Acetylene, ammonia, hydrogen, propane, propylene and methane are all flammable gases referred to as fuel gases. When these gases are mixed with an oxidant and provided with an ignition source they will burn.
The diagram below shows the flammability concentration limits for an atmosphere comprised of air. The red bands show the percentage range of fuel gas that represents a particular danger of fire or explosion.
As the percentage of the fuel gas increases, the risk of fire becomes greater. When the concentration has exceeded the higher value, the air becomes saturated by the gas and ignition becomes less likely.
Gray - air content vol %; red - gas content vol %