Study on power load adjustment capacity of a 300MW cogeneration unit in heating season

After the formulation of the "3060" double carbon target, the installed capacity of new energy has increased rapidly and the proportion of power generation has become higher and higher. In the heating season, under the operation mode of "determining electricity by heat" of cogeneration units, in order to meet the consumption of renewable energy and the heating guarantee of people's livelihood, the contradiction with power grid peak shaving is increasing. This paper mainly introduces the load capacity of a 300MW cogeneration unit in the heating season. Through the test method, find out the load capacity of the unit under different heating capacity, which can be used as a reference for power generation enterprises and power grid


Background
The cogeneration unit uses high-grade energy for power generation, and the low-grade steam after power generation is used for industrial production or heating. It can not only meet the needs of production and life, but also improve the comprehensive utilization efficiency of energy, and realize the policy of "proper distribution, different needs, corresponding temperature and cascade utilization" for energy. The advantage of cogeneration is to turn the hierarchical utilization of energy into reality. The cogeneration [1] unit itself has high energy efficiency, but limited by the operation mode of "determining electricity by heat", it reduces the peak shaving capacity of the power system. In order to meet the consumption of renewable energy and the heating guarantee of people's livelihood, the contradiction with power grid peak shaving [2][3][4] is increasing. Therefore, it is necessary to find out the actual load capacity of cogeneration units in the heating season and break the restrictions of the original heating working diagram. Taking a 300MW cogeneration unit as an example, its load capacity is obtained through test [5]

Overview of boiler equipment
The boiler is SG-1025/17.4-M851subcritical, intermediate primary reheat, natural circulation and coalfired drum boiler. The positive pressure direct blowing four corner tangential combustion is adopted, the main steam temperature is regulated by two-stage water spray desuperheating, and the reheat steam temperature is regulated by flue gas baffle and water spray desuperheating. Table 2 shows the main design parameters of the boiler.

Introduction to heating system
The heating steam extraction of the four units in the plant is the exhaust steam extraction of intermediate pressure cylinder, and there is no industrial steam. The steam extraction of the steam turbine is realized by adjusting the pressure through the butterfly valve of the connecting pipe of the medium and low pressure cylinder. The heating steam extraction pipeline is arranged in a unit system. The steam extraction of the unit is sent to the corresponding heat network heater through the steam extraction pipeline to heat the circulating water of the heat network, and the circulating water pump of the heat network is used for external heating. There are 2 initial heating stations in total, including 4 heating network heaters and 0 heat network circulating pumps in the initial heating network station of phase I; The initial station of phase II heat supply network has 5 heat supply network heaters, 9 heat supply network circulating pumps, all of which are electric pumps. The return water of the heat supply network first enters the initial station of the phase II heat supply network. After the pressure head is raised by the circulating water pump of the heat supply network, it enters the phase I and phase II heat supply network heaters respectively. The temperature is heated from 60 ℃ to about 120 ℃ and sent out.

Minimum output test when heating extraction steam is 250 t/h
Gradually reduce the unit load to about 150MW. At this time, the heating extraction flow of the unit is about 250t/h, and the load reaches 50% of the rated capacity required by the dispatching agreement. During the test, the heating extraction steam flow of the unit is 250.93t/h, and the average active power of the generator is 150.02MW.

Maximum output test when heating extraction steam is 350 t/h
Gradually increase the main steam flow to 950t/h, close to the maximum steam inlet of 979t/h, and then bring the heating extraction flow of the unit to 350t/h. During the test, the heating extraction flow was 346.87t/h, and the average active power of the generator was 262.41MW.

Minimum output test when heating extraction steam is 350 t/h
Firstly, bring the heating extraction flow of the unit to 350t/h, gradually close the steam inlet butterfly valve of the low-pressure cylinder, reduce the steam inlet of the unit and maintain the heating extraction flow unchanged. When the unit load is reduced to about 203MW, the opening of the steam inlet butterfly valve of the lowpressure cylinder is 16.04%. Due to the jamming of the butterfly valve, the butterfly valve cannot be closed continuously, and the test is started under the current state. During the test, the heating extraction steam flow of the unit is 348.60t/h, and the average active power of the generator is 203.02MW.

Limiting factors of unit output
The maximum electric load test of the unit is limited by the maximum steam inlet of the unit. During the test, the main steam flow is about 950t/h, which is close to the maximum steam inlet of the unit of 979t/h. In the minimum electric load test, when the heating extraction steam flow is 347.74t/h and 439.11t/h, the steam inlet butterfly valve of low-pressure cylinder has been closed to the minimum opening (it can only be reduced to 16% due to valve jamming). In order to maintain the heating extraction steam flow, it is impossible to continue to reduce the unit load; When the heating extraction flow is 250.49t/h, it reaches 50% of the rated capacity required by the dispatching agreement. In order to ensure the safety of the unit, the load of the power plant will not be reduced; When the heating extraction steam flow is 145.42t/h, in order to ensure that the heating extraction steam flow reaches the test value and maintain the middle exhaust pressure greater than 0.08MPa, the measures of reducing the heating quick closing valve shall be taken during load reduction. Due to the logical relationship that the heating butterfly valve of the connecting pipe of the medium and low pressure cylinder can be adjusted only when the heating quick closing valve is fully open, the unit load cannot be further reduced by adjusting the opening of the heating butterfly valve, Can only maintain the current state. It can be seen from the above table and the heating working condition diagram that the maximum output of the test is slightly lower than the upper limit of the design output, mainly because the main steam flow of the test is about 950t/h , which does not reach 979t/h under the design working condition; The minimum output of the test is lower than the lower limit of the design output when the heating extraction flow is 145.42t/h and 250.49t/h, and higher than the lower limit of the design output when the heating extraction flow is 347.74t/h and 439.11t/h. When the heating extraction steam flow is 347.74t/h and 439.11t/h, the lower limit of load is higher than the design output, mainly because the opening of butterfly valve of connecting pipe of medium and low pressure cylinder is about 16%, which does not reach the minimum steam inlet of low pressure cylinder.

Upper and lower limit fitting formula of output
According to the heating working condition diagram, the upper and lower limit formulas of output are fitted, as shown in the table below.Where, x is the heating extraction capacity；y is Unit load.

Conclusion
After long-term operation and heating transformation [6] of cogeneration units and the needs of heat users, and the current difficult situation of peak regulation of thermal power units is not considered in the early design stage, the original design heating working condition diagram can not reflect the real electric load adjustment range of the unit; At the same time, in the heating season, the contradiction between power grid peak shaving and cogeneration units continues to deepen. Under the new situation of vigorously developing new energy, cogeneration units are bound to break through the operation range of the original heating working condition diagram. Through tests, find out the actual electrical load adjustment range of the unit, remove the margin left for safe operation, and reflect the real peak shaving level of the unit.