Solids and liquids do not burn. First they vaporize to a gas, then the gas burns. When wood is burned, the wood itself vaporizes. Much of this vapor is gaseous wood alcohol and water/steam. If too much vapor is trapped inside the wood, it will exert pressure and cause the wood to pop which causes the crackle.
"Distilling Wood" can be done by heating wood chips in a test tube and condensing the effluent vapor. This produces water, wood alcohol and activated charcoal.
Answered by: Christopher Ingram, B.S., Independent thinker