Creeping flow relative to permeable spheres

Drag coefficients of irregularly shaped particles

Permeability of Fractal Aggregates

The Ca-looping process shows a considerable potential for reducing postcombustion CO2 emissions from power plants in the short-term as demonstrated by the recent success of a 1.7 MWt pilot plant. This process involves the carbonation reaction of CaO to capture CO2 and the subsequent calcination of limestone (CaCO3) to regenerate the sorbent. Yet the capture capacity of natural limestones decreases with the increasing number of calcination/carbonation cycles, which is mainly attributed to a decrease of the reactive surface area with the number of cycles as a result of material sintering during calcination. A number of techniques have been developed in the last few years to improve the durability of Ca-based sorbents and minimize their loss in adsorption capacity. The goal is to increase the active surface area and the stability of the pore structure of the sorbent, which would enhance their efficiency for CO2 capture. Material chemistry methods oriented to this objective are generally focused on the use of rigid porous materials as carriers of the Ca-based sorbents, use of additives to improve the sorbent thermal stability, reduction of the sorbent particle size down to the nanometer scale, and use of synthetic precursors to produce novel sorbents with a rich micropore structure. Besides enhancing the thermal stability in newly developed synthetic sorbents, an issue of concern is to promote their mechanical stability. Attrition of natural limestone particles is a main problem affecting the sorbent performance in the Ca-looping process. This paper is devoted to a critical review on the novel Ca-based sorbents developed in the last few years with improved thermal and mechanical stability.

Ca-based synthetic materials with enhanced CO2 capture efficiency

Highly porous microbial aggregates occur in a variety of environments ranging from marine snow in oceans to floes in biological reactors. For permeable aggregates undergoing gravitational settling or subjected to fluid shear, predicted velocities of intra-aggregate flow range from 1 to 100 pm s I. Estimation of intra-aggregate velocities requires spccihcation of the aggregate size, porosity, and permeability, as well as a mean fluid shear rate characterizing the fluid turbulence. We have examined substrate removal by microorganisms in permeable aggregates with an analysis of mass transfer. The overall uptake by bacteria in aggregates can be up to 60% greater than uptake by dispcrscd bacteria. In general, a model of substrate uptake based on advective transport is more appropriate than a model based on diffusive transport for cells in large, permeable aggregates that utilize substrates with diffusivities < 1 Op6 cm2 s-l.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1987.32.5.1034

C.T. Tan

Papers

Sedimentation of a porous sphere