There are a lot of gases. Most are the same that can be found in the air. There’s bacteria in your gut that creates gas. There has to be some methane in your gas that makes a fart flammable. Only one third of people produce methane. Both methane and hydrogen are flammable, but only methane will produce a blue flame when lit.
It’s because the stuff that forms the outer layer of the vegetables are complex carbohydrates that your enzymes may not be able to break down on your own. So, you rely on your intestines to help you break it down.