Reduction of Na and K contents in bio-heavy oil using micro-/nano-sized CO2 bubbles

Co-combustion of bunker-C oil and bio-heavy oil for power generation provides a strategic way for mitigating the environmental burdens (climate change) due to the intrinsic carbon neutrality of bio-heavy oil. Prior to the co-combustion process, the high content of Na and K in heavy oil should be removed for the operational safety. Given that the washing process has been currently practiced for metal removal, it is desirable to develop the chemical-free washing process. Note that H2SO4 was commonly used during the washing process. To this end, this study laid great emphasis on the possible use of CO2 for the washing process. To increase the removal efficiency of Na and K, this study particularly employed micro-/nano-sized CO2 bubbles. To access the in-depth insights, a computational fluid dynamics (CFD) simulation was conducted prior to a lab-scale experimental work. The CFD simulation suggests that CO2 bubbling expedited the mixability during the washing process. Also, the CFD simulation suggested that the CO2 bubbling effects could be enhanced because the high viscosity of bio-heavy oil cancelled out the buoyancy force. A lab-scale experimental work was also in good agreement with the CFD simulation. In the presence of CO2 bubbles, 80% of Na and K was removed when the volumetric ratio of water and bio-heavy oil was 8. Even under the volumetric ratio condition of 0.125, the metal removal efficiency was not substantially reduced. Thus, all experimental findings signify that metal removal by micro-/nano-sized CO2 bubbling should be done by a cascade system.