Hydrodynamics of sorbents in a semi-dry circulating fluidised bed riser for flue gas purification

Abstract

Nowadays, the increasing stringent environmental regulations results in coal-fired power plants struggling to comply with these regulations. The attention toward technologies for the abatement of pollutant gases has increased significantly over the last few decades. The focus of the industry primarily lies with the removal of sulphur and carbon from the flue gasses being emitted to the atmosphere. Both Flue gas desulphurisation (FGD) and Carbon capture and storage (CCS) technologies are very mature technologies, but factors relating to gas-particle interactions are continuously researched in an attempt to further improve the efficiency thereof. Accordingly, research was done on a laboratory scale semi-dry circulating fluidised bed (CFB) using hydrated lime and potassium carbonate as sorbents respectively.Although extensive research on both FGD and CCS technologies has been done in the past, limited information could be found pertaining to the direct influence of gas temperature and gas relative humidity as two independent variables on the hydrodynamics in a CFB. The aim of this study was to describe the hydrodynamics of hydrated lime and potassium carbonate under varying inlet gas temperatures as well as varying inlet gas relative humidity. The aim was achieved by describing the influence of both independent variables on two-phase flow in a CFB using both experimental results and numerical method The method chosen for increasing the inlet gas temperature and inlet gas relative humidity was adapted from literature to be suitable for this study. Operating parameters were chosen to ensure operation in the fast fluidisation regime to enable the ability to compare with past and future CFB research. Measured and calculated data responses were recorded experimentally and numerically, including pressure differentials, temperature profiles and local and average solids volume fraction at different riser sampling ports. Each response was addressed by discussing the influence of varying operating conditions and therefore an extensive amount of experimental work as well as CFD simulations were required.The inlet gas temperature and inlet gas relative humidity were the varying parameters in this study. The inlet gas relative humidity showed the most significant influence between the two. Although captured slightly, the limited operating range of the equipment resulted in a less prominent influence than expected. The experimental results indicated a pressure differential across the riser with negative pressure readings as expected. It was also observed that preferential particle flow occurred near to the riser wall opposite the gas inlet. The inlet-outlet configuration of the riser could be the reason for this asymmetric flow profile.In addition to the extensive amount of experimental work done in this study, CFD simulations were done toaid in discussions pertaining to the hydrodynamics in a CFB riser. The geometry of the riser was constructed using Siemens NX-12® software andsimulations were carried out in STAR-CCM+® CFD software. Moreover, a Eulerian-Lagrangian with two-way coupling approach was followed. The average particle sizes were based on the D[4,3] retrieved from PSD analysis and the Schiller-Naumann homogenous drag model was employed. The models were validated against experimental results and was found to successfully describe the hydrodynamics of the two sorbents in a CFB riser, despite the inability to capture the exact values. In addition, the aim was to investigate the influence of inlet gas temperature and inlet gas relative humidity on the hydrodynamics, and this was achieved using the experimental and modelling results.