Bioresource Technology 183 (2015) 195–202
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Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor Feng Duan a, Chien-Song Chyang b,⇑, Li-hui Zhang a, Siang-Fong Yin b a b
School of Energy and Environment, Anhui University of Technology, Maanshan 243002, Anhui Province, PR China Department of Chemical Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC
h i g h l i g h t s Combustion of rice straw was conducted in a VFBC to study bed agglomeration behavior. Eutectics with low melting point materials promote defluidization at high temperatures. The alkali concentration has the highest value at the upper bubbling zone. Coal ash inhibits the formation of low melting compounds of rice straw combustion.
a r t i c l e
i n f o
Article history: Received 6 December 2014 Received in revised form 9 February 2015 Accepted 10 February 2015 Available online 16 February 2015 Keywords: Agglomeration Defluidization time Vortexing fluidized bed Pelletized rice straw
a b s t r a c t To investigate bed agglomeration characteristics, the combustion of pelletized rice straw was conducted in a bench-scale vortexing fluidized bed. Effects of bed temperature, superficial velocity, secondary gas velocities, and mass blended ratio of coal on the defluidization time were investigated. The alkali concentrations in different sections of the bed zone were also studied. The bed materials and agglomerates were analyzed using SEM/EDX to obtain the surface morphology and the compositions. The results revealed that the defluidization time is increased with superficial gas velocity and is decreased with bed temperature. Eutectic composition with low melting point materials promote defluidization at high temperatures. Effect of the secondary gas velocity on the defluidization time indicates different trends at different bed temperatures. The highest value of alkali concentration appears at upper bubbling zone. Coal ash can avoid the existence of a certain eutectic composition, and increases its melting point. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Rice straw is the most abundant agricultural by-product during the rice harvesting. The harvested rice straw to paddy ratio is approximately 1.0. Based on the data from the Food and Agriculture Organization (FAO), the global production of rice straw is approximately 720 million tons per year. Rice straw is known for its low density and a variety of lengths that cause high shipping and handling costs. The market for pellets from agriculture residues in Asia is rapidly expanding (Chaivatamaset et al., 2014; Kupka et al., 2008). A fluidized bed combustor is often used in burning biomass residues because the fluidized bed combustor has high fuel flexibility, uniform bed temperature, and low pollutant emissions. However, some operating issues still remain with the biomass
⇑ Corresponding author. Tel.: +886 3 2654119; fax: +886 3 4636242. E-mail addresses: [email protected], [email protected] (C.-S. Chyang). http://dx.doi.org/10.1016/j.biortech.2015.02.044 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
combustion using the current fluidized bed technology. Agglomeration is one of the problems that occur during biomass combustion in fluidized bed combustors (Scala and Chirone, 2005), and this problem can be attributed to the large amount of alkali in ash. Quartz sand is commonly used as the bed material due to its high availability, lower price, and high heat capacity. However, silicon in quartz sand reacts with the alkali in the ash and forms low melting point materials, causing agglomeration and eventually leading to defluidization, which results in unscheduled shutdowns and unnecessary expenses. The hydrodynamics and heat transfer inside the combustor cannot be observed directly by the naked eye. Measured data are needed to understand the phenomena. The event of defluidization can be estimated by the change in pressure drop because the bed pressure drop is proportional to the bed inventory (Chaivatamaset et al., 2011). The pressure drop fluctuates around an average when the FBC running steady, but the pressure drop falls rapidly after defluidization starts (Atakül et al., 2005; Billen et al., 2014; Lin et al., 2011). Scala and Chirone (2005) used three
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thermocouples, located vertically at various heights in the bed, to measure the temperature difference. The results indicate that when defluidization occurs, the temperature measured by the lower thermocouple falls rapidly, while the temperatures measured by the other two thermocouples tend to increase, before the fuel feeding stopped. This trend is related to the decrease in the heat transfer when the bed is defluidized and the segregated fuel combustion in the upper bed region. The effects of different operating parameters such as bed temperature, bed material sizes, superficial gas velocity, and static bed height, etc., on defluidization time were discussed in the literatures (Bartels et al., 2008; Silvennoinen and Hedman, 2013). We can prolong the defluidization time, defined as the period of time from the start of fuel feeding to defluidization, by changing the operating parameters. Bed temperature is the key factor for defluidization. The most convenient and effective method of preventing defluidization is to keep the bed temperature from rising above the melting point of the eutectic compositions, which prevents the melting phenomenon (Skrifvars et al., 2005; Yu et al., 2011). As the bed temperature decreases, CO emission also increases to much higher levels than the norms of the regulation (Bahillo et al., 2004; Baron et al., 2002). Superficial gas velocity directly affects the quantity of bottom ash and the heat transfer of burning char (Yu et al., 2011). Defluidization time increases with superficial gas velocity, but the effect of superficial gas velocity on defluidization is minimal at higher bed temperatures (Liu et al., 2010). For a certain superficial gas velocity, defluidization time is inversely proportional to the particle size of the bed material. The fluidization number for large particles is less than the fluidization number for small particles, leading to a poor quality of fluidization and hot spots in the bed. The surface-to-volume ratio of the particle decreases with increasing particle size. The larger particles require less coating to agglomerate, and the agglomerated particles take less ash to cause defluidization (Chaivatamaset et al., 2011, 2013; Kuo et al., 2008; Lin and Wey, 2004; Lin et al., 2003). In addition to changing operating parameters, selecting bed material type and adding inhibitors such as clay and kaolinite are also discussed in the literature. The methods mentioned above can only prolong the defluidization time but still cannot avoid agglomeration and defluidization. Some previous studies have shown that biomass co-combustion with peat or coal can be an inexpensive measure for preventing bed agglomeration (Arvelakis and Frandsen, 2007; Brus et al., 2004; Gogebakan et al., 2009; Lundholm et al., 2005). The main mechanism for preventing the formation of viscous coatings is the retention of alkali by chemisorptions or physical adsorption by minerals and ash constituents in coals (Lundholm et al., 2005). The concept of vortexing fluidized bed combustion (VFBC) was introduced to create a vortex by injecting a secondary gas tangentially into the freeboard to increase the combustion intensity. VFBC has been used for biomass or coal combustion studies in recent years (Duan et al., 2013, 2014b). In these studies, biomass combustion remained steady while quartz sands were used as the bed material. The bed temperatures were kept below 800 °C to avoid agglomeration. The vortexing effect caused by the secondary gas would capture the small particles including ash back to the bed zone. Therefore, the agglomeration behavior may differ from a conventional FBC at this condition. Reports on the agglomeration characteristics at different secondary gas velocities are few. It is meaningful to screen the operation parameters to avoid agglomeration before designing combustion scenarios in a VFBC. The objective of this work is to investigate the effect of operating conditions such as bed temperature, superficial gas velocity and secondary gas velocities on the defluidization time in a VFBC. Coal is also used as an additive
to quantify the potential positive effect in terms of defluidization time by co-combustion. The alkali concentration distributions in the bed zone under various operating conditions were also studied. 2. Methods 2.1. Fuel and bed materials Quartz sand (99.5% SiO2) with a mean diameter of 540 lm was used as an inert bed material in this study. Cylindrical pelletized rice straw was used as the fuel in the present work. The mean diameter of cylindrical pelletized rice straw is 6 mm and its length ranges from 5 to 15 mm with a mean length of 9.6 mm. The coal was used as an additive in this study. The mean diameter of the coal particles is 3.1 mm. The properties of pelletized rice straw and coal are shown in Table 1. During the co-combustion test, the feeding materials were prepared according to the mass blended ratio. In the mineral analyses, apart from silica content, the potassium content of the pelletized rice husks is the highest. This composition was expected to be problematic for agglomeration in biomass fluidized-bed combustion. 2.2. Vortexing fluidized-bed facility Fig. 1 shows the schematic diagram of the experimental setup of the vortexing fluidized bed combustion (VFBC) system in this study. The combustor is assembled with a wind box, distributor, combustion chamber, and freeboard. The combustion chamber is 0.22 0.11 m2 in area and is 0.678 m in height. The freeboard with an inner diameter of 0.154 m and 4 m in total height is fabricated with SUS310 pipe. The combustion chamber and freeboard are equipped with an electrical heating system and wrapped with 0.025 m thick ceramic fiber for thermal isolation. The feeding system is an assembly of a feedstock hopper and a screw feeder, with the feeding point at 0.5 m above the distributor. The temperatures within the combustor are measured with Ktype thermocouples. Bed temperature is measured with three ther-
Table 1 Properties of pelletized rice straw and coal. Analyses
Pelletized rice straw
Coal
Proximate analysis (wt%, as received) Moisture 9.29 Volatiles 64.24 Fixed carbon 13.21 Ash 13.26
16.00 32.94 40.49 10.57
Ultimate analysis (wt%, dry and ash free) C 44.40 H 7.40 O 47.07 N 1.13 S 0.00
66.93 4.6 26.10 1.49 0.88
Mineral analysis (mg/kg, dry basis) Na 860.80 Mg 4074.00 Al 6203.00 Si 39310.00 P 940.60 S ND K 19350.00 Ca 4917.00 Ti 120.20 Fe 1401.00
1328 7764 4609 148,200 813.3 7692 1612 3174 172.6 3575
Heating value (kcal/kg) HHV(WB) LHV(WB) Bulk density (kg/m3)
6192.32 5832.08 1350.46
3593.00 3244.76 693.63
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197
Fig. 1. Schematic diagram of bench scale VFBC system.
mocouples (0.07, 0.15, and 0.25 m above the distributor), installed vertically along the combustion chamber to measure the temperatures at each height. The mean value of the temperatures detected from these three thermocouples is considered the bed temperature. The primary and secondary air is supplied by an air compressor. In addition, a nitrogen supply system is used to mix nitrogen into the primary and secondary air. Four equally spaced secondary gas injection nozzles (0.013 m in diameter) are installed tangentially 0.796 m above the distributor. The flue gas from the combustor passes through a cyclone to remove most of the unburned char and ash particles and is then cooled by a heat exchanger. Finally, the flue gas is released to the atmosphere, and fly ash is collected in a bag house. 2.3. Experimental procedures The steady state combustion is carried out under atmospheric conditions according to the set working conditions. The operating conditions for the test are summarized in Table 2. The test is started by preheating the combustor chamber using electric heaters
Table 2 Operating conditions. Operating parameter
Units
Values
Fuel feeding rate Coal mass blending ratio Bed temperature Freeboard temperature Superficial gas velocity Stoichiometric oxygen in the combustion chamber Primary gas flow rate Secondary air flow rate Excess oxygen ratio Bed material Mean particle size of bed material Density of bed material Static bed height Bed material weight
kg/h % °C °C m/s % NL/min NL/min % – lm kg/m3 cm kg
2.78 25, 50 750–850 850 0.6–1.0 100 212–353 78–195 50 Quartz sand 540 2500 20 7
and the continuous secondary air injection. Primary air was injected into the combustor occasionally to stir the sand. The fuel is fed into the combustor via a screw feeder while the bed temperature reaches 500 °C, and the gas flow rate is adjusted to the set value. The elapsed time is recorded while the fuel begins to feed. Defluidization is considered to occur when the temperature difference in the bed is over 50 °C, and we stop the fuel feeding and switch off the primary and secondary gas at this point. During the test, the fly ash is collected from the cyclone. After the test, the windbox is pulled down after the combustion chamber is cooled to the room temperature, and then the bed materials and agglomerates are collected. The combination of SEM/EDX is used to characterize the surface morphology and composition of the particles and agglomerates. The potassium concentration profile in the bed is examined by inductively coupled plasma-mass spectrometry (ICP-MS). 3. Results and discussion 3.1. Defluidization time Fig. 2 shows the effect of the superficial gas velocity on the defludization time and elutriated fly ash. The historical bed temperature profiles at various superficial gas velocities are also shown in this figure. In this study, bed temperature profiles are monitored to determine the onset of defluidization. Fig. 2A–C shows that the temperature of the upper bed zone (z = 0.25 m) is always slightly higher than the temperatures in the other parts of the bed zone (z = 0.07 m, z = 0.15 m). This difference can be attributed to the combustion of the pelletized rice straw, which occurs mainly in this section. With the increase of superficial velocity, the temperature differences decrease due to the well mixed bed material (Fig. 2D). When defluidization occurs, the change in the temperature profile is similar under different superficial gas velocities. In these figures, the temperature at z = 0.25 m increases sharply, while the temperature at z = 0.07 m drops quickly when defluidization happens. This temperature drop can be caused by
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1100
1100
A
B
T1 ( 7 cm)
U = 0.6 m/s
T2(15 cm)
1000
1000
T1 ( 7 cm)
Temperature ( C)
o
o
Temperature ( C)
T3(25 cm) 900 800 700
U = 0.7 m/s T2(15 cm)
900
T3(25 cm)
800 700
Defluidization
Defluidization 600
600 500
500
0
1000
2000
3000
4000
5000
0
1000
2000
1100
1100
C
T2(15 cm)
1000
5000
6000
7000
T2(15 cm) T3(25 cm)
900 800
4000
T1 ( 7 cm)
U =1.0 m/s
1000
T3(25 cm)
Temperature (ºC)
Temperature (ºC)
D
T1 ( 7 cm)
U = 0.8 m/s
3000
Time (sec)
Time (sec)
Defluidization
700
900 800 700
Defluidization 600
0
1000
2000
3000
4000
5000
6000
500
7000
0
3000
6000
Time (sec)
9000
120
500
E Defluidization time (min)
12000 15000 18000 21000
Time (sec)
Defluidization time Fly ash elutriated
400
100 80
300 60 200
Eo = 50% Sb = 100% o
Tb = 850 C
100
Qsec = 78 NL/min
0
40
0.6
0.8
1.0
Fly ash elutriated (g/hr)
500
600
20 0
Superficial gas velocity, U (m/s) Fig. 2. Effect of superficial gas velocity on the defluidization time and fly ash elutriated. (Eo = excess oxygen ratio; Sb = stoichiometric oxygen ratio in the bed; Tb = bed temperature; Qsec = flow rate of secondary gas.)
the melting of low melting point materials and generates agglomerate at the same time. The agglomerate of varying sizes brings down fluidization and eventually leads to defluidization, which results in the decrease of temperature in the lower bed zone (z = 0.15 m) because burned fuel particles cannot migrate from the bed surface to the bottom. According to the data shown in Fig. 2A–D, the defluidization time is increased with the superficial gas velocity as shown in Fig. 2E, which is in agreement with the published literatures
(Chou and Lin, 2012; Kuo et al., 2008; Liu et al., 2010). As the superficial gas velocity increases, the collision and mixing of particles in the bed zone becomes vigorous. The effect of the adhesive force appears to decrease, while the erosion rate of the coating layer on the bed particle surface increases with increasing superficial gas velocity. The higher superficial velocity causes higher mixing of bed material and burned fuel particles, which results in a uniform bed temperature and prevents hot spots. The data shown in Fig. 2E also reveal that the elutriation of fly ash is increased with
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superficial gas velocity, which results in the decrease in the amount of bottom ash in the bed. Therefore, the chance of agglomeration in the bed zone decreases. The effects of bed temperature, secondary gas velocity, and coal mass blend ratio on the defluidization time are shown in Fig. 3. As seen in Fig. 3A, the defluidization time is decreased from 300 to 45 min when the bed temperature is increased from 800 to 900 °C. The extent of melting of low melting point materials increases with bed temperature, which results in the increase of the viscous force among the particles. This increase of the viscous force among the particles causes the agglomeration and eventually leads to the defluidization. The defluidization did not occur throughout the 10 h when the test was operated at 750 °C. This result is different from the findings reported in the literature, which are 2 h for combustion of rice straw and 1.3 h for combustion of wheat straw at 750 °C in a bubbling fluidized bed (Lin
Defluidization time (min)
1200
A
no defluidization Eo = 50%
1000
Sb = 100%
800
U = 0.6 m/s o Tf= 850 C
600
Qsec = 78 NL/min
400 200 0
740
760
780
800
820
840
860
880
900
920
o
Bed temperature ( C)
B
Defluidization time (min)
300 250 200
o
Eo = 50%
o
Tf= 850 C
Tb=850 C
150
Tb=800 C
o
Sb = 100% U = 0.6 m/s
100
et al., 2009; Liu et al., 2009). The comparison of the operating conditions in this study and other literature, concerning the combustion of rice straw and wheat straw, are summarized in Table 3. The fluidization number used in this study, which represents the extent of fluidization, is larger than the fluidization number of Lin et al. (2009) and Liu et al. (2009), and the defluidization time is proportional to the fluidization number. The long defluidization time, >10 h, obtained in this study can be attributed to the high superficial gas velocity. In this study, the secondary gas was introduced tangentially into the freeboard, resulting in a swirl flow in the freeboard, and the elutriated fly ash was trapped back to the bed. As the secondary gas flow rate is increased, the vortexing effect is more significant. The amount of elutriated fly ash is decreased with the secondary gas flow rate, resulting in increasing amount of bottom ash (Duan et al., 2014a). Fig. 3B shows the effects of the secondary gas flow rate on the defluidization time. In this figure, the defluidization time obtained at different temperatures shows the different trends. The defluidization time is decreased with secondary gas flow rate when the bed temperature is 800 °C, while the defluidization time is stayed nearly constant at a bed temperature of 850 °C. These observations can be explained by the fact that the defluidization time decreases with bed temperature and ash concentration. The agglomeration tends to occur at a higher concentration of the fly ash in the bed. The fly ash trapped by swirl flow increases with the flow rate of secondary gas. Therefore, the defluidization time is decreased with the flow rate of the secondary gas at the bed temperature of 800 °C. The effect of the fly ash concentration in the bed is overwhelmed by the effect of the temperature at high temperatures. The agglomeration occurring at 850 °C needs only a small amount of ash accumulation in the bed. Therefore, the effect of the secondary gas flow on the defluidization time is minimal at 850 °C. The agglomeration characteristics of pelletized rice husks and coal in different proportions were also investigated in this study. Fig. 3C shows the effect of the coal mass blended ratio on the defluidization time. As discussed above, a higher bed temperature is prone to agglomeration, and the defluidization time is increased with the coal mass blending ratio. In this test, the steady state combustion was carried out continuously for 20 h at a fixed coal mass blending ratio of 50%, and no agglomeration was observed. The results indicate that the ash from coal combustion can inhibit agglomeration of bed material particles. Therefore, co-combustion of biomass with coal is suggested. 3.2. K concentration profile
50 60
80
100
120
140
160
180
200
Secondary gas flow rate (NL/min) 1200
C 1000
Defluidization time (min)
199
no defluidization
800 600
Eo = 50% Sb = 100% o
400
Tb=850 C o
Tf=850 C
200
Qsec= 135 NL/min U = 0.6 m/s
0 -10 -5 0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Coal mass blending ratio (%) Fig. 3. Effect of operating conditions on the defluidization time. (Tf = freeboard temperature; U = superficial gas velocity.)
According to the temperature distributions detected by three thermocouples as shown in Fig. 2, we can presume that agglomerates are generated at the upper part of the bed zone. However, the relationship between defluidization and accumulation of alkali concentration in the different sections of the bed is unknown. In this study, we divided the bed into three sections from the distributor to the bed surface. The schematic diagram shown in Fig. 4 represents these three sections. The sections are the upper bubbling zone, the lower bubbling zone, and the grid zone. As the combustion chamber is cooled down, the top half of static material is defined as upper bubbling zone, while the bottom half is defined as lower bubbling zone. The grid zone is defined as the zone between the air distributor and the blast cap. The upper bubbling zone is composed primarily of agglomerates, and the sizes of most agglomerates are larger than 2000 lm. The lower bubbling zone is composed of agglomerates and bed materials, and the size of the agglomerates is much less than the size of the agglomerates in the upper bubbling zone. The color of bed material is milky white in these two bubbling zones. The grid zone is close to the dis-
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Table 3 A comparison of operating conditions for rice straw combustion experiments. Parameters
Unit
This study
Lin et al. (2003)
Liu et al. (2009)
Fuel Combustor dimension Bed temperature Bed material Mean particle size Superficial gas velocity Fluidization number Defluidization time
– m °C – lm m/s – min
Rice straw 0.22 0.11 750 Quartz sand 540 0.60 5.83 1200 (no defluidization)
Wheat straw w0.068 1.2 750 Quartz sand 275 0.06 2.23 78
Rice straw w0.032 0.6 750 Quartz sand 250 0.10 4.50 120
10
K concentration (wt. %)
A o
8
850 C o 800 C o 750 C
6
Eo = 50%
4
2
Sb = 100% U = 0.6 m/s Qsec = 78 NL/min
0 Grid zone
Lower-bubbling zone
Upper-bubbling zone
8
tributor, which consists of bed materials with an orange color. The color may be caused by coating some metals on the surface of the bed material. The depth of these three sections varies with operating conditions. As the combustion chamber is cooled down, agglomerates and bed materials in three different sections are collected and examined for alkali concentration by ICP-MS. The potassium, problematic elements in agglomeration, is a dominant alkali in rice straw (Lin et al., 2003). Only the potassium concentration is discussed in this study. The distribution of potassium concentrations in different sections of the bed with various operating parameters such as bed temperature, ratio of primary gas velocity to secondary gas velocity, and coal mass blending ratio is shown in Fig. 5. Fig. 5A shows that the potassium concentration in the upper bubbling zone is higher than the potassium concentration in the other sections and strongly dependent on the bed temperature. The potassium concentration of the bed material at the surface zones is decreased significantly while the bed temperature is increased from 750 to 850 °C. This decrease can be explained by fact that only a limited amount of accumulated fly ash can induce bed agglomeration at a higher temperature. The alkali metal oxide content of the ash from biomass combustion can react with the silica to form the eutectic compositions with lower melting points. The reaction is:
K2 O þ nSiO2 ! K2 O nSiO2
ðreaction1Þ
When n is 3 or 4, the melting temperature of the eutectic compositions is lower than 800 °C (Nuutinen et al., 2003; Wu et al., 1993). The data from temperature and potassium concentration distribution demonstrate that as accumulation of potassium con-
6
P = Primary gas flow rate S = Secondary gas flow rate Eo = 50% Sb = 100% o
4
Tb = 850 C
P/S=1.57 P/S=2.12 P/S=2.71 P/S=3.62 P/S=4.52
Average K concentration in surface zone
2
0
Grid zone
Lower-bubbling zone Upper-bubbling zone
8
C 6
K concentration
Fig. 4. Schematic representation of the bed structure.
K concentration (wt. %)
B
100% Rice straw 75% Rice straw +25% Coal 50% Rice straw +50% Coal
Eo = 50% 4
Sb = 100% o
Tb = 850 C 2
0 Grid zone
Lower-bubbling zone Upper-bubbling zone
Fig. 5. Potassium concentration in the bed under various operation conditions.
centration in the upper bubbling surface zone reaches the critical value at higher bed temperatures, the agglomeration at the bed surface results and leads to defluidization. Fig. 5B shows that the potassium concentration in the upper bubbling zone is remained at 3–3.8 wt% at a fixed bed temperature
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F. Duan et al. / Bioresource Technology 183 (2015) 195–202 Table 4 EDX analyses of selected zone of the particles (wt%, oxygen-free basis). Su.Ma.
Point
Na
Mg
Al
Si
K
Ca
Fe
C
a b c d
– – 1.49 –
– – 3.21 2.87
– 1.03 8.78 1.87
63.91 64.37 65.58 74.74
25.29 25.52 20.94 20.52
10.80 9.08 – –
– – – –
D
e f
– –
– –
– 3.41
86.46 81.99
13.54 14.60
– –
– –
E
g h
– –
– –
13.98 12.69
81.39 82.25
4.63 5.06
– –
– –
F
i
–
–
2.29
97.71
–
–
–
G
j k l
– – –
– 2.18 –
3.37 10.73 –
64.17 47.85 83.20
18.62 19.83 16.80
5.77 4.62 –
8.07 15.25 –
H
m n o
– – –
1.89 1.93 1.53
21.45 19.06 19.56
51.05 51.33 55.08
19.83 16.54 15.70
– 4.75 –
5.78 6.38 8.14
of 850 °C. The effect of operating parameters on potassium concentration distribution in various sections of the bed is not obvious. The data shown in Fig. 5B are obtained from the experiment operated at 850 °C with various primary/secondary gas flow rate ratios. This vortexing effect increases as the ratio decreases, and the fly ash from pellet combustion with higher potassium is carried back to the upper bubbling zone. Meanwhile, the fly ash captured by the vortexing effect decreases with the increase in this ratio, resulting in a lower potassium concentration. However, more pellet particles will combust in the upper part of the bed zone at a higher primary gas flow rate and lower secondary gas flow rate, increasing the reaction rate of pellet particles in the bed zone and resulting in higher local temperatures, thus accelerating the potassium release. Therefore, the potassium concentration in the upper bubbling zone increases gradually with the ratio of primary gas flow rate to secondary gas flow rate. According to the data from Fig. 5B, it can be said that the effect of the secondary air on the agglomeration is minimal at a high temperature of 850 °C. Fig. 5C shows the potassium concentrations in the three sections at various coal mass blending ratios. The upper bubbling section has the highest potassium concentration, and the value of the blended fuel combustion is nearly two times the value of the blended fuel combustion from rice straw in individual combustion. It can be interpreted by the fact that the defluidization time increases with the coal mass blending ratio and the accumulation of potassium with its concentration gradually increasing with the operating time.
3.3. The compositions of particles The agglomerates obtained in this study are fragile and light. During the test, the composition and morphology of the particles are observed by SEM/EDX. Table 4 presents the EDX analyses of the particles from selected zones. The surface of the unused fresh bed material is rough and sharp and is composed of 97.71% silicon and 2.29% aluminum. In the grid zone, aluminum is increased to approximately 13%, while silicon is reduced to approximately 82.00%, and the potassium is approximately 5.0%. In the lower bubbling zone, the composition of the bed materials consists of aluminum, silicon, and potassium with the morphology of a rough surface, and in this zone the potassium is increased to approximately 14.00%. The color of the bed materials in the grid zone and lower bubbling zone is different, possibly because of the content of aluminum, as the aluminum content in the grid zone is significantly increased.
The inorganic ash remaining in the upper bubbling zone includes two extreme types of agglomeration. One type of agglomerate has a uniform coating, formed on the surface of the bed material grains. The other type is glossy and exhibits liquid-like holes. It can be attributed to the existence of bubbles which can be observed on the surface. This type of agglomerate dominates this installation. During the test, the coating layer starts to melt, and the bed material surface becomes viscous when the thickness of the coating layer reaches a certain value, resulting in the merging of particles at collision. The sizes of the agglomerates are not uniform, leading to a poor quality of fluidization. The composition of the melted materials consists mainly of approximately 10% calcium, 64% silicon, and 25% potassium. The second type of agglomerate is designated as ‘‘melt-induced’’ agglomeration. In this case, the bed materials are bonded together by a melting phase, which roughly matches the ash chemical composition and is produced at normal bed temperatures. In this figure, the composition of the bonded points consists of aluminum, magnesium, calcium, silicon, and potassium. Compared with the first type, the potassium is reduced up to 20% in the second type of agglomerate. However, the SEM analysis data from the co-combustion test show that the agglomerates, found on the surface of the bed material have a uniform coating and in some cases, a porous coating. Results from the EDS analysis also showed that the elemental composition of the coating is different with individual rice straw combustions depending on the mixed coal mass, implying that the mechanism of bed agglomeration may be different for co-combustion. Therefore, the metal oxides such as Al2O3 and Fe2O3 in the bottom ash will react with alkaline compounds to form K2Fe2O4 and K2Al2O4. These metal oxides can also react with K2OnSiO2 (reactions ((2)–(4)), and the melting point of these reaction products are all higher than 955 °C (Lin et al., 2003).
K2 O nSiO2 þ Fe2 O3 ! K2 O Fe2 O3 nSiO2
ðreaction2Þ
K2 O nSiO2 þ Al2 O3 ! K2 O Al2 O3 nSiO2
ðreaction3Þ
K2 O nSiO2 þ Al2 O3 ! K2 O Al2 O3 ðn 1ÞSiO2 þ SiO2
ðreaction4Þ
These products with higher melting points therefore form on the surface of the fixed carbon of biomass, biomass ash, and the eutectic compositions with a lower melting point. This formation prevents the generation and migration of eutectic compositions with lower melting points and decreases the chances of agglomeration.
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4. Conclusions Defluidization time increases with superficial gas velocity and decreases with bed temperature. Eutectic compositions with low melting point materials promote defluidization at the high temperatures. Effect of the secondary gas flow rate on the defluidization time is minimal at high bed temperatures (850 °C) and obvious at a lower temperature (800 °C). The alkali concentration of the upper bubbling zone has the largest value. Coal ash can avoid the existence of a certain eutectic composition, and increases its melting point. Acknowledgements The financial support from the Open Foundation of Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, and the Nature Science Research Project of Anhui province (No. 1508085ME73) are greatly acknowledged. References Arvelakis, S., Frandsen, F.J., 2007. Melting behavior of ashes from the co-combustion of coal and straw. Energy Fuels 21, 3004–3009. Atakül, H., Hilmiog˘lu, B., Ekinci, E., 2005. The relationship between the tendency of lignites to agglomerate and their fusion characteristics in a fluidized bed combustor. Fuel Process. Technol. 86, 1369–1383. Bahillo, A., Armesto, L., Cabanillas, A., Otero, J., 2004. Thermal valorization of footwear leather wastes in bubbling fluidized bed combustion. Waste Manage. 24, 935–944. _ W., Kandefer, S., Pilawska, M., 2002. Combustion Baron, J., Bulewicz, E.M., Zukowski, of hydrocarbon fuels in a bubbling fluidized bed. Combust. Flame 128, 410–421. Bartels, M., Lin, W., Nijenhuis, J., Kapteijn, F., van Ommen, J.R., 2008. Agglomeration in fluidized beds at high temperatures: mechanisms, detection and prevention. Prog. Energy Combust. 34, 633–666. Billen, P., Costa, J., van der Aa, L., Westdorp, L., Van Caneghem, J., Vandecasteele, C., 2014. An agglomeration index for CaO addition (as CaCO3) to prevent defluidization: application to a full-scale poultry litter fired FBC. Energy Fuels 28, 5455–5462. Brus, E., Öhman, M., Nordin, A., Boström, D., Hedman, H., Eklund, A., 2004. Bed agglomeration characteristics of biomass fuels using blast-furnace slag as bed material. Energy Fuels 18, 1187–1193. Chaivatamaset, P., Sricharoon, P., Tia, S., 2011. Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed. Appl. Therm. Eng. 31, 2916–2927. Chaivatamaset, P., Sricharoon, P., Tia, S., Bilitewski, B., 2014. The characteristics of bed agglomeration/defluidization in fluidized bed firing palm fruit bunch and rice straw. Appl. Therm. Eng. 70, 737–747. Chaivatamaset, P., Sricharoon, P., Tia, S., Bilitewski, B., 2013. A prediction of defluidization time in biomass fired fluidized bed combustion. Appl. Therm. Eng. 50, 722–731.
Chou, J.D., Lin, C.L., 2012. Inhibition of agglomeration/defluidization by different calcium species during fluidized bed incineration under different operating conditions. Powder Technol. 219, 165–172. Duan, F., Chyang, C.S., Lin, C.W., Tso, J., 2013. Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR). Bioresour. Technol. 134, 204–211. Duan, F., Chyang, C.S., Wang, Y.J., Tso, J., 2014a. Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor. Bioresour. Technol. 154, 201–208. Duan, F., Zhang, J.P., Chyang, C.S., Wang, Y.J., Tso, J., 2014b. Combustion of crushed and pelletized peanut shells in a pilot-scale fluidized-bed combustor with flue gas recirculation. Fuel Process. Technol. 128, 28–35. Gogebakan, Z., Gogebakan, Y., Selçuk, N., Selçuk, E., 2009. Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite. Bioresour. Technol. 100, 1033–1036. Kuo, J.H., Wey, M.Y., Lin, C.L., Chiu, H.M., 2008. The effect of aluminum inhibition on the defluidization behavior and generation of pollutants in fluidized bed incineration. Fuel Process. Technol. 89, 1227–1236. Kupka, T., Mancini, M., Irmer, M., Weber, R., 2008. Investigation of ash deposit formation during co-firing of coal with sewage sludge, saw-dust and refuse derived fuel. Fuel 87, 2824–2837. Lin, C.L., Kuo, J.H., Wey, M.Y., Chang, S.H., Wang, K.S., 2009. Inhibition and promotion: the effect of earth alkali metals and operating temperature on particle agglomeration/defluidization during incineration in fluidized bed. Powder Technol. 189, 57–63. Lin, C.L., Peng, T.H., Wang, W.J., 2011. Effect of particle size distribution on agglomeration/defluidization during fluidized bed combustion. Powder Technol. 207, 290–295. Lin, C.L., Wey, M.Y., 2004. The effect of mineral compositions of waste and operating conditions on particle agglomeration/defluidization during incineration. Fuel 83, 2335–2343. Lin, W., Dam-Johansen, K., Frandsen, F., 2003. Agglomeration in bio-fuel fired fluidized bed combustors. Chem. Eng. J. 96, 171–185. Liu, H., Feng, Y., Liu, H., Wu, S., 2010. Study of the bed agglomeration in the strawfired fluidized bed. Energy Sources Part A 32, 1470–1478. Liu, H., Feng, Y., Wu, S., Liu, D., 2009. The role of ash particles in the bed agglomeration during the fluidized bed combustion of rice straw. Bioresour. Technol. 100, 6505–6513. Lundholm, K., Nordin, A., Öhman, M., Boström, D., 2005. Reduced bed agglomeration by co-combustion biomass with peat fuels in a fluidized bed. Energy Fuels 19, 2273–2278. Nuutinen, L.H., Tiainen, M.S., Virtanen, M.E., Enestam, S.H., Laitinen, R.S., 2003. Coating layers on bed particles during biomass fuel combustion in fluidized-bed boilers. Energy Fuels 18, 127–139. Scala, F., Chirone, R., 2005. Characterization and early detection of bed agglomeration during the fluidized bed combustion of olive husk. Energy Fuels 20, 120–132. Silvennoinen, J., Hedman, M., 2013. Co-firing of agricultural fuels in a full-scale fluidized bed boiler. Fuel Process. Technol. 105, 11–19. Skrifvars, B.J., Yrjas, P., Laurén, T., Kinni, J., Tran, H., Hupa, M., 2005. The fouling behavior of rice husk ash in fluidized-bed combustion. 2. Pilot-Scale and fullscale measurements. Energy Fuels 19, 1512–1519. Wu, P., Eriksson, G., Pelton, A.D., 1993. Optimization of the thermodynamic properties and phase diagrams of the Na2O–SiO2 and K2O–SiO2 systems. J. Am. Ceram. Soc. 76, 2059–2064. Yu, C., Qin, J., Nie, H., Fang, M., Luo, Z., 2011. Experimental research on agglomeration in straw-fired fluidized beds. Appl. Energy 88, 4534–4543.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor Feng Duan a, Chien-Song Chyang b,⇑, Li-hui Zhang a, Siang-Fong Yin b a b
School of Energy and Environment, Anhui University of Technology, Maanshan 243002, Anhui Province, PR China Department of Chemical Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan, ROC
h i g h l i g h t s Combustion of rice straw was conducted in a VFBC to study bed agglomeration behavior. Eutectics with low melting point materials promote defluidization at high temperatures. The alkali concentration has the highest value at the upper bubbling zone. Coal ash inhibits the formation of low melting compounds of rice straw combustion.
a r t i c l e
i n f o
Article history: Received 6 December 2014 Received in revised form 9 February 2015 Accepted 10 February 2015 Available online 16 February 2015 Keywords: Agglomeration Defluidization time Vortexing fluidized bed Pelletized rice straw
a b s t r a c t To investigate bed agglomeration characteristics, the combustion of pelletized rice straw was conducted in a bench-scale vortexing fluidized bed. Effects of bed temperature, superficial velocity, secondary gas velocities, and mass blended ratio of coal on the defluidization time were investigated. The alkali concentrations in different sections of the bed zone were also studied. The bed materials and agglomerates were analyzed using SEM/EDX to obtain the surface morphology and the compositions. The results revealed that the defluidization time is increased with superficial gas velocity and is decreased with bed temperature. Eutectic composition with low melting point materials promote defluidization at high temperatures. Effect of the secondary gas velocity on the defluidization time indicates different trends at different bed temperatures. The highest value of alkali concentration appears at upper bubbling zone. Coal ash can avoid the existence of a certain eutectic composition, and increases its melting point. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Rice straw is the most abundant agricultural by-product during the rice harvesting. The harvested rice straw to paddy ratio is approximately 1.0. Based on the data from the Food and Agriculture Organization (FAO), the global production of rice straw is approximately 720 million tons per year. Rice straw is known for its low density and a variety of lengths that cause high shipping and handling costs. The market for pellets from agriculture residues in Asia is rapidly expanding (Chaivatamaset et al., 2014; Kupka et al., 2008). A fluidized bed combustor is often used in burning biomass residues because the fluidized bed combustor has high fuel flexibility, uniform bed temperature, and low pollutant emissions. However, some operating issues still remain with the biomass
⇑ Corresponding author. Tel.: +886 3 2654119; fax: +886 3 4636242. E-mail addresses: [email protected], [email protected] (C.-S. Chyang). http://dx.doi.org/10.1016/j.biortech.2015.02.044 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
combustion using the current fluidized bed technology. Agglomeration is one of the problems that occur during biomass combustion in fluidized bed combustors (Scala and Chirone, 2005), and this problem can be attributed to the large amount of alkali in ash. Quartz sand is commonly used as the bed material due to its high availability, lower price, and high heat capacity. However, silicon in quartz sand reacts with the alkali in the ash and forms low melting point materials, causing agglomeration and eventually leading to defluidization, which results in unscheduled shutdowns and unnecessary expenses. The hydrodynamics and heat transfer inside the combustor cannot be observed directly by the naked eye. Measured data are needed to understand the phenomena. The event of defluidization can be estimated by the change in pressure drop because the bed pressure drop is proportional to the bed inventory (Chaivatamaset et al., 2011). The pressure drop fluctuates around an average when the FBC running steady, but the pressure drop falls rapidly after defluidization starts (Atakül et al., 2005; Billen et al., 2014; Lin et al., 2011). Scala and Chirone (2005) used three
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thermocouples, located vertically at various heights in the bed, to measure the temperature difference. The results indicate that when defluidization occurs, the temperature measured by the lower thermocouple falls rapidly, while the temperatures measured by the other two thermocouples tend to increase, before the fuel feeding stopped. This trend is related to the decrease in the heat transfer when the bed is defluidized and the segregated fuel combustion in the upper bed region. The effects of different operating parameters such as bed temperature, bed material sizes, superficial gas velocity, and static bed height, etc., on defluidization time were discussed in the literatures (Bartels et al., 2008; Silvennoinen and Hedman, 2013). We can prolong the defluidization time, defined as the period of time from the start of fuel feeding to defluidization, by changing the operating parameters. Bed temperature is the key factor for defluidization. The most convenient and effective method of preventing defluidization is to keep the bed temperature from rising above the melting point of the eutectic compositions, which prevents the melting phenomenon (Skrifvars et al., 2005; Yu et al., 2011). As the bed temperature decreases, CO emission also increases to much higher levels than the norms of the regulation (Bahillo et al., 2004; Baron et al., 2002). Superficial gas velocity directly affects the quantity of bottom ash and the heat transfer of burning char (Yu et al., 2011). Defluidization time increases with superficial gas velocity, but the effect of superficial gas velocity on defluidization is minimal at higher bed temperatures (Liu et al., 2010). For a certain superficial gas velocity, defluidization time is inversely proportional to the particle size of the bed material. The fluidization number for large particles is less than the fluidization number for small particles, leading to a poor quality of fluidization and hot spots in the bed. The surface-to-volume ratio of the particle decreases with increasing particle size. The larger particles require less coating to agglomerate, and the agglomerated particles take less ash to cause defluidization (Chaivatamaset et al., 2011, 2013; Kuo et al., 2008; Lin and Wey, 2004; Lin et al., 2003). In addition to changing operating parameters, selecting bed material type and adding inhibitors such as clay and kaolinite are also discussed in the literature. The methods mentioned above can only prolong the defluidization time but still cannot avoid agglomeration and defluidization. Some previous studies have shown that biomass co-combustion with peat or coal can be an inexpensive measure for preventing bed agglomeration (Arvelakis and Frandsen, 2007; Brus et al., 2004; Gogebakan et al., 2009; Lundholm et al., 2005). The main mechanism for preventing the formation of viscous coatings is the retention of alkali by chemisorptions or physical adsorption by minerals and ash constituents in coals (Lundholm et al., 2005). The concept of vortexing fluidized bed combustion (VFBC) was introduced to create a vortex by injecting a secondary gas tangentially into the freeboard to increase the combustion intensity. VFBC has been used for biomass or coal combustion studies in recent years (Duan et al., 2013, 2014b). In these studies, biomass combustion remained steady while quartz sands were used as the bed material. The bed temperatures were kept below 800 °C to avoid agglomeration. The vortexing effect caused by the secondary gas would capture the small particles including ash back to the bed zone. Therefore, the agglomeration behavior may differ from a conventional FBC at this condition. Reports on the agglomeration characteristics at different secondary gas velocities are few. It is meaningful to screen the operation parameters to avoid agglomeration before designing combustion scenarios in a VFBC. The objective of this work is to investigate the effect of operating conditions such as bed temperature, superficial gas velocity and secondary gas velocities on the defluidization time in a VFBC. Coal is also used as an additive
to quantify the potential positive effect in terms of defluidization time by co-combustion. The alkali concentration distributions in the bed zone under various operating conditions were also studied. 2. Methods 2.1. Fuel and bed materials Quartz sand (99.5% SiO2) with a mean diameter of 540 lm was used as an inert bed material in this study. Cylindrical pelletized rice straw was used as the fuel in the present work. The mean diameter of cylindrical pelletized rice straw is 6 mm and its length ranges from 5 to 15 mm with a mean length of 9.6 mm. The coal was used as an additive in this study. The mean diameter of the coal particles is 3.1 mm. The properties of pelletized rice straw and coal are shown in Table 1. During the co-combustion test, the feeding materials were prepared according to the mass blended ratio. In the mineral analyses, apart from silica content, the potassium content of the pelletized rice husks is the highest. This composition was expected to be problematic for agglomeration in biomass fluidized-bed combustion. 2.2. Vortexing fluidized-bed facility Fig. 1 shows the schematic diagram of the experimental setup of the vortexing fluidized bed combustion (VFBC) system in this study. The combustor is assembled with a wind box, distributor, combustion chamber, and freeboard. The combustion chamber is 0.22 0.11 m2 in area and is 0.678 m in height. The freeboard with an inner diameter of 0.154 m and 4 m in total height is fabricated with SUS310 pipe. The combustion chamber and freeboard are equipped with an electrical heating system and wrapped with 0.025 m thick ceramic fiber for thermal isolation. The feeding system is an assembly of a feedstock hopper and a screw feeder, with the feeding point at 0.5 m above the distributor. The temperatures within the combustor are measured with Ktype thermocouples. Bed temperature is measured with three ther-
Table 1 Properties of pelletized rice straw and coal. Analyses
Pelletized rice straw
Coal
Proximate analysis (wt%, as received) Moisture 9.29 Volatiles 64.24 Fixed carbon 13.21 Ash 13.26
16.00 32.94 40.49 10.57
Ultimate analysis (wt%, dry and ash free) C 44.40 H 7.40 O 47.07 N 1.13 S 0.00
66.93 4.6 26.10 1.49 0.88
Mineral analysis (mg/kg, dry basis) Na 860.80 Mg 4074.00 Al 6203.00 Si 39310.00 P 940.60 S ND K 19350.00 Ca 4917.00 Ti 120.20 Fe 1401.00
1328 7764 4609 148,200 813.3 7692 1612 3174 172.6 3575
Heating value (kcal/kg) HHV(WB) LHV(WB) Bulk density (kg/m3)
6192.32 5832.08 1350.46
3593.00 3244.76 693.63
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197
Fig. 1. Schematic diagram of bench scale VFBC system.
mocouples (0.07, 0.15, and 0.25 m above the distributor), installed vertically along the combustion chamber to measure the temperatures at each height. The mean value of the temperatures detected from these three thermocouples is considered the bed temperature. The primary and secondary air is supplied by an air compressor. In addition, a nitrogen supply system is used to mix nitrogen into the primary and secondary air. Four equally spaced secondary gas injection nozzles (0.013 m in diameter) are installed tangentially 0.796 m above the distributor. The flue gas from the combustor passes through a cyclone to remove most of the unburned char and ash particles and is then cooled by a heat exchanger. Finally, the flue gas is released to the atmosphere, and fly ash is collected in a bag house. 2.3. Experimental procedures The steady state combustion is carried out under atmospheric conditions according to the set working conditions. The operating conditions for the test are summarized in Table 2. The test is started by preheating the combustor chamber using electric heaters
Table 2 Operating conditions. Operating parameter
Units
Values
Fuel feeding rate Coal mass blending ratio Bed temperature Freeboard temperature Superficial gas velocity Stoichiometric oxygen in the combustion chamber Primary gas flow rate Secondary air flow rate Excess oxygen ratio Bed material Mean particle size of bed material Density of bed material Static bed height Bed material weight
kg/h % °C °C m/s % NL/min NL/min % – lm kg/m3 cm kg
2.78 25, 50 750–850 850 0.6–1.0 100 212–353 78–195 50 Quartz sand 540 2500 20 7
and the continuous secondary air injection. Primary air was injected into the combustor occasionally to stir the sand. The fuel is fed into the combustor via a screw feeder while the bed temperature reaches 500 °C, and the gas flow rate is adjusted to the set value. The elapsed time is recorded while the fuel begins to feed. Defluidization is considered to occur when the temperature difference in the bed is over 50 °C, and we stop the fuel feeding and switch off the primary and secondary gas at this point. During the test, the fly ash is collected from the cyclone. After the test, the windbox is pulled down after the combustion chamber is cooled to the room temperature, and then the bed materials and agglomerates are collected. The combination of SEM/EDX is used to characterize the surface morphology and composition of the particles and agglomerates. The potassium concentration profile in the bed is examined by inductively coupled plasma-mass spectrometry (ICP-MS). 3. Results and discussion 3.1. Defluidization time Fig. 2 shows the effect of the superficial gas velocity on the defludization time and elutriated fly ash. The historical bed temperature profiles at various superficial gas velocities are also shown in this figure. In this study, bed temperature profiles are monitored to determine the onset of defluidization. Fig. 2A–C shows that the temperature of the upper bed zone (z = 0.25 m) is always slightly higher than the temperatures in the other parts of the bed zone (z = 0.07 m, z = 0.15 m). This difference can be attributed to the combustion of the pelletized rice straw, which occurs mainly in this section. With the increase of superficial velocity, the temperature differences decrease due to the well mixed bed material (Fig. 2D). When defluidization occurs, the change in the temperature profile is similar under different superficial gas velocities. In these figures, the temperature at z = 0.25 m increases sharply, while the temperature at z = 0.07 m drops quickly when defluidization happens. This temperature drop can be caused by
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1100
1100
A
B
T1 ( 7 cm)
U = 0.6 m/s
T2(15 cm)
1000
1000
T1 ( 7 cm)
Temperature ( C)
o
o
Temperature ( C)
T3(25 cm) 900 800 700
U = 0.7 m/s T2(15 cm)
900
T3(25 cm)
800 700
Defluidization
Defluidization 600
600 500
500
0
1000
2000
3000
4000
5000
0
1000
2000
1100
1100
C
T2(15 cm)
1000
5000
6000
7000
T2(15 cm) T3(25 cm)
900 800
4000
T1 ( 7 cm)
U =1.0 m/s
1000
T3(25 cm)
Temperature (ºC)
Temperature (ºC)
D
T1 ( 7 cm)
U = 0.8 m/s
3000
Time (sec)
Time (sec)
Defluidization
700
900 800 700
Defluidization 600
0
1000
2000
3000
4000
5000
6000
500
7000
0
3000
6000
Time (sec)
9000
120
500
E Defluidization time (min)
12000 15000 18000 21000
Time (sec)
Defluidization time Fly ash elutriated
400
100 80
300 60 200
Eo = 50% Sb = 100% o
Tb = 850 C
100
Qsec = 78 NL/min
0
40
0.6
0.8
1.0
Fly ash elutriated (g/hr)
500
600
20 0
Superficial gas velocity, U (m/s) Fig. 2. Effect of superficial gas velocity on the defluidization time and fly ash elutriated. (Eo = excess oxygen ratio; Sb = stoichiometric oxygen ratio in the bed; Tb = bed temperature; Qsec = flow rate of secondary gas.)
the melting of low melting point materials and generates agglomerate at the same time. The agglomerate of varying sizes brings down fluidization and eventually leads to defluidization, which results in the decrease of temperature in the lower bed zone (z = 0.15 m) because burned fuel particles cannot migrate from the bed surface to the bottom. According to the data shown in Fig. 2A–D, the defluidization time is increased with the superficial gas velocity as shown in Fig. 2E, which is in agreement with the published literatures
(Chou and Lin, 2012; Kuo et al., 2008; Liu et al., 2010). As the superficial gas velocity increases, the collision and mixing of particles in the bed zone becomes vigorous. The effect of the adhesive force appears to decrease, while the erosion rate of the coating layer on the bed particle surface increases with increasing superficial gas velocity. The higher superficial velocity causes higher mixing of bed material and burned fuel particles, which results in a uniform bed temperature and prevents hot spots. The data shown in Fig. 2E also reveal that the elutriation of fly ash is increased with
F. Duan et al. / Bioresource Technology 183 (2015) 195–202
superficial gas velocity, which results in the decrease in the amount of bottom ash in the bed. Therefore, the chance of agglomeration in the bed zone decreases. The effects of bed temperature, secondary gas velocity, and coal mass blend ratio on the defluidization time are shown in Fig. 3. As seen in Fig. 3A, the defluidization time is decreased from 300 to 45 min when the bed temperature is increased from 800 to 900 °C. The extent of melting of low melting point materials increases with bed temperature, which results in the increase of the viscous force among the particles. This increase of the viscous force among the particles causes the agglomeration and eventually leads to the defluidization. The defluidization did not occur throughout the 10 h when the test was operated at 750 °C. This result is different from the findings reported in the literature, which are 2 h for combustion of rice straw and 1.3 h for combustion of wheat straw at 750 °C in a bubbling fluidized bed (Lin
Defluidization time (min)
1200
A
no defluidization Eo = 50%
1000
Sb = 100%
800
U = 0.6 m/s o Tf= 850 C
600
Qsec = 78 NL/min
400 200 0
740
760
780
800
820
840
860
880
900
920
o
Bed temperature ( C)
B
Defluidization time (min)
300 250 200
o
Eo = 50%
o
Tf= 850 C
Tb=850 C
150
Tb=800 C
o
Sb = 100% U = 0.6 m/s
100
et al., 2009; Liu et al., 2009). The comparison of the operating conditions in this study and other literature, concerning the combustion of rice straw and wheat straw, are summarized in Table 3. The fluidization number used in this study, which represents the extent of fluidization, is larger than the fluidization number of Lin et al. (2009) and Liu et al. (2009), and the defluidization time is proportional to the fluidization number. The long defluidization time, >10 h, obtained in this study can be attributed to the high superficial gas velocity. In this study, the secondary gas was introduced tangentially into the freeboard, resulting in a swirl flow in the freeboard, and the elutriated fly ash was trapped back to the bed. As the secondary gas flow rate is increased, the vortexing effect is more significant. The amount of elutriated fly ash is decreased with the secondary gas flow rate, resulting in increasing amount of bottom ash (Duan et al., 2014a). Fig. 3B shows the effects of the secondary gas flow rate on the defluidization time. In this figure, the defluidization time obtained at different temperatures shows the different trends. The defluidization time is decreased with secondary gas flow rate when the bed temperature is 800 °C, while the defluidization time is stayed nearly constant at a bed temperature of 850 °C. These observations can be explained by the fact that the defluidization time decreases with bed temperature and ash concentration. The agglomeration tends to occur at a higher concentration of the fly ash in the bed. The fly ash trapped by swirl flow increases with the flow rate of secondary gas. Therefore, the defluidization time is decreased with the flow rate of the secondary gas at the bed temperature of 800 °C. The effect of the fly ash concentration in the bed is overwhelmed by the effect of the temperature at high temperatures. The agglomeration occurring at 850 °C needs only a small amount of ash accumulation in the bed. Therefore, the effect of the secondary gas flow on the defluidization time is minimal at 850 °C. The agglomeration characteristics of pelletized rice husks and coal in different proportions were also investigated in this study. Fig. 3C shows the effect of the coal mass blended ratio on the defluidization time. As discussed above, a higher bed temperature is prone to agglomeration, and the defluidization time is increased with the coal mass blending ratio. In this test, the steady state combustion was carried out continuously for 20 h at a fixed coal mass blending ratio of 50%, and no agglomeration was observed. The results indicate that the ash from coal combustion can inhibit agglomeration of bed material particles. Therefore, co-combustion of biomass with coal is suggested. 3.2. K concentration profile
50 60
80
100
120
140
160
180
200
Secondary gas flow rate (NL/min) 1200
C 1000
Defluidization time (min)
199
no defluidization
800 600
Eo = 50% Sb = 100% o
400
Tb=850 C o
Tf=850 C
200
Qsec= 135 NL/min U = 0.6 m/s
0 -10 -5 0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Coal mass blending ratio (%) Fig. 3. Effect of operating conditions on the defluidization time. (Tf = freeboard temperature; U = superficial gas velocity.)
According to the temperature distributions detected by three thermocouples as shown in Fig. 2, we can presume that agglomerates are generated at the upper part of the bed zone. However, the relationship between defluidization and accumulation of alkali concentration in the different sections of the bed is unknown. In this study, we divided the bed into three sections from the distributor to the bed surface. The schematic diagram shown in Fig. 4 represents these three sections. The sections are the upper bubbling zone, the lower bubbling zone, and the grid zone. As the combustion chamber is cooled down, the top half of static material is defined as upper bubbling zone, while the bottom half is defined as lower bubbling zone. The grid zone is defined as the zone between the air distributor and the blast cap. The upper bubbling zone is composed primarily of agglomerates, and the sizes of most agglomerates are larger than 2000 lm. The lower bubbling zone is composed of agglomerates and bed materials, and the size of the agglomerates is much less than the size of the agglomerates in the upper bubbling zone. The color of bed material is milky white in these two bubbling zones. The grid zone is close to the dis-
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Table 3 A comparison of operating conditions for rice straw combustion experiments. Parameters
Unit
This study
Lin et al. (2003)
Liu et al. (2009)
Fuel Combustor dimension Bed temperature Bed material Mean particle size Superficial gas velocity Fluidization number Defluidization time
– m °C – lm m/s – min
Rice straw 0.22 0.11 750 Quartz sand 540 0.60 5.83 1200 (no defluidization)
Wheat straw w0.068 1.2 750 Quartz sand 275 0.06 2.23 78
Rice straw w0.032 0.6 750 Quartz sand 250 0.10 4.50 120
10
K concentration (wt. %)
A o
8
850 C o 800 C o 750 C
6
Eo = 50%
4
2
Sb = 100% U = 0.6 m/s Qsec = 78 NL/min
0 Grid zone
Lower-bubbling zone
Upper-bubbling zone
8
tributor, which consists of bed materials with an orange color. The color may be caused by coating some metals on the surface of the bed material. The depth of these three sections varies with operating conditions. As the combustion chamber is cooled down, agglomerates and bed materials in three different sections are collected and examined for alkali concentration by ICP-MS. The potassium, problematic elements in agglomeration, is a dominant alkali in rice straw (Lin et al., 2003). Only the potassium concentration is discussed in this study. The distribution of potassium concentrations in different sections of the bed with various operating parameters such as bed temperature, ratio of primary gas velocity to secondary gas velocity, and coal mass blending ratio is shown in Fig. 5. Fig. 5A shows that the potassium concentration in the upper bubbling zone is higher than the potassium concentration in the other sections and strongly dependent on the bed temperature. The potassium concentration of the bed material at the surface zones is decreased significantly while the bed temperature is increased from 750 to 850 °C. This decrease can be explained by fact that only a limited amount of accumulated fly ash can induce bed agglomeration at a higher temperature. The alkali metal oxide content of the ash from biomass combustion can react with the silica to form the eutectic compositions with lower melting points. The reaction is:
K2 O þ nSiO2 ! K2 O nSiO2
ðreaction1Þ
When n is 3 or 4, the melting temperature of the eutectic compositions is lower than 800 °C (Nuutinen et al., 2003; Wu et al., 1993). The data from temperature and potassium concentration distribution demonstrate that as accumulation of potassium con-
6
P = Primary gas flow rate S = Secondary gas flow rate Eo = 50% Sb = 100% o
4
Tb = 850 C
P/S=1.57 P/S=2.12 P/S=2.71 P/S=3.62 P/S=4.52
Average K concentration in surface zone
2
0
Grid zone
Lower-bubbling zone Upper-bubbling zone
8
C 6
K concentration
Fig. 4. Schematic representation of the bed structure.
K concentration (wt. %)
B
100% Rice straw 75% Rice straw +25% Coal 50% Rice straw +50% Coal
Eo = 50% 4
Sb = 100% o
Tb = 850 C 2
0 Grid zone
Lower-bubbling zone Upper-bubbling zone
Fig. 5. Potassium concentration in the bed under various operation conditions.
centration in the upper bubbling surface zone reaches the critical value at higher bed temperatures, the agglomeration at the bed surface results and leads to defluidization. Fig. 5B shows that the potassium concentration in the upper bubbling zone is remained at 3–3.8 wt% at a fixed bed temperature
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F. Duan et al. / Bioresource Technology 183 (2015) 195–202 Table 4 EDX analyses of selected zone of the particles (wt%, oxygen-free basis). Su.Ma.
Point
Na
Mg
Al
Si
K
Ca
Fe
C
a b c d
– – 1.49 –
– – 3.21 2.87
– 1.03 8.78 1.87
63.91 64.37 65.58 74.74
25.29 25.52 20.94 20.52
10.80 9.08 – –
– – – –
D
e f
– –
– –
– 3.41
86.46 81.99
13.54 14.60
– –
– –
E
g h
– –
– –
13.98 12.69
81.39 82.25
4.63 5.06
– –
– –
F
i
–
–
2.29
97.71
–
–
–
G
j k l
– – –
– 2.18 –
3.37 10.73 –
64.17 47.85 83.20
18.62 19.83 16.80
5.77 4.62 –
8.07 15.25 –
H
m n o
– – –
1.89 1.93 1.53
21.45 19.06 19.56
51.05 51.33 55.08
19.83 16.54 15.70
– 4.75 –
5.78 6.38 8.14
of 850 °C. The effect of operating parameters on potassium concentration distribution in various sections of the bed is not obvious. The data shown in Fig. 5B are obtained from the experiment operated at 850 °C with various primary/secondary gas flow rate ratios. This vortexing effect increases as the ratio decreases, and the fly ash from pellet combustion with higher potassium is carried back to the upper bubbling zone. Meanwhile, the fly ash captured by the vortexing effect decreases with the increase in this ratio, resulting in a lower potassium concentration. However, more pellet particles will combust in the upper part of the bed zone at a higher primary gas flow rate and lower secondary gas flow rate, increasing the reaction rate of pellet particles in the bed zone and resulting in higher local temperatures, thus accelerating the potassium release. Therefore, the potassium concentration in the upper bubbling zone increases gradually with the ratio of primary gas flow rate to secondary gas flow rate. According to the data from Fig. 5B, it can be said that the effect of the secondary air on the agglomeration is minimal at a high temperature of 850 °C. Fig. 5C shows the potassium concentrations in the three sections at various coal mass blending ratios. The upper bubbling section has the highest potassium concentration, and the value of the blended fuel combustion is nearly two times the value of the blended fuel combustion from rice straw in individual combustion. It can be interpreted by the fact that the defluidization time increases with the coal mass blending ratio and the accumulation of potassium with its concentration gradually increasing with the operating time.
3.3. The compositions of particles The agglomerates obtained in this study are fragile and light. During the test, the composition and morphology of the particles are observed by SEM/EDX. Table 4 presents the EDX analyses of the particles from selected zones. The surface of the unused fresh bed material is rough and sharp and is composed of 97.71% silicon and 2.29% aluminum. In the grid zone, aluminum is increased to approximately 13%, while silicon is reduced to approximately 82.00%, and the potassium is approximately 5.0%. In the lower bubbling zone, the composition of the bed materials consists of aluminum, silicon, and potassium with the morphology of a rough surface, and in this zone the potassium is increased to approximately 14.00%. The color of the bed materials in the grid zone and lower bubbling zone is different, possibly because of the content of aluminum, as the aluminum content in the grid zone is significantly increased.
The inorganic ash remaining in the upper bubbling zone includes two extreme types of agglomeration. One type of agglomerate has a uniform coating, formed on the surface of the bed material grains. The other type is glossy and exhibits liquid-like holes. It can be attributed to the existence of bubbles which can be observed on the surface. This type of agglomerate dominates this installation. During the test, the coating layer starts to melt, and the bed material surface becomes viscous when the thickness of the coating layer reaches a certain value, resulting in the merging of particles at collision. The sizes of the agglomerates are not uniform, leading to a poor quality of fluidization. The composition of the melted materials consists mainly of approximately 10% calcium, 64% silicon, and 25% potassium. The second type of agglomerate is designated as ‘‘melt-induced’’ agglomeration. In this case, the bed materials are bonded together by a melting phase, which roughly matches the ash chemical composition and is produced at normal bed temperatures. In this figure, the composition of the bonded points consists of aluminum, magnesium, calcium, silicon, and potassium. Compared with the first type, the potassium is reduced up to 20% in the second type of agglomerate. However, the SEM analysis data from the co-combustion test show that the agglomerates, found on the surface of the bed material have a uniform coating and in some cases, a porous coating. Results from the EDS analysis also showed that the elemental composition of the coating is different with individual rice straw combustions depending on the mixed coal mass, implying that the mechanism of bed agglomeration may be different for co-combustion. Therefore, the metal oxides such as Al2O3 and Fe2O3 in the bottom ash will react with alkaline compounds to form K2Fe2O4 and K2Al2O4. These metal oxides can also react with K2OnSiO2 (reactions ((2)–(4)), and the melting point of these reaction products are all higher than 955 °C (Lin et al., 2003).
K2 O nSiO2 þ Fe2 O3 ! K2 O Fe2 O3 nSiO2
ðreaction2Þ
K2 O nSiO2 þ Al2 O3 ! K2 O Al2 O3 nSiO2
ðreaction3Þ
K2 O nSiO2 þ Al2 O3 ! K2 O Al2 O3 ðn 1ÞSiO2 þ SiO2
ðreaction4Þ
These products with higher melting points therefore form on the surface of the fixed carbon of biomass, biomass ash, and the eutectic compositions with a lower melting point. This formation prevents the generation and migration of eutectic compositions with lower melting points and decreases the chances of agglomeration.
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