Modelling and simulation of different gasifiers with respect to deposition behaviour

Abstract

Consulta en la Biblioteca ETSI Industriales (Riunet)

[EN] In this thesis, the gasification process parameters and the gasifier geometry are modified with the purpose of determining how they influence on the ash deposition behaviour in en-trained-flow gasifiers and their effects on the process performance. A parameter study in order to find factors that influence the deposition behaviour has been obtained. Ash deposition and the formation of a slag layer on the gasifier’s wall affect the heat transfer through the wall, the operating maintenance and the service life of the units, and the amount of fly-ashes in the resulting syngas, among other harmful effects. This leads to the reduction of the gasification process efficiency. Therefore, the gasifier design has to be made focusing on the ash deposition. That is the reason why the knowledge of how the process parameters and geometric aspects influence this mechanism is essential. In order to achieve this purpose, a three-dimensional geometry of an entrained-flow Prenflo gasifier is created, where the gasification conditions are simulated. For this, a gas-ification model describing these conditions is implemented in ANSYS Fluent. The Eddy Dis-sipation Concept (EDC) model is used to set the necessary gasification reactions and the Discrete Phase Model (DPM) is activated to predict the particle wall interaction. Further-more, the multiple surface reactions model is selected in order to define how the coal parti-cles interact with the oxidant, resulting in ash particles. These particles impact with the gas-ifier’s wall following an elastic-plastic model, implemented in Fluent through a User-Defined Function (UDF). Finally, typical values of process parameters have been selected for the study. Certain simplifications have had to be made in this work with the purpose of making computation possible in a reasonable period of time with limited computing and memory resources. These simplifications have been made both in gasification reactions and in the consideration of clean walls, focusing in the prediction of the first slag layer, and not in the slag flow. It has been verified that the adhesion probability of the melted ash particles on the wall and the ash concentration on the wall have a direct relationship with the increase of the injected coal particle diameter and with the amount of ash content in the coal. On the other hand, they have an indirect behaviour with the increase in the injected oxidant and fuel flow velocity, with the increase in the wall temperature and in the operating process pressure, and with the temperature of critical viscosity of the coal ash. Regarding the geometry vari-ations, the adhesion probability and ash deposition on the wall have an indirect relationship with the higher location of the burners. We can also verify that the more the bottom is in-clined, the thinner is the slag layer on it.