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

Abstract

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.

Performance of a bauxite waste as oxygen-carrier for chemical-looping combustion using coal as fuel

“…The product stream was analyzed during 120 min and no significant amounts of carbonaceous products (CO, CO 2 ) were detected. Therefore it was concluded that the solid-solid contribution to the carbon conversion was negligible in this case, similarly to what was found before by other authors [21,38,52]. Char gasification must proceed as an intermediate step in the coal conversion process using bauxite waste as oxygen carrier.…”

“…The H 2 and CO removal from the gasification products reduced the inhibition effect that both gases, especially H 2 , could have on the gasification process [51]. Similar conclusions were reached before in previous studies using Fe-based oxygen carriers [21,38,52]. The decrease in the inhibition of char gasification can be observed in Figure 5 (B), where the char conversion evolution with time is represented for both sand and bauxite waste experiments.…”

Progress in Chemical-Looping Combustion and Reforming technologies

“…Dennis et al [118,119] have proposed the use of a cycling fed-batch operation for solid fuels in order to reduce the attrition problems associated with the conveying of large quantities of solids in the interconnected fluidized beds configuration. In this operation mode, three consecutive periods of time (fuel feeding, char combustion without fuel feed, and oxygen-carrier regeneration with air) are carried out in only one fluidized reactor.…”

“…CO 2 can be re-circulated from the flue gases. The use of CO 2 has been proposed for highly reactive solid fuels, such as low-rank coals or biomass [119]. Otherwise, special considerations should be given when using CO 2 as gasification agent depending on the design configuration because of the slow gasification rate with CO 2 , as will be discussed later.…”

“…Most of the works demonstrating the iG-CLC technology have been carried out using two interconnected fluidized-bed reactors [54,187,206]. Another alternative is to use a batch of an appropriate oxygencarrier in a single fluidized bed with three stages [118,119]. The process would consist of several fluidized-bed reactors operating in parallel and out of phase with one another to achieve a continuous supply of energy.…”