Biogas production from seaweed biomass : a biorefinery approach

Abstract

As the demand for energy is increasing worldwide, many countries are becoming increasingly dependent on fossil fuel consumption, leading to a rapid increase in carbon dioxide and reduction of petroleum reserves. Alternative and viable options to replace fossil fuels, improve energy security and reduce greenhouse emissions have been proposed worldwide. Marine macroalgae (seaweed) has emerged as an alternative feedstock for the production of a myriad of renewable fuels, such as biogas. The implementation of the anaerobic digestion (AD) process of seaweed requires optimisation before commercialisation is feasible. This PhD study, therefore, aimed to establish a seaweed-based biorefinery approach to produce biogas as main commodity. The study initially focused on exposing two seaweed species common in Irish waters (Laminaria digitata and Saccharina latissima) to chemical, mechanical, enzymatic and physical pretreatment methods in order to enhance the release of macromolecules (lipids, protein, total carbohydrate and reducing sugars) and, ultimately, increase biodigestibility to produce biogas. Results showed that, among all chemical pretreatment conditions tested in this study, dilute acid hydrolysis (4% HNO3 at 130ºC for 2 hrs) had the greatest effect in releasing macromolecules from L. digitata and S. latissima. The environmentally friendly pretreatments (freezer milling, oxalic acid and the enzymatic product Cellulase) improved the recovery of reducing sugars. The two seaweed species were subjected to AD to investigate their suitability to generate biogas as source of renewable energy in 120 ml and 1.0 L size reactors. Pretreatments inhibited the anaerobic digestion (AD) process and only a 6% increase in biogas production was obtained when the biomass was subjected to a combination of 2.0% citric acid and Cellulase. For an economically viable digester operation, digester temperature setting is one of the most critical factors. Reactors incubated at a mesophilic temperature were more 5 effective for biogas and methane production efficiency than either thermophilic or psychrophilic digesters during the AD of L. digitata The AD of different seaweed species commonly found in Irish and the Northern Atlantic Ocean was compared in order to evaluate their potential to produce biogas. The lowest concentration of biogas was achieved from the AD of Fucus serratus. S. latissima, Saccorhiza polyschides and L. digitata produced the highest biogas yields, making the three species prospective candidates for the production of biogas as a renewable source of energy. The seaweed-based biorefinery model integrates the AD of by-products from the biodiesel (glycerol) and the livestock industry (bovine slurry) to produce biogas. The anaerobic co-digestion of these waste streams with either L. digitata or S. latissima increased biogas and methane yields when compared to AD of the seaweed alone. Results show that the process could be a promising approach to integrating these byproducts in order to generate biogas. During experiments to investigate the scaling up of the process, in 10 L pilot plants, 217 and 305 ml g/VS of methane were produced from the anaerobic digestion of L. digitata and S. latissima, respectively. The low volatile solid destruction, high alkalinity and accumulation of H2S caused a reduction in methane production. The organic residue (digestate) generated after the AD of L. digitata was shown to be a source of biofertiliser that can be used to enhance the growth rate of two biofuel crops, ryegrass and sunflower. The results obtained from this study provided essential data to support the scale-up of anaerobic digestion of seaweed in order to generate biogas as a source of renewable energy. A seaweed-based biorefinery approach achieved the extraction of macromolecules, the co-digestion of waste products, production of biogas and digestate re-use as source of fertiliser.