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Ind. Eng. Chem. Res. 2002, 41, 2888-2894
Bed Agglomeration Problems in Fluidized-Bed Biomass Combustion Go1 ran Olofsson,* Zhicheng Ye, Ingemar Bjerle, and Arne Andersson Department of Chemical Engineering II, Lund University, SE-221 00 Lund, Sweden
Bed material agglomeration was studied experimentally in a fluidized-bed biomass combustor. Four biomass fuels, four bed materials, three bed additives, and three NOx additives were tested in a temperature range of 670-870 °C and at two pressure levels of 1.0 and 1.5 MPa. Two types of agglomeration were observed, a homogeneous and a heterogeneous type. The first occurred at low temperature and could be partly compensated for by erosion of the bed. The second took place at high temperatures and often involved interaction between the fuel ash and the bed material. The immobility of the bed particles made the heterogeneous agglomeration a selfaccelerated process. The occurrence of hot spots in the bed was the precondition for heterogeneous agglomeration being induced. When silicon was present, alkali metals were the main contributors to heterogeneous agglomeration. Aluminum and iron compounds were able to suppress agglomeration through the high melting point of the eutectics that were formed. Introduction Limited energy resources and environmentally unfriendly emissions of SO2, NOx, CO2, and particulate matter from the combustion of fossil fuels have forced modern society to search for alternatives sufficient to meet the demands for energy production but releasing smaller amounts of pollutants. Considerable research has been directed at techniques for the thermochemical conversion of biomass and at their application and commercialization. One of these techniques, fluidizedbed combustion, has the advantage of more effective heat transfer, greater fuel flexibility, lower operational temperatures, and lower emissions of SO2-NOx as compared with other technologies.1 Although fluidized-bed combustion is a mature technique for use with coal, frequent operational problems have been encountered in its application to biomass. One of these is agglomeration of the bed material. Biomass ash is relatively rich in alkali and alkalineearth metals, causing it to melt at relatively low temperatures. Fouling of the heat exchangers is an additional problem. The following important aspects of agglomeration and agglomeration-induced defluidization have been discussed in the literature: When the bed agglomerates, fluidization decreases and may even cease. The point at which the bed ceases to fluidize is marked by a sharp decrease in pressure and by segregation of the temperatures over the bed.3 The problem of agglomeration is basically related to the content of the fuel ash, the operating temperature, and the type of bed material employed. At elevated temperatures,1,6 the presence of Na and K compounds in larger amounts can lead to rapid agglomeration of the fluidized bed. The higher the temperature, the greater the agglomeration tendency2-5,7 and the size of the agglomerates become. At sufficiently high temperatures, a fraction of the ash-forming compounds is volatilized.2 The potassium can be released as chloride if chlorine is present in the fuel, or it can also be released as a hydroxide, oxide, sulfate, or carbonate.3 In the combustion of biomass, the ash compounds can adhere to the bed particles through diffusion of volatile ash compounds into the bed particles and the chemical reactions
that follows. This leads to the bed particles being quite regular in shape and constant in size.2,7 Potassium, sodium, silicon, and calcium are often found in the molten phase.7 The eutectic [K2SO4-(KPO3)2], which has a melting point as low as 718 °C, can also be found if the ash is rich in phosphorus.5 S and Cl do not generally participate in the final agglomeration mechanism.4 Several alternatives for reducing the agglomeration tendency have been described in the literature. The addition of Al2O3 or Fe2O3 to the SiO2-K2O system represses the agglomeration tendency through compounds of high melting point being formed.1,3,4 Another alternative is to use particles composed of a hard, essentially nongrindable material (such as sand) mixed with particles of a porous readily grindable material (such as limestone or dolomite)5,10 which become pulverized during fluidization while at the same time binding the ash to itself. The pulverized particles need to frequently be recycled and refreshed.10 Still another solution is to cofire the biomass with another fuel.1,3,4,9,10 The second fuel should either contain less alkali or be rich in transition metals, so that the overall alkali content is reduced or a eutectic of higher melting point is formed. If no phosphorus is present, injection of SO2 helps to convert K2O to K2SO4, which has a higher melting point (1069 °C).3 Agglomeration tends to be greater when the bed particles are large because larger particles have a higher minimum fluidization velocity and a lower specific surface area. The size of fuel, however, does not affect the agglomeration tendency.3 Although bed agglomeration has been studied for years, the phenomenon is far from being adequately understood. Because agglomeration is the result of complex interactions between many different compounds, it cannot be ascribed simply to the presence and behavior of potassium, silicon, calcium, chlorine, or any other single element6 and probably also not to the ash or to the bed material alone. Its chemistry needs to be studied in greater detail.7 A precise and quantitative account of the bed agglomeration process has yet to be presented.4 The present paper deals with an agglomeration experiment conducted in a fluidized-bed biomass com-
10.1021/ie010274a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/11/2002
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2889 Table 2. Compositions (in %) of Bed Materials and Additives Fyle sand magnesite mullite bone ash calcite Na2O MgO Al2O3 SiO2 P2O3 SO2 K2O CaO TiO2 Fe2O3
Figure 1. Schematic graph of the biomass PFBC facilities. Table 1. Fuel Characterizationa fuel
sawdust
MBM
straw
willow
HHV, MJ/kg LHV, MJ/kg volatile, % fixed carbon, % water, % wt % dry ash C H N O S Cl Na K
19.31 17.90 76.03 14.87 8.36
na na na na 2
16.9 na 73.4 na 6.9
18.3 76 na 10.1
0.76 51.27 6.21 0.25 41.56 0.01 0.01 0.01 0.76
43 na na 1.26 na 0.2 0.12 0.59 0.19
8.2 45 6.1 0.54 na na 0.365 0.025 0.65
3.25 48.6 6 0.6 41.5 0.04 0.005 0.028 0.34
a
na ) not analyzed.
bustor, with the formation of agglomerated particles at elevated pressure being discussed in terms of the boundaries of the particles. Efforts are made, through quantification, to relate the agglomeration that occurs to the parameters frequently considered in this connection. Experiments The experiments were carried out in a pressurized fluidized-bed combustor with a fuel feeding capacity of 90 kWth, shown in Figure 1. It consists of three main parts: the reactor, a filter, and a catalytic reactor. Each of these three parts is placed in a separate cylindrical pressure vessel with an inner diameter of 0.5 m. The combustor can perform at a fuel feeding rate of 10-20 kg/h. A detailed description of the facility and of the operating procedures was provided in an earlier publication.11 Four biomass or biomass-derived fuels and four types of bed materials were tested. The four types of fuels were sawdust, straw, willow, and meat and bone meal (MBM). The compositional analyses are shown in Tables 1 and 2. The temperatures tested were in the range of 670-870 °C. Two pressure levels were selected: 1.0 and 1.5 MPa. Both the flow rate of the carrier gas and the fuel feeding rate were adjusted so that an oxygen concentration of approximately 6 vol % in the product gas could be maintained. The gas velocity in the reactor was 0.25-0.30 m/s, and the particles of the bed materi-
0.03 0.01 0.45 98.2 0.0015 0 0.06 0.04 0.49 0.21
0.0035 84.4 0.34 3.93 0.05 0.16 0.007 7.55 0.013 3.07
0.2 0.03 75.2 24.5 0.0003 0 0.01 0.02 0.01 0.03
3 1.7 0.7 8 28.8 0.6 1.67 44.82 0.03 4.8
clay
0.006 0.07 0.21 0.82 0.05 18.2 0.25 38.45 0.005 0.005 0.012 0.014 0.014 1.28 53.8 0.29 0.003 0.40 0.035 7.56
als were nominally 200 µm in size. Na2CO3, urea, and NH4HCO3 were added to the fuel input as pure chemicals in a ratio of 5 wt % for suppressing NOx emissions. The additives used for capturing the alkali were mullite, calcite, clay, and a mixture of clay and calcite, added in a ratio of 10 wt %, with their compositions being shown in Table 2. An orthogonal experimental design was adopted to be able to investigate the influence of different parameters on agglomeration in an effective way with use of only a limited number of experiments. The fuel, the bed material, the alkali additive, and the NOx additive selected served as experimental parameters,with each parameter having four levels. The extent of the bed agglomeration occurring was evaluated and was related to the parameters selected. The combustion gas was analyzed online by means of mass spectrometry (Balzers QMG 420 MS) and Fourier transform infrared spectrometry (Gasmet FT-IR gas analyzer). The used bed material from the bottom of the reactor and the fly ash from the filter were also sampled and analyzed by means of atomic absorption spectroscopy (GBC 908AA) and scanning electron microscopy (JEOL JSM-840A SEM). Results Observation. Two types of the bed agglomeration were observed: (1) homogeneous agglomeration in which particles of small and uniform size were formed and (2) heterogeneous agglomeration involving the formation of large and irregular particles. In the former case, the particles, less than 2 mm in size, were observed “growing” throughout the active bed, and fluidization was not obviously affected. The latter case, in contrast, was characterized by local melting of the bed particles, with the agglomerated particles sometimes being as large as 60 mm in size. Agglomeration of this latter type was initiated by hot spots in which there was an abrupt increase in the local temperature, with the highest temperatures observed being over 1000 °C. The bed then was sometimes totally blocked by the melts. A typical case of homogeneous agglomeration was observed when the bed material consisted of MgO added together with clay and the fuel consisted of straw. After that experiment, the bed particles had a mean size of 420 µm, as compared with their original size of 180 µm (Figure 2). When the bed was in operation, the size of the bed particles tended normally to decrease because of erosion. Because of the combined effects of erosion and agglomeration, the bed remained in an undisturbed state. Heterogeneous agglomeration resulting in the bed in a pressurized fluidized-bed combustion plant being totally defluidized was observed when a combination of
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Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 Table 4. Parameter Significance on Agglomeration
1 2 3 4 k1 k2 k3 k4 R Figure 2. Comparison of bed particle size affected by erosion and agglomeration. Table 3. Extent of Agglomeration fuel
bed material
alkali additive
NOx additive
agglomeration extent
sawdust sawdust straw MBM willow straw straw
Fyle sand Fyle sand Fyle sand Fyle sand Fyle sand MgO bone ash
mullite none calcite clay calcite, clay clay mullite
Na Na urea urea none Na none
4 3.5 3.5 3.5 2 2 2
straw, Fyle sand, and calcite was employed. Large agglomerates of the bed material up to 50 mm in size were formed, with some of them being glasslike, indicating a history of high temperatures. A white spot was found atop the melted cores of the large agglomerates. The severe agglomeration revealed the importance of the silicon oxide content of the bed material being high in the case of heterogeneous agglomeration. It also indicated that, even without the addition of external alkali, the alkali contained in the straw was sufficient to cause problems of agglomeration. Results for the Orthogonal Experimental Matrix. To quantify the results to make them comparable, the following criteria for evaluating the extents of agglomeration were established: 1 ) very slight and homogeneous agglomeration involving clusters up to 1 mm in size without noticeable changes in the fluidization indicators of a drop in temperature or pressure. 2 ) a slight degree of homogeneous agglomeration, with clusters up to 2 mm in size, slight fluctuations in temperature, and/or a slight drop in pressure, without fluidization being terminated. 3 ) heterogeneous agglomeration involving clusters up to 50 mm in size, no obvious melting on the surface of the clusters, large fluctuations in temperature, and a large pressure drop, with fluidization being terminated sooner or later. 4 ) heterogeneous agglomeration, with clusters of up to 50 mm in size or greater, melting clearly occurring on the cluster surfaces, along with sudden changes in temperature and/or drops in pressure, and the rapid termination of the fluidization. Table 3 lists all of the experiments in which it could be recognized visually that the bed was melted or agglomerated. As can be seen in the table, Fyle sand, which contains more than 98% silicon oxide, was involved in all of the experiments in which heavy agglomeration (g3) occurred. In these experiments, either extra sodium was added or the fuel was rich in potassium and/or sodium. Because the melting point for potassium silicate and sodium silicate is in the range of the operating temperature window, it is reasonable
fuel
bed material
alkali additive
NOx additive
A straw MBM willow sawdust 1.00 0.70 0.50 0.80 0.50
B MgO sand bone ash mullite 0.50 2.63 0.40 0.00 2.63
C clay calcite mullite C1 + C2 1.30 0.17 1.00 0.67 1.13
D Na2CO3 NH4HCO3 none OdC(NH2)2 1.75 0.20 0.67 0.70 1.55
to assume that agglomeration was initiated by the melting of these alkali silicates. The results for evaluation of the experiments are listed in matrix form in Table 4. The impact of each parameter (j) at each level (i) was calculated as the average extent of agglomeration (ki,j; where i ) 1-4 and j ) A-D) for all experiments in which the parameter level of concern was employed. The impact of the Fyle sand, for example, is the average extent of agglomeration for all of the experiments in which Fyle sand was employed as the bed material. The importance of each parameter (fuel, bed material, NOx additive, or alkali additive) for agglomeration was indicated by the maximum difference in the ki,j value for it in column Rj. The relative importance of different parameters can be quantified then by comparing their Rj values. Of the four parameters selected, the most significant one for agglomeration was that of the bed material, followed by the NOx additive and the alkali additive in that order. Of the four bed materials tested, Fyle sand was the most troublesome because of its high content of SiO2, a key compound in forming the low-meltingpoint “glue” that consists of K2O‚nSiO2 and Na2O‚nSiO2. The other bed materials showed a much smaller tendency to agglomerate. No agglomeration took place when mullite was employed as the bed material. The parameter of the NOx additive would presumably not have had as noticeable effect as it did if sodium carbonate had not been employed because the two other NOx additives, urea and ammonia bicarbonate, made essentially no contribution to agglomeration. Already at relatively low temperatures, the addition of sodium promoted the melted phase being formed. This hindered the mobility of the bed particles, which in turn increased the local bed temperature at the top of the bed and led to further compounds being included in the melts. Of the alkali additives, clay is the one most closely related to agglomeration. It has a high content of alkali (1.28% K2O), silicon (38.5% SiO2), aluminum (18.2% Al2O3), and iron (7.56% Fe2O3). Although its own alkali was probably bound in the K-O-Al-Si system, free SiO2 was available for attracting and binding the alkali from the fuel ashes. As can be seen in the table, the lowest level of agglomeration is attained with use of willow as fuel, mullite as the bed material, calcite as the alkali additive, and ammonium bicarbonate as the NOx additive. The worst combination would be that of straw, Fyle sand, clay, and Na2CO3. Bed Additives. Clay, calcite, and mullite were selected as additives to capture the alkali compounds so that they would not be emitted in the gas phase. An additive is useful if it can bind alkali and form a eutectic that does not melt at operating temperature. When mullite was used as the bed material, no agglomeration
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2891
Figure 3. EDX analysis for an agglomerated sample from straw/ sand combustion. The atomic ratio of Si/Ca/K was 5/1.5/1.
Figure 4. EDX analysis for agglomerated sample at the feeding place from straw/sand combustion The atomic ratio of Si/Ca/K was 9.7/2.2/1.
tendency was found, whereas clay had some negative effect, as indicated above. Calcite was involved in the melted agglomerates because white glassy spots were frequently found in them. It has been reported that limestone and straw ash can form a brittle yet sufficiently hard agglomerate to resist attrition of the bed. The eutectic temperature of a mixture of K2CO3 and CaCO3 is about 750 °C.7 SEM Analysis. For SEM/EDX analysis, the samples were either used as they were or were embedded in a peroxy polymer material followed by fine polishing. Both homogeneous and heterogeneous agglomerates were examined here. EDX analysis was performed on selected elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, and Ti. The results given below are based on the assumption that the samples only contained these elements. Figures 3-5 show SEM photographs and the relative distribution of the respective elements on the surface of the sample. The white dots show the existence of the respective elements. In the SEM photographs, it could be seen in most cases that the agglomerates were grains “glued” together by a melted phase, one also found in the homogeneously agglomerated particles. Although it is known that agglomeration is caused by the fusion of bed-fuel ash due to the alkali-silicate mechanism,1,4,6,10 exactly how the agglomeration process is started and
Figure 5. EDX analysis for agglomerated sample from straw/ MgO combustion. MgO added with 10 wt % clay and straw mixed with 0.5 wt % sodium carbonate.
develops is, nevertheless, unclear. X-ray analysis showed that the melted phase in the heterogeneous agglomerates that resulted from the fusion of straw ash and sand had an even distribution of Si, Ca, and K (Figure 3). The atomic ratio of Si/Ca/K was 5/1.5/1. The large amount of Si present made it possible for both K and Ca to combine with Si to form a uniform melt. The thermophysical properties of these compounds suggest potassium silicates to melt first and then stick the surrounding calcium silicates together, with a loose agglomerate being formed if the amount of potassium is clearly less than that of calcium and if the bed temperature is not so high as to melt the calcium silicates. In the present sample, in which the amounts of potassium and of calcium are similar, the agglomerate becomes a uniform melt. Another sample from the same experiment showed a different atomic ratio of Si/Ca/K, one of 9.7/2.2/1. The amount of potassium was small compared with that of calcium and was insufficient to cover the calcium silicate particles. This is evident in Figure 4, in which the potassium can be seen as a separate piece, consisting of Si and K, stacked on top of the melted phase. The melted phase was found to consist primarily of silicon and potassium, even when the bed material was not sand. Figure 5 shows the agglomerate obtained in the experiment in which the bed material was MgO together with 10 wt % clay, with straw mixed with 0.5 wt % sodium carbonate being combusted. In the melted part, potassium silicates were clearly the main component. The silicon came either from the fuel, from the clay that was added, or from the Fyle sand bed material remaining from the previous experiment. In contrast, the calcium in the bed material came from its being attached to the melted phase. A closer analysis was made of the embedded samples. The location and the composition of the molten phase were of interest. Both SEM and EDX analyses were performed. Figure 6 shows SEM photographs of the agglomerated bed material when the fuel consisted of sawdust to which Na2CO3 had been added and the bed material consisted of sand. A molten phase rich in Si, Na, K, and Ca was found at the edges (Figure 6a), in the cracks of sand particles (Figure 6b) and between adjacent particles (Figure 6c). X-ray analysis showed that the bulk (points marked as 1) of the agglomerates were mainly sand consisting of nearly 100% of Si. Si
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Figure 6. SEM photographs for an agglomerated sample from sawdust/sand combustion: (a) melts on the particle edge; (b) melts inside particle cracks; (c) melts in gaps between particles.
Figure 8. SEM photographs for an agglomerated sample from straw/MgO combustion: (a) bed particles surrounded by melts; (b) bed particles glued together by melted Na and K compounds.
Figure 7. Suggested mechanism of agglomerate formation. Table 5. Elemental Compositions (%) at Different Locations location
Na Mg
Si
S
Cl
K
Ca
Figure 9. Effect of temperature and pressure on the agglomeration extent. Ti
Fe
edge on particle 7.5 3 83.5 0.56 1.5 4 between particles 14 3.1 72 0.3 3.3 5.7 2.06 inside cracks 9.5 2.1 68 0.5 0.5 3.4 8.5 2.7 4.8
was also dominant in the molten phase, ranging there from 65 to 85 wt %. Other elements present were Na in the range of 7.5-14 wt %, Ca of 4-8 wt %, and K of 1.5-3.5 wt %. In terms of the composition of the fuel, the bed material, and the additive, it can be assumed that the Na in the molten phase came from the additive and that the K and Ca came from the fuel ash. Because the Si content of the molten phase was more than the amount needed for formation of the eutectics Na2O‚ nSiO2 and K2O‚nSiO2 (n ) 1-4), it is reasonable to assume that very fine sand particles are involved in the molten phase. In the gas phase, Na2O and K2O first attack the surface of the very fine ash particles (that contain SiO2 and CaO), covering these particles with a molten phase. Such tiny ash particles surrounded by a molten layer are very sticky and can thus easily bind together with other similar tiny particles and/or stick to larger particles. This mechanism, which supposedly functions at relatively low temperatures and elevated pressures, can be discerned in Figure 7. The compositions (in wt %) of the molten phase in the three positions there are shown in Table 5. In the molten phase at the outer edge of the agglomerate, smaller amounts of Na, K, and Ca and greater amounts of Si were observed. Figure 8 shows the SEM photographs for the same sample as that in Figure 5. For the polished surfaces, the composition of the agglomerate could be analyzed locally. In Figure 8a a small particle (marked as 1) together with surrounding material (marked as 2) can be seen. Between the two is a transition area marked as 3. Analysis showed the small particle to consist mainly of Mg, Fe, and Si. The ratio of Mg to Fe matched very well the composition of the raw MgO bed material. The surrounding material was found to be rich in Si, K, Al, and Fe. The composition indicated aluminum potassium silicates to be present, a composition typical for the molten phase. The alkali additive (clay) con-
tained 18 wt % Si, 9.6 wt % Al, and 1 wt % K. This corresponds to the amounts of Si and Al found in the molten phase. The K, on the other hand, came mostly from the fuel, where it accounted for 0.6 wt %. The transition area was rich in Mg, Si, K, and Fe, indicating there to be a possible interaction between the molten phase and the bed material. Four small particles (marked as 1-4) were found in the sample shown in Figure 8b. X-ray analysis indicated particles 1 and 2 to be MgO bed particles. In particle 3, only Cl (97 wt %) and Si (3 wt %) could be identified. As was already pointed out, it can be difficult to measure Na by means of SEM though the excitation energy of Na is at the lower end of the measuring range. Such a particle should consist of NaCl because Na2CO3 was added to the fuel as a NOx additive. Particle 4 had about 47 wt % Cl, 28 wt % Si, and 20 wt % K and smaller amounts of the other elements. This composition corresponded to a mixture of NaCl (mp 801 °C) and K2O‚nSiO2 at high temperature. At points 5 and 6, found in the space between the particles, the material consisted, as expected, of 43 wt % Si, 41 wt % K, and 7.3 wt % Mg, with the remainder being Fe and Cl. At these points, the composition clearly corresponded to that of potassium silicates, which typically occurred in the molten phase of the agglomerates. Discussion Effect of the Temperature. High temperature has been found to be the most important cause of agglomeration being initiated.2,3,5,7 Generally, the higher the bed temperature, the more severe the extent of agglomeration becomes. This general trend was likewise found in the present study. Although the composition of the fuel ash and bed material determined the chemical characteristics of the agglomerate, the bed temperature has a more direct effect, as shown in Figure 9. Effect of the Pressure. Pressure was one of the parameters investigated. The effect of pressure and that of temperature were found in the present study, how-
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2893 Table 6. Changes of Gas Composition with Temperature and Pressure (%) (Base Case: 1100 K, 10 bar) substance
+ 5 bar
+5K
+ 10 K
+ 20 K
CO2 H2O N2 Σ moles
-0.6066 0.0735 0.0020 0.1286
-0.4765 0.4928 0.0011 -0.0169
-1.1457 1.1531 0.0108 -0.0102
-2.416 2.0662 0.0063 -0.0427
ever, to be related. To retain a given gas velocity in the bed, the air input needs to be increased at elevated pressure. Because the extra inert nitrogen increases the cooling effect of the gas on the fluidized bed, the test was performed in such a way that an increase in the operating pressure in the reactor also resulted in a reduction in the bed temperature. Theoretical calculations of the equilibrium composition of the product gas at 10-15 bar and 1100 K showed the increase in pressure to have the same effect on agglomeration as an increase in temperature of less than 10 K; see Table 6. This shows an increase in pressure to have only a marginal effect on agglomeration of the bed. Hot Spot. An abrupt increase in temperature, a hot spot, was found to be an indicator of heterogeneous agglomeration having started. What causes the formation of a hot spot is not clear generally, however, although in the present case it appears to be caused by small fluctuations in the feeding rate. Small fluctuations in the fuel entering the combustor that can create disturbances in the fluidization around the feeding point can be assumed to lead to hot spots being formed. Unstable fuel feeding, temporary channeling of the carrier gas through the bed, or other momentary disturbances can also be thought to produce hot spots. In practical terms, the occurrence of hot spots both in pilot studies and in industrial-scale plants appears unavoidable. The hot spot temperature can easily reach the melting point of various alkali silicates. Because the molten silicates are sticky, the grains of the bed material become covered by a thin layer of the molten phase to form particle clumps. These clumps limit the mobility of other unbound particles and result in a local rise temperature, which in turn melts the compounds of higher melting point. When the hot spot temperature decreases, the silicates solidify and the bed particles glue together to form agglomerates. The presence of agglomerates in the bed leads to poorer fluidization there, thus resulting in differences in temperature in different parts of the bed. The higher temperatures found at the top of the bed enhance the agglomeration process further. Agglomeration of this type can thus be regarded as an autoaccelerated process. Two Types of Agglomeration. Homogeneous agglomeration took place when the bed material itself was inert to the alkali-silicon reaction. Here, both the alkali and the silicon came either from the fuel ash or from the additive. Because of their amounts being relatively low as compared with the bed material, the sticky phase formed by their reaction was only able to cover the bed particles with a very thin layer. After a long run, the fluidized bed became agglomerated. If the gas velocity succeeded, nevertheless, in keeping the agglomerated particles fluidized, fluidization was not seriously affected. Because of the limited input of silicon oxide along with the fuel feeding, large clusters of alkali silicates of low melting point are unlikely to be formed then, with the agglomerated particles of the bed material thus not necessarily leading to defluidization and to the consequent shutdown. In contrast, heterogeneous agglomera-
tion that led to defluidization of the bed was found to mainly be due to the formation of large irregular agglomerated particles. These were possibly formed in areas in which the fluidization was relatively inactive but in which the temperature was high. If the temperature is sufficiently high, these large agglomerates can melt and a hard block of the bed material can be formed. In agglomeration of this type, the bed material was found to often be in the melting phase. Effect of Different Elements. Agglomeration is reported to mainly be induced by the formation of alkali silicates (mostly K2O‚nSiO2) that have melting points similar to the operating temperature of the FBC (mp K2O‚SiO2 ) 976 °C, mp K2O‚2SiO2 ) 1015 °C,12 mp K2O‚3SiO2 ) 740 °C,7 mp K2O‚4SiO2 ) 764 °C1). In the combustion of biomass, the alkali content of the fuel ash is mainly potassium. When Na2CO3 is used as an NOx additive, there is a risk of sodium silicates being formed that join in the melting phase because they also have low melting points, around 800 °C (mp Na2O‚SiO2 ) 1088 °C, mp Na2O‚2 SiO2 ) 874 °C,12 mp 3Na2O‚8SiO2 ) 793 °C13). The occurrence of CaO in the agglomerated bed was likewise found in the present study. The function of CaO in the agglomeration mechanism can be deduced from the phase diagrams. Pure CaO does not melt at temperatures lower than 2500 °C, and CaO‚ SiO2 transfers to the liquid phase at 1600 °C. Both temperature ranges are far from those under FBC conditions. When CaO is added to the Na2O-SiO2 system, a eutectic of Na2O-CaO‚5SiO2 and 3Na2O‚ 8SiO2 can, nevertheless, be formed, one with a melting point of as low as 755 °C.13 Note that the CaO content in this eutectic system is only 5 wt %. Thus, the addition of CaO to the bed can be dangerous. Published data indicate that transition metals (such as Fe) readily react with alkali metals as follows, where X can be either Na or K:
Fe2O3 + X2O f X2Fe2O4 Fe2O3 + X2CO3 f X2Fe2O4 + CO2 The products formed have a minimum melting point of 1135 °C. Such reactions are very desirable because they result in a lesser availability of alkaline to react with SiO2. Selection of the Bed Material. Alkali-induced defluidization/agglomeration is an interaction between fuel ash and bed material. SiO2-based bed material reacts more readily with alkali compounds than the other two bed materials, magnesite and mullite. The alkali source can be inherent in the fuel, such as in willow and straw, or in a NOx additive, such as Na2CO3. The agglomeration tendency of the bed material is dependent upon its silicon and aluminum contents. Silicon sand contains more than 98% SiO2, compared with 4% magnesite and 24.5% mullite. Mullite is prevented from agglomerating by its 75% Al2O3. The alkali aluminum silicates generally have a higher melting point than the alkali silicates. By the selection of magnesite and mullite, the SiO2 content in the bed material can be kept low. However, if the fuel ash is rich in both alkali and SiO2 (just as straw is), agglomeration of the bed material is unavoidable. Effect of NOx Additives on Agglomeration. Of the NOx additives that were tested, urea and ammonia bicarbonates were found to decompose particularly rapidly in the bed and to merge into the gas phase. The
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decomposition products, NH3, HNCO, N2, and CO2, are unlikely to contribute to bed agglomeration. The Na2CO3 compound added to the fuel may possibly cause agglomeration or be involved in it. As discussed above, two requirements need to be fulfilled in order for agglomeration to be induced: (1) a sufficiently high local temperature and (2) use of a compound that melts at that temperature. Na2CO3 can affect the agglomeration process in different ways: (1) At a sufficiently high temperature, of around 1000 °C, Na2CO3 decomposes to Na2O and CO2. Na2O is known to have a tendency to agglomerate with silicon oxides. (2) In a combustion environment in which water vapor accounts for 10% of the flue gas, Na2CO3 can react with the water, at above 400 °C, to form NaOH. NaOH melts at 310 °C and at a higher temperature can release Na2O3. Na2CO3 reacts with chlorine from fuel to form NaCl, which has a melting point of 801 °C. The existence of NaCl in the presence of sand particles has been shown to be problematical.14 It is scarcely surprising, therefore, that the addition of Na2CO3 to the bed can lead to problems of agglomeration. Conclusions On the basis of the experimental and analytical results and the consideration discussed above, the following conclusions concerning biomass combustion in a fluidized bed at elevated pressure can be drawn: 1. Both a homogeneous form of bed agglomeration in which the particles are nearly uniform, being less than 2 mm in size, and a heterogeneous form in which particles are irregular, being up to 60 mm in size, are found. Homogeneous agglomeration occurs when the bed particles interact only slightly, if at all, with the fuel ash, thus requiring that the bed temperature not be too high. Heterogeneous agglomeration, in which the bed particles are in a sticky melt phase, often leads to total defluidization. 2. Heterogeneous agglomeration is mainly initiated by hot spots that result from factors of somewhat uncertain character. Unstable fuel feeding or temporary channeling of the carrier through the bed can lead to the creation of hot spots. Agglomeration of this type is a self-accelerated process, although erosion reduces the size of the agglomerates to a certain extent. 3. Potassium, sodium, and silicon are the most troublesome elements in the fuel ash-bed material interaction, but the presence of calcium in the bed of fuel ash also enhances the agglomeration tendency. Magnesium, iron, and aluminum are refractory elements that can repress agglomeration at temperatures no higher than 1000 °C. 4. Of the fuel types tested, sawdust was best with respect to bed agglomeration. Straw always showed agglomeration if silicon sand, which had a significant effect on the agglomeration tendency, was used as the bed material. Mullite was the most desirable bed material of those that were tested.
Acknowledgment The work presented received financial support from the project “Improved Energy Generation Based on Biomass FBC with Minimum Emission (JOR3-CT980200)” sponsored by the European Commission, to which the authors are grateful. Thanks are also given to The Netherlands Energy Research Foundation (ECN), which conducted part of the elemental analysis of the raw materials. Literature Cited (1) Grubor, B. D.; Oka, S. N.; Ilic, M. S.; Dakic, D. V.; Arsic, B. T. Biomass FBC combustionsbed agglomeration problems. Proc. 13th Int. Conf. Fluid Bed Combust. 1995, 1, 515-522. (2) Lind, T. Ash formation in circulating fluidised bed combustion of coal and solid biomass. VTT Publ. 1999, 378, 1-166. (3) Lin, W.; Dam-Johansen, K. Agglomeration in fluidized bed combustion of biomasssMechanisms and co-firing with coal. Proc. 15th Int. Conf. Fluid Bed Combust. 1999, 1188-1191. (4) Ohman, M.; Nordin, A. In Impact of Mineral Impurities on Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 259270. (5) Ghaly, A. E.; Ergudenler, A.; Laufer, E. Agglomeration characteristics of alumina sand-straw ash mixtures at elevated temperatures. Biomass Bioenergy 1993, 5, 467-80. (6) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences. Fuel Process. Technol. 1998, 54, 47-78. (7) Moilanen, A.; Kurkela, E.; Laatikainen-Luntama, J. In Impact of Mineral Impurities on Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 555-567. (8) Manzoori, A. R.; Agarwal, P. K. The role of inorganic matter in coal. Fuel 1993, 72, 1069-1075. (9) Heikkinen, R. E. A.; Virtanen, M. E.; Patrikainen, H. T.; Laitinen, R. S. In Impact of Mineral Impurities on Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 333-339. (10) Latva-Somppi, J.; Kauppinen, E. I.; Valmari, T.; Ahonen, P.; Burav, A. S.; Kodas, T. T.; Johanson, B. The ash formation during co-combustion of wood and sludge in industrial fluidized bed boilers. Fuel Process. Technol. 1998, 54, 79-94. (11) Padban, N.; Wang, W.; Ye, Z.; Bjerle, I.; Ordenbrand, I. Tar formation in pressurised fluidized bed air gasification of woody biomass. Energy Fuels 2000, 14, 603-611. (12) Weast, R. C. Handbook of Chemistry and Physics, 56th ed.; CRC Press: Cleveland, OH, 1975. (13) Roth, R. S.; Negas, T.; Cook, L. P. Phase diagram for ceramists; The American Ceramic Society, Inc.: Columbus, OH, 1981; Vol. IV. (14) Padban, N.; Ye, Z.; Bjerle, I. In Developments in thermochemical biomass conversion; Bridgewater, A. V., Boocock, D. G. B., Eds.; Blackie Academic and Professional: London, 1997; pp 1301-1041.
Resubmitted for review November 12, 2001 Revised manuscript received March 18, 2002 Accepted March 21, 2002 IE010274A
