Browsing by Author "Fuls, Wim"
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- ItemOpen AccessA methodology for implementing a water balance of ESKOM power stations using the online condition monitoring software EtaPRO(2021) Sewlall, Preetha; Fuls, WimEskom produces approximately 90% of the electricity used in South Africa of which approximately 90.8% is from fossil fuel power plants. The process of electricity generation requires a significant quantity of raw water; therefore, Eskom is considered a strategic water user in South Africa. Water management is a growing focus area due to the increase in water usage and requires continuous improvement. Water management has been identified as an area lagging behind on the advanced analytics initiatives in Eskom. Excel based tools were used for the development of water balance models and water performance calculations in Eskom. This was attributed to the user-friendly functionality and availability to all users. However, the Excel tool posed challenges in allowing for standardisation and validation of calculations, tracking of model changes, continuous trending and storage of data as well as structured graphical user interfaces for screens and dashboard developments. There was therefore a need to develop a methodology on how to structure a water balance model for coal-fired power plants with standard calculation templates that allowed for customisation by each power plant within Eskom. It was required that the water balance model be implemented on a performance and monitoring tool allowing for comparison of power plant targets to actual online data in real time, enhancing the monitoring capabilities. It should have the ability to generate real time water performance data creating an opportunity for improved water management across the generation fleet. The approach adopted in this dissertation was to learn from existing Eskom Excel water balance tools and develop a standard mathematical model in the form of EtaPRO calculation templates. These templates are be structured such that they function as process components to develop water balances at power plants. The mathematical verification of the Excel calculations were to be conducted using Mathcad. The access to real time data, performance monitoring capabilities and availability at all Eskom Power plants, led to the selection of EtaPRO as the modelling platform. The research conducted led to the development of a methodology for setting up a water balance model for a wet-cooled coal-fired power plant. Calculation templates developed into EtaPRO were validated against the Mathcad mathematical model. The results included a well-documented mathematical model of a water balance in Mathcad and the development of 19 calculation templates that perform the function of standard process components. In addition to calculation templates, multiple Non Volatile (NV) records were created to allow the power plants to capture and track permanent data inputs. NV records also allow for creation of case studies, improving the process monitoring capabilities. A water balance model for a selected power plant was simulated in EtaPRO using the developed calculation templates and user defined formulae. Test screens and dashboards were created to illustrate how the calculation templates and water balance framework would be used to develop a typical water balance model and monitoring system. In conclusion, it is possible to develop process models within the EtaPRO software from well-defined mathematical models to address the performance monitoring concerns on water systems within Eskom.
- ItemOpen AccessApplication of process data reconciliation in power plants(2017) Mathebula, Muhluri Calvin; Fuls, WimThe operation of power plants and chemical processes requires process measurements for optimal operations. Process measurements are essential for plant performance optimization, process monitoring and process control. It is vital to have reliable and accurate process data to achieve process optimization. However, process measurements are inevitably subject to measurement errors. These measurement errors are classified as random and gross errors. Data reconciliation technique is an effective data treatment method that is used in chemical processes to enhance the quality of process data. The purpose of data reconciliation is to reduce random errors to achieve measurements which are as accurate and reliable as possible. Data reconciliation technique uses available process measurements to produce consistent and accurate estimates, so close to the true values that they satisfy model constraints. Further, data reconciliation technique depends on measurement redundancy to perform reconciliation and produce reliable estimates. In addition, data reconciliation can also provide estimates of unmeasured observable variables. Process data reconciliation is not complete without a gross error detection strategy that can effectively detect and eliminate gross errors in measurements. Data reconciliation is applied to linear and nonlinear steady state processes with measured and partially measured variables. Heat exchanger and steam generator models with nonlinear mass and energy constraints are used. The reconciliation process is applied in a feed water flow measurements model to illustrate the applicability of data reconciliation.
- ItemOpen AccessAsymmetric flow measurement in space constrained cooling water ducts using a traversing probe(2015) Mutshinya, Rudzani Allan; Fuls, WimThe aim of this research was to identity and recommend a type of flow measurement technique suitable for flow measurements in the CW ducts. After conducting a literature survey, the author restricted research to a local flow measuring technique as a result of physical constraints on site. In addition, the flow profile in the CW ducts was unknown so that existing flow rate measurement standards could not be applied. Thus the focus of this research was to predict the expected accuracy of local flow measurements in asymmetric flow conditions.
- ItemOpen AccessBoiler feed pump low load – leak off recirculation study(2020) van Tonder, Daniël; Fuls, WimFor power plants that make use of high energy boiler feed pumps, there is a risk that the boiler feed pump may experience cavitation and overheating at low load and start-up conditions. These plants make use of a leak off or recirculation system that diverts some of the flow back to the feed water tank, ensuring that a minimum flow through the pump is maintained at low load and start-up operating conditions. The recirculation valve, also known as a leak off valve, experiences a very high pressure difference and cavitation pitting is common due to the water being close to saturation. There are various ways in which the recirculation flow is controlled in the industry such as open orifice, on/off binary type control valves, automatic recirculation valves (ARC) or modern modulating leak off systems. The valves themselves can also be simple plug type or make use of pressure staging to reduce the risk of cavitation. This project involves modelling the flow system around the boiler feed pump and its control for the various architectures employed in Eskom. This is to assist in understanding the reasons for cavitation damage that is found in some recirculation valves as well as the low load capability of the system. Single stage components with extremely high pressure drops are singled out as components with the highest risk of cavitation in the systems. Although extremely high pressure drops are found across the leak off valves themselves, the majority of the valves are multistage valves which are specifically designed to accommodate cavitation development and are therefore not of major concern. Some of the findings of the study are: The rule of thumb used within Eskom to determine the amount of pressure reducing stages on leak off valves could be more conservative. The specification of new valves and components for the leak off systems requires accurate specification based on detailed process models, such as the ones developed for this study. The full range of all possible operational cases must also be considered during the design.
- ItemOpen AccessBoiler system modelling using Flownex®(2016) Rossouw, André; Fuls, WimThe objective of this project is to develop a boiler modelling methodology, specifically using Flownex, which is capable of running transient simulations for a large variety of coal-fired boiler designs typically used in Eskom. Flownex has been identified as the key software to accomplish the global objective of the Centre for Energy Efficiency under EPPEI at the University of Cape Town, which is to develop a software model of a complete coal-fired power station which includes all the main systems required for independent transient simulation. The boiler model captures the true geometric layout and flow orientation with associated characteristics of a wide variety of boiler designs utilised by Eskom. In order to achieve this, boilers and heat exchangers are grouped according to common physical properties which simplify the modelling process and optimise results. This is preceded by an investigation into the types of boiler designs currently operational in Eskom including available associated geometrical and process characteristics. A study into heat transfer mechanisms applicable to coal-fired boiler heat exchangers was done to ensure fundamental theoretical principles are adhered to during the development of the analytical models, the first step in the modelling process. The Flownex solving methodology is evaluated against the analytical models in a simplified heat exchanger before full detail modelling of heat exchangers are done. The component and method used in Flownex requires convection and radiation heat transfer to be accounted for separately and thus heat exchangers are classified sequentially according to their location in the boiler, this process relies heavily on data obtained in the boiler study. Heat exchangers and auxiliary systems are then integrated into a single system used to obtain steady-state results. The steady-state boiler model is evaluated against actual boiler design data for various loads to prove applicability to various boiler designs and operating conditions.
- ItemOpen AccessDetermining appropriate loss coefficients for use in the nozzle-model of a stage-by-stage turbine model(2019) Marx, Alton Cadle; Fuls, WimA previously developed turbine modelling methodology, requiring minimal blade passage information, produced a customizable turbine stage component. This stage-by-stage turbine nozzlemodel component was derived from the synthesis of classical turbine theory and classical nozzle theory enabling the component to accurately model a turbine stage. Utilizing Flownex, a thermohydraulic network solver, the turbine stage component can be expanded to accurately model any arrangement and category of turbine. This project focused on incorporating turbine blade passage geometrical information, as it relates to the turbine specific loss coefficients, into the turbine stage component to allow for the development of turbine models capable of predicting turbine performance for various structural changes, anomalies and operating conditions. The development of turbine loss coefficient algorithms as they relate to specific blade geometry data clusters required the investigation of several turbine loss calculation methodologies. A stage-by-stage turbine nozzle-model incorporating turbine loss coefficient algorithms was developed and validated against real turbine test cases obtained from literature. Several turbine models were developed using the loss coefficient governed turbine stage component illustrating its array of capabilities. The incorporation of the turbine loss coefficient algorithms clearly illustrates the correlation between turbine performance deviations and changes in specific blade geometry data clusters.
- ItemOpen AccessDeveloping a minimal-input one-dimensional methodology for modelling axial compressors using a diffuser analogy(2023) Barlin, Daniel; Fuls, WimDespite notable advances in Computational Fluid Dynamics as a tool for gas turbine design, the analysis and performance prediction of turbomachinery still benefits greatly from the use of one- dimensional mean-line modelling, as it can provide results of acceptable accuracy while requiring fewer geometric inputs, as well as comparatively lower computational expenses. However, even mean-line codes are often limited by the amount of geometric and operational data available, often withheld by Original Equipment Manufacturers for proprietary reasons. As a result, a large number of tuning factors are often employed in mean-line simulations to achieve more accurate results. This report describes the development of a one-dimensional modelling methodology which requires minimal geometric inputs and tuning factors for predicting the performance of a transonic axial compressor stage, for use in a gas turbine. The minimal input model is achieved through development of a diffuser analogy, whereby the stage's rotor and stator rows are modelled as simple diffusers with passage flow areas approximately equal to those in the compressor stage. A design point calibration operation is first performed to estimate all diffuser flow path areas using a set of industry knowledge-based assumptions, and blade angles are determined using widely adopted correlations from open literature. The estimated geometries are then used as inputs for an off-design model built in Flownex SE. Using correlations of total pressure loss from the open literature, the Flownex model is then able to predict the stage's performance over a range of off-design operating conditions. The model was validated against a well-documented axial compressor stage from NASA through four successive models. First, Flownex's built-in Designer function was used to determine the rotor losses that would result in good agreement with experimental data of stage total pressure ratio. The second model adopted a simple total pressure loss correlation from the literature. The third model employed a single calibration factor for adjusting flow path areas, in order to obtain better agreement with the experimental data. Further definition of the area calibration factor was achieved by applying the third model to three other NASA compressor stages. Finally, correlations determined from this exercise were evaluated. The methodology presented an efficient means of predicting compressor stage performance with acceptable accuracy, while requiring minimal geometric inputs and only a single calibration factor, thus allowing gas turbine operators and analysts to gain greater insight into the compressor's off-design behaviour with the limited geometric and operational data available to them.
- ItemOpen AccessDevelopment of a performance-based high pressure feed water heater model for Flownex SE(2014) Thakaso, Matete; Fuls, WimFeed-water heaters (FWHs) are an integral part of the power plant with respect to the overall heat recovery, and thus its efficiency. While the main purpose of the FWHs is to heat up the feed-water before being sent to the boiler, there are more added advantages owing to the installation of this equipment to the power plant such as less heat being removed to the atmosphere at the condenser. This report aims at understanding the different types of FWHs in operation and understanding how heat is transferred from the bled steam into the feed-water, and using the understanding to model a transient FWH. The FWH model is a building block towards building a complete transient power plant model. Process operations are all subject to constraints of some sort. The limitations can be in the form of physical geometry, process parameters or lack of correlations that relate known data. The constraints vary in their complexity, and depending on the type of constraints a different mathematical modelling technique can be applied to implement the FWH model. The grey-box modelling technique was chosen to be the appropriate one for analysis as it captures the dynamics that depend on the first principles and correlations whilst still using the global inlet and outlet properties of the FWH. The FWHs have three zones; de-superheating, condensing and sub-cooling zones. The heat transfer characteristics of the three zones are different, and thus their heat duties with the condensing taking a significant amount of the heat duty.
- ItemOpen AccessDynamic clearance modelling of steam turbines(2022) Ross, Michael Anthony Jared; Fuls, WimWith the desire for conventional coal-fired power plants to perform flexible operations, the impact of this operation has become important to the field of steam turbine modelling. This study sought to develop a computationally inexpensive turbine model with minimal OEM intervention in order to predict the internal clearances of high-pressure and intermediate-pressure turbines from Eskom's current turbine fleet. The study saw the utilisation of the Nozzle Analogy theory to develop a 1D multistage turbine thermofluid model as well as the development of a representative 1D turbine process model in order to predict the internal temperature gradients promoted within a steam turbine during transient operation. From this model a further 3D FEA turbine model of both the HP and IP turbine units were developed from simple turbine diagrams to apply the predicted temperature boundaries and predict the thermal and structural response of turbine components during transient loading during a full Cold Start procedure. The result of this study was the successful validation of the 1D and 3D Turbine models against plant data from the candidate unit. This was in the form of known process data of unit performance, as well as thermocouple and differential expansion data taken from sensors housed on the turbine unit itself. Through the validation of these parameters, various calibrations techniques were developed over the course of the study with these techniques allowing investigators to gain insight into turbine aging, operator intervention as well as brought turbine component response. The successful establishment of the paired turbine model allowed investigators to evaluate the cold clearances defined during construction and maintenance of these turbine units in industry, which contributes greatly to the availability and efficiency of the unit during these transient operations. Additionally, the establishment of this model allowed for the investigation of the role that start up speed has on turbine component response. This study demonstrated that the development of such a modelling methodology was possible and yielded results with were accurate and insightful in understanding turbine component responses which are otherwise impossible to measure during real-world operation.
- ItemOpen AccessEvaluation of nuclear spent fuel dry storage casks and storage facility designs(2017) Hartnick, Megan Donna; Fuls, WimKoeberg Nuclear Power Station (KNPS) is the only nuclear power station in Africa and it stores its spent nuclear fuel (SNF) onsite in the spent fuel pool (SFP). Additional aged SNF assemblies are stored in dry storage casks in a facility located on the KNPS site. This minor research dissertation aims at evaluating various dry storage cask found in open literature. The dissertation provides an overview of cask types, heat transfer, radiation shielding and storage facility types. Specific criteria are required in the selection of casks and the storage facility to house the casks on site. The selection criteria for casks and the storage facility were determined and technically evaluated in this dissertation. The selected casks were evaluated in terms of SNF criticality, radiation shielding, decay heat removal and heat transfer. Other aspects also determined by calculation were the seismic stability of casks and the cask footprint. The results obtained show the relationship of the spent fuel (SF) packing density between the different casks. Different shielding materials are used in the casks and it aided the heat transfer process to take place with some casks having additional features which included cooling fins and air vents for adequate cooling of the SNF. Through these some trends could be identified which could be used in the selection or design of new storage casks. Recommendations for further study are to evaluate a greater range of casks to verify and improve upon the relationship of evaluated parameters that were shown in the technical evaluation. These casks should all have similar means of maintaining sub-criticality, shielding and heat removal in order to generate comparable results.
- ItemOpen AccessImpact of degradation of the moisture separators on the overall performance of the moisture separator reheater in a nuclear power plant(University of Cape Town, 2020) Saaymans, Natalie; Fuls, WimThe moisture separator forms part of the moisture separator reheater (MSR) component used in a steam cycle in a nuclear power plant, to reduce the risk of erosion of the low-pressure (LP) turbine and to improve cycle efficiency. The performance and optimisation of moisture separators is well studied in literature; however, there have been few investigations on the impact of moisture separator degradation on MSR performance. To investigate this impact a mathematical model, representing the steam flow through the MSR, is developed and used to simulate and analyse the impact of degradation conditions. The mathematical model was developed for design conditions, calibrated and validated against manufacturer specifications. The model was then augmented to include two moisture separator degradation conditions. The first degradation condition is the partial blockage of separator vane channels due to fouling, and the second is separator material deterioration resulting in steam bypass of the moisture separator. The model uses known properties of the MSR inlet steam and predicts the properties of steam exiting the MSR, given the simulated degradation of the moisture separator. The outcomes of the model simulations demonstrated that partial blockage of moisture separator vane channels increases steam velocity though the separator and consequently improves MSR performance, but with a noted pressure drop. The velocity increased until a theoretical upper limit, above which re-entrainment of droplets back into the steam flow reduces MSR performance. It was concluded that there is margin in the separator surface area design, where a minimal reduction in separator surface area (represented in the model as blockage of the vane channels) would improve the performance of the MSR, while still allowing for a buffer against the re-entrainment velocity upper limit. Equally, an unexpected improvement in MSR performance may be an indication of blockage of separator vane channels that, if not monitored and managed, could surpass the critical velocity limit where re-entrainment adversely affects the MSR performance. The simulation results demonstrated that steam bypass of the moisture separator is a credible degradation condition which affects MSR performance. It was found that steam bypass of the moisture separator leads to a decline in the quality of steam exiting the separator and a decline in MSR performance. The simulation of a fully bypassed moisture separator showed that the MSR performance declines by more than three times the design value when compared to the scenario where there is no bypass of the moisture separator.
- ItemOpen AccessImpact on heat rate and subsequent emissions due to varying operation of coal fired power plants(2019) Akpan, Patrick Udeme-Obong; Fuls, WimEnergy mix modellers often use a constant emissions factor model, which more or less implies a constant heat rate, when trying to show the emissions reduction benefits of integrating renewable power generation system on the grid. This approach does not consider the fact that there is a deterioration in the heat rate with load for the Coal Fired Power Plants that need to accommodate the additional renewable supply. If varying heat rate were to be included in a study, it is often limited to plant specific cases. This PhD presents a novel Variable Turbine Cycle Heat Rate (V-TCHR) model for predicting the part load Turbine cycle heat rate (TCHR) response of various Coal Fired Power Plant (CFPP) architectures, without detail knowledge of the entire steam cycle parameters. A total of 192 process models of representative CFPP architectures were developed using a Virtual Plant software. The models had different combinations of the degree of reheat; the throttle temperature; throttle pressure; and condenser cooling technology. The part load response of all the models were simulated using the software.
- ItemRestrictedImprovements to a key contributor of frequency control : the co-ordination of guide vane operation in a pumped storage plant(2014) Kippie, Mu'azzam; Fuls, WimThe South African grid has been experiencing small reserve margins from 2008 and this will continue until the new power stations are integrated onto the grid. This is a time where the frequency control performance has become an important indicator to see how the power plants are able to deal with the restoration of frequency specifically during peak times. A study was done to identify the systems/aspects which at the time contribute a considerable amount to frequency control or could affect frequency in the medium to long term. From this, the co-ordination of the guide vanes at pumped storage plants was selected. Pumped storage units exhibit a non-minimum phase characteristic which can negatively affect the frequency response on the grid. Attempts were made to improve this system’s contribution to frequency control. To achieve this, the main waterway, including surge tank and guide vanes of a pumped storage station were modelled in a thermal-hydraulic simulation environment. With the model it was possible to realise the reverse power phenomenon inherent in pumped storage plants. The model was validated with similar studies in the literature and pumped storage plant data. A selection of scenarios with various guide vane operation techniques were proposed to improve the performance of two connected units during load changes. Some of the techniques produced improvement in the reverse power characteristic while others produced deterioration in the performance due to coupling of the unit using a common penstock but enjoyed an improvement in the plant net output. The study showed that it is possible to reproduce the non-minimum phase characteristic of a pumped storage plant using thermal-hydraulic models, and that various control schemes can be tested using the model. This paves the way for more elaborate control scheme evaluations, including those that look at coordination of all the pumped storage plants on a network.
- ItemOpen AccessLow load operation of turbine-driven boiler feed pumps(2019) Clark, John Shaun; Fuls, WimBoiler feed pump turbines (BFPTs) are in use at a number of Eskom power stations. They utilise bled steam extracted from the main turbine in order to drive multistage centrifugal pumps which supply the boilers with feedwater. With an increase of renewables in the energy mix, the need for Eskom’s coal-fired power stations to run for extended periods at very low loads has arguably never been this great. Various systems affect the ability of these generation units to run economically at low loads. One such system is the boiler feed pump turbine and its associated pumps. A station was selected from Eskom’s fleet based on access to information and the station being a relatively typical plant. The Unit (a boiler and turbogenerator set) selected for study was one with the most thorough instrumentation available for remote monitoring. The BFPT system of this Unit was modelled in Flownex, a one-dimensional thermofluid process modelling package. The model included individual pump stages, steam admission valves and a stage-by-stage turbine model utilising custom stage components. These turbine stage components represent each stage with nozzles and other standard Flownex components. The boundary conditions of the system were set as functions of generator load in order to represent typical values for use in case studies. The relationships between load and boundary conditions were based on large samples of data from the station’s data capture system (DCS). A corresponding standby electric feed pump system was also modelled in Flownex for a comparative case study. After model validation, a number of case studies were performed, demonstrating the functionality of the model and also providing specific results of value to the station in question. These results include the minimum generator load possible with different steam supplies; maximum condenser back pressure before plant availability is affected; the viability of changing the pump leak-off philosophy; and the effect of electric feed pump use on power consumption. The main recommendations from the case studies were as follows: i. to stroke the steam admission valves as per the design charts, ii. to test the operation of the BFPT down to 40 % generator load, iii. to keep the pump leak-off philosophy unchanged, iv. to maintain the cooling water system and condensers sufficiently to avoid poor condenser vacuum, v. to reconsider the decommissioning of the “cold reheat” steam supply, vi. and, to favour use of the BFPT over the electric feed pumps at all generator loads.
- ItemOpen AccessModelling of the deaerator system in Flownex(2015) Banda, Richard Bobby; Fuls, WimThe study of the steady-state and dynamic behaviour of thermal power plants is of interest and significant benefit in different engineering fields ranging from research and design, to the assistance of operator training, plant optimization, fault finding and failure analysis. In light of the these benefits, and the increasing electrical energy demand in South Africa, the Eskom Power Plant Engineering Institute Centre for Energy Efficiency intends to build a transient simulation model of a coal fired power plant. The software identified for this task is Flownex SE. Flownex is a one-dimensional thermal-hydraulic solver that solves user defined networks by obtaining a numerical solution of the governing equations of fluid dynamics and heat transfer. The software contains a vast library of low level standard industrial components such as valves and pipes that can be linked together to form networks. Due to the overall size and complexity of the intended plant model, it was suggested that individual plant components be modelled separately and then integrated together to form the complete model. The primary objective of this study was to develop one such model, of a deaerator, in Flownex. In addition to being a building block for the complete plant model, the deaerator model will also be used as a standalone model to predict the steady state, transient and non-condensable gas extraction characteristics of the equipment. The first activity performed was to establish the types and operating principles of the deaerators used in industry, particularly in Eskom power stations. This was achieved through a literature survey complemented by six power station visits and a review of some assets owned by Eskom. It was established that the tray and spray type deaerators were the most commonly used deaerator types, and that their operating principle was based on the temperature-solubility relationship of gases in water and Henry's Law. Based on this knowledge, an analytical model of a deaerator was developed. The purpose of this analytical model was to serve as a verification tool for the final Flownex Model. The analytical model was developed by writing a Mathcad algorithm that solved the steady state one-dimensional mass and energy conservation equations around the deaerator boundary together with the oxygen component continuity equation. The model was successfully validated by comparing its predictions to acceptance tests data from an Eskom's Plant 1 power station. The final step was the development, verification and validation of the Flown ex model. The Flownex model was developed and successfully verified by comparing its predictions to that of the analytical model. Three case studies were then performed as a validation exercise in order to demonstrate the integrity of the model in simulating both steady state and transient scenarios. In all three studies the model predicted the unknown values satisfactorily and within acceptable error margins. It was therefore concluded that the primary objective of the study had been met.
- ItemOpen AccessSolar Augmentation of Process Steam Boilers for Cogeneration(2021) Rwezuva, Onekai Adeliade; Fuls, WimIn this study, the techno-economic feasibility of converting an existing process steam plant into a combined heat and power plant, using an external solar thermal field as the additional heat source was studied. Technical feasibility entailed designing a suitable heat exchanger, which uses hot oil from the solar field to raise the steam conditions from dry saturated to superheated. The solar field was sized to heat a selected heat transfer fluid to its maximum attainable temperature. A suitable turbine-alternator was chosen which can meet the required plant power demand. For this to be a success, the processes which require process steam were analysed and a MathCAD model was created to design the heat exchanger and check turbine output using the equations adapted from various thermodynamics and power plant engineering texts, together with the Standards for the Tubular Exchanger Manufacturer's Association. The U.S. National Renewable Energy Laboratory system advisor model was used to size the suitable solar field. A financial model was developed in Excel to check the economic feasibility of the project, using discounted payback period as the economic indicator. It was found out that amongst loan interest rates, variation of system output and the electricity output, the profitability of the project was largely influenced by the electricity tariff. An optimum size for the heat exchanger of 30ft was established from the sensitivity analysis and it was concluded that the project is currently not economically viable on an independent investor financing model, unless either the electricity tariff improves or the solar thermal energy and turbine technology costs decrease.
- ItemOpen AccessStudying water-wedging as a cause for short term overheating in the boiler of a coal-fired power plant(2018) Basson, Nicol; Fuls, WimA common failure occurrence on fossil fuel power plant boiler systems is referred to as short term overheating (STO). This phenomenon occurs when the tube is heated to higher than its design temperature in a short period of time, causing a ductile failure of the tube material. The superheaters are particularly susceptible to STO. Such a failure can be caused by various conditions, where most of these are condition-based, i.e. based on the physical condition of the pipes or boiler. However, there are some cases which are process-related, i.e. based on the thermo-physical process occurring inside the pipe. Very often a water blockage or water wedge is recorded to be the root cause of the short term overheating in superheaters when no condition-based indicators can be found. It then is claimed to be the result of over-attemperation spray by the operator. This type of failure tends to happen at the outlet of vertical (pendant-type) superheaters. This study aims to find thermo-physical conditions where such a conclusion is valid by studying the transient behaviour of a representative superheater segment under postulated conditions. The specific geometry chosen is one for which short term overheating due to water wedging has been recorded in the past. A transient flow model was constructed and verified by comparing its results with plant data, as well as some results from a numerical model developed from fundamental principles. Once the simulation modelling methodology was confirmed, the model was modified to resemble the geometry of the final superheater outlet leg to facilitate direct comparison with a pendant boiler component as found on a power plant. A number of scenarios were executed in transient state on the model at different boiler loads. The temperature evolution of the pipe wall was tracked over time, and together with calculated equivalent stresses, was compared to the yield strength of the material. A temperature vs yield strength curve was obtained from material testing using new and aged tube material. The results showed that short term overheating at the superheater outlet tubes due to water blockages alone is unlikely to occur, even at low loads and substantial over firing. The stresses exerted over the tube wall and throughout the tube length is not enough to overcome the yield stress of the superheater tube material, even for aged material. Thus, the claim of overattemperation as the root cause of a short term overheating failure is improbable, and other explanations for the failure must be observed. Even though it is possible for water-wedging to occur, the phenomenon alone is unlikely to be the root cause for the occurrence of short term overheating.
- ItemOpen AccessTechno-economic feasibility of a solar assisted coal power plant(2014) Govender, Anthony; Bennett, Kevin; Fuls, WimThe use of solar heat in conventional coal-fired power plants has been demonstrated to reduce the coal consumption of the plant. A reduction in the amount of coal that is burnt by the power plant, means that less greenhouse gases are emitted by the power plant. Hence, the plant has a smaller impact on global warming. Countries such as Australia and the USA have implemented this concept of adding solar heat to a coal-fired power plant. This study investigates if solar heat addition to Duvha Power Station in Mpumalanga, South Africa, is technically and economically feasible. Duvha Power Station is one of the largest coal-fired power stations in Eskom. Two solar heat integration options were examined in this study i.e. the use of solar heat to heat feed water or to produce superheated steam. A market assessment of concentrated solar power (CSP) technologies was performed to establish the maximum water/steam conditions (temperature and pressure) that can be produced by each CSP technology. The CSP technologies assessed were the parabolic trough collector (PTC), the linear Fresnel reflector (LFR) and the central receiver (CR).By using the results of the market assessment, a suitable CSP technology was selected for each integration option. The technical capabilities of each plant area of Duvha Power Station, such as the boiler, turbines, feed water pump etc., was also assessed by reviewing original equipment manufacturer (OEM) data sheets. The solar field size of each integration option was determined through an iterative method, such that none of the technical capabilities of the power station were exceeded once solar heat was added. The annual hourly heat output of each solar field was thereafter predicted by using the System Advisor Model (SAM).The annual hourly heat output of each solar field was then used with a thermodynamic model of Duvha Power Station(referred to as the Duvha Virtual Plant TM model),to calculate the hourly project benefits. The hourly benefits are coal savings, greenhouse gas emission reduction, solar electricity etc. The capital expenditure (CAPEX) and operating expenditure (OPEX) of each integration option was calculated by using cost models provided in SAM. The benefits and costs of each integration option were used in an economic life-cycle assessment (LCA) model, to determine the most economically feasible integration option. It was found that the integration options that produced high-temperature steam have the highest integration effectiveness, such as the steam supply to the high-pressure turbine etc. The LCA revealed that the supply of steam, by using the LFR,to the highest pressure feedwater heater (HPH6), is the most economical option.This is because the LFR technology has the lowest CAPEX and fixed OPEX cost amongst the 3 CSP technologies.This integration option has a discounted payback period of 14,6years and a real Levelised cost of electricity (LCOE) of R1.64/kWhe.
- ItemOpen AccessTemperature reconstruction and acoustic time of flight determination for boiler furnace exit temperature measurement(2018) Raikes, Geoff; Mouton, Hennie; Fuls, WimThe furnace exit gas temperature (FEGT) is one of the fundamental parameters necessary to determine the energy balance of the boiler in a coal-fired power plant, and is thus beneficial to the production of reliable thermo-fluid models of its operation and the operation of the systems down and upstream. The continuous measurement of the FEGT would also be a useful indicator to predict, prevent and diagnose faults, optimize boiler operation and aid the design of heat transfer surfaces. Acoustic pyrometry, a technique that measures temperature based on the travel time of an acoustic wave in a gas, is investigated as a viable solution for continuous direct measurement of the FEGT. This study focuses specifically on using acoustic pyrometry to reconstruct the temperature profile at the furnace exit including methods for accurately determining the time of flight (TOF) of acoustic waves. An improved reconstruction technique using radial basis functions (RBF) for interpolation and a least squares algorithm is simulated and its performance was compared to cubic spline interpolation, regression and Lagrange interpolation by evaluating its reconstruction accuracy in terms of mean and root-mean-squared (RMS) error when reconstructing set temperature profiles. Various parameters including transceiver positions, grid divisions and time of flight error, are investigated in terms of how they inform acoustic pyrometry implementation. The improved RBF interpolation function managed to reconstruct complex temperature profiles and had a greater reconstruction accuracy than compared interpolation methods, improving on the accuracy of previous work done. Random acoustic path error was found to not be additive with reconstruction error however repeating acoustic TOF readings improved reconstruction accuracy to mitigate this effect. In general, it was also found that symmetrical transmitter/receiver positions produced more accurate reconstructions as well as positioning receivers/transceivers and grid lines closer to the furnace walls, where the greatest temperature change occurs. In addition to testing reconstruction methods, a low-cost experimental set-up was constructed to measure the time of flight. The focus of this study was on using various signal processing methods to determine the time of flight and evaluating their accuracy in the presence of noise. Methods such as threshold detection with bandpass filtering, cross correlation, generalized cross-correlation (GCC) and a new method developed employing variable notch filters with locations and widths based on repetitive frequencies identified in the noise with cross correlation. The performance of methods was experimentally tested under varying signal to noise ratios (SNR) and noise conditions. These SNR tests showed that cross-correlation methods produced more reliable TOF readings under lower SNRs than threshold detection methods. Under white noise the smooth coherent transform (SCOT) GCC variation proved to produce the most accurate results producing an average TOF error of 0.84 % up until a SNR of 1.4 before reducing in accuracy. In coloured noise (generated based on previous boiler recordings) the variable notch filter method with crosscorrelation was able to identify repetitive noise frequencies filter them out and ultimately produced results with an average TOF error of 1.99 % up until a SNR of 0.67, where the noise level exceeds that of the signal.
- ItemOpen AccessThe development of a test rig to determine fouling factors of feedwater heaters(2019) Hallatt, Nicolaas; Fuls, WimFeed water heaters are large shell and tube heat exchangers. They from part of the Rankine cycle used in coal fired power plants with the main purpose being the improvement of the overall cycle efficiency. Like most heat exchangers, feed water heaters suffer from fouling. Fouling is defined as “any undesirable deposit on heat exchanger surfaces that increases resistance to heat transmission”. In the design of heat exchangers, fouling is accommodated by adding additional surface area to the heat exchanger. The amount of additional area is determined by the use of fouling factors. Although this is the only wide-spread method accepted in industry, the fouling factors in use are outdated, generally considered conservative and lead to oversized heat exchangers. The purpose of this study was to design and build a test rig that can accurately measure fouling factors of feed water heater tubes that has been in service for a full life cycle. A comprehensive literature study was performed to decide on the most effective test method, as well as the required instrument type and accuracy. The best method was found to be where the overall heat transfer coefficient for a fouled tube, outside cleaned tube (half clean) and clean tube was measured. The measured values are then converted to the internal, external and overall fouling factors. Validation test were done on the test rig. These included energy balance tests, theoretical comparison tests and repeatability tests. The results of all tests were acceptable and within measurement uncertainty limits. Five sample test tubes, obtained from a 30 year old LP heater at an Eskom power station, were tested. The results indicated that the average measured fouling factors were less than 20% of the commonly used HEI fouling factors. This is significantly lower and confirms that the fouling factors in use for this specific case are conservative. The test rig proved to be accurate and effective in measuring the fouling factors. Although the tests shows promising results, the small amount of tubes tested from only one heat exchanger are not sufficient to make meaningful conclusions. The test rig is now ready for a future study where a large sample of tubes can be tested.