Hydrodynamics inside a laboratory-scale semi-dry flue gas desulphurisation riser

Abstract

Since power plants were initially designed without SO2 emission control in mind, power generating facilities are struggling to comply with the increasingly stringent emissions legislation. One possible approach to address this problem is using sulphur capturing technology. From the available methods, the characteristics of semi-dry flue gas desulphurisation (FGD) technology makes it a promising avenue to explore. Accordingly, a laboratory scale circulating fluidised bed (CFB) riser was designed with the intention of performing FGD research. Although much research was performed on fast fluidisation and the role of CFB’s in FGD, little is known about the influence of varying riser heights on the hydrodynamics. Given the novelty that was associated with the new system, the aim of this study was to describe the hydrodynamics of hydrated lime inside the riser. This aim was achieved by discussing the influence of design and operating conditions on the two-phase flow inside the system using both experimental findings and CFD modelling. Similarity parameters were applied to literature data to determine a suitable range of operating conditions and the desired particle size for this study. This ensured that the fast fluidisation regime was obtained inside the riser such that the results remained relevant for future work. The hydrodynamics were quantified through a few measured or calculated responses, including the local and average solids volume fraction at several sampling ports, the radial non-uniformity index and the pressure differentials associated with the riser. To discuss the influence of the varying design and operating conditions on each response, extensive experimental work was required. Therefore, the required time and resources were minimised by making use of a 23 full factorial as well as a central composite design approach. This enabled the construction of linear and quadratic regression models which supplemented the discussion regarding the hydrodynamics inside the riser. The inlet air velocity, solids feed rate and riser height settings were the three varying factors and among these, the solids feed rate exhibited the weakest correlation with the responses mentioned before. The experimental results indicated that preferential particle flow occurred near the riser wall on the opposite side of the air inlet. This non-uniformity could be reduced by increasing the inlet air velocity, decreasing the solids feed rate or decreasing the riser height.

Throughout literature, prominent exit effects were reported on risers with T-shaped outlets. In this study, however, it was found that such an effect was only present inside the medium and tall risers if the gas velocity was smaller than 6.5 m/s. Above 6.5 m/s, an exit effect was only present in the medium sized riser. In addition, the pressure differential across the cylindrical riser sections with respect to the overall pressure differential could be reduced by increasing the inlet air velocity. It was further found that the exit assembly at the top of the medium height riser provided the least resistance to flow given that the particles had to travel a shorter distance to the downstream cyclone. Apart from the experimental work, two-phase CFD models were created to supplement the discussions regarding the riser hydrodynamics. The riser geometries were constructed in Siemens NX-12TM and the simulations were performed in STAR-CCM+TM . An Eulerian-Lagrangian approach with two-way coupling was selected based on the fluidisation regime inside the riser. In addition, the homogeneous Schiller-Neumann drag model was employed and the D[4,3] of the sorbent was chosen as the average particle size. The models successfully described the trends that were observed in the experimental work, despite the inability thereof to capture exact values. In addition, the CFD data revealed flow swirling at the top of the riser and confirmed that lower riser aspect ratios (or height-to-diameter – H/D) ratios were associated with more uniform particle distributions. The latter finding contradicts literature and should be explored in future work. It was found that several experimental observations regarding the riser hydrodynamics could be explained with the assistance of the CFD models which was the desired outcome for this study.