Carburisation behaviour of direct reduced iron in complex CH ₄ -H ₂ -CO-CO ₂ gas mixtures

Current research on gas-based direct reduced iron carburisation primarily focuses on simple CH 4 -H 2 or CO-H 2 systems, with limited attention given to carburisation behaviour under complex gas mixtures. This study examined the carburisation of direct reduced iron in CH 4 –H 2 –CO–CO 2 gas mixture across 550–950 °C combining thermogravimetry, carbon speciation (XPS, carbon-sulphur analyser) and strength/swelling measurements. Complementary density functional theory simulations examined CH 4 and CO adsorption on α-Fe(110). The results indicate that under identical carburisation duration, significant weight gain of direct reduced iron occurred at 550 and 950 °C, but was negligible between 750 and 850 °C. Within the temperature range where weight gain occurs, increasing H 2 ratio accelerates the carburisation process, while CO 2 exhibits a strong inhibitory effect. Elevating CH 4 ratio promotes carbon absorption above 850 °C, and higher CO enhances low-temperature uptake. The carbon exists primarily as graphite below 650 °C, while cementite becomes dominant at elevated temperatures. Fe 3 C generates above 850 °C, triggering the volumetric expansion and 50% compressive strength loss versus direct reduced iron. Carburising at 950 °C for 20–30 min ensures adequate Fe 3 C without inducing severe swelling or compressive strength degradation. Simulations reveal that CH 4 and CO adsorb on the α-Fe (110) surface via physical and chemical absorption, respectively. However, CH 4 transitions to chemical absorption after dehydrogenation. Electron depletion between C–H and C–O bonds reveal the carburisation pathways of CH 4 and CO on the Fe surface.