Hemicellulose, cellulose and lignin interactions on Arundo donax steam assisted pyrolysis

Slow Steam pyrolysis of Arundo donax is proposed as possible process for the recovery of a solid material suitable to be used as biochar. In view of the evaluation of the feasibility of such a process, products yields, surface area of char and energetic content of gaseous products needed to assist energetically the process are relevant data to be evaluated in dependence on process operating conditions and biomass characteristics. Biomass main components (hemicellulose, cellulose and lignin) contribute to a different extent to the determination of products yield and characteristics both for their own intrinsic chemical nature and for the onset of possible interactions due to their simultaneous presence in a real biomass. Moreover, it is known that inorganic elements present in the biomass can affect pyrolysis mechanisms and consequently products yields and characteristics. In the present paper the influence of inorganic species and of possible interactions between biomass main components on pyrolysis of A. donax has been studied. To this aim steam assisted pyrolysis tests have been carried out on a mixture of xylan, cellulose and lignin resembling the composition of A. donax canes in a proper experimental apparatus up to 873 K, at pressure P = 5 × 105 Pa and heating rate HR = 5 K/min. Products yields, gas releasing rate as function of the temperature, gas composition and specific internal surface of char have been compared to the data computed from the superposition of the results obtained for the single components and to the behavior of untreated and demineralized samples of A. donax canes processed in the same experimental apparatus in the same operating conditions. Results obtained from this study show that primary pyrolysis of holocellulose is affected by the presence of inorganic species that depress the devolatilization of heavier compounds in favor of cracking reactions determining a higher release of light compounds, CO2 and CO. Moreover, interactions between lignin and cellulose are relevant in operating conditions where mass transfer phenomena cannot be neglected. Interesting results have been obtained for the solid residue as for the development of its internal surface that seems to be reduced from both interactions between biomass components and presence of inorganic ions.