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AbstractA pure two-4uid model, instead of the Eulerian gas-Lagrangian particle models (particle trajectory models), is used for simulatingthree-dimensional (3-D) turbulent reactive gas–particle 4ows and coal combustion. To improve the simulation of the 4ow 9eld and NOXformation, a modi9ed k– –kp two-phase turbulence model and a second-order-moment (SOM) reactive rate model are proposed. Theproposed models are used to simulate NO formation (fuel NO produced by NH3) of methane–air combustion, and the prediction results arecompared with those using the pure presumed PDF-9nite-reaction-rate model and experimental data. The modi9ed k– model and SOMmodel are more reasonable than the standard k– model and the pure presumed PDF-9nite-reaction-rate model. The proposed models arealso used to predict the coal combustion and NO formation at the exit of a double air register swirl pulverized-coal burner. The predictedresults indicate that a pulverized-coal concentrator installed in the primary-air tube of burner has a strong e>ect on the coal combustionand NOX formation.? 2003 Elsevier Ltd. All rights reserved.43244
Keywords: NOx formation; Combustion; Pure two-4uid model; Modi9ed k– –kp model; Second-order-moment model; Pulverized-coal concentrator 1. IntroductionPulverized coal combustion is widely used in utilityboilers, industrial boilers, furnaces, kilns, and other energyconversion devices. The e@cient and clean utilization of pul-verized coal is a major problem in combustion processes, es-pecially in burning low-grade coal. It is well known that theemission of nitrogen oxides (NOX ) during coal combustionis a major environmental problem. Fortunately, the emissionof NOX , in contrast to SOX emission, can be reduced signif-icantly by the modi9cation of the combustion process. Toreduce the NOx formation, di>erent techniques, such as airstaging, reburning, 4ue-gas recirculation, are used in pulver-ized coal burners and furnaces (Rackler, 1995;Wroblewska,1995; Istberg et al., 1998; FD ortsch et al., 1998). It is alsorecognized that numerical simulation is a useful tool foroptimizing the results of these techniques. Di>erent turbu-lence and chemistry models for modeling coal combustionand NOX formation in complex turbulent 4ows have beenproposed. However, it is most di@cult to model the reactingcoal particles and their e>ect on the gas phase. In treating the particle phase for modeling pulverized coal, most of theexisting models are based on a Lagrangian treatment of theparticles (Sommerfeld et al., 1993; Lockwood et al., 1988;Smoot and Smith, 1985; Papadakis and Bergeles, 1994;Coimbra et al., 1994; Costen et al., 2000). By using theparticle trajectory model, it is easy to simulate the combust-ing coal particle history. However, in order to obtain a de-tailed distribution of particle velocity and concentration forcomparison with experimental data, a large amount of parti-cle trajectories are needed. Some models using the Euleriantreatment of particle phase are based on a single-4uid model(no-slip model) (Fiveland and Wessel, 1988) and two-4uidmodel (Guo et al., 1998; Zhou, 1988). The no-slip modelneglects the velocity slip between the gas phase and coalparticle phase, and assumes that the temperature of the coalparticle phase is equal to the temperature of the gas phase,and the temperature distribution of the gas–particle mixturecan be obtained by solving the overall energy equation. Thepure two-4uid (PTF) model uses a comprehensive
Euleriantreatment for both gas and particle phases. Both velocity andtemperature slips between coal particles and gas phase arecalculated by solving the momentum equations and energyequations of the gas and particle phases. The PTF can con-veniently describe all particle history e>ects—the particlemass change due to moisture evaporation, devolatilization and char combustion, and particle temperature change dueto convection, di>usion and heat transfer between the twophases. However, it is inaccurate for the standard k– –kpmodel to simulate turbulent 4ow, especially swirling 4ow(Zhou and Chen, 2001; Sloan et al., 1986). In order to de-crease inaccuracy, the k– –kp model should be modi9ed.The numerical simulation of NOX formation in turbulentcombustion is frequently used in the optimization design oflow NOX burners and furnaces. The direct numerical simula-tion (DNS) (Tanahashi et al., 2000), large-eddy simulation(LES) (Park et al., 2000), probability density distributionfunction (PDF) transport equation model (Pope, 1985) andconditional moment closure (CMC) (Bilger, 1993), devel-oped in recent years, can well simulate the detailed 9nite-ratekinetics of NOX formation, but these re9ned models needrather large computation time and computermemory. Hence,they are used mainly for very simple 4ows for fundamentalstudies, such as jet and channel 4ows, and are very di@cultto use in complex engineering 4ows, such as 3-D recircu-lating and swirling 4ows. For engineering NOX formationmodeling, some investigators use the EBU–Arrhenius (EA)model (Mueller and Kremer, 1995), but the EA model actu-ally cannot take into account the 9nite reaction rate. Manyinvestigators, including some commercial codes, adapt thepresumed PDF-9nite-reaction-rate model (DalSecco et al.,1995; Faltsi-Saravelon and Wild, 1995), using a product ofseveral single variables PDFs instead of the joint PDF. Ex-perimental studies point out that thismodel underpredicts theaveraged reaction rate. And turbulence is assumed to haveno e>ect on the reaction rate in the model. Alternatively, thesecond-order moment (SOM) turbulence-chemistry modelsfor NOX formation, based on the idea of SOM turbulencemodels (Liao et al., 1996), will be more reasonable than theEA model and the presumed PDF models.In order to reduce NOX formation during coal com-bustion, the authors designed a new type of swirl burnerwhich is installed in a pulverized-coal concentrator in theprimary-air tube (Li et al., 2000). In this paper, a modi9edk– –kp two-phase turbulence model, a second-order mo-ment turbulence-chemistry model for modeling NOX for-mation, a general model of pulverized-coal devolatilizationand a general model of char combustion are incorporatedinto the comprehensive model for predicting the validationof the pulverized-coal concentrator.2. Description of the models