Effect of alkali metal ions presence on the products of xylan steam assisted slow pyrolysis

Biomass pyrolysis has been extensively investigated during the last decades both as a stand-­alone process for the production of bio-­ based fuels and for the implications as intermediate stage in other thermochemical processes such as gasification and combustion. The comprehension of biomass pyrolysis mechanisms can greatly benefit from the study of the pyrolysis behavior of separated biomass organic components as well as of the effect of metal ions on their decomposition pathways. Only few works reported about the metal ions effect on the pyrolysis of hemicellulose. In the present paper the influence of metal ions on hemicellulose pyrolysis was studied choosing commercial xylan as reference. The thermal behavior of raw and demineralized xylan samples was studied in a thermogravimetric apparatus at atmospheric pressure under nitrogen from 323 K up to 973 K with a heating rate of 5 K/min. Then steam assisted pyrolysis tests were carried out up to two different final temperatures, 873 and 973 K, at pressure P = 5 × 105 Pa and heating rate HR = 5 K/min. Products yields, gas releasing rates as a function of the temperature, gas and liquid compositions were compared for raw and demineralized samples. The raw and demineralized xylan exhibited different pyrolytic behaviors both in nitrogen and steam atmospheres. Raw xylan produced a higher amount of char compared to the demineralized one in nitrogen atmosphere, whereas negligible differences in char yields were observed in presence of steam. Demineralization pretreatment promoted the production of a large amount of furfural and depressed the production of gas in both the pyrolysis and the gasification stages.