Flow Simulation and Optimization of Plasma Reactors for Coal Gasification

Plasma Science Technology, Vol 5, No 5(2003Flow Simulation and Optimization of Plasma reactorsfor Coal Gasification04五 Chunjun(冀春俊), hang yingzi(张英姿), Ma tengcai(马腾才)2gineering Department, Dalian University of Technology, Dalian 116023China2 Material Modification National Key Laboratory, Dalian University of Technology,Dalian 116023. Chinaabstract This paper reports a 3-d numerical simulation system to analyze the complicatedflow in plasma reactors for coal gasification, which involve complex chemical reaction, two-phaseflow and plasma effect. On the basis of analytic results, the distribution of the density, tempera-ture and components'concentration are obtained and a different, plasma reactor configuration isto optimize the fow parameters. The numerical simulation results show an improvedconversion ratio of the coal gasification. Different kinds of chemical reaction models are used tomulate the complex flow inside the reactor. It can be concluded that the nunerical simulationsystem can be very useful for the design and optimization of the plasma reactorKeywords: numerical simulation, plasma reactor, coal gasification, two-phase flowPACS:5250;8240.R1 Introductionconversion ratio of coal to gas. The plasma reac-or modeling has advanced to the extent that three-Coal gasification is one of the new clean coal techdimensional simulations of reactors are possiblenologies for industry. It can reduce energy consump- nowadays (1. Traditionally, the plasma equipmenttion and at the same time be more friendly to envi-modeling has been focused on discharge physicsronmental protection. And the coal gasification tak- power coupling, plasma generation and other issues,ing an arc-plasma as an auxiliary heat source is quitereasdifferent from traditional coal conversion technologythe gas flow related issues are usually neglected. Recently, experimental evidences show thatA high-temperature arc-plasma is a kind of typical the flow field and gas distribution strongly affectthermal plasma. Containing a large number of active plasma uniformity and the efficiency of the plasmaparticles such as various kinds of charge ions, neutralreactor as well l4. It is therefore necessary to simuparticles and electrons, the plasma is regarded as the late the flow in the plasma reactor to have an insightmost ideal means for coal gasification in recent yearsinto the effect of the gas flow on the reactor and toThe key equipment for coal gasification with theoptimize the design parametersare-plasma is a plasma reactor, which will determinethe thermal efficiency of the plasma source and thecind of plasma reactorThe project supported by the National 973 Project of China(中国煤化工CNMHGJi Chunjun et al. Flow Simulation and Optimization of Plasma Reactors for Coal GasificationThe plasma flowD=02mThe coal powderThe main part of the plasma reactorD=16mand air mixtureig.1 A schematic illustration of the plasma reactorfor coal gasification is modeled using the STAR-CDbe absenta flow-analyzing package to obtain the temperatureb. The plasma is a continuous ideal gas. It isvelocity and concentration distribution inside the re- optically thin in local thermodynamic equilibriumactor. A different plasma Aow configuration scheme (LTEis put forward on the basis of the analytical results.c. The flow in the reactor is in a steady stateNumerical simulations of the new geometry model isAs a result, the governing equations are not timeperformed under different plasma flow velocities inan attempt to explore the effect of the plasma flowsumpulparameters on the efficiency of the gasification. Dif-in comparison with the real case and will not intro-ferent kinds of chemical reaction models are used toduce big computing error. At this step the plasma issimulate the fow inside the plasma reactor so as totreated in the same way as the coal powder carryingtudy the thermodynamic balance of the chemicalair because there will be little effect of its propertieIn the future, however, for a more accurate simulModetion, the effect of the plasma properties on the fowshould be taken account of in the model. Under these2.1 Basic model selectionsgiven assumptions, the numerical models for the flowesented as followsA schematic illustration of the plasma reactor for2.1.1 Gas phase modelcoal gasification is shown in Fig. 1The flow model is very complicated due to theThe three-dimensional governing equations shouldbe employed to model the flow in the reactor dactions and the plasma effect involved. according tothe strong three-dimensional characteristic caused bythe two stream fows converging together. In order tothe working conditions of the reactor and the plasmasimulate the turbulence Aow, the low-Reynold num-properties, the following assumptions are introducedber K-e model, which had been successfully applieda. There is no electric current in the reactor be-flow 4, was adpoted according to the flohind the DC plasma generator ( 31.Therefore, thecondition concernedtion and electric potential eqtion are not included in the flow-governing equations2.1.2as-solid two-phase mAnd the Lorentz force in the momentum equationmodel provided byand the Joule heat in the energy equation will also中国煤化工e model is of the la-CNMHG1988Plasma Science Technology, VoL5, No 5(2003)Outlet yFig. 2 The calculating domain and boundary locationFig 3 The mesh gridgrangian/ Eulerian kind, in which the conservashown in Fig. 2. The mesh grid, being of hexahedronequations of mass, momentum and energy for solid cell type, is shown in Fig. 3. And embedded refine-phase are written for each individual element. The ment is adopted in such areas where the two streamsgoverning equations for the gas phase are expressed of flow converging together and where the reactionin Eulerian form and are suitably modified to take is mainly taken place, to achieve both high accuracyaccount of the presendand high efficiency of the numerical simulationthe flow in the reactor is turbulent the random walkThe boundary conditions assigned for the case 1technique is employed to introduce the fluctuatingare as followsnature of the turbulent velocity field, which resultsa.Inlet 1: vr=0, v0=0, v=-5 m/in turbulent dispersion of the solid particles.0vz/Or=0,amv2/06=0,T=293K2.1.3 Chemical reaction modelb.nlet2:=0,v=0,v=-5m/s,Coal combustion and gasification reactions aredv,/ artaken place in this reactor. But it is acceptable to useC. Outlet: using the standard flow split condi-the coal combustion model to simulate the chemicaln,and assigning l to the split factor.reactive flow because of the similarity between thesed. Other boundaries are adiabatic watwo kinds of reactions (61, The model is divided intothree sub-models of the coal devolatilization, the hoThe coal particles with a diameter of 50 um are ofmogeneous combustion of gaseous volatiles and theparabolic distribution in the radial direction on therogeneous oxidation of solid char particles reinlet section according to the experimental measure-spectively. This model has taken account of the fol-ments. To form such a distribution, the parcels inlowing effects: the devolatilisation of the particles;he inlet have to be distributed as evenly as possi-the effect of the mass transfer process on the turbuble, i.e. each parcel covers about the same area, thenlence of the gas phase; the modeling of the reactivethe particle number being contained in the parcels isparticles themselves the effect of the turbulent flowgiven as a parabolic function of its radius. Altogetheron the reaction of the particles 6.there are eight layers of parcels in the radial direc-tion, each of which contains 8, 8, 16, 16, 32, 32, 32, 482.2 Mesh grid and the boundaryparcels respectively. And each parcel in different layconditionsers contains6409521,6302144,614106.2,592629.85604142,517460.5,474506.3,420814.1 particles reThe calculating domain and boundary location areTHhis distribution is shown in Fig. 4中国煤化工l989CNMHGJi Chunjun et al.: Flow Simulation and Optimization of Plasma Reactors for Coal GasificationMAGNITOF338Fig. 6 The velocity vectors on the middle sectionFig 4 The distribution of coal particles in the inletThe compositions of the coal in use for industrialanalysis are given in Table 1Table 1. The coal compositionsVolatile(%)Water(%)Fixed carbon(%)Ash (%)31.35472.3 Numerical procedureFiz. o The te e ature wot. L Mr tn the tiddle sext ior.It is proper to adopt the SIMPLE algorithm for thesolution of the gas governing equations since the Aowin the reactor is steady (71. The chemical reactions下款品are calculated by the models provided by STAR-CDAnd the differencing schemes for gas governing equations and the scalars of the chemical constituents areall UD (Upwind differencing)3 Numerical results andFig. 7 The temperature distribution on the outlet sec-analysestionThe distributions of the temperature velocity andgenerated under the condition. So this is the regionconcentrations have been obtained and the distribu- where the combustion is mainly taken placetions on the typical sections are given belowFig. 7 also shows that the high temperature flameIt can be seen from Fig. 5, Fig. 6 and Fig. 7 that area, i. e. the main reacting place, extents to the out-occur mainly in the upper region close let of the reactor, which means the reactions haveto the plasma inlet. According to the combustion not taken place completely in the reactor. And thetheory, the two necessary conditions for combustion chemical reactions have been confined in a sector re-are the flammable temperature and the proper pro-gion,of which the area is only one forth of that ofportion of combustible substance and oxygen, which the whole cross-section. Those two factorsmay be easily satisfied in that upper region by thehigh temperature plasma and the coal volatile gasI中国煤化工man attemptCNMHGPlasma Science Technology, Vol5, No5(2003)improve the fow and reaction pattern inside thereactor4 The numerical results of theInleoptimized design and theanalysesAccording to the simulation results of theeactor, three more plasma inlets have beenFig 8 The mesh grid for the new designpattern inside the reactor. For this design the reacting region area will be greatly augmented and theconversion ratio will be increased correspondinglywhich can be shown from the numerical results be-The calculating domain extended upstream anddownstream and its mesh grids are shown in Fig. 8The embedded refinement technique is also employedfor this case, which can be seen from Fig.8.Theboundary locations and definitions are all the sameFig 9 Velocity distribution on middle section for theas the first case except that the number of plasmaalets is four instead of only one. The boundary con-ditions for the case 2 are given as followsa. Inlet 1: vr=0, ve=0, v=5 m/s/0r=0,v2/06=0,T1=293K.b. Inlet 2 N 5: vr=0, v0=0, v=5 m/sOutlet: using the standard flow split condi-tion, and assigning 1 to the split factorFig10 Temperature distribution on the middle sectiond. Other boundaries are adiabatic wallsfor the case 2In order to see the influence of the plasma flow ve-locity, additional case 3 and case 4 were run with anreacting regions and the termperature field downinlet plasma flow velocity of 10 m/s and 15 m/s,re-stream is more uniform, which also indicate the in-spectively, while all the other parameters were keptcrease in the chemical reactions. But there is still athe same as in the case 2cool flow region between the two main reactinggions as shown in the Fig. 10, indicating that someThe temperature and velocity distributions in thecoal powder have not yet been reacted until the outreactor are shown in Fig. 9 to Fig. 14let of the reactor. This means that theFig. 9 and Fig. 10 show that the plasma flows inthe circumferential direction cause more chemically中国煤化T3 some coal powdersthe reactorCNMHGJi Chunjun et al.: Flow Simulation and Optimization of Plasma Reactors for Coal Gasificationwig.l1 Velocity dislribution. on aiddle ection kcr theFig 12 Tempa:sture distribation on the middle ectiono tae cese 3DDFix. 13 Velocity distritution cr midol: sec ion or theFig14 Temperature d cr n the middle sectionfor tbe case 4casecase 3case 4Fig 15 The temperature distribution on the downstream cross-sectionThe velocity of the plasma flow will affect the Howbigger the flow velocity is, the better thefield and the temperature field notably, which is in re-will mir with the coal powder and airsponse to the simulated results by Chen Xi 9, WhatTHE中国煤化工acting regions willCNMHGPlasma Science Technology, Vol5, No 5(2003)be, which willwing aspects. Firstly, we have to take account ofreactor when the speed is 15 m/sthe actual flow situation, including the variation ofFig. 15 shows the temperature distributionsthe plasma properties with temperature, the morethe cross-sections for the cases 2 N 4. The more andaccurate chemical reaction formulas along with themore uniform temperature distributions suggest that Lorentz force and Joule heat caused by the electricthe chemical reactions will have distributed more and current in the plasma generator. Secondly, we havemore evenly. The coal powder loss without reactioto analyze more cases with different reactor designswill thereby decrease greatly. It can be drawn from and different boundary conditions, such as the swirlthe decrease in the high temperature regions beingnumber of the plasma flow and the influence of thecoupled with the temperature distributions on the plasma inlet directions, to get more optimized de-middle section that the chemically reacting regions signs.ill be adjacent to the main inlet when the velocityof the plasma is large. This will allow comparablysufficient reactions to take place within the reactorhich will also remarkably increase the conversionratioReferences5 Discussions1 Kushner M J, Collison W Z, Grapperhaus JThe numerical simulation of the actual situation ofet al. A three-dimensional model for inductivelthe fow in the plasma reactor is considerably morepled plasma etching reactors: Azimuthal sym-accurate than that of the three-dimension flow char-metry, coil properties, and comparison to experi-acteristics and the coal combustion effect added inment.J.App.Phys,1996,80(3):1337~13442 Khater M H, Overzet. L J, Cherrington B EEtchthe flow simulation. The numerical simulation re-uniformity optimization in low pressure induc-sults and their analyses show that: firstly, the chem-tively coupled plasmas: Source design and gasically reacting regions with the four plasma inlets indistribution effects, in Proc. Plasma Etch Userthe circumferential direction of the reactor are relaGroup, Northern California Chapter/Amer,Vactively uniform; secondly, the reacting region withinSoc.1998.97the reactor can extend to the whole cross-section if3 Chen X Mass flow of high temperature ionizedhe velocity of the plasma flow is reasonably high,gas. Beijing: Science Press, 1993(in Chinese)4 Dilawari A H, Szekely J. The Role of Transportfor which there can be found an optimized value bythe numerical analyPhenomena in the Plasma Synthesis of Fine Par-Some assumptions and their simplifications em-lurgical Transaction B, 20B: 243loyed in this paper will cause some inaccuracy in the5 Smoot L D, Smith PJ. Coal combustion and Gassimulated results of the reactor, albeit those errorsfication, Plenum Press, 1985re limited and won't completely spoil the simula6 Zhou L. Numerical simulation of two-phase turbu-tion. Moreover, the simulation we have made is stilllent flow and combustion. Tsinghua Univ. Pressvery limited, many other factors, which will change7 Paik S, Chen Xi, Kong P, et al. Plasma Chemistrythe flow pattern and reaction pattern within the re-and Plasma Processing, 1991, 11(2):299actor,such as the plasma fow directions etc, still8 Lee Y C, Hsu K C, Pfender E. ISPC-5, 1981.2need to be explored thoroughlyTherefore, further studies can be done on the fol中国煤化工E.IsPC6,1983,1CNMHGJi Chunjun et al.: Flow Simulation and Optimization of Plasma Reactors for Coal Gasification10 Dilawari A H, Szekely ]. Fluid Flow and Heat(Manuscript received 2 April 2003)Transfer in Plasma Reactors- I. Calculation of Ve.mailaddressofZhangYinzitinazyz@sohu.comlocities, Temperature Profile and Mixing, Int.JHeat Mass Transfer, 30(11):2357中国煤化工CNMHG