Agbo, Simon
(2024)
Sustainable aviation fuels (SAF) production. process and techno-economic assessment modelling of gas fermentation with SCWG heat integration.
PhD thesis, University of Nottingham.
Abstract
This dissertation investigates the potential of producing sustainable aviation fuel (SAF) through the integration of aerobic gas fermentation and supercritical water gasification of lignin-content, aiming to address increasing environmental challenges and energy supply uncertainties in the aviation industry. The industry currently accounts for 2% of global CO2 emissions, a figure projected to rise due to increasing air traffic. Fuel makes up about 23% of operating costs, highlighting the need for an economically viable, environmentally friendly solution.
This study evaluates the feasibility of heat-integrating aerobic gas fermentation of H2 and CO2 with supercritical water gasification (SCWG) of black liquor for a proposed SAF plant in China, and SCWG of pot ale draft for a UK-based plant. A comprehensive techno-economic assessment (TEA) compares the production of SAF (C16 fractions) via three routes: acetaldehyde (C2 heat-integrated) and isobutanol (C4 non-heat and heat-integrated) pathways assessing their economic viability. The entire process scenarios are simulated in Aspen HYSYS v12 with the integration of Cell Designer, OptFlux, and Excel enabling the accurate modelling of the gas fermentation bioreactor. This methodology uniquely links systems biology to a typical chemical engineering process simulation.
The evaluation using various TEA methods shows that the C2 heat-integrated SAF plant requires a total capital investment (TCI) of $101-$102 million, with annual fixed operating costs (FOC) around $6.42-$6.87 million and variable operating costs (VOC) of $1.76 million. Despite generating a net 160 GWh of electricity sold at $0.1085/kWh and 7.7 kt of SAF sold at $611/ton, this plant records a negative cumulative NPV of about -$3 million over a 25-year period. Break-even occurs at year 25 with electricity sold at $0.1120/kWh or at year 12 with SAF and electricity prices at $771/ton and $0.134/kWh, respectively. Comparatively, using the same black liquor in a steam-turbine powered electricity plant yields a $70 million NPV and breaks even within the initial 4 years.
For the proposed C4 route-to-SAF plants, two crucial experiments were conducted to inform the modelling of the upgrading units. In the oligomerization of isobutene experiment, trimers (C12) and tetramers (C16) were identified as significant SAF fractions, constituting approximately 90% of the product distribution over the Amberlyst-35 catalyst. Optimal conditions for the highest yield of C12 and C16 were determined at 70°C. A residence time of 45 minutes was also recorded. The oligomerised isobutene product undergoes hydrogenation reaction. Results indicated that increasing the pressure to 20 bar with a 3:1 catalyst to substrate ratio (1 wt.% Pd on Al2O3) significantly accelerated the reaction rate. Reducing the catalyst concentration to 1 wt.% Pd on Al2O3 and a 1:1 ratio showed a slightly reduced but notably faster reaction than initial low-pressure conditions. Data from the hydrogenation experiment were used for kinetic fitting modelling, revealing second-order kinetics for the hydrogenation reaction and determining the kinetic constant. Additionally, parsimonious flux balance analysis (pFBA) of gas fermentation in OptFlux helped determine the molar ratio H2:CO2 as (5:1) and CO2:O2 as (1:1), with key stoichiometric equations derived for modelling the bioreactor in ASPEN HYSYS. Oxygen transfer coefficients (KLA) were also found to be 323.13 [1/h] and 329.72 [1/h] for C4 heat-integrated and C4 non-heat-integrated cases, respectively.
Results from the experiments and pFBA modelling informed the TEA of both C4 cases. Investment estimations revealed that the C4 heat-integrated route-to-SAF plant requires a total TCI of $117.35 million, compared to $66.31 million for the C4 non-heat-integrated case. FOC were estimated at $7.35 million for the C4 heat-integrated case, compared to $6.5 million for the C4 non-heat-integrated case. VOC analysis showed that the C4 heat-integrated process incurs lower costs due to the absence of a need for cooling water for the bioreactor, unlike the C4 non-heat-integrated case, which incurs about 1.4 times higher costs. The C4 heat-integrated plant generates a net 142.47 GWh/annum of electricity, while its counterpart generates 61.90 GWh/annum. Initially, the heat-integrated process shows a lower cumulative NPV (-$139.61 million) compared to the non-heat-integrated process (-$78.99 million) in the second year. However, over time, the cumulative NPV of the heat-integrated process increased to around $20 million by the 25th year, showing improved profitability whereas the non-heat-integrated case stayed at -$52.28M at the same point. Despite this, the C4 heat-integrated has a longer payback period of 16 years, which might impact investor interest. To break even, the C4 heat-integrated scenario requires an electricity selling price of $0.123/kWh, assuming a constant SAF price of $611/ton. Conversely, the non-heat-integrated scenario requires a much higher electricity price of $0.241/kWh to break even, representing a significant 95% increase in selling price. Break-even analysis shows the lowest required selling prices for SAF ($694.65/ton) and electricity (0.163 kWh/$) in the 12th year for the C4 heat-integrated route. Monte Carlo simulations reveal uncertainties in NPV calculations. The C4 heat-integrated case demonstrates a 69% likelihood of achieving a net cumulative NPV between $5 million and $65 million, with an 11% risk of loss. Initial IRR stands at 12% for the C4 heat-integrated process at a $611/ton SAF price. Sensitivity analysis revealed that doubling the SAF price raises NPV by $65 million with a 16% IRR and a 10-year payback. Tripling the SAF price boosts NPV by $110 million,achieving a 20% IRR and an 8-year payback, which is much higher when compared with the conventional electricity plant.
The proposed UK SAF mandate buy-out price (£2567/tonne) was introduced and utilized to determine the impact on the proposed SAF plants. For the C4 heat-integrated route, NPV increases from $21 million to $210 million (based on $611/ton SAF), elevating IRR from 12% to 27% and reducing payback from 16 to 6 years. More so, implementing the buy-out price significantly improved the C2 heat-integrated case with NPV reaching $110M from -$3M, IRR at 25%, and a shortened payback period to 7 years.
Overall, the heat-integrated approaches, especially the C4 heat-integrated route-to-SAF, emerged as the most economically viable option for SAF production followed by the C2-heat-integrated case. Outperforming both C2 heat-integrated and C4 non-heat-integrated scenarios, the C4 heat-integrated route exhibited promising NPV and minimal selling price requirements. The insights generated seek to support the design, execution, and evaluation of policies that foster the growth of SAF, aiding the transition to a more sustainable aviation industry.
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