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Energy & Fuels 2004, 18, 1108-1117
On the Relevance of Axial and Transversal Fuel Segregation during the FB Combustion of a Biomass Riccardo Chirone, Francesco Miccio, and Fabrizio Scala* Istituto di Ricerche sulla Combustione - CNR, P. le V. Tecchio 80, 80125 Napoli, Italy Received November 7, 2003. Revised Manuscript Received April 28, 2004
The fluidized bed combustion of a typical Mediterranean biomass (pine-seed shells) was investigated in a pilot-scale bubbling FB combustor (200 kWth) at different operating conditions. Both over- and under-bed fuel feeding options were considered. The focus of the experiments was to study the extent of volatile matter mixing/segregation in the bed and the subsequent postcombustion in the splashing zone and freeboard. To this end, temperature profiles along the combustor axis as well as transversal gas concentration profiles above the bed surface were measured. Experiments highlighted that complete conversion of the fuel within the combustor was obtained, even if a significant volatiles postcombustion above the bed occurred. This results in an appreciable overheating of the freeboard. Both the fuel feeding option and the excess air affect the extent of volatiles postcombustion. Gas concentration profiles were not uniform along the combustor diameter because of the single-point fuel feeding and the incomplete fuel mixing in the bed. The measured carbon-containing gaseous species profiles along the bed diameter were used to estimate the fuel lateral dispersion coefficient in the fluidized bed, by means of a simplified dispersion/pyrolysis model. Comparison of experimental profiles with model results at over- and under-bed feeding conditions allowed us to evaluate both the fuel lateral dispersion coefficient inside the bed and on the bed surface, which were determined to be of the order of 0.01 m2/s and 0.1 m2/s, respectively.
Introduction The exploitation of alternative energy resources consisting of nonfossil solid fuels such as biomass and wastes has recently highlighted the key role played by the evolution and fate of the volatile matter during the combustion process. In fact, these fuels are usually characterized by a large volatile matter content, whose associated heat of combustion may account for more than 70% of the fuel heating value. As a consequence, location and efficiency of volatiles combustion exerts a large influence on the combustion behavior and heat distribution in the boiler as well as on pollutants formation. Fluidized bed combustion (FBC) technology has been indicated to be one of the best options to burn this class of fuels with high efficiency and low environmental impact.1-3 However, operational experience of stationary FBC of high-volatile fuels has shown some problems, connected mainly to the location of the volatiles combustion. In fact, whereas char combustion takes place smoothly and efficiently in the bed thanks to its high reactivity and the good mixing within the dense phase, the combustion of volatile matters poses serious problems in order to obtain uniform and well-distributed burning profiles. * Corresponding author: Ph.: +39 081 7682969. Fax: +39 081 5936936. E-mail:
[email protected]. (1) La Nauze, R. D. J. Inst. Energy 1987, 60, 66-76. (2) Saxena, S. C.; Jotshi, C. K. Prog. Energy Combust. Sci. 1994, 20, 281-324. (3) Anthony, E. J. Prog. Energy Combust. Sci. 1995, 21, 239-268.
Poor axial uniformity is commonly experienced during FBC of biomass and refuse-derived fuels.4-8 Experimental evidences have shown that once injected in the boiler the fuel particles rapidly tend to segregate at the bed surface during the devolatilization stage.9,10 As a consequence a significant amount of volatile matter bypasses the bed and burns directly in the freeboard, resulting in a considerable temperature increase in this region. This issue is strictly connected to the extension and location of heat exchange surfaces, to the pathways to pollutants formation, and to the reliability and safety of the combustor operation. The way by which fuel feeding is accomplished is crucial in this concern. An improper design of the feeding line is a major source of local overheating and consequent difficulties in keeping the temperature uniform. Under-bed fuel feeding was considered to be more effective than over-bed feeding for coal-fired bubbling FB boilers, because of an improved bed retention of fines present in the fuel feedstock. This option, (4) Peel, R. B.; Santos, F. J. Inst. Energy Symp. Ser. 1980, 4, IIB2-1. (5) Achara, N.; Horsley, M. E.; Purvis, M. R. I.; Teague, R. H. Proc. 3rd Int. Fluidiz. Conf.; The Institute of Energy: London, 1984, DISC/ 25/215. (6) Gulyurtlu, I.; Cabrita, I. Proc. 3rd Int. Fluidiz. Conf.; The Institute of Energy: London, 1984, DISC/11/80. (7) Andersson, B. A.; Leckner, B.; Amand, L. E. Proc. 8th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1985; pp 1019-1029. (8) Irusta, R.; Antolin, G.; Velasco, E.; De Miguel, R. Fluidization VIII; Engineering Foundation: New York, 1995; pp 855-862. (9) Yates, J. G.; MacGillivray, M.; Cheesman, D. J. Chem. Eng. Sci. 1980, 35, 2360-2361. (10) Fiorentino, M.; Marzocchella, A.; Salatino, P. Chem. Eng. Sci. 1997, 52, 1909-1922.
10.1021/ef034084j CCC: $27.50 © 2004 American Chemical Society Published on Web 06/05/2004
Fuel Segregation during FB Combustion of a Biomass
however, becomes questionable when high-volatile fuels are used in FB combustors. In addition, radial segregation of volatile matter is usually associated with a single or limited number of fuel feeding points in large-scale FB boilers.11-13 It leads to poor transversal mixing between fuel and air, which in turn favors the establishment of radial profiles of concentration and delays fuel conversion. Both the location and number of the fuel entry ports as well as the bed fluid-dynamics influence the extent of the transversal segregation.14,15 A major contribution to the study of radial segregation of volatile matter in FBC was provided by the work of Stubington and co-workers,16-19 that led to the formulation of the so-called “multiple discrete diffusion flame model” for FBC of volatiles.20,21 This mechanistic model of coal particle devolatilization and motion in a fluidized bed assumes that the volatiles are released during different stages (lift-up, residence at surface, and recirculation of the fuel particle) and predicts that a large fraction of volatiles may be directly released in the freeboard. More recently, on the base of the theoretical analysis by Fiorentino et al.,22 a model of a bubbling FBC operated with high-volatile solid fuel feedings was presented,23 accounting for volatile matter segregation and postcombustion above the bed. One way to overcome transversal maldistribution of combustibles in a fluidized bed is to increase the number of fuel feeding points.13,24 To design the optimal number (and position) of the feeding points it is necessary to know the lateral mixing rate of the fuel in the bed at the selected operating conditions. This is not, however, a simple task because this mixing rate is difficult to determine experimentally in large-scale units. On the other hand, available experimental data and predictive correlations for the mixing rate (usually expressed as a lateral dispersion coefficient) are typically derived from lab-scale experiments, and the possibility to scale-up these values to large-scale units is questionable.13 Further, most of these experiments are carried out in conditions much different from those relevant to the FBC of a biomass. It has been emphasized, for example, that mixing/segregation patterns relevant to devolatilizing biomass particles are much different from those of inert particles of the same size fluidized under (11) de Kok, J. J.; Nieuwesteeg, M. W. C. M. A.; van Swaaij, W. P. M. Proc. 8th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1985; pp 105-114. (12) Ljungstro¨m, E. B. Proc. 8th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1985; pp 853-864. (13) Niklasson, F.; Thunman, H.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2002, 41, 4663-4673. (14) Stephens, E. A. Proc. 8th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1985; pp 1248-1257. (15) Schu¨tte, K.; Wittler, W.; Rotzoll, G.; Schu¨gerl, K. Fuel 1989, 68, 1499-1502. (16) Stubington, J. F. J. Inst. Energy 1980, 53, 191-195. (17) Stubington, J. F.; Davidson, J. F. AIChE J. 1981, 27, 59-65. (18) Stubington, J. F.; Chan, S. W. Fuel 1990, 69, 678-683. (19) Stubington, J. F.; Clough, S. J. Proc. 14th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1997; pp 1111-1122. (20) Stubington, J. F.; Chan, S. W.; Clough, S. J. AIChE J. 1990, 36, 75-85. (21) Stubington, J. F.; Chan, S. W. Proc. 12th Int. Conf. Fluidized Bed Combust.; ASME: New York, 1993; pp 167-173. (22) Fiorentino, M.; Marzocchella, A.; Salatino, P. Chem. Eng. Sci. 1997, 52, 1893-1908. (23) Scala, F.; Salatino, P. Chem. Eng. Sci. 2002, 57, 1175-1196. (24) Highley, J.; Merrick, D. AIChE Symp. Ser. 1971, 67 (116), 219227.
Energy & Fuels, Vol. 18, No. 4, 2004 1109
comparable operating conditions.10,22,23,25 Dispersion of the devolatilizing particles inside the bed appears to be influenced by the formation of “endogenous” volatile bubbles around the particles themselves, giving rise to forces that would be absent when considering inert particles.22,26 In this respect, it was shown that in the case of high-volatile fuels, devolatilizing particles are lifted up at the bed surface much more rapidly than would be predicted for an inert particle of the same size and density.10,25 This stresses the importance of evaluating the mixing rate of fuel particles under the same conditions that would be encountered in a full-scale plant. Although several studies on segregation phenomena associated with volatile matter release in bubbling fluidized beds are available in the literature for fossil and biomass fuels, a lack of information still persists concerning the distribution and dispersion of chemical species along the cross-section, both inside the bed and in the splashing region, when changing the operating conditions. The aim of the present work is to provide some insight on this issue. An experimental activity has been carried out using a pilot-scale bubbling fluidized bed facility fed with a biomass fuel. Results in terms of temperature profiles, heat distribution, and transversal gas concentration are presented in the paper. A theoretical assessments on the role played by dispersion mechanisms is also proposed and discussed, on the basis of a simplified dispersion/pyrolysis model. This theoretical analysis is directed to evaluate the fuel lateral dispersion coefficient in the bed from the measured gas radial concentration profiles. This parameter has a crucial importance to correctly design a new fuel feeding system or check the validity of an existing one, on the basis of criteria available in the literature.13,24 Experimental Section Pilot Scale Apparatus. A bubbling fluidized bed combustor (FBC-370) has been used to carry out steady-state combustion tests under atmospheric conditions. The pilot-scale 200 kW facility is sketched in Figure 1. The AISI 310 stainless steel fluidization column has a circular section (370 mm ID) and a total height of 4.65 m. The lower section of the column contains the plenum chamber and the pipe-type distributor equipped with 55 vertical tuyeres. The distributor sustains bed material allocated in the intermediate section, which is also equipped with several access ports. The heat exchange is accomplished thanks to an array of horizontal bayonet-type tubes whose adjustable penetration into the bed controls the heat removal rate. The higher section of the fluidization column (freeboard) provides disengagement of elutriated solids and is also fitted with several ports for temperature, pressure, and gas concentration probes. Two cyclones, the first with medium and the second with high efficiency, are used for flue gas de-dusting. The entire vessel is thermally insulated by a ceramic wool blanket in order to ensure a safe temperature at the external surface. FBC-370 is equipped with a continuous over-bed/under-bed feeding system. Fuel particles are dosed by means of a screwtype metering device. They are passed in sequence to a second rotating screw, which allows the entrance of particles into the bed at an elevation z ) 125 mm, for the U-bed feeding option. (25) Bruni, G.; Solimene, R.; Marzocchella, A.; Salatino, P.; Yates, J. G.; Lettieri, P.; Fiorentino, M. Powder Technol. 2002, 128, 11-21. (26) Solimene, R.; Marzocchella, A.; Salatino, P. Powder Technol. 2003, 133, 79-90.
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Chirone et al.
Figure 1. Scheme of the experimental facility (FBC-370). Otherwise, particles fall down directly on the bed surface at z ) 941 mm after the passage throughout an inclined tube (O-bed option). A horizontal water-cooled probe is installed at an elevation z ) 1024 mm from the distributor and allows gas suction for chemical analysis at different positions along the diameter. The probe is equipped with a stainless steel sintered tip to prevent capture of fine particulate. Materials. Silica sand (725 µm nominal size) has been used as inert material during the combustion tests. The bed holdup was set at 85 kg, corresponding to a static bed height of 0.5 m. A biomass fuel, shells of pine-seeds (Pinus pinea), was used for the experimental campaign. The fuel particles have an irregular and drop-like shape (approximately 30 × 10 mm in size), good mechanical resistance, and anisotropic properties. Table 1 summarizes the characteristic properties of the fuel. The biomass can be considered as a typical high-volatile and low-ash fuel. Procedures. The combustor start-up is accomplished thanks to a propane premixed burner that directly discharges hot gases inside the bed. After the bed reaches a temperature high enough to ignite fuel particles (e.g., 600 °C), the feeding of fuel is started and propane is switched off. Adjustments of operating variables are required in order to achieve the desired steady-state condition, which is maintained for at least 40 min. Temperatures, pressures, and flue gas concentrations (O2, CO2, CO, CH4, N2O, and NO) in various points of the fluidization column are on-line monitored and recorded using a data acquisition system supervised by a personal computer. Gas concentrations are also measured through the horizontal probe at different radial positions. The flow rate of cooling water as
Table 1. Properties of Pine-seed Shells particle size, mm density, kg m-3 LHV (dry basis), kJ kg-1
10 ÷ 30 1200 16300
proximate analysis volatiles, % fixed carbon, % ash, % moisture, %
59.6 26.6 0.8 13.0
ultimate analysis (dry basis) carbon, % hydrogen, % nitrogen, % sulfur, % oxygen, %
48.5 6.1 0.2 t* fuel particles dispersion is modeled exactly as for the over-bed feeding case (eqs 1-3) with the only difference that the initial condition for the dispersion equation is that calculated using eq 6 at time t*, that is C(x,t*).
Figure 10. Comparison between experimental and calculated normalized transversal concentration profiles of carboncontaining volatiles for U-bed fuel feeding (Tbed ) 850 °C, U ) 0.97, e ) 1.25).
When a continuous feed of fuel is considered, the steady volatiles concentration profile is calculated similarly to the over-bed case (eq 4) as
CV(x) ) F/Q(E h (x,t*) +
∫t*∞ E(x,t) dt)
(8)
In eq 8 the first term between the brackets (given by eq 7) accounts for the contribution to volatiles due to the fuel particles rising phase (0 < t < t*), while the second term accounts for the contribution due to the surface dispersion phase (t* < t < ∞). The value of the fuel particles rise velocity (vZ) at the operating conditions considered, was estimated from experimental data of Fiorentino et al.,10 using the procedure described by Scala and Salatino.23 This value, divided by the expanded bed height relevant to the pilotscale experiments, provided a fuel particles rise time t* ≈ 1 s at the actual operating conditions. For the surface dispersion mechanism (that is for t > t*) all the calculations were carried out with a value DS ) 0.1 m2/ s. This leaves DR as the only unknown model parameter. Also for the under-bed case, comparison between experimental and model data was carried out by normalizing both the sets of data to give an average concentration of unity along the bed diameter. Figure 10 reports the comparison between the normalized under-bed experimental data from Figure 8 and the normalized total volatiles concentration profiles calculated for different values of the lateral dispersion coefficient DR. Inspection of the figure indicates that the DR value that best represents the measured under-bed concentration profile (at U ) 0.97 m/s) is lower than that found for DS, and in particular slightly larger than 0.01 m2/s. Again, this value should be considered only as an order of magnitude evaluation. It is interesting to note, however, that in accordance to the above result, several authors have reported that lateral dispersion coefficients within the bed are lower by about 1 order of magnitude than those measured at the bed surface.13,30 (30) Leckner, B. Prog. Energy Combust. Sci. 1998, 24, 31-61.
Fuel Segregation during FB Combustion of a Biomass
Conclusions A typical Mediterranean biomass (pine-seed shells) was burned in a pilot-scale bubbling FB combustor (200 kWth) at different operating conditions and using both over- and under-bed fuel feeding options. Temperature profiles along the combustor axis and transversal gas concentration profiles above the bed surface were measured to study the extent of volatile matter mixing/ segregation phenomena in the bed and the subsequent postcombustion in the splashing zone and freeboard. The freeboard temperature was always higher than the bed one as a consequence of volatile matter postcombustion, with a measured maximum temperature increase of 125 °C. The freeboard over-temperature was enhanced by adopting over-bed feeding and by lowering the excess air. Thermal measurements indicated that about 80-90% of the energy was released/recirculated in the bed, the remaining being released in the freeboard and leading to the overheating of this section. Analysis of transversal profiles of gaseous species in the splashing region showed that the single-point fuel injection determined a significant maldistribution of volatiles along the bed diameter, that in turn was responsible for the incomplete mixing of fuel with oxygen in the bed. This fuel segregation was enhanced by adopting under-bed feeding as a consequence of a less uniform spreading of the fuel particles along the combustor section. The use of over-bed feeding, however, appeared to bring about a lower fraction of volatiles burned in the bed, consistently with the temperature profiles results. The decrease in the excess air factor emphasized the loss of transversal uniformity, while a change of the fluidization velocity or the bed temperature did not alter significantly the shape of concentration profiles. The fuel lateral dispersion coefficient in the fluidized bed was estimated by comparing the experimental transversal profiles of the gaseous species with the calculations of a simplified dispersion/pyrolysis model. The comparison was carried out both at over- and underbed feeding conditions, and allowed the separate evalu-
Energy & Fuels, Vol. 18, No. 4, 2004 1117
ation of the lateral dispersion coefficients inside the bed and on the bed surface. Lateral dispersion coefficients of the order of 0.01 m2/s inside the bed and 0.1 m2/s on the bed surface were estimated. An estimate of the lateral dispersion coefficient under real FBC operating conditions is essential to the application of existing tools for the design of the fuel feeding system. The dispersion coefficient values reported in this work could be used as a first estimate when experimental gaseous radial profiles are not available in a real fluidized bed boiler. Acknowledgment. The authors gratefully acknowledge Ms. A. Silvestre, Mr. A. Cante, and Mr. L. Ferrante for their support during the experimental campaign. Glossary C CV Dbed DR DS e E E h F Hbed Q t t* Tbed Tmax U Umf vZ W x z ∆Tmax
normalized concentration of the fuel particles, 1/m steady volatiles concentration, kgvol/m3 bed inner diameter, m lateral dispersion coefficient within the bed, m2/s lateral dispersion coefficient at the bed surface, m2/s excess air factor, dimensionless specific volatiles release rate per unit mass of fuel, kgvol/kgfuel m s specific volatiles release per unit mass of fuel, kgvol/ kgfuel m fuel feed rate, kgfuel/s expanded bed height, m total gas flow rate along one diameter, m2/s time, s fuel particles rise time, s bed temperature, °C maximum value of temperature in the freeboard, °C total fluidization velocity, m/s minimum velocity for fluidization, m/s fuel particles rising velocity, m/s normalized volatiles yield rate, kgvol/kgfuel s lateral coordinate, m axial coordinate, m maximum temperature increase in the freeboard, °C EF034084J
