This paper presents the results of standard fuel analyses of 112 different fuels, along with the results from chemical fractionation through selective leaching. The samples, obtained from boilers during the period 1995−2010, represent the following fuel classes: coal, peat, wood-derived fuels, agricultural wastes, and sewage sludge. The leaching results show that coal consists mainly of insoluble ash forming matter, dominated by different silicates. The ash-forming matter in wood-derived fuels is mainly soluble and is dominated by potassium, calcium, and phosphorus; the insoluble ash-forming matter originates from soil contamination. The ash-forming matter in peat has the characteristics of both wood-derived fuels and coal. In agricultural waste, the ash-forming matter is mainly soluble, even though it may contain some silicon, and is dominated by potassium, chlorine, and phosphorus. The ash-forming matter in sludge is mainly acid-soluble or insoluble. It is also important to note that the aluminum silicates present in sludge may capture volatile potassium during combustion, thereby reducing the risk of deposit and corrosion. The ash-forming matter in sludge is often dominated by phosphorus precipitating agents such as iron sulfate and aluminum sulfate. The study demonstrates the value of chemical fractionation through selective leaching as a tool for explaining ash behavior in FBC.

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Deposit formation, that is, fouling and slagging, corrosion, and sintering are often cause for unwanted shutdowns of boilers. These problems are ash related. The composition and physical state of ash determines if problems may occur and where and when. Ash properties depend on the way ash‐forming matter is present in a fuel, how it is released from the fuel, and process conditions in the furnace. This chapter is dedicated to the more challenging fuels like biomass‐derived fuels that nowadays are used as a replacement for coal. Examples are taken from 92 fuels extracted from the Åbo Akademi fuel database, a collection of fuel analyses carried out at Åbo Akademi University during the last decade. Fuel characterization and analysis of ash‐forming matter in fuels will be discussed. Here, the analyses of the association of ash‐forming matter by chemical fractionation are emphasized. Release of ash‐forming matter and the formation of ash will be examined. Here, knowledge obtained from the fuel analyses plays an important role. Formed ash may cause deposits on heat exchanger surfaces, agglomeration of bed material in a fluidized bed, or corrosion. Nowadays, biomass fuels are often co‐fired, either with positive or negative effects on boiler availability. All these above‐mentioned phenomena can be seen from a chemistry point of view, where understanding of interaction of ash forming and melting behavior of ashes is seen to be vital.

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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.

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The bed agglomeration characteristics resulting from the combustion of 11 mixtures of rapeseed cake and spruce bark were studied in a bench-scale bubbling fluidized-bed reactor (5 kW). The objective was to determine the defluidization temperatures and the prevailing bed agglomeration mechanism as functions of the fuel mixture. Controlled fluidized-bed agglomeration tests were performed for each mixture with quartz sand as the bed material. The total defluidization temperatures and the initial defluidization temperatures were determined based on the measured pressure and temperature profiles in the bed. After combustion, bottom ash samples, agglomerates, and fly ash samples were analyzed by means of scanning electron microscope combined with energy dispersive X-ray detector (SEM-EDX). The composition of the ash-forming matter produced by the combustion of rapeseed cake is significantly different from that produced by the combustion of bark, resulting in different bed agglomeration tendencies. Bark contains ash-forming matter dominated by calcium, with some silicon and potassium, whereas rapeseed cake is rich in phosphorus, potassium, and sodium. The total defluidization temperature for pure bark was above 1045 °C, whereas, for rapeseed cake, defluidization occurred during combustion (800 °C). During the combustion of bark, the formation of a potassium-rich layer on the silica-bed grains was found to be a crucial for the formation of agglomerates. The low defluidization temperature for the rapeseed cake can be attributed to the formation of sticky ash, which is dominated by phosphates. Two main phosphate forms were observed in the neck between the silica grains: calcium–potassium/sodium phosphates, and magnesium–potassium phosphates. As the proportion of bark increased, the Ca/P ratio increased in the fuel mixture, and the formation of high-temperature melting phosphates in the ash was favored. However, the addition of bark also favored the formation of a potassium-rich layer on the silica bed material, leading to the coexistence of both bed agglomeration mechanisms. In the present work, mixtures with a minimum of 60 wt % bark resulted in significantly increased defluidization temperatures and reduced bed agglomeration tendencies, compared to what occurs in rapeseed cake monocombustion.