Analysis of heating transformation of different types of thermal power units

. This paper introduces the total heating capacity, total installed capacity, heating area change trend and unit transformation of direct regulating heating units in Shandong power grid in recent years. According to the operation data of condensing pumping units, back pressure heating units and cylinder cutting units in previous heating seasons, the heating and peak shaving performance of different types of cogeneration units of the same type, grade and grade in heating seasons are analyzed, It is convenient for the power grid dispatching department to more reasonably and orderly arrange the unit mediation plan of the province's thermal power plants according to the online situation of new energy and external power. At the same time, it is a reference for power generation enterprises to carry out flexible transformation according to the actual situation of units during the 14th Five Year Plan period, and take into account the low load peak shaving performance of units or the whole plant while increasing heating capacity.


Background
Cogeneration unit mainly includes regulating extraction condensing heating unit (hereinafter referred to as extraction condensing unit), back pressure heating unit (including high back pressure circulating water heating unit, optical axis transformation [1] unit, pure back pressure heating unit, etc.), low-pressure cylinder zero output heating [2][3] unit (mainly cylinder cutting unit), etc. As of June 2021, Shandong power grid has directly transferred 154 public thermal power units with a rated capacity of 57305MW, including 137 thermal power units with a rated capacity of 49840MW, accounting for 86.97%. Among the thermoelectric units, the number of condensate pumping units, back pressure units and cylinder cutting units is 88, 33 and 16 respectively, accounting for 79%, 11.79% and 9.21% of the capacity; According to the capacity level, the number of units below 300MW, 300MW, 600MW and 1000MW is 46, 62, 21 and 8 respectively, accounting for 14.23%, 41.37%, 27.99% and 16.41% of the capacity. After the "carbon peaking and carbon neutralization" double carbon goal was put forward, the power system, as an important link of energy transformation, has accelerated the clean trend of power generation side, and the installed capacity of renewable energy has been increasing; At the same time, Shandong Province issued a notice on flexibility transformation to improve the regulation capacity of power system through flexibility transformation. Under the heating pressure brought by the consumption of renewable energy, urban development and the shutdown of small thermal power, coal-fired power units are facing the dual test of heating and peak shaving [4][5][6].

Heating trend and transformation of thermoelectric units
In the heating season from 2018 to 2019, the total capacity of the public fire electric motor assembly unit directly transferred by Shandong Province is 48215MW, the total heating capacity is 158580701GJ, which is equivalent to 20490 t/h of heating extraction flow and 337084442 ㎡ of heating area; In the heating season from 2019 to 2020, the total installed capacity is 49450MW, the total heating capacity is 167966636GJ, which is equivalent to the heating extraction flow of 21703

Condensate extraction heating unit
Condensate extraction heating unit can supply one or more parameters of steam to the outside world, and its pressure is controlled by the pressure regulating system. This unit can meet the different requirements of external heat load and electric load in a large range, and has flexible operation, so it is widely used. The heating performance of condensate extraction unit can be determined by three methods: test method [7] , working condition diagram analysis method and thermal calculation method. The test method can accurately reflect the actual heating capacity of the thermal power unit. The unit needs to carry out the load capacity test in the heating season, but there are many test conditions and it is difficult to summarize and sort out due to the limitations of on-site conditions. Thermodynamic calculation method is to calculate the variable working conditions of thermal power units under specific constraints. The calculation process of this method is complex. The working condition diagram method is a method to query the heating capacity of the unit according to the design thermoelectric characteristic curve of the unit. This method is more intuitive and easy to obtain the corresponding results. The working condition diagram is the relationship curve between the active power of steam turbine generation, main steam flow and regulated steam extraction under different combinations. By digitizing and dimensional conversion of the working condition diagram, the trend diagram of peak regulation upper and lower limits under different steam extraction of the unit can be obtained, and then the calculation model of peak regulation upper and lower limits formula and the calculation model of steam extraction upper limit formula of condensing unit can be obtained.The upper and lower limits of the unit's output under the current steam extraction and the upper limit of the unit's steam extraction under the current output can be obtained by selecting an appropriate section according to the formula [8] . Like the high back pressure unit, the optical axis unit operates stably throughout the heating season with almost no adjustment. Among the two optical axis units, one unit has a rated capacity of 110MW, an operating load of 62MW in the heating season, and an average steam extraction capacity of 280t/h; The other unit has a rated capacity of 225MW, an operating load of 113MW in the heating season, and an average steam extraction capacity of 345t/h. After the optical axis transformation, the daily operation load of the unit in the heating season is at a low level, but it has strong heating capacity.

Low pressure cylinder zero output heating unit
The low-pressure cylinder zero output heating unit is mainly transformed by cylinder cutting. After the strength check of the last two stages of blades, the original steam inlet pipe of the low-pressure cylinder is cut off, a bypass pipe is added in the medium and low-pressure connecting pipe, and a small amount of steam after check and calculation is used to take away the blast heat generated by the rotation of the low-pressure rotor after "cylinder cutting". The "zero output" operation of the low-pressure cylinder is realized by cutting off the low-pressure cylinder, which greatly reduces the electrical load of the unit and meets the deep peak shaving demand of the power grid, so as to realize "thermoelectric decoupling". The operation data of some cylinder cutting units in cylinder cutting and non cylinder cutting states are shown in Table 5. With the decrease of volumetric flow, the efficiency of the stage will become worse. When the volume flow rate decreases to a certain value, the wheel circumference power of the stage is equal to zero; If the volume flow is further reduced, the stage will change from the original work stage to the blast power consumption stage, and the stage efficiency will change from positive to negative. At this time, the stagnation enthalpy of exhaust steam will be higher than the inlet stagnation enthalpy, blast friction will heat the steam flow, and the temperature of blades will also rise, causing cylinder deformation and excessive blade stress. When the low-pressure cylinder enters the blowing condition, the exhaust water spray device is usually set to reduce the exhaust temperature of the last stage. Under small volume flow, the last blade root works with negative reaction, and the water spray from LP cylinder will be sucked back into the moving blade through the last blade root, and with the vortex movement, the water droplet will be sucked back to erode the last blade root; The small flow rate greatly increases the outlet angle of the stationary blade, resulting in water droplets scouring to the back of the steam inlet side of the moving blade, resulting in erosion. The above two flow modes of water droplets do great damage to the blade and weaken the strength of the blade. At the same time, under the condition of small volume flow, the large negative angle of attack at the inlet of the last stage moving blade causes large-scale steam flow separation and induces self-excited vibration, resulting in the sudden increase of dynamic stress level and stall flutter. In relative volume , In the process of flow reduction, when the relative volume flow is low to a certain value, the blade vibration stress begins to increase rapidly, then reaches the maximum value, further reduces the volume flow, the vibration stress decreases gradually, and the vibration stress changes non monotonically with the relative volume flow. Therefore, there is a large dynamic stress amplification interval for the last stage blade at the relative volume flow of 0.1 ~ 0.2. During operation, try to avoid long-term operation in this working condition interval.

Conclusion
During the "14th five year plan" period, Shandong direct coal-fired power generation unit is required to carry out flexible transformation according to 20% of the capacity every year. The heating season of the existing condensate extraction unit is limited by 50% of the thermal power ratio, and the minimum output should reach 30% and 40% of the rated capacity. With the increase of heat load and the fluctuation of coal price in recent years, the contradiction between unit heating and peak shaving in heating season in Shandong Province is gradually deepened, and there are more flexible modifications of unit heating. The power generation enterprise shall flexibly transform according to the actual situation of the unit, and take into account the low load peak shaving performance of the unit or the whole plant while increasing the heating capacity.