Demonstrating the applicability of chemical looping combustion for fluid catalytic cracking unit as a novel CO2 capture technology

Güleç, Fatih (2020) Demonstrating the applicability of chemical looping combustion for fluid catalytic cracking unit as a novel CO2 capture technology. PhD thesis, University of Nottingham.

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Abstract

Fluid Catalytic Cracking (FCC) units are responsible for ~25% of CO2 emissions released from the oil refineries, which themselves account for 4-6% of total global CO2 emissions. Although post- and oxy-combustion technologies have been suggested to capture the CO2 released from the regenerator of FCC, Chemical Looping Combustion (CLC) may also be a potential approach to capture the CO2 released from the regenerators in FCC units with lower energy consumption.

In this study, the applicability of CLC for the FCC unit was investigated as a novel approach for CO2 capture. In order to demonstrate the applicability of CLC, four main aspects were studied. A refinery FCC catalyst (Equilibrium Catalyst–ECat) was firstly modified with reduced oxygen carriers e.g. Copper (Cu), Copper (I) oxide (Cu2O), Cobalt (II) oxide (CoO), and Manganese (II, III) oxide (Mn3O4) and oxidised oxygen carriers e.g. Copper (II) oxide (CuO), Cobalt (II, III) oxide (Co3O4), and Manganese (II) oxide (Mn2O3) using mechanical mixing and wet-impregnation methods. Secondly, to identify any detrimental effects of the oxygen carriers on cracking, both reduced and oxidised oxygen carrier modified ECat formulations were tested for n-hexadecane cracking using the standard test method of FCC catalysts. Then, to investigate the CLC behaviour of coke with oxidised oxygen carriers (CuO, Co3O4 and Mn2O3), thermogravimetric analyses (TGA) were conducted on a low volatile semi-anthracite Welsh coal, which has a similar elemental composition to actual FCC coke. Finally, the CLC of coke deposited on the reduced oxygen carrier impregnated ECat was investigated with the stoichiometrically required amount of oxidised oxygen carrier impregnated ECat. The CLC tests were investigated in a fixed-bed and a fluidised-bed reactors equipped with an online mass spectrometer to monitor CO2 release.

The results demonstrated that mechanical mixing of Cu with ECat was shown to have a negative impact on the cracking of n-hexadecane. However, the mixing of Cu2O, CoO, and Mn3O4 with ECat had no significant effect on gas, liquid and coke yields on product selectivity. Additionally, wet-impregnation of Cu, MnO, and Mn3O4 had a negligible impact on the cracking of n-hexadecane in terms of conversion, yields and product selectivity. In terms of the CLC tests of coke, complete combustion of the model coke was achieved with bulk CuO, Co3O4, and Mn2O3 when the stoichiometric ratio of oxygen carrier to coke was maintained higher than 1.0 and sufficient time was provided. Furthermore, although 90 vol.% combustion efficiency of the coke deposited on ECat was reached with bulk CuO and Mn2O3, the regeneration temperature required (800 °C) was not realistic in terms of commercial regenerator conditions. However, a relatively high combustion efficiency, (> 94 vol.%) of the coke deposited on reduced Cu and Mn3O4 impregnated ECat was achieved with the stoichiometrically required amount of CuO and Mn2O3 impregnated ECat at the conditions used in conventional FCC regenerators, 750 °C for 45 min. According to these results, CLC is a promising technology to incorporate into the next generation of FCC units to optimise CO2 capture.

In addition to the application of CLC for FCC, the selective low temperature CLC of higher hydrocarbons was discovered during the cracking tests of n-hexadecane over oxidised oxygen carriers mixed ECat. Therefore, CLC of n-hexadecane and n-heptane with CuO and Mn2O3 was investigated in a fixed bed reactor to reveal the extent to which low temperature CLC can potentially apply to hydrocarbons. The effects of fuel to oxygen carrier ratio, fuel feed flow rate and fuel residence time on the extent of combustion were reported. Methane did not combust, while near complete conversion was achieved for both n-hexadecane and n-heptane with excess oxygen carrier for CuO. For Mn2O3, total reduction to Mn3O4 occurred, but the slower reduction step to MnO controlled the extent of combustion. Although the extent of cracking is negligible in the absence of cracking catalysts, for the mechanism to be selective for higher hydrocarbons suggests that the reaction with oxygen involves radicals or carbocations arising from bond scission. Sintering of bulk CuO occurred after repeated cycles, but this can easily be avoided using alumina support. The fact that higher hydrocarbons can be combusted selectively at 500 °C and below, offers the possibility of using CLC to remove these hydrocarbons and potentially other organics from hot gas streams.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Snape, Colin E.
Meredith, Will
Sun, Cheng-Gong
Keywords: CO2 capture, Fluid catalytic cracking, FCCU, Chemical looping combustion, CLC, Mn-based oxygen carrier, Cu-based oxygen carrier, Co-based oxygen carrier, Zeolite catalysts
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UK Campuses > Faculty of Engineering
UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 63764
Depositing User: Güleç, Fatih
Date Deposited: 07 Jan 2021 15:46
Last Modified: 07 Jan 2021 16:00
URI: http://eprints.nottingham.ac.uk/id/eprint/63764

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