Application of exergy analysis in understanding the performance of a coal-fired steam power plant 12

Abstract

Keywords: Coal; Captive; Energy; Exergy; Steam Power Plant; Feed Water Heater

Introduction

In recent years, coal-based steam power plant (SPP) is being eliminated from the energy generation matrix in a phase-wise manner, and it is because of its low efficiency and high environmental degradation [1]. Hence, many research activities are being promoted in the area of renewables [2]. Of course, it cannot be denied the fact that renewable source of energy has the potential to impede climate change. However, the issue of its capability to meet the

growing demand for energy specially in the form of power is still in the research stage [3]. Further, data released by IEA regarding power production referring to 2015 shows that the contribution of renewable account for 23.1% in the energy generation matrix, while fossil-fuel based thermal power plant accounts for 76.9% (oil 4.1%; nuclear 10.6%; natural gas 22.9%; coal 39.3%) [4]. Hence, it can be inferred, that still the majority of electricity production

*Corresponding author. *E-mail address: sankalp.arpit815@gmail.com This paper was recommended for publication in revised form by Regional Editor Tolga Taner Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

comes from SPP, and in the coming decades also, it will dominate the graph. Further wide availability of coal, ease of transportation, and suitability for baseload make SPP a suitable choice [5]. Seeing the importance of SPP in global energy matrix, efforts are being taken to improve the performance of SPP through thermodynamic analysis. The most common method adopted to assess the performance of SPP is through energy analysis. However, due to scarcity of natural resources, a rigorous analysis where not only the quantity of energy is judged, rather issues like quality also becomes important, and this “quality of energy” is well gauged by exergy analysis. It represents the true potential of an energy system assessing not only the effective utilization of energy, but also its degradation by different processes. Hence, there is an increasing need for considering both energy and exergy analysis for the performance assessment [6]. Many authors have applied exergy for different energy system. Ibrahim et al. [7] investigated about gas turbine, and concluded combustion chamber to be a major source of exergy destruction among all components. Topal et al. [8] investigated circulating fluidized bed with olive pits co-firing and concluded that with co-firing, there is an increase in exergy efficiency by 0.51% and a decrease in dust emission. Another author, Kamate et al. [9] investigated about backpressure steam turbine, condensing steam turbine and boiler installed in a sugar factor, and concluded that backpressure steam turbine requires least improvement potential, while boiler requires highest improvement potential. Apart from all these, exergy analysis has been applied in context of combined cycle power plant [10], diesel based cogeneration [11], trigeneration [12], Kalina cycle [13], absorption systems [14], transportation [15] and coal based SPP [16]. Few other authors are also cited in context of SPP. Regulagadda et al. [17] assessed 32 MW coal-fired power plant and introduces boiler as a major contributor for exergy destruction. Fadhil et al. [18] also assessed exergy analysis of a 120 MW coal-fired power plant, and introduces boiler as the main source of exergy destruction, and this result was supported by Erdem et al. [19]. Further, the pattern of exergy destruction shown by various components in SPP does not change irrespective of load [20], ambient temperature [21] and fuel used [22]. Hence, in order to improve the performance assessment, some methods are suggested by various researchers. In this context, Rashidi et al. [23] carried out performance assessment of SPP and concluded that performance is dependent on the condenser pressure and boiler exit temperature. Further, Li et al. [24] suggested to operate the plant at full load. Similarly, Hou et al. [25] assessed the performance of SPP using ASPEN, and suggested to increasing the combustion chamber temperature which ultimately increases the temperature of main steam, causing a rise in the power output from turbine. Similarly, Xiong et al. [26] investigated

the application of oxy-combustion SPP instead of the conventional one, and appreciated the application oxycombustion boiler. Recently, many authors are suggesting the incorporation of solar for reheating. Alotaibi et al. [27] incorporate solar by removing low pressure turbine extractions, and showed increment of power by 9.8 MW. Further, many researchers have suggested the incorporation of other cycles with SPP. In this regard, Khankari et al. [28] incorporated kalian cycle to recapture from the condenser of 500 MW thermal power plant, and produced electricity. It need not be exaggerated that though the available analyses can give some broad guidelines about the performance of a power plant however the performance is dependent on its configurations and operating parameters. It may further be noted that so far energy and exergy analysis for utility based power plants have been considered, captive power plants (CPP) have rarely been considered. A CPP is typical industrial facilities which provides a localized source of power to the entire complex. Hence, seeing this aspect we cannot neglect the fact that CPP are very important for the large industrial establishment, particularly where energy security and flexibility is an important factor [29]. Further, assessing losses in energy system through energy and exergy analysis, helps in reducing the fossil fuel consumption, which indirectly helps in reducing economic cost, water footprint etc. As far as the author knows, so far no efforts have been made for the performance analysis of any SPP, configured into CPP mode. The present work aims at a thorough assessment of the plant and tries to explore the scope of further improvement in plant performance. Power Plant Description This plant (Unit 4) is situated in the Jojobera (Jamshedpur, India), and it is owned by Tata power. Figure 1 presents the schematic diagram of SPP with 120 MW capacity, with coal as fuel. Technical specifications of unit 4 are presented in Table 1. It is modelled as improved Rankine cycle with reheating and regenerative feed water heating, with two high pressure heaters (HPH6 and HPH5), three low pressure heaters (LPH3, LPH2 and LPH1) and one open feed water heater called deaerator. However, without the description of coal handling plant, it would be very improper to start the system description. Coal, when received from collieries through train, is unloaded by wagon tippler technology. It is then transported to dead storage site through belt conveyor system. After that coal is prepared for combustion through crushers. After that the crushing, coal is transported to coal storage area through belt conveyor and kept as an inventory, and it is injected into the boiler furnace (33) along with preheated air (32), to convert feed water to superheated steam. The feed water after converting into superheated steam is allowed to expand in HPT

Figure 1. Schematic representation of a coal fired steam power plant with 120 MW capacity.

(4), and there after one part of steam is sent through CRH for reheating (5), and another part of steam (19) is sent to HPH 6 for feed water heating. CRH after reheating in boiler is converted to HRH, and allowed to expand in IPT (6), where after expansion in IPT, one part is sent towards HPH5 (20), and other one towards deaerator (21). Further, the stream coming towards LPT (7) from IPT is allowed to expand in LPT, after which it gets divided into four streams, and sent towards LPH3 (22), LPH2 (23) and LPH1 (24) and condenser (8), respectively. The steam after producing useful work in all three turbines i.e. HPT, IPT and LPT is routed to condenser, where it is condensed by cooling water. After that, the condensate is sent by CEP towards LPH1 through DC. From the last LPH1, the condensate is routed to the deaerator (14), via LPH2 and LPH3 respectively. After getting preheated in the deaerator, BFP supplies this condensate from deaerator to HPH5 (16) and HPH6 (17) respectively. After HPH6, the condensate enters the boiler section through economizer (18), thereby entering boiler section. Hence, following the

Table 1. Operating conditions of Unit 4 Operating conditions

route the cycle is completed. It should be noted that all the number in parenthesis represents the reference points, which is used for designation in SPP, and it is represented in Table 2.

Methodology

Energy analysis To carry thermodynamic modelling of each component, the thermophysical properties of the working fluid must be known. Thermodynamic properties of fluid have been obtained from the EES [30] while for gas mixtures the Table 2. Nomenclature of various points present in SPP (Figure 1) Reference points

Nomenclature

law of average has been used knowing the mole fraction of each component [31]. Appendix A presents the property of the various working fluids. Further, for carrying out analysis of SPP, following assumptions have been made, and they are listed below: • The system operates under steady-state conditions. • Combustion is assumed to be complete. • The reference condition for the ambient is considered as P0 = 101.235 kPa and T0 = 303 K. • The fuel coal has following composition: C (33.1%), H2 (2.95%), Sulphur (0.65%), O2 (12.298%), N2 (0.6%), Ash (50.679%), H2O (43%) with a LHV of 14070 kJ/kg. These values are provided by Tata power Boiler In the boiler, the chemical energy of coal is used to change the feed water into superheated steam. For heat loss, Eq. (1) is used [32]  33 LHV + m  32 h32 = m  34 h34 + m  1 (h4 − h1 ) m   (h − h ) + Ql +m 6

In Eq. (1), ṁ33 represents the mass of coal. Further ṁ32, ṁ34, ṁ1, ṁ6 represents the mass flow rate of air, flue gas, water through economizer and HRH. Similarly h32, h34, h1, h6 represents corresponding enthalpy of air, flue gas, water inlet through economizer and hot reheat. Further, Eq. (2) presents the formulation of boiler efficiency, where LHV represents lower heating value of fuel. Before assessing the performance of entire steam cycle, it is very important to find out the mass of steam extracted from HPT, IPT and LPT. To determine these flow rate through various extraction points, Eq. (3) is used, and this equation is valid for ṁ19 only, i.e. HPH6. In this regard, Figure 2 presents the schematic diagram of HPH6. The mass flow rate from BFP is already know to us i.e. 102.2 kg/sec. Further pressure and temperature of all point is known to us. Hence, applying basic mass equation for HPH6, we have Eq. (3). Similarly, procedure is followed for other extraction. Table 3 presents the mass of steam extracted from different turbines. In Eq. (3), ṁ19, ṁ17, ṁ27 and ṁ18 denotes mass flow of extracted steam, feed water, drip and economizer. Further h19, h17, h27 and h18 denotes corresponding enthalpy.  19 h19 + m  17 h17 = m  27 h27 + m  18 h18 m

Turbine The power generated in this plant depends on the work produced by HPT, IPT and LPT. The term ẆHPT, ẆIPT and

Table 3. Mass of extracted steam from different turbines Mass of steam extracted

In Eq. (4a), ṁ6 represents the flow rate of steam through HRH to IPT turbine and nomenclature of ṁ20 is explained in Table 3. Similarly h6 and h20 represents the corresponding enthalpy. LPT  LPT = m  7 (h7 − h22 ) + (m  7 −m  22 )(h22 − h23 ) W  7 −m  22 − m  23 )(h23 − h24 ) + + (m

 LPT ,isen = m  7 (h7 − h22 s ) + (m  7 −m  22 )(h22 s − h23 s ) W ẆLPT represent the power output from HPT, IPT and LPT respectively. The equations for deriving work from each turbine is presented from Eq. (3a) – (5c). HPT  HPT = m  4 (h4 − h19 ) + (m  4 −m  19 )(h19 − h5 ) W

 HPT ,isen = m  4 (h4 − h19 s ) + (m  4 −m  19 )(h19 s − h5 s ) (3b) W    Ql HPT = Wisen , HPT − WHPT

In Eq. (3a), ṁ4 represents the mas flow rate of steam to HPT turbine. Further the nomenclature of ṁ19 is presented in Table 3. Similarly h4 and h19 represents its corresponding enthalpy value. IPT  IPT = m  6 (h6 − h20 ) +(m  6 −m  20 )(h20 − h21 ) + W  6 −m  20 − m  21 )(h21 − h7 ) (m  IPT ,isen = m  6 (h6 − h20 s ) +(m  6 −m  20 )(h20 s − h21s ) + W  6 −m  20 − m  21 )(h21s − h7 s ) (m    Ql IPT = Wisen , IPT − WIPT

 7 −m  22 − m  23 − m  24 )(h24 s − h8 s ) (m    Ql LPT = Wisen , LPT − WLPT

In Eq. (5a), ṁ7 and ṁ8 represents the flow rate of steam through LPT turbine and condenser. Further the nomenclature of ṁ22, ṁ23 and ṁ24 is explained in Table 3. Similarly h7, h22, h23, h24 and h8 represents the corresponding enthalpy. Pump The energy equation for condensate extraction pump (CEP) and boiler feed pump (BFP) is presented by Eq. (6a) and Eq. (7a). CEP  CEP = (P10 − P9 )n 9 W

In Eq. (6a), P10 represents pressure at the outlet of CEP, whereas P9 represents the pressure at the inlet of CEP. BFP

In Eq. (7a), P16 represents pressure at the outlet of BFP, whereas P15 represents the pressure at the inlet of BFP.

Condenser The condenser is provided with cooling water to transform low-pressure steam to condensed liquid. Respective equation [32] to calculate heat loss from the condenser is given Eq. (8)   8 h8 + m  25 h25 + m  31h31 + Ql   m cond = m9 h9 + m26 h26

In the present equation, ṁ8 and ṁ9 denotes the mass of steam at the inlet and outlet of the condenser, whereas ṁ25 and ṁ26 denotes the mass of cold water. Further, ṁ31 represents the mass flow rate of water fed to condenser through LPH1. Similarly, h8 and h9 denotes the enthalpy of steam at inlet of condenser and outlet of condenser h25 and h26 represents the corresponding enthalpy of the cooling water. Drain cooler Drain cooler is a heat exchanger, which is used to preheat the condensate from CEP, and related equation for drain cooler is presented by Eq. (8a).  =m  10 h10 + Ql  11h11 m DC

Deaerator   28 h28 + m  21h21 + m  14 h14 = m15 h15 + Ql m dea

Exergy Analysis Exergy [6] is a property of the system-environment combination, and it represents the work potential of a given amount of energy for a given environmental condition. For the present performance assessment, potential and kinetic exergy related to the exergy is neglected: only physical and chemical exergies are considered for calculations. The first and second laws of thermodynamics, when applied to a thermodynamic system at a steady state, result in the following equations:  x ,heat + ∑ m  x ,w + Idest (11a)  i e x ,i = ∑ m  e e x ,e + Ex Ex i

In Eq. (11a), E˙xheat is exergy flow due to heat generated, i and e represent inlet and exit conditions of the system respectively, whereas E˙xw shows the exergy flow generated with work done by system. In the case of water and steam, the exergy is carried out by Eq. (12) where h0 and s0 are enthalpy and entropy values at dead state condition. e x , physical = (h − h0 ) − T0 (s − s0 )

Further, for calculating the chemical exergy of dry solid fossil fuels such as coal, Eq. (13a) to be used [33], f=

Feed water heaters Energy analysis of a closed feed water heater is presented by Eq. (9a). The Eq. (9a) is applicable for HPH6, however the analysis and calculation part of all heaters i.e. HPH5, LPH3, LPH2 and LPH1 remains same. Further, deaerator is an example of an open feed water heater. The energy equations related to the deaerator is presented by Eq. (10a). HPH6   17 h17 + m  19 h19 = m  27 h27 + m  18 h18 + Ql m HPH 6

Where f = 1.0437 + 0.1882  + 0.0610  + 0.0404   and c, h, o and n are mass fraction of carbon, hydrogen, oxygen and nitrogen, respectively. The value of f is 1.06. Chemical exergy of flue gases In the system analysed, the chemical exergies of combustion products have important roles, and it is calculated by Eq. (13b) [34]. n

Where xi represents the mole fraction of each component of the gaseous fuel, and ex,chemical,i is the specific chemical exergy of each component. The value of specific exergy of each component is taken from [35]. Table 4 presents the chemical exergy of flue gas. The mass fraction is obtained from combustion equation with 17% of excess air. Energy and Exergy Efficiency of Steam Power Plant The energy efficiency of SPP is obtained by Eq. (14a), and it is based on the LHV of fuel and net power obtained [36]. hI =

Further, the exergy efficiency of SPP can be represented by Eq. (14b) respectively.

Table 4. Chemical exergy of flue gas at ambient pressure Composition

Results

The performance assessment of SPP is being analyzed using various equations applied for each componenets.

Using the approach mentioned in Eq. (14a), the overall efficiency comes around 34.7%, with fuel input as 351 MW, and power output of 36.78 MW by HPT, 42.58 MW by IPT,

Conclusion And Recommendations

In the present paper, performance assessment of SPP configured with reheating and regenerative configuration is carried out, and it might be able to help in understanding the most inefficient component, requiring highest improvement potential. For carrying out this assessment, real time data were collected from the control unit of power plant under full load condition. The main equipment on which stress has been given includes the boiler, turbine, condenser, deaerator and pump. Some of the important conclusion that can be drawn are as follows: • Energy efficiency of SPP comes out to be 34.7 %, and introduces condenser as a major source of heat loss. • Exergy efficiency of SPP comes out to be 32%, and introduces boiler as a major source of exergy destruction, followed by steam turbine. Therefore, combustion process should be optimized by creating suitable fuel and air mixing operations, preheating air with waste heat, utilizing solar power as air preheater during daylight hours etc. • In turbine, particularly Low pressure turbine (10.87 MW) has highest exergy destruction among HPT (3.6 MW) and IPT (2.8 MW). • The performance of SPP can be increased by increasing main steam temperature and reheating temperature. Apart from all these, some more practical steps could be introduced on improving power plant efficiency, seeing the economic and social aspect. As we could observe from

Figure 8. Effect of condenser pressure (bar) on the exergy efficiency of SPP.

Figure 7 and Figure 8, that the condenser pressure has a significant role in the performance assessment of SPP. Hence, removing the obstructions around the cooling towers, installing walls directing the airflow are few strategies that could be incorporated to increase efficiency [41]. Other suggestions included are as follows: • Repowering of SPP, which means changing the configuration of present plant by incorporating gas turbine units [42]. • Several other methods like hot wind box repowering, feed water heating etc. could also be implemented, keeping eyes of various parameters like original investment, payback time, social acceptance part.

Nomenclature

Specific heat at constant pressure [kJ/kgK] Specific exergy of fuel [kJ/kg] Exergy rate [kW] Specific exergy [kJ/kg]

Exergy destruction rate [kW] Mass flow rate [kg/sec] Pressure [kPa] Heat loss [kW] Specific entropy [kJ/kgK] Temperature [°C, K] Power [kW] Specific enthalpy [kJ/kg] mole fraction of each component

Greek letters hI Thermal efficiency of steam power plant hII Second law efficiency Subscripts a air o dead state c Mass of coal

Abbreviations

BFP boiler feed pump CPP captive power plant CRH cold reheat CEP condensate extraction pump Cond. Condenser SPP steam power plant DC Drain cooler F/W feed water GSC gland steam cooler HPH high pressure heater HRH high reheat LPH low pressure heater LHV lower heating value HPT high pressure turbine IPT intermediate pressure turbine LPT low pressure turbine LDO light diesel oil TPH Ton per hour

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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