Chemical-looping gasification for the production of aviation fuel with negative emissions: Full chain process modeling and techno-economic analysis
Examensarbete för masterexamen
Sustainable energy systems (MPSES)
Saeed, Muhammad Nauman
The greenhouse gases from the conversion of fossil fuels are the main culprits for the increase in the planet's temperature which is reflected in the global warming context. The aviation sector is more dependent on fossil fuels as there is less possibility to find a viable alternative for fuel requirements for this sector. Chemical Looping Gasification with biomass as a fuel combined with downstream Fischer-Tropsch (FT) synthesis for aviation fuel production is a possible way to decarbonize transportation sectors like aviation. Chemical Looping Gasification (CLG) is like indirect gasification in a circulating fluidized bed, except that instead of inert bed material, particles containing metal-oxides, called oxygen carriers (OCs) are used as the bed material. CLG process has advantage of unnecessary use of an expensive and energy-intensive air separation unit (ASU). Also, due to the presence of a more oxidizing environment in CLG and the catalysing property of OC, the tar yield drops substantially resulting in improved syngas yield and better biomass to syngas conversion compared to the conventional gasification processes. Moreover, all produced CO2 is concentrated in the fuel reactor, with no or limited emissions from the air reactor. This means that it could be a very good process for combined fuel production and capture of CO2, something which would result in net-negative emissions. The study is based on modeling the full chain process of biomass to liquid fuel (BtL) using Aspen Plus software. The model is designed for a gasifier load of around 80 MWth and includes drying of biomass followed by a CLG unit using different oxygen carriers (LD slag and Ilmenite). The circulation rate of an oxygen carrier is adjusted to achieve the desired autothermal CLG operation with temperature of 935oC in the fuel reactor (FR). The resulting syngas from CLG goes through syngas cleaning and conditioning units to meet the requirements for FT synthesis. The steam to biomass ratio is adjusted to 0.7 to achieve an H2/CO ratio of 2.1 before the FT reactor. In the FT reactor with cobalt as a catalyst and at temperature of 220oC, the syngas gets converted into hydrocarbons with carbon numbers ranging from 1 to 40 using the Anderson-Schulz-Flory distribution. Since the aim of the model is to produce aviation fuel, the FT synthesis process combined with a reformer in the recycle loop is adjusted for maximizing the yield of paraffin with carbon numbers ranging from 8 to 16. Based on the optimized model, the clean syngas after syngas cleaning units has an energy content of 8.68 MJ/Nm3 (LHV basis) with a cold-gas efficiency of 77.86 %. FT synthesis model with a reformer estimates an FT crude production of around 647 bbl/day with 154 kilo-tonne of CO2 captured every year and conversion efficiency of biomass to FT-crude of 38.98 %. The calculated levelized cost of fuel (LCOF) is 35.19 $ per GJ of FT crude, with an annual plant profit (cash inflow) of 11.09 M$ and a payback period of 11.56 years for the initial investment.