IS CARBON SEQUESTRATION A POSSIBLE APPLICATION OF BIOCHAR?

Adsorption on solid matrix represents a well assessed method for the separation and quantification of CO2 from gas exhausts. An ideal CO2 sorbent: i) should exhibit high selectivity toward CO2 over N2 and other exhaust components (CO, NH3 and light hydrocarbons), ii) should be produced via inexpensive and low-energy consumption methods and by using renewable resources as precursors and iii) should exhibit flexible morphologies, pore structures and functionalities. 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. As a general rule, high CO2 uptakes correspond to samples with high micropore volumes. Biochar is a stable microporous carbon-rich by-product produced through pyrolysis/carbonization of plant- and animal-based biomass and could represent a promising candidate for environmental remediation. Aim of this work is to investigate the applicability of biochar in carbon sequestration area, also taking into account the great structural and compositional variability of biomasses after controlled thermal conversion (surface area, pore size distribution, ash content). Wet-chemical treatments, aimed to ameliorate the CO2 uptakes by biochar chemical modification were also explored. Cellulose fibers and Populus nigra wood were treated at different temperatures in a steam assisted slow pyrolysis lab-scale plant and the resulting biochars were analyzed as raw carbon-based materials and after wetchemical treatments. The goal of these adsorption tests was the identification of the optimal shape, size, geometry and chemistry of the pores as ought to be for CO2 capture. The gas storage ability of the two sets of samples was evaluated with volumetric analyzers at various temperatures (from cryogenic to near ambient conditions) from vacuum up to ambient pressures, under equilibrium conditions by using N2, CO2, CH4 as probe molecules. The adsorption data were analyzed and modeled to have a complete characterization of the textural properties of all the materials. Between the two set of materials investigated in this work, the biochars obtained through the pyrolysis of cellulose fibers exhibited the highest sorption capacities and the highest CO2/N2 and CO2/CH4 selectivities. In general, for these biochars, increasing values of surface area were detected as the pyrolysis temperature is raised. Morevoer, the analysis of CO2 isotherms at 273 K revealed that the adsorbed CO2 volume by these biochars increased with pyrolysis temperature, indicating that a higher pyrolysis temperature allowed the development of a narrower microporosity. Results indicate that the chemical modifications of biochars did not lead to a great improvement of the CO2 sorption capacity. Only in the case of the sample after base-leaching treatment, a slight CO2 sorption increase was detected and explained with the partial removal of the no-adsorbing matrix (ashes). In some cases the chemical modification caused a dramatic surface area drop and a complete blockage of pores with a significant reduction of CO2 sorption capacity.