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Energy and exergy analyses of a fluidized bed coal combustor steam plant in textile industry
Fuel 183 (2016) 441–448
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Full Length Article
Energy and exergy analyses of a fluidized bed coal combustor steam plant in textile industry N. Filiz Tumen Ozdil a,⇑, Atakan Tantekin a, Zafer Erbay b a b
Department of Mechanical Engineering, Adana Science and Technology University, 01180 Adana, Turkey Department of Food Engineering, Adana Science and Technology University, 01180 Adana, Turkey
a r t i c l e
i n f o
Article history: Received 19 July 2015 Received in revised form 20 May 2016 Accepted 20 June 2016
Keywords: Exergy Fluidized bed Thermodynamic analysis
a b s t r a c t In this study, the analyses of first and second laws of thermodynamic are presented for a 6.5 MW power plant located in Adana, Turkey. The system components, examined in the present study, are listed as a fluidized bed coal combustor (FBCC), a heat recovery steam generator (HRSG), an economizer (ECO), fans, pumps, a cyclone and a chimney. All of the system components are examined one by one and the energy and exergy analyses are carried out for all of the system components. The highest value of irreversibility is observed in the FBCC, about 93% of the entire system irreversibility tracked by HRSG and ECO with 3% and 1%, respectively. The high excess air value, which is the primary origin of irreversibility, causes the heat losses from the FBCC, due to the increment in mass flow rate of the combustion gas. Moreover, the high excess air value gives rise to occurrence of low combustion efficiency in FBCC which can be decreased through decreasing flow rate of air with decreasing oxygen. Secondly, changes in the energy and exergy efficiencies are examined employing different ambient temperature. As the ambient temperature increases, the second law efficiencies of FBCC and HRSG increases but efficiency of ECO decreases. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction One of the most prominent theme is energy in conjunction with consuming of fossil fuels and environmental contamination. Presently, the leading energy resource is fossil fuels and the world has finite resources of fossil fuels. It forms an emergency for energy misery in the future. However, consumption of fossil fuels is an important reason of environmental pollution. The mentioned two problems are examined to solve by two ways stated as Ref. [1]: i. growing alternative energy resources and applications (especially renewable energy sources), ii. progress in the energy efficiency of systems which use fossil fuels. A cheap, fast and easy way to overcome develops the efficiency of systems. For this, the analysis of the performance, such as the losses and irreversibility analyses, is the core of the development actions. In this context, exergy term and exergy analysis have staminal. Exergy analysis is a useful appliance for design, analyses,
⇑ Corresponding author. E-mail addresses: [email protected] (N.F. Tumen Ozdil), atantekin@ adanabtu.edu.tr (A. Tantekin), [email protected] (Z. Erbay). http://dx.doi.org/10.1016/j.fuel.2016.06.091 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
and optimization of thermal systems. It is successfully applied to a wide variety of energy systems and beneficial information about the selection of proper component model and process procedure is provided by the application of exergy analyses in the scientific literature. Sizes and status of irreversibilities for the entire system may be recognized by exergy analysis, while potential improvements for energetic efficiency can be suggested [2–7]. There are several studies about thermodynamic analysis on fluidized bed coal combustor (FBCC) steam plant systems. Thermodynamic performance analysis of a FBCC power plant located in Turkey was performed by Eskin et al. [8]. They performed analyses for the system and subsystem one by one and created a model of the FBCC. According to the results, obtained from the developed and validated model, they identified component having the major irreversibility as FBCC. Furthermore, the first and second law efficiencies of the FBCC increased, when the ambient temperature enhanced. In another study of Eskin and Kılıc [9], the exergy analysis of a FBCC with different cooling tubes arrangements was investigated. The second law efficiency of the FBCC was examined changing the heat transfer coefficient in their study. Two important parameters which were defined in their study, called as the volume ratio and height ratio. They emphasized that increment in the height ratio caused the decrement of the effectiveness in FBCC. Moreover, as volume ratio decreased, effectiveness of the FBCC also decreased. Besides, height ratio had no major effect on
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Nomenclature ex E˙xD h LHV _ m P Q_ s T _ W
specific exergy (kJ/kg) exergy destruction (kW) specific enthalpy (kJ/kg) lower heating value of coal (kJ/kg) mass flow rate (kg/s) pressure (Pa) rate of heat transfer (W) specific entropy (kJ/kg K) temperature (K) rate of work (W)
Subscripts AF air fan CH chimney
the effectiveness of the FBCC for the high value of the volume ratio. Based on the studies of Eskin et al. [10], the effects of various parameters (steam pressure, excess air) on FBCC steam power plant were examined using the thermodynamic laws by a developed model. The model results had similarity with plant operational data in their study. They showed that when the excess air increased, efficiency of both energy and exergy decreased and the major irreversibility taken place on the FBCC component. Ozdemir et al. [11] investigated an exergoeconomic analysis of a FBCC power plant to optimize the energy for the power plant. Quantitative exergy cost balance was considered in the system and subsystem. In their study, the exergoeconomic analysis results were exhibited and the highest exergy destruction was occurred in FBCC and tracked by HRSG, VF, ECO, AF, CH and P. As a result, they demonstrated more information than exergy analysis to understand deeply the performance of the FBCC power plant. Aljundi [12] presented an experimental study, including the energy and exergy analyses of a power plant, located in Jordan. In his study, he evaluated the system components one by one. The major energy and exergy losses of power plant were determined. Furthermore, the system performance was investigated for the different ambient temperatures. He obtained that there is no efficacy of minor change in the ambient temperature on the power plant’s performance. Kaushik et al. [13] expressed that the real useful energy losses could not be expressed by only first law of thermodynamics. The study was deal with the comparison of energy and exergy analysis of coal and gas power plants. They concluded that the major energy losses occurred in boiler and combustion chamber for coal power plant and gas power plant, respectively. Although there are some studies focused on the exergetic performance of coal combustion systems [14,15], there is still required information and studies performed with industrial data that is obtained from an existing industrial plant. In this study, an elaborate exergy analysis was performed for an industrial fluidized bed coal combustor system used in an existing textile plant. The exergetic performance assessment for the system components were done in parts. Furthermore, the Grassmann diagrams, which provide quantitative data concerning the proportion of the exergy input to the FBCC system, were demostrated. 2. Steam power plant description The system that is analyzed thermodynamically is called as fluidized-bed coal combustor (FBCC) steam plant. The plant, involving a FBCC, a HRSG, an ECO, a CY, two VF, an AF, a CH and two P, has a 6.5 MW capacity. The examined fluidized-bed coal combustor steam plant is established in Adana, in southern of
comb. CY destr. ECO FBCC HRSG P VF 0
combustion gas cyclone destruction economizer fluidized bed coal combustor heat recovery steam generator pump ventilation fan reference state
Greek symbols first law efficiency second law efficiency
gI gII
Turkey. The schematic diagram of the investigated FBCC plant is represented in Fig. 1. The FBCC, which is the major part of the steam plant system, has steam volume of 10 t/h, steam pressure of 10 bar with dimensions 3 m 2 m cross-section and 12 m height. Two ventilation fans which are used as a distributor to provide the combustion air for the system, are called as primary and secondary air fan. Their capacities are with 8456 m3/h (45 kW) and 1845 m3/h (4 kW), respectively. The FBCC plant is employed using Sßırnak Asphaltite as solid fuel and it’s compounds are listed in Table 1. The solid fuel is received into the bed with a screw conveyor feeder. The FBCC component has vertical and horizontal heat exchangers, which are placed throughout the bed elevation and round the broader side of the bottom zone, respectively. The plant operating data are given in Table 2. The capacities of the feed water pump and aspiration fan which are used in the system are 11 kW and 15,000 m3/h (37 kW), respectively. In the FBCC steam plant, the feed water is pumped inside the economizer and arrives at the HRSG in which steam is generated by the agency of the heat exchanger tubes located in the HRSG. The assumptions made here can be listed as; i. a steady state plant case, ii. the ideal gas principles are kept in view for air and combustion gas, iii. the exergy of the ash is disregarded thanks to minor contribution, iv. the vicissitudes in the kinetic and potential energy are paid no attention. 3. Analysis The aim of this study is to carry out an energy and exergy analysis in order to grasp and show how the FBCC power plant works more effective and efficient. Energy and exergy balance equations generate the principal of thermodynamic relationship. Namely, the thermodynamic efficiency of the FBCC steam plant is identified with two methods which are energy and exergy efficiencies basis on first and second law of thermodynamic. The inspection of the feed water and the steam production are provided by the HRSG which includes the saturated steam in its top zone and the saturated water in its bottom zone. Meanwhile the water grade remains stable in the HRSG. When the feed water enters in HRSG, the water temperature contained in the HRSG achieves the satiation temperature. The thermodynamic tables are used to obtain properties of water, steam and combustion gases. Even, the reference surround-
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Fig. 1. Schematic diagram of the power plant.
Table 1 The properties of the solid fuel.
Moisture (w, %) Ash (%) Volatile matter (%) Sulphur (s, %) Hydrogen (h, %) Oxygen (o, %) Nitrogen (n, %) Carbon (c, %) GCV/HHV (kcal/kg) LHV (kcal/kg)
Table 3 The combustion gases’s mole fractions. Sßırnak Asphaltite
Combustion gas
Mole fraction (%)
8.15 36.67 20.03 5.36 4 2 1 22.79 4637 4426
SO2 H2 CO O2
81.60 0.42 11.16 6.82
Table 4 The component’s of the reference ambient mole fraction.
Table 2 Operating conditions of the power plant. Mass flow rate of coal Steam flow rate Steam pressure Steam temperature Comb. gas flow rate Air flow rate Water flow rate
0.5 kg/s 1.2 kg/s 750 kPa 177 °C 1.3 kg/s 1.38 kg/s 1.2 kg/s
ings are accepted as T0 = 25 °C, P0 = 101.3 kPa. TESTO 435 measurement equipment that utilizes NiCr-Ni, K-type thermocouple, is employed with the following features ranges 60, +300 °C and sensitivity 0.5 °C for the temperature measurements. Besides, the compound of the combustion gases, existing from the chimney, is measured with portable TESTO 350 device. It is working on electrochemical principles. Table 3 illustrates the mole fraction of the measured combustion gases while Table 4 shows the mole fraction of the reference surroundings.
Reference component
Mole fraction (%)
SO2 H2 CO O2
0.00020 0.00005 0.00070 20.3500
3.1. First law of thermodynamic The first law of thermodynamic remarks that energy may not be generated or exterminated. The first law analysis is performed using mass and energy balance equations as can be seen from Eqs. (1) and (2):
_ in ¼ Rm _ out Þ Mass Input ¼ Mass OutputðRm
ð1Þ
Energy Input Energy Output _ out hout Rm _ in hin Þ ¼ Net EnergyðQ W ¼ Rm
ð2Þ
The schematic diagram for all of the parts of the analyzed FBCC steam plant is demonstrated in Fig. 2.
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3.2. Second law of thermodynamic The exergy indicates the prospective work capacity of the system. Exergy is destructed while it isn’t preserved as energy. Exergy destruction is expressed as irreversibility which refers to the performance of system. Namely, exergy is described as the maximum amount of work that may be generated by a system. Exergy balance equations may be created as Eq. (6), presented in Ref. [8]:
Exergy Input Exergy Output Exergy Consumed Reversibility Exergy Destruction ¼ Net Exergy
ð6Þ
A sweeping exergy analysis of the FBCC steam plant has been exerted to underline the definitive thermodynamic performance. The exergy analysis of the steam plant is carried out analyzing the components of the system individually because of the complexity of the plant. Furthermore, the presented study encloses the computation of the exergy destruction (E˙xD) that happens in a system owing to irreversibility. The physical exergy of the solid fuel is neglected due to the minor effect of it while the chemical exergy of the solid fuel used in the FBCC is computed by Eq. (7):
exch fuel ¼ ½ðLHVÞo þ 2442wudry þ 9417s
ð7Þ
where (LHV) is the lower heating value of the solid fuel, u is the chemical exergy factor, w is the mass fraction of moisture in the fuel and s is the mass fraction of sulphur in the fuel. The chemical exergy factor of the solid fuels is computed with Eq. (8), stated as Ref. [7], utilizing the data in Table 1:
udry ¼ 1:0437 þ 0:1882ðh=cÞ þ 0:0610ðo=cÞ þ 0:0404ðn=cÞ
ð8Þ
where h is hydrogen, c is carbon, o is oxygen and n is nitrogen amount in the fuel. The physical exergy calculation can be performed using general exergy formula, given as Eq. (9):
exph ¼ ðh h0 Þ T0 ðs s0 Þ
ð9Þ
The chemical exergy of the combustion gas is reckoned with the help of Eq. (10) which is described as Ref. [16]. e exch comb ¼ RT0 Inðy=y Þ
Fig. 2. Schematic diagram of the components of the steam plant.
(a) Energy Balance equation for fluidized bed coal combustor can be figured out using Eq. (3):
_ 2 h2 þ m _ 3 h3 þ m _ 4 h4 þ m _ 6 h6 ¼ m _ 5 h5 þ m _ 7 h7 þ m _ 8 h8 þ Q_ loss;3 m ð3Þ (b) Energy Balance equation for heat recovery steam generator may be specified as Eq. (4):
_ 5 h5 þ m _ 7 h7 þ m _ 8 h8 þ m _ 16 h16 m _ 4 h4 þ m _ 6 h6 þ m _ 9 h9 þ m _ 17 h17 þ Q_ loss;4 ¼m
ð4Þ
(c) Energy Balance equation for Economizer may be reckoned with Eq. (5):
_ 9 h9 þ m _ 15 h15 ¼ m _ 10 h10 þ m _ 16 h16 þ Q_ loss;5 m
ð5Þ
ð10Þ
R and T0 refer to general gas constant and the ambient temperature. Moreover, y and ye refer mole fraction value of combustion gas component and mole fraction value of the component in the definition of the ambient that are given in Tables 3 and 4, respectively. The performance of the system is stated with Entropy generation (Sgen), described as the ratio of the exergy destruction to the ambient temperature. For much more certain results, entropy generations, mentioned as a recent approach, are computed. The entropy generation is computed with the help of Eq. (11):
_ D =T0 Þ Sgen ¼ ðEx
ð11Þ
When second law of thermodynamic is applied to the system, (a) Exergy Balance equation for Fluidized bed coal combustor may be computed with Eq. (12):
_ 2 ex2 þ m _ 3 ex3 þ m _ 4 ex4 þ m _ 6 ex6 m _ D _ 5 ex5 þ m _ 7 ex7 þ m _ 8 ex8 þ Q_ loss;3 ð1 T0 =TS Þ þ Ex ¼m ð12Þ (b) Exergy Balance equation for Heat recovery steam generator may be reckoned with Eq. (13):
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_ 7 ex7 þ m _ 8 ex8 þ m _ 16 ex16 _ 5 ex5 þ m m _ D _ 4 ex4 þ m _ 6 ex6 þ m _ 9 ex9 þ m _ 17 ex17 þ Q_ loss;4 ð1 T0 =TS Þ þ Ex ¼m ð13Þ
(b) The exergy efficiency for heat recovery steam generator is given by Eq. (18):
gII;HRSG ¼ ½ðm_ 17 ex17 m_ 16 ex16 Þ=½ðm_ 5 ex5 m_ 4 ex4 Þ
(c) Exergy Balance equation for Economizer may be computed with Eq. (14):
_ 6 ex6 Þ þ ðm _ 9 ex9 m _ 8 ex8 Þ _ 7 ex7 m þ ðm
ð18Þ
_ D _ 9 ex9 þ m _ 15 ex15 ¼ m _ 10 ex10 þ m _ 16 ex16 þ Q_ loss;5 ð1 T0 =TS Þ þ Ex m
(c) The exergy efficiency of economizer is specified as Eq. (19):
ð14Þ
ð19Þ
gII;ECO ¼ ðm_ 16 ex16 m_ 15 ex15 Þ=ðm_ 9 ex9 m_ 10 ex10 Þ (d) The exergy efficiency of cyclone is written as Eq. (20):
3.3. Overall energy efficiency
gII;CYC ¼ ðm_ 11 ex11 þ m_ 18 ex18 Þ=m_ 10 ex10
Proportion of the energy which is provided to the system is accepted as energy efficiency of the FBCC steam plant. Increment in the energy of the feed water is determined as the product for the FBCCSP.
gI;FBCCSP ¼ ðm_ 17 h17 m_ 14 h14 Þ=ðm_ 3 h3 þ WVF þ WP þ WAF Þ
ð15Þ
3.4. Exergy efficiencies The second law efficiency, described as exergy efficiency, is mentioned as the effectiveness of the system. When determining the second law efficiency of the system, the exergy proportion should be taken into consideration. The ratio of the exergy output to the exergy input is described as second law efficiency [8]. The general exergy efficiency may be specified as Eq. (16):
ð20Þ
(e) The exergy efficiency of chimney is given by Eq. (21):
gII;CH ¼ ðm_ 13 ex13 þ Q_ CH ð1 T0 =TS ÞÞ=m_ 12 ex12
ð21Þ
(f) Overall exergy efficiency Proportion of the exergy, which is ensured to the system, is adopted as exergy efficiency of the FBCC steam plant. Increment in the exergy of the feed water is described as the product for the FBCCSP.
gII;FBCCSP ¼ ðm_ 17 ex17 m_ 14 ex14 Þ=ðm_ 3 ex3 þ m_ 1 ex1 þ WVF þ WP þ WAF Þ ð22Þ
gII ¼ exergy output=exergy input ¼ 1 ðexergy loss=exergy inputÞ
ð16Þ
(a) Fluidized bed coal combustor The exergy transfer through the heat exchangers, is utilized as exergetic product to calculate the exergy efficiency in the FBCC. The supplied air to the FBCC and the difference between exergy rate of combustion gas and the sum of the exergy rates of solid fuel are used as exergetic fuel. The exergy efficiency of the FBCC may be computed with Eq. (17):
gII;FBCC ¼ ½ðm_ 5 ex5 m_ 4 ex4 Þ þ ðm_ 7 ex7 _ 6 ex6 Þ=½ðm _ 2 ex2 þ m _ 3 ex3 m _ 8 ex8 Þ m
ð17Þ
Fig. 3. Exergy destruction for the major parts of the FBCCSP.
Table 5 The properties of streams coded in Fig. 1 and calculated exergy rate values at the dead state conditions of 298.15 K and 101.325 kPa. #
Type
Mass flow rate (kg/s)
Temperature (K)
Pressure (kPa)
E˙x (kW)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 FBCC HRSG ECO CH
Air Air Coal Water-liquid Water-mix Water-liquid Water-mix Comb. gas Comb. gas Comb. gas Comb. gas Comb. gas Comb. gas Water-liquid Water-liquid Water-liquid Steam Coal (Ash) ExLOSS ExLOSS ExLOSS ExLOSS
1.38 1.38 0.5 1.4 1.4 1.2 1.2 1.3 1.3 1.3 1.3 1.3 1.3 1.2 1.2 1.2 1.2 0.15 – – – –
291 319 291 420 420 420 420 1020 634 378 373 398 395 372 374 410 450 393 – – – –
101.325 108 0 750 750 750 750 101.325 101.325 101.325 101.325 101.325 101.325 900 900 900 750 0 – – – –
0.63 1.57 10402.60 117.64 804.33 100.84 753.40 1233.92 938.85 827.07 826.42 831.58 830.83 40.79 42.65 86.66 943.29 0 2952.35 567.54 22.75 0.75
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4. Results and discussion The thermodynamic analysis is fulfilled utilizing first and second law of thermodynamic to compute the effectiveness of components of the FBCC steam plant. Moreover, the energy and exergy efficiencies of the FBCC, HRSG and ECO with the varied ambient temperatures are observed. In addition to this, entropy generation is considered due to the relation with system performance directly. The properties of streams used in exergy calculations are given in Table 5 for the FBCC steam plant. Based on the thermodynamic analysis, exergy destruction rates, energy and exergy efficiencies of main parts of FBCCSP are displayed in Figs. 3–5, respectively meantime the dispersion of exergy destruction is shown in Fig. 6. Considering the balance equations, given above, the exergy destruction, energy and exergy efficiencies
Fig. 4. Energy efficiency for the major parts of the FBCCSP.
Fig. 5. Exergy efficiency for the major parts of the FBCCSP.
Table 6 The leading results of exergy analysis for the components in FBCCSP. Components
E˙xD (kW)
gI (%)
gII (%)
FBCC HRSG ECO CY CH VF + AF + P FBCCSP
4878.71 210.15 45.01 0.65 0 81.94 5216.46
44.71 72.18 65.91 93.69 96.79 81.61 30.43
14.60 82.04 39.38 99.92 100 14.13 8.60
are reckoned and shown in Table 6. As seen in Table 6, the energy efficiencies of FBCC, HRSG, ECO, CY and CH are found as 44.71%, 72.18%, 65.91%, 93.69% and 96.79%, respectively. However, the exergy efficiencies of FBCC, HRSG, ECO, CY and CH are computed as 14.60%, 82.04%, 39.38%, 99.92%, and 100%, respectively. Fig. 4 shows that energy efficiencies for most of the components are over 0.6 except FBCC as stated Ref. [10]. The first and second law efficiencies of the FBCCSP are 30.43% and 8.60%, respectively whereas the exergy destruction in the FBCCSP is determined as 5216.46 kW. As can be seen in Fig. 3, the highest exergy destruction of the system occurs in the FBCC component. The causes of the irreversibility, occurring in FBCC, may be presented as below. i. The amount of supplied excess air, entering to the FBCC which causes the higher heat loss. The enhancement on the amount of the excess air gives rise to the rising on the combustion gas’s mass flow rate. Owing to the enhancement in mass flow rate of the combustion gas and the decrement of the FBCC temperature, heat losses from the FBCC increases and the efficiency of the FBCC decreases. The typical excess air value of the coal in the stoker can be suggested between 20% and 25% as mentioned Ref. [17]. On the other hand, the excess air value in FBCC is measured 48%, in this study. Namely, one of the main reasons of the inefficiency is high amount of excess air ratio in this steam plant. ii. The chemical reaction can be presented as the second reason due to the combustion process, happening in FBCC for the irreversibility. As seen from composition of combustion gas existing from chimney, combustion efficiency is low due to the high value of CO. High excess air ratio causes low reaction rate of coal and high amount of unburnt carbon content which means CO formation instead of CO2 formation. Namely, the reason of the low combustion efficiency is high excess air ratio which causes the increment of the heat losses from the FBCC. iii. The third one is explained as the type of the coal using as solid fuel. The change in the coal using in FBCC causes the changing of the exergy destruction due to the different calorific value of the coal. As the calorific value of the coal increases, the exergy efficiency of the FBCC increases because of the high internal energy of the coal.
Table 7 Variations of energy and exergy efficiencies with different ambient temperatures (20 and 25 °C). Temperature (°C)
Fig. 6. Distribution of exergy destruction rates for the components of FBCCSP.
gI,FBCC gII,FBCC gI,HRSG gII,HRSG gI,ECO gII,ECO
Total changes (%)
20
25
0.4298 0.1452 0.7053 0.7812 0.9277 0.4863
0.4471 0.1460 0.7218 0.8204 0.6591 0.3938
4.03 0.54 2.35 5.02 28.95 19.03
N.F. Tumen Ozdil et al. / Fuel 183 (2016) 441–448 Table 8 Variations of energy and exergy efficiencies with different ambient temperatures (25 and 30 °C). Temperature (°C)
gI,FBCC gII,FBCC gI,HRSG gII,HRSG gI,ECO gII,ECO
Total changes (%)
25
30
0.4471 0.1460 0.7218 0.8204 0.6591 0.3938
0.4644 0.1462 0.7270 0.8397 0.5156 0.3360
3.87 0.09 0.72 2.35 21.78 14.66
Fig. 7. Entropy generation of the main components.
In this study, the ambient temperature changes between 20 °C and 30 °C with 5 °C increment while keeping the pressure at 101.3 kPa in order to demonstrate the effect of the different ambient temperatures on energy and exergy efficiency of the FBCC, HRSG and ECO as can be seen in Tables 7 and 8. The variation of ambient temperature plays a decisive role on the efficiency of the FBCC, HRSG and ECO. The exergy efficiency of the FBCC and HRSG increases, while exergy efficiencies of the ECO goes down as the ambient temperature increases. These results indicate that the increment of ambient temperature has a positive impact on the efficiencies of the FBCC and HRSG. Moreover, entropy generation is calculated to show the performance of the components. The entropy generation of the components is illustrated as can be seen in Fig. 7. The high entropy generation is taken place in FBCC while the entropy generations in the other components are considerably moderate. Uncertainty analyses are implemented for the first and second law efficiencies of system, described as Ref. [4] with the help of Eqs. (23) and (24);
wg1
g1
" ¼
w 2 w 2 w 2 w 2 w 2 _ WAF WVF WP Dh m þ þ þ þ _ m Dh WVF WP WAF
#12
447
5. Conclusions The thermodynamic analysis is investigated basis on the thermodynamic first and second laws for a real FBCC steam power plant in this paper. As can be seen from the results, the highest amount of exergy destruction occurs in the FBCC component by 4878.71 kW. The fundamental justifications of the inefficiency happened in the FBCC are high amount of excess air, low combustion efficiency and type of the solid fuel. The amount of the excess air influences directly amount of the combustion gas flow rate. As the amount of the excess air, heat losses in FBCC increases due to the decrement of temperature in FBCC. Moreover, high amount of excess air which means high amount of oxygen, has a negative effect on combustion efficiency. In this study, it is understood that the combustion process is incomplete due to the unburnt carbon content of combustion gas existing from chimney. Lastly, lower heating value of the solid fuel influences the efficiency of the system. As the calorific value of the coal increases, the efficiency of the FBCC enhances due to the high internal energy of the coal. Moreover, the entropy generation results show that the highest entropy generation is occurred in FBCC like exergy destruction. The impact of the different ambient temperature on the efficiency of FBCC, HRSG and ECO is also observed using first and second law of thermodynamic. The increment of the FBCC temperature causes the increment of water quality in the FBCC due to the increment of heat transfer rate from the ambient temperature to feed water in FBCC and HRSG. The results indicate that the increment of the ambient temperature causes negative effect on the exergy efficiency of ECO, while it causes positive effect on the efficiency of FBCC and HRSG. As a common tendency, the calculated results for energy efficiency have a great similarity with the calculated results for exergy efficiency for overall system. It can be suggested that i. the implementation of the thermoeconomic analysis of the FBCC power plant systems can be investigated in the future, ii. decreases excess air amount in order to increase efficiency, iii. combustion process in the FBCC is the source of irreversibility that can be decreased through preheating combustion gas, iv. different solid fuel (coal) type which has higher calorific value, can be used in FBCC in order to increase efficiency.
Acknowledgements The authors would like to thank Kıvanç Textile Inc. which is Turkey’s major textile factory for providing operational data of the FBCC steam plant as well as their courteous help. This study is supported by the Adana Science and Technology University Research Fund under project no: MÜHDBF.MM.2015-10. It is gratefully acknowledged for the continuous incentive support.
ð23Þ 2 wm_ wDh 2 wDs 2 wT 2 ¼ þ þ þ _ m g2 Dh Ds T 2 2 2 #12 wWAF wWVF wWP þ þ þ WVF WP WAF
References
wg2
ð24Þ
‘‘w” is described as the uncertainty amount of dependent variable for the energy and exergy efficiencies. The results of the uncertainty analyses are found as 0.8% and 2.32% for energy and exergy efficiencies, respectively. The results are coherent with the results of Ozdil et al. [4].
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[13] Kaushik SC, Reddy VS, Tyagi SK. Energy and exergy analyses of thermal power plants: a review. Renew Sustain Energy Rev 2011;15:1857–72. [14] Som SK, Datta A. Thermodynamic irreversibilities and exergy balance in combustion processes. Prog Energy Combust Sci 2008;34:351–76. [15] Lee U, Park K, Jeong YS, Lee S, Han C. Design and analysis of a combined Rankine cycle for waste heat recovery of a coal power plant using LNG cryogenic exergy. Ind Eng Chem Res 2014;53:9812–24. [16] Caliskan H, Tat ME, Hepbasli A. Performance assessment of an internal combustion engine at varying dead (reference) state temperatures. Appl Therm Eng 2009;29:3431–6. [17] Energetic processes and equipments. Syllabus for FBC Lab. Bureau of Energy Efficiency; 2015. [accessed 04.05.15].
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Energy and exergy analyses of a fluidized bed coal combustor steam plant in textile industry N. Filiz Tumen Ozdil a,⇑, Atakan Tantekin a, Zafer Erbay b a b
Department of Mechanical Engineering, Adana Science and Technology University, 01180 Adana, Turkey Department of Food Engineering, Adana Science and Technology University, 01180 Adana, Turkey
a r t i c l e
i n f o
Article history: Received 19 July 2015 Received in revised form 20 May 2016 Accepted 20 June 2016
Keywords: Exergy Fluidized bed Thermodynamic analysis
a b s t r a c t In this study, the analyses of first and second laws of thermodynamic are presented for a 6.5 MW power plant located in Adana, Turkey. The system components, examined in the present study, are listed as a fluidized bed coal combustor (FBCC), a heat recovery steam generator (HRSG), an economizer (ECO), fans, pumps, a cyclone and a chimney. All of the system components are examined one by one and the energy and exergy analyses are carried out for all of the system components. The highest value of irreversibility is observed in the FBCC, about 93% of the entire system irreversibility tracked by HRSG and ECO with 3% and 1%, respectively. The high excess air value, which is the primary origin of irreversibility, causes the heat losses from the FBCC, due to the increment in mass flow rate of the combustion gas. Moreover, the high excess air value gives rise to occurrence of low combustion efficiency in FBCC which can be decreased through decreasing flow rate of air with decreasing oxygen. Secondly, changes in the energy and exergy efficiencies are examined employing different ambient temperature. As the ambient temperature increases, the second law efficiencies of FBCC and HRSG increases but efficiency of ECO decreases. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction One of the most prominent theme is energy in conjunction with consuming of fossil fuels and environmental contamination. Presently, the leading energy resource is fossil fuels and the world has finite resources of fossil fuels. It forms an emergency for energy misery in the future. However, consumption of fossil fuels is an important reason of environmental pollution. The mentioned two problems are examined to solve by two ways stated as Ref. [1]: i. growing alternative energy resources and applications (especially renewable energy sources), ii. progress in the energy efficiency of systems which use fossil fuels. A cheap, fast and easy way to overcome develops the efficiency of systems. For this, the analysis of the performance, such as the losses and irreversibility analyses, is the core of the development actions. In this context, exergy term and exergy analysis have staminal. Exergy analysis is a useful appliance for design, analyses,
⇑ Corresponding author. E-mail addresses: [email protected] (N.F. Tumen Ozdil), atantekin@ adanabtu.edu.tr (A. Tantekin), [email protected] (Z. Erbay). http://dx.doi.org/10.1016/j.fuel.2016.06.091 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
and optimization of thermal systems. It is successfully applied to a wide variety of energy systems and beneficial information about the selection of proper component model and process procedure is provided by the application of exergy analyses in the scientific literature. Sizes and status of irreversibilities for the entire system may be recognized by exergy analysis, while potential improvements for energetic efficiency can be suggested [2–7]. There are several studies about thermodynamic analysis on fluidized bed coal combustor (FBCC) steam plant systems. Thermodynamic performance analysis of a FBCC power plant located in Turkey was performed by Eskin et al. [8]. They performed analyses for the system and subsystem one by one and created a model of the FBCC. According to the results, obtained from the developed and validated model, they identified component having the major irreversibility as FBCC. Furthermore, the first and second law efficiencies of the FBCC increased, when the ambient temperature enhanced. In another study of Eskin and Kılıc [9], the exergy analysis of a FBCC with different cooling tubes arrangements was investigated. The second law efficiency of the FBCC was examined changing the heat transfer coefficient in their study. Two important parameters which were defined in their study, called as the volume ratio and height ratio. They emphasized that increment in the height ratio caused the decrement of the effectiveness in FBCC. Moreover, as volume ratio decreased, effectiveness of the FBCC also decreased. Besides, height ratio had no major effect on
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Nomenclature ex E˙xD h LHV _ m P Q_ s T _ W
specific exergy (kJ/kg) exergy destruction (kW) specific enthalpy (kJ/kg) lower heating value of coal (kJ/kg) mass flow rate (kg/s) pressure (Pa) rate of heat transfer (W) specific entropy (kJ/kg K) temperature (K) rate of work (W)
Subscripts AF air fan CH chimney
the effectiveness of the FBCC for the high value of the volume ratio. Based on the studies of Eskin et al. [10], the effects of various parameters (steam pressure, excess air) on FBCC steam power plant were examined using the thermodynamic laws by a developed model. The model results had similarity with plant operational data in their study. They showed that when the excess air increased, efficiency of both energy and exergy decreased and the major irreversibility taken place on the FBCC component. Ozdemir et al. [11] investigated an exergoeconomic analysis of a FBCC power plant to optimize the energy for the power plant. Quantitative exergy cost balance was considered in the system and subsystem. In their study, the exergoeconomic analysis results were exhibited and the highest exergy destruction was occurred in FBCC and tracked by HRSG, VF, ECO, AF, CH and P. As a result, they demonstrated more information than exergy analysis to understand deeply the performance of the FBCC power plant. Aljundi [12] presented an experimental study, including the energy and exergy analyses of a power plant, located in Jordan. In his study, he evaluated the system components one by one. The major energy and exergy losses of power plant were determined. Furthermore, the system performance was investigated for the different ambient temperatures. He obtained that there is no efficacy of minor change in the ambient temperature on the power plant’s performance. Kaushik et al. [13] expressed that the real useful energy losses could not be expressed by only first law of thermodynamics. The study was deal with the comparison of energy and exergy analysis of coal and gas power plants. They concluded that the major energy losses occurred in boiler and combustion chamber for coal power plant and gas power plant, respectively. Although there are some studies focused on the exergetic performance of coal combustion systems [14,15], there is still required information and studies performed with industrial data that is obtained from an existing industrial plant. In this study, an elaborate exergy analysis was performed for an industrial fluidized bed coal combustor system used in an existing textile plant. The exergetic performance assessment for the system components were done in parts. Furthermore, the Grassmann diagrams, which provide quantitative data concerning the proportion of the exergy input to the FBCC system, were demostrated. 2. Steam power plant description The system that is analyzed thermodynamically is called as fluidized-bed coal combustor (FBCC) steam plant. The plant, involving a FBCC, a HRSG, an ECO, a CY, two VF, an AF, a CH and two P, has a 6.5 MW capacity. The examined fluidized-bed coal combustor steam plant is established in Adana, in southern of
comb. CY destr. ECO FBCC HRSG P VF 0
combustion gas cyclone destruction economizer fluidized bed coal combustor heat recovery steam generator pump ventilation fan reference state
Greek symbols first law efficiency second law efficiency
gI gII
Turkey. The schematic diagram of the investigated FBCC plant is represented in Fig. 1. The FBCC, which is the major part of the steam plant system, has steam volume of 10 t/h, steam pressure of 10 bar with dimensions 3 m 2 m cross-section and 12 m height. Two ventilation fans which are used as a distributor to provide the combustion air for the system, are called as primary and secondary air fan. Their capacities are with 8456 m3/h (45 kW) and 1845 m3/h (4 kW), respectively. The FBCC plant is employed using Sßırnak Asphaltite as solid fuel and it’s compounds are listed in Table 1. The solid fuel is received into the bed with a screw conveyor feeder. The FBCC component has vertical and horizontal heat exchangers, which are placed throughout the bed elevation and round the broader side of the bottom zone, respectively. The plant operating data are given in Table 2. The capacities of the feed water pump and aspiration fan which are used in the system are 11 kW and 15,000 m3/h (37 kW), respectively. In the FBCC steam plant, the feed water is pumped inside the economizer and arrives at the HRSG in which steam is generated by the agency of the heat exchanger tubes located in the HRSG. The assumptions made here can be listed as; i. a steady state plant case, ii. the ideal gas principles are kept in view for air and combustion gas, iii. the exergy of the ash is disregarded thanks to minor contribution, iv. the vicissitudes in the kinetic and potential energy are paid no attention. 3. Analysis The aim of this study is to carry out an energy and exergy analysis in order to grasp and show how the FBCC power plant works more effective and efficient. Energy and exergy balance equations generate the principal of thermodynamic relationship. Namely, the thermodynamic efficiency of the FBCC steam plant is identified with two methods which are energy and exergy efficiencies basis on first and second law of thermodynamic. The inspection of the feed water and the steam production are provided by the HRSG which includes the saturated steam in its top zone and the saturated water in its bottom zone. Meanwhile the water grade remains stable in the HRSG. When the feed water enters in HRSG, the water temperature contained in the HRSG achieves the satiation temperature. The thermodynamic tables are used to obtain properties of water, steam and combustion gases. Even, the reference surround-
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Fig. 1. Schematic diagram of the power plant.
Table 1 The properties of the solid fuel.
Moisture (w, %) Ash (%) Volatile matter (%) Sulphur (s, %) Hydrogen (h, %) Oxygen (o, %) Nitrogen (n, %) Carbon (c, %) GCV/HHV (kcal/kg) LHV (kcal/kg)
Table 3 The combustion gases’s mole fractions. Sßırnak Asphaltite
Combustion gas
Mole fraction (%)
8.15 36.67 20.03 5.36 4 2 1 22.79 4637 4426
SO2 H2 CO O2
81.60 0.42 11.16 6.82
Table 4 The component’s of the reference ambient mole fraction.
Table 2 Operating conditions of the power plant. Mass flow rate of coal Steam flow rate Steam pressure Steam temperature Comb. gas flow rate Air flow rate Water flow rate
0.5 kg/s 1.2 kg/s 750 kPa 177 °C 1.3 kg/s 1.38 kg/s 1.2 kg/s
ings are accepted as T0 = 25 °C, P0 = 101.3 kPa. TESTO 435 measurement equipment that utilizes NiCr-Ni, K-type thermocouple, is employed with the following features ranges 60, +300 °C and sensitivity 0.5 °C for the temperature measurements. Besides, the compound of the combustion gases, existing from the chimney, is measured with portable TESTO 350 device. It is working on electrochemical principles. Table 3 illustrates the mole fraction of the measured combustion gases while Table 4 shows the mole fraction of the reference surroundings.
Reference component
Mole fraction (%)
SO2 H2 CO O2
0.00020 0.00005 0.00070 20.3500
3.1. First law of thermodynamic The first law of thermodynamic remarks that energy may not be generated or exterminated. The first law analysis is performed using mass and energy balance equations as can be seen from Eqs. (1) and (2):
_ in ¼ Rm _ out Þ Mass Input ¼ Mass OutputðRm
ð1Þ
Energy Input Energy Output _ out hout Rm _ in hin Þ ¼ Net EnergyðQ W ¼ Rm
ð2Þ
The schematic diagram for all of the parts of the analyzed FBCC steam plant is demonstrated in Fig. 2.
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3.2. Second law of thermodynamic The exergy indicates the prospective work capacity of the system. Exergy is destructed while it isn’t preserved as energy. Exergy destruction is expressed as irreversibility which refers to the performance of system. Namely, exergy is described as the maximum amount of work that may be generated by a system. Exergy balance equations may be created as Eq. (6), presented in Ref. [8]:
Exergy Input Exergy Output Exergy Consumed Reversibility Exergy Destruction ¼ Net Exergy
ð6Þ
A sweeping exergy analysis of the FBCC steam plant has been exerted to underline the definitive thermodynamic performance. The exergy analysis of the steam plant is carried out analyzing the components of the system individually because of the complexity of the plant. Furthermore, the presented study encloses the computation of the exergy destruction (E˙xD) that happens in a system owing to irreversibility. The physical exergy of the solid fuel is neglected due to the minor effect of it while the chemical exergy of the solid fuel used in the FBCC is computed by Eq. (7):
exch fuel ¼ ½ðLHVÞo þ 2442wudry þ 9417s
ð7Þ
where (LHV) is the lower heating value of the solid fuel, u is the chemical exergy factor, w is the mass fraction of moisture in the fuel and s is the mass fraction of sulphur in the fuel. The chemical exergy factor of the solid fuels is computed with Eq. (8), stated as Ref. [7], utilizing the data in Table 1:
udry ¼ 1:0437 þ 0:1882ðh=cÞ þ 0:0610ðo=cÞ þ 0:0404ðn=cÞ
ð8Þ
where h is hydrogen, c is carbon, o is oxygen and n is nitrogen amount in the fuel. The physical exergy calculation can be performed using general exergy formula, given as Eq. (9):
exph ¼ ðh h0 Þ T0 ðs s0 Þ
ð9Þ
The chemical exergy of the combustion gas is reckoned with the help of Eq. (10) which is described as Ref. [16]. e exch comb ¼ RT0 Inðy=y Þ
Fig. 2. Schematic diagram of the components of the steam plant.
(a) Energy Balance equation for fluidized bed coal combustor can be figured out using Eq. (3):
_ 2 h2 þ m _ 3 h3 þ m _ 4 h4 þ m _ 6 h6 ¼ m _ 5 h5 þ m _ 7 h7 þ m _ 8 h8 þ Q_ loss;3 m ð3Þ (b) Energy Balance equation for heat recovery steam generator may be specified as Eq. (4):
_ 5 h5 þ m _ 7 h7 þ m _ 8 h8 þ m _ 16 h16 m _ 4 h4 þ m _ 6 h6 þ m _ 9 h9 þ m _ 17 h17 þ Q_ loss;4 ¼m
ð4Þ
(c) Energy Balance equation for Economizer may be reckoned with Eq. (5):
_ 9 h9 þ m _ 15 h15 ¼ m _ 10 h10 þ m _ 16 h16 þ Q_ loss;5 m
ð5Þ
ð10Þ
R and T0 refer to general gas constant and the ambient temperature. Moreover, y and ye refer mole fraction value of combustion gas component and mole fraction value of the component in the definition of the ambient that are given in Tables 3 and 4, respectively. The performance of the system is stated with Entropy generation (Sgen), described as the ratio of the exergy destruction to the ambient temperature. For much more certain results, entropy generations, mentioned as a recent approach, are computed. The entropy generation is computed with the help of Eq. (11):
_ D =T0 Þ Sgen ¼ ðEx
ð11Þ
When second law of thermodynamic is applied to the system, (a) Exergy Balance equation for Fluidized bed coal combustor may be computed with Eq. (12):
_ 2 ex2 þ m _ 3 ex3 þ m _ 4 ex4 þ m _ 6 ex6 m _ D _ 5 ex5 þ m _ 7 ex7 þ m _ 8 ex8 þ Q_ loss;3 ð1 T0 =TS Þ þ Ex ¼m ð12Þ (b) Exergy Balance equation for Heat recovery steam generator may be reckoned with Eq. (13):
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_ 7 ex7 þ m _ 8 ex8 þ m _ 16 ex16 _ 5 ex5 þ m m _ D _ 4 ex4 þ m _ 6 ex6 þ m _ 9 ex9 þ m _ 17 ex17 þ Q_ loss;4 ð1 T0 =TS Þ þ Ex ¼m ð13Þ
(b) The exergy efficiency for heat recovery steam generator is given by Eq. (18):
gII;HRSG ¼ ½ðm_ 17 ex17 m_ 16 ex16 Þ=½ðm_ 5 ex5 m_ 4 ex4 Þ
(c) Exergy Balance equation for Economizer may be computed with Eq. (14):
_ 6 ex6 Þ þ ðm _ 9 ex9 m _ 8 ex8 Þ _ 7 ex7 m þ ðm
ð18Þ
_ D _ 9 ex9 þ m _ 15 ex15 ¼ m _ 10 ex10 þ m _ 16 ex16 þ Q_ loss;5 ð1 T0 =TS Þ þ Ex m
(c) The exergy efficiency of economizer is specified as Eq. (19):
ð14Þ
ð19Þ
gII;ECO ¼ ðm_ 16 ex16 m_ 15 ex15 Þ=ðm_ 9 ex9 m_ 10 ex10 Þ (d) The exergy efficiency of cyclone is written as Eq. (20):
3.3. Overall energy efficiency
gII;CYC ¼ ðm_ 11 ex11 þ m_ 18 ex18 Þ=m_ 10 ex10
Proportion of the energy which is provided to the system is accepted as energy efficiency of the FBCC steam plant. Increment in the energy of the feed water is determined as the product for the FBCCSP.
gI;FBCCSP ¼ ðm_ 17 h17 m_ 14 h14 Þ=ðm_ 3 h3 þ WVF þ WP þ WAF Þ
ð15Þ
3.4. Exergy efficiencies The second law efficiency, described as exergy efficiency, is mentioned as the effectiveness of the system. When determining the second law efficiency of the system, the exergy proportion should be taken into consideration. The ratio of the exergy output to the exergy input is described as second law efficiency [8]. The general exergy efficiency may be specified as Eq. (16):
ð20Þ
(e) The exergy efficiency of chimney is given by Eq. (21):
gII;CH ¼ ðm_ 13 ex13 þ Q_ CH ð1 T0 =TS ÞÞ=m_ 12 ex12
ð21Þ
(f) Overall exergy efficiency Proportion of the exergy, which is ensured to the system, is adopted as exergy efficiency of the FBCC steam plant. Increment in the exergy of the feed water is described as the product for the FBCCSP.
gII;FBCCSP ¼ ðm_ 17 ex17 m_ 14 ex14 Þ=ðm_ 3 ex3 þ m_ 1 ex1 þ WVF þ WP þ WAF Þ ð22Þ
gII ¼ exergy output=exergy input ¼ 1 ðexergy loss=exergy inputÞ
ð16Þ
(a) Fluidized bed coal combustor The exergy transfer through the heat exchangers, is utilized as exergetic product to calculate the exergy efficiency in the FBCC. The supplied air to the FBCC and the difference between exergy rate of combustion gas and the sum of the exergy rates of solid fuel are used as exergetic fuel. The exergy efficiency of the FBCC may be computed with Eq. (17):
gII;FBCC ¼ ½ðm_ 5 ex5 m_ 4 ex4 Þ þ ðm_ 7 ex7 _ 6 ex6 Þ=½ðm _ 2 ex2 þ m _ 3 ex3 m _ 8 ex8 Þ m
ð17Þ
Fig. 3. Exergy destruction for the major parts of the FBCCSP.
Table 5 The properties of streams coded in Fig. 1 and calculated exergy rate values at the dead state conditions of 298.15 K and 101.325 kPa. #
Type
Mass flow rate (kg/s)
Temperature (K)
Pressure (kPa)
E˙x (kW)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 FBCC HRSG ECO CH
Air Air Coal Water-liquid Water-mix Water-liquid Water-mix Comb. gas Comb. gas Comb. gas Comb. gas Comb. gas Comb. gas Water-liquid Water-liquid Water-liquid Steam Coal (Ash) ExLOSS ExLOSS ExLOSS ExLOSS
1.38 1.38 0.5 1.4 1.4 1.2 1.2 1.3 1.3 1.3 1.3 1.3 1.3 1.2 1.2 1.2 1.2 0.15 – – – –
291 319 291 420 420 420 420 1020 634 378 373 398 395 372 374 410 450 393 – – – –
101.325 108 0 750 750 750 750 101.325 101.325 101.325 101.325 101.325 101.325 900 900 900 750 0 – – – –
0.63 1.57 10402.60 117.64 804.33 100.84 753.40 1233.92 938.85 827.07 826.42 831.58 830.83 40.79 42.65 86.66 943.29 0 2952.35 567.54 22.75 0.75
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4. Results and discussion The thermodynamic analysis is fulfilled utilizing first and second law of thermodynamic to compute the effectiveness of components of the FBCC steam plant. Moreover, the energy and exergy efficiencies of the FBCC, HRSG and ECO with the varied ambient temperatures are observed. In addition to this, entropy generation is considered due to the relation with system performance directly. The properties of streams used in exergy calculations are given in Table 5 for the FBCC steam plant. Based on the thermodynamic analysis, exergy destruction rates, energy and exergy efficiencies of main parts of FBCCSP are displayed in Figs. 3–5, respectively meantime the dispersion of exergy destruction is shown in Fig. 6. Considering the balance equations, given above, the exergy destruction, energy and exergy efficiencies
Fig. 4. Energy efficiency for the major parts of the FBCCSP.
Fig. 5. Exergy efficiency for the major parts of the FBCCSP.
Table 6 The leading results of exergy analysis for the components in FBCCSP. Components
E˙xD (kW)
gI (%)
gII (%)
FBCC HRSG ECO CY CH VF + AF + P FBCCSP
4878.71 210.15 45.01 0.65 0 81.94 5216.46
44.71 72.18 65.91 93.69 96.79 81.61 30.43
14.60 82.04 39.38 99.92 100 14.13 8.60
are reckoned and shown in Table 6. As seen in Table 6, the energy efficiencies of FBCC, HRSG, ECO, CY and CH are found as 44.71%, 72.18%, 65.91%, 93.69% and 96.79%, respectively. However, the exergy efficiencies of FBCC, HRSG, ECO, CY and CH are computed as 14.60%, 82.04%, 39.38%, 99.92%, and 100%, respectively. Fig. 4 shows that energy efficiencies for most of the components are over 0.6 except FBCC as stated Ref. [10]. The first and second law efficiencies of the FBCCSP are 30.43% and 8.60%, respectively whereas the exergy destruction in the FBCCSP is determined as 5216.46 kW. As can be seen in Fig. 3, the highest exergy destruction of the system occurs in the FBCC component. The causes of the irreversibility, occurring in FBCC, may be presented as below. i. The amount of supplied excess air, entering to the FBCC which causes the higher heat loss. The enhancement on the amount of the excess air gives rise to the rising on the combustion gas’s mass flow rate. Owing to the enhancement in mass flow rate of the combustion gas and the decrement of the FBCC temperature, heat losses from the FBCC increases and the efficiency of the FBCC decreases. The typical excess air value of the coal in the stoker can be suggested between 20% and 25% as mentioned Ref. [17]. On the other hand, the excess air value in FBCC is measured 48%, in this study. Namely, one of the main reasons of the inefficiency is high amount of excess air ratio in this steam plant. ii. The chemical reaction can be presented as the second reason due to the combustion process, happening in FBCC for the irreversibility. As seen from composition of combustion gas existing from chimney, combustion efficiency is low due to the high value of CO. High excess air ratio causes low reaction rate of coal and high amount of unburnt carbon content which means CO formation instead of CO2 formation. Namely, the reason of the low combustion efficiency is high excess air ratio which causes the increment of the heat losses from the FBCC. iii. The third one is explained as the type of the coal using as solid fuel. The change in the coal using in FBCC causes the changing of the exergy destruction due to the different calorific value of the coal. As the calorific value of the coal increases, the exergy efficiency of the FBCC increases because of the high internal energy of the coal.
Table 7 Variations of energy and exergy efficiencies with different ambient temperatures (20 and 25 °C). Temperature (°C)
Fig. 6. Distribution of exergy destruction rates for the components of FBCCSP.
gI,FBCC gII,FBCC gI,HRSG gII,HRSG gI,ECO gII,ECO
Total changes (%)
20
25
0.4298 0.1452 0.7053 0.7812 0.9277 0.4863
0.4471 0.1460 0.7218 0.8204 0.6591 0.3938
4.03 0.54 2.35 5.02 28.95 19.03
N.F. Tumen Ozdil et al. / Fuel 183 (2016) 441–448 Table 8 Variations of energy and exergy efficiencies with different ambient temperatures (25 and 30 °C). Temperature (°C)
gI,FBCC gII,FBCC gI,HRSG gII,HRSG gI,ECO gII,ECO
Total changes (%)
25
30
0.4471 0.1460 0.7218 0.8204 0.6591 0.3938
0.4644 0.1462 0.7270 0.8397 0.5156 0.3360
3.87 0.09 0.72 2.35 21.78 14.66
Fig. 7. Entropy generation of the main components.
In this study, the ambient temperature changes between 20 °C and 30 °C with 5 °C increment while keeping the pressure at 101.3 kPa in order to demonstrate the effect of the different ambient temperatures on energy and exergy efficiency of the FBCC, HRSG and ECO as can be seen in Tables 7 and 8. The variation of ambient temperature plays a decisive role on the efficiency of the FBCC, HRSG and ECO. The exergy efficiency of the FBCC and HRSG increases, while exergy efficiencies of the ECO goes down as the ambient temperature increases. These results indicate that the increment of ambient temperature has a positive impact on the efficiencies of the FBCC and HRSG. Moreover, entropy generation is calculated to show the performance of the components. The entropy generation of the components is illustrated as can be seen in Fig. 7. The high entropy generation is taken place in FBCC while the entropy generations in the other components are considerably moderate. Uncertainty analyses are implemented for the first and second law efficiencies of system, described as Ref. [4] with the help of Eqs. (23) and (24);
wg1
g1
" ¼
w 2 w 2 w 2 w 2 w 2 _ WAF WVF WP Dh m þ þ þ þ _ m Dh WVF WP WAF
#12
447
5. Conclusions The thermodynamic analysis is investigated basis on the thermodynamic first and second laws for a real FBCC steam power plant in this paper. As can be seen from the results, the highest amount of exergy destruction occurs in the FBCC component by 4878.71 kW. The fundamental justifications of the inefficiency happened in the FBCC are high amount of excess air, low combustion efficiency and type of the solid fuel. The amount of the excess air influences directly amount of the combustion gas flow rate. As the amount of the excess air, heat losses in FBCC increases due to the decrement of temperature in FBCC. Moreover, high amount of excess air which means high amount of oxygen, has a negative effect on combustion efficiency. In this study, it is understood that the combustion process is incomplete due to the unburnt carbon content of combustion gas existing from chimney. Lastly, lower heating value of the solid fuel influences the efficiency of the system. As the calorific value of the coal increases, the efficiency of the FBCC enhances due to the high internal energy of the coal. Moreover, the entropy generation results show that the highest entropy generation is occurred in FBCC like exergy destruction. The impact of the different ambient temperature on the efficiency of FBCC, HRSG and ECO is also observed using first and second law of thermodynamic. The increment of the FBCC temperature causes the increment of water quality in the FBCC due to the increment of heat transfer rate from the ambient temperature to feed water in FBCC and HRSG. The results indicate that the increment of the ambient temperature causes negative effect on the exergy efficiency of ECO, while it causes positive effect on the efficiency of FBCC and HRSG. As a common tendency, the calculated results for energy efficiency have a great similarity with the calculated results for exergy efficiency for overall system. It can be suggested that i. the implementation of the thermoeconomic analysis of the FBCC power plant systems can be investigated in the future, ii. decreases excess air amount in order to increase efficiency, iii. combustion process in the FBCC is the source of irreversibility that can be decreased through preheating combustion gas, iv. different solid fuel (coal) type which has higher calorific value, can be used in FBCC in order to increase efficiency.
Acknowledgements The authors would like to thank Kıvanç Textile Inc. which is Turkey’s major textile factory for providing operational data of the FBCC steam plant as well as their courteous help. This study is supported by the Adana Science and Technology University Research Fund under project no: MÜHDBF.MM.2015-10. It is gratefully acknowledged for the continuous incentive support.
ð23Þ 2 wm_ wDh 2 wDs 2 wT 2 ¼ þ þ þ _ m g2 Dh Ds T 2 2 2 #12 wWAF wWVF wWP þ þ þ WVF WP WAF
References
wg2
ð24Þ
‘‘w” is described as the uncertainty amount of dependent variable for the energy and exergy efficiencies. The results of the uncertainty analyses are found as 0.8% and 2.32% for energy and exergy efficiencies, respectively. The results are coherent with the results of Ozdil et al. [4].
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