Characteristics of Organic Rankine Cycles with Zeotropic Mixture for Heat Recovery of Exhaust Gas of Boiler

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1 Available online at ScienceDirect Energy Procedia 75 (2015 ) The 7 th International Conference on Applied Energy ICAE2015 Characteritic of Organic Rankine Cycle with Zeotropic Mixture for Heat Recovery of Exhaut Ga of Boiler Xinming Xi, Yingyan Zhou, Cong Guo, Lijun Yang, Xiaoze Du* MOE Key Lab of Condition Monitoring and Control for Power Plant Equipment (North China Electric Power Univerity), Beijing , China Abtract In order to overcome the hortage of pure with contant phae change temperature, zeotropic mixture were employed to contruct the organic Rankine cycle (ORC) ytem for wate heat recovery of exhaut ga of boiler in coal-fired power plant. The temperature glide of zeotropic mixture can reduce the mimatch of temperature in order to make it approach ideal Lorenz cycle. Different type of hydrocarbon (HC) were elected a baic according to their phyical propertie, which were of high energy efficiency, low cot and environmental friendline for organic Rankine cycle. To promote the ecurity level, high-effective fire retardant R13I1/R245fa were added to form binary zeotropic mixture. Thermodynamic model of ORC with and without internal heat exchanger (IHX) uing zeotropic mixture a working were etablihed, by which the power generating capability per unit heat tranfer area, exergy efficiency and net power output a function of proportion of pure were obtained to acce the performance of ORC in recovering the wate heat of exhaut ga of boiler. In addition, the factor affecting performance of ORC, including volume flow rate, expanion ratio, and the preence of IHX in ytem, were invetigated to optimize the ytem deign The Author. Publihed by Elevier Ltd. Thi i an open acce article under the CC BY-NC-ND licene ( The Author. Publihed by Elevier Ltd. Peer-review Selection and/or under reponibility peer-review of under Applied reponibility Energy Innovation of ICAE Intitute Keyword: organic Rankine cycle; zeotropic mixture; exhaut ga of boiler; wate heat recovery 1. Introduction Boiler i one of the mot important equipment in coal-fired power plant, of which the heat lo of flue ga account for about 4%-8% of overall thermal energy. Organic Rankine cycle (ORC) i a promiing technology for medium or low grade heat recovery for it imple configuration, trong practicality and miniaturization. Typical heat ource of ORC application include geothermal energy [1], olar energy [2], bioenergy [3] and ocean thermal energy [4-5] etc.. The potential application of ORC in recovering * Correponding author. Tel.: ; fax: addre: duxz@ncepu.edu.cn The Author. Publihed by Elevier Ltd. Thi i an open acce article under the CC BY-NC-ND licene ( Peer-review under reponibility of Applied Energy Innovation Intitute doi: /j.egypro

2 1094 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) exhaut heat of boiler in power plant may improve energy utilization efficiency and reduce environmental pollution. The propertie of ORC working medium are the dominant factor influencing ytem performance. Numerou tudie have been carried on electing a working matching well with heat ource, which i one of the mot important tep in building organic Rankine cycle. [6-8]. Mot reearche concentrate in ingle component and few concern binary or ternary. However, no ingle pure ha been identified a optimal working medium for ORC, which i mainly due to the trong interdependence among optimal working, working condition and cycle architecture [9-10]. Another important limitation of pure i the contant evaporating temperature which i not uitable for the variabletemperature heat ource. Recently, three kind of pure and one mixture R141b/RC318 were elected a working and their parametric change were calculated by Li et al. [11] without conidering the relationhip between heat ource and working in evaporator. The reult howed that ORC with mixture working ha lower ytem efficiency than that with pure, neverthele it can extend the election of working. Wang and Zhao made a theoretical analyi of zeotropic mixture R245fa/R152a ued in low-temperature olar Rankine cycle. The thermal efficiencie with internal heat exchanger were alo dicued [12]. In further, they validated the uperiority of R245fa/R152a by experimental method [13]. The imulation of ChemCAD [14] and Cycle Tempo [15] alo diplay that zeotropic mixture can achieve higher thermal efficiencie than pure. Kai et al. [16] invetigated the cycle performance baed on experimental reult aiming at recovering exhaut energy for a dieel engine. Chy et al. [17] tudied the effect of uing mixture a working in ORC and found a potential increae of 16% and 6% in cycle increae for heat ource temperature at 150 and 250. In the preent tudy, novel HC/fire-uppreant mixture are invetigated a working for heat recovery from exhaut ga of boiler in coal-fired power plant. A a function of proportion of pure, the power generating capability per unit heat tranfer area, exergy efficiency and work output are obtained, which i beneficial for electing optimal working of ORC ytem. Different from traditional Rankine cycle, turbine outlet temperature of ORC i higher than inlet temperature of evaporator, thu internal heat exchanger (IHX) i conidered in ytem, and it performance i invetigated. 2. Thermodynamic model of ORC with zeotropic mixture Conidering the previou experimental data and environmental factor, Hydrocarbon (HC) are the bet choice a working of organic Rankine cycle, with high energy efficiency, low cot and high environmental friendline. The property of high molecular weight alkane are alway table, hence R600, R600a, R601, R601a, benzene and cyclohexane can be elected a the baic component. Due to the flammable and exploive characteritic of HC, fire retardant i added for obtaining a more environmental friendly and afety ubtitute. In thi paper, R245fa and R13I1 were ued a uppreant. R13I1 ha been found to be non-ozone depleting, micible with mineral oil and compatible with refrigeration ytem material. It alo ha an extremely low GWP value and acute toxicity. Therefore, it i conidered a a promiing alternative, epecially a a component in mixture, for HFC/HCFC. The propertie of zeotropic mixture largely depend on their compoition and concentration. Lower exergy lo can be obtained when the working temperature profile matche well with that of heat ource. Therefore, aturated vapor preure of two baic hould be bigger to get a more ignificant temperature glide. After primary creening, four mixture, R245fa+R601a, R245fa+R601, R13I1+R601a and R13I1+R601, were elected for the ubequent analyi. Garg et al. [18] ha put forward a method analyzing the flammability limit baed on Zabetaki work[19]. According to their analyi, flammability

3 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) of R601a/R601 could be uppreed by a mole fraction of 0.18 of R245fa, or 0.1 of R13I1. In thi paper, a more extenive fraction range ha been conidered for overall analyi. Phyical model of ORC wa etablihed for wate heat recovery of exhaut ga of coal-fired boiler in power plant. Two type of cycle configuration are conidered with the only difference in preence of internal heat exchanger (IHX). And two typical thermodynamic cycle with working of R245fa and R13I1+R601( ) and with the tate point, repectively, were hown in Fig. 1. (a) (b) Fig. 1. T-S diagram of ORC with pure and zeotropic mixture, repectively. (a) R245fa; (b) R13I1+R601( ) Referring to the traditional refrigerating ytem, the cycle highet temperature of the zeotropic mixture i et to be K with 10K pinch temperature for evaporator. 5K or no uperheat i ued for different working, in order to avoid entering into two-phae region at the outlet of expander and leading to team hydraulic hammer hazard. Then the temperature profile in evaporator can be obtained by the local thermal equilibrium between exhaut ga and working under the given ma flow rate. In thi tudy, the deign heat ink temperature i aumed to be K on the bai of the environmental condition and the local annual average temperature of power plant in north China. The pinch temperature in condener i et to be 5K. For the major purpoe of the preent tudy i dicuing the performance of different mixture on ORC uing recycled heat from exhaut ga. Thu it mainly focued on heat ource ide. Auming that the ientropic efficiency of turbine and pump i et to be 0.8, repectively, working ma flow driven by exhaut ga can be acquired by, m ga (h 07 -h 08 )=m (h 04 -h 03 ) (1) m mga( h07 h08) (2) h h The power generating capability of exhaut ga, W ga, Wga w net m (3)

4 1096 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) The exergy efficiency from ga to organic working, g-f, can be then obtained by, m [( h04 h02t ) T0 ( 04 02t )] g f (4) m [( h h ) T ( )] ga 07 The overall exergy efficiency, II, i, Wga II (5) m h h ) T ( )] ga[( where the ubcript 0 repreent the environmental condition, i.e., K and MPa for the preent tudy. An ORC ytem hould not only conider cycle efficiency but alo economy invetment. According to Cayer tudy [20], the overall heat tranfer coefficient, U, of heat tranfer equipment uch a preheater, evaporator, and condener multiplied heat tranfer area, A, a a normal ytem economic indicator. The value of UA can be calculated from the following model with aumed heat exchanger of counter-flow configuration. Evaporation preure, p, and aturation temperature, T,04, of working are aumed. The change of temperature of working in evaporator i obtained a, T f (UA) (6) The inlet temperature of heat ource, T,05, i known. Giving the outlet temperature, T,06, the temperature of heat ource in evaporator with UA can be expreed a, T, UA T,06 dq c m QUA T t,06 0 p, 0 c p, m c p, m dt (7) of which, T i the temperature of working with UA in evaporator. When the calculated temperature, T,UA, equal to T,05, the integrating proce come to the end, under which condition, the UA of the evaporator can be acquired by, Q UA dq UA (8) 0 t t The ratio of net power output to the product of overall heat tranfer coefficient by the area i expreed by, W = (9) ga UA

5 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) Thermodynamic model wa built and olved by MATLAB2010a and REFPROP Verion 8.0, with which, phyical propertie of working could be acquired. Ma flow rate of exhaut ga, m ga, i aumed 1kg. The REFPROP data bae ued for the propertie of the preent come from the empirical equation of tate for mixture baed on experimental data. Thee data are ued to determine the tructure, coefficient, and parameter of the correlation equation and to evaluate the behavior of the equation of tate in different region. The Kunz et al. [21] and Lemmon and Jacoben [22] model are ued for calculating the thermodynamic data in REFPROP. The imulating reult in the preent tudy are compared to the calculation of Wang et al. [12] for thermal efficiency and work output. The accuracy range between 0.21% and 0.54% for thermal efficiency, and between 0.73% and 1.19% for work output, which i acceptable for the calculation in thi paper. 3. Reult with dicuion 3.1. Power output of ORC without IHX Fig. 2. Net work output per unit working ma of ORC without IHX. Fig. 3. T-S diagram of R245fa+R601a cycle with different ma fraction. Fig. 2 how the variation of work output with different concentration of zeotropic mixture. It i obviou that HC get a higher w net than that of other under the ame ma fraction. It can be found that the work output almot increae linearly with HC ma fraction. On the apect of w net, zeotropic mixture are uperior to R245fa and R13I1, while no advantage over hydrocarbon. A hown in Fig. 3, from the ORC curve in T-S diagram, taking R245fa+R601a a example, it can be found that when the fraction of R601a increae, the aturated liquid line and aturated vapor line move left, and the aturated liquid line move larger. At the ame time, the evaporating temperature of mixture increae. All thee lead to a larger area encloed in T-S diagram and more work output, a howed in Fig. 2 and Fig Economic analyi of ORC without IHX Uing zeotropic mixture a working can ignificantly improve the work output of ORC, neverthele the total UA of evaporator, condener and regenerator increae ignificantly. Fig. 4 how

6 1098 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) the total UA of evaporator under different HC ma fraction. The UA value are affected by exhaut temperature and temperature difference between heat ource and working. A a reult, curve have a maximum, which how that UA value rie with HC ma fraction at the beginning and then decreae. Fig. 4. Total UA at different HC ma fraction.. Fig. 5. Variation of with HC ma fraction. Achieving maximum benefit from the power output and ytem invetment i critical to organic Rankine cycle ytem. The optimization of the ytem can be done by maximizing the compoite economic performance index,, which i ummarized in Fig. 5. The reult how that, in mot cae, zeotropic mixture prevail over the HC. HC have the propertie of flammable and exploive and it dangerou to be ued olely. Fire-uppreant material can ignificantly increae the ytem ecurity level with an increae of value at the ame time. Bae on comprehenive conideration of energy, economy, environmental kindne and afety, zeotropic mixture are deemed to poe a broad application propect The factor affecting the performance of ORC Fig. 6 how the pecific volume value V 4 at the turbine inlet. It can be found that the curve ha a maximum with the HC ma fraction. For ome mixture, the value of V 4 i higher than that of pure. On the other hand, another factor, volume ratio,, ha a minimum a given in Fig. 7. The uing of mixture i favorable becaue of the low value of for a majority of HC/fire-uppreant mixture compared with pure. Fig. 8 illutrate that the evaporating and condening preure decreae with increaing of HC concentration. Zeotropic mixture can reduce the material requirement and overall equipment invetment. What more, the condening preure of high HC concentration for mixture with R13I1 and mixture with R245fa are near atmopheric preure. The cooling water operate in the condener at the atmopheric preure. For the reaon that a maller preure difference for two ide in condener could reduce it cot, atmopheric preure at the working ide can increae the performance of the condener. In order to take full advantage of wate heat, reduce heat exchange area in condener, the internal heat exchanger (IHX) i uually added to the ytem. Taking HC/fire-uppreant mixture R13I1+R601a a

7 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) example, working at turbine outlet point 05t i cooled to dew point 05a (P 5a =P 5t and T 5a =T 5t ), a hown in Fig. 1(b). Similarly, 02-02a i the preheat proce before evaporator. Fig. 6. Variation of pecific volume at the inlet of turbine with HC ma fraction. Fig. 7. Variation of outlet/inlet volume flow ratio with HC ma fraction Fig. 8. Variation of evaporating and condening preure with HC ma fraction. Fig. 9. Sytem efficiencie of ORC with and without IHX. Fig. 10. Exhaut ga temperature with and without IHX. The ue of IHX make no difference to W ga. However, the available energy from heat ource i reduced. Thi phenomenon reult in an exergy efficiency increaing, hown in Fig. 9. IHX cycle alo ha

8 1100 Xinming Xi et al. / Energy Procedia 75 ( 2015 ) the benefit of increaing exhaut ga temperature for wate heat recovery ytem, which i illutrated in Fig. 10. A i known, low exhaut ga temperature will caue acid corroion to the boiler. By adding IHX, ome working with high performance, while unfortunately with low exhaut temperature, can be ued in engineering application, which greatly extend the election of working for ORC. 4. Concluion Zeotropic mixture have the character of temperature glide during phae change, which can match the heat ource well and reduce the overall ytem exergy lo. An invetigation on ORC with and without IHX ha been performed baed on novel HC/fire-uppreant mixture with different proportion. The following concluion could be made. (1) Uing zeotropic mixture a working can ignificantly increae work output, W ga, but the total UA and ytem equipment invetment alo improve. (2) Different mixture with fraction of pure lead to different cycle performance, which can extend the range of election according to requirement. (3) The variation trend of mixture with R601or R601a are approximately the ame. While the performance parameter,, V 4, and condening preure of mixture with R601a are better than that of R601. (4) By adding IHX in the ORC ytem, the UA increae and the outlet temperature of heat ource rie, which can avoid acid corroion of exhaut ga to the boiler. (5) The work output per unit UA of zeotropic mixture with low HC ma fraction are higher than thoe of pure hydrocarbon. And fire-uppreant material can ignificantly increaed the ecurity level of the entire ytem. The proportion of R245fa in mixture ha a relatively maller impact than that of R13I1 on cycle performance. Acknowledgement The financial upport from the national Natural Science Foundation of China (Grant No. U ) i gratefully acknowledged.. Reference [1] Saleh B, Koglbauer G, Wendland M, Ficher J. Working for low-temperature organic Rankine cycle. Energy. 2007;32(7): [2] Manolako D, Papadaki G, Kyriti S, Bouziana K. Experimental evaluation of an autonomou low-temperature olar Rankine cycle ytem for revere omoi dealination. Dealination. 2007;203(1 3): [3] Drecher U, Brüggemann D. Fluid election for the Organic Rankine Cycle (ORC) in bioma power and heat plant. Applied Thermal Engineering. 2007;27(1): [4] Moore FP, Martin LL. A nonlinear nonconvex minimum total heat tranfer area formulation for ocean thermal energy converion (OTEC) ytem. Applied Thermal Engineering. 2008;28(8 9): [5] Wu C, Burke TJ. Intelligent computer aided optimization on pecific power of an OTEC Rankine power plant. Applied Thermal Engineering. 1998;18(5): [6] Shengjun Z, Huaixin W, Tao G. Performance comparion and parametric optimization of ubcritical Organic Rankine Cycle (ORC) and trancritical power cycle ytem for low-temperature geothermal power generation. Applied Energy. 2011;88(8): [7] Chen H, Gowami DY, Stefanako EK. A review of thermodynamic cycle and working for the converion of lowgrade heat. Renewable and Sutainable Energy Review. 2010;14(9): [8] Wang J, Sun Z, Dai Y, Ma S. Parametric optimization deign for upercritical CO2 power cycle uing genetic algorithm and artificial neural network. Applied Energy. 2010;87(4): [9] Borukiewicz-Gozdur A, Nowak W. Comparative analyi of natural and ynthetic refrigerant in application to low temperature Clauiu Rankine cycle. Energy. 2007;32(4):

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