Bond cleavage and reactive radical intermediates in heavy tar thermal cracking

•Reactive radicals of different activities are distinguished by hydrogen donor solvents.•Low activity radicals were generated at a slower rate before high activity radicals.•Apparent Ea of bond cleavage is 31–39 kJ/mol, lower than that of mass loss.•More asphaltenes and a higher substitutive degree may result in more bond cleavage.•Highly reactive radicals are mainly responsible for the formation of stable radicals.