An Improved System for Utilizing Low-temperature Waste Heat of Flue Gas from Coal-Fired Power Plants

In this paper, an improved system to efficiently utilize the low-temperature waste heat (WHUS) from the flue gas of coal-fired power plants is proposed based on heat cascade. The essence of the proposed system is that the waste heat of exhausted flue gas is not only used to preheat air for assisting coal combustion as usual but also to heat up feedwater and the low-pressure steam extraction. Preheated by both the exhaust flue gas in the boiler island and the low-pressure steam extraction in the turbine island, thereby part of the flue gas heat in the air preheater can be saved and introduced to heat the feedwater and the high-temperature condensed water. Consequently, part of the high-pressure steam is saved for further expansion in the steam turbine, which obtains additional net power output. Based on the design data of a typical 1000 MW ultra-supercritical coalfired power plant in China, in-depth analysis of the energy-saving characteristics of the optimized WHUS and the conventional WHUS is conducted. When the optimized WHUS is adopted in a typical 1000 MW unit, net power output increases by 19.51 MW, exergy efficiency improves to 45.46%, and net annual revenue reaches 4.741 million USD. In terms of the conventional WHUS, these aforementioned performance parameters are only 5.83 MW, 44.80% and 1.244 million USD, respectively. The research of this paper can provide a feasible energy-saving option for coal-fired power plants.


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for feedwater preheating, thus allows more steam to expand throughout the whole turbine and to generate more power. Consequently, the net efficiency of the whole plant can be improved.
So far, the academic research on efficient utilization of flue-gas waste heat mainly concentrates on novel layouts of the recovery system and special heat exchanger meeting the demanding conditions of flue gas from coal combustion. For example, Espatolero et. al. [19] explored the effects of the temperature of exhausted flue gas and the heat-exchanger performance on waste heat recovery, and evaluated the energy-saving effect for the boiler cold-end. Chen et. al. [20] investigated several new technologies to exploit low-grade heat recovery from humid flue gas, taking the latent heat of water vapor condensation into account. Xu et. al. [21,22] proposed a novel waste heat recovery system by dividing the air preheater into high-temperature and low-temperature preheaters. An additional low-temperature economizer is placed between the electrostatic precipitator and the lowtemperature air preheater. This proposal achieves a net additional power output of 9 MW for a 1000 MW coal-fired power plant. Wang et. al. [23] developed an advanced waste heat and water recovery technology by nanoporous ceramic membrane to extract a portion of the water vapor and its latent heat from flue gases.
For the industrial applications of flue-gas waste heat recovery, several projects have been launched or are currently ongoing. For example, the German Schwarze Pumpe power plant (2×800 MW lignite generation unit) implemented a flue gas division system after the electrostatic precipitator and recovered low-grade heat to heat up the condensed feedwater. Significant energysaving effect has been reported [24,25]. In China, several 1000 MW scale coal-fired power plants, such as the most efficient coal-fired power plant in the world (Waigaoqiao 3rd power plant in Shanghai) [26], have adopted the waste heat utilization system (WHUS) to heat up the condensed water. The employed low-temperature economizer promotes the boiler efficiency by 2 percentage points and the net plant efficiency by 0.8 to 0.9 percentage point [27][28][29].
There have been two major issues to be addressed for low-grade heat recovery from flue gas: 1). Most of the proposed concepts depend on a low-temperature economizer, which mostly work near or even below the acid dew point and thus suffers from severe material corrosion. 2) The temperature of exhaust flue gas has been rather low and the small temperature difference of heat transfer leads to large heat exchanger area and investment cost. Thus, in-depth research of the waste heat utilization is of great importance to find the best trade-off design between the efficiency improvement and the capital investment.
With the above context and following our previous research on low-grade waste heat for power plant, we propose originally a novel waste heat recovery system and improve the origin system based on the comprehensive understanding of the performance relevance between air preheating process in the boiler island and feedwater preheating process in the turbine island. Cascade utilization of heat is realized between different working fluids (flue gas, steam extraction, air, etc.). The paper is organized as follows: In section 2, the basic concept of waste heat recovery and its evaluation criteria are introduced. Then, in section 3, the conventional heat recovery system is analyzed to highlight the existing bottleneck for in-depth utilization of potential waste heat. Subsequently, in section 4, we propose an improved WHUS system and evaluate the system by both thermodynamic and economic criteria. Finally, the conclusions are drawn (section 5).

Waste heat recovery and the evaluation criteria
The basic concept of recovering waste heat from exhausted flue gas to heat up the lowtemperature feedwater is illustrated in Figure 1. With the recovered heat, the requirement of steam extraction for heating up feedwater preheater is reduced, thus more steam can expand fully to the condensate pressure for additional power generation.

Thermodynamics of Waste heat recovery
The amount of heat recovered from the exhausted flue gas is calculated as follows: is the flow rate of the flue gas (kg/s); ℎ , and ℎ , refer to the input and output flue gas enthalpy, respectively (kJ/kg). Note that the enthalpy change in Eq. 1 has already taken the latent heat during water condensation into account.
The energy balance of a feedwater preheater can be expressed by Eq. 2, if no external heat is introduced to the feedwater preheater considered: where , ℎ, and are the mass, enthalpy, and the heat recovered from the flue gas, respectively. We assume that the power plant considered operates at steady states with or without the heat recovery system and we consider only the system design but no partial-load operation. Thus, the following equations are established:

Additional work
The additional work generated by the expansion of the extra stream can be calculated as: where ℎ and ℎ stand for the enthalpy of the steam extraction considered and the exhaust steam, respectively (kJ/kg).
Taking the change of power consumption of the auxiliary devices, ∆ , into account, the net additional power output of the whole power plant can be expressed as follows: The increment of auxiliary power consumption is mainly due to the induced draft fans configured in the boiler rear flue gas duct to compensate the pressure loss of flue gas when flowing through the additional heat exchangers for heat recovery. The increase in the fan power can be calculated as follows: where ∆ is the increase in the pressure drop of exhausted flue gas (kPa), is the isentropic efficiency of the induced draft fan ( = 0.85) [31,32] and D is the volumetric flow rate of the flue gas (m 3 /s).

Heat rate reduction
The power industry usually uses the heat rate as a common indicator to evaluate the performance of a power generation unit. The heat rate represents the amount of fuel energy input per 1 kWh net electricity output. Given the net additional power generation ∆ , the reduction in the plant heat rate is thus represented by Eq. 8 [33]: where the refers to the total input energy (MW), while the is the net electricity output (MW).

Reference coal-fired power generation unit
In the presented paper, we select a typical 1000 MW ultra-supercritical power generation unit in China as a case study to quantify the benefit from the waste heat recovery from flue gas. The net power output reaches 942 MW. At full load, the plant operates with the pressure of main steam of 26.25 MPa, the temperature of main and reheated steam at 600 °C. The plant is designed for a bituminous coal with the element analysis of 56.26% carbon, 3.79% hydrogen, 12.11% oxygen, 0.82% nitrogen, 0.17% sulfur and 18.1% water. The layout of the power plant has been given in Figure 2 and the stream data of all regenerative heaters (RHs), when the heat recovery system is not considered, is listed in Table 2 for the operating condition THA (THA refers to the turbine heat rate acceptance condition and it is a design condition without water supply.).

Conventional waste heat recovery system
In the coal-fired power generation unit, a large amount of steam with different parameters needs to be extracted to heat the feedwater and the condensed water, that is, the regenerative process. In this process, the temperatures of the feedwater and the condensed water will be increased, which is beneficial to improve the thermodynamic cycle efficiency. However, the working ability of the steam, which is extracted from the turbine to heat the feedwater and the condensed water, will be destructed since it can no longer continue to expand in the steam turbine. In the conventional WHUS, the exhaust energy of the flue gas is utilized to heat the condensed water, part of the steam extraction is thus saved and can be continue to expand for more power output. As a result, it will raise the gross power output and improve the thermal conversion efficiency. Figure 2 depicts the configuration of the conventional WHUS. The LTE is arranged in the downstream of the air preheater in flue gas duct, which is parallel to the RH6. Part of the condensed water at the inlet of the RH6 will enter the LTE and return to the regenerative system after absorbing the flue gas waste heat. Afterward, the condensed water will converge with the main condensed water at the outlet of the RH6. In this way, the 6th-stage steam extraction can be partly saved. Table 2 presents the thermodynamic analysis results of the conventional WHUS. The inlet flue gas temperature of the LTE is equal to that of the exhaust flue gas from the air preheater, which is 131 °C. Meanwhile, due to the relatively low sulfur content of the coal (approximately 0.17%), as well as the acid steam wraparound effect brought by the flying ash, the outlet flue gas temperature of the LTE can be reduced to 100°C without serious corrosion problem.. According to the relevant thermodynamics theories, the smaller the temperature difference between the working mediums, the smaller heat transfer exergy destruction. In this case, higher condensed water temperature is preferred, given the fixed flue gas temperature range. In related heat transfer and techno-economic theories, however, a small heat transfer temperature difference increases the heat transfer area and the volume of the heat exchange device. As a result, investment in the heat exchanger is heightened. To balance the thermodynamic performance and equipment investment in the conventional WHUS, LTE adopts the counter-current arrangement and is connected in parallel to RH6. By this arrangement, on the one hand, provided that the engineering constraint is allowed, the condensed water temperature is enhanced as high as possible. As seen in Table 2, considering the flue gas temperature of the LTE is only 131-100 °C, which can only be used to heat the condensed water of RH6 at most (83.3-122.1 °C). On the other hand, the outlet condensed water temperature of LTE is set to 116 °C, slightly lower than 122.1 °C, which ensures the minimum heat transfer temperature difference of the LTE is maintained over 15 °C [17]. Overall, the total investment of the conventional WHUS could be maintained at a relatively acceptable level. Meanwhile, the net power output is increased by 5.83MW, whereas the heat rate of the generation unit is reduced by 42.56 kJ /kWh.
In the LTE, the energy donor is the exhaust flue gas and the energy acceptor is the condensed water of the regenerative system. Therefore, WHUS performance is affected not only by the characteristic of the flue gas, but also by the parameters of the steam cycle. Specifically, power output Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2017 doi:10.20944/preprints201707.0029.v1 and economic benefits are not only affected by the quantities of heat released by the flue gas, but also by the parameters of saved steam extraction. In the conventional WHUS, the LTE is installed in the outlet of the air preheater, the inlet flue gas temperature of the LTE is only 131 °C, which can replace the 6th-stage steam extraction at most, as shown in Figure 3. The 6th-stage steam extraction is characterized by a relatively low working ability since its pressure is only 0.23 MPa, which is the limited factor for improving the energy-saving effects of recycling the flue gas waste heat.

Description of the optimized WHUS
According to the analysis above, to further improve the energy conservation effects of the WHUS, it is essential to enhance the flue gas temperature that entering the LTE. Meanwhile, noting that the logarithmic mean temperature difference of the air preheating process is relatively large (over 60 °C). Thus, to utilize the energy rationally, an optimized WHUS is proposed in this section. Figure 4 illustrates the optimized WHUS. This system adds a bypass flue gas duct which is paralleled with the main air preheater. In the bypass flue gas duct, two gas-water heat exchangers are successively installed, approximately one third of the outlet flue gas of the economizer enters the Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2017 doi:10.20944/preprints201707.0029.v1 high-temperature gas-water heat exchanger and the low-temperature gas-water heat exchanger of the bypass flue gas duct in sequence, to heat the feedwater (189.9-290 °C) and the condensed water (83.3-153.3 °C), respectively. Since the heat of the flue gas entering the main air preheater reduces in the optimized WHUS, two additional heat exchangers are added to maintain the inlet air temperature of the furnace. Among them, the first-stage heat exchanger utilizes the low-pressure steam extraction to heat the air, while the second one applies the waste flue gas (131-100 °C) to heat the air. The parameters of main heat exchange equipment are shown in Table 3.   Figure 5 presents the heat transfer curve of the optimized WHUS. As indicated both in this figure and in Table 3, the optimized WHUS fully realizes the energy grade match among the exhaust flue gas, air and the condensed water. By adopting two additional heat exchangers, the 7th-stage steam extraction and low-temperature flue gas are utilized to heat the air before it enters the main air preheater, which guarantee the logarithmic mean temperature difference of the air preheating process can be controlled within 36 °C. Subsequently, approximately one third of the flue gas with the temperature of 372-131 °C is saved and introduced into the bypass flue gas duct to heat the feedwater and the condensed water. Part of 1-3th, 5th and 6th-stage steam extractions could be saved and continued to expand for more power output in the steam turbine. Evidently, the energy saving effects of the optimized WHUS is improved remarkably.

Thermodynamic performance results
The thermodynamic analysis comparison between the conventional WHUS (as shown in Figure  2) and the optimized WHUS (as presented in Figure 4) is conducted in Table 4. The gross work output of the optimized WHUS increases by 22.01 MW. This increase is mainly attributed to that the temperature of the flue gas used to heat the feedwater and the condensed water reaches 372-131 °C in the optimized WHUS, which is much higher than that of the conventional WHUS (131-100 °C). The high-grade steam extraction can thus be replaced. As a result, gross work output improves significantly.
However, since several additional heat exchangers are adopted in the optimized WHUS, some pumps and fans are required to overcome the resistance of the water, air and flue gas. As indicated in Table 4, the auxiliary power in the optimized WHUS increases by 2.28 MW. Overall, the increment in net work output is 19.51 MW in the optimized WHUS, and the reduction in heat consumption rate is 143.35 kJ/kWh; whereas for the conventional WHUS, the aforementioned performance parameters are only 5.83 MW and 42.56 kJ/kWh, respectively. Thus, the thermal efficiency of the optimized WHUS is significantly improved.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2017 doi:10.20944/preprints201707.0029.v1  Figure 6 shows the effects of waste heat utilization on the steam extraction and the work output of different systems. The column chart with slash line represents the variation in the multistage steam extractions of the regenerative heaters. When the steam extraction is reduced, the column is located above the x axis; conversely, the column is located below the x axis if steam extraction is increased. The column chart with shadow denotes the variation in work, if there is an increment in work, the column is located above the x axis, and vice versa. The following conclusions can be drawn from Figure 6: 1. In the conventional WHUS, by adopting the LTE, the flue gas with the temperature of 131-100 °C is utilized to heat the condensed water from the inlet of the RH6, as a consequence of which, the 6th-stage steam extraction is saved by 14.06 kg/s and the power output is increased by 7.37 MW. Meanwhile, it has to be noted that the 7th and 8th-stage steam extractions show a slight increase, this can be mainly attributed to the fact that the reduction in the 6th-stage steam extraction limits the drainage water flowing into RH7 and RH8 accordingly. However, considering the variation in the 7th and 8th-stage steam extractions is relatively small, the resultant power output variation can almost be neglected. In summary, the total steam extraction of the conventional WHUS decreases by 13.12 kg/s whereas the power output increases by 7.15 MW. 2. In the optimized WHUS, there are obvious changes in the 1-3th, 5th and 6th-stage steam extractions. The reason is that the gas-water heat exchangers arranged in the bypass flue gas duct utilize part of the flue gas with the temperature of 372-131°C to heat the feedwater of RH1-RH3 and the condensed water of RH5-RH6. As a result, the steam extractions of these regenerative heaters reduce considerably. The 7th-stage steam extraction is increased by 13.72 kg/s, which is utilized to preheat the air in the first-stage heat exchanger. Besides, the steam extraction of DEA is increased whereas the 8th-stage steam extraction is reduced, this is because the drainage water flowing into DEA and RH8 is affected by the steam extraction of prior stage regenerative heater, which will further affect the steam extraction of DEA and RH8. Overall, in the optimized WHUS, the total reduction of steam extraction is 11.87 kg/s whereas the power output increases by 22.01 MW. 3. In the heat regenerative system, there is a huge working ability difference between the steam extractions from different stages of regenerative heaters. For instance, the working abilities of the 1-3th, 5th and 6th-stage steam extractions are obviously higher than that of the 7th-stage steam extraction. As can be seen from Figure 6, by saving 1 kg steam extraction of RH1, RH2 and RH3, the corresponding additional power outputs are 1.21MW, 1.15MW, and 0.94MW, respectively. Whereas saving 1 kg 6th-stage steam extraction can only improve the power output by 0.45 MW, as for RH7, the power output is only decreased by 0.25 MW if the steam extraction consumption is increased by 1 kg. 4. The overall reductions in the steam extractions of the conventional WHUS and optimized WHUS varied slightly (13.12 kg/s vs. 11.87 kg/s). And the exhaust flue gas temperature of these two systems is equally set to 100 °C, which means the same amount of waste heat is recovered. Nevertheless, in the conventional WHUS, the flue gas waste heat is used to save the 6th-stage steam extraction, and the results show that its gross work output increment is 7.15 MW. However, the 1-3th, 5th and 6th-stage steam extractions are significantly reduced in the optimized WHUS despite the increase in the 7th-stage steam extraction. Finally, the gross work output increment reaches 22.01 MW, which is approximately three times as that of the conventional WHUS. In conclusion, with the reasonable utilization of the low-grade energy from both the boiler island and the turbine island, more high-grade steam extraction is saved in the optimized WHUS, better thermodynamic and waste heat recycling performances can be obtained, given that the same amount of waste heat is recovered in two systems.

Exergy analysis
To reveal the internal phenomena of the optimized WHUS [18][19][20][21], an exergy analysis is performed in this section for both the optimized WHUS and the conventional WHUS. The results are listed in Table 5.
As shown in Table 5, the exergy efficiency of the optimized WHUS is 45.46%, which is 0.66% higher than that of the conventional WHUS. Comparing the exergy distribution of the optimized WHUS with the conventional WHUS, it can be found that, the exergy destruction of the optimized WHUS is reduced by 10.9MW in the boiler island and 3.96MW in the turbine island. Hence, the reduced exergy destruction of the optimized WHUS is mainly attributed to the boiler island.
To be specific, the exergy destruction in the boiler island is significantly affected by the air preheating process. This influence is ascribed to the fact that the optimized WHUS utilizes lowpressure steam extraction and low-temperature flue gas to heat the air in sequence. Therefore, the heat transfer temperature difference decreases significantly in the air preheating process. As a result, the heat transfer exergy destruction decreases by 14.43 MW. However, by adopting the bypass flue gas duct, the exergy destruction in the boiler island increases by 4.39 MW. By taking the exergy Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2017 doi:10.20944/preprints201707.0029.v1 destruction of other parts in the boiler island into account, the exergy destruction in the boiler island of the optimized WHUS is reduced by 10.9 MW compared to that of the conventional WHUS. As for the turbine island, the variation in exergy destruction mainly takes place in the regenerative process. The reason accounting for this is that: in the optimized WHUS, more feedwater and condensed water is heated via the gas-water heat exchangers adopted in the bypass flue gas duct in the boiler island, thereby the water volume flowing through the regenerative system is reduced significantly, and the exergy destruction is reduced by 3.97 MW accordingly. Besides, with consideration of the exergy destruction in other parts such as condenser and pipeline etc, the total exergy destruction in the turbine island of the optimized WHUS is reduced by 3.96 MW, compared to that of the conventional WHUS.
From the analysis above, it is obvious that the optimized WHUS utilizes the low temperature energy in both the boiler island and the turbine island reasonably, thereby realizes the energy grade improvement of the waste heat utilization process. Essentially speaking, the exhaust flue gas temperature of the optimized WHUS keeps the same with that of the conventional WHUS, but the exergy destruction of the air preheating is significantly reduced. Finally, the exergy efficiency of the optimized WHUS is improved by 0.66%, which seems very small numerically, noting that the denominator of efficiency calculation is extremely large (2248.06MW), thereby the resultant energysaving effects are actually rather considerable. As presented in Table 4, given the same fuel input, the additional power output of the optimized WHUS is 19.51 MW, reaching over 3 times as the conventional WHUS (5.83MW), reflecting the remarkable energy-saving benefits of the optimized WHUS.

Techno-economic analysis
To further evaluate the energy-saving benefits of the optimized WHUS in actual engineering application, the techno-economic analysis is conducted in the section, the following assumptions are adopted during the analysis: (1)the on-grid power tariff is set at 0.061 USD/kWh; (2) the annual operation hours of the power generation unit is 5000 hours [22]. Here, the annual operation hours stand for the equivalent operation hours of the power generation unit under the rated capacity. Hence, for the power unit that operates below the rated capacity constantly, its annual operation hours are relatively low in spite of the high actual operation hours. Considering that nowadays it is very common for the large-scale coal-fired power units in China to participate in peak load regulation, which means that they are operated below the rated capacity in a long term, thus the annual operation hours of the coal-fired power units in China are comparatively low; (3) the operation and maintenance (O&M) cost accounts for 4% of the total investment annually [23,24]; and (4) the exchange rate is set to 6.25 CNY/USD.

Estimation of the total investment cost
Based on the scaling up method [5,25,26], the investment of the new added equipment and the related pump are estimated by the following equation: Size TIC GDP CE I K Size (9) where TIC is the total investment cost of system optimization; IInstall,b is the investment cost for the benchmark equipment; Sizea and Sizeb are the size parameters of the equipment and the benchmark equipment, respectively; f is the size factor; GDP is the variation factor; CE is the price index factor for the chemical equipment; K is the region factor. The detailed reference data are listed in Table 6 [27]. b: Cost is taken from a feasibility study of flue gas waste heat recovery project in China 2009 [28]. c: Cost is quoted from Moaseri [29]. d: The parameters are based on [15].e: The parameters are based on [26,30].
The specific investment costs for added equipment of the optimized WHUS are listed in Table  7, the costs for pipeline and engineering installation are estimated to be 5% and 17% of the total equipment investment cost [31], respectively. For the conventional WHUS, introducing the LTE adds 2.993 million USD to the original total investment, with consideration of other investments such as pumps, pipeline, construction and installation, its TIC is 3.765 million USD. While for the optimized WHUS, as the logarithmic heat transfer temperature difference of the air preheater decreases because of the increasing inlet air temperature, its heat transfer area and investment cost will be increased, thus extra 0.632 million USD is required for the air preheater, as shown in Table 7. Moreover, adopting the gas-water heat exchangers and two-stage heat exchangers introduces 1.911 million USD and 4.175 million USD, respectively. Taking other relevant investments into account, the TIC of the optimized WHUS reaches 8.536 million USD. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2017 doi:10.20944/preprints201707.0029.v1

Economic performance index
Based on the investment estimation results, this section analyzes the feasibility of the optimized WHUS from the perspective of economic benefits. The net annual revenue (NAR) is calculated based on the dynamic analysis, the construction investment and the operation cost estimation. The specific formula is as follows： where EAI is the additional income per year generated by the system optimization, which is calculated as: net eq e EAI P h C = Δ (11) where heq is the equivalent operation hours per year and is the on-grid power tariff. In addition, the annualized investment capital cost ( ) can be calculated as follows [32,33]: where i refers to the fraction interest rate per year, which is set at 8%; and n represents the system lifespan, which is presumably 20 years. Table 8 provides the techno-economic analysis results. The EAI of the optimized WHUS is almost 5.951 million USD, which is more than three times of the conventional WHUS. The CTIC and CO&M of the optimized WHUS are larger than that of the conventional WHUS, reaching 0.869 million USD and 0.341 million USD, respectively. Nevertheless, the net additional power output in the optimized WHUS is much higher than that in the conventional WHUS and it will affect the NAR majorly. Consequently, the NAR of the optimized WHUS reaches 4.741 million USD per year, which presents its excellent economic performance.

Conclusion
In this study, an optimized low-temperature flue gas waste heat utilization system is proposed based on the energy cascade utilization principles. In-depth analyzes on the thermodynamic and techno-economic characteristics of the optimized WHUS are conducted. The following conclusions can be drawn: 1. In the conventional WHUS, in order to recycle the flue gas waste heat, LTE is adopted and arranged in the downstream of the air preheater in the flue gas duct. Since the inlet flue gas temperature of the LTE is 131 °C, which can replace part of the 6th-stage steam extraction. Combined with the engineering constraints, the heat rate of the power generation unit is only reduced by 42.56 kJ/(kW·h) . Furthermore, the energy-saving effects are limited. 2. In the optimized WHUS, the low-temperature heat from both the boiler island and the turbine island is utilized reasonably to preheat the air. In this way, not only the inlet air temperature of the air preheater is increased, also the saved high temperature flue gas (372-131 °C) can be introduced to the bypass flue gas duct to heat the feedwater and the condensed water, as a consequence of which, part of the high-pressure steam extraction is saved, leading to the net work output of the optimized WHUS is increased by 19.51 MW, while the heat rate is reduced by 143.35 kJ/(kW·h). The energy-saving effects of the optimized WHUS are remarkable. 3. In the conventional WHUS, the 6th-stage steam extraction is saved, while in the optimized WHUS, the 1-3th, 5th and 6th-stage steam extractions are saved. In general, the working ability of the high pressure steam extraction is much larger than that of the low pressure steam extraction. Therefore, the resultant energy-saving effects differ distinctly although the total amounts of steam saved by both systems are almost similar. 4. For the conventional WHUS, the logarithmic mean temperature difference in the air preheating process reaches 60 °C. However, in the optimized WHUS, the logarithmic mean temperature difference is less than 36 °C because the air is successively heated by low-pressure steam extraction and low-temperature flue gas. In this case, the exergy destruction of the air preheating process is reduced by 14.43 MW, which becomes the main reason for decreasing the total exergy destruction of the optimized WHUS. Ultimately, the exergy efficiency of the optimized WHUS improves to 45.46%. 5. Techno-economic analysis results show that the total investment of the optimized WHUS is 8.536 million USD, which is doubled compared to that of the conventional WHUS. Nevertheless, the net additional power output in the optimized WHUS is 19.51 MW, which is over three times of the conventional WHUS. Consequently, the net annual revenue of the optimized WHUS can reach 4.741 million USD per year, which is approximately four times as large as the conventional WHUS.