Algae to kerosene: the green wake

Matteo Prussi, Nicolae Scarlat, JRC, Directorate C - Energy, Transport and Climate Energy Efficiency and Renewables

Among HEFA routes, producing jet fuels from algae looks promising for setting the path towards a more environmental friendly aviation.

According to the evaluation before the COVID pandemic, the international civil aviation was consuming 160 megatons (Mt) of fuel, and emitting approximately the 2.6% of total GHG emissions from fossil fuel combustion [1]. According to projections, by 2045 the fuel consumption is expected to increase from 2.2 to 3.1 times compared to 2015 [2]. Even though the COVID-19 pandemic has heavily impacted the sector, aviation keeps considering the environmental impact mitigation as a pillar for its development. Some initiatives are already in place, at international level, such as the global CO2 standards regulating the fuel efficiency for new aircrafts from 2020 [3].

ALTERNATIVE FUELS FOR AVIATION

To define a proper strategy towards decarbonisation, in 2016 the United Nation’s International Civil Aviation Organization (ICAO) Assembly agreed on the adoption of a global market-based scheme to tackle international aviation GHG emissions: the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) [4]. CORSIA requires airline operators to offset GHG emissions, expressed in CO2 equivalent, with respect to a baseline set for 2019. There are several ways in which an operator can offset its emissions, and the use of alternative fuels is considered fundamental to achieve a carbon neutral growth. A new Long Term Aspiration Goal is under development, at ICAO level [5], to allow setting the path towards a more environmental friendly aviation. In spite of the interest in Sustainable Aviation Fuels (SAF) as a suitable solution to achieve decarbonisation in the short- to medium-term, their current penetration is very low. According to Chiaramonti et al. [6], under the current policy framework, aviation is not expected to be supplied with a significant amount of biofuels in 2030. At European level, an important initiative is promoted by the European Commission: the REfuel EU Aviation [7]. This initiative aims to boost the supply and demand for sustainable aviation fuels in the EU, to reduce European aviation’s environmental footprint and enable its contribution to the EU’s climate goals. As of today, 8 alternative fuel production pathways have been certified by ASTM for commercial flights. Among the approved technologies, co-processing and Hydroprocessed Fatty Acid Esters and Free Fatty Acid (HEFA) are certainly the most mature. In HEFA, lipid feedstocks, such as vegetable oils, used cooking oils, tallow, etc. are converted into green diesel using Hydrogen. Green diesel can be further isomerized and separated to obtain a jet fraction. The pathway was approved in 2011 to be blended at 50% present with fossil jet fuel. Europe has already an interesting installed capacity for HEFA [8], with an upper technical limit of approximately 2.4 Mt/yr. As lipid feedstocks are already used for road alternative fuel production, and also meet the interest of the maritime sector, the investigation of alternatives is crucial to achieve a real deployment of SAF. Recently, ASTM approved the HEFA route from algae [9]. Micro and macro algae are feedstocks which have been largely studied for biofuels. Yang et al. [10] investigated four main pathways to produce jet fuels from algae, concluding about the maturity of HEFA technology, and highlighting that high lipid content in algae is required to run the process economically. Production costs are usually identified as the main bottleneck for large-scale deployment of algae to fuel plant, however an economic analysis based on large scale plants is still missing, and the potential of this crop is not yet fully investigated. It is worth remarking that in terms of productivity, algae can deliver 10 t/ha per year, under a conservative estimation, and a significant amount

Algae culture Algae paste Oil

Aviation Alternative Fuel

Figure 3 - Comparison on emission from various alternative fuels options, for the aviation sector.

of proteins, which is remarkable also when compared with effective cultivation such as palm. Other relevant advantages of algae productions is their capability of being grown on non-arable land and fed with salty water. Additionally, the rate of nutrients uptake is really high, when compared to standard agriculture. Finally, algae have to be considered as an effective way to biologically fix CO2 from air, and at the current stage of knowledge, this is an interesting possibility, as other “air-capture” options are costly and highly energy demanding [11]. In 2019, IHI [12] with NEDO developed a process based on an special strain of Botryococcus braunii (Hyper-Growth Botryococcus Braunii (HGBb)) claimed of being able to reach an oil content above 50% on dry basis, rich in hydrocarbons. This algae has been investigated by many authors, among others Ranga Rao et al [13], which confirmed the high lipids and hydro-carbon accumulation potential. NEDO claimed to have developed a low energy harvesting process, with open air drying and nutrient recycling. IHI Corporation runs a 1.5 ha pilot plant in Thailand, and the resulting synthesized paraffinic kerosene was approved ASTM D7566 in 2020 through the fast track process, as a 10% blend [14]. The GHG saving potential for algae to kerosene pathways has been evaluated by the DoE-ANL: their study reported a potential life-cycle energy consumption reduction by 55% and carbon emission by 45% [15]. EC-JRC, which co-leads the Core-LCA of the CORSIA Fuel Task group, estimated the current emissions associated with algaeto kerosene pathways. Even if it is worth remarking that this exercise has been performed out of the FTG work, the CORSIA LCA methodology has been used as guideline.

MODELLING AN ALGAE TO KEROSENE PATHWAY

In order to estimate the potential GHG saving of the algae to kerosene pathway, a processbased attributional LCA approach [16] was designed, accounting for mass and energy flows, along the whole fuel supply chain. Data have been gathered from the available literature.

The proposed algae-to-kerosene 1ha plant is constituted by an inoculum production stage, followed by the ponds for massive cultivation. Once ready, the cultivation batch is harvested and pumped to downstream processes. Bioflocculation and sun drying have been considered to obtain pumpable algae wet paste. A cell disruption stage is followed by a first phase separation, where to collect the oil, and the water phase for nutrients recycle. The solid phase is processed in a solvent extraction plant, to recover additional oil and the protein cake. The oil is then converted to HEFA bio-kerosene, while the cake is used to recover energy and nutrient for the process.

As average biomass productivity and oil contest are related to many factors (e.g. cultivation strategy, nutrient management, wheatear conditions, etc.), three cases and two scenarios for energy inputs have been considered. The average yields range between 8.5 g/m²d (28 t/yr ha) and 14.1 g/ m²d (42 t/yr ha) and the respective average oil content between 50% of 30%. Two scenarios have been defined for the energy input. In the first one, the emission factor per kWhel is based on the European 2020 electric mix [17]. In the second one, the electrical need is supplied by dedicated wind and solar plant.

ESTIMATING THE GHG SAVING POTENTIAL

The GHG emissions range between 72.3 CO2e/MJ, for the base case, and 31.8 CO2 e /MJ in the best scenario with renewables supplying energy demand. These values correspond to a saving of 19-68% in GHG emissions, with respect to fossil kerosene (estimated level at 89 CO2e/MJ). In terms of contribution to the final MJ of fuel, fertilisers and energy input for RWPs are the major items. Likewise for other crop based alternative fuels, the cultivation phase is significant. As the modelled plant targets the aviation fuel market, it is worth comparing the results with the default values proposed in the ICAO/CORSIA documentation [18]. In CORSIA, the emission levels for HEFA alternative fuels are estimated at13.9 CO2e/MJ for Used Cooking Oil and 47.4 CO2e/MJ for rapeseed oil, produced in Europe.

CONCLUSIONS

The best case scenario shows a 68% GHG emission saving in comparison to the reference fossilbased kerosene. If compared with the GHG saving potential of kerosene from other traditional bio-based feedstocks, the results confirm algae tobe an interesting alternative. In order to achieve relevant savings, appropriate conditions for their cultivation have to be adopted, such as high process optimisation, nutrient recycling and use of renewable energy to meet input demand.

The views expressed here are purely those of the authors and may not, under any circumstances, be regarded as an official position of the European Commission.

References available at page 51.

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