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A model of wood flash pyrolysis in fluidized bed reactor
Renewable Energy 30 (2005) 377–392 www.elsevier.com/locate/renene
A model of wood flash pyrolysis in fluidized bed reactor Zhongyang Luo , Shurong Wang, Kefa Cen Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China Received 13 December 2002; accepted 23 March 2004
Abstract With a view of exploiting renewable biomass energy as a highly efficient and clean energy, liquid fuel from biomass pyrolysis, called bio-oil, is expected to play a major role in future energy supply. At present, fluidized bed technology appears to have maximum potential in producing high-quality bio-oil. A model of wood pyrolysis in a fluidized bed reactor has been developed. The effect of main operation parameters on wood pyrolysis product distribution was well simulated. The model shows that reaction temperature plays a major important role in wood pyrolysis. And a good agreement between experimental and theoretical results was obtained. It was shown that particles less than 500 lm could achieve a high heating-up rate to meet flash pyrolysis demand. # 2004 Published by Elsevier Ltd. Keywords: Wood; Pyrolysis; Model; Fluidized bed reactor
1. Introduction With concerns over energy shortage and CO2 emission, biomass is now being considered as an inexhaustible and clean energy resource all over the world. Being the world’s second largest country of CO2 emission after the USA, China’s recoverable fossil fuel reserves have a CO2 emission potential of some 225 Gt, which would all be released to the atmosphere by 2040 [1]. This emission may cause a disruption of the climate and ecological balance. At the same time, biomass is abundant in China, and accounts for about 17% of the nation’s primary energy structure. Of late, thermochemical conversion is considered to be the most
Corresponding author. Tel.: +86-571-8795-2440; fax: +86-571-8795-1616. E-mail address: [email protected] (Z. Luo).
0960-1481/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.renene.2004.03.019
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Nomenclature A B Bi C d D E Fo g H J k DL m M Nu p Pr q Q r R Re S T u v V z a q e s k r n w l
pre-exponential factor, s1 viscous effusion degree, m2 Biot number heat capacity, J/(kg K) particle size, m diffusivity, m2/s activation energy, J/mol Fourier number acceleration of gravity, m/s2 volume enthalpy, J/m3 mass transfer rate, kg/(m2 s) reaction rate coefficient, s1 particle distance between two steps, m mass, kg molecular weight Nusselt number pressure, Pa Prandtl number reaction heat, J/kg reaction heat rate per volume, J/(m3 s) particle radius, m gas constant, 8.314 J/(mol K) Reynolds number cross-sectional area, m2 reaction temperature, K flow velocity, m/s volumetric flow rate, m3/s volume, m3 reactor height, m heat transfer coefficient, W/(m2 K) density, kg/m3 voidage of inner particle time, s thermal conductivity, W/(m K) 5:67 108 W/(m2 K4) emissivity of particle surface drag coefficient kinetic viscosity, kg/(m s)
Subscripts 0 initial time
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c d e f g G i l m n o r R s v w x
379
char heat convection reaction end molecular diffusion gas overall gas out particle corresponding pyrolysis product fluidizing gas dense bed viscous flow bio-oil radiation heat transfer particle boundary solid particle volatile wood suspension bed
common and convenient method for the biomass being utilized as an energy resource. This includes combustion, gasification, liquefaction and carbonization. In all these processes, liquefaction is expected to play a major role in future biomass utilization. Liquefaction can increase volumetric heat content, decrease transportation costs, and increase the use of liquid fuels in more equipment. There are several ways available for biomass thermochemical liquefaction. However, flash pyrolysis appears to be the most promising method of producing bio-oil. Fluidized bed technology is one of the most efficient and economic methods of actualizing flash pyrolysis, as it offers high heating rate, rapid devolatilization, convenient char collection, and re-utilization. A fluidized bed flash pyrolysis reactor, operating at atmospheric and nitrogen atmosphere, has been successfully developed at the Zhejiang University in China. Sawdust, wood, and rice straw were fed continuously at a maximum rate of about 3 kg/h directly into the reactor by way of a unique solid feeder. At optimum condition, a maximum bio-oil yield of up to 60% and 50%, respectively, was obtained [2]. Besides experimental research, numerical simulation is another extremely important tool for reactor design and experimental data interpretation. Therefore, a model was developed in this paper to simulate biomass pyrolysis behavior in our fluidized bed reactor.
2. Theory In the model, wood particle is considered to be spherical in shape. When the wood particle is exposed to medium temperature when being fed into the dense bed
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in a fluidized bed reactor, heating occurs immediately by conduction, radiation, and convection. As the temperature of the particle rises rapidly from the outer surface to core, pyrolysis decomposition reaction is likely to take place at the front at first, and then progress inwards until it reaches the core. The generated vapors diffuse through pore and out of particle to the bulk of the gas. Meanwhile, homogeneous secondary cracking occurs inside the porous particle and in the bulk of gaseous phase. Along with pyrolysis, particle shrinkage takes place, which may influence the transport phenomenon. Generally, to maximize bio-oil production, the temperature of the dense bed is to be set at an optimized and medium value. The temperature of the suspension bed is to be lower than that of the dense bed so as to restrain homogeneous secondary cracking. It is reasonable to develop corresponding equations for wood pyrolysis both is dense bed and suspension bed. On the basis of the above discussion, the model is developed with the following assumptions: 1. The wood particle maintains the shape of a sphere during pyrolysis. 2. Fresh wood particle has uniform distributions in porosity, density, conductivity, and specific heat capacity. 3. Reaction temperature variation of dense bed and suspension bed is neglected, since the temperature is accurately adjusted to a predetermined value in pyrolysis progress. 4. Bulk parameters are only functions of an axial location, but are uniform along the radial direction in a fluidized bed reactor. 5. Wood or char particles have far larger specific heat capacity than volatile temperatures released during particle thermo-decomposition. Therefore, the volatile temperature leaving the particle is considered to be readily heated to the temperature of wood or char that it flows. At the same time, the solid temperature is also constant, while the volatile temperature flows through the wood particle. 6. Nitrogen can be heated up to reaction temperature in a very short distance less than 5 mm from air-distributor. Therefore, the wood particle is exposed to hot nitrogen atmosphere immediately, as soon as it is fed into the dense bed, with the feeding entrance to the reactor being 10 mm apart from the gas-distributor. 7. Wood particle has no free water as a pre-drying procedure is adopted before the experiment.
3. Model equations 3.1. Chemical reaction kinetics The raw material of wood is considered homogeneous, while the reaction products are grouped into three main components: gas, bio-oil and char. Therefore, a two-stage and semi-global model is adopted to describe the wood thermal decomposition. Wood undergoes thermal degradation to gas, bio-oil, and char by way of primary reactions (k1, k2, k3); then the bio-oil may be changed either to
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gas by cracking (k4), or to char by repolymerization (k5) in the following secondary reactions. The Primary reactions are represented as first order in wood mass and having an Arrhenius type. The Secondary reactions are assumed to occur in gas phase within the pores of solid material and bulk gas phase in reactor, which are mainly related to bio-oil concentration. For gas and bio-oil, the corresponding kinetic expressions are associated with mass transfer phenomenon, which would be described better in mass diffusion equation. the kinetic equations for wood and char can be written as: @mw ¼ ðk1 þ k2 þ k3 Þmw @s
ð1Þ
@mc ¼ kl mw þ k5 mo @s
ð2Þ
kl ¼ Al expðEl =RTÞ
ð3Þ
e ¼ Vv =V ¼ ðVg þ Vo Þ=V
ð4Þ
where mw ¼ qw V , mc ¼ qc V , mo ¼ eqo V and mg ¼ eqg V . qw and qc are the apparent solid-phase densities that are correlated with particle volume, while qo and qg are determined on the basis of gaseous volume. Thus, Eqs. (1) and (2) could be expressed in other ways: @ðqw V Þ ¼ ðk1 þ k2 þ k3 Þqw V @s
ð5Þ
@ðqc V Þ ¼ k1 qw V þ k5 eqo V @s
ð6Þ
3.2. Enthalpy balance The Wood particle is heated up to rapidly to the reaction temperature when exposed to hot atmosphere in the dense bed by way of intensive convection, conduction, and radiation. Along with wood pyrolysis progress, endothermic and exothermic reactions may take place. The solid particle undergoes shrinkage with devolatilization Simultaneously, which also influences enthalpy balance. All these can be described as: X DH H @V ¼ divðk grad TÞ þ Qi Ds V @s i¼1;5
ð7Þ
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It is obvious that the enthalpy flow of solid substances is zero; thus, the enthalpy balance equation for particle pyrolysis in dense bed can be written in detail as: @ @ ½ðqw Cw þ qc Cc þ eqo Co þ eqg Cg ÞT þ u ½ðqo Co þ qg Cg ÞT @s @r |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
ð1Þ ð2Þ 1 @ @T r2 k ¼ 2 þ ðk1 q1 þ k2 q2 þ k3 q3 Þqw þ ðk4 q4 þ k5 q5 Þeqo |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} r @r @r |fflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflffl} ð4Þ ð3Þ
ðqw Cw þ qc Cc þ eqo Co þ eqg Cg ÞT @V @T V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
ð8Þ
ð5Þ
where (1) represents enthalpy change rate, (2) describes enthalpy flow, (3) figures thermal conduction flux, (4) refers to heat effect during particle thermal-degradation, and (5) reflects the effect of shrinkage on enthalpy. The related initial and boundary conditions are listed as: Ts¼0 ¼ T0 ; @T ¼0 @r r¼0 @T ¼ ðad þ ar ÞðTm TR Þ k @r
0 r rR ð9Þ
r¼rR
where ad and ar correspond to heat transfer coefficients of convection and radiation, respectively. So far, no generally accepted equation is available for convection coefficient of the bulk gaseous phase to the fine particle in the dense bed. Agarwal et al. [3] provided an equation (Eq. (10)) to calculate the convection coefficient in the dense bed. However, if the Reynolds number is very small, the Nusselt number will be found below the lower limit of 2 when using Eq. (10), which means that at this time, convection in the dense bed would be weaker than that in packed bed. At this time, Scott and Piskorz [4] recommended that Nu value could be chosen as 2 or 4. Nu ¼ 0:03Re1:3 Nu ¼ 2:0 þ 0:6Re1=2 Pr1=3
Re < 100 Re > 100
ð10Þ
The Radiation coefficient of ar in Eq. (9) can be calculated by the following expression:
ar ¼ nr Tm2 þ TR2 ðTm TR Þ ð11Þ When the wood particle is fed into the dense bed, the char layer will immediately appear to surround particles due to thermo-degradation. The radiation of char is very close to that of black-body and an emissivity of 1 could be selected [5]. The Above-mentioned enthalpy balance equation could also be applied in the suspension bed. However, heat transfer in the suspension bed is really weaker than
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that in the dense bed, where the Biot number is only about 103–102. Thus, a particle can be considered isothermal, whose temperature can be written as: T Tx ¼ eðBiFoÞ Tm Tx
ð12Þ
3.3. Component diffusion @ eq @V ðeqo Þ þ divðqo u þ J fo Þ ¼ k2 qw ðk4 þ k5 Þeqo o |fflfflfflfflfflfflfflfflfflffl ffl {zfflfflfflfflfflfflfflfflfflffl ffl } |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl {zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl } @s @sffl} V ffl{zfflfflfflffl |fflfflfflffl |fflfflfflffl{zfflfflfflffl}
ð13Þ
eqg @V @ ðeqg Þ þ divðqg u þ J fg Þ ¼ k3 qw þ k4 eqo @s V @s
ð14Þ
ð2Þ
ð1Þ
ð3Þ
ð4Þ
where (2) represents mass convection and diffusion, (3) refers to chemical reaction, while (4) materializes the effect of volume shrinkage on component diffusion. The parameter u in above equations can be expressed as: Bo pMv Jnv ¼ qv u ¼ rp ð15Þ RTlv where B is a coefficient relevant to porosity [6]: B ¼ 4 1011 e3 =ð1 eÞ2
ð16Þ
Gaseous products are assumed to accord with the ideal gas law: p¼
qv RT Mv
ð17Þ
Thus, u can be deduced as: u¼
B @ðr2 pÞ lv r2 @r
ð18Þ
Volatile diffusion flux is expressed in the Fick law: Jfv ¼
Dv @ðr2 qfv Þ @r r2
ð19Þ
The related initial and boundary conditions are listed as: qo ¼ qg ¼ 0; 0 r R @qg @qo ¼0 ¼ @r r¼0 @r r¼0
s¼0 ð20Þ
3.4. Particle volume shrinkage Particle volume shrinkage will occur during thermo-degradation. Chan et al. [5] and Lee et al. [7] pointed out that marked shrinkage would take place during pyrolysis, especially in the case of larger particles. Here, particle voidage is
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assumed to vary with the apparent densities of char and wood, which is written as: e ¼ e0 þ
qs qw0 ðee e0 Þ qce qw0
ð21Þ
In the same way, particle volume is calculated as follows: V ¼ V0 þ
ms mw0 ðVe V0 Þ mce mw0
ð22Þ
3.5. Residence time of gaseous products and solid particle Along with reactor height from inlet to outlet during wood pyrolysis, the particle undergoes thermo-degradation, and massive volatile temperature is released from the inner particle to bulk gaseous phase through the solid surface continuously. Therefore, the bulk gas flow will become larger and larger due to additive volatile temperature released from the wood particle, which will reduce gas residence time. Scott and Piskorz [4] pointed out that this phenomenon would lead to a decrease of gaseous product residence time by at least 15%. Here, a plug-flow model is suitable for describing the phenomenon: DsG ¼
e S DL ðvl þ vv Þ
ð23Þ
Up to now, no generally accepted equation for particle residence time is available. Conti et al. [8] indicated that the solid biomass was unlikely to participate in aggregation or back-mixing but was directly elutriated. Thus, the wood particle is assumed to move in the direction of gas flow, with only a slip between gas flow [9]: wqg q s qg dus us dqs ¼ ðuG us Þ2 g ds ds qs qs qs ds
ð24Þ
where 8 < 9:689Re0:78 ð1 þ 0:147Re0:82 Þ 0:1 Re 5 w ¼ 9:689Re0:78 ð1 þ 0:227Re0:55 Þ 5 Re 40 : 9:689Re0:78 ð1 þ 0:838Re0:82 Þ 40 Re 400
ð25Þ
3.6. Secondary cracking in bulk gaseous phase When the volatile temperatere is released from particle into bulk gaseous phase, it still undergoes secondary cracking due to the hot atmosphere in the reactor. However, the repolymerization of volatile to char could be ignored because of the not so high reaction temperature in the reactor. Boroson et al. [10] drew a conclusion that reforming probability in the gaseous phase was too small to be conv sidered even at high temperatures about 500–800 C. Meanwhile, the average freedom course between volatile molecules was so large that mutual collision hardly took place, thus reducing reforming probability. Therefore, secondary
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cracking of the volatile molecule could be described as follows: @ðqo Þ @ 2 qo uG @qo ¼ Do k6 qo @s @z2 e @z
ð26Þ
@ðqg Þ @ 2 qg uG @qg þ k6 q o ¼ Dg @z2 e @z @s
ð27Þ
4. Parameter selection Many researchers have obtained various chemical reaction kinetic parameters of wood pyrolysis. The cited kinetic parameters applied in our model are listed in Table 1. In the following sections, the wood pyrolysis behavior in our developed fluidized bed reactor (Fig. 1) is simulated in detail. The fluidized bed reactor has a diameter of 80 mm and a height of 1200 mm, which is operated within temperatures v between 450 and 700 C. Nitrogen flow at about 3.5 N m3/h is used to fluidize sand in the bed, and sweep the volatile temperature quickly off the hot zone. The Wood particle has 660 kg/m3 density, 2.4 kJ/kg K specific heat capacity, and 0.3 W/m K conduction coefficient [11]. Meanwhile, the char has 0.84 kJ/kg K in specific heat capacity, and 0.1 W/m K in conduction coefficient [7]. Drummond and Drummond [12] indicated that the primary reaction was slightly endothermic v when the temperature was about 500–550 C, while the secondary reaction tended to be exothermic. Generally, the endothermic stage mainly takes place during levoglucosan generation, while that of exothermic stage in the secondary reaction. The wood devolatilization reaction heat might range from 418 to 418 kJ/kg, while a reaction heat about 42 kJ/kg would be liberated in the following secondary cracking [5,13].
Table 1 Kinetic parameters selection Pre-exponential factor
Value (s1)
Activation energy
Value (kJ/mol)
References
A1 A2 (s1) A3 (s1) A4, A6 (s1) A5 (s1)
1:08 107 2:0 108 1:3 108 3:09 106 1:48 106
E1 E2 E3 E4, E6 E5
121 133 140 108 144
Chan et al. [5] Chan et al. [5] Chan et al. [5] Boroson et al. [10] Chan et al. [5]
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Fig. 1. Fluidized bed reactor system for flash pyrolysis of biomass.
5. Results and discussion 5.1. Effect of dense bed temperature There is no doubt that reaction temperature plays an active role in wood pyrolysis, as it does in a fluidized bed reactor. Fig. 2 shows the effect of dense bed temperature on pyrolysis product distribution along with reactor height. With the dense bed temperature increase, it is wood that will mostly undergo thermov degradation in the dense bed. At a dense bed temperature of 500 C, about 80% of wood was decomposed to gas, char, and bio-oil, While it was almost completed v when the temperature rose up to 600 C. Therefore, higher temperatures will promote wood thermo-decomposition. However, the reaction of bio-oil secondary cracking to gas will become more violent when the temperature increases beyond some value. Though wood pyrolysis takes place more rapidly and completely at a v dense bed temperature of 700 C, all bio-oils were decomposed to gas for secondv ary cracking. Thus, a temperature of 500 C is more suitable for maximizing biooil production, which will not only efficiently promote wood pyrolysis rate but also restrain secondary cracking to a certain extent. it is also obvious that gas production will increase with temperature all along. This is the reason why gasification is often operated at higher temperatures.
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Fig. 2. Wood pyrolysis product distribution vs. dense bed temperature (suspension bed temperature: v 450 C; feed rate: 1 kg/h; particle size: 200 lm; black dot: dividing point). (a) dense bed temperature v v v 450 C; (b) dense bed temperature 500 C; (c) dense bed temperature 600 C; (d) dense bed temperature v 700 C.
5.2. Effect of suspension bed temperature When compared with the dense bed temperature, the suspension bed temperature has minor effects on wood pyrolysis behavior. At an optimum dense bed v temperature of about 500 C, only a small fraction of wood undergoes thermal degradation within suspension bed. However, bio-oil secondary cracking may take place violently within the suspension bed if the temperature is higher. Therefore, with a view of restraining bio-oil secondary cracking, it is important to control the suspension bed temperature at a reasonable value. As shown in Fig. 3, secondary cracking of bio-oil becomes more acute with an increase of suspension bed temperature, also accompanied by an increase of gas production. In their detailed research on secondary cracking, Stiles and Kandiyoti [14] indicated that more than 30% bio-oil would take part in secondary cracking v when volatile temperatures stayed at 600 C for 3.5 s than for Q.25s. Fig. 3(b) has shown that nearly half of the bio-oil was transformed to gas caused by secondary v cracking at 600 C. However, secondary cracking could be neglected when the
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Fig. 3. Wood pyrolysis product distribution vs. suspension bed temperature (dense bed temperature: v 500 C; feed rate: 1 kg/h; particle size: 200 lm; black dot: dividing point). (a) suspension bed temperav v ture: 550 C; (b) suspension bed temperature: 600 C. v
temperature is below 450 C according to Fig. 2(b). In accordance with the result proposed by Samolada and Vasalos [15]. However, it is not reasonable to set the temperature of the suspension bed to a value lower than the setting point of biooil, in order to avoid the bio-oil being unexpectedly pre-cooled before entering into the quencher. Generally, according to the experimental results in our fluidized bed reactor, it is more suitable to set suspension bed temperature within the range of v about 400–450 C. 5.3. Effect of particle size Generally, particle size has mutual effects on pyrolysis behavior. On the one hand, the fine particle will be carried by gas flow more easily, which will lead to a short residence time and incomplete decomposition. On the other hand, bio-oil production will be lower if adopting excessive large wood particles because of slow heating up. Therefore, a wood particle of a moderate size should be more reasonable for bio-oil production. As shown in Fig. 4, wood particle size is often set in the order of microns so as to meet flash pyrolysis and pneumatic transportation. Within this range, particle size has little influence on product distribution. After all, the non-sensitivity of product distribution on particle size will lower pretreatment complexity of the raw material. 5.4. Effect of feed rate Until now, the effect of feed rate on pyrolysis behavior had not been reported. The adjustment of feed rate will change gas flow and the volatile temperature in the reactors which will influence wood decomposition and bio-oil secondary cracking. Fig. 5 indicates that bio-oil production increases with feed rate. If wood could be completely decomposed, more volatile temperature will be yielded at the same interval when the feed rate becomes higher, which will enhance gas flow. Therefore, volatile residence at the hot atmosphere will be shortened and secondary cracking
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v
Fig. 4. Wood pyrolysis product distribution vs. wood particle size (dense bed temperature; 550 C; v suspension bed temperature: 450 C; feed rate: 1 kg/h; black dot: dividing point). (a) particle size: 100 lm; (b) particle size: 300 lm.
v
Fig. 5. Wood pyrolysis product distribution vs. feed rate (dense bed temperature: 600 C; suspension v bed temperature: 450 C; particle size: 200 lm; black dot: dividing point). (a) feed rate: 0.5 Kg/h; (b) feed rate: 5 Kg/h.
will be restrained to some extent. Unlike reactor height which influences wood pyrolysis behavior mostly in suspension bed, feed rate plays an active role both in dense bed and in suspension bed. Because the temperature of the dense bed is generally higher than that of the suspension bed, feed rate will restrain the bio-oil secondary cracking more efficiently. However, it is not advantageous to increase the feed rate when incomplete wood decomposition occurs at lower temperatures. 5.5. Verification of model Based on the above analysis, it is clear that the reaction temperature plays the most important role in pyrolysis behavior. Fig. 6 shows the experimental result carried out on our fluidized bed reactor, which is compared with model calculation. As shown in Fig. 6, the modeling result is in good consonance with the
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Fig. 6. Experimental and theoretical results of ultimate product distribution (particle size: 200 lm; feed rate: 1 kg/h; black dots: experimental data). (a) bio-oil production; (b) gas production.
v
experimental results, while a dense bed temperature of 500 C, and lower suspension bed temperature are optimum for bio-oil production.
6. Study of heating-up rate During pyrolysis process, no matter what the reaction temperature is, the wood particle should go through proper temperature areas. Rapid heating-up will make the particle pass low temperature area more quickly in order to reduce charring and maximize bio-oil production. However, the heating-up rate is affected by many operational parameters, such as particle characteristics and heat flux. Therefore, if the reactor type is pre-determined, temperature and particle size will be the two most important factors influencing heating-up rate. Kothari and Antal [16] suggested that flash pyrolysis could be divided into two steps in succession: heating-up stage and volatile release stage. At heating-up stage, the weight loss of particle could be neglected and the particle is rapidly heated. As the particle temperature reaches up to a predetermined value, the volatile temperature begins to be released out of the particle, which is called the volatile release stage. In fact, the above two stages could not be completely separated. However, when compared with the reaction period and the residence time of particle, the time for the particle being heated up to 95% of reaction temperature is too short to consider pyrolysis process, especially in the case of high flux. Therefore, wood decomposition could be ignored during the calculation of heating-up rate in fluidized bed reactor. Only in the situation higher reaction temperature or larger particle is it necessary to the above two stages consider simultaneously. In this paper, particle core, where heating-up rate is the slowest, is selected as the basis to study the effect of temperature and particle size on the heating-up rate. As shown in Table 2, the heating-up rate of a fine particle could achieve up to v an order of 104 C/s, which is also verified in other research. In some cases, fine v particles might attain a very high heating-up rate, up to the order of 105 C/s [8,17]. However, the actual heating-up rate depends not only on heat transfer but
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Table 2 Heating-up rate vs. particle size and temperature Size (lm)
100 300 500
v
v
450 C
v
550 C
700 C
UT95% v ( C/s)
UT350 v ( C/s)
UT95% v ( C/s)
UT350 v ( C/s)
UT95% v ( C/s)
UT350 v ( C/s)
2:2 104 6:3 102 2:1 102
3:4 104 1:0 103 3:4 102
2:9 104 8:4 102 2:8 102
5:3 104 1:6 103 5:5 102
4:1 104 1:2 103 4:0 102
8:1 104 2:5 103 8:6 102
also on pyrolysis progress, which implies that the actual heating-up rate is smaller than the values listed in Table 2. Generally, the heating-up is often calculated on the basis of 95% of reaction temperature. However, the essential effect of heating-up rate on flash pyrolysis is to shorten particle residence time at low temperatures. This will decrease the charring opportunity and increase the bio-oil production. Thus, it is more reasonable to select a moderate temperature value but not a reaction temperature as the baseline v for calculating the heating-up rate. Here, the temperature of 350 C is selected as the calculation base, at which mostly volatile temperature is released out of the particle. The related result is also listed in Table 2, which shows that all particles less than 500 lm are located in flash pyrolysis range. When one Compares the results in Table 2 with those shown in the above figures, it is clear that the pyrolysis rate is far slower than the heating-up rate. Therefore, it is more reasonable to modify the name ‘‘flash pyrolysis’’ as ‘‘pyrolysis at flash heating-up’’.
7. Conclusions An integrated model, combined reaction kinetics with mass and heat transfer, was proposed to predict pyrolysis behavior in a fluidized bed reactor. The Modeling results showed that the reaction temperature played the most important role in v wood pyrolysis behavior. A moderate dense bed temperature of 500 C, and lower suspension bed temperature were more suitable for optimizing bio-oil production, at which the wood particle could undergo a complete decomposition and bio-oil secondary cracking be well restrained. Besides this, particle size and feed rate have mutual effects on pyrolysis behavior. The Fine particle will enhance rapid heating-up, but may lead to incomplete particle decomposition at lower temperatures. In the case of higher dense bed temperature, volatile secondary cracking will be efficiently restrained when the feed rate is increased. However, an excessive large feed rate might cause incomplete wood particle decomposition. Rapid heating-up will make particle passing low temperature area more quickly so as to reduce charring and maximize bio-oil production. The study of heating-up rate showed that it is more suitable to choose a moderate temperature, and not
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normal reaction temperature as the evaluation criterion. It was proved that particles less than 500 lm could meet the flash pyrolysis requirement. Acknowledgements This work was supported by National Natural Science Foundation of China, under Project Nos. 50025618, 29976039 and 50176046. References [1] Bach W, Fiebig S. China’s key role in climate protection. Energy 1998;23(4):253–70. [2] Luo ZY, Wang SR, Dong LJ, Fang MX, Hong J, Gu YL, et al. Research on flash pyrolysis of biomass for bio-oil in a fluidized bed reactor. Reno: Proceedings of the 16th International Conference on Fluidized Bed Combustion. 2001, p. 5. [3] Agarwal PK, Genetti WE, Lee YY. Models for devolatilization of coal particles in fluidized beds. Fuel 1984;63:1157–65. [4] Scott DS, Piskorz J. The continuous flash pyrolysis of biomass. The Canadian Journal of Chemical Engineering 1984;62:404–12. [5] Chan WR, Kelbon M, Krieger BB. Modelling and experimental verification of physical and chemical process during pyrolysis of a large biomass particle. Fuel 1985;64:1505–15. [6] Cen KF, Ni MJ, Luo ZY, Yan JH, Chi Y, Fang MX, et al. Theory, design and operation on circulating fluidized bed boilers. Beijing: Chinese Electric Power Press; 1998 [in Chinese]. [7] Lee CK, Chaiken RF, Singer JM. Charring pyrolysis of wood in fires by laser simulation. 16th International Symposium on Combustion. 1976, p. 1459–70. [8] Conti L, Scano G, Boufala J. Bio-oils from arid land plants: flash pyrolysis of EUPHORBIA CHARACIAS bagasse. Biomass and Energy 1994;7(1–6):291–6. [9] Hastaoglu MA, Hassam MS. Application of a general gas-solid reaction model to flash pyrolysis of wood in a circulating fluidized bed. Fuel 1995;74(5):697–703. [10] Boroson ML, Howard JB, Longwell JP, Peters WA. Product yields and kinetics from the vapor phase cracking of wood pyrolysis tars. AICHE Journal 1989;35(1):120–8. [11] Dubianzhiren. Wood application. Shanghai: Shanghai Science and Technology Press; 1986 [Zhang QL, Zhang QS, Zhang SY, Trans.; in Chinese]. [12] Drummond AF, Drummond IW. Pyrolysis of sugar cane bagasse in a wire-mesh reactor. Industrial and Engineering Chemistry 1996;35:1263–8. [13] Koufopanos CA, Papayannakos N, Maschio G, Lucchesi A. Modelling of the pyrolysis of biomass particles. Studies on kinetics, thermal and heat transfer effects. The Canadian Journal of Chemical Engineering 1991;69:907–15. [14] Stiles HN, Kandiyoti R. Secondary reactions of flash pyrolysis tars measured in a fluidized bed pyrolysis reactor with some novel design features. Fuel 1989;68:275–82. [15] Samolada MC, Vasalos IA. A kinetic approach to the flash pyrolysis of biomass in a fluidized bed reactor. Fuel 1991;70:883–9. [16] Kothari V, Antal MJJ. Numerical studies of the flash pyrolysis of cellulose. Fuel 1985;64:1487–93. [17] Scott DS, Piskorz J. The flash pyrolysis of aspen-poplar wood. The Canadian Journal of Chemical Engineering 1982;60:666–74.
A model of wood flash pyrolysis in fluidized bed reactor Zhongyang Luo , Shurong Wang, Kefa Cen Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China Received 13 December 2002; accepted 23 March 2004
Abstract With a view of exploiting renewable biomass energy as a highly efficient and clean energy, liquid fuel from biomass pyrolysis, called bio-oil, is expected to play a major role in future energy supply. At present, fluidized bed technology appears to have maximum potential in producing high-quality bio-oil. A model of wood pyrolysis in a fluidized bed reactor has been developed. The effect of main operation parameters on wood pyrolysis product distribution was well simulated. The model shows that reaction temperature plays a major important role in wood pyrolysis. And a good agreement between experimental and theoretical results was obtained. It was shown that particles less than 500 lm could achieve a high heating-up rate to meet flash pyrolysis demand. # 2004 Published by Elsevier Ltd. Keywords: Wood; Pyrolysis; Model; Fluidized bed reactor
1. Introduction With concerns over energy shortage and CO2 emission, biomass is now being considered as an inexhaustible and clean energy resource all over the world. Being the world’s second largest country of CO2 emission after the USA, China’s recoverable fossil fuel reserves have a CO2 emission potential of some 225 Gt, which would all be released to the atmosphere by 2040 [1]. This emission may cause a disruption of the climate and ecological balance. At the same time, biomass is abundant in China, and accounts for about 17% of the nation’s primary energy structure. Of late, thermochemical conversion is considered to be the most
Corresponding author. Tel.: +86-571-8795-2440; fax: +86-571-8795-1616. E-mail address: [email protected] (Z. Luo).
0960-1481/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.renene.2004.03.019
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Nomenclature A B Bi C d D E Fo g H J k DL m M Nu p Pr q Q r R Re S T u v V z a q e s k r n w l
pre-exponential factor, s1 viscous effusion degree, m2 Biot number heat capacity, J/(kg K) particle size, m diffusivity, m2/s activation energy, J/mol Fourier number acceleration of gravity, m/s2 volume enthalpy, J/m3 mass transfer rate, kg/(m2 s) reaction rate coefficient, s1 particle distance between two steps, m mass, kg molecular weight Nusselt number pressure, Pa Prandtl number reaction heat, J/kg reaction heat rate per volume, J/(m3 s) particle radius, m gas constant, 8.314 J/(mol K) Reynolds number cross-sectional area, m2 reaction temperature, K flow velocity, m/s volumetric flow rate, m3/s volume, m3 reactor height, m heat transfer coefficient, W/(m2 K) density, kg/m3 voidage of inner particle time, s thermal conductivity, W/(m K) 5:67 108 W/(m2 K4) emissivity of particle surface drag coefficient kinetic viscosity, kg/(m s)
Subscripts 0 initial time
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c d e f g G i l m n o r R s v w x
379
char heat convection reaction end molecular diffusion gas overall gas out particle corresponding pyrolysis product fluidizing gas dense bed viscous flow bio-oil radiation heat transfer particle boundary solid particle volatile wood suspension bed
common and convenient method for the biomass being utilized as an energy resource. This includes combustion, gasification, liquefaction and carbonization. In all these processes, liquefaction is expected to play a major role in future biomass utilization. Liquefaction can increase volumetric heat content, decrease transportation costs, and increase the use of liquid fuels in more equipment. There are several ways available for biomass thermochemical liquefaction. However, flash pyrolysis appears to be the most promising method of producing bio-oil. Fluidized bed technology is one of the most efficient and economic methods of actualizing flash pyrolysis, as it offers high heating rate, rapid devolatilization, convenient char collection, and re-utilization. A fluidized bed flash pyrolysis reactor, operating at atmospheric and nitrogen atmosphere, has been successfully developed at the Zhejiang University in China. Sawdust, wood, and rice straw were fed continuously at a maximum rate of about 3 kg/h directly into the reactor by way of a unique solid feeder. At optimum condition, a maximum bio-oil yield of up to 60% and 50%, respectively, was obtained [2]. Besides experimental research, numerical simulation is another extremely important tool for reactor design and experimental data interpretation. Therefore, a model was developed in this paper to simulate biomass pyrolysis behavior in our fluidized bed reactor.
2. Theory In the model, wood particle is considered to be spherical in shape. When the wood particle is exposed to medium temperature when being fed into the dense bed
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in a fluidized bed reactor, heating occurs immediately by conduction, radiation, and convection. As the temperature of the particle rises rapidly from the outer surface to core, pyrolysis decomposition reaction is likely to take place at the front at first, and then progress inwards until it reaches the core. The generated vapors diffuse through pore and out of particle to the bulk of the gas. Meanwhile, homogeneous secondary cracking occurs inside the porous particle and in the bulk of gaseous phase. Along with pyrolysis, particle shrinkage takes place, which may influence the transport phenomenon. Generally, to maximize bio-oil production, the temperature of the dense bed is to be set at an optimized and medium value. The temperature of the suspension bed is to be lower than that of the dense bed so as to restrain homogeneous secondary cracking. It is reasonable to develop corresponding equations for wood pyrolysis both is dense bed and suspension bed. On the basis of the above discussion, the model is developed with the following assumptions: 1. The wood particle maintains the shape of a sphere during pyrolysis. 2. Fresh wood particle has uniform distributions in porosity, density, conductivity, and specific heat capacity. 3. Reaction temperature variation of dense bed and suspension bed is neglected, since the temperature is accurately adjusted to a predetermined value in pyrolysis progress. 4. Bulk parameters are only functions of an axial location, but are uniform along the radial direction in a fluidized bed reactor. 5. Wood or char particles have far larger specific heat capacity than volatile temperatures released during particle thermo-decomposition. Therefore, the volatile temperature leaving the particle is considered to be readily heated to the temperature of wood or char that it flows. At the same time, the solid temperature is also constant, while the volatile temperature flows through the wood particle. 6. Nitrogen can be heated up to reaction temperature in a very short distance less than 5 mm from air-distributor. Therefore, the wood particle is exposed to hot nitrogen atmosphere immediately, as soon as it is fed into the dense bed, with the feeding entrance to the reactor being 10 mm apart from the gas-distributor. 7. Wood particle has no free water as a pre-drying procedure is adopted before the experiment.
3. Model equations 3.1. Chemical reaction kinetics The raw material of wood is considered homogeneous, while the reaction products are grouped into three main components: gas, bio-oil and char. Therefore, a two-stage and semi-global model is adopted to describe the wood thermal decomposition. Wood undergoes thermal degradation to gas, bio-oil, and char by way of primary reactions (k1, k2, k3); then the bio-oil may be changed either to
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gas by cracking (k4), or to char by repolymerization (k5) in the following secondary reactions. The Primary reactions are represented as first order in wood mass and having an Arrhenius type. The Secondary reactions are assumed to occur in gas phase within the pores of solid material and bulk gas phase in reactor, which are mainly related to bio-oil concentration. For gas and bio-oil, the corresponding kinetic expressions are associated with mass transfer phenomenon, which would be described better in mass diffusion equation. the kinetic equations for wood and char can be written as: @mw ¼ ðk1 þ k2 þ k3 Þmw @s
ð1Þ
@mc ¼ kl mw þ k5 mo @s
ð2Þ
kl ¼ Al expðEl =RTÞ
ð3Þ
e ¼ Vv =V ¼ ðVg þ Vo Þ=V
ð4Þ
where mw ¼ qw V , mc ¼ qc V , mo ¼ eqo V and mg ¼ eqg V . qw and qc are the apparent solid-phase densities that are correlated with particle volume, while qo and qg are determined on the basis of gaseous volume. Thus, Eqs. (1) and (2) could be expressed in other ways: @ðqw V Þ ¼ ðk1 þ k2 þ k3 Þqw V @s
ð5Þ
@ðqc V Þ ¼ k1 qw V þ k5 eqo V @s
ð6Þ
3.2. Enthalpy balance The Wood particle is heated up to rapidly to the reaction temperature when exposed to hot atmosphere in the dense bed by way of intensive convection, conduction, and radiation. Along with wood pyrolysis progress, endothermic and exothermic reactions may take place. The solid particle undergoes shrinkage with devolatilization Simultaneously, which also influences enthalpy balance. All these can be described as: X DH H @V ¼ divðk grad TÞ þ Qi Ds V @s i¼1;5
ð7Þ
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It is obvious that the enthalpy flow of solid substances is zero; thus, the enthalpy balance equation for particle pyrolysis in dense bed can be written in detail as: @ @ ½ðqw Cw þ qc Cc þ eqo Co þ eqg Cg ÞT þ u ½ðqo Co þ qg Cg ÞT @s @r |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
ð1Þ ð2Þ 1 @ @T r2 k ¼ 2 þ ðk1 q1 þ k2 q2 þ k3 q3 Þqw þ ðk4 q4 þ k5 q5 Þeqo |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} r @r @r |fflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflffl} ð4Þ ð3Þ
ðqw Cw þ qc Cc þ eqo Co þ eqg Cg ÞT @V @T V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
ð8Þ
ð5Þ
where (1) represents enthalpy change rate, (2) describes enthalpy flow, (3) figures thermal conduction flux, (4) refers to heat effect during particle thermal-degradation, and (5) reflects the effect of shrinkage on enthalpy. The related initial and boundary conditions are listed as: Ts¼0 ¼ T0 ; @T ¼0 @r r¼0 @T ¼ ðad þ ar ÞðTm TR Þ k @r
0 r rR ð9Þ
r¼rR
where ad and ar correspond to heat transfer coefficients of convection and radiation, respectively. So far, no generally accepted equation is available for convection coefficient of the bulk gaseous phase to the fine particle in the dense bed. Agarwal et al. [3] provided an equation (Eq. (10)) to calculate the convection coefficient in the dense bed. However, if the Reynolds number is very small, the Nusselt number will be found below the lower limit of 2 when using Eq. (10), which means that at this time, convection in the dense bed would be weaker than that in packed bed. At this time, Scott and Piskorz [4] recommended that Nu value could be chosen as 2 or 4. Nu ¼ 0:03Re1:3 Nu ¼ 2:0 þ 0:6Re1=2 Pr1=3
Re < 100 Re > 100
ð10Þ
The Radiation coefficient of ar in Eq. (9) can be calculated by the following expression:
ar ¼ nr Tm2 þ TR2 ðTm TR Þ ð11Þ When the wood particle is fed into the dense bed, the char layer will immediately appear to surround particles due to thermo-degradation. The radiation of char is very close to that of black-body and an emissivity of 1 could be selected [5]. The Above-mentioned enthalpy balance equation could also be applied in the suspension bed. However, heat transfer in the suspension bed is really weaker than
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that in the dense bed, where the Biot number is only about 103–102. Thus, a particle can be considered isothermal, whose temperature can be written as: T Tx ¼ eðBiFoÞ Tm Tx
ð12Þ
3.3. Component diffusion @ eq @V ðeqo Þ þ divðqo u þ J fo Þ ¼ k2 qw ðk4 þ k5 Þeqo o |fflfflfflfflfflfflfflfflfflffl ffl {zfflfflfflfflfflfflfflfflfflffl ffl } |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl {zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl } @s @sffl} V ffl{zfflfflfflffl |fflfflfflffl |fflfflfflffl{zfflfflfflffl}
ð13Þ
eqg @V @ ðeqg Þ þ divðqg u þ J fg Þ ¼ k3 qw þ k4 eqo @s V @s
ð14Þ
ð2Þ
ð1Þ
ð3Þ
ð4Þ
where (2) represents mass convection and diffusion, (3) refers to chemical reaction, while (4) materializes the effect of volume shrinkage on component diffusion. The parameter u in above equations can be expressed as: Bo pMv Jnv ¼ qv u ¼ rp ð15Þ RTlv where B is a coefficient relevant to porosity [6]: B ¼ 4 1011 e3 =ð1 eÞ2
ð16Þ
Gaseous products are assumed to accord with the ideal gas law: p¼
qv RT Mv
ð17Þ
Thus, u can be deduced as: u¼
B @ðr2 pÞ lv r2 @r
ð18Þ
Volatile diffusion flux is expressed in the Fick law: Jfv ¼
Dv @ðr2 qfv Þ @r r2
ð19Þ
The related initial and boundary conditions are listed as: qo ¼ qg ¼ 0; 0 r R @qg @qo ¼0 ¼ @r r¼0 @r r¼0
s¼0 ð20Þ
3.4. Particle volume shrinkage Particle volume shrinkage will occur during thermo-degradation. Chan et al. [5] and Lee et al. [7] pointed out that marked shrinkage would take place during pyrolysis, especially in the case of larger particles. Here, particle voidage is
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assumed to vary with the apparent densities of char and wood, which is written as: e ¼ e0 þ
qs qw0 ðee e0 Þ qce qw0
ð21Þ
In the same way, particle volume is calculated as follows: V ¼ V0 þ
ms mw0 ðVe V0 Þ mce mw0
ð22Þ
3.5. Residence time of gaseous products and solid particle Along with reactor height from inlet to outlet during wood pyrolysis, the particle undergoes thermo-degradation, and massive volatile temperature is released from the inner particle to bulk gaseous phase through the solid surface continuously. Therefore, the bulk gas flow will become larger and larger due to additive volatile temperature released from the wood particle, which will reduce gas residence time. Scott and Piskorz [4] pointed out that this phenomenon would lead to a decrease of gaseous product residence time by at least 15%. Here, a plug-flow model is suitable for describing the phenomenon: DsG ¼
e S DL ðvl þ vv Þ
ð23Þ
Up to now, no generally accepted equation for particle residence time is available. Conti et al. [8] indicated that the solid biomass was unlikely to participate in aggregation or back-mixing but was directly elutriated. Thus, the wood particle is assumed to move in the direction of gas flow, with only a slip between gas flow [9]: wqg q s qg dus us dqs ¼ ðuG us Þ2 g ds ds qs qs qs ds
ð24Þ
where 8 < 9:689Re0:78 ð1 þ 0:147Re0:82 Þ 0:1 Re 5 w ¼ 9:689Re0:78 ð1 þ 0:227Re0:55 Þ 5 Re 40 : 9:689Re0:78 ð1 þ 0:838Re0:82 Þ 40 Re 400
ð25Þ
3.6. Secondary cracking in bulk gaseous phase When the volatile temperatere is released from particle into bulk gaseous phase, it still undergoes secondary cracking due to the hot atmosphere in the reactor. However, the repolymerization of volatile to char could be ignored because of the not so high reaction temperature in the reactor. Boroson et al. [10] drew a conclusion that reforming probability in the gaseous phase was too small to be conv sidered even at high temperatures about 500–800 C. Meanwhile, the average freedom course between volatile molecules was so large that mutual collision hardly took place, thus reducing reforming probability. Therefore, secondary
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cracking of the volatile molecule could be described as follows: @ðqo Þ @ 2 qo uG @qo ¼ Do k6 qo @s @z2 e @z
ð26Þ
@ðqg Þ @ 2 qg uG @qg þ k6 q o ¼ Dg @z2 e @z @s
ð27Þ
4. Parameter selection Many researchers have obtained various chemical reaction kinetic parameters of wood pyrolysis. The cited kinetic parameters applied in our model are listed in Table 1. In the following sections, the wood pyrolysis behavior in our developed fluidized bed reactor (Fig. 1) is simulated in detail. The fluidized bed reactor has a diameter of 80 mm and a height of 1200 mm, which is operated within temperatures v between 450 and 700 C. Nitrogen flow at about 3.5 N m3/h is used to fluidize sand in the bed, and sweep the volatile temperature quickly off the hot zone. The Wood particle has 660 kg/m3 density, 2.4 kJ/kg K specific heat capacity, and 0.3 W/m K conduction coefficient [11]. Meanwhile, the char has 0.84 kJ/kg K in specific heat capacity, and 0.1 W/m K in conduction coefficient [7]. Drummond and Drummond [12] indicated that the primary reaction was slightly endothermic v when the temperature was about 500–550 C, while the secondary reaction tended to be exothermic. Generally, the endothermic stage mainly takes place during levoglucosan generation, while that of exothermic stage in the secondary reaction. The wood devolatilization reaction heat might range from 418 to 418 kJ/kg, while a reaction heat about 42 kJ/kg would be liberated in the following secondary cracking [5,13].
Table 1 Kinetic parameters selection Pre-exponential factor
Value (s1)
Activation energy
Value (kJ/mol)
References
A1 A2 (s1) A3 (s1) A4, A6 (s1) A5 (s1)
1:08 107 2:0 108 1:3 108 3:09 106 1:48 106
E1 E2 E3 E4, E6 E5
121 133 140 108 144
Chan et al. [5] Chan et al. [5] Chan et al. [5] Boroson et al. [10] Chan et al. [5]
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Fig. 1. Fluidized bed reactor system for flash pyrolysis of biomass.
5. Results and discussion 5.1. Effect of dense bed temperature There is no doubt that reaction temperature plays an active role in wood pyrolysis, as it does in a fluidized bed reactor. Fig. 2 shows the effect of dense bed temperature on pyrolysis product distribution along with reactor height. With the dense bed temperature increase, it is wood that will mostly undergo thermov degradation in the dense bed. At a dense bed temperature of 500 C, about 80% of wood was decomposed to gas, char, and bio-oil, While it was almost completed v when the temperature rose up to 600 C. Therefore, higher temperatures will promote wood thermo-decomposition. However, the reaction of bio-oil secondary cracking to gas will become more violent when the temperature increases beyond some value. Though wood pyrolysis takes place more rapidly and completely at a v dense bed temperature of 700 C, all bio-oils were decomposed to gas for secondv ary cracking. Thus, a temperature of 500 C is more suitable for maximizing biooil production, which will not only efficiently promote wood pyrolysis rate but also restrain secondary cracking to a certain extent. it is also obvious that gas production will increase with temperature all along. This is the reason why gasification is often operated at higher temperatures.
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Fig. 2. Wood pyrolysis product distribution vs. dense bed temperature (suspension bed temperature: v 450 C; feed rate: 1 kg/h; particle size: 200 lm; black dot: dividing point). (a) dense bed temperature v v v 450 C; (b) dense bed temperature 500 C; (c) dense bed temperature 600 C; (d) dense bed temperature v 700 C.
5.2. Effect of suspension bed temperature When compared with the dense bed temperature, the suspension bed temperature has minor effects on wood pyrolysis behavior. At an optimum dense bed v temperature of about 500 C, only a small fraction of wood undergoes thermal degradation within suspension bed. However, bio-oil secondary cracking may take place violently within the suspension bed if the temperature is higher. Therefore, with a view of restraining bio-oil secondary cracking, it is important to control the suspension bed temperature at a reasonable value. As shown in Fig. 3, secondary cracking of bio-oil becomes more acute with an increase of suspension bed temperature, also accompanied by an increase of gas production. In their detailed research on secondary cracking, Stiles and Kandiyoti [14] indicated that more than 30% bio-oil would take part in secondary cracking v when volatile temperatures stayed at 600 C for 3.5 s than for Q.25s. Fig. 3(b) has shown that nearly half of the bio-oil was transformed to gas caused by secondary v cracking at 600 C. However, secondary cracking could be neglected when the
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Fig. 3. Wood pyrolysis product distribution vs. suspension bed temperature (dense bed temperature: v 500 C; feed rate: 1 kg/h; particle size: 200 lm; black dot: dividing point). (a) suspension bed temperav v ture: 550 C; (b) suspension bed temperature: 600 C. v
temperature is below 450 C according to Fig. 2(b). In accordance with the result proposed by Samolada and Vasalos [15]. However, it is not reasonable to set the temperature of the suspension bed to a value lower than the setting point of biooil, in order to avoid the bio-oil being unexpectedly pre-cooled before entering into the quencher. Generally, according to the experimental results in our fluidized bed reactor, it is more suitable to set suspension bed temperature within the range of v about 400–450 C. 5.3. Effect of particle size Generally, particle size has mutual effects on pyrolysis behavior. On the one hand, the fine particle will be carried by gas flow more easily, which will lead to a short residence time and incomplete decomposition. On the other hand, bio-oil production will be lower if adopting excessive large wood particles because of slow heating up. Therefore, a wood particle of a moderate size should be more reasonable for bio-oil production. As shown in Fig. 4, wood particle size is often set in the order of microns so as to meet flash pyrolysis and pneumatic transportation. Within this range, particle size has little influence on product distribution. After all, the non-sensitivity of product distribution on particle size will lower pretreatment complexity of the raw material. 5.4. Effect of feed rate Until now, the effect of feed rate on pyrolysis behavior had not been reported. The adjustment of feed rate will change gas flow and the volatile temperature in the reactors which will influence wood decomposition and bio-oil secondary cracking. Fig. 5 indicates that bio-oil production increases with feed rate. If wood could be completely decomposed, more volatile temperature will be yielded at the same interval when the feed rate becomes higher, which will enhance gas flow. Therefore, volatile residence at the hot atmosphere will be shortened and secondary cracking
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v
Fig. 4. Wood pyrolysis product distribution vs. wood particle size (dense bed temperature; 550 C; v suspension bed temperature: 450 C; feed rate: 1 kg/h; black dot: dividing point). (a) particle size: 100 lm; (b) particle size: 300 lm.
v
Fig. 5. Wood pyrolysis product distribution vs. feed rate (dense bed temperature: 600 C; suspension v bed temperature: 450 C; particle size: 200 lm; black dot: dividing point). (a) feed rate: 0.5 Kg/h; (b) feed rate: 5 Kg/h.
will be restrained to some extent. Unlike reactor height which influences wood pyrolysis behavior mostly in suspension bed, feed rate plays an active role both in dense bed and in suspension bed. Because the temperature of the dense bed is generally higher than that of the suspension bed, feed rate will restrain the bio-oil secondary cracking more efficiently. However, it is not advantageous to increase the feed rate when incomplete wood decomposition occurs at lower temperatures. 5.5. Verification of model Based on the above analysis, it is clear that the reaction temperature plays the most important role in pyrolysis behavior. Fig. 6 shows the experimental result carried out on our fluidized bed reactor, which is compared with model calculation. As shown in Fig. 6, the modeling result is in good consonance with the
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Fig. 6. Experimental and theoretical results of ultimate product distribution (particle size: 200 lm; feed rate: 1 kg/h; black dots: experimental data). (a) bio-oil production; (b) gas production.
v
experimental results, while a dense bed temperature of 500 C, and lower suspension bed temperature are optimum for bio-oil production.
6. Study of heating-up rate During pyrolysis process, no matter what the reaction temperature is, the wood particle should go through proper temperature areas. Rapid heating-up will make the particle pass low temperature area more quickly in order to reduce charring and maximize bio-oil production. However, the heating-up rate is affected by many operational parameters, such as particle characteristics and heat flux. Therefore, if the reactor type is pre-determined, temperature and particle size will be the two most important factors influencing heating-up rate. Kothari and Antal [16] suggested that flash pyrolysis could be divided into two steps in succession: heating-up stage and volatile release stage. At heating-up stage, the weight loss of particle could be neglected and the particle is rapidly heated. As the particle temperature reaches up to a predetermined value, the volatile temperature begins to be released out of the particle, which is called the volatile release stage. In fact, the above two stages could not be completely separated. However, when compared with the reaction period and the residence time of particle, the time for the particle being heated up to 95% of reaction temperature is too short to consider pyrolysis process, especially in the case of high flux. Therefore, wood decomposition could be ignored during the calculation of heating-up rate in fluidized bed reactor. Only in the situation higher reaction temperature or larger particle is it necessary to the above two stages consider simultaneously. In this paper, particle core, where heating-up rate is the slowest, is selected as the basis to study the effect of temperature and particle size on the heating-up rate. As shown in Table 2, the heating-up rate of a fine particle could achieve up to v an order of 104 C/s, which is also verified in other research. In some cases, fine v particles might attain a very high heating-up rate, up to the order of 105 C/s [8,17]. However, the actual heating-up rate depends not only on heat transfer but
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Table 2 Heating-up rate vs. particle size and temperature Size (lm)
100 300 500
v
v
450 C
v
550 C
700 C
UT95% v ( C/s)
UT350 v ( C/s)
UT95% v ( C/s)
UT350 v ( C/s)
UT95% v ( C/s)
UT350 v ( C/s)
2:2 104 6:3 102 2:1 102
3:4 104 1:0 103 3:4 102
2:9 104 8:4 102 2:8 102
5:3 104 1:6 103 5:5 102
4:1 104 1:2 103 4:0 102
8:1 104 2:5 103 8:6 102
also on pyrolysis progress, which implies that the actual heating-up rate is smaller than the values listed in Table 2. Generally, the heating-up is often calculated on the basis of 95% of reaction temperature. However, the essential effect of heating-up rate on flash pyrolysis is to shorten particle residence time at low temperatures. This will decrease the charring opportunity and increase the bio-oil production. Thus, it is more reasonable to select a moderate temperature value but not a reaction temperature as the baseline v for calculating the heating-up rate. Here, the temperature of 350 C is selected as the calculation base, at which mostly volatile temperature is released out of the particle. The related result is also listed in Table 2, which shows that all particles less than 500 lm are located in flash pyrolysis range. When one Compares the results in Table 2 with those shown in the above figures, it is clear that the pyrolysis rate is far slower than the heating-up rate. Therefore, it is more reasonable to modify the name ‘‘flash pyrolysis’’ as ‘‘pyrolysis at flash heating-up’’.
7. Conclusions An integrated model, combined reaction kinetics with mass and heat transfer, was proposed to predict pyrolysis behavior in a fluidized bed reactor. The Modeling results showed that the reaction temperature played the most important role in v wood pyrolysis behavior. A moderate dense bed temperature of 500 C, and lower suspension bed temperature were more suitable for optimizing bio-oil production, at which the wood particle could undergo a complete decomposition and bio-oil secondary cracking be well restrained. Besides this, particle size and feed rate have mutual effects on pyrolysis behavior. The Fine particle will enhance rapid heating-up, but may lead to incomplete particle decomposition at lower temperatures. In the case of higher dense bed temperature, volatile secondary cracking will be efficiently restrained when the feed rate is increased. However, an excessive large feed rate might cause incomplete wood particle decomposition. Rapid heating-up will make particle passing low temperature area more quickly so as to reduce charring and maximize bio-oil production. The study of heating-up rate showed that it is more suitable to choose a moderate temperature, and not
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