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The chemical reaction is going on in the substance being burned, and in the air above it as the heat of combustion vaporizes some of the substance being burned. That's why coals glow and flames seem to leap into the air. If your reaction does not have a flame, then either it is not producing visible light or the reaction does not occur in the air above the substance.
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S of course
it is because of the chemical reaction taking place between the substance burning and the air just above it . this is because of the heat combustion. so the material can burn with or even with out fire. this happen in many cases when the substance like sulfur because of the process called combustion of heat .

hope so my answer is useful  ^_^
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