Enviro-economic assessment of thermochemical polygeneration from microalgal biomass

This paper presents a model-based assessment of the thermochemical conversion of microalgal biomass into Fischer-Tropsch liquids, hydrogen and electricity through polygeneration. Two novel conceptual plants are investigated, which are both comprised of the same operation units (gasification, water-gas shift, Fischer-Tropsch synthesis, upgrading, separation, Rankine cycle and gas turbines) and mainly differ in the location of the water-gas-shift unit. Both plants are found to present a carbon efficiency greater than conventional biomass-to-liquid processes. The most profitable plants in terms of the saleable products yields ca. 0.23 m3 (1.4 bbl) of liquid transportation fuels (gasoline, kerosene and diesel), ca. 16 kg of hydrogen (716.8 scm), and ca. 1.55 × 109 J (430 kW·h) of electricity per 1000 kg of dried microalgae. The corresponding displaced fossil fuels could offset the carbon emissions by 0.56 kg of carbon dioxide for every kg of processed dried microalgae. Nevertheless, predicted break-even prices are lower than 40 USD per ton of dried microalgae in the base case scenario, which is at least 10 times cheaper than the current best scenario for microalgal biomass production. These low prices are a major impediment to the viability of these thermochemical polygeneration plants, albeit presenting a good potential toward cleaner liquid fuel production.