Haha you’re forgetting the second part of the reaction: oxygen. You would need to somehow transport oxygen in addition to fossil fuels in space to burn em. You would need to expend a lot more energy in their transportation than what you would get from them.

Hence, this idea has merit only to be used for propulsion (for now). Once we crack fusion (which I’m sure we will within this century), fossil fuels lose this application in space too.