Is it safe?
Although CNG is flammable, it has a narrow flammability range, according to the U.S. Environmental Protection Agency, making it an inherently safe fuel. Strict safety standards make CNG vehicles as safe as gasoline-powered vehicles. In the event of a spill or accidental release, CNG poses no threat to land or water, as it is nontoxic. CNG also disperses rapidly, minimizing ignition risk when compared to gasoline. Natural gas is lighter than air and will not pool as a liquid or vapor. Nevertheless, indoor leaks can form a flammable mixture in the vicinity of an ignition source.
CNG is primarily methane, which is a greenhouse gas that could contribute to global climate change if leaked. Methane is slightly soluble in water and under certain anaerobic conditions does not biodegrade. If excess amounts accumulate, the gas can bubble in water creating a possible risk of fire or explosion.
Reported incidents of CNG bus fires are related to engine failures, not the use of natural gas. Natural gas buses have onboard gas detectors and other safety devices such as tank safety valves that only allow fuel flow when the engine is on. Also, the tanks must be periodically inspected by the U.S. Department of Transportation.
There are, however, some safety concerns with CNG buses compared to diesel fuel buses, such as greater breaking distance due to increased fuel storage system weight. This is a relatively small concern, however, because the fuel system is a small fraction of a bus’ total weight. CNG buses also might accelerate more slowly than their diesel counterparts.
