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Dynamic modelling of a CFB boiler including the solids, gas and water-steam system
Abstract
Due to the widespread deployment of non-flexible renewable energy sources, an increasing trend is to use coal-fired boilers as support to the base energy production. High flexibility is usually required, and consequently, it is fundamental to be able to accurately predict and study dynamic behavior of the boilers. This point is particularly crucial for Circulating Fluidized Bed (CFB) technology. Indeed, while bringing excellent fuel flexibility and emission control, different case specific boiler configurations and arrangements induce significant variations in the dynamic behavior. This calls for a computational tool capable for accurate and specific dynamic studies of the entire system. One-dimensional process modelling and dynamic simulation, including the relevant automation functionalities, offers the best solution. Modelling and simulation of CFB boiler plants includes, however, many challenges, including modelling of combined volumes of gas and solid particles, complex fluidization phenomenon inside the furnace and in the circulating loop(s), strong coupling with the heat transfer to cooling systems, and the reactions for the combustion and sulfur capture. In addition, the model build-up phase and calculation speed shall not become limiting factors for applicability in engineering projects. This paper presents the latest enhancements in the Apros simulation environment, including improved managing of the furnace solid balance, heat exchange calculation from the circulating solids, and integration of dynamic behavior of the solid material looping. These developments were evaluated in a modelling study for a 750 MWth bituminous coal-fired CFB boiler. The modelling methodology is presented and the boiler model capability is demonstrated with simulation results.
... The process simulation in this study was carried out in APROS software, which is a multifunctional simulation program developed by Fortum and the Technical Research Centre of Finland (VTT) (Haenninen, 2009;Lappalainen et al., 2014;Lappalainen et al., 2017;Hiidenkari et al., 2019;Alobaid et al., 2020). The program can simulate thermal combustion power plants, energy, and industrial processes as well as safety analysis by using specialised component libraries for the process, automation, and electrical systems. ...
This study presents a comprehensive dynamic process simulation model of a 1 MWth circulating fluidized bed test facility applied for lignite and refuse-derived fuel co-combustion. The developed dynamic process simulation model describes the circulating fluidized bed riser and the supplying system with a high level of detail considering heat transfer, gas-solid interaction, combustion, and fluid dynamics. The model was first tuned at two steady-state operation points and was then validated by the measured data from a long-term test campaign of the 1 MWth circulating fluidized bed test facility at various loads (60%–80% to 100%). During the load changes, the simulated pressure and temperature profiles along the combustor as well as the flue gas concentrations agree very well with the measurement data. Finally, increasing the proportion of waste-derived fuel in the co-combustion process was investigated to evaluate the flexibility of its use in power generation to further reduce CO2 emissions.
... Bearing in mind the aforementioned, this work aims to develop a 1-D dynamic process model to simulate the operation of a 1 MW th pilot CFB combustor integrated with a TES system. The model is developed in the commercial software APROS, which has been used in the past for the modeling of CFB boilers (Lappalainen et al., 2017). The TES system is comprised of a BFB, which stores particles from the boiler during ramp/down operation, and returns them during ramp up. ...
In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the pilot 1 MWth CFB boiler of the Technical University of Darmstadt. Experiments have been performed with the same unit, the data of which are utilized for the model validation. The examined conditions correspond to the steady-state operation of the boiler at 100, 80, and 60% heat loads, as well as for transient conditions for the load changes from 80 to 60% and back to 80%. Fair agreement is observed between the simulations and the experiments regarding the temperature profiles in the riser, the heat extracted by the cooling lances, as well as the concentration of the main species in the flue gases; a small deviation is observed for the pressure drop, which, however, is close to the results of a CFD simulation run. The validated model is extended with the use of a thermal energy storage (TES) system, which utilizes a bubbling fluidized bed to store/return the particles during ramp up/down operation. Simulations are performed both with and without the use of TES for the load path 100–80–60–80–100%, and the results showed that the TES concept proved to be superior in terms of changing load flexibility, since the ramp up and down times proved to be much faster, and lower temperature drops between the loads are observed in this case.
... The model of the pilot CFB furnace is created with the software APROS (Advanced Process Simulation). APROS was developed by Fortum and the Technical Research Centre of Finland (VTT) [21,[24][25][26][27][28]. Process components such as pumps, fans, heat exchangers, or piping can be selected from a component library of the programme to model the process realistically. ...
The share of power from fluctuating renewable energies such as wind and solar is increasing due to the ongoing climate change. It is therefore essential to use technologies that can compensate for these fluctuations. Experiments at 1 MWth scale were carried out to evaluate the operational flexibility of a circulating fluidized bed (CFB) combustor during transient operation from 60% to 100% load. A typical load following sequence for fluctuating electricity generation/demand was reproduced experimentally by performing 4 load changes. The hydrodynamic condition after a load change depends on if the load change was in positive or negative direction due to the heat stored in the refractory/bed material at high loads and released when the load decreases. A 1.5D-process simulation model was created in the software APROS (Advanced Process Simulation) with the target of showing the specific characteristics of a CFB furnace during load following operation. The model was tuned with experimental data of a steady-state test point and validated with the load cycling tests. The simulation results show the key characteristics of CFB combustion with reasonable accuracy. Detailed experimental data is presented and a core-annulus approach for the modeling of the CFB furnace is used.
... The Apros CFB model has been developed for some years, so plenty of modelling work had already been done before this contribution [3][4]. Importantly, just before this study, the CFB model was redesigned to some extent, most notably by implementing the core-annulus approach in the solids balance of the model. ...
This work presents a dynamic core-annulus model of circulating fluidized bed (CFB) boilers which is implemented in the Apros simulation software. In describing and verifying the model, the focus is on the solids balance and heat transfer inside the furnace. The model is to be used as a training simulator and engineering tool for power plants using CFB boilers. The model behavior was verified in different simulation cases. While the model behaved realistically in most of them, also topics for model development were revealed. Further development of the model is ongoing.
... For the general cases of the dynamic simulation of the CFB furnace as listed above, [8][9][10][11][12][13] immediate interest of the simulation codes was on the steam dynamics in association with the load control system of the generated power. From the point of steam generation, the furnace side was simplified as the source of energy to the water-steam side, reflecting the lack of interest or the modeling challenges on the specific behavior of the furnace solid dynamics. ...
In a circulating fluidized bed boiler, the large thermal mass and flow characteristics of the solids strongly affect the transient response of the circulating fluidized bed loop temperature, which determines the heat transfer rate to steam flow. Therefore, it is essential to interpret the dynamic response of the solid behavior in the circulating fluidized bed loop for the stable and efficient operation of the circulating fluidized bed boiler. In this study, the dynamic simulation of the solid behavior along with the flue gas flow in a circulating fluidized bed loop was performed by applying the core-annulus approach for the solid-gas flow inside the furnace and selected models for other physical phenomena of the fluidized bed. The circulating fluidized bed loop of a commercial boiler was selected as the target system. Especially, the model simulates the characteristics of the solid behavior, such as the local solid mass distribution, and the solid flow inside the furnace and the circulating solid according to the various operating conditions. These aspects are difficult to measure and quantify in a real power plant. In this paper, the simulated furnace temperature behavior as the representative performance parameter of the circulating fluidized bed loop was discussed along with the qualitative operation experiences reported in the literature. The operating conditions include the feed rate of fuel and air, the particle size, the solid inventory and the solid circulation rate. Furnace temperature behavior was reproduced through the simulation for each operating case in the literature and was analyzed with the solid behavior along with the combustion rate and heat transfer rate of the circulating fluidized bed loop. The simulation enables quantitative evaluation of the effect of the solid behavior on the temperatures of the furnace and return part in the various operating conditions.
In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the 300 MWth circulating fluidized bed power plant of Sumitomo SHI FW. The operation of the plant is simulated under steady-state at two boiler loads: 100% and 60%, and the results for the distribution of temperature, pressure, mass and heat flow of the water/steam as well as for the flue gas composition and its thermodynamic properties are compared against performance data provided by the company, showing good agreement. Following this, the validated model is utilized to investigate the effectiveness of a thermal energy storage concept, which utilizes a bubbling fluidized bed to store particles during ramp down operation and return them to the fluidized bed during ramp up. Three dynamic scenarios are simulated for the load transition 100%-60%-100%: (a) a case without the use of thermal energy storage, (b) a case with thermal energy storage removing 20% of the solid inventory of the fluidized bed, and (c) a case with thermal energy storage removing 40% of the solid mass of the bed. The results of the three simulations are compared between them and the achieved ramp down/up rates (calculated based on the 90% method) are equal to: (a) 5.09/5.06%/min, (b) 5.64/5.73%/min and (c) 6.19/6.12%/min, respectively. These rates are higher than the current state-of-the-art rates of circulating fluidized bed power plants. The highest rates were achieved with the 3rd simulation scenario, in which, however, the live steam pressure fluctuations exceeded the technical limits of ±10 bar.
In this study, a sophisticated dynamic process model of a circulating fluidized bed furnace has been developed. The model describes the 1 MWth test facility, erected at the Technical University of Darmstadt, with a high level of detail including the air supply, the circulating fluidized bed, the flue gas path, the water-cooling system, and the control structures. The developed model was tuned using the experimental data at 82% load. After that, the load is decreased to 63% and the obtained numerical results were compared with the measurement data. Then, the dynamic load increases from 63% to 88% to 100%, followed by dynamic load decreases from 100% to 89% to 68% were simulated. During the load change, the pressure and temperature profiles along the riser were compared with measurement data, showing good agreement. Furthermore, the flue gas concentrations at the outlet of the cyclone agree very well with the values of the test facility. Using the validated model, the transient behaviour of the process variables (pressure, temperature, composition, and mass flow rates) of solids and gas flows during the combustion can be determined. Furthermore, the solid circulating, the inertia, and a variety of parameters that cannot be fully measured in the test facility were numerically obtained.
In large and complex processes use of simulators will take place more and more in the future to avoid and learn about transients and unexpected situations. There are plenty of dynamic simulation tools in the markets but most of them have been developed and used for quite specific and limited application areas. At the moment there are just few simulation tools which are capable to handle dynamically in the same simulation case challenging process areas like combustion, distillation, chemistry and control of very large process areas. For this reason, in this study, there was a need to carry out co–simulation where two dynamic simulation tools Apros and Aspen Plus Dynamics were combined together by using Matlab Simulink as an interface. The co–simulation approach was a way to construct large dynamic process simulation environment, where typical operational transients and failure situations of an oxy fired power plant were possible to test and analyze.
Under EU 7th Framework Programme (FP7) the FLEXI BURN CFB project an oxy firing concept based on circulated fluidized bed (CFB) combustion and supercritical once-through (OTU) water steam cycle was developed and successfully demonstrated [1]. This project also generated a dynamic simulation model of the 1st generation oxy firing concept, as we call it. In the EU 7th Framework Program’s project O2GEN (Optimization of Oxygen-based CFBC Technology with CO2 capture), the 2nd generation concept of CFB based oxy firing was developed. The target of the project was, at first, to increase O2 content in recirculated flue gas to see what kind of effect it has to the heat exchange and corrosion in the boiler. Secondly, the target was to carry out exergy analysis to increase the whole concept efficiency by decreasing energy consumption in the boiler, Air Separation Unit (ASU) and CO2 -purification unit (CPU). Based on these steady state analysis means the refined concept was then modelled for dynamic simulations which were carried out by using Apros and Aspen Plus Dynamics dynamic modelling tools in a co–simulation mode.
This paper deals with dynamic simulation of the 2nd generation 600 MW oxy fired CFB once trough boiler. In this paper we will present the dynamic model which includes oxy fired CFB boiler, turbine island, ASU and CPU with all needed equipment like heat exchangers, pipes, pumps, air and fuel feeding systems, distillation columns and sequestration vessels. Automation like measurements, controllers and actuators are included in the model. The simulation case selected for this paper presents an analysis what happens when one of the oxidant fans trips on boiler full load operation and how to handle this kind of failure by decreasing the boiler load to sufficient level of regarding the available oxidant feeding capacity. There were several items which were taken into account in the boiler, turbine island, ASU and CPU side when this kind of failure takes impact. The main findings in the failure analysis will be presented. Finally, discussion and future aspects of the simulation approach described will be given.
Dynamic process simulation provides a tool to evaluate operational issues of a new process concept before the plant construction. This paper studies a carbon capture and storage (CCS) capable power plant concept with a model including a supercritical once-through CFB boiler with gas and water steam sides, a turbine island, an interface from the air separation unit (ASU) and the control system to manage typical operational transients. Switching between the air and oxy firing modes is one of the key operations in oxy combustion processes. The selected mode switching strategy uses simultaneous linear ramps for the mass flows of the primary and secondary air, oxygen, and recirculated flue gas. The results show that the firing mode can be successfully switched within 25–37 min. The flue gas path difference between the air-firing and oxy-firing modes due to the flue gas recirculation causes significant differences in dynamic behaviour. The simulations emphasize importance of good control and coordination of the gas flows. Feedback control of the flue gas and/or oxidants O2 content during the mode switching is suggested to improve robustness against disturbances, for example, in oxygen delivery, flow measurements, fuel feeding and combustion.
A model describing the entrainment of solid particles in the freeboard region of a fluidized bed is proposed. Published experimental data have been used to correlate the entrainment rate near the bed surface and the elutriation rate above the bed. It has been shown that the column diameter can have an important influence on the amount of fines elutriated.
A modeling method for a large-scale circulating fluidized bed (CFB) boiler is presented. The method uses the cell approach to simulate the axial distribution of solid and gas phases in the bed and uses ‘core–annular’ construction to describe the radial distributions of solid particles and particle clusters. At the same time, it takes into account the influence of the wide size distribution of both bed and feed materials on hydrodynamics, chemical reactions and heat transfer. The steam side dynamic characteristics of the boiler are also considered based on the modular modeling method. Based on the method, simulations of static and dynamic performances of a 410t/h Pyroflow CFB boiler are carried out. The results show that the model is capable of predicting the complete performance of the CFB boiler. The modeling method has important theoretical and engineering values for the research of combustion technology, large-scale design and operating performances of a circulating fluidized bed.
VTT PublicationsVTT Publications 720 This thesis focuses on the development of the two-fluid model of the APROS simulation program. The system of constitutive equations and how equations are related to basic equations have been presented and discussed. The new non-condensable gas model, which was implemented to the two-fluid model, has been described in detail. The extension of the non-condensable gas model to the two-fluid system and the validation of the model have also been presented. The changes made to the six-equation model when the model has been applied to supercritical pressure calculation have been depicted. Finally, the author describes how the whole complicated system is verified and validated. Through the simulations, the applicability of the two-phase model for the analyses of real plant applications is substantiated and verified. In addition to this summary, the thesis consists of four publications. The first paper deals with how the CCFL (Counter Current Flow Limitation) correlations have been implemented to the code and how these correlations have been veri-fied. In the second paper, the non-condensable gas model and its implementation to the two-fluid model have been presented. The third paper describes how the sharp temperature distribution can be maintained in the liquid flow through the aid of simple higher order discretization. In the fourth paper, the modifications carried out to the two-fluid model when applied to the calculation of the super-critical pressure flow are described and discussed. This thesis focuses on the development of the two-fluid model of the APROS simulation program. The system of constitutive equations and how equations are related to basic equations have been presented and discussed. The new non-condensable gas model, which was implemented to the two-fluid model, has been described in detail. The extension of the non-condensable gas model to the two-fluid system and the validation of the model have also been presented. The changes made to the six-equation model when the model has been applied to supercritical pressure calculation have been depicted. Finally, the author de-scribes how the whole complicated system is verified and validated. Through the simulations, the applicability of the two-phase model for the analyses of real plant applications is substantiated and verified. In addition to this summary, the thesis consists of four publications. The first paper deals with how the CCFL (Counter Current Flow Limitation) correlations have been implemented to the code and how these correlations have been veri-fied. In the second paper, the non-condensable gas model and its implementation to the two-fluid model have been presented. The third paper describes how the sharp temperature distribution can be maintained in the liquid flow through the aid of simple higher order discretization. In the fourth paper, the modifications carried out to the two-fluid model when applied to the calculation of the super-critical pressure flow are described and discussed.