Integration of Anaerobic Digestion into a Catalytic Lignocellulosic Biorefinery

Hurst, George (2023) Integration of Anaerobic Digestion into a Catalytic Lignocellulosic Biorefinery. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

During the acid catalysed production of platform chemical such as levulinic acid, furfural and 5-hydroxymethylfurfural from lignocellulosic biomass, significant amounts of insoluble carbonaceous solid residue is produced as by-product. The solid residue by-product represents a significant waste stream that contributes to the biorefinery inefficiency which requires characterisation and valorisation for the efficient use of the whole biomass feedstock. The solid residue was composed of a matrix of unreacted biomass, inert biomass fractions, acid catalysed humins from the cross-polymerisation of reactive intermediates and the carbonised chars, that vary in composition and properties due to both catalysts type and reaction conditions. Solid residue produced with homogenous catalysis under aqueous conditions primarily consists of oxygen-rich surface properties due to humin condensation, similar to that of hydrochar formed under hydrothermal carbonisation. While solid residue formed with heterogeneous catalysts showed markedly different properties that were more akin to unreacted biomass, with minimal surface or bulk modification. Response surface methodology was determined to be sufficient to model solid residue yields alongside high value product yields, most notable levulinic acid. The reaction temperature was found to have the largest effect on both surface properties and yields with lesser effects due to reaction temperature and catalyst concentration. The modelling of the aqueous by-product found that formic acid was consumed during the acid catalysis process and may also contribute to the solid residue condensation products. While the addition of trace hydrochloric acid was found to significantly improve levulinic acid yields with sulphated zirconia suggesting synergistic catalysis could improve a range of heterogeneous catalysts. The solid residue properties where characterised in depth with the homogenous catalyst related solid residue found to have similar properties to that of hydrochar sufficient to improve anaerobic digestion. During the anaerobic digestion of animal manures, the effects of solid residue was found to reduce ammonium inhibitor concentrations and improve microbial diversity sufficient to increase 14-day methane yields. The application of solid residue from lignocellulosic biorefineries as an anaerobic digestion supplement could provide a low-cost adsorbent material for the improvement of a wide range of nitrogen-rich feedstocks that would otherwise be under-utilised and present environmental concern. Such integration would reduce the carbon emissions from green chemical production while adding increasing the value of the largest waste product.