Particle to Gas Heat Transfer in a Circulating Fluidized Bed Riser

Transcription

1 Enineerin Conference International ECI Diital Archive 10th International Conference on Circulatin Fluidized Bed and Fluidization echnoloy - CFB-10 Refereed Proceedin Srin Particle to Ga Heat ranfer in a Circulatin Fluidized Bed Rier Yair Makkawi Heriot-Watt Univerity Follow thi and additional work at: htt://dc.enconfintl.or/cfb10 Part of the Chemical Enineerin Common Recommended Citation Yair Makkawi, "Particle to Ga Heat ranfer in a Circulatin Fluidized Bed Rier" in "10th International Conference on Circulatin Fluidized Bed and Fluidization echnoloy - CFB-10",. Knowlton, PSRI Ed, ECI Symoium Serie, (2013. htt://dc.enconfintl.or/cfb10/46 hi Conference Proceedin i brouht to you for free and oen acce by the Refereed Proceedin at ECI Diital Archive. It ha been acceted for incluion in 10th International Conference on Circulatin Fluidized Bed and Fluidization echnoloy - CFB-10 by an authorized adminitrator of ECI Diital Archive. For more information, leae contact franco@bere.com.

2 PARICLE O GAS HEA RANSFER IN A CIRCULAING FLUIDIZED BED (CFB RISER Yair. Makkawi Euroean Bioenery Reearch Intitute (EBRI, School of Enineerin and Alied Science, Aton Univerity, Birminham B4 7E, United Kindom : +44 ( ; E: y.makkawi@aton.ac.uk ABSRAC he main objective of thi tudy wa to meaure the heat tranfer from article to a in a dilute CFB rier ( 0. 8 and to derive a redictive equation for the local article-a heat tranfer coefficient. hi coefficient wa found to be a tron function of the article velocity, concentration and lenth of the heat tranfer ection. Accordinly, a new correlation in term of thee arameter i rooed. INRODUCION Studie on heat tranfer in a Circulatin Fluidized Bed (CFB have been mainly focued on two characteritic heat exchane mechanim (i exchane between the wall to a or article (ii exchane between the a to article. While coniderable amount of reearch ha been conducted on the firt mechanim, which i relevant to fluidized bed boiler, limited attention ha been aid to the econd mechanim. Particle to a heat tranfer i of articular interet in CFB thermo-chemical rocee, uch a bioma thermal converion and the new emerin technoloy of chemical looin. In bioma aification/yrolyi, circulatin hot article, uch a and and char, are ued to drive the thermochemical converion of freh bioma feed via article-a heat exchane. While in chemical looin, hot metal article are circulated between two reactor a a mean for carryin oxyen. hi roce i alo relevant to dryin and catalytic reaction. MOIVAION OF HIS SUDY Review of the ublihed literature on heat tranfer in dilute-hae uenion, uch a in a CFB rierndicate coniderable uncertaintie with reect to heat tranfer coefficient. able 1 how two of the mot widely ued correlation and two recently develoed one. he literature contain a fairly lare number of tudie, uin the correlation of Ranz and Marhall (1 and Gunn (2n the imulation of article-a heat tranfer in a CFB, e.. Xie et al. (3 and Watanabe and Otaka (4. We recently carried out a numerical imulation, Fluent code (5, to invetiate the article-a heat tranfer in dene uenion (bubblin bed and dilute uenion (CFB rier. Uin Gunn correlation, the latter cae ha hown oor areement with exerimental meaurement, while excellent areement ha been noticed for the cae of dene uenion a hown in Fi. 1. he ame wa oberved when alyin Ranz 1

3 and Marhall correlation (1. It hould be noted thatn both cae, the hydrodynamic were in ood areement with the exerimental meaurement (not hown here. able 1. Particle-a heat tranfer correlation Correlation Author Rane (a Nu Nu Re Pr Ranz & Marhall ( Re Pr Re 10< Re<10 4 Pr (b '' (c Nu Re Fm Fe '' (d Nu Re Fm Fe Pr Gunn (2 0.3< <1.0 Rajan et al. (6 Rajan et al. (6 dilute hae dene hae Fiure 1. Simulation and exeriment reult of uenion temerature in (a bubblin bed with initial cold bed fluidized by hot air and (b CFB with hot article inlet and cold fluidizin air. Rajan et al. (6 recently develoed a new correlation for article-a heat tranfer coefficient in a vertical neumatic conveyer oeratin at low article to a flow ratio in the rane of F m he reorted correlationven in Eq. (c and Eq. (d in able 1, were derived from data collected at the inlet and exit of the conveyer, thu rereentin the overall averae erformance acro the whole conveyer heiht. he coefficient were cateorized to dilute and dene flow reime, however the author did not ive clear cut boundarie between what they call dene and dilute. In thi tudy, the main objective wa to carry out localized meaurement in a CFB rier oerated at low uenion denity within the rane of 0. 8, thi correond to a article-a flow ratio of 0.5 F m 10. he data wa then ued to derive a imle correlation for article-a heat tranfer coefficient. 2

4 EXPERIMENAL Set-u and Procedure Fi. 2 how a chematic diaram of the circulatin fluidized bed, which mainly comrie a 163 cm heiht and 5.2 cm diameter rier, connected to a rimary and econdary cyclone and a article feedin/receivin tank. he fluidizin air flow rate wa meaured by two rotametervin a maximum flow rate of 1300 lit/min. he averae uenion temerature and reure alon the rier were meaured by eiht thermocoule and reure tranducer. wo electric heatin element, wraed around the article feedin tank, were ued for heatin the article u to a maximum temerature of 100 ºC. he rier, downer and article tank were all thermally inulated to minimize heat loe. Gla bead and and in the rane of 235 m to 700m were ued a the bed material. he exeriment tart by loadin the feedin tank with around 10 k of article. he article are then heated to the deired temerature uin the electric heater at a elected et oint. At fluidization, the reure and temerature of the uenion alon the rier Fiure 2. Circulatin fluidized bed were loed at the rate of 1 Hz. he article ma flow rate wa determined by relatin the article ma to collectin time. o enure comrehenive data, a total of 48 different combination of oeratin condition were conidered. Data Analyi Method If we aume uniform cro-ectional article and a ditribution and neliible downfall of article at the wall, then local heat tranfer coefficient alon the rier can be etimated by dividin the rier to a number of axial ement. Fi. 3 how a chematic rereentation of thi method. he heat tranfer in each ection wa calculated uin the meaured a/article flow rate and temerature at the end of each ection uch that: q m C ( 1 mc, ( 1,, i (1 he averae void fraction ( for each bed ection of lenth ( z wa determined from reure dro meaurement ( P uch that, P 1 (2 z 3

5 With the available information on temerature and reure variation alon variou ection of the rier, the local article-a heat tranfer coefficient wa determined by relatin the localized heat tranfer ( q to the article urface area ( A and the temerature difference between the two hae ( a follow: q (out (out h (3 A he article urface area wa related to the etimated void fraction, uch that, A 6(1 Ac z d (4 where A c i the rier cro-ectional area and d i the article diameter. he heat tranfer drivin force ( wa iven in term of the lo mean temerature a follow: ( ( lo[( (, i 1 1, i1, i1 ] (5 In obtainin the article temerature, we tart by aumin that the meaured temerature alon the rier rereent the uenion temerature or bed temerature (a and article mixture. Hence, thi can be exreed a volume-weihted averae temerature: mix ( 1 (6 he exit air temerature in each ection i obtained from Eq. 6 a follow: 1 mix, i1 (1 i1 (1 i1, i1 (7 Equatin both ide of Eq. 1, the exit article temerature in each ection i iven by: m C, 1 ( 1 mc, m C, mc, (8 akin, and ubtitutin Eq. 7 into Eq. 8ve 1 (1 ( 1, i i mix, i1, i i i i (9 (i+1 (i+1 Section (i (i (i z (in m (in, m Fiure 3. Data analyi method Fiure 4. Ga and article temerature alon the CFB rier. Inlet article tem=70 o Cnlet a tem=18 o C 4

6 RESULS AND DISCUSSION Variation of emerature, Preure and Particle Concentration Examle of the variation in the temerature, reure and article concentration alon the CFB rier are hown in Fi. 4 and 5. hee fiure tyically demontrate the reat comlexity and vat variation in the hydrodynamic and heat tranfer in uch ytem. he temerature rofile hown in Fi. 4 indicate tee variation at the bottom art of the rier, within the rane of ~40 cm above the article entrance (thermal entrance lenth. It i interetin to note that both hae are at thermal equilibrium at the heiht of ~140 cm, jut few centimeter below the rier exit. However, for hiher article temerature and article loadin, thermal equilibrium at exit wa never reached. hi i exected for uch a relatively hort rier. he reure and article volume fraction alon the rier are hown in Fi. 5. Similarly, the variation are tee at the dene bottom zone. It i alo interetin to note the inificant entrance effect on article concentration near the article feedin oint (10 cm from bottom. hi i of inificant imortant ince mot of the heat tranfer take lace in thi reion. Fiure 5. Preure and article concentration variation alon the rier for la bead of 700 m diameter at two different oeratin condition. Particle-Ga Heat ranfer Coefficient Particle-a heat tranfer coefficient in a dilute CFB i believed to be dominated by article/a axial convection. hi behavior i rather different than what i oberved in CFB boiler and combutor, where the roce i controlled by radial convection and conduction between the bed and wall. hereforen term of oeratin condition, we are intereted in etablihin relation between the local heat tranfer coefficient and article/a axial velocitie and article concentration. 5

7 For the rane of oeratin condition conidered in thi tudy the article-a heat tranfer coefficient wa found to fall within the rane of Fi. 6 how an examle of the variation in heat tranfer alon the rier. A exected, raid heat tranfer take lace at the bottom reion, which then decreae toward the to. he thermal entrance lenth increae with increain the article to a flow ratio. Fi. 7 how the local heat tranfer coefficient a function of the article concentration and axial velocity for data collected from different ection of the rier and at all rane of oeratin condition conidered. he cro-ectional local averae article concentration wa obtained from reure dro meaurement (Eq.2 and the averae local article axial velocity wa obtained from article ma flux meaurement uch that: u G Fiure 6. article-a heat tranfer a function of rier heiht. Inlet article tem=70 o Cnlet a tem= 18 o C ( 1 (10 In Fi. 7at i oberved that the heat tranfer coefficient decreae with increain article concentration. hi i exactly ooite to CFB wall-bed heat tranfer, where the heat tranfer coefficient i directly roortional to article concentration, mainly due to the dominated radial conduction and convection throuh the dene wall layer. In dilute CFB, the heat tranfer mechanim i rather different, dominated by axial convection. A the article concentration decreae, a-article convection diminihe, but at the Fiure 7. Local heat tranfer coefficient a function of article concentration and velocity 6

8 ame time the a convection comonent become imortant. hi i conitent with the obervation reorted by Rajan et al. (6 and Bandrowki et al. (7, where it i wa hown that the heat tranfer coefficient decreae with increain article concentration within the limit of In Fi. 7b the local heat tranfer coefficient i oberved to conitently increae with increain the axial article velocity. PROPOSED CORRELAION FOR HEA RANSFER COEFFICIEN he analyi hown in ection 4.2 uet that the local article-a heat tranfer coefficient in the CFB rier can be bet correlated with article velocity, concentration and lenth of the heat tranfer ection. Hence, dimenional analyi lead to the followin function: (1 z h f Re ', d (11 ' where the Reynold number, Re exreed in term of the article velocity (i.e. ud. Uin rereion analyi, the followin rooed correlation for heat tranfer coefficient i obtained: h ' 0.871(1 z 8.4(Re d (12 hi relation i valid for 0. 8 and ' 0.1 Re 200. Fi. 8 how the meaured heat tranfer coefficient for all rane of oeratin condition conidered in the tudy aaint the value obtained from Eq. 12. Fiure 8. Correlation for heat tranfer coefficient Comarion of the meaured heat tranfer coefficient and value obtained from literature correlation iven in able 1 i hown in Fi. 9. Generally, the data i cattered; however, Rajan correlation for dene hae (Eq. d aear to ive the cloet match with our exerimental meaurement. he other two Fiure 9. Prooed heat tranfer coefficient 7

9 correlation, of Gunn (Eq. b and Rajan for dilute hae (Eq. c, coniderably deviate from our meaurement. Reult from Eq. (a are omitted a it ive almot the ame value a that from Gunn correlation. CONCLUSION Particle-a heat tranfer coefficient in a dilute CFB rier ( 0. 8 ha been found to be a tron function of the article axial velocity, concentration and lenth of the heat tranfer ection. A new correlation equationn term of thee three arameter ha been develoed. he reent exerimental data for article-a heat tranfer coefficient ha alo been comared with elected correlation from the literature. Work i in rore to extend thi correlation to hiher temerature and to incororate it in Fluent CFD code for the imulation of a CFB aifier. ACKNOWLEDGEMEN he author acknowlede the hel of Mr. Joeh Eke in the exerimental run. NOAION A c cro-ectional area of the rier (m 2 Re Reynold number, = u d / A urface are of article (m 2 Re Reynold number, = Ud / C ecific heat caacity (J k -1 K -1 temerature ( o C d article diameter (m u velocity (m -1 F e Fedorov number,=d [4 2 ( / -1/3 2 ] 0.3 (- U uerficial a velocity (m/ F m article to a flow ratio (- z lenth of heat tranfer ection (m G Particle flux (k m -3 Greek ymbol h article-a heat tranfer coefficient ( W m -2 K denity (k m -3 m ma flow (k/ void fraction (- Nu Nuelt number, =hd /k (- P reure (N m -2 Subcrit Pr Parndtl number, =C /k (- a Q heat tranfer (W article REFERENCES 1. Ranz, W. E. Marhall, W. R. (1952. Evaoration from dro, Chem. En. Pro. 48 (Part I ( Gunn, D. J. (1978. ranfer of Heat or Ma to Particle in Fixed and Fluidied Bed. Int. J. Heat Ma ranfer, 21, Xie, D., Bowen, B. D., Grace, J. R., Lim, C. J. (2004. wo-dimenional model of heat tranfer in circulatin fluidized bed. Part I: Model develoment and validation, Int. J. of Heat and Ma ranfer 46 ( Watanabe, H. Otaka, M. (2006. Numerical imulation of coal aification in entrained flow coal aifier, Fuel 85, Fluent 6.3, Fluent 6.3 Uer Guide. Lebanon, NH, 1996, Any Inc. 6. Rajan, K. S., Dhaandhan, K., Srivatava, S.N., Pitchumani, B. (2008. Studie on a olid heat tranfer durin neumatic conveyin, Int. J. of Heat and Ma ranfer 51, Bandrowki, J., Kaczmarzyk, G. (1976. Ga-to-article heat tranfer in vertical neumatic conveyin of ranular material, Chem. En. Sci.,