Energy Fuels 2010, 24, 3890–3897 Published on Web 06/25/2010
: DOI:10.1021/ef100422y
Study of a Fixed-Bed Biomass Combustor: Influential Parameters on Ignition Front Propagation Using Parametric Analysis J. Porteiro,* D. Pati~ no, J. Moran, and E. Granada Escuela T ecnica Superior de Ingenieros Industriales, Universidad de Vigo, Lagoas-Marcosende s/n 36200, Vigo, Pontevedra, Spain Received April 5, 2010. Revised Manuscript Received June 11, 2010
This paper presents a study of the parameters influencing the combustion behavior of an experimental counter-current fixed-bed combustor. An innovative parametric analysis has been conducted to examine a wide range of parameters from very different biomasses. This analysis based on the parametrization of front propagation velocity versus the airflow rate curve not only allows fuel properties to be compared in terms of maximum or minimum values of ignition or air flux but also enables the shape of their performance curve to be examined. The study of the data has identified the moisture content and particle size as the most influential parameters. These factors strongly determine the operational range of the fuel in terms of air mass flux and ignition front propagation velocity. Furthermore, a new parameter called char stoichiometry is defined to examine fuel combustion regimes. This factor represents the stoichiometric ratio of the air and char fraction (assuming the fuel is pure carbon). The results obtained show that the transition from an oxygen-limited regime to a reaction-limited phase takes place when the air mass flow rate reaches char stoichiometry values. This may suggest that the first phase of combustion is determined mainly by the behavior of the char matter in the fuel. On the other hand, certain parameters, such as porosity of the bed or particle shape, did not show identifiable effects in our tests, suggesting that their influence in the process seems to be less decisive than that of the other parameters previously mentioned.
fields. The main parameters influencing the process are identified in the references and can be gathered in a simple three-group classification, as shown in Figure 1. In the light of this arrangement, some studies have chiefly analyzed operating conditions,3-6 while others have focused on fuel influence.7-10 However, because of the high number of influential variables, a further study of certain parameters is still required. The contribution of the total air supplied has already been investigated. Fatehi and Kaviany3 suggested two main stages in combustion depending upon the availability of oxygen. At a low airflow rate, in sub-stoichiometric conditions, the process is limited to surface reactions and velocity is controlled by the amount of oxygen supplied to the bed. At higher gas flow rates, the combustion regime is limited by the overall fuel reaction rate. Shin and Choi4 have subsequently extended the classification, noting three different stages. The oxygen-limited and fuel-reaction-limited phases are similar to those reported by Fatehi and Kaviany, but a final stage of extinction by convection is included. Although other papers5 have measured incomplete oxygen consumption in the flue gases at low airflow rates, the classification made by Shin and Choi is probably the most widely used.6,8,13 Other operating conditions have been analyzed, such as air preheating, secondary airflow rate, and turbulence generation. Wiinikka et al.14,15 tested the influence of primary and secondary air in a continuous wood pellet burner. Swirling flow has also been shown
1. Introduction Several assays are currently being conducted in experimentalpilot combustors;1-10 their accurate in-bed data, easy handling, and high operability have helped to further develop this kind of system. The conclusions inferred from these studies are interesting and contribute extensively to a fuller understanding of the process that takes place in full-scale combustion plants.11,12 Nevertheless, the results should be carefully applied straightforward because of the intrinsic difficulty of extrapolating experimental data from one system to another.1,2 Far-reaching research into fixed-bed biomass combustors has been conducted in both the experimental and theoretical *To whom correspondence should be addressed. Telephone: þ34-986812604. E-mail:
[email protected]. (1) Thunman, H.; Leckner, B. Fuel 2001, 80, 473–481. (2) Van Kessel, L. B. M.; Arendsen, A. R. J.; de Boer-Meulman, P. D. M.; Brem, G. Fuel 2004, 83, 1123–1131. (3) Fatehi, M.; Kaviany, M. Combust. Flame 1994, 99, 1–17. (4) Shin, D.; Choi, S. Combust. Flame 2000, 121, 167–180. (5) R€ onnb€ ack, M.; Axell, M.; Gustavsson, L.; Thunman, H.; Leckner, B. In Progress in Thermochemical Biomass Conversion, 1st ed.; Bridgwater, A. V., Ed.; Blackwell Science Ltd.: Oxford, U.K., 2001; pp 743-757. (6) Porteiro, J.; Pati~ no, D.; Collazo, J.; Granada, E.; Moran, J.; Miguez, J. L. Fuel 2010, 89 (1), 26–35. (7) Horttanainen, M.; Saastamoinen, J.; Sarkomaa, P. Energy Fuels 2002, 16 (3), 676–686. (8) Ryu, C.; Yang, Y. B.; Khor, A.; Yates, N. E.; Sharifi, V. N.; Swithenbank, J. Fuel 2006, 85, 1039–1046. (9) Yang, Y. B.; Yamauchi, H.; Nasserzadeh, V.; Swithenbank, J. Fuel 2003, 82, 2205–2221. (10) Yang, Y. B.; Ryu, C.; Khor, A.; Sharifi, V. N.; Swithenbank, J. Fuel 2005, 84, 2026–2038. (11) Pati~ no, D.; Moran, J.; Porteiro, J.; Collazo, J.; Granada, E.; Miguez, J. L. Energy Fuels 2008, 22, 2121–2128. (12) Porteiro, J.; Collazo, J.; Pati~ no, D.; Granada, E.; Moran, J. C.; Miguez, J. L. Energy Fuels 2009, 23, 1067–1075. r 2010 American Chemical Society
(13) Zhao, W.; Zhengqi, L.; Wang, D.; Zhu, Q.; Sun, R.; Meng, B.; Zhao, G. Bioresour. Technol. 2007, 99 (8), 2956–2963. (14) Wiinikka, H. High temperature aerosol formation and emission minimization during combustion of wood pellets. Ph.D. Thesis, Energy Technology Centre (ETC), Pitea, Sweden, 2005. (15) Wiinikka, H.; Gebart, R. Biomass Bioenergy 2004, 27, 645–652.
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the size of the particles is increased. In addition, mixing small and large particles is reported to be an advantageous process for the combustion rate. Ryu et al.8 have found a similar decrease in the burning rate when increasing the particle size of pine particles. Other research has reached similar conclusions from a modeling approach, whose main premises are based on the experimental references cited. Fewer conclusions are forthcoming in the research on the effects of particle shape, bed porosity, or fuel density on propagation front velocity. In ref 19, results seem to point to a narrowing of the air range when fuel density decreases. In ref 8, Ryu et al. have observed a decrease in the burning rate as the bulk density increases. In ref 7, Horttanainen et al. have also reported similar trends, as well as an inverse relationship between particle sphericity and bed porosity. The aim of this work is to extend the knowledge of the parameters with an influence on ignition front propagation, including new biomasses and combustion conditions. When the similarity of the fuel behavior versus the air mass flow rate is exploited, a parametric analysis will be made, leading to a more detailed study. The results obtained will help to assess the parameters influencing the combustion behavior of biomass particles under fixed-bed conditions.
Figure 1. Common variables influencing fixed-bed combustion.
to be an interesting solution for improving the combustion process. Van der Lans and co-workers experimented with straw,16 recording a slight increase in the propagation rate when preheating the air. Similar trends were found by Zhou et al.17 On the other hand, several papers have addressed the influence of fuel composition. For example, the influence of moisture on front propagation has been experimentally investigated by refs 6, 7, and 18-20. All of these papers present analogous results and trends, even though different biomasses were used. The general conclusion appears to be that, as moisture content increases, the propagation velocity of the front falls sharply. Especially significant are the conclusions reached by Horttanainen et al.,7 who also emphasize the influence of moisture by narrowing the range of possible airflow rates. Volatile and ash content are also very important for defining the fuel combustion behavior. Nevertheless, it is difficult to find experimental data directly relating these variables with the measured propagation rate. One easy way to consider these two parameters is by analyzing their influence on the lower heating value (LHV) of the fuel. In refs 4 and 6, a direct relationship was found between the flame propagation speed and LHV. Other papers, such as ref 21, have found a similar trend through a theoretical model. Special care must be taken to relate the morphological parameters of the fuel to its combustion behavior. The high interdependence of the variables makes it extremely difficult to isolate the real influence that each one has. Previous papers have singled out particle size and bulk density as the main ones. In ref 4, experimental data point to a drop in the flame speed as particle size increases. Their measurements support these findings and also the widening of the range of the airflow rate for larger particles. However, certain contradictory conclusions were reached by R€ onnb€ ack and co-workers,5 who could not find any special influence of size on front velocity, except for a slight variation in the range of air. An extensive analysis of this parameter has been conducted in ref 7. Experiments with the pellet showed that the range of the usable airflow rate increases as
2. Experimental Section 2.1. Experimental Facility. The experimental plant used in this work is based on that presented and suitably described in ref 6. The tube combustor and air plenum are the main parts of the burner. A steel grate supports the packed bed, which is introduced in batches over it, forming the fixed bed. Air is supplied from the bottom of the arrangement, through the plenum and the grate, providing an adequate distribution over the whole transverse area. Ignition is started in the upper surface of the bed and is followed by a downward propagation of the front in a counter-current mode. Figure A in the Supporting Information depicts the scheme of the rig, while Video 1 in the Supporting Information features a graphical description of the structure of the plant. K-type thermocouples are used in this plant to measure the bed temperature and also to provide the velocity of propagation of the different reaction fronts (drying, devolatilization, or char oxidation). The time taken to reach a specific predetermined temperature between two adjacent thermocouples enables the front propagation velocity to be calculated. This will also allow for the thickness of the drying and volatilization phases to be measured if a threshold temperature is defined. However, the char combustion region that spreads from inside the bed to its surface cannot be determined by this method because the bed surface position is unknown. During the experimental process, each propagation velocity for a certain air mass flow rate was obtained as the average of several successive measurements. To reduce the statistical uncertainty of measurements, one airflow rate is used for each batch of fuel; therefore, at least seven measurements were averaged to obtain each experimental point. 2.2. Fuels. The results analyzed in this work correspond to 12 different biomasses. Fuel selection was made to cover a wide range of particle shapes, sizes, and compositions according to their different origin. Eight of these biomasses have already been presented and characterized in ref 6 (Figure B in the Supporting Information). This paper includes four new fuels characterized using the methodology already applied in ref 6. In fact, new fuels are derived from the wood pellet formerly employed, albeit with certain modifications. Composition and morphology were artificially varied to create new controlled parameters in our tests to include certain areas not covered by any of the fuels used originally.
(16) Van der Lans, R. P.; Pedersen, L. T.; Jensen, A.; Glarborg, P.; Dam-Johansen, K. Biomass Bioenergy 2000, 19, 199–208. (17) Zhou, H.; Jensen, A. D.; Glarborg, P.; Jensen, P. A.; Kavaliauskas, A. Fuel 2005, 84, 389–403. (18) Saastamoinen, J. J.; Taipale, R.; Horttanainen, M.; Sarkomaa, P. Combust. Flame 2000, 123, 214–226. (19) Saastamoinen, J. J.; Horttanainen, M.; Sarkomaa, P. Combust. Sci. Technol. 2001, 165, 41–60. (20) Liang, L.; Sun, R.; Fei, J.; Wu, S.; Liu, X.; Dai, K.; Yao, N. Bioresour. Technol. 2008, 99, 7238–7246. (21) Yang, Y. B.; Ryu, C.; Khor, A.; Yates, N. E.; Sharifi, V. N.; Swithenbank, J. Fuel 2005, 84, 2116–2130.
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Table 1. Composition of New Biomass Fuels proximate analysis (wb, %) fuel type
formula (daf)a
moisture
volatile
char
ash
LHV (MJ kg-1)
Sab
wood pellet 3 (wp3) dried wp3 (wp3d) crushed wp3 (wp3c) crushed and dried wp3 (wp3cd)
CH1.71O0.70 CH1.71O0.70 CH1.71O0.70 CH1.71O0.70
7.3 2.3 7.3 2.3
69.0 72.6 69.0 72.6
23.0 24.2 23.0 24.2
0.70 0.80 0.70 0.80
16.6 17.6 16.6 17.6
6.12 6.12 6.12 6.12
a
Dry and ash-free basis. b Kilograms of dry air per kilogram of fuel burnt.
Figure 2. Photograph and detail of the shape and size of the crushed fuel particles. Table 2. Packing Bed Properties of the New Biomass Fuels FP F req (kg m-3) (kg m-3) (mm)
fuel type wood pellet 3 (wp3) dried wp3 (wp3d) crushed wp3 (wp3c) crushed and dried wp3 (wp3cd)
1240 1178 1240 1178
690 650 570 540
4.4 4.4 1.0 1.0
ψ
ε
0.84 0.84 0.69 0.69
0.45 0.45 0.54 0.54
The first modification consisted of drying the particles. Table 1 shows the modified moisture content of wood pellet number 3 after partial drying (coded wp3d) compared to the initial fuel wood pellet 3 (wp3). The second modification refers to morphology. The original pellet was crushed into small particles and dust without modifying the composition (wp3c). Figure 2 presents a photograph of a sample of the crushed material, focusing on the geometry of the new particles. Table 2 shows the differences in packing bed properties. Finally, both modifications are combined to create wp3cd by crushing the dried matter. The aim of the artificial modifications applied to the new fuels is to produce new experimental levels for some of the variables usually controlled. It will also help to validate or reject forecasted trends in the behavior of the fuels in the combustor.
Figure 3. Ignited mass flow versus air mass flow rate for the modified fuels.
composition (and thus the same stoichiometry) and operating with the same airflow rate, the air excess ratio will be higher for the fuel burning more slowly.
3. Results and Discussion 3.1. Modified Fuels. The air mass flow rate supplied is normally the most determinant parameter influencing the behavior of a combustor, whereas the maximum ignition front velocity defines the fuel consumption rate or the maximum heat (power) that the burner could release. The relationship between the two parameters for the fuels tested in this work can be seen in Figure 3. Exact values of these tests are shown in Table 3. The maximum temperature is measured in the middle of the bed at the thermocouple tip. Air excess can be calculated following eq 1. It should be remembered that air excess refers to the amount of fuel burned and, therefore, depends upon the total velocity of the front. Hence, for two different fuels with the same
n ¼
m_ 00 air Sa m_ 00 ig
ð1Þ
As can be inferred from Figure 3, the shapes of the curves are quite similar to those for the fuels employed in previous work, where the three reaction zones had already been shown. The initially linear increase in the ignited mass flux with an air mass flow rate is characteristic of the oxygenlimited zone, with a more or less flat zone in the middle range, where maximum propagation velocities are recorded (fuellimited or reaction-limited zone), and finally, a steep decrease until extinction in the convection zone. In Figure 3, filled squares and circles represent wet fuels. It should be noted that 3892
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Table 3. Test Results for the Artificially Modified Fuels ignited mass fuel type
wood pellet 3 (wp3)
dried wp3 (wp3d)
crushed wp3 (wp3c)
crushed and dried wp3 (wp3cd)
airflow rate (kg m-2 s-1)
flux (kg m-2 s-1)
uncertainty (%)
air excess ratio (n)
maximum temperature (°C)
0.025 0.050 0.100 0.151 0.200 0.250 0.300 0.352 0.402 0.452 0.477 0.050 0.150 0.250 0.350 0.450 0.500 0.530 0.050 0.080 0.100 0.150 0.200 0.250 0.300 0.050 0.100 0.150 0.200 0.250
0.032 0.034 0.049 0.060 0.064 0.063 0.059 0.063 0.063 0.036 0.000 0.038 0.065 0.064 0.061 0.058 0.065 0.000 0.040 0.046 0.057 0.064 0.070 0.074 0.055 0.039 0.067 0.073 0.071 0.065
5.49 5.24 5.32 4.69 4.71 4.41 4.47 5.35 5.48 6.20
0.13 0.24 0.33 0.41 0.51 0.65 0.83 0.96 1.04 2.07
850 860 1000 1110 1140 1170 1180 1160
4.75 4.59 4.26 4.14 6.86 6.24
0.22 0.38 0.64 0.95 1.28 1.26
830 1000 1110 1140
7.52 5.83 6.74 7.04 4.98 4.52 6.27 4.75 4.59 4.26 4.14 6.86
0.20 0.27 0.29 0.38 0.47 0.56 0.89 0.21 0.24 0.34 0.46 0.63
1040 830 790 910 1140 1000 680 750 880 990
the final extinction zone for the crushed fuels is not so clearly defined. Tests could not be performed in that air zone because the small size of the fuel particles generates a very high pressure drop that the fan of the plant cannot overcome. When the curves of the two pellet fuels are compared (Figure 3), differences are not excessively large but it seems that dried wood pellet (wp3d) has a slightly broader air range and also a marginally higher propagation velocity. It could therefore be said that the total area or work area covered by the dried pellet is slightly bigger. When the curves of crushed fuels are examined, differences are not so clear. It is extremely difficult to draw conclusions, but it seems that wp3cd has a higher number of points in the zone of faster front velocity. With regard to usable airflow, the difference in limits is not obvious. The differences that have just been analyzed highlight the significance of composition and morphological factors (water and particle size) in the final behavior of the fuel. They also indicate the existence of two opposite behaviors. Fuels could present a flat long curve with a medium propagation velocity (medium power) that is very stable and less dependent upon the air mass flow rate. On the other hand, if the fuel follows a sharp profile, higher velocities can be achieved (high power), albeit extremely dependent upon the total air supplied. If the data obtained are plotted against the air excess ratio, as shown in Figure 4, the initial part of the curves follows an almost vertical trend, indicating the proportional relationship between the increase in total air supplied and the increase in the velocity of propagation. This idea was presented previously in ref 6. Figure 4 highlights the fact that the stable work area of the fuels in the fixed-bed counter-current process lies in the
1120
1100
Figure 4. Ignited mass flow versus air excess.
sub-stoichiometric range. The physical interpretation of this finding in real-scale plants could mean that primary air combustion zones in boilers need only a small amount of air to achieve optimum conditions. Driving all of the combustion air through this zone could lead to excessive cooling by convection or even bed quenching. The air excess ratio can also be defined in relation to the stoichiometric air of the char instead of the stoichiometric air of the particle. Although char may have small amounts of hydrogen, nitrogen, and oxygen, carbon normally accounts (22) Peters, B. Combust. Flame 2002, 131, 132–146. (23) Di Blasi, C. AIChE J. 2002, 48, 2386–2397. (24) Thunman, H.; Leckner, B.; Niklasson, F.; Johnson, F. Combust. Flame 2002, 129, 30–46.
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Figure 6. Typical fuel behavior used as the basis for the parametric study. Sample experimental points extracted from Brassica pellet (bp) data in ref 6.
Figure 5. Ignited char mass flux versus air excess referred to the stoichiometric ratio of air and char fraction.
Table 4. Parametric Data on Fuels
for over 90% of it.22-24 Hence, char can be considered pure carbon. Measures are obtained only when steady state is achieved. This means that the propagation velocities of all fronts (drying, devolatilization, and char oxidation) are steady; hence, the ignited char rate can be easily obtained with m_ 00 char = m_ 00 igXchar. Figure 5 depicts the velocity of char consumption versus the air excess referred to the char (eq 2). Results are proportional to those shown in Figure 4; however, the importance of this graph lies in the region where the oxygen-limited phase takes place. Data show that the end of this phase coincides with the moment when char stoichiometry is achieved. This important aspect seems to somehow emphasize the relevance of the char oxidation and heterogeneous reactions in the first combustion phase. Front velocity increases in this stage as char oxygen requirements are fulfilled, while air mass flux and temperature requirements are above a limiting initial threshold. Although the whole particle is under overall sub-stoichiometric conditions, only when char oxidation exceeds its stoichiometric point does the second phase of combustion (fuel-limited reaction) start. In this case, the velocity of the front is controlled by the fuel energy balance in the reaction front, which is mainly determined by the heat-transfer rate inside the bed. The behavior shown in Figure 5 is not isolated and has been supported by the fuels previously analyzed in ref 6, as can be seen in Figure C in the Supporting Information. In fact, during the oxygen-limited regime, the gases emitted by the front are ignited as soon as they emerge into the atmosphere outside of the combustor, revealing their lack of oxygen. nC ¼
m_ 00 air ðSa ÞC m_ 00 char
fuel type 6
wood pellet 1 (wp1) wood pellet 2 (wp2)6 Brassica pellet (bp)6 poplar pellet (pp)6 RDF pellet (rdfp)6 olive stone (os)6 almond shell (as)6 pine shavings (ps)6 wood pellet 3 (wp3) dried wp3 (wp3d) crushed wp3 (wp3c) crushed and dried wp3 (wp3cd) a
ξ1
ξ2
ξ3
υ1
υ2
Atot
_ 00 av m
0.175 0.201 0.176 0.176 0.100 0.101 0.176 0.126 0.100 0.150 0.100 0.100
0.427 0.427 0.327 0.377 0.326 0.301 0.427 0.327 0.402 0.500 0.250 0.250
0.527 0.553 0.402 0.452 0.402 0.352 0.502 0.427 0.477 0.530 0.350a 0.360a
0.073 0.065 0.061 0.061 0.032 0.046 0.053 0.073 0.049 0.065 0.057 0.067
0.061 0.059 0.065 0.068 0.041 0.056 0.039 0.093 0.063 0.065 0.074 0.065
0.026 0.024 0.017 0.021 0.011 0.014 0.018 0.026 0.022 0.029 0.016 0.017
0.050 0.044 0.043 0.046 0.028 0.040 0.035 0.061 0.046 0.054 0.047 0.047
Extrapolated from data and curve trend.
combustor and the real plant. Accordingly, the geometry of Figure 6 is divided into three different zones defined by the aforementioned combustion phases. The entire set of parametric values derived from the experimental data is gathered in Table 4. Once the parameters have been defined, other parameters derived from them can be calculated to characterize the shape and size of the graph. Total area (Atot) or work area of the curve can be defined, giving an idea of the availability of the fuel. Fuels with higher total area may represent fuels that operate over a wide range of airflow rates or with high ignition velocities. The average mass flux (eq 3) is defined as the ratio of the total area to the maximum range of usable airflow rate. A _ 00 av ¼ tot ð3Þ m ξ3
ð2Þ
The influence of the composition variables is then analyzed through parametric approximation. Figure D in the Supporting Information depicts the total area of each fuel as a function of the moisture content. The trend in that graph seems to corroborate a decrease in the area as long as the moisture content is increased. This idea has been presented previously in refs 6, 7, and 18-20, as commented in the Introduction. However, this graph does not provide any information about the way this area is minimized. It could mean a general drop in the velocities or the air range. To accomplish that task, Figure 7 plots average mass flux versus the moisture content for each fuel. In this case, an almost
3.2. Parametric Analysis of All Biomasses. In this section, the whole set of results from both this paper and from ref 6 are studied through their parametrization. Given the similarity of the behavior of the fuels versus the air mass flow rate, they will be generalized in the manner shown in Figure 6. In this plot, the experimental points derived from the data are shown, as well as the artificial geometry into which we attempt to fit them. The main idea is to analyze not only the maximum values of the graph but also the shape that can determine the performance of the fuel in the experimental 3894
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Figure 7. Average fuel mass flux versus the moisture content. Figure 9. Maximum temperature measured for each fuel as a function of the moisture content. Data from Saastamoinen et al.18 and present work.
Figure 8. Air excess for maximum velocity of propagation versus the moisture content.
linear tendency is shown. The total area drop due to moisture is greater than the decrease in usable airflow values. The explanation could be that the drop in total area is due mainly to a decrease in overall front velocities, although ξ3 is also slightly diminished. Figures E and F in the Supporting Information also support this argument, showing the decrease in the maximum propagation velocity and the variation in the usable airflow limit. Figure 8 reveals an interesting aspect of the process. As the moisture of the fuel increases, the air excess for maximum propagation velocity increases as well. Despite the scatter, the trend seems to indicate that increasing the water content in the process slows the reaction rate, as previously reported by Yang et al.9 Consequently, maximum velocities are also lowered, and combustion takes place with a higher air excess. The probable influence of the moisture content on the maximum temperature measured in the bed has not been appreciated. This idea was already posited by Saastamoinen and co-workers in ref 18. Plotting their data alongside those derived from this work, Figure 9 shows the maximum bed temperature versus the water content for fuels with less than 30% moisture. It has been proven that moisture content shows the degradation of a particle and, consequently, propagation front velocity; however, once water has evaporated, the ignited front (char oxidation) seems to achieve its maximum temperature with no significant influence of
Figure 10. (A) Total area under the curves as a function of the ash and moisture content. (B) Total area covered by the curve versus the LHV of each fuel.
moisture. In short, it can be said that, although the net power released by the process decreases with the presence of moisture in the fuel, the maximum temperature reached in the core of the bed is unaffected. Other important composition variables are ashes and the quality of the fuel represented by the heating value. Figure 10A 3895
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Figure 11. Average mass flux versus the mean particle size.
Figure 12. Maximum front propagation velocity versus the mean particle size.
shows the total area under the parametric curve as a function of the ash content. This parameter could be expected to have a small influence when there is a low ash content. Nevertheless, for higher rates, it becomes decisive. This statement should be made with care because the fuel that best confirms the trend also has the highest moisture content, as represented by the bubble area in the graph. On the other hand, as shown in Figure 10B, an increase in the LHV of the fuel promotes an increase in the area covered by the curve.4,6,21 This could be due to higher velocities or a wider airflow rate range, because the higher heat delivered by the combustion made the process generally quicker and also less sensitive to cooling by air convection. In terms of morphological parameters, as mentioned in the Introduction, the mean particle size is one of the most important variables. The determination of the mean particle size is explained in ref 6, and the same methodology has been followed here, with the representative equivalent radius (req) being obtained for each fuel (Table 2). References suggest that the rate of propagation of the reaction front through the fuel bed is lower for larger particles than for smaller ones4,25 because large particles are thermally thicker with slow drying and devolatilization rates and less effective heat transfer to nearby particles.8 Yang et al.21 also found that the rate is highly influenced by fuel size, and smaller fuels have a higher combustion rate because of the increased reacting surface area and enhanced gas-phase mixing in the bed. Figure G in the Supporting Information presents the evolution of the total area covered by the curves depending upon the particle size. At first sight, there is no obvious link. Nevertheless, an analysis of average mass flux versus the equivalent radius (Figure 11) reveals quite a clear trend. The average mass flux is reduced for larger particles. Because the total area seems to be independent, this means that the decrease in the average mass flux is due to an increase in the air range available (ξ3). In other words, larger particles seem to be less sensitive to air convection cooling, achieving higher airflow rates, as suggested by Horttanainen et al.7 On the other hand, Figure 12 shows how maximum velocities are achieved by small particles, in very similar terms to Figure 11. In counter-current processes, thermal radiation is the most significant heat-transfer mechanism supporting
ignition wave propagation, with opposing convection. Increasing the radiation intensity leads to a shorter ignition time.19 Although the total area is similar in both cases, the results in Figures 11 and 12 endorse the idea of different curve shapes for each case. As already mentioned, small particles, presenting a high surface/volume ratio, are highly susceptible to surface reactions. Accordingly, the main heat-transfer mechanism (radiation) records a higher rate for small particles, promoting faster devolatilization and higher propagation velocities. This higher surface/volume ratio is also decisive in the extinction by convection phase, promoting the premature quenching of the process in small particle fuels. As a result, large particle curves are long in the usable air range, while small particles have a sharper behavior or a tall shape with high power release (high propagation velocities), although they are extremely sensitive to the influence of the airflow rate. Particle shape is another important morphological parameter that can be analyzed by means of sphericity (eq 4). However, as shown in refs 6 and 7 and also in Figure H in the Supporting Information, there is a close relationship between sphericity and bed porosity. Porosity depends upon the ratio between bulk and particle densities, ε = 1 - (F/FP); therefore, its influence can be studied through the effects of these two latter parameters. ψ ¼
π1=3 ð6VP Þ2=3 SP
ð4Þ
Previous papers5 have detected an increase in the extension of the reaction-limited zone when particle density was increased, but ref 25 stated that an increase in fuel density does not have any significant effect on the conversion rate when the terms of the conversion rate fluctuate, therefore being expressed as the mass reaction rate per cross-sectional area of the bed. In ref 7, denser particles have been related to a wider range of possible airflow rates. Our results have not been at all conclusive in this regard. No influence of the particle density has been detected on the total area covered by the curve, the maximum velocity achieved, or even the average mass flux. Likewise, no influence of porosity was observed on the maximum available airflow rate or curve stability. Neither any influence of air availability nor any effects on overall bed temperature were appreciated. Although a physical interpretation could endorse the fact that high-density
(25) Thunman, H.; Leckner, B. Proc. Combust. Inst. 2005, 30, 2939– 2946.
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fuels should record slower drying and devolatilization rates and a higher resistance to air cooling by convection, this work did not obtain significant results in this regard. Therefore, it appears that the influence of particle density or porosity is less significant than other aspects, such as moisture content and particle size.
Acknowledgment. The authors acknowledge the financial support from the project ENE 2009-14104-C02-01 of the Ministry of Science and Innovation and also the project 08DPI003303PR of the Xunta de Galicia (Spain). Supporting Information Available: Simplified scheme of the fixed-bed combustor (Figure A), samples of biomass fuels used in (Figure B), ignited char mass flux versus air excess referred to char from ref 6 (Figure C), total area versus moisture content for all fuels (Figure D), maximum propagation velocity versus moisture content for all fuels (Figure E), maximum range of usable airflow versus moisture content for all fuels (Figure F), total area covered by the curves of all fuels versus the mean particle size (Figure G), sphericity versus porosity graph (Figure H), and video showing the 3D design of the combustor (Video 1). This material is available free of charge via the Internet at http://pubs.acs.org.
4. Conclusions This paper presents a study of the experimental parameters influencing combustion behavior of an experimental-pilot fixed-bed combustor. Artificially modified biomasses allowed for a more detailed study of such important variables as moisture content and mean particle size. An analysis of the propagation rate as a function of air excess has confirmed the general idea that, in a counter-current combustion process, the greater working range is located in the sub-stoichiometric zone. This finding ratifies the widely reported adverse effect of excessive air convection through the front. If propagation velocity is expressed as a function of air excess related to char stoichiometry, the change in front transition from oxygen limited to fuel reaction limited is produced at char stoichiometry (nC =1). This means that the oxygen-limited phase is determined by the oxygen transport to the char surface. In other words, the first phase of combustion is determined mainly by the behavior of the char matter of the fuel. Data for 12 different biomasses were parametrically examined, highlighting the main variables of the process through the shape of their propagation curves. In this study and in agreement with the relevant literature, an increase in the moisture content was found to reduce the total work area of the fuels, especially minimizing the maximum propagation velocities achieved. However, it had no appreciable influence over the maximum temperature measured inside the bed. On the other hand, the particle size seems to largely determine the curve shape. Smaller particles promote sharp fuel behaviors that lead to high maximum velocities and a narrower available airflow rate. This may lead to the important conclusion for real-scale plants that the size of the particles could be tailored depending upon the application needed (stability and power requirements). Finally, no effects from other morphological parameters, such as porosity and particle density, were appreciated, or at least, any such effects were not clearly distinguished, which probably means that the importance of these variables is lower than that of the others analyzed in the present work.
Nomenclature Atot = total area or work area of the parametric curve (kg2 m-4 s-2) LHV = lower heating value on a water basis with ash (as received) (kJ kg-1) 00 m air = air mass flux (kg m-2 s-1) m_ 00 av = average mass flux of the parametric curve (kg m-2 s-1) 00 m char = ignited char mass flux (kg m-2 s-1) m00 ig = ignited fuel mass flux (kg m-2 s-1) n = air excess ratio (referred to as the stoichiometric air of the particle) nC = air excess ratio (referred to as the stoichiometric air of the char) req = radius of the equivalent sphere (mean particle size) (mm) Sa = stoichiometric ratio of air and fuel (kilograms of dry air per kilogram of fuel burnt) (Sa)C = stoichiometric ratio of air and char fraction (kilograms of dry air per kilogram of char burnt) Xchar = char mass fraction from proximate analysis Greek Letters ε = bed porosity ξ = parametric value for the air mass flow rate (kg m-2 s-1) F = bulk density (kg m-3) FP = particle density (kg m-3) υ = parametric value for the ignited mass rate (kg m-2 s-1) ψ = sphericity
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