Saturday, April 20, 2019
CNG

CNG

Compressed Natural Gas Fueling Stations

Unlike gasoline or diesel stations, compressed natural gas stations (CNG) are not "one size fits all." Building a CNG station for a retail application or a fleet requires calculating the right combination of pressure and storage needed for the types of vehicles being fueled. Making the right choices about the size of compressor and the amount of storage at the station will make a big difference in the cost of fuel and range for vehicles.

Types of Stations

There are two types of CNG infrastructure: time-fill and fast-fill. The main structural differences between the two systems are the amount of storage capacity available and the size of the compressor. These factors determine the amount of fuel dispensed and time it takes for CNG to be delivered.

    • Fast-Fill CNG Station


      Fast-fill: Generally, fast-fill stations are best suited for retail situations where light-duty vehicles, such as vans, pickups, and sedans, arrive randomly and need to fill up quickly. The space needed to store the equipment measures about the size of a parking space. CNG can also be delivered via dispensers alongside gasoline or other alternative fuels dispensers. Fast-fill stations receive fuel from a local utility line at a low pressure and then use a compressor on site to compress the gas to a high pressure. Once compressed, the CNG moves to a series of storage vessels so the fuel is available for a quick fill-up.

       

      Drivers filling up at a fast-fill station experience similar fill times to a conventional gasoline fueling station—less than 5 minutes for a 20 gallon equivalent tank. CNG at fast-fill stations is often stored in the vessels at a high service pressure (4,300 psi), so it can deliver fuel to a vehicle faster than the fuel coming directly from the compressor, which delivers fuel at a lower volume. Drivers use a dispenser to transfer CNG into the tank. The dispenser uses sensors to calculate pressure and measure the number of GGEs delivered to the tank, taking temperature into account.

      Learn more about filling CNG tanks.

 

  • Time-Fill CNG Station


    Time-fill: Time-fill stations are used primarily by fleets and work best for vehicles with large tanks that refuel at a central location every night. Time-fill stations can also work well for small applications, such as a fueling appliance at a driver's home. At a time-fill station, a fuel line from a utility delivers fuel at a low pressure to a compressor on site. Unlike fast-fill stations, vehicles at time-fill stations are generally filled directly from the compressor, not from fuel stored in tanks. The size of the compressor needed depends on the size of the fleet. Although there is a small buffer storage tank, its purpose is not to fill vehicles, but to keep the compressor from turning off and on unnecessarily—wasting electricity and causing undue wear and tear on the compressor. The storage tanks are sometimes used to "top off" vehicle tanks during the day.

    The time it takes to fuel a vehicle depends on the number of vehicles, compressor size, and the amount of buffer storage. Vehicles may take several minutes to many hours to fill. The advantage of using a time-fill station is that the heat of recompression is less, so you usually get a fuller fill then with a fast-fill station. Also, with a time-fill station you can control when you fill the vehicles. This means you can instead choose to run the compressor during off-peak hours (like at night), to achieve lower electricity rates

    Time-fill stations are carefully architected based on the application they will be used for. For example, a transit bus company may need a larger compressor that can deliver 8 to 9 gallons per minute, while a refuse truck company can make due filling trucks at 3 gallons per minute using a smaller compressor. A consumer application may need far less—such as, less than half of a gallon an hour. These differences account for the large variance in the cost of installation.

 

Natural Gas Benefits and Considerations

Compressed natural gas (CNG) and liquefied natural gas (LNG) are clean, domestically produced alternative fuels. Using these fuels in natural gas vehicles increases energy security and can lower emissions. Using renewable natural gas provides even more benefits. Like any alternative fuel, there are some considerations to take into account when contemplating the use of CNG or LNG.

Energy Security

In 2013, the United States imported about 33% of the petroleum it consumed, and transportation accounted for more than 70% of total U.S. petroleum consumption. With much of the world's petroleum reserves located in politically volatile countries, the United States is vulnerable to supply disruptions. However, because U.S. natural gas reserves are abundant, this alternative fuel can be domestically produced and used to offset the petroleum currently being imported for transportation use.

Vehicle Performance

Natural gas vehicles (NGVs) are similar to gasoline or diesel vehicles with regard to power, acceleration, and cruising speed. The driving range of NGVs is generally less than that of comparable gasoline and diesel vehicles because, with natural gas, less overall energy content can be stored in the same size tank as the more energy dense gasoline or diesel fuels. Extra natural gas storage tanks or the use of LNG can help increase range for larger vehicles.

In heavy-duty vehicles, dual-fuel, compression-ignited engines are slightly more fuel-efficient than spark-ignited dedicated natural gas engines. However, a dual-fuel engine increases the complexity of the fuel-storage system by requiring storage of both types of fuel.

Lower Emissions

Compared with vehicles fueled by conventional diesel and gasoline, natural gas vehicles can produce lower levels of some emissions. And because CNG fuel systems are completely sealed, CNG vehicles produce no evaporative emissions.

Infrastructure and Vehicle Availability

A wide variety of new, heavy-duty natural gas vehicles are available from U.S. original equipment manufacturers (OEMs). Although the number of available light-duty natural gas vehicles from original equipment manufacturers is limited, the choices are steadily growing. For availability, see the Alternative Fuel and Advanced Vehicle Search or the Clean Cities 2015 Vehicle Buyers Guide (Coming Soon).

Fleets and consumers also have the option of reliably converting existing gasoline or diesel vehicles for natural gas operation using qualified system retrofitters. It is critical that all vehicle and engine conversions meet the emissions and safety regulations and standards instituted by the U.S. Environmental Protection Agency, the National Highway Traffic Safety Administration, the National Fire Protection Agency's NFPA 52 Vehicular Gaseous Fuel Systems Code, and state agencies like the California Air Resources Board.

Although the United States has an extensive natural gas distribution system in place, vehicle fueling infrastructure is limited. Therefore, fleets may need to install their ownnatural gas infrastructure, which can be costly. Finding partners who will commit to use the infrastructure can improve the payback period.

 

Natural Gas Fuel Basics

Photo of a natural gas fuel pump.

Natural gas is an odorless, nontoxic, gaseous mixture of hydrocarbons—predominantly methane (CH4). It accounts for about a quarter of the energy used in the United States. About one-third goes to residential and commercial uses, such as heating and cooking; one-third to industrial uses; and one-third to electric power production. Although natural gas is a clean-burning alternative fuel that has long been used to power natural gas vehicles, only about one-tenth of 1% is used for transportation fuel.

Between 80%-90% of the natural gas used in the United States is domestically produced. Most natural gas is drawn from wells or extracted in conjunction with crude oil production. Natural gas can also be mined from subsurface porous rock reservoirs through extraction processes, such as hydraulic fracturing (see a list of supplemental sources from the Energy Information Administration (EIA)). Renewable natural gas is an emerging fuel produced from decaying organic materials, such as waste from plants, landfills, wastewater, and livestock.

CNG and LNG as Alternative Fuels

Two forms of natural gas are used in vehicles: CNG and LNG. Both are clean-burning, domestically produced, relatively low priced, and widely available. Because of the gaseous nature of this fuel, when stored onboard a vehicle, it must be in either a compressed gaseous (CNG) or liquefied (LNG) state. CNG and LNG are considered alternative fuels under the Energy Policy Act of 1992.

Natural gas is sold in units of diesel or gasoline gallon equivalents (DGEs or GGEs) based on the energy content of a gallon of gasoline or diesel fuel.

Compressed Natural Gas

To provide adequate driving range for a vehicle, CNG is stored in cylinders at a pressure of 3,000 to 3,600 pounds per square inch. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a GGE basis. A GGE equals about 5.66 pounds of CNG. CNG is used in light-, medium-, and heavy duty applications.

Liquefied Natural Gas

LNG is produced by purifying natural gas and super-cooling it to -260°F to turn it into a liquid. Because it must be kept at cold temperatures, LNG is stored in double-walled, vacuum-insulated pressure vessels. LNG is good for trucks needing a longer range because liquid is more dense than gas (CNG) and, therefore, more energy can be stored by volume in a given tank. LNG is typically used in medium- and heavy duty vehicles; a GGE equals about 1.5 gallons of LNG.

 

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