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 a 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 a 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) as well as HCl and SO₂ were measured upstream of the convection pass, where deposit samples were also collected with a deposit probe. The deposit samples were analyzed semiquantitatively by means of SEM/EDX. The elemental mass balances show accumulation of alkali metals and phosphorus in the boiler. Analyses of bed material particle cross sections show the presence of phosphorus compounds within a K-silicates matrix between the agglomerated sand particles, indicating a 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 an 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, an increase of potassium chloride upstream of the convection pass and a 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.