Gasification and Chemical-Looping Combustion of a Lignite Char in a Fluidized Bed of Iron Oxide

Gasification and chemical-looping combustion experiments with a lignite (Hambach) char are reported, using an electrically heated fluidized bed in a 25 mm diameter tube at about 1073 K. The fluidizing gas was CO₂ in nitrogen, with either batch or continuous feed of char into a bed of sand or particles of Fe2O3. The experiments were also modeled using the two-phase theory of fluidization; a well-mixed particulate phase is assumed, and the bubble flow is augmented by gasification products. This was combined with Langmuir?Hinshelwood (L?H) kinetics of gasification deduced from the experiments on gasification, with CO₂, of this char in sand. These L?H kinetics are complex; the rate constants are different for two ranges of partial pressure of CO₂: (i) from 0 to 0.05 bar and (ii) from 0.05 to 0.9 bar. The theory gives good predictions of (i) off-gas concentrations of CO and CO₂ and (ii) accumulation of carbon in the bed. Combustion also occurred when the char was fed into a bed of Fe2O3 particles fluidized by nitrogen, giving the appearance of a solid?solid reaction, i.e., char oxidized by Fe2O3. The occurrence of a solid?solid reaction is however very unlikely, and it is believed that the reaction actually occurs via gaseous intermediates, CO and CO₂, and is triggered by small amounts of oxygen in the char or air entrained with the char. This hypothesis is well-supported by the theoretical model. The results are particularly relevant for the chemical-looping combustion of char in Fe2O3.