An Improved Flexible Solar Thermal Energy Integration Process

energies

Article An Improved Flexible Solar Thermal Energy Integration Process for Enhancing the Coal-Based Energy Efﬁciency and NOx Removal Effectiveness in Coal-Fired Power Plants under Different Load Conditions

Yu Han 1,2, Cheng Xu 1,*, Gang Xu 1,*, Yuwen Zhang 2 ID and Yongping Yang 1

1 Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation, North China Electric Power University, Beijing 102206, China; [email protected] (Y.H.); [email protected] (Y.Y.) 2 Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO 65211, USA; [email protected] * Correspondence: [email protected] (C.X.); [email protected] (G.X.)

Received: 8 August 2017; Accepted: 18 September 2017; Published: 25 September 2017

Abstract: An improved flexible solar-aided power generation system (SAPG) for enhancing both selective catalytic reduction (SCR) de-NOx efficiency and coal-based energy efficiency of coal-fired power plants is proposed. In the proposed concept, the solar energy injection point is changed for different power plant loads, bringing about different benefits for coal-fired power generation. For partial/low load, solar energy is beneficially used to increase the flue gas temperature to guarantee the SCR de-NOx effectiveness as well as increase the boiler energy input by reheating the combustion air. For high power load, solar energy is used for saving steam bleeds from turbines by heating the feed water. A case study for a typical 1000 MW coal-fired power plant using the proposed concept has been performed and the results showed that, the SCR de-NOx efficiency of proposed SAPG could increase by 3.1% and 7.9% under medium load and low load conditions, respectively, as compared with the reference plant. The standard coal consumption rate of the proposed SAPG could decrease by 2.68 g/kWh, 4.05 g/kWh and 6.31 g/kWh for high, medium and low loads, respectively, with 0.040 USD/kWh of solar generated electricity cost. The proposed concept opens up a novel solar energy integration pattern in coal-fired power plants to improve the pollutant removal effectiveness and decrease the coal consumption of the power plant.

Keywords: solar aided power generation; selective catalytic reduction (SCR) de-NOx; coal consumption; solar generated electricity cost; various loads

1. Introduction Coal is the primary energy source for electricity production, and this will continue for a long period in the future [1]. However, as the major emitters of NOx, SOx, dust and CO2, coal-ﬁred power plant have negative effects on the environment. Using environmentally-friendly alternative energy sources is an effective way to reduce pollutant emissions. As a good fossil fuel replacement, solar energy has attracted great attention amongst the R&D and industry communities. However, its instability, relatively low intensity and high investment nature constrain the commercial application of solar thermal energy. Integrating solar energy into coal-fired power plants, normally referred to solar-aided power generation (SAPG), offers better performance and lower cost [2]. SAPG was first proposed by Zoschak and Wu in 1975 [3]. In recent

Energies 2017, 10, 1485; doi:10.3390/en10101485 www.mdpi.com/journal/energies Energies 2017, 10, 1485 2 of 18 years, Yang et al. [4,5] analyzed the influence of regenerative systems with the integration of solar energy and revealed the coupling mechanism of SAPG. Yan et al. [6,7] analyzed the thermal performance of SAPG systems in different coal-fired units, and evaluated the energy-saving effect with multi-level solar energy use. Popov [8] analyzed the influence of inlet and outlet heated water temperature on the energy saving effect of SAPG. Suresh et al. [9] conducted an energy, exergy, environment and economic (4-E) analysis for SAPG power plants to comprehensively evaluate the thermal and economic performance. Zhong et al. [10] proposed an optimized operation strategy of a SAPG system based on the changeable integrate mode and different solar irradiance. Zhai et al. [11,12] evaluated SAPG systems based on thermo-economic structural and life cycle theory. Peng et al. [13] conducted thermodynamic analysis of SAPG systems under variable loads. Bakos et al. [14] proposed to integrate solar energy into a lignite-fired power plant and analyzed the resulting thermal and economic performance. Hou et al. [15] proposed a new evaluation method of the solar contribution in a SAPG system based on exergy analysis. Pierce et al. [16] compared SAPG and stand-alone concentrating solar power, and concluded that SAPG has great performance advantages. Xu et al. [17] proposed a new system based on integrating solar energy into a low-rank coal pre-drying process, which could significantly improve the boiler efficiency. To the best of our knowledge, in the previous studies solar energy was generally used to decrease the coal consumption and achieve energy savings effects. It seems that improving thermal performance is the sole function of integrated solar energy in coal-fired power plants. As we know, not only thermal performance but also pollutant removal effectiveness is important for coal-fired power generation, but the integration of solar energy to improve pollutant removal efficiency has not drawn any attention. In most coal-fired power plants, the SCR de-NOx device is located downstream of the economizer and upstream of the air preheater. Although the temperature of the SCR de-NOx unit is in the optimal operation range for the base load in coal-fired power plants, it shows a downtrend as the load decreases, and such as, the SCR de-NOx temperature is always far below the optimal operation range under low load and medium load conditions. It is common for coal-fired power plants to work under low and medium load, especially in China. It should be noted that a parabolic trough-based solar energy system, which is the most commercially available technology, could heat thermal oil to a temperature of around ◦ 400 C[8,17]. The optimal temperature range of selective catalytic reduction (SCR) de-NOx units in ◦ a coal-fired power plant is 340–390 C[18–20]. High-efficiency SCR de-NOx could thus be achieved by integrating solar energy to keep the SCR de-NOx process at an optimal operation temperature. Obviously, the energy level of parabolic trough-based solar energy and SCR de-NOx are beneficially matched, so the collected solar energy could increase the de-NOx process temperature to improve the de-NOx efficiency. In this paper, a new solar energy integrated system, which could improve both SCR de-NOx efficiency and coal saving effects under various loads, was proposed. The integration pattern changes for different power loads. The integrated solar energy is used to increase the SCR de-NOx efficiency and combustion air temperature at low load while it saves extraction steam under high load conditions, thus offering both energy-saving and environmental protection effects. The thermodynamic and economic performance of the proposed system were determined, and the improvement of SCR de-NOx performance was also discussed.

2. System Proposal

In most coal-fired power plants, the SCR de-NOx device is placed in the flue between the economizer and the air preheater (AP). Generally, the SCR de-NOx temperature is in the optimal operation range under full load, but presents a downtrend as the load decreases. Figure1 illustrates the connection between the temperature and efficiency of a SCR de-NOx unit based on the operation curve in a typical coal-fired power plant, as reported in [20]. Energies 2017, 10, 1485 3 of 18 Energies 2017, 10, 1485 3 of 18 Energies 2017, 10, 1485 3 of 18

85 85 80 80 75 75 Optimal temperatureOptimal 70 temperatureranges 70 ranges 65 65 SCR deNOx efficiency (%) deNOx SCR

SCR deNOx efficiency (%) deNOx SCR 60 26060 280 300 320 340 360 380 400 420 440 260 280 300 320 340 360 380 400 420 440 o Temperature ( Co ) Temperature ( C) Figure 1. SCR de-NOx efficiency variation with the temperature. Figure 1. SCR de-NOx efficiency variation with the temperature. Figure 1. SCR de-NOx efficiency variation with the temperature. In order to keep the SCR de-NOx temperature stable in the optimal operation range, the required In order to keep the SCR de-NOx temperature stable in the optimal operation range, the required solar energyIn order for to SCR keep de-NO the SCRx varies de-NO forx temperature different loads stable of in a the coal-fired optimal unit:operation (1) no range, need the for required high load; solar energy for SCR de-NOx varies for different loads of a coal-fired unit: (1) no need for high load; (2) asolar small energy amount for neededSCR de-NO for mediumx varies for load different and (3) loads a large of a amount coal-fired needed unit: (1) for no low need load. for Control high load; of the (2) a small amount needed for medium load and (3) a large amount needed for low load. Control of distribution(2) a small of amount solar energy needed under for medium different load loads and is(3) required, a large amount because needed it is difficult for low toload. change Control the of total the thedistribution distribution of ofsolar solar energy energy under under different different loads loads is is required, required, becausebecause itit is difficult toto changechange the the solar energy collected from collectors with the load variation. One possibility is to use the feed water at totaltotal solar solar energy energy collected collected from from collectors collectors with with the the load load variation. variation. OneOne possibilitypossibility isis toto useuse thethe feed feed the inlet of the No. 1 high pressure regenerative heater (RH) to absorb the extra solar energy under high waterwater at the at the inlet inlet of theof the No. No. 1 high1 high pressure pressure regenera regenerativetive heater heater (RH) (RH) toto absorbabsorb the extraextra solarsolar energy energy load and medium load conditions, saving high-pressure extraction steam (No. 1 extraction steam). underunder high high load load and and medium medium load load conditions, conditions, savingsaving high-pressurehigh-pressure extraction steamsteam (No.(No. 11 extractionextractionTherefore, steam). steam). a novel SAPG system concept for improving both the SCR de-NOx efficiency and provide coal savings effects is proposed. Figure2 illustrates the concept of the proposed system for Therefore,Therefore, a novela novel SAPG SAPG system system conc conceptept for for improving improving bothboth thethe SCRSCR de-NOxx efficiencyefficiency andand differentprovideprovide coal loads: coal savings (1)savings For effects aeffects low is power ispr proposed.oposed. load, Figure theFigure solar 2 2illustrates illustrates energy is the the used conceptconcept to heat of the the flue proposed gas before systemsystem the for for SCR de-NOdifferentdifferentx device loads: loads: to(1) increase(1) For For a low a thelow power SCRpower de-NOload, load, thex thetemperature solar solar en energyergy up is is used toused its to optimalto heatheat thethe operation flue gas beforebefore range, thethe providing SCR SCR ade-NO greatde-NO environmentalx devicex device to increaseto increase protection the the SCR SCR effect de-NO de-NO (increasex temperaturex temperature of SCR up up de-NO to to its itsx optimal optimalefficiency) operationoperation as well range,range, as additional providingproviding flue gasa greata heat great environmental for environmental preheating protection theprotection air. Theeffect effect additional (increase (increase of flue of SCR SCR gas de-NO de-NO heat,xx whichefficiency) efficiency) is obtained asas well asas from additionaladditional solar energyflue flue (injectedgasgas heat heat afterfor for preheating the preheating economizer), the the air. air. could The The additional be additional released fl toflueue the gas gas combustion heat, heat, whichwhich air isis in obtainedobtained the following from solarsolar air preheating energyenergy process.(injected(injected Theafter after additionalthe the economizer), economizer), energy could could absorbed be be released released by the to to the combustion the combustion combustion air air air immediately inin thethe following increases airair preheatingpreheating the boiler furnaceprocess.process. energy The The additional input, additional which energy energy could absorbed decreaseabsorbed by theby the coalthe combustion combustion consumption airair and immediatelyimmediately bring about increases a great the coalthe boilerboiler savings furnace energy input, which could decrease the coal consumption and bring about a great coal effect;furnace (2) energy For high input, load, which the SCRcould de-NO decreasex temperature the coal consumption is in the optimal and bring operation about range a great and coal does savings effect; (2) For high load, the SCR de-NOx temperature is in the optimal operation range and notsavings need effect; to be increased.(2) For high The load, collected the SCR solar de-NO energyx temperature is then used is in tothe heat optimal the feed operation water range before and RH1, does not need to be increased. The collected solar energy is then used to heat the feed water before savingdoes not No. need 1 extraction to be increased. steam. TheThe savedcollected extraction solar energy steam is could then passused throughto heat the the feed following water stagesbefore of RH1, saving No. 1 extraction steam. The saved extraction steam could pass through the following theRH1, turbines saving and No. increase1 extraction the outputsteam. power,The saved bringing extraction a great steam coal could savings pass effect; through (3) At the medium following load, stages of the turbines and increase the output power, bringing a great coal savings effect; (3) At thestages collected of the solar turbines energy and is increase used to boththe output heat the po fluewer, gas bringing before a the great SCR coal de-NO savingsdevice effect; (increasing (3) At medium load, the collected solar energy is used to both heat the flue gas beforex the SCR de-NOx medium load, the collected solar energy is used to both heat the flue gas before the SCR de-NOx SCRdevice de-NO (increasingx efficiency SCR as de-NO well asx increasingefficiency as the well boiler as increasing furnace energythe boiler input) furnace and energy heat the input) feed and water beforedeviceheat RH1(increasing the feed (saving water SCR No. before de-NO 1 extraction RH1x efficiency (saving steam). No. as well1 extraction as increasing steam). the boiler furnace energy input) and heat the feed water before RH1 (saving No. 1 extraction steam).

Boiler flue No.1 extraction No.2 extraction Boilergas flue No.1 extractionsteam No.2 steamextraction gas steam steam

Economizer Solar energy Economizer Solar energy Flue gas heater RH1 RH2 Flue gas heater RH1 RH2 SCR de-NOx Air SCR de-NOx preheater Air preheater

(a)

(a) Figure 2. Cont.

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Boiler flue No.1 extraction No.2 extraction gas steam steam

Economizer

RH1 RH2

SCR de-NOx Feed water heater Air preheater Solar energy

(b) Boiler flue No.1 extraction No.2 extraction gas steam steam

Economizer

RH1 RH2 Flue gas heater

SCR de-NOx Solar energy Feed water heater Air preheater Solar energy

(c)

FigureFigure 2. 2.Concept Concept of of the the proposed proposed SAPG. ( (aa)) Low Low load; load; (b (b) High) High load; load; (c) ( Mediumc) Medium load. load.

3. Application3. Application of of the the Proposed Proposed Integrated Integrated System in in a a 1000 1000 MW MW Coal-Fired Coal-Fired Power Power Plant Plant

3.1.3.1. Description Description of of the the Reference Reference Plant Plant A 1000A 1000 MW MW ultra-supercritical ultra-supercritical coal-ﬁredcoal-fired power pl plantant is is selected selected as as a reference a reference case. case. The The live live steamsteam is producedis produced at at 26.38 26.38 MPa MPa and and 600600 ◦°C,C, and a a single single reheater reheater operates operates at 620 at 620 °C ◦andC and 5.38 5.38 MPa. MPa. The ultimate and proximate analyses of the raw coal are listed in Table 1. The overall performance of The ultimate and proximate analyses of the raw coal are listed in Table1. The overall performance of the the reference power plant and the main thermal parameters of the regenerative system in design/off- reference power plant and the main thermal parameters of the regenerative system in design/off-design design cases are shown in Tables 2 and 3, respectively. The steam turbine expansion lines on the casesMollier are shown diagram in of Tables all loads2 and are3 shown, respectively. in Figure The3. steam turbine expansion lines on the Mollier diagram of all loads are shown in Figure3. Table 1. Proximate analysis of the coal. Table 1. Proximate analysis of the coal. Items Mar (%) Aar (%) Car (%) Har (%) Oar (%) Nar (%) Sar (%) Qnet,ar (MJ·kg−1) Parameters 18.50 10.24 57.33 3.26 9.43 0.61 0.63 21.23 −1 Items Mar (%) Aar (%) Car (%) Har (%) Oar (%) Nar (%) Sar (%) Qnet,ar (MJ·kg )

Parameters Table18.50 2. Thermodynamic 10.24 57.33 performance 3.26 of the reference 9.43 1000 MW 0.61 coal-fired 0.63 power plant. 21.23

100% (High 75% (Medium 50% (Low Items Table 2. Thermodynamic performance of the referenceLoad) 1000 MWLoad) coal-ﬁred power plant.Load) Coal input rate (kg/s) 100.4 76.2 52.4 Items 100% (High Load) 75% (Medium Load) 50% (Low Load) Total energy of coal input (MWth) 2134.3 1619.7 1115.0 Live steam/reheatCoal input steam rate (kg/s) mass flow rate (kg/s) 100.4 748.2/620.4 541.8/460.876.2 354.1/309.2 52.4 Total energy of coal input (MW ) 2134.3 1619.7 1115.0 SCR de-NOx temperatureth (°C) 350.0 330.0 311.0 Live steam/reheat steam mass ﬂow rate (kg/s) 748.2/620.4 541.8/460.8 354.1/309.2 SCR de-NO temperature (◦C) 350.0 330.0 311.0 x Energies 2017, 10, 1485 5 of 18 Energies 2017, 10, 1485 5 of 18

Exhaust flue gas temperature (°C) Table 2. Cont 121.0. 115.0 109.0 Combustion air temperature (°C) 320.0 304.0 290.0 ExcessItems air coefficient 100% (High 1.15 Load) 75% (Medium1.24 Load) 50% (Low1.35 Load) Gross power output (MW) 1000.0 750.0 500.0 Exhaust flue gas temperature (◦C) 121.0 115.0 109.0 StandardCombustion coal airconsumption temperature rate (◦C) (g/kWh) 320.0 262.06 304.0265.17 273.80 290.0 Excess air coefficient 1.15 1.24 1.35 GrossTable power 3. output Primary (MW) thermal parameter of 1000.0 the regenerative system750.0 of the case unit. 500.0 Standard coal consumption rate (g/kWh) 262.06 265.17 273.80 Load Items 1# 2# 3# DEA 5# 6# 7# 8# 9# Extraction steam temperature (°C) 406.0 359.3 302.5 376.6 297.1 246.4 191.2 88.1 63.2 TableExtraction 3. Primary steam pressure thermal (MPa) parameter 8.09 of5.78 the regenerative2.29 1.09 system 0.60 of the0.38 case 0.23 unit. 0.07 0.02 100% Extraction steam flow rate (kg/s) 40.8 87.0 36.0 23.3 16.5 16.3 31.8 22.6 21.7 (design LoadOutlet feed water Itemstemperature (°C) 299.2 1# 273.7 2# 216.3 3# 180.6 DEA 153.8 5# 137.5 6# 120.8 7# 84.0 8# 59.3 9# load) OutletExtraction feed water steam pressure temperature (MPa) (◦ C)32.20 406.0 32.21 359.3 32.22 302.5 1.02 376.6 1.02 297.1 1.03 246.4 191.21.04 1.05 88.1 63.21.06 ExtractionWater flow steam rate pressure (kg/s) (MPa) 748.2 8.09 748.2 5.78 748.2 2.29 748.2 1.09 561.1 0.60 561.1 0.38 561.1 0.23 496.6 0.07 496.6 0.02 100% ExtractionExtraction steam steam temperature flow rate (kg/s)(°C) 411.0 40.8 365.0 87.0 283.0 36.0 382.4 23.3 301.5 16.5 250.7 16.3 196.4 31.8 83.222.6 21.758.6 (design load) ◦ ExtractionOutlet feed steam water pressure temperature (MPa) ( C) 6.05 299.2 4.32 273.7 1.73 216.3 0.84 180.6 0.46 153.8 0.29 137.5 120.80.18 0.05 84.0 59.30.02 75% Outlet feed water pressure (MPa) 32.20 32.21 32.22 1.02 1.02 1.03 1.04 1.05 1.06 Extraction steam flow rate (kg/s) 25.2 55.7 25.3 16.81 11.2 11.2 22.5 15.4 13.5 (medium Water flow rate (kg/s) 748.2 748.2 748.2 748.2 561.1 561.1 561.1 496.6 496.6 Outlet feed water temperature (°C) 280.0 255.6 202.2 169.6 143.8 128.5 112.8 77.5 54.7 load) Extraction steam temperature (◦C) 411.0 365.0 283.0 382.4 301.5 250.7 196.4 83.2 58.6 Outlet feed water pressure (MPa) 23.81 23.82 23.83 0.78 0.78 0.79 0.80 0.81 0.82 Extraction steam pressure (MPa) 6.05 4.32 1.73 0.84 0.46 0.29 0.18 0.05 0.02 75% ExtractionWater flow steam rate flow (kg/s) rate (kg/s) 541.8 25.2 541.8 55.7 541.8 25.3 541.8 16.81 418.7 11.2 418.7 11.2 418.7 22.5 373.9 15.4 373.9 13.5 (medium load)ExtractionOutlet feed steam water temperature temperature (°C) (◦ C) 416.6 280.0 371.5 255.6 258.7 202.2 388.6 169.6 306.3 143.8 257.1 128.5 202.9 112.8 88.177.5 54.752.0 ExtractionOutlet feed steam water pressure pressure (MPa) (MPa) 4.07 23.81 2.91 23.82 1.18 23.83 0.59 0.78 0.32 0.78 0.20 0.79 0.12 0.80 0.04 0.81 0.820.01 50% (low ExtractionWater steam flow flow rate rate (kg/s) (kg/s) 13.5 541.8 31.3 541.8 15.6 541.8 10.6 541.8 418.76.7 418.7 6.8 418.714.2 373.9 9.4 373.9 6.4 load) OutletExtraction feed water steam temperature temperature (°C) (◦C) 255.9 416.6 233.1 371.5 184.4 258.7 155.3 388.6 130.9 306.3 116.7 257.1 102.3 202.9 68.988.1 52.048.1 OutletExtraction feed water steam pressure pressure (MPa) (MPa) 15.97 4.07 15.98 2.91 15.99 1.18 0.55 0.59 0.55 0.32 0.56 0.20 0.57 0.12 0.58 0.04 0.010.59 Extraction steam flow rate (kg/s) 13.5 31.3 15.6 10.6 6.7 6.8 14.2 9.4 6.4 50% (low load) Water flow rate (kg/s) 354.1 354.1 354.1 354.1 283.0 283.0 283.0 255.3 255.3 Outlet feed water temperature (◦C) 255.9 233.1 184.4 155.3 130.9 116.7 102.3 68.9 48.1 Outlet feed water pressure (MPa) 15.97 15.98 15.99 0.55 0.55 0.56 0.57 0.58 0.59 Water flow rate (kg/s) 354.1 354.1 354.1 354.1 283.0 283.0 283.0 255.3 255.3

4000

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Enthalpy (kJ/kg) Enthalpy 1000

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0 012345678 Entropy (kJ/(kg⋅K)) (a) 4000

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Enthalpy (kJ/kg) 1000

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0 012345678 Entropy (kJ/(kg⋅K)) (b) Figure 3. Cont.

Energies 2017, 10, 1485 6 of 18 Energies 2017, 10, 1485 6 of 18

4000

3500

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Enthalpy (kJ/kg) Enthalpy 1000

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0 012345678 Entropy (kJ/(kg⋅K)) (c) FigureFigure 3. 3.Steam Steam turbine turbine expansion expansion lines lines onon thethe Mollier diagram diagram of of all all loads. loads. (a ()a High) High load; load; (b) ( bMedium) Medium load;load; (c )(c Low) Low load. load.

3.2. System Description 3.2. System Description Figure 4a shows a schematic of the integrated power plant using the proposed concept under Figure4a shows a schematic of the integrated power plant using the proposed concept under different load scenarios. As the most commercially available technology, parabolic trough collectors different load scenarios. As the most commercially available technology, parabolic trough collectors are used to collect solar energy. An oil-flue gas heater (OFGH) located before the SCR de-NOx device are used to collect solar energy. An oil-flue gas heater (OFGH) located before the SCR de-NOx device is adopted to heat the flue gas to increase the SCR de-NOx temperature for low and medium loads is adoptedand an oil-water to heat heater the flue (OWH) gas to is increaseused to heat the the SCR feed de-NO waterx temperaturebefore RH1 for for high low and and medium medium loads. loads andA anflue oil-water gas-air heater heater (FGAH) (OWH) is is used used to to recover heat the the feed extra water flue gas before heat RH1 (increased for high by andthe solar medium energy) loads. A flueand gas-airincrease heater the combustion (FGAH) is air used temperature to recover for the low extra and flue medium gas heat loads. (increased Valves 1 byand the 2 control solar energy) the andsolar increase energy the contribution combustion by aircontrolling temperature the heat for co lownduction and medium oil flow loads.rate for Valves different 1 and loads. 2 control Valve 3 the solaris used energy to control contribution the flow by rate controlling of feed water. the heat Valv conductiones 4 and 5 are oil employed flow rate forto control different the loads. air flow Valve rate. 3 is used to control the flow rate of feed water. Valves 4 and 5 are employed to control the air flow rate. 1. For a partial load (below 50% of the base load) scenario, valves 1 and 5 open while valves 2–4 1. Forclose. a partial The schematic load (below of the 50% prop ofosed the baseSAPG load) is shown scenario, in Figure valves 4b. 1All and the 5 collected open while solar valves energy 2–4 close.is used The to schematic heat the flue ofthe gas proposed in the OFGH SAPG and is the shown SCR inde-NO Figurex operation4b. All the temperature collected solar increases energy correspondingly. The additional flue gas heat is recovered in the FGAH and the combustion air is used to heat the flue gas in the OFGH and the SCR de-NOx operation temperature increases temperature is increased. correspondingly. The additional flue gas heat is recovered in the FGAH and the combustion air 2. For the baseload, valves 2–4 open while valves 1 and 5 are closed. The schematic of the proposed temperature is increased. SAPG is shown in Figure 4c. All the collected solar energy is used to heat the feed water in OWH 2. For the baseload, valves 2–4 open while valves 1 and 5 are closed. The schematic of the proposed and save high-grade extraction steam. The SCR de-NOx temperature is in the optimal operation SAPGrange is and shown there in is Figure no need4c. to All increase the collected it, so the solar OFGH energy does isnot used work to (valve heat the 1 in feed Figure water 4a closes). in OWH andThe save FGAH high-grade in high load extraction is bypassed steam. and The not SCR in op de-NOerationx temperaturebecause there is is in no the additional optimal flue operation gas rangeheat and(valve there 5 in is Figure no need 4a closes). to increase it, so the OFGH does not work (valve 1 in Figure4a closes). 3. TheFor FGAH the medium in high power load is load, bypassed all the and valves not inopen operation and thebecause schematic there of the is no proposed additional SAPG flue is gas heatshown (valve in Figure 5 in Figure 4d. The4a closes). solar energy is used to heat both the flue gas in OFGH and the feed 3. Forwater the mediumin OWH, powerincreasing load, SCR all de-NO the valvesx temperature open and and the saving schematic high-grade of the extraction proposed steam. SAPG is shownBesides, in Figurethe FGAH4d. Theis employed solar energy to recover is used the toaddi heattional both flue the gas flue heat, gas while in OFGH part of and the the air feed is waterbypassed in OWH, to keep increasing the exhaust SCR flue de-NO gas temperaturex temperature in a andreasonable saving range. high-grade extraction steam. Besides,To evaluate the FGAH the performance is employed of the to recoverproposed the SAPG, additional the following flue gas parameters heat, while are part assumed of the and air is designed:bypassed (1) toFor keep all power the exhaust loads, fluethe SCR gas de-NO temperaturex temperatures in a reasonable of the proposed range. system are kept as 350 °C after integrating solar energy (same as full load); (2) The total collected solar energy is equal To evaluate the performance of the proposed SAPG, the following parameters are assumed and to the heat required for SCR de-NOx at 50% load (low load); (3) The boiler exhaust flue gas designed:temperatures (1) For of allthe power proposed loads, SAPG the SCRin all de-NOloads arex temperatures same as those of of the the proposed case unit; system (4) Theare average kept as ◦ 350directC after normal integrating irradiation solar (DNI) energy of solar (same energy as full is load); set to (2) 0.61 The kW/m total2 collected(average data solar in energy Northwest is equal of to theChina) heat required [2,17]. for SCR de-NOx at 50% load (low load); (3) The boiler exhaust flue gas temperatures of the proposed SAPG in all loads are same as those of the case unit; (4) The average direct normal irradiation (DNI) of solar energy is set to 0.61 kW/m2 (average data in Northwest of China) [2,17].

Energies 2017, 10, 1485 7 of 18

Economizer DEA COND

OFGH SCR deNOx RH1 RH2 RH3 RH5 RH6 RH7 RH8 RH9

V3 V1 AP

V2 OWH

FGAH

Heat V5 V4 conduction oil Solar collectors Air Flue gas (a)

Economizer DEA COND

OFGH SCR deNOx RH1 RH2 RH3 RH5 RH6 RH7 RH8 RH9

FGAH Heat conduction oil Solar collectors

Air Flue gas (b)

Economizer DEA COND

SCR deNOx RH1 RH2 RH3 RH5 RH6 RH7 RH8 RH9

OWH

Heat conduction oil Solar collectors Air Flue gas (c)

Economizer DEA COND

OFGH SCR deNOx RH1 RH2 RH3 RH5 RH6 RH7 RH8 RH9

OWH

FGAH

Heat conduction oil Solar collectors Air Flue gas (d)

FigureFigure 4. Schematic 4. Schematic of the of proposed the proposed SAPG. SAPG. (a) Overall (a) Overall schematic; schematic; (b) Low (b load;) Low (c )load; high ( load;c) high (d) load; medium (d) load. medium load.

Energies 2017, 10, 1485 8 of 18

4. Methodology

4.1. Thermodynamic Evaluation Energies 2017, 10, 1485 8 of 18

4.1.1. System4. Methodology Simulation The thermodynamic cycle and energy equilibrium of the proposed SAPG based on the case unit is 4.1. Thermodynamic Evaluation simulated using the EBSILON Professional software, which is widely used for the design, evaluation and optimization4.1.1. System of power Simulation plants [21,22]. The model details of the main components are listed in Table4. The thermodynamic cycle and energy equilibrium of the proposed SAPG based on the case unit Table 4. Models details of the primary components in EBSILON Professional. is simulated using the EBSILON Professional software, which is widely used for the design, evaluation and optimization of power plants [21,22]. The model details of the main components are Components Models listed in Table 4. Steam generator Dry ash extraction and single reheat is modeled as a black box Table 4. ModelsInlet pressure details of is definedthe primary for the components steam turbine. in EBSILON In most Professional. cases, the outlet pressure is defined by the inlet pressure of the following turbine stage. For the last turbine stage, Steam turbinesComponents Models the outlet pressure is defined by the inlet pressure of the condenser (0.00575 MPa). Steam generator Dry ash extraction and single reheat is modeled as a black box Mechanical Efficiency = 0.998 Inlet pressure is defined for the steam turbine. In most cases, the outlet pressure is Upperdefined terminal by the temperatureinlet pressure of difference the following and turbine the lower stage. terminal For the last temperature turbine difference of Steam turbines RHs thestage, after-cooler the outlet are pressure to be specified.is defined by Pressure the inlet losspressure = 3.3–5% of the incondenser the steam (0.00575 extraction of differentMPa). Mechanical RHs, Heat Efficiency loss = 0% = 0.998 InletUpper temperature terminal temperature (20 ◦C) and difference pressure and (0.1 the MPa) lower ofterminal the cooling temperature medium is specified, CondenserRHs difference of the after-cooler are to be specified. Pressure loss = 3.3–5% in the steam upper temperature difference = 5 ◦C, pressure loss = 0.005 Mpa extraction of different RHs, Heat loss = 0% Pumps IsentropicInlet temperature efficiencies (20 °C) = 0.8, and mechanical pressure (0.1efficiency MPa) of the = cooling 0.998 medium is specified, Condenser Electric generator Generatorupper temperature efficiency difference = 0.99 = 5 °C, pressure loss = 0.005 Mpa Pumps Isentropic efficiencies = 0.8, mechanical efficiency = 0.998 Solar collector, OFGH, Electric generator The Generator heat transfer efficiency capacity = 0.99 is calculated using the heat balance equation in Section 4.1.1 OWHSolar and collector, FGAH OFGH, The heat transfer capacity is calculated using the heat balance equation in Section OWH and FGAH 4.1.1

FigureFigure5 compares 5 compares the the simulation simulation results results andand designed data data of ofextraction extraction steam steam flow rate ﬂow in ratethe in the case unit,case basedunit, based on all on loads all loads and and the the designed designed coal input input rate. rate.

100

87 87.1 Designed data 80 Simulation data

40.8 41 40 36 36.2 31.8 31.8

23.3 23.3 Flow rate (kg/s) 22.6 22.621.7 21.4 20 16.5 16.516.3 16.3

0 RH1 RH2 RH3 DEA RH5 RH6 RH7 RH8 RH9 Extraction steam (a) 60 55.7 55.8 Designed data 50 Simulation data

30 25.2 25.4 25.3 25.1 22.5 22.5

20 16.8 16.8 15.4 15.5

Flow rate (kg/s) 13.5 13.3 11.2 11.311.2 11.2 10

0 RH1 RH2 RH3 DEA RH5 RH6 RH7 RH8 RH9 Extraction steam (b) Figure 5. Cont. Energies 2017, 10, 1485 9 of 18 Energies 2017, 10, 1485 9 of 18

40 Designed data 31.3 31.4 Simulation data 30

15.6 15.5 14.2 14.2 13.5 13.6

10.6 10.6 9.4 9.5 Flow rate rate (kg/s) Flow 10 6.7 6.8 6.8 6.8 6.4 6.3

0 RH1 RH2 RH3 DEA RH5 RH6 RH7 RH8 RH9 Extraction steam (c)

FigureFigure 5. Comparison 5. Comparison of theof the simulation simulation datadata and and desi designedgned data data of extraction of extraction steam steam flow rate. ﬂow (a)rate. High (a) High load; (load;b) Medium (b) Medium load; load; (c) ( Lowc) Low load. load.

The highThe high agreement agreement between between the the simulatedsimulated results results and and thethe realreal data dataproves proves that the that models the modelsare are reliable to simulate the thermal progress of the proposed SAPG. In order to make the simulation reliable to simulate the thermal progress of the proposed SAPG. In order to make the simulation reliable and simple, the solar collector, OFGH, OWH and FGAH are regarded as heat injection reliablesources and simple, of the coal-fired the solar power collector, plant. OFGH, Based OWHon the and energy FGAH balance, are regardedin the proposed as heat SAPG, injection the sources of therequired coal-ﬁred energy power for plant.SCR de-NO Basedx is on equal the to energy the additional balance, available in the proposed flue gas heat SAPG, for air the preheating required energy for SCRand de-NO the additionalx is equal heat to absorbed the additional by the co availablembustion ﬂueair, which gas heat can be for expressed air preheating as: and the additional

00 heat absorbed by the combustion air,=−=== which can be expressed⋅ α as: − QBIIQQBIISCR j() p c fg a j 0 ( p c ) (1)

0 0 where Qfg and Qa representQSCR the= additionalBj(Ip − Iavailablec) = Qfg flue= gasQa heat= B andj · α 0the(Ip additional− Ic ) heat absorbed by (1) the combustion air (kW). Ip and Ic denote the flue gas enthalpies (kJ/kg-coal) at the SCR de-NOx 0 0 wheretemperatureQfg and Q ofa represent the proposed the SAPG additional (350 °C) and available case unit, ﬂue respectively. gas heat andIp and the additionalIc are the theoretical heat absorbed by thehot combustion air enthalpies air (kJ/kg-coal) (kW). Ip and in theIc propdenoteosed theSAPG ﬂue and gas the enthalpies case unit, respectively. (kJ/kg-coal) Bj represents at the SCR the de-NOx ◦ 0 0 temperaturecoal combustion of the proposed rate (kg/s) SAPGand α0 denotes (350 C) the and excess case air unit, ratio. respectively. Ip and Ic are the theoretical The total required solar energy is equal to the sum of required energy for SCR de-NOx and the hot air enthalpies (kJ/kg-coal) in the proposed SAPG and the case unit, respectively. Bj represents the heat absorbed by feed water before RH1, which can be expressed as: coal combustion rate (kg/s) and α0 denotes the excess air ratio. QQ=+ Q The total required solar energy is equals to the SCR sum RH1 of required energy for SCR de-NO(2)x and the heat absorbed by feed water before RH1, which can be expressed as: 4.1.2. Heat Transfer Area Calculation Qs = Q + Q (2) The area of the solar collectors could be calculatedSCR as [17]:RH1 Q 4.1.2. Heat Transfer Area Calculation = s As (3) DNI ⋅ϕ The area of the solar collectors could be calculated as [17]:

where Qs denotes the total required solar energy of the proposed SAPG (kW); φ is the solar collector efficiency and DNI represents the average direct normalQs irradiation of solar energy. As = (3) The solar collector efficiency φ, which is relativeDNI ·toϕ the inlet fluid temperature, ambient air temperature and DNI, could be calculated as [23]: where Qs denotes the total required solar energy of the proposed SAPG (kW); φ is the solar collector ΔΔTT2 efficiency and DNI representsϕ=73.3 the−⋅Δ−⋅−⋅ average0.007276 directT 0.496 normal ( irradiation ) 0.0691 ( of solar ) energy. (4) The solar collector efficiency φ, which is relativeDNI to the inlet DNI fluid temperature, ambient air temperaturewhere Δ andT is DNI, receiver could fluid be temperature calculated above as [23 ambient]: air temperature (°C). The overall heat transfer coefficient of the FGAH can be given as follows [24]: ∆T ∆T2 ϕ = 73.3 − 0.007276 · ∆T − 0.496 · ( ) − 0.0691 · ( ) (4) DNI DNI ◦ where ∆T is receiver fluid temperature above ambient air temperature ( C). The overall heat transfer coefficient of the FGAH can be given as follows [24]:

ξ · C K = n (W/(m2 · K)) (5) FGAH 1 1 + xfg · αfg xa1 · αa1 + xa2 · αa2 Energies 2017, 10, 1485 10 of 18

where αfg, αa1 and αa2 denote the convective heat transfer coefficients of the flue gas, primary air and secondary air, respectively; ξ denotes the utilization factor (0.9); Cn represents the factor reflecting the rotational speed (Cn = 1) [24]; and xfg (0.46), xa1 (0.14) and xa2 (0.28) represent the share of the flue gas, primary air and secondary air in the air preheater, respectively. The overall heat transfer coefficient of OFGH and OWH can be calculated as follows [25]:

1 K = (6) 1 1 + + Rth1 + Rth2 α1 α2 where α1 and α2 denotes the convective heat transfer coefﬁcients of hot source and cold source, respectively. Rth1 and Rth2 represents the thermal resistance of the tube wall and fouling thermal resistance, respectively. The heat transfer area can be calculated as follows: Q A = (7) K · ∆t where Q represents the heat transfer capacity and ∆t denotes the logarithmic mean temperature difference.

4.2. SCR de-NOx Performance Evaluation

Operation temperature is the most important parameter in the SCR de-NOx process of a coal-fired power plant. Generally, the SCR de-NOx efficiency is high at full load but presents downtrend as the load decreases, which is due to the decreased operation temperature. The increase of the SCR de-NOx efficiency by incorporating the proposed SAPG is calculated based on the operation curve in a typical coal-fired power plant, which is illustrated in Figure1[ 20]. The total NOx production could be calculated as follows [26]:

−6 GNOx = 1.63Bj · (β · Nar + Vy · 10 · CNOx) (8) where β represents the transform ratio of fuel-nitrogen to NOx; Vy denotes the ﬂue gas ﬂow rate 3 3 (m /kg-coal); and CNOx is the thermal NOx produced by coal (m /kg-coal).

4.3. Brieﬂy Economic Evaluation To evaluate the economic performance of the proposed SAPG, the cost of the generated electric power possible after introducing the solar energy is selected as the evaluation criteria and the cost of electricity (COE) is calculated as [17,21]:

FCI · CRF + O&M COE = (9) Es where O&M represents the annual operating and maintenance costs; Es is the additional solar electricity output, which is equal to the electricity output increase of the proposed SAPG. The capital recovery factor (CRF) can be calculated as [17,27]:

(1 + r)n · r CRF = (10) (1 + r)n − 1 where r denotes the discount rate and n represents the expected plant lifetime. The ﬁxed capital investment (FCI) of the solar collecting device and FGAH can be calculated using the scaling up method, which is expressed as [17,27]:

S f FCI = FCIr · (11) Sr Energies 2017, 10, 1485 11 of 18

where FCIr is the reference FCI of the equipment; S and Sr denote the scale parameter of the equipment in the proposed SAPG and the reference equipment, respectively; f is the scaling factor. The main parameters for COE and FCI calculations are shown in Tables5 and6, which can be obtained from [2,17,28–31].

Table 5. Basic parameters for economic analysis.

Items Values Annual effective interest rate (r) 8% Plant economic life (n) 25 years O&M 4% of FCI Annual operating hours 3037 h/year Annual high load (100%) operating hours 1263 h/year Annual medium load (75%) operating hours 1120 h/year Annual low load (50%) operating hours 1867 h/year

Table 6. Basic capital cost data of facilities.

Component Reference FCI (Million USD) Reference Scale Scale Unit Scaling Factor Solar collector 34.78 186,000 Area (m2) 0.90 OFGH 0.69 13,149 Heat transfer area (m2) 0.68 OWH 0.80 8372 Heat transfer area (m2) 0.68 FGAH 6.24 353,949 Heat transfer area (m2) 0.68

5. Results and Discussion

5.1. Thermal Performance Table7 shows the parameters of the main heat exchangers in the proposed SAPG. The following observations are obtained from Table7: (1) In the proposed SAPG, the total collected solar energy in all loads is 24.5 MWth; (2) the utilization of 24.5 MWth of solar energy varies in different load; and (3) the heat transfer area of all heat exchangers is not large, which is reasonable and practical in engineering.

Table 7. Thermal parameters of the primary heat exchangers in the proposed SAPG.

Heat 75% Items 100% (High Load) 50% (Low Load) Exchangers (Medium Load) Inlet/outlet oil temperature (◦C) 380.0/338.0 380.0/338.0 380.0/338.0 Oil flow rate (kg/s) 244.0 244.0 244.0 Solar collector Efficiency 70.8 70.8 70.8 Heat transfer capacity (MWth) 24.5 24.5 24.5 Heat transfer area (m2) 56,729 56,729 56,729 Inlet/outlet oil temperature (◦C) -/- 380.0/338.0 380.0/338.0 Inlet/outlet flue gas temperature (◦C) -/- 330.0/350.0 311.0/350.0 Oil/flue gas flow rate (kg/s) -/- 169.3/761.6 244.0/566.6 OFGH Overall heat transfer coefficient (W/m2 K) - 40.0 33.7 Heat transfer capacity (MWth) - 17.0 24.5 Heat transfer area (m2) - 25,545 25,545 Inlet/outlet oil temperature (◦C) 380.0/338.0 380.0/338.0 -/- Inlet/outlet feed water temperature (◦C) 273.7/280.6 255.6/349.9 -/- Oil/water flow rate (kg/s) 244.0/745.7 74.7/15.4 -/- OWH Overall heat transfer coefficient (W/m2 K) 279.6 132.8 - Heat transfer capacity (MWth) 24.5 7.5 - Heat transfer area (m2) 1087 1087 - Inlet/outlet flue gas temperature (◦C) -/- 135.1/115.0 147.5/109.0 Inlet/outlet air temperature (◦C) -/- 28.8/90.5 34.6/83.1 Flue gas/air flow rate (kg/s) -/- 804.5/257.4 607.4/501.5 FGAH Overall heat transfer coefficient (W/m2 K) - 10.3 13.5 Heat transfer capacity (MWth) - 17.0 24.5 Heat transfer area (m2) - 26,172 26,172 Energies 2017, 10, 1485 12 of 18

Inlet/outlet air temperature (°C) -/- 28.8/90.5 34.6/83.1 Flue gas/air flow rate (kg/s) -/- 804.5/257.4 607.4/501.5 Overall heat transfer coefficient (W/m2 K) - 10.3 13.5 Heat transfer capacity (MWth) - 17.0 24.5 Heat transfer area (m2) - 26,172 26,172 Energies 2017, 10, 1485 12 of 18 The thermal performance of the proposed SAPG is illustrated in Table 8. It can be seen that the proposedThe SAPG thermal could performance reduce the of standard the proposed coal consumption SAPG is illustrated rate by in 2.68 Table g/kWh,8. It can 4.05 be g/kWh seen that and the 6.31proposed g/kWh under SAPG high, could medium reduce and the standardlow load, coalrespectively. consumption The increase rate by 2.68of the g/kWh, output 4.05electric g/kWh power and varies6.31 from g/kWh 10.4 under MWe high, to 11.9 medium MWe under and low different load, respectively. loads. In general, The increase the proposed of the output SAPG electrichas a great power coalvaries saving from effect 10.4 for MW alle loads.to 11.9 The MW coale under saving different effect shows loads. an In general,uptrend thewith proposed the decrease SAPG of hasload. a great coal saving effect for all loads. The coal saving effect shows an uptrend with the decrease of load. Table 8. Thermal performance of the proposed SAPG.

Table 8. Thermal performance of the proposed75% SAPG. (Medium 50% (Low Items 100% (High Load) Load) Load) Combustion air temperatureItems increase (°C) 100% (High - Load) 75% (Medium 28.3 Load) 50% (Low 50.6 Load) ◦ SavedCombustion high-grade air temperature extraction increasesteam (kg/s) ( C) - 12.3 28.3 3.2 50.6 - Saved high-grade extraction steam (kg/s) 12.3 3.2 - Gross power generated (MWe) 1010.4 761.7 511.9 Gross power generated (MWe) 1010.4 761.7 511.9 Gross additional power generated (MWe) 10.4 11.7 11.9 Gross additional power generated (MWe) 10.4 11.7 11.9 StandardStandard coal coal consumption consumption raterate (g/kWh)(g/kWh) 259.40 259.40 261.11 261.11 267.48 267.48 StandardStandard coal coal consumption consumption rate decreasedecrease (g/kWh) (g/kWh) 2.68 2.68 4.05 4.05 6.31 6.31

FigureFigure 6 6compares compares the the coal-based coal-based thermal efﬁciencyefficiency ofof the the proposed proposed system system and and the the original original case caseunit unit under under different different loads. loads. As indicated,As indicated, the coal-basedthe coal-based thermal thermal efﬁciency efficiency of the of proposed the proposed is 47.34%, is 47.34%,47.03% 47.03% and 45.01% and 45.01% under under high, mediumhigh, medium and low and load, low respectively, load, respectively, which is which 0.49%, is 0.72% 0.49%, and 0.72% 1.07% andhigher 1.07% than higher that than of the that original of the caseoriginal unit. case unit.

47.5

47.0

46.5

46.0

45.5 Original case unit 45.0 Proposed system

Coal-based thermal efficiency (%) 44.5 50 60 70 80 90 100 Load (%)

Figure 6. Coal-based thermal efficiency of the proposed system and original case unit. Figure 6. Coal-based thermal efﬁciency of the proposed system and original case unit.

The energy flow of the proposed SAPG under different loads are illustrated in Figure 7. The followingThe observations energy ﬂow can of be the derived: proposed (1) SAPGthe solar under energy different inputs loadsfor all areloads illustrated are the same in Figure (24.57 . MWth);The following (2) For high observations loads, the cansolar be energy derived: enters (1) the the regenerative solar energy cycle inputs by forheating all loads the feed are thewater, same (24.5 MWth); (2) For high loads, the solar energy enters the regenerative cycle by heating the feed which could save 24.5 MWth of heat from the steam turbine bleeds; (3) For low loads, solar energy increaseswater, whichthe combustion could save air 24.5 temperature, MWth of heat which from leads the steamto 24.5 turbine MWth bleeds; of immediate (3) For lowenergy loads, input solar increaseenergy of increases the boiler the and combustion could produce air temperature, more live steam; which (4) leadsFor medium to 24.5 MWthloads, 17.0 of immediate MWth of solar energy energyinput enters increase the ofboiler the boiler by increasing and could the produce combustion more air live temperature steam; (4) For while medium 7.5 MWth loads, of 17.0that MWthenters of thesolar regenerative energy enters cycle theby heating boiler by the increasing feed water; the (5) combustion Compared airwith temperature the saved No. while 1 extraction 7.5 MWth steam of that (406enters °C, 8.09 theregenerative MPa) under cycle highby load, heating the energy the feed le water;vel of (5)additional Compared live with steam the (600 saved °C, No. 26.38 1 extraction MPa) ◦ ◦ producedsteam (406 by solarC, 8.09 energy MPa) under highlow load load, is the higher energy. Obviously, level of additional the energy live level steam of (600 solarC, energy 26.38 MPa) is upgradedproduced by bythe solar new energyutilization under method low loadin low is load. higher. It explains Obviously, why the the energy energy levelsaving of effect solar for energy low is loadsupgraded is significantly by the new greater utilization than that method for high in low loads. load. It explains why the energy saving effect for low loads is signiﬁcantly greater than that for high loads. Energies 2017, 10, 1485 13 of 18 Energies 2017, 10, 1485 13 of 18

(a)

(b)

(c)

Figure 7. Energy flow of the proposed SAPG. (a) High load; (b) low load; (c) medium load. Figure 7. Energy ﬂow of the proposed SAPG. (a) High load; (b) low load; (c) medium load.

5.2. SCR de-NOx Efficiency Increase 5.2. SCR de-NOx Efficiency Increase Table 9 illustrates the SCR de-NOx performance of the proposed SAPG. Figure 8 compares the Table9 illustrates the SCR de-NO x performance of the proposed SAPG. Figure8 compares the SCR de-NOx efficiency in the proposed SAPG and the original case unit. As can be seen, for all loads SCR de-NOx efficiency in the proposed SAPG and the original case unit. As can be seen, for all loads of the proposed SAPG, the SCR de-NOx temperature and efficiency are kept at 350 °C◦ and 82.9%, of the proposed SAPG, the SCR de-NOx temperature and efficiency are kept at 350 C and 82.9%, respectively. The SCR de-NOx efficiency increases by 3.1% and 7.9% under medium load and low respectively. The SCR de-NO efficiency increases by 3.1% and 7.9% under medium load and low load, load, respectively, as comparedx with the reference case. In general, the proposed SAPG shows great respectively, as compared with the reference case. In general, the proposed SAPG shows great NOx NOx removal benefits under low load. removal benefits under low load.

Table 9. SCR de-NOx performance improvement of the proposed SAPG. Table 9. SCR de-NOx performance improvement of the proposed SAPG. 100% (High 75% (Medium 50% (Low Items Items 100% (HighLoad) Load) 75% (MediumLoad) Load) 50% (LowLoad) Load) ◦ SCRSCR de-NO de-NOx temperaturex temperature of theof the referenced referenced unit unit ( C) (°C) 350.0 350.0 330.0 330.0 311.0 ◦ SCRSCR de-NO de-NOx temperaturex temperature of theof the proposed proposed SAPG SAPG ( C) (°C) 350.0 350.0 350.0 350.0 350.0 SCR de-NO temperature increase (◦C) - 20.0 39.0 SCR de-NOx x temperature increase (°C) - 20.0 39.0 SCR de-NOx efficiency of the original case unit (%) 82.9 79.8 75.0 SCR de-NOx efficiency of the original case unit (%) 82.9 79.8 75.0 SCR de-NOx efficiency of the proposed SAPG (%) 82.9 82.9 82.9 SCR de-NOx efficiency of the proposed SAPG (%) 82.9 82.9 82.9 SCR de-NOx efficiency increase (%) - 3.1 7.9 SCR de-NOx efficiency increase (%) - 3.1 7.9

Energies 2017, 10, 1485 14 of 18 Energies 2017, 10, 1485 14 of 18

74 Proposed SAPG Original case unit SCR deNOx efficiency deNOx (%) SCR SCR deNOx efficiency deNOx (%) SCR 72

70 50 60 70 80 90 100 Load (%)

FigureFigure 8. 8. ComparisonComparison of of the the SCR SCR de-NO de-NOxx efficiencyefﬁciency in in the the proposed proposed SAPG SAPG and and original original case case unit. unit.

FigureFigure 99 illustrates illustrates the the contribution contribution of NO of xNOemissionx emission decrease decrease in the proposedin the proposed SAPG. As SAPG. indicated, As indicated,the improvement the improvement of SCR de-NO of SCRx efﬁciency de-NOx efficiency could bring could 128.7 bring kg/s 128.7 and 74.0kg/s kg/sand 74.0 of NO kg/sx emission of NOx emissiondecrease underdecrease low under loadand low medium load and load, medium respectively. load, respectively. The NOx emission The NO decreasesx emission caused decreases by coal causedsavings by vary coal from savings 5.6 kg/s vary to 9.6from kg/s 5.6 under kg/s differentto 9.6 kg/s loads. under Obviously, different the loads. improvement Obviously, of SCRthe improvementde-NOx efﬁciency of SCR provides de-NOx theefficiency main contributionprovides the main for the contribution NOx emission for the decrease NOx emission at low loaddecrease and atmedium low load load, and which medium accounts load, which for over accounts 90%. for over 90%.

140 Caused by de-NOx efficiency increase 120 Caused by coal consumption decrease

100

20 NOx emission decreaseNOx emission (kg/s) NOx emission decreaseNOx emission (kg/s) 0 50% 75% 100% Load

FigureFigure 9. 9. ContributionContribution to to NO NOxx emissionemission decrease decrease in in the the proposed proposed SAPG. SAPG.

5.3.5.3. Sensitivity Sensitivity Analysis Analysis

InIn the the proposed proposed system, system, the the solar solar in intensitytensity will will influence influence the SCR de-NOx performanceperformance and and thermalthermal performance. performance. The The off-design off-design behavior behavior of of the the solar solar field field is is performed performed in in this this section. section. As As we we know,know, the DNI DNI of of solar solar energy energy may may show show abrupt abrupt change changes.s. In In the the proposed proposed flexible flexible system, system, the the abrupt abrupt changeschanges of of DNI DNI could could be be dealt dealt by by controlling controlling the the so solarlar energy energy contribution contribution (by (by controlling controlling the the flow flow raterate of of the the heat heat conduction conduction oil oil by by valves). valves). (1) (1) Wh Whenen DNI DNI increases, increases, the the additional additional solar solar energy energy is is used used toto heatheat feedfeed water water in in all all loads, loads, which which could could bring bring good energygood energy savings savings effects; (2)effects; When (2) DNI When decreases, DNI decreases,the solar energy the solar injected energy to injected the feed to water the decreasesfeed water first. decreases The solar first. energy The solar injected energy to the injected flue gas to willthe fluedecrease gas will when decrease there is when no solar there energy is no in solar the feedener watergy in the heating feed process.water heating The flexible process. operation The flexible mode operationcould weaken mode the could influence weaken of abrupt the influence changes of of ab DNIrupt and changes keep great of DNI SCR and de-NO keepx greatperformance SCR de-NO of thex performanceproposed system. of the proposed system. FigureFigure 1010 illustratesillustrates the the variations variations of of the the solar solar energy energy contribution contribution as as a a function function of of changes changes in in solarsolar intensity. TheThe following following observation observation are are derived: derived: (1) Under(1) Under high high load, load, the solar the energysolar energy for heating for heatingfeed water feed shows water an shows uptrend an uptrend as DNI increases;as DNI increases; (2) For medium (2) For medium and low loads,and low as DNIloads, increases, as DNI increases,the solar energythe solar for energy SCR for de-NO SCRx de-NOperformancex performance improvement improvement firstly firstly shows shows an uptrend, an uptrend, and thenand then remains constant. The solar energy for heating feed water shows an uptrend with the increase of DNI, when DNI is higher than 424 W/m2 and 610 W/m2 under medium and low loads, respectively.

Energies 2017, 10, 1485 15 of 18

remains constant. The solar energy for heating feed water shows an uptrend with the increase of DNI, 2 2 EnergiesEnergieswhen 2017 DNI2017, ,10 is10, ,higher1485 1485 than 424 W/m and 610 W/m under medium and low loads, respectively.1515 of of 18 18

2626 4040 24 24 High load 3535 High load 22 Medium Medium load load 22 30 30 Low Low load load 20 20 2525 18 18 2020 15 1616 15 Medium load 10 1414 Medium load 10 Low load Low load 5 Solar energy for SCR de-NOx de-NOx SCR for energy Solar 5

Solar energy for SCR de-NOx de-NOx SCR for energy Solar 12

performance improvement (MW) improvement performance 12 performance improvement (MW) improvement performance 00 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Solar energy for heating feed water (MW) 300 400 500 600 700 800 900 1000 2 Solar energy for heating feed water (MW) 2 SolarSolar intensity intensity (W/m (W/m2)) SolarSolar intensity intensity (W/m (W/m)2)

((aa)) ((bb))

x FigureFigureFigure 10.10. 10. InfluenceInfluenceInﬂuence ofof of solarsolar solar intensityintensity intensity onon on solarsolar solar energyenergy energy contribution.contribution. (( aa)) ForFor SCRSCR de-NOde-NO x performanceperformanceperformance improvement;improvement;improvement; ((b (b))) forfor for heatingheating heating feedfeed feed water.water.

Figure 11 shows the variations of the SCR de-NO efficiency and standard coal consumption FigureFigure 11 11 shows shows thethe variationsvariations ofof thethe SCR SCR de-NOde-NOxx efficiency efficiencyx andand standardstandard coalcoal consumptionconsumption raterate rate decrease as a function of changes in solar intensity. As indicated in Figure 11, the SCR de-NO decreasedecrease asas aa functionfunction ofof changeschanges inin solarsolar intensity.intensity. AsAs indicatedindicated inin FigureFigure 11,11, thethe SCRSCR de-NOde-NOxx efficiencyefficiencyefficiency ofof of thethe the proposedproposed proposed SAPGSAPG SAPG atat at highhigh high loadload load remarema remainsinsins atat at 82.9%82.9% 82.9% withwith with thethe the changeschanges changes ofof of solarsolar solar intensity.intensity. intensity. For medium and low load, the SCR de-NO efficiency will reduce slightly when the solar intensity ForFor mediummedium andand lowlow load,load, thethe SCRSCR de-NOde-NOxx efficiencyxefficiency willwill reducereduce slightlyslightly whenwhen thethe solarsolar intensityintensity isis 2 2 lowerloweris lower thanthan than 424424 424 W/mW/m W/m22 andand and 610610 W/m 610W/m W/m22,, respectively.respectively., respectively. InIn thethe In pointpoint the pointofof thermalthermal of thermal performance,performance, performance, asas thethe as solarsolar the intensityintensitysolar intensity increases,increases, increases, thethe standardstandard the standard coalcoal consumptionconsumption coal consumption raratete decreasedecrease rate decrease showsshows anshowsan uptrend.uptrend. an uptrend. InIn general,general, In general, underunder under a relatively large variation of solar intensity, the proposed SAPG could keep high SCR de-NO aa relativelyrelatively largelarge variationvariation ofof solarsolar intensity,intensity, thethe proposedproposed SAPGSAPG couldcould keepkeep highhigh SCRSCR de-NOde-NOxx efficiencyefficiencyefficiency andand greatgreat energyenergy savingsaving effects effects for for all all loads. loads.

1010 83.0 83.0 9 High High load load 9 Medium load 82.5 8 Medium load 82.5 8 Low Low load load 77 82.082.0 66 81.581.5 55 81.081.0 44 High High load load 3 80.5 3 80.5 Medium Medium load load rate decrease (g/kWh) rate decrease 2 Low load (g/kWh) rate decrease 2

Low load coal consumption Standard

SCR de-NOx efficiency(%) 80.0 Standard coal consumption Standard

SCR de-NOx efficiency(%) 80.0 11 79.579.5 00 300300 400 400 500 500 600 600 700 700 800 800 900 900 1000 1000 300300 400 400 500 500 600 600 700 700 800 800 900 900 1000 1000 2 2 SolarSolar intensity intensity (W/m(W/m2)) SolarSolar intensityintensity (W/m(W/m2)) ((aa)) ((bb))

Figure 11. Inﬂuence of (a) solar intensity on SCR de-NOx x efﬁciency and (b) standard coal consumption FigureFigure 11. 11. Influence Influence of of ( (aa)) solar solar intensity intensity on on SCR SCR de-NO de-NOx efficiency efficiency and and ( (bb)) standard standard coal coal consumption consumption rateraterate decrease.decrease. decrease.

5.4.5.4.5.4. EconomicEconomic Economic PerformancePerformance Performance TableTableTable 1010 providesprovides the the economic economic performanceperformance performance ofof of thethe the proposedproposed proposed SAPG.SAPG. SAPG. AsAs As indicatedindicated indicated inin in TableTable Table 10, 10,10 , thethethe COECOE COE ofof solarsolar generatedgenerated electricityelectricity isis 0.040 0.040 USD/kWh, USD/kWh, whichwhich which isis is lowerlower lower thanthan than thethe the currentcurrent current parabolicparabolic parabolic troughtroughtrough SAPGSAPG SAPG (0.09–0.11(0.09–0.11 (0.09–0.11 USD/kWh)USD/kWh) [2,17].[2,17]. [2,17]. TheThe The reasonsreasons reasons mainlymainly mainly comecome come fromfrom from thethe the combinedcombined combined effectseffects effects ofof of thethe the highhighhigh coalcoal coal savingssavings savings effecteffect andand thethe affordableaffordable FCI FCI of of the the additional additional equipment. equipment.

TableTable 10.10. EconomicEconomic performancesperformances ofof thethe proposedproposed SAPG.SAPG.

ItemsItems Proposed Proposed SAPGSAPG FCIFCI ofof thethe additionaladditional equipmentequipment SolarSolar collectorcollector (million(million USD)USD) 11.95 11.95 OFGHOFGH (million(million USD)USD) 1.08 1.08 OWHOWH (million(million USD)USD) 0.21 0.21

Energies 2017, 10, 1485 16 of 18

Table 10. Economic performances of the proposed SAPG.

Items Proposed SAPG FCI of the additional equipment Solar collector (million USD) 11.95 OFGH (million USD) 1.08 OWH (million USD) 0.21 FGAH (million USD) 1.06 Total additional FCI (million USD) 14.3 Additional operation and maintenance cost (million USD) 0.57 Additional solar electricity output (GWh) 48.5 COE of solar generated electricity (USD/kWh) 0.040

6. Conclusions An improved flexible solar-aided power generation system (SAPG) for enhancing both the selective catalytic reduction (SCR) de-NOx efficiency and coal-based energy efficiency of coal-fired power plants under different power loads was proposed. This opens up a novel solar energy integration pattern in solar-aid coal-fired power plants. The SCR de-NOx efficiency of the proposed SAPG could increase by 3.1% and 7.9% under medium load and low load, respectively, by integrating the solar energy to increase the operation temperature of SCR de-NOx. Under high load, the proposed SAPG could reduce the standard coal consumption rate by 2.68 g/kWh by heating the feed water, and under low load, 6.31 g/kWh of standard coal consumption rate is reduced by reheating the combustion air. Under medium load, both the combustion air and feed water are heated and the resulting reduction of standard coal consumption is 4.05 g/kWh.

Acknowledgments: This study was partially supported by the National Major Fundamental Research Program of China (No. 2015CB251504), the National Nature Science Fund of China (No. 51706065, No. 51476053) and the Fundamental Research Funds for the Central Universities (No. 2017MS013, No. 2017ZZD003, No. 2015XS78). Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. Yu Han proposed the original ideas, carried out simulation, conducted calculation and wrote the paper. Cheng Xu, Gang Xu and Yuwen Zhang checked the paper, revised the paper and provided language support. Cheng Xu, Gang Xu and Yongping Yang provided technical and financial support. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

Abbreviations: Full name AP Air preheater COE Cost of electricity COND Condenser CRF Capital recovery factor DEA Deaerator DNI Direct normal irradiation FCI Fixed capital investment FGAH Flue gas-air heater OFGH Oil-ﬂue gas heater OWH Oil-water heater RH Regenerative heater SAPG Solar aided power generation SCR Selective catalytic reduction Symbols: Content Bj Coal combustion rate CNOx Thermal NOx produced by coal Cn Rotational speed factor Energies 2017, 10, 1485 17 of 18

Es Additional solar electricity output f Scale factor GNOx Total produced NOx I Enthalpy of ﬂue gas I0 Enthalpy of air K Heat transfer coefﬁcient n Number of years O&M Ratio of annual operating and management Q Heat transfer capacity

Rth Thermal resistance r Fraction interest rate per year S Scale parameter Vy Flue gas flow rate x Proportion α Convective heat transfer coefficients α0 Excess air ratio β Transform ratio of fuel-nitrogen to NOx ξ Utilization factor Receiver fluid temperature above ambient air ∆T temperature ∆t Logarithmic mean temperature difference ϕ Solar collector efficiency Subscripts: Content a Air a1 Primary air a2 Secondary air c Case unit fg Flue gas p Proposed system r Reference device s Solar energy

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