Years ago the fire service was warned that the primary fuel for vehicles was changing. Motors for modern passenger vehicles have been changed to accommodate today’s E-10, E-15 and E-85 ethanol and gasoline blends. Many fire departments were warned that unless their foam concentrates were alcohol treated types, they would be ineffective on spills or fires involving this new blend of vehicle fuel. During that time several other alternative fuels have been developed and are becoming more prevalent today. These alternative fuel systems include compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), and Hydrogen.
The use of these alternative fuels has been somewhat limited by the presence of refueling stations. On the west coast where alternative fuel refueling stations are more prevalent, more vehicles using these types of fuels exist. Vehicles powered by these alternative fuels are understandably not common in areas where they cannot refuel. However, we can now find medium- and heavy-duty vehicles designed for long transports, such as tractor-trailers, that are making use of these alternative fuels. Additionally, if you notice fixed fueling stations that offer CNG, LNG, LPG or hydrogen fuel in your jurisdiction, it is likely that you will soon begin to see an increase in these types of vehicles.
What’s the Difference?
Let’s start with the difference between Natural Gas, Liquid Petroleum Gas and Hydrogen. Whether it is compressed or liquefied, natural gas is composed mostly of methane, which has a vapor density of 0.554 — making it lighter than air — a vapor pressure of 466,000 mm Hg — which we will discuss more in a minute — a LEL of five percent, a UEL of 15 percent — a fairly narrow explosive range — and a flash point of -306.4 degrees Fahrenheit. Liquid Petroleum Gas is composed mostly of propane, has a vapor density of 1.56 — making it heavier than air — a vapor pressure of 7,150 mm Hg, a LEL of 2.1 percent, a UEL of 9.5 percent, and a flash point of -156 degrees Fahrenheit. Hydrogen’s vapor density is 0.069 — much lighter than air — a Vapor Pressure of 124,000 mm Hg, a LEL of four percent and a UEL of 75 percent — a much broader explosive range than natural gas and liquefied petroleum gas.
Compressed Natural Gas or CNG is stored on the vehicle in high-pressure tanks — 3,000 to 3,600 psi. Recall the extremely high vapor pressure of natural gas — 466,000 mm Hg. This is equal to around 9,011 psi. To liquefy natural gas using pressure alone would require a vessel or container capable of holding more than this pressure — 9,011 psi. Because the pressure vessels transporting CNG are only 3,000 to 3,600 psi, the state of the CNG inside the vessel is a gas — similar to your SCBA cylinder. A Sulphur-based odorant, such as ethyl mercaptan, is normally added to CNG to facilitate leak detection.
Four Types of CNG Containers
There are four types of CNG containers. A type 1 CNG tank is constructed of all seamless steel making them the strongest and heaviest type. A type 2 CNG tank is made of steel but is hoop wrapped with a composite, making it 25 percent lighter than a Type 1 CNG tank. A type 3 CNG tank has an aluminum liner with a composite shell making it lighter than a Type 2 CNG tank. Finally, a type 4 CNG tank has a non-metallic or plastic liner, such as polyethylene, that is wrapped with a composite shell. Type 4 CNG tanks are about 60 percent lighter than a Type 1 CNG tank of comparable size. Type 3 and 4 CNG tanks are better at diffusing heat from fire than their steel counterparts, but the carbon fiber wraps are combustible and ignite at approximately 650 degrees Fahrenheit. Furthermore, the composite wraps in Types 2, 3, and 4 can be degraded by sunlight. As a result, they are typically shielded on the vehicle. Because composite cylinders are better at diffusing heat and do not experience similar heat rise as their steel counterparts, they do not utilize typical pressure relief valves. Instead, they use temperature activated pressure relief devices (TPRD). These devices have a fusible material that is set to release at 220 to 240 degrees Fahrenheit. Once activated, the TPRD will release the entire contents of the tank — which can take several minutes — and there could be up to two TPRDs on each cylinder. There is no set direction in which TPRDs vent, and a vehicle on its side will also change the normal vent direction. Remember that natural gas is lighter than air and will rise and dissipate.
Liquefied Natural Gas or LNG is natural gas stored as a super-cooled (cryogenic) liquid. Due to the extremely high vapor pressure of natural gas, pressure alone is normally not sufficient to convert the gas to a vapor. But by reducing the temperature, natural gas can be converted to a liquid — similar to cooling steam below 212 degrees Fahrenheit, which would make water. The temperature required to condense natural gas depends on its precise composition, but it is typically between -184 and –274°F). The disadvantage, however, is the high cost of cryogenic storage on vehicles and the major infrastructure requirement of LNG dispensing stations, production plants and transportation facilities. An LNG tank uses a double walled stainless steel vacuum space similar to a cross between an SCBA cylinder and a thermos bottle. Because of the insulation and construction, an LNG tank is good at diffusing heat. Unlike CNG tanks, LNG tanks use spring mounted pressure relief valves, one that typically operates around 230 psi and a secondary that operates at around 350 psi. Both are designed to relieve pressure and prevent container failure. Unlike the TPRDs used in CNG tanks, the pressure relief valves used in LNG tanks can open and close multiple times, opening until the pressure in the vessel drops below the valve’s operating pressure. Under ambient conditions, an LNG tank can hold pressure for seven to 10 days before the relief valve operates. Remember that natural gas is lighter than air and will rise and dissipate. However, because LNG is a cryogenic liquid, no odorant, such as ethyl mercaptan, can be added, meaning it will not smell.
Liquefied Petroleum Gas or LPG is also called Autogas. Given the much lower vapor pressure of liquefied petroleum gas, pressure alone is capable of liquefying the material. At normal temperatures and pressures, LPG will evaporate. Because of this, LPG is stored in pressurized single walled steel bottles — similar to the LP tank connected to a gas grill. Unlike natural gas, LPG has a vapor density of greater than one and is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements. Such accumulations can cause explosion hazards, and are the reason that LPG fueled vehicles are prohibited from indoor parking garages in many jurisdictions. Due to the vapor density of LPG, tanks are typically not roof mounted, like some CNG vehicles. LPG, like LNG, use standard pressure relief valves, which can open and close multiple times, relieving pressure as needed.
Five Types of Hydrogen Tanks
There are five types of Hydrogen tanks. The first four types approximate the construction of the four types of CNG tanks. The fifth type of Hydrogen tank is all composite and liner less. Unlike CNG tanks, hydrogen tanks are capable of operating at pressures of 2,500 to almost 10,000 psi. This enables the hydrogen to be partially converted to liquid through the application of pressure. Most hydrogen tanks utilize a pressure relief device, not a valve, which will melt at 225 degrees Fahrenheit. Like CNG, the entire contents of the tank will be released once the device activates. Complete venting may take several minutes. Hydrogen does not smell, is much lighter than air, and due to its purity burns with a pale blue flame, making it invisible to the naked eye in daylight conditions. Normally the fire will ignite nearby combustibles on the vehicle, which will emit visible flame and smoke.
Some garbage trucks have these alternative fuel tanks mounted at the top of the truck body. The tanks are also found in the rear of buses and in trunks of passenger vehicles. Fires and motor vehicle collisions involving these types of alternative fuel tanks pose a greatly increased risk to responding firefighters. As in all hazardous materials incidents, identification of the hazard involved is the first step. First, you can ask the occupant of the vehicle if their vehicle utilizes alternative fuels. If they are not available or do not know, you can attempt to find alternative fuel markings, labels or badges on the vehicle. This assumes that the vehicle is conspicuously marked and the markings have not already been destroyed. In most cases, ethyl mercaptan is added to both LP and natural gases as an odorant. While relying on your sense of smell is the least desirable identification technique at a hazardous materials incident, it is possible that while working at the scene of a motor vehicle collision involving an unmarked vehicle, you may begin to smell rotten eggs, garbage, sewage or a dead animal. These smells are synonymous with ethyl mercaptan. However, in some cases, CNG and LPG may not contain the odorant additive. Additionally, LNG and Hydrogen do not contain the odorant, which means it will not smell. Even where ethyl mercaptan is present, responders will quickly experience olfactory fatigue. Firefighters may believe that the leak is subsiding, but they are instead losing their ability to smell the leak. The only definitive method for identification is to utilize air monitoring equipment.
When dealing with CNG, LNG, and LPG, cooling the tank is essential to preventing a BLEVE. Should the relief valve fail to function; a BLEVE is a real possibility. This was the case in January of 2015 when the Indianapolis Fire Department responded to a garbage truck fire. On arrival, the fire was confined to the hopper body/compactor. Efforts to dump the load of the trash truck were unsuccessful. As lines were stretched, the driver of the truck told the crew that the “gas tanks” were located on top of the truck. This was the first indication that the truck was powered by something other than diesel fuel. Less than 15 minutes from the engine’s arrival, several of the CNG tanks on the roof of the garbage truck BLEVEied, spraying shrapnel in a 360-degree pattern and striking one firefighter in the helmet with a force that knocked him to the ground. One of the tanks traveled a quarter mile and landed about 40 feet in front of an elementary school.
In cases where a temperature activated pressure relief device (TPRD) operates as designed and fire or an ignition source is present, it is also possible to have a high-powered gas jet. These burning vapors under pressure have the potential to ignite nearby combustibles and cause burn injuries to anyone in their path, including firefighters wearing appropriate PPE and SCBA. The tank mounting position and relief valve location will drive in which direction these gas jets will vent. Note that once this TPRD operates, the entire contents of the tank will be vented. This is unlike LNG and LPG tanks, which utilize pressure relief valves that are capable of opening and closing multiple times.
When dealing with hydrogen tanks, small fires that are remote in relation to the tanks should be extinguished using standard tactics. However, if the hydrogen tanks are involved or the fire is in the area of the hydrogen tanks, care should be taken to avoid applying water to the hydrogen tanks. Applying water to burning or heated hydrogen tanks decreases the ability of the TPRDs from operating properly and could actually increase the change of a BLEVE. When a significant fire occurs in a vehicle powered by hydrogen, or the vehicle is fully involved, do not approach the vehicle. Establish a safe perimeter of 80 to 100 feet around the vehicle and protect exposures while allowing the vehicle to burn. Any attempts to extinguish the fire could prevent the TPRD from working properly and could result in a BLEVE, especially if water is applied to the hydrogen tanks.
We must now consider alternative fuels as being yet another danger that may be present at vehicle fires. Keep an eye out. Many of us can recall the gas pumps before they sold E-85 next to regular gasoline. It won’t be long before we are remembering when they didn’t sell LNG and Hydrogen at the gas station. In the meantime, the long distance vehicles are out there. One of the major shipping companies recently had a motor vehicle collision on Interstate 95 in South Carolina. Although the damage was limited, LNG powered the vehicle. These alternative fuels will change our departments. What once were a routine, 30-minute call, will become research- intensive incidents.
Be safe and do good!