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

Abstract

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.

Use of chemical fractionation to understand partitioning of biomass ash constituents during co-firing in fluidized bed combustion

“…Any cross contamination of the ash streams from previous test runs can be easily detected, especially when using woody fuels with low mineral content. This type of problem was already identified in literature, where ash mass recovery reached 160% [10]. Nevertheless, for most of the combustion tests this contamination was not observed, and when took place appeared to be correlate with the more reactive elements, Na, K and Ca, in accordance with the chemical fractionation (Fig.…”

“…Typically, the more soluble compounds leached out by water are alkali sulphates, carbonates and chlorides. Recent studies showed that phosphates are also leached out in the water step [10]. NH 4 Ac generally leaches out the organically associated elements, especially Ca and Mg and some K and Na.…”

Bed Agglomeration Characteristics in Fluidized Quartz Bed Combustion of Phosphorus-Rich Biomass Fuels

“…19 Piotrowska and co-authors also found agglomerate necks consisting mainly of potassium, calcium, and phosphorus in co-combustion of rapeseed cake and wood. 20 Bari si c and co-authors found that the addition of limestone to the mixtures of rapeseed cake and wood prevented bed agglomeration because of the formation of bed particle coatings containing high-temperature melting phosphates. 21 During full scale combustion of wood and grain waste (oat seed) in a 75 MW th BFB boiler, Silvennoinen and Hedman showed that the formed bed particle layers consisted mainly of P, K, Ca, and Mg. 22 In another study, Fryda et al, co-fired meat and bone meal (MBM) with olive bagasse residues and concluded that the phosphorus in the MBM contributed to the rapid bed agglomeration.…”

“…[16][17][18][19][20] Relatively little research regarding bed agglomeration characteristics/mechanisms during combustion or gasification of phosphorus-rich biomasses could be found in the literature. [19][20][21][22][23][24] Bostr€ om et al, employing different mixtures of bark and rapeseed meal (RM) observed clear differences in the bed agglomeration characteristics between phosphorus-rich and phosphorus-poor biomass fuels and fuel mixtures. The quartz bed grains with continuous inner reaction layers, observed in fluidized bed combustion of woody biomass fuels, were not seen when the fuel (bark) was co-fired with the rapeseed meal containing high concentration of phosphorus.…”

Halide aerosols in circulating fluidised bed co-combustion. Role of coal bound kaolin

“…Replacing conventional fuels with waste or biomass tends to lead to various ash related problems in boiler furnaces [4][5][6]. Regarding biomass combustion the main initiators of ash melting and corrosion are attributed to the salts of Na, K, Ca, S and Cl [7][8][9] as well as P [10][11][12][13]. For waste incineration the aforementioned list of main contributors is commonly complemented with Zn, Pb and also Sn [14][15][16][17][18].…”