Spontaneous condensation of carbon dioxide in flue gas at supersonic state

Abstract

A cleaning method of removing CO2 from flue gas by supersonic flow is investigated. A condensation flow model is developed based on the classical droplet nucleation and growth theory. The non-equilibrium phase transition in supersonic flow is predicted by adding transport equations. The effects of converging-diverging nozzle, inlet subcooling and gas composition on CO2 spontaneous condensation in flue gas are studied. The results show that the nozzle with Witoszynski curve is the first to nucleate, at x = 0.121 m. Without considering the backpressure, the shrinkage ratio and diverging angle only affect the flow behavior behind the throat. With the increase of shrinkage ratio or diverging angle, the liquid mass fraction increases from 0.062 to 0.085 and 0.056 to 0.089 respectively, but there are hazards such as increased pressure loss and reduced nozzle flow rate. The diverging length has no significant effect on the flow behavior but the increase of length can prolong the droplet growth time. The higher inlet subcooling not only makes the supersonic flow reach the critical state in advance, from x = 0.121 m to x = 0.111 m, but also significantly improves the droplet growth rate, from 0.649 m/s to 0.854 m/s. The increase of CO2 concentration greatly enhances the mass and heat transfer process.