High temperature phenomena occurring during reactions of agricultural wastes in electric arc furnace steelmaking: interactions with gas and slag phases

Abstract

Iron and steel making is an energy intensive industrial sector using mainly coal as the heat source and reduction agent. The industry gives rise to about 10 % of the anthropogenic CO2 emissions in the world. Due to the challenge for CO2 mitigation, interest for agricultural waste (palm and coconut shells) use as a renewable energy and carbon source as heating agent and reducing agent contributes to energy conservation and emission reduction, and can partially replace coal and coke. In the present study, the conventional material investigated was metallurgical coke which was blended with different proportions of palm and coconut shells as well as agricultural waste chars in order to reduce the waste in the landfill. Metallurgical coke, palm shell/coke blends and coconut shell/coke blends were combusted in a drop tube furnace (DTF) at 1200 °C under 20% O2 and 80% N2 gas mixture while palm char was devolatilized at 450 °C under N2 atmosphere. Subsequently, the residual materials were put in contact with an EAF iron oxide rich slags and their interfacial reactions and phenomena have been studied at 1550 °C in a horizontal tube furnace under inert atmosphere (1 L/min Ar) with off gases measured using an IR analyser. The initial devolatilization and the subsequent step of combustion of these samples are conducted in a Drop Tube Furnace (DTF) and in a Thermogravimetric Analyser (TGA), respectively, while the sessile drop approach was used to investigate the interfacial reactions taking place in the slag/carbon region. A Thermogravimetric Analyser coupled with Mass Spectrometer (TGA-MS) was also used to study the behavior of coke and agricultural wastes at high temperatures in order to understand the thermal behavior and gas products that evolved at high temperatures. The weight loss profiles, gas formation and products distribution were significantly different between the coke and agricultural waste samples. It was found that more gases were released from agricultural waste than from coke that participated in the subsequent carbon/slag reactions. In the gas phase reaction studies, the blends containing agricultural waste materials indicated higher combustion efficiencies compared to coke alone with an improved surface area resulted from volatile matter removal. The role of chemical structure and properties, as well as inorganic matter in agricultural waste blends also influenced the combustion performance. The rate of devolatilization appears to improve the coke/palm shell blends burnout as well as its foaming behavior when put in contact with an iron oxide rich slag. For carbon/slag interactions, experiments were conducted using the sessile drop technique (1550 °C) with off gases (CO, CO2) measured using an IR analyzer; the wetting behaviour was determined from contact angle measurements and estimation of slag foam volumes were calculated using specialized software. Off gas analyses following the carbon/slag interfacial reactions have been measured for all the carbonaceous materials and significantly different gas concentrations have been observed. The rates of total gas generation (CO+CO2) from palm char was comparable to those seen in coke; however the gases released from palm chars were extent over a longer period of time and allowed their entrapment in the slag matrix, enhancing the volume of the slag. A slower rate of FeO reduction is seen when coke reacted with the Electric Arc Furnace (EAF) slag, while the palm shell blends showed a faster reduction. Independent of the carbon material used as a substrate, the final stage of reaction reveals comparable contact angles due to similar extents of reduction and Fe deposition at the interface. The steady gas generation seen in palm char compared to coke allows the formation of a highly porous particle, promoting gasification and allowing more gases to be trapped in the slag phase. These results indicate that partial replacement of coke with palm shells is not only viable, but efficient leading to improved/sustained interactions with EAF slag. Optimization between the two phenomena, reduction and foaming is required for improved EAF process performance.