Here are pictures showing carbon grease easily catching fire when ignited with a flame. An executive of a national KEC cleaning company told a Facilities manager for a national chain that they only clean down to the "Black stuff" as the "Black stuff aka carbon grease does not burn".He went on to say someone will have to start the carbon on fire in a duct work to prove that it burns. HERE IS HIS PROOF!!!! This goes to show that all grease and carbon MUST BE REMOVED EVERY TIME!! NFPA96 clearly states that cleaning companies must remove all combustible material from the grease exhaust systems and this is certainly combustible!!