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

The reaction of CO2 with porous particles of CaO in was studied, along with its reverse reaction, for chicken eggshells, mussel shells, and limestone. Reaction I is a promising way of removing CO2, e.g., from the exhaust of a power station, so that a pure stream of CO2 can subsequently be produced for sequestration by calcining (roasting) the solid CaCO3 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 N2. Typically, a sample (2 g) of cleaned calcareous material (sieved to 600 ?m) was added to the hot bed, and the CO2 produced was measured, while the material was fully calcined. Next, enough CO2 was added to the fluidizing N2 to raise [CO2] 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 CO2 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 CO2 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 CO2 to absorb and the empty volume to accommodate the product, CaCO3. The resistance of eggshells to attrition was broadly comparable to that of Purbeck (U.K.) limestone.