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 þ NaCl) as well as HCl and SO 2 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.

Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are techniques for abating nitrogen oxides (NOx ) NO + NO 2 ) from boilers and engines. Each of these techniques has a limited range of applicability, within which 50% to 70% NOx reduction is attained. Higher reductions are achieved by methods that use the aforesaid techniques in sequence, thus cumulating their reduction ability, but also collecting their respective limits. In this paper, we describe a new method that we call "combined staging" (CS). This method combines FS, AS, and SNCR in synergy rather then in sequence. In CS, the fuel is first staged for converting NOx precursors to hydrogen cyanide (HCN). Then, the air is staged for reducing HCN to N 2 . Further reduction is achievable by optional SNCR. In the followings the basics of FS, AS, SNCR, and their sequential applications are reviewed first. Then, the combined staging is introduced, its chemical details are elucidated via kinetic modeling, and options for its application are illustrated. Finally, assumptions and limits of the kinetic models are discussed. The present work reveals that CS can reduce over 40% NOx at lower temperatures and within shorter residence time than required by other techniques and methods. Thus, CS could reduce NOx effectively in devices where other techniques fails, e.g., in kraft recovery boilers, fluidized bed combustors, low-grade fuel combustors, small and domestic boilers, and fast engines.

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This paper is part 2 in a series of two papers describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets with different fuels in a 12 MW th CFB boiler. In the first part (