EFFECT OF Na+ IONS ON THE SLOW PYROLYSIS OF XYLAN

Biomass pyrolysis was extensively studied for its peculiar flexibility with respect to the desired products - bio-oil, bio- gas and char - that can be used as fuels [1, 2], chemicals sources [2], and low cost materials for a wide range of applications [1]. Given the high variability in biomass composition and the presence of inherent metal ions in the biomass, demineralization and metal ions doping procedures were applied to different biomasses and cellulose to study the influence of inorganics on products yield, gas and liquid composition, and structural properties of char [3, 4]. In this framework, less work was done on hemicellulose and lignin. It was found that alkali and earth alkali metals (AAEMs) in the biomass undergoing pyrolysis diverted the decomposition mechanisms of cellulose from depolymerisation reactions to monomers fragmentation, thus altering products distribution in favour of gaseous and condensable lower molecular weight compounds [5]. Considering its polysaccharide nature, hemicellulose was expected to react similarly to cellulose. However, only few works reported about the effect of AAEMs on extracted hemicelluloses or proxy macromolecules [6, 7] despite of their high content in both annual and perennial plants. The chemical variety of hemicelluloses and the difficulty to apply the standard washing or impregnation procedures usually adopted for biomasses and cellulose make this kind of approach difficult to handle. Moreover, despite of the many efforts directed to the numerical modelling of the kinetics of biomass pyrolysis, little attention was given to the impact of AAEMs on the pyrolysis process in terms of reactions involved. Based on the work of Leng et al [8], Ferreiro et al. [9] proposed a modified kinetic mechanism for the pyrolysis of cellulose based on the Bio-PoliMI cellulose sub-mechanism. The modified kinetic mechanism was capable of reproducing the increase of char and gas yield induced by the addition of KCl. However, gas speciation was not well captured. Recently, Ranzi et al. [10] proposed an adjustment in their previous kinetic mechanism [11] to capture the catalytic effect of the ash during the pyrolysis process of biomass. The effect of ash was included in both cellulose and hemicellulose mechanisms. Similarly to cellulose, the presence of ashes in hemicellulose pyrolysis increased the formation of char and dehydration products while the formation of higher molecular weight species was inhibited. In the present work, an experimental campaign on the effect of Na+ on hemicellulose pyrolysis was conducted with the aim of producing a database for the further validation of the updated version of Bio-PoliMI mechanism, more specifically the hemicellulose sub-mechanism. To carry out the experimental work xylan was used as proxy compound for hardwood hemicellulose. Xylan was demineralized through a cation exchange resin in order to reduce the presence of inherent inorganics. Then, the demineralized sample was doped with three different Na+ concentrations, namely 0.1, 0.3 and 0.6 wt. %. Thermogravimetric analyses (TGA) of the samples were performed under inert atmosphere at 7 K/min up to 973 K to obtain TG and DTG curves. Then, pyrolysis tests of the same samples were performed under the same operating conditions as TGA to obtain products yields, gas composition and yields of the main condensable species [9]. The results show that doped and demineralized xylan samples exhibited different pyrolytic behaviours. The gas production was favoured at the expense of pyrolysis liquids and the evolution of the releasing rate of permanent gases (mainly CO2 and CO) along the temperature was greatly altered by the presence of Na+.