To study the influence of alkali metal on coke formation

a coking test was carried out on the inner surface of a used 25Cr35Ni cracking tube at 850℃ using potassium acetate as coking inhibitor.Its inner surface and cokes deposited in the coking tests were characterized by scanning electron microscopy(SEM)

field-emission scanning electron microscopy(FE-SEM)

X-ray energy dispersion spectrometry(EDS)

X-ray diffraction(XRD）and Raman spectroscopy.The results showed that the inner surface of the used 25Cr35Ni cracking tube was mainly composed of Cr1.3Fe0.7O3，FeCr2O4，Mn1.5Cr1.5O4

the intermetallic compound (Fe and Ni）and the needle-like deposition phase (intermetallic compound of Cr and Fe).Potassium acetate could inhibit coke formation effectively.When its concentration was 200 μg·g-1

deposition of coke was reduced by 51.54% in 1h.Potassium acetate did not change the length of coke filament

but it made filament diameter into nanometer level.The mechanism of inhibiting coking by potassium acetate was that gasification reaction between coke and water was catalyzed by alkali metal.The carbon in cokes deposited on the inner surface was of sp2 and sp3 bonds structure.Potassium acetate increased also graphitization degree of coke deposited.