Why combustion need oxygen?

Question

Why actually oxygen is needed for combustion?, can combustion take place with other element? Or oxygen is the only gas involving in this chemical reaction?

Answer 1

This is really a terminology issue. Some folks like to define combustion as an exothermic reaction between fuel and oxygen that requires initial energy to start (the classic fire triangle). If that's the definition of the word "combustion," then the reason why it needs oxygen is clear -- because that's what the word means.

However, there is a much broader view of what constitutes combustion. If we want to stick with the view of combustion that it is an exothermic reaction between a fuel and something plus heat, we can replace oxygen with any oxidizer, which in chemistry has a specific meaning: an atom that can accept electrons from other atoms. In that view, combustion can occur with methane and chlorine, or ammonia and hydrogen peroxide, or any number of combinations of some fuel component and some oxidizing component.

We can zoom out a bit more and say that combustion occurs between a fuel and oxidizer with or without a heat source. This is what happens with hypergolic propellants -- spontaneous decomposition when fuel and oxidizer are brought into contact without an external heat source, resulting in strong exothermicity.

Of course, we could take a broader-still view of combustion and drop the requirement for two different things (fuel and oxidizer) and define it just as an exothermic decomposition process. This then can include monopropellants that are used as rocket fuels, as well as solid propellants (although those are generally self-oxidizing, in the sense that the solid is both fuel and oxidizer in one molecule that breaks apart when it turns to a gas, so it oxidizes itself).

To put it all together, ultimately what somebody means when they say "combustion" depends on who is saying it. For common English or folks who think "combustion is fire," then fuel, oxygen, and heat are all required by definition. But for propulsion-minded folks, particularly rocket-propulsion folks, combustion is a much broader and richer set of possibilities.