An analytical model for freeboard and in-bed limestone sulfation in fluidized-bed coal combustors

✍ Scribed by D.C. Fee; K.M. Myles; G. Marroquin; Liang-Shih fan

• Publisher: Elsevier Science
• Year: 1984
• Tongue: English
• Weight: 525 KB
• Volume: 39
• Category: Article
• ISSN: 0009-2509
• DOI: 10.1016/0009-2509(84)80180-3

✦ Synopsis

A new model, which combines in-bed and freeboard sulfation, significantly improves the ability to predict sulfur capture by limestone sorbents in fluidized-bed coal combustors. In this model, the in-bed hydrodynamics are described in terms of a bubble phase and an emulsion phase while the freeboard region has only a diluted emulsion phase. The solids, which are in the emulsion phases, are considered to be completely back-mixed; the gaseous bubble phase travels in plug-flow but exchanges with the emulsion phase. The sulfation reaction occurs principally in the emulsion phase and the reaction rate is a direct function of the sulfur dioxide concentration, the extent of the calcium oxide conversion (as measured by a thermogravimetric analyzer), and the amount of limestone present in the bed and in the freeboard. The amount present, or holdup, in the free-board is calculated from empirical correlations for elutriation and from particle-time trajectories as predicted from equations of motion.

The model indicates that a significant amount of sulfur capture can occur in the freeboard region, especially with high superficial gas velocities and small particle sizes, both of which lead to increased freeboard holdup. For an overall sulfur retention of 90x, approximately 9% of the sulfur is predicted to be captured within the freeboard above a combustor bed 1.8 m by 1.8 m by 1.2 m high of 1200 pm diameter limestone particles that are fluidrzed at 2.4 m/s. The model also predicts that the sulfur captured by the entrained particles is negligible. These predictions compare favorably with the actual sulfur retentions experienced in similar sized pilot-scale combustors.