Adjustment Of The Nonlinear Parameters In Dynamic Simulations Of Steam Generators

In the long term, due to political decisions and changes in the market of power generation, the position of German coal-fired power plants as base load plants is in question. This fact requires further consideration for the use as highly dynamic power plant in the range of peak load coverage. Dynamic behavior assessment of steam generators related to load change operation, start-up and shut-down processes is in the focus of power plant operators also with regard to possible wear and tear of the components. Moreover, an innovative technology is being investigated. In the near future, the oxyfuel process, depicting a climate-friendly combustion technology of lignite and hard coal, will be ready for production and applied in large power plants. The use of simulation tools is a low-cost option to gain knowledge from analyses about the behavior of steam generators in dynamic operation or untypical situations without any component damage. The use of dynamic simulations is also reasonable if in addition a new combustion technology is used that is not applied in large power plants yet. Content of this paper is the modelling of power plant components for the use in complex dynamic systems and the simulation of these highly dynamic processes. The way of modelling is explained by means of an example. NOMENCLATURE t [s] time m [kg] mass p [bar] pressure ṁ [ kg s ] massflow h [ kJ kg ] enthalpy ξ [ kg kg ] mass concentration θ [◦C] temperature ρ [ kg m3 ] density v [ m kg ] specific volume g [ m s2 ] acceleration of gravity c [ J kg·K ] specific heat capacity V [ m ] volume z [m] heigth d [m] diameter L [m] length s [m] wall thickness r [m] radius A [ m ] flow cross-section α [ W m2·K ] heat-transfer coefficient ζ [−] pressure loss coefficient Q̇ [W ] heat flow T [s] time constant K [−] amplification factor x [−] output data u [−] input data Proceedings 26th European Conference on Modelling and Simulation ©ECMS Klaus G. Troitzsch, Michael Möhring, Ulf Lotzmann (Editors) ISBN: 978-0-9564944-4-3 / ISBN: 978-0-9564944-5-0 (CD) in inlet out outlet W wall F fluid INTRODUCTION Dynamic Simulations The use of dynamic simulations has rapidly increased during the last years in many fields of science and technology. The temporal transient behavior of many instationary processes is analyzed in computer models and used for optimization. Interventions like setpoint controlled operation or process control procedures often provide solutions for transient processes which are economically more efficient and also more sparing for the component. The dynamic behavior of the overall system which consists of a combination or overlap of several instationary sub-processes, can simply be depicted by the application of dynamic simulation tools which was usually impossible in the conventional way by expert knowledge. In power plant technology the focus is placed on the analysis of accident situations or simulated load changes as well as the start-up and shut-down of complete plants. The analysis may avoid possible destructions of particular plant components by incorrect parameters. Thus, the occurring economic costs caused by the blackout of large plants e.g. power plants can be minimized or even avoided. Another point is efficiency improvement in power plant operation and the consideration of aspects during the design phase of large plants. Innovative solutions are investigated and tested by means of dynamic simulation models. The low cost option for checking the dynamic behavior during the dimensioning of power plants is indispensable at vast investment costs like new building of large power plants. In the near future, part-load operation will mainly be used in conventional steam generators. For the operators, this change will raise several questions in terms of availability and economic efficiency which can simply be analyzed by dynamic simulations. Oxyfuel Process Being an innovative combustion technology, the oxyfuel process can be categorized into the line of combustion processes of lignite and hard coals. Carbon dioxide CO2, which is harmful for the climate and which arises from the combustion with pure oxygen and from the exhaust gases during distilling separation of water in high concentration, is being prepared and compressed and shall finally be stored in the underground. The oxyfuel process proceeds according to the scheme depicted in figure (fig: 1) and is described as follows: In Figure 1: OxyFuel process (6) the air separation facility, pure oxygen is provided as an oxidant. The coal is combusted with a material mixture of oxidant and recirculating flue gas. Due to the recirculation of dust and ash-less flue gas, the combustion temperature decreases to parameters permitted for steam generators with regard to component wear and tear of the heat transfer heating surfaces. The inert nitrogen missing in the oxidant is replaced by the recirculation of flue gas so that the mass flow of the combustion gas changes only marginally compared to the conventional combustion with air. Here, the composition of flue gas and its associated specific values and properties are determining for the transmission properties in the steam generator. After dust separation the flue gas is partly recirculated. The remaining exhaust gas consists basically of carbon dioxide and is cooled for water separation. It is then compressed and stored underground. Simulation Software Dynstar With the help of Dynstar, an in-house developed simulation tool of Zittau/Goerlitz University, simple control sections up to complex systems can be modeled. On a graphical surface, function blocks are placed and interconnected in a way the process simulation becomes possible. The various function block libraries contain many different function blocks which offer multiple possibilities for process modelling. For the simulation of power plant processes plant components like heat exchanger, pump, turbine stage, condenser, tube, valve, mixing or distribution elements are modeled. Special property libraries have been implemented for Figure 2: Simulation Software Dynstar (5) modelling thermodynamic processes and the application of special media like water and combustion gases. Library LibIF97 (1) is applied for water or steam and for ideal gases is used library LibIDGas (2). MODELLING Structure Of The Models For the modelling of a combustion process using a new innovative technology no measuring values are available for the user. ”Black-box” modelling is thus excluded. The lack of expert knowledge or operating point dependent design data of the steam generator components for various load cases and dynamic switching operation of a start-up process of the plant with air and a later switching to oxyfuel process makes the modelling by means of the ”grey box” model impossible as well. Thus, to the modeler must be given complete constructive and geometric data to simulate the physical laws, just as in the ”glass box” model in figure (fig: 3) (3). The function blocks T · ẋ + x = K · u T = f (a, b, c) K = f (a, b, c) ”’glass box”’