Properties and behaviour of SO2 adsorbents for CFBC
AUTHOR: Irene M. Smith
DATE: November 2007
Sulphur capture in CFBC boilers usually relies on addition of limestone, which is not used efficiently. Limestones with lower sulphation capacities are undesirable because they increase the raw materials and the residues requiring management. This increases operating costs and CO2 emissions. It is preferable to use less limestone or shells instead. The performance of adsorbents varies widely and depends mainly on their morphology and microstructure rather than on their chemical properties. Adsorption mechanisms are investigated. Fragmentation and attrition are important during calcination. Sulphation is affected by boiler size, configuration, operating conditions, oxidising and reducing zones, reactions with compounds in coal ash, and attrition. Preparation and handling of limestone at the plant is examined and care has to be taken to avoid moisture in all systems.
Recently designed plants achieve an SO2 capture efficiency at or over 95% with addition of adsorbent at a Ca:S molar ratio of 2–2.5, depending on the sulphur content of the fuel and the reactivity of the adsorbent. Various methods of enhancing SO2 capture are reviewed.Sorbent reactivation by pressurised steam hydration or grinding/hydration appear to be most promising but are not yet used on full-scale CFBC. Fuels and operating conditions affect emissions control and the production of residues. The adsorbent requirement may be reduced by cofiring with low sulphur high calcium fuels and by optimising the furnace temperature for different fuels. The effect of sulphur capture on other emissions is examined. NOx and N2O emissions may be reduced by choice of fuels and adsorbent. A decrease in CO2 emissions results from minimising use of limestone or by carbonation/calcination to capture CO2 before sulphation.