Question

why the wood Stat to burn with a flame but after sometimes it stops to produce flame but give off heat and light energy......? Explanation

Answer 1

Now, wood itself can't vaporize. When you heat wood hot enough, the organic molecules the wood is made of (mostly cellulose and lignin) chemically decompose to form simpler molecules, such as methanol and formaldehyde. Those vaporize under the heat of the flame, mix with air, and burn. That burning process releases heat, which continues to heat the wood, releasing more organic gasses, and the process continues. If you were to heat wood without oxygen present, you could actually collect and condense those gasses (a process known as 'destructive distillation').

Now, this whole process also reduces some of the wood down to elemental carbon, which we call charcoal. Charcoal, being a solid, can't burn, in the sense of producing a flame, but it can react with oxygen if you heat it up enough. A hot pile of charcoal can react with oxygen on its surface, producing heat, which keeps the charcoal hot enough to react with more oxygen. As long as the charcoal stays hot and air keeps moving through it, it can continue to smolder. If you stop the air from moving (like by smothering it), or cool it down (either by adding water or splitting it up into smaller chunks), then the reaction will stop.