Integration of Hydrothermal Conversion and Anaerobic Fermentation for the Valorisation of High Ash Feedstocks

The generation of bioenergy is widely regarded as a key factor in reducing global GHG emissions. Alternative, high-ash feedstocks could provide an abundant source of sustainable biomass, which do not compete for valuable arable land with food or feed crops. However, thermal conversion of these biomass sources can prove problematic, due to a high moisture content, low bulk density and unfavourable ash chemistry. Biological conversion can also prove challenging, as recalcitrant biochemical structures limit the efficiency of biodegradation. The aim of this thesis was to investigate the potential for integrating hydrothermal and biological conversion technologies to improve the energy conversion efficiency (ECE) of alternative, high-ash biomass, compared to biological processing alone. This work focusses on the valorisation of two lignocellulosic feedstocks: water hyacinth (WH) and grass (GR), and three brown macroalgal species: S. latissima (SL), F. serratus (FS) and L. digitata (LD). A number of integration strategies exist between hydrothermal carbonisation (HTC) and anaerobic digestion (AD); including [i] AD of hydrochars, [ii] combustion of hydrochars and AD of process waters and [iii] AD of HTC slurry. The suitability of each integration strategy was assessed across a range of HTC reaction temperatures (150°C, 200°C and 250°C). The results show the separation of hydrochars for combustion and process waters for anaerobic digestion (AD) provides the greatest improvement in ECE. Higher HTC processing temperatures are associated with a lower energy output and lower ECE, due to decreased hydrochar yields and formation of inhibitory compounds in the process waters. However, hydrochar quality is compromised at lower processing temperatures, due to limited energy densification or removal of problematic ash species. This integration strategy proved particularly effective for SL, FS and GR. The ECE of SL (64%) was improved to between 71-90%, FS (31-35%) to between 57-91% and GR (50%) to between 63-82%. Although, HTC processing temperatures of between 200-250°C are recommended. The application of this HTC-AD integration strategy for LD appears to be seasonally variable. Winter-harvested LD shows an improvement in ECE from 59% to between 64-87%. However, the ECE of summer-harvested LD was not significantly improved using HTC-AD. WH-derived hydrochars showed no improvement in ash behaviour, potentially limiting the application as a combustion fuel. AD of WH-HTC slurry produced at 150°C improved the ECE of WH from 25% to 48% during one-stage digestion or 62% during two-stage digestion. Therefore, low-temperature hydrothermal pre-treatment could be a suitable valorisation strategy for WH. Overall, integrated hydrothermal and biological processing appears an effective strategy to improve the ECE of high ash feedstocks; overcoming the problems associated with thermochemical conversion. However, the optimised integration strategy and reaction conditions vary between different types of biomass.