BIOCHARS AS LOW-COST CO2 ADSORBENTS: PRELIMINARY RESULTS AND PERSPECTIVES

In a view of exploiting adsorption on solid matrix as diagnostic tool for the monitoring of gaseous pollutants emission in combustion, this study evaluates the differences in affinity of selected gases in meso-nanoporous materials and the interactions in the confined pore voids to maximize physisorption and/or separation. The adsorption tests have been performed on carbonaceous materials produced by steam assisted slow pyrolysis experiments on cellulose fibers and Populus nigra wood at different temperatures. The goal of these tests was the identification of the optimal shape, size, geometry and chemistry of the pores as ought to be for a given application i.e. the monitoring of pollutants emission. The gas storage ability of the materials was sampled by using N2, CO2, CH4 and various temperatures (from cryogenic to near ambient conditions) and from vacuum up to ambient pressures, under equilibrium conditions. The adsorption data were analyzed and modeled to have a complete characterization of the textural properties of all the materials. The prediction of the CO2/N2 and CO2/CH4 selectivities of selected samples was performed from the single component experimental adsorption data using the Ideal Adsorbed Solution Theory (IAST). All the investigated chars, with the exception of one char obtained from Populus nigra wood, were classified as microporous materials and increasing values of surface area were detected as the pyrolysis temperature is raised. Accordingly, the volume of micropores grew with the increase of temperature. In all the samples the total pore volume roughly corresponded to the micropores total volume, which indicated a low contribution of mesoporosity to the overall porous structure. The analysis of CO2 isotherms at 273 K revealed that the CO2 uptake of the chars increased with pyrolysis temperature, indicating that a higher pyrolysis temperature allowed the development of a narrower microporosity. These structural features are relevant in view of the adsorbent selection and optimization since the main parameters governing both the CO2 uptake capacity and selectivity are micropore volume and size and chemical functionalization of the pores. In general, high CO2 uptakes correspond to samples with high micropore volumes. CO2/N2 and CO2/CH4 selectivities were also evaluated only for selected samples and a good affinity toward the adsorption of CO2 and low affinities toward the adsorption of CH4 and N2 was found in all the cases.