Browsing by Author "Jestin, Louis"

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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.
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    Open Access
    A thermofluid network-based methodology for integrated simulation of heat transfer and combustion in a pulverized coal-fired furnace
    (2020) van Der Meer, Willem Arie; Rousseau, Pieter Gerhardus; Jestin, Louis
    Coal-fired power plant boilers consist of several complex subsystems that all need to work together to ensure plant availability, efficiency and safety, while limiting emissions. Analysing this multi-objective problem requires a thermofluid process model that can simulate the water/steam cycle and the coal/air/flue gas cycle for steady-state and dynamic operational scenarios, in an integrated manner. The furnace flue gas side can be modelled using a suitable zero-dimensional model in a quasi-steady manner, but this will only provide an overall heat transfer rate and a single gas temperature. When more detail is required, CFD is the tool of choice. However, the solution times can be prohibitive. A need therefore exists for a computationally efficient model that captures the three-dimensional radiation effects, flue gas exit temperature profile, carbon burnout and O2 and CO2 concentrations, while integrated with the steam side process model for dynamic simulations. A thermofluid network-based methodology is proposed that combines the zonal method to model the radiation heat transfer in three dimensions with a one-dimensional burnout model for the heat generation, together with characteristic flow maps for the mass transfer. Direct exchange areas are calculated using a discrete numerical integration approximation together with a suitable smoothing technique. Models of Leckner and Yin are applied to determine the gas and particle radiation properties, respectively. For the heat sources the burnout model developed by the British Coal Utilisation Research Association is employed and the advection terms of the mass flow are accounted for by superimposing a mass flow map that is generated via an isothermal CFD solution. The model was first validated by comparing it with empirical data and other numerical models applied to the IFRF single-burner furnace. The full scale furnace model was then calibrated and validated via detailed CFD results for a wall-fired furnace operating at full load. The model was shown to scale well to other load conditions and real plant measurements. Consistent results were obtained for sensitivity studies involving coal quality, particle size distribution, furnace fouling and burner operating modes. The ability to do co-simulation with a steam-side process model in Flownex® was successfully demonstrated for steady-state and dynamic simulations.
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    Open Access
    Investigation into methods for the calculation and measurement of pulverised coal boiler flue gas furnace exit temperature
    (2016) Tootla, Naeem Ebrahim; Jestin, Louis
    The boiler flue gas furnace exit temperature (FET) is a key operating parameter of coal fired steam boilers. From the design perspective, the FET is vital for materials selection and sizing of heat transfer surfaces. From an operating perspective, it is a major indicator of the rate of combustion and heat transfer that is occurring within the furnace. Downstream of the furnace, the FET has a significant impact on both the performance and reliability of the boiler heat exchangers, which ultimately impacts on both boiler efficiency and availability. Monitoring of the FET can advise operating and engineering corrective actions which will ultimately result in better efficiency, reliability and availability together with the associated economic benefits. Therefore, methods of determining FET are investigated. Two methods are focused on for this study, one indirect and one direct. The indirect method studied is a mass and energy balance method which begins with a global boiler mass and energy balance to calculate the major boiler flow rates of coal, air and flue gas which are difficult to measure online. These parameters are then used as inputs into a furnace or backpass mass and energy balance to calculate the furnace exit temperature. The method is applied to a case study, and is evaluated in terms of the measurement uncertainties which are propagated on the intermediate parameters calculated, as well as on the final calculated FET. The main conclusions are that this indirect method contains various uncertainties, due to parameters which have to be assumed such as (i) the distribution of ingress air (also called tramp air) in the different sections of the boiler and (ii) the estimation of the share of water evaporation heat transfer occurring in the water walls of the furnace part of the boiler. The method is however still useful and can be easily applied to any boiler layout and can be used as a reference tool to verify other measurements. The direct method studied is acoustic pyrometry. The work specifically focuses on the sources of error in determining the temperature from the measurement of the time of flight of sound, the impact of particle concentration on the speed of sound through a gas-particle mixture, and the temperature profile reconstruction from acoustic time of flight measurements. A limited set of physical testing was also carried out using one acoustic generator and receiver to take measurements on a real coal power plant. As part of this physical testing, the detection of time of flight from acoustic signals was explored. Already installed radiation pyrometers were also used as a reference for interpreting the acoustic measurements. The indications are that the acoustic pyrometer provides a more representative temperature measurement than the radiation pyrometers. The uncertainty of the acoustic measurement for the same case study as the indirect method was determined and compared with the calculated result. While many aspects still need to be researched further, this initial study and experimental testing produced very promising results for future application of acoustic pyrometry for better monitoring of the coal combustion processes in power plant boilers.
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    Open Access
    Transient boiler heat exchanger thermal behaviour analysis
    (2014) Gwebu, Excellent Zibhekele; Jestin, Louis
    Coal fired power plants that were built in the past four decades are aging. The main aging mechanisms are creep and thermal fatigue. Creep results from the high temperatures at which the components operate. Thermal fatigue is due to thermal stresses and these stresses result from temperature gradients within the material. Cycling of these thermal stresses accelerate the creep in a process called creep- fatigue aging. The boiler and its final heat exchangers and headers are the main components that are affected by these mechanisms. The aging of these components results in high maintenance costs, reduction of the plant reliability and availability, and contribute to increased safety risks for the plant and personnel. Therefore, there is a need to understand the steady state and dynamic behaviour of the components of these plants in order to predict the stresses that the material experience. This report discusses an investigation to the possibility of modelling the thermal dynamic behaviour of typical boiler heat exchanger components which have to withstand the highest temperature of a Pulverised Fuel Rankine cycle power plant. Thus, illuminating the issues that need to be addressed in modelling such heat exchangers. Modelling approaches of heat exchangers are systematically presented, starting with the use of exact analytical solutions. This is followed by the application of finite volume numerical method. Finishing off with the use of the Flownex software. The exact analytical solutions are used to characterise the transient temperature distribution in solid materials with simplified heat transfer, highlighting the dependence of the solutions on the Fourier number and Biot number. These solutions are further used to calculate thermal stresses generated in the material, illustrating the relationship between thermal stresses and temperature gradients. Furthermore, a finite volume solution is applied to modelling an infinitely long tube. I t is illustrated that for transient conduction heat transfer problems, the solution depends on both physical space discretisation and time- wise discretisation. The numerical solution is verified against the exact analytical solution. Finally, the Flownex software is used to illustrate the issues that need to be addressed when modelling the transient behaviour of a heat exchanger . For this purpose only the average area discretisation scheme is used since it allows for any generic solid structure to be modelled, provided that the appropriate level of discretization is applied. The Flownex modelling starts by modelling transient conduction heat transfer within an infinitely long tube. The Flownex solution is verified against the finite volume numerical solution. The Flownex solution depends on thickness discretisation, especially for thick cylindrical components. Finite tubes are also modelled on Flownex including axial discretisation and layout simplification of the tubes. Flownex is also used to model a heat exchanger bundle using two methods; a tube by tube method and a method that involves the combination of all ii the tubes into one tube. The product of the thermal resistance and the capacitance of the system governs the transient simulations for both methods.