Power plant stability at lower loads is becoming ever more important, highlighting the increasing requirement for the development of advanced models and tools to analyse and design systems. Such tools enable a better understanding of the thermo-fluid processes and their dynamics, which improves the ability to specify and design better control algorithms and systems. During low load operation and transients, such as start-up and shutdown, the required water flow rate through the evaporator tubes of once-though boilers must be significantly higher than the evaporation rate to protect against overheating of the tubes until once-through operation is reached. Controlling the minimum required water flow rate through the evaporator and economiser is notoriously difficult. Within industry, strong emphasis is placed on maintaining the minimum required flow through the economiser and evaporator without adequate consideration of the potential thermal fatigue damage on the economiser, evaporator and superheater components and the risk of turbine quenching incidents. The purpose of this study was to develop an integrated process and control model that can be used to study transient events. The model developed in Flownex can simulate the complex thermo-fluid processes and associated controls of the feedwater start-up system. This includes the waterrecirculation loop, and allows for detailed transient analysis of the complete integrated system. The model was validated using data from an actual power plant in steady state as well as a transient cold start-up, up to once-through operation. Transient results from the model are also compared to the power plant unit during start-up for the addition or loss of mills using the existing control strategy. The model results compare well with the actual process behaviour. A new control strategy was then proposed and tested using the model. The results indicated significant improvement in control performance and overall controllability of the start-up system, and the large temperature fluctuations currently experienced at the economiser inlet during transients were significantly reduced. The new control strategy was also implemented on a real power plant unit undergoing commissioning. During all modes of start-ups (cold, warm and hot), as well as transients, the performance of the control system showed significant improvement, with a notable decline in instabilities of the feedwater flow. As predicted in the model, the large temperature fluctuations are significantly reduced. The new model therefore enabled the development of an improved control strategy that reduces damaging thermal fatigue. The general controllability of transients is also significantly improved, thereby minimizing risks of water carry-over, quenching and unit trips during start-up.

This analysis calculates the velocity profile and recirculation ratio in the aspirator region of an enhanced once-through steam generator of the Babcock & Wilcox design. This information is important to the development of accurate RELAP5 models, steam generator level calculations, steam generator downcomer models, and flow induced vibration analyses. The OpenFOAM CFD software package was used to develop the three-dimensional model of the EOTSG aspirator region, perform the calculations, and post-process the results. Through a series of cases, each improving upon the modeling accuracy of the previous, insight is gained into the importance of various modeling considerations, as well as the thermal-hydraulic behavior in the steam generator downcomer. Modeling the tube support plates and tube nest is important for the accurate prediction of flow rates above and below the aspirator port, but has little affect on the aspirator region itself. Modeling the MFW nozzle has minimal influence on the incoming steam velocity, but does create a slight azimuthal asymmetry and alter the flow pattern in the downcomer, creating recirculation patterns important to inter-phase heat transfer. Through the development of a two-phase solution that couples the aspirated steam and liquid feedwater, it was found that the ratio of droplet surface area to volume plays the most important role in determining the rate of aspiration. Calculations of the velocity profile and recirculation ratio are compared against those of historical calculations, demonstrating the possibility that these parameters were previously underpredicted. Such a conclusion can only be confidently made once experimental data is made available to validate the results of this analysis. / Master of Science

Heat recovery steam generators (HRSG) play a very important role in combined cycle (CC) power plants, where steam is generated from a gas turbine exhaust and supplied at the appropriate pressure and temperature to steam turbines for further power generation. The power plants achieve an overall efficiency above 55% and are ideally suited for combined heat and power generation in utility systems. The performance of energy conversion is improved by reducing exergy losses which implies reducing the temperature difference between the combustion gas and the steam cycle. Thus recent HRSG designs include up to three pressure levels with reheat in the steam cycle for maximum energy recovery and the use of high pressure, high temperature superheater and reheater in CC plants. Super critical boilers are also conceivable. Since HRSG performance has a large impact on the overall efficiency of the CC power plant, an accurate simulation of the performance of the HRSG is necessary. We present a steady state HRSG model to support design and rating simulations of vertical units. The simulation model, called FELVAL, divides the boiler in its rows. The row model can also be divided several times following the tube length, to better estimate the fumes temperature distribution across the hot gas path. Another model, called SUFVAL, carries out the design as well as the automatic generation of the FELVAL units and all the needed connections. The log mean temperature difference (LMTD) method and the effectiveness-NTU (ε -NTU) method are alternatively used to compute the overall heat transferred in each part of the HRSG. The problem of convergence of boiler models with more than one row in parallel is discussed. Good initialisation of the different variables is crucial to obtain convergence. The models are tested on 2 references HRSG. The first one is an assisted circulation boiler that operates at 3 subcritical pressure levels. The second is a once through boiler able to operate above the critical pressure of water. These new models were introduced into a commercial software of data reconciliation (VALI of Belsim sa) already used by the engineering and design departments of a HRSG manufacturer. They thus have a general-purpose package enabling them to make design, data reconciliation and simulation with the same software. Moreover, the use of FELVAL model will enable them to simulate any type of boiler and to obtain informations on the change of the temperatures inside the heat exchangers. This information is crucial for well monitoring closely the operation of a boiler, and better understanding its behaviour. This knowledge improvement allows to limit the overdesign and the safety margins and to reduce the investment costs.

As the importance of water use in the power generation sector increases across the nation, the ability to obtain and analyze real power plant data is pivotal in understanding the water energy nexus. The Navajo Generating Station in Arizona and the Browns Ferry Nuclear Plant in Alabama are examples of where water shortages have threatened the operation of power generators. The availability of freshwater in the United States is beginning to dictate how and where new power plants are constructed. The purpose of this study is to provide and analyze cooling system parameters using 2008 data provided by the Energy Information Administration. Additionally, the cost of water saved among different categories of power plants is calculated. In general, the conditions which cause cooling systems to withdraw less water are not necessarily the more expensive conditions, and vice versa. While not all the variability in the cost of cooling systems is being accounted for, the results from this study prove that nameplate capacity, capacity factor, age of power plant, and region affect the costs of installed cooling systems. This study also indicates that it would be most cost effective for once-through cooling systems to be replaced with recirculating- pond instead of recirculating- tower systems. The implications of this study are that as power plant owner's struggle in balancing cost with water dependence, several parameters must first be considered in the decision-making process. / Master of Science

The optimal estimation theory of the Wiener-Kalman filter is extended to cover the situation in which the number of memory elements in the estimator is restricted. A method, based on the simultaneous diagonalisation of two symmetric positive definite matrices, is given which allows the weighted least square estimation error to be minimised. A control system design method is developed utilising this estimator, and this allows the dynamic controller in the feedback path to have a low order. A 12-order once-through boiler model is constructed and the performance of controllers of various orders generated by the design method is investigated. Little cost penalty is found even for the one-order controller when compared with the optimal Kalman filter system. Whereas in the Kalman filter all information from past observations is stored, the given method results in an estimate of the state variables which is a weighted sum of the selected information held in the storage elements. For the once-through boiler these weighting coefficients are found to be smooth functions of position, their form illustrating the implicit model reduction properties of the design method. Minimal-order estimators of the Luenberger type also generate low order controllers and the relation between the two design methods is examined. It is concluded that the design method developed in this thesis gives better plant estimates than the Luenberger system and, more fundamentally, allows a lower order control system to be constructed. Finally some possible extensions of the theory are indicated. An immediate application is to multivariable control systems, while the existence of a plant state estimate even in control systems of very low order allows a certain adaptive structure to be considered for systems with time-varying parameters.

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The master thesis deals with hydraulic characteristics of once-through boilers and describes the problems which may arise during the flow of water through the evaporator. Especially the formation of aperiodic instability of the evaporator and hydraulic uniformity amongst every steam generating tube. The thesis describes parameters, which cause these problems, and solutions how to deal with them. The calculation has been done for evaporator of the boiler called K2 located in power station in Chvaletice.