# Interaction between a Turbulent Flow and Reaction under Various Conditions in Oxygen Blown HYCOL Gasifiers.

код для вставкиСкачатьDev. Chem. Eng. Mineral Pmcess., 11(5/6), pp. 557-577, 2003. Interaction between a Turbulent Flow and Reaction under Various Conditions in Oxygen Blown HYCOL Gasifiers Hao Liua9b,Caixia Chenb and Toshinori Kojimaa* a Department of Applied Chemistry, Seikei University, 3-3- I Kichijojikita-machi, Musashino-shi, Tokyo 180-8633, Japan bResearch Fellow of NED0 (New Energy and Industrial Development Organization), Japan With a three-dimensional comprehensive computer simulation model, the effect of mixture fraction fluctuations on the overall gasijiication characteristics due to a turbulent flow was studied for gasification in oxygen blown HYCOL entrained-flow coal gasifiers. The method compared the results with or without fluctuation of the gas-mixture fraction. The theoretically predicted results were also compared with experimental data obtained from a 50 ton/day pilot-scale HYCOL gasifier. Zt was seen that the fluctuation in mixture fraction of gases produced from devolatilization is just as important as that of char-02 reaction, i.e. neither of them can be neglected. They have an important influence not only on the temperature and gas composition distributions, but also on the cold gas efficiency and product gas composition. However, the fluctuations of gas-mixture fractions produced from char-C02 and char-steam reactions have only a limited effect. This conclusion is valid f o r a wide range of conditions of pressure, particle size and O2 concentration during entrained-flow coal gasification. Based on these results, the simulation model can be simplified without significantly influencing the validity of the predicted results. The predictions that include fluctuations of mixture fractions show better agreement with the experimental data. * Author f o r correspondence (kojima@ch.seikei.ac.jp). 557 H. Liu, C.Chen and T Kojima Introduction In recent years, entrained-flow coal gasification has been highlighted as a promising clean and efficient coal utilization technology for the future. Gasification in an entrained-flow gasifier is very complex, mainly due to turbulent flow, elevated temperature, etc. Theoretical studies on entrainedflow gasification are becoming more important due to these problems and the high cost of experimental work. Several mathematical models have been developed for entrained-flow gasifiers (e.g. Wen et al., 1979; Govind et al., 1984; Villiams, 1995; Smoot et al., 1985; Hill et al., 1993). Those models have the common features of solving the mass, momentum and energy conservation equations with similar submodels by almost identical mathematical methods (e.g. Smoot, 1993). Some models have been applied to bench-scale entrained-flow gasifiers (e.g. Smoot 1987), but the simulation capability for gasification was quite limited because of their oversimplification of the chemistry. In this work, a comprehensive three-dimensional coal gasification model with a multi-solid progress variables (MSPV) method has been developed by us and extended to the simulation of oxygen-blown HYCOL gasifiers. The validity of the model has been shown by comparing the predicted results and the experimental data from our previous studies for an IGCC gasifer (see Chen el al., 2000a; Chen et al., 1999; Chen et al., 2000b; Chen et al., 2001). Owing to the complexity of gasification in an entrained-flow gasifier, it is very important to make appropriate simplifications to a comprehensive threedimensional coal gasification model, without losing the validity of the predicted result. In a HYCOL entrained-flow gasifier, oxygen instead of air (as in an IGCC gasifier) is used as the oxidizer. Furthermore, in a HYCOL entrained-flow gasifier, the flow field is highly turbulent and leads to fluctuations of gas-mixture fractions. Accordingly, these fluctuations of mixture fractions due to a turbulent flow may affect the overall gasification characteristics of a gasifier, which can only be clarified through simulation. In our previous work (Liu et al., 2002), we clarified the interaction between a turbulent flow and reaction in an air-blown IGCC gasifier and identified that fluctuations of gas-mixture fractions produced from char-C02 and char-steam reactions could be neglected without significant influence on the predicted results. However the conditions in a HYCOL gasifer can be very different from an IGCC gasifer, e.g. in O2 concentration, temperature, reaction rates, intensity of turbulence, etc. Conclusions for IGCC gasification are not necessarily applicable to HYCOL gasification. Furthermore, in our previous 558 TurbulentFlow and Reaction in Oxygen Blown HYCOL Gasifiers work investigations only covered limited conditions and the effects concerning devolatilization and char-02 reactions were not separated. Due to the high O2 concentration, intensive turbulence, etc., in a HYCOL gasifier, then a finer simulation grid is required than for an IGCC gasifier in order to reach a converged solution. Accordingly, calculation for a HYCOL gasifier through a three-dimensional model takes a longer time than an IGCC gasifier and it would be advantageous if the calculation time could be shortened. In this work, the interaction between a turbulent flow and reaction, i.e. the effects of fluctuations of mixture fractions on the temperature and gas composition distributions, cold gas efficiency and product gas composition, were examined for a wide range of conditions of oxygen-blown HYCOL gasification. Based on these results, simplification on the model can be achieved without significantly losing the validity of the predicted results. This work not only helps to provide an understanding of the characteristics and mechanism of entrained flow gasification, but it also helps to build a valid theoretical model and simplify it appropriately. Our investigation is essentially different from the work of Smoot and Smith (1985) in that we studied oxygenblown entrained-flow coal gasification, but they studied a lifted coal combustion flame. Model Description and Simulation Method The numerical methods and the sub-models recommended for the entrainedflow coal combustion and gasification process (Smoot and Smith, 1985) were used in the present model. The gas phase is assumed to be a steady-state, reacting, continuum field that can be described by general conservation equation as follows: where, CDrefers to any quantity of mass, velocity components (u, v and w), turbulent kinetic energy (k), turbulent kinetic energy dissipation rate (e), gas enthalpy (h), mixture fractions cfi) and their variances (gi). S, is the source term. S,,, is an additional source term representing the interaction between gas and particle phases (Smoot et al., 1985). The corresponding expressions of #, 559 H.Liu, C.Chen and 1: Kojima S,, S, a n d p are listed in Table 1. The reactions and rate expressions are listed in Table 2. Table 1. The source term S, and Sopfor different Variable z s, (€J U V W .”) ( ”1 +a( ”(e] a +a( ”)a( $) ”( ”(a -E a + a $2) a +L( q $2) a +a - E +d p -E+ Q (p &) Q p 2)+ a p p + p azcaaQ8caz Q variables in equation (I). S *P -;dn,u,) - ;c,v,, -~dn,W,)-m,a. 0 0 h -sdn,h,) dt With a Lagrangian method, the pulverized coal particles are tracked. The particle-source-in-cell model (Crowe et al., 1977) is used to deal with the interaction between gas and particle phases through various particle source terms. The net difference in the particle properties between leaving and entering any given cell provides the particle source term S, for the gas-flow equations. Turbulence in the gas-phase is modeled by Favre-averaging of the 560 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasifiers gradient diffusion processes with the two-equation k - E model (Launder ei al., 1972) for closure. The standard constants of the k - & model (Launder et al., 1974) were used in the simulation. The particle stochastic trajectory model (Shuen et al., 1983) based on instantaneous gas velocities is used to simulate the particle motion and turbulent dispersion. The values of the fluctuating velocities are assumed to follow a Gaussian distribution and are determined in a stochastic manner within a turbulent eddy. Table 2 Reactions and the rate expressions. Parameter Devolatilization dY - d(Y, + Y 2 ) _ = k, (Y,' - Y , )+ k,(Y,' - Y,) dt dt k, = A, exp(-E, I RT,) Value k , = A, exp(-E, I RT,) A1 (s-2 El (kJ mol") A2 ( s - 9 E~ (H mol-') Yl * Y2* Char-02,H 2 0 and C02 reactions k, =A, expCE, I R T ) A , (kg Pa-'.' s-' m-') C HzO c - coz c-0 2 E, (kJ mol-') C - H20 c - co2 c-0 2 3.7~10' 7.4~ lo4 1.5 x1ol3 2.5~10' 0.565 0.565 0.0782 0.0732 0.052 1.15x lo5 1.125x 10' 0.61 x 10' Coal devolatilization is modeled by a simple, two-step mechanism (Ubhayakar et al., 1977). The particle surface reactions are characterized by the 0.5 order, multiple, parallel reaction rate formulation of Smith (1982).An extended version of the statistical coal-gas mixture fraction model with the multiple solid progress variables (MSPV) method (Brewster et al., 1988) is 561 H.Liu, C.Chen and Z Kojima used. Four components of coal off-gas, i.e. four coal-gas mixture fractions are used to track the reaction products. These mixture fractions at a point are defined as the mass ratio of coal off-gas to the total gas product and written as (Brewster et al., 1988): j 4 where mi (i = 1-4) represents the mass of gas originating from devolatilization (i = l), ~ h a r - 0(i~ = 2), char-steam and char-C02 reactions (i = 3, 4) respectively, mi, is the mass of inlet gas, andfi (i = 1-4) is the conserved mixture fraction calculated by transport equations. The origin of char-steam reaction is a result of the inherent moisture present in the coal. From the general approach of the MSPV method, the other four mixture fractions are defined by: F,= mi I (3) j=I From these definitions, each mixture fraction Fi (i = 1-4) varies independently between zero and unity and, to a first approximation, is assumed to be statistically independent of the other mixture fractions. Thus Fi is related tofi by: e.= (4) j=i+l The average value of any dependent fluctuating gas property P(gas species, temperature, density or viscosity) is a function of F1,F2,F3 and F4,and can be calculated by convoluting the instantaneous value over the probability density functions (PDF) of the independent mixture fractions: 562 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasijiers Table 3. Property of the coal. Proximate analysis (wt %) Moisture Ash VM FC 5.3 12.1 46.7 35.8 I Ultimate analysis (dry, ash free, wt %) IC H N 0 S I 77.6 6.5 1.13 13.9 0.22 Table 4. Operating conditions (base conditions). Parameters Particle size distribution (pm) (wt % for each size classification) 10% 10% 20% 20% 20% 20% Mass mean diameter (pm) Gas flow rate (kg s-I) Lower coal burner Higher coal burner Particle loading (kg s") Lower coal burner Higher coal burner Pressure (MPa) Values 150 100 40 20 10 4 39.8 0.284 0.180 0.289 0.289 3 .O The local variances of the mixture fractions are calculated from gi transport equations similar to those of Launder and Spalding (1972). The PDFs are assumed to have the form of a clipped-Gaussian distribution, adjusted to account for turbulent intermittency (Smoot et al., 1985). The governing partial differential equations for all the quantities were reduced to their finite difference analogues by integrating over the computational cells. The conservation equations are Favre-averaged and solved by the SIMPLER method (Patankar 1980). A line-by-line iteration technique was used to solve the finite difference equations. Solution of the particle-phase conservation equations is coupled with the gas-phase. An 563 H. Liu, C.Chen and T.Kojima overall convergence of the two phases is achieved using an iteration procedure. Simulation is based on a 50 todday oxygen-blown entrained-flow HYCOL gasifier (Ueda et al., 1994; Nogita et al., 1986). The coal property is based on Taiheiyo bituminite (see Table 3). The operating conditions used in the simulations are listed in Table 4 (base conditions). To obtain a relatively general conclusion, simulations were also conducted under other conditions, in addition to the base conditions of Table 4 (while keeping other parameters the same as the base conditions). The chemical reaction-rate constants from the literature (Smoot et aZ., 1987) for a similar coal were used in the present calculation. To obtain a steady convergence in the calculations, the model tracked 15360 particle trajectories. A ‘ 4 3 x 4 3 ~ 6 8 ’ grid mesh was adopted. A converged solution is defined when the global energy balances are within 0.3% deviation of the total combustion energy, and the normalized residual (mean residuaymean inlet velocity) for each velocity component is less than 0.3%. Results and Discussions (0 Effect on the Temperature and Gas Composition Distributions The effects of fluctuations of mixture fractions on the temperature and gas composition distributions, due to a turbulent flow, were investigated for gasification in a HYCOL gasifier as shown in Figures 1 and 2. The calculated results are compared when the fluctuations of mixture fractions are included with those results when the fluctuations of mixture fractions are not included. Figure 3 shows the effect of fluctuations of mixture fractions on the temperature distribution. In Cases (a), (a*), (b) and (c), no fluctuation of mixture fraction, the fluctuation of mixture fraction F1, the fluctuations of mixture fractions F1 and Fz, the fluctuations of mixture fractions F1 through Fq, were included respectively. Thus, in Case (b), the fluctuations of mixture fractions concerning devolatilization and char-O2 reactions were included. In Case (c), in addition to devolatilization and char-02 reactions, the fluctuations concerning gasification reactions were also considered. For all the calculations in this paper, all conditions are the same as the base conditions listed in Table 4 unless otherwise specified. All of the figures illustrate the horizontal crosssectional distributions (one-quarter of a circle). A jagged edge is used to approximate the curved boundary. From Figure 3 it can be seen that the results 564 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasifiers n U (1) upper burner (2) lower burner (3) throat Heat recoveq section t Gasification section (1)- -4- (2) = e r f A Slag quenching section Figure 1. fa) Schematic diagram of a HYCOL gas$er. (8) Schematic overhead view of nozzle geomem. of Cases (a), (a*) and (b) are significantly different, with Cases (b) and (c) nearly the same. These results suggest that the effect of fluctuations of gasmixture fractions produced from devolatilization on temperature distribution is just as important as the fluctuation concerning char-02 reaction, i.e. neither of 565 H. Liu, C. Chen and T.Kojima Gasifier Coal OxyP Nitrogen Tank n Ash treatment Figure 2. Schematicflow diagram of a HYCOL gasifier. them can be neglected. However, no significant influence of the fluctuations of gas-mixture fractions produced from char-CO2 and char-steam reactions on the temperature distribution is observed. Figure 4 shows the effect of fluctuations of gas-mixture fractions produced from various reactions on CO concentration distribution. The fluctuations of mixture fractions concerning devolatilization and char-02 reactions significantly influence the distribution of CO concentration, but the effect of the fluctuations of mixture fractions concerning gasification reactions (charCOz and char-steam reactions) is limited. This is most likely due to the high reaction rates of devolatilization and char-02 reactions, and the low reaction rates of gasification reactions (char-COz and char-steam reactions). Figure 5 shows the effect of the fluctuations of mixture fractions on H2 concentration distributions. Similarly, the results of Cases (b) and (c) are nearly the same, while Cases (a), (a*) and (b) are very different. 566 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasijiers Figure 3. EIfect of fluctuations of mixture fractions on the distribution of temperature (K). cross-sectional distribution (onequarter of the cross section), under base conditions. (a) No fluctuation, (a *) Fluctuation with FI.(6) Fluctuation with Fl and F2. (c) Fluctuation with F1, F2, F3 and F,. Figure 4. EIfect of fluctuations of mixture fractions on the distribution of CO concentration, cross-sectional distribution (one-quarter of the cross section), under base conditions. (a) No fluctuation, (a *) Fluctuation with FI. (6) Fluctuation with Fl and F2. (c) Fluctuation with FI. F2,F3 and F4, 567 ff. tiu, C Chen and Z Kojima Figure 5. Efect of fluctuations of mixture fractions on the distribution of H2 concentration, cross-sectional distribution (one-quarter of the cross section), under base conditions. (a) No fluctuation, (a 8, Fluctuation with Fl. (b) Fluctuation with Fl and F2-(c) Fluctuation with FI, F2,Fj and F,. Figure 6. Efect of fluctuations of mixture ffactions on the distribution of CO concentration for fine particles, cross-sectional distribution (one-quarter of the cross section), diameter: 4 pn. (a) No fluctuation, (b) Fluctuation with F, and F2, (c) Fluctuation with FI, F2,Fj and F,. 568 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasifiers (ii) Effect of Fluctuation under Various Conditions To obtain a general conclusion concerning the interaction between a turbulent flow and reaction for entrained-flow gasification, the investigation was conducted over a wide range of conditions. Figures 6 and 7 show the concentration distributions of CO and H2 for fine particles (diameter 4 pm). We also studied the case when the reaction rate pre-exponential for the charC 0 2 and H20 reaction rates were increased by a factor of 10 (see Figures 8 and 9). Moreover, Figures 10 and 11 show the results at 1.5 MPa. Combining our previous results for air-blown IGCC gasifiers (Liu et al., 2002) and the results shown in Figures 3 through 11, it was identified that during entrainedflow gasification, over a wide range of conditions of particle size, pressure, O2 concentration, etc., then the fluctuation of gas-mixture fraction produced from devolatilization is just as important as that of char-02 reaction. However, the fluctuations concerning gasification reactions can be neglected. We also investigated the case of low volatile matter (VM) content. Calculation was performed at low volatile matter conditions, i.e. by changing the VM content of the coal from 46.7% into 30% (wt% as received) while keeping the other parameters the same as listed in Tables 3 and 4. The results are shown in Figure 12. The conclusions derived for a high VM-content coal is also applicable to the case of a coal with relatively low VM-content. However, the influence of fluctuation of gas-mixture fraction produced from devolatilization and char-O2 reaction is pronounced for a high VM-content coal. (iii) Effect on General Characteristics of a HYCOL Gasifier and Comparison with Experimental Data Figure 13 shows the effect of fluctuation of mixture fractions on the general characteristics of a HYCOL gasifier and the comparison with experimental data. Case (d) represents the experimental results measured on a 50 todday pilot-scale oxygen-blown entrained-flow HYCOL gasifier. The cold gas efficiency and concentrations of CO and H2 in product gas for Case (b) are nearly the same as those for Case (c). However, the results for Case (a) are different from Cases (b) or (c). These findings show that the fluctuations of gas-mixture fractions produced from devolatilization and char-02 reactions influence not only the distributions of temperature and gas composition, but also the general characteristics of a gasifier, i.e. cold gas efficiency and product gas composition, during gasification in an oxygen-blown HYCOL gasifier. 569 H. Liu, C.Chen and Z Kojima Figure 7. Efect of fluctuations of mixture fiactions on the distribution of H2 concentration for fine particles, cross-sectional distribution (onequarter of the cross section), diameter: 4 p. (a) No fluctuation, (b) Fluctuation with FI and F2, (c) Fluctuation with F,, F2,F3 and F,. Figure 8. Eflect offluctuations of mixture fiactions on the distribution of CO concentration when gasification rates were increased by a factor of 10, crosssectional distribution (onequarter of the cross section). (a) No fluctuation, @) Fluctuation with Fl and F2, (c) Fluctuation with FI, F2, F3 and F4. 5 70 Wbulent Flow and Reaction in Oxygen Blown HYCOL Gasifiers Figure 9. Efect of fluctuations of mixture fiactions on the distribution of Ht concentration when gasijcation rates were increased by a factor of lo, crosssectional distribution (one-quarter of the cross section). (a) No fluctuation, (3) Fluctuation with Fl and F2, (c) Fluctuation with F,,F2,F3 and F,, Figure 10. Efect of fluctuations of mixture fiactions on the distribution of CO concentration at 1S MPa, cross-sectional distribution (one-quarter of the cross section). (a) No fluctuation, (b) Fluctuation with FI and Fa (c) Fluctuation with F,, F2, Fj and F4. 5 71 H. Liu, C. Chen and I: Kojima Figure 11. Eflect of fluctuations of mixture fractions on the distribution of H2 concentration at 1 .S MPa, cross-sectional distribution (one-quarter of the cross section). (a) No fluctuation, @) Fluctuation with F, and F2,(c) Fluctuation with F,, Fz,FJ and Fd. The comparison between the predictions for various cases and the experimental data (Case (d)) identified that the predicted CO and H2 concentrations in the product gas for Cases (b) and (c) agree better with the experimental data than Case (a), i.e. including fluctuations of mixture fractions gives a better prediction. This result is in agreement with OUT previously simulated results for an IGCC gasifier as summarized in Table 5 (LIU et al., 2002). Conclusions With a three-dimensional comprehensive computer simulation model, the effects of mixture fraction fluctuations on the overall gasification characteristics due to a turbulent flow were studied for gasification in an oxygen-blown HYCOL entrained-flow coal gasifier. The method compared the results with and without the fluctuation of gas-mixture fraction. The theoretically predicted results were also compared with experimental data obtained from a 50 todday pilot-scale HYCOL gasifier. It was shown that the fluctuation of gas-mixture fraction produced from devolatilization is just as important as that of ~ h a r - 0reaction, ~ i.e. neither of them can be neglected. 5 72 Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasijiers Figure 12. Eflect of fluctuations of mkture fiactions on the distribution of temperature (K), cross-sectional distribution (one-quarter of the cross section), under low volatile matter condition, i.e., 30% VMcontent (wt% as received). (a) No fluctuation,(b) Fluctuation with FI and Fz, (c) Fluctuation with F], F2, F3 and F,. They have important influences not only on the distributions of temperature and gas composition, but also on cold gas efficiency and product gas composition. However, the fluctuations of gas-mixture fractions produced from char-CO2 and char-steam reactions have only a limited effect. This conclusion is valid for a wide range of conditions of pressure, particle size and O2 concentration during entrained-flow coal gasification. Based on these results, the simulation model can be appropriately simplified without significantly influencing the validity of the predicted results, and hence reducing the calculation time. The predictions that include fluctuations of mixture fractions show better agreement with the experimental data. 5 73 H. Liu, C. Chen and Z Kojima Figure 13. Comparison between predictions and experimental results, under base conditions. (a) No fluctuation, (b) Fluctuation with Fl and F2, (c) Fluctuation with Fl, F2, F3 and F4 , (d) Experimental. Table 5. A summary of previously simulated results for IGCC gasifier: a comparison between predicted and measured heating values of product gas (Liu et al., 2002). Heating value of product gas (kcaVm3) Case No fluctuation 1014.1 Fluctuations with F1 and Fz 1046.7 Fluctuations with F,, Fz, F3 and F4 1056.6 Experimental measurement 1055.8 Acknowledgement The authors would like to thank NEDOKCUJ for financial support of this work under BRAIN-Cprogram. Nomenclature pre-exponential factors, s-’ pre-exponential factors, kg m-2s-’ Pa -03 acceleration due to gravity, m s-’ turbulent model constants activation energies of devolatilization reactions, kJ mar' apparent activation energy, k~ mol” overall mixture fraction, the mass ratio of total coal off-gas to the total gas product mixture fractions defined in equations (2) and (3) Turbulent Flow and Reaction in Oxygen Blown HYCOL Gasijiers variance of fi thermal enthalpy, J kg-' turbulent kinetic energy, m2 s-2 devolatilization rate constants, s-l rate constant, kg m-'s-' Pa-'.' mass of gas originating from gas solid reactions, kg mass of inlet gas, kg mass of particle, kg static pressure, Pa particle concentration, kg m-3 radiative heat flux, J m-' generalized source term source term arising from particles time, s particle temperature, K gas velocity components, m s-' particle velocity components, m s" coordinate of three directions, m volatile yield volatile yield for low activation devolatilization reaction maximum of Yl volatile yield for high activation devolatilization reaction maximum of Y, average value of any gas property dissipation rate of turbulent kinetic energy, m2se3 turbulent model constants generalized variable gas density, kg m-3 turbulent viscosity, kg m-' s-' References [ l ) Brewster, B.S., Baxter, L.L., Smoot, L.D., 1988. 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