Why is there no explosion when the percentage of Methane exceeds 15%?

When the concentration of methane exceeds 15%, there is typically insufficient oxygen available for rapid combustion to occur. For combustion to take place effectively, a specific balance of fuel (in this case, methane) and oxidizer (oxygen) is required. If methane levels are too high without a corresponding increase in oxygen, the mixture can become fuel-rich, which means there isn't enough oxygen to support the exothermic reaction that characterizes combustion.

This fundamental principle of combustion suggests that only a certain range of fuel-to-oxygen ratios will sustain a flame or explosion. When the mixture becomes too rich in fuel, it can hinder the combustion process, preventing an explosion from happening despite the presence of significant amounts of methane. This situation emphasizes the critical role that oxygen plays in combustion and how deviations from the optimal ratio impact the reaction dynamics.