Metrics for sustainability analysis of post-combustion abatement of CO2 emissions: Microalgae mediated routes and CCS (carbon capture and storage)

The capture and utilization of CO2 emissions from power plants is an important aspect of the transition to a low-carbon economy. In this context, microalgae are widely recognized for their potential to capture and biochemically convert CO2, with promising applications as raw material for a variety of products. Therefore, the energy penalty conventionally presented by CCS (carbon capture and storage) could be replaced by an alternative that offers added revenues, the biorefinery. The productive arrangement is approached from a process engineering standpoint and relies on sustainability metrics to assess environmental and economic performance. The work presents a sustainability assessment procedure, applied to tracking sustainability of alternative routes for mitigating CO2 emitted by a NGCC (natural gas combined cycle) power plant, a post-combustion capture concept. The evaluated CO2 destination routes are: a) cultivation of microalgae with flue gas, harvesting of microalgae and downstream processing of biomass to a chemical commodity (ammonia), evaluating the possibility of recycling part of the intermediary syngas stream as renewable fuel to the gas turbine; and b) CO2 capture by chemical absorption, followed by compression and storage steps. The proposed Sustainability Degree indicates biofixation of CO2 with total conversion of syngas to ammonia as the most sustainable among the evaluated alternatives.