Producing synthetic natural gas from microalgae via supercritical water gasification: A techno-economic sensitivity analysis

A techno-economic sensitivity analysis of the production of synthetic natural gas (SNG) via catalytic supercritical water gasification (SCWG) of microalgae produced in raceway ponds (RP), tubular-, or flat-panel-airlift photobioreactors (FPA-PBR) has been perfomed. The aim of combining microalgae production with SCWG is to close material flows with respect to water and nutrients, the so called SunCHem process. The sensitivity analysis is based on an annual production of 86,500 t of microalgae biomass yielding 1.14 PJ of methane per year. The sensitivity analysis showed that with an annual algae productivity of 38.5 t per hectare of RP an energy return on energy invested (EROEI) of 1.84 can be achieved for the self-sufficient base case scenario. An SNG production cost of 194 € GJ−1 was obtained for RP. An EROEI of 0.08 was calculated for tubular PBR with a productivity of 75.1 t ha−1 a−1 in the base case scenario and thus was found to be inappropriate for SNG production. EROEI for FPA-PBR with an assumed microalgae productivity of 79 t ha−1 a−1 was found to be 1.01 in the base case scenario and an SNG production cost of 266 € GJ−1. With significantly more optimistic assumptions concerning microalgae productivity, energy input and capital requirement with respect to microalgae cultivation, an EROEI of 3.6–5.8 and SNG production costs of 53–90 € GJ−1 were found for RP, whereas for FPA-PBR an EROEI of 2–3.7 and SNG production costs of 30–103 € GJ−1 were obtained.

► Complete integrated algae to SNG production pathways were assessed.
► Energy return on energy invested is clearly positive.
►  Scenarios are based on an annual production rate of 86,500 tonne algae.
► Open ponds, flat panels, or tubular cultivation systems were considered.
► Sensitivity analysis cover a large range of process parameters.