We are seeing accelerating interest in the use of compressed natural gas (CNG) as a vehicle fuel in the U.S. This is being spurred by developments including 1) new methods of drilling that have greatly increased gas reserves and 2) by environmental pressure to reduce greenhouse gases. The purpose of this article is to acquaint potential CNG station developers with some of the issues to be dealt with and to provide an overall understanding of the workings of the station.
The following terms should be understood by anyone considering installing a CNG fueling station:
1) CNG: This is natural gas that has been highly compressed to reduce its volume to make it practical for application as a vehicle fuel. New CNG vehicles store natural gas at 3,600 psig. Older ones may use 3,000 psig gas.
2) Standard cubic foot (Scf): Since natural gas can be delivered or used at different pressures, its volume is usually corrected to standard conditions (60 degrees F and 0 psig) for sales purposes. Therefore, 1 Scf of natural gas will have the same fuel value regardless of the actual pressure.
3) Gallons of gasoline equivalent (GGE): In order to fairly compare the cost of CNG with gasoline, CNG is usually sold as GGE. This is the amount of natural gas that would have the same energy content as one gallon of gasoline.
Why use natural gas as vehicle fuel? The reasons can be summarized:
2. Clean burning
3. Low cost
Abundance: This must be viewed as availability relative to need. In 2009, the U.S. consumed 94.6 quads (quadrillion Btu = 1×1015 Btu). Of this, 23.4 quads were natural gas. Of the total usage, 28% was used in transportation of which 12% of that was used in aviation. Assuming that aviation would be the least likely transportation segment to switch to natural gas, then the total fuel usage for ground and water transport was 94.6(.28)(1-.12) = 23.3 quads.
Oil usage was 37% of the total energy usage, or 35 quads, of which 57% was imported (20 quads). Now, 70% of oil is used for transportation or 24.5 quads. Furthermore, coal usage was 21% of the total energy consumption or 19.9 quads.
A reasonable assumption might be that we wish to replace half of the non-aviation transportation segment and half of the coal consumption with natural gas. Then the demand for natural gas would be 23.3(.5) + 19.9(.5) = 21.6 quads added to the current demand of 23.4 quads or 45 quads. For simplicity, we have assumed a one-for-one replacement of natural gas for oil or coal.
The supply of natural gas within the U.S. is estimated at 2,590 Stcf (Tcf) . This includes both unproved and proved reserves. At roughly 1,000 Btu/Stcf, then we have 2,590 quads available. Thus, using the future demand scenario described above, we have a 2,590/45 = 58 year supply of natural gas.
Clean Burning: The transportation sector (particularly cars, trucks, and buses) is one of the greatest contributors to air pollution in the U.S. Emissions from vehicles contribute to smog, low visibility, and various greenhouse gas emissions. According to the Department of Energy (DOE), about half of all air pollution and more than 80% of air pollution in cities are from cars and trucks in the U.S.
Natural gas can reduce these high levels of pollution from gasoline and diesel powered vehicles. According to the EPA, compared to traditional vehicles, vehicles operating on compressed natural gas have reductions in carbon monoxide emissions of 90-97%, and reductions in carbon dioxide (greenhouse gas) emissions of 25%. Nitrogen oxide emissions can be reduced by 35-60%, and other non-methane hydrocarbon emissions could be reduced by as much as 50-75%. In addition, because of the relatively simple makeup of natural gas in comparison to traditional vehicle fuels, there are fewer toxic and carcinogenic emissions from natural gas vehicles, and virtually no particulate emissions .
Low Cost: In addressing the cost of CNG, to be fair, one must look at not just the cost of natural gas, but the costs of compression and dispensing and the added cost of the CNG vehicle. The gas itself can be purchased by the station at around $10 per thousand Stcf. However, taxes and equipment costs increase the price at the dispenser beyond this value. For the first quarter of 2011, average pricing for fuel in the U.S. was :
This represents a savings of $1.63/gal over gasoline and $1.98/gal over diesel fuel.
In considering whether a CNG vehicle would be cost-effective for the consumer, the main consideration is how much fuel will be burned annually. Currently, the added cost of CNG power is such that, on economic considerations alone, only vehicles with high annual fuel usage can justify the cost. Additionally, since there are relatively few CNG filling stations available, vehicles with a predictable daily travel pattern are best considered for CNG. These two criteria, high fuel consumption and predictability are best met by fleet vehicles such as transit buses, trash compactors, delivery trucks, airport shuttles, taxis, etc. and such vehicles often show paybacks of less than two years on the added cost of the vehicle.
As more fleet vehicles are converted and fleet filling stations allow for public fill options, we believe the criterion of predictable travel pattern will diminish. Furthermore, as CNG becomes more the norm for vehicle fuels, the added cost of the vehicles will be reduced. Considering the supply situation discussed earlier, CNG prices at the pump are expected to be more stable than gasoline well into the future. Government – both state and federal – subsidies for CNG fueling stations and CNG vehicles are aimed at addressing these issues, providing incentives to enable crossover to a CNG-based infrastructure.
Safety: CNG is one of the safest transport fuels available, generally regarded as safer than gasoline or diesel. In its natural state, methane is odorless. As a safety measure, the gas is odorized with mercaptans prior to distribution to provide a ready means of leak detection.
CNG has a high ignition temperature, about 1,200 degrees F, compared with about 600 degrees F for gasoline. It also has a narrow range of flammability, that is, in concentrations in air below about 5% and above about 15%, natural gas will not burn. The high ignition temperature and limited flammability range make accidental ignition or combustion of CNG unlikely. A leak, causing fuel contact with hot engine surfaces is much less likely to result in a fire if CNG is the fuel.
CNG has no known toxic or chronic physiological effects (it is not poisonous). Exposure to a moderate concentration may result in a headache or similar symptoms due to oxygen deprivation but it is likely that the smell would be detected well in advance of concentrations being high enough for this to occur.
CNG fuel systems are sealed which prevents any spills or evaporative losses. If a leak occurs due to an accident, a maintenance procedure or a fitting malfunction in an NGV fuel system, the natural gas will dissipate into the atmosphere because it is lighter than air. Natural gas is not toxic or corrosive and will not contaminate ground water. CNG combustion produces no significant air toxins, which are a concern in gasoline and some other alternative fuels. CNG fuel storage cylinders are much, much stronger than diesel or gasoline tanks, with the result that they are less likely to rupture in an accident.
In most circumstances, natural gas is delivered via underground pipeline networks. This method eliminates the need for road tankers to deliver fuel from the refinery, further enhancing the overall safety of CNG as a fuel .
Note, however, that the fact that natural gas is lighter than air leads to special problems in vehicle repair facilities that must be addressed. The vapors of liquid fuel are denser than air and tend to hover near the floor. Therefore, safety issues are related to eliminating ignition sources close to floor level. If natural gas-fueled vehicles are to be repaired, then the emphasis is on eliminating ignition sources near the ceiling or to maintaining proper ventilation. The heating system may have to be replaced to eliminate hot surfaces that could become ignition sources.
General Fueling Approaches
There are two general approaches to fueling vehicles with compressed natural gas – time fill and fast fill.
Time Fill: As implied by the name, time fill takes place over a period of hours, say overnight. The main reason for using this approach is to reduce the cost of equipment. One to several dozen vehicles are simply connected to the compressor output and left to fill overnight. This approach is generally applicable to small fleets which are returned to a central location daily. For larger fleets, the compressor size may approach that of a fast-fill system and the need for multiple dispensers may reduce or eliminate the cost savings in the dispensers. Still, logistics may favor time-fill. When a large number of vehicles – such as trash compactors – return to home base at the same time, they simply can be parked, connected to the fill system and left for the night, eliminating the need for queuing at the fast-fill dispensers.
Fast-Fill: Fast-fill systems are, from the consumer’s point of view, very similar to gasoline-dispensing systems. The customer scans a credit card, waits for approval, then removes the nozzle from the dispenser body and connects it to the vehicle fill port. Filling takes a few minutes, then stops automatically when the required vehicle pressure is reached.
There are two types of fast-fill systems – buffer and cascade. A qualified engineering partner can help identify which system is more appropriate for a given operation.
Equipment Selection And Station Design
Many factors must be considered when selecting and specifying the equipment and infrastructure for a CNG fueling station. All equipment should be designed and selected by an experienced engineering partner and in accordance with applicable codes. The major equipment components to be considered in planning a new CNG fueling station include the odorizer, gas dryers, compressors, storage vessels, dispensers, and point of sale systems. Gas delivery pressure from the gas supplier will vary due mainly to demand from all users on given supply line. However, the gas company can usually provide gas at a given normal pressure, and this is the point for which the equipment is selected.
The piping design should be handled by a qualified professional engineer. Generally, the piping scope of work starts at the meter. Once the gas has been compressed, it is usually transported via stainless steel tubing to reduce corrosion and erosion. Tubing to the dispensers is usually buried in MDPE pipe conduit.
Design of a CNG station is best accomplished in a collaborative process between the prospective owner and an experienced engineering team. The owner best understands his or her needs and the engineer can best translate those needs into an orderly and cost-effective installation.
The first step in the design process should be to develop a “Design Criteria” document that lists all of the expectations of the owner. This will provide a basic road map for the project so that engineers and designers are not reliant on word-of-mouth instructions or randomly sent emails. The next step is to develop a preliminary layout, based on assumed equipment sizing.
Meanwhile, equipment specifications are developed and put out for bid. Once equipment has been selected and certified drawings are received, the actual equipment drawings are placed on the layouts. These will show piping and electrical connection locations. Now, piping, electrical and concrete drawings can be completed.
Various permits are required for CNG stations. The geographical location of the facility and the “Authority Having Jurisdiction” will affect the number and types of permits required. Each site will have different issues requiring different permits. Most permits will be those required by state and local entities. A qualified engineering partner will know the various permitting requirements.
Overall, construction of a CNG fueling facility is a relatively clean process and requires fewer permits than a liquid fuel operation. As previously stated, the accidental release of natural gas would not pollute the ground or water as would a spill from a liquid fuel stations. Liquid fuel facilities require permitting of the discharge of water used for setting the underground liquid fuel tanks. CNG fueling facilities use aboveground tanks and do not require water to ballast the tanks, negating the need for that permit.
There are several incentives and grants available designed to stimulate an infrastructure conversion that will make owning CNG vehicles more attractive, thereby reducing atmospheric pollution and dependence on foreign oil. Assuming success of the programs, it can be assumed they will be phased out as the conversion takes hold.
State incentives vary. Louisiana offers a 50% refundable tax credit with no limit for the incremental cost of CNG fueling equipment and vehicle conversion or purchase. Dual fuel and dedicated both qualify.
The federal government offers a 30% tax credit up to a limit for purchase or conversion of a vehicle to dedicated CNG. Dual-fuel vehicles do not qualify. Some credit for fueling facilities is available, but more important, 100% depreciation is allowed in 2011 for CNG fueling equipment purchased and put in service in 2011. That drops to 50% for 2012. There is other significant legislation pending at the federal level. In addition, there may be grant money available from DOE and DNR (Department of Natural Resources).
Time To Develop
Generally, the time required to develop a CNG station will be about eight months from the time the engineer is give the green light to proceed until startup. Time is often limited by equipment delivery, which may be four to six months after the order is placed. Allow about six weeks at the beginning to prepare bid packages, sending them out and evaluating.
The balance of the station should be complete by the time the equipment is delivered so that all that is required is to set the equipment in place, hook it up and go through the start-up and training procedures. Allow one month after delivery for equipment installation and connection.
A CNG fueling station can, in some situations, be an attractive opportunity for return on investment. Current economics and infrastructure favor stations designed primarily to service fleet vehicles that use large quantities of fuel and have a predictable daily circuit.
A public option can add to the income of a station while – at the same time – making CNG vehicle ownership more attractive to some private vehicle owners. State and federal incentives are available to help jump-start the industry by providing the infrastructure funding needed to make a transition from an oil-based to a natural-gas-based economy possible.
An experienced engineering partner can help to ensure the optimal selection of equipment, careful layout and design and proper permitting. All of these items are vital for a safe, efficient, and successful CNG fueling station project.
Allen Wiley is a registered professional engineer with over 31 years of mechanical design and engineering experience. He has served as a senior mechanical engineer at Hunt, Guillot and Associates (HGA) for over six years and is the resident expert on CNG fueling station design.
Trotter Hunt is a registered professional engineer and responsible for marketing at HGA. The firm is one of the largest engineering design and project management firms in the South and a leading provider of services to the natural gas industry.
 EIA. 2009 Annual Energy Review.
 EIA. Energy Outlook 2010.
 NGSA. http://naturalgas.org/environment/naturalgas.asp#pollution
 Altech Echo Corp. http://altecheco.com/pages/Safety.htm
 NGV America. http://www.ngvamerica.org/about_ngv/