Why does fine sawdust burn far more rapidly and intensely than a dense log, assuming both are wood fuel?

Answer

The sawdust has a vastly greater surface area relative to its mass, allowing for much faster heat transfer and vaporization.

The physical form of the fuel, specifically its surface area relative to mass, dictates the rate of combustion. Greater surface area facilitates faster heat transfer and vaporization of constituent materials.

Why does fine sawdust burn far more rapidly and intensely than a dense log, assuming both are wood fuel?

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