Research on influence of steam extraction parameters and operation load on operational flexibility of coal-fired power plant

https://doi.org/10.1016/j.applthermaleng.2021.117226Get rights and content

Highlights

  • A novel steam extraction system is proposed for power plant flexible operation.

  • The extracted steam heat is stored in molten salts and phase change materials.

  • The stored heat is reused to heat pressured water at the peak power consumption.

  • The output power can be reduced below 50% rated power effectively.

Abstract

The operational flexibility of coal-fired power plant is very important for the integration of large-scale renewable energy to the grid. In order to increase the operational flexibility of coal-fired power plant, a 600 MW subcritical coal-fired power plant was selected as research example to analyze the influence of steam extraction parameters and operation load by simulation. A novel main steam extraction system was proposed to adjust the power output under the low load period. The sensible and latent heat of the extracted steam was stored in molten salts and phase change materials, respectively, during the load reduction process. While the stored heat was reused again to heat the feed water and condensate water during the load raising process. The 50% rated load (300.03 MW) of the unit was used as the research benchmark to further adjust the load. The results show that the proposed steam extraction system can improve the operational flexibility of coal-fired power plant. The power output can be reduced to 204.51 MW from 300.03 MW when the main steam extraction mass flow rate is 250 t/h during the load reduction process. The sensible heat storage power factor and the operation load of the unit can affect the power output increment of coal-fired power plant during the load raising process. Under the condition of the maximal sensible heat storage power factor is 0.2462, the power output can be increased to 325.47 MW from 300.03 MW when the bypassed feed water mass flow rate of 251 t/h was heated by the stored heat during the load raising process. Using such operation model, the coal-fired power plant can achieve the maximum deep peak shaving time of 8.4 h/d, and its equivalent thermal efficiency is increased by 2.28%. With the same stored thermal energy to heat the bypassed feed water of 308.65 t/h, the power output can be increased to 394.2 MW from 360.07 MW (60% rated load) at the same sensible heat storage power factor of 0.2462 during the load raising process, and the coal-fired power plant can achieve the maximum deep peak shaving time of 9.86 h/d with its equivalent thermal efficiency increase of 2.55%. The results show that the proposed steam extraction method can make the unit operate in the lowest stable load and provide a wider renewable energy access space below the minimum stable load. The research provides a theoretical guidance for the flexibility design of coal-fired power plant, especially suitable for the large-scale renewable energy access to the grid in China.

Introduction

The environmental problems caused by carbon emissions can be effectively solved by reducing the consumption of fossil energy [1], [2]. The development of renewable energy is an inevitable choice for different countries, such as in China, the 3060 double carbon target has been proposed to formulate the future energy policies. Developing the renewable energy will be the future energy utilization direction in China. However, the large-scale renewable energy (e.g. wind and solar energy) has the characteristics of intermittency and instability [3], which directly affects the safety of the grid operation [4], [5]. The operational flexibility of coal-fired power plant (CFPP) [6] should be proposed for the future power grid regulation. The function of the CFPP has to change from the main power supply mode to the peak load regulation mode to make more and more renewable energy power access to the grid. There are two approaches to achieve the operational flexibility of CFPP. One is improving the load response rate [7], the other is adjusting the unit load range [8].

Adjusting the load response rate of CFPP was proposed to achieve the operational flexibility of CFPP. Wang et al. [9] realized the operational flexibility of CFPP at the range of 50–100% rated load by changing the water coal ratio. Zhou et al. [10] added a throttle valve in the extraction system of regenerative heater of the CFPP and proposed an improved coordinated control technology to realize the improvement of dynamic response ability and the operational flexibility of variable operation. Wang et al. [11], [12] changed successively the control logic of feed water bypass system and the coordinated boiler turbine heating mode. The load response rate and capacity of the unit were increased by 2 times and 2.9% rated power, respectively. The above studies show that the operational flexibility of CFPP can be improved by adjusting the load response. However, adjusting the load response rate of CFPP is only suitable for accepting more unstable renewable energy, and cannot provide more space for renewable energy.

The utilization of thermal energy storage (TES) can realize the highly realizable decouple of the power generation in space and time [13], [14], and improve the operational flexibility of CFPP to give more space for renewable energy. Lai et al. [15] added the hot water storage system into the cogeneration unit. The exhaust steam of the intermediate pressure turbine was extracted to heat the stored hot water. When the heat load is 300 MW, the unit load can be continued to decrease by 12%. Richter et al. [16] proposed that added the steam extraction system and the hot water storage devices into thermal power plant to reduce the unit load. The unit load was decreased by 7% in the TES process, and increased by 4.3% in the thermal energy release (TER) process. Trojan et al. [17] stored the pressured water of deaerator in hot water storage tank during the low load period. The unit load was decreased by 21.96 MW, and the lower load of the unit can be reached 67.5%. Li et al. [18] investigated the flexible regulation capacity of 600 MW supercritical CFPP by using TES of phase change materials. The unit load can be decreased by 13.3% and increased by 7.4% under the rated condition, respectively. Cao et al. [19] studied that the excessively electrical power of the unit was used to heat the molten salt under the rated condition. The stored thermal energy of molten salt was used to heat water to generate steam and to drive the auxiliary steam turbine. During the TES and TER process, the unit load was decreased by 16.67% and increased by 6.1% respectively. The previous researches on the operational flexibility of CFPP is only studied under the high load or the rated load. There is no literature on the operational flexibility characteristics of CFPP under low load. If the large-scale renewable energy is access to the grid, the power out of CFPP should be reduced lower, and even lower than the minimum stable load of the units. Thus, the deep peak shaving of CFPP should be urgently proposed to meet the demand of continuous load reduction. In addition, the excessive heat is stored in pressurized water and the generated surplus power is transferred to heat, and the stored heat is reused in the peak load process. Which results in the lower energy efficiency for the low heat density of pressure water, and for the heat from coal transferred to electricity and electricity transferred to heat again. So, the energy conversion efficiency of TES should be improved.

In addition, due to the difference between the coal used in the actual operation and design operation, the minimum operating load of the unit should not be less than 50% turbine heat acceptance condition (THA). The extracted steam with the higher temperature has the great vapor sensible heat and latent heat. If the total heat of the extracted steam is stored by sensible material, which has the disadvantages of pinch point temperature difference [20] and large latent heat of steam condensation [21], so that the temperature of thermal storage material is only about steam saturation temperature. If the total heat of extracted steam is stored by a kind of phase change material, the temperature of phase change material is not higher than the steam saturation temperature, and higher steam temperature is easy to cause the decomposition of phase change material. The higher temperature of the working medium results the higher energy efficiency in CFPP. So, the sensible and latent heat of extracted steam is stored in sensible material and phase change material, respectively, which will improve the energy efficiency of the CFPP.

Therefore, the unit load of 50 %THA is chosen as minimum stable load to reduce the power output through the main steam extraction, and the heat of extracted steam is stored in molten salts and phase change materials, respectively. The stored heat is used to heat the bypassed feed water and condensate water to reduce the steam extraction of regenerate heaters, so that the power output is increased.

The innovation of this research is to use the main steam extraction system to reduce the power output of the CFPP under the minimum stable load, and the heat of the extracted steam is stored and used again, so that its thermal energy efficiency is higher. The advantage of such steam extraction is that on the one hand, the boiler may operate in the lowest stable combustion load, and on the other hand, the power output of CFPP can be lowed below the minimum stable load and provide more space for renewable energy to the grid. The main research of this manuscript is to investigate the feasibility of operational flexibility of CFPP using main steam extraction to reduce the minimum operating load, and analyze the parameters influences of the extracted steam and the operation load on its operation range. The influences of the heat ratio of sensible heat to the total heat of extracted steam and the operation load of unit on the operational flexibility were studied and the energy efficiency was analyzed.

Section snippets

Studied system

The 600 MW subcritical CFPP was selected as a research object. The power output of CFPP is 600.18 MW at 100% THA.

Methodology for operational flexibility of CFPP

In this paper, the first law of thermodynamics is used to calculate the thermodynamic parameters of operational flexibility system of CFPP.

Model validation

According to Fig. 3, the parameters of CFPP at 100% THA, 60% THA and 50% THA are obtained. The errors between designed value and simulation value is analyzed. As shown in Table 3. The maximum error is reheat steam pressure under 50% THA, which is 2.11%. The changes of reheat steam pressure has little effect on the enthalpy of reheat steam. It can be ignored. Other errors are less than 2%, which meets the engineering accuracy requirements. So this model [26] can be used to investigate the

Conclusion

The 600 MW subcritical coal-fired power plant with 50% rated load (300.03 MW) is selected as the research model to adjust the unit load. The integrating of the main steam extraction system with coal-fired power plant to realize the operational flexibility is studied. The impacts of the extracted steam parameters and the operation load on the coal-fired power plant are analyzed during the load reduction and raising process. The main conclusions are listed as follows:

(1) The novel main steam

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 52076006).

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