In September 2020, China clearly proposed the goals of “carbon peak” by 2030 and “carbon neutral” by 2060. Developing green and renewable new energy such as wind power and solar power is an effective means to promote energy transformation (Wang Y. et al., 2021; Liu et al., 2023; Lei et al., 2024). With the increasing proportion of new energy, the randomness, intermittency and volatility of new energy will bring significant challenges to the reliability of power supply of power system (Ou et al., 2024). In 2022, the power generation of thermal power plants accounted for 66.5% in China, to absorb new energy, which requires thermal power plants need to further tap the peak shaving capacity, and give full play to the role of “ballast stone” (He et al., 2022; Tong et al., 2022; Yan et al., 2022). The National Development and Reform Commission of China and the National Energy Administration jointly issued the Notice on the Transformation and Upgrading of National Coal power Units, which puts forward the requirements of flexible transformation to meet the requirements of deep peak regulation, so as to absorb new energy on a larger scale, and the deep peak regulation of coal-fired power plants will become normal (Pan et al., 2022).

The conventional operating load of coal-fired units is 50%–100% of rated load, and the load is reduced to 30% or even lower when deep peak shaving (Gu et al., 2016; Hui et al., 2024). When coal-fired units operate at low load, their safety and economy are significantly reduced. Re-searchers have conducted a large number of studies on the performance of coal-fired units in deep peak shaving (Wang J. et al., 2021). Cong et al. studied the operation characteristics of the 660 MW ultra-supercritical secondary reheating unit at 100%, 75%, 50% and 30% rated load through experimental methods and found that the operation parameters were stable and the emission of flue gas pollutants reached the standard, but the economy was significantly reduced (Cong et al., 2021). Liu et al. showed that the load can be adjusted to 30% of rated load and operate stably without equipment modification through experimental re-search (Liu et al., 2019). Shi et al. conducted an experimental study on the performance of a 600 MW supercritical air-cooled coal-fired unit under different depth peak shaving load, and the results showed that when the load of the unit was reduced from 100% to 30% of the rated load, the heat of the unit increased by 15.33%, the coal consumption increased by 51.33 g/(kW·h), and the plant power rate also increased significantly (Shi et al., 2023). Zhao et al. studied the thermal economy of the unit when the load of the unit is reduced from 50% to 30%, and the results showed that the economy is better when the peak shaving depth is more than 40% of the load of the unit (Bin et al., 2022). Wang et al. conducted an experimental study on the main equipment characteristics and unit economy of 1050 MW ultra-supercritical unit in the process of peak shaving operation at 30% and 20% load depth. The results show that when the load of the unit is below 30%, the boiler efficiency, steam turbine efficiency, plant electricity rate, and heat and coal consumption of the unit show a significant downward trend with the increase of depth peak shaving amplitude (Wang et al., 2023).

The above research results show that the deep peak shaving in the literature can achieve the goal of 20% deep target. In order to help achieve the “double carbon” goal and further tap the potential of deep peak shaving, this paper takes the 630 MW super-critical unit of a power plant as an example. The thermal characteristics of the unit under the normal operation range of 100 %–50% rated load and the deep peak shaving operation conditions of 50%–17% rated load are studied, which provides a reference for the wide load flexibility adjustment of coal-fired units and the improvement of new energy consumption capacity.

A 630 MW ultra-supercritical reheat coal-fired power plant in China is taken as the reference unit. The system diagram of the reference unit is shown in Figure 1. The boiler of the 630 MW unit adopts the supercritical transformer DC boiler with the model of HG-1970/25.4-PM18, the burner adopts the swirling pulverized coal burner with the front and back wall combustion mode, and the preparation system adopts the pulverized system of double inlet and double outlet steel ball mill. The steam turbine is a super-critical, single intermediate reheat, condensing steam turbine with model N630-24.2/566/566. The parameters under the different turbine heat acceptance (THA) condition of this stage group are shown in Table 1.

In order to fully understand the performance of the 630 MW coal-fired unit, the wide load performance test of the unit was carried out, which included seven pure condensation conditions: 630 MW, 475 MW, 300 MW, 240 MW,180 MW,140 MW and 105 MW. During the test, we should maintain the coal quality stable and meet pollutant emission standards. The main and auxiliary equipment were put into operation normally, the low temperature economizer was put into operation, and the heater was withdrawn from operation. In order to ensure the safety of the unit, deep peak shaving is carried out, that is, when the unit load starts to decrease from 300 MW, the load drop rate should be strictly controlled, each load condition operated stably for at least 2 h, and relevant data are collected until the load condition reaches 105 MW finally. Analysis of the unit under different operation load of boiler and steam turbine thermal efficiency indicators to explore the sliding pressure operation way the impact on the economy. Boiler efficiency was calculated using the Performance Test Code (PTC) developed by the American Society of Mechanical Engineers (ASME): ASME PTC-4 2013 (ASME, 2013). Steam turbine efficiency was calculated using the ASME PTC-6 2004 (ASME, 2004).

According to the standards, the boiler efficiency calculation method is expressed as: η b = 100 − Q l o s s Q r = 100 − q 2 − q 3 − q 4 − q 5 − q 6 where η _b (%) expresses the boiler efficiency, Q _loss (kJ/kg) denotes the energy losses in the daily operation of a coal-fired boiler. Q _r (kJ/kg) represents the low calorific value of 1 kg coal. q ₂∼q ₆ (%) stand for the percentage corresponding to the heat loss of flue gas, unburnt carbon monoxide (CO), unburnt carbon, boiler surface and sensible heat loss from hot cinder respectively.

Turbine cylinders efficiency calculation method is expressed as: η i = h 0 − h 1 h 0 − h 1 ′ where η _i (%) expresses the turbine cylinders (HPT, IPT, LPT) efficiency, h ₀ (kJ/kg) represents the enthalpy at the entrance state, h ₁ (kJ/kg) represents the enthalpy at the exit state for the actual process, h′₁ (kJ/kg) represents the enthalpy at the exit state for the isentropic process.

Heat rate value calculation method is expressed as: H R = W m s − W s h × h m s − h f w + W h r − W r h × h h r − h c r + W s h × h m s − h s h + W r h × h h r − h r h P e where HR (kJ/kg) stands for the power output, P _e (MW) stands for the power output; W _ms (t/h), W _sh (t/h), W _hr (t/h), W _rh (t/h) stand for the mass flow of main steam, superheated spray water, reheated steam, reheat spray water; h _ms (kJ/kg), h _fw (kJ/kg)), h _hr (kJ/kg), h _cr (kJ/kg), h _sh (kJ/kg), h _rh (kJ/kg) stand for the enthalpy of main steam, feed water, reheated steam, main steam, superheated spray water, reheated steam, reheat spray water, superheated spray water, reheat spray water.

Absolute electrical efficiency η _el (%) is calculated by: η e l = 3600 H R

Power consumption rate η _au (%) is calculated by: η a u = P a u P e where P _au (MW) stands for the auxiliary power consumption.

Standard coal consumption for power generation b g s (g·kW⁻¹ · h⁻¹) is calculated by: b g s = 1000 H R Q n e t , a r s η b η p where Q n e t , a r s (kJ/kg) stands for the low heating value of the standard coal, η _p (%) stands for piping efficiency.

Power supply standard coal consumption b s s (g·kW⁻¹ · h⁻¹) is calculated by: b s s = b g s 100 − η a u

According to the Henan Provincial Energy Regulatory Office’s 2019″Henan Power Peak Balancing Auxiliary Service Trading Rules (Trial)” and the “Notice on Revising and Improving some provisions of the Provincial Power Peak Balancing Auxiliary Service Rules” issued in 2024: In the principle of deep peak load compensation in Henan Province, when the unit load rate is 40 %–45% (excluding 45%), the upper limit of the quotation is 0.2 yuan/(kW·h); When the unit load rate is 30%–40%, the upper limit of the quotation is 0.5 yuan/(kW·h); When the unit load rate is less than 30%, the quotation limit is 0.7 yuan/(kW·h). The benchmark on-grid electricity price in Henan province is 0.3779 yuan/(kW·h), and the depth peak charging period is 15 min. According to the above criteria, the peak-regulating benefits of the unit in different peak-regulating intervals for 1 h were analysed.

Figure 6 Test results show that as the load decreases, the coal consumption for power generation and power supply of the unit increases significantly. Therefore, when analysing the benefit of deep peak regulation, besides calculating the deep regulation service fee, the influence of coal consumption cost should also be calculated, and the unit price of standard coal is calculated at 1,000 yuan/ton. Deep adjustment income (yuan/h) is calculated based on the decrease of power supply cost caused by service fee subsidy minus the increase of power supply cost caused by the increase of coal consumption in deep adjustment. Since there is no subsidy when the load is 45%, the calculation is based on 45%. The increase of coal consumption under deep adjustment load affects the in-crease of power supply cost according to the following formula: S = B s t − B 45 × S c 1000000

Where, S is the effect of increasing coal consumption, yuan/(kW·h); B _st is the coal consumption of deep adjustment load, g/(kW·h); B ₄₅ is the coal consumption of 45% load, g/(kW·h); S _c is the unit price of standard coal, yuan/t.

According to the test results in Figure 5, the coal consumption change of power supply in different intervals is analyzed according to the linear change. When the unit load is in the range of 240 MW–300 MW, the coal consumption increases by 1.44 g/(kW·h) when the load is reduced by 1%. When the load of the unit is in the range of 180 MW–240 MW, the coal consumption increases by 2.03 g/(kW·h) for each 1% reduction in load, and the corresponding cost is 0.0343 yuan/(kW·h). When the unit load is in the range of 140 MW–180 MW, the coal consumption increases by 2.56 g/(kW·h) for each 1% reduction in load. When the load of the unit is in the range of 105 MW–140 MW, the coal consumption increases by 5.58 g/(kW·h) for every 1% reduction in load. The comprehensive benefit of deep peak regulation is calculated, and the results are shown in Table 3. It can be seen from the table that when the coal price is 1,000 yuan/ton and the peak regulation service fee is subsidized according to the upper limit, the greater the depth of peak regulation, the greater the benefit.

For supercritical 630 MW coal-fired unit at 16.67%–100% rated load wide load tests are carried out to explore the thermal efficiency under different operating conditions, the main conclusions are as follows:

1. When the unit load is greater than 40%, the boiler efficiency changes little, and achieves 93.58% of the maximum boiler efficiency when the load is 475 MW. When the unit load is less than 40% for depth peak shaving, the thermal efficiency of the boiler decreases continuously. When the unit load is 105 MW, the boiler efficiency is only 89.61%.