The effect of sulfur on chemical looping combustion with iron oxides
Creators: name: Zhang, Zili
Publisher: name: Imperial College London
Year: 2014

description: Advancing climate change poses an increasing threat to humanity, together with the higher demand for energy around the world. New ‘cleaner’ technologies for the production of energy from solid fuels (coal or biomass) via thermo-chemically viable routes are required. Chemical looping combustion (CLC) is a process concept using metal oxide (oxygen carrier) for transportation of oxygen from air for electrical power generation and inherent production of a pure stream of CO2. CLC has been generally applied to gaseous fuel; however, by integrating chemical looping and gasification, the combined process shows great potential for producing H2 and power from solid fuels. Coal and biomass contain significant quantities of sulfur. Upon gasification, the sulfur is released in the form of H2S, which will be then be introduced into the iron-based chemical looping process, followed by further gasification. Iron oxides are known to form stable sulfides under reducing conditions. The performance of chemical looping using iron-based oxygen carrier could therefore be adversely affected by the introduction of H2S in a real system. The overarching aim of this thesis is to assess the effect of H2S on chemical looping combustion using iron (III) oxide in a laboratory scale spouted bed reactor. A closed-system spouted bed reactor has been designed and constructed to study the solid looping system with gaseous fuel. A model of the bed was developed, from which the bed could reasonably be assumed to be a well-mixed bubbling fluidised bed reactor at certain conditions. The reactor was used for the kinetic study of reduction of Fe2O3 to Fe3O4 with a CO/CO2 mixture under isothermal condition at the temperature range of 723K – 973K. The oxygen carrier before and after thermal cycling was characterised using SEM, mercury porosimetry, BET surface area analysis. Using a nominal particle size of Fe2O3, the rate of reduction was controlled mainly by intrinsic chemical reaction kinetics with a high effectiveness factor. The intrinsic rate constant was estimated with an activation energy of 73 kJ mol-1, which is comparable to values reported in the literature. The reactor was modified for use in the quartz internal, together with acid-washed, calcined sand (Quartz-T) to be able to significantly reduce interactions between sulfur and the inert material used in the construction of the reactor and the spouted bed. The fate of H2S in the chemical looping cycling was determined and the effect of H2S on reduction of Fe2O3 to Fe3O4 over multiple cycles was studied. There were two major mechanisms of reaction between H2S and Fe2O3 that were found to affect the rate of reduction of Fe2O3 and main sulfur product distribution: (1) production of SO2 as a main sulfur product, (2) production of FeS as a main sulfur product. The dominating mechanism was found to depend on the thermodynamic potential of S2, the thermodynamic potential of O2 (which depends on the extent of reduction), and the temperature. The effect of the H2S on the kinetics of reduction was found to be due to the structural change of Fe2O3 particles that is governed by the reaction between H2S and Fe2O3. A mathematical simulation based on a grain model under chemical reaction control was used to satisfactorily describe the relationship between rate of reduction and the extent of the reaction in the presence of sulfur.