Production of Syngas Suitable to be Used in Fermentation to Obtain Biochemical Added-Value Compounds

Abstract

Lignin-rich residues are a side stream in advanced ethanol biorefineries and they are usually used for heat and power production. However, an alternative is the lignin valorisation by thermochemical and biochemical processes that might be economically more suitable. In this work lignin-rich residues were firstly thermochemical converted to syngas in a fluidised bed gasifier, using steam and oxygen as gasifying agents. The obtained results so far showed that the rise of steam/lignin-rich residues ratio favoured the release of H2 and CO2 and led to a decrease in CO and hydrocarbons content, as steam reforming reactions were favoured. On the other hand, the rise of the equivalent ratio clearly favoured partial oxidation reactions and thus the release of CO and CO2, while H2 and hydrocarbons contents were reduced, due to their partial combustion. The increase of temperature favoured the formation of H2, at the expenses of hydrocarbons and tar release, as both decreased. However, it is fundamental to control gasification temperature to prevent ash melting and consequent bed agglomeration, due to the high ash content in lignin-rich residues, generally with high content of silica and alkali metals. The results led to the selection of gasification conditions (800 °C, equivalent ratio of about 0.13 and steam/lignin ratio of around 0.4) that headed to the following gas ratio CO/CO2/H2=1:0.7:0.7, which was tested for further bacterial fermentation. Fermentation tests with Butyribacterium methylotrophicum showed that the above syngas composition strongly influenced the production levels of acetic and butyric acids. Marked differences in the CO, H2 and CO2 consumption rates were also observed and were correlated with cell growth and metabolites production.