Ultra-high CO2 capture efficiency in CFB oxyfuel power plants by calcium looping process for CO2 recovery from purification units vent gas

Abstract

This work presents a new option for the recovery of the CO2 losses from CO2 purification units in oxyfuel plants, by means of the Ca-looping process. The idea is to capture the CO2 in the vent stream from purification units by reaction with CaO sorbent in a carbonator reactor, where CaCO3 is formed. Sorbent is then regenerated in a calciner reactor by oxyfuel combustion of a fraction of the coal fed to the power plant. Since the Ca-looping process requires a continuous purge of exhaust sorbent and make-up of fresh limestone, the system is best coupled with a CFB boiler, where the exhausted Ca-rich sorbent can be used for in-furnace sulfur absorption.In this work, detailed mass and energy balances of the system proposed are reported, including a preliminary sizing of the reactors of the Ca-looping unit. A sensitivity analysis was also performed, by considering two types of coal as feed (mainly differing in sulfur content), two levels of non-condensable gases in the impure CO2 stream to be purified and different behaviors of the exhausted Ca-based sorbent injected in the CFB boiler, where it can experience different levels of recarbonation.Interesting results were obtained for this new system, which can capture about 90% of the CO2 vented from the purification unit in a reasonably compact reactors system, allowing an overall CO2 avoidance of the order of 99% with respect to conventional coal-fired steam plants without capture. As far as energy penalties are concerned, they were evaluated by the specific primary energy consumption for CO2 avoided index (SPECCA). Small differences with respect to reference oxyfuel plants without CO2 recovery were obtained, with either slightly better or slightly worse performances, depending on the sulfur content of the coal used. Penalties are associated to the export of CaO in the final exhausted sulfated sorbent from the CFB boiler, which increases when a higher sulfur coal is used. However, experimental analysis on the recarbonation level which can be attained by the CaL exhaust sorbent in the CFB boiler and further process optimization are needed to correctly account for these penalties and possibly minimize them.