In the first fermentation stage of the BioSFerA project, the resulting syngas (CO2/CO/H2) is converted into acetate by acetogenic bacteria. Several anaerobic bacteria, including Clostridium, Acetobacterium and Eubacterium, have shown the ability to ferment single carbon gases such as CO and CO2 plus H2 into chemicals, usually acetate, through the acetyl-CoA pathway. These bacteria are named acetogenic bacteria. The acetyl-CoA pathway (Wood–Ljungdahl pathway) can use both CO and H2 as a source of electrons and CO and CO2 as a source of carbon. Since acetogenic bacteria express a mixture of compounds (acetate, ethanol, lactate, 2,3-butanediol) increase of acetate production and reduction of the spectrum of unwanted byproducts can be obtained using metabolic engineering and synthetic biology . The main product of that gas fermentation, i.e. acetate, can be further used as an alternative carbon source for a second fermentation process.
One of the main advantages of the BioSFerA concept is the limited gas cleaning requirements, fact which minimizes the investment cost of that part of the system. To achieve this, specialized bacterial strains resistant to the contaminants of the gasification process will be selected. The performance and robustness of the targeted acetogenic bacteria, including native and metabolically engineered strains, will be validated under actual contaminants present in the real syngas. The use of the tolerant bacteria facilitates the gasification of different feedstock in different operational conditions that can result in variable amounts of contaminants. This will allow to reduce the need for purification of syngas obtained by gasification during the scale-up stage.
Gas fermentation processes for production of acetate will be demonstrated in parallel by BBEPP and CARTIF. In particular, BBEPP will test Moorella thermoacetica and CARTIF will test the best acetate-producing Clostridium strains. In a later phase, the optimized acetogenic strains obtained by CSIC will be evaluated. Experiments will be carried out in parallel in order to identify the most appropriate acetogenic bacteria in terms of productivity (g/L/d), yield (%) and titre (g/L) acetate concentration to feed the subsequent fermentation process. Fully equipped bioreactors of 1L and 10L, which can be pressurised to up to 10 bar, will be used for these tests.