Improved Gas–Solids Mass Transfer in Fluidized Beds: Confined Fluidization in Chemical-Looping Combustion

Abstract

Fluidized bed processes with reactive bed material have become increasingly popular as research topics, with applications such as chemical-looping technologies, oxygen carrier aided combustion, and fluidized bed gasification being extensively investigated. When used at commercial scale the performance of such processes may be limited not by gas–solid reactivity, but by mass transfer of reactants from bubbles to the emulsion phase. In an effort to break down the two-phase flow structure, and thereby increase the bubble–emulsion mass transfer coefficient, spherical packing material was added to a fluidized bed using ilmenite as bed material. Two types of packing were tested: expanded clay aggregate (ECA) and aluminum silicate balls (ASB). Both packings had a diameter of about 12 mm but drastically different bulk densities of 240 kg/m3 and 1400 kg/m3, respectively. These were tested in chemical-looping-combustion batch experiments using a stainless-steel reactor with a diameter of 78 mm, with syngas or carbon monoxide as fuel at 915 °C. The lighter packing formed a floating plug while the heavier remained stationary at the reactor bottom. To compare the confined fluidized bed to a reference conventional one, a simple reaction model was implemented based on the experiments. It showed that in the confined fluidized bed the associated effective reaction rate constant increased by up to a factor of 2 for a given bed mass. Further, up to 4 times less oxygen carrier bed mass was needed to achieve the same gas conversion, at a lower total pressure drop. Experiments with only carbon monoxide showed similar gains when using aluminum silicate balls, indicating that catalysis of the water gas shift reaction was not the main factor for improved gas conversion. It can be concluded that the concept of confined fluidization has great potential to increase mass transfer in fluidized beds with active bed material.

Chemical Looping Combustion in a Packed Fluidized Bed Reactor─Fundamental Modeling and Batch Experiments with Random Metal Packings

“…In the case of syngas, the inlet and outlet flows of H 2 must be accounted for as water is condensed prior to the analysis. Thus, eqs and can be obtained from a mass balance between inlets and outlets The conversion of H 2 is calculated using eq …”

“…Previous studies showed that, in a BFB, the mass transfer rate of gas between the bubble and emulsion phase decreases with an increase in the bubble size. − While small bubbles are desirable for effective mass transfer, large bubbles can have the opposite effect by causing gas bypass and slugging . One effective method to eliminate bubble growth in BFBs is applying the concept of packed fluidized beds. ,, In this method, inert stagnant packings of a much larger size than the fluidized particles are applied to inhibit bubble growth and breakdown larger bubbles into smaller bubbles, as illustrated in Figure .…”

“…In contrast to normal fluidized bed combustion, it cannot be expected that residual combustible components can be converted in the freeboard. Aronsson et al investigated the effect of using spherical aluminum silicate balls (ASBs) and expanded clay aggregate (ECA) as packings during CLC batch experiments . They observed that these packings can improve fuel conversion.…”

“…They observed that these packings can improve fuel conversion. However, they could also result in higher pressure drops inside the bed and particle segregation phenomena. , Recently, Nemati et al compared the effect of evolved RMSR packings, simple spherical ASB packings, and a bubbling bed with no packings. It was reported that RMSR can successfully increase the fuel conversion rate compared to ASB and beds with no packings while having limited effect on particle inventory, pressure drop, and solid flux.…”

Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas

“…The air density (ρ air ) is assumed to be negligible compared to the particle density (ρ p ) and the voidage of the fixed bed (ε f ixed ) is assumed to be 0.4, whereas the voidage in the bubbling bed (ε BFB ) is assumed to be 0.6. Sand could be assumed to be an inert bed material, but to make sure that this is the case, the reactivity of sand was tested in a separate experiment at high temperature where syngas was mixed with nitrogen [29]. This experiment showed that the sand was inert and did not show any tendencies of gas-solid reactions with the CO or H2.…”