In Photofuel three groups competed in biocatalyst development targeting different fuel compounds. The first group, Uppsala University targeted the production of butanol in the cyanobacterium Synechocystis 6803. A four-step engineering led to a biocatalyst that exceeds the project’s productivity objective of 100 mg/L/day. Imperial College London optimised the cultivation conditions in a continuous laboratory system to 600 mg/L and a maximum concentration of 2.35 g/L. This is more than ever reported before for butanol from a photosynthetic organism. Further upscaling proved to be difficult and stable production in 180L and 1200L outdoor systems at A4L is still challenging due to contamination of the photobioreactor with other organisms. Adding the capability to utilise phosphite as P-source improved the situation but did not lead to the production of large butanol amounts.
The second group, Imperial College London developed a production system for octanol and the related compounds octyl acetate and free fatty acids. Free fatty acids are no drop in fuel but can be used for the production of HVO, a renewable diesel with high performance as shown in engine tests for cars and trucks by VW and Volvo. The octanol biocatalyst reached a productivity of about 60 mg/L per day. An organic overlay reduced the toxicity accumulated up to 4 g/L octanol. Addition of a further gene to the biocatalyst converts the octanol to the less toxic octyl acetate, or free fatty acids to FAME, which may be efficiently skimmed from the surface.
The third group, University of Bielefeld used the terpene synthesis pathway common to all plants for bisabolene production in the microalgae Chlamydomonas reinhardtii. The bisabolene synthethase of the grand fir was the only gene which had to be transferred. Its very large size typically limits the production in Chlamydomonas, which was overcome by spreading introns throughout the gene. The 5 mg/L bisabolene per day are not competitive for fuel production but a breakthrough for production of high-value pharmacologically active terpenes as e.g. the anti-cancer drug taxol.
The University of Florence implemented the production of hydrocarbons excreted by Botryococcus braunii in an open pond system. The organism is a natural reference for biocatalytic fuel production since over 250 million years. The biocrude is very heavy and requires intensive upgrading as determined by Neste.
IFPEN and Neste prepared fuel blends for engine testing at VW, Volvo and Fiat. The diesel blends contained 30% HVO or 20% HVO and 10% 1-butanol. Both fuels had a high engine performance and some emission categories were even improved in tests for cars and trucks. The HVO blend achieved the fuel objective, the butanol blend not due to a drop in flash point. Octanol might be added up to 20-25 % without compromising fuel safety or engine performance as determined on base of chemical properties. Gasoline fuel was blended with 1-butanol and iso-butanol. Engine performance would profit most from a high octane rating, which is better in iso-butanol than 1-butanol but significantly lower than in ethanol.
LCA was used by KIT and IFPEN to determine the environmental impact of biocatalytic butanol production. Climate Change is the most relevant category as new biofuels have to achieve a GHG avoidance of at least 65% compared to fossil fuels, which is 957 gCO2eq. per kg of biobutanol. The best of the modelled scenarios assumes a 100 ha plant, biomass recycling to nutrients and low impact power resulting in 2.35 kg CO2 - about 2.5-fold higher than permitted. Improvement options are: Higher butanol to biomass ratio, low-impact cultivation system, higher share of renewable power and a more efficient product separation system.
Similarly, the costs of biobutanol production were calculated by IFPEN to 2183 EUR/t for 8400 tonnes per year from a 100 ha plant. However, the costs for butanol separation by pervaporation add another 9 to 10 EUR/kg.
A major potential is seen for business case development, the technology development can be concluded to be feasible. The high efforts for butanol separation may be alleviated by production of substance like octyl acetate or FAME which can be skimmed from the surface. Their productivity was below the objective, but the system is based on the same chassis strain as butanol and could profit of several of its optimisation steps.
