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Thermal Power Plant Design
Group members: Lucía Carballo, Liam Corbel, Erin Mar, Riley McConaughey, Joel Osuna, Tyler Rhoads, Joshua Warden
The purpose of this project was to go over a conceptual power plant design developed by Team 7 for San Diego State University's ME 555 Thermal Analysis and Design course. The goal of the design is to develop a viable thermodynamic power plan that satisfies the following conditions: Net power output of 150 MW, a minimum thermal efficiency of 45%, and steam quality at the exit of the final turbine prior to steam entering the condenser between 97% and 100%. Our design has an efficiency of 47.36%, and produces 150 MW while burning 8.783 kg/s of ethane fuel.
Design Requirements
As previously listed the primary requirements for our design are that the net power output must be a minimum of 150 MW, the thermal efficiency must be a minimum of 45%, and the steam quality prior to entering the condenser must be between 97% and 100%. In order to achieve these requirements, there were a set of constraints such as a maximum turbine temperature and a minimum condenser pressure, which could be changed. The exact figures for our requirements and limitations are presented below in table 1. The steam generator is to be a water-tube furnace, and a tube-banck array (cross flow) bolier, superheater and economizer. If the thermal efficiency is surpassed by 5% of the original 45%, management will consider an increase in fuel consumption to a maximum fuel flow rate of 9 kg/s.
Design Proposal
Shown on the next page is the schematic of design 1, in Figure 1. Design 1 was selected over another design, which can be seen in our interim report. Design 1 features a power generation set composed of two turbine sets, two pumps, a condenser, a closed feedwater heater, an open feedwater heater and a steam trap. Both turbine sets consist of two turbines, one at a higher pressure and one at a lower pressure in order to siphon off mass flow rate for the feedwater heaters. This design achieves the required 150 MW power output when running at a steam mass flow rate of 100 kg/s. At the electric generator this design has an efficiency of 47.36% which is above the required efficiency of 45%.
Steam will enter the first turbine at stage 1, with a temperature of 600 °C and a pressure of 8 MPa. Then at stage 2, the pressure is 2.5 MPa, and 0.0035% of the mass flow is siphoned off to the closed feedwater heater, with the rest of the steam going to the second turbine. At stage 3, coming out of the second turbine the pressure decreased down to 1 MPa, with a temperature of 216.5 °C. The steam is then reheated to 570 °C, before being sent to the third turbine at stage 4. At stage 5 the pressure has decreased to 0.16 MPa, where 0.0035% of the mass flow rate is siphoned off to the open feedwater heater, while the rest is sent to the fourth turbine. Then at stage 6, coming out of the fourth turbine the pressure is 0.008 MPa as a saturated vapor with a quality of 0.989. Then at stage 7, the condenser cools down the steam to a saturated liquid, with a temperature of 41.51 °C. The complete written state analysis for design can be found in appendix 5.1.
This power plant design meets the design requirements of producing 150MW with at least an efficiency of 45%. Our power plant design has an efficiency of 47.36%, and produces 150 MW while burning 8.783 kg/s of fuel. Initially we had some error with how the feedwater heaters worked, which led to a decrease in efficiency. Thus, the design originally reached over 50% efficiency thus was able to be designed to work with the upper limit of 9 kg/s of fuel burn. Therefore, our powerplant does not meet the management's allowed use of 9 kg/s of fuel burn, but we completed the design and report as if we were still able to.
Link to Design Proposal: https://docs.google.com/document/d/1Nx5hU8GCFkfCVrYfMq0qdaJAkIwuth5PK62HDoKHirI/edit?usp=sharing
State Analysis
The written state analysis for design 1 can be found in appendix 5.2. The values obtained by the state analysis are provided below in Table 3. Other important values obtained from state analysis are provided below in table 4. This design uses turbines at 93% isentropic efficiency, and pumps at 80% efficiency. Steam enters the first turbine at 8 MPa, and 600°C. This analysis meets all requirements provided by management. The max pressure of 8 MPa was chosen for its ability to have a greater entropy at the maximum temperature allotted. This leads to an overall greater power output since the enthalpies in this region are greater than those at higher pressures.
Figure 1
Figure 2
Combustion Analysis
In the combustion analysis for design 1, it is assumed that the fuel entering the furnace enters at standard temperature and pressure, and air enters at atmospheric pressure. The pitch-point temperature difference is 25°C and the approach point temperature difference is 30°C. It is also assumed that the fuel is burned with 180% excess air, and with complete combustion, there is no unburned carbon present in the combustion products, the fuel is clean-burning, no ash residue is created, there is no sulfur present in the fuel, no sorbent is required and no additional moisture is present in the fuel. The air entering the furnace has a moisture content of 0.0013 lb water/lb dry air. The losses due to convection and radiation are 0.95% and other losses are 1.95%.
The fuel that was chosen for design 1 was Ethane for combustion with 180% theoretical air. The mass flow rate of fuel and wet gas were found to be 58970.87 Btu/hr and 2750431.6 Btu/hr, respectively. The combustion efficiency found is 69.45% taking in consideration important assumptions such as the temperature of air leaving the heater is 77 ℉ and has a moisture content of of 0.013 lb water/lb dry air. It is also assumed that the reference temperature is 77 ℉ and Ethane enters the furnace at 77 ℉ and 1 atm. Table 5 shows the combustion analysis results with the most important parameters being highlighted in yellow.
Conclusion
The power plant design reached the required parameters of producing 150 MW at at least 45% efficiency. Our design produces 150 MW at 47.36% efficiency. However, it uses 8.783 kg/s of fuel and management states that they would consider increasing fuel flow rate above 8 kg/s when 50% efficiency is reached. Originally our proposed design had reached over 50% efficiency. However, due to errors in calculations of the thermodynamic state analysis the efficiency was lowered.