Fly ash produced by coal combustion using two types of desulphurizatio
n process were studied: a conventional pulverized coal boiler equipped
with lime injection (PCL ash), and a circulating fluidized bed combus
tion boiler with limestone injection (CFBC ash). The ashes were charac
terized completely: granulometry, morphology, mineralogy, chemical com
position and behaviour to water contact.Both PCL ash and CFBC ash pres
ent similar features: fine granulometry, presence of anhydrite phase a
nd sulphate content. However, PCL ash also shows lots of spherical par
ticles, unlike CFBC ash, and a much higher lime content, due to the lo
wer desulphurization rate in PC boilers. Unlike CFBC ash, most of the
trace elements in PCL ash show an inverse concentration-particle size
dependence. Leachates obtained from both samples are rich in soluble s
alts [CaSO4 and Ca(OH)(2)] and arsenic and selenium are prevented from
solubilizing by high lime content. In wetted PCL ash, the formation o
f ettringite crystals stabilizes calcium and sulphate ions. Simultaneo
usly, arsenate, selenate and chromate anions are trapped in the crysta
l. CFBC ash does not really harden because the lime content is too low
. However, the leached selenium concentration is cut down in wetted CF
BC ash samples. (C) 1996 ISWA