Numerical Study of Waste Heat Recovery from Tunnel Kiln Utilized to Produce Rare Earth Phosphor

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1 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) Numercal Study of Waste Heat Recovery from Tunnel Kln Utlzed to Produce Rare Earth Phosphor Lnsheng We* 1, Langyn Guo*, Zhang Yafang*, Hu Zhaoj* and Tan Zhhong* Abstract An ndrect heat recovery method s adopted to recover waste heat from tunnel kln suppled by electrcty utlzed to produce rare earth phosphor. To ensure the qualty of products and the lfe of tunnel kln, a water tank s nstalled surroundng the coolng regon of tunnel kln as a heat exchanger. Meanwhle, a computatonal flud dynamcs model s developed to estmate and optmze the performance of ths waste heat recovery. The predcted temperatures are n good agreement wth measured results, and the error s wthn a reasonable range. In addton, a relatonshp between the nlet mass flow rate and the nlet water temperature s obtaned n terms of desred outlet water temperature. Usng water tank as a heat exchanger s an effectve and reasonable method to recover waste heat of tunnel kln utlzed to further process of rare earth phosphor. The 4443 kg standard coal would be saved a year when nlet water temperature s 285 K. Keywords Computatonal flud dynamcs, heat recovery, rare earth phosphor, tunnel kln, water tank. 1. INTRODUCTION Chna has an abundance of rare earth resources, and tunnel kln suppled by electrcty s wdely appled n rare earth phosphor producton. The loss of heat n a tunnel kln s manly attrbuted to hot flue gas and kln shell [1] [6]. Therefore, waste heat recovery from hot gases and hot kln surfaces s known as a potental way to reduce energy consumpton of tunnel kln. There are two prncpal methods of waste heat recovery: drect extracton of hot flue gas and ndrect heat exchange through kln surfaces n coolng regon. The former method has been fully researched [7] [9]. However, there are some problems for drect extracton of hot flue gas n a tunnel kln suppled by electrcty as follows: 1) More energy would be consumed, whch would greatly mpact the temperature of reacton regon so that reduce the product qualty. 2) It s not practcal because there s lttle use for the waste heat on ste and ts transportaton cost s too expensve. Therefore, ndrect heat recovery s regarded as a more effectve and reasonable method to recover waste heat of tunnel kln. Wth the development of computer scence and technology, Computatonal Flud Dynamcs (CFD) has been extensvely used n many felds of ndustral applcatons ncludng heat recovery from kln [10] [17]. A dynamc model, whch descrbed the behavor of a rotary kln for waste ncneraton, was presented to follow large varatons n process condtons and to be of practcal value from a control pont of vew [18]. Some numercal heat transfer models were employed for estmatng the performance of radant heat recovery exchangers appled to rotary klns suppled by fuel combuston for cement producton [19] [21]. A *School of Resources Envronment and Chemcal Engneerng, Nanchang Unversty, Nanchang , Chna. 1 Correspondng author: Tel: E-mal: welnsheng@ncu.edu.cn mathematcal model was developed to nvestgate mportant parameters varables along the coolng regon of tunnel kln and to optmze waste heat recovery system n a brck makng process suppled by coal combuston [22]. To sum up, numercal method whch used to analyss the waste heat recovery from kln has been demonstrated as an effectve approach. The qualty of rare earth phosphor materal made n Chna has reached the top level of world. The further processng of rare earth phosphor materal s a hgh energy-consumpton process. Unfortunately, the energy consumpton n ths process s almost neglected. The energy effcency of tunnel kln s very low, whch s utlzed to further process of rare earth phosphor materal. Only about 5 % s effectvely used, and the rest s lost accordng to statstcs. The am of ths paper s to nvestgate the heat recovery from a tunnel kln suppled by electrcty whch s utlzed to rare earth phosphor producton, and to obvously reduce the energy consumpton of ths process. 2. METHODS 2.1 Expermental Set-up The tunnel kln suppled by electrcty s dvded nto preheatng regon, reacton regon, and coolng regon. The man work of ths paper s to study the waste heat recovery from the coolng regon, whch s composed of the extenson secton, the connecton secton, and the ext secton, as shown n Fgure 1. In rare earth phosphor producton progress, crucbles are flled of raw materal at frst, and then move wth strpper plate n tunnel kln from tunnel nlet to outlet. The components of the rare earth raw materal are very complex ncludng Y 2 O 3, Eu 2 O 3, CeO 2, Tb 4 O 7 and so on. Its physcal propertes s qute smlar to Al 2 O 3, whose densty, specfc heat capacty, thermal conductvty and mass are 3970 kg/m 3, kj/kg K, W/m K, 1.0 kg, respectvely. The crucble s made of alumnum oxde ceramcs, whose densty, specfc heat capacty, thermal conductvty and mass are 3900

2 168 kg/m 3, kj/kg K, W/m K, 0.5 kg, respectvely. The materal of strpper plate s mullte, whose densty, specfc heat capacty, thermal Lnsheng We et al. / Internatonal Energy Journal 14 (2014) conductvty and mass are 3160 kg/m 3, 1.1 kj/kg K, W/m K, 2 kg, respectvely. (a) Tunnel nlet (b) Water Tank (c) Coolng Regon Model (d) Coolng Regon of Tunnel Kln Fg. 1. Expermental set-up.

3 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) Plenty of nsulaton brcks are nstalled around the tunnel kln n order to reduce energy loss. The structure of nsulaton layer n the coolng regon s shown n Fgure 1 (c). It can be seen that the structures of nsulaton layer n dfferent part of coolng regon are not the same. In extenson secton, the nsulaton layer s composed of seven layers, ther materals and propertes are shown n Table 1. Compared wth the extenson secton, the dfference s only that the connecton secton s lack of the frst layer, and there s no the frst and second layer n the ext secton. The temperature of rare earth phosphor at outlet of preheatng regon s about 1600 K and then ncreases to about 1800 K n the reacton regon. Water tank s arranged at the ext secton of coolng regon n order to ensure the temperature of reacton regon, and thus guarantee the product qualty and lfe of tunnel kln. The temperature n tunnel kln was measured by some hgh-temperature thermocouples (WRR-130), whch were nstalled on the outer surface of the nsulaton layer of tunnel kln. The hot electrode of thermocouples penetrated the nsulaton layer and then reaches the nner channel of the tunnel kln. The actual temperature was shown va control panel. Table 1. The materal propertes of extenson secton. Layers Materal Densty kg/m 3 Specfc heat capacty J/(kg K) Thermal conductvty W/(m K) Inner layer Alumna hollow brck Frst layer Mullte brck Second layer Mullte brck Thrd layer Hgh alumna brcks Fourth layer Lght hgh alumna Ffth layer Lght hgh alumna Sxth layer Ordnary alumnum-slcate refractor fber Numercal Smulaton The smplfed expermental devces and the precse equatons make t possble to study the heat transfer n the tunnel kln. In ths paper, some assumptons are made as follows: 1) The physcal propertes of crucble, strpper plate, and raw materal are constant. 2) The varaton of gas flow rate n the axal drecton and the effect of ar humdty are not taken nto account n the model n order to smplfy the calculaton. 3) Heat s transferred between the gas and products manly by natural convecton and radaton. 4) Steady state workng condtons. Based on the actual structure and the smplfed prncples, the physcal model s created as shown n Fgure 1(c). Fnte volume method s employed to solute equatons of mass, momentum, and heat energy conservaton. These equatons are as follows: Contnuty ρ + u (ρ ) = S t Momentum ( ρ u ) ( ρuu j ) p σ j + = + t j j C j + S u u 2 u σ = µ µδ M (1) (2) 3 j ( + ) (3) j j 3 Energy ( ρe) ( u ( ρe + p)) T + = ( k + u jσ j ) + S t 3. RESULTS AND DISCUSSION E (4) 3.1 Axal Temperature Dstrbuton wthout Water Tank Fgure 2 shows axal temperature dstrbuton of rare earth phosphor n coolng regon when the water tank s not nstalled. Obvously, t can be seen that the temperature steadly decreases along drecton of movement, meanwhle the smulated and measured results agree very well wth each other. In addton, the energy whch s emtted through the tunnel kln surface s W accordng the calculaton of the smulaton results. It shows that a lot of potental energy can be recycled. 3.2 Influence of Water Tank Length In the study, the outlet water temperature s one of the most sgnfcant characterstcs. In the numercal model, outlet water temperature can be calculated va equaton 5. T t A A( ρ Tt v n ) da = (5) ( ρ v n ) da Fgure 3 shows outlet water temperature as functon of water tank length at varous flow rates when nlet water temperature s 290 K. It can be observed that the outlet water temperature slghtly ncreases wth

4 170 ncreasng water tank length. The smulated outlet water temperature ncreases from 347 K to 353 K at flow rate Lnsheng We et al. / Internatonal Energy Journal 14 (2014) of m 3 /h. Ths s to say that water tank length has lttle effect on the outlet water temperature. Temperature (K) 2000 Smulated 1800 Measured 1600 (Wthout water tank) X-coordnate (m) (a) (b) Fg. 2. Axal temperature dstrbuton of rare earth phosphor n coolng regon wthout water tank.

5 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) Outlet water temperature (K) m 3 /h (smulated) m 3 /h (smulated) m 3 /h (smulated) m 3 /h (measured) m 3 /h (measured) m 3 /h (measured) Water tank length (mm) Fg. 3. Outlet water temperature varaton wth dfferent water tank length at varous flow rates. 3.3 Influence of Flow Rate Inlet Water Temperature For smplfcaton, water tank length s set to 2.0 m n followng secton because water tank length has lttle nfluence on outlet water temperature when water tank length ncreases from 1.8 m to 2.8 m. The factory s located n Ganzhou, Jangx provnce of Chna, nlet water temperature ranges from about 270 K to 300 K all the year round. Therefore, the varaton of nlet water temperature s taken nto account n the smulaton. Fgure 4 depcts the outlet water temperature varaton wth dfferent nlet water flow rate and temperatures when the water tank length s 2.0 m. It can be easly observed that outlet water temperature ncreases wth ncreasng nlet water temperature and decreasng nlet water flow rate. The outlet water temperature reaches the hghest value of K at nlet water flow rate of m 3 /h and nlet water temperature of 300 K. Outlet water temperature s desred to reach approxmately 343 K, the heat water can be used to wash the rare earth phosphors obtaned n tunnel kln. In order to gan desred outlet water temperature, a functon of thrd order between the nlet mass flow rate V (g/s) and the nlet water temperature T (K) s obtaned as shown n Fgure 5 as follows: V = T T T The nlet water flow rate can be determned accordng nlet water temperature va Equaton 6 n practcal rare earth phosphor producton process. 3 (6) Fg. 4. Outlet water temperature varaton wth dfferent nlet water flow rates and temperatures.

6 172 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) Inlet water mass flow rate (g/s) Inlet water temperature (K) Fg. 5. Optmzaton relatonshp for nlet water temperature and nlet water flow rate. 3.4 Axal Temperature Dstrbuton wth Water Tank The nstallaton of water tank has lttle effect on rare earth phosphor temperature, and furthermore has no effect on the qualty of products and the lfe of tunnel kln as shown n Fgure 6. Thus, water tank nstalled around the coolng regon of tunnel kln as a heat exchange s a feasble way to recovery waste heat. The smulated results and the measured results agree very well wth each other, whch shows the model s enough relable. The nlet water flow rate s m 3 /h when nlet water temperature s 285 K n terms of the relatonshp of Equaton 6. The heat water of 1468 kg s obtaned every day at desred temperature of 343 K. The power transferred to water s 4070 W accordng the optmzaton results. The energy of J would be saved a year, and the equvalent quanttes of standard coal s 4443 kg a year. Ths shows that usng water tank as a heat exchanger s an effectve and reasonable method to recover waste heat of tunnel kln utlzed to further process of rare earth phosphor. Temperature (K) Smulated (wth water tank) Measured (wth water tank) Measured (wthout water tank) X (m) (a)

7 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) (b) Fg. 6. Axal temperature dstrbuton of rare earth phosphor n coolng regon wth water tank. 4. ERROR ANALYSIS In order to test the accuracy of numercal smulaton results, error analyss s necessary. The smulaton errors are manly conssts of physcal modellng errors, dscretzaton errors, computer round-off errors and teratve convergence errors. 4.1 Physcal Modellng Errors Physcal modellng errors arse from mathematcal model form assumptons, boundary condtons and ntal condtons. In ths smulaton, the adopted ntal condtons and boundary condtons are close to the actual workng condtons. These assumptons and smplfcatons made n the smulaton are reasonable, and ther effects are too small to be consdered. 4.2 Dscretzaton Errors Dscretzaton errors are defned as the dfference between exact soluton to the dscrete equatons and analytcal soluton to the partal dfferental equatons. These errors occur from the algebrac expressons of the governng flow equatons and other physcal models n a dscrete doman of space and tme. Ths type of error arses n all numercal methods. Therefore dscretzaton errors are also termed as numercal errors. It s related to the approxmate representaton of a parameter whch vares contnuously n space by some polynomal functon for the varaton across a mesh cell. The dscretzaton errors wll tend to zero when the number of grd ponts ncreases and the sze of the grd spacng tends to zero. As the mesh s refned, the soluton should become less senstve to the grd spacng and approach the contnuum soluton. Ths s grd convergence. In ths smulaton, the sze of the grd spacng s small enough. 4.3 Computer Round-Off Errors Computer round-off errors occur when exact soluton cannot be extracted from dscrete equatons. They result from precson floatng-pont numbers whch can only represent dscrete ponts on the real number lne. These errors are unmportant, but sometmes they cause major naccuracy or may prevent convergence. In ths smulaton, the computer round-off errors are mnmzed by double precson method. 4.4 Iteratve Convergence Errors Iteratve convergence errors exst because the teratve methods used n smulaton must have a stoppng pont eventually. The tme consumng teratons are generally truncated close to the fnal soluton. In ths smulaton, the teratve convergence errors reduce wth ncreasng teratve computng tme. 4.5 The Inspecton of Smulaton Errors In order to further test the error of the model, the relatve smulaton error s ntroduced, whch s calculated by equaton as follow: C M ε = 100% M (7) Fgure 7 shows the dstrbuton of the relatve error between smulaton and measurement results. It can be seen that the relatve smulaton error s enough small, and only a few errors are greater than 15 %. The average relatve smulaton errors wthout water tank and wth water tank are 7.71 % and 8.82 %, respectvely. The maxmum average relatve error between smulaton and measure s less than 10 %, whch s wthn a reasonable range. In summary, the results of error analyss ndcate that the CFD model s feasble and relable.

8 174 Lnsheng We et al. / Internatonal Energy Journal 14 (2014) Relatve smulaton error (%) Wthout water tank (Fguer 2 (a)) Wth water tank (Fguer 6 (a)) X-coordnate (m) Fg. 7. Dstrbuton of the error between smulaton and measurement results. 5. CONCLUSIONS A water tank as a heat exchanger s nstalled n coolng regon of tunnel kln utlzed to further process of rare earth phosphor n order to recover waste heat of tunnel kln. Meanwhle, a CFD model s developed to estmate and optmze the performance of ths ndrect heat recovery. 1) The numercal smulaton results are n good agreement wth the expermental data. The maxmum average relatve error between the smulaton and experment s 8.82 %, whch s wthn a reasonable range. The numercal smulaton method s very smple and sutable for ths heat recovery applcaton. 2) The outlet temperature ncreases wth ncreasng nlet water temperature and decreasng water flow rate. In order to gan desred outlet water temperature, a relatonshp between the nlet mass flow rate and the nlet water temperature s obtaned. And the nlet water flow rate can be determned accordng nlet water temperature n practcal rare earth phosphor producton process. 3) Usng water tank as a heat exchanger s an effectve and reasonable method to recover waste heat of tunnel kln utlzed to further process of rare earth phosphor. The 4443 kg standard coal would be saved a year when nlet water temperature s 285 K. In addton, the qualty of products and the lfe of tunnel kln are guaranteed. CFD NOMENCLATURE Equatons 1 to 4: ρ densty, kg/m 3 t u σ j p Computatonal Flud Dynamcs tme, s x -drecton velocty component, m/s stress tensor statc pressure, pa µ dynamc vscosty, N s/m 2 k T thermal conductvty, W/(m K) temperature, K E total energy per unt mass, J S mass source term C S momentum source term M energy source term S E Equaton 5: outlet water temperature, K A area of the outlet, m 2 velocty vector v ρ flud densty, kg/m 3 a unt vector normal to the surface T t n Equaton 7: C M ε smulated result measured result relatve smulaton error ACKNOWLEDGEMENT The project was supported by the Scence and Technology Pllar Program of Jangx Provnce, Chna (20122BBG ). REFERENCES [1] Chakrabart B.K., Investgatons on heat loss through the kln shell n magneste dead burnng process: a case study. Appled Thermal Engneerng 22: [2] Madlool N.A., Sadur R., Hossan M.S. and Rahm N.A., A crtcal revew on energy use and savngs n the cement ndustres. Renewable and Sustanable Energy Revews 15: [3] Sh D.L., Vargas W.L. and McCarty J.J

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