Impact of oxygen transport on char conversion in dual fluidized bed systems

Other conference contribution, 2015

In the context of thermochemical conversion of solid fuels in dual fluidized bed (DFB) reactors, oxygen carrier bed materials have been found to enhance the rate of char gasification.Char gasification is typically the rate limiting step in the conversion process, which plays a role on the performance of the overall process and constrains the design of the reactor. The quantity of oxygen carried by the bed material in relation to the fuel feeding rate is likely to influence char conversion. This parameter is commonly characterized in literature by the oxygen-carrier-to-fuel ratio (ϕ), and it can be adjusted by either varying the fuel feed and/or the circulation rate of bed material in the DFB. The residence time of the fuel is typically altered by these changes, which results in an unclear role of the oxygen-carrier-to fuel ratio on the final char conversion. The present work investigates the impact of oxygen transport on char conversion in the 2-4 MWth DFB gasifier at Chalmers University of Technology. An iron ore is used as oxygen carrier and steam as fluidizing agent. A parametric study is conducted, in which a wide range of oxygen-carrier-to-fuel ratio (ϕ=3-12) are achieved by varying fuel flow and bed material circulation. Char conversion is calculated on the basis of gas analysis and mass balance calculations. The results confirm that oxygen transport enhances char conversion, in line with previous research in smaller units. Both fuel feed and circulation of bed material had an impact on the final char conversion under the conditions tested, which differs from previous investigations. For a given residence time of the fuel particles in the reactor, char conversion was found proportional to the oxygen-carrier-to-fuel ratio.