Research on Coordinated Control Strategy of Thermal Heating and Melting Depth of Steam Heating and Melting Salt Reservoir

Abstract

The implementation of the upgrading of the national coal electric power unit has provided a clear proposal to promote the clean and low-carbon transformation of the power industry. With the power of large-scale intermittent renewable energy and power generation, the electric crew should be flexible enough to adjust resources to achieve a depth of 35% THA. This article aims to propose a heat extracting and heat storage system for fire power plants, to realize the coordinated control strategy of the deep peak, and to explore the coordinated control strategy of the steam–molten salt heat exchanger, molten salt and water exchanger, and the turbine’s main control. The simulation results reveal that the coordinated control of the steam–molten salt heat exchanger, molten salt and water heat exchanger, and steam turbine control could reduce the depth of the fire power unit by 10% THA. The output power response speed of the thermal power unit is enhanced by utilizing the heat turbine, which could effectively enhance the output power response speed of the thermal power unit and increase the output power response speed pertinent to 302.55 s by 75.60%.

1. Introduction

At present, the capacity of coal-fired units in China is estimated to be 59.22%, and the primary duty of the coal-fired plant was designed in the early stages of electric power design, so it is difficult to participate in the deep load adjustment in the later stages [1,2]. In the northern region, some thermoelectric production units are set up to provide the user with a heat source during the heating season. The electric and electric combination production unit is frequently utilized in the mode of “heat determination” mode, which is influenced by the heating load of the heating season, and it is difficult to realize the flexibility of grid scheduling and variable load operation in the low load-interval [3,4,5].

In order to resolve the problems mentioned above, domestic and international investigators have conducted relevant research on the adjustment of the load flexibility of the unit. For pure set units whose energy consumption is low, the peak ability is small, and the promotion is not strong, hot water for storage of heat or an electric boiler for storage of water can only be employed as a water supply. In order to fully exploit the thermal performance characteristics of the unit, the valley electric period is stored in the heat storage period, the heat peak of the unit is stored in the heat peak of the unit, and the flexibility of the load is adjusted, which is commonly not controlled by the type, location, climate condition, and the load limit of the unit, and the addition of the unit is moderately strong. Herein, the coordination and control of the heat and heat reservoir of the heat ignition unit are systematically planned to be rationally examined. Wang et al. [6] established a coordinated control strategy based on the multi-scale signal decomposition theory, which could fully exploit the rapid adjustment ability of battery energy storage systems in the power of the power grid and could successfully resolve the peak problem in new energy power systems. The validity of this approach was then checked by the simulation example. Rongqiang et al. [7] proposed a decoupling prediction control strategy with input constraint to control the heat vapor temperature of the ignition unit, and the simulation results reveal that the response performance of grid scheduling is better than that of proportional integration and differentiation. In a study by Wei and Fang [8], the rapid response of the large range of load changes specified that the coordinated control of the boiler and steam turbine could be utilized, and a large number of distributed equipment would be required to work together based on the typical coal plant model. A coordinated control scheme with H∞ performance was also developed by exploiting the linear quadratic regulator. The performed analysis indicated that the design control system has good performance in an extensive running range and exhibits good machine furnace coordination and control ability. In other works conducted by Zhang et al. [9], Yuan et al. [10], and Fan et al. [11], a frequency control method of the fire power unit (FPU) based on the frequency difference signal was proposed. The frequency modulation performance of the unit was also optimized by introducing multi-step positive feedback and differential feedback on the basis of the steam turbine side, boiler side, and auxiliary control side. The experimental results revealed that the control strategy could effectively enhance the response speed of the frequency. Xugang et al. [12] developed a multi-objective coordinated combustion optimization control strategy based on the implicit generalized predictive control for the large inertia, great lag, and strong coupling of the coal-fired boiler system of the FPU. In the case of the main steam pressure, the pressure of the furnace, the amount of oxygen in the steam, the amount of fuel, as well as the wind quantity and quantity of the air supply were the controls, and the design was performed based on the implicit generalized predictive control uncoupled and a multi-input multi-output combustion optimization control system. By decoupling the multi-input multi-output prediction model by PID neural network, the error-based feedback correction algorithm was introduced, and the achieved results showed that compared with the conventional PID and implicit generalized predictive control undetectable control strategy, the main steam pressure, the steam oxygen content, and the pressure adjustment time of the furnace could lessen the time of the pressure adjustment by 141 s, 210 s, and 162 s, and the hyper-adjustment volume leads to a reduction of about 18.2%, 18.2%, and 9.3%, respectively. In order to resolve the problem of high water level deviation of the high-pressure heater in the flexible peak of the thermal power unit, Zhanzhou et al. [13] investigated a 600 MW thermal power unit, and a forward feed control strategy based on the high-pressure heater level of the least square method was established. Using this approach, the variation of the rapid response disturbance condition by the feedforward compensation was reduced. Additionally, the deviation of the high-pressure heater water level in the process of flexible adjustment was also lessened. The obtained results of the field experiment indicated that the average water level error, the maximum error, and the error mean of the forward feed were decreased by about 50%.

In conclusion, the existing research results are chiefly focused on the control strategy of the thermal power unit, and the lack of the coupling study of the molten salt reservoir heat and the FPU is obvious. Hence, it is urgently needed to explore the coordination and control of the heat and heat depth of fire plant steam [14]. By implementing MATLAB software, this paper aims to construct the coordination control model of the FPU that appropriately draws the steam heating of the heat power plant and the corresponding heat regulating system, and is running in the 45% turbine heat acceptance (THA) condition of the FPU. The achieved results indicate that the coordinated control strategy between the steam molten salt heat exchanger and the molten salt feedwater heat exchanger lessens the deep peak shaving of thermal power units by 10%, and the turbine master control is also examined. The effectiveness of the coordinated control approach for thermal power units in the steam extraction coupled molten salt heat storage system is verified, and the influence on the automatic generation control (AGC) response of thermal power units were quantitatively analyzed.

The goal of this research is to offer a ground-breaking method for retrofitting thermal power units so they may respond in real-time to load changes enabling grid access to intermittent renewable energy sources at a THA of 35% or less. The contributions of this paper are as follows: (1) To develop a coordinated control model for the main control of the turbine and the deep peaking molten salt heat storage system for thermal power plants; (2) The entire output power of a thermal power unit system coupled with molten salt heat storage is managed according to the state amount of grid frequency fluctuations using a series double closed-loop PID algorithm; (3) AGC reaction time of thermal power units can be significantly increased, with a 75.60% faster output power response at 302.55 s, using steam-extraction-based coordinated control for thermal power plants with related molten salt heat storage systems.

2. Modeling of the Coordination Control System

2.1. Unit Profile and System Model

This paper aimed to examine supercritical coal units of 660 MW grade in a domestic power plant, and the turbine type is specified by the following characteristics: supercritical, one middle reheat, single axis, three-cylinder two lines of steam, indirect cold air, and reactionary steam type. The whole flow of the steam turbine consists of three cylinders, namely a high-pressure cylinder, a double-flow middle cylinder, and a double-flow low-pressure cylinder. The boiler system adopts a single furnace, once reheat, balanced ventilation, tight closed, solid slag, all-steel frame, and full suspension structure boiler of $\Pi$ type. The steam heating melting salt reservoir of the fire plant has been demonstrated in Figure 1.

2.2. Coal Powder System Model of the FPU

The mathematical model of the coal powder system model of the FPU satisfies Equation (1), where $r_m$ denotes the fuel instruction, $r_B$ represents the actual fuel, $k_m$ is the time constant of the powder system, and $\tau$ is the lag time. The delay time includes the transfer time on the coal machine, the time on the coal mill, and the transit time in a wind powder pipeline [15,16].

$$r_B=\frac{e^{-\tau s}}{k_{m}s+1}{r}_m$$

(1)

2.3. The Model of the Steam Turbine

Based on the method of mechanism modeling, Liu et al. [17] established the turbine model by employing the flow and specific enthalpy. However, some steam flow and enthalpy values are not measurable in practice, and these factors can only be evaluated by approximate estimation, which inevitably affects the accuracy of the model. In order to avoid this problem, herein, we apply the input–output data to the model, which is entered into the product of valve opening and main steam pressure and is utilized to represent the steam flow of the steam turbine. Because the amount of steam is determined by the valve opening and the main steam pressure, the output is the power of the unit, and the overall differential equation model satisfies Equation (2), where $P_e$ represents the output power of the generator, $p_T$ is the main steam pressure actual fuel, ${\mathsf{\mu }}_T$ is the turbine regulator valve opening, and $k_e$ and $k_t$ denote the corresponding correction coefficients.

$$P_e+k_e\frac{d{P}_e}{dt}=k_t{p}_T{\mathsf{\mu }}_T$$

(2)

2.4. Coordinate Control System Model

In this paper, MATLAB software is implemented to analyze the coordination control strategy of the heat and heat reservoir of the steam heating in a fire power plant. The developed model has been schematically illustrated in Figure 2.

Figure 2 depicts the coupled control model of the coordinated control system and the digital electro-hydraulic control systems that are connected by valve opening. The coordinated control system is a model for the coordinated control of regulating valve opening, coal combustion, and heat storage power. The digital electro-hydraulic control system is a power control model. Here, the transfer function is modeled as a simplified coordinated control system. The digital electro-hydraulic control system model is coupled to using this transfer function model via the opening of the regulating valve. Facilitates simulation on MATLAB to analyze the primary frequency function of the coordinated control of coupled thermal storage systems [18,19].

Based on a steam heating molten salt heat storage/heat release system with a series double closed-loop PID control, the series double closed-loop PID algorithm regulates the combined output power of the thermal power unit and the fused salt heat storage in accordance with the frequency variation of the power grid. In the operation of the 45% THA operating condition of the FPU, the demand for a depth of 35% of the THA FPU depth is appropriately met. Concerning the steam volume of the molten salt heat exchanger system vapor, the molten salt heat exchanger lessens the unit by 10% THA. By the same principle, when the heat exchanger needs to release the corresponding heat, the heat turbulence is caused by the steam turbine. Combined with the actual condition of the operation unit, the heat capacity of the hot salt reservoir in the model is small at 80 MWh, and the heat storage and the heat period are no more than 12 h [20].

3. Coordinated Control Strategy of the Peak Shaving System for Extraction Steam Heating and Molten Salt Heat Storage Depth

The dynamic power distribution strategy of molten salt heat storage and turbine boiler master control coordinated control for real-time dispatching of the power grid has been demonstrated in Figure 3.

When the power grid load dispatching center is issued by the power grid load dispatching center, the power distribution control center calculates whether the power of the heating system can adjust the power requirement. This process is the rapid response of the heat storage process, and the power of the unit is quickly lessened by pumping the steam. If the demand power is not adjusted, the corresponding regulation of the heat storage system and the FPU is, respectively, evaluated. The unit coordinates of the control system and the reservoir heat system simultaneously act to meet the power change requirements of the grid [21].

The coordinated control logic of the thermo-energy unit of the coupled melting salt storage system has been illustrated in Figure 4.

When the power grid load scheduling center sends down the load instruction, the storage system’s heat storage process quickly responds, and by adjusting the range of the power increase, the steam and the steam instructions start to act. First, the steam valve is opened, and the power of the unit is swiftly reduced by the opening degree of the regulating valve door. The power distribution control center is evaluated by the dynamic power distribution control center by simultaneously evaluating the regulation power of the energy storage system and the FPU, the unit coordination control system, and the reservoir heat system. The coordinated control system is aimed to adjust the main steam pressure, temperature, and other parameters to meet the load setting value, so as to meet the power response of the AGC demand [22,23,24].

When the power grid load scheduling center sends up the load instruction, the heat release process of the energy storage system quickly responds, and the high plus water and water order instructions begin to act. Because of the increase in the water flow of the high-pressure heater, the steam is reduced by controlling the steam valve, and the power of the generator is enhanced, the water supply and the main water supply are mixed, and the heating surface is heated by the boiler. The power distribution control center of the dynamic power distribution control center evaluates the regulation power of the energy storage system and the FPU, and the coordinated control system adjustment system and the heat release system respond simultaneously, and the actual power loss of the grid is satisfied [25].

The turbine main control and the integrated salt reservoir system coordinate control of the associated scientific apparatus fabricators have been illustrated in Figure 5.

In the process of molten salt heat storage, the steam extraction valve of the steam molten salt heat exchanger is opened, and the middle cylinder and low-pressure cylinder are lessened. The extraction port is in the middle of the hot section of the main valve, which is in the middle of the heat and the low-pressure valve, and the steam is trapped and enters the condenser. The excess steam heat is stored in a molten salt heat tank by the steam and molten salt heat exchanger, and the output power of the generator decreases [26,27].

In the salt melting process, the high-plus water heat exchanger and the regenerative system are parallel, and the main steam flow is constant. The water supply of the high-pressure heater is exchanged for energy exchange and high water supply temperature through the melting salt and water heat exchanger as well as the high temperature melting salt. The molten salt of the heat is stored in the cryogenic salt tank, and the water supply and the high-pressure heater of the original system are then mixed into the boiler and continue to heat up to high-temperature superheated steam [28]. Using molten salt to heat the water, the pressurized water replaces the condensed water to enter the deaerator and reduces the steam flow of the steam turbine reheat system, so as to increase the load of the unit and improve the load response rate of the unit [29].

4. Coordinated Control Performance of the Steam Heating Molten Salt Storage Heat Depth Peaking System and Its Influence on the AGC in Thermal Power Units

In the case of the operation unit of a power plant in Mongolia, this paper analyzes the coordination and control characteristics of the thermal power unit of the coupled molten salt reservoir based on the actual coal instruction and the melting salt reservoir heat/release heat capacity. In the present work, the AGC response of the thermal depth modulation system is analyzed, and the discrepancies between the actual output powers of the molten salt reservoir are compared.

4.1. Coordination of the Steam Heating and Melting Salt Reservoir to Control the Input Characteristics

The simulation value of the coal quantity for the heat and heat reservoir is presented in Figure 6. The values of the coal quantity are set from 146.62 (t/h) to 169.60 (t/h).

The heat/release capacity of molten salt is shown in Figure 7; considering a certain heat loss, the actual molten salt is the highest heat storage heat of 72.49 MWh, and the lowest storage heat is 78.54 MWh.

4.2. The Output Power Response of the Molten Salt Reservoir and the Influence of AGC

The simulation amount and the actual amount of the coal load based on the actual coal instruction have been presented in Figure 8. Concerning the validity of the simulation value and the actual value, the alteration of the coal from 146.09 (t/h) to 171.50 (t/h) is predicted, and the consistency of the variation trend presents the rationality of the model and the speed of the response.

The actual power given and the simulation power response have been illustrated in Figure 9. As seen from the plotted results, the change in the power response from 223.66 MW to 303.60 MW is detectable at just 103.63 s. Additionally, the actual power value and the simulation power response error reach their maximum value, i.e., 0.4 MW, such that the deviation rate is 0.13%, and the rest of the time is consistent.

The effect of the thermal system on the output power has been illustrated in Figure 10. After the molten salt heat storage system is connected, the power response capability of the thermal power unit could be substantially enhanced. The power set value could be reached in 73.32 s, and the power response speed is enhanced by 75.60% after arriving at a steady state.

4.3. Data Analysis

The stage of the unit is about 1500 s in which the output power varies from 231 MW to 297 MW. When reducing the output power, the valve can be directly reduced to decrease the steam turbine input. On the other hand, it can extract the excess steam to store the heat in the molten salt. When the output power of the unit is enhanced, it is not only to increase the opening degree of the valve to ensure that the load is increased, but also to increase the flow of the main steam by the melting salt and heat system, so that the power improvement is faster. The data of the heat and heat loss of the steam heating of the fired electric unit due to steam heating and molten salt reservoir are presented in

Table 1. The heat effect of the power units on the steam heating and melting salt storage heat.

Response Time/s	Output Power /MW	Reservoir Heat	No Storage Heat	Percentage Increase/%
73.32	237.6	0.7959	0.007175	99.10
302.55	237.6	0.8101	0.1977	75.60
500	237.6	0.8123	0.3658	54.97
600	237.6	0.8114	0.4358	46.29

.

As demonstrated in Table 1, when the output power increases from 231 MW to 297 MW, the output power is 237.6 MW. The output power associated with 73.32 s is predicted to be 236.38 MW. In order to reach the chamber with the target power of 99.49%, the heat power response of the less molten salt is increased by 99.10%. The power pertinent to 302.55 s is estimated to be 240.60 MW, and for the target power of 101.26%, the heat power response of the less bonded salt is increased by 75.60%, and after the super-adjustment, it could quickly stabilize to the set value. It is seen that the molten salt storage heat system could effectively enhance the power response speed of the depth of the heat discharge unit.

5. Conclusions

This paper proposes a method to store excess or missing heat from thermal units during deep peaking in a molten salt thermal storage system by extracting or adding steam. Based on a MATLAB software simulation platform, a coordinated control simulation model of the thermal power unit of the coupled molten salt reservoir is established, and the coordination control strategy of the steam heating and heat depth modulation system of the fire power plant is developed. The influence of the control ability and the AGC is verified. The contributions and conclusions of this work are summarized by the following:

(1)

The model can precisely analyze the coordinated control system of the heat depth peak regulation of the molten salt storage for steam extraction heating.

(2)

The coordinated control strategy of the steam-molten salt heat exchanger, molten salt and water heat exchanger, and steam turbine main control could reduce the depth of the FPU by 10% THA.

(3)

The coordinated control approach of the thermal power units with a steam extraction system coupled with a molten salt thermal storage system could effectively improve the AGC response speed of the thermal power units, and the output power response speed of 302.55 s could be increased by 75.60%.

By contrasting various thermal storage techniques for thermal units, we plan to study the corresponding potential for deep peaking of thermal units as well as primary frequency control in our upcoming work. We are also interested in examining whether developing molten salt heat storage technology will enable us to lower the fire power unit’s deep peak-regulating capacity to 25% THA.