Bed Agglomeration Characteristics of Wood-Derived Fuels in FBC

818

Energy & Fuels 2006, 20, 818-824

Bed Agglomeration Characteristics of Wood-Derived Fuels in FBC Maria Zevenhoven-Onderwater,*,† Marcus O ¨ hman,‡ Bengt-Johan Skrifvars,† § Rainer Backman, Anders Nordin,‡ and Mikko Hupa† Process Chemistry Centre, A° bo Akademi UniVersity, c/o Biskopsgatan 8, FIN-20500 A° bo, Finland, and Energy Technology and Thermal Process Chemistry, Umea˚ UniVersity, SE-90187 Umea˚ , Sweden ReceiVed October 24, 2005. ReVised Manuscript ReceiVed January 20, 2006

The agglomeration tendency of five Scandinavian forest-derived biomass fuels was studied using an advanced fuel analysis, i.e., a combination of chemical fractionation analysis, controlled bed defluidization tests, and SEM/EDX analysis of bed samples. It is shown that all five fuels have a tendency to form bed agglomerates when fired in a fluidized bed with silica sand as the bed material. The agglomeration appeared to proceed by formation of a sticky layer on bed particles gluing them together. The layers on the bed particles contained Si, Ca, and K, and, in some cases, P. The combination of advanced fuel analysis by SEM/EDX showed that the soluble fraction of Ca and K (i.e., leachable from the fuel with water and acetate) may be responsible for the formation of the layer. Silicon may mainly come from the bed particles.

Introduction Fluidized bed combustion of biomass fuels is a common energy conversion technique in Scandinavia nowadays. Although much experience has been gained over the years, problems still arise when firing unconventional fuels. Many of these problems are ash related and can lead to unscheduled shutdowns due to fouling, slagging, and/or bed defluidization as a result of agglomeration. Extensive research has been carried out to understand and predict these phenomena in fluidized bed combustion of biomass fuels.1-5 Chemical fractionation of fuels in combination with the use of global equilibrium analysis, referred to as thermodynamic multiphase multicomponent equilibrium calculations, has been shown to be able to predict problems related to deposit formation in the flue gas channel.6-8 Also, the usefulness of * To whom correspondence should be addressed. Phone: +358 2 215 4718. Fax: +358 2 215 4962. E-mail: [email protected]. †A ° bo Akademi University. ‡ University of Umeå. §A ° bo Akademi University and University of Umea˚. (1) O ¨ hman, M. Experimental studies on bed agglomeration during fluidized bed combustion of biomass fuels. Academic Dissertation, University of Umeå, Sweden, 1999. (2) Lind, T. Ash formation in circulating fluidized bed combustion of coal and solid biomass. Academic Dissertation, VTT Technical Research Centre of Finland, Espoo, Finland, 1999. (3) Valmari, T. Potassium behavior during combustion of wood in circulating fluidized bed power plants. Academic Dissertation, VTT Technical Research Centre of Finland, Espoo, Finland, 2000. (4) Skrifvars, B.-J.; Blomquist, J.-P.; Hupa, M.; Backman, R. Presented at the 15th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 1998. (5) Brus, E. Bed agglomeration during combustion and gasification of biomass fuelssmechanisms and measure prevention. Academic Licentiate Thesis, Umeå University, Sweden, 2004. (6) Zevenhoven-Onderwater, M. F. J.; Blomquist, J.-P.; Skrifvars, B.J.; Backman, R.; Hupa, M. Fuel 2000, 79 (7), 1353-1361. (7) Zevenhoven, M.; Skrifvars, B.-J.; Yrjas, P.; Hupa, M.; Nuutinen, L.; Laitinen, R. Presented at the 16th International Conference on Fluidized Bed Combustion, Reno, NV, 2001; Paper 73. (8) Skrifvars, B.-J.; Zevenhoven, M.; Backman, R.; Hupa, M. Presented at the 16th International Conference on Fluidized Bed Combustion, Reno, NV, 2001; Paper 113.

these thermodynamic calculations for predicting agglomeration under gasification conditions has been studied.9,10 Although the distribution of ash-forming elements in the fuel was not taken into account, these studies demonstrated the predicting power of thermodynamic calculations. Ash-forming matter in biomass species consists of soluble and less soluble compounds. Soluble compounds may represent volatile compounds that may be released to the gas phase and become involved in deposit formation. However, they may interact with the bed material as well and thus become involved in bed agglomeration. The chemistry involved in the formation of agglomerates under both combustion and gasification conditions is only partly understood. It is clear that the elements responsible are Ca, K, and Si and, to a lesser extent, P.2,3,5 It is still unclear from where these elements originate and how they are involved in the chemical reactions in the bed that lead to agglomeration. Also the correlation between fuel composition and the occurrence of agglomeration in FBC is still not clear. In this study the chemical fractionation analysis, based on selective leaching as originally developed by Benson and Holm,11 Baxter,12 and Zevenhoven-Onderwater13 is used to give a better understanding of how ash-forming elements are bound in five Scandinavian wood-derived biomass fuels. Controlled bed defluidization tests are used to measure the defluidization tendencies for the fuels under combustion conditions. Scanning electron microscopy studies of bed agglomerates sampled from the test rig are used to detect and study how the composition of agglomerates varies depending on the fuel composition. (9) Zevenhoven, M.; Backman, R.; Skrifvars, B.-J.; Hupa, M. Fuel 2001, 80, 1489-1502. (10) Zevenhoven, M.; Backman, R.; Skrifvars, B.-J.; Hupa, M. Fuel 2001, 80, 1503-1512. (11) Benson, S. A.; Holm, P. L. Ind. Chem. Eng. Prod. Res. DeV. 1985, 24, 145-149. (12) Baxter, L. L. Task 2. Pollutant emission and deposit formation during combustion of biomass fuels; Sandia National Laboratories: Livermore, CA, 1994. (13) Zevenhoven-Onderwater, M. Ash-forming matter in biomass fuels. Academic Dissertation, A° bo Akademi University, Finland, 2001.

10.1021/ef050349d CCC: $33.50 © 2006 American Chemical Society Published on Web 02/18/2006

Bed Agglomeration of Wood-DeriVed Fuels in FBC

Figure 1. Schematic view of the chemical fractionation analysis technique, such as it was developed for coals (refs 10 and 11).

Thermodynamic multiphase multicomponent equilibrium calculations are used to estimate the melting behavior of the coatings. Results from the chemical fractionation, the defluidization tests, the SEM/EDX analyses, and the equilibrium calculations are combined and used in an attempt to explain the formation of agglomerates. Experimental Section Five fuels were studied, these being a bark, forest residue type 1, forest residue type 2, sawdust, and a construction residue wood. All five fuels were subjected to the four different experimental parts as described below. The fuels were partly selected to represent the interests of Scandinavian heat and power producers, i.e., forest-derived fuels with as many different characteristics in combustion behavior as possible. Chemical Fractionation and Analysis. Chemical fractionation of the five solid fuels was carried out according to a modified procedure7 of Benson and Holm,11 Baxter,12 and ZevenhovenOnderwater13 as illustrated in Figure 1. The chemical fractionation technique distinguishes different types of inorganic matter in the fuel according to their solubility in different solvents. Increasingly strong solvents, i.e., water (H2O), 1 N ammonium acetate (NH4Ac), and 1 N hydrochloric acid (HCl) produced three aqueous samples and four solid samples for characterization. All solid and aqueous fractions were analyzed for Si, Al, Ti, Fe, Ca, Mg, Na, K, S, P, and Cl. Typical ash-forming compounds that are leached out by water are alkali salts. Compounds leached out by ammonium acetate are ion exchangeable, such as Ca and Mg, and are typically more organically associated. The hydrochloric acid leaches carbonates and sulfates. Silicates and other minerals remain in the insoluble residue. It is assumed that the ash-forming elements leached out by the water and ammonium acetate represent elements in the fuel that may form the volatile reactive species, which could form fine ash particles after condensation in the flue gas channel or that could interact with the bed material. Moreover, it is assumed that ash-forming elements extracted by hydrochloric acid or being present in the residue represent the less volatile ash-forming elements. Controlled Fluidized Bed Tests. O ¨ hman and Nordin14 have previously described the controlled fluidized bed defluidization method. The bench-scale reactor (5 kW) (see Figure 2) is constructed from stainless steel, being 2 m high and 100 and 200 mm in bed and freeboard diameters, respectively. For every experiment fresh sand with a particle size of 200-250 µm was (14) O ¨ hman, M.; Nordin, A. Energy Fuels 1998, 12, 90-94.

Energy & Fuels, Vol. 20, No. 2, 2006 819 used as bed material. The statisitic bed height was 50 mm. To obtain isothermal conditions in the bed, and to minimize the significant influence of cold walls in such a small-scale unit, the reactor is equipped with electrical wall-heating elements, which equalize the wall and bed temperatures. The tests were initiated by loading the bed with a certain ash to bed material ratio, under normal FBC conditions, by burning the biomass fuel, i.e., fuel pellets with a diameter of 6 mm and a length of 10-15 mm. At an ash amount corresponding to approximately 6 wt % ash in the bed, the fuel feeding was stopped and the operation was switched to external heating. The excess oxygen concentration was controlled to 6 vol % (wet gases). A fluidization velocity of 0.3 m/s, 4 times the minimum fluidization velocity, was used, and the initial bed temperature was maintained at 800 °C for all fuels. After the fuel feeding was stopped the bed was heated at a rate of 3 °C/min to the point where it defluidized. To maintain a combustion atmosphere in the bed during the external-heating phase, propane was mixed with the primary air in a chamber prior to the air distributor. Monitoring of differential pressures shows the onset of bed defluidization and temperatures in the bed.1 Defluidization can clearly be seen as a drop in the bed pressure. Earlier studies14 have shown that only 1.5 wt % of ash in the bed is sufficient for defluidization to occur eventually, and no significant influence of the variables “amount of bed material” (350-450 mL), “heating rate” (1-5 °C/min), and “fluidization velocity” (U/Umf 3-5) or “air-to-fuel ratio” (excess O2 8-12% dry) on the determined defluidization temperature, Tdef, has been found. SEM/EDX Analysis of Bed Samples. Throughout the experimental runs samples of the bed material were collected using an air-cooled suction probe equipped with a cyclone separator. Three samples were taken for each run, one sample at the end of the combustion period, another sample during the bed temperature increase phase, and a third bed sample at the defluidization temperature, Tdef. SEM pictures were taken of all samples. Only the third sample was analyzed with SEM/EDX point analyses. The samples were mounted in epoxy, cut with a diamond saw, and polished. The resulting cross-sectional area was then examined. A number of spot analyses were taken from selected points in the sample, such as agglomeration necks between particles and layers on particles Melting Behavior of Layers on Bed Particles. On the basis of SEM/EDX spot analyses of the layer on bed particles the melting behavior was estimated by extracting phase behavior data from phase diagrams and by thermodynamic multiphase multicomponent equilibrium calculations.15 The melting behavior calculations were based on the average chemical composition of the layer on the analyzed bed particles. The equilibrium calculations are based on Gibbs energy minimization. The input amounts of each element considered were calculated to represent a practical case with realistic amounts of layer-forming matter, taken from the SEM/EDX analyses, and air. Thus, the results obtained from the calculations corresponded directly to a realistic layer in an FBC environment.

Results and Discussion Fractionation Experiments. Results from the fractionation of the different fuels are shown in Figure 3a-e. The figures show the distribution of Si, Al, Fe, Ti, Ca, Mg, Na, K, S, P, and Cl over the fractions as mg/kg ds. A clear bar represents the amount of ash-forming elements leached out by water, a striped bar the amount of ash-forming elements leached by the acetate, and a gray bar that leached by the HCl. A black bar represents the amount of ash-forming elements not leached out by any of the leaching agents. (15) Eriksson, G.; Hack, K. Metall. Trans. B 1990, 21B, 1013-1022.

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ZeVenhoVen-Onderwater et al.

Figure 2. Illustration of the bench-scale fluidized bed reactor. T1 and T2 are used for determination of Tdef (T1 ) 25 mm and T2 ) 50 mm above the distributor plate).

The ash content of the bark sample was 5.5 wt % in the dry fuel. It contained much silica, some 12 g/kg in the dry fuel. The two other main ash-forming elements were calcium and potassium. As shown in Figure 3a the main part of the silicon and also of the aluminum and iron were found in the rest fraction. This indicated a significant presence of minerals, probably soil contamination of the fuel. Both calcium and magnesium were mainly found in the water and acetate fractions. Only some 20% of the total amount present could be found in the HCl fraction and solid residue. Up to 90% of the sodium present was found in the HCl fraction and solid residue, whereas about half of the potassium was found in the water and acetate fractions. Almost all sulfur was found in the HCl and rest fractions. Chloride was found in the water fraction almost completely. Figure 3, parts b and c, show the results for forest residue types 1 and 2. The forest residue samples had a much lower content of ash-forming elements in the fuel (some 2 wt % in the dry fuel) than the bark. The main ash-forming elements were silicon, calcium, magnesium, and potassium. The main part of the silicon, aluminum, and iron was present in the solid residue and HCl fraction. A small amount was found in the acetate fraction. Calcium and magnesium were found in all fractions. Calcium was distributed more or less evenly over the water-acetate and HCl-residue fractions; the major part of magnesium was found in the water and acetate fraction. For type 1 about 40% of sodium was found in the residue, whereas this was up to 70% for type 2. For both forest residues the major part of potassium was found in the water and acetate fractions. The major part of the sulfur was found in the HCl and residue fractions. The construction residue wood sample was expected to contain ash-forming elements that are interconnected differently when compared to the other fuels due to the possible presence of anomalous chemicals, such as paint. This also led to the highest amount of ash-forming elements in the fuel, i.e., 7 wt % in the dry fuel. The three main ash-forming elements were silicon, iron, aluminum, and in lesser amounts calcium. Titanium

was also detected, apparently present as small amounts of coloring agents present in the construction residue wood. As shown in Figure 3d silicon was found in the solid residue and the HCl fraction. About half of the aluminum was found in the residue, the rest in the HCl fraction. The major part (approximately 60%) of the calcium was found in the acetate and water fractions, whereas for magnesium this was only approximately 25%. Potassium and sodium were present in all fractions, with 20-30% of the alkali metals present in the water and acetate fractions; the rest was found in the HCl fraction and residue. The main part of sulfur was found in the acetate and water fraction. Chloride leached out of the fuel in the water step completely. The sawdust sample contained the lowest amount of ashforming elements with only 0.7 wt % in the dry fuel. The fuel ash was relatively high in silicon, calcium, potassium, and sulfur. Upon fractionation of this fuel, silica was found in the water and acetate fraction. Calcium and magnesium were present in all fractions, whereas the main part of potassium was present in the water fraction. Approximately 70% of the sulfur present was found in the HCl fraction and solid residue, whereas this was some 50% for the phosphorus in the fuel. Again all chloride was found in the water fraction (Figure 3e). Figure 4 shows the distribution of ash-forming elements over the soluble fraction and the less soluble fractions as defined earlier.5 Soluble species are considered to originate form ashforming matter leached by water and NH4Ac and may be of special interest from an agglomeration point of view. Less soluble matter may originate from the ash-forming matter leached by HCl or retained in the solid residue. A comparison of forest residue type 2 with sawdust indicates that the distributions of the ash-forming elements in the fuels were much alike. Also forest residue type 1 showed a similarity with these two, showing, however, a higher amount of ashforming elements in the water and acetate fractions. This was rather surprising, since type 2 contained also green needles and was a fuel expected to contain a higher amount of soluble matter.


Energy & Fuels, Vol. 20, No. 2, 2006 821

Figure 3. (a) Distribution of ash-forming elements in the four different fractions of the dry bark. (b) The distribution of ash-forming elements in the four different fractions of the forest residue type 1. (c) The distribution of ash-forming elements in the four different fractions of the forest residue type 2. (d) The distribution of ash-forming elements in the four different fractions of the construction residue. (e) The distribution of ash-forming elements in the four different fractions of the sawdust.

Both bark and sawdust showed that the major amount of the ash-forming elements was found in the HCl fraction and solid residue. However, neglecting the high amount of soil contamination in the bark (represented by silicon, aluminum, iron, and sodium) the sulfur in bark was most abundant in the solid residue and HCl fraction. A substantial amount of sulfur was found to be present in the water and acetate fraction in sawdust. In bark, more magnesium than calcium was found in the residue and HCl fractions. This was just the opposite of which was found in the sawdust samples. Construction residue wood may be difficult to compare with the other four fuels. It should be regarded as a waste-derived fuel containing a wide range of contaminants. Controlled Fluidized Bed Tests. The controlled defluidization tests of the different fuels resulted in defluidization at temperatures in the range of 930-980 °C. These results are presented in Table 2.

The lowest Tdef, around 930 °C, was found for bark and for the forest residue type 1. Sawdust followed with a Tdef of approximately 940 °C and construction residue wood with a Tdef of 950 °C. The highest Tdef was found for the forest residue type 2, 980 °C. SEM/EDX Analysis of Bed Samples. Figure 5 shows a typical SEM/EDX, backscattered image, of a bed agglomerate sample from sawdust firing. The light colored layer on the dark sand bed particles was 1-10 µm thick, as for all fuels. Figure 6 summarizes the spot analyses made on the layers of the bed particles retrieved from each firing case. It shows that the layers of all bed samples, except for the construction residue wood, contained silicon, calcium, and potassium as major elements. Phosphorus was usually found in the layers as well but at a clearly lower extent than calcium or potassium. In the construction residue wood more sodium than potassium and more iron and less phosphorus were found in the layers.



Table 1. Composition of Fuels as Used in the Thermodynamic Equilibrium Calculations sand bed moisture C H O Cl ash

wt % % db % db % db % db % db

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 rest

wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash wt % in ash

98.9 0.2 0.1 0.1 0.1 0.0 0.1

bark

forest residue type 1

forest residue type 2

construction residue wood

sawdust

6.6 50.3 5.9 38.3 0.2 5.5

12.9 52.2 6.1 39.8 1.4 1.8

13.7 52.0 6.0 39.6 0.6 2.2

12.0 49.4 5.8 30.7 1.5 7.2

11.1 52.8 6.1 40.2 0.2 0.7

46.7 6.6 4.3 25.0 3.2 1.2 4.8 1.1 1.6 5.6

34.3 4.9 2.1 30.7 5.1 1.7 10.3 4.7 6.3 1.4

35.1 5.6 2.8 30.7 4.6 1.4 9.5 2.9 4.2 3.8

52.4 9.6 9.7 12.1 2.5 3.9 2.8 2.0 0.7 4.5

28.6 6.7 4.5 31.0 5.1 3.1 10.1 1.3 2.7 6.5

Table 2. Defluidization Temperature and First Melting Point and the Maximum Amount of Melt of the Layer as Calculated for the Five Fuels experiment

bark forest residue (type 1) forest residue (type 2) construction residue wood sawdust

SEM/EDS spot analysis

Tdef (°C)

T0 (°C)

amount %

930 930 980 950 940

775 775 775 825 775

35 39 35 32 50

The first melting point, based on the spot analyses of the layer, as determined with the thermodynamic equilibrium calculations was the same for all five fuels, i.e., around 775 °C. Large amounts of silicon present in all layers determined this first melting point, of the most stable phase, i.e., low-melting potassium silicates. Also the amount of melt did not differ much when comparing the fuels. This gives an indication of the rather invariant composition of the layer on the bed particles for all fuels fired. The SEM/EDX analyses indicated that presence of a sticky layer is a prerequisite for the formation of agglomerates. Agglomerates were present in all bed samples taken at temperatures above 800 °C, i.e., already at the starting point of each defluidization test. This indicates that agglomeration takes place at far lower temperatures than the defluidization temperature. Apparently furnace conditions and other physical phenomena such as attrition, abrasion, and fragmentation in the bed can withhold the bed from defluidization. At a certain temperature,

Figure 4. Sum of ash-forming elements in the reactive fine and less reactive particle fraction of each fuel as they were analyzed with the chemical fractionation analysis. The × at each bar indicates the analyzed amount of ash, recalculated to elements in the dry fuel.

however, the amount of coated particles seems to exceed a critical concentration causing an increase in the number of agglomerates, which can lead to total defluidization. This defluidization temperature Tdef seems to be fuel dependent. Formation of agglomerates is considered to take place in three steps:1 (1) buildup of a layer enriched in Si, K, and Ca, (2) melting of and (3) formation of agglomerates due to the stickiness of the formed layer. Role of Ash-Forming Matter in Bed Agglomeration. Neither the chemical interaction between bed material and soluble ash-forming matter nor the less soluble fraction alone can explain the formation of a layer and occurrence of defluidization. However, the formation of a layer on bed material, a prerequisite for agglomeration and defluidization, may be explained. The SEM/EDX analyses quantified the average composition of the layer formed on bed particles, but it could not reveal where the elements found came from. Some 98.9 wt % of the bed consisted of Si as SiO2. This means that the silicon, as found in the agglomerates, could originate both from the fuel and bed material. Calcium and potassium could originate from the fuel, i.e., both from the soluble and the less soluble ash-forming matter. It is not likely that the calcium or potassium originates from the bed material since this contains only 0.12 wt % given as CaO and 0.06 wt % given as K2O (see Table 1).

Figure 5. SEM pictures (backscatter image) of bed particles and agglomerates from the controlled bed agglomeration test of sawdust: magnification 120×.


Energy & Fuels, Vol. 20, No. 2, 2006 823

Figure 6. Semiquantitative SEM/EDS analyses of the composition of bed particle layers from the controlled bed agglomeration tests of the five fuels.

There may be three possible ways to obtain a 10 µm thick layer: (1) The layer grows outward onto the particle, assuming the bed particle acts as an inert carrier for the layer material. In this case all the elements in the layer originate from the fuel. (2) The layer grows inward into the particle. In this case reactive elements may react with the bed particle. In both cases growth is limited by either erosion of the layer or diffusion limitation or both. (3) A combination of 1 and 2. By comparing the amount of bed material after the controlled experiments with the amount of bed before the experiments and the amount of fuel fed, the total weight of the layer present could be calculated and distributed over the three elements of major importance. After this the distribution of the elements over the soluble and less soluble ash-forming matter could be combined with the quantity of the elements needed to build the layer.

Up to 50% of the potassium fed into the furnace was found in the layers. This may be the easily soluble potassium present alone. The rest of the soluble potassium could escape the bed in gaseous or fine particle forms. An exception may be formed for the construction residue wood where all the soluble potassium may be caught on the bed material. It was found that 8-30% of the calcium present in the fuel ended up in the layers. This means that up to 92% of the calcium might react to form other components, such as calcium sulfates, phosphates, and oxides. The easily soluble calcium could in principle be responsible for the layer formation. However, since calcium was evenly distributed over all the fractions in all fuels, it is difficult to say which part is responsible for the layer formation. Calcium may be present as included minerals (CaC2O4) in some fuels, which are only leached with HCl.6 Even these calcium minerals may be considered reactive when released from the fuel. The calcium leached with the water and acetate fractions is considered to form submicrometer-sized ash


particles of, for example, CaSO4 and Ca3(PO4)2. Such components were identified in some of the layers by SEM/EDX point analyses during earlier studies at our laboratory. Here, the layer on the bed particles appeared to be homogeneous, indicating ash compounds have reacted into a homogeneous molten layer. Up to 30-65% of the total silicon entering the boiler with the fuel could be found in the layer. This means that the rest of the silicon is not involved in the layer formation. Assuming that a fine particle fraction of the silica, as found leached with H2O and NH4Ac, will play a role in the bed agglomeration this amount should be enough to form the layer that is found. This is the case only for the forest residue type 1. However, when compared with the other fuels forest residue type 1 was not more susceptible for defluidization than the other fuels. Thus, it could also mean that the bed particle itself is involved in layer formation. The less soluble silicon in most cases represents a contamination of soil particles that enters the furnace with the biomass fuel. These soil particles may be considered as alternative bed particles, which might obtain a layer upon their surface as well. Layer formation may take place by reaction between Si, Ca, and K. In practice components containing these three elements will not arrive at the same time at the same place. There are two possibilities: (1) Silicon reacts with potassium forming potassium silicates with a first melting point as low as some 750 °C. This may be the first layer formed on bed particles, which catches other small particles released from the fuel, such as calcium phosphates, sulfates, and oxides. After this first capture all components present can interact forming a sticky layer with a melting point below 800 °C. (2) Silicon reacts with calcium forming calcium silicates with a first melting point above 1000 °C. This second route is considered unlikely to cause agglomeration, due to the high first melting point, making capture of potassium components by gluing less probable at the low temperatures as in FBC.


However, presence of a small amount of potassium may reduce the first melting point considerably. Conclusions All five fuels studied here show a defluidization tendency in a narrow temperature range from 930 to 980 °C. This temperature range is well in line with earlier experiences of firing forestry type fuels. Defluidization is preceded by the formation of agglomerates at temperatures as low as 800 °C. In all cases the composition of the layer formed was similar and contained mainly potassium, calcium, and silica as well as small amounts of phosphorus. Silicon, necessary for buildup of a layer, probably originates from the bed material. Potassium, found in a bed particle layer, may originate from the soluble ash-forming matter. Calcium may originate from either the soluble or less soluble ash-forming matter caused by the possible presence of calcium-containing included minerals in the fuel. The less soluble mineral matter could play a role as an alternative sand bed particle or in the formation of more heterogeneous agglomerates consisting of both bed material and remaining ash after char burnout. The first and critical step in the buildup of a layer on bed material is probably interaction of potassium containing easily soluble (reactive) components with the silica rich bed. Acknowledgment. The work presented in this paper was part of the Va¨rmeforsk Project No. B8-803. Partners in this project were Brista Kraft, Sigtuna Energi, Ra¨vsta, Ma¨rsta, Sweden; Skellefteå Kraft AB, Skellefteå, Sweden; Falun Energi, Falun, Sweden; So¨derenergi, So¨derta¨lje, Sweden; and C-4 Energi, Kristianstad, Sweden and are kindly acknowledged for thought-provoking discussions during the project. EF050349D

Bed Agglomeration Characteristics of Wood-Derived Fuels in FBC

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