Anaerobic Digestion Of Sargassum Horueri For Biogas And Pyrolysis Of Digestate For Phenol Production

Anaerobic digestion is an effective technology for agricultural residue and biomass treatments,which has been widely applied both in domestic and abroad,because of its low power consumption,cleanliness and sustainability.The sargassum horueri is a biomass used for the ecological restoration of the sea.However,large-scale decay of mature seaweeds in the sea has caused a series of environmental pollution problems.Anaerobic digestion of sargassum horueri for biogas production is a resource utilization method of sargassum horueri.At present,with the development of large-scale and industrialization of biogas project,scientists have paid more attention to the hazard-free treatment and resource utilization of digestate.Traditional utilization of digestate such as aquaculture and land application have caused secondary pollution of water and soil.Besides,these methods are unable to handle large-scale digestate.Therefore,digestate problem has restricted the development of large-scale biogas engineering.Recently,researchers have been committed to developing new industrial methods of digestate utilization and obtaining high value-added products.In this paper,the mesophilic anaerobic digestion experiment of sargassum horueri was conducted to investigate biogas potential and methane content in biogas,in order to analyze the feasibility of raw material anaerobic digestion and found a solid basis for biogas project technology of marine biomass in the future.Results showed that the biogas production of intermittent fermentation was 110.6mL/g TS and methane content was above 60%.The gas production in continuous fermentation was 108ml/g TS with methane content around 55%.Compared with other macroalgae and land biomass,sargassum horueri had no advantage on biogas production,because of the high content of metal ions,nitrogen and crystal structure of cellulose in feedstock.Firstly,high content of metal ions in raw material inhibited the activity of fermentation microorganisms.The second reason came from the high content of nitrogen,which made the carbon-nitrogen ratio of the fermentation system exceeded the theoretical optimum value.Moreover.the crystal structure of cellulose in raw material was dense and regular,which increasing the difficulty of decomposition by microorganism and reducing the utilization of cellulose.Pyrolysis of sargassum horueri based digestate was conducted for phenol production.The reaction conditions such as temperature,pressure,retention time,and addition of activated carbon(AC)were investigated.Results indicated that increasing temperature enhanced the bio-oil yield below 450℃,while which decreased after that temperature.The maximum bio-oil yield was about 31% at 450℃.The maximum bio-oil yield was about 31% at 450℃.On contrary,reaction pressure and retention time had no significant effect on bio-oil yield.Adding AC greatly promotes the pyrolysis reaction by increasing the bio-oil yield to 38.78%.The bio-oil obtained mainly contains mainly phenolics,nitrogen-containing compounds and ketones,with a small amount of alcohols,acids,esters and hydrocarbons.The content of phenolic compounds gradually increased with temperature rose below 500℃.Meanwhile,the reaction pressure promoted the formation of phenolic compounds below 8Mpa.The retention time of 2min was chosen as the optimal condition.Besides,AC has no significant effect on composition of bio-oil.In sum,the maximum phenolic compounds content of 72.2% was achieved at 450℃,5Mpa,with AC and digestate ratio of 1:1.The mechanism and reaction pathway from digestate to phenol production was also investigated in this project.Mechanism indicated the cleavage of β-O-4 bond and decarboxylation of side chains occurred below 350℃,by which small molecular compounds and CO2 were generated.The C-C bonds cleavage,demethoxylation,demethylation,Cα-Cβ dehydrogenation,and methylation reactions took place in the system accompany with temperature rise,resulting in generation of simple phenolic compounds.At the same time,CO and CH4 gas generated along with the decarburization of the structure and the cleavage reaction of the side chains methylene.In addition,the reaction pressure promoted the demethoxylation reaction,which converted guaiacols and syringols into phenol production significantly.