Carbon Monoliths in Adsorption-based Post-combustion CO2 Capture

Abstract

The development of adsorption-based technologies for post-combustion CO2 capture requires finding an adsorbent with adequate equilibrium and transport properties. Structured adsorbents are appealing for fixed-bed TSA processes, because they present lower pressure drop and higher thermal conductivity than conventional adsorbent beds, which facilitates the use of higher flowrates and shorter cycle times, maximizing throughput. In this work, the equilibrium of adsorption of the main flue gas components, CO2, N2, O2 and H2O over two carbon honeycomb monoliths with different textural development has been measured in a pressure and temperature range of interest for post-combustion CO2 capture: between 0°C and 70°C and up to 120 kPa for CO2, N2, and O2, and between 30°C and 70°C up to the corresponding saturation pressure for H2O. The maximum adsorption capacity and isosteric heat of adsorption follows the order: H2O > CO2 > N2 ≈ O2. The carbon monoliths present equilibrium selectivity towards CO2 and H2O over N2 and O2 at typical flue gas conditions. Moderate activation is preferred to maximize the CO2 adsorption capacity and selectivity in these conditions. The Toth model was employed to fit the equilibrium data for the adsorption of CO2, N2, and O2 with highly satisfactory results. The adsorption isotherms of H2O presents the characteristic “s” shape of hydrophobic adsorbents, with low uptakes at low relative pressures, which will facilitate H2O desorption during cyclic operation. These were fitted using the extended CMMS model, which describes satisfactorily the experimental data in the full relative humidity range. The adsorption kinetics were preliminary evaluated by measuring the rate of mass uptake from a mixture with 10% CO2 (balance N2) at 30°C, 50°C and 70°C, and the data were fitted to the linear driving force model to obtain the kinetic rate constants.