The Influence of Nitrogen on Catalytic Methanation
Research output: Thesis › Doctoral Thesis
Steel production is largely carried out via blast furnace/basic oxygen route (integrated steel plant), where the by-product gases from the different production units are one of the largest CO2 contributors to the global GHG emissions. In order to achieve the climate goals set in the Paris agreement, the integration of renewable energies and reduction of the CO2 emissions is one of the key points that have to be implemented in the existing steel production infrastructure. Blast furnace gas (BFG) and basic oxygen furnace gas (BOFG) are, due to their high CO, CO2 and N2 content and poor heating value, showing great potential as a carbon source for the implementation of Power-to-Gas (PtG) technology. In the present thesis, the behaviour of the methanation of BFG and BOFG gas at different operating conditions was investigated. The influence of N2 on the methanation process was determined, as well as GHSV, pressure and the H2-surplus variations to achieve the complete conversion of carbon oxides were carried out. The parameter variations and the N2 influence were explored on a complementary basis using the simulation tool Aspen Plus®. The simulation results are compared with experimental data. Experimental tests have shown that the complete conversion of the CO and CO2 in BFG and BOFG is achieved with and without the presence of N2 in the feed gas, with already upwards of 5% H2 surplus for both process gases, with three-stage methanation. High pressures resulted in higher COx conversion, whereas the increase of the GHSV inhibited the conversion on account of the residence time. The N2 in the feed gas therefore only has a significant influence on the higher heating value of the CH4-rich product gas, resulting in the case of BFG in 19.2–19.8 MJ m-3 (ratio H2/COx 1.09–1), and for BOFG in 26.7–28.8 MJ m-3 (ratio H2/COx 1.09–1). However, the enriched BFG and BOFG, when utilised in the integrated steel plant as lean gases, contribute to a decrease in natural gas and electrical energy demand. Simulation results were obtained in Aspen Plus®, with application of kinetic reactors as well as Gibbs reactors, and compared with the experimental data. From the three chosen kinetic models from the open literature, the kinetic model from Rönsch predicted the trend of the conversions and yields correctly, with and without present N2 as well as over a wide temperature range between 250-650 °C. The slight deviations of the CO2 concentration between the Rönsch kinetic model and experimental data led to the assumption of thermodynamic limitations in the three reactors, connected in series. The assumption was confirmed by the application of Gibbs reactors. It is shown that an equilibrium based on the reactor outlet temperature described the experimental data well, thus confirming the thermodynamically dominated reactions in the used polytropic reactors.
|Translated title of the contribution||Der Einfluss von Stickstoff auf die katalytische Methanisierung|
|Publication status||Published - 2021|