Fate of Alkali Metals and Phosphorus of Rapeseed Cake in Circulating Fluidized Bed Boiler Part 1: Cocombustion with Wood

This paper is part I in it series of two describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets in a 12 MW thermal CFB boiler. In paper I the results of using the mixture of wood chips and wood pellets as a base fuel are described. Up to 45% on energy basis of rapeseed cake was cocombusted during it 4 h test. Two approximately 12 h tests with energy fractions of rapeseed cake of 12 and 18% were performed with limestone as a varying parameter. Fuels were characterized by means of chemical fractionation and standard methods. Elemental mass balances were calculated for ingoing and Outgoing streams of the boiler. In addition SEM/EDX analyses of ashes were performed. Gaseous (KCl + NaCl) its well its HCl and SO2 were Measured upstream of the convection pass, where deposit samples were also collected with deposit probe. The deposit samples were analyzed semiquantitatively by means of SEM/EDX. The elemental mass balances Show accumulation of alkali metals and phosphorus ill the boiler. Analyses of bed material particle cross sections show the presence of phosphorus Compounds within it K-silicates matrix between the agglomerated sand particles, indicating it direct attack of gaseous potassium compounds on the bed surface followed by adhesion of ash particles rich in phosphorus. Build-up of deposit during the cocombustion tests mainly took place on the windward side of the probe; where in increase of K, Na, and P has been observed. Addition of limestone prevented formation of K-silicates and increased retention of phosphorus in the bed, most probably due to formation of high-melting calcium phosphates. During the tests with limestone,,in increase of potassium chloride upstream of the convection pass and it decrease of phosphorus in the fly ash fraction could be noticed. Agglomeration and slagging/fouling when cofiring wood with rapeseed cake may be linked to its high content of organically bonded phosphorus-phytic acid salts-together with high contents of water-soluble alkali metals chlorides and sulfates in the fuel mixture.