Effect of Natural Ilmenite on the Solid Biomass Conversion of Inhomogeneous Fuels in Small-Scale Bubbling Fluidized Beds

Abstract

The application of oxygen carriers as alternative bed material in fluidized bed combustion originates from chemical lopping processes. They serve as oxygen transport agents undergoing consecutive redox cycles. Thereby, oxygen carriers can provide surplus oxygen in oxygen-lean areas of fluidized bed combustion processes. In turn, re-oxidation takes place in oxygen-rich reactor parts. A more homogeneous combustion and reduced CO emissions follow during steady-state operation. However, especially regarding solid biomass conversion, inhomogeneous fuel qualities result in transient combustion conditions. Therefore, this research deals with the influence of the oxygen carrier ilmenite on solid biomass conversion. Separated batch experiments with methane (volatile), char and wood pellets took place in a laboratory bubbling fluidized bed reactor. They reveal that ilmenite enhances the in-bed CO2 yield by up to 63% during methane combustion. Batch char experiments confirm that solid–solid reactions with ilmenite are negligible. However, heterogeneous gas–solid reactions reduce the O2 partial pressure and limit the char conversion rate. The batch wood pellet experiments show that the ilmenite oxygen buffering effect is mitigated due to high local oxygen demand around the pellets and limited pellet distribution in the bed. Finally, the continuous operation in a 100 kWth BFB with inhomogeneous fuel input indicates a higher in-bed fuel conversion and confirms lower CO emissions and less fluctuation in the flue gas during inhomogeneous fuel supply.

Chemical Looping Combustion of a Biomass Char in Fe2O3-, CuO-, and SrFeO3−δ-Based Oxygen Carriers

“…The disappearance of CO from both stages of combustion was expected, because Fe 2 O 3 is capable of reacting with emitted VM and CO from char combustion. Consequently, CO 2 was formed, resulting in a higher concentration of CO 2 in the off-gas compared to the inert bed, which aligned with the findings by Schneider et al 35 when a natural ore, ilmenite, FeTiO 3 , was used as a bed material during the combustion of solid biomass in a bubbling fluidized bed. Because, here, Fe 2 O 3 reacted with CO but was also rapidly reoxidized, owing to O 2 present in the particulate phase, the observed concentration of O 2 measured at the reactor outlet was lower than in the SiO 2 experiments, as shown by an enlargement of the relevant parts of the traces in Figure 3 given in Figure S14 of the Supporting Information.…”