Simulation of coal pyrolysis by solid heat carrier in a moving-bed pyrolyzer

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Fuel 87 (2008) 435–442 www.fuelfirst.com

Simulation of coal pyrolysis by solid heat carrier in a moving-bed pyrolyzer Peng Liang a b

a,b,*

, Zhifeng Wang a, Jicheng Bi

a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, PR China Received 28 November 2006; received in revised form 26 June 2007; accepted 26 June 2007 Available online 26 July 2007

Abstract A one-dimensional, steady state, numerical model for coal pyrolysis by solid heat carrier in moving-bed has been developed. The multiple-reaction model of coal pyrolysis and the gas–solid–solid three phases heat transfer theory in packed bed have been applied to account for the pyrolysis process. The results show that the axial temperature distribution of the coal particles increase with a heating rate more than 600 K/min. Coal particle size has significant influence on the heating rate, while blending ratio is the determinant factor of pyrolysis temperature. Given the main operating parameters, product distributions (H2, CO, CH4, tar, etc) are calculated by the model. The modeling results are found to agree the experimental data using a moving-bed pyrolyzer with processing capacity 10 kg h1 of coal.  2007 Elsevier Ltd. All rights reserved. Keywords: Poly-generation; Coal pyrolysis; Solid heat carrier

1. Introduction Pyrolysis and combustion are the two common types for coal utilization. The utilization efficiency of coal will be improved greatly when the two types are integrated reasonably. Because the circulating fluidized bed (CFB) combustion technique has developed quickly recently, it provides a base for the integration. High-temperature ash from the CFB boiler is used as solid heat carrier to provide heat for the coal pyrolysis, getting gas and tar. The semi-coke from the moving-bed pyrolyzer returns to the CFB boiler to generate electricity and provide heat. Thus, poly-generation of heat, power, gas and tar can be realized in a system by coupling CFB combustion with coal pyrolysis [1]. Modeling work of coal pyrolysis has been widely investigated, and many useful results have been obtained. With the deeper understanding of coal molecular structure in * Corresponding author. Address: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China. Tel./fax: +86 351 4072379. E-mail address: [email protected] (P. Liang).

0016-2361/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.06.022

recent years, some models on coal pyrolysis have been reported, such as CPD model, Flash Chain model and FG-DVC model, etc. [2–4]. Moving-bed pyrolyzer of coal pyrolysis with solid heat carrier is one of the most important parts of the poly-generation system, as it determines whether the combinatorial system runs steady or not. Considerable experimental investigation on coal pyrolysis with solid heat carrier has been conducted over the years [5–7], however the relative simulation work is scarcely reported. He [8] reported the calculation and analysis of heat and mass transfer on lignite pyrolysis with solid heat carrier. A moving-bed reactor model based on the theory of flow and heat transfer in non-sintered porous media was developed [9]. However, ash and coal was considered as one unit to describe the heat transfer between solid and pyrolysis gas in the model, which was not an accurate assumption. As a result, the reliability of the prediction on coal particle temperature remains questionable. The present model is based on the heat transfer theory of gas–solid–solid three phases. The multiple-reaction model is applied in the model to forecast the volatile evolution, gas, tar yield and temperature distribution in the pyrolyzer. The model is shown to

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Nomenclature A cpa cpc cpf dac dp dpa dpc Ej h hag hcg ka k0j kc R Rac Ta Ta0 Tc Tc0 Tg ug us wj

cross area of the moving-bed pyrolyzer, m2 heat capacity of the ash, J/kg K heat capacity of the coal, J/kg K heat capacity of the fluid, J/kg K diameter of the contact area of adjacent particles, m diameter of the particle, m diameter of ash particle, m diameter of coal particle, m activation energy of reaction forming product j, J/mol heat transfer coefficient of particle and gas, W/ m2 K heat transfer coefficient of ash and gas, W/m2 K heat transfer coefficient of coal and gas, W/ m2 K thermal conductivity of ash, W/m K preexponential factor of reaction forming product j, 1/s thermal conductivity of coal, W/m K ideal gas constant, J/mol K ratio of contact area to coal surface area temperature of ash, K initial temperature of ash, K temperature of coal, K initial temperature of coal, K temperature of gas, K superficial velocity of gas, m/s velocity of solid, m/s value of wj at t = 1

be capable of predicting the axial temperature distribution of gas, coal and ash, as well as the evolution process of products from coal particles under different operating parameters, such as the coal particle size, blending ratio and residence time. This work will enable a better understanding on the process and provide a theoretic foundation for the further design of moving-bed pyrolyzer in the polygeneration system. 2. Experimental section The coal used in this study is Fugu coal from Shannxi Province. Table 1 shows the proximate and ultimate analyses for this coal. The coal sample (mean diameter 3 mm) was dried at 100 C in an oven for 6 h prior to the tests. The physical parameters of CFB ash used as solid heat carrier are shown in Table 2. The tests were carried out in a moving-bed apparatus with a capacity 10 kg h1 of coal feeding, shown in Fig. 1. The system mainly consists of a solid heat carrier hopper, a coal hopper and a moving-bed pyrolyzer with 0.15 m internal diameter and about 1.8 m height. A dust

wj aa ac Sa

Sc

mass fraction of product j evolved up to time t ash surface area per unit volume, defined by aa = 6(1  e)/dpa, m2/m3 coal surface area per unit volume, defined by ac = 6(1  e)/dpc, m2/m3 ash surface area per unit volume in the movingbed pyrolyzer, defined by Sa = aah/(h + 1), m2/m3 ash surface area per unit volume in the movingbed pyrolyzer, defined by Sc = ach/(h + 1), m2/m3

Greek symbols qa density of the ash, kg/m3 qc density of the coal, kg/m3 qc0 density of the raw coal, kg/m3 qg density of pyrolysis gas, kg/m3 r Stefan–Boltzman constant, W/m2 K4 h volume ratio of ash to coal b emissivity of coal particle e the void ratio l viscosity of the gas, kg/m s v radiation shape factor kg thermal conductivity of gas, W/m K DH reaction heat, J/kg Dimensionless numbers Re Reynolds number Nu Nusselt number

collector is equipped at outlet of the pyrolyzer to prevent fine ash particles from carrying out by pyrolysis gas. In the experiments, 40 kg CFB ash was preheated to 820 C by electric furnace in the heat carrier hopper. The moving-bed pyrolyzer was heated to a certain temperature to make up heat loss. After the ash was heated to desired temperature, the hot ash and the coal were fed into the moving-bed pyrolyzer by screw feeders. High-temperature ash and coal particles were mixed under gravity by the baffles in the mixing section which was equipped at the upside of the pyrolyzer. Temperature of the bed mixture was detected by seven K-type thermocouples equipped along the pyrolyzer inner wall. To prevent the condensation of heavy tar in the dust collector, it was heated to the pyrolysis temperature electrically. While the mixed particles moving downwards in the pyrolyzer, the coal particles were heated by ash particles. Thermal decomposition occurred when coal particles were heated. The volatile products including gas, tar and water flowed into a water-cooled heat exchanger and a gas–liquid separator. The uncondensable gas at ambient temperature kept on flowing into a gas collector after metering. The residual solid (char and ash)

P. Liang et al. / Fuel 87 (2008) 435–442

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Table 1 Analyses of coal sample Proximate analysis (wt.%, as received base)

Ultimate analysis (wt.%, dry and ash free base)

Moisture

Ash

Volatile

Fixed-carbon

C

H

N

S

Oa

10.24

3.67

33.66

52.43

76.71

6.01

1.51

0.51

15.26

a

By difference.

Table 2 Parameters for calculation Bed cross-sectional area Reaction heat Bed porosity Diameter of ash particle Viscosity of the gas Density of coal Density of ash Initial temperature of coal Initial temperature of ash Heat capacity of coal Heat capacity of ash Thermal conductivity of gas Thermal conductivity of ash Thermal conductivity of coal Heat capacity of the fluid Emissivity of coal particle Radiation shape factor

A DH e dpa l qc0 qs Tc0 Ta0 cpc cpa kg ka kc cpf b v

m2 J/kg m kg/m s kg/m3 kg/m3 K K J/kg K J/kg K W/m K W/m K W/m K J/kg K

0.018 3 · 105 0.4 6.70 · 104 3.79 · 105 1250 1550 298 1093 1520 840 0.0742 0.29 0.19 1214 0.8 1

Heat carrier hopper Coal hopper

Dust collector Hot water Mixing section

Water cooler

Moving-bed pyrolyzer Cold water

Gas collector Gas metering Quenching tank

Gas-liquid separator

Fig. 1. Schematic diagram of the moving-bed apparatus.

was discharged from the moving-bed pyrolyzer via a screw feeder continuously. Composition of the gaseous product was analyzed by gas chromatography. The tar remained in the heat exchanger and the gas–liquid separator was rinsed by tetrahydrofuran (THF). The tar yield was obtained after the THF and water was removed by a rotary evaporator. 3. Analysis and modeling In the moving-bed pyrolyzer, high-temperature ash and coal particles are mixed fleetly in the mixing section. While the mixed material moving downwards in the pyrolyzer, the coal particles are heated by ash and pyrolysis gas evolves from the under layer. Decomposition occurs when coal particles are heated. The char and ash are returned to the CFB boiler, while the pyrolysis gas is emitted from the outlet of the moving-bed pyrolyzer. The following simplifying assumptions are cited in the present model: 1. Insignificance of the heat transfer in the mixing section is assumed, because of the mixing action between coal and ash particles completes instantaneously (within a few seconds) compared with the holding time (4– 8 min) in the moving-bed pyrolyzer. 2. It is assumed that the moving-bed pyrolyzer is operated at a one-dimensional steady state, and the governing parameters are only relevant to the height of the pyrolyzer. 3. Coal and ash are assumed as spherical particles with the diameter dpc and dpa. The bed material (coal and ash) is perfect homogeneous, and no internal temperature gradient of coal and ash particles is assumed. Heat transfer Biot modulus is taken as a criterion to judge the applicability of the present isothermal assumption. As the Biot number less than 0.1, the coal particle may be expected to be isothermal during pyrolysis [10]. According to the data of present model, it is accepted that temperature gradient within coal particles to be neglected as the coal particle size less than 3 mm. Without internal temperature gradient considered, however, the visible error will be caused by increasing the coal particle size. 4. The volume of coal particle is assumed a constant. Density of the particle decreases with the devolatilization behavior. Thus, the density of coal particle decreases in proportion to the total volatile matter loss, according to qc = qc0(1  w).

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5. The density of ash particle is assumed a constant in the moving-bed pyrolyzer. Furthermore, the chemical reaction of ash and volatile matter from coal is not considered in the present model.

3.1. Pyrolysis reactions

H2 CH4

CO2 C2H4 C2H6 Tar

Peak 1 2 3 1 2 1 2

1 2

Ej (J/mol)

k0j (1/s)

wj wt.% of coal (db)

93,214 129,580 129,580 147,554 75,240 126,236 81,510 96,140 139,612 139,612 156,332 314,754

20 1.7 · 105 2.8 · 104 3.0 · 104 55 2.5 · 103 550 230 2.3 · 106 1.7 · 106 7.6 · 1011 2.0 · 1017

1.81 2.07 2.32 2.07 2.50 5.83 2.78 2.36 0.28 1.05 3.22 3.78

ð1Þ

The total Pconversion rate of coal pyrolysis can be described as w ¼ j wj . When the moving-bed flows downwards at a constant rate us, the pyrolysis time t of coal particle can be expressed as a function of z, t = z/us. The independent variable z is the axial dimension in moving-bed pyrolyzer. Therefore, Eq. (1) becomes dwj k 0j expðEj =RT c Þðwj  wj Þ ¼ us dz

Product

CO

Multiple independent parallel first-order reactions are used here to describe the evolution of the volatile products (including H2, CO, CO2, CH4, tar, etc). For each product, assuming the rate constant to be kj = k0j exp(Ej/RTc), the kinetic equation of the product is dwj ¼ k 0j expðEj =RT c Þðwj  wj Þ dt

Table 3 Kinetic parameters for calculation

ð2Þ

Since coal pyrolysis is not a single reaction but rather a complex reaction scheme, any set of parameters cannot be expected to represent kinetic data accurately over a wide range of conditions. According to the functional-group model of coal pyrolysis [11], individual species evolve from certain functional group, both the activation energy, Ej, and the frequency factor, k0j, of different products are considered independent of coal type approximately. Therefore, the kinetic parameters of each product in present model are assumed constants, namely Ej and k0j are independent of coal type and the operating parameters, shown in Table 3 [12,13]. However, the final volatile product yields, wj , are relevant to coal type and particle size. They were determined by the fixed-bed experiments of coal pyrolysis by solid heat carrier theoretically [7]. 3.2. Mechanisms of heat transfer The moving-bed pyrolyzer involves complex heat transfer mechanisms. It is presumed that the coal particles are surrounded by ash particles. Five heat transfer mechanisms in moving-bed pyrolyzer are discussed as follows (a) Conduction between coal and ash particles through the contact area. (b) Convection heat transfer between pyrolysis gas and coal particles. (c) Convection heat transfer between pyrolysis gas and ash particles. (d) Radiation heat transfer between surfaces of adjacent particles (coal and ash) at different temperature.

(e) Heat carried by volatile releasing from coal, due to the decomposition of coal particles. Mechanism (a): The coal particles is surrounded by the adjacent ash particles in the moving-bed pyrolyzer. Contact area between two spherical particles (coal and ash) in the moving-bed is influenced by particle size, deformation during heating, operating parameter etc. [14,15]. It should be noted that the contact diameter, dpa, between a coal and an ash particle is difficult to calculate. The Luikov’s approach [16] provides a reference for evaluating the contact diameter, namely dac = 0.2 dpa. Building on this foundation, the ratio of contact area to coal surface area can be calculated and expressed as Rac. For each calculating unit volume in the moving-bed, the surface area of ash and coal h 1 could be written as S a ¼ aa 1þh and S c ¼ ac 1þh approximately. Where, h represents the volume ratio of ash to coal in the moving-bed pyrolyzer. The conduction heat transfer between ash and coal particles is given by the equation dqcod ¼

2k a k c Ta  Tc S c Rac A dz; ka þ kc d pa þ d pc

ð3Þ

where, the thermal conductivity between ash and coal parkc ticles is written as kkaaþk . c Mechanism (b) and (c): In the moving-bed pyrolyzer, the void between adjacent particles is filled with pyrolysis gas. The convection surface area between coal and gas per calculating unit is expressed as Sc(1  Rac)Adz and the surface area between ash and gas can be given by Sa(1  ScRac/Sa)Adz. Because of ScRac/Sa  1, then the surface area between ash and gas is transformed as SaAdz. The heat convection mechanism (b) and (c) are in parallel with each other. The convection heat transfer between pyrolysis gas and coal particles can be described by dqcg ¼ hcg S c ð1  Rac ÞAðT g  T c Þdz

ð4Þ

Similarly, heat transfer between pyrolysis gas and ash particles can be calculated using dqag ¼ hag S a AðT a  T g Þdz

ð5Þ

P. Liang et al. / Fuel 87 (2008) 435–442

Under convective flow conditions, the convective heat transfer coefficient, h, can be calculated using the correlation reported by Achenbach [17]. þ

0:25 0:0028Re3h Þ =ðd p =kÞ;

eqg ug d p l

ð6Þ

Re , 1e

and Reh ¼ respectively. where, Re ¼ By transforming the above equation, the heat transfer coefficients between gas and the ash/coal particle (hcg and hag) can be expressed as the functions of the above parameters and variables. Mechanism (d): Radiation heat transfer between surface of ash and coal particles depends on the difference of temperature. At the upper section of the moving-bed pyrolyzer, where the difference in temperature is big, radiation is the predominant heat transfer mechanism. Radiation heat transfer equation between ash and coal particles is presented as dqrad ¼ brvS c ð1  Rac ÞAðT 4a  T 4c Þdz

X us Að1  eÞ qc0 ð1  wj Þcpc dT c hþ1 j ¼ dqcod þ dqrad þ dqcg þ dqch

ð11Þ

3.3. Calculation The parameters Tg, Tc, Ta and wj may be calculated from the Eq. (2), and the Eqs. (9)–(11). With the boundary conditions z ¼ 0;

wj ¼ 0;

dT g =dz ¼ 0;

T c ¼ T c0 ;

T a ¼ T a0

700

ð7Þ

Mechanism (e): Since coal pyrolysis is an endothermic process, the required reaction heat of coal pyrolysis in the calculating unit can be expressed as i Ph k 0j expðEj =RT c Þðwj  wj Þ 1e j P  DH dz dqch ¼ Aqc0 hþ1 wj

ð10Þ

us Ahð1  eÞ qa cpa dT a ¼ dqcod  dqag  dqrad hþ1

600

Temperature, oC

h ¼ ð1:9388Re

2:32

439

500

400

ash/coal=6.0 ash/coal=4.2 ash/coal=3.2 ash/coal=2.5

100

j

ð8Þ 0

Based on the above heat transfer mechanisms, heat balance equations of pyrolysis gas, coal and ash are expressed as X us Að1  eÞ qc0 ðwj  wj Þcpf dT g ¼ dqag  dqcg hþ1 j

ð9Þ

0.0

0.1

0.2

0.3

0.4

0.5

z/m Fig. 3. Axial distribution of coal temperature at different blending ratio (coal particle size 3 mm, us = 0.125 m/min).

700

800 600

500 o

Temperature, C

Temperature,oC

700

600

500

gas coal ash

100

0

400

300

200

100

0

0.0

0.1

0.2

0.3

0.4

1mm 3mm 5mm 8mm

0.5

z/m Fig. 2. Axial temperature distribution of pyrolysis gas, coal and CFB ash (coal particle size 3 mm, us = 0.125 m/min, blending ratio 6.0).

0.0

0.1

0.2

0.3

0.4

0.5

z/m Fig. 4. Axial distribution of coal temperature of different coal particle size (us = 0.125 m/min, blending ratio 6.0).

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The characteristic parameters of pyrolysis gas, coal, ash and the moving-bed pyrolyzer used in the present model are shown in Table 2 [18,19].

The ordinary differential equations presented above are solved by four stage Runge–Kutta scheme, with the step size 0.02 m. The resulting parameters, including Tg, Tc,

4 0.20

H2 Computational Experimental

3

Yield / wt% of coal

0.16

Yield / wt% of coal

CH 4

0.12

0.08

Computational Experimental

2

1

0.04

0

0.00 520

560

600

640

520

600

640

600

640

3.0

2.0

CO

C2H4 and C2H6 1.5

2.5

Computational Experimental

Yield / wt% of coal

Yield / wt% of coal

560

1.0

0.5

Computational Experimental

2.0

1.5

1.0

0.5

0.0

0.0 520

560

600

640

520

560

4 8

Yield / wt% of coal

Yield / wt% of coal

3

2

CO2 1

Computational Experimental

0

6

4

Tar 2

Computational Experimental

0 520

560

600

Pyrolysis temperature / oC

640

520

560

600

640

Pyrolysis temperature / oC

Fig. 5. Influence of pyrolysis temperature on the yield of products (coal particle size 3 mm, us = 0.125 m/min, blending ratio 6.0).

P. Liang et al. / Fuel 87 (2008) 435–442

Ta and wj, are expressed as functions of dimension z at different operating conditions.

441

accurate when the coal particle is larger than 3 mm. It is admitted that visible error will be caused in the model prediction when the coal particle increases to 5 mm and 8 mm.

4. Results and discussion 4.2. Product distribution 4.1. Temperature distribution The axial temperature distributions of gas, coal and ash are shown in Fig. 2. The computational results indicate that the coal particle temperature increases with a heating rate more than 600 K/min at the upside of the movingbed pyrolyzer. The temperature of coal particle increases dramatically as z < 0.1 m, while the temperature of ash and coal appears to be uniform at about 0.1 m. The temperature of gas is close to the ash and decrease with the height of the moving-bed pyrolyzer. Fig. 3 illustrates the axial temperature distribution of coal particle at different blending ratio of ash to coal. Ash is the heat resource for the pyrolysis. The more heat carrier, the higher the temperature of the moving-bed pyrolyzer. Heat transfer is enhanced at higher blending ratio, especially the convection between pyrolysis gas and ash particle becomes prominent. Thus, the calculating results show that temperature of the coal particle increases with the blending ratio of solid heat carrier to coal. Fig. 4 illustrates the axial temperature distribution of coal particle with different diameter. It is obvious that the coal diameter influences the heating rate significantly in the model predictions. As an important parameter, coal particle size determines the area of heat transfer. Smaller particle size results higher heat transfer area. Heat transfer mechanisms, including mechanism (a), (b) and (d), are related to the coal particle size evidently. Therefore, at the same height of the moving-bed pyrolyzer (or at the same holding time), smaller coal particle would reach higher temperature. In the present model, no internal gradient within coal particle is assumed. However, it is not

The effect of pyrolysis temperature on the product yields is shown in Fig. 5. The predicted yields of the products at different pyrolysis temperatures are compared with the experimental data respectively. With the increase of pyrolysis temperature, yields of H2, CH4 and CO tend to increase significantly, both for the computational values and for the experimental data. The maximum yield of tar is around 7 wt.% of the raw coal. Fig. 6 represents the computational and experimental data of the pyrolysis products at different height of moving-bed pyrolyzer. With increase in the height of movingbed from 0.5 m to 1 m, the holding time of the bed material increases from 4 min to 8 min at a constant us of 0.125 m/ min. Apparently an increase in the holding time from 4 min to 8 min results in only a slight difference on the product yields both for calculated values and experimental data. It can be concluded that the volatile products approach the maximum value at the holding time of 4 min, which is consistent to the previous research work [7]. Regardless of the secondary reaction on volatile, maximal yield of tar is obtained at z of 0.1 m. The present model is shown to be effective in predicting the product yields. 5. Conclusions A one-dimensional numerical model for coal pyrolysis by solid heat carrier in a moving-bed pyrolyzer is developed based on a laboratory scale pyrolyzer. The temperature distribution and gas, tar yields are predicted, according to the experimental operating parameters. Temperature of coal particle increases with a heating rate more 9

4.0 3.5

Computational Experimental

H2 H2

CH4 CH4

C2H4+C2H6 C2H4+C2H6

Yield, wt% of coal

Yield, wt% of coal

CO2 CO2

Tar Tar

7

3.0 2.5 2.0 1.5 1.0

6 5 4 3 2

0.5 0.0

CO CO

Computational Experimental

8

1 0 0.0

0.2

0.4

0.6

z /m

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

z/m

Fig. 6. Yield of the products at different height of moving-bed pyrolyzer (coal particle size 3 mm, pyrolysis temperature 600 C, blending ratio 6.0).

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than 600 K/min at the upside of the moving-bed. Coal particle size has significant influence on the heating rate, while blending ratio is the determinant factor of pyrolysis temperature. The results show that the simulation results are close to the experiment ones. It is concluded that the model may provide basic engineering data for the reactor design of CFB combustion combined with coal pyrolysis in large scale. Acknowledgement The authors are grateful to Fugu Power Plant of Shannxi for financial support and Innovation Fund of Institute of Coal Chemistry, Chinese Academy of Sciences. References [1] Cen KF, Fang MX, Luo ZY. J Eng Thermophys 1995;16:499–502 [in Chinese]. [2] Charpenay S, Serio MA, Bassilakis R. Energ Fuel 1996;10:26–38.

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