Browsing by Subject "Superheater"

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    Open Access
    A methodology for integrated thermofluid modelling of radiant superheaters in steady state and transient operations
    (2019) Gwebu, Excellent Zibhekele; Rousseau, Pieter G.; Malan, Arnaud G.; Jestin, Louis M.
    Critical components in coal-fired power plants such as final superheater heat exchangers experience severe conditions associated with high metal temperatures and high temperature gradients during base load and transient operations. Such adverse conditions could significantly reduce the life span of the components, especially due to the requirement of greater plant flexibility that is an essential part of the global power system transformation. Integrated thermofluid process models can be employed to obtain a better understanding of the relationship between the operational conditions and the metal temperatures. Thus, a methodology was developed to model radiant superheater heat exchangers in steady state and transient operations. The methodology is based on a network approach which entails solving the transient one-dimensional forms of the conservation equations for mass, energy and momentum. The model building blocks account for the convective thermal resistance on the steam side, the conductive thermal resistances of the tube wall and scaling or fouling on the tube walls, as well as the convective and radiative thermal resistances and direct radiation on the flue gas side. The model captures the physical layout of the tube passes in a tubesheet via the arrangement of the network building blocks. It is also possible to connect tubesheets together across the width of the boiler as per the arrangement in a real plant. The modelling methodology was first used to develop a process model of a convective cross-flow primary superheater heat exchanger with complex flow arrangement. The dual-tube 12-pass superheater was discretized along the flue gas flow path as well as along the steam flow path. The model was qualitatively validated using real plant data from literature and for reference purposes also systematically compared to conventional lumped parameter models. The ability of the model to analyse the effect of ramp rate during load changes on the tube metal temperature was demonstrated, as well as the ability to determine the maldistribution of flow and temperature on the steam and flue gas sides. The methodology was also applied to model a U-shaped radiant superheater heat exchanger. Due to the challenges associated with obtaining comprehensive real plant data in an industrial setting, a validation methodology was proposed that is based on a combination of plant design C-schedules and a boiler mass and energy balance, as well as limited plant measurements. The consistent comparisons with C-schedule data provide evidence of the validity of the model, which was further demonstrated via the comparisons with real plant data. The model allows prediction of the steam mass flow and temperature distribution going into the outlet stub headers as well as the main outlet headers for different inlet flow and temperature distributions on the steam and flue gas sides. These results were compared to detail real-plant measurements of the outlet header temperatures. The model also allows prediction of the metal temperatures along the length of the tubes which cannot readily be measured in the plant. The model was applied to demonstrate the impact of different operational conditions on the tube metal temperatures. Such integrated process models can be employed to study complex thermofluid process phenomena that may occur during intermittent, transient and low load operation of power plants. In addition, such models could be useful for predictive and preventative maintenance as well as online condition monitoring.
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    Open Access
    A methodology to investigate the cause of quenching in once-through tower type power plant boilers
    (2020) De Klerk, Gary; Rousseau, Pieter; Jestin, Louis
    Due to the penetration of variable renewable energy (VRE) sources, conventional coal fired power plants need to operate with greater flexibility via two-shifting or low load operation whilst remaining reliable and conserving the lifetime of components. Thick sectioned components are prone to thermal fatigue cracking as a result of through-wall temperature gradients during start up and shutdown. These temperature gradients can be significantly amplified during quenching when components at high temperature are unintentionally exposed to colder liquid or steam. Such quench events are known to occur during two-shift operation of a large once-through coal fired tower type boiler, which is the subject of this study. The purpose of this study is to develop and demonstrate a methodology to determine the root cause of quenching in a once-through tower type boiler and provide information that can be used to predict the impact on thick-walled components by estimating the through-wall temperature gradients. The first modelling element in the methodology is a simplified transient heat transfer model for investigating condensation of steam in the superheater. The model is presented and verified by comparison with real plant data. The second element is a liquid tracking model that approximates the liquid level in the superheater as a function of time to predict the location and magnitude of through-wall temperature gradients. The complex geometry of the superheater was divided into a number of control volumes and a dynamic thermo-fluid process model was developed to solve the transient conservation of mass and energy equations for each volume using a semi-implicit time wise integration scheme. The liquid tracking model was verified by comparison with a similar model constructed in Flownex and also by comparison with plant data. Varying levels of discretisation were applied to a particular quench event and the results are presented. The third modelling element is a two-dimensional transient pipe wall conduction model that is used at selected localities to evaluate the temperature gradients within the pipe wall. The temperature gradients and internal heat flux were verified by temperature measurements from the outer surface of a main steam pipe undergoing quenching. The stresses associated with the temperature gradients were also briefly considered. The real plant quenching problem is analysed in detail and found to be caused by liquid overflow from the separators. A particular plant configuration creates a previously unidentified siphon of water from the separating and collecting vessel system into the superheater. This situation is not recognised by plant operators and thus persists for some time and causes flooding of the superheater. Analysis of the resultant through-wall temperature gradients show that quenching causes significant stresses which can be avoided. By understanding the causes and preventing the occurrence of quenching, the life of thick-walled high temperature components can be conserved.