Microalgae photobioreactors integrated with biogas cogeneration plants: preliminary analysis for CO2 capture, nitrogen removal and energy recovery

Introduction Some of the issues open for cogeneration plants fueled with biogas from anaerobic digestion of manure are: - CO2 capture from the exhaust gases, in order to reduce greenhouse gases emissions; - nitrogen removal from the manure after anaerobic digestion, for a perspective agronomic use in sensitive areas like the Po Valley. The chemical and physical processes nowadays adopted for these tasks are expensive and feasible for large installations only. Fixing CO2 by bioreactors for algae production seems to be an alternative methodology better suitable for distributed cogeneration plants, more over this also allow to considerably reduce nitrogen loads from manure anaerobic digestion. The aim of this study is to propose a multidisciplinary approach to bioprocess engineering aspects, providing indications of system design. Methods The proposed technology for cogeneration system consists of an internal combustion engine and an anaerobic digester fed with sewage and manure from cattle, and supplemented vegetable biomass. The plant is 200 kW nominal electric power. It is supposed to produce exhaust gases with CO2 contents for about 5100 kg/day loading and a liquid digestate flow as about 50 m3/day. The simulation model is expected to use the Spirulina microalgae for the algal biomass production in photobioreactors for CO2 sequestering and nitrogen removal. The algal produced biomass is dried up to 15% moisture content and successively sent to a gasification unit. Results The system analysis allows to estimate a projected capacity of CO2 fixation by about 136 kg/h and a nitrogen removal rate by about 4 kg/h, according to the considered size of cogeneration plant. The analysis also simulates the thermodynamic behavior of a downdraft gasification reactor fed with dried Spirulina sp. Two alternative gas mixtures were considered as gasifying agents: Oxygen with Water Vapor and Air with Water Vapor. Equivalence ratio was respectively fixed to 0.25 and 0.30 in the two cases. The simulation model provided the following results: Oxygen + Water Vapor: gas flow = 157 Nm3/h; LHV = 8814 KJ/Nm3 Air + Water Vapor: gas flow = 266 Nm3/h; LHV = 4782 KJ/Nm3 Conclusions The preliminary carried out analysis provides a tool for plant engineering evaluating choices in terms of environmental sustainability and energy efficiency.