In the future, a large number of road vehicles will not be powered by fossil fuels, and in order to prevent incidents in connection with such a change in the transportation sector, regulations and practices should stay one step ahead. This project was funded by the Nordic Road Association (NVF), and was intended to review and update current knowledge regarding alternative fuels. Another aim was to provide guidelines for the operations of rescue services, and to offer recommendations for the creation of regulations. Road tunnels and underground garages constitute particularly high-risk environments with regard to fires and explosions. The project focused on Swedish commercial gaseous fuels and electric vehicles: liquefied petroleum gas (LPG), Dimethyl ether (DME), methane (CNG and LNG), and hydrogen gas.
Sweden has the greatest depth of experience with vehicles powered by methane gas, i.e., CNG. The number of electric vehicles has increased enormously in recent years, particularly in Norway and, to a lesser extent, Sweden. Although the use of alternative fuels often entails risks, these should not be exaggerated, as all vehicle fuels have the potential to cause fire or an explosion. As compared to liquid fuels, however, these new fuels inarguably introduce new forms of risk, such as different types of gases being released and different types of explosions that can occur. This in turn requires a different rescue service approach and an upgrade of regulation and standards.
Associated Risks
Gaseous fuels can be compressed, liquefied-compressed (which is to say liquefied and compressed, expressed in this way so as to avoid confusion) or take the form of a cryogenically frozen gas, wherein it has been so heavily cooled that the gas condenses into a liquid. The storage and handling of methane takes place in the form of both compressed gas (CNG) and cryogenic gas (LNG). Hydrogen gas is primarily handled in compressed form, whereas LPG and DME are normally handled in liquefied-compressed form.
An explosion is a rapid release of gas under pressure. The keyword here is ‘rapid’, as this quick release results in a blast wave. Pertinent examples of such an explosion include the rupturing of a pressurised tank, i.e., a pressure vessel explosion, and a chemical reaction (combustion, for example) that results in a rapid increase in pressure such as occurs when a combustible gas-air mixture is ignited. An explosion can thus be physical – a gas tank that ruptures due to excessive pressure – or chemical (exothermic reaction) – as a result of ignition, for example.
The portion of the gas that is in liquid phase during a pressure vessel explosion may lead to a BLEVE when the liquefied gas rapidly evaporates in the warmer environment outside of the tank. For a BLEVE to occur, the liquid (i.e., only relevant for LPG, DME or LNG) must be heated around 100 K above normal storage temperature, e.g. by a fire. A BLEVE can inflict fatal damage in not only the immediate vicinity of the vehicle but further away, as parts of the tank can be propelled a great distance.
According to European guidelines, gas containers are to be inspected at regular intervals, but these are currently ignored entirely in Sweden. Two pressure vessel explosions have recently occurred during refuelling at 230 bars for Swedish CNG vehicles which is about half of the design pressure for the gas container. There could be many more Swedish vehicle gas containers that currently operate with narrow safety margins.
Standardised testing should ensure that pressure relief devices activate in case of fire. Despite, there are many cases when they have been unable to prevent a pressure vessel explosion due to the increased pressure inside the container and the weakened material following the fire. One reason is that the fire can be either more powerful or local compared to the fire in the standard. Another is poor maintenance and inspection of gas containers and systems. Taken altogether a pressure vessel explosion followed by a BLEVE (for LPG, LNG, DME) or fire ball (for CNG, hydrogen) as a result of a vehicle fire must be accounted for. The already weakened tank is weakened further once heated by the fire. At the same time the pressure increases due to the heating of the gas. If the pressurerelief valve does not activate in time, an explosion that may be lethal to anyone in the immediate vicinity will occur.
Electric Vehicle Risks
At least part of the energy that powers electric vehicles is stored in a battery. Li-ion-based technologies are the most common on the market at present, and will likely continue to be for the foreseeable future. The energy that is released during the combustion of a battery is moderate in relation to that of the rest of a vehicle, and contributes less to the fire load as compared to traditional petrol. In order to prevent battery failure as a result of both external impact and internal error, batteries are equipped with technical safety systems. If the damage sustained nevertheless causes high temperatures or internal short circuits, the battery may suffer failure and undergo thermal runaway.
The fire load of an electric vehicle is thus no greater than that of one with a more conventional fuel, but does involve different risks. The electric system of a traction battery must be taken into account during a rescue operation, particularly when a car is charging. Traction batteries do not increase fire-risk if appropriate tactic is taken by rescue services. During a thermal runaway, however, the production of highly flammable and toxic gases may become considerable. When thermal runaway takes place in connection with a fire, the gases produced do not exacerbate the situation as the fire gases from the fire are themselves toxic. If no fire occurs, however, the production of large amounts of toxic gas, such as hydrogen fluoride, may occur and go unnoticed.
Fires in batteries are very difficult to extinguish due to the extensive insulation of batteries, and a great deal of cooling is required to stop a thermal runaway. Thus, a fire suppression operation involving an electric vehicle should focus on extinguishing the fire around the battery, and preventing fire propagation from it. The thermal runaway process of damaged Li-ion batteries may re-start and/or continue for more than 24 hours after the damage occurred. This can lead to re-ignition or a new fire starting.
One of the greatest dangers posed by electric vehicles at present is arguably not the technologies that constitute them and the possible adverse consequences of their use, but uncertainty regarding how to handle them. The technology is relatively new, constantly changing, and differs significantly from conventional fuels. This may lead to uncertainty during a rescue operation, and thus a greater degree of risk.
Underground Garages
Garage fires are relatively common. Most often they are caused either by arson or electrical failures. Most often the fire is limited to the vehicle of origin. The conditions for rescue operations in underground garages is already a great challenge with long entrance distances and heavy smoke resulting in problems of identifying the fire location. Air supply becomes a limiting factor and the risk of collapsing beams or ceiling equipment must be taken into account. Altogether, this means defensive tactics are chosen when possible.
Several government authorities can be considered to be responsible for safety in garages. The most obvious of these is the Swedish National Board of Housing, Building and Planning, which is responsible for regulations regarding the construction of buildings (BBR). BBR does not, however, take into consideration the fuels of the vehicles that may be parked in a garage within a building. Safety for vehicles that use alternative fuels in underground garages fall between the cracks, with the Swedish National Board of Housing, Building and Planning, the Swedish Civil Contingencies Agency, and the Swedish Traffic Administration all pointing to one another. Several gas explosions have occurred that seriously affected building structures. For an explosion in an underground garage most force will be directed against the ceiling and thereby threatening the stability of the structure, depending on how it is constructed.
Road Tunnels
The European Union has issued regulation regarding safety in road tunnels of 500m or more on the European road network. Swedish Traffic Administration ratifies regulation regarding safety in road tunnels in Sweden. These regulations do not factor in alternative fuels in vehicles, and the responsibility for managing these risks is placed on tunnel managers. In general tunnels are built to resist fire. The most challenging situation concerns evacuation in smoke, in particular for two-way tunnels. A number of studies have investigated the risk of vapour cloud explosions in tunnels due to release of gas from a vehicle. In sum, the large tunnel cross section and mechanical ventilation limits the size of explosive gas clouds from vehicle gas systems. There are no reported incidents of gas-cloud explosions inside road tunnels. In all known cases of bombs detonating inside tunnels, the structure have resisted explosion well. This is not surprising considering that they are enclosed by matter and have two openings where the pressure can be released.
Emerging Risks
Although accurately predicting which types of fuel will dominate in the future is difficult, rules and regulations must be carefully designed so as to ensure safety for likely scenarios. Risks need to be understood and evaluated in order for efficient regulations to be introduced at an early stage.
Current handling of petrol and diesel are well-tested and relatively safe. However, not least petrol contributes to many vehicle fires and fatalities. Alternative fuels, in the form of either gas or electricity, will likely lead to fewer leaks that are ignited, and thus fewer vehicle fires in general. However, electric and gas-powered vehicles introduce new risks, e.g., release by toxic gases and explosions.
Little knowledge could be found concerning possible structural consequences from a vehicle gas container explosion inside an underground garage. Existing evidence suggests that such an event could be devastating, at least for some types of structures. Future research is required to clarify the extent of damage that would result on different types of buildings with underground garages. This is the key emerging risk that was identified.
For road tunnel constructions and road users the emerging risk from the alternative fuels that were studied are judged to be small. Following a gas release a small or no explosion can be expected. A pressure vessel explosion most likely resulting from a fire but is not expected to lead to additional casualties (assuming road users either perish or evacuate due to the fire in the first place) or any significant tunnel damages.
A critical aspect for underground garages and road tunnels is the rescue service operations, and the new dangers that vehicles with alternative fuels pose to them. At the moment there is much uncertainty concerning the best extinguishing tactic to avoid pressure vessel explosions.
Vehicle gas systems could be designed to better resist a pressure vessel explosion as the result of a fire. Related safety standards and practice should be improved. Automatic ventilation in garages and road tunnels must be adapted to detect gases emitted by electric and gas-powered vehicles.
