Simulation numérique d’un four d’incinération d’ordures industriel couplé à une chaudière à circulation naturelle

In the context of resource conservation and the rational production and use of energy, there is growing interest in Switzerland, as elsewhere, in the incineration of domesttic wastes. One of the characteristics of waste incineration cogeneration power plants is their frequent partial or excess loading. In order to describe their behavior at these non-nominal loads, there is a need for simulation tools, which predict with accuracy the behavior of the principal components of the plant at these loads. The paper presents a simulation model of an industrial waste incineration furnace coupled to a natural convection boiler. The model provides information on the internal behavior of the system, which leads to the creation of windows of operation for the incinerator/boiler system. The model was validated using measurements taken at an industrial site and forms the basis of a robust and accurate simulation tool for the description of this kind of system. With this tool, a multitude of different parametric studies can be done on geometery, waste composition, non-nominal loads, combustion gas recirculation, etc. Simulation results are given as well as results from the direct use of this model in evaluating the addition of a topping cycle to an existing waste incineration cogeneration power plant.

Radiative heat transfer in an industrial waste incinerator: Application of the zone method | Transfert de chaleur par rayonnement dans un four d'incinération industriel: Application de la méthode des zones

Daniel Favrat

In the context of resource conservation, incineration of domestic wastes is a technique allowing an interesting potential in energy valorization. However, because of the variability (annual, monthly) in quality and quantity of raw material, one of the characteristics of waste incineration cogeneration power plants is their frequent partial or excess loading. In order to describe their behaviour at these non-nominal loads, there is a need for simulation tools which predict with accuracy the behaviour of the principal components of the plant at these loads. This paper presents an original approach for predicting the heat transfer in a waste incinerator. In particular, the model developed is applied to and validated with measurements from a waste incinerator located at the Cottendart waste incineration cogeneration power plant in Switzerland. The part of the incinerator studied is characterized by high gas temperatures (1 000-500°C) for which radiative heat transfer represents the most important part of the global heat exchange. A three-dimensional model using nodal analysis is proposed. The radiative heat exchange is treated using the zone method. With a fine enough mesh, the behaviour of the system in regions far from the nominal point can be predicted with very high accuracy. This paper describes the model and the simulation results. © Elsevier.

1997

Solid oxide fuel cell stack simulation and optimization, including experimental validation and transient behavior

Diego Larrain

This thesis presents the development of models for the simulation and optimization of the design of a planar solid oxide fuel cell (SOFC) stack. Fuel cells produce electric power directly from a fuel by electrochemical reactions. The high efficiencies demonstrated make them a promising technology for energy conversion. The main challenges lie with reliability and cost reduction. Some applications demand at the same time strong requirements on the compactness of the system and its ability to load following. The models have been developed to represent the novel stack proposed by HTceramix SA (Yverdon, Switzerland) which is tested and partly developed at the Laboratoire d'Energétique Industrielle (LENI). The model has been created in a way which allows its use for design optimization: this requires detailed and validated outputs to gain insight in the behavior of a new stack design and computational efficiency to allow sensitivity studies and optimization. Electrochemistry, mass and heat transfer phenomena are combined with a 2D fluid motion description to obtain a generalized model which can be applied to a large range of geometries. An efficient stack modeling approach is proposed. Validation of the model has been carried out with measurements and a 3D computational fluid dynamics model. A methodology based on parameter estimation has been used to identify kinetic parameters and other uncertain parameters. Local temperature measurements and a local current density measurement have been performed and also used for model validation. The 2D model has been successfully validated showing good agreement with both the experiments and the detailed 3D model. Simulation of the novel stack geometry (counter-flow) has allowed to identify the main problems arising from this compact geometry where the non-homogeneous velocity field creates stagnant zones which limit the operation at high efficiency. The simulated temperatures are characterized by important gradients and excessive level values (>850°C) for an intermediate temperature SOFC (700-800°C). This motivated to work on an alternative geometry, which based on simulation results, solves most of the problems previously identified. The thesis presents several examples of the influence of design on the system performance and reliability. Transient simulations have been performed and the design choice had only a small impact on the transient behavior which presents intrinsically an important thermal inertia. On the contrary, degradation behavior is dependent on the design. Stack degradation has been simulated by including the metal interconnect degradation into the stack model. The approach has allowed to identify a new criterion to express degradation consistently for different test conditions. To assist stack design, new approaches are necessary. The geometry of a stack was initially determined by a number of decision variables (such as cell area, thickness of the channels and interconnects) on which extensive sensitivity analysis were conducted. This method is of limited use as each of the objectives on stack design led to different solutions. To overcome this limitation, multi-objective optimization has been applied to the stack design problem. Application of this method is new in this field and different optimization strategies are tested. The results from the optimization allow to identify a clear trade-off between the compactness of the stack and the temperature level (and therefore the degradation).

EPFL2005

An Approach for the Time–Dependent Thermoeconomic Modeling and Optimization of Energy System Synthesis, Design and Operation.

Daniel Favrat

From the standpoint of waste elimination and the design and operation of the systems installed, the cogeneration of electricity and heat from the incineration of wastes has become today an important issue in Switzerland. Decision models developed to minimize the total costs associated with such systems respond only partially to the problems associated with the choice of configuration (synthesis), component design, and the operation of the waste incineration/cogeneration unit. In fact, the time factor which greatly affects certain key parameters such as the amount of wastes and the electrical and heating demands placed on the system (to name just two) renders the problem of synthesis, design and operation very complex. The thermoeconomic methodology presented in this article is a general approach which can be used to respond to the questions: when to invest and reinvest and which technology, configuration, component capacity and performance and modes of operation to adopt. The problem consists of developing the thermoeconomic models (typically very non-linear, continuous and/or non- contiguous) which can be used to minimize the time integral of total costs, optimizing simulateously the configuration, design and operation of the installation over its entire economic lifetime.

1997

Solid oxide fuel cell stack simulation and optimization, including experimental validation and transient behavior

Diego Larrain

EPFL2005