Energy Fuels 2010, 24, 2991–2998 Published on Web 04/14/2010
: DOI:10.1021/ef9015813
Heat Transfer Inside Particles and Devolatilization for Coal Pyrolysis to Acetylene at Ultrahigh Temperatures Yue Shuang, Changning Wu, Binhang Yan, and Yi Cheng* Department of Chemical Engineering, Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Tsinghua University, Beijing 100084, P. R. China Received December 22, 2009. Revised Manuscript Received March 25, 2010
Coal pyrolysis to acetylene in thermal plasma provides a direct route to make chemicals from coal resources, where the rapid heating and release of volatile matters in coal particles play the dominant role in the overall reactor performance. A mechanism model incorporating the heat conduction in solid materials, diffusion of released volatile gases, and reactions was proposed for a deep understanding of the heat transport inside a coal particle under extreme environmental conditions such as high temperatures greater than 2000 K and milliseconds of reaction time. The two competing rates model, known as the Kobayashi model, was applied to describe the devolatilization kinetics, which was verified by comparing the predicted yield of volatiles with the experimental data in the literature. Thermal balance between coal particles and the hot carrier gas was established, and the four influencing factors including the heating rate, particle size, reactants flow ratio, and heat of devolatilization were paid attention when analyzing the heating profile inside the particles and the yield of volatiles. The results showed that the inherent resistance due to the volatiles released from coal particles seriously impeded the thermal energy transportation from heating gas to the particle. This led to a weakened heating rate, i.e., a long heating up time, and thereafter a low yield of volatiles, especially when the particle size was large (e.g., >40 μm). Meanwhile, the heat conduction inside the coal particle also imposed additional resistance to reduce the heat transportation rate from heating gas to the particle, especially when the particle size was larger than 80 μm. The predicted yield of volatiles considering the mechanism of the two resistances agreed reasonably with the reported experimental data under different operating conditions but was smaller than that which could be obtained when neither resistance is considered. It can be concluded that the proposed heat transport mechanism inside coal particles works well in understanding the coal pyrolysis process at ultrahigh temperatures.
then, a large number of investigations were carried out worldwide, e.g., in the United Kingdom,3 the United States,4,5 India,6,7 Germany,8,9 the former Soviet Union, France,10,11 Poland,12 and China.13 Several reports5,7 pointed out that this new route was more economical than the partial oxidation of methane method for acetylene. It has been acknowledged that coal pyrolysis to acetylene in plasma would open up a direct and clean means to convert coal to chemicals. However, the coal pyrolysis process is still far from being understood for the extreme operating conditions at ultrahigh temperatures and integrated multiple processes in milliseconds. For example, the inlet temperature of the heat carrier gas (i.e., hydrogen) is greater than 3000 K on average, and the gas velocity in the reactor is about 100-1000 m/s. Practical measurements in such a severe environment are not realistic, so that neither physical nor chemical information can be obtained on the microscale of the reactor. In our earlier work, we have proposed a novel design for the jet nozzles of coal particles, which has been successfully applied in a reactor unit
1. Introduction Acetylene is widely used as a fuel and a chemical building block as well. Nowadays, acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from the cracking of hydrocarbons in the world. However, 70-80% of acetylene is produced by the calcium carbide method in China, i.e., a high-energy-consuming and heavy-polluting route started from coal resources. In the 1960s, Bond et al.1 and Nicholson and Littlewood2 successfully produced acetylene from coal using a plasma jet. When coal was heated to temperatures above 1800 K in milliseconds, a gaseous mixture was produced with the acetylene as the principal hydrocarbon constitute. This proved the possibility of a one-step process from coal to acetylene. Since *To whom correspondence should be addressed. Fax: 86-1062772051. Email:
[email protected]. (1) Bond, R. L.; Ladner, W. R.; McConnell, G. I. T. Nature 1963, 200, 1313–1314. (2) Nicholson, F.; Littlewood, K. Nature 1972, 236, 397–400. (3) Littlewood, K. In Mat. Res. Soc. Symp. Proc., Vol. 30; Elsevier Science Ltd.: Boston, MA, 1984; pp 127-132. (4) Wragg, J. G.; Kaleel, M. A.; Kim, C. S. Coal Proc. Tech. 1980, 6, 186–192. (5) Kushner, L. M. In Radio Frequency/Radiation and Plasma Processing; Technomic: Lancaster, PA, 1985; pp 193-207. (6) Chakravartty, S. C.; Dutta, D.; Lahiri, A. Fuel 1976, 55, 43–46. (7) Wilming, H.; Peuckert, C.; Mueller, R. In SynFuels 2nd Worldwide Symposium; McGraw-Hill Public: New York, 1982; pp 25. r 2010 American Chemical Society
(8) Klotz, H. D.; Drost, H.; Schulz, G.; Spangenberg, H. J. Chem. Techn. 1987, 39, 480–483. (9) Baumann, H.; Bittner, D.; Beier, H. G.; Klein, J.; Juntgen, H. Fuel 1988, 67, 1120–1123. (10) Amourox, J. Rev. Gen. Elect. 1980, 89, 647–653. (11) Fauchais, P. Int. Chem. Eng. 1980, 20, 289–305. (12) Plotczyk, W. W.; Resztak, A.; Szymanski, A. Int. J. Mater. Prod. Tec. 1995, 10, 530–540. (13) Dai, B.; Fan, Y. S.; Yang, J. Y.; Xiao, J. D. Chem. Eng. Sci. 1999, 4, 957–959.
2991
pubs.acs.org/EF
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
14
with 2-MW power input to the plasma. A systematic thermodynamic study was carried out to understand the effects of solid carbon, the gaseous C/H ratio, and the additions of water and methane on the production of acetylene from coal pyrolysis, with referenced pilot-plant data from the 2-MW plasma reactor.15 Further, a CFD-DPM model with the consideration of the complex reacting flow features was first established to describe the practical process of coal pyrolysis.16 Although the major properties of the 2-MW plasma reactor can be predicted, the basic features of the heating and devolatilization of coal particles were modeled in less detail. To our best knowledge, no accurate mechanism to model this complex process has been reported in the literature. As is generally known, volatile matter, i.e., the gaseous product of coal devolatilization, is always the first chemical conversion step in any coal utilization process.17 Thermal energy is the driving force for devolatilization, which means that the behavior of inward flow of thermal energy to coal is the key issue for coal utilization processes. Heat conduction resistance in coal particles and the mass transfer resistance of generated volatiles would be the two main resistances impeding the inward heat flow. Each of the influences of these two resistances on the heating rate of coal particles and the rate of devolatilization has been reported.18-23 On the basis of the experiments of coal devolatilization by mixing with hot carrier gas, Peters and Bertling18 indicated that the outward flow of the degasification products hindered the transmission of heat carriers to the coal and the endothermic degasification process. Kalendt and Nettleton19 confirmed these two kinds of resistances by measuring the temperature profile at both the surface and the center of coal particles. Field et al.20 found a simple formula to calculate the maximum temperature gradient within a particle, in which the result was proportional to the heat flux and the radius of the particle, while inversely proportional to the thermal conductivity of the matter. Suuberg21 further pointed out that the internal heat-transfer limitations would have a significant impact only at average heating rates above 105 K/s for particle diameter of 50100 μm. Wutti et al.22 proposed a one-dimensional mathematical model to determine the influence of mass and heat transport processes on the overall devolatilization rate of a coal particle, with the pyrolysis temperature below 1000 C, the gas-particle slip velocity up to 10 m/s, and the particle diameter at the range of 0.01-40 mm. Fletcher et al.23 predicted the total volatile increase in yield when the coal was heated up to 1500 K with heating rates from 1 K/s to l05 K/s. As the average gas temperature can reach 4000 K in thermal plasma and the heating rate can reach 106 K/s, the whole pyrolysis process could finish in a few milliseconds. In this work, a heat transfer reaction coupled mathematical model was established for a single pulverized coal particle. The influences
of four parameters, i.e., the heating rate, particle size, mass flow rate of the reactants, and heat of the devolatilization process, on the coal particle temperature and the devolatilization performance throughout the heating process were studied. 2. Mathematical Model In this section, a mechanism model incorporating the heat conduction in solid materials, diffusion of released volatiles, and reactions was presented to understand the characteristics of the process of coal devolatilization. Hydrogen was selected as the working gas for the thermal plasma in the simulation because of its good thermodynamic and transport properties. Although this study focused on hydrogen plasma, the mathematical model proposed in this study could be applied to other types of working gas for the thermal plasma. 2.1. Model Assumptions. The mathematical model was developed on the basis of the following assumptions: (1) The thermal plasma is produced only by hydrogen. (2) The coal particles are assumed to be dry, porous spheres with constant diameter, uniform boundary conditions (on the surface), and centrosymmetrical variations of local temperature and porosity. (3) The coal particles are ideally mixed with the thermal plasma, no collision and radiation between particles. (4) The internal mass diffusion of the released volatiles in coal particles is ignored but considered with an effective particle-based correction for the effect of mass transfer on heat transfer. (5) The solids and the released volatiles at any local position inside a coal particle are assumed to have the same temperature due to strong interphase heat transfer. (6) The temperature of the environmental thermal plasma is assumed to be uniform. (7) The thermal effect of ash melting on the temperature of a coal particle is neglected due to the relative small mass of the ash in the coal (e.g., Ad = 2-6% in our pilot plant test). 2.2. Mathematical Formulation. Simplified geometry and environmental conditions around a coal particle were adopted in the simulations, instead of considering the complex hydrodynamics of multiphase reacting flow in the coal pyrolysis process with detailed reactor geometry. The heat transfer model within a coal particle (as shown in Figure 1) is established on the basis of the conduction equation with consideration of the solid material and the volatile gases: DTs ðr, tÞ ðφs Fs cp, s þ ð1 - φs ÞFvol cp, vol Þ Dt 1 D DTs ðr, tÞ - Δr H 3 γvol ðr, tÞ ð1Þ ¼ 2 λeff r2 r Dr Dr
(14) Cheng, Y.; Chen, J. Q.; Ding, Y. L.; Xiong, X. Y.; Jin, Y. Can. J. Chem. Eng. 2008, 86, 413–420. (15) Wu, C. N.; Chen, J. Q.; Cheng, Y. Fuel Process. Technol. 2009, DOI: 10.1016/j.fuproc.2009.09.013. (16) Chen, J. Q.; Cheng, Y. J. Chem. Eng. Jpn. 2009, 42, s1–s8. (17) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591– 601. (18) Peters, W.; Bertling, H. Fuel 1965, 44, 317–331. (19) Kalendt, A. S.; Nettleton, M. A. Erd€ or Kohle 1966, 19, 354–356. (20) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of Pulverized Coal; Br. Coal Util. Res. Assoc.: Leatherland, England, 1967. (21) Suuberg, E. M. Energy Fuels 1988, 2, 593–595. (22) Wutti, R.; Petek, J.; Staudinger, G. Fuel 1996, 75, 843–850. (23) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54–60.
where Ts(r,t) represents the local temperature at any radial position r and time t; φs is the local volume fraction of the solid phase; Fs and Fvol are the densities of the solid material and the volatile phases, respectively; cp,s and cp,vol are the specific heat capacities of the solid material and volatile phases, respectively; λeff is the effective local thermal conductivity; ΔrH is the reaction heat of devolatilization, as set to 1350 kJ/kg according to the experimental result by Hertzberg et al.;24 and γvol(r,t) is the reaction rate of devolatilization (kg/s). (24) Hertzberg, M.; Zlochower, I. A. Combust. Flame 1991, 84, 15–37.
2992
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
The heat balance equation between a coal particle and the environmental heating gas is given as RR dðm_ pl Hpl Þ dð 0 φs Fs Hs 4πr2 drÞ dðm_ vol Hvol Þ ¼ dt dt dt Z R ðΔr H 3 γvol Þ4πr2 dr ð3Þ 0
where R is the radius of the coal particle; m_ pl and m_ vol represent the mass flow rates of the hydrogen plasma and the released volatiles, respectively; Hpl, Hs, and Hvol represent the enthalpies of the hydrogen plasma, the coal residue, and the volatiles, respectively. In calculation of the enthalpies, the temperature of the volatiles outside the particles, Tvol(t), is assumed to be transiently raised to the temperature of the environmental heating gas, Tb(t), that is, the temperature of the hydrogen plasma Tpl(t). The initial conditions are given as
Figure 1. Schematic of a coal particle with environmental heating gas.
Ts ðr, 0Þ ¼ Tc, 0 , φs ðr, 0Þ ¼ φc, 0 ð0 e r e RÞ Tb ð0Þ ¼ Tb, 0
The effective local thermal conductivity is expressed by λeff ¼ φs λs þ ð1 - φs Þλvol
ð2Þ
ð4Þ
where Tc,0 is the initial temperature of the coal particle, Tb,0 is the initial temperature of the environmental heating gas, and φc,0 is the initial volume fraction of the solid phase inside the coal particle, which is estimated to be 0.7 according to the experimental data in this paper. The boundary conditions are given as
where λs and λ vol represent the thermal conductivities of the solid material and volatile phases, respectively. The volatiles are treated as an ideal mixture of CH 4, CO, CO 2 , C 2 H4 , C 3 H8 , etc., empirically. 25
8 > DT > s > > ð4πR2 Þλeff ¼ 4πðR þ 0:5δf Þ2 hðTb - Tw Þθ þ σSB εð4πR2 ÞðTb4 - Tw4 Þ < Dr r ¼R > DT > s > > ¼0 : Dr
ð5Þ
r ¼0
where Tw is the temperature at the surface of the coal particle, σSB is the Stefan-Boltzmann constant, ε is the emissivity of the surface for the pulverized coal, δf is the thickness of the gas film around the coal particle (estimated to be 2R at a relatively small Reynolds number in this study), h is the gasparticle heat transfer coefficient, and θ is a factor related to the effect of volatiles’ release on heat conduction. The gas-particle heat transfer coefficient could be calculated from the Nusselt number, Nu = (hdp)/λg, which is estimated as a function of the operating conditions and material properties,26 !0:15 Fb μf 1=2 1=3 Nu ¼ ð2:0 þ 0:6Ref Prf Þ Ff μb
plasma, which decreases gas-particle slip velocity rapidly. Taking this into account, the velocity used is an averaged value of 20 m/s in this work. The factor θ reported by Spalding27 was adopted in this study, B θ ¼ B e -1 ð7Þ cp , f dmvol B ¼ 2πdp λf dt where (dmvol)/(dt) denotes the formation rate of volatiles from coal (kg/s). 2.3. Kinetic Model of Coal Devolatilization. Many kinetic models have been reported for the devolatilization process of coal, varying greatly in their complexity.28-34 At a low level of complexity, there are the single kinetic rate model,28 the two competing rates (Kobayashi) model,29 and the distributed
Ff dp jusl j ð6Þ μf μ cp, f Prf f λf where dp represents the diameter of the coal particle, μ is the fluid viscosity, usl is the gas-particle slip velocity, and the subscripts f and b denote that the parameters should be calculated with respect to the temperatures of the film (Tf = (Tw þ Tb)/2) and bulk fluid, respectively. The pulverized coal particles are rapidly accelerated by the high-speed thermal Ref ¼
(27) Spalding, D. B. Some Fundamentals of Combustion; Butterworths Scientific Publications: London, England, 1955. (28) Badzioch, S.; Hawksley, P. G. Ind. Eng. Chem. Proc. Des. Dev. 1970, 9, 521–528. (29) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. Sixteenth Symp. (Int.) Combust./Combust. Inst. 1976, 22, 625–656. (30) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22 (4), 625–656. (31) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405–422. (32) Niksa, S.; Kerstein, A. R. Energy Fuels 1991, 5 (5), 647–665. (33) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels 1989, 3, 175–186. (34) Jupudi, R. S.; Zamansky, V.; Fletcher, T. H. Energy Fuels 2009, 23, 3063–3067.
(25) Gaschnitz, R.; Krooss, B.; Gerling, P.; Faber, E.; Littke, R. Fuel 2001, 80, 2139–2153. (26) Boulos, M. I.; Fauchais, P.; Pfender, E. Thermal plasma; Plenum Press: New York, 1994; Vol. 1.
2993
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
Figure 2. Schematic diagram of the two competing rates model.
activation energy model.30 At a high level of complexity, there are the functional group-devolatilization vaporization cross-linking model (FG-DVC),31 the FLASHCHAIN model,32 and the chemical percolation devolatilization (CPD),33,34 which are based on fundamental processes and applicable over a wide range of coal types. In this study, the two competing rates model was chosen to describe the devolatilization kinetics. As shown in Figure 2, the devolatilization process is governed by two one-order parallel competing reactions, which have different volatile yields and activation energies at the same temperature.29 According to this model, coal would be partly released volatiles V1 and partly released volatiles V2 with residual chars R1 and R2, respectively. This model had been widely applied in coal combustion and pyrolysis, showing good agreement with the experimental results.35 The reaction rate of devolatilization (kg m-3 s-1) is expressed by 1 dmvol , γvol ¼ Vp dt
Figure 3. Comparison of predicted yields of volatiles with the experimental data at different given growth rates of particle temperatures to 2500 K.
vided into 100 shells equally spaced along the radial direction. The time step (∼0.1 μs) is determined by the largest diameter of particles employed in this study. Thus, the good convergence could be ensured for the numerical solutions. The material properties, λs, cp,s, and φs, are estimated with the varying temperature at each local position and each time step. λs is treated as a linear fitted function based on the experimental data,37 as λs = 0.23(1 þ 0.0033Ts); cp,s is calculated by a method R t developed by Merrick38 as a function F0 γ dt 0 vol of Ts, and φs ¼ The material properties of hydroFs gen plasma come from Boulos et al.26 as a function of the temperature, while those of volatiles are calculated by kinetic theory. The material properties of the mixture in the environmental heating gas are calculated by the mass-weighted approach.
dmvol, 1 dmvol, 2 dmvol ¼ þ dt dt dt Z t ðk1 þ k2 Þ dtÞ ð8Þ ¼ ðR1 k1 þ R2 k2 Þ½ðmc, 0 - mash Þ expð 0
where R1 and R2 are mass stoichiometric coefficients of the volatile product with respect to the two competing reactions, respectively; Vp is the particle volume; mc,0 is the initial particle mass, mash is the mass of ash in the particle. The reaction rate coefficients, k1 and k2, are given by
3. Results and Discussion
where A1 and A2 are the pseudo-frequency factors, E1 and E2 are the pseudo-activation energies, and T is the local temperature inside the particles. The first reaction is dominant at low temperatures. But at high temperatures, the second reaction is much faster than the first one. A set of kinetic parameters (A1 = 2.0 105 s-1, E1 = 104.6 kJ/mol, R1 = 0.3, A2 = 1.3 107 s-1, E2 = 167.4 kJ/mol, and R2 = 1.0) was given by Kobayashi et al.29 based on the experimental pyrolysis data of one kind of bituminous coal particles and another kind of lignitous ones (400-325 Tyler mesh in diameter) at a heating rate of 1 104 to 2 105 K/s. The set of kinetic parameters is used in this study, where the types of coal approximate to the above two types. However, it should be mentioned that using a more accurate kinetic model will be favorable to achieve a more reliable analysis on the process of coal devolatilization. 2.4. The Details of Numerical Simulation. The differential eq 1 is discretized with second-order discretization in space and first-order discretization in time.36 The particle is di-
3.1. Validation of Devolatilization Kinetics at Ultrahigh Temperatures. The predicted yields of volatiles using the two competing rates model at different given growth rates of particle temperature are plotted in Figure 3. At infinite growth rate, the particle temperature is assumed to be a constant value of 500, 800, 1100, 1400, 1700, 2000, 2300, or 2600 K, and the yield of volatiles is estimated after a long enough time for complete devolatilization. At other growth rates of particle temperature varying from 1 K/s to 107 K/s, the particle temperature is raised from 300 to 2500 K. The predictions, with neither heat conduction nor volatiles diffusion being considered, are compared with the experimental data at a wide range of reaction temperatures (up to 2300 K) reported by Merrick,39 Kobayashi et al.,29 and AVCO Systems Division.40 It can be seen from Figure 3 that the maximum yields under the constant reaction temperature are obtained at the infinite growth rate, which are increased evidently with the increase of reaction temperature. This result is reasonable because of the overestimated reaction temperature based on the infinite growth rate of particle temperature. On the other hand, at limited growth rates, both the predicted and experimental yields of volatiles will be increased when the growth rate is raised. This trend was also found to be similar in the literature.23 This phenomenon could be explained by the fact that, at a relative low reaction
(35) Cen, K. F.; Yao, Q.; Gao, X.; Luo, Z. Y. Theory of Combustion and Pollution Control; China Machine Press: Beijing, China, 2004; pp 275 (in Chinese). (36) Versteeg, H. K.; Malalaserkera, W. An Introduction to Computational Fluid Dynamics - The Finite Vol. Method; Prentice Hall: New Jersey, 1995; pp 168-190.
(37) Badzioch, S.; Field, M. A.; Gregory, D. R. Fuel 1964, 43, 267– 280. (38) Merrick, D. Fuel 1983, 62, 540-546. (39) Arc-coal acetylene process development program. Phase 1B. Final technical progress report; AVCO Systems Division: Wilmington, MA, 1980. (40) Merrick, D. Fuel 1983, 62, 534–539.
k1 ¼ A1 e - E1 =RT , k2 ¼ A2 e - E2 =RT
ð9Þ
2994
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
(2) Model-II: only the factor due to mass diffusion is considered with the assumption of λs = ¥ (3) Model-III: only the factor due to heat conduction is considered with the assumption of θ = 1 (4) Model-IV: both of the two factors are considered.
temperatures, the first reaction in the Kobayashi model plays the dominant role, leading to the parallel formations of volatiles and the residual char. However, the second reaction in the Kobayashi model becomes more effective with the formation of volatiles at high reaction temperatures. In other words, the growth rate of particle temperature plays an important role on the maximum yield of volatiles, which will be achieved in a short pyrolysis process at a relative high particle heating rate or in a long pyrolysis process at a relatively low particle heating rate. In summary, the two competing rates model is qualified for describing the devolatilization process at a reaction temperature as high as 2000 K. 3.2. Heat and Mass Transfer Inside the Particle. In a real process of coal devolatilization, as the energy input and the flow rates of hydrogen and coal are fixed, the temperature of the hydrogen surrounding each particle would decrease in time with the progress of heat conduction to the coal particles accompanying the devolatilization process (see eq 3). As described in eqs 1 and 5, both heat conduction and mass diffusion inside the particle will affect the particle temperature and the yield of volatiles. Four simulation models are carried out to investigate the influences of the above two resistance factors on the particle temperature and the yield of volatiles: (1) Model-I: neither of the two resistance factors is considered with the assumptions of θ = 1 and λs = ¥
Figure 4 plots the predicted yields of volatiles with time under the operating conditions (see Table 1) reported by Kobayashi et al.,29 AVCO Systems Division,40 and Chen and Cheng,16 and from our 4-MW plasma reactor. It can be found that the predictions by Model-IV are much different from the ones by Model-I, and the former is closer to the experimental results. This result indicates that the effect of two resistance factors on the coal devolatilization should be paid enough attention. The initial temperature of heating gas (i.e., hydrogen plasma before mixing with the coal particles) is estimated from the power input of the plasma generator with energy conversion efficiency, considering the material and energy balances with respect to the plasma generator and the thermal plasma as a thermodynamic equilibrium state. The mean pyrolysis time of particles is evaluated from the operating conditions based on the gas-particle drag force )usl/Fpd2p, by considering expression, 18 μ(1 þ 0.15 Re0.687 f one-dimensional (vertical) acceleration of a particle with the mean diameter and effect of released volatiles on the material properties of the gas. The yields of volatiles in our 2-MW and 4-MW plasma reactors are at the level of 40 wt % (daf), which might indicate the uncompleted devolatilization due to the relative short pyrolysis time of particles. In summary, the predicted yields of volatiles considering the mechanism of the two resistances agree reasonably with the reported experimental data under different operating conditions, but smaller than that could be obtained when neither resistance has been considered. The following discussion is based on the simulations that are carried out under a set of typical operating conditions from our 4-MW plasma reactor unit for coal pyrolysis. The power input of the plasma generator is 4-MW with an energy conversion efficiency of about 0.60. The hydrogen is fed into the plasma generator at a mass flow rate of 131 kg/h and a thermodynamic equilibrium temperature of 3386 K. The pulverized coal particles (∼80 μm in mean diameter) are then injected into the plasma reactor at a temperature of 300 K and a mass flow rate of 1300 kg/h, which is decreased from 1700 kg/h in the last case, as listed in Table 1, to improve the coal devolatilization. The reactor is operated at a pressure of 115 kPa. The variations of predicted average particle temperature and yield of volatiles with time by using different models are plotted in Figure 5. For Model-I, the fresh coal particle is
Figure 4. Variations of predicted yield of volatiles with time under the operating conditions reported by AVCO Systems Division,40 Baumann et al.,9 and Chen and Cheng16 and from our 4-MW plasma reactor.
Table 1. Operating Conditions for the Experimental Data Plotted in Figure 4 and the Estimated Mean Pyrolysis Time of Particles operating conditions
AVCO Systems Division40
Baumann et al.9
Chen and Cheng16
4-MW plasma reactor
internal diameter of reactor (m) reactor length for pyrolysis (m) operating pressure (kPa) power input (kW) energy conversion efficiency (%) hydrogen feeding rate (kg/h) coal feeding rate (kg/h) mean particle diameter (μm) ash content in dry coal (wt%) initial temperature of heating gas (K) initial temperature of particles (K) initial particle velocity (m/s) mean pyrolysis time of particles (ms)
0.197 ∼0.300 120 807 60 34.55 204.25 70 6.45 3200 300 ∼20 8.0-10.0
0.023 0.300 115 15 60 1.62 0.60 100 5.80 2673 300 ∼10 6.0-8.0
0.100 0.500 115 1800 50 50 700 80 10.93 3378 300 ∼10 5.0-7.0
0.150 0.300 115 4000 60 131 1700 80 5.00 3306 300 ∼10 3.5-5.5
2995
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
heated up extremely fast before about 1.0 ms. An evident drop of growth rate of the particle temperature could be observed after about 2000 K, which is due to the rapidly raised endothermic reaction rate of coal devolatilization. After about 4.0 ms, corresponding to the complete devolatilization, the particle temperature continues to be raised to the common temperature of the gas and particle phases, with the yield of volatiles approaching a maximum value of 73 wt %. When the resistance factor due to mass diffusion is considered (i.e., Model-II), the growth rate of particle temperature undergoes a similarly evident drop with the predicted result of Model-I, but the particle temperature is decreased to a degree of about 500 K. This is reasonable because the large amount of volatiles released needs to disperse out of the particle. Although the particle temperature reaches the gasparticle common temperature after about 7.0 ms, the yield of volatiles is much less than the one predicted by Model-I during the devolatilization process, with a longer time for complete devolatilization (6.0 ms vs 4.0 ms) and a smaller maximum value for the yield of volatiles (73 wt % vs 70 wt %). This is mainly due to the irreversible conversion of coal to residual char when the particle temperature is decreased. When only the resistance factor due to heat conduction is considered (i.e., Model-III), in comparison with the predicted average particle temperature by Model-I, the change
appears in the time range of 0-2.0 ms, with a maximum difference of up to 500 K, which is due to the additional resistance imposed by the heat conduction inside the coal particle and hence the reduced heat transportation rate from heating gas to the particle. The reduced heat transportation rate leads to considerable reduction of the yield of volatiles during the whole process of coal devolatilization. Under the present operating conditions, the resistance factor due to heat conduction shows less of an effect on the devolatilization process than the one due to mass diffusion. However, the effect of heat conduction will become more remarkable when the particle diameter is increased or the energy supplied for each kilogram of coal particles is decreased. For Model-IV, similar trends could be observed with respect to the simulation results by Model-II, which indicates that the released volatiles traveling outward from the porous coal particle make up the dominant resistance for particle heat-up and devolatilization under the present operating conditions. It could be also found that the predicted results by Model-IV are cocontributed by the two resistance factors mentioned above. In an ultrafast process of coal pyrolysis, it is hard to ensure the pyrolysis time of coal particles long enough to facilitate the complete devolatilization because that the formation of acetylene will suffer from the remarkably reduced temperature of the heating gas due to the gasparticle heat transfer, the gas-wall heat transfer, and the endothermic reaction of coal devolatilization. For the current simulation case, if the reaction is stopped at 2.0 ms, the yield of volatiles is just 33 wt % (daf) by Model-IV, much less than the value of 60 wt % (daf) by Model-I. As discussed above, the resistance factors due to mass diffusion and heat conduction should be considered in modeling the process of coal devolatilization. 3.3. Effect of Particle Size on the Heating Process. Different particle sizes directly lead to different characteristics of heat conduction and mass diffusion inside coal particles with wide variations of average particle temperature and yield of volatiles with time, as shown in Figure 6. For Model-I, the devolatilization rate is relatively low for a large-size particle with a relatively low average particle temperature at the same reaction time, which is mainly caused by the decreased heat transfer coefficient between the particle and the environmental heating gas (h d-1 p ).
Figure 5. Variations of (a) average particle temperature and (b) yield of volatiles with time under the typical operating conditions from our 4-MW plasma reactor.
Figure 6. Variations of (1) average particle temperature and (2) yield of volatiles with time at different particle diameters. (a) Model-I, (b) Model-II, (c) Model-III, and (d) Model-IV.
2996
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
Figure 7. Variations of (a) temperatures of the coal particle and hydrogen and (b) yield of volatiles with time at different radial positions (dp = 80 μm).
Figure 8. Variations of (a) average temperatures of coal particle and hydrogen and (b) yield of volatiles with time at different coal feeding rates (dp = 80 μm, hydrogen flow rate =131 kg/h).
Comparing the model predictions between Model-I and Model-II, it can be seen that the effects of mass diffusion inside coal particles on the particle temperature profiles and maximum yield of volatiles become more distinct when the particle diameter is larger than 40 μm. Comparing the model predictions between Model-I and Model-III, it can be seen that the effects of heat conduction become more distinct when the particle diameter is larger than 80 μm. For ModelIV, the particle heating and devolatilization process are hindered severely due to the integrated effect of the two resistance factors, which leads to a much longer heating time and a considerable drop in the final yield of volatiles in comparison with the ones predicted by Model-I, especially when the particle size is large (e.g., > 40 μm). For further illustration of the effects of the two resistance factors on the particle heating and devolatilization process, variations of local particle temperature and yield of volatiles with time at different radial positions are plotted in Figure 7, together with the variation of the temperature of the environmental heating gas. The trend in Figure 7a is similar to the reported experimental data in the literature.41 In the first 1.0 ms, heat could not be transferred efficiently into the center region in the coal particle because of the resistance of heat conduction. This results in the accumulation of energy at the particle surface, leading to very strong particle heating, even faster than that in Model-I. As the particle temperature grows fast, the reaction rate becomes high. The large amount of released volatiles slows down the heat transfer from the environmental heating gas to the particle surface, which affects everywhere in the whole particle. Because the surface of the particle was heated up earlier than the center part, the reaction also finishes earlier than at the center, seen from Figure 7b. After about 3.0 ms, the devolatilization rate gradually becomes slow and approaches zero, with a rapid drop of resistance of mass diffusion. It can be also seen that the yields of volatiles are little decreased at the center of the particle, compared with the values at the surface and the average. This is mainly because of the difference in heating rates. If the reaction is stopped also at 2.0 ms, the surface shell has a yield of volatiles of 54 wt % (daf), while the core shell just has 0.8 ms for devolatilization with a much lesser yield of volatiles of 12 wt % (daf). It should be indicated that the heat conduction inside the particle plays a key role in impeding the heat transfer inside the particle, while the mass diffusion hinders the heat transfer from the environmental heating gas to the particle.
Figure 9. Variations of (a) the average temperatures of the coal particle and hydrogen and (b) the yield of volatiles with time at different reaction heats of devolatilization.
3.4. Effect of Coal Feeding Rate. As mentioned above, the energy input and the flow rates of hydrogen are fixed to mimic a 4-MW pilot plant plasma reactor. Coal particles are assumed to disperse uniformly in the system. With the increase of the coal feeding rate to this system, the final temperature of hydrogen will decrease, which follows the energy balance with respect to the system. And also, the temperature of environmental heating gas surrounding each particle would decrease in time with the progress of heat transfer and devolatilization of the coal particles. In order to reveal the influence of the coal-hydrogen feed ratio, only the coal feeding rate under typical operations varies from 600 to 2000 kg/h. The profiles of the average temperature of coal particles and hydrogen and the average yield of volatiles with the coal feeding rate are shown in Figure 8. The coal feeding rate has a tremendous impact on the temperature of the particles and the yield of volatiles. A larger coal feeding rate corresponds to a lower final temperature and a longer heating time, which causes a reasonable low final yield of volatiles and a longer time for complete devolatilization. It can be concluded that the flow ratio of heating gas to coal plays an important part in shortening the required reaction time and increasing the product yield. 3.5. Effect of Heat of Devolatilization. Figure 9 shows the variation of average particle temperature with time at different reaction heats of devolatilization, with an environmental temperature much higher than 2000 K. The heat of
(41) Tomeczek, J.; Kowol, J. Can. J. Chem. Eng. 1991, 69 (1), 286–293.
2997
Energy Fuels 2010, 24, 2991–2998
: DOI:10.1021/ef9015813
Shuang et al.
devolatilization is set to 400 kJ/kg and 1350 kJ/kg, according to the reported results by Freihaut42 and Hertzberg and Zlochower,24 respectively. It can be seen that the reaction heat of devolatilization shows little effect on the average particle temperature and heating time, which is consistent with the experimental work by Sampath et al.43 and Fu44 under temperatures of 2000 K. Therefore, it can be concluded that the reaction heat of devolatilization has a negligible effect on the coal pyrolysis under a wide range of reaction temperatures. It should be noted that all the simulations are based on some assumptions, one of which is the ideal gas-particle mixing in the plasma reactor. In a real process, however, the contact efficiency between coal particles and the hot hydrogen gas is to be dominant so that the coal particles would experience different pyrolysis conditions. It is expected that the mechanism model in this paper should be integrated with the governing equations for describing the complex multiphase reacting flows in the practical reactor.
Acknowledgment. Financial support from the National Natural Science Foundation of China (NSFC) under grants No. 20976091 and No. 20806045 are acknowledged.
Nomenclature A = pseudo frequency factor Ad = ash in dry coal CFD = computational fluid dynamics cp = isobaric heat capacity (J kg-1 K-1) dp = diameter of the coal particle (m) DPM = discrete phase method E = pseudo activation energy (J mol-1) h = gas-particle heat transfer coefficient (W m-2 K-1) H = enthalpy (J kg-1) k = reaction rate coefficient (s-1) m_ = mass flow rate (kg s-1) Nu = Nusselt number Pr = Prandtl number r = radial position (m) R = molar gas constant (8.314 J mol-1 K-1) Re = Reynolds number R = the radius of the coal particle (m) t = time (s or ms) T = temperature (K) usl = gas-particle slip velocity (m s-1) Vp = particle volume (m3)
4. Conclusion Different from any other conventional coal conversion processes, coal pyrolysis to acetylene in thermal plasma takes place under ultrahigh reaction temperatures with millisecond(s) of reaction time. Understanding of the heat transport and devolatilization inside a coal particle becomes one of the most crucial fundamentals for determining the optimum operating conditions such as the energy input, coal feeding rate, particle size, and its distribution. In this work, a mechanism model incorporating the heat conduction in solid materials, diffusion of released volatile gases, and reactions has been established and validated successfully using the available data in the literature. All the results demonstrated that both of the resistance factors would impede thermal energy transportation into the particle. Due to the obstacle of internal heat conduction, the inward heat transport rate would be reduced and also cause different yields of volatiles in different r directions. This effect would become more obvious when the particle size was larger. The volatiles formed from devolatilization process also caused both the heating rate of the coal and the yield of volatiles to be seriously reduced. This resistance would be weakened as the reaction rate decreased. It can be concluded that both the heat conduction inside coal particles and diffusion of volatiles should be considered for such an ultrahigh temperature and ultrafast pyrolysis process.
Greeks δf = thickness of the gas film around the coal particle (m) ΔrH = reaction heat of devolatilization (kJ kg-volatiles-1) ε = emissivity of the pulverized coal (0.9) j = local volume fraction of the solid phase γvol = reaction rate of devolatilization (kg m-3 s-1) λ = thermal conductivity (W m-1 K-1) λeff = efficient local thermal conductivity (W m-1 K-1) μ = fluid viscosity (Pa s) F = density (kg m-3) σSB = Stefan-Boltzmann constant (5.67 10-8 W m-2 K-4) θ = a factor relative to the effect of volatiles’ release on heat conduction Subscripts 0 = initial state ash = ash contained in coal b = thermal plasma gas c = coal f = gas film around the coal particle pl = plasma s = solid phase in coal vol = volatile phase in coal w = surface of the coal particle
(42) Freihaut, J. D. Ph. D. thesis, Pennsylvania State University: University Park, PA, 1980. (43) Sampath, R.; Maloney, D. J.; Zondlo, J. W. Twenty-Seventh Symp. (Int.) Combust./Combust. Inst. 1998, 2, 2915–2923. (44) Fu, W. B. Combustion Theory and Macro-general Laws of Coal; Tsinghua University Press: Beijing, China, 2003 (in Chinese).
2998
