Design and Performance Analysis of New Ultra-Supercritical Double Reheat Coal-Fired Power Generation Systems

energies

Article Design and Performance Analysis of New Ultra-Supercritical Double Reheat Coal-Fired Power Generation Systems

Ming Yang, Liqiang Duan * and Yongjing Tong

School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China; [email protected] (M.Y.); [email protected] (Y.T.) * Correspondence: [email protected]; Tel.: +86-10-6177-1443

Abstract: In order to solve the existing problems of large mean heat transfer temperature differences of regenerative air heaters and high superheat degrees of regenerative extraction steam in double reheat coal-ﬁred power generation systems, two new design schemes of ultra-supercritical double reheat cycles are proposed, which can realize the deep boiler-turbine coupling among the heat transfer processes of air, feeding water and regeneration extraction steam on the base of the principle of energy level matching. A typical 1000 MW ultra-supercritical double reheat cycle system is selected as the reference system and the overall system model is built by using the Ebsilon simulation software. The performances of two new systems are analyzed by using both the exergy method and energy equilibrium method. The results show that net output powers of both new systems 1 and 2 increase by 6.38 MW and 6.93 MW, respectively, and the standard coal consumptions of power generation decrease by 1.65 g/kWh and 1.79 g/kWh, respectively. The off-design performances of new systems and the reference system are analyzed, and the results show that performances of two new systems are better than that of the reference system. The system ﬂow of the new system 2 is more complex compared with that of the new system 1. Generally speaking, the performance of new system 1 is better than that of new system 2. On the basis of new system 1, new system 3 is further optimized and its full operating condition performance characteristics are analyzed. The standard coal consumption rate of new system 3 is reduced about 1 g/kWh at higher load, and around 0.2 g/kWh at low load.

Citation: Yang, M.; Duan, L.; Tong, Y. Keywords: ultra-supercritical double reheat cycle; boiler-turbine coupling; exergy method; thermal Design and Performance Analysis of performance analysis; system optimization New Ultra-Supercritical Double Reheat Coal-Fired Power Generation Systems. Energies 2021, 14, 238. https://doi.org/en14010238 1. Introduction

Received: 9 December 2020 At present, coal is still the main energy source in many countries all over the world. Accepted: 30 December 2020 However, in the next five years, the contribution of coal to the global energy structure will Published: 5 January 2021 decline from 27% to 25%. China’s economy is in the period of structural transformation, and coal demand will gradually decline. The International Energy Agency (IEA) predicts Publisher’s Note: MDPI stays neu- that China’s coal consumption will show a structural decline of less than 1% per year on tral with regard to jurisdictional clai- average [1]. The annual development report of China’s power industry in 2019 pointed out ms in published maps and institutio- that the installed capacity of non-fossil energy power generation in China accounted for nal affiliations. 40.8% of the total installed capacity of power generation in the country, which was 2.1 per- centage points higher than the previous year. Power generation from non-fossil energy sources has accelerated the growth, and the proportion of coal-fired power generation to the total power generation capacity is decreasing gradually [2]. In order to further develop Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. the coal-fired power generation, it is imperative to improve the power generation efficiency This article is an open access article and reduce the coal consumption. distributed under the terms and con- In order to further improve the efficiency of power generation, many energy saving ditions of the Creative Commons At- and consumption reduction measures have been proposed. Increasing steam parameters tribution (CC BY) license (https:// not only reduces the heat transfer loss of the boiler, but also increases the generating creativecommons.org/licenses/by/ capacity of the unit, which can directly and effectively improve the efficiency of the unit. 4.0/). Nowadays, the main steam parameters of ultra-supercritical units have reached up to

Energies 2021, 14, 238. https://doi.org/10.3390/en14010238 https://www.mdpi.com/journal/energies Energies 2021, 14, 238 2 of 22

600 ◦C/31 MPa, and the ultra-supercritical technology with higher parameters is also in progress. Now the development goal is to increase the initial steam parameters of coal-fired power plants to 700 ◦C/35 MPa, and its thermal efficiency can reach more than 50% [3]. The waste heat utilization of flue gas can effectively reduce the flue gas energy loss and improve the efficiency of thermal power plants [4–8]. Min Yan et al. [4] proposed a novel boiler cold-end optimization system, in which both bypass flue gas waste heat after the wet flue gas desulfurizer and steam extraction heat were used to preheat the air, resulting more flue gas flowing into the bypass flue and high-pressure steam extraction reduced. Compared with the bypass flue waste heat recovery system, Yu Han et al. [5] added a steam-air heater between the air preheater and the pre-positioned air preheater, resulting in better energy-saving effects. Based on the data from an existing 1000 MW typical power generation unit in China, Xu et al. [6] analyzed four typical flue gas heat recovery schemes quantitatively from the thermodynamics perspective. Wang et al. [7] proposed that both the flue gas waste heat recovery exchanger bundled by the polytetra- fluoroethylene tube (FGC1) and the dewatering heat exchanger (FGC2) could recycle heat and water respectively. Four integration schemes of FGC1 and FGC2 with the regenerative system were proposed and analyzed. Multi-pressure condenser could effectively improve the thermal efficiency of the cycle. Xu et al. [8] applied the thermodynamic law to obtain the guiding principle for the optimum design of multi-back pressure condenser. The above schemes improved the overall system efficiency by reducing the heat loss to the environment. Improving the fuel quality and utilizing other heat sources can effectively reduce the coal consumption in power plants. Xu et al. [9] improved the thermal efficiency of coal-fired power plants by removing the water content in some coal types. The results showed that coal’s mass flow rate would increase by 0.6–1.5% as 0.1 kg moisture was removed per kg raw coal. The net efficiency of the power plant could increase in the range of 0.6–0.9%. Wang et al. [10] proposed a general system integration optimization method (GIOM) considering various possible integration schemes. The results showed that at the relatively higher heliostat field power (HFP) work conditions, the heat energy should be integrated into the highest pressure feedwater heater (FHs) as a priority. While at the relatively lower HFP and higher turbine power work conditions, the heat should be distributed into two highest pressure FHs in a certain proportion instead of fully distributed into the highest pressure FHs. The performances of both the receiver and the power block were improved. According to the Second Law of Thermodynamics, reducing the temperature difference of heat transfer can effectively reduce the heat transfer exergy loss. Zhou et al. [11,12] proposed a steam cycle optimization process with 10 regenerative heaters, which further improved the thermal performance of the double reheat power generation plant. To solve the problem of too high superheat degree of the extraction steam in 1000 MW ultra-supercritical (USC) units, the energy-saving effects of two systems with one or two outside steam coolers (OSC) respectively were analyzed and compared with that of the small turbine regenerative system in the cases of 85% and 90% internal efficiency of the small steam turbine. Meanwhile, the energy-saving effects were analyzed for various utilization modes of the superheat degree under different loading conditions. Breaking the boundary between the steam turbine system and the boiler system, further reducing the heat transfer temperature difference of the working medium, improving the heat transfer characteristics of different working mediums on both sides of the steam turbine system and the boiler system, can further reduce the heat transfer exergy loss. Yang et al. [13] put forward a heat integration system based on the boiler-turbine coupling method for large-scale coal-fired power plants. Low-pressure steam extraction at the turbine side is used to preheat the low temperature air, the absorbed heat of the air during the air preheating process is reduced, and the replaced part of high-temperature flue gas is used to heat the feed water and condensate water, and then, the heat transfer exergy losses are reduced obviously. Finally, the mass flow of the high-pressure steam extraction Energies 2021, 14, 238 3 of 22

is reduced and the output power of the overall power plant is increased. Fan et al. [14] put forward another scheme of boiler-turbine coupling. An additional economizer is used to replace the air preheater. The flue gas is used to heat part of the feed water, while the air is heated by the heat of the superheat of 9-stage extraction steams. The air preheating process reduces the superheat degrees of 9-stage extraction steams and the temperature differences of heat transfer in the thermal system, and finally reduces the coal consumption of the unit. In order to further optimize the double reheat coal-fired power plant system, this paper takes a 1000 MW advanced ultra-supercritical double reheat coal-fired power generation unit as a reference system. Based on the principle of energy level matching and considering the heat transfer characteristics of steam, air and water, new schemes of boiler- turbine coupling are proposed. The thermodynamic model is established, and the energy consumption analysis is carried out to reveal the energy-saving potential and thermal performance of the new systems. The new systems proposed in this paper have the following features: (1) The low temperature feed water and the superheat heat of extraction steam are used to heat part of the air. (2) The mass flow rate of the flue gas required for the air preheater is reduced, and the remaining high temperature flue gas replaces part of high temperature steam extraction to heat both the condensate water and feed water, so as to increase the output power of steam turbine. (3) Compared with systems of other literatures, the superheat degrees of extraction steams and the temperature differences of the air heat transfer process are reduced together in the new system 1 and new system 2. In addition, the performance of the optimized system (new system 3) is obtained based on the further optimization of the new system 1. The optimized system uses the feed water to preheat the air and then the heated air enters into the air preheater to be heated further, which will reduce the temperature differences of heat transfer in the thermal system.

2. Description of Reference Double Reheat Cycle and Heat Transfer Process Analysis A reference USC double reheat power plant is selected for analysis in this paper. The elemental analysis of design coal of the power plant is shown in Table1, the net calorific value of design coal is 23,440 kJ/kg. The reference unit flowchart is shown in Figure1. The initial parameters of the power plant are 30.1 MPa/605 ◦C/613 ◦C/613 ◦C. Ten-stage regenerative heaters and two-stage outside steam coolers are adopted in the regenerative extraction steam system. The temperature of the feed water entering into the boiler is 314 ◦C. Exhaust flue gas temperature is 117 ◦C, and the boiler efficiency is 94.70%. The coal consumption rate of power generation is 257.38 g/kWh. The overall performance of the plant is relatively high, however, there is still room for further energy saving and energy consumption reduction.

Table 1. Elemental analysis of coal.

Element Mass Fraction/%

1 Car 61.7 2 Har 3.67 3 Oar 8.56 4 Nar 1.12 5 Sar 0.6 6 Aar 8.8 7 Mar 15.55 8 Vdaf 34.73 Energies 2021, 14, x FOR PEER REVIEW 4 of 26

Table 1. Elemental analysis of coal.

Element Mass Fraction/% 1 Car 61.7 2 Har 3.67 3 Oar 8.56 4 Nar 1.12 5 Sar 0.6 6 Aar 8.8 Energies 2021, 14, 238 4 of 22 7 Mar 15.55 8 Vdaf 34.73

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OSC

Air Heater H1 H2 H3 H4 H6 H7 H8 H9 H10 OSC

Energies 2021, 14, x FOR PEER REVIEW 5 of 26 FigureFigure 1. 1. FlowchartFlowchart of of the the reference reference double double rehe reheatat coal-fired coal-ﬁred power generation system.

The heat transfer characteristics of both the air preheating system and the feeding water regenerative system are analyzed to reveal the potential of energy saving of the power plant. InIn thethe airair preheater,preheater, the the mass mass flow flow rate rate of of flue flue gas gas is 900is 900 kg/s kg/s and and the the mass mass flow flow rate rate of airof air is 819 is 819 kg/s. kg/s. The The specific specific heat heat capacity capacity of the of fluethe flue gas isgas larger is larger than than that ofthat air. of As air. the As heat the transferheat transfer process process proceeds, proceeds, the heat the transferheat transfer temperature temperature difference difference between between the fluethe gasflue andgas and the airthe will air will increase. increase. As shown As shown in Figure in Figure2, the 2, high the high temperature temperature terminal terminal difference differ- ofence the of air the preheater air preheater is 40 ◦ isC, 40 but °C, the but low the temperature low temperature terminal terminal difference difference is around is around 87 ◦C, which87 °C, makeswhich themakes average the average heat transfer heat temperaturetransfer temperature differences differences of the whole of the air preheaterwhole air reachpreheater up to reach 60 ◦C. up The to 60 heat °C. transfer The heat exergy transfer loss exergy in the loss air preheater in the air ispreheater large. is large.

FigureFigure 2.2. Heat transfer curvecurve ofof airair preheater.preheater.

As shown in Figure1, 10-stage regenerative heaters and two-stage outside steam

coolers are adopted to improve the temperature of the feed water entering into the boiler. Figure3 shows the heat transfer process curve of the regenerative heating process. Applying two-stage outside steam coolers effectively reduces the superheat degrees of the second and fourth stage extraction steam. However, superheat degrees of other stages’ extraction steam are still high at around 100–250 ◦C, which will inevitably result in the great exergy losses.

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As shown in Figure 1, 10-stage regenerative heaters and two-stage outside steam coolers are adopted to improve the temperature of the feed water entering into the boiler. Figure 3 shows the heat transfer process curve of the regenerative heating process. Ap- plying two-stage outside steam coolers effectively reduces the superheat degrees of the Energies 2021, 14, 238 second and fourth stage extraction steam. However, superheat degrees of other stages’ 5 of 22 extraction steam are still high at around 100–250 °C, which will inevitably result in the great exergy losses.

FigureFigure 3. Heat 3. transferHeat transfer process curve process of the curve regenerative of the regenerative heating process. heating process.

◦ Generally,Generally, in the air preheater, in the air the preheater, air temperature the air temperaturechanges from 30 changes to 347 °C, from and 30 to 347 C, and the feed thewater feed temperature water temperature changes from changes 30 to 314 from °C. 30Their to 314temperature◦C. Their change temperature ranges change ranges are similar.are The similar. outlet The temperature outlet temperature of the air is higher of the than airis that higher of the than feed thatwater, of and the the feed water, and the temperaturetemperature difference difference between the between air and thethe airextraction and the steam extraction is smaller. steam After is smaller. heated After heated by by 9-, 10-stage9-, 10-stage steam steamextraction, extraction, the feed the water feed temperature water temperature is about 80 is °C. about Comparing 80 ◦C. Comparing with with the theinlet inlet air of air the of air the preheater, air preheater, the wate ther heated water by heated 9-, 10-stage by 9-, steam 10-stage extraction steam extractionis is more more matchedmatched with with the flue the gas. ﬂue gas.

3. New Boiler-Turbine3. New Boiler-Turbine Coupling System Coupling System 3.1. Optimizing3.1. Optimizing Methods Methods Usually, the heat transfer processes are carried out independently in both steam tur- Usually, the heat transfer processes are carried out independently in both steam turbine bine side and boiler system side in the conventional coal-fired power generation system side and boiler system side in the conventional coal-fired power generation system (as (as shown in Figure 4), which results in great heat transfer exergy loss and low energy utilizationshown efficiency. in Figure Yang4 ),et whichal. [13] results proposed in great the boiler-turbine heat transfer coupling exergy loss system and which low energy utilization broke theefficiency. boundary of Yang the unit et al. and [13 reduced] proposed the heat the transfer boiler-turbine exergy loss coupling of the system. system It which broke the reduced boundarythe temperature of the difference unit and of reduced the air heat the transfer heat transfer at the boiler exergy side, loss however, of the system. it It reduced did not solvethe temperature the problem of difference high superheat of the degree air heat of steam transfer extraction at the at boiler the steam side, tur- however, it did not bine side.solve Fan et the al. problem[14] proposed of high another superheat system degree that used of steam9-stage extractionextraction superheat at the steam turbine side. heat, notFan the flue et al. gas [14 heat,] proposed to heat the another air, and system used the that flue used gas 9-stageheat to heat extraction part of superheatthe heat, not feed water.the The flue system gas heat, effectively to heat utilized the air, th ande extraction used the steam flue gassuperheat heat to heat heat and part re- of the feed water. duced theThe heat system transfer effectively temperature utilized difference the of extraction the air preheating steam superheat process. However, heat and it reduced the heat used parttransfer of the vaporization temperature latent difference heat of of the the extraction air preheating steam process.to heat the However, air, which it used part of the needed to change the internal structure of the regenerative heater. Due to a certain amount vaporization latent heat of the extraction steam to heat the air, which needed to change the of steam extraction, the amount of heat to heat the air was constant. When the load internal structure of the regenerative heater. Due to a certain amount of steam extraction, the amount of heat to heat the air was constant. When the load changed, the excess air coefficient changed and the air temperature could not be adjusted. In order to solve this problem,

different from the traditional heat exchange process between the boiler side and the turbine side, a new boiler-turbine coupling idea is proposed in this paper. As shown in Figure5, the system uses both the low temperature feed water and the superheat heat of extraction steam to heat part of the air, and reduces the mass flow rate of the air passing through the air preheater. The mass flow rate of the flue gas required for the air preheater is reduced, and the remaining high temperature flue gas can be used to heat both the condensate water and the feed water, thus, the mass flow rate of the regenerative steam extraction can be reduced. Preheating the air with the low temperature feed water reduces the heat transfer temperature difference of the air heat transfer. Heating the air with the superheat heat of the extraction steam reduces the heat transfer exergy loss of the regenerative extraction steam system. The remaining high temperature flue gas replaces part of high temperature steam Energies 2021, 14, x FOR PEER REVIEW 7 of 26

changed, the excess air coefficient changed and the air temperature could not be adjusted. In order to solve this problem, different from the traditional heat exchange process between the boiler side and the turbine side, a new boiler-turbine coupling idea is proposed in this paper. As shown in Figure 5, the system uses both the low temperature feed water and the superheat heat of extraction steam to heat part of the air, and reduces the mass flow rate of the air passing through the air preheater. The mass flow rate of the flue gas required for the air preheater is reduced, and the remaining high temperature flue gas can be used to heat both the condensate water and the feed water, thus, the mass flow rate of the regenerative steam extraction can be reduced. Preheating the air with the low temper- Energies 2021, 14, 238 ature feed water reduces the heat transfer temperature difference of the air heat transfer.6 of 22 Heating the air with the superheat heat of the extraction steam reduces the heat transfer exergy loss of the regenerative extraction steam system. The remaining high temperature flue gas replaces part of high temperature steam extraction to heat both the condensate waterextraction and feed to heat water, both so the as condensateto increase waterthe output and feed power water, of steam so as toturbine. increase By the reducing output thepower temperature of steam turbine.difference By of reducing heat transfer the temperature in each heat difference transfer ofprocess, heat transfer the cascade in each utili- heat transfer process, the cascade utilization of energy is realized, the exergy loss is reduced, and zation of energy is realized, the exergy loss is reduced, and the power generation effi- the power generation efficiency of the unit is improved. ciency of the unit is improved.

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FigureFigure 4. 4. HeatHeat exchange exchange process process between between traditiona traditionall boiler boiler side side and and steam steam turbine turbine side. side.

FigureFigure 5. 5. SchematicSchematic diagram diagram of of a anew new boiler-turbine boiler-turbine coupling. coupling. 3.2. New Boiler-Turbine Coupling System

Based on the integration idea of Section 3.1, two new boiler-turbine coupling systems are proposed in this paper. Figure6 shows the flowchart of new system 1. As shown in Figure6, two subsystems are integrated to new system 1 compared with the original system: the air heating subsystem for heating part of the air mainly is composed of one air-water heat exchanger (AH), and five gas-steam heat exchangers (AH1-AH5). This subsystem uses the steam superheat heat to heat the air. The flue gas bypass subsystem for heating the feed water with high temperature flue gas is composed of high and low temperature economizers (Eco1, Eco2). Figure7 shows the flowchart of new system 2. New system 2 replaces two-stage outside steam coolers with the air heater on the basis of

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3.2. New Boiler-Turbine Coupling System Based on the integration idea of Section 3.1, two new boiler-turbine coupling systems are proposed in this paper. Figure 6 shows the flowchart of new system 1. As shown in Figure 6, two subsystems are integrated to new system 1 compared with the original system: the air heating subsystem for heating part of the air mainly is composed of one air- water heat exchanger (AH), and five gas-steam heat exchangers (AH1-AH5). This subsystem uses the steam superheat heat to heat the air. The flue gas bypass subsystem for heat- Energies 2021, 14, 238 7 of 22 ing the feed water with high temperature flue gas is composed of high and low temperature economizers (Eco1, Eco2). Figure 7 shows the flowchart of new system 2. New system 2 replaces two-stage outside steam coolers with the air heater on the basis of new system 1.new Since system two-stage 1. Since outside two-stage steam outside coolers steam have been coolers cancelled, have been new cancelled, system 2 newneeds system to add 2 aneeds pre-economizer to add a pre-economizer (Eco3) to ensure (Eco3) the inlet to ensure temperature the inlet of temperaturethe feed water. of theNew feed system water. 2 improvesNew system the 2volume improves flow the rate volume of the ﬂowair preh rateeated of the by air the preheated extraction by steam, the extraction however, steam, heat transferhowever, temperature heat transfer differences temperature of flue differences gas-steam of heat ﬂue exchangers gas-steam heatat all exchangers levels are larger at all thanlevels those are larger of new than system those 1. of new system 1.

SHP HP IP IP LP LP LP LP G Economizer 1st RH1 2nd RH1 1st RH3 2nd RH3 OSC SH2 OSC AH1 AH2 AH3 AH4 AH5 1st RH2 2nd RH2 SH1

H5 Eco1 Air Heater H1 H2 H3 H4 H6 H7 H8 H9 H10 Eco2 AH

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Figure 6. FlowchartFlowchart of new system 1.

Economizer SHP HP IP IP LP LP LP LP G 1st RH1 2nd RH1 1st RH3 2nd RH3 SH2 1st RH2 2nd RH2 AH1 AH2 AH3 AH4 AH5 AH6 AH7 SH1

Eco3 H5

Air Heater Eco1 H1 H2 H3 H4 H6 H7 H8 H9 H10

Eco2 AH

Figure 7. Flowchart of new system 2. Figure 7. Flowchart of new system 2. TwoTwo new new systems systems follow follow with with the the principle of of energy cascade utilization through redistributionredistribution of of heat. heat. Before Before heating heating the the feed feed water water and and the the condensate condensate water, water, the the regen- regen- erativeerative extraction extraction steam steam heats heats the the air air because because the the air air energy energy level level is is higher higher than than the the feed feed waterwater energy energy level. level. Using Using the the flue ﬂue gas gas to to heat heat part part of of the the feed feed water water and and condensate condensate water water cancan reduce reduce the mass ﬂowflow ofof thethe regenerativeregenerative steam steam extraction. extraction. Eventually, Eventually, the the regenerative regenera- tiveextraction extraction steam steam with with high high energy energy level level is reduced, is reduced, the unitthe unit power power output output is increased is increased and andthe coalthe coal consumption consumption of power of power generation generation is reduced. is reduced.

4. Calculation and Analysis of Thermal Performance 4.1. Model Simulation Design and Assumptions In order to evaluate the thermodynamic performance advantages of two novel systems, EBSILON Professional is used to build thermodynamic models. The model is based on the heat balance diagram of steam turbine and boiler manufacturer. The turbine heat acceptance (THA) load is selected as the design point of the system. In the reference system model, the relevant definitions and assumptions for the model calculation are as follows: (1) The mass flow rate of live steam is 2533 t/h. (2) Regenerative steam extraction pressures and pipeline pressure drops at all levels are based on the thermal balance diagrams of the power plant. (3) The pressure drop coefficient of the 1st reheater and pipelines is 4.3%. (4) The pressure drop coefficient of the 2nd reheater and pipelines is 4.2%. (5) The absolute pressure drop of the boiler inlet feed water is 1.5 MPa. For the adequate and comprehensive comparative analysis, the new system 1 model is improved as follows: (1) In order to compare the variations of extraction steam at all pressure levels, the main steam mass flow of new system 1 at the design point is kept the same as that of the reference system. (2) The initial parameters are 30.1 MPa/600 °C/610 °C. (3) The condenser pressure is 4.5 kPa. (4) The pressure drop of each air heater at the air side is 0.2 kPa. (5) The pressure drop of each air heater drop at the steam side is 2 kPa. (6) The heat loss of the air heater is ignored. (7) Air outlet temperature of AH is 150 °C.

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4. Calculation and Analysis of Thermal Performance 4.1. Model Simulation Design and Assumptions In order to evaluate the thermodynamic performance advantages of two novel systems, EBSILON Professional is used to build thermodynamic models. The model is based on the heat balance diagram of steam turbine and boiler manufacturer. The turbine heat acceptance (THA) load is selected as the design point of the system. In the reference system model, the relevant definitions and assumptions for the model calculation are as follows: (1) The mass flow rate of live steam is 2533 t/h. (2) Regenerative steam extraction pressures and pipeline pressure drops at all levels are based on the thermal balance diagrams of the power plant. (3) The pressure drop coefficient of the 1st reheater and pipelines is 4.3%. (4) The pressure drop coefficient of the 2nd reheater and pipelines is 4.2%. (5) The absolute pressure drop of the boiler inlet feed water is 1.5 MPa. For the adequate and comprehensive comparative analysis, the new system 1 model is improved as follows: (1) In order to compare the variations of extraction steam at all pressure levels, the main steam mass flow of new system 1 at the design point is kept the same as that of the reference system. (2) The initial parameters are 30.1 MPa/600 ◦C/610 ◦C. (3) The condenser pressure is 4.5 kPa. (4) The pressure drop of each air heater at the air side is 0.2 kPa. (5) The pressure drop of each air heater drop at the steam side is 2 kPa. (6) The heat loss of the air heater is ignored. (7) Air outlet temperature of AH is 150 ◦C.

4.2. Analysis of Heat Transfer Performance of Tail Flue Gas in New Boiler-Turbine Coupling System Due to the reduction of high-pressure steam extraction, the mass flow rates of both the 1st reheat steam and 2nd reheat steam in two new systems increase. The increase of heat absorbed by the reheat steam in the boiler results in a lower flue gas inlet temperature of the air preheater than that of the reference system. As shown in Figure8, the heat transfer temperature difference of the air preheater in new system 1 is 43 ◦C. In the flue gas bypass subsystem, the heat transfer temperature difference of the Eco1 is 30 ◦C, and that of Eco2 is 36 ◦C. As shown in Figure9, the heat transfer temperature difference of the air preheater in new system 2 is 43 ◦C, the heat transfer temperature difference of Eco1 is 11 ◦C, that of Eco2 is 36 ◦C, and that of Eco3 is 26 ◦C. The heat transfer temperature differences of the tail flue gas in two new systems are much smaller than that of the air preheater in the reference system, which reduces the heat transfer exergy loss of flue gas and improves the thermal efficiency. Energies 2021, 14, x FOR PEER REVIEW 11 of 26

4.2. Analysis of Heat Transfer Performance of Tail Flue Gas in New Boiler-Turbine Coupling System Due to the reduction of high-pressure steam extraction, the mass flow rates of both the 1st reheat steam and 2nd reheat steam in two new systems increase. The increase of heat absorbed by the reheat steam in the boiler results in a lower flue gas inlet temperature of the air preheater than that of the reference system. As shown in Figure 8, the heat transfer temperature difference of the air preheater in new system 1 is 43 °C. In the flue gas bypass subsystem, the heat transfer temperature difference of the Eco1 is 30 °C, and that of Eco2 is 36 °C. As shown in Figure 9, the heat transfer temperature difference of the air preheater in new system 2 is 43 °C, the heat transfer temperature difference of Eco1 is 11 °C, that of Eco2 is 36 °C, and that of Eco3 is 26 °C. The heat transfer temperature differences of the tail flue gas in two new systems are much smaller than that of the air preheater Energies 2021, 14, 238 9 of 22 in the reference system, which reduces the heat transfer exergy loss of flue gas and improves the thermal efficiency.

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FigureFigure 8.8. HeatHeat transfertransfer analysisanalysis ofof thethe tailtail ﬂueflue inin newnew systemsystem 1.1.

Figure 9. Heat transfer analysis of the tail flue ﬂue in new system 2.

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4.3. Exergy Utilization Diagram of the Air Preheating Subsystem In order to comprehensively analyze the energy utilization law of the air preheating subsystem and find out key influence factors of exergy loss in the process of energy transfer, the exergy utilization diagram (EUD) is adopted. The exergy loss reduced by the energy cascade utilization when the air is heated by the regenerative steam extraction is analyzed in depth, and Energies 2021, 14, x FOR PEER REVIEW 10 of 22 its heat transfer characteristics are analyzed. Different from the traditional exergy analysis method, the exergy utilization diagram can reveal the relationship between the energy change and energy grade change in thermal process 4.3.in theExergy form Utilization of diagram Diagram [15–17]. of In the EUD, Air Preheating∆H is the energySubsystem change of the heat transfer process, whichIn order embodies to comprehensively the first law of thermodynamics, analyze the energy and utilization A represents law the of energythe air preheating level, which subsystemembodies theand second find out law key of influence thermodynamics. factors of The exergy area between loss in the curves process of the of exothermicenergy trans- side fer,and the the exergy endothermic utilization side diagram is the exergy (EUD) loss is of adopted. the heat The transfer exergy process. loss reduced Therefore, by the the image en- ergyanalysis cascade can utilization visually and when clearly the show air is the heated exergy by loss the in regenerative the process ofsteam energy extraction transfer. is an- alyzedEnergy-level in depth, and “A” its isheat a dimensionless transfer characteristics quantity, are which analyzed. is the ratio of exergy change ∆ε to energyDifferent change from∆ theH intraditional thermal processexergy anal [17].ysis It is method, defined the as follows:exergy utilization diagram can reveal the relationship between the energy change and energy grade change in ther- ∆ε ∆S mal process in the form of diagramA [15–17].= In= 1EUD,− T ∆𝐻 is the energy change of the heat(1) ∆H 0 ∆H transfer process, which embodies the first law of thermodynamics, and A represents the energywhere, level,A is thewhich energy embodies level; ∆ Stheis thesecond entropy law change,of thermodynamics. J/(kg·K); ∆ε is The the area exergy between change, curvesMW; ∆ofH theis theexothermic energy change,side and MW; the endothermiT0 is the environmentc side is the temperature, exergy loss of K. the heat transfer process.In an idealTherefore, heat transferthe image process, analysis A cancan bevisually defined and as clearly follows: show the exergy loss in the process of energy transfer. T Energy-level “A” is a dimensionlessA quantity,= 1 − 0which is the ratio of exergy change ∆𝜀(2) T to energy change ∆𝐻 in thermal process [17]. It is defined as follows: where, T is the temperature of energy release∆𝜀 side (heat∆𝑆 source) or energy absorption side 𝐴 = =1𝑇 (1) (cold source), K. ∆𝐻 ∆𝐻 where,When A is the analyzing energy thelevel; heat ∆𝑆 transfer is the entropy process change, of air preheating J/(kg·K); subsystem,∆𝜀 is the exergy it is simplified change, as an ideal heat transfer process. Figure 10 shows the EUD of the heat transfer processes of MW; ∆𝐻 is the energy change, MW; 𝑇 is the environment temperature, K. seven-stageIn an ideal regenerative heat transfer extraction process, steam A can in thebe defined reference as system follows: and new systems. It can be seen that the energy level of regenerative extraction steam in the reference 𝑇 system is high, while the energy level of𝐴 the=1 feed water is low. The energy level difference(2) between them is large, resulting in a great exergy𝑇 loss in the heat transfer process. In new where,systems, T is the the air temperature temperature of is energy higher release than the side feed (heat water source) temperature, or energy and absorption the energy side level (coldof the source), air is higher K. than that of the feed water. The air heating subsystem can effectively reduceWhen the analyzing heat transfer the heat exergy transfer loss. Newprocess system of air 2 preheating has two more subsystem, stages of it steam-flue is simplified gas asheat an ideal exchangers heat transfer than new process. system Figure 1, but 10 the shows reduced the exergyEUD of loss the inheat each transfer stage isprocesses less than ofthat seven-stage of new system regenerative 1. extraction steam in the reference system and new systems.

(a) Reference system

Figure 10. Cont.

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Energies 2021, 14, x FOR PEER REVIEW 11 of 22

(b) New system 1

(c) New system 2 Figure 10. EUD diagrams of seven-stage regenerative steam extraction heat transfer processes of original system and new systems.

It can be seen that the energy level of regenerative extraction steam in the reference system is high, while the energy level of the feed water is low. The energy level difference between them is large, resulting in a great exergy loss in the heat transfer process. In new systems, the air temperature is higher than the feed water temperature, and the energy level of the air is higher than that of the feed water. The air heating subsystem can effectively reduce the heat(c )transfer New system exergy 2 loss. New system 2 has two more stages of steamflue gas heat exchangers than new system 1, but the reduced exergy loss in each stage is Figure 10. EUD diagrams of seven-stage regenerative steam extraction heat transfer processes of lessFigure than 10. thatEUD of diagrams new system of seven-stage 1. regenerative steam extraction heat transfer processes of originaloriginal system system and and new new systems. systems. 4.4. Analysis of Steam Extraction Mass Flow and Power Generation Capacity in 4.4.It Analysis can be ofseen Steam that Extraction the energy Mass level Flow of andregenerative Power Generation extraction Capacity steam in in Regenerative the reference Regenerative System systemSystem is high, while the energy level of the feed water is low. The energy level difference betweenHeating them the is large, air air with with resulting the the superheat superheat in a great heat exer of gy extraction extraction loss in the steam heat will transfer increase process. the required In new systems,mass flow flow the of of air extraction extraction temperature steam steam is at higher at all all pressure pressure than the levels,levels, feed while water while heating temperature, heating thethe feed feedand water the water with energy with the levelfluethe fluegasof the will gas air willreduce is higher reduce the thanmass the massthatflow of flowof the the offeed rege the water.nerative regenerative The steam air heating steamextraction. extraction. subsystem Figure Figure11acan showseffec- 11a tivelytheshows power reduce the generation power the heat generation transfercapacity capacity exergyof each loss. ofstage each New ex stagetraction system extraction steam. 2 has As steam.two can more Asbe seenstages canbe from of seen steam- Figure from flue11b,c,Figure gas in 11heat twob,c, exchangers innew two systems, new than systems, the new mass system the flow mass 1,rate but flows ofthe rates1–4 reduced stages of 1–4 exergyof stages extraction loss of extraction in steam each stage reduce. steam is lessOnreduce. thanthe contrary, that On theof new contrary,the systemmass theflow 1. mass rates flow of 5–8 rates stages of 5–8 of extraction stages of extractionsteam increase. steam The increase. mass flowThe massrates flowof 9–10 rates stages of 9–10 are stagesbasically are unchanged. basically unchanged. Generally speaking, Generally the speaking, total mass the flow total 4.4.ratemass Analysis of flow steam rate of extractionSteam of steam Extraction extractionof new Mass system of Flow new 1 andin systemcreases Power 1 byincreasesGeneration 2.86 kg/s by Capacity and 2.86 that kg/s in of and new that system of new 2 Regenerativeincreasessystem 2 increasesby System 3.11 kg/s. by 3.11 kg/s. Heating the air with the superheat heat of extraction steam will increase the required mass flow of extraction steam at all pressure levels, while heating the feed water with the flue gas will reduce the mass flow of the regenerative steam extraction. Figure 11a shows the power generation capacity of each stage extraction steam. As can be seen from Figure 11b,c, in two new systems, the mass flow rates of 1–4 stages of extraction steam reduce. On the contrary, the mass flow rates of 5–8 stages of extraction steam increase. The mass flow rates of 9–10 stages are basically unchanged. Generally speaking, the total mass flow rate of steam extraction of new system 1 increases by 2.86 kg/s and that of new system 2 increases by 3.11 kg/s.

(a) Power generation capacity of different regenerative extraction steam.

Figure 11. Cont.

EnergiesEnergies 20212021,, 1414,, x 238 FOR PEER REVIEW 1512 of of 26 22

(b) Reduced mass flow rates of extraction steam at different stages in new system 1.

(d) Power output changes of steam extraction in new system 1.

(e) Power output changes of steam extraction in new system 2.

FigureFigure 11. 11. SteamSteam extraction extraction mass mass flow ﬂow and and power power generation generation capacity. capacity.

Energies 2021, 14, 238 13 of 22

As shown in Figure 11d,e, there is a great difference in power output of different stages of steam extraction. Power generation capacity of high-pressure extraction steam is higher than that of low-pressure extraction steam. Reducing the mass flow rate of high-pressure extraction steam and increasing the mass flow rate of low-pressure extraction steam can increase the power output of the overall power plant. For the new system 1, the mass flow rate reduction of 1–4 stages of extraction steam makes the power output increase by 22.17 MW, while the mass flow rate increase of 5–8 stages of extraction steam results in the power output decrease of around 14.69 MW. Finally, the net power output of new system 1 increases by 7.56 MW. For the new system 2, the mass flow rate reduction of 1–4 stages of extraction steam makes the power output increase by 25.75 MW, while the mass flow rate increase of 5–8 stages of extraction steam results in the power output decrease of around 15.88 MW. Finally, the net power output of new system 1 increases by 9.23 MW.

4.5. Analysis of Steam Extraction Mass Flow and Power Generation Capacity in Regenerative System Table2 shows main performance data and calculation results of three systems.

Table 2. Main performance data and calculation results of three systems.

Reference New System New System System 1 2 Mass t/h 2533 2533 2533 Flow Main steam Press. bar 301 301 301 Temp. ◦C 600 600 600 Mass t/h 2342 2358 2365 Flow Reheat steam Press. bar 10.1 10.1 10.1 Temp. ◦C 610 610 610 Mass t/h 2014 2057 2068 Flow Double reheat steam Press. bar 30.8 30.8 30.8 Temp. ◦C 610 610 610 Coal ﬂow rate t/h 319 319 319 Air mass ﬂow t/h 2948 2948 2948 Air heated by the steam extraction t/h - 1252 2035 Total power output MW 992.56 999.92 1001.57 Power consumption of fans MW - 0.98 2.08 Net output power MW 992.56 998.94 999.49 Standard coal consumption rate g/kWh 257.38 255.73 255.59

The standard coal consumption rate (bs) is calculated as follows:

s bs = bQnet,ar/Qnet,ar (3)

where, b is the coal consumption rate, g/kWh; Qnet,ar refers to the net caloriﬁc value of s design coal, kJ/kg; Qnet,ar refers to the net caloriﬁc value of standard coal, kJ/kg.

b = 1000B/Pe (4)

where, B is coal mass ﬂow rate, t/h; Pe denotes to the net output power, MW. Energies 2021, 14, 238 14 of 22

Compared with the reference system, the total power output of new system 1 increases by 7.36 MW, and that of new system 2 increases by 9.15 MW. However, due to the pressure loss of air passing through multi-stage heaters in air preheating subsystem of two new systems, it is necessary to increase fans to overcome the air ﬂow resistance. The power consumption of fans in new system 1 is about 0.98 MW, and that of new system 2 is about 2.08 MW. In summary, the net output power of new system 1 increases by 6.38 MW and the standard coal consumption decreases by 1.65 g/kWh. The net output power of new system 2 increases by 6.93 MW and the standard coal consumption rate decreases by 1.79 g/kWh. Replacing two-stage outside steam coolers with the air heater decreases the standard coal consumption under the design condition.

5. Off-Design Performance Analysis In order to reveal the actual operation performances of two new systems. The off- design performances of two systems are deeply investigated. When analyzing the off- design performance, the model is set in the form of a ﬁxed power output. Table3 shows the data of three systems under off-design conditions.

Table 3. Data of three systems under off-design conditions.

Reference System THA 75% THA 50% THA 40% THA 30% THA Main steam flow t/h 2533 1866.23 1211.74 971.68 738.1 Air flow t/h 2947.82 2217.91 1608.96 1478.44 1197.33 Flue gas flow t/h 3233.58 2436.85 1759.88 1602.73 1294.4 Exhaust flue gas ◦C 117.13 109.24 98.85 94.62 89.18 temp. Hot air temp. ◦C 347 334.78 311.23 304.05 288.13 Total power MW 986.79 727.14 481.07 384.52 283.85 Coal flow t/h 318.49 240.01 165.48 136.29 106.43 Standard coal g/kWh 258.47 264.33 275.47 283.85 300.27 consumption rate New System 1 THA 75% THA 50% THA 40% THA 30% THA Main steam flow t/h 2516.39 1833.81 1189.19 959.68 731.11 Air flow t/h 2927.67 2320 1718.39 1501.91 1189.88 Ratio of air heated % 42.63 42.65 37.71 37.29 38.66 by steam Flue gas flow t/h 3216.61 2537.57 1868.42 1625.44 1286.34 Bypass flue gas % 36.81 37.68 33.62 31.69 35.87 ratio Exhaust flue gas ◦C 116.79 107.17 93.88 88.12 81.92 temp. Hot air temp. ◦C 346.95 333.19 305.37 294.29 276.13 Total power MW 986.78 727.14 481.07 384.52 283.85 output Fans power MW 0.97 0.48 0.13 0.08 0.05 consumption Coal flow t/h 316.82 238.61 164.5 135.44 105.77 Standard coal g/kWh 257.37 262.96 273.92 282.14 298.45 consumption rate Coal consumption g/kWh 1.1 1.37 1.55 1.71 1.82 reduction Energies 2021, 14, 238 15 of 22

Table 3. Cont.

New System 2 THA 75% THA 50% THA 40% THA 30% THA Main steam flow t/h 2510.41 1832.23 1190.27 960.47 726.96 Air flow t/h 2924.15 2316.62 1718.79 1499.7 1190.28 ratio of air heated % 67.23 63.91 55.76 51.45 48.97 by steam Flue gas flow t/h 3212.74 2533.92 1868.85 1623.04 1286.78 Bypass flue gas % 65.78 63.1 57.12 52.09 55.35 ratio Exhaust flue gas ◦C 116.86 106.12 90.44 85.33 81.29 temp. Hot air temp. ◦C 346.65 332.15 303.177 292.75 268.23 Total power MW 986.79 727.14 481.06 384.52 283.85 output Fans power MW 2.03 0.87 0.25 0.12 0.07 consumption Coal flow t/h 316.44 238.26 164.54 135.24 105.81 Standard coal g/kWh 257.33 262.72 274.05 281.76 298.59 consumption rate Coal consumption g/kWh 1.14 1.61 1.42 2.09 1.68 reduction

Energies 2021, 14, x FOR PEER REVIEW It can be seen that the proportion of air heated by the extraction steam varies18 of under 26 different load conditions in two new systems, and the mass flow of the flue gas displaced by the extraction steam will also change. Therefore, the boiler bypass flue gas mass flow needs to be adjusted with the change of working conditions. The boiler flue gas exhaust temperaturestemperatures of ofnew new systems systems are are low low under under the the low low load. load. The The possible possible low low temperature temperature corrosioncorrosion problem problem of of tail tail heaters heaters should should be be considered considered and the excessiveexcessive lowlow loadload operation opera- tionshould should be be avoided avoided as as far far as as possible. possible. FigureFigure 12 12 shows shows the the changes changes of ofthe the standard standard coal coal consumption consumption rates rates under under different different workingworking conditions conditions of new of new systems systems and andrefere referencence system. system. The results The results show that show the that per- the formancesperformances of two of new two systems new systems are superior are superior to that of to the that reference of the system reference under system different under loaddifferent conditions, load conditions,and the standard and the coal standard consumption coal consumption rates of new rates systems of new under systems different under loadsdifferent are 1.1–2.0 loads g/kWh are 1.1–2.0 lower g/kWh than that lower of thanthe reference that of the system. reference system.

Figure 12. Coal consumption rates of new systems and reference system. Figure 12. Coal consumption rates of new systems and reference system. Generally speaking, the reduced coal consumption rates of two new systems are approximate.Generally speaking, However, the the reduced ﬂowchart coal of consumption new system 2 rates is more of two complex new systems due to the are substi- ap- proximate. However, the flowchart of new system 2 is more complex due to the substitu- tion of two-stage outside steam coolers with the air heater and the adding of one-stage pre-economizer. A large amount of air is heated by the extraction steam in new system 2. When the load changes, the wet steam is easily produced at the outlet of steam-air heat exchanger, which will affect the safety of unit. From this point, new system 1 is superior to new system 2.

6. Re-Optimization of New System 1 6.1. Energy-Saving Potential Analysis of New System 1 New system 1 uses the superheat heat of the extraction steam to heat part of air, which reduces the heat transfer exergy loss of extraction steam and part of air. As shown in Figure 13, in the air preheater of new system 1, the air inlet temperature is 30 °C, and the flue gas exhaust temperature is 117 °C. A large heat transfer temperature difference between them still results in a large heat transfer exergy loss of the air preheater. Air preheater in new system 1 still has the further energy-saving potential.

Energies 2021, 14, 238 16 of 22

tution of two-stage outside steam coolers with the air heater and the adding of one-stage pre-economizer. A large amount of air is heated by the extraction steam in new system 2. When the load changes, the wet steam is easily produced at the outlet of steam-air heat exchanger, which will affect the safety of unit. From this point, new system 1 is superior to new system 2.

6. Re-Optimization of New System 1 6.1. Energy-Saving Potential Analysis of New System 1 New system 1 uses the superheat heat of the extraction steam to heat part of air, which reduces the heat transfer exergy loss of extraction steam and part of air. As shown in Figure 13, in the air preheater of new system 1, the air inlet temperature is 30 ◦C, and the ﬂue gas exhaust temperature is 117 ◦C. A large heat transfer temperature difference Energies 2021, 14, x FOR PEER REVIEW 19 of 26 between them still results in a large heat transfer exergy loss of the air preheater. Air preheater in new system 1 still has the further energy-saving potential.

VHP HP IP IP LP LP LP LP G Economizer 1st RH1 2nd RH1 1st RH3 2nd RH3 OSC SH2 OSC AH1 AH2 AH3 AH4 AH5 1st RH2 2nd RH2 SH1

H5 Eco1 Air Heater H1 H2 H3 H4 H6 H7 H8 H9 H10 Eco2 AH

AH6

Figure 13. FlowchartFlowchart of of new new system 3.

Therefore,Therefore, new new system system 3 3 is is proposed by optimizing the air preheater subsystem of newnew system system 1. 1. New New system system 3 3 uses uses the the feed feed wate waterr to to preheat preheat the the air air and and then then the the heated heated air air entersenters into the airair preheaterpreheater toto be be heated heated further, further, which which will will reduce reduce the the heat heat transfer transfer exergy ex- ergyloss ofloss the of air the preheater. air preheater. The ﬂowchart The flowchart of new of system new system 3 is shown 3 is shown in Figure in Figure 13. After 13. theAfter air theis heated air is heated to 100 ◦toC 100 by the°C by air-feed the air-feed water heatwate exchangerr heat exchanger (AH), part(AH), of part air enters of air enters into the into air thepreheating air preheating subsystem subsystem and the and remaining the remaining air enters air into enters the into air preheater, the air preheater, and ﬁnally, and they finally,jointly they enter jointly into theenter furnace. into the furnace.

6.2.6.2. Analysis Analysis of of Optimization Optimization Results Results InIn order order to to analyze analyze optimization optimization results, results, the the thermodynamic thermodynamic model model of of new new system system 3 3 isis also also established established by by usin usingg the the EBSILON EBSILON Professional Professional ParametersParameters of of equipment equipment added added in in the the new new system system 3 are set as follows: ◦ (1)(1) TheThe outlet outlet air air temperature temperature of of AH AH is is 100 100 °C.C. (2) Pressure drop values on both the water side and the air side of AH are 0.2 kPa. (2) Pressure drop values on both the water side and the air side of AH are 0.2 kPa. (3) The outlet air temperature of AH8 is 150 ◦C. (3) The outlet air temperature of AH8 is 150 °C. The heat transfer process curve of the air preheater in new system 3 is shown in The heat transfer process curve of the air preheater in new system 3 is shown in Fig- Figure 14. Inlet and outlet temperatures of air and flue gas are very close. As part of the ure 14. Inlet and outlet temperatures of air and flue gas are very close. As part of the feed feed water is used to heat the air, the absorbed heat by air in the air preheater decreases water is used to heat the air, the absorbed heat by air in the air preheater decreases and and the mass flow rate of the bypass flue gas increases. With the mass flow increase of the mass flow rate of the bypass flue gas increases. With the mass flow increase of the feeding water heated by the flue gas heater, the high-pressure steam extraction is reduced, resulting in the increase of the 1st reheat steam and 2nd reheat steam flow, and the increase of the absorbed heat by the steam. The temperature of flue gas entering into the air preheater is further reduced to 357.74 °C. The inlet temperature of air entering the air preheater rises from 30 °C to 100 °C. The heat transfer temperature difference of the air preheater decreases significantly and the heat transfer exergy loss decreases.

Energies 2021, 14, 238 17 of 22

the feeding water heated by the flue gas heater, the high-pressure steam extraction is reduced, resulting in the increase of the 1st reheat steam and 2nd reheat steam flow, and Energies 2021, 14, x FOR PEER REVIEWthe increase of the absorbed heat by the steam. The temperature of flue gas entering17 of into22 the air preheater is further reduced to 357.74 ◦C. The inlet temperature of air entering the Energies 2021, 14, x FOR PEER REVIEW 20 of 26 air preheater rises from 30 ◦C to 100 ◦C. The heat transfer temperature difference of the air preheater decreases significantly and the heat transfer exergy loss decreases.

Figure 14. Heat transfer curve of air preheater. FigureFigure 14. 14. HeatHeat transfer transfer curve curve of of air air preheater. preheater. 6.3. Variation of Steam Extraction at Different Levels in New System 3 6.3. Variation of Steam Extraction at Different Levels in New System 3 Due to the addition of an air-water heat exchanger (AH) in new system 3, the heat Due to the addition of an air-water heat exchanger (AH) in new system 3, the heat transfer amount between the air and the feed water increases, which results in the increase transfer amount between the air and the feed water increases, which results in the increase of steam extraction mass flow rates of 6–8 stage low-pressure heaters. At the same time, of steam extraction mass flow rates of 6–8 stage low-pressure heaters. At the same time, as as the absorbed heat by air in the air preheater decreases, the mass flow of the bypass flue the absorbed heat by air in the air preheater decreases, the mass flow of the bypass flue gas gas increases, and the steam extraction flow of high temperature economizer decreases. increases, and the steam extraction flow of high temperature economizer decreases. As shown in Figure 15, compared with new system 1, the total mass flow rate of 1–4 As shown in Figure 15, compared with new system 1, the total mass flow rate of stage extraction steam in new system 3 decreases by 8.58 kg/s and the power output in- 1–4 stage extraction steam in new system 3 decreases by 8.58 kg/s and the power output creases by 12.41 MW. The total mass flow rate of 5–8 stages of extraction steam increases increases by 12.41 MW. The total mass flow rate of 5–8 stages of extraction steam increases byby 10.95 10.95 kg/s kg/s and and the the power power output output decreases decreases by by7.88 7.88 MW. MW. The The mass mass flow flow rate rate of ofthe the lowlow- pressurepressure turbine turbine exhaust exhaust steam steam increases increases by by 2.167 2.167 kg/s, kg/s, and and the the total total unit unit power power output output increasesincreases by by 4.58 4.58 MW. MW.

(a) Reduced flow rates of extraction steam at different stages.

Figure 15. Cont.

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(b) Power output changes of extraction steam at different stages.

FigureFigure 15. 15. VariationVariation of of steam steam extraction extraction at at different different levels. levels.

6.4.6.4. Performance Performance Analysis Analysis of of New New System System 3 3 TheThe performance performance parameters parameters of of new new system system 1 1 and and new new system system 3 3 are are compared compared as as shownshown in in Table Table 44..

Table 4. Table 4. ComparisonComparison of of main main performance performance parameters parameters of of new new system system 1 1 and and new system 3.

NewNew System System 1 1 New New System System 3 MassMass Flow Flow t/h t/h 2533 2533 2533 2533 MainMain steamsteam Press.Press. bar bar 301 301 301 301 Temp. ◦C 600 600 Temp. °C 600 600 MassMass Flow Flow t/h t/h 2358 2358 2368.57 2368.57 1st reheat steam Press. bar 10.1 10.1 1st reheat steam Temp.Press. bar◦C 10.1 610 10.1 610 Temp. °C 610 610 Mass Flow t/h 2057 2085.02 2nd reheat steam MassPress. Flow t/h bar 2057 30.8 2085.02 30.8 2nd reheat steam Temp.Press. bar◦C 30.8 610 30.8 610 Coal flowTemp. °C t/h 610 319 610 319 CoalAir flowflow t/h t/h 319 2948 2947.82 319 Flow of airAir heated flow by steam t/h t/h 2948 1252 2947.82 1259.67 Total power output MW 999.92 1004.45 Flow of air heated by steam t/h 1252 1259.67 Power consumption of fans MW 0.98 0.61 TotalNet outputpower poweroutput MW MW 999.92 998.94 1004.45 1003.84 StandardPower consumption coal consumption of fans rate g/kWh MW 0.98 255.73 254.480.61 Net output power MW 998.94 1003.84 StandardAfter the coal further consumption optimization rate of new g/kWh system 1, the heat 255.73 transfer temperature 254.48 difference of the air preheater is further reduced, and the proportion of bypass flue gas increases fromAfter 37% tothe 49%. further The optimization mass flow rates of new of 1–3 system stage high-pressure1, the heat transfer extraction temperature steam decrease, differ- encethe 1stof the reheat air preheater steam flow is ratefurther increases reduced, by 2.83and kg/sthe proportion and the 2nd of bypass reheat steamflue gas increases increases by from7.7 kg/s. 37% Theto 49%. net powerThe mass output flow increases rates of by1–3 5.5 stage MW high-pressure and the standard extraction coal consumption steam decrease,rate decreases the 1st reheat by 1.25 steam g/kWh. flow rate increases by 2.83 kg/s and the 2nd reheat steam increases by 7.7 kg/s. The net power output increases by 5.5 MW and the standard coal con- sumption6.5. Off-Design rate decreases Performance by Analysis1.25 g/kWh. A constant power model is set up to calculate the off-design performance of new 6.5.system Off-Design 3. Table Performance5 shows the Analysis performance data of new system 3 under variable operating conditions.A constant As shown power in model Table 5is, theset bypassup to ca fluelculate gas proportion the off-design of new performance system 3 is of higher new systemthan that 3. Table of new 5 systemshows the 1 under performance all working data conditions; of new system the ratio3 under of air variable heated operating by steam conditions.increases, and As shown the power in Table consumption 5, the bypass of the flue fan gas increases. proportion Flue of gas new exhaust system temperatures 3 is higher thanof both that new of new system system 1 and 1 newunder system all working 3 are similar. conditions; the ratio of air heated by steam

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Energies 2021, 14, 238 increases, and the power consumption of the fan increases. Flue gas exhaust temperatures19 of 22 of both new system 1 and new system 3 are similar.

Table 5. Analysis of main parameters under variable parameters. Figure 16 shows the changes of the standard coal consumptions in both new systems 1 New System 3 and 3 under off-design THA conditions. 75% THA It can be 50% seen THA that after the 40% further THA optimization 30% THA of new Main steam flow system t/h 1, the 2507.06 coal consumption 1823.23 rate decreases 1182.16 by 1 g/kWh under 953.66 both THA and 725.37 75% THA Air flow conditions. t/h Under 2918.34 low load2311.66 conditions, the1715.18 reduction amount1499.91 of coal consumption1189.28 rate Ratio of air heated by decreases. Especially under 30% THA condition, only 0.2 g/kWh is saved. New system 3 % 42.93 44.38 49.55 52.97 48.77 steam performs poorly at low load, which needs to be noticed and improved. Flue gas flow t/h 3206.36 2528.49 1864.92 1623.27 1285.7 Bypass flue gas ratio Table % 5. Analysis 49 of.43 main parameters52.17 under variable57.517 parameters. 60.59 59.89 Exhaust flue gas temp. °C 116.66 107.38 96.19 90.74 83.83 New System 3 THA 75% THA 50% THA 40% THA 30% THA Hot air temp. °C 346.61 328.98 297.83 285.73 262.45 Total power output MWMain steam990.96 flow t/h731.1 2507.06 1823.23 483.95 1182.16387.96 953.66287.41 725.37 Air flow t/h 2918.34 2311.66 1715.18 1499.91 1189.28 Fans’ power consump- Ratio of air heated MW 0.6 %0.33 42.93 44.380.18 49.550.15 52.970.05 48.77 tion by steam Coal flow t/h Flue gas flow 315.81 t/h237.75 3206.36 2528.49164.19 1864.92135.26 1623.27 105.72 1285.7 Standard coal consump- Bypass flue gas ratio % 49.43 52.17 57.517 60.59 59.89 g/kWhExhaust flue256.29 gas 261.84 273.33 281.7 298.26 ◦C 116.66 107.38 96.19 90.74 83.83 tion rate temp. Coal consumption rate Hot air temp. ◦C 346.61 328.98 297.83 285.73 262.45 g/kWh 1.08 1.12 0.59 0.44 0.19 reduction Total power output MW 990.96 731.1 483.95 387.96 287.41 Fans’ power MW 0.6 0.33 0.18 0.15 0.05 Figureconsumption 16 shows the changes of the standard coal consumptions in both new systems Coal flow t/h 315.81 237.75 164.19 135.26 105.72 1 and Standard3 under coaloff-design conditions. It can be seen that after the further optimization of g/kWh 256.29 261.84 273.33 281.7 298.26 newconsumption system 1, the rate coal consumption rate decreases by 1 g/kWh under both THA and 75% THACoal conditions. consumption Under low load conditions, the reduction amount of coal consumption g/kWh 1.08 1.12 0.59 0.44 0.19 rate decreases.rate reduction Especially under 30% THA condition, only 0.2 g/kWh is saved. New system 3 performs poorly at low load, which needs to be noticed and improved.

Figure 16. The standard coal consumption comparison of three systems under variable conditions. Figure 16. The standard coal consumption comparison of three systems under variable conditions. 7. Conclusions 7. Conclusions This paper takes a 1000 MW ultra-supercritical double reheat coal-ﬁred power gen- erationThis systempaper takes as a referencea 1000 MW system. ultra-supercritical Based on the double comprehensive reheat coal-fired consideration power gen- of the erationutilization system of ﬂueas a gasreference waste heatsystem. and Based the extraction on the comprehensive steam superheat consideration heat, different of the new utilizationheat integration of flue systemsgas waste based heat on and the the boiler-turbine extraction steam coupling superheat idea are heat, proposed. different The new main features of new systems are as follows: (1) The low temperature feed water and the superheat heat of extraction steam are used to heat part of the air in new system 1 and new system 2. The superheat degrees of Energies 2021, 14, 238 20 of 22

extraction steams and the temperature differences of the air heat transfer process are reduced together. (2) New system 2 replaces two-stage outside steam coolers with the air heater on the basis of new system 1, and thermal performances of two systems are deeply compared. (3) New system 3 uses the feed water to preheat the air on the basis of new system 1, and the heat transfer temperature difference of air preheater is further reduced. Thermodynamic models are established and their thermodynamic performances under different operation conditions are investigated and compared. The results are as follows: (1) The new systems follow with the energy level matching principle. Through the boiler- turbine coupling, the exergy losses of heat transfer process are obviously reduced, and the thermal efficiency of the overall system are improved. The results show that the power output of new system 1 increases by 6.38 MW and the standard coal consumption rate decreases by 1.65 g/kWh compared with the reference system. The power output of new system 2 increases by 6.93 MW and the standard coal consumption rate decreases by 1.79 g/kWh compared with the reference system. (2) The performances of new system 1 and new system 2 are better than that of reference system under all working conditions. When the load decreases, the coal consumption rate of two new systems increases more slowly, and the inlet air temperature of boiler can be maintained to ensure the safe operation of the boiler. However, the exhaust temperatures of flue gas in two new systems are slightly lower than that of original system. When the load is too low, the exhaust flue gas temperature will be too low, and the tail heaters may be corroded at lower temperature. Therefore, two new systems should keep the load rate above 50% THA as far as possible. (3) The standard coal consumption rates of new systems 1 and 2 are similar. However, new system 2 is more complex than new system 1. In addition, the wet steam may appear at the outlet of steam-air heat exchanger of new system 2 under variable operating conditions, which affects the safety of the system. Considered comprehensively, new system 1 is superior to new system 2. (4) New system 3 further reduces the heat transfer temperature difference of air preheater, and improves the power generation efficiency of the overall system. The standard coal consumption rate is further reduced by 1 g/kWh under high load compared with new system 1. The energy-saving effect is obvious. However, the reduced coal consumption rate decreases at low load conditions. Especially under the 30% THA condition, the reduced standard coal consumption rate is only 0.2 g/kWh compared with new system 1. In this paper, the waste heat utilization of flue gas is not considered. The waste heat of flue gas could be considered in future work. Additionally, the low reduced coal consumption rate at low load conditions in new system 3 is worth further studying. Moreover, the comprehensive economic performances of new systems should be further studied in the future.

Author Contributions: Conceptualization, M.Y. and L.D.; methodology, Y.T.; software, L.D.; valida- tion, M.Y.; resources, L.D. and Y.T.; data curation, L.D.; writing—original draft preparation, M.Y.; writing—review and editing, L.D.; visualization, M.Y.; supervision, L.D.; project administration, L.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R&D Program of China (grant number 2017YFB0602101) and the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No. 51821004). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Energies 2021, 14, 238 21 of 22

Acknowledgments: This study has been supported by the National Key R&D Program of China (grant number 2017YFB0602101) and the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No. 51821004). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclatures Abbreviation AH Air heater H Regenerative heater IEA International energy agency IP Intermediate-pressure cylinder LP Low-pressure cylinder OSC Outside steam cooler RH Reheater SH Superheater SHP Super high-pressure cylinder THA Turbine heat acceptance condition USC Ultra-supercritical HP High-pressure cylinder HRSG Heat recovery steam generator HT High-pressure turbine Symbols b Coal consumption, g/kWh bx Standard coal consumption rate, g/kWh A Energy level B Coal flow rate, t/h Pe Net output power, MW Qnet,ar Net calorific value of design coal, kJ/kg s Q net,ar Net calorific value of standard coal, kJ/kg T0 Environment temperature, K ∆S Entropy change, J/(kg·K) ∆ε Exergy change, MW ∆H Energy change, MW

References 1. International Energy Agency. Global Coal Market Report (2018–2023); International Energy Agency: Paris, France, 2018. 2. China Electricity Council. China Electric Power Industry Annual Development Report; China Electricity Council: Beijing, China, 2017. 3. Liu, R.; Xiao, P.; Zhong, L. Research progress of advanced 700 ◦C ultra-supercritical coal-fired power generation technology. Therm. Power Gener. 2017, 9, 1–7. 4. Yan, M.; Zhang, L.; Shi, Y.; Zhang, L.; Li, Y.; Ma, C. A novel boiler cold-end optimization system based on bypass flue in coal-fired power plants: Heat recovery from wet flue gas. Energy 2018, 152, 84–94. [CrossRef] 5. Han, Y.; Xu, G.; Zheng, Q.; Xu, C.; Hu, Y.; Yang, Y.; Lei, J. New heat integration system with bypass flue based on the rational utilization of low-grade extraction steam in a coal-fired power plant. Appl. Therm. Eng. 2017, 113, 460–471. [CrossRef] 6. Xu, G.; Huang, S.; Yang, Y.; Wu, Y.; Zhang, K.; Xu, C. Techno-economic analysis and optimization of the heat recovery of utility boiler flue gas. Appl. Energy 2013, 112, 907–917. [CrossRef] 7. Wang, J.; An, L.; Zhang, X. Performance analysis on heat and water recovery from flue gas of coal-fired boiler. Therm. Power Gener. 2018, 47, 48–54. 8. Xu, Z.; Yang, S.; Yang, M. Optimization of multi-pressure condenser. J. Chin. Electr. Eng. Sci. 1994, 14, 14–19. 9. Xu, C.; Xu, G.; Zhao, S.; Dong, W.; Zhou, L.; Yang, Y. A theoretical investigation of energy efficiency improvement by coal pre-drying in coal fired power plants. Energy Convers. Manag. 2016, 122, 580–588. [CrossRef] 10. Wang, J.; Duan, L.; Yang, Y. Study on the general system integration optimization method of thesolar aided coal-fired power generation system. Energy 2019, 169, 660–673. [CrossRef] 11. Zhou, L.; Xu, G.; Zhao, S.; Xu, C.; Yang, Y. Parametric analysis and process optimization of steam cycle in double reheat ultra-supercritical power plants. Appl. Therm. Eng. 2016, 99, 652–660. [CrossRef] 12. Zhou, L.; Xu, G.; Bai, P.; Xu, C.; Yang, Y. Thermodynamic Analysis on the Superheating Degree Utilization Modes of 1 000 MW Ultra-supercritical Units. J. Chin. Soc. Power Eng. 2017, 37, 495–500. 13. Yang, Y.; Zhang, C.; Xu, G. A Heat Integration System Based on Boiler-Turbine Coupling for Large Scale Coal-fired Power Plants. Proc. CSEE 2015, 35, 375–381. Energies 2021, 14, 238 22 of 22

14. Fan, C.; Pei, D.; Wei, H. A novel cascade energy utilization to improve efﬁciency of double reheat Cycle. Energy Convers. Manag. 2018, 171, 1388–1396. [CrossRef] 15. Jin, H.; Ishida, M. Graphical exergy analysis of complex cycles. Energy 1993, 18, 615–625. [CrossRef] 16. Zheng, D.; Uchiyama, Y.; Ishida, M. Energy-utilization diagrams for two types of LNG power-generation systems. Energy 1986, 11, 631–639. [CrossRef] 17. Zheng, D.; Ishida, M.; Ikawa, T. Graphic exergy analysis for methanol synthesis. Part 1. Evaluation of steam reforming process system: Evaluation of Steam Reforming Process System. Sekiyu Gakkaishi 1986, 29, 47–53. [CrossRef]