UNDERSTANDING THE EFFECT OF METAL IONS ON RAW AND DEMINERALIZED XYLAN SAMPLES DURING PYROLYSIS

Biomass pyrolysis is considered a route to the production of both smart energy carriers and a valuable material, the biochar, from waste residues thanks to its high flexibility with respect to the desired product: bio-oil, bio-gas or char. Biomass chemical composition greatly affects both product yields and characteristics given the different chemical nature and distribution of its main organic components, namely cellulose, hemicellulose and lignin. The comprehension of biomass pyrolysis mechanisms benefits from the study of biomass components pyrolysis behavior as well as of the effect of metal ions on their decomposition pathways. Since only few works report about the metal ions effect on the pyrolysis of xylan, recognized as representative of hemicellulose in pyrolysis experiments, in this work we performed a quite complete demineralization of commercial beechwood xylan (ash content 4.4 wt.%, mainly alkali and earth alkali metals). The pyrolysis behavior of demineralized xylan (DX), both in nitrogen and in steam, was compared to the raw xylan (X) in order to understand more about the effect of metals ions on xylan pyrolysis. The thermal behavior of X and DX samples was studied at first in a thermogravimetric apparatus and then by carrying out steam assisted pyrolysis tests up to two different final temperatures (873 and 973 K). Products yields, gas releasing rates as a function of the temperature, gas and liquid compositions were compared for the two samples. X and DX samples exhibited different pyrolytic behaviors both in nitrogen and steam atmospheres. Under nitrogen it was observed that metal ions in X were responsible of a slight anticipation of the initial decomposition temperature and of the presence of a second important event (peaked at 550 K) in the devolatilization curve that is only slightly visible at higher temperature in the DTG curve of DX. Moreover, in the case of X, the devolatilization of lighter species leaves higher amount of solid residue compared to the demineralized sample. On the other hand, negligible differences in char yields for X and DX were observed in presence of steam. Results obtained in presence of steam indicated that metal ions promoted cracking reactions producing permanent gases (CO and CO2) and ring opening instead of dehydration reactions, depressing the production of furfural and of CO.