Cementite decomposition and coke gasification in He and H2–He gas mixtures

Abstract Cementite decomposition and coke gasification were investigated by first carburisation of iron samples at 700 °C in a 50%CO–49.81%H2–0.19%H2O gas mixture for 4 h and then further treatment in He and He–H2 gas at the same temperature for different time periods. After carburisation cementite is formed at the iron surface covered by a coke layer. Scanning electron microscopy (SEM) shows carbon filaments and columnar layered bulk graphite in the carbon deposits. X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveal only cementite as the iron-containing phase in the coke. Cementite in the iron specimen and cementite in the coke show a different decomposition behaviour during treatment in He and He–H2 gases. In the iron specimen cementite decomposition proceeds fast and is comparable in both He and He–H2 gases. However, cementite in the coke is much more stable than that at the sample surface especially in the case of only He gas. By using thermogravimetric analysis (TGA) no gasification is detected in He gas but very significant gasification in He–H2 gas mixture until all the coke is removed. By adding some H2O into the He–H2 gas the gasification process is accelerated.