Biogas production from seaweed biomass : a biorefinery approach

摘要

As the demand for energy is increasing worldwide, many countries are becomingudincreasingly dependent on fossil fuel consumption, leading to a rapid increase in carbonuddioxide and reduction of petroleum reserves.udAlternative and viable options to replace fossil fuels, improve energy security andudreduce greenhouse emissions have been proposed worldwide. Marine macroalgaeud(seaweed) has emerged as an alternative feedstock for the production of a myriad ofudrenewable fuels, such as biogas. The implementation of the anaerobic digestion (AD)udprocess of seaweed requires optimisation before commercialisation is feasible. ThisudPhD study, therefore, aimed to establish a seaweed-based biorefinery approach toudproduce biogas as main commodity.udThe study initially focused on exposing two seaweed species common in Irish watersud(Laminaria digitata and Saccharina latissima) to chemical, mechanical, enzymatic andudphysical pretreatment methods in order to enhance the release of macromoleculesud(lipids, protein, total carbohydrate and reducing sugars) and, ultimately, increase biodigestibilityudto produce biogas.udResults showed that, among all chemical pretreatment conditions tested in this study,uddilute acid hydrolysis (4% HNO3 at 130ºC for 2 hrs) had the greatest effect in releasingudmacromolecules from L. digitata and S. latissima. The environmentally friendlyudpretreatments (freezer milling, oxalic acid and the enzymatic product Cellulase)udimproved the recovery of reducing sugars.udThe two seaweed species were subjected to AD to investigate their suitability toudgenerate biogas as source of renewable energy in 120 ml and 1.0 L size reactors.udPretreatments inhibited the anaerobic digestion (AD) process and only a 6% increase inudbiogas production was obtained when the biomass was subjected to a combination ofud2.0% citric acid and Cellulase.udFor an economically viable digester operation, digester temperature setting is one of theudmost critical factors. Reactors incubated at a mesophilic temperature were moreud5udeffective for biogas and methane production efficiency than either thermophilic orudpsychrophilic digesters during the AD of L. digitataudThe AD of different seaweed species commonly found in Irish and the NorthernudAtlantic Ocean was compared in order to evaluate their potential to produce biogas. Theudlowest concentration of biogas was achieved from the AD of Fucus serratus. S.udlatissima, Saccorhiza polyschides and L. digitata produced the highest biogas yields,udmaking the three species prospective candidates for the production of biogas as audrenewable source of energy.udThe seaweed-based biorefinery model integrates the AD of by-products from theudbiodiesel (glycerol) and the livestock industry (bovine slurry) to produce biogas. Theudanaerobic co-digestion of these waste streams with either L. digitata or S. latissimaudincreased biogas and methane yields when compared to AD of the seaweed alone.udResults show that the process could be a promising approach to integrating these byproductsudin order to generate biogas.udDuring experiments to investigate the scaling up of the process, in 10 L pilot plants, 217udand 305 ml g/VS of methane were produced from the anaerobic digestion of L. digitataudand S. latissima, respectively. The low volatile solid destruction, high alkalinity andudaccumulation of H2S caused a reduction in methane production. The organic residueud(digestate) generated after the AD of L. digitata was shown to be a source of biofertiliserudthat can be used to enhance the growth rate of two biofuel crops, ryegrass andudsunflower.udThe results obtained from this study provided essential data to support the scale-up ofudanaerobic digestion of seaweed in order to generate biogas as a source of renewableudenergy. A seaweed-based biorefinery approach achieved the extraction ofudmacromolecules, the co-digestion of waste products, production of biogas and digestateudre-use as source of fertiliser.