Modelling of Gasification of a Single Coal Char Particle*

Transcription

1 Modelling of Gasification of a Single Coal Char Particle* R. ŽAJDLÍK, Ľ. JELEMENSKÝ, J. MARKOS**, and B. REMIAROVÁ Departent of Cheical and Biocheical Engineering, Faculty of Cheical and Food Technology, Slovak University of Technology, SK Bratislava eail: arkos@cvt.stuba.sk Received 27 April 2001 A atheatical odel of gasification of a large single coal char particle in a carbon dioxide atosphere was developed. The odel forulation took into account the heterogeneous reaction echanis and the solid phase structure. The preexponential factor of the gassolid reaction of carbon with carbon was estiated in this study whereas the values of activation energies and other odel paraeters were taken fro literature. The odel was used to fit the experiental data obtained by TG easureents that were perfored using coal char particles prepared by degasification of Slovak brown coal fro the coal ine Cígeľ at low and ediu teperatures and at atospheric pressure with different inlet ixtures of carbon dioxide and argon. Cobustion and gasification are the ain coercial technologies of coal processing where the heterogeneous reaction between carbon and carbon dioxide takes place. A successful design and optial operation of coercial cobustors/gasifiers would not be possible without a proper understanding of the cobustion/gasification reaction echanis and kinetics. The echanis of coal cobustion/gasification consists of various processes, which proceed at various rates. This behaviour can be explained by the single coal particle history. Once the coal particle is placed into the cobustion/gasification device, iediate and quick release of volatile copounds takes place. Then, a slower step of the process, naely oxidation of carbon, is started. Four ain cobustion reactions are assued, which include three heterogeneous reactions of carbon oxidation and one hoogeneous reaction of carbon onoxide oxidation to carbon dioxide [1, 2]. However, only the reaction of carbon with carbon dioxide is necessary for the forulation of a odel of coal gasification [1]. Both ore coplex reaction schees [3, 4] and odels of coal cobustion/gasification based on a siplified reaction schee and for stationary conditions [4] are available in literature. This siplification is achieved by the iniization of the influence of other factors, for exaple, transport liitations when kinetic paraeters are deterined. Our task was the forulation of a odel able to predict the single coal char particle behaviour during its cobustion/gasification. This odel includes siultaneous intraparticle diffusion and reaction, internal heat transfer by conduction, and external heat transfer by radiation and convection. Fro the cheical engineering point of view, cobustion or gasification of coal char can be characterized as noncatalytic gassolid reaction. The two liiting echaniss, naely the hoogeneous and the shell progressive one [2, 5], are used to describe this process. However, the real behaviour of the coal particle during its cobustion/gasification is ore coplex and could be assued as superposition of the two echaniss entioned above. Careful selection of the echanis should reflect the reaction rate agnitude and the rate of intraparticle ass transport. In our previous study [1], the evidence for hoogeneous echanis of coal gasification in a wide range of teperature and carbon dioxide concentration was found. THEORETICAL During the odel developent the following assuptions were adopted: nonisotheral conditions, constant pressure, constant particle size during reaction, particle free of volatile copounds at the beginning of the process, increase of the particle porosity during its oxidation, no solid products fored in the reaction course, and heat exchange realized by conduction through *Presented at the 28th International Conference of the Slovak Society of Cheical Engineering, Tatranské Matliare, May **The author to who the correspondence should be addressed. 364 Che. Pap. 55(6) (2001)

2 MODELLING OF GASIFICATION OF COAL CHAR the lainar layer, and by radiation. Applying the aboveentioned assuptions, the reaction rate is given by The effective diffusion coefficients and the theral conductivity vary with the porosity, too. These dependences are expressed as follows Č = *oc exp (I^C^CSo, (J) Dest [ел ß (6) The ass balance for gas coponents (г = 1 for ССЬ, г = 2 for CO) inside the coal particle is given by the following diensionless equation d(e p Y si ) дт (2) The carbon ass balance in the diensionless for is given by dy c дт = KTh2 v c Rt (3) The enthalpy balance for the coal particle in the diensionless for can be expressed as ar The initial conditions were as follows for t = 0: Y si = Y? = 0 Yc =YS = 1 fcp fcp ßesi^si e s = e s Boundary conditions applied for t > 0 and ip = 0: dy c /d<p = dy si /d<p = d0 s /d<r = 0 for t > 0 and ip = 1: (4a) (4b) (4c) (4d) (4e) (4f) Ms) Ъ = *5 (4h) d0 s /dy> = {*i h (fl e 0 S S ) + ((0 g ) 4 (ö s s ) 4 )} (4i) During gasification/cobustion both the particle internal structure and its porosity are changed. Due to the porosity variation also other paraeters are changed. These changes correspond to decrease of the particle ass. Assuing the aount of other solid coponents negligible, the porosity variation is proportional to the changes of carbon aount in the coal particle. If the linear relation between the carbon conversion and the particle porosity is assued [6], the porosity change could be expressed by the following diensionless equation e p = e p + i>(ly sc ) (5) *H i ( i ^) (i yc) ] where ß is equal to 2 for rando pore size distribution [7]. The effective diffusion coefficient of gas coponents in solid phase at initial conditions can be calculated by the following equation Desi = DiS p The binary diffusion coefficient, Dj, was calculated using the equation fro literature [8] where Di = Г 175 PM&W Mi,Ar J_ 1 Ml* M AT + Zg]' 1 (7) (8) (9) (10) The values of diffusivity volues iľ» were taken fro tables available in [1]. EXPERIMENTAL Coal fro the coal ine Cígeľ (Handlová, Slovakia) was used in our experients. Coal char was prepared by slow degassing of the original coal particle in nitrogen up to 1100 K. Gasification experients were carried out in an equipent described previously [9]. Atosphere with different carbon dioxide concentrations (5 100 vol. %) in argon was used. The initial gasification teperatures varied within the range fro 1000 to 1300 K. The coal particle diaeter was in the range of The voluetric flow rate of the inlet feed was 100 d 3 h _1. Gasification of the particle was conducted up to the desired ass of the particle (50 % 100 % of carbon burnout). The ass of the particle, teperature in the particle centre, at the particle surface, and the teperature of gas close to the geoetrical surface of the particle were continuously easured. The overall reaction rate of the coal char particle gasification depends on the accessibility of its internal surface to the gaseous reactant (CO2). The particle internal surface accessibility for gases is closely related to the particle pore structure. In our previous work [10] we have found that during the release of volatile copounds an intraparticle pore structure is Che. Pap. 55(6) (2001) 365

3 R. ŽAJDLÍK, L. JELEMENSKÝ, J. MARKOS, B. REMIAROVÁ created, in which the diffusion of gaseous reactants takes place. Therefore, the pore structure of original, devolatilized, partially and copletely burnt particles was easured using the gas adsorption (SORP TOMATIC 1900, Fisons, Rodano, Milan, Italy) and ercury penetration (Porosieter P2000, Fisons, Rodano, Milan, Italy) ethods. RESULTS AND DISCUSSION The atheatical odel represented by eqns (2 4) is a syste of nonlinear differential equations, which were transfored into a set of ordinary differential equations using the orthogonal collocation ethod. This syste was solved by Gear's ethod with tie as independent variable. The odel was siplified applying the assuption of negligible resistance of the lainar fil to the ass transfer (copared to the internal ass transfer and the reaction kinetics). Thus, the concentration of gaseous coponent i at the particle external surface is equal to its concentration in the gas bulk flow. The heat transfer between the coal char particle and the surrounding gas phase is assued to occur by eans of radiation and convection throughout the lainar layer. The heat transfer coefficient, a, depends on the experiental setup, and was calculated by a seiepirical equation fro literature [5]. The initial values of porosity and coal char density were easured experientally. The coal char particle density as well as the specific heat capacity and eissivity were assued to be independent of teperature. Only one kinetic paraeter, the preexponential factor, was estiated. The activation energy value (143.0 kj ol 1 ) was published elsewhere [9], the reaction order with respect to carbon and carbon dioxide equal to their stoichioetric coefficients was assued. The estiation of odel paraeter was based on the leastsquare ethod eploying a odified Levenberg Marquardt algorith. 19 records of experiental particle ass variation vs. tie were fitted in order to get the optial value of the odel preexponential factor ( ol 1 s 1 kg 1 ). However, it is alost ipossible to copare this value with other values available in literature. This paraeter is strongly influenced by the origin and the type of coal used in experients [11]. Moreover, the value of the preexponential factor could be related to other particle properties (internal surface area, surface area accessible for the reaction, particle ass, solid reactant ass, etc.) and depends on the for of the rate equation (reaction order). The coparison of experiental and coputed dependences of particle ass and teperature in the centre of particle vs. tie showed a good agreeent (Figs. 1 and 2, respectively). A slight difference in dependence of the particle ass decrease vs. tie in Fig. 1 (first the coputed reaction rate is higher, then " bo 1 15 e \ Ч \Ч "^w 1 i 1 1 L ŕ/in Fig. 1. Coparison of experiental (solid line) and calculated (dashed line) variation of coal char particle ass during gasification in a ixture of carbon dioxide and argon vs. tie (content of CO2 25 %, initial teperature 1220 K) * ^ ) //in Fig. 2. Coparison of experiental (solid line) and calculated (dashed line) variation of teperature in the centre of coal char particle during gasification in a ixture of carbon dioxide and argon vs. tie (content of CO2 25 %, initial teperature 1220 K). it becoes lower copared to that easured) could be explained by that the carbon reaction order was probably lower than unity. Figs. 3 and 4 present the calculated reactant concentration profiles (C and CO2, respectively) inside the coal char particle at different reaction ties. The approxiately constant carbon concentration inside the whole particle shown in Fig. 3 confired the suitability of the use of the reactiondiffusion echanis ' 366 C/ie/71. Pap. 55(6) (2001)

4 MODELLING OF GASIFICATION OF COAL CHAR Fig. 3. Calculated carbon concentration profile inside the coal char particle at different reaction ties (J. 0 in, in in, in, in, in). The ain goal of this work was to develop a siple odel for the prediction of the large coal char particle gasification, able to describe the variation of the coal char particle ass and the teperature inside the coal char particle with tie. The value of ol 1 s 1 kg 1 for the only estiated paraeter of the odel developed in this study (the preexponential factor of the heterogeneous reaction between carbon and carbon dioxide) was obtained by coparison of experiental and odelled data. All other odel paraeters were taken fro literature. The fit of experiental data by odel is good. Other results (the reactant concentration and teperature profiles) confired the validity of our assuption that the hoogeneous echanis fits heterogeneous carbon carbon dioxide reaction well. Based on the results we can conclude that this heterogeneous gassolid reaction takes place within the entire volue of the large coal char particle. Coputed reactant concentration profiles do not show any sharp changes and are approxiately the sae inside the whole coal char particle, already in the first third of the gasification process. Acknowledgeents. This work was supported by the Grant VEGA No. 1/7351/20 fro the Slovak Scientific Grant Agency. SYMBOLS Fig. 4. Calculated ole fraction profiles of carbon dioxide inside the coal char particle at different reaction ties (1. 1 s in in, in, in, in). for the coal char gasification odelling. Moreover, the carbon dioxide concentration inside the coal char particle reached the inlet CO2 concentration already at carbon conversion of 0.3 (Fig. 4, curve 5). Therefore, it could be concluded that the intraparticle difiusion of carbon dioxide was faster than its consuption. CONCLUSION С Cp U D e Е л Д Г Я< M n P R RT *P Г Th T t x Yi Yc diensionless paraeter = ar p /X u olar concentration ol ~ 3 specific heat capacity of coal char J kg" 1 K" effective diffusion coefficient 2 s activation energy J ol reaction enthalpy J ol 6 ol preexponential factor olar ass reaction order for solid reactant reaction order for gas reactant pressure "I cl s x kg" kg ol" gas constant J ol 1 diensionless reaction rate = f/ particle radius particle radial coordinate diensionless paraeter, teperature tie ole fraction diensionless concentration of the gas coponent = Cgi/C^j diensionless concentration of carbon = Cc/C% Greek Letters a heat transfer coefficient W " 2 ß epirical paraeter E diensionless paraeter = a e (T^) 3 Rp/X u e e eissivity p porosity of particle Pa " 1 s 2 iri СЛвш. Pap. 55(6) (2001) 367

5 R. ŽAJDLÍK, Ĺ. JELEMENSKÝ. J. MARKOS. B. REMIAROVÁ в K Л Au P v é e. a Ľ 'P 4> ф diensionless paraeter = = > е 81 С (Д г Я )/Л и Т е diensionless teperature = T/T? diensionless paraeter = C /CQ diensionless paraeter defined by eqn (7) effective theral conductivity of coal char W " 1 K effective theral conductivity of ash W " 1 K" 1 stoichioetric coefficient density kg " 3 3 olar volue ol" 1 reaction rate based on the ass of solid phase ol kg * s diensionless paraeter = D^JD^ Stefan Boltzan constant W ~ 2 diffusivity volue diensionless tie = td^/щ Subscripts Ar С CO2 g i p s tortuosity diensionless radius = r/r p diensionless paraeter = C c vc diensionless paraeter = PsCpuD^/X^ argon carbon carbon dioxide gas phase, bulk gas flow nuber of gas coponent particle solid phase Superscripts S geoetrical surface of particle 0 initial condition REFERENCES 1. Žajdlík, R., Markos, J., Jeleenský, Ľ., and Reiarová, В., Che. Pap. 54, 467 (2000). 2. Markos, J., Žajdlík, R.. Reiarová, В., and Jeleenský, Ľ., Che. Pap. 55, 359 (2001). 3. Liu, G.S., Täte, A. G. : Bryant, G. W. ľ and Wall, T. F., Fuel 79, 1145 (2000). _4 4. Žajdlík, R.j Markos, J.. Reiarová, В.. and Jeleenský, Ľ., Petroleu Coal 42, 70 (2000). 5. Doraisway, L. K. and Shara, M. M., Heterogeneous Reactions, Vol. 1, p Wiley, New York, Froent, G. F. and Bischoff,. В., Cheical Reactor Analysis and Design, in Noncatalytic GasSolid Reaction. P Wiley, New York, Wen, C. Y., Ind. Eng. Che. 60, 34 (1968). 8. Reid, R. C., Prausnitz, J. M., and Poling, В. E., The Properties of Gases and Liquids. McGrawHill, Singapore, Žajdlík, R., Jeleenský, Ľ., Reiarová, В., and Markos, J., Che. Eng. Sei. 56, 1355 (2001). 10. Reiarová, В., Žajdlík, R., Markos, J., and Jeleenský, Ľ., in Proceedings of the 25th International Conference of the Slovak Society of Cheical Engineering, Jasná, Deänovská dolina, Slovak Republic, Xu, C. R. and Fu, W. В., Cobust. Sei. Technol. 124, 167 (1997). 368 Che. Pap. 55(6) (2001)