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The relevance of attrition to the fate of ashes during fluidized-bed combustion of a biomass
Proceedings of the Combustion Institute, Volume 28, 2000/pp. 2279–2286
THE RELEVANCE OF ATTRITION TO THE FATE OF ASHES DURING FLUIDIZED-BED COMBUSTION OF A BIOMASS RICCARDO CHIRONE, PIERO SALATINO and FABRIZIO SCALA Dipartimento di Ingegneria Chimica Universita` degli Studi di Napoli “Federico II” Istituto di Ricerche sulla Combustione—C. N. R. P.le Tecchio, 80-80125 Napoli, Italy
The fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacacia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700 ⬚C to 850 ⬚C at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration. SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles’ burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon afterburning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkalisilicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration.
Introduction Exploitation of biomass resources as an alternative energy source is attractive because of their availability, renewability, and the possibility of achieving clean and CO2-neutral conversion. Fluidized-bed combustion stands as one of the best technologies for biomass utilization due to its intrinsic advantages such as fuel flexibility, high combustion efficiency, and low pollutant emissions. On the other hand, it has been recognized that the principal disadvantage is related to possible agglomeration and defluidization issues often encountered during operational experience with these fuels [1,2]. Indeed, the large alkali content in the ash residues of most biomass fuels leads to the formation of low-melting-point compounds in combination with the inert bed material (usually silica sand). The mechanisms of deposition of biomass ashes on the bed particles’ surface and formation of sticky melts responsible for agglomeration are not as yet well understood, despite a considerable research effort [3–5]. The fluidized-bed combustion of a biomass fuel
(Robinia pseudoacacia) has been extensively investigated in the last years with a focus on the fate of fixed carbon and on the relevance of comminution phenomena [6–8]. Experimental results showed that extensive char comminution characterizes biomass conversion. Fixed carbon conversion occurs to a comparable extent via direct coarse char combustion and via fines generation from coarse char and their subsequent postcombustion in the bed. This is the result of two factors: (1) the tendency of high-volatile fuels to give rise to highly porous—or even incoherent—chars that easily yield elutriable fines upon impact or surface wear, and (2) the relatively large intrinsic combustion reactivity of fines from these chars [9], which makes their postcombustion more effective. Even if both pathways end up with almost complete carbon conversion, they are not at all equivalent. Char entrainment and elutriation, the main location of char burnout, char particle timetemperature history, and related ash deposition issues are crucially dependent on the relative importance of the combustion pathways associated with fine versus coarse char combustion.
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Fig. 1. Experimental apparatus: 1, gas preheating section; 2, electrical furnaces; 3, ceramic insulator; 4, gas distributor; 5, thermocouple; 6, fluidization column; 7, head with three-way valve; 8, sintered brass filters; 9, hopper; 10, SO2 scrubber; 11, stack; 12, cellulose filter; 13, membrane pump; 14, gas analyzers; 15, personal computer; 16, manometer; 17, digital mass flowmeters; 18, air dehumidifier (silica gel).
Experimental
40 mm i.d. and 1 m high (Fig. 1). The gas distributor was a 2 mm thick perforated plate with 55 holes, 0.5 mm in diameter, disposed in a triangular pitch. The fluidization column and the 0.6 m high stainless steel preheating section were heated by two electric furnaces. The temperature of the bed, measured 40 mm above the distributor, was kept constant by a PID controller. The freeboard was kept unlagged in order to minimize fines postcombustion in this section. Gases were fed to the column via two highprecision digital mass flowmeters. The top flange of the fluidization column was fitted to a two-exit brass head equipped with a three-way valve. By operating this valve, it was possible to convey flue gases alternately to two removable filters made of sintered brass. Fuel particles could be fed to the bed via a hopper connected sideways to the upper part of the freeboard. A paramagnetic analyzer and two NDIR analyzers were used for on-line measurement of O2, CO, and CO2 concentrations, respectively, in the exhaust gases.
Experimental Apparatus The experiments were carried out in a stainless steel atmospheric bubbling fluidized-bed combustor,
Materials The bed material consisted of 180 g of pure quartz, corresponding to an unexpanded bed height
Experimental results reported by Chirone et al. [10] and Scala et al. [8] provided some insight into the combustion mechanism of Robinia biomass char fines. It was inferred from experimental results that the formation of a solid carbon phase by adhesion of char fines onto bed solids might be responsible for the very small carbon elutriation rate observed during steadily burning feedings of Robinia fine particles. The purpose of this paper is to study both theoretically and experimentally the mechanism by which attrited fines generation and their subsequent postcombustion affect the ultimate fate of mineral matter. The relevance of fines adhesion onto inert bed particles during biomass fluidized-bed combustion is assessed and found to be relevant to the formation of the potassium-silicate melts responsible for bed agglomeration and defluidization problems.
FLUIDIZED BED COMBUSTION OF BIOMASS TABLE 1 Properties of Robinia pseudoacacia Fuel density, Kg/m3 Char density, Kg/m3 LHV, kJ/Kg
380 100 15,600
Proximate analysis (dry basis), %w Volatiles Fixed carbon Ash
79.2 19.3 1.5
Ultimate analysis (dry basis), %w Carbon Hydrogen Nitrogen Sulfur Ash Oxygen (diff)
43.9 7.8 0.02 – 1.5 46.78
Ash composition, %w CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 SO4
76.2 11.8 9.2 0.78 0.25 0.69 0.07 ⬍0.1
of 0.1 m. The quartz was double sieved in the nominal size range 0.6–0.85 mm, with a Sauter mean diameter of 0.76 mm. Minimum fluidizing velocity was 0.27 m/s at 850 ⬚C. Experiments were carried out with Robinia pseudoacacia, a ligneous biomass fuel whose properties are reported in Table 1. Robinia branches were cut into particles of approximately cylindrical shape with diameter and length both 10 mm. Each particle was surrounded by bark. The fluidization gas consisted of technical grade air and nitrogen or mixtures of the two. Inlet oxygen concentration in the combustor was varied between 1% and 21% on a volume basis. Procedures Two kinds of experimental tests were performed in the fluidized-bed combustor, namely, batch and steady combustion tests. Whatever the type of the test, the reactor was charged with a bed of quartz (180 g) and heated to the required temperature prior to each experiment. Batch experiments Batches of five particles of Robinia (corresponding to 0.33 g of fixed carbon) were fed overbed to the reactor kept at 700 ⬚C or 850 ⬚C, while keeping the
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bed slightly beyond incipient fluidization in a stream of nitrogen. When devolatilization was over, superficial gas velocity was increased to 0.8 m/s, and nitrogen and air feedings were adjusted to establish the required oxygen concentration in the inlet fluidizing gas (1% or 3%). Elutriated fines were collected by means of the two-exit head by letting the flue gas flow alternately through sequences of filters (one was in use while the previous one was replaced) for definite periods of time (ranging from 0.5 to 10 min). The difference between the weights of the filters before and after operation, divided by the time interval during which the filter was in operation, gave the average fines elutriation rate relative to that interval. Fines collected in the filters were further analyzed to determine their fixed carbon and ash content. This procedure allowed time-resolved measurement of carbon and ash elutriation rates. A minimum of five tests were repeated under the same experimental conditions, and results indicated that the fines elutriation rate could be measured with an accuracy of 10%. The attrition rate of quartz was separately quantified in “blank” experiments in which only quartz was charged in the reactor and fluidized under the same operating conditions. Results of blank tests indicated that the contribution of fines produced by attrition of quartz was negligible. Steady experiments The tests consisted of the steady combustion of a fixed total amount of 100 g of Robinia. Fuel particles were continuously fed to the bed, kept at 700 ⬚C, 750 ⬚C, 800 ⬚C, or 850 ⬚C and fluidized at 0.8 m/s with a mixture of air and nitrogen at inlet oxygen concentration of 2% or 21%. The fuel feeding rate was varied in order to keep the difference between the inlet and outlet oxygen concentration within 30% of the value at the inlet. The reactor was left open at the top, and the occurrence of defluidization was detected by visual inspection of the bed. In the event that defluidization occurred, the bed particle agglomerates were mechanically destroyed by stirring the bed with a stainless steel bar, and the run continued until the entire 100 g of Robinia was burned. After the end of the run, the bed material was discharged from the reactor for further characterization. The morphology of quartz samples, before and after operation, was characterized by the use of a scanning electron microscope (SEM) (Philips XL30 with LaB6 filament) at magnifications up to 1600. SEM observations were complemented by elemental analysis of the particle surface by means of an EDX probe (EDAX DX-4). Theoretical Assessment of Char Particles’ Time-Temperature History The time-temperature history of biomass fine and coarse particles in a fluidized-bed combustor has
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been simulated by means of a single-particle model. Consistent with experimental operating conditions, differential behavior of the combustor with respect to oxygen has been assumed. The model is based on the energy balance on a burning particle accounting for convective and radiative heat transfer as well as for heat release associated with particle burning. The latter contribution was assessed under the hypothesis that the burning rate is controlled by oxygen diffusion in the particle boundary layer. This assumption, justified by the extremely large intrinsic reactivity of Robinia char (about 3 orders of magnitude larger at 850 ⬚C than that of char from a bituminous coal [9]), has been checked by order of magnitude evaluation of different resistances (intraparticle and external diffusion, intrinsic kinetics) with reference to both fine and coarse char particles. The energy balance around a spherical burning char particle reads q
d dT c 12KcCO2 DH hc (Tbed T) 6 dt re (T4bed T4)
(1)
where Kc
DO2 Sh k Nu and hc g d d
(2)
are the mass and heat transfer coefficients. For fine char particles, the Sherwood number has been assumed to be equal to 2. For coarse char particles, Sh has been evaluated according to the Prins et al. [11] correlation. The Nusselt number for coarse char particles has been evaluated according to the Prins et al. [12] correlation for mobile particles in a fluidized bed. As regards the way heat transfer between fine particles and the fluidized bed takes place, three cases have been considered: 1. Fines burn in the interstices of the dense bed without colliding with bed particles. In this case the Nusselt number has been assumed to be equal to 2. 2. Fines collide with bed particles without adhering. Accordingly, fines experience cyclic behavior consisting of the following alternating stages: (a) the particle is freely dragged by the gas flow in the interstices of the emulsion phase; (b) the particle is in contact with a bed particle. It is assumed, for the sake of simplicity, that the durations of these phases are equal to each other. An estimate of the cycle time can be calculated as the ratio of the mean path between two bed particles—of the order of the bed particle diameter—and the interstitial gas velocity. For bed particles of about 1mm, the cycle time is about 0.005 s. During stage (a), Nu 2 has been assumed. During stage (b), the heat transfer coefficient has been calculated according to Schlu¨nder [13].
Fig. 2. Robinia biomass particles time-temperature history during fluidized-bed combustion. (a) T 700 ⬚C, xO2 0.02; (b) T 700 ⬚C, xO2 0.12.
3. Fines irreversibly adhere to the bed particle upon collision. Accordingly, after the first 0.005 s period, the relevant mechanism of heat transfer between the two particles is assumed to be conduction. Thermal conductivity of the partially molten fines has been evaluated from the literature [14]. Figure 2a and b report the time-temperature history of 0.1 mm char fines burning in a bed at 700 ⬚C. Two different in-bed oxygen concentrations were considered, namely, 2% (Fig. 2a) and 12% (Fig. 2b). In the same figure, the steady temperature of a typical coarse particle of 10 mm size burning at the same bed operating conditions is reported for comparison. Typical burnout times for biomass fine and coarse particles in these conditions are of the order of a few seconds or hundreds of seconds, respectively. Calculation results show that extremely high temperatures are reached by the fine particles in very short times independently of the mechanism assumed, especially at the larger oxygen concentration. Upon collision with a bed particle, temperatures at
FLUIDIZED BED COMBUSTION OF BIOMASS
Fig. 3. Average fines residence time in a fluidized-bed combustor as a function of fines carbon loading in the bed for three different fuels. Worked out from Refs. [10] and [18].
TABLE 2 Results of batch experiments R T 700 ⬚C 1% O2 T 700 ⬚C 3% O2 T 850 ⬚C 1% O2 T 850 ⬚C 3% O2
0.53 0.53 0.68 0.67
Burn out time (min) 39 17 30 15
the contact point might be well beyond typical melting temperatures of alkali-silica compounds (around 750 ⬚C [1,5]). Even at a bed temperature as low as 700 ⬚C and at in-bed oxygen concentrations as low as 2%, such conditions can be reached. On the other hand, coarse particles burn at temperatures only slightly higher than the bed temperature, in this case below relevant melting temperatures. The fact that, unlike coals, biomass fines burn hotter than coarse particles is consistent with previous experimental findings [15,16] and is mostly related to the large reactivity of biomass char [17]. Experimental Results and Discussion Figure 3 gives the average residence times of fines s Wc/Ec in the fluidized-bed combustor as a function of fines carbon loading in the bed. Data points were obtained by working out experimental data reported by Chirone et al. [10] and Massimilla et al. [18] for steady combustion of fine particles (about 0.1 mm feed particle size) of three different fuels: Robinia biomass, a Tyre-derived fuel, and a South African bituminous coal. It is noteworthy that fines’ average residence times in the bed are much longer for Robinia (20–25 min) than for the other two fuels
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(2–3 min), despite that the sizes and densities of the fine particles differed negligibly from one another. This finding provides indirect evidence that adhesion of char fines onto bed solids might be at work. Occurrence of adhesion is directly highlighted in the purposely designed experimental program of the present work. Table 2 reports the fraction, R, of ashes retained in the bed (that is, not elutriated) during batch combustion experiments of coarse Robinia particles. Temperatures of 700 ⬚C and 850 ⬚C and two oxygen concentrations (1% and 3% by volume) were considered. In the same table, the particle burnout times are reported. Results show that at 700 ⬚C and at 850 ⬚C, about 50% and 65%, respectively, of the total fuel-ash fed to the reactor was retained in the bed, regardless of oxygen concentration. The complement is represented by the fly-ashes cumulatively elutriated throughout burnoff. No coarse ash particles could be retrieved from the bed material at the end of the experiments; the ash retained in the bed was intimately associated with inert bed particles. Steady combustion experiments of Robinia coarse particles were carried out at four different temperatures ranging from 700 ⬚C to 850 ⬚C. At any bed temperature, visual observation indicated that, after devolatilization, Robinia particles appeared at the bed surface from time to time, spending most of their burnout time in the upper zone of the bed. The burning particle appeared to be surrounded by a brighter boundary layer. No adhesion of bed quartz particles on the burning char could be observed. Onset of bed agglomeration and defluidization, if any, could be clearly assessed by the sudden stop of the burning particle (it rested at a stable position) and by the lack of bed surface motion and bubble bursting. During the steady experiments carried out with 100 g total biomass feeding, agglomeration/defluidization was observed only at the two higher temperatures, namely 800 ⬚C and 850 ⬚C, regardless of oxygen concentration. No agglomeration was noticed in combustion experiments carried out at 700 ⬚C and 750 ⬚C. At the end of each run, the bed was completely discharged for further morphological characterization. At all temperatures, the discharged bed showed a slightly yellowish color, in contrast to the white fresh quartz. The bed weight after operation increased by about 1–2 g. This result is in line with ash retention data found in batch experiments (Table 2). Figure 4 compares SEM micrographs of bed quartz samples before utilization and after biomass combustion at 700, 750, and 850 ⬚C (samples at 800 ⬚C were similar to those at 850 ⬚C). Differences between the samples can be interpreted in the light of theoretical curves from the single-particle combustion model (case 3). Fresh quartz showed a very smooth surface with some sharp edges. The phenomenology can be summarized as follows:
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Fig. 4. SEM micrographs of bed quartz samples before utilization and after biomass combustion at 700 ⬚C, 750 ⬚C, and 850 ⬚C, and corresponding fines’ time-temperature histories calculated with the single-particle combustion model.
Fig. 5. Fixed carbon and ash material balances of fine char particles in a fluidized bed.
• Bed material resulting from experiments at 700 ⬚C has the same sharp and angular appearance as the fresh quartz, indicating that melting has not occurred. This situation corresponds to curve A in Fig. 4. The surface of the particle presents some roughness, not present in the original quartz, with small protuberances probably due to the accumulation of fine ash deposits. • Morphology of the bed sample from experiments carried out at 750 ⬚C is similar to the previous sample, but evidence of the localized formation of a melt is clearly observed on the surface. It is inferred that eutectic temperatures might have been
reached locally. This situation corresponds to curve B in Fig. 4. • The sample of bed material from experiments carried out at 850 ⬚C displays a completely different morphology: several particles are agglomerated by solid bridges of fused material. Sharp edges have been rounded off by extensive melting at the bed particle surface. This situation corresponds to curve C in Fig. 4. EDX analysis has been carried out on each sample in different zones of the particles. A strong enrichment of calcium and potassium and, to a lesser extent, of magnesium and phosphorus was observed
FLUIDIZED BED COMBUSTION OF BIOMASS
for all the samples. No major difference in the quantities of these elements on the surface of particles of different samples has been found. It is noteworthy that the molten zones presented a larger amount of potassium compared with calcium; other elements were present only in trace quantities. Typical proportions found are Si 60%, K 30%, Ca 10%. On the whole, theoretical results and experimental findings suggest the following mechanism of fines postcombustion and ash deposition onto bed particles. The mechanism is summarized by the networks of Fig. 5 that express the balance on fixed carbon and on mineral matter associated with fine particles in the bed. According to these networks, both fixed carbon and ash are present in fine char particles either freely moving in the interstices of the bed (free fines: Wf, Wf,ash) or attached onto the surface of inert bed solids (Wa, Wa,ash). During combustion of coarse biomass, extensive char attrition and fines generation take place, as discussed by Salatino et al. [7] and Scala et al. [8]. Upon detachment from the coarse char particles, the fines’ temperature quickly rises as a consequence of carbon postcombustion, possibly beyond the melting temperature of silica-potassium compounds. Attrited fines experience collisions with bed particles. Upon impact with bed particles, the formation of a softened or fluid phase at the contact point may be responsible for fines’ adhesion on the bed particles. In the event that the bed particle temperature is itself higher than the relevant eutectic temperature, fused potassium-silicate is permanently formed, yielding a sticky surface layer responsible for bed agglomeration and defluidization. If the bed particle temperature is, instead, below the eutectic melting temperature, fines adhesion can still take place, but the formation of a surface melt on the bed solids is prevented, and so is bed agglomeration. To complete the scenario, attrition of bed ash agglomerates possibly detaching adhered fines should be taken into consideration.
Conclusions The fate of ashes during the fluidized-bed combustion of a biomass fuel has been studied by a combination of theoretical computations and experimental results. The simulation of the time-temperature history of attrited fine char suggests that extremely high temperatures can be reached as a consequence of carbon postcombustion, strongly dependent on oxygen concentration or excess air. Fines’ peak temperatures are possibly higher than those at which softening and melting of silica-potassium compounds take place. Accordingly, generation of fines by attrition and combustion-induced fine particle overheating are responsible for the formation of ash-layered bed material even at bed temperatures below the
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potassium-silicates melting point. This occurrence has been confirmed by steady combustion experiments at different bed temperatures. The further fate of the ash-layered bed material depends on the bed temperature: beyond the ash-melting range, fused potassium-silicate is formed, eventually leading to bed agglomeration and defluidization; below the ash-melting range, formation of fused material is prevented, but the thickness of the ash layer can steadily increase due to continuous adhesion of fine material. It is concluded that attrition phenomena, whose consideration has been mostly restricted so far to the fate of fixed carbon and to its implications on combustion efficiency, play a key role in determining the ultimate fate of ash components in the fluidized-bed combustion of biomass fuel. Nomenclature c CO2 d DO2 Eash Ec Fash hc Kc kg Nu R Sh t T Tbed Wa Wa,ash Wc Wf Wf,ash xO2 DH e q r s
heat capacity of the char particle, kJ/kg K oxygen concentration, kmol/m3 char particle diameter, m oxygen molecular diffusivity, m2/s ash elutriation rate, kg/s fixed carbon elutriation rate, kg/s ash feed rate, kg/s convective heat transfer coefficient, kW/K m2 convective mass transfer coefficient, m/s gas thermal conductivity, kW/K m Nusselt number fraction of fed ashes retained in the bed Sherwood number time, s char particle temperature, K bed temperature, K carbon loading in attached fines, Kg ash loading in attached fines, Kg (Wf Wa), Kg carbon loading in free fines, Kg ash loading in free fines, Kg oxygen mole fraction heat of combustion of fixed carbon, kJ/kg effective emissivity char particle density, kg/m3 Stefan-Boltzmann constant, kW/m2 K4 (Wc/Ec), s Acknowledgments
The support of Mrs. C. Zucchini and Mr. S. Russo for SEM/EDX analysis is gratefully acknowledged. REFERENCES 1. Grubor, B. D., Oka, S. N., Ilic, M. S., Dakic, D. V., and Arsic, B. T., Proceedings of the Thirteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1995, pp 515–522.
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2. Natarajan, E., Ohman, M., Gabra, M., Nordin, A., Liliedahl, T., and Rao, A. N., Biomass Bioenergy 15:163– 169 (1998). 3. Skrifvars, B., Backman, R., Hupa, M., Sfiris, G., Abyhammar, T., and Lyngfelt, A., Fuel 77:65–70 (1998). 4. Latva-Somppi, J., Kurkela, J., Tapper, U., Kauppinen, E. I., Jokiniemi, J. K., and Johanson, B., Proceedings of the International Conference on Ash Behavior Control in Energy Conversion Systems, SCEJ, Japan, 1998, pp. 119–126. 5. Lin, W., and Dam-Johansen, K., Proceedings of the Fifteenth International Conference on Fluidized Bed Combustion, paper FBC99-0120, ASME, New York, 1999. 6. Chirone, R., Greco, G., Salatino, P., and Scala, F., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp. 145–150. 7. Salatino, P., Scala, F., and Chirone, R., Proc. Combust. Inst. 27:3103–3110 (1998). 8. Scala, F., Salatino, P., and Chirone, R., Energy Fuels 14:781–790 (2000). 9. Masi, S., Salatino, P., and Senneca, O., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp 135– 143.
10. Chirone, R., Russo, S., Serpi, M., Salatino, P., and Scala, F., Combust. Sci. Technol. 153:83–93 (2000). 11. Prins, W., Casteleijn, T. P., Draijer, W., and Van Swaaij, W. P. M., Chem. Eng. Sci. 40:481 (1985). 12. Prins, W., Draijer, W., and Van Swaaij, W. P. M., Heat and Mass Transfer in Fixed and Fluidized Beds, Hemisphere, Washington, 1986, pp. 317–331. 13. Schlu¨nder, E. U., Int. Chem. Eng. 20:550–553 (1980). 14. Inumaru, J., Ashizawa, M., Kurimura, M., Chikuma, H., and Ohtaka, M., Proceedings of the International Conference on Ash Behavior Control in Energy Conversion Systems, SCEJ, Japan, 1998, pp. 60–67. 15. Hernberg, R., Stenberg, J., and Zethraeus, B., Combust. Flame 95:191–205 (1993). 16. Joutsenoja, T., Heino, P., Hernberg, R., and Bonn, B., Combust. Flame 118:707–717 (1999). 17. Palchonok, G. I., Breitholtz, C., Thunman, H., and Leckner, B., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp. 871–878. 18. Massimilla, L., Chirone, R., D’Amore, M., and Salatino, P., Carbon Attrition during the Fluidized Combustion and Gasification of Coal, final technical report, DOE grant no. DE-FG22-81PC40796, Cuen, Napoli, 1985.
COMMENTS E. M. Bulewiez, Cracow University of Technology, Poland. 1. The biomass fuel particles (wood) were rather large, in comparison with the dimensions of the apparatus and the bed material particle size. Under these conditions, the fuel particles would tend to float on top of the bed until practically the end of the burning of the volatiles. The remaining charcoal would then undergo both fragmentation and attrition. Was this taken into account? 2. Were any model experiments attempted (e.g., reacting suitable potassium compounds with silica over extended times)? This could elucidate the problem, since the gas phase is most unlikely to play a part in agglomeration. It was satisfying to note that the authors also employed cyclical bed-temperature variation. We found this technique to be extremely useful in conducting small scale experiments [1].
REFERENCE 1. Pilowski M., Kandefon S., in Fourth International Conference on Combustion Technologies for a Clean Environment, Lisbon, Portugal, 1995, pp. 29, 4, 22–28. Author’s Reply. Yes. Segregation of biomass particles was observed only during devolatilization. In this stage, attrition was negligible. Devolatilized (char) particles appeared instead to be thoroughly mixed with bed solids. The interaction between attrited fines and inert bed particles pos-
tulated in the colliding fines model is likely to take place during char burn-off. No such model experiment was attempted. The authors believe that the peculiarity of mineral matter being closely associated with highly reactive carbon (as happens within attrited char fines) cannot easily be reproduced in model experiments. This feature is the key for the large overheating of mineral matter with respect to bed inert solids. ● Joseph J. Helble, University of Connecticut, USA. Is attachment treated as being irreversible at low temperatures? Did you consider attachment-detachment, which would retard the motion of the fires with the bed? Author’s Reply. In the long term, fine attachment by adhesion and detachment by attrition play a combined role and are responsible for the much longer fines residence times observed when feeding biomass with respect to other fuels characterized by lower propensity to adhesion. Further discussion on this point can be found in Ref. [1], where results of experiments consisting of continuously feeding pulverized fuels to a fluidized-bed combustor are presented and discussed. REFERENCE 1. Chirone, R., Russo, S., Serpi, M., Salatino, P., and Scala, F., Combust. Sci. Technol. 153:83 (2000).
THE RELEVANCE OF ATTRITION TO THE FATE OF ASHES DURING FLUIDIZED-BED COMBUSTION OF A BIOMASS RICCARDO CHIRONE, PIERO SALATINO and FABRIZIO SCALA Dipartimento di Ingegneria Chimica Universita` degli Studi di Napoli “Federico II” Istituto di Ricerche sulla Combustione—C. N. R. P.le Tecchio, 80-80125 Napoli, Italy
The fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacacia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700 ⬚C to 850 ⬚C at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration. SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles’ burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon afterburning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkalisilicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration.
Introduction Exploitation of biomass resources as an alternative energy source is attractive because of their availability, renewability, and the possibility of achieving clean and CO2-neutral conversion. Fluidized-bed combustion stands as one of the best technologies for biomass utilization due to its intrinsic advantages such as fuel flexibility, high combustion efficiency, and low pollutant emissions. On the other hand, it has been recognized that the principal disadvantage is related to possible agglomeration and defluidization issues often encountered during operational experience with these fuels [1,2]. Indeed, the large alkali content in the ash residues of most biomass fuels leads to the formation of low-melting-point compounds in combination with the inert bed material (usually silica sand). The mechanisms of deposition of biomass ashes on the bed particles’ surface and formation of sticky melts responsible for agglomeration are not as yet well understood, despite a considerable research effort [3–5]. The fluidized-bed combustion of a biomass fuel
(Robinia pseudoacacia) has been extensively investigated in the last years with a focus on the fate of fixed carbon and on the relevance of comminution phenomena [6–8]. Experimental results showed that extensive char comminution characterizes biomass conversion. Fixed carbon conversion occurs to a comparable extent via direct coarse char combustion and via fines generation from coarse char and their subsequent postcombustion in the bed. This is the result of two factors: (1) the tendency of high-volatile fuels to give rise to highly porous—or even incoherent—chars that easily yield elutriable fines upon impact or surface wear, and (2) the relatively large intrinsic combustion reactivity of fines from these chars [9], which makes their postcombustion more effective. Even if both pathways end up with almost complete carbon conversion, they are not at all equivalent. Char entrainment and elutriation, the main location of char burnout, char particle timetemperature history, and related ash deposition issues are crucially dependent on the relative importance of the combustion pathways associated with fine versus coarse char combustion.
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Fig. 1. Experimental apparatus: 1, gas preheating section; 2, electrical furnaces; 3, ceramic insulator; 4, gas distributor; 5, thermocouple; 6, fluidization column; 7, head with three-way valve; 8, sintered brass filters; 9, hopper; 10, SO2 scrubber; 11, stack; 12, cellulose filter; 13, membrane pump; 14, gas analyzers; 15, personal computer; 16, manometer; 17, digital mass flowmeters; 18, air dehumidifier (silica gel).
Experimental
40 mm i.d. and 1 m high (Fig. 1). The gas distributor was a 2 mm thick perforated plate with 55 holes, 0.5 mm in diameter, disposed in a triangular pitch. The fluidization column and the 0.6 m high stainless steel preheating section were heated by two electric furnaces. The temperature of the bed, measured 40 mm above the distributor, was kept constant by a PID controller. The freeboard was kept unlagged in order to minimize fines postcombustion in this section. Gases were fed to the column via two highprecision digital mass flowmeters. The top flange of the fluidization column was fitted to a two-exit brass head equipped with a three-way valve. By operating this valve, it was possible to convey flue gases alternately to two removable filters made of sintered brass. Fuel particles could be fed to the bed via a hopper connected sideways to the upper part of the freeboard. A paramagnetic analyzer and two NDIR analyzers were used for on-line measurement of O2, CO, and CO2 concentrations, respectively, in the exhaust gases.
Experimental Apparatus The experiments were carried out in a stainless steel atmospheric bubbling fluidized-bed combustor,
Materials The bed material consisted of 180 g of pure quartz, corresponding to an unexpanded bed height
Experimental results reported by Chirone et al. [10] and Scala et al. [8] provided some insight into the combustion mechanism of Robinia biomass char fines. It was inferred from experimental results that the formation of a solid carbon phase by adhesion of char fines onto bed solids might be responsible for the very small carbon elutriation rate observed during steadily burning feedings of Robinia fine particles. The purpose of this paper is to study both theoretically and experimentally the mechanism by which attrited fines generation and their subsequent postcombustion affect the ultimate fate of mineral matter. The relevance of fines adhesion onto inert bed particles during biomass fluidized-bed combustion is assessed and found to be relevant to the formation of the potassium-silicate melts responsible for bed agglomeration and defluidization problems.
FLUIDIZED BED COMBUSTION OF BIOMASS TABLE 1 Properties of Robinia pseudoacacia Fuel density, Kg/m3 Char density, Kg/m3 LHV, kJ/Kg
380 100 15,600
Proximate analysis (dry basis), %w Volatiles Fixed carbon Ash
79.2 19.3 1.5
Ultimate analysis (dry basis), %w Carbon Hydrogen Nitrogen Sulfur Ash Oxygen (diff)
43.9 7.8 0.02 – 1.5 46.78
Ash composition, %w CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 SO4
76.2 11.8 9.2 0.78 0.25 0.69 0.07 ⬍0.1
of 0.1 m. The quartz was double sieved in the nominal size range 0.6–0.85 mm, with a Sauter mean diameter of 0.76 mm. Minimum fluidizing velocity was 0.27 m/s at 850 ⬚C. Experiments were carried out with Robinia pseudoacacia, a ligneous biomass fuel whose properties are reported in Table 1. Robinia branches were cut into particles of approximately cylindrical shape with diameter and length both 10 mm. Each particle was surrounded by bark. The fluidization gas consisted of technical grade air and nitrogen or mixtures of the two. Inlet oxygen concentration in the combustor was varied between 1% and 21% on a volume basis. Procedures Two kinds of experimental tests were performed in the fluidized-bed combustor, namely, batch and steady combustion tests. Whatever the type of the test, the reactor was charged with a bed of quartz (180 g) and heated to the required temperature prior to each experiment. Batch experiments Batches of five particles of Robinia (corresponding to 0.33 g of fixed carbon) were fed overbed to the reactor kept at 700 ⬚C or 850 ⬚C, while keeping the
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bed slightly beyond incipient fluidization in a stream of nitrogen. When devolatilization was over, superficial gas velocity was increased to 0.8 m/s, and nitrogen and air feedings were adjusted to establish the required oxygen concentration in the inlet fluidizing gas (1% or 3%). Elutriated fines were collected by means of the two-exit head by letting the flue gas flow alternately through sequences of filters (one was in use while the previous one was replaced) for definite periods of time (ranging from 0.5 to 10 min). The difference between the weights of the filters before and after operation, divided by the time interval during which the filter was in operation, gave the average fines elutriation rate relative to that interval. Fines collected in the filters were further analyzed to determine their fixed carbon and ash content. This procedure allowed time-resolved measurement of carbon and ash elutriation rates. A minimum of five tests were repeated under the same experimental conditions, and results indicated that the fines elutriation rate could be measured with an accuracy of 10%. The attrition rate of quartz was separately quantified in “blank” experiments in which only quartz was charged in the reactor and fluidized under the same operating conditions. Results of blank tests indicated that the contribution of fines produced by attrition of quartz was negligible. Steady experiments The tests consisted of the steady combustion of a fixed total amount of 100 g of Robinia. Fuel particles were continuously fed to the bed, kept at 700 ⬚C, 750 ⬚C, 800 ⬚C, or 850 ⬚C and fluidized at 0.8 m/s with a mixture of air and nitrogen at inlet oxygen concentration of 2% or 21%. The fuel feeding rate was varied in order to keep the difference between the inlet and outlet oxygen concentration within 30% of the value at the inlet. The reactor was left open at the top, and the occurrence of defluidization was detected by visual inspection of the bed. In the event that defluidization occurred, the bed particle agglomerates were mechanically destroyed by stirring the bed with a stainless steel bar, and the run continued until the entire 100 g of Robinia was burned. After the end of the run, the bed material was discharged from the reactor for further characterization. The morphology of quartz samples, before and after operation, was characterized by the use of a scanning electron microscope (SEM) (Philips XL30 with LaB6 filament) at magnifications up to 1600. SEM observations were complemented by elemental analysis of the particle surface by means of an EDX probe (EDAX DX-4). Theoretical Assessment of Char Particles’ Time-Temperature History The time-temperature history of biomass fine and coarse particles in a fluidized-bed combustor has
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been simulated by means of a single-particle model. Consistent with experimental operating conditions, differential behavior of the combustor with respect to oxygen has been assumed. The model is based on the energy balance on a burning particle accounting for convective and radiative heat transfer as well as for heat release associated with particle burning. The latter contribution was assessed under the hypothesis that the burning rate is controlled by oxygen diffusion in the particle boundary layer. This assumption, justified by the extremely large intrinsic reactivity of Robinia char (about 3 orders of magnitude larger at 850 ⬚C than that of char from a bituminous coal [9]), has been checked by order of magnitude evaluation of different resistances (intraparticle and external diffusion, intrinsic kinetics) with reference to both fine and coarse char particles. The energy balance around a spherical burning char particle reads q
d dT c 12KcCO2 DH hc (Tbed T) 6 dt re (T4bed T4)
(1)
where Kc
DO2 Sh k Nu and hc g d d
(2)
are the mass and heat transfer coefficients. For fine char particles, the Sherwood number has been assumed to be equal to 2. For coarse char particles, Sh has been evaluated according to the Prins et al. [11] correlation. The Nusselt number for coarse char particles has been evaluated according to the Prins et al. [12] correlation for mobile particles in a fluidized bed. As regards the way heat transfer between fine particles and the fluidized bed takes place, three cases have been considered: 1. Fines burn in the interstices of the dense bed without colliding with bed particles. In this case the Nusselt number has been assumed to be equal to 2. 2. Fines collide with bed particles without adhering. Accordingly, fines experience cyclic behavior consisting of the following alternating stages: (a) the particle is freely dragged by the gas flow in the interstices of the emulsion phase; (b) the particle is in contact with a bed particle. It is assumed, for the sake of simplicity, that the durations of these phases are equal to each other. An estimate of the cycle time can be calculated as the ratio of the mean path between two bed particles—of the order of the bed particle diameter—and the interstitial gas velocity. For bed particles of about 1mm, the cycle time is about 0.005 s. During stage (a), Nu 2 has been assumed. During stage (b), the heat transfer coefficient has been calculated according to Schlu¨nder [13].
Fig. 2. Robinia biomass particles time-temperature history during fluidized-bed combustion. (a) T 700 ⬚C, xO2 0.02; (b) T 700 ⬚C, xO2 0.12.
3. Fines irreversibly adhere to the bed particle upon collision. Accordingly, after the first 0.005 s period, the relevant mechanism of heat transfer between the two particles is assumed to be conduction. Thermal conductivity of the partially molten fines has been evaluated from the literature [14]. Figure 2a and b report the time-temperature history of 0.1 mm char fines burning in a bed at 700 ⬚C. Two different in-bed oxygen concentrations were considered, namely, 2% (Fig. 2a) and 12% (Fig. 2b). In the same figure, the steady temperature of a typical coarse particle of 10 mm size burning at the same bed operating conditions is reported for comparison. Typical burnout times for biomass fine and coarse particles in these conditions are of the order of a few seconds or hundreds of seconds, respectively. Calculation results show that extremely high temperatures are reached by the fine particles in very short times independently of the mechanism assumed, especially at the larger oxygen concentration. Upon collision with a bed particle, temperatures at
FLUIDIZED BED COMBUSTION OF BIOMASS
Fig. 3. Average fines residence time in a fluidized-bed combustor as a function of fines carbon loading in the bed for three different fuels. Worked out from Refs. [10] and [18].
TABLE 2 Results of batch experiments R T 700 ⬚C 1% O2 T 700 ⬚C 3% O2 T 850 ⬚C 1% O2 T 850 ⬚C 3% O2
0.53 0.53 0.68 0.67
Burn out time (min) 39 17 30 15
the contact point might be well beyond typical melting temperatures of alkali-silica compounds (around 750 ⬚C [1,5]). Even at a bed temperature as low as 700 ⬚C and at in-bed oxygen concentrations as low as 2%, such conditions can be reached. On the other hand, coarse particles burn at temperatures only slightly higher than the bed temperature, in this case below relevant melting temperatures. The fact that, unlike coals, biomass fines burn hotter than coarse particles is consistent with previous experimental findings [15,16] and is mostly related to the large reactivity of biomass char [17]. Experimental Results and Discussion Figure 3 gives the average residence times of fines s Wc/Ec in the fluidized-bed combustor as a function of fines carbon loading in the bed. Data points were obtained by working out experimental data reported by Chirone et al. [10] and Massimilla et al. [18] for steady combustion of fine particles (about 0.1 mm feed particle size) of three different fuels: Robinia biomass, a Tyre-derived fuel, and a South African bituminous coal. It is noteworthy that fines’ average residence times in the bed are much longer for Robinia (20–25 min) than for the other two fuels
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(2–3 min), despite that the sizes and densities of the fine particles differed negligibly from one another. This finding provides indirect evidence that adhesion of char fines onto bed solids might be at work. Occurrence of adhesion is directly highlighted in the purposely designed experimental program of the present work. Table 2 reports the fraction, R, of ashes retained in the bed (that is, not elutriated) during batch combustion experiments of coarse Robinia particles. Temperatures of 700 ⬚C and 850 ⬚C and two oxygen concentrations (1% and 3% by volume) were considered. In the same table, the particle burnout times are reported. Results show that at 700 ⬚C and at 850 ⬚C, about 50% and 65%, respectively, of the total fuel-ash fed to the reactor was retained in the bed, regardless of oxygen concentration. The complement is represented by the fly-ashes cumulatively elutriated throughout burnoff. No coarse ash particles could be retrieved from the bed material at the end of the experiments; the ash retained in the bed was intimately associated with inert bed particles. Steady combustion experiments of Robinia coarse particles were carried out at four different temperatures ranging from 700 ⬚C to 850 ⬚C. At any bed temperature, visual observation indicated that, after devolatilization, Robinia particles appeared at the bed surface from time to time, spending most of their burnout time in the upper zone of the bed. The burning particle appeared to be surrounded by a brighter boundary layer. No adhesion of bed quartz particles on the burning char could be observed. Onset of bed agglomeration and defluidization, if any, could be clearly assessed by the sudden stop of the burning particle (it rested at a stable position) and by the lack of bed surface motion and bubble bursting. During the steady experiments carried out with 100 g total biomass feeding, agglomeration/defluidization was observed only at the two higher temperatures, namely 800 ⬚C and 850 ⬚C, regardless of oxygen concentration. No agglomeration was noticed in combustion experiments carried out at 700 ⬚C and 750 ⬚C. At the end of each run, the bed was completely discharged for further morphological characterization. At all temperatures, the discharged bed showed a slightly yellowish color, in contrast to the white fresh quartz. The bed weight after operation increased by about 1–2 g. This result is in line with ash retention data found in batch experiments (Table 2). Figure 4 compares SEM micrographs of bed quartz samples before utilization and after biomass combustion at 700, 750, and 850 ⬚C (samples at 800 ⬚C were similar to those at 850 ⬚C). Differences between the samples can be interpreted in the light of theoretical curves from the single-particle combustion model (case 3). Fresh quartz showed a very smooth surface with some sharp edges. The phenomenology can be summarized as follows:
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Fig. 4. SEM micrographs of bed quartz samples before utilization and after biomass combustion at 700 ⬚C, 750 ⬚C, and 850 ⬚C, and corresponding fines’ time-temperature histories calculated with the single-particle combustion model.
Fig. 5. Fixed carbon and ash material balances of fine char particles in a fluidized bed.
• Bed material resulting from experiments at 700 ⬚C has the same sharp and angular appearance as the fresh quartz, indicating that melting has not occurred. This situation corresponds to curve A in Fig. 4. The surface of the particle presents some roughness, not present in the original quartz, with small protuberances probably due to the accumulation of fine ash deposits. • Morphology of the bed sample from experiments carried out at 750 ⬚C is similar to the previous sample, but evidence of the localized formation of a melt is clearly observed on the surface. It is inferred that eutectic temperatures might have been
reached locally. This situation corresponds to curve B in Fig. 4. • The sample of bed material from experiments carried out at 850 ⬚C displays a completely different morphology: several particles are agglomerated by solid bridges of fused material. Sharp edges have been rounded off by extensive melting at the bed particle surface. This situation corresponds to curve C in Fig. 4. EDX analysis has been carried out on each sample in different zones of the particles. A strong enrichment of calcium and potassium and, to a lesser extent, of magnesium and phosphorus was observed
FLUIDIZED BED COMBUSTION OF BIOMASS
for all the samples. No major difference in the quantities of these elements on the surface of particles of different samples has been found. It is noteworthy that the molten zones presented a larger amount of potassium compared with calcium; other elements were present only in trace quantities. Typical proportions found are Si 60%, K 30%, Ca 10%. On the whole, theoretical results and experimental findings suggest the following mechanism of fines postcombustion and ash deposition onto bed particles. The mechanism is summarized by the networks of Fig. 5 that express the balance on fixed carbon and on mineral matter associated with fine particles in the bed. According to these networks, both fixed carbon and ash are present in fine char particles either freely moving in the interstices of the bed (free fines: Wf, Wf,ash) or attached onto the surface of inert bed solids (Wa, Wa,ash). During combustion of coarse biomass, extensive char attrition and fines generation take place, as discussed by Salatino et al. [7] and Scala et al. [8]. Upon detachment from the coarse char particles, the fines’ temperature quickly rises as a consequence of carbon postcombustion, possibly beyond the melting temperature of silica-potassium compounds. Attrited fines experience collisions with bed particles. Upon impact with bed particles, the formation of a softened or fluid phase at the contact point may be responsible for fines’ adhesion on the bed particles. In the event that the bed particle temperature is itself higher than the relevant eutectic temperature, fused potassium-silicate is permanently formed, yielding a sticky surface layer responsible for bed agglomeration and defluidization. If the bed particle temperature is, instead, below the eutectic melting temperature, fines adhesion can still take place, but the formation of a surface melt on the bed solids is prevented, and so is bed agglomeration. To complete the scenario, attrition of bed ash agglomerates possibly detaching adhered fines should be taken into consideration.
Conclusions The fate of ashes during the fluidized-bed combustion of a biomass fuel has been studied by a combination of theoretical computations and experimental results. The simulation of the time-temperature history of attrited fine char suggests that extremely high temperatures can be reached as a consequence of carbon postcombustion, strongly dependent on oxygen concentration or excess air. Fines’ peak temperatures are possibly higher than those at which softening and melting of silica-potassium compounds take place. Accordingly, generation of fines by attrition and combustion-induced fine particle overheating are responsible for the formation of ash-layered bed material even at bed temperatures below the
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potassium-silicates melting point. This occurrence has been confirmed by steady combustion experiments at different bed temperatures. The further fate of the ash-layered bed material depends on the bed temperature: beyond the ash-melting range, fused potassium-silicate is formed, eventually leading to bed agglomeration and defluidization; below the ash-melting range, formation of fused material is prevented, but the thickness of the ash layer can steadily increase due to continuous adhesion of fine material. It is concluded that attrition phenomena, whose consideration has been mostly restricted so far to the fate of fixed carbon and to its implications on combustion efficiency, play a key role in determining the ultimate fate of ash components in the fluidized-bed combustion of biomass fuel. Nomenclature c CO2 d DO2 Eash Ec Fash hc Kc kg Nu R Sh t T Tbed Wa Wa,ash Wc Wf Wf,ash xO2 DH e q r s
heat capacity of the char particle, kJ/kg K oxygen concentration, kmol/m3 char particle diameter, m oxygen molecular diffusivity, m2/s ash elutriation rate, kg/s fixed carbon elutriation rate, kg/s ash feed rate, kg/s convective heat transfer coefficient, kW/K m2 convective mass transfer coefficient, m/s gas thermal conductivity, kW/K m Nusselt number fraction of fed ashes retained in the bed Sherwood number time, s char particle temperature, K bed temperature, K carbon loading in attached fines, Kg ash loading in attached fines, Kg (Wf Wa), Kg carbon loading in free fines, Kg ash loading in free fines, Kg oxygen mole fraction heat of combustion of fixed carbon, kJ/kg effective emissivity char particle density, kg/m3 Stefan-Boltzmann constant, kW/m2 K4 (Wc/Ec), s Acknowledgments
The support of Mrs. C. Zucchini and Mr. S. Russo for SEM/EDX analysis is gratefully acknowledged. REFERENCES 1. Grubor, B. D., Oka, S. N., Ilic, M. S., Dakic, D. V., and Arsic, B. T., Proceedings of the Thirteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1995, pp 515–522.
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2. Natarajan, E., Ohman, M., Gabra, M., Nordin, A., Liliedahl, T., and Rao, A. N., Biomass Bioenergy 15:163– 169 (1998). 3. Skrifvars, B., Backman, R., Hupa, M., Sfiris, G., Abyhammar, T., and Lyngfelt, A., Fuel 77:65–70 (1998). 4. Latva-Somppi, J., Kurkela, J., Tapper, U., Kauppinen, E. I., Jokiniemi, J. K., and Johanson, B., Proceedings of the International Conference on Ash Behavior Control in Energy Conversion Systems, SCEJ, Japan, 1998, pp. 119–126. 5. Lin, W., and Dam-Johansen, K., Proceedings of the Fifteenth International Conference on Fluidized Bed Combustion, paper FBC99-0120, ASME, New York, 1999. 6. Chirone, R., Greco, G., Salatino, P., and Scala, F., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp. 145–150. 7. Salatino, P., Scala, F., and Chirone, R., Proc. Combust. Inst. 27:3103–3110 (1998). 8. Scala, F., Salatino, P., and Chirone, R., Energy Fuels 14:781–790 (2000). 9. Masi, S., Salatino, P., and Senneca, O., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp 135– 143.
10. Chirone, R., Russo, S., Serpi, M., Salatino, P., and Scala, F., Combust. Sci. Technol. 153:83–93 (2000). 11. Prins, W., Casteleijn, T. P., Draijer, W., and Van Swaaij, W. P. M., Chem. Eng. Sci. 40:481 (1985). 12. Prins, W., Draijer, W., and Van Swaaij, W. P. M., Heat and Mass Transfer in Fixed and Fluidized Beds, Hemisphere, Washington, 1986, pp. 317–331. 13. Schlu¨nder, E. U., Int. Chem. Eng. 20:550–553 (1980). 14. Inumaru, J., Ashizawa, M., Kurimura, M., Chikuma, H., and Ohtaka, M., Proceedings of the International Conference on Ash Behavior Control in Energy Conversion Systems, SCEJ, Japan, 1998, pp. 60–67. 15. Hernberg, R., Stenberg, J., and Zethraeus, B., Combust. Flame 95:191–205 (1993). 16. Joutsenoja, T., Heino, P., Hernberg, R., and Bonn, B., Combust. Flame 118:707–717 (1999). 17. Palchonok, G. I., Breitholtz, C., Thunman, H., and Leckner, B., Proceedings of the Fourteenth International Conference on Fluidized Bed Combustion, ASME, New York, 1997, pp. 871–878. 18. Massimilla, L., Chirone, R., D’Amore, M., and Salatino, P., Carbon Attrition during the Fluidized Combustion and Gasification of Coal, final technical report, DOE grant no. DE-FG22-81PC40796, Cuen, Napoli, 1985.
COMMENTS E. M. Bulewiez, Cracow University of Technology, Poland. 1. The biomass fuel particles (wood) were rather large, in comparison with the dimensions of the apparatus and the bed material particle size. Under these conditions, the fuel particles would tend to float on top of the bed until practically the end of the burning of the volatiles. The remaining charcoal would then undergo both fragmentation and attrition. Was this taken into account? 2. Were any model experiments attempted (e.g., reacting suitable potassium compounds with silica over extended times)? This could elucidate the problem, since the gas phase is most unlikely to play a part in agglomeration. It was satisfying to note that the authors also employed cyclical bed-temperature variation. We found this technique to be extremely useful in conducting small scale experiments [1].
REFERENCE 1. Pilowski M., Kandefon S., in Fourth International Conference on Combustion Technologies for a Clean Environment, Lisbon, Portugal, 1995, pp. 29, 4, 22–28. Author’s Reply. Yes. Segregation of biomass particles was observed only during devolatilization. In this stage, attrition was negligible. Devolatilized (char) particles appeared instead to be thoroughly mixed with bed solids. The interaction between attrited fines and inert bed particles pos-
tulated in the colliding fines model is likely to take place during char burn-off. No such model experiment was attempted. The authors believe that the peculiarity of mineral matter being closely associated with highly reactive carbon (as happens within attrited char fines) cannot easily be reproduced in model experiments. This feature is the key for the large overheating of mineral matter with respect to bed inert solids. ● Joseph J. Helble, University of Connecticut, USA. Is attachment treated as being irreversible at low temperatures? Did you consider attachment-detachment, which would retard the motion of the fires with the bed? Author’s Reply. In the long term, fine attachment by adhesion and detachment by attrition play a combined role and are responsible for the much longer fines residence times observed when feeding biomass with respect to other fuels characterized by lower propensity to adhesion. Further discussion on this point can be found in Ref. [1], where results of experiments consisting of continuously feeding pulverized fuels to a fluidized-bed combustor are presented and discussed. REFERENCE 1. Chirone, R., Russo, S., Serpi, M., Salatino, P., and Scala, F., Combust. Sci. Technol. 153:83 (2000).