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Alternately, an approach was proposed by MET – the original FGD manufacturer, to consider its Absorber Liquid Redistribution Device (ALRD®), a patented and proprietary technology to enhance the mass transfer characteristics in the absorber. This technology offsets the phenomena known as “gas sneakage” which represents a fractional part of the flue gas that passes through an open spray tower, near the vessel wall, due to the physical limitations in the arrangement of the spray headers and nozzles near the circumference. This results in relatively low spray density concentrations at the circumference of the absorber. This improvement is accomplished with a relatively simple retrofit having a negligible increase in pressure drop across the tower. The ALRD technology was developed and patented by MET (US Patent 6,550,751 B1) and is included in its OEM designs.
One of the primary design criteria for achieving a desired level of SO2 reduction is the liquid-to-gas (L/G) ratio or the quantity of liquid sprayed relative to the volume of flue
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The B.L. England Unit 2 absorber was originally designed to operate with a recycle slurry density of 15 wt% solids. Under the revised operating conditions and improved SO2 removal efficiency, the concentration of solids in the recycle slurry increased to 18-20 percent, most of which was in equipment. The evaluation of the existing equipment to meet the revised operating conditions indicated that none of the existing equipment required modification. The increase in the percentage of solids in the slurry and higher SO2 removal efficiency was within the capability of the absorber recycle pumps, absorber agitators, oxidation air compressors, bleed pumps, hydroclones and centrifuges.
Upgrade approach
The selected upgrade option was to install two ALRD levels to meet the ACO requirements in the most cost-effective, and schedule-effective approach, while meeting the wide range of operating conditions required. Another benefit of this option was a negligible increase in system pressure drop, in contrast with the increased L/G option. The ALRD technology was incorporated into the Unit 2 FGD during two short outages. As illustrated in Figure 1, the ALRD technology is designed to modify the annular portion of the absorber cross-sectional area near the absorber wall, and re-direct the falling liquid film from the absorber wall back into the absorber spray zone for further interaction. This slight decrease in the cross-sectional area of the tower has a minimal effect on the flue gas pressure loss in the tower, but a major effect on gas-liquid contact.
