Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia

Energy & Fuels 2000, 14, 781-790

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Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia) Fabrizio Scala and Piero Salatino* Dipartimento di Ingegneria Chimica, Universita` degli Studi di Napoli “Federico II”, P. le Tecchio, 80-80125 Napoli, Italy

Riccardo Chirone Istituto di Ricerche sulla Combustione, CNR, P. le Tecchio, 80-80125 Napoli, Italy Received August 4, 1999

Fluidized bed combustion of char from a biomass, Robinia pseudoacacia, was investigated in a bench scale combustor. Different experimental techniques have been adopted to characterize the combined role of combustion and comminution phenomena (primary, secondary, and percolative fragmentations, attrition by abrasion) in determining fixed carbon conversion and the rate of carbon elutriation. Comparison of experimental results obtained under steadily oxidizing conditions and under alternating oxidizing/inert conditions suggested mechanistic aspects of the fluidized bed combustion of biomass char. Fixed carbon combustion was almost always complete. Conversion occurred to a large extent via the generation of carbon fines followed by postcombustion during their residence time in the bed. Approximately half of the initial fixed carbon followed this pathway, the remainder being directly burnt as coarse char. The prevailing mechanism of carbon fines generation in the bed was percolative fragmentation rather than attrition by abrasion. In spite of the extensive generation of elutriable carbon fines, the combined effect of high fuel reactivity and of relatively long fines residence times in the reactor determined the large combustion efficiency. It is inferred from experimental results that char fines adhesion onto bed solids might be relevant to the observed phenomenology.

Introduction The growing interest for the exploitation of biomass fuels is mostly related to the consideration that they provide a renewable energy source, neutral with respect to greenhouse-compounds generation. The application of fluidized bed combustion (FBC) technologies stems out as a natural choice for biomass combustion, beyond the demonstration stage to the point of being currently looked at as a viable technological option. Nonetheless, a number of issues, mostly related to the fate of the ash components, are still open to further investigation. The goal is that of overcoming or at least of controlling operational problems (corrosion, bed agglomeration) related to ash behavior and of achieving broader operational ranges over which FBC can be effectively applied, especially with regard to combustion temperature. The use of biomass, as well as of other high-volatile fuels, has stimulated the progressive reconsideration of the role of fundamental processes in fluidized bed combustion of solid fuels. On the one hand, the large contribution to the overall heat release coming from homogeneous combustion of volatile matter emphasizes the importance of volatile/fluidizing gas mixing/segregation processes.1 On the other hand, the loosely connected or even incoherent structures of chars left behind by * Corresponding author. Phone: +39 081 7682258. Fax: +39 081 5936936. E-mail: [email protected].

devolatilization and their large intrinsic combustion reactivities dramatically change the fate of fixed carbon and associated ash from high-volatile fuels with respect to low-volatile ones. It has long been recognized the importance of comminution phenomena in the fluidized bed combustion of solid fuels. These include primary fragmentation of fuel particles upon devolatilization, char particles attrition by abrasion, and secondary and percolative fragmentations. Chirone et al.2 provided a comprehensive review on comminution of carbon particles during fluidized bed combustion, focused on coals and other low-volatile solid fuels. Arena et al.3 showed that the generation of carbon fines by attrition of coal particles is relevant to the combustion efficiency and to the environmental impact of the operation, but it provides only a minor route to overall carbon conversion. Most experimental studies on high-volatile fuel combustion disregarded particle attrition.4-6 Gulyurtlu and Cabrita7 (1) Chirone, R.; Marzocchella, A.; Salatino, P.; Scala, F. Proceedings of the 15th International Conference on Fluidized Bed Combustion; ASME: New York, 1999; paper FBC99-0021. (2) Chirone, R.; Massimilla, L.; Salatino, P. Prog. Energy Combust. Sci. 1991, 17, 297-326. (3) Arena, U.; Cammarota, A.; Chirone, R. Fluidization VIII; Engineering Foundation: New York, 1995; pp 437-444. (4) Masson, H. A. Proceedings of the 8th International Conference on Fluidized Bed Combustion; ASME: New York, 1985; pp 1058-1067. (5) La Nauze, R. D. J. Inst. Energy 1987, 60, 66-76 (6) Lin, J. L.; Keener, H. M.; Essenhigh, R. H. Combust. Flame 1995, 100, 271-282.

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and Gulyurtlu et al.8 incidentally noted that carbon fines are formed in a larger quantity when alternative solid fuels are fired instead of coal. Arena et al.9-11 and Salatino et al.12 examined the combustion-comminution behavior of three waste-derived fuels, namely, ebonite, a tire-derived and a refuse-derived fuel. Experimental data showed that the relevance of comminution phenomena is emphasized in the case of high-volatile fuels due to the propensity of such fuels to give rise upon devolatilization either to highly porous, friable chars or to a multitude of fragments of very small size. The authors pointed out that combustion efficiency is strongly dependent on how the combustion time scale compares with the residence time of char fines in the bed, which in turn is related to elutriation phenomena. Experimental results reported by Leckner et al.13 and Andersson et al.14 indicate that large combustion efficiencies can be achieved with alternative fuels provided that particles experience a sufficiently long residence time in the bed. This result is not surprising if one considers that alternative fuels usually have a much higher intrinsic reactivity than fossil ones.15 A framework for the assessment of the fate of fixed carbon in a fluidized bed combustor was provided by Arena et al.11 The proposed series-parallel network of attrition-combustion-elutriation processes was based on the assumption that fixed carbon in the bed could be divided into a coarse phase and a fine phase made respectively of nonelutriable and of elutriable particles. Mechanistic aspects of fixed carbon conversion during the fluidized bed combustion of a biomass fuel are addressed in the present paper. The analysis integrates several procedures and experimental techniques developed in order to quantify the relative contributions of the different pathways to fixed carbon conversion. The fuel under consideration is Robinia pseudoacacia (R. pseudoacacia), a fast-growing ligneous biomass common in the Mediterranean area and suitable for short rotation crops, with potential interest for energy production. Experimental activity has been carried out in a laboratory-scale fluidized bed facility. The key mechanisms of carbon attrition during the combustion of the biomass fuel as well as the importance of the fines generationpostcombustion vs direct coarse char combustion routes in the overall carbon conversion have been assessed. (7) Gulyurtlu, I.; Cabrita, I. Proceedings of the 3rd International Fluidization Conference; The Institute of Energy: London, 1984, DISC/ 11/80. (8) Gulyurtlu, I.; Reforco, A.; Cabrita, I. Proceedings of the 11th International Conference on Fluidized Bed Combustion; ASME: New York, 1991; pp 1421-1424. (9) Arena, U.; Cammarota, A.; Chirone, R.; D’Anna, G. Proceedings of the 13th International Conference on Fluidized Bed Combustion; ASME: New York, 1995; pp 943-949. (10) Arena, U.; Chirone, R.; D′Amore, M.; Miccio, M.; Salatino, P. Powder Technol. 1995, 82, 301-316. (11) Arena, U.; Chirone, R.; Salatino, P. Proceedings of the of 26nd Symposium (International) on Combustion; The Combustion Institute: 1996; pp 3243-3251. (12) Salatino, P.; Scala, F.; Chirone, R.; Pollesel, P. Proceedings of the 4th International Conference on Technology Combustion for Clean Environment; Lisbon, Portugal, 1997; pp 23-27. (13) Leckner, B.; Andersson, B. A.; Vijil, J. Proceedings of the Conference on Combustion of Tomorrow’s Fuels-II; Engineering Foundation: New York, 1984; pp 401-410. (14) Andersson, B. A.; Leckner, B.; Amand, L. E. Proceedings of the 8th International Conference on Fluidized Bed Combustion; ASME: New York, 1985; pp 1019-1029. (15) Masi, S.; Salatino, P.; Senneca, O. Proceedings of the 14th International Conference on Fluidized Bed Combustion; ASME: New York, 1997; pp 135-143.

Scala et al.

Experimental Section Experimental Apparatus. The experiments were carried out in a stainless steel (AISI 312) atmospheric bubbling fluidized bed combustor with 40 mm i.d. and 1 m height. The fluidization gas distributor is a 2 mm thick perforated plate with 55 holes 0.5 mm in diameter disposed in a triangular pitch. A 0.6 m high stainless steel column, filled with steel scrap for gas preheating and mixing, is placed under the distributor. The fluidization column and the preheating section are heated by two semicylindrical 2.2 kW electric furnaces. The temperature of the bed, measured by means of a chromelalumel thermocouple placed 40 mm above the distributor, is kept constant by a PID controller. The freeboard is kept unlagged in order to minimize fines postcombustion in this section. Gases are fed to the column via two high-precision digital mass flowmeters (Brooks). The same reactor is used in two different configurations for the experimental tests. In the first configuration (Figure 1A), used for particle fragmentation experiments, the top section of the fluidization column is left open to the atmosphere. A stainless steel circular basket can be inserted from the top in order to retrieve fragmented and unfragmented particles from the bed. The tolerance between the column walls and the basket is limited to reduce as much as possible the amount of carbon left in the bed when pulling out the basket. The basket mesh is of 0.8 mm, so that the bed material can pass through the net openings. In the second configuration (Figure 1B), used for fines elutriation rate experiments, the top flange of the fluidization column is fitted to a two-exit brass head equipped with a threeway valve. By operating this valve, it is possible to convey flue gases alternately to two removable filters made of sintered brass. Batches of material can be fed to the bed via a hopper connected sideways to the upper part of the freeboard. A paramagnetic analyzer and two NDIR analyzers (Hartmann & Braun) are used for on-line measurement of O2, CO, and CO2 concentrations, respectively, in the exhaust gases. Materials. The bed material consisted of 180 g of silica sand, corresponding to an unexpanded bed height of 0.1 m. Sand was double sieved in the nominal size range of 300-400 µm with a Sauter mean diameter of 0.36 mm. Minimum fluidizing velocity was 0.05 m/s at 850 °C. Experiments have been carried out with Robinia pseudoacacia, a high-volatile biomass fuel whose properties are reported in Table 1. Robinia branches were cut into particles of approximately cylindrical shape with both diameter and length of 10 mm. Each particle was surrounded by the bark. Fluidization gas consisted of compressed air from the laboratory distribution line, technical grade nitrogen from cylinders, or a mixture of the two. Inlet oxygen concentration in the combustor was varied between 0 and 3% on a volume basis. The total gas flow rate was 900 Nlt/h, corresponding to a fluidization superficial velocity of 0.8 m/s at 850 °C. Procedures. Two kinds of experimental tests were performed in the fluidized bed combustor, namely, particle fragmentation experiments (primary and secondary) and fines elutriation rate experiments. Whatever the type of test, the reactor was charged with a bed of sand (180 g) and heated either to 700 or 850 °C prior to each experiment. Particle Fragmentation Experiments. Experiments were performed using the basket equipped configuration (Figure 1A). For primary fragmentation experiments the bed of sand (300-400 µm) was fluidized with nitrogen at 0.8 m/s. During the run the basket rested on the distributor. Following the procedure proposed by Chirone et al.2 experiments were carried out by injecting single fuel particles (approximately 0.4 g) into the bed kept at 850 °C from the top of the column. After about 3 min, required to completely devolatilize the fuel particle, the resulting char was retrieved by means of the basket in order to investigate the number and size of the

Fluidized Bed Combustion of Robinia pseudoacacia

Energy & Fuels, Vol. 14, No. 4, 2000 783

Figure 1. Experimental apparatus: (A) basket equipped configuration; (B) two-exit head configuration. Table 1. Properties of Robinia pseudoacacia particle density, g/cm3 char density, g/cm3 LHV, kJ/kg proximate anal (dry basis), % (w/w) volatiles fixed carbon ash ultimate anal (dry basis), % (w/w) carbon hydrogen nitrogen sulfur ash oxygen (diff) ash composn, % (w/w) CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 SO4

0.38 0.10 15 600 79.2 19.3 1.5 43.9 7.8 0.02 1.5 46.78 76.2 11.8 9.2 0.78 0.25 0.69 0.07 dp > 75 Å has been measured by means of a highpressure mercury porosimeter Carlo Erba 2000 equipped with a Macropore unit. Cross-sections of char particles were observed under a scanning electron microscope (Philips XL30 with LaB6 filament) at magnifications up to 333 times.

Experimental Results The conceptual framework of the present analysis is reported in Figure 2A. The graph is that proposed by Arena et al.11 to represent the fixed carbon mass balance in the bed. It is assumed that fixed carbon is present in the bed as two phases: a coarse phase (C), made of relatively large nonelutriable char particles; a fine phase

Figure 3. SEM micrographs of Robinia char particles at two different magnifications (reproduced at 50% of the original): (A) 333×; (B) 64×.

(F), made of char fines of elutriable size. Comminution phenomena and parallel combustion determine the relative importance of the different pathways of carbon consumption. The fate of fixed carbon is followed from feeding (F0) to its final destiny, through primary fragmentation (F0C,F0F), coarse char combustion (FCP) and attrition (FCF), and fine char postcombustion (FFP) and elutriation (Ec). Primary Fragmentation. Experimental results showed that Robinia particles exhibit negligible primary fragmentation upon devolatilization in spite the relatively large volatile matter content (about 75% by weight) of the fuel. The particle multiplication factor n1 at the end of devolatilization resulted in a value of 1. According to the scheme reported in Figure 2A, F0F ) 0 and F0C ) F0 in this case. Analysis of the char particles obtained after devolatilization showed that even if they do not undergo fragmentation, they exhibit local structural collapses, possibly related to internal overpressures, and assume a barrellike shape. Parts A and B of Figure 3 report SEM micrographs at two different magnifications of coarse Robinia char particles. The micrographs show that Robinia char exhibits a highly anisotropic pore structure characterized by parallel channels all running in the axial direction. The cumulative pore size distribution on a volume basis, reported in Figure 4, indicates that the high overall char porosity (0.91) consists predominantly of macropores (>1 µm). It is likely that the high level of anisotropy of the pore structure as well as of the mechanical resistance of the



Figure 4. Cumulative pore size distribution of Robinia char particles.

Figure 5. Carbon conversion degree during char combustion as a function of time.

material results in preferential volatile matter release along the axial direction. On the other hand the mechanical stresses associated with devolatilization mainly apply in the orthogonal direction and are relaxed via local particle failure. However, the integrity of the particle is preserved upon devolatilization because of the presence of the bark. The role of the bark to increase the mechanical resistance of the particle has been confirmed by feeding the fluidized bed with a limited number of Robinia particles from which the bark had been mechanically removed. These particles underwent significant primary fragmentation with a particle multiplication factor of n1 ) 5. Char Conversion Rate. Figure 5 reports the degree of carbon conversion by combustion as a function of time continuously monitored during batch combustion experiments at two temperatures (700 and 850 °C) and oxygen concentrations (1 and 3% by volume). The degree of carbon conversion ξ was calculated as

ξ(t) )

∫0t(CCO + CCO )Q dr′

12

2

WC,0

(1)

by working out CO and CO2 concentrations at the combustor exhaust. Q and WC,O are the fluidizing gas flow rate and the initial carbon loading in the reactor, respectively. The following features can be noted:

Figure 6. Secondary fragmentation particle multiplication factor as a function of the apparent carbon conversion degree under inert and oxidizing conditions.

(a) ξ vs time profiles at different combustion temperatures and for given oxygen concentration are very close to each other. The limited dependence on temperature suggests that the carbon combustion rate is largely controlled by oxygen diffusion in the particle boundary layer rather than by intrinsic combustion kinetics. (b) The ultimate carbon conversion degree is always smaller than unity. The deviation is larger, the longer the burn-out time. This finding, together with results of carbon attrition experiments, suggests that a small but detectable amount of solid carbon leaves the reactor unburnt, contributing to the overall loss of carbon combustion efficiency. Secondary Fragmentation. Secondary fragmentation experiments have been carried out by monitoring the number of particles produced by a single feed char particle under different oxidizing conditions. The same analysis has been carried out also under inert conditions to shed light on the relevance of mechanical stresses on particle fragmentation. In order to compare tests with so different time scales, the particle multiplication factor (n2) for secondary fragmentation experiments is reported (Figure 6) as a function of the apparent degree of carbon conversion (ξa) defined as

ξa(t) )

∫0tEc dt′ + 12∫0t(CCO + CCO )Q dt′ 2

WC,0

(2)

where Ec is the carbon elutriation rate. The apparent degree of carbon conversion accounts for combustion as well as for attrition. Plotting n2 as a function of ξa in Figure 6 allows the direct comparison of experiments conducted in inert and oxidizing conditions and in turn of the relative importance of mechanical and combustion-assisted fragmentations. Robinia particles produced a relatively large number of fragments, up to 8, under inert conditions, i.e. in the absence of the weakening effect of combustion on the particle structure. This result might be a consequence of the local structural failures observed after particle devolatilization. Analysis of data obtained under oxidizing conditions indicate a decrease of n2 with oxygen concentration. This behavior is different from that observed with coals and other low-volatile fuels2 but similar to that found with waste-derived fuels.9 The

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Figure 7. Normalized carbon elutriation rate as a function of time for batchwise experiments under steady inert and oxidizing conditions. T ) 700 °C.

Figure 8. Normalized carbon elutriation rate as a function of time for batchwise experiments under steady inert and oxidizing conditions. T ) 850 °C.

explanation of this result is not straightforward and must take into account several competitive factors. On the one hand, combustion should promote the production of fragments by weakening of the char structure, but at the same time fragments may be quickly burned to a small size and could be lost through basket openings or as elutriated material. On the other hand, longer exposure of particles to mechanical stresses occurs as the oxygen concentration decreases from 3 to 0%, and this should result in a larger extent of char fragmentation. Separate evaluation of coarse char particles temperature during combustion indicated that overheating with respect to bed temperature was negligible (less than 10 °C under the operating conditions tested). As a consequence, possible effects of sintering on particle secondary fragmentation should be ruled out. The presence of a more resistant external layer of the particle (the bark), that has to be consumed by combustion and/or attrition, may explain the fact that n2 departs from 1 only after a time lag. The removal of this external layer discloses the inner core, less resistant under the action of mechanical stresses. Fines Elutriation Rate during Steadily Oxidizing Experiments. Carbon elutriation rates from the bed as a function of time are reported in Figures 7 and 8 for batchwise experiments carried out at 700 and 850 °C, respectively. Purely mechanical attrition and com-

Scala et al.

Figure 9. Normalized carbon elutriation rate as a function of time for a batchwise experiment under alternating inert and oxidizing conditions. T ) 700 °C.

bustion assisted attrition have been investigated by operating the fluidized bed both in inert and in oxidizing conditions (1 and 3% inlet oxygen concentration). Carbon elutriation rates have been normalized on the basis of the mass of carbon initially fed to the bed. The carbon elutriation rate under inert conditions (0%) at both temperatures is characterized by a first large elutriation rate period caused by particle rounding off followed by a gradual reduction with time. Differently from coals, curves do not level off but continue to decrease with time even after long time periods. Fines generation occurred at a rate more than 1 order of magnitude larger and over a time scale much shorter than for a medium rank coal char. The integration of Ec vs time curves over the first 40 min indicates that about 20% of the fixed carbon fed into the combustor escapes the bed as elutriated fines. Over the same time interval, the total elutriated carbon would only be of the order of a few percent of the initial carbon charge in the case of a medium rank coal.2 The carbon elutriation rate under purely mechanical attrition is much larger than under combustion-assisted attrition conditions and decreases as the oxygen concentration increases. Curves obtained under oxidizing conditions do not present the nonmonotonical shape with a maximum typical of low-volatile solid fuels.2 Very large fixed carbon combustion efficiencies were obtained with Robinia char in oxidizing conditions, even at the lower temperature of 700 °C. In particular combustion efficiencies of 0.941 and 0.980 (at 700 °C) and of 0.974 and 0.988 (at 850 °C) were found when oxygen inlet concentrations were 1 and 3%, respectively. Fines Elutriation Rate in Experiments with Alternating Oxidizing and Inert Conditions. Elutriated carbon fines have been collected during some experimental runs where char particles experienced alternating inert-oxidizing conditions. Figures 9 and 10 report normalized fixed carbon elutriation rates as a function of time for experiments carried out respectively at 700 and 850 °C. Each filter for fines collection was in use for a fixed period of 3 min in which the bed was fluidized alternately with nitrogen or with a nitrogen-oxygen mixture having an oxygen content of 1%. Reference curves relative to experiments with steadily inert or oxidizing conditions (1% oxygen concentration, Figures 7 and 8) have been also reported in



it was generally observed2 that the carbon elutriation rate under oxidizing conditions is larger than that measured under inert conditions. This was interpreted in light of the combustion-assisted attrition concept: values of β as large as 30 are reported in the fluidized bed combustion of low-volatile fuels,2 whereas the degree of fines postcombustion ξF is significantly smaller than 1. Figures 7 and 8, on the contrary, suggest that

Ec , Ec,i ) FCF,i

Figure 10. Normalized carbon elutriation rate as a function of time for a batchwise experiment under alternating inert and oxidizing conditions. T ) 850 °C.

the figures. Experimental data fall between the reference curves and show an alternating behavior, the carbon elutriation rate under inert conditions (0%) being closer to (but not coincident with) the upper reference curve, the others (1%) being closer to (but not coincident with) the lower reference curve. At the higher temperature the difference between the elutriation rate values obtained in inert and oxidizing conditions is larger throughout the run. The finding that elutriation rates under oxidizing and inert conditions do not fall on curves relative to steadily oxidizing (Ec = 0) and steadily inert (Ec ) FCF,i) conditions, respectively, indicates that the system is not able to instantaneously switch from one to the other of the two limiting curves as oxidizing conditions change stepwise. Discussion Analysis of results in Figures 7 and 8 in the light of the combustion-comminution network of Figure 2A is now in order. Under inert atmosphere, both FCP and FFP vanish. At steady state the balance on fixed carbon yields

Ec,i ) FCF,i ) F0

(3)

where subscript i reminds that variables are referred to inert conditions. Under oxidizing atmosphere, both FCP and FFP are nonzero and the carbon elutriation rate Ec can be expressed as

Ec ) FCF(1 - ξF) ) FCF,iβ(1 - ξF)

(4)

where ξF ) FFP/FCF represents the degree of carbon fines postcombustion over their residence time in the bed. FCF is expressed in eq 4 as the product of the fines generation rate in inert conditions FCF,i and a factor β that accounts for enhancement of attrition due to the progress of combustion. Under oxidizing conditions, carbon elutriation rates can be either smaller or larger than FCF,i, depending on whether the product β(1 - ξF) is smaller or larger than unity, respectively. Analysis of Figures 7 and 8 reveals a scenario which is rather different from that encountered in fluidized bed combustion of low-volatile fuels. In the latter case

(5)

that is, differently from coal, the enhancement of carbon fines generation under combustion-assisted attrition conditions is largely overcome by extensive postcombustion of the attrited fines during their residence time in the bed. This result is not surprising considering that the intrinsic combustion reactivity of char from Robinia is much larger than that of char from a bituminous coal char (by almost 3 orders of magnitude at 850 °C, for example15). Furthermore, it can be observed that, even at inlet oxygen concentrations as low as 1% by volume and at a bed temperature as low as 700 °C, it is

FFP = FCF

(6)

or equivalently, ξF = 1 and β(1 - ξF) = 0. The quantitative assessment of carbon attrition during fluidized bed combustion has been typically accomplished2 by neglecting fines postcombustion (ξF = 0) and assuming the attrition rate to be equal to the elutriation rate (FCF = Ec). This approach, however, cannot be extended to the attrition of char from Robinia biomass or whenever carbon fines postcombustion is relevant. A method for the quantitative assessment of the fines generation rate under conditions of nonnegligible fines postcombustion was recently developed by Salatino et al.16 Experimental results were expressed in terms of the ratio R ) FCF/FCP of the fines generation rate to the coarse char combustion rate. Two main conclusions were drawn from application of the method to Robinia char particles: (a) Values of the parameter R were around 1 for all the experimental conditions, i.e. about 2 orders of magnitude larger than the corresponding ratios for a medium rank coal. This finding confirms the much higher propensity of the biomass char to fines generation when compared with low-volatile fuels. (b) The parameter R for Robinia was practically insensitive to the operating conditions (fluidization velocity, oxygen concentration, bed particle size). The relative constancy of R suggested that the relevant fines generation mechanism was peripheral percolative fragmentation rather than attrition by surface abrasion. The dominance of peripheral percolative fragmentation implies that the ratio of the carbon consumption rate by fines generation to that by combustion is a constant determined solely by fuel particle properties17-20 (16) Salatino, P.; Scala, F.; Chirone, R. Proceedings of the 27th Symposium (International) on Combustion; The Combustion Institute: 1998; pp 3103-3110. (17) Walsh, P. M.; Dutta, A.; Cox, R. J.; Sarofim, A. F.; Beer, J. M. Proceedings of the 22nd Symposium (International) on Combustion; The Combustion Institute: 1988; pp 249-258. (18) Walsh, P. M. Proceedings of the 10th International Conference Fluidized Bed Combustion; ASME: New York, 1989; pp 765-773. (19) Walsh, P. M.; Li, T. Combust. Flame 1994, 99, 749-757.

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Figure 12. Comminution behavior of Robinia char in a fluidized bed combustor.

Figure 11. Normalized carbon elutriation rate as a function of the apparent carbon conversion degree under oxidizing conditions. T ) 850 °C.

according to the relationship

R)

(

)

1 - Vash - θcr θcr - θ0

(7)

where θ0 and θcr are the char initial porosity and the porosity at the percolative threshold and Vash is the volume fraction of ash in the unconverted char particle. The char critical porosity at the percolation threshold can be calculated for Robinia, given a value of the initial porosity θ0 ) 0.91, from the experimental data reported by Salatino et al.16 A value θcr ) 0.955 has been found. The validity of this conclusion was further checked by analyzing fines elutriation rates collected at 850 °C (Figure 8). At the steady state, in oxidizing conditions, the carbon mass balance on fines in the bed reads

Ec + FFP ) FCF

(8)

Expressing Ec ) kelWF and FFP ) kcFWF, where WF is the actual carbon fines loading in the bed, kel is the fines elutriation rate constant, and kcF is a fines combustion rate constant, it is

Ec ) kelFCF/(kel + kcF)

(9)

Experimental results suggest that under oxidizing conditions the rate of fines that burn in the bed is much larger than the fines elutriation rate (FFP . Ec w kcFWF . kelWF). This implies

kel/kcF = 0.01 w kcF . kel

(10)

Ec = kelFCF/kcF

(11)

Accordingly

Expressing FCF ) RFCP and FCP ) kcCWC, where WC is the actual coarse char loading in the bed and kcC is a coarse char combustion rate constant:

Ec ) kelRWCkcC/kcF

(12)

Figure 11 reports normalized elutriation rates ob(20) Miccio, F.; Salatino, P. Proceedings of the 24th Symposium (International) on Combustion; The Combustion Institute: 1992; pp 1145-1151.

tained under oxidizing conditions at 850 °C replotted as functions of the apparent fixed carbon conversion degree ξa. Analysis of Figure 11 suggests that Ec/WC,O is independent of oxygen concentration, throughout carbon conversion ξa. This result, together with the observation that the ratio kcC/kcF should be itself barely dependent on oxygen concentration, implies, in light of eq 12, that the parameter R is a constant. To summarize, the effect of fines generation/postcombustion on carbon conversion rate can be looked at as a network of two reactions in series:

(1 + R)Ccoarse char + O2 f CO2 + RCgenerated fines (13) RCgenerated fines + RO2 f RCO2

(14)

whose ultimate product is CO2. In the first reaction fixed carbon in coarse char particles generates both gaseous products by combustion and fixed carbon as detached fines. The second reaction represents combustion of the fixed carbon in the generated fines. The sequence of the two reactions is equivalent to the overall reaction

(1 + R)C + (1 + R)O2 f (1 + R)CO2

(15)

Because reaction 14 is faster than 13 (there is about 1 order of magnitude difference between the particle diameters), the latter can be considered the rate-limiting step. Accordingly, the observed enhancement of particle burning rate can be explained by assuming the apparent stoichiometric coefficient for overall carbon oxidation of 1 + R. This is the same as assuming that the char particles burn with an apparent rate enhanced by the same coefficient 1 + R. Following the experimental results, Figure 12 shows a qualitative representation of the series-parallel phenomena determining the size reduction of Robinia char particles in a fluidized bed combustor. Comparison of results obtained under steadily (Figures 7 and 8) and alternating (Figures 9 and 10) oxidizing conditions provide further insight on the relevant time scales of the system. Results of the alternating oxidizing-inert experiments can be analyzed in light of a simple model based on transient material balances on carbon fines in the reactor, under the combined effect of fines generation, combustion, and elutriation; that is,

dWF,i + kelWF,i ) FCF,i dt

(16)



Figure 13. Computed carbon elutriation rate profile under alternating inert and oxidizing (1% O2) conditions, compared to typical steady-state carbon flow rates. T ) 850 °C.

under inert conditions and

dWF + kelWF + kcFWF ) FCF dt

(17)

under oxidizing conditions. Considering the inequality expressed by eq 10, eq 17 is modified into

dWF + kcFWF ) FCF dt

(17′)

Multiplication of eqs 16 and 17′ by kel and integration subject to the initial conditions

WF,i,0 ) 0 at the beginning of the inert stage and

WF,0 = FCF,i/kel at the beginning of the oxidizing stage, yield

Ec,i = FCF,i(1 - exp(-kelt))

(18)

during the inert stage and

Ec = FCF,i exp(-kcFt)

(19)

during the oxidizing stage. Equations 18 and 19 can be used to calculate carbon elutriation rates during inert and oxidizing intervals. It must be noted that FCF,i and FCF have been taken as constants in the model for the sake of simplicity. In actual experiments, due to the batchwise nature of tests, both FCF,i and FCF decrease with time (Figures 9 and 10) as a consequence of char consumption. Figure 13 reports the fines elutriation rate as a function of time under alternating oxidizing-inert conditions at T ) 850 °C computed using 1/kel = 1/kcF = 1 min. For comparison typical steady-state carbon flow rates worked out from experimental results are also reported in the figure. Ec vs time profiles can be further averaged over each oxidizing-inert stage and compared with experimental values (Figure 10). Matching eqs 18 and 19 versus data points in Figure 10 suggests that both time constants

1/kel and 1/kcF, relative to inert and oxidizing periods, respectively, are of the order of 1 min. The conclusion that kel = kcF = 1 min-1 apparently contradicts that previously reported on the basis of results from steadily oxidizing experiments (eq 10); i.e., kel/kcF = 0.01. It is believed that this discrepancy is a consequence of the inadequacy of the theoretical framework expressed by eqs 16 and 17. It can be hypothesized that occurrence of char fines adhesion onto inert bed particles, not taken into account in eqs 16 and 17, should be called for to explain the discrepancy between results of steady and unsteady combustion experiments. Two factors might cooperate in determining fines adhesion: (a) the presence of significant amounts of alkali metal (especially potassium, see Table 1) in the biomass ashes, possibly leading to formation of low-melting eutectics; (b) significant combustion-induced fine particle overheating with respect to the fluidized solids might bring about ash softening and melting, thus enhancing adhesion. The occurrence of adhesion phenomena in connection with biomass fuel combustion has been reported in the literature, assessed by monitoring either bed agglomeration21-25 or ash deposition onto bed material.21-23 Char fines adhesion onto bed particles, if any, would result in an average fines residence time in the combustor longer than expected on the basis of classical elutriation mechanisms, consistent with the large carbon combustion efficiencies recorded in the experiments. On the other hand experiments in alternating inertoxidizing conditions, by their very nature, yield values of kel = 1 min-1 valid for elutriation of the “free” fines. The formation of a relatively nonelutriable phase consisting of “adhered” fines would be masked in these experiments. On the other hand adhesion, in combination with the large char intrinsic reactivity, would increase the relevance of boundary layer oxygen diffusion to the apparent kinetics of carbon fines combustion. The reported figure of kcF = 1 min-1, that is, about 1 order of magnitude smaller than the kinetic rate constant predicted assuming fines freely burning in the bed,15 is consistent with the hypothesis that fines adhesion is at work. The proposed mechanism agrees with observations of Chirone et al.26 who performed steady combustion experiments of powdered Robinia in a fluidized bed. Very small fines elutriation rate constants kel were reported by these authors when the elutriation rate was referred to the total amount of char fines in the bed rather than to the “free” carbon fines only. In light of the above discussion Figure 2A might be modified into Figure 2B where an additional carbon (21) Grubor, B. D.; Oka, S. N.; Ilic, M. S.; Dakic, D. V.; Arsic, B. T. Proceedings of the 13th International Conference on Fluidized Bed Combustion; ASME: New York, 1995; pp 515-522. (22) Lin, W.; Dam-Johansen, K. Proceedings of the 15th International Conference on Fluidized Bed Combustion; ASME: New York, 1999; paper FBC99-0120. (23) Latva-Somppi, J. Experimental studies on pulp and paper mill sludge ash behavior in fluidized bed combustors, ESPOO 1998, Technical Research Centre of Finland, VTT Publications 336; 1998. (24) Natarajan, E.; Ohman, M.; Gabra, M.; Nordin, A.; Liliedahl, T.; Rao, A. N. Biomass Bioenergy 1998, 15, 163-169. (25) Skrifvars, B.; Backman, R.; Hupa, M.; Sfiris, G.; Abyhammar, T.; Lyngfelt, A. Fuel 1998, 77, 65-70. (26) Chirone, R.; Russo, S.; Serpi, M.; Salatino, P.; Scala, F. Proceedings of the Mediterranean Combustion Symposium; The Combustion Institute: 1999; pp 906-915.

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phase (A), namely, carbon fines adhered onto inert bed particles, has been considered. Conclusions The fluidized bed combustion of a biomass fuel char (Robinia pseudoacacia) has been investigated with a focus on the fate of fixed carbon and on the relevance of comminution phenomena. Negligible primary fragmentation followed by extensive comminution of the char characterize the biomass conversion. Fixed carbon conversion occurs partly via direct coarse char combustion and partly via fines generation of coarse char and their subsequent postcombustion in the bed. Even if both pathways end up with almost complete carbon conversion to CO or CO2, they are not at all equivalent. Char entrainment and elutriation, the main location of char burn out, char particle time-temperature history, and related ash deposition issues are all crucially dependent on the relative importance of the combustion pathways associated with fine versus coarse char combustion. Combustion-induced fines generation is mostly due to peripheral percolative fragmentation. This mechanism implies that the ratio of the carbon leaving a

Scala et al.

coarse char particle as attrited fines and that converted to combustion products is a constant with respect to operating conditions. Comparison of results of steadily oxidizing experiments and of alternating oxidizing-inert experiments provided some insight in the combustion mechanism of char fines. It is inferred from experimental results that the formation of a “captive” solid carbon phase by adhesion of char fines onto bed solids might be responsible for the large carbon combustion efficiencies. Despite only indirect evidence given here of the occurrence of fines adhesion, results are consistent with observations of other groups who underlined the crucial role of ash-bed solids agglomeration. Incipient softening/melting of high alkali metal ashes, possibly enhanced by the fuel particle overheating by combustion, might cooperate in determining adhesion. Acknowledgment. The support of Mrs. C. Zucchini and Mr. S. Russo in SEM analysis, of Mr. S. Masi in porosimetric analysis, and of Mr. A. Cammarota and Mr. G. Greco in fluidized bed experiments is gratefully acknowledged. EF9901701

Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia

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