Morphologies of coke deposited on surfaces of pure Ni and Fe-Cr-Ni-Mn alloys during pyrolysis of propane

Morphologies of coke deposited on pure Ni and Fe-Cr-Ni-Mn alloy surfaces during pyrolysis of propane at 750–1000°C have been investigated in detail. It is found that surface scales developed initially on pure Ni and Fe-Cr-Ni-Mn alloy surfaces have no catalytic effect on deposition of filamentary coke. But metal or alloy substrates under cracked scales strongly catalyze nucleation and growth of filamentary coke along the cracks. Ni is more efficient to catalyze the growth of filamentary coke than Fe-Cr-Ni-Mn alloys. The structure of oxide scales has marked influence on distribution and size of filamentary coke deposited on alloy surfaces. Coking morphology is closely dependent of coking temperature and time. Either increasing coking temperature or prolonging coking time results in coking morphology changes from filamentary to spherical. Both dissolution/precipitation mechanism and direct nucleation and growth mechanism may make a contribution to the development of graphitic film coke.