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Effect of Moisture Content on Devolatilization Times of Pine Wood Particles in a Fluidized Bed L. F. de Diego, F. Garcı´a-Labiano, A. Abad, P. Gaya´n, and J. Ada´nez* Instituto de Carboquı´mica (CSIC), Department of Energy and Environment, Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain Received July 3, 2002
This work analyzes the effect of moisture content on devolatilization times of pine wood particles in a fluidized bed combustor. The devolatilization process was followed by measuring the CO2 and O2 concentrations obtained after the complete combustion of the volatiles. The devolatilization rate decreased and was more uniform along the devolatilization time as the moisture content of the wood particles increased. The devolatilization times increased almost linearly with moisture, and the slope slightly increased when the bed temperature decreased. The devolatilization times were correlated by a power-law relationship, which related the devolatilization time to the fuel particle diameter and shape factor [tv ) a(dp,eqφ)n]. The values of exponent n were between 1.5 and 1.7 and were almost unaffected by the bed temperature or the moisture content. The values of the constant a decreased with increasing the bed temperature and with decreasing the moisture content of the wood particles. To predict the devolatilization times of wood particles as a function of their moisture content, a modification of the power-law relationship is proposed.
Introduction Wood and other lignocellulosic materials are becoming interesting for energy production because they are renewable fuels that reduce CO2 and sulfur emissions. The thermal decomposition of lignocellulosic materials has been widely studied,1-9 and several works5-9 have shown the influence of operating conditions on the weight loss and product formation. Gasification, methanol production, and direct combustion are three possible routes by which energy from lignocellulosic materials can be extracted. This paper deals with direct combustion. Fluidized bed combustion (FBC) is a suitable technology for this purpose. In recent years, there has been an increasing research emphasis on the application of fluidized bed boilers to the combustion of coal, biomass, organic waste and mixtures of them. However, the designs of contemporary fluidized bed boilers for biomass and organic waste combustion are mainly based * To whom correspondence should be addressed. Phone: (34) 976733977. Fax: (34) 976733318. E-mail:
[email protected]. (1) Palchonok, G. I.; Dikalenko, V. A.; Stanchits, L. K.; Borodulya, V. A.; Werther, J.; Leckner, B. Proceedings of the 14th International Conference on Fluidized Bed Combustion; ASME: Fairfield, NJ, 1997; pp 125-133. (2) Chan, W. R.; Kelbon, M.; Krieger, B. B. Fuel 1985, 64, 15051513. (3) Di Blasi, C. Ind. Eng. Chem. Res. 1996, 35, 37-46. (4) Grønli, M. G.; Melaaen, M. C. Energy Fuels 2000, 14, 791-800. (5) Chan, W. R.; Kelbon, M.; Krieger-Brockett, B. Ind. Eng. Chem. Res. 1988, 27, 2261-2275. (6) Bilbao, R.; Millera, A.; Murillo, M. B. Ind. Eng. Chem. Res. 1993, 32, 1811-1817. (7) Bilbao, R.; Arauzo, J.; Salvador, M. L. Ind. Eng. Chem. Res. 1995, 34, 786-793. (8) Miller, R. S.; Bellan, J. Combust. Sci. Technol. 1996, 119, 331373. (9) Di Blasi, C.; Gonzalez-Hernandez, E.; Santoro, A. Ind. Eng. Chem. Res. 2000, 39, 873-882.
on experience from coal combustion1 because the complex mechanism of combustion of these solids in fluidized beds is insufficiently known. When biomass particles are introduced into a fluidized bed combustor (750-950 °C) undergo different processes: drying, devolatilization and finally combustion of the resultant char. At slow heating rates, moisture evaporation takes place, almost completely, before biomass pyrolysis; however, at high heating rates, such as those present in fluidized beds, moisture evaporation and biomass pyrolysis take place simultaneously, though at different positions along the particle radius.1,9 Devolatilization and volatile combustion are the main steps in the combustion of biomass. The volatile fraction of biomass particles contributes a significant proportion to the total amount of heat released during combustion. In some cases, moisture contents as high as 50% can be found; as a consequence, the drying of the particles is very important in the global process of devolatilization. The distribution of volatiles throughout the bed will depend on the balance between the rate of devolatilization and the rate of dispersion of fuel particles within the fluidized bed. This balance determines the locations of fuel volatiles release to the bed for subsequent mixing with oxygen and combustion. There are many studies on the volatiles burn-out time under FBC conditions analyzing the effect of reacting atmosphere,10-13 bed temperature,11-16 fluidizing velocity,15 fuel particle (10) Salam, T. F.; Shen, X. L.; Gibbs, B. M. Fuel 1988, 67, 414419. (11) Vural, H. Proceedings of the 11th International Conference on Fluidized Bed Combustion; ASME: Fairfield, NJ, 1991; pp 1145-1150. (12) Jung, K.; Stanmore, B. R. Fuel 1980, 59, 74-80. (13) Ross, D. P.; Heidenreich, C. A.; Zhang, D. K. Fuel 2000, 79, 873-883.
10.1021/ef0201477 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/31/2002
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Figure 2. Typical O2 concentration profile obtained during devolatilization tests. (s) Measured. (- -) Corrected.
Figure 1. Experimental setup used for devolatilization tests. (a) visual, (b) continuous gas analysis.
size,13-16 and type of coal.13-14,16-17 However, the review of Agarwal and La Nauze18 shows that the effect of the moisture on the devolatilization of large fuel particles has received little attention. Major research efforts have centered on the devolatilization and combustion of predried solid fuels and limited research has been reported on the coupled drying and devolatilization of high-moist fuels. In the combustion of wet solid fuels some delay in the solid pyrolysis have been observed.5,9,13-15,19 So, combustion of wet biomass may prevent an abrupt release of volatiles near the feeding point, thus enhancing the uniformity of the distribution of combustible volatiles over the cross-sectional area of fluidized beds; however, it is also widely recognized that moisture largely affects the reactor efficiency and product quality.20 The goal of the present work was to study the effect of moisture content on the devolatilization times of wood particles in a fluidized bed combustor. Experimental Section The devolatilization of wood particles was studied in a fluidized bed reactor of 50 mm i.d. and 0.5 m height, with a perforated steel plate distributor, as shown in Figure 1. The fluidizing gas (air) was preheated in a ceramic fixed bed located below the distributor plate. The entire system was inside an electrically heated furnace. The air flow rate and the bed temperature were measured with a mass flow controller and a thermocouple, respectively. The fluidized bed was composed of 300 g of silica sand with a particle size of 0.50-0.63 mm. The superficial gas velocity inside the fluidized bed was kept (14) Urkan, M. K.; Arikol, M. Fuel 1994, 73, 768-772. (15) Prins, W.; Siemons, R.; van Swaaij, W. P. M.; Radovanovic, M. Combust. Flame 1989, 75, 57-79. (16) Pillai, K. K. J. Inst. Energy 1981, 54, 142-150. (17) Stubington, J. F.; Chui, T. Y. S.; Saisithidej, S. Fuel Sci. Technol. Int. 1992, 10(3), 397-419. (18) Agarwal, P. K.; La Nauze, R. D. Chem. Eng. Res. Des. 1989, 67, 457-480. (19) Ouedraogo, A.; Mulligan, J. C.; Cleland, J. G. Combust. Flame 1998, 114, 1-12. (20) Bryden K. M.; Ragland, K. W.; Rutland, C. J. Biomass Bioenergy 2002, 22, 41-53.
constant at 40 cm/s in all experiments, and four temperatures between 650 and 950 °C were used. The experiments were carried out with pine wood particles with well-defined size and shape. The dimensions of the particle sizes used were 15 × 15 × 15, 20 × 9 × 20, 10 × 16 × 15, 10 × 10 × 10 and 20 × 4 × 20 mm3. Elemental analysis showed that the pine (Pinus Sylvestris) wood contained 52.9% C, 6.8% H, and 0.1% N with respect to dry mass. The moisture and ash contents of ambient-dried particles were 8.3 and 1.3 wt % respectively. To obtain particles with different moisture contents, up to 50 wt %, water was added to the original samples (moisture 8.3 wt %) and each wood particle was kept for a week in a sealed vessel to allow its stabilization. The moisture content was determined by weighing the wood particle before water addition and after stabilization. Some original samples were also dried in an oven at 108 °C to obtain particles without moisture (moisture content ) 0%). The devolatilization process was analyzed by two methods: (1) visual observation of volatiles flame, and (2) continuous gas analysis of O2 and CO2. For the visual technique (Figure 1a), the devolatilization experiments were recorded on video through a mirror located over the reactor. The second technique consisted of measuring the combustible volatiles evolution as a function of the time by means of gas analyzers. For these experiments (Figure 1b), the fluidized bed was sealed with a lid and the individual particles were dropped into the bed through a ball valve connected vertically to the lid. The combustible volatiles evolution was followed by means of the continuous measurement of the CO2 and O2 concentrations obtained after the complete combustion of the volatiles. A nondispersive infrared analyzer for CO2 (and CO) and a paramagnetic analyzer for O2 were used. The gas concentrations were logged as a function of time via a PC-based datalogging system. If necessary, the volatiles were burned with air addition in a fixed bed reactor (900 °C) placed downstream from the fluidized bed. In this way, the CO concentration was negligible in all of the experiments. This technique allowed us to determine both the total devolatilization times and the evolution of the devolatilization rate with time. For better data analysis, the CO2 and O2 concentrations versus time profiles were corrected for gas flow and dispersion in the sampling line and analyzers. The system dispersion was determined, in previous experiments, by adding CO2 pulses to the bed and analyzing the results. Figure 2 shows the measured and corrected evolution of the O2 concentration for a typical devolatilization test. The end of the devolatilization was considered as the time when the CO2 or O2 concentration reached the first local minimum. After this time, the CO2 and O2 concentrations remained almost constant for a long time
Devolatilization Times of Pine Wood Particles
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while the remaining char burnt. Figure 2 also illustrates the point for the end of devolatilization, which was defined as the devolatilization time tv. For each experiment, one particle was dropped into the hot fluidized bed. For each experimental condition, three to five separate experiments were performed. The reproducibility of these measurements was around 6%.
Results and Discussion In preliminary experiments with the top cover removed, it was visually observed that when a wood particle was introduced into the hot (>650 °C) fluidized bed of inert particles, first the particle heats to some temperature lower than the bed temperature, visually indicated by the fact that the particle looks black against the glowing bed surface. Later, a luminous flame surrounds the particle. Depending on the original moisture content and size of the particle and the bed temperature, the characteristic luminous flame surrounded the wood particle almost immediately or a few seconds after their introduction into the fluidized bed combustor. During volatiles combustion, the wood particle, which was still clearly cooler than the surrounding bed (the particle remains black relative to the bed surface), was observed to remain floating on the bed surface and only occasionally submerged completely in the bed. The extinction of the flame of volatiles was accompanied by a rapid increase in the temperature of the particle, which turned yellow and clearly much brighter than the surrounding bed particles. This temperature increase was first observed along the edges of the particle and progressively extended, during char combustion, to whole surface of the particle. This general behavior was in agreement with observations of previous investigators working with coals14-16 and balsa wood.21 During char combustion, there was a gradual reduction in particle size and in most of the tests the particle fractured to form smaller particles, which burnt decreasing its size and sometime themselves broke up further. However, during devolatilization, the original shape of the wood particles was essentially preserved, and no fragmentation was observed for any of the wood particles in the different tests. In devolatilization tests carried out with low-moist content (oven-dried or ambient-dried) wood particles, a big, tall, bright luminous flame was observed, and most of the volatiles of the large wood particles were burnt in the freeboard region of the fluidized bed and not around the wood particle. Only at the end of the devolatilization process was the flame observed to be formed around the particle, which was floating on the bed surface. With increasing moisture content, the intensity of the volatile flame decreased, and at 50 wt % of moisture content a weak volatile flame was observed, which extinguished after a long time. Figure 3 shows the effect of the moisture content on the variation of devolatilization rates with time at two different temperatures. As the moisture content of the wood particles increased, more water must be evaporated and therefore the devolatilization rate of combustible volatiles decreased and was more uniform along (21) Saito, M.; Amagai, K.; Ogiwara, G.; Arai, M. Fuel 2001, 80, 1201-1209.
Figure 3. Effect of particle moisture content on the devolatilization rate.
the devolatilization time, which increased, confirming the visual observations. It can be also observed that, for the same moisture content, the amount of combustible volatiles released in the first seconds of the reaction increased with increasing temperature and the devolatilization time increased slightly with decreasing temperature. Experimental studies on the devolatilization of fuel particles in combustion systems suggest that the devolatilization times can be correlated by a power-law relationship, which relates the devolatilization time tv to the fuel particle diameter dp.
tv ) adpn
(1)
However, there are large discrepancies in the numerical values of the exponent n among different researchers and/or different fuels. Pillai16 in a broad study of 12 carbonaceous fuels, found n to vary with bed temperature and coal type, over the range 0.32 < n
