Microalgae cultivation with valorisation of CO2 separated from CH4 in a biogas upgrading process: a life cycle assessment

In accordance with the circular economy concept, new trends in industry are addressed to the integration of different production systems in order to minimise waste flows to the environment and resource depletion. The biogas and biomethane industry clearly offers a great potential from this point of view: biogas production is rising worldwide, and the possibility of obtaining a much more valuable energy carrier such as biomethane is pushing towards the further processing of biogas with an upgrading phase, thus obtaining two separated gaseous flows. The main one is indeed a concentrated flow of biomethane, but the process inherently determines the production of another relevant flow, the off gas containing mostly CO2 deriving from the partial oxidation of the organic substrate. The technical difficulties related to storage, as well as the scarce economic value compared to the main product, induce in most cases to consider the off-gas stream as a waste and to release it into the atmosphere. Although merchant CO2 commonly used in various industrial applications has a higher purity grade (>99.5% v/v), the off-gas from biogas upgrading, despite a CO2 concentration around 90% v/v, may equally have potential uses, without the need for further treatment; one of these can be algal cultivation for the production of bioenergy or high added value compounds. Therefore, in the present work the valorisation of a waste CO2 stream from a biogas upgrading process is considered, through its direct blowing in a microalgae cultivation system, aimed at the production of bioactive compounds with high quality standards. The main objective is to quantify the environmental benefits due to the switch from merchant CO2 to waste CO2. In order to do this, the Life Cycle Assessment methodology is used, according to the ISO14040 standards. The microalgal cultivation process is assessed at pilot scale, considering two different scenarios: in the first one the biomass is fed with a stream of merchant CO2 of high purity grade, while in the second one waste CO2 from biogas upgrading is used for the same purpose. The results show that a better environmental performance is achieved when using a stream of waste CO2 rather than pure merchant CO2, thanks to a slight increase in productivity and to lower impacts related to the background process of CO2 production. In conclusion, the present study suggests that there are interesting possibilities to integrate the processes of biogas upgrading and microalgae cultivation, and this symbiosis can bring to positive results in terms of production yields of microalgae cultivation systems and improvement of overall environmental performance. However, further research is needed to ensure that algal biomass quality standards are not compromised when waste CO2 stream is used in the cultivation process.