Improving the primary steam parameters is one of the most direct ways to improve the cycle efficiency of a power generation system. In the present study, the typical problem connected to the excessively high superheat degree of extraction steam in an ultra-supercritical (USC) double-reheat unit is considered. Using a 1000 MW power plant as an example, two systems (case 1 and case 2) are proposed, both working in combination with a regenerative steam turbine. The thermal performances of these two systems are compared with that of the original system through a heat balance method and an exergy balance strategy. The results reveal that the two coupled systems can significantly reduce the superheat degree of extraction steam, turbine heat rate, and coal consumption of the unit and improve the energy utilization efficiency. These results will provide useful theoretical guidance to future investigators wishing to address the general problem relating to energy conservation and modelling of the coupled extraction steam regenerative system of USC double-reheat units.
Improving the primary steam parameters is one of the most direct ways of improving the cycle efficiency of a power generation system. In recent years, the operating parameters of double-reheat units have reached 31 MPa/600°C/620°C/620°C [
Although the double-reheat system with an ESC is widely used, the extraction stage cannot be changed, and the ESC is simply added to the original extraction stage. The steam in an ESC does not have phase changes, which leads to a low heat transfer coefficient, large heat exchange area, and high installation cost [
However, the above research only focused on 10-stage double-reheat systems. In this study, two system schemes for reducing extraction superheating degrees were developed for 12-stage regenerative units and higher steam parameters. The influence of different extraction stages of regenerative steam turbines was analyzed, and the thermal performance of the three system schemes were compared.
In this study, the thermodynamic cycles of the system were simulated with Thermoflex, which is widely used for the system simulation and calculation of different units. The following assumptions were made about the system: 1) The operation of the unit was in a steady state; 2) The isentropic expansion efficiencies are equal to 0.89, 0.92, 0.93 and 0.88 during the different stages of the VHP, HP, IP, and LP turbines, respectively; 3) The efficiency of the regenerative steam turbine was 0.89; 4) the boiler efficiency was 0.95; and 5) the generator efficiency was 0.99.
The heat rate and efficiency of the power plant are used in the electric power industry to evaluate the thermal performance of coal-fired units [
The unit efficiency is expressed as follows:
The definition of the standard coal consumption of the power plant is expressed in the following form:
where
Exergy analysis is a thermodynamic analysis technique based on the second law of thermodynamics, which reflects the substantial change in heat transfer. For the steady flow process, physical exergy is expressed as follows:
Here, the subscript 0 refers to the specified stream flow and physical values at the reference pressure and temperature. Here,
where
The exergy efficiency of the RH is expressed as follows:
where Δ
A state-of-the-art 1000 MW USC double-reheat power plant is selected as the original system in this study. The main steam parameters were 35 MPa/615°C/630°C/630°C. The main components of the original system were the VHP, HP, IP, and LP turbine groups, a boiler with two reheaters, a DEA, and high- and low-pressure RHs. 12 stages extraction steams were involved in the system.
Items | Pressure (MPa) | Temperature (°C) | Saturated temperature (°C) | Steam superheat (°C) |
---|---|---|---|---|
RH1 | 10.642 | 415.2 | 315.6 | 99.6 |
RH2 | 7.705 | 588.7 | 292.4 | 296.3 |
RH3 | 5.525 | 531.8 | 270.26 | 261.54 |
RH4 | 3.528 | 459.3 | 243 | 216.3 |
RH5 | 2.176 | 569.9 | 216.69 | 353.21 |
DEA | 1.182 | 472.2 | 187.28 | 284.92 |
RH7 | 0.750 | 405.4 | 167.21 | 238.19 |
RH8 | 0.411 | 324.7 | 144.6 | 180.1 |
RH9 | 0.222 | 260.7 | 123.5 | 137.2 |
RH10 | 0.116 | 192.0 | 103.8 | 88.2 |
RH11 | 0.053 | 118.1 | 82.8 | 35.3 |
RH12 | 0.021 | 60.6 | 60.6 | 0 |
For the high superheat degree of extraction steam, this study proposes two schemes: a double-turbine system with six-stage extraction steam (case 1) and a double-turbine system with seven-stage extraction steam (case 2). Here the number of the extraction steam stage is referring to that extracted from regenerative steam turbine and the main difference of the two cases is the number of extraction steam stages of the regenerative turbine. The process flow diagrams of the two schemes are shown in
Items | Pressure (MPa) | Temperature (°C) | Saturated temperature (°C) | Steam superheat (°C) |
---|---|---|---|---|
RH1 | 13.241 | 447.3 | 332.28 | 115.02 |
RH2 | 9.403 | 399.7 | 306.48 | 93.22 |
RH3 | 6.391 | 346 | 279.7 | 66.3 |
RH4 | 4.219 | 293.3 | 253.6 | 39.7 |
RH5 | 2.661 | 240.6 | 227.28 | 13.32 |
RH6 | 1.518 | 198.9 | 198.92 | 0 |
RH7 | 0.930 | 176.8 | 176.76 | 0 |
RH8 | 0.489 | 347 | 151.8 | 195.2 |
RH9 | 0.217 | 253.4 | 123.25 | 130.15 |
RH10 | 0.120 | 190 | 104.78 | 85.22 |
RH11 | 0.058 | 121.8 | 85.9 | 35.9 |
RH12 | 0.020 | 60.6 | 60.6 | 0 |
Items | Pressure (MPa) | Temperature (°C) | Saturated temperature (°C) | Steam superheat (°C) |
---|---|---|---|---|
RH1 | 13.224 | 447.3 | 332.17 | 115.13 |
RH2 | 9.906 | 406.8 | 310.34 | 96.46 |
RH3 | 6.791 | 353.6 | 283.78 | 69.82 |
RH4 | 4.529 | 301.4 | 259.18 | 42.22 |
RH5 | 2.905 | 249.6 | 232.18 | 17.42 |
RH6 | 1.802 | 207.2 | 207.12 | 0 |
RH7 | 1.033 | 181.29 | 181.17 | 0 |
RH8 | 0.460 | 148.7 | 148.7 | 0 |
RH9 | 0.242 | 258.6 | 126.07 | 132.53 |
RH10 | 0.157 | 211.2 | 113.3 | 97.9 |
RH11 | 0.065 | 127.9 | 88 | 39.9 |
RH12 | 0.022 | 62.2 | 60.06 | 2.14 |
Items | Ordinary system | Case 1 system | Case 2 system |
---|---|---|---|
Main steam flow rate (Kg.h) | 2428.6 | 2723.6 | 2809.8 |
Main steam pressure (MPa) | 35 | 35 | 35 |
Main steam temperature (°C) | 615 | 615 | 615 |
First reheat steam pressure (MPa) | 9.897 | 12.453 | 12.579 |
First reheat steam temperature (°C) | 630 | 630 | 630 |
Second reheat steam pressure (MPa) | 3.175 | 3.337 | 3.166 |
Second reheat steam temperature (°C) | 630 | 630 | 630 |
Power generation output (MW) | 1000 | 1000 | 1000 |
Hate rate (kJ/kWh) | 6996 | 6937 | 6943 |
Coal consumption of unit (g/kWh) | 254.15 | 252 | 252.23 |
Coal consumption reduce (g/kWh) | ---- | 2.15 | 1.92 |
Exergy loss of RH (kW) | 93252 | 68020 | 68443 |
Exergy efficiency of RH | 0.897 | 0.922 | 0.915 |
This study proposed two thermodynamic optimization schemes (case 1 and case 2) for a double-turbine regenerative system of a USC double-reheat unit. Their thermal performance and exergy were analyzed and compared with those of the original system. The results are as follows: The double-turbine regenerative system can significantly reduce the superheat degree of the extraction steam of RHs in the double-reheat system, improve the energy utilization efficiency, and reduce the heat rate and coal consumption of the system. However, with an increasing of the extraction stage numbers of the regenerative steam turbine, the main steam flow and the exergy loss of the double-turbine regenerative system also increase. In case 2 system, the exergy loss of RH1–RH8 was the lowest, and the average superheat degree of extraction steam had the greatest decrease. However, the heat rate, coal consumption of the unit, and total exergy of RHs were greater than those in case 1 system. This indicates that the reduction in superheat degree is not directly proportional to the thermal economy of the unit. Case 1 system has the best thermal performance among the three systems. Compared with the those of original system, the heat rate, coal consumption, and irreversible loss were reduced by 59 kJ/kWh, 2.15 g/kWh, and 25232 kW, respectively, while the average exergy efficiency of the heater increased to 92.2%. Meanwhile, the case 2 system has one additional extraction steam stage of the regenerative steam turbine compared with the case 1 system. The heat rate and coal consumption increased by 4 kJ/kWh and 0.23 g/kWh, respectively. The total exergy loss of the heater also increased.