft. Since there are 7.48 gallons per cubic foot, at 3600 psi about 1.92 GGEs would fit in the same space as 7.48 gallons of liquid fuel. It takes about 3.89 times the volume to store a GGE than a gallon of liquid fuel.
These variables makes it difficult to compare CNG with gasoline or diesel economically, even when the relative costs per volume are known, which vary with supply and demand, taxes and tariffs, and whatever economic incentives might exist in your area. The economics of a particular NGV also depends on its mileage, just like a petroleum burning vehicle. In terms of GGE equivalents, the EPA estimates for a 2009 Honda Civic GX are 24 mpg (city), 36 mpg (highway) and 28 mpg (combined). The range per fill is somewhere around 200 to 250 miles.
According to the US Dept. of Energy (DOE) in October 2006, the national average for gasoline was $2.22/gal, and the average price for CNG was $1.77/GGE (80% of gasoline). By July 2007 gasoline had risen to $3.03/gal while CNG was $2.10/GGE (69% of gasoline). A look at CNG price websites in October 2008 show prices in southern California ranging from $1.75 to $3.00 per GGE. Assuming the same GGE mileage the cost per mile for an NGV clearly beats that for a gasoline vehicle.
Natural gas vehicles have many environmental benefits over gasoline and diesel, including:
Where natural gas (and "biogas"), i.e. methane, is plentiful, produce natural gas vehicles (NGVs), and where feasible convert existing vehicles that run on petroleum products (gasoline and deisel) to natural gas. Concentrate these efforts on fleet vehicles owned by large organizations such as governments.
