JCoyote: The problem isn't keeping a pilot light going... It's, with medieval technology, creating an apparatus to spray it across the pilot light, without having it spark back down into the holding tank and ignite the whole lot of it, which, explosive or not, would cause an explosion due to pressure buildup in the holding tank.

This is not generally a good thing.

Whereas, if the chemical is inert except when combined with water, it's very simple to create a device that sprays water and the chemical at the same time, and they mix outside the nozzles, creating ignition with zero chance of burning back down the tubes.