Application Research on Zero Output of Double Low Pressure Cylinder of 600MW Unit

: Under the dual carbon target, the rapid development of new energy promotes the increasing peak shaving capacity of thermal power units as basic power sources. Taking 600MW unit as an example, this paper studies the feasibility of zero output of low pressure cylinder for heat supply transformation of 600MW unit, and analyzes the peak shaving capacity of the unit by cutting off double low pressure cylinder and single low pressure cylinder.


Introduction
The National Development and Reform Commission and the National Energy Administration issued the Fourteenth Five Year Modern Energy System Plan (the Plan). Compared with the Fourteenth Five Year Plan for National Economic and Social Development of the People's Republic of China and the Outline of Vision Goals for 2035 previously released, there are many changes in the expression of coal power. At the same time, the Plan also puts forward specific goals for the thermal power flexibility transformation and "three transformation linkage" of coal power: strive to achieve a cumulative flexibility transformation scale of more than 200 million kilowatts of coal motor units by 2025, focus on the flexibility transformation of 300000 kilowatts and below coal power units, and research and promote the flexibility transformation of 600000 kilowatts subcritical coal motor units in areas with difficult peak shaving. By 2025, the proportion of flexibly regulated power supply will reach about 24%, and the power demand side response capacity will reach 3%~5% of the maximum power load. We will vigorously promote the "three transformation linkage" of coal power energy conservation and carbon reduction transformation, flexibility transformation and heat supply transformation, and the scale of energy conservation transformation during the "Fourteenth Five Year Plan" period will not be less than 350 million kilowatts.

Analysis of heat supply transformation
The Notice on Improving the Flexibility of Directly Dispatched Utility Coal fired Power Generation Units in the Province issued by the Provincial Energy Administration clearly pointed out that during the "Fourteenth Five Year Plan" period, the flexibility of coal fired power generation units should be implemented according to 20% of the capacity every year, in which the minimum technical output of pure condensing units and condensing units should reach 30% and 40% of the rated capacity respectively under the condition of stable combustion, and the new pure condensing units and condensing units should reach 30% and 40% respectively, The condensing unit with the heat electricity ratio less than 50% shall be modified according to the standard of the condensing unit. The steam turbine of a factory is a supercritical pressure extraction condensing steam turbine, with the model of C660/543-24.2/0.45/566/566 supercritical, one intermediate reheat, single shaft, four cylinders and four exhaust, and extraction condensing. The maximum continuous output is 710MW, the rated output is 660MW, the rated heating steam extraction is 560t/h, and the maximum steam extraction is 810t/h. The unit adopts compound variable pressure operation mode, and the turbine has seven stages of non adjustable regenerative steam extraction and one stage of regulated heating steam extraction. With the continuous expansion of urban areas and the continuous development of industry, the heating steam consumption for residents and industrial steam consumption are increasing; Considering the number of users, the nature of steam consumption, the law of steam consumption, and the pipeline loss in the central heating area, the design heat load of steam in the industrial area will be 355t/h at the maximum, 296t/h at the average, and 190t/h at the minimum in 2021; In 2022, the maximum is 515t/h, the average is 446t/h, and the minimum is 350t/h; In 2023, the maximum is 655t/h, the average is 576t/h, and the minimum is 510t/h. The extraction temperature of industrial steam extraction is 260 ℃ -290 ℃, and the extraction pressure is 2.30MPa-2.55MPa。 According to the current heating and heating load and industrial steam extraction load, under the most unfavorable conditions in the heating season, the maximum heating capacity of the plant needs to reach 2844t/h. With the annual development of the heating and heating load and industrial steam extraction load in the city, the long-term heating and heating area and industrial steam extraction demand show an increasing trend year by year。According to the current operation of 5 units, the maximum total steam extraction capacity can only reach 2056t/h; If there is no relevant transformation to improve the heating capacity, it is necessary to reduce the steam supply in the industrial zone to ensure the heating demand of people's livelihood during the winter heating period. For this reason, the characteristics of typical technical schemes for flexibility transformation of heat supply units are analyzed, and the comparison of various transformation schemes is shown in Table 1. Comprehensively compare the technical advantages and disadvantages of each scheme, and in combination with the unit operation status and transformation requirements of the plant, it is recommended to implement the transformation scheme with zero output technology of low pressure cylinder as the route.

Analysis of steam extraction capacity of unit under zero output condition
3.1 Analysis of heating and steam extraction capacity of single cut off B low-pressure cylinder When the low pressure cylinder is put into zero output operation, the cooling steam flow of the single low pressure cylinder is considered as 30t/h, and the heat supply and steam extraction capacity of the unit is calculated under the zero output condition when the low pressure cylinder B is cut off alone and the low pressure cylinders A and B are cut off simultaneously. See Table 2 for the check results of heat supply and steam extraction capacity under zero output working condition of the unit with single removal of low pressure cylinder B.  Table 3 for the check results of heat supply and steam extraction capacity under zero output condition when the unit cuts off low pressure cylinders A and B.

Comparison and analysis of two cylinder cutting methods
For two different cylinder cutting methods, three typical working conditions are selected for analysis and comparison, as shown in Table 4. Note: QG100MS in the table refers to 100% rated main steam flow. It can be seen from the above table that under the same steam flow, the peak shaving capacity of the unit when switching double cylinders is about 1W higher than that when switching single cylinders; At the end of the load month, the higher the coal consumption is, the higher the heat rate of power generation is. The heat rate of power generation under the double cylinder switching mode and the horizontal bar switching mode are lower, and the lower the load is, the greater the difference in the heat rate of power generation is.

Conclusion
Under the zero output condition of single removal of low pressure cylinder B, when the rated main steam flow is 100% (QG100MS single cylinder removal condition), the heat supply and steam extraction capacity of the unit is 953.23t/h, and the coal consumption for power generation is 202.32g/kWh. Compared with the theoretical maximum heat supply capacity of 859.70t/h before the unit transformation, the heat supply capacity is increased by 93.54t/h, and the coal consumption for power generation is reduced by 12.81g/kWh; When the rated main steam flow is 30% (QG30MS single cylinder switching condition), the heat and steam extraction capacity of the unit is 243.40t/h, the generating power is about 157.13MW, and the electric peak shaving capacity reaches 23.81%. Under the zero output condition of simultaneously cutting off the low pressure cylinders A and B, when the rated main steam flow is 100% (QG100MS double cylinder cutting condition), the heat supply and steam extraction capacity of the unit is 1048.63t/h, and the coal consumption for power generation is 187.70g/kWh, which is 188.94t/h higher than the theoretical maximum heat supply capacity of 859.70t/h before the unit transformation, and the coal consumption for power generation is 27.44g/kWh lower; When the rated main steam flow is 30% (QG30MS double cylinder switching condition), the heat and steam extraction capacity of the unit is 334.89t/h, the generating power is about 147.39MW, and the electric peak shaving capacity reaches 22.33%. The implementation of flexible heating transformation can greatly improve the peak shaving capacity and low load heating capacity of units, meet the needs of flexible transformation and heating capacity increase, and enhance the competitiveness and profitability of units in the peak shaving market of electric auxiliary services.