The Role of Hydrothermal Treatment in the Biorefining of Macroalgae

Bioenergy is a key part of the renewable energy sector which has yet to reach its full potential. Macroalgae as a biomass feedstock has many benefits over the first and second generation biofuels. The organic-rich aqueous phase resulting from the hydrothermal processing of macroalgae could additionally provide a means of supplying commodity chemicals or feedstock for anaerobic digestion (AD) as a by-product from the biofuel production. The work for this PhD has focussed on the hydrothermal carbonisation of some key species of seaweed: Saccharina latissima (SL) and Fucus serratus as well as Miscanthus giganteus. Hydrothermal Carbonisation (HTC) was performed on the samples along with some key model compounds and co-processed mixes of the terrestrial and algal biomass/model compounds in a 80:20 ratio, respectively. HTC was performed at 150◦C, 200◦C or 250◦C, for one hour. The hydrochars and aqueous phase products were characterised via ultimate and proximate analysis and XRF analysis along with mass balance calculations. The aqueous phase samples were also analysed via NMR, TOC, HPLC, pH, and GC-FID analysis. Anaerobic Digestion (AD) was also performed for the two 100% seaweed samples using a AMPTS(II) digester. A higher heating value of 24.2 MJ/kg was achieved for the co-processed mix at 250◦C, and the effect of co-processing was promotive in increasing the hydrochar yield up 4% at this temperature. The main findings indicate that the sugars in the pure biomass feedstock tend to decompose more at higher temperatures, as expected due to the hydrolysis of the polysaccharides, and produce more shorter chain acids at higher temperatures, eventually degrading into more gases at the 250◦C temperature. The abundance of VFAs increases with temperature, due to acidogenesis and degradation of the amino acids and fatty acids. The co-processed hydrochars shows significantly reduced heavy metals content compared
to the 100% seaweed hydrochars, due to the lower proportion of seaweed feedstock. However, the hydrochar from the co-processed mix falls remains just short of the slagging and fouling limits. It is recommended that further research on co-processing uses a 85:15 Miscanthus:Saccharina latissima ratio to increase the likelihood that the hydrochar produced adheres to slagging and fouling limits. Anaerobic digestion tests of the seaweed aqueous phase feedstocks yielded successful BMP production levels, achieving 160 ml CH4/gCOD for the SL 250◦C sample. The proposed biorefinery concept involving AD post-HTC treatment is therefore possible with the qualities of the aqueous phase suitable for anaerobic digestion, in addition to other potential further uses. The relatively new technology of Hydrothermal Carbonisation, coupled with the use of seaweed as part of the feedstock mix, is hence shown to be an important consideration for further development, to help bridge the gap between our current renewable energy capacity and the UK’s future renewable energy targets, to enable a more sustainable future.