Abstract
This study investigates the integration of hydrothermal carbonisation (HTC) with anaerobic digestion (AD) as a valorisation route for two macroalgae species; S. latissima (SL) and F. serratus (FS). HTC reactions were conducted at temperatures of 150 °C, 200 °C and 250 °C, with resulting hydrochars, process waters and hydrothermal slurries assessed for biomethane potential yields. Un-treated SL generated similar biomethane levels compared to all SL slurries. Whereas all FS slurries improved biomethane yields compared to un-treated FS. Hydrochars represent a greater energy carrier if used as a solid fuel, rather than a feedstock for anaerobic digestion. Integrating HTC and AD, through hydrochar combustion and process water digestion has a greater energetic output than anaerobic digestion of the un-treated macroalgae. Treatment at 150 °C, with separate utilisation of products, can improve the energetic output of S. latissima and F. serratus by 47% and 172% respectively, compared to digestion of the un-treated macroalgae.
Highlights
In the shift towards a bio-based economy there is an ever-increasing demand for the generation of renewable biofuels from feedstocks which do not compete for land with terrestrial food and feed crops (Kraan, 2013)
Macroalgae char yields are significantly lower than alternative feedstocks, such as lignocellulose 40–70% (Smith et al, 2016), corn stover 36–63% (Machado et al, 2018), orange pomace 37–54% (Erdogan et al, 2015) and cassava rhizome 51–57% (Nakason et al, 2018)
This study investigates the potential for biomethane generation from the hydrothermal products derived from two species of macroalgae; S. latissima and F. serratus for each of these integration approaches
Summary
In the shift towards a bio-based economy there is an ever-increasing demand for the generation of renewable biofuels from feedstocks which do not compete for land with terrestrial food and feed crops (Kraan, 2013). The use of macroalgae to produce third generation biofuels could overcome inherent disadvantages of utilising first and second generation crops (Montingelli et al, 2015). Despite the advantages of using macroalgae over terrestrial biomass, inherent physiochemical properties hinder the use of seaweeds in conventional thermal conversion technologies. Such properties include a high moisture content and a high inorganic content, resulting in a lower heating value than terrestrial crops and problems with corrosion and fouling (Ross et al, 2008). The presence of high concentrations of alkali metals within macroalgae creates a high tendency for slagging, fouling and corrosion during utilization during combustion, pyrolysis or gasification (Smith and Ross, 2016)
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