Dynamic Studies of Integrated Power Plants Mikko Jegoroff a, Hannu Mikkonen a, Matti Tähtinen a, Timo Leino a, Sami Tuuri b

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1 Dynamic Studies of Integrated Power Plants Mio Jegoroff a, Hannu Mionen a, Matti Tähtinen a, Timo Leino a, Sami Tuuri b VTT Technical Research Centre of Finland (a) Box 1603, Jyväsylä, Finland (Tel: ; Mio.Jegoroff@vtt.fi, Hannu.i.Mionen@vtt.fi, Matti.Tahtinen@vtt.fi, Timo.Leino@vtt.fi) Fortum, Nuclear and Thermal Power Division (b) Box 100, Espoo, Finland (Tel: ; Sami.Tuuri@fortum.com) Abstract: The integration of concentrated solar and conventional power plants is one of the most promising and recently growing renewable technologies for electricity and heat generation in large scale. The hybrid concepts enable reduction in the consumption of fossil fuels and greenhouse gas emissions as the solar field produces part of the plant s heat/steam load. The objective of this wor was to study how the thermal stresses in a superheater tube fluctuate during a day in a combined CSP-CFB power plant with dynamical modelling in Apros. Secondary aim was to find the dynamical events taing place in the studied process concept. The thermal stresses in a superheater tube were found to fluctuate during a day in a combined CSP-CFB power plant. The variation in stress was found to be the greatest when the CSP plant was connected to the CFB steam circuit in the morning. However, the changes in stress can be decreased by better control methods. These can be developed using the Apros CSP- CFB model. The platform provides excellent base to study dynamical behaviour of the integrated energy production and to develop and optimize control strategies, upper level controls, and operational practices. Material issues can also be studied with the new material stress module. Keywords: Apros, CFB, CSP, measurement

2 1. INTRODUCTION The integrations of solar heat/power and conventional power plants have increased during the last 10 years. These hybrid concepts enable reduction in the consumption of fossil fuels and greenhouse gas emissions as the solar field produces part of the plant s heat/steam load. In general information on different plant combinations is needed. Due to the cyclic and volatile character of solar energy, both couplings and control methods of the plant must be carefully designed to achieve stable energy production in different conditions. This leads to shortened start-up curves, fast load transients, and decreased minimum load levels for the hybrid plant boiler. At the same time the tightened flexibility requirements and reduced emission limits have to be taen into account. Unsteady operating conditions such as start, stop and load level changes cause thermal transitions in all boiler heat surfaces. In the thic tube wall sections thermal stress level may rise to many times higher than during stable load. The thin-walled parts of the boiler, there is a ris that the maximum permissible absolute temperatures are exceeded. The highest riss for this are in the beginning of start-up, wherein, there is small steam flow in the superheater tubes and the temperatures of flue gas can be already significantly high. Boiler material temperatures are in danger of being exceeded even when the entered fuel power - feed water balance is disturbed as a result of a large disturbance in the process. Such a situation can be as a result of a feed water or a fuel feed disturbance. The pressure parts are subjected to cyclic or repeated stresses, caused by fluctuating loads, which taen together over the design life, shall form the fatigue loading. The fatigue loading shall comprise different loading events, i.e. well defined loading sequences which may be repeated during the design life. As solar power fluctuates constantly, the boiler part must compensate the load changes which lead to greater variation in material temperatures and heat stresses. This might decrease the lifetime of the boiler. Similar problems can also occur if the boiler controller does not always wor optimally due to the lac of information. The existing temperature and pressure measurements can directly be used to point out areas with high material stress. In addition, approximate methods which calculate material stresses from changes in temperature and pressure can be found [1]. However, the determined values cover only a few locations in the plant which are usually not enough to find the high ris fracture areas. Apros is multifunctional software for modelling and dynamic simulation of different processes. It provides easy on-line access for configuring and running the simulation models, solution algorithms and model libraries for processes such as conventional power plants, nuclear power plants, and pulp and paper mills. A dynamic simulation model allows examining the plant and process behaviour. Apros can simulate fast transients and different states of the systems without difficulties [2]. A time dependent modelling platform with a 430 MWe CFB power plant and a 70 MW solar field has been developed to Apros to analyse the performance of different integrated plant layouts. Different integration methods has been tested with the tool. The dynamic simulations have shown that the developed platform wors well [3]. Recently a module which calculates material stresses in cylindrical bodies from temperature differences and internal pressure has been implemented to Apros.

3 The objective of this wor was to study how the thermal stresses in a superheater tube fluctuate during a day in a combined CSP-CFB power plant with dynamical modelling in Apros. Secondary aim was to find the dynamical events taing place in the studied process concept. 2. TEST PLATFORM DESCRIPTION The Apros simulation environment provides easy on-line access for full-scale modelling and dynamic simulation of different processes. The model libraries have been comprehensively validated against data from physical process experiments. Besides the process, also automation and electrical systems can be modelled in detail. The simulation environment aims at meeting the requirements for testing, design, analysis, and training simulator applications [2]. Figure 1 shows a various use possibilities of Apros. Figure. 1. Various uses of Apros. Several types of solvers for thermal hydraulic networs can be used, depending on the demanded fidelity level. By using very short time steps even fast transient phenomena can be studied. Usually the user lets the time step vary based on process conditions. These results are increased in accuracy whenever needed, i.e., during fast transients, but fast calculation during normal, more stable process conditions. A powerful feature is that the simulation run can be continued promptly after any configuration change. Also, the complete model information can be saved into a model snapshot file containing the full model configuration and its momentary state data at the specified time instant. Accordingly, at any time, the user can bactrac to a snapshot once saved in the past [4]. In the solution, the model is regarded as a networ of nodes, i.e., control volumes, and branches, i.e., connections between the nodes. This calculation level networ is managed automatically by the process component level, which is the level where user operates. The primary state variables of the thermal hydraulic nodes are pressure, enthalpy and component mass fractions, and flow velocity for branches. The main physical principles are in the mass, momentum and energy equations

4 ( ) ( v ) t z (1) ( v ) ( v ) v t z 2 i g F ( h ) ( v h ) p t z t h i w F i F fl Q i Q w P F w v F i v i (2) (3) where the subscript refers to the phase, either liquid or gas. So, if written separately for both phases, six equations are obtained as the flow model name indicates. The subscript i refers to interface and w to wall. The term G describes the mass transfer between the phases, i.e. evaporation or condensation. The term F describe the different forms of friction, i.e. due to the flow channel wall, interfacial forces, valve position and form losses. P is the pressure increase by the head of a pump. The term Q denotes the heat flow between wall and fluid or the heat flow between phases. Enthalpy h is total enthalpy, so it includes the inetic energy. The terms G, F and Q are calculated from empirical correlations, taing into account the phases separately and the prevailing process conditions, such as flow regimes and wall temperature. Material property functions are used in calculating various quantities, such as density, viscosity and heat capacity according to the state variables. The equation solver provides tools for solving large systems of linear equations arising from the discretisation and linearization of partial differential equations with respect to space and time [4]. The reuse of model specification is extensively supported. The openness allows the inclusion of the user's own models in the calculation, as well as easy connection to external models, automation systems or control room equipment. The simulation session management interface enables an instructor to develop new scenarios of interest, and to follow up the training sessions in simulation training. Another type of simulation management functionality is provided to execute automation testing [5]. In addition to model configuration the modelling interface provides tools to manage simulation experiments, and to visualize the dynamic behaviour of the simulated system. The user can freely select any component variables to be displayed on the flow sheet diagrams as numerical values (monitor fields) or as trends in separate windows. The trend graphs and dynamically updated flow sheet diagrams can be easily printed out. Also, any variable data can be logged to file for post-processing purposes. At simulation time, the flow sheet diagrams can be directly used for modifying component properties on-line, e.g. controller set values, controller tuning parameters, or starting/stopping devices [5] Modelled CFB-boiler power plant The fuel flexibility is one of the most important benefits of the CFB combustion technology. Therefore it is widely used in the energy production. In the platform CFB model the air feed is divided into two phases: primary and secondary air. Primary air is fed through the grate, and secondary air is fed to furnace. Both air feeds are pre-heated with flue gas. The fed primary air fluidizes bed material into fuel particle suspension. Particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper part of the furnace and

5 then into a cyclone. In the cyclone the main part of the bed material is separated from the gas and returned to the furnace via loop seal. The hot gases from the cyclone pass heat transfer surfaces and flow out of the boiler [6]. Coal crushed to 3-6 mm size is used in CFB boilers. From storage hoppers conveyer and feeders transport the fuel to feed chutes in the furnace. Fuel combustion taes place both in the furnace and the cyclone [7]. Ash conveyors which are located under the bed, remove ash from the bottom of the furnace [6]. Figure 1 shows the main components of the CFB-boiler part of the model. Figure 2 shows a schematic of the modelled CFB power plant which produces 430 MWe electricity with live steam values 560 C and 275 bar. The fuel in the model is bituminous coal with moisture content 15 %. The furnace is modelled using the novel Apros circulating fluidized bed (CFB) module [4]. The evaporator is modelled as a once through type in which the water/steam flows through the boiler directly without recirculating it via drum. The model includes low pressure- and high pressure preheating of feed water, feed water pump and tan and all the heat exchangers needed to produce superheated steam including reheating system. After three-stage turbine island the steam flows to a condenser. All the equipment needed to control and optimize water-, steam-, air- and flue gas systems are included in the model. This provides possibilities to mae load changes, stabilize the steam values and electricity production and also controlling the intermittent CSP power production. Figure 1. Flow chart of CFB-boiler power plant [6].

6 Figure 2. The schematic of a modelled CFB-boiler power plant Modelled parabolic through solar field and integration to CFB-boiler The solar field model contains weather modules and two sets of collectors. The weather models calculate solar radiation and tracing of the collectors. The modelled solar field uses parabolic lens technique. Figure 3 shows the schematic of a modelled parabolic through solar field. The first set of collectors is used for preheating and boiling of feed water, which is connected to a steam separation drum. Separated steam is connected to superheating sections which leads the steam to a manifold. The output steam temperature from the solar field is controlled by a spray such it can be handled at the turbine island. The separated water is connected to a preheating section input to adjust the solar field feed water temperature. The feed water to the solar field is taen from the boiler condenser before pre-heating due to low temperature. The steam is used for boiler feed water pre-heating of to replace steam flow from the turbine tapping. One solar field is modelled and stream variables are scaled. The model does not contain any energy storage to stable changes of solar field steam production. When inserting steam from solar field into to the boiler process the performance of the boiler heat exchangers has to be re-evaluated [8]. Figure 3. The schematic of a modelled parabolic through solar field and CFB-boiler.

7 3. THERMAL STRESS CALCULATION 3.1. Material stress in a boiler The formation of thermal stresses in different parts of the boilers water-steam circuit is the limiting factor of the boiler start-up, load changes and shut-downs. The heat transfer coefficient between the flowing medium and the wall control the rate of the temperature change in the wall surface. When flowrate increases, heat transfer improves and therefore the wall surface temperature follows more rapidly changes in the temperature of medium. The largest thermal stresses can therefore be found in places with greatest fluid velocity differences e.g. pipes interfaces to the headers. The geometry of the structure may increase material stresses. For example, in the sharp bends of pipelines stress can be 2-3 times greater compared to the straight tube due to the resistance effect of the pressure and differences in heat transfer [1]. Insulation has also an essential effect on the material stress rates. If it is considered as ideal, the outer surface temperature follows the inner surface temperature slowly and sees steady state to the same temperature. When the outer surface is non-insulated, the outer surface temperature and inner surface temperature deviate from each other in stationary state, thus there is thermal stress also in stationary state. Practically materials are always non-ideally insulated in the power plant environment. Degree of insulation in the outer surface has a significant impact to the maximum thermal stress in all load change situations [1]. The accurate selection of the measurement points is essential to avoid material fatigue. They have to be in places where load changes have the greatest effect i.e the most critical parts of the process. Critical parts are headers, especially superheater headers and in particular the areas of pipe connection in the immediate; boiler cylinder, areas located close to the nozzles. Depending on the situation, pressure affects either the opposite or the same direction with the thermal stresses. Naturally, material stresses are the worst as they affect in the same direction and net stresses are the greatest [1]. In order to optimize the life time, at least the following issues are worth noting: the design material temperatures must not be exceeded, the nominal temperature values must not be exceeded continuously, unnecessary load changes should be avoided, the allowed rate for plant start-up and shut-down must not be exceeded, heat shocs (rapid changes in temperature) should be avoided Stress calculation in Apros A new stress solver module was made to Apros in order to facilitate the definition of the stress calculation. The foundation for the calculation is in the measurements of temperature differences and internal pressure [9-10]. In this paper these are obtained from the generated Apros model. The tangential thermal stress in a cylindrical body e.g. tube can be determined from [1] 2 r o α E σ tt = 1 μ [ r 2 + r i r 2 (r o2 r 2 i ) r θ dr + 1 r θ dr θ] (4) r2 r i r i r

8 radial thermal stress from 2 r o α E σ tr = 1 μ [ r 2 + r i r 2 (r 2 o r 2 i ) r θ dr + 1 r θ dr] (5) r2 r i and axial thermal stress from σ tz = α E 1 μ [ 2 r 2 2 o r i where α = coefficient of thermal expansion, E = elasticity modulus, μ = Poisson s ratio, r = radius, r i = radius of the inner surface of cylindrical shell, r o = radius of the outer surface of cylindrical shell, θ = temperature r o r i r r θ dr θ] (6) r i The pressure stress components are can be defined similarly [1]. The tangential pressure stress is σ pt = p (r o 2 + r 2 ) r 2 (r 2 o + r 2 i ) 2 r i (7) radial pressure stress σ pr = p (r o 2 + r 2 ) r 2 (r 2 o + r 2 i ) 2 r i (8) and axial pressure stress where p = pressure, r i 2 σ pz = p r 2 2 o r (9) i The net effect is the combination of the local thermal and pressure stresses [1]. This can stated for tangential direction σ t = σ tt + σ pt (10) radial direction σ r = σ tr + σ pr (11) and axial direction σ z = σ tz + σ pz (12)

9 3.3. Demonstration of the module: Material stress in superheater The water-steam flow circuit is modelled one dimensionally in each the calculation spaces i.e. thermal-hydraulic nodes in the Apros. In addition, the tube or wall material can be modelled if needed. The modeller can choose how many tube materials are distributed in the radial direction (thicness + material). The standard APROS thermal structure calculation resolves the inner and outer surface wall temperatures. Figure 4. The schematic of an implemented thermal stress calculation blocs before and after 1 st superheater. The stress solver module is connected to the chosen thermal-hydraulic node and the associated thermal structure. The module determines the shear stresses caused by temperature differences in the tangential, radial and axial direction the shear stresses caused by medium pressure in the tangential, radial and axial direction the net effect in the tangential, radial and axial direction the average net stress (average of the tangential, radial and axial net stress values) When flow rate increases, heat transfer improves and therefore the wall surface temperature follows more quicly changes of the medium temperature. Because of this, the largest thermal stresses are susceptible to the points where the fluid velocity on the surface of piece is the biggest. Points lie these are in the superheater tubes. The schematic of an implemented thermal stress calculation blocs before and after the superheater 1 is shown in the Figure 4. The target of the boiler control is to achieve as smooth and efficient energy production as possible. The control system must react fast enough to the load changes or other preventable disturbances that may occur. The computing capacity of the current automation and information systems provide better opportunities to tae into account the impacts of the dynamic state. In order to understand the hybrid power plant dynamics and recognize factors which affect the process dynamics and material stresses, it is important to carry out and analyse diverse process changes. The simulation studies usually include typical operation transients, such as load changes, to verify the feasibility, control strategies and operability of the process.

10 The hybrid power plant simulated is assumed to be located in Morocco and the simulation day is set to 21 st of December. The master controller set point for the produced electricity was set to constant. The solar field is connected to the CFB power plant circuit when the steam pressure and temperature generated by the CSP is high enough and vice versa. 4. RESULTS 4.1. The hybrid power plant dynamic simulation The main process measurements are shown in the Figures 5 6 during the simulated one day period. The master controller set point, which is produced electricity, is set as constant. Produced power stay relatively constant during day and it is controlled by fuel feed (Fig. 5). At control point of view fuel feed controls also feed water flow and steam pressure to the turbine section. This causes small change in the steam pressure, but temperature of the steam remains controlled (Fig. 5). The solar field is connected to the CFB steam cycle after the steam pressure and temperature from the CSP field is high enough (Figs. 7 8). The control connects the CSP field to production before 8 o cloc which decrease rapidly the fuel mass flow to the CFB. The greater the change in the solar irradiance, the faster is the change in the fuel power of the CFB (Fig. 5). Oscillation in the power production can be seen when the high pressure preheating tapping is closing and solar field steam is starting to tae place as a pre-heater (Fig. 5). Figure 5. Left: Simulated energy production of generator (MW) and fuel flow (g/s). Right: Simulated main steam temperature ( C) and pressure (MPa). Figure 6. Simulated oxygen content of flue gas (%).

11 The other characteristic is a slope in the middle of the day (Figs. 7 8). This is caused by limitations of tracing at the parabolic trough solar field and declination of the sun. The location at northern hemisphere outside the Tropic of Cancer causes increase of incidence angle which affects the produced power. The effects of date and tracing to steam generation are more precisely explained by Tähtinen et al [8]. Figure 7. Left: Simulated feed water temperature ( C) and rate of flow (g/s) from CFB-boiler to CSP field. Right: Simulated steam temperature ( C) and rate of flow (g/s) from CSP field to CFB-boiler. Figure 8. Left: Simulated temperatures ( C) of the feed water flow to CSP and the steam flow from CSP-field to the CFB-boiler. Right: Simulated pressures (MPa) of the feed water flow to CSP and the steam flow from CSP-field to the CFB-boiler. In this study heat stresses were calculated before and after 1 st superheater. The main steam values in parallel with material average net stress responses for the one day period are shown in the Figures The frequency of the stress variation correlates with the material fatigue. The net stress can be seen to oscillate when the high pressure preheating tapping is closing and the solar field steam is starting to tae place as a pre-heater (Figs. 9 11). A decrease in the stress level of the 1 st superheater tube can be seen when the solar field tae part to the energy production. This was expected because steam flow in the superheater decreases. The greater the amount of the disturbances in the process, the greater is their effect on thermal stress in the boiler materials. This can be seen in the Figures 9 11, where main steam pressure and temperature controls do not wor optimally. There is oscillation at 3-hour intervals. The oscillation is not great, but during long time this will have an effect to the life time of boiler materials. The boiler control system must react fast enough to prevent the minor load changes or other disturbances that may occur.

12 Figure 9. Left: Simulated main steam pressure (MPa) in parallel with thermal stress before and after superheater 1 (MPa). Right: Simulated main steam temperature ( C) in parallel with thermal stress before and after superheater 1 (MPa). Figure 10. Left: Simulated main steam pressure (MPa) in parallel with thermal stress before and after superheater 2 (MPa). Right: Simulated main steam temperature ( C) in parallel with thermal stress before and after superheater 2 (MPa). Figure 11. Left: Simulated main steam pressure (MPa) in parallel with thermal stress before and after intrex (MPa). Right: Simulated main steam temperature ( C) in parallel with thermal stress before and after intrex (MPa).

13 5. CONCLUSIONS The integration of a concentrated solar (CSP) and a conventional power plant is one of the most promising and recently growing renewable technology for electricity and heat generation in large scale. The hybrid concepts enable reduction in the consumption of fossil fuels and greenhouse gas emissions as the solar field produces part of the plant s heat/steam load. The model provides excellent base to study dynamical behaviour of the integrated energy production and to develop and optimize control strategies, upper level controls, and operational practices. Material technical issues are particularly important to consider in such a process. The thermal stresses in a superheater tube were found to fluctuate during a day in a combined CSP-CFB power plant. The variation in stress was found to be the greatest when the CSP plant was connected to the CFB steam circuit in the morning. However, the changes in stress can be decreased by better control methods. These can be developed using the Apros CSP- CFB model. The platform offers tools and methods to analyse and further improve interactions between intermittent renewable power generation and traditional power plants but also methods, controls and optimization strategies for hybrid power plant concepts. The main challenge in the tas is the rather low level of available design/reference data. The next step in further studies could be to find the optimised way to integrate CSP solar power into a conventional power plant. The efficiency increases by control and automation of the process. Also full model validation is needed with existing measurement data. However, both CFB-boiler plant and solar field control development must be continued. As a final result of this study, it must be noted that the direct steam generation with CSP solar field and its integration into traditional combustion power plant will set new demands for the boiler dynamic operation. The rapid load changes due to solar steam integration can cause unsteady combustion conditions, which can further lead to unexpected emission formation, gasification reactions, local hot spots and de-fluidization etc. It means that the boiler control has to achieve as smooth and efficient energy production as possible. Control system must react fast enough to the load changes or other preventable disturbances that may occur.

14 NOMENCLATURE CFB CSP Circulating fluidised bed Concentrated solar power REFERENCES [1] Härönen, S.; Lämpöjännitysten mittaaminen osana höyryattilan automaatiota. Licenciate examination, University of Tampere, Finland, [2] Lappalainen, J.; Blom, H.; Juslin, K., Dynamic process simulation as an engineering tool A case of analysing a coal plant evaporator, VGB Powertech, 1/2 2012, pp [3] Jegoroff, M.; Tähtinen, M.; Mionen, H.; Leino, T., Dynamic modelling platform for integrated power plants. 4 th Solar Integration Worshop, International Worshop on Integration of Solar Power into Power Systems, Berlin, Germany, November [4] Lappalainen, J.; Mionen, H.; Jegoroff, M.; Sanchez-Biezma, A.; Kovacs, J.; Tourunen, A., Simulation studies on oxy-cfb boiler dynamics and control, 3rd Oxyfuel Combustion Conference, Ponferrada, Spain, September 9-13, [5] Juslin, K., 2005, A companion model approach to modelling and simulation of industrial processes, VTT Publications 574, 155p+app.15p. [6] Huhtinen, M.; Kettunen, A.; Nurminen, P.; Paanen, H., Höyryattilateniia edition. Helsini, Edita. Pages 316. [7] Basu, P., Combustion of coal in circulating fluidized-bed (CFB) boilers: a review. Chem Eng Sci 1999;54: [8] Tähtinen, M.; Kannari, L.; Weiss, R.; Mionen, H., Dynamic modelling of concentrated solar power and power plant integration. 4th solar worshop, Berlin, Germany, November , 2014 [9] Boley B. A.; Weiner J. H.; Theory of Thermal Stresses. New Yor, London. Sidney, John Wiley & Sons Inc., Pages 586. [10] Fridman Y. B.; Strength and Deformation in Non-uniform Temperature Fields. New Yor, Concultanta Bureau Enterprises Inc., Pages 169.