Everyone's right. Combustible is a technical term and is used much more frequently than "burnable". In fact, I have seldom seen the word burnable. Technically speaking, combustible is often defined as something that will burn at or above 38 degrees C (100 degrees F). However, materials that will burn at temperatures below 38 degrees C (100 F) are usually referred to as "Flammable". Some flammable materials (usually liquids and gasses) burn even at temperatures as low as [-43 degrees C gasoline] and [-60 C butane]. Years ago, English speakers used the word "inflammable" to mean "flammable". Strange, because one would think that "inflammable" might mean "not flammable" due to the prefix "in". The word "inflammable" is not used very often any more.

agree with Eliot , news , newspaper , published stuff always use combustible, that might explain why could google a lot both are okay , but burnable is just more easy for lazy people like me

I can't say for sure but I'd think "combustible" is more common. Googling both words as search terms also turns up about 25 million hits for "combustible" versus about 200,000 for "burnable."

"combustible" is the technical term. "burnable" is very general. So when talking technically, you say "combustible".