Analytical evaluation of the coupling of hydrothermal carbonization and pyrolysis (HTC-Py) for the obtainment of bioavailable products

Fir sawdust was firstly pre-treated through hydrothermal carbonization (HTC) at two different temperatures: 150 and 200 degrees C. The resulting hydrochars were then subjected to intermediate pyrolysis at 550 degrees C. The main focus of this study was to investigate the impact of HTC on the pyrolysis products in terms of both energy and mass yield, considering both HTC and pyrolysis for delivering bioavailable and fermentable substances. HTC, in fact, can dissolve most of hemicellulose, providing an hydrochar which produces high (up to 8.7% w/w with 200 degrees C HTC) yield of levoglucosan. Such yield, which is close to that achieved from cellulose pyrolysis with the same pyrolysis apparatus, suggests that HTC is an effective pre-treatment for enhancing the selectivity of pyrolysis toward anhydrosugars. To highlight the overall potential of products from the hydrothermal carbonization-pyrolysis (HTC-Py) scheme, further characterizations were conducted on the HTC liquids (HTC-L) and aqueous phase liquids of obtained bio-oils (APL) using various techniques, including targeted GC-MS and size exclusion chromatography (SEC-RID). Sankey diagrams with overall yield were built to evaluate the overall selectivity of HTCPy in producing bioavailable substances and sugars. Results showed that HTC-Py, through hemicellulose fractionation and improved pyrolysis selectivity, allows increasing from 52% to 58% the yield of bioavailable organics and, more importantly, HTC pre-treatment allowed to increase the yield products that can be fermented with biotechnological workhorse (namely sugars, dissolved hemicellulose, and carbon monoxide) from 4% to 16%.