Bioresource Technology 175 (2015) 275–283
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Prediction of product distribution in fine biomass pyrolysis in fluidized beds based on proximate analysis Sung Won Kim ⇑ Global Technology, SK Innovation, 325 Exporo, Yuseong-gu, Daejeon 305-712, Republic of Korea
h i g h l i g h t s A model was developed to predict product distribution from biomass pyrolysis. The model is based on proximate analysis and hydrodynamics in fluidized beds. Relationships between product yields and fluidization condition were derived. Gas and char yields are a strong function of temperature and vapor residence time.
a r t i c l e
i n f o
Article history: Received 4 September 2014 Received in revised form 18 October 2014 Accepted 20 October 2014 Available online 28 October 2014 Keywords: Biomass pyrolysis Fluidized bed Prediction Product distribution Proximate analysis
a b s t r a c t A predictive model was satisfactorily developed to describe the general trends of product distribution in fluidized beds of lignocellulosic biomass pyrolysis. The model was made of mass balance based on proximate analysis and an empirical relationship with operating parameters including fluidization hydrodynamics. The empirical relationships between product yields and fluidization conditions in fluidized bed pyrolyzers were derived from the data of this study and literature. The gas and char yields showed strong functions of temperature and vapor residence time in the pyrolyzer. The yields showed a good correlation with fluidization variables related with hydrodynamics and bed mixing. The predicted product yields based on the model well accorded well with the experimental data. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Pyrolysis is one of the most promising technologies of biomass utilization, which converts the biomass to bio-oil, char and gasses depending on the pyrolysis conditions. The pyrolysis is a thermal degradation of materials in the absence of oxygen. The pyrolysis can be a promising option for lignocellulosic biomass conversion because bio oils derived from biomass pyrolysis could act as feedstocks for producing hydrocarbons that may be readily integrated into existing petroleum refineries or future bio-refineries (Kim et al., 2013a). In past decades, significant progress has been made in developing various pyrolysis technologies, and successfully testing them on laboratory, pilot and industrial scales. Recently, extensive research has been focused on fast pyrolysis using the fluidized bed process because of its high heat transfer and the short residence time for vapor to obtain a high yield of bio oil. Many studies ⇑ Tel.: +82 42 609 8314; fax: +82 42 609 8804. E-mail address: [email protected] http://dx.doi.org/10.1016/j.biortech.2014.10.107 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
have been carried out to determine the parametric influence of operating conditions on the yield of pyrolysis products of various biomasses for the design and optimum operating conditions of fluidized bed pyrolyzers. They found that the product distribution is affected by operating parameters as well as biomass type (Garcia-Perez et al., 2008; Lee et al., 2008; Heo et al., 2010). The description of the pyrolysis process including product distribution is particularly challenging because it involves a great deal of physical and chemical transformations and produces a large number of product species. In spite of the technological progress and large amount of experimental data reported in the literature, there is still considerable debate over the reaction mechanism controlling the distribution of pyrolysis products (Garcia-Perez et al., 2008). As a result, existing models aiming to predict the rates or yields of the released pyrolytic volatiles are still supported by empirical data (Neves et al., 2011). Neves et al. (2011) suggested an empirical model to approximate the pyrolysis behavior of most biomass, where the product distribution is a function of temperature. However, the product distribution is affected by reactor type and geometry in addition to the chemical structure of the biomass
276
S.W. Kim / Bioresource Technology 175 (2015) 275–283
in experiments of this study and literature, which address pyrolysis of fine biomass less than 2 mm to minimize the impact of internal heat transfer inside feedstock. A comparison between model results and experimental data was done to show its predictive capability for pyrolysis of various biomasses in fluidized beds.
Bio oil product Moisture
Biomass
Organics
Condensable liquid
Gas
Gas product Char product
Ash
Char
2. Methods
Fig. 1. Biomass pyrolysis concept for the predictive model in this study.
2.1. Methodology and model structure
(Cao et al., 2004). Especially, the pyrolytic oil yield in a fluidized bed is affected by the fluidizing conditions, such as gas velocity, the physical properties of bed material and static bed height or vapor residence time (Cui and Grace, 2007). Therefore, biomass thermochemical conversion development requires methodologies for prediction of the product distribution, whereby an appropriate balance between empiricism and fundamentals related with characteristics of biomass and flow dynamics should be considered to describe the complex range of interrelated phenomena (Ioannidou et al., 2011). When considering biomass thermal conversion, proximate analysis is one of the most important characterization methods. This consists of determining the moisture, ash, volatile matter and fixed carbon contents of the raw biomass. These values are essential as certain moisture, volatile matter and fixed carbon affect both thermal behavior and plant design (Garcia et al., 2013). Recently, a thermogravimetric model for pyrolysis product distribution in a captive batch reactor was proposed based on the proximate analysis of three agricultural residues (Ioannidou et al., 2011). The model enabled the possibility to evaluate a mass balance during the thermal treatment of the residue. However, the model did not take into account the possible activity of a second reaction (Ioannidou et al., 2011) which can possibly come from a variation in operating conditions. Also, it is still far from being applied in a fluidized bed pyrolyzer due to different operating principles between reactor types. A predictive model was developed to describe the general trends of product distribution in fluidized bed for biomass pyrolysis. The model was made of mass balance based on proximate analysis and an empirical relationship with operating parameters. The empirical relationships between product yields and fluidization conditions in fluidized bed reactors were derived from the data
The biomass pyrolysis concept for this study is based on the fact that the precise mechanism is not clearly known because of extremely complex reactions that take place during the biomass pyrolysis process (Ioannidou et al., 2011). A simplified two-step concept of the pyrolysis was introduced to consider possible secondreaction which is affected by operating condition as Fig. 1. The concept assumes that biomass is composed of three components, i.e., organics (volatile and fixed carbon), moisture and ash based on proximate analysis. It is assumed that all organics and water are devolatilized from biomass and ash is left as part of solid residue as a first step in the pyrolysis process. The organics are partly converted into gas and char, depending on operating conditions in the second step of the pyrolysis process. Finally, unconverted organics, pyrolytic water and moisture are produced as bio oil or a liquid product from the fluidized bed reactor (Neves et al., 2011). The whole solid fraction of ash and pyrolytic char is produced as char, assuming that all the ashes in the feed material remain in the char. Taking into account the simplified concept and the proximate analysis results of the input material (moisture: M, volatile matter: VM, fixed carbon: FC and ash: A), it is possible to calculate mass balance as Eqs. (1)–(3).
Feedstock ¼ M þ OR þ A
ð1Þ
where OR is organics (volatiles and fixed carbon) as Eq. (2).
OR ¼ VM þ FC
ð2Þ
Product ¼ bio oil þ gas þ char
ð3Þ
Assuming that each feedstock for pyrolysis in this model has the same content of moisture as the sample used in proximate analysis, the mass of bio oil is calculated as the sum of the moisture content and mass of unconverted organics obtained from char and gas yields as Eq. (4).
Table 1 Proximate analysis of a set of biomasses used in this study. Raw material
Jatropha seedshell cake Mallee Eucalyptus loxophleba (woody fraction) Cassava stalk Cassava rhizome Jatropha seedshell waste Quercus acutissima Miscanthus sinensis Palm kernel shell Jatropha seedshell cake Miscanthus sinensis var. purpurascens Sugar cane bagasse Corn cob Corn stover Eucalyptus grandis Cassava rhizome Cassava stalk Japanese larch Radiata pine sawdust Oriental white oak a
Organics (sum of volatile and fixed carbon).
Proximate analysis [wt%]
Reference
Moisture
Volatile
Fixed carbon
Ash
2.7 0.0 0.0 0.0 2.7 8.3 10.2 5.9 2.7 8.0 6.8 4.6 8.5 6.2 1.8 2.4 8.8 7.6 10.4
79.8 81.9 79.9 77.7 79.8 73.9 71.2 71.3 79.8 74.9 76.9 79.9 76.7 80.9 81.5 81.2 91.0a 92.2a 87.6a
14.1 17.6 14.1 18.2 14.1 16.7 15.2 17.8 14.1 15.7 10.8 13.7 8.2 12.4 13.1 11.2
3.4 0.5 6.0 4.1 3.4 1.0 3.4 5.0 3.4 1.4 5.3 1.6 6.1 0.5 3.5 5.1 0.2 0.2 2.0
This study Garcia-Perez et al. (2008) Pattiya (2011) Kim et al. (2013a) Lee et al. (2008) Bok et al. (2013) Kim et al. (2013b) Heo et al. (2010) Carrier et al. (2013)
Pattiya and Suttibak (2012) Park et al. (2008a) Park et al. (2008b) Park et al. (2009)
Table 2 Experimental details used in this study. References
Material
This study
418, 764
1.00
Garcia-Perez et al. (2008) Pattiya (2011) Kim et al. (2013a) Lee et al. (2008)
303a 428a 750, 1700 1113a
1.00 0.105 1.00 0.30
Bok et al. (2013)
1500
2.0, 0.20
Silicon carbide Sand Sand Sand Fused alumina Sand
Kim et al. (2013b) Heo et al. (2010) Carrier et al. (2013)
763a 700 408, 573, 868, 1129 125, 338, 406, 513
0.94 0.15 0.85
SiC Zirconia Alumina Sand
0.40
Silica sand
300, 700, 1000 300, 700, 1000
0.15 0.15, 0.18 0.15
Emery Emery Emery
c
300, 700, 1000
Particle diameter [lm]
Fluidizing gas Particle density [kg/ m3]
Gas
Gas flow rate [liter/min]
Temperature [°C]
Reactor
Symbols in figure
Diameter [m]
Height [m]
190
3210
Nitrogen
33, 41
390–512
0.102
0.97
376 428 278 40
2600b 2600b 2600b 3930
Nitrogen Nitrogen Nitrogen Nitrogen
38–53 5.9–6.6 30 2.5–8.2
350–580 437–537 500 300–550
0.102 0.0381 0.10 0.076
0.518 0.30 0.85 0.80
j N d
200
2600b
Nitrogen
30, 70
400–550
3210 3800 2500 2670
Nitrogen Nitrogen Nitrogen
33 5 40
392–511 350–550 500
0.17c 0.1 0.102 0.08 0.075
0.30c 0.3 0.97 0.30 0.68
}
b
Nitrogen
7
393–550
0.05
0.45
h
40 40
3930 3930
Nitrogen Nitrogen
2–4 2–4
400–550 400–550
0.08 0.08
0.30 0.30
4 s
40
3930
Nitrogen
2–4
400–550
0.08
0.30
⁄
190 186.5 40 320 338
2600
S.W. Kim / Bioresource Technology 175 (2015) 275–283
Feed rate [kg/hr]
Park et al. (2009) b
Fluidized bed material
Average size [lm]
Pattiya and Suttibak (2012) Park et al. (2008a) Park et al. (2008b)
a
Feedstock
Calculated from range of feedstock size. Based on Kunii and Levenspiel (1991). Rectangular type.
277
278
S.W. Kim / Bioresource Technology 175 (2015) 275–283
Table 3 Operating parameters of fluidized bed and their correlations used in this study. Name of parameter, symbol
Equation or correlation
Reference
kg/m s
Eb ¼ 0:1qp ð1 emf ÞðU g U mf Þ
Pemberton and Davidson (1986)
Gas flux, Eg
kg/m2s
where qp is particle density, emf is voidage at minimum fluidizing condition and Ug is gas velocity Eg ¼ qg U g
Superficial gas velocity at minimum fluidizing condition, Umf
m/s
Bed entrainment flux, Eb
Unit 2
where qg is gas density i l h 1=2 g 33:7 for dp P 100 lm ð33:72 þ 0:0408ArÞ dp q
U mf ¼
g
1:8
U mf ¼
ðqp qg Þ0:934 g 0:934 dp 1110lg0:87 qg0:066
for dp < 100 lm
Wen and Yu (1966) Baeyens and Geldart (1974)
where lg is gas viscosity, Ar is Archimedes number, g is acceleration of gravity and dp is particle size of bed material U
Bubble fraction in fluidized bed, db
–
db ¼ U gb
Bubble rising velocity, Ub
m/s
U b ¼ ðU g U mf Þ þ U br
Single bubble’s rising velocity, Ubr
m/s
U br ¼ 0:711ðgdb Þ
Average bubble diameter, db
m
db ¼ 0:138hb ðug umf Þ0:42 expð2:5 105 ðug umf Þ2 103 ðug umf ÞÞ where hb is bed height
Nusselt number for gas-particle heat transfer, Nu
–
1=2
0:8
Nu ¼
Kunii and Levenspiel (1991) Kunii and Levenspiel (1991) Kunii and Levenspiel (1991) Cai et al. (1994)
h p dp kg
where hp is heat transfer coefficient between gas and particle and kg is thermal conductivity of gas
Bio oil ¼ M þ ORð1 Y RC Y RG Þ
ð4Þ
In Eq. (4), YRC indicates the relative char yield, which is the char yield converted from organics, and YRG indicates the relative gas yield, which is the gas yield converted from organics based on the simplified concept in Fig. 1. YRC and YRG can be expressed as Eqs. (5) and (6).
Y RC ¼ ðChar AÞ=OR
ð5Þ
Y RG ¼ Gas=OR
ð6Þ
The char and gas yields in pyrolysis products can be obtained from Eqs. (5) and (6), if YRC and YRG are known. 2.2. Experimental method A sample of Jatropha (Jatropha curcas L.) seedshell cake was acquired from an oil extraction plant in Indonesia. Proximate analysis was carried out with a Thermostep (ELTRA) analyzer according to the ASTM 5142 standard test method. The proximate analysis showed volatiles (mainly organic) of 79.80 wt% as shown in Table 1. The Jatropha seedshell cake sample was ground and sieved to an average size of 418 and 764 lm using an electric mixer and standard sieves. Silicon carbide (dp = 190 lm, qs = 3210 kg/m3) particles were used as bed material in this study. The pyrolysis system consisted of a mass flow controller (MFC), main column, screw feeder, cyclone, condensers, and accumulative flowmeter. The system was previously described in detail (Kim et al., 2014). The flow rate of nitrogen (99.9%) for fluidization was controlled by the MFC and the volume of the product gas was measured by the accumulative flowmeter (G6R, GTEC Industry). Before entering the stainless steel fluidized bed reactor (0.102 m id. and 0.97 m high), the fluidizing N2 gas was preheated in the air plenum to 450–600 °C. To prevent condensation of the pyrolysis vapor, the top of the reactor and the pipe connecting the reactor to the first condenser were maintained at 400 °C. The refrigerant was circulated in the shell of the condensers by a chiller system. The gas was sampled by a gas sampler to analyze its composition. Pyrolysis tests were conducted at 390–512 °C with N2 flow rates of 33 and 41 L/min at 25 °C (superficial gas velocities of 0.15–0.21 m/s depending on temperature), corresponding to ca. 7.5–10.4 Umf and a bubbling fluidization regime. A static bed height
of 0.2 m was maintained in the test. Biomass particles were fed at 1.0 kg/h. After completion of the feeding of biomass in the feeder, heating of the reactor was discontinued. Thereafter, the oil sampling pots, condensers, oil filter and the content of the reactor were weighted. The total yield of liquid products was calculated as the sum of the increase of mass in the sampling pots, condensers and filter. The mass of char was determined by weight, and the weight of the bed materials was subtracted from the total mass of solids for calculation of the char mass. The gas yield was calculated by overall mass balance. 2.3. Collection of literature data A number of studies on lignocellulosic biomass pyrolysis in fluidized beds have been reported in the literature. However, many works focused on biomass conversion in certain operating conditions in fluidized bed reactors, and essential information to evaluate fluidization quality such as the details of the bed material were not fully included in the report. Data from a set of investigations with detailed information, including 18 different biomass samples and specific information about the operating conditions of fluidized bed reactor, were collected as shown in Tables 1 and 2. The collected data are limited to biomass smaller in size than 2 mm to minimize the effect of internal heat transfer inside feed particles on the product distribution from pyrolysis (Kersten et al., 2005; Isahak et al., 2012). The experimental data on the pyrolytic product distribution were analyzed and correlated with the information concerning the experimental rigs, operating parameters and feed properties based on previous findings in the literature. 3. Results and discussion 3.1. Relations between product yields and operating parameters The major operating parameters of a pyrolysis process influencing the product distribution include pyrolysis temperature, sweeping gas flow rate, residence time of vapor, biomass heating rate, mineral matter and size of biomass particles (Akhtar and Amin, 2012). In addition, fluidization variables, affecting hydrodynamics and particle mixing, are the most important parameter from the viewpoint of a fluidized bed pyrolyzer (Cui and Grace, 2007). The
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S.W. Kim / Bioresource Technology 175 (2015) 275–283 1
Relave gas yield, YRG [-]
Relave gas yield, YRG [-]
1
0.1
0.1
(a)
(b)
0.01
0.01
0.1
1
10
100
100
dBM/dp [-] 1
1
Relave gas yield, YRG [-]
Relave gas yield, YRG [-]
1000
Bed entrainment flux/Gas flux [-]
0.1
0.1
(c)
(d) 0.01
0.01 0.1
1
1
10
Ug/Umf [-]
Relave Temperature [-]
Relave gas yield, YRG [-]
1
0.1
(e)
0.01 0.1
1
10
Relave residence me of vapor [-] Fig. 2. Effect of operating parameters on relative gas yield: (a) size ratio of biomass to bed material, (b) flux ratio of bed entrainment to gas, (c) relative temperature, (d) fluidization number and (e) relative residence time of vapor (for symbol identification, see Table 2).
mixing of binary mixtures such as biomass and bed materials is mainly affected by the particle size ratio and gas velocity (Wu and Baeyens, 1998). Also, the heat transfer and hydrodynamics in the fluidized bed are governed by bubble behavior (Kim et al., 2003, 2013b). To assess the relationships between the relative gas and char yields and operating parameters for Eqs. (4)–(6) in the model, the collected data from this study and literature (Table 2) were plotted against various operating parameters as summarized in
Table 3. The dimensional parameters, temperature and vapor residence time were modified into dimensionless terms considering reactor design and scale-up because the yields are dimensionless. A concept of relative temperature (TR) was introduced as in Eq. (7) to consider that the organics are devolatilized over 100 °C (373.15 K) at atmospheric conditions.
TR ¼
T py 373:15 373:15
ð7Þ
280
S.W. Kim / Bioresource Technology 175 (2015) 275–283
1
Relave char yield, YRC [-]
Relave char yield, YRC [-]
1
0.1
0.1
(b)
(a) 0.01 0.1
1.0
0.01 0.3
Bubble fracon, δ [-]
3
Relave Temperature [-]
Relave char yield, YRC [-]
Relave char yield, YRC [-]
1
0.1
(c) 0.01
0.1
(d) 0.01
0.1
1
10
0.01
Relave residence me of vapor [-]
0.1
Ash content [-]
Fig. 3. Effect of operating parameters on relative char yield: (a) bubble fraction in bed, (b) relative temperature, (c) relative residence time of vapor and (d) ash content (for symbol identification, see Table 2).
where Tpy is the pyrolysis temperature in Kelvin (K). Also, the vapor residence time (tvapor) was modified into a relative residence time of vapor (trv), where tvapor is divided by 2 s as in Eq. (8).
t rv ¼
t vapor 2
ð8Þ
Two seconds was chosen as a normalization factor because it is an optimal vapor residence time to prevent the secondary cracking of volatiles in pyrolysis (Carlson et al., 2008; Kim et al., 2013b), indicating the gas or char yield changes possibly before and after the 2 s of vapor residence time. Finally, several parameters to predict the relative gas and char yields were determined by comparison of correlation coefficients from the correlation diagram on a trial-and-error strategy based on previous findings about the effect of operating parameters in the literature (Neves et al., 2011). The results with comparatively good correlation were plotted against each other in the correlation diagrams as shown in Fig. 2 for the relative gas yield and in Fig. 3 for the relative char yield. The relative temperature and the relative residence time of vapor commonly showed a strong correlation with the gas and char yields. The pyrolysis temperature is a major parameter in determining the yields of char, gas and liquid. At the lower temperature (
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Prediction of product distribution in fine biomass pyrolysis in fluidized beds based on proximate analysis Sung Won Kim ⇑ Global Technology, SK Innovation, 325 Exporo, Yuseong-gu, Daejeon 305-712, Republic of Korea
h i g h l i g h t s A model was developed to predict product distribution from biomass pyrolysis. The model is based on proximate analysis and hydrodynamics in fluidized beds. Relationships between product yields and fluidization condition were derived. Gas and char yields are a strong function of temperature and vapor residence time.
a r t i c l e
i n f o
Article history: Received 4 September 2014 Received in revised form 18 October 2014 Accepted 20 October 2014 Available online 28 October 2014 Keywords: Biomass pyrolysis Fluidized bed Prediction Product distribution Proximate analysis
a b s t r a c t A predictive model was satisfactorily developed to describe the general trends of product distribution in fluidized beds of lignocellulosic biomass pyrolysis. The model was made of mass balance based on proximate analysis and an empirical relationship with operating parameters including fluidization hydrodynamics. The empirical relationships between product yields and fluidization conditions in fluidized bed pyrolyzers were derived from the data of this study and literature. The gas and char yields showed strong functions of temperature and vapor residence time in the pyrolyzer. The yields showed a good correlation with fluidization variables related with hydrodynamics and bed mixing. The predicted product yields based on the model well accorded well with the experimental data. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Pyrolysis is one of the most promising technologies of biomass utilization, which converts the biomass to bio-oil, char and gasses depending on the pyrolysis conditions. The pyrolysis is a thermal degradation of materials in the absence of oxygen. The pyrolysis can be a promising option for lignocellulosic biomass conversion because bio oils derived from biomass pyrolysis could act as feedstocks for producing hydrocarbons that may be readily integrated into existing petroleum refineries or future bio-refineries (Kim et al., 2013a). In past decades, significant progress has been made in developing various pyrolysis technologies, and successfully testing them on laboratory, pilot and industrial scales. Recently, extensive research has been focused on fast pyrolysis using the fluidized bed process because of its high heat transfer and the short residence time for vapor to obtain a high yield of bio oil. Many studies ⇑ Tel.: +82 42 609 8314; fax: +82 42 609 8804. E-mail address: [email protected] http://dx.doi.org/10.1016/j.biortech.2014.10.107 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
have been carried out to determine the parametric influence of operating conditions on the yield of pyrolysis products of various biomasses for the design and optimum operating conditions of fluidized bed pyrolyzers. They found that the product distribution is affected by operating parameters as well as biomass type (Garcia-Perez et al., 2008; Lee et al., 2008; Heo et al., 2010). The description of the pyrolysis process including product distribution is particularly challenging because it involves a great deal of physical and chemical transformations and produces a large number of product species. In spite of the technological progress and large amount of experimental data reported in the literature, there is still considerable debate over the reaction mechanism controlling the distribution of pyrolysis products (Garcia-Perez et al., 2008). As a result, existing models aiming to predict the rates or yields of the released pyrolytic volatiles are still supported by empirical data (Neves et al., 2011). Neves et al. (2011) suggested an empirical model to approximate the pyrolysis behavior of most biomass, where the product distribution is a function of temperature. However, the product distribution is affected by reactor type and geometry in addition to the chemical structure of the biomass
276
S.W. Kim / Bioresource Technology 175 (2015) 275–283
in experiments of this study and literature, which address pyrolysis of fine biomass less than 2 mm to minimize the impact of internal heat transfer inside feedstock. A comparison between model results and experimental data was done to show its predictive capability for pyrolysis of various biomasses in fluidized beds.
Bio oil product Moisture
Biomass
Organics
Condensable liquid
Gas
Gas product Char product
Ash
Char
2. Methods
Fig. 1. Biomass pyrolysis concept for the predictive model in this study.
2.1. Methodology and model structure
(Cao et al., 2004). Especially, the pyrolytic oil yield in a fluidized bed is affected by the fluidizing conditions, such as gas velocity, the physical properties of bed material and static bed height or vapor residence time (Cui and Grace, 2007). Therefore, biomass thermochemical conversion development requires methodologies for prediction of the product distribution, whereby an appropriate balance between empiricism and fundamentals related with characteristics of biomass and flow dynamics should be considered to describe the complex range of interrelated phenomena (Ioannidou et al., 2011). When considering biomass thermal conversion, proximate analysis is one of the most important characterization methods. This consists of determining the moisture, ash, volatile matter and fixed carbon contents of the raw biomass. These values are essential as certain moisture, volatile matter and fixed carbon affect both thermal behavior and plant design (Garcia et al., 2013). Recently, a thermogravimetric model for pyrolysis product distribution in a captive batch reactor was proposed based on the proximate analysis of three agricultural residues (Ioannidou et al., 2011). The model enabled the possibility to evaluate a mass balance during the thermal treatment of the residue. However, the model did not take into account the possible activity of a second reaction (Ioannidou et al., 2011) which can possibly come from a variation in operating conditions. Also, it is still far from being applied in a fluidized bed pyrolyzer due to different operating principles between reactor types. A predictive model was developed to describe the general trends of product distribution in fluidized bed for biomass pyrolysis. The model was made of mass balance based on proximate analysis and an empirical relationship with operating parameters. The empirical relationships between product yields and fluidization conditions in fluidized bed reactors were derived from the data
The biomass pyrolysis concept for this study is based on the fact that the precise mechanism is not clearly known because of extremely complex reactions that take place during the biomass pyrolysis process (Ioannidou et al., 2011). A simplified two-step concept of the pyrolysis was introduced to consider possible secondreaction which is affected by operating condition as Fig. 1. The concept assumes that biomass is composed of three components, i.e., organics (volatile and fixed carbon), moisture and ash based on proximate analysis. It is assumed that all organics and water are devolatilized from biomass and ash is left as part of solid residue as a first step in the pyrolysis process. The organics are partly converted into gas and char, depending on operating conditions in the second step of the pyrolysis process. Finally, unconverted organics, pyrolytic water and moisture are produced as bio oil or a liquid product from the fluidized bed reactor (Neves et al., 2011). The whole solid fraction of ash and pyrolytic char is produced as char, assuming that all the ashes in the feed material remain in the char. Taking into account the simplified concept and the proximate analysis results of the input material (moisture: M, volatile matter: VM, fixed carbon: FC and ash: A), it is possible to calculate mass balance as Eqs. (1)–(3).
Feedstock ¼ M þ OR þ A
ð1Þ
where OR is organics (volatiles and fixed carbon) as Eq. (2).
OR ¼ VM þ FC
ð2Þ
Product ¼ bio oil þ gas þ char
ð3Þ
Assuming that each feedstock for pyrolysis in this model has the same content of moisture as the sample used in proximate analysis, the mass of bio oil is calculated as the sum of the moisture content and mass of unconverted organics obtained from char and gas yields as Eq. (4).
Table 1 Proximate analysis of a set of biomasses used in this study. Raw material
Jatropha seedshell cake Mallee Eucalyptus loxophleba (woody fraction) Cassava stalk Cassava rhizome Jatropha seedshell waste Quercus acutissima Miscanthus sinensis Palm kernel shell Jatropha seedshell cake Miscanthus sinensis var. purpurascens Sugar cane bagasse Corn cob Corn stover Eucalyptus grandis Cassava rhizome Cassava stalk Japanese larch Radiata pine sawdust Oriental white oak a
Organics (sum of volatile and fixed carbon).
Proximate analysis [wt%]
Reference
Moisture
Volatile
Fixed carbon
Ash
2.7 0.0 0.0 0.0 2.7 8.3 10.2 5.9 2.7 8.0 6.8 4.6 8.5 6.2 1.8 2.4 8.8 7.6 10.4
79.8 81.9 79.9 77.7 79.8 73.9 71.2 71.3 79.8 74.9 76.9 79.9 76.7 80.9 81.5 81.2 91.0a 92.2a 87.6a
14.1 17.6 14.1 18.2 14.1 16.7 15.2 17.8 14.1 15.7 10.8 13.7 8.2 12.4 13.1 11.2
3.4 0.5 6.0 4.1 3.4 1.0 3.4 5.0 3.4 1.4 5.3 1.6 6.1 0.5 3.5 5.1 0.2 0.2 2.0
This study Garcia-Perez et al. (2008) Pattiya (2011) Kim et al. (2013a) Lee et al. (2008) Bok et al. (2013) Kim et al. (2013b) Heo et al. (2010) Carrier et al. (2013)
Pattiya and Suttibak (2012) Park et al. (2008a) Park et al. (2008b) Park et al. (2009)
Table 2 Experimental details used in this study. References
Material
This study
418, 764
1.00
Garcia-Perez et al. (2008) Pattiya (2011) Kim et al. (2013a) Lee et al. (2008)
303a 428a 750, 1700 1113a
1.00 0.105 1.00 0.30
Bok et al. (2013)
1500
2.0, 0.20
Silicon carbide Sand Sand Sand Fused alumina Sand
Kim et al. (2013b) Heo et al. (2010) Carrier et al. (2013)
763a 700 408, 573, 868, 1129 125, 338, 406, 513
0.94 0.15 0.85
SiC Zirconia Alumina Sand
0.40
Silica sand
300, 700, 1000 300, 700, 1000
0.15 0.15, 0.18 0.15
Emery Emery Emery
c
300, 700, 1000
Particle diameter [lm]
Fluidizing gas Particle density [kg/ m3]
Gas
Gas flow rate [liter/min]
Temperature [°C]
Reactor
Symbols in figure
Diameter [m]
Height [m]
190
3210
Nitrogen
33, 41
390–512
0.102
0.97
376 428 278 40
2600b 2600b 2600b 3930
Nitrogen Nitrogen Nitrogen Nitrogen
38–53 5.9–6.6 30 2.5–8.2
350–580 437–537 500 300–550
0.102 0.0381 0.10 0.076
0.518 0.30 0.85 0.80
j N d
200
2600b
Nitrogen
30, 70
400–550
3210 3800 2500 2670
Nitrogen Nitrogen Nitrogen
33 5 40
392–511 350–550 500
0.17c 0.1 0.102 0.08 0.075
0.30c 0.3 0.97 0.30 0.68
}
b
Nitrogen
7
393–550
0.05
0.45
h
40 40
3930 3930
Nitrogen Nitrogen
2–4 2–4
400–550 400–550
0.08 0.08
0.30 0.30
4 s
40
3930
Nitrogen
2–4
400–550
0.08
0.30
⁄
190 186.5 40 320 338
2600
S.W. Kim / Bioresource Technology 175 (2015) 275–283
Feed rate [kg/hr]
Park et al. (2009) b
Fluidized bed material
Average size [lm]
Pattiya and Suttibak (2012) Park et al. (2008a) Park et al. (2008b)
a
Feedstock
Calculated from range of feedstock size. Based on Kunii and Levenspiel (1991). Rectangular type.
277
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S.W. Kim / Bioresource Technology 175 (2015) 275–283
Table 3 Operating parameters of fluidized bed and their correlations used in this study. Name of parameter, symbol
Equation or correlation
Reference
kg/m s
Eb ¼ 0:1qp ð1 emf ÞðU g U mf Þ
Pemberton and Davidson (1986)
Gas flux, Eg
kg/m2s
where qp is particle density, emf is voidage at minimum fluidizing condition and Ug is gas velocity Eg ¼ qg U g
Superficial gas velocity at minimum fluidizing condition, Umf
m/s
Bed entrainment flux, Eb
Unit 2
where qg is gas density i l h 1=2 g 33:7 for dp P 100 lm ð33:72 þ 0:0408ArÞ dp q
U mf ¼
g
1:8
U mf ¼
ðqp qg Þ0:934 g 0:934 dp 1110lg0:87 qg0:066
for dp < 100 lm
Wen and Yu (1966) Baeyens and Geldart (1974)
where lg is gas viscosity, Ar is Archimedes number, g is acceleration of gravity and dp is particle size of bed material U
Bubble fraction in fluidized bed, db
–
db ¼ U gb
Bubble rising velocity, Ub
m/s
U b ¼ ðU g U mf Þ þ U br
Single bubble’s rising velocity, Ubr
m/s
U br ¼ 0:711ðgdb Þ
Average bubble diameter, db
m
db ¼ 0:138hb ðug umf Þ0:42 expð2:5 105 ðug umf Þ2 103 ðug umf ÞÞ where hb is bed height
Nusselt number for gas-particle heat transfer, Nu
–
1=2
0:8
Nu ¼
Kunii and Levenspiel (1991) Kunii and Levenspiel (1991) Kunii and Levenspiel (1991) Cai et al. (1994)
h p dp kg
where hp is heat transfer coefficient between gas and particle and kg is thermal conductivity of gas
Bio oil ¼ M þ ORð1 Y RC Y RG Þ
ð4Þ
In Eq. (4), YRC indicates the relative char yield, which is the char yield converted from organics, and YRG indicates the relative gas yield, which is the gas yield converted from organics based on the simplified concept in Fig. 1. YRC and YRG can be expressed as Eqs. (5) and (6).
Y RC ¼ ðChar AÞ=OR
ð5Þ
Y RG ¼ Gas=OR
ð6Þ
The char and gas yields in pyrolysis products can be obtained from Eqs. (5) and (6), if YRC and YRG are known. 2.2. Experimental method A sample of Jatropha (Jatropha curcas L.) seedshell cake was acquired from an oil extraction plant in Indonesia. Proximate analysis was carried out with a Thermostep (ELTRA) analyzer according to the ASTM 5142 standard test method. The proximate analysis showed volatiles (mainly organic) of 79.80 wt% as shown in Table 1. The Jatropha seedshell cake sample was ground and sieved to an average size of 418 and 764 lm using an electric mixer and standard sieves. Silicon carbide (dp = 190 lm, qs = 3210 kg/m3) particles were used as bed material in this study. The pyrolysis system consisted of a mass flow controller (MFC), main column, screw feeder, cyclone, condensers, and accumulative flowmeter. The system was previously described in detail (Kim et al., 2014). The flow rate of nitrogen (99.9%) for fluidization was controlled by the MFC and the volume of the product gas was measured by the accumulative flowmeter (G6R, GTEC Industry). Before entering the stainless steel fluidized bed reactor (0.102 m id. and 0.97 m high), the fluidizing N2 gas was preheated in the air plenum to 450–600 °C. To prevent condensation of the pyrolysis vapor, the top of the reactor and the pipe connecting the reactor to the first condenser were maintained at 400 °C. The refrigerant was circulated in the shell of the condensers by a chiller system. The gas was sampled by a gas sampler to analyze its composition. Pyrolysis tests were conducted at 390–512 °C with N2 flow rates of 33 and 41 L/min at 25 °C (superficial gas velocities of 0.15–0.21 m/s depending on temperature), corresponding to ca. 7.5–10.4 Umf and a bubbling fluidization regime. A static bed height
of 0.2 m was maintained in the test. Biomass particles were fed at 1.0 kg/h. After completion of the feeding of biomass in the feeder, heating of the reactor was discontinued. Thereafter, the oil sampling pots, condensers, oil filter and the content of the reactor were weighted. The total yield of liquid products was calculated as the sum of the increase of mass in the sampling pots, condensers and filter. The mass of char was determined by weight, and the weight of the bed materials was subtracted from the total mass of solids for calculation of the char mass. The gas yield was calculated by overall mass balance. 2.3. Collection of literature data A number of studies on lignocellulosic biomass pyrolysis in fluidized beds have been reported in the literature. However, many works focused on biomass conversion in certain operating conditions in fluidized bed reactors, and essential information to evaluate fluidization quality such as the details of the bed material were not fully included in the report. Data from a set of investigations with detailed information, including 18 different biomass samples and specific information about the operating conditions of fluidized bed reactor, were collected as shown in Tables 1 and 2. The collected data are limited to biomass smaller in size than 2 mm to minimize the effect of internal heat transfer inside feed particles on the product distribution from pyrolysis (Kersten et al., 2005; Isahak et al., 2012). The experimental data on the pyrolytic product distribution were analyzed and correlated with the information concerning the experimental rigs, operating parameters and feed properties based on previous findings in the literature. 3. Results and discussion 3.1. Relations between product yields and operating parameters The major operating parameters of a pyrolysis process influencing the product distribution include pyrolysis temperature, sweeping gas flow rate, residence time of vapor, biomass heating rate, mineral matter and size of biomass particles (Akhtar and Amin, 2012). In addition, fluidization variables, affecting hydrodynamics and particle mixing, are the most important parameter from the viewpoint of a fluidized bed pyrolyzer (Cui and Grace, 2007). The
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S.W. Kim / Bioresource Technology 175 (2015) 275–283 1
Relave gas yield, YRG [-]
Relave gas yield, YRG [-]
1
0.1
0.1
(a)
(b)
0.01
0.01
0.1
1
10
100
100
dBM/dp [-] 1
1
Relave gas yield, YRG [-]
Relave gas yield, YRG [-]
1000
Bed entrainment flux/Gas flux [-]
0.1
0.1
(c)
(d) 0.01
0.01 0.1
1
1
10
Ug/Umf [-]
Relave Temperature [-]
Relave gas yield, YRG [-]
1
0.1
(e)
0.01 0.1
1
10
Relave residence me of vapor [-] Fig. 2. Effect of operating parameters on relative gas yield: (a) size ratio of biomass to bed material, (b) flux ratio of bed entrainment to gas, (c) relative temperature, (d) fluidization number and (e) relative residence time of vapor (for symbol identification, see Table 2).
mixing of binary mixtures such as biomass and bed materials is mainly affected by the particle size ratio and gas velocity (Wu and Baeyens, 1998). Also, the heat transfer and hydrodynamics in the fluidized bed are governed by bubble behavior (Kim et al., 2003, 2013b). To assess the relationships between the relative gas and char yields and operating parameters for Eqs. (4)–(6) in the model, the collected data from this study and literature (Table 2) were plotted against various operating parameters as summarized in
Table 3. The dimensional parameters, temperature and vapor residence time were modified into dimensionless terms considering reactor design and scale-up because the yields are dimensionless. A concept of relative temperature (TR) was introduced as in Eq. (7) to consider that the organics are devolatilized over 100 °C (373.15 K) at atmospheric conditions.
TR ¼
T py 373:15 373:15
ð7Þ
280
S.W. Kim / Bioresource Technology 175 (2015) 275–283
1
Relave char yield, YRC [-]
Relave char yield, YRC [-]
1
0.1
0.1
(b)
(a) 0.01 0.1
1.0
0.01 0.3
Bubble fracon, δ [-]
3
Relave Temperature [-]
Relave char yield, YRC [-]
Relave char yield, YRC [-]
1
0.1
(c) 0.01
0.1
(d) 0.01
0.1
1
10
0.01
Relave residence me of vapor [-]
0.1
Ash content [-]
Fig. 3. Effect of operating parameters on relative char yield: (a) bubble fraction in bed, (b) relative temperature, (c) relative residence time of vapor and (d) ash content (for symbol identification, see Table 2).
where Tpy is the pyrolysis temperature in Kelvin (K). Also, the vapor residence time (tvapor) was modified into a relative residence time of vapor (trv), where tvapor is divided by 2 s as in Eq. (8).
t rv ¼
t vapor 2
ð8Þ
Two seconds was chosen as a normalization factor because it is an optimal vapor residence time to prevent the secondary cracking of volatiles in pyrolysis (Carlson et al., 2008; Kim et al., 2013b), indicating the gas or char yield changes possibly before and after the 2 s of vapor residence time. Finally, several parameters to predict the relative gas and char yields were determined by comparison of correlation coefficients from the correlation diagram on a trial-and-error strategy based on previous findings about the effect of operating parameters in the literature (Neves et al., 2011). The results with comparatively good correlation were plotted against each other in the correlation diagrams as shown in Fig. 2 for the relative gas yield and in Fig. 3 for the relative char yield. The relative temperature and the relative residence time of vapor commonly showed a strong correlation with the gas and char yields. The pyrolysis temperature is a major parameter in determining the yields of char, gas and liquid. At the lower temperature (
