Modelling and simulation of anaerobic digestion of various lignocellulosic substrates in batch reactors: Influence of lignin content and phenolic compounds II

Hydrolysis of lignocellulosic substrates is impeded by the lignin polymer, acting as a seal around the cellulose and hemicellulose polymer. To facilitate hydrolysis and improve biomethane production, pretreatment of the substrate is required. However harsh pretreatments prior to anaerobic digestion can cause a release of inhibitory phenolic compounds such as vanillic acid, p-coumaric acid, ferulic acid and hydroxybenzoic acid. In this study the developed anaerobic digestion model takes the substrate lignin concentration as well as the concentration of such phenolic compounds into account. The biomethane production and hydrolysis rate of seven different substrates was described and simulated. A good agreement between simulations and measurements was obtained, as the maximum Theil's inequality coefficient for the different substrates was 0.14. The impact of higher concentrations of the phenolic compounds, up to 2000 mg/l, was simulated for two of the substrates namely, hemp straw and miscanthus. As significant inhibition only occurred for the anaerobic digestion of miscanthus, a global sensitivity analysis and parameter estimation (assessing all the processes in the model) was done for this substrate. The global sensitivity analysis showed the great importance of the hydrolysis rate and the need to research factors, i.e. inhibitors and substrate types, influencing this hydrolysis step.