In order to cope with the construction of a new power system, a large number of new energy grids have caused a lack of system regulation capacity, on the one hand, hardware technologies such as energy storage, pumped storage, and charging piles have developed rapidly, and on the other hand, a series of new economic forms such as virtual power plants and load aggregators have also emerged. These new technologies have further improved the flexibility and response speed of the power system, and provided strong support for the dynamic balance and efficient operation of the system. However, we should also recognize that such resources are very costly. Among the countries that have clearly established and implemented the goal of carbon neutrality, although the mainland has a huge economy, there is still a certain gap between the economic level and some developed countries. The level of foreign electricity prices is high, and users have a strong ability to resist electricity price fluctuations, so high-cost electrochemical energy storage and other technologies can be developed through market-oriented cultivation; The mainland must explore a development model that is more in line with the mainland's national conditions, achieve the "dual carbon" goal more economically, and minimize the impact of rising energy prices on the overall economic development.
How much potential does coal power have to be regulated in China?
The traditional fossil energy resource endowment of mainland China is characterized by "rich in coal, poor in oil and low in gas". Coal resources are abundant, and coal power has long been an important part of the mainland's power supply structure, with relatively mature technology and many existing units. Therefore, the economic development of regulatory resources can start with coal power. Although the start-stop of coal-fired power units is usually regarded as an anomaly or a small probability phenomenon, if coal-fired power has the ability to start and stop multiple times, its hidden regulation potential will be huge.
According to the length of shutdown, thermal power can be divided into cold start, warm start, hot start, extremely hot start and other different starting conditions. Generally speaking, the shutdown time of coal-fired units is extremely hot within one hour, 1 hour to 10 hours is hot start, 10 hours to 72 hours is warm start, and more than 72 hours of downtime is cold start. In the case of spot operation in many provinces across the country, the time period when the spot price appears is usually 2-4 hours at noon, that is, the shutdown time for the real need for thermal power to start and stop adjustment is about 2-4 hours, and the corresponding unit state is hot or extremely hot.
The unit under extremely hot and hot working conditions has the following characteristics. First, it can start quickly: due to the short downtime, the temperature of the equipment has not dropped significantly, and the start-up speed is fast, which reduces the start-up preparation time; Second, the start-up cost is low: the heat in the equipment is still high after the shutdown, and the heat storage of the thermal system can be fully utilized, which can reduce the fuel consumption of the boiler; Third, compared with other working conditions, the start and stop are safer: the cycle process of cooling and heating for a long time is avoided, the thermal stress of metal materials is reduced, and the life of the unit is improved.
If through technical research, the normalization of the extremely hot state and hot state start and stop of coal-fired units can be realized, how much regulation capacity can be excavated by mainland coal-fired units? According to the national power industry statistics released by the National Energy Administration, as of the end of April 2024, the country's cumulative installed power generation capacity is about 2.99 billion kilowatts. According to the "2023-2024 National Electricity Supply and Demand Situation Analysis and Forecast Report" released by the China Electricity Council on the 30th, by the end of 2023, the proportion of coal-fired power installed capacity will be 39.9%, and the installed capacity of coal-fired power can be roughly calculated to be about 1.165 billion kilowatts. After deducting a certain amount of cold standby, the grid-connected coal power capacity is about 1 billion kilowatts. Assuming that the proportion of thermal power under the minimum operation mode of the power grid is 50%, and after flexible transformation, it meets the requirements of the "Guiding Opinions of the National Development and Reform Commission and the National Energy Administration on Strengthening the Construction of Power Grid Peak Regulation and Energy Storage and Intelligent Dispatching Capacity", and the minimum technical output is 30% of the rated load, on this basis, 1/5 of the capacity is reserved for the basic frequency regulation and voltage regulation needs of the power system, and the rest is adjusted by start and stop, the new regulation capacity can reach 120 million kilowatts. If all of these regulatory capacities are used to absorb PV, a conservative estimate based on the PV synchronization rate of 60% (which cannot actually reach 60%) is equivalent to an additional 200 million kilowatts of PV consumption without the need to develop additional regulation resources. And as the electricity load continues to rise, the installed capacity of coal-fired power will also increase accordingly, and this consumption will also rise. This shows that there is still a huge amount of potential for regulation in the mainland.
What technical support is needed for the normalized start-stop regulation of coal-fired power?
Different from the common cold state and temperature state start-stop of the unit, the normalized start-stop adjustment requires the boiler to maintain the operation of about two coal mills after the unit is shut down, maintain the steam temperature and pressure within a certain parameter range, and maintain a certain speed of the steam turbine. This ensures that when the system needs it, the units can be connected to the grid as quickly as possible. If we want to truly realize the normalized start-stop adjustment, we must conduct in-depth consideration from an all-round perspective and carry out a series of targeted technical research.
From the perspective of coal-fired power units, the operating conditions of the units change dramatically under frequent start and stop, so it is necessary to develop new material technologies and optimize the logic of the unit system to solve or reduce various problems caused by temperature changes, mainly due to increased stress.
For the steam turbine, firstly, the rotor of the medium and medium pressure cylinder should be kept at a high temperature during the short-term start and stop process, so as to avoid the temperature difference between the upper and lower cylinders being too large, and to prevent the rotor from bending caused by friction with the steam seal when the large shaft rotates; Second, it is necessary to prevent the deformation of the rotor or shaft seal caused by the frequent switching of the steam source supplied by the shaft seal of the steam turbine and the violent fluctuation of the temperature at the shaft seal, which will cause the disappearance of the dynamic and static gap, resulting in dynamic and static friction and vibration climbing; Thirdly, it is necessary to consider the high exhaust temperature caused by factors such as the friction between steam and the final blades of the low-pressure cylinder and the blast effect caused by steam flow under low flow conditions. Fourth, the frequent start and stop of the unit will lead to the continuous operation of the valve stem of the inlet valve and the fluctuation of the inlet temperature, and it is necessary to solve the problem of valve jamming caused by the thickening of the oxide scale and peeling off from the surface of the valve stem and stuck in the annular gap between the valve stem and the valve sleeve. For generators, the increase of the adjustment amplitude and start-stop frequency of the unit accelerates the thermal expansion and contraction effect of the generator rotor coil, and the alternating stress increases significantly. For boilers, frequent start-stop and shutdown need to consider the problem of low-cycle fatigue damage in the stress concentration area of pressure components such as boiler steam drums. The saturated steam pressure and temperature in the steam drum have large changes, and due to the different thermal conductivity of steam and water and the influence of the structural factors of the steam drum, there is a temperature difference in different parts of the steam drum wall, and thermal stress is generated. The periodic thermal stress caused by start-up also acts on economizers, water walls, and superheaters, which can easily cause low-cycle fatigue and crack initiation of these equipment. The occurrence of such problems should be reduced by improving the structure of equipment such as steam drums, using materials with good durability under alternating stress, and strengthening maintenance.
From the perspective of power system, it is necessary to increase the performance of the basic frequency modulation unit and improve the system inertia to suppress the frequency fluctuation problem when the coal machine starts and stops.
On the hour-level time scale, coal-fired power units can enhance their own regulation capacity through start-stop. However, on the minute-level time scale, the adjustment ability of a coal-fired power unit in the process of "starting" or "stopping" is poor. Due to the deviation between the actual output and the start-stop curve, the coal machine even needs to consume part of the adjustment capacity of the system during the start-stop process. Other units that do not participate in start-stop regulation must assume the responsibility of basic frequency modulation, which brings new challenges to the stability and regulation ability of basic frequency modulation units. In addition, coal-fired power units absorb more new energy through starting and stopping, and the inertia of the system as a whole will decrease accordingly, and the fluctuation degree of frequency will increase accordingly. It is necessary to increase the system inertia and improve the stability of the system by optimizing the rotor structure, adding external accessories of the motor, and developing virtual inertia technology.
From the perspective of planning and design, it is necessary to gain insight into the general trend of the evolution of the role of coal power in the context of energy transition, and accurately grasp and optimize the design ideas of coal power.
For the existing coal-fired units, under the premise of ensuring the safe and stable operation of the units, the flexible transformation is carried out with the goal of reducing the lower limit of the minimum technical output, increasing the climbing rate, and fast starting and stopping, and at the same time improving the technical level of the operating personnel, focusing on improving the start-stop operation ability of the unit and the adjustment ability of parameters under changing working conditions. For incremental thermal power units, the current idea of blindly pursuing the economy of coal consumption should be changed. With the steady progress of the "dual carbon" goal, the integration of a large number of new energy power generation into the grid, and the introduction of the electricity price mechanism for coal-fired power capacity, the positioning of coal-fired power in the power market has changed from the traditional main energy source to a supporting and regulating power source. This means that coal-fired power cannot guarantee the number of hours of utilization, and large units cannot reach their rated load most of the time, and coal economy is no longer a priority. The research direction of incremental units should be to reduce the lower limit of minimum technical output, increase the climbing rate, and improve the rapid start-stop ability. Specifically, it is manifested in improving the resistance of metal materials such as steam turbine cylinder rotors, generator coils, and boiler pipes to temperature changes, and at the same time, optimizing the start-stop logic, simplifying operation, and realizing the intra-day adjustment function of coal-fired power in the design process.
What is the economical regulation of coal power start-stop?
Due to the start and stop time of the unit is concentrated in the period of photovoltaic power generation and serious oversupply of electricity, the spot price of electricity is extremely low or even negative at this time (considering the environmental premium of green electricity), at present, Shandong, Zhejiang and other provinces have had negative prices during the new energy generation period, and with the increase of new energy installed capacity in the future, the spot negative price of new energy output will be more frequent. Assuming that the spot price is -0.1 yuan/kWh during the start and stop time, it is an economic account.
(1) Unit cost analysis
(1) Unit cost analysis
The cost of start-stop adjustment of the unit is divided into two parts: the cost of maintaining a short-term grid-connected state after the shutdown and the start-up cost. Taking a 600 MW unit as an example, the cost of maintaining a short-term grid-connected state of the unit mainly includes coal consumption and steam consumption. Considering that in order to be quickly connected to the grid after shutdown, the corresponding auxiliary equipment needs to be put into operation. In this state, the steam consumption of the hot steam required by the unit is about 30 tons/hour, and the coal consumption of 20,000 kJ/kg at the low level is about 70 tons/hour. The steam and coal are converted into standard coal, and the standard coal price is 900 yuan/ton, and the cost is about 137,770.2 yuan.
The start-up cost of the unit needs to take into account the coal consumption, water consumption, and fuel consumption to maintain the stable combustion of the boiler. The total time from flushing to grid connection to the unit recovery to 30% of the rated capacity (minimum technical output) is about 1 hour, about 1 ton of fuel is consumed, and coal consumption is changed from about 70 tons/hour to about 100 tons/hour, while considering steam consumption, power consumption, etc., after conversion, the starting cost is about 100,000 yuan, and the total cost is about 237,770.2 yuan.
The operating cost of the unit under the condition of 30% rated load: because the coal consumption of the unit will increase significantly under the operation of low load, the coal consumption is 330 g/kWh, and the variable cost only considers the coal cost, and the variable cost of the unit is about 213,840 yuan for four hours of operation.
From the cost comparison of the two situations, it can be found that the cost of the shutdown mode has increased slightly compared with maintaining the minimum output, but it has certain economic advantages considering the start-stop compensation of the unit.
(2) Analysis of the unit's electrical energy income
From the comparison of electricity market revenue, assuming that the medium and long-term contract ratio is 80% of the unit capacity, that is, the medium and long-term contract volume during the shutdown period is 480 MW*4 hours, and the medium and long-term transaction price is set at 0.4 yuan/kWh, the unit revenue is calculated separately in the two cases. According to the second calculation method, when the unit is shut down, the electric energy cost is:
480000*4*0.4+（0-480000）*4*（-0.1）=960000（元）
When the unit maintains 30% of the rated load condition, the unit output is 180 MW, and the cost at this time is:
480*4*0.4+（180-480）*（-0.1）*4=888000（元）
From the comparison of the benefits of the two situations, the way of downtime earns more profits through medium and long-term contracts, and the overall income increases, which is more economically advantageous.
In addition, at present, some thermal power units are used as heating units, and in order to ensure stable heat supply, they cannot be adjusted to start and stop. However, the idea can be changed: by installing an electric boiler on the user side for heating, the generator set can start and stop on demand, which not only improves the flexibility of the unit itself, but also increases the power demand and further increases the system's absorption capacity. With low-cost electric heating during this period, its economy can be briefly analyzed. If the heating area is 40 watts/square meter, the heat consumption per square meter per hour is 144 kJ/h. The thermal efficiency of the boiler is 75%, and the low calorific value of coal is calculated as 7000 kcal/kg, and the standard coal consumption per square meter per hour for heating is about 0.00655 kg/square meter. If it is a heating area of 5 million square meters, the total coal consumption per hour is about 32.756 tons / hour. If an electric boiler is installed on the load side, the comprehensive cost is approximately 0 during the start-stop adjustment period due to the negative spot price of the coal-fired unit during the start-stop period, and other expenses such as transmission and distribution are superimposed. For heating demand only, the installation of electric boilers on the user side can save about 29,480.4 yuan per hour during the start-stop adjustment period.
Compared with the two situations of unit shutdown and maintenance of minimum output during the new energy boom period, the unit shutdown has freed up more space for new energy consumption, which is conducive to the green transformation of the energy structure and the realization of the "double carbon" goal. However, multiple start-stops are also accompanied by huge operating volume and safety risks, which affect the life of the unit to a certain extent, so it is necessary to stimulate the active participation of coal-fired power units from the perspective of start-stop compensation, and when conditions are met, they can achieve two start-stops a day.
Coal power has played a vital role in ensuring the stable operation of the mainland's power system, and despite a period of stigma and capacity cuts, it has proven to be an indispensable support for the entire power system. Technically speaking, China's coal turbine is Europe's gas turbine. Europe and the United States and other countries have sufficient and stable gas supply and have adapted to the high cost of energy, so they choose to develop gas-fired units for adjustment. The mainland's coal turbines can also operate in the form of gas turbines through technological upgrading, replacing the status of foreign gas turbines, and at the same time escorting the mainland's energy security and economic development. In terms of future potential, coal-fired units still have massive potential for regulation. Whether it is technically or economically, the short-term start-stop adjustment of the coal machine is feasible. Moreover, the adjustment capacity of the coal machine will continue to rise with the increase of load and the increase of the installed capacity of the coal machine. From the perspective of strategic needs, the realization of the "dual carbon" goal is inseparable from the completion of the transformation of coal power to regulate the power supply. The power industry must achieve carbon neutrality ahead of schedule in order to help the mainland successfully complete the "dual carbon" goal, which will inevitably require coal-fired power to transform from a major power source to a supporting and regulating power source, and truly play a role in regulation. In the future, coal-fired power will inevitably increase the ramp-up rate and achieve normalized start-stop regulation at extremely low utilization hours, so as to ensure the stability of energy supply and promote the development of the energy sector in a cleaner and low-carbon direction.
Editor: Weng Shuang, Wang Xinyao
Review: Li Liping
This article is an exclusive contribution of China Electric Power Enterprise Management.
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