Comparison of Different Natural Sorbents for Removing CO₂ from Combustion Gases, as Studied in a Bench-Scale Fluidized Bed

Abstract

The reaction of CO 2 with porous particles of CaO in CaO (cr) + CO 2(g) f CaCO 3(cr) was studied, along with its reverse reaction, for chicken eggshells, mussel shells, and limestone. Reaction I is a promising way of removing CO 2 , e.g., from the exhaust of a power station, so that a pure stream of CO 2 can subsequently be produced for sequestration by calcining (roasting) the solid CaCO 3 from reaction I. The reverse of reaction I regenerates the sorbent, which can thus be used cyclically. The forward and reverse steps of reaction I were investigated using a small electrically heated bed of sand at ∼750 °C, fluidized by N 2 . Typically, a sample (∼2 g) of cleaned calcareous material (sieved to ∼600 µm) was added to the hot bed, and the CO 2 produced was measured, while the material was fully calcined. Next, enough CO 2 was added to the fluidizing N 2 to raise [CO 2 ] to above the value for equilibrium; thus, the CaO was carbonated. This forward step of reaction I is shown to exhibit an apparent final conversion, the carrying capacity of the sorbent, below unity. This carrying capacity reduces after several cycles of calcination and carbonation, because blockage of pores denies access of CO 2 to part of the CaO. After several such cycles, particles were removed from the reactor, either in their partially carbonated or fully calcined states, for studies using gas adsorption analysis, X-ray diffraction, and mercury porosimetry. Interestingly, it was found for all three sorbents that the carrying capacity of CaO for CO 2 degraded at a similar rate. The carrying capacity was roughly proportional to the volume of pores narrower than ∼100 nm, as measured by Barrett-Joyner-Halenda (BJH) gas adsorption analysis. Evidently, these narrow pores contain both the surface area for CO 2 to absorb and the empty volume to accommodate the product, CaCO 3 . The resistance of eggshells to attrition was broadly comparable to that of Purbeck (U.K.) limestone.

The calcium looping cycle for CO2 capture from power generation, cement manufacture and hydrogen production

“…These properties depend on impurities and the conditions / age of formation. Dolomites have shown a reduced decay rate upon cycling, despite a lower initial reactivity owing to a lower amount of CaO per unit mass (Dobner et al, 1977;Silaban et al, 1996); natural sorbents such as shells have also been studied and have been shown to exhibit mildly increased reactivity in comparison to natural limestone in some cases (Ives et al, 2008;Li et al, 2009). Doping of natural limestones has been investigated; however the results have been variable (Salvador et al, 2003;Fennell et al, 2007b;Florin and Harris, 2008b;Sun et al, 2008), even when the same dopant has been used.…”

Ca looping technology: current status, developments and future directions

“…Second, there is vast experience in handling lime products, and any spent material from a lime-based process can, in principle, either be used or disposed of relatively easily. 10,19 Grasa and Abanades 20 carried out an extended series of trials and found that there is a residual carrying capacity remaining aft er 500 cycles. 17 Yong et al 18 represent the ideal properties of a sorbent in terms of fi ve distinct characteristics (Fig.…”

High activity of acid-treated quail eggshell catalysts in the transesterification of palm oil with methanol

“…Since the eggshells of chicken and quail are composed of calcium carbonates, their calcination at above 700°C produces basic calcium oxide which can be used as a basic adsorbent for carbon dioxide (Ives et al, 2008) and as a catalyst for the production of biodiesel (Wei et al, 2009). These eggshells are composed of three layers: a thin cuticle outer layer, a thick palisade middle layer and a thin mammillary inner layer.…”