Modeling and Experimental Studies of the Effects of Volume

Sep 22, 2014 - Shrinkage on the Pyrolysis of Waste Wood Sphere ... shrinkage model was coupled with a one-dimensional unsteady wood pyrolysis model to...
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Modeling and Experimental Studies of the Effects of Volume Shrinkage on the Pyrolysis of Waste Wood Sphere Qun X. Huang,* Ru P. Wang, Wen J. Li, Yi J. Tang, Yong Chi, and Jian H. Yan State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: When the shape of single wood spheres was captured and processed under different pyrolysis environments instantaneously, the evolution of sample geometry was obtained and a shrinkage model was proposed in this paper. The proposed shrinkage model was coupled with a one-dimensional unsteady wood pyrolysis model to predict the temperature profiles and mass variation as well as product distribution within wood spheres. A one-step drying mechanism and three parallel primary decomposition reactions as well as three secondary cracking reactions were used to describe the entire pyrolysis process. Experiments were carried out to assess the efficiency of the model prediction, and the effects of volume shrinkage on the temperature, weight loss, and product distribution were analyzed and discussed. Experimental results show that the shrinkage rate is proportional to the furnace temperature, and the average shrinkage rate increases from 0.39 to 0.53 mm/min for 20 mm wood spheres and from 0.26 to 0.55 mm/min for 30 mm wood spheres when the furnace temperature increases from 673 to 973 K. However, the final shrinkage ratio is inversely proportional to the furnace temperature and decreases with the increased sphere diameter. The peaks of simulated temperature profiles are in good agreement with experimental results when shrinkage is considered. Simulated mass loss profiles with shrinkage agree well with experimental data. On the contrary, if a constant particle size is used, the deviation between simulated and measured residual fractions is about 24% for the wood spheres studied in this paper.

1. INTRODUCTION Because of the decrease of fossil fuels, biomass, especially waste wood spheres, characterized by its renewability and abundant reserves, has been widely used in energy and chemical industries as feedstock for replacing fossil fuels.1−3 In addition to direct combustion, wood pyrolysis/gasification has received intensive interest recently, and many projects have been demonstrated or commercially applied using different reactors, including updraft/downdraft fixed bed, fluidized bed, and moving bed.4 Because of the poor understanding of the complicated thermal reactions within wood particles under a reducing environment, many systems are reported to suffer from a low carbon conversion efficiency and high yield of undesirable tar in syngas.5 When heated under reducing conditions, a wood particle experiences a series of homogeneous and heterogeneous reactions, accompanied by heat/mass transfer, momentum interaction, and energy conversion. The reaction kinetics will be influenced by wood composition, density, moisture content, pore structure, etc.6,7 In the past few years, researchers have been devoted to explore pyrolysis kinetics and product distribution,8 for characterizing the pyrolysis of different samples,9 aiming to develop an accurate degradation mechanism from single stage to multi-stage.10 Sadhukhan et al.11 studied the isothermal pyrolysis of wood particles in a tubular reactor and obtained their mass loss data, which were used to calculate the decomposition coefficients for different wood components. The ̂ model proposed by Vijeu et al.12 consists of three parallel primary reactions and one secondary reaction. Wood particles are considered to decompose into gas, tar, and char first, and then tar is cracked into gas and other tar species. Okekunle et al.13,14 proposed a secondary decomposition model considering that the liquid primary tar would be converted into gas, secondary tar, and char. They investigated the effects of the © 2014 American Chemical Society

heating rate, reaction temperature, and wood composition on the pyrolysis time and product yield. The interaction between the primary products was also studied, and the primary volatiles and char were assumed to react with each other to produce new species.15,16 Grieco and Baldi17 proposed a kinetic model by taking the effect of the secondary reaction within the particle into consideration. They classified the primary tar into three types with respect to their molecular weight and found that the tar with heavy molecular weight undergoes a cross-linking reaction, giving rise to gas and char before escaping wood particles. One of the most notable deficiencies of many previous developed models is the assumption of a constant sphere size,13,14,17 which has been observed changing under different conditions. Labscale experiments have proven that the volume shrinkage during thermal reaction will affect the decomposition rate and inner temperature significantly.6,18 Di Blasi19,20 has once proposed a shrinkage model for wood particles using three empirical variables, and the particle volume is updated according to the local medium density, porosity, and pyrolysis products. The model has predicted that, when shrinkage is considered, the volatile residence time will decrease.21,22 However, they did not provide direct experimental results on the shrinkage behavior. Davidsson and Pettersson23 studied the shrinkage of different-shaped wood pellets, and they used three shrinking types, i.e., uniform shrinking, shell shrinking, and cylinder shrinking to examine the effect on the pyrolysis rate. Unfortunately, the temperature distribution, the most important parameter inside the wood pellets, was not concerned. In the present study, a high-resolution charge-coupled device (CCD) camera equipped with a macro lens is used to record Received: July 4, 2014 Revised: September 19, 2014 Published: September 22, 2014 6398

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Figure 1. Schematic drawing of the experimental apparatus. studies show that the influence of wood type and sample size on the pyrolysis model is relatively small;7,9 therefore, and two sphere sizes (diameter of 20 and 30 mm) are studied in this research. To build the shrinkage model, the temporal photos of different sized spheres at different pyrolysis temperatures (673, 773, 873, and 973 K) are captured with the camera, as shown in Figure 2. The changes of the sphere size are retrieved through an image processing method, and the instantaneous size of the wood sphere in terms of radius is defined as

the volume shrinkage behavior of a single wood sphere under different pyrolysis conditions. The size variation of the sphere is deduced with the image processing method, and the inner temperature profiles are recorded with fine thermocouple probes. A new mathematic model for describing sphere size evolution is proposed and coupled with a two-step pyrolysis model. Numerical simulations are carried out to investigate the effect of shrinkage on the temperature, mass loss, and product generation in the sphere during pyrolysis under different heating temperatures.

1 − ω = r /r0 × 100%

2. EXPERIMENT SETUP

(1)

where ω, r, and r0 are the shrinkage ratio and instantaneous and initial radii of the wood sphere, respectively. Here, the shapes of different sized wood spheres are assumed to maintain sphere during the whole process. The calculated shrinkage ratio, furnace temperature, and sphere size are shown in Figure 3. Because we can notice that the evolution profiles of all wood sphere size are similar, in this study, a mathematical function based on Boltzmann fitting is proposed to describe the changes of the sphere size

As shown in Figure 1, the experimental setup was built to obtain the sphere shrinkage profiles during pyrolysis. An electrically heated furnace and a quartz tube with a diameter of 8 cm and a length of 60 cm were used as the fixed-bed reactor. The left of the reactor made of high-transparency quartz glass worked as an observing window, which was sealed with nitrogen flow (400 L/h) to avoid contamination. The nitrogen flow maintaining the reducing environment exits from the other side of the tube carrying gaseous products. Samples were placed in a silica crucible fixed to a glass rod. The whole reactor was placed on an electronic scale (Sartorius BS4202S) to measure the weight loss. A high-resolution CCD camera equipped with a macro lens was employed to capture the image of wood spheres, and an image processing method was developed to retrieve the shape instantaneously. The inner temperatures of the spheres during pyrolysis were measured with fine thermocouple probes (size of 500 μm) intruded in the spheres, as shown in Figure 1. The center (r = 0), middle (r = 0.5r0), and surface (r = r0) temperatures were recorded simultaneously with the image and mass loss. During the test, the reactor was heated to and maintained at the desired temperature first, and the residence time of wood spheres in the reactor was controlled around 30 min to achieve complete reaction. The temperature of the gas near the wood sphere was also measured by a thermocouple for heat-exchanging calculation. Moreover, the pyrolysis gas and tar of the wood sphere were collected, and their components were analyzed by gas chromatography (Agilent 490 Micro GC) and gas chromatography−mass spectrometry (GC−MS, TRACE GC 2000−TRACE MS), respectively. Duplicate experiments were carried out to eliminate measurement uncertainty.

⎡ ψ1 − ψ2 ⎤ 1 − ω = ⎢ψ2 + ⎥ /100 ⎣ 1 + e(t − ψ3)/ ψ4 ⎦

ψγ = α1Tf 3 + α2Tf 2 + α3Tf + α4

(2)

(γ = 1, 2, 3, 4)

(3)

where ψ1, ψ2, ψ3, and ψ4 are pyrolysis temperature-based polynomials, as shown in eq 3. Coefficients of α1, α2, α3, and α4 depend upon the wood material and initial radius. Their values for the wood spheres used in current paper are given in Table 1. From eq 2, we can estimate wood shrinkage behavior for the experimental temperatures, as shown in Figure 3. The predicted values fit experimental data very well. For wood spheres with the same diameter, the shrinking rate is proportional to the furnace temperature but the final shrinkage ratio decreases with the temperature. The average shrinkage rate changes from 0.39 to 0.53 mm/min for 20 mm wood spheres and from 0.26 to 0.55 mm/min for 30 mm wood spheres when the furnace temperature increases from 673 to 973 K. However, the shrinkage ratios are found decreasing from 26.0 to 16.0% for 20 mm wood spheres and from 17.2 to 15.4% for 30 mm wood spheres with the increased temperature. This may be because the higher the temperature, the larger the inner pressure. Then, the higher inner pressure enlarges the wood sphere expanding during pyrolysis. At the same temperature, the shrinkage ratio is inversely proportional to sphere diameters. The rapid temperature increasing rate inside small spheres may account for this phenomenon.

3. SHRINKAGE MODEL In this paper, different sized spherical particles made of schima superba wood were used as samples. Schima superba wood, an abundant typical biomass resource, is an important feedstock for biomass briquette, which has been widely used in China. Moreover, previous 6399

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Figure 2. Evolution of the sample appearance during pyrolysis under different temperatures. and one-dimensional. The two-step unsteady pyrolysis model proposed by Park et al.24 is improved to describe the pyrolysis behavior by changing the constant sample size into a variable. As depicted in Figure 4a, the first step of the model is the primary decomposition of wood components, yielding volatile gas species, primary tar, and solid intermediates. As the temperature is increased, the primary tar will be cracked into gas species and char and the intermediate solid will be converted into char in the second step. The constant volume size used in the model by Park et al. is replaced with instantaneous size deduced from the upper proposed shrinkage model, as illustrated in Figure 4b. Using the improved model, numerical calculations have been carried out to quantitatively investigate the effect of shrinkage on pyrolysis reactions. The detailed kinetics and governing equations are described in the following section. 4.1. Reaction Kinetics. Before the pyrolysis reaction, the moisture evaporation can be described as woodW → χ H 2O + (1 − χ )wood w

Figure 3. Instantaneous size of wood spheres (dotted lines, experimental results; solid lines, fitted results).

(4)

where χ is the initial moisture content in the wood sample. Because of the large heat consumption, evaporation has a great influence on wood pyrolysis/gasification. In this paper, the first-order Arrhenius kinetic function25 is used

4. PYROLYSIS MODELING WITH SHRINKAGE CONSIDERED When the wood sphere is heated under inert conditions, endothermic water evaporation takes place first, followed by a series of homogeneous and heterogeneous pyrolysis reactions. In current study, the pyrolysis of the spherical sample is assumed to be homogeneous

⎛ Evap ⎞ k vap = A vapT −0.5 exp⎜ − ⎟ρ χ ⎝ RT ⎠ W 6400

(5)

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Table 1. Expression of Shrinkage Parametersa α1 (×107) α2 (×104) α3 α4 a

ψ1

ψ2

ψ3

ψ4

−25.85r0 + 29.88 62.46r0 − 71.09 −4.984r0 + 5.623 1314.3r0 − 1377.2

2.252r0 − 2.37 −4.658r0 + 4.321 0.25r0 − 0.136 −10.02r0 + 25.67

217.2r0 − 351.4 −531r0 + 889.5 42.37r0 − 74.727 −10934r0 + 20848

−155r0 + 136.9 384r0 − 339.5 −31.42r0 + 27.77 8556r0 − 7513

Here, the unit of initial radius is centimeters.

Figure 4. Pyrolysis model with shrinkage considered.24 where ρW is the density of the initial wood sphere. Avap and Evap are the pre-exponential factor and activation energy, respectively. When the temperature increases, pyrolysis reactions described by a two-step model will be accelerated. The first step is the endothermic primary decomposition, including three parallel reactions (r1, r2, and r3), yielding gas species, primary tar, and intermediate solid, respectively. The cracking of primary tar into gas (r4) and char (r5) and the conversion of intermediate solid into char (r6) constitute the main reactions of the second step. All reaction rates (r1−r6) are formulated with the first-order Arrhenius kinetic function

⎛ E ⎞ ki = Ai exp⎜− i ⎟ρW (1 − χ ) ⎝ RT ⎠

⎛ E ⎞ ki = Ai exp⎜ − i ⎟ρt ⎝ RT ⎠

(i = 1, 2, 3)

Table 2. Source Terms of Continuity Equations

(i = 4, 5)

(7)

∂t

+

⎤ ⎡ BR ⎢ 1 ∂(ρvap T ) 1 ∂(ρg T ) 1 ∂(ρt T ) ⎥ + + μ ⎢⎣ M vap ∂r Mg ∂r M t ∂r ⎥⎦

(8)

(9) 1 ∂ 2 (r vρj ) = Sj r 2 ∂r

(13)

where BW0 and Bc are the permeabilities of original wood and char, respectively. η is the pyrolysis degree characterizing the progress of the pyrolysis reaction and is defined as

(10)

η = 1 − (ρw + ρis + ρm )/ρW0

where j is the reactant/product species, including dry wood, intermediate solid, char, and liquid water for eq 9 and water vapor, pyrolysis gas, and primary tar for eq 10. Sj is the source term, which is listed in Table 2. Here, ε and v are the porosity of wood spheres and velocity of gaseous species, respectively. Porosity is related with the densities of the solid phase and pure wood ingredients. The gaseous species flow velocity within the wood sphere is calculated through Darcy’s law,26 as shown in eq 12

ε = 1 − ρp (1 − εW0)/ρW0

(12)

where ρp, defined as ρp = ρw + ρm + ρis + ρc, is the instantaneous density of all solid-phase species. ρW0 and εW0 are the density and porosity of original wood on a wet basis, respectively. Mvap, Mg, and Mt are molecular weights of water vapor, pyrolysis gas, and primary tar, respectively. In this study, pyrolysis gas is considered to be an ideal gas mixture. The components of gas and tar are analyzed by gas chromatography and GC−MS, respectively. The molecular weight of the mixture is equal to the weighted average of the molecular weight of each component in the mixture. B is the permeability of the solid phase, defined as eq 1319

B = (1 − η)B W0 + ηBc

= Sj

∂(ερj )

−(k1 + k2 + k3) k3 − k6 k5 + k6 −kvap kvap k1 + k4 k2 − k4 − k5

(6)

where Ai and Ei are the pre-exponential factor and activation energy of the ith reaction, respectively. In this study, values of kinetic parameters are obtained from previous published literature.24 4.2. Control Equations. According to the two-step pyrolysis model, the mass conservation equations of solid and gaseous phases are given in eqs 9 and 10

∂t

source term

w is c m vap g t

v=−

⎛ E ⎞ k6 = A 6 exp⎜ − 6 ⎟ρis ⎝ RT ⎠

∂ρj

reactant/product j

(14)

From eqs 10 and 12, it can be seen that radius shrinkage will influence the producing rate of gaseous products and the velocity within wood spheres. The energy conservation equation is given in eq 15, the first item of which is the unsteady term, accounting for the absorbed heat. The second item is the convective term. The last two are conductive and source terms in turn, which are caused by thermal conductivity and pyrolysis reactions, respectively. In this study, it is considered that

(11) 6401

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gas and solid phases in wood spheres are always in local thermal equilibrium

adopted for convective terms, and the harmonic mean was used to calculate the equivalent thermal conductivity in the surface between two grids. Spatial and temporal steps were set to 0.2 mm and 0.0001 s, respectively. In the simulation, reaction kinetic parameters were first calculated after grid building and initial condition setting. Then, mass and energy conservation equations for solid and gaseous phases were solved. The spatial grid was assumed to be uniform and was updated according to the instantaneous radius calculated from eq 2. The material properties and coefficients used during simulation were obtained by measurement or cited from literature, as listed in Table 3.

(Cwρw + Ccρc + C isρis + Cmρm + εC pgρg + εC ptρt ∂T ∂T + (C pgρg + C ptρt + C pvapρvap )v ∂t ∂r 1 ∂ ⎛⎜ 2 ∂T ⎞⎟ = 2 +Q rλ r ∂r ⎝ ∂r ⎠ + εC pvapρvap )

(15)

where λ is the thermal conductivity in the radial direction and defined by eq 16.19 Q is the heat generation source term during wood pyrolysis and is formulated as eq 17 λ = (1 − η)λ w + ηλc + ελ v + 13.5σT 3d /ξ

(16)

Table 3. Values of Material Properties Used in the Simulation

6

Q=

∑ kiHi

(T < 373 K)

or

Q = k vapH vap

i=1 6

+

∑ kiHi

(T ≥ 373 K) (17)

i=1

where σ, d, and ξ are the Stefan−Boltzmann constant, pore size, and emissivity. Hi is reaction heat released or absorbed by pyrolysis reactions, and their values are obtained from previous literature.24 Hvap is the latent heat of vaporization. In this study, the proposed shrinkage model is coupled into control equations to calculate the instantaneous radius of the wood spheres through eqs 2 and 3. The spatial grid size in the control equations is updated according to new sphere radii during calculation. 4.3. Boundary Conditions. Boundary conditions are described in eqs 18 and 19. In the current study, convective and radiative heat exchanges are taken into account and adiabatic boundary conditions are used at the central position λ

∂T = h(Ta − Ts) + σξs(Tf 4 − Ts 4) ∂r

∂T =0 ∂r

(r = r0)

(r = 0)

(18) (19)

where ξs is the surface emissivity, which is continuously changing with the progressing of pyrolysis. According to the study by Galgano and Di Blasi,27 surface emissivity can be defined as eq 20 by referring to the sphere images shown in Figure 2. ⎧ ξw (T < 480) ⎪ ⎪ ξs = ⎨ ξw + (Ts − 480)(ξc − ξw)/120 (480 ≤ T ≤ 600) ⎪ ⎪ ξc (T > 600) ⎩ (20) On the basis of upper improved reaction kinetics and heat/mass balance equations, the effect of shrinkage on wood pyrolysis behavior can be predicted. However, as heterogeneous materials, the real pyrolysis reactions are far more complex than upper models. Therefore, it is assumed that (a) wood spheres are homogeneous and both heat and mass transfer occur only in the radial direction, (b) the spherical shape of the wood sample is maintained during the pyrolysis process, (c) the effect of the moisture content on thermal conductivity is negligible, and (d) gaseous and solid phases are in thermal equilibrium in each discrete unit.

parameter

value

source

ρW0 (kg/m3) χ (%) εW0 Mt (kg/mol) Mg (kg/mol) Cpvap (J kg−1 K−1) Mvap (kg/mol) Cw (J kg−1 K−1) Cis (J kg−1 K−1) Cc (J kg−1 K−1) Cpt (J kg−1 K−1) Cpg (J kg−1 K−1) d (m) ξw ξc λw (W m−1 K−1) λc (W m−1 K−1) σ (W m−2 K−4) h (W m−2 K−1) BW0 (m2) Bc (m2) Cm (J kg−1 K−1) Hvap (J/kg) Avap (s−1 K−0.5) Evap (J/mol) ξ μ (kg m−1 s−1) λv (W m−1 K−1) R (J mol−1 K−1)

560 10 0.39 0.105 0.027 1850 0.018 1500 + T 420 + 2.1T + 6.85 × 10−4T2 420 + 2.1T + 6.85 × 10−4T2 −100 + 4.4T − 1.57 × 10−3T2 770 + 0.629T − 1.91 × 10−4T2 5 × 10−5(1 − η) + 1 × 10−4η 0.6 1.0 0.21 0.071 5.67 × 10−8 20 5.0 × 10−16 1.0 × 10−13 4200 −2.7 × 106 6.0 × 105 48220 0.95 3.0 × 10−5 0.0258 8.314

measured measured measured measured measured estimated estimated 31 31 31 31 31 31 27 27 27 27 24 24 26 26 32 32 25 25 15 33 34

The experimental and numerical deduced temperatures at the center (r = 0), middle (r = 0.5r0), and surface (r = r0) for wood spheres pyrolyzed at 773 K are plotted in Figures 5, 6, and 7, respectively. The mass loss and mass loss rate profiles are shown in Figures 8 and 9, respectively. It is observed that the maximum temperatures at center and middle positions in the wood sphere are delayed for about 240 s when the sphere size changes from 20 to 30 mm. This is because the increased particle size will extend the residence time of pyrolysis gas species and inhibit the heat and mass transfer within wood spheres.7,14 Similarly, the weight loss behavior is also affected strongly by the particle size. Subjected to the heat-transfer rate and gas species releasing rate during pyrolysis, the 30 mm wood sphere requires more time to achieve complete reaction and the maximum mass loss rate is delayed for 98 s compared to that of the 20 mm sphere.

5. RESULTS AND DISCUSSION 5.1. Model Evaluation. To verify the accuracy of the shrinkage model, numerical simulations using the improved pyrolysis model were carried out for different sized wood spheres (20 and 30 mm diameters). Control volume integration was used to discretize the conservation equations for solid and gaseous phases and energy. The central difference scheme was 6402

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Figure 5. Center temperatures of different sized wood spheres. Figure 8. Residual mass fractions of different sized wood spheres.

Figure 6. Middle temperatures of different sized wood spheres. Figure 9. Mass loss rates of different sized wood spheres.

temperature peaks when shrinkage is considered. However, the peaks of two different sized wood spheres are prolonged by 61 and 114 s if shrinkage is ignored. A similar phenomenon is also found by Chaurasia and Kulkarni.28 It suggests that the proposed shrinkage model is accurate and necessary for describing the pyrolysis behavior. The constant size of the wood sphere makes the appearance of temperature peaks later, and the influence increases when the sphere size changes from 20 to 30 mm. Weight losing behavior are also greatly affected by volume shrinkage. When the sphere size is constant, weight loss becomes very slow and the final residual fraction is increased by about 24% compared to experimental results. Mass loss rates without shrinkage are much lower than experiment results, and the maximum rates are also delayed. This is because the release of pyrolysis gas from wood spheres is slowed down, and thereby, the mass loss rate is decreased when the sphere size is constant.29,30 It should be noted that endothermic water evaporation accounts for the constant temperature (373 K in Figures 5 and 6) at the center and middle positions. The time required for water evaporation is proportional to the sphere size and lasts for about half of the pyrolysis time, which is adverse for wood pyrolysis. Another short plateau appears at about 660 K for center temperature curves and it is also observed by Park et al.,24 who have concluded that the endothermic decomposition of cellulosic materials at this temperature is the main reason. After 660 K,

Figure 7. Surface temperatures of different sized wood spheres.

To evaluate the accuracy of the improved model, numerical deduced temperatures with and without shrinkage are compared. When volume shrinkage is considered, simulated results are in good agreement with experimental data. However, if the particle size is assumed to be constant, great deviation between simulated and experimental results is observed. The peaks of simulated temperature profiles (center and middle temperatures) are observed at 300 and 550 s for 20 and 30 mm wood spheres, respectively, and agree well with experimental 6403

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center and middle temperatures increase steeply and reach a peak value because of the carbonization reaction. 5.2. Effect of Shrinkage on Wood Pyrolysis Products and Pore Structure. Although it is well-known that shrinkage will accelerate the pyrolysis reaction, quantitative analysis of the shrinkage effect on product generation is very limited. To provide an insightful investigation of the impact of particle shrinkage on wood pyrolysis, the instantaneous concentration of four product species and the differences between the concentration of each product with and without shrinkage at 773 K for the 20 mm wood sphere are calculated with eqs 21 and 22, respectively ρϑ × 100% Kϑ = ρp + ρg + ρt (21)

ΔEϑ = K ϑ − K ϑ′

(22)

where ρϑ represents the local density of product ϑ, which can be gas, primary tar, intermediate solid, or char. ρp is the instantaneous density of all solid-phase species as defined above. Kϑ and K′ϑ are the local concentrations of product ϑ with and without shrinkage, respectively. The differences between the instantaneous concentrations of each product with and without shrinkage are plotted in Figures 10−13.

Figure 11. Differences between the instantaneous concentrations of primary tar with and without shrinkage considered. The difference value is positive from 190 to 300 s but negative from 300 to 380 s. The maximum concentration difference at 0.48% appeared at 350 s.

Figure 10. Differences between the instantaneous concentrations of pyrolysis gas with and without shrinkage considered. The difference value is positive from 190 to 300 s but negative from 300 to 380 s. The maximum concentration difference at 0.08% appeared at 350 s. Figure 12. Differences between the instantaneous concentrations of intermediate solid with and without shrinkage considered. The difference value is positive from 200 to 300 s but negative from 300 to 380 s. The maximum concentration difference at 70% appeared at 340 s.

For pyrolysis gas and tar shown in Figures 10 and 11, the influence of volume shrinkage is illustrated by the positive peak and negative valley before and after 300 s, respectively. The peak and valley clearly indicate that the generation of pyrolysis gas and tar are underestimated before 300 s and overestimated after that when shrinkage is ignored. This is caused by the fact that, for the pyrolysis reaction before 300 s, the calculated temperatures with shrinkage in the center and middle positions are 80 and 75 K higher than the prediction without shrinkage, respectively. Approximate symmetric difference profiles, whose peak values are 49 and −70% at 290 and 340 s for intermediate solid, respectively, are observed in Figure 12. It indicates that

the generation and consumption of intermediate solid are greatly under- and overestimated before and after 300 s because of the underrated heating rate. Because of the incorrectly predicted intermediate solid density, the production of char is also underrated from 220 to 380 s, as shown in Figure 13. Moreover, the difference increases from the surface to the center, implying that the calculated error caused by ignoring volume shrinkage will accumulate from the surface to the center along the spatial coordinate. 6404

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Figure 15. Pore size evolution with shrinkage (solid line) and without shrinkage (dash line).

Experimental results show that the radius shrinkage of the wood sphere discussed in this paper is in a range of 15.4−26.0% and greatly influenced by the furnace temperature and original sphere size. The shrinkage rate is proportional to the furnace temperature, but the final shrinkage ratio is opposite. Besides, wood spheres with small size give rise to the large final shrinkage ratios. Endothermic evaporation has a significant influence on the heating rate of wood particles, and reducing the water content can effectively accelerate the wood pyrolysis reaction. The shrinkage model proposed in this study is effective, and it can significantly improve the accuracy of the wood pyrolysis model. The rise of the temperature within the wood sphere is overestimated by 61 s when shrinkage is neglected. Keeping the sphere size as a constant makes the final residual fraction overestimated by 24%. The pyrolysis rate is accelerated by particle shrinkage, making the generation of products, including gas, tar, intermediate solid, and char, in advance. The maximum generation rates of gas and tar are observed to be delayed because of the underestimated porosity and pore size when shrinkage is ignored.

Figure 13. Differences between the instantaneous concentrations of char with and without shrinkage considered. The difference value is positive from 220 to 380 s, and the maximum concentration difference at 99% appeared at 320 s.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87952834. Fax: +86-571-87952438. E-mail: [email protected].

Figure 14. Porosity evolution with shrinkage (solid line) and without shrinkage (dash line).

Notes

The variation of the pore structure might be another reason why the mass loss rate was small when shrinkage was ignored. Figures 14 and 15 show the porosity and pore size at five different positions along the radical direction, respectively. It could be observed that simulation with a constant sphere size will underestimate the porosity and pore size and slow the releasing rate of gas and tar, thereby retarding the mass loss without shrinkage.

The authors declare no competing financial interest.

6. CONCLUSION To investigate the effect of shrinkage of the wood particle on its pyrolysis behavior, a new shrinkage model was proposed in this paper from the instantaneous recorded images. A onedimensional unsteady model coupled with the proposed shrinkage model was used to describe the pyrolysis behavior of a single wood sphere. The inner temperature distribution and weight loss as well as product distribution were examined.

■

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ACKNOWLEDGMENTS Acknowledgment is gratefully extended to the National Basic Research Program of China 973 Program (Grant 2011CB201500), the National Science and Technology Pillar Program (2012BAB09B03), the National High Technology Research and Development Program (2012AA063505), and the Creative Team Project of Solid Waste Treatment of Zhejiang Province (A2009R50049) for their financial support.

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NOMENCLATURE A = pre-exponential factor (s−1) B = permeability (m2) C = specific heat capacity (J kg−1 K−1) Cp = specific heat capacity at a constant pressure (J kg−1 K−1) d = pore diameter (m) dx.doi.org/10.1021/ef501504k | Energy Fuels 2014, 28, 6398−6406

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e = natural constant E = activation energy (J/mol) h = convective heat-transfer coefficient (W m−2 K−1) H = reaction heat (J/kg) k = reaction rate (kg m−3 s−1) M = molecular weight (kg/mol) P = pressure (Pa) Q = heat generation in control volume (J m−3 s−1) r = radius (m) R = ideal gas constant (J mol−1 K−1) t = time (s) T = temperature (K) v = velocity (m/s) Greek Letters

χ = moisture content in original wood (%) ε = porosity η = pyrolysis degree ω = shrinkage ratio (%) λ = thermal conductivity (W m−1 K−1) μ = viscosity (kg m−1 s−1) ρ = density (kg/m3) ξ = emissivity σ = Stefan−Boltzmann constant (W m−2 K−4)

Subscripts

0 = initial status a = atmosphere c = char f = furnace g = pyrolysis gas is = intermediate solid m = liquid water s = particle surface t = tar v = volatiles vap = water vapor w = wood on a dry basis W = wood on a wet basis

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dx.doi.org/10.1021/ef501504k | Energy Fuels 2014, 28, 6398−6406