CLG-G3BioF starts with the functional materials selection through two series of experimentations. The first series is with a lab-scale reactor where the reacting environment for CLG is periodically changed by switching the gases entering the reactor. In this primary selection, the manganese ore (MnGBhne) is discarded because of its low syngas yield and moderate gasification rate as compared to the other three materials (ilmenite ore, LD slag (from steel-making industry) and iron sand (from copper smelting)). Therefore, the ilmenite, LD slag and iron sand are further used in the second series of experiments with a pilot unit that has interconnected fluidized bed reactors and can take a fuel flow corresponding to 1.5 kWth thermal power. This reactor configuration mimics well a real CLG process and can assure an accurate material selection. In the 1.5 kWth experiments, the fuel rate is kept constant while the temperature (860 and 920°C), oxygen-to-fuel ratio (0.12-0.5) and gasifying agent (steam, steam-CO2 mixture and CO2) are varied to study their effect on the syngas yield. It is clearly seen that the steam gasifies better the microalgae than the steam-CO2 mixture and the CO2. A higher oxygen-to-fuel ratio results in a lower syngas yield while the temperature doesn’t have a significant effect on the gasification. Through this step, the ilmenite and LD slag are found the most suitable as oxygen carriers.
The ilmenite is then subject to a 50 kWth unit for the proof-of-concept and optimization of the microalgae-CLG process at the TRL5 level. 7 hours of non-stop microalgae-CLG campaign is managed and run successfully when varying key operation parameters (temperature and oxygen-to-fuel ratio). This campaign produces a stable stream of syngas and stands out as the world’s first demonstration for the microalgae-CLG process at TRL5. In this campaign, we figure out that an autothermal operation can be achieved when the oxygen-to-fuel ratio is kept close to 0.3 and in this condition the increase of temperature results in more syngas. At 950 ºC and oxygen-to-fuel ratio of 0.3 a gas yield of 0.7 Nm3 syngas per kg microalgae and a cold gas efficiency of 42% were obtained.
Following the experimentations for material selection and process demonstration, the project carried out a series of simulations to explore the integration of the microalgae-CLG with upstream and downstream processes for the production of biofuels. The simulation model is consisted by equilibrium reactors (RGibbs), yield reactors (RYield) and stoichiometric reactors (RStioc), and considers a 25 MWth input of the microalgae. Key operating parameters are studied with regard to their impact on the process sensitivity over heat management, gas yield and biofuel yield. The simulation confirms that the autothermal condition is achievable and in this condition the microalgae-CLG can be well integrated with the Fischer-Tropsch process to produce bio- gasoline and diesel. Around 0.13 kg biofuel (of which is almost half gasoline and half diesel) is expected from 1 kg of microalgae in the CLG process.
Lastly, a “cradle-to-grave” method is used to study the process’s potential impact on the carbon footprint, global warming (GWP), acidification (AP) and eutrophication (EP). In this work, the generation of 1 kg bio-jet fuel is considered as a reference, and the Life Cycle Assessment (LCA) model included the microalgae cultivation, ilmenite ore mining, catalysts syntheses for the Fischer-Tropsch and Water-Gas shift units, the oxygen carrier reuse/disposal, and CO2 capture and storage, and so on. Based on the LCA simulation, the microalgae drying and transport has the highest GWP factor and these processes can release 3.87 kg CO2, while the microalgae cultivation has a negative CO2 emission of -1.89 kg CO2. The microalgae harvesting has 0.075 kg Phosphate release which contributes 92% to the process EP potential of the whole system. The ilmenite mining and transport are the main streams for the sulphur dioxide emission, and these streams contribute 96% to the AP potential. The microalgae-CLG unit has little environmental impact and so the future optimization should be focused mainly on the upstream processes.
