Thermodynamics Analysis
of a Steam Power Plant
Introduction
This analysis is done as part of an (unreal) project during the advanced thermodynamics course that I have undertaken during my bachelor studies. The analysis and the presented outcomes were done in collaboration with my fellow classmates Abdulaziz Abdulla, Ahmed Almallah and Sayed Mohamed Almoosawi.
A large Bahraini Aluminium company called ALBA (Aluminium Bahrain) have an installed power capacity of over 2000MW in order to supply its four power plants. One of these plants contains a steam turbine and five gas turbines. Newly, the steam turbine system was upgraded by adding a feedwater heater to it. ALBA’s mangers gave this team the task of exploring the effects of this upgrade on the thermal efficiency of the cycle. Moreover, knowing that the last steam turbine upgrade was done ten years ago, the mangers want the team to propose enhancements to the steam turbine system and confirm with evidences that the upgrade would be successful.
Aims and objectives
This project’s objective is to calculate the efficiency of the previous and current steam power cycle at ALBA. Moreover, after comparing both of the efficiencies, recommendations must be given in order to increase the efficiency and reduce the waste of energy. The aims will be achieved by the following:
- Applying energy and mass principles
- Applying the knowledge of fundamental and applied thermodynamics
- Comparing real engines cycle with the ideal power cycles by describing their basic work and energy conservation principles and mentioning their applications in advanced thermodynamics cycles
- Using the second law of thermodynamics for real life industrial applications and considering the real cycle efficiency limitations
- Evaluating alternative energy sources and their role in the sustainability of the environment
Analysis
The following diagram shows how the steam power cycle looks like before adding the Feed Water Heater.
Figure 1. Original Steam Power Cycle.
This cycle describes the original Rankine cycle were the pumps push the liquid water to the boiler. The water will then receive the thermal energy, which would change its state to superheated steam. The steam is then transferred to a high pressure turbine. Ideally, the expansion in the turbine is assumed to be isentropic which means that entropy will be constant throughout the expansion. The resulted steam should be superheated before entering the low pressure turbine. The expansion at the second turbine will be assumed to result in saturating the fluid into vapor, in order to obtain as much work as possible from the low pressure turbine (the expansion process is not isentropic in this case). The condenser then condenses the fluid completely back to saturated liquid, before completing the cycle at the first pump. By obtaining the efficiency of this cycle, it would be useful to establish a baseline of the original efficiency, and how the upgrade of adding the feed water heater actually affected the cycle’s efficiency.
The following diagram shows the modified cycle after adding the free water heater. Heat is added to the steam at the heat exchanger, which turns it to superheated steam just like the previous case. However, after the isentropic expansion at the high pressure turbine, minor part of the flow (which remains superheated) is directed to the free water heater. The remaining flow continuous its expansion at the low pressure turbine and leaves it as saturated vapor, while producing some work. The flow is then directed to the condenser, to reject heat from the system while the state changes to compressed liquid.
Figure 2. Current steam power cycle after upgrading it with the FWH.
When the flow is pumped through the first pump to the feed water heater, it gets mixed with the minor flow of superheated steam which is directed from the high pressure turbine. This mixing causes the fluid to change its phase to saturated liquid, before the second pump compresses it. Theoretically, the saturated liquid would enter the second pump at a higher temperature in this case when compared to the previous cycle. The higher temperature means that less heat input would be required from the boiler and as a result, the efficiency of the cycle should increase. Based on this theoretical analysis, the calculations that will be carried out in the following section will be aimed towards proving the validity of this theory.
Results
A. Original Cycle
The following diagram shows the original cycle before adding the feed water heater, and assuming that the entire flow goes through the low pressure turbine.
Figure 3. Original steam power cycle with the given data.
From the previous diagram, it is clear that the state of the working fluid at the exit of the low pressure turbine is assumed to be saturated vapor, whereas the state at the exit of the condenser is assumed to be saturated liquid.
After the first pump, the state changes to compressed liquid, which stays in this state in the second pump before entering the boiler. In the boiler, the fluid is assumed to be superheated, and then sent to the high pressure turbine to complete the cycle. These assumptions are in ideal situations and they are used to obtain the required data to calculate the efficiency.
By applying mass and energy conservation, and with the use of thermodynamics tables, the remaining unknown variables have been calculated, and they are plotted in the following diagram.
Figure 4. Original steam power cycle with the calculated variables.
The efficiency of the cycle can be determined by the following equation:
= 23.4 %
B. Enhanced Cycle (with feed water heater)
The following diagram shows the cycle after implementing the feed water heater:
Figure 5. Enhanced steam power cycle with the given data.
From the given data and parameters, the following can be assumed:
- HP pump expands isentropically
- The highest state of energy to enter both pumps must be saturated liquid otherwise the flow will cause cavities
- Turbines, pumps and feed water heater are adiabatic
- Water leaving the feed water heater must be saturated liquid
- Steam leaving the LP turbine must be saturated vapour
- The Enthalpy of the steam streams going to the feed water heater and LP turbine must be the same
- Kinetic and potential are negligible
By following the previous assumptions and applying mass and energy conservation laws, the remaining unknown variables have been calculated and they are plotted in the following diagram.
Figure 6. Enhanced steam power cycle with the calculated variables.
The efficiency of the cycle can be determined by the following equation:
= 24.7 %
Enhancing the cycle’s efficiency further
By changing mass flow rate distribution for the FWH and LP turbine
From the previous calculations, the mass flow rate distribution was given as 1.3t/hr to FWH and 8.7t/h for the low pressure turbine. We can change the distribution to obtain more work from the turbine:
Assumptions:
The enthalpy of the water leaving the condenser must not be more than the enthalpy of saturated water at 50 kPa. This ensures that water entering the pump is liquid, thus no cavities will attempt to destroy the pump.
Distribution calculation
The enthalpy of the water leaving the HP turbine will not change. Neither will the enthalpy of the saturated liquid leaving the FWH. However, we have set a new enthalpy going in the FWH from the condenser’s side. By applying energy and mass conservation, the parameters of the cycle after altering the flow rate was calculated and it is shown in the following figure.
Figure 7. Enhanced steam power cycle with the calculated variables after altering the flow rate.
In this case, the calculated efficiency was found to be 24.8%, which is an increase of 0.1% with no additional cost as changing the flow rate distribution requires no extra equipment.
Summary
The aims of this project were calculating the efficiencies of the previous and current power cycle of ALBA and compare both of them, in addition to providing recommendations to increase the efficiency of the current cycle. What is presented on this page is a brief summary of the findings of the project that was carried out by my fellow classmates Abdulaziz Abdulla, Ahmed Almallah, Sayed Mohamed Almoosawi and myself. I would like to emphasize that the work presented here is reasonably brief, as I tried to reduced the amount of presented calculations as much as possible, and to provide just an overview of the project. Further details can be discussed upon request. Thanks for stopping by!