Alkali metals, mainly potassium, together with other ash forming inorganic components in biomass are believed to be responsible for bed agglomeration in fluidized bed boilers burning biomass. A solution to the problem is to capture alkali vapor released from the fuel during combustion by inorganic solid additives such as kaolin. In this study, the capture of vaporous potassium chloride was investigated in a fixed bed reactor equipped with an on-line alkali detector. The detector, which is based on surface ionization, is capable of operating at alkali metal concentrations as low as those encountered after reaction with kaolin during biomass combustion (of the order of 1 ppb). Various experiments of KCl capture by kaolin powder were made at a reactor temperature of 850 degreesC. Kaolin removed up to 99% of alkali species in gas phase. The efficiency slightly decreased when KCl concentration decreased. The effect of kaolin addition on the release of alkali metals during wood combustion was studied at 650 degreesC in air. During pyrolysis, the alkali metal release increased slightly when kaolin was mixed with the wood due to the release of alkali metal impurities in the kaolin additive. The alkali metal release during char combustion was reduced to approximately 50% of the original value. The reduction increased as the amount of kaolin addition increased. Overall, the addition of kaolin suppressed the alkali metal release from the wood by approximately 20%. (C) 2004 Elsevier Ltd. All rights reserved.