Final answer:
Lower flammability is the reason for the lower explosion risk when the CMM percentage is greater than 15%, because the mixture becomes too rich to support combustion, resulting in increased stability.
Step-by-step explanation:
One reason why there is a much lower risk of an explosion if the percentage of Coal Mine Methane (CMM) is greater than 15% is lower flammability. When CMM concentrations are above 15%, the mixture is too rich to burn. Flammability is a chemical property that describes how easily a substance can ignite or burn when exposed to a flame or heat. For a flammable substance to burn, it needs to be within its flammable range, which includes the right mixture of fuel and oxygen. Above this range, the mixture is too 'rich' and below it, too 'lean' to support combustion. This is why having a higher percentage of CMM (above the upper explosive limit) reduces the risk of explosion, as there is not enough oxygen present for the gas to ignite and thus leads to increased stability.