Could pyrolysis substitute hydrolysis in 2nd generation biomass valorization strategies? A chemical oxygen demand (COD) approach

Pyrolysis converts a wide array of feedstock into a mixture of products which can be used by microorganisms in hybrid thermochemical-biological (HTB) processes. Such approach can be very efficient in the valorization of lignocellulosic waste streams. In this study, a method based on calculation of Chemical Oxygen Demand (a measure of chemical energy) was proposed in order to assess the maximum potential of HTB process from lignocellulose. A deep literature survey on papers dealing with pyrolysis of lignocellulosic feedstock ended up with a database of bioavailable matter (e.g. water-soluble) obtained under different reactor configurations and various biomass pre-treatment methods. Such evaluation suggests that, at the present state of the research, intermediate and non-optimized fast pyrolysis is able to deliver bioavailable products with 30–40% yield. Considering the downstream processes typical of HTB (e.g. detoxification or use of specific microorganisms), this depolymerization performance is attractive only using a feedstock that is not already suitable for hydrolysis. Innovative combination of pre-treatment and fast pyrolysis can transform more than 60% of the feedstock chemical energy into bioavailable products. This value, higher than those obtained through conventional hydrolysis-based strategies, confirms the great promise of HTB processing of lignocellulose into valuable intermediates or final bioproducts.