Fate of phosphorus during co-combustion of rapeseed cake with wood

Abstract

Recent studies show that deposit formation and agglomeration in fluidized bed boilers may be aggravated by a high phosphorus content besides alkali metals, chlorine and sulphur in a fuel. This paper presents the fate of phosphorus during co-combustion of wood chips and wood pellets with rapeseed cake pellets, a high phosphorus fuel in a 12MW CFB boiler. 12 hour tests with 12% and 18% (energy basis) of rapeseed cake with wood were performed with and without limestone addition. All fuels were characterised by means of standard fuel analyses combined with chemical fractionation. Retrieved ash samples were analysed using wet chemical analysis complemented with SEM/EDXA. Gaseous alkali metal chlorides as well as HCl and SO2 were measured upstream of the convective pass at a flue gas temperature of 800°C where also the deposit samples were collected with a deposit probe. The composition of deposits was studied with SEM/EDXA. Analyses of bed material particle cross-sections showed phosphorus compounds present within a K-silicates matrix between the agglomerated sand particles, indicating direct attack of gaseous potassium compounds on the bed surface followed by adhesion of rich in phosphorus ash particles. Build-up of the deposits took place mainly on the windward side of the probe; where up to 9 wt-% of phosphorus was present. SEM/EDXA shows that rapeseed cake addition caused an increase of K, Na besides P indicating presence of low melting phosphate salts in the deposits. During limestone addition in the deposit samples the increase of Cl could be noticed however no significant change in P content was observed. This paper shows that agglomeration and fouling when co-firing rapeseed cake may be linked to its high content of organically bonded phosphorus - phytic acid salts; together with high content of water soluble fraction of alkali metals chlorides and sulphates in the fuel mixture.