1. Chemical Reactions: This is the most common source of combustion. It involves a chemical reaction between a fuel and an oxidant (usually oxygen). This reaction releases heat and light energy, which we perceive as fire. Examples include burning wood, gas, or oil.

2. Nuclear Reactions: This type of combustion involves nuclear reactions, specifically nuclear fission. In nuclear fission, the nucleus of an atom is split, releasing immense amounts of energy. This energy can be used to heat water and generate steam, which then drives turbines to produce electricity. This is how nuclear power plants work.