Gas Flow Distribution in Pelletizing Shaft Furnace

Availableonlineatwww.sciencedirect.com8c■Nc【@RcT·JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2006. 13(6): 16-20Gas Flow Distribution in Pelletizing Shaft FurnaceCAI Jiu-ju, DONG Hui, WANG Guo-sheng, YANG Jur(1. SEPA Key Laboratory on Eco-Industry, Northeastern University, Shenyang 110004, Liaoning, China; 2. Department ofChemical Engineering, Shenyang Institute of Chemical Technology, Shenyang 110141, Liaoning, China)Abstract: Through thermal test, cold state experiment, analysis and simulation of thermal process, the gas flow ditribution in pelletizing shaft furnace (PSF)was discussed. The results show that there are five flowing trends; a-mong them, the downward roasting gas and the upward cooling gas are the most unsteady, which influence flow distribution greatly, Among the operating parameters, the ratio of inflow is a key factor affecting the flow distribution.The roasting and cooling gases will entirely flow into the roasting zone and internal vertical air channels (IVAC),re-spectively, if the ratio of inflow is critical. From such a critical operating condition increasing roasting gas flow or de-creasing cooling gas flow, the roasting gas starts flowing downwards so as to enter the inside of IvAC; the greaterthe ratio of inflow, the larger the downward flowrate, Among constructional parameters, the width of roasting zoneb, width of ivac b, and width of cooling zone b3, and the height of roasting zone h, height of soakinheight of cooling zone h are the main factors affecting flow distribution. In case the ratio of b /h, or h3/h2, or h1/hzincreased, the upward cooling gas tends to decrease while therd roasting gas tends to increase with a gradual decrease in the ratio of inflowKey words: pellet; shaft furnace; fixed bed; packed bed gas flowPelletizing shaft furnace(PSF)with internal verti- er, and the heat exchange between the pellets andtional pellet roasting equipment in China. For many tenaceA gases or cooling gases takes place in the in-I air channels (IVAC) and a drying bed is a conven- roastingamong the pellets in PSF, so the gasyears, PSf has dominated the production of acidic flow distribution is the main factor affecting the heatpellets. With a sharp increase in iron and steel out- exchange of PSF. Generally, the region where theput, more and more pellets are required. However. gas flow distributes densely is always the spot whereit is more difficult to meet the requirement of PSF the heat exchange takes place intensively, and vicebecause of its inefficient output; therefore, its effi- versa. Therefore the study on direction, flow rateciency should be improved. In essence, PSF isand distribution of gas flow in PSF is of great signifkind of moving bed between fixed and fluidized icance for intensifying the heat exchange and enhanpellets are pre-dried and dried in cing the output and quality of pellets. 2Jthe drying bed, then dropped into the hearth, andAt the present time, there are few PSf abroadmoved downward at a speed of 0. 001. 001 7 m/s. and few domestic researches on the gas flow distri-During this process, pellets exchange heat with hot bution of PSF, which is just restricted to the levelroasting gases produced from combustion chambers 1970s or 1980s.3. Therefore, the gas flow distribu-and conduct oxidizing and roasting reaction in roast- tion were studied by the aid of thermal test, colding zone firstly. Then the roasted pellets are cooled stateanalsis and simulation ofby cooling air in cooling zone. Finally, the cooled thermaets are discharged. In essence, PSF is a sort oforsH中国煤化工 main influencing facgas-particle two-phase countercurrent heat exchang- the high efficiency production for PSE ndation forCN MH Glays a foFoundation Item: Item Sponsored by National Natural Science Foundof China(50334020): National Key Fundamental Research andDevelopment Project of China(2000026300)BiographyCAIJiu-ju(1948-),Male,Doctor,Professor:E-maildhdjc@126.com:RevisedDate:July20,2005Gas Flow Distribution in Pelletizing Shaft FurnaceI Theoretical Principle and methodand cold state experiment in this study.(2)Simulation of flow fieldThe gas flow distribution in PSF is mainlyer building a mathebased on gas dynamics of gas-particle two-phase mining the stream functions, control equations, andon is relatively authori- definite conditions, the pressure and velocity fieldstative to calculate the gas flow pressure drop can be obtained, thus the gas flow distribution andthrough packed beds 8. Ergun's equation isinfluencing factors can be achieved 1DJVp=-gradki e+io d-EIvllv 2 Gas Flow Distribution in PSFdp=( d, )'eThe gas flow in PSF is more complex than thatwhere p is static pressure; v is velocity vector; u is in equal-height and uniform section bulk materialcoefficient of gas dynamic viscosity; pr is density of beds due to the internal exit and inlet in PSF. Fig. 1gas; e is void fraction of bulk material beds; is is the schematic of flow trend deduced from the reshape coefficient of pellets; d is diameter of pellets; sults of cold state experiments and thermalkI, k2 are coefficients tested in experimenttestL4,5. 11. From Fig. 1, the basic flow trends in PSFAccording to Erguns equation, the gas flowing are as followsthrough the bulk material beds depends on neither(1)Roasting gas (ws) spouting out from thethe kinetics nor the potential energy of the gas flow, bocas to the bulk beds may be divided into twobut the static pressure difference. The gas flow dis- parts: one is upward roasting gas (wg), whichtribution can be determined by the energy loss flows into roasting zone or flows downwards intothrough bulk material beds, and the mainstreams of soaking zone, across preheating zone and dry bed inalong the routes with the least resisturn, finally enters the exhaust gases pipe; the othance loss from the region at higher static pressure to er (w g) is downward roasting gas, which flowsthe region at lower static pressure; moreover, the downwards into the soaking zone, and then intoroute doesn t vary due to the complexity of gas flow IVAC, and finally mixes with cooling air in IVACaround a block of pellets in PSF(2) Cooling air(w )may be divided into threeSince the gas flow in bulk material beds is influ- parts: one(w.) flows across the cooling zone, andenced by the shape and packing mode of pellets, it's then flows into IVAC; the second (u a) is upwardch as flow distribution and flow rate prob- cooling air, which enters the soaking zone, and mixesellet. however, if the bulk material beds are con-sidered as an integral porous medium, it's possibleto solve macroscopical problems such as flow direction, flowrate and distribution when gas flowsthrough a certain zone of bulk material bedsTo be true, the path line of gas flowing throughbulk material beds is flexuous. However, if the gasflowing through certain zone of bulk material beds istaken for the target object, certain beelines orcurves can beduced to reltrace. These certain beelines or curves are calledstreamlines. The flowrate between two discretionalstreamlines next to each other is equal at all timesThe methods of research are as follows中国煤化工(1) Measurement of static pressure differenceCNMHGaAccording to the relation between static pressure difference and gas flow distribution, gas flowrate and distribution can be judged through testingthe static pressure difference of typical positions inPSFL3, 9. which is theoretical basis of thermal testFig 1 Flow trend in pseJournal of Iron and Steel Research, Internationalwith the roasting gas near IVAC; the third (w a) isIt can be seen from Fig. 2 that (1) When thedownward cooling air is negligible, there are threedownwards and is introduced into the re-cooling flow trends synchronously in PSF, thereinto, aboutcooler at the bottom of PSF84% of the cooling air flows into the internal chanAmong the five flow trends mentioned above, nels, and the other part of cooling air flows into thethe downward roasting gas and upward cooling air roasting zone near IAVC while all the roasting gasesare rather unsteady, and they influence the gas flow flow through roasting zone and dry bed, and thenand static pressure distribution greatly. However, are introduced into the exhaust gas pipe. The inflocold state experiments and simulation of flow field ratio of volume flow rate of the roasting gas to cool-indicated that the two cases can not exist at the same ing air is 1:1.3, and the critical inflow ratio is 1:time because of their confronting each other, so only 0. 8one case is possible to occur synchronously, other(2)There is upward cooling air but no downwise both are impossible. If the two flow trends ward roasting gas. Gas flow in soaking zone is spare.don t exist, the roasting gas and cooling air don t(3) The gas flow distribution is approximatelydisturb each other; at this point, PSF is in a critical uniform along the section of PSF except soakingoperating condition, and the ratio of volume flow zone in PSF. When flowing through the roastingrate of the roasting gas to cooling air is called critical zone (including preheating zone), the cooling zoneflow rate ratio k. kand soaking zone, the gas flow moves at a speed ofare volume flow rate of the roasting gas and cooling 0.85,0 15 and 0. 93 m/s respectively( void columnair respectively in a critical operating condition, and velocitythe critical flow rate ratio is determined by the relative position of the inlets of roasting gas, the inlets of 3 Factors Affecting Gas Flow Distributioncooling air, IVAC and the inflow ratio of w, to wg, etc.Through cold state experiments, thermal testAccording to the simulation of flow field andand simulation of flow field.s, Il, it can be concluthermal test of JISC No. 2 PSFLII, the gas flow dis- ded that constructional and operating parameters oftribution can be obtained, as shown in Fig. 2, whose PSF are the main factors affecting flow directionleft part depicts bundles of streamlines and right flow rate and distribution of every gas flow trend.parts depicts the velocity vectors in y plane in the 4 For convenience, the ratio of standard volume flowtypical sections of PSF. The thermal test operating rate of roasting gas to cooling air is defined as inflowconditions of JISC No. 2 PSF are: capacity coeffi- ratio k,k=wgwa, and the proportion of upwardcient of 6.3(t.m 2.h), mean volume flow rate roasting gas in total roasting gas is defined as f,f=of cooling air of 11.67 m/s, mean volume flow rate wk/wgi the proportion of cooling air flowing intoof combustion supporting air of 4. 10 m"/s, mean vol- IVAC in total cooling air is defined as m, m=w/wume flow rate of blast furnace gas of 4.88 m/s(lowerAmong operating parameters, the inflow ratio kcalorific value is 3 200 kJ/m), and mean temperature at is the key factor influencing gas flow (Fig 3). If kthe end of combustion chamber of about 1 150 Ck, PSf is under a critical operating condition, theroasting gas and cooling air flow into roasting zoneand IVAC respectively, and then there are neitherupward cooling air nor downward roasting gas, i.e.hoperating condition for increasing roasting gas flowHImrate or decreasing cooling air flow rate, the roastingdodsid中国煤化工 e ratio k, the largerill biCNMHG,m=l(Fig 3). Ondecreasing the roasting gas or increasing the coolingair, the cooling air starts flowing downwards so asto enter roasting zone; the smaller the ratio k, the larFig. 2 Gas flow distribution of JISC No. 2 PSFger will be the upward flow rate, but f=l(Fig 3)Gas Flow Distribution in Pelletizing Shaft Furnace110110内2/2Fig3 Influence of operating parameters on gas flow distributionAmong constructional parameters, the ratio ofIVAC width to roasting zone width b2/b,, the ratiof cooling zone height to soaking zone height h3/h2and the ratio of roasting zone height h/h2are themain factors influencing gas flow distribution, asshown in Fig 4 and Fig. 5(1) Under a certain operating condition, k0, m<1. If theratio b2/b increases, the upward cooling air gradual(2) When the ratio h3/h2 increases, the uplyd m increases till psf is underward cooling air tends to increase while the downI operating condition; if b2/b1 continues to in- ward roasting gas tends to decrease, so the operatcrease, there is downward roasting gas which begins ing condition of kk and f