energies
Article Experimental Study on a Flue Gas Waste Heat Cascade Recovery System under Variable Working Conditions
Jiayou Liu 1,2,3, Xiaoyun Gong 3, Wenhua Zhang 3, Fengzhong Sun 2,* and Qingbiao Wang 1,3,4,* 1 College of Resources, Shandong University of Science and Technology, Tai’an 271019, China; [email protected] 2 School of Energy and Power Engineering, Shandong University, Jinan 250061, China 3 Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation, College of Civil Engineering and Architecture, Shandong University of Science and Technology, Qingdao 266590, China; [email protected] (X.G.); [email protected] (W.Z.) 4 National Engineering Laboratory for Coalmine Backfilling Mining, Shandong University of Science and Technology, Tai’an 271019, China * Correspondence: [email protected] (F.S.); [email protected] (Q.W.); Tel.: +86-531-8839-5691 (F.S.); +86-538-3076-026 (Q.W.) Received: 23 December 2019; Accepted: 7 January 2020; Published: 9 January 2020
Abstract: Recovering flue gas waste heat is beneficial to improving the unit efficiency in power plants. To obtain the change rules of performance parameters of a flue gas waste heat cascade recovery system (FWCRS) under variable working conditions, an experiment bench was designed and built. The variation laws of the inlet temperature and exhaust flue gas temperature of a low temperature economizer (LTE), the inlet and outlet air temperature of an air preheater (AP), the heat exchange quantities of the AP, LTE, and front-located air heater and an additional economizer (AE), as well as the waste heat recovery efficiency, the system exergy efficiency, and the energy grade replacement coefficient were obtained as the flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and circulating water flow in AE changed. Using an orthogonal test, the flue gas temperature, bypass flue gas ratio and air temperature were proved to be the significant factors affecting the performance parameters of FWCRS, and the bypass flue gas ratio was suggested as an adjusting parameter of FWCRS under variable working conditions.
Keywords: flue gas waste heat; cascade recovery; variable working conditions; performance parameters
1. Introduction With the rapid development of the world economy and increase of the population, the demand for energy sources is growing. In China, most of the electricity still comes from coal-fired power plants, and about half of the produced coal was consumed by thermal power stations over the past decade [1], which brings about greenhouse effect, environmental pollution, and other issues. Despite non-fossil energy being used as much as possible, coal-fired power will still occupy a dominant position in the foreseeable future in China [2]. Reducing coal consumption and improving coal use efficiency on the existing coal-fired units are very necessary for solving the above problems. For coal-fired power plant units, the exhaust flue gas temperature of boilers can reach more than 120 ◦C, and about 50–80% of boiler heat loss is due to the exhaust flue gas energy loss, which accounts for 3–8% total energy input of the units [3]. To reduce exhaust heat loss, reducing exhaust flue gas temperature is one of the effective measures to decrease coal consumption and improve unit efficiency, and the temperature can be reduced by recovering the waste heat from high-temperature flue gas. The recovered flue gas waste heat is generally used to heat condensed water from condenser, heat boiler feed water, or preheat air for coal combustion in boilers on coal-fired units [4–9], as well as used
Energies 2020, 13, 324; doi:10.3390/en13020324 www.mdpi.com/journal/energies Energies 2020, 13, 324 2 of 19 for drying boiler fuel, waste heat power generation, seawater desalination, organic Rankine cycle, etc. [10–14]. The waste heat in sulfur-reduced flue gas can also be recovered by absorption heat pump to save energy and reduce pollution emissions [15]. The flue gas waste heat is usually recovered by heat exchangers located at tail flue of power plant boilers. Wang [16] assessed the coal-saving effect of a 600 MW unit installed a low pressure economizer (LPE) in three cases, and 2–4 g/(kW h) standard coal was saved. Xu [17] proposed a novel flue gas · waste heat recovery system, in which a low temperature economizer (LTE) was located between a high-temperature air preheater (AP) and a low temperature AP, and 5.6 MW of exergy destruction was saved. Stevanovic [18] studied the unit efficiency improvement when a high pressure economizer (HPE) used for recovering flue gas waste heat was supplied cold or hot feed water on an aged 620 MW coal-fired unit, and the case of HPE fed with cold water proved to be competitive. To recover more waste heat effectively from flue gas, many flue gas waste heat depth use systems were put forward and analyzed. The systems were more complex and can heat feed water and combustion air at the same time. Han [19] proposed a new heat integration system, 4 g/(kW h) standard coal was saved · by adding a steam-air heater on a bypass flue gas waste heat recovery system of a 1000 MW unit. Yan [20] optimized a bypass flue gas waste heat use system on a 1000 MW unit by recovering waste heat from flue gas after wet flue gas desulphurization, the net standard coal consumption was reduced 5.38 g/(kW h). Fan [21] developed a cascade energy use system by combining flue gas waste heat · recovery and bleeding steam energy use, the net efficiency of a double reheat cycle was improved 0.61%. Yang [22] proposed a conceptual waste heat recovery system on a 1000 MW coal-fired unit, in which the AP was divided into high-temperature AP, main AP and low temperature AP, the flue gas waste heat was used to heat condensed water by LTE and heat the combustion air by the APs, and 13.3 MW additional net power output was produced. The above-mentioned flue gas waste heat use systems all have good energy-saving and coal-saving effects; however, the analysis of energy-saving effects of the systems were based only on rated conditions, without analyzing the systems’ performance under variable working conditions. The performance of flue gas waste heat recovery system under off-design conditions was also studied in the literature. Song [23] used the thermodynamic analysis method to study the off-design operating performance of an LTE system, and the results indicated that the energy-saving effect of the system was stable under high-load conditions. Song [24] also analyzed the energy-saving effect of a deep waste heat use system on a 1000 MW ultra-supercritical unit under variable operating conditions by thermodynamic calculation, and the result showed that the energy-saving effect of the unit was obviously reduced under low-load conditions. Zhang [25] studied the energy-saving potential of an exhaust gas heat recovery system on a 600 MW unit by developing a calculation model of steam turbine and heat exchangers under off-design conditions, and the ratio of bypass flue gas was proved to have the greatest influence on the saved coal amount of the unit. Han [26] simulated the thermal efficiency of a lignite-fired 600 MW unit with a flue gas drying and waste heat use system, and the improvement of plant thermal efficiency was reduced from 1.51% to 1.36% as the operating condition changed from benchmark condition to 50% full load condition. The dynamic model of heat exchangers used for waste heat use and large scale coal-fired power plants were also studied in the literature [27,28]. However, in the study of the performance of the waste heat use systems under variable conditions mainly focused on model analysis, theoretical calculations, and simulation calculations, little experimental research was conducted, and the influence of operating parameters on performance of waste heat use system has not been studied experimentally. The flue gas from coal-fired boiler generally contains acid gases such as SO2 and NOx, the water vapor in the flue gas will condense and form solution, which may cause corrosion, ash deposit and fouling on the tail flue and heat exchanger surface, affecting the safe and reliable operation of the unit [29–32]. Therefore, the exhaust flue gas temperature of flue gas waste heat recovery system should not be reduced too much before desulfurization, generally reduced to about 80 ◦C[33]. Energies 2020, 13, 324 3 of 19 Energies 2020, 13, x FOR PEER REVIEW 3 of 20
DuringDuring the the operation operation of of the the power power station station unit, unit, the the unit unit load load often often changeschanges duedue toto thethe influenceinfluence of electricityof electricity demand, demand, the the operation operation of of the the flue flue gas gas waste waste heatheat recoveryrecovery system system may may deviate deviate from from the the designdesign working working condition condition and and run run underunder variablevariable working conditions, conditions, and and the the flue flue gas gas waste waste heat heat recoveryrecovery eff effectect will will be be a ffaffected.ected.To To obtain obtain betterbetter waste heat recovery recovery effect effect and and guide guide the the operation operation andand regulation regulation of of flue flue gas gas waste waste heat heat recoveryrecovery system,system, studying the the variation variation laws laws of of performance performance parametersparameters of of the the system system under under variable variable operatingoperating conditions is is necessary. necessary. However, However, as as the the operating operating parametersparameters of of a fluea flue gas gas waste waste heat heat recoveryrecovery systemsystem may affect affect system system performance performance and and cannot cannot be be adjustedadjusted arbitrarily arbitrarily on on anan actual unit, unit, it it is is difficult difficult to toobtain obtain the theoperating operating rules rules of the of system the system by on- by on-sitesite test. test. InIn this this study, study, an an experimental experimental benchbench waswas builtbuilt to study study the the variation variation laws laws of of performance performance parametersparameters such such as as the the inlet inlet flue flue gasgas temperaturetemperature and exhaust exhaust flue flue gas gas temperature temperature of ofan an LTE, LTE, the the inlet and outlet air temperature of an AP, the heat exchange quantity of the AP, LTE, front-located inlet and outlet air temperature of an AP, the heat exchange quantity of the AP, LTE, front-located air air heater (FAH) and additional economizer (AE), the waste heat recovery efficiency, the system heater (FAH) and additional economizer (AE), the waste heat recovery efficiency, the system exergy exergy efficiency, and the energy grade replacement coefficient of a flue gas waste heat cascade efficiency, and the energy grade replacement coefficient of a flue gas waste heat cascade recovery recovery system (FWCRS) as the flue gas flow, flue gas temperature, bypass flue gas ratio, air system (FWCRS) as the flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and temperature, and circulating water flow in AE changed, and the main factors affecting the circulatingperformance water parameters flow in AEof FWCRS changed, were and determined the main by factors orthogonal affecting test. the The performance obtained experimental parameters ofresults FWCRS can were provide determined a reference by for orthogonal the operation test. adjustment The obtained of actual experimental units. results can provide a reference for the operation adjustment of actual units. 2. Experimental Section 2. Experimental Section 2.1. Description of FWCRS 2.1. Description of FWCRS FWCRS is a flue gas waste heat depth use system. The technical process of the system is shown in FWCRSFigure 1. is Most a flue of gasthe wastehigh-temperature heat depth useexhaust system. flue Thegas technicalfrom boiler process economizer of the flows system through is shown an in FigureAP to1. heat Most combustion of the high-temperature air, and small part exhaust of flue flue gas gas passes from through boiler economizer an AE to heat flows boiler through feed water an AP toor heat condense combustion water. air, After and the small two part parts of of flue flue gas gas passes are cooled through down, an AEthe tolow heat temperature boiler feed flue water gas or condensemerges water.into an AfterLTE to the heat two circulating parts of flue medium. gas are Then, cooled the down,heated the medium low temperature enters into a flue FAH gas to mergesheat intothe an air LTE flowing to heat into circulating the AP. In medium. the system, Then, the thehigh-temperature heated medium flue enters gas is into used a FAHto heat to the heat boiler the air flowingfeed water into theor condensed AP. In the system,water with the matching high-temperature temperature, flue the gas low is used temperature to heat theflue boiler gas is feed used water to orheat condensed the air with water matching with matching temperature temperature, by LTE-FAH the system, low temperature the cascade flue recovery gas is of used exhaust to heat flue the gas air withis achieved. matching temperature by LTE-FAH system, the cascade recovery of exhaust flue gas is achieved.
Figure 1. Figure 1.Technical Technical process process of of flue flue gasgas wastewaste heatheat cascade recovery recovery system. system. AP: AP: Air Air Preheater; Preheater; AE: AE: AdditionalAdditional Economizer; Economizer; LTE: LTE: Low Low Temperature Temperature Economizer;Economizer; FAH: Front-located Air Air Heater. Heater.
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The FWCRS has been applied to a 1000 MW ultra-supercritical double reheat coal-fired unit [34]. The technical process of the system is displayed in Figure 2 [35]. Up to 30% of high-temperature flue gas flows through an HPE and an LPE in sequence to heat boiler feed water and condensed water respectively, and the bypass flue gas valve is used to adjust the flue gas flow. The rest flue gas flows through an AP and heat primary air and secondary air in it. Then the cooled flue gas flows through electrostatic precipitator and enters into an LTE with the action of an induced fan. In an LTE, the low temperature flue gas heats the circulating medium and enters into desulfurization process. A feed Energiesfan drives2020, 13 the, 324 cold air into a FAH, in which the air is heated by the heated circulating medium. Then4 of 19 the heated air is further heated in the AP. The primary air flows to coal mill, and the secondary air flows to boiler hearth. TheIn FWCRSthe process has of been flue applied gas heat to use a 1000 in Figure MW ultra-supercritical2, the cold air is heated double in the reheat FAH coal-fired by low energy unit [34 ]. Thegrade technical flue gas process waste ofheat the transferred system is from displayed the LTE, in Figuresaving 2high[ 35 energy]. Up to grade 30% flue of high-temperature gas heat. The saved flue gasheat flows is used through to heat an boiler HPE feed and water an LPE and in condense sequenced water, to heat reducing boiler feedthe stream water for and heating condensed the water, water respectively,and the saved and stream the bypass can be flue used gas to valvedrive steam is used turbin to adjuste to generate the flue electricity. gas flow. The unit rest flueefficiency gas flows is throughimproved, an AP and and the heat coal primary consumption air and and secondary exhaust air gas in temperature it. Then the cooledare reduced. flue gas When flows the through air electrostatictemperature precipitator meets the unit and operation enters into requirements, an LTE with th thee more action flue of gas an waste induced heat fan. is recovered In an LTE, in HPE, the low temperatureLPE, and LTE, flue gasthe higher heats the is the circulating unit efficiency. medium However, and enters as intothe unit desulfurization often operates process. under Avarying feed fan drivesloads, the the cold operating air into aconditions FAH, in whichof the theFWCRS air is heatedwill also by change. the heated To circulatingguide the system medium. operation Then the heatedadjustment air is further and optimize heated insystem the AP. design, The primary as well airas to flows obtain to coalhigher mill, waste and heat the secondary use efficiency, air flows it is to boilernecessary hearth. to study the operating performance of the system under variable working conditions.
FigureFigure 2. 2.Technical Technical process of of FWCRS FWCRS on on a 1000 a 1000 MW MW ultra- ultra-supercriticalsupercritical double double reheat reheatcoal-fired coal-fired unit. unit.AP: AP: Air Air Preheater; Preheater; HPE: HPE: High High Pressure Pressure Econom Economizer;izer; LPE: LPE: Low Low Pressure Pressure Economizer; Economizer; LTE: LTE: Low Low TemperatureTemperature Economizer; Economizer; FAH: FAH: Front-located Front-located Air Air Heater.Heater.
2.2.In Experimental the process Setup of flue gas heat use in Figure2, the cold air is heated in the FAH by low energy grade flue gas waste heat transferred from the LTE, saving high energy grade flue gas heat. The saved A prototype experimental bench was designed and built based on the technical process of the heat is used to heat boiler feed water and condensed water, reducing the stream for heating the water, FWCRS in Figure 1 and the actual system on a 1000 MW double reheat ultra-supercritical coal-fired and the saved stream can be used to drive steam turbine to generate electricity. The unit efficiency unit in Figure 2. The flow chart of the flue gas waste heat cascade recovery experimental bench is is improved,illustrated in and Figure the 3. coal The consumption high-temperature and flue exhaust gas was gas produced temperature by aare 465 reduced.kW oil-fired When hot blast the air temperaturestove, and the meets stove the fuel unit is operationdiesel oil. The requirements, temperature the and more the flueflow gas of the waste flue heat gas can is recovered be adjusted in by HPE, LPE,combustion and LTE, thecontrol higher system. is the The unit initial efficiency. design However, flue gas as temperature the unit often was operates 400 °C. under The hot varying flue gas loads, theentered operating into conditionsan AP and an of AE the in FWCRS two ways, will respecti also change.vely. The To AE guide consists the system of an HPE operation and an adjustmentLPE, and andthey optimize are double system H-finned design, tube as well heat as exchangers. to obtain higher The flue waste gas heat flew use outside efficiency, the tubes, it is necessary and the to studycirculating the operating water flew performance inside the oftubes. the systemThe AP under is a spiral variable finned working tube heat conditions. exchanger, in which the
2.2. Experimental Setup A prototype experimental bench was designed and built based on the technical process of the FWCRS in Figure1 and the actual system on a 1000 MW double reheat ultra-supercritical coal-fired unit in Figure2. The flow chart of the flue gas waste heat cascade recovery experimental bench is illustrated in Figure3. The high-temperature flue gas was produced by a 465 kW oil-fired hot blast stove, and the stove fuel is diesel oil. The temperature and the flow of the flue gas can be adjusted by combustion control system. The initial design flue gas temperature was 400 ◦C. The hot flue gas entered into an AP and an AE in two ways, respectively. The AE consists of an HPE and an LPE, and they are double H-finned tube heat exchangers. The flue gas flew outside the tubes, and the Energies 2020, 13, 324 5 of 19 Energies 2020, 13, x FOR PEER REVIEW 5 of 20 circulatingflue gas flew water inside flew the inside tubes, the and tubes. the air The flew AP ou istside a spiral the finned tubes. tubeThe gas heat flow exchanger, into the in AP which and theAE fluecan be gas adjusted flew inside by plug thetubes, valves. and The the cooled air flew flue outside gas mixed the tubes.and flowed The gas into flow an intoLTE. theIn an AP LTE, and AEthe canflue begas adjusted exchanged by plugheat with valves. circulating The cooled medium flue gas water, mixed and and was flowed further into cooled an LTE. and Indischarged an LTE, theby fluean induced gas exchanged fan. The heatinlet withflue gas circulating temperature medium and water,exhaust and flue was gas further temperature cooled of and the discharged LTE were set by anat 115 induced °C and fan. 85 The°C, inletrespectively. flue gas The temperature cold air was and exhaustsent to FAH flue gasby a temperature feed fan and of heated the LTE by were the setcirculating at 115 ◦ Cmedium and 85 water◦C, respectively. from the LTE. The The cold LTE air an wasd FAH sent are to elliptical FAH by afinned feed fan tube and heat heated exchangers, by the circulatingin which the medium gas flew water outside from the the tubes, LTE. The and LTE the andcirculating FAH are water elliptical flew finned inside tube the heattubes. exchangers, The LTE, inFAH, which circulating the gas pipeline, flew outside and water the tubes, pump and make the up circulating an LTE-FAH water system. flew inside In the thesystem, tubes. the The flue LTE, gas FAH,heat was circulating transferred pipeline, from andthe waterflue gas pump side maketo the upair anside LTE-FAH by the circulating system. In water. the system, Then thethe flue heated gas heatair flowed was transferred into AP and from was the heated flue continuously. gas side to the Finally, air side the by hot the air circulating was discharged water. Thenoutside, the and heated the airoutlet flowed air temperature into AP and was was set heated at 350 °C. continuously. The induced Finally, fan and the the hot feed air fan was are discharged centrifugal outside,fans. Driven and theby a outlet circulating air temperature pump, the was circulating set at 350 water◦C. Theente inducedred into fanthe andLPE theand feed HPE fan in aresequence, centrifugal and fans.was Drivenheated by the a circulating high-temperature pump, the flue circulating gas. The waterwater enteredflow into into the the HPE LPE and and LPE HPE can in be sequence, adjusted and by wasvalves heated on water by the pipeline. high-temperature A water-water flue gas. tube-shell The water heat flow exchanger into the HPEwas set and on LPE the can circulation be adjusted loop. by valvesOn one on side water of pipeline.the heat A exchanger, water-water the tube-shell circulating heat water exchanger temperature was set on was the reduced circulation to loop.prevent On onevaporization. side of the On heat the exchanger, other side the of circulatingthe heat exchange water temperaturer, a cooling wastower reduced was located to prevent on the vaporization. roof of the Onlaboratory the other and side used of theto reduce heat exchanger, the temperature a cooling of circulating tower was locatedwater to on ensure the roof theof safe the operation laboratory ofand the usedsystem. to reduceThe on-site the temperature layout of flue of gas circulating waste heat water cascade to ensure recovery the safe experimental operation ofbench the system.is shown The in on-siteFigure 4. layout of flue gas waste heat cascade recovery experimental bench is shown in Figure4.
Figure 3. Flow chart of flueflue gas waste heat cascade recovery experimental bench. AP: Air Preheater; FAH: Front-locatedFront-located AirAir Heater; Heater; HPE: HPE: High High Pressure Pressu Economizer;re Economizer; LPE: LPE: Low Low Pressure Pressure Economizer; Economizer; LTE: LowLTE: TemperatureLow Temperature Economizer. Economizer.
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Figure 4. ExperimentalExperimental setup setup of of flue flue gas gas waste waste heat heat cascade cascade recovery recovery system. system. AP: AP: Air Air Preheater; Preheater; FAH: FAH: Front-located Air Heater; HPE: High Pressure Econ Economizer;omizer; LPE: Low Low Pressure Pressure Economizer; Economizer; LTE: LTE: Low Temperature Economizer.Economizer. 2.3. Data Acquisition and Processing 2.3. Data Acquisition and Processing On the above experimental bench, the thermal resistances were set at the inlet and outlet of On the above experimental bench, the thermal resistances were set at the inlet and outlet of the the heat exchangers to obtain the temperatures of the flue gas, air, and water. The gas flow meter heat exchangers to obtain the temperatures of the flue gas, air, and water. The gas flow meter and and water flow meter were used to measure the flow of flue gas, air, and water, respectively. The water flow meter were used to measure the flow of flue gas, air, and water, respectively. The measured data were all transmitted by an industrial switch typed X108 to an industrial computer measured data were all transmitted by an industrial switch typed X108 to an industrial computer typed IPC-610H in a control room for subsequent processing. The data acquisition system was set up typed IPC-610H in a control room for subsequent processing. The data acquisition system was set up based on WinCC software, and Origin software was used for data subsequent processing. The models based on WinCC software, and Origin software was used for data subsequent processing. The models and accuracies of the measuring instruments are displayed in Table1. For every test condition, the and accuracies of the measuring instruments are displayed in Table 1. For every test condition, the data acquisition system collects data every minute. As all data fluctuations did not exceed 1%, five sets data acquisition system collects data every minute. As all data fluctuations did not exceed 1%, five of data were selected for subsequent calculations. Affected by the accuracies of measuring instruments, sets of data were selected for subsequent calculations. Affected by the accuracies of measuring the experimental parameters have uncertainties. Assuming the expression of the tested parameters is instruments, the experimental parameters have uncertainties. Assuming the expression of the tested parameters is Y = f(x1, x2, …, xn), and x1, x2, …, xn are independent variables, the uncertainty transmission of the experimental parameter Y can be obtained using Equation (1).
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Y = f (x1, x2, ... , xn), and x1, x2, ... , xn are independent variables, the uncertainty transmission of the experimental parameter Y can be obtained using Equation (1). s !2 !2 !2 ∂ f 2 ∂ f 2 ∂ f 2 σY = (δx1 ) + (δx2 ) + + (δxn ) (1) ∂x1 ∂x2 ··· ∂xn
x x ∆x max i max δx = | | 100% = − 100% (2) x × x ×
Table 1. Measuring instruments used in the experiments.
Measuring Instruments Manufacturer Model Accuracy Hong De Control Technology Thermal resistance (Shanghai) Co., Ltd., Shanghai, HD-WZPK-238 0.1 ◦C China Xi’an Zhongwang Measurement Gas flow meter and Control Instrument Co., Ltd., FCY-110D-AMYG 1% ± Xi’an, China Hebei Feigerise Automation Water flow meter Technology Co., Ltd., Langfang, PMFG-S-32-FAPAC0116EIASAR 0.5 China Hong De Control Technology Smart pressure (Shanghai) Co., Ltd., Shanghai, HD3051GP3S22M3B3C1 0.1 transmitter China
Where σY is the uncertainty of Y, δx1, δx2, δxn are the relative errors of x1, x2, and xn, respectively, and can be calculated by Equation (2). |∆x|max, xi, x are the maximum absolute error, test value and average value of the measured x, respectively. In the experiment, the uncertainties of the calculated parameters are not more than 5%.
3. Performance Parameters of FWCRS Under variable working conditions of FWCRS, the inlet and outlet temperature of each heat exchangers change constantly. Too low exhaust flue gas temperature may cause corrosion and ash fouling in the tail flue, and too high exhaust flue gas temperature will reduce the waste heat recovery efficiency. Too high or too low inlet combustion air temperature of boiler will affect the safe and reliable running of the unit. The inlet flue gas temperature of LTE is an important parameter affecting system efficiency [36]. The lower outlet air temperature of FAH may lead to ash deposit and corrosion in AP. Therefore, the exhaust flue gas temperature to,LTE and the inlet flue gas temperature ti,LTE of LTE, the outlet air temperature to,FAH of FAH, and the outlet air temperature to,AP of AP were studied as important operating parameters of FWCRS in the experiment. The temperatures can be measured using thermal resistances in the experiment. For FWCRS, the heat exchange quantities of AP, AE, LTE, and FAH directly related to the flue gas waste heat recovery effect. The heat exchange quantities Q are also used as performance parameters of the system and are calculated using Equation (3).
Q = ρGcp(T T ) (3) 1 − 2 where Q is the heat exchange quantity of each heat exchangers, ρ is the density of flue gas, water, or air, G is the flow rate of flue gas, water, or air in unit time, cp is the specific heat of flue gas, water, or air, T1 and T2 are the initial temperature and final temperature of flue gas, water and air, respectively. The Energies 2020, 13, 324 8 of 19 energy grade replacement coefficient ε is defined to evaluate the increase degree of energy grade of flue gas waste heat, which can be obtained using Equation (4).
λs λr ε = − (4) λr where ε is the energy replacement coefficient, λr is the energy grade of recovered low-grade flue gas waste heat, and λs is the energy grade of replaced high-grade flue gas waste heat. The energy grade of flue gas waste heat can be calculated by Equation (5) [36]. T1 E cρG T1 T2 T0 ln T T T λ = = − − 1 = 1 0 ln 1 (5) q cρG(T T ) − T1 T2 T2 1 − 2 − where λ is the energy grade of flue gas waste heat, E is the exergy released by flue gas, q is the heat released by flue gas, c is the average specific heat of flue gas, m is the flow of flue gas, T1 and T2 are the initial temperature and final temperature of flue gas, T0 is the ambient temperature. According to Figure2, the recovered flue gas waste heat is finally used to save steam for heating boiler feed water and condensate from condenser, improving the working capacity of the unit. The more flue gas waste heat is recovered, the more steam is saved. The waste heat recovery efficiency is used to evaluate the amount of the recovered waste heat. Due to the exergy loss caused by the irreversibility of energy conversion, the recovered waste heat cannot be completely converted into useful work. The exergy is defined as the available energy in the recovered waste heat that can be converted into work. The system exergy efficiency is introduced to evaluate the amount of the available energy in the input energy of FWCRS. The waste heat recovery efficiency ηr and the system exergy efficiency ηe of FWCRS are determined by the following Equations, respectively.
QAE + QFAH ηr = 100% (6) Q f ×
EAE + EAP ηe = 100% (7) E f + W × where ηr and ηe are the waste heat recovery efficiency and system exergy efficiency, respectively, Qf is the heat supplied by flue gas to the system, QAE is the heat released by flue gas in AE, QFAH is the heat obtained from the air from FAH, Ef is the exergy released by flue gas in FWCRS, EAE is the exergy obtained from the water in AE, EAP is the exergy obtained from the air in AP, W is the electric power consumption of fans and water pumps in FWCRS in unit time. In Equation (6), Qf, QAE, and QFAH are determined by Equation (3), EAE, and EAP are calculated by Equation (8) [36]. T1 E = ρGcp (T1 T2) T0 ln (8) − − T2 where E is the exergy, ρ is the density of flue gas, water, or air, G is the flow rate of flue gas, water, or air in unit time, cp is specific heat of flue gas, water, or air, T1 and T2 are the initial temperature and final temperature of flue gas, water and air, respectively, T0 is ambient temperature.
4. Results and Discussion
4.1. Effect of Flue Gas Flow on Performance Parameters of FWCRS
The flue gas temperature was set at 380 ◦C, the initial air temperature was 24.3 ◦C, and the bypass flue gas ratio was about 15% of the total flue gas flow. As the flue gas flow varied from 125 to 250 Nm3/h, the relationships between the flue gas flow and the performance parameters of FWCRS such as main node temperatures, heat exchange quantities of exchangers, waste heat recovery efficiency ηr, Energies 2020, 13, 324 9 of 19 Energies 2020, 13, x FOR PEER REVIEW 9 of 20 systemηr, system exergy exergy effi efficiencyciency ηe ,η ande, and energy energy grade grade replacement replacement coe coefficientfficient ε εare are shown shown inin FigureFigure5 5 with with error bars.
(a)360 (b) 24
320 to, AP 20 280 ti, LTE t Q 240 o, LTE 16 AP to, FAH QAE
200 kJ QLTE 4 12 °C
/ QFAH /10 t 160 Q 8 120 115°C 85°C 80 4 40 0 0 120 140 160 180 200 220 240 260 120 140 160 180 200 220 240 260 Flue gas flow/Nm3/h Flue gas flow/Nm3/h (c) 100 ηr 1.8 ηe 80 ε 1.6
60 1.4 ε
40 1.2 Efficiency/%
1.0 20
0.8 0 120 140 160 180 200 220 240 260 Flue gas flow/Nm3/h Figure 5. Effect of flue gas flow on FWCRS: (a) Relationship between flue gas flow and main node temperatures; (b) Relationship between flue gas flow and heat exchange quantities of heat exchangers; Figure 5. Effect of flue gas flow on FWCRS: (a) Relationship between flue gas flow and main node (c) Relationship between flue gas flow and η , η , and ε. temperatures; (b) Relationship between flue rgase flow and heat exchange quantities of heat exchangers; As(c) Relationship shown in Figure between5a, withflue gas the flow flue and gas η flowr, ηe, and increased, ε. the outlet air temperature of AP, the inlet flue gas temperature of LTE, the outlet air temperature of FAH, and the exhaust flue gas temperature As shown in Figure 5a, with the flue gas flow increased, the outlet air temperature of AP, the of LTE increased to different degrees. When the flue gas flow was about 220 Nm3/h, the inlet flue inlet flue gas temperature of LTE, the outlet air temperature of FAH, and the exhaust flue gas gas temperature of LTE was nearly 117 C, slightly higher than the design temperature, the exhaust temperature of LTE increased to different◦ degrees. When the flue gas flow was about 220 Nm3/h, the flue gas temperature was about 90 C, higher than the design value, and the outlet air temperature of inlet flue gas temperature of LTE was◦ nearly 117 °C, slightly higher than the design temperature, the FAH and AP were close to the design conditions. The increasing flue gas flow led to higher exhaust exhaust flue gas temperature was about 90 °C, higher than the design value, and the outlet air flue gas temperature, and measures should be taken to reduce the temperature to improve waste temperature of FAH and AP were close to the design conditions. The increasing flue gas flow led to heat recovery. Figure5b indicates the heat exchange quantities of AP, AE, LTE, and FAH increased higher exhaust flue gas temperature, and measures should be taken to reduce the temperature to with the flue gas flow added. The heat exchange quantity of AP increased greatly because more improve waste heat recovery. Figure 5b indicates the heat exchange quantities of AP, AE, LTE, and high-temperature flue gas flowed across the AP. In Figure5c, the waste heat recovery e fficiency and FAH increased with the flue gas flow added. The heat exchange quantity of AP increased greatly system exergy efficiency increased slightly, which revealed the added flue gas flow was beneficial to because more high-temperature flue gas flowed across the AP. In Figure 5c, the waste heat recovery improve waste heat recovery. However, the energy grade replacement coefficient decreased with the efficiency and system exergy efficiency increased slightly, which revealed the added flue gas flow flue gas flow increased, the main reason was the increased exhaust flue gas temperature and inlet flue was beneficial to improve waste heat recovery. However, the energy grade replacement coefficient gas temperature of LTE reduced the energy grade improvement of flue gas waste heat. For an actual decreased with the flue gas flow increased, the main reason was the increased exhaust flue gas coal-fired unit, as the unit load rises, the flue gas flow increases, the variation of theoretical calculation temperature and inlet flue gas temperature of LTE reduced the energy grade improvement of flue results such as some node temperatures and heat exchange quantities of HPE and LPE on a 1000 MW gas waste heat. For an actual coal-fired unit, as the unit load rises, the flue gas flow increases, the unit in [24] is consistent with the experiment results. A large amount of flue gas flow will cause the variation of theoretical calculation results such as some node temperatures and heat exchange exhaust flue gas temperature to increase. As the unit load rises, the FWCRS should be adjusted to quantities of HPE and LPE on a 1000 MW unit in [24] is consistent with the experiment results. A reduce the exhaust flue gas temperature to ensure the waste heat recovery effect. large amount of flue gas flow will cause the exhaust flue gas temperature to increase. As the unit load rises, the FWCRS should be adjusted to reduce the exhaust flue gas temperature to ensure the waste heat recovery effect.
Energies 2020, 13, 324 10 of 19 Energies 2020, 13, x FOR PEER REVIEW 10 of 20
4.2. Effect Effect of of Flue Flue Gas Gas Temperature Temperature on Performance Parameters of FWCRS
3 The flue flue gas flow flow was set at 250 Nm /h,/h, the initial air temperature was 24.5 °C,◦C, and the bypass flueflue gas gas ratio ratio was was about about 15% 15% of of the the total total flue flue gas gas flow. flow. As As the the flue flue gas gas temperature temperature varied varied from from 320 320 °C to◦C 400 to 400 °C,◦ C,the the relationship relationship between between the the flue flue ga gass temperature temperature and and the the performance performance parameters parameters of FWCRS are displayed in Figure 6 6 with with errorerror bars.bars.
400 28 (a) (b) 360 24 320 to, AP 20 Q 280 ti, LTE AP Q to, LTE AE 16 240 t kJ QLTE o, FAH 4
/°C Q t 200 /10 FAH Q 12
160 8 120 115°C 85°C 4 80
40 0 320 340 360 380 400 320 340 360 380 400 Flue gas temperature/°C Flue gas temperature/°C
(c) 80 0.90
70
ηr 0.85 60 ηe ε 50
0.80 ε 40 Efficiency/%
30 0.75
20
10 0.70 320 340 360 380 400 Flue gas Temperature/°C Figure 6. Effect of flue gas temperature on FWCRS: (a) Relationship between flue gas temperature and Figure 6. Effect of flue gas temperature on FWCRS: (a) Relationship between flue gas temperature main node temperatures; (b) Relationship between flue gas flow and heat exchange quantities of heat and main node temperatures; (b) Relationship between flue gas flow and heat exchange quantities of exchangers; (c) Relationship between flue gas temperature and ηr, ηe, and ε. heat exchangers; (c) Relationship between flue gas temperature and ηr, ηe, and ε. In Figure6, as the flue gas temperature rose, the main node temperatures, and the heat exchange In Figure 6, as the flue gas temperature rose, the main node temperatures, and the heat exchange quantities of the exchangers increased. The waste heat recovery efficiency and the system exergy quantities of the exchangers increased. The waste heat recovery efficiency and the system exergy efficiency rose, and the energy grade replacement coefficient decreased. The change trends of the efficiency rose, and the energy grade replacement coefficient decreased. The change trends of the parameters were basically the same as the trends when the flue gas flow was added. This is consistent parameters were basically the same as the trends when the flue gas flow was added. This is consistent with the actual operation of a power station unit. As the unit load increases, the flue gas flow and with the actual operation of a power station unit. As the unit load increases, the flue gas flow and the the flue gas temperature will increase at the same time, leading to the increased exhaust flue gas flue gas temperature will increase at the same time, leading to the increased exhaust flue gas temperature and more heat recovery. However, excessive exhaust flue gas temperature will result in temperature and more heat recovery. However, excessive exhaust flue gas temperature will result in greater waste heat loss. As the flue gas temperature and flue gas flow increase, the FWCRS should greater waste heat loss. As the flue gas temperature and flue gas flow increase, the FWCRS should be adjusted to transfer the flue gas waste heat to the boiler’s feed water and condensate as much as be adjusted to transfer the flue gas waste heat to the boiler’s feed water and condensate as much as possible by AE to decrease the exhaust flue gas temperature and improve the waste heat use efficiency, possible by AE to decrease the exhaust flue gas temperature and improve the waste heat use saving more steam for power generation. efficiency, saving more steam for power generation. 4.3. Effect of Bypass Flue Gas Ratio on Performance Parameters of FWCRS 4.3. Effect of Bypass Flue Gas Ratio on Performance Parameters of FWCRS 3 The flue gas flow was set at 250 Nm /h, the initial air temperature was 27.1 ◦C, and the flue gas The flue gas flow was set at 250 Nm3/h, the initial air temperature was 27.1 °C, and the flue gas temperature was 380 ◦C. As the bypass flue gas ratio varied from 10% to 30% of the total flue gas flow, temperature was 380 °C. As the bypass flue gas ratio varied from 10% to 30% of the total flue gas
Energies 2020, 13, x FOR PEER REVIEW 11 of 20 flow, the relationship between the bypass flue gas ratio and the performance parameters of FWCRS are shown in Figure 7 with error bars. Figure 7 indicates the main node temperatures decreased as the bypass flue gas ratio increased. When the ratio was about 30%, the exhaust flue gas temperature of LTE was close to the designed value. As the bypass flue gas flow added, more heat was transferred to the circulating water by the HPE and LPE, the bypass flue gas temperature decreased, and at the same time, the flue gas used to heat the air in AP was reduced, the outlet flue gas temperature of AP also decreased. Then the inlet flue gas temperature of LTE was reduced. The heat exchange quantities of AP, LTE, and FAH decreased, and the heat exchange quantity of AE increased. As the flue gas flowed through the AP was reduced, the heat exchange quantity of AP decreased. More flue gas flowed through AE increased heat exchange quantity of AE. The lower node temperature caused the decreased heat exchange quantity in LTE and FAH. As the bypass flue gas ratio added, the inlet flue gas temperature of LTE was greatly reduced, the exergy released by flue gas increased, the reduction of exergy obtained from the air flowing through the AP was dominant, the total exergy decreased, so the system exergy efficiency was reduced. However, more waste heat was recovered in AE, leading to increased waste heat recovery efficiency. The energy grade replacement coefficient increased because the large
Energiesbypass2020 flue, 13 gas, 324 ratio reduced the inlet flue gas temperature and exhaust flue gas temperature of11 LTE, of 19 increasing the energy grade of recovered waste heat. It can be seen that changing the bypass flue gas flow can effectively adjust the exhaust temperature and the waste heat recovery efficiency of FWCRS. Sothe the relationship bypass flue between gas flow the ratio bypass can flue be gasused ratio as an and important the performance adjustment parameters parameter of FWCRSfor FWCRS are operation.shown in Figure 7 with error bars.
(a) 360 (b) 28
320 24 280
to, AP 20 240 ti, LTE Q to, LTE 16 AP 200 kJ 4 Q °C to, FAH AE / /10 t 160 QLTE Q 12 QFAH 120 115°C 85°C 8 80 4 40
0 0 10 15 20 25 30 10 15 20 25 30 Bypass flue gas ratio/% Bypass flue gas ratio/%
(c) 90 1.2
ηr 80 1.1 ηe ε 70 1.0 60 0.9
50 ε 0.8
Efficiency/% 40
0.7 30
20 0.6
10 0.5 10 15 20 25 30 Bypass flue gas ratio/% Figure 7. Effect of bypass flue gas ratio on FWCRS: (a) Relationship between bypass flue gas ratio and main node temperatures; (b) Relationship between bypass flue gas ratio and heat exchange quantities Figure 7. Effect of bypass flue gas ratio on FWCRS: (a) Relationship between bypass flue gas ratio and of heat exchangers; (c) Relationship between bypass flue gas ratio and η , η , and ε. main node temperatures; (b) Relationship between bypass flue gas ratio rande heat exchange quantities ofFigure heat 7exchangers; indicates ( thec) Relationship main node between temperatures bypass decreasedflue gas ratio as and the η bypassr, ηe, and flue ε. gas ratio increased. When the ratio was about 30%, the exhaust flue gas temperature of LTE was close to the designed value.
As the bypass flue gas flow added, more heat was transferred to the circulating water by the HPE and LPE, the bypass flue gas temperature decreased, and at the same time, the flue gas used to heat the air in AP was reduced, the outlet flue gas temperature of AP also decreased. Then the inlet flue gas temperature of LTE was reduced. The heat exchange quantities of AP, LTE, and FAH decreased, and the heat exchange quantity of AE increased. As the flue gas flowed through the AP was reduced, the heat exchange quantity of AP decreased. More flue gas flowed through AE increased heat exchange quantity of AE. The lower node temperature caused the decreased heat exchange quantity in LTE and FAH. As the bypass flue gas ratio added, the inlet flue gas temperature of LTE was greatly reduced, the exergy released by flue gas increased, the reduction of exergy obtained from the air flowing through the AP was dominant, the total exergy decreased, so the system exergy efficiency was reduced. However, more waste heat was recovered in AE, leading to increased waste heat recovery efficiency. The energy grade replacement coefficient increased because the large bypass flue gas ratio reduced the inlet flue gas temperature and exhaust flue gas temperature of LTE, increasing the energy grade of recovered waste heat. It can be seen that changing the bypass flue gas flow can effectively adjust the exhaust temperature and the waste heat recovery efficiency of FWCRS. So the bypass flue gas flow ratio can be used as an important adjustment parameter for FWCRS operation. Energies 2020, 13, x FOR PEER REVIEW 12 of 20
4.4. Effect of Air Temperature on Performance Parameters of FWCRS
The flue gas temperature was set at 380 °C, the flue gas flow was 250 Nm3/h, and the bypass flue gas ratio was about 15% of the total flue gas flow. The experiment was carried out respectively when the air temperature was −5.1 °C, 6.5 °C, 15.2 °C, 24.3° C, and 28.4 °C. The relationship between the air temperature and performance parameters of FWCRS such as main node temperatures, heat exchange quantities of the exchangers, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient are shown in Figure 8 with error bars. In Figure 8, as the air temperature increased, the main node temperatures were raised. When the air temperature was −5.1 °C, the exhaust flue gas temperature was about 80 °C, below the designed temperature. The lower air temperature may lead to too low exhaust flue gas temperature and too low inlet air temperature of AP, low−temperature corrosion or ash fouling will occur in the tail flue and cold end of AP, affecting the safe and reliable operation of the unit. Therefore, as the air temperature is low, measures must be taken to control the exhaust flue gas temperature above the safe temperature in the operation of FWCRS. The higher air temperature was not good for reducing exhaust flue gas temperature. The higher air temperature can cause high exhaust flue gas temperature, so the higher air temperature was not good for reducing exhaust flue gas temperature. The heat exchange quantities of LTE and FAH increased slightly and the heat exchange quantities of AE and AP decreased. The main reason was the heat transfer temperature difference of AE and AP Energieswas reduced2020, 13 ,and 324 the temperature difference of LTE and FAH was added because of the increased12 of air 19 temperature. The exergy released from the flue gas was reduced, and the exergy obtained from the air in FAH and AP and the water in AE also decreased, but the obtained exergy reduced more, leading 4.4. Effect of Air Temperature on Performance Parameters of FWCRS to decreasing system exergy efficiency. The increased air temperature reduced the heat released by 3 the flueThe gas, flue the gas recovered temperature heat was in set AE at and 380 FAH◦C, the changed flue gas slightly, flow was resulting 250 Nm in/h, rising and the of bypasswaste heat flue gasrecovery ratio wasefficiency. about 15%The ofincreased the total inlet flue gasair flow.temperature The experiment and exhaust was carriedflue gas out temperature respectively of when LTE thereduced air temperature the energy wasgrade5.1 replacementC, 6.5 C, coefficient, 15.2 C, 24.3 decreasingC, and 28.4 the energyC. The relationshipgrade promotion between effect. the For air − ◦ ◦ ◦ ◦ ◦ temperatureFWCRS on an and actual performance unit, the parameterssystem should of FWCRS be adjusted such asto mainmake node the exhaust temperatures, flue gas heat temperature exchange quantitieshigher than of the exchangers,safe operating waste temperature heat recovery of tail effi heatingciency, systemsurface exergy in cold e ffiseason,ciency, and and to energy reduce grade the replacementexhaust flue coegasffi temperaturecient are shown as much in Figure as possible8 with to error improve bars. the waste heat use effect in hot season.
(a)360 (b) 28
320 24
280 t kJ o, AP 4 t 20 Q 240 i, LTE AP t o, LTE QAE t 16 200 o, FAH QLTE
/°C Q
t FAH 160 12
120 115°C 85°C 8 80 Heat exchange quantity/10 4 40
0 0 −10 −5 0 5 10 15 20 25 30 −10 −5 0 5 10 15 20 25 30 Energies 2020, 13, x FOR PEER REVIEW 13 of 20 Air temperature/°C Air temperature/°C
(c) 80 0.95
ηr
70 ηe ε 0.90 60
0.85 50 ε 40 0.80 Efficiency/%
30 0.75 20
10 0.70 −10 −5 0 5 10 15 20 25 30 Air temperature/°C
Figure 8. Effect of air temperature on FWCRS: (a) Relationship between air temperature and main node temperatures; (b) Relationship between air temperature and heat exchange quantities of heat Figure 8. Effect of air temperature on FWCRS: (a) Relationship between air temperature and main exchangers; (c) Relationship between air temperature and η , η , and ε. node temperatures; (b) Relationship between air temperaturer e and heat exchange quantities of heat Inexchangers; Figure8, as(c) theRelationship air temperature between increased, air temperature the main and η noder, ηe, and temperatures ε. were raised. When the air temperature was 5.1 C, the exhaust flue gas temperature was about 80 C, below the designed 4.5. Effect of Circulating− Water◦ Flow in AE on Performance Parameters of FWCRS ◦ temperature. The lower air temperature may lead to too low exhaust flue gas temperature and too low inlet airThe temperature flue gas temperature of AP, low wastemperature set at 380 °C, corrosion the flue gas or ash flow fouling was 250 will Nm occur3/h, and in the the tail bypass flue andflue − coldgas ratio end ofwas AP, about affecting 15% theof the safe total and flue reliable gas flow. operation The circulating of the unit. water Therefore, flow in as AE the was air temperatureadjusted from is low,5.0 to measures 7.1 t/h. mustThe relationship be taken to control between the the exhaust circulating flue gas water temperature flow in above AE and the safethe performance temperature inparameters the operation of FWCRS of FWCRS. are illustrated The higher in airFigure temperature 9 with error was bars. not good for reducing exhaust flue gas temperature.In Figure The 9, as higher the circulating air temperature water canflow cause in AE high increased, exhaust the flue main gas node temperature, temperatures so the decreased higher air temperatureslightly. The wasinlet not flue good gas temperature for reducing of exhaust LTE and flue the gas exhaust temperature. flue gas The temperature heat exchange were quantities higher than of LTEdesign and value. FAH increasedThe node slightlytemperatures and the were heat not exchange sensitive quantities to circulating of AE flow and APchanges decreased. in AE. The The main heat reasonexchange was quantity the heat of transfer AE increased temperature and the di ffheaterence exchange of AE andquantities AP was of reducedAP, LTE and and the FAH temperature decreased dislightly,fference mainly of LTE because and FAH the was added added water because flow of reduced the increased the inlet air temperature.flue gas temperature The exergy of LTE released and fromdecreased the flue the gas heat was exchange reduced, quantity. and the exergy The changes obtained in fromcirculating the air water in FAH flow and of AP AE and had the a watersmall ineffect AE alsoon heat decreased, exchange but quantities the obtained of heat exergy exchangers. reduced more,The reduced leading exhaust to decreasing flue gas system temperature exergy eincreasedfficiency. exergy released from flue gas. However, less exergy obtained in AP and AE increased, leading to decreasing system exergy efficiency. The recovered waste heat in AE increased slightly, and the recovered heat in FAH decreased, the total recovered heat changed a little, but the released heat by flue gas in FWCRS increased more because of the decreased exhaust flue gas temperature, the waste heat recovery efficiency was reduced slightly. The decreased inlet flue gas temperature and exhaust flue gas temperature of LTE promoted the energy grade replacement coefficient, improving the energy grade of waste heat recovered. The changes of circulating water flow in AE have less effect on node temperatures, heat exchange quantities, and waste heat recovery efficiency. The inlet and outlet water temperature of AE are restricted by the water temperature required by other heat exchangers on an actual unit, adjusting the circulating water flow in AE plays a limited role in regulation of FWCRS.
Energies 2020, 13, 324 13 of 19
The increased air temperature reduced the heat released by the flue gas, the recovered heat in AE and FAH changed slightly, resulting in rising of waste heat recovery efficiency. The increased inlet air temperature and exhaust flue gas temperature of LTE reduced the energy grade replacement coefficient, decreasing the energy grade promotion effect. For FWCRS on an actual unit, the system should be adjusted to make the exhaust flue gas temperature higher than the safe operating temperature of tail heating surface in cold season, and to reduce the exhaust flue gas temperature as much as possible to improve the waste heat use effect in hot season.
4.5. Effect of Circulating Water Flow in AE on Performance Parameters of FWCRS
3 The flue gas temperature was set at 380 ◦C, the flue gas flow was 250 Nm /h, and the bypass flue gas ratio was about 15% of the total flue gas flow. The circulating water flow in AE was adjusted
Energiesfrom 5.0 2020 to, 13 7.1, x tFOR/h. ThePEERrelationship REVIEW between the circulating water flow in AE and the performance14 of 20 parameters of FWCRS are illustrated in Figure9 with error bars.
(a)360 (b) 28
320 24
280 to, AP t i, LTE 20 QAP 240 t o, LTE QAE t o, FAH 16 QLTE 200 kJ 4
°C QFAH / /10 t 160 Q 12
120 115°C 85°C 8 80 4 40
0 0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 4.55.05.56.06.57.07.5 Circulating water flow in AE/t/h Circulating water flow in AE/t/h (c) 80 0.90 ηr η 70 e ε 0.85
60 0.80
50
0.75 ε 40 Efficiency/% 0.70 30
0.65 20
10 0.60 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Circulating water flow in AE/t/h
Figure 9. Effect of circulating water flow in AE onFWCRS: (a) Relationship between circulating water b Figureflow in 9. AE Effect and of main circulating node temperatures; water flow in ( AE) Relationship on FWCRS: ( betweena) Relationship circulating between water circulating flow in AE water and heat exchange quantities of heat exchangers; (c) Relationship between circulating water flow in AE and flow in AE and main node temperatures; (b) Relationship between circulating water flow in AE and η , η , and ε. heatr eexchange quantities of heat exchangers; (c) Relationship between circulating water flow in AE
and ηr, ηe, and ε. In Figure9, as the circulating water flow in AE increased, the main node temperatures decreased slightly. The inlet flue gas temperature of LTE and the exhaust flue gas temperature were higher 5. Orthogonal Test of Influence Factors than design value. The node temperatures were not sensitive to circulating flow changes in AE. The heatIt can exchange be seen from quantity 4.1–4.5, of AE for increased FWCRS, the and main the heat node exchange temperatures, quantities heat ofexchange AP, LTE quantities and FAH indecreased heat exchangers, slightly, mainly waste becauseheat recovery the added effici waterency, flowsystem reduced exergy the efficiency, inlet flue gasand temperature energy grade of replacementLTE and decreased coefficient the of heat the exchange system were quantity. affected The tochanges different in degrees circulating by the water operating flowof parameters AE had a suchsmall as eff theect flue on heat gas exchangeflow, flue quantitiesgas temperature, of heat exchangers.bypass flue gas The ratio, reduced air temperature, exhaust flue gasand temperature circulating water flow in AE. However, in the actual operation of FWCRS, the operating parameters usually change at the same time. To obtain the variation laws of the main node temperatures, heat exchange quantities in heat exchangers, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient under variable working conditions by comprehensive experiments, more experiments and more time are required. To determine the main factors affecting the system’s performance, and to guide the operation adjustment of FWCRS, orthogonal test was carried out using exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS as test factors. In the orthogonal test, five level data of each test factors was selected, and the data is shown in Table 2. The orthogonal table L25 (56) was used in the test. The level data of each test factors was chosen individually based on the orthogonal table for 25 tests, and 25 sets of test results of exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS under different flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and circulating water flow in AE were obtained. The influence of the test factors on system performance can be got by range analysis and variance analysis.
Energies 2020, 13, 324 14 of 19 increased exergy released from flue gas. However, less exergy obtained in AP and AE increased, leading to decreasing system exergy efficiency. The recovered waste heat in AE increased slightly, and the recovered heat in FAH decreased, the total recovered heat changed a little, but the released heat by flue gas in FWCRS increased more because of the decreased exhaust flue gas temperature, the waste heat recovery efficiency was reduced slightly. The decreased inlet flue gas temperature and exhaust flue gas temperature of LTE promoted the energy grade replacement coefficient, improving the energy grade of waste heat recovered. The changes of circulating water flow in AE have less effect on node temperatures, heat exchange quantities, and waste heat recovery efficiency. The inlet and outlet water temperature of AE are restricted by the water temperature required by other heat exchangers on an actual unit, adjusting the circulating water flow in AE plays a limited role in regulation of FWCRS.
5. Orthogonal Test of Influence Factors It can be seen from 4.1–4.5, for FWCRS, the main node temperatures, heat exchange quantities in heat exchangers, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of the system were affected to different degrees by the operating parameters such as the flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and circulating water flow in AE. However, in the actual operation of FWCRS, the operating parameters usually change at the same time. To obtain the variation laws of the main node temperatures, heat exchange quantities in heat exchangers, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient under variable working conditions by comprehensive experiments, more experiments and more time are required. To determine the main factors affecting the system’s performance, and to guide the operation adjustment of FWCRS, orthogonal test was carried out using exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS as test factors. In the orthogonal test, five level data of each test factors was selected, and the data is shown in 6 Table2. The orthogonal table L 25 (5 ) was used in the test. The level data of each test factors was chosen individually based on the orthogonal table for 25 tests, and 25 sets of test results of exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS under different flue gas flow, flue gas temperature, bypass flue gas ratio, air temperature, and circulating water flow in AE were obtained. The influence of the test factors on system performance can be got by range analysis and variance analysis.
Table 2. Level data of test factors in orthogonal test.
Test Factors Level Data Flue Gas Flue Gas Bypass Flue Air Circulating Water Flow/Nm3/h Temperature/°C Gas Ratio /% Temperature/°C Flow in AE/t/h 1 125 320 10 5.1 5 − 2 156 340 15 6.5 5.5 3 188 360 20 15.2 6 4 219 380 25 24.3 6.5 5 250 400 30 28.4 7.1
Figure 10 indicates the influence of test factors on the performance parameters of FWCRS. It can be seen the variation rules of exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS were basically consistent with the laws in 4.1–4.5. According to the range analysis, the range value and important order of the main factors affected the systems performance parameters were determined and are displayed in Table3. It can be seen the initial flue gas temperature has the greatest effect on exhaust flue gas temperature to,LTE, the bypass flue gas flow has the most important effect on waste heat recovery efficiency ηr and system exergy efficiency ηe, the air temperature is the most important factor affecting the energy grade replacement coefficient ε. In addition, the flue gas flow is also an important factor affecting Energies 2020, 13, x FOR PEER REVIEW 15 of 20
Table 2. Level data of test factors in orthogonal test.
Test Factors Level Bypass Circulating Flue Gas Flue Gas Air Data Flue Gas Water Flow in Flow/Nm3/h Temperature/℃ Temperature/℃ Ratio /% AE/t/h 1 125 320 10 −5.1 5 2 156 340 15 6.5 5.5 3 188 360 20 15.2 6 4 219 380 25 24.3 6.5 5 250 400 30 28.4 7.1
Figure 10 indicates the influence of test factors on the performance parameters of FWCRS. It can be seen the variation rules of exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS were basically consistent with the laws in 4.1–4.5. According to the range analysis, the range value and important order of the main factors affected the systems performance parameters were determined and are displayed in Table 3. It can be seen the initial flue gas temperature has the greatest effect on exhaust flue gas temperature to,LTE, the bypass flue gas flow has the most important effect on waste heat recovery efficiency ηr and Energies 2020, 13, 324 15 of 19 system exergy efficiency ηe, the air temperature is the most important factor affecting the energy grade replacement coefficient ε. In addition, the flue gas flow is also an important factor affecting performance parameters such as to,LTE and ηe. The circulating water flow in AE has a small impact on performance parameters such as to,LTE and ηe. The circulating water flow in AE has a small impact onthe the performance performance parameters parameters of FWCRS. of FWCRS. In the In theactual actual operation operation of unit, of unit,as the as flue the gas flue flow, gas flue flow, gas flue gastemperature, temperature, and and air air temperature temperature are are mainly mainly affect affecteded by by fuel fuel properties, properties, unit unit loads loads and and ambient ambient environment, they are generally difficult to be adjusted arbitrarily. Therefore, when the unit runs environment, they are generally difficult to be adjusted arbitrarily. Therefore, when the unit runs under under variable working conditions, adjusting the bypass flue gas ratio is an important way to variable working conditions, adjusting the bypass flue gas ratio is an important way to improve flue improve flue gas recovery effect, adjusting circulating water flow in AE can be used as an auxiliary gas recovery effect, adjusting circulating water flow in AE can be used as an auxiliary adjustment mean. adjustment mean.
(a) 100 80 1.1 (b) 110 80
t to, LTE o, LTE 1.2 η 105 η r r 70 η 70 η 95 e e ε 100 ε 1.0 60 1.1 60 95 90 50 50 90 1.0 ε 0.9 ε /°C /°C t t 85 40 85 40 Efficiency/% Efficiency/% 0.9 80 30 30 0.8 80 75 20 0.8 70 20 10 75 0.7 65 0.7 120 140 160 180 200 220 240 260 320 340 360 380 400 Flue gas flow/Nm3/h Flue gas temperature/°C (c) 100 70 1.05 (d) 100 80 1.1
to, LTE ηr 70 60 1.00 ηe
95 to, LTE 95 ε 1.0 60 ηr 50 0.95 ηe ε 50
90 40 0.90 ε 90 0.9 /°C t 40 ERC Efficiency/% Efficiency/%
30 0.85 Temperature/°C 30 85 85 0.8 20 0.80 20
80 10 0.75 80 10 0.7 10 15 20 25 30 ?5 0 5 10 15 20 25 30 Energies 2020, 13, x FORBypass PEER flue gas REVIEW ratio/% Air Temperature/°C 16 of 20
(e) 100 70 1.10 to, LTE η r 1.05 η e 60 ε 95 1.00
50 0.95
90 0.90 ε /°C t 40
Efficiency/% 0.85
30 85 0.80
0.75 20
80 0.70 5.05.56.06.57.07.5 Circulating water flow in AE/t/h FigureFigure 10. 10.Orthogonal Orthogonal analysis analysis of the of factorsthe factors affected affected exhaust exhaust flue gas flue temperature, gas temperature, waste heatwaste recovery heat effirecoveryciency, system efficiency, exergy system efficiency, exergy and efficiency energy, gradeand energy replacement grade replacement coefficient: (coefficient:a) Flue gas ( flow;a) Flue (b )gas Flue gasflow; temperature; (b) Flue gas (c) Bypasstemperature; flue gas (c) ratio;Bypass (d )flue Air gas temperature; ratio; (d) Air (e) temperature; Circulating water(e) Circulating flow in AE. water flow in AE.
Table 3. Importance order of the influencing factors.
Range Value Test Factors Importance Order FGF 1 FGT 2 AT 3 BFR 4 CWF 5 to,LTE 8.4 31.4 2.0 6.2 2.2 FGT > FGF > BFR > CWF > AT ηr 0.0083 0.0162 0.0041 0.1799 0.0046 BFR > FGT > FGF > CWF > AT ηe 0.0302 0.0102 0.0203 0.1316 0.0186 BFR > FGF > AT > CWF > FGT ε 0.0839 0.4204 0.4645 0.1738 0.1058 AT > FGT > BFR > CWF > FGF Notes: 1 FGF: Flue gas flow; 2 FGT: Flue gas temperature; 3 AT: Air temperature; 4 BFR: Bypass flue gas ratio; 5 CWF: Circulating water flow in AE.
6. Conclusions An experiment bench was designed and built, and using experiments the variation rules of performance parameters of FWCRS such as main node temperatures, heat exchange quantities of AP, AE, LTE, and FAH, as well as waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS were obtained. The orthogonal test was carried out using exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS as test factors, and the main factors affecting the system’s performance were determined. The conclusions are as below. As the flue gas flow varied from 125 to 250 Nm3/h, the outlet air temperature of FAH and AP, the inlet flue gas temperature of LTE, and the exhaust flue gas temperature increased, the heat exchange quantities of AP, AE, LTE, and FAH also increased, the waste heat recovery efficiency and system exergy efficiency rose slightly, and the energy grade replacement coefficient decreased. As the flue gas temperature varied from 320 °C to 400 °C, the main node temperature including the outlet air temperature of FAH and AP, inlet flue gas temperature of LTE, and the exhaust flue gas temperature all increased, the heat exchange quantities of exchangers increased, the waste heat recovery efficiency and system exergy efficiency rose slightly, and the energy grade replacement coefficient decreased. As the bypass flue gas ratio varied from 10% to 30% of the total flue gas flow, the main node temperatures such as the outlet air temperature of FAH and AP, the inlet flue gas temperature of LTE, and the exhaust flue gas temperature decreased, the heat exchange quantities of AP, LTE, and FAH decreased, and the heat exchange quantity of AE increased, the system exergy efficiency was reduced, and the waste heat recovery efficiency and the energy grade replacement coefficient increased. As the air temperature rose from −5.1 to 28.4 °C, the main node temperatures increased, the heat exchange quantities of LTE and FAH increased slightly and the heat exchange quantities of AE and
Energies 2020, 13, 324 16 of 19
Table 3. Importance order of the influencing factors.
Range Value Test Factors Importance Order FGF 1 FGT 2 AT 3 BFR 4 CWF 5
to,LTE 8.4 31.4 2.0 6.2 2.2 FGT > FGF > BFR > CWF > AT ηr 0.0083 0.0162 0.0041 0.1799 0.0046 BFR > FGT > FGF > CWF > AT ηe 0.0302 0.0102 0.0203 0.1316 0.0186 BFR > FGF > AT > CWF > FGT ε 0.0839 0.4204 0.4645 0.1738 0.1058 AT > FGT > BFR > CWF > FGF Notes: 1 FGF: Flue gas flow; 2 FGT: Flue gas temperature; 3 AT: Air temperature; 4 BFR: Bypass flue gas ratio; 5 CWF: Circulating water flow in AE.
6. Conclusions An experiment bench was designed and built, and using experiments the variation rules of performance parameters of FWCRS such as main node temperatures, heat exchange quantities of AP, AE, LTE, and FAH, as well as waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS were obtained. The orthogonal test was carried out using exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS as test factors, and the main factors affecting the system’s performance were determined. The conclusions are as below. As the flue gas flow varied from 125 to 250 Nm3/h, the outlet air temperature of FAH and AP, the inlet flue gas temperature of LTE, and the exhaust flue gas temperature increased, the heat exchange quantities of AP, AE, LTE, and FAH also increased, the waste heat recovery efficiency and system exergy efficiency rose slightly, and the energy grade replacement coefficient decreased. As the flue gas temperature varied from 320 ◦C to 400 ◦C, the main node temperature including the outlet air temperature of FAH and AP, inlet flue gas temperature of LTE, and the exhaust flue gas temperature all increased, the heat exchange quantities of exchangers increased, the waste heat recovery efficiency and system exergy efficiency rose slightly, and the energy grade replacement coefficient decreased. As the bypass flue gas ratio varied from 10% to 30% of the total flue gas flow, the main node temperatures such as the outlet air temperature of FAH and AP, the inlet flue gas temperature of LTE, and the exhaust flue gas temperature decreased, the heat exchange quantities of AP, LTE, and FAH decreased, and the heat exchange quantity of AE increased, the system exergy efficiency was reduced, and the waste heat recovery efficiency and the energy grade replacement coefficient increased. As the air temperature rose from 5.1 to 28.4 C, the main node temperatures increased, the heat − ◦ exchange quantities of LTE and FAH increased slightly and the heat exchange quantities of AE and AP decreased, the waste heat recovery efficiency rose, and the system exergy efficiency and energy grade replacement coefficient decreased. As the circulating water flow in AE was adjusted from 5.0 to 7.1 t/h, the main node temperatures decreased, the heat exchange quantity of AE increased and the heat exchange quantities of AP, LTE and FAH decreased slightly, the waste heat recovery efficiency rose, and the system exergy efficiency were reduced, and the energy grade replacement coefficient rose. The orthogonal test proved the variation laws of the exhaust flue gas temperature, waste heat recovery efficiency, system exergy efficiency, and energy grade replacement coefficient of FWCRS were basically consistent with the experiment results. The flue gas temperature, bypass flue gas ratio and air temperature were the significant factors affecting the performance of FWCRS. The bypass flue gas ratio was suggested to be a main adjusting parameter of FWCRS under variable working conditions.
Author Contributions: J.L. designed the experimental bench, carried out the experiments and wrote the manuscript, X.G. and W.Z. processed the experiment data, F.S. provided experimental conditions, reviewed the manuscript and provided valuable suggestions, Q.W. assisted in conducting experiments and made recommendations. All authors have read and agreed to the published version of the manuscript. Energies 2020, 13, 324 17 of 19
Funding: This work was supported by the National Development and Reform Commission Fund of the Power Industry Low–carbon Technology Innovation and Industrialization Demonstration Projects [Development and Reform Office High–tech (2013) 1819] and SDUST Research Fund (2018TDJH101). Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
AE Additional Economizer AT Air temperature AP Air Preheater BFR Bypass flue gas ratio CWF Circulating water flow in AE FAH Front located Air Heater FGF Flue gas flow FGT Flue gas temperature FWCRS Flue gas Waste heat Cascade Recovery system HPE High Pressure Economizer LPE Low Pressure Economizer LTE Low Temperature Symbols c Average specific heat of flue gas, kJ/(kg K) · cp Specific heat of flue gas, water, or air, kJ/(kg K) · E Exergy released by flue gas, kJ/kg EAE Exergy obtained by water in AE, kJ EAP Exergy obtained by air in AP, kJ Ef Exergy released by flue gas in FWCRS, kJ G Flow of flue gas, water, or air in unit time, m3/s Q Heat exchange quantity of each heat exchangers, kJ QAE Heat released by flue gas in AE or heat exchange quantity in AE, kJ QAP Heat exchange quantity in AP, kJ Qf Heat supplied by flue gas to the system, kJ QFAH Heat obtained by air from FAH or heat exchange quantity in FAH, kJ QLTE Heat exchange quantity in LTE, kJ q Heat released by flue gas, kJ/kg T1 Initial temperature, K T2 Final temperature, K T0 Ambient temperature, K ti,LTE Inlet flue gas temperature of LTE, ◦C to,LTE Exhaust flue gas temperature of LTE, ◦C to,FAH Outlet air temperature of FAH, ◦C to,AP Outlet air temperature of AP, ◦C W Electric power consumption of fans and water pumps in FWCRS in unit time, kJ xi Single value of measured x x Average value of measured x |∆x|max Maximum absolute error of measured x δx Relative errors of x ε Energy replacement coefficient ηe System exergy efficiency, % ηr Waste heat recovery efficiency, % λ Energy grade of flue gas waste heat λr Energy grade of recovered low-grade flue gas waste heat λs Energy grade of replaced high-grade flue gas waste heat ρ Density of flue gas, water, or air, kg/m3 σY Uncertainty of Y Energies 2020, 13, 324 18 of 19
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