Simulation of Particle-Gas Flow in a Cyclone Using URANS

Particle-gas flow in a cyclone separator used in a circulating fluidized-bed boiler is simulated using computational fluid dynamics software Fluent 6.2.36 and an Unsteady Reynolds-Averaged Navier-Stokes (URANS) method. A Lagrangian method is used for particle simulation and a one-way coupling between particles and gas is assumed. The effect of the turbulence model is studied using several turbulence models. Only the Reynolds stress model gives a physically reasonable flow field without adjusting parameters unknown beforehand

Study of Recarbonation in Circulating Fluidized Bed Combustion

Oxy-fuel circulating fluidized bed combustion (CFBC) can use calcium based sorbents, primarily limestone, for the in-situ capture of much of the sulfur dioxide in the fuel. Under oxy-fuel CFBC conditions, the CO2 content is usually high, and at high combustor temperatures sulfur capture can occur in two steps, calcination and then sulfation. The typical Ca utilization ratio in oxy-fuel CFBCs is less than half. When temperature is below the calcination temperature while remaining exposed to a high CO2 environment, recarbonation of unused CaO may occur. This reaction between calcium oxide and carbon dioxide has the potential to create serious operational problems and boiler maintenance issues. The main purposes of this study were to design a test method to study recarbonation of limestone under oxy-firing fluidized bed conditions using a test reactor and to carry out test runs using this method. The test runs were carried out in a test reactor at Metso Power Research and Development Center in Tampere, Finland

Circulating Fluidized Bed Combustion-Build-Up and Validation of a Three-Dimensional Model

This paper presents the validated simulation of a full-scale circulating fluidized bed boiler as obtained via a comprehensive three-dimensional CFB process model. The model is utilized in boiler design and scale-up as well as to study and optimize boiler performance. Feedstock characterization tests, which are also presented, are used to provide data for those parts of the process where up-to-date modeling is not fully reliable, thus enabling the model to provide accurate results

A comprehensive model of CFB combustion

This paper presents a new comprehensive model for circulating fluidized bed boilers to be used to support design and scale up as well as a learning tool of the process. The model covers the entire circulating loop and provides a 3-dimensional description of the in-furnace processes in mixing, combustion and heat transfer. The model is unique in that the input data are limited to operational data and that a large number of experimental boiler data has been used to verify the different submodels as well as the overall model. The model is built up by combination of several submodels, each covering main phenomena in the three key fields of CFB boiler modeling: fluiddynamics including gas and solids mixing, char and gas combustion and heat transfer. The modeling of the gas mixing (which governs the gas combustion, assumed to be transport controlled) has a dynamical basis in the modeled fluctuations in velocity and concentration values of the gas phase originating from the dense bottom bed dynamics. This unique feature enables verification of the model by means of time-averaged gas concentration measurements also in the lower region of the furnace. Such measurements sense fluctuations between high-velocity oxidizing gas phase and low velocity oxygen lean phase, yielding results biased towards low oxygen concentrations. The model includes phenomena such as solids backflow effect at the riser exit and corner effects in wall layers and can handle input data on fuel fragmentation. Model main output data consists of spatial distributions of the following parameters: pressure, temperature, heat flux, concentration and flow of main gas species and solid fractions (i.e. inert solids and fuel particles) and particle size distribution of solid fractions. The model gives generally good agreement with experiments from large CFB units for which required input data is available

Modeling of the solids inventory in a CFB boiler

This work presents a model for prediction of the solids inventory in CFB boilers. The model has as input the operational strategy undertaken to control the solids inventory and accounts for the internal solids size segregation in the circulating loop and solids attrition. The model is transient in that it resolves changes in the size distribution of the bed inventory over time. Model results are compared to experimental data from the Chalmers 12 MWth CFB boiler, showing good agreement

Modeling of the solids inventory in a CFB boiler

This work presents a model for prediction of the solids inventory in CFB boilers. The model has as input the operational strategy undertaken to control the solids inventory and accounts for the internal solids size segregation in the circulating loop and solids attrition. The model is transient in that it resolves changes in the size distribution of the bed inventory over time. Model results are compared to experimental data from the Chalmers 12 MWth CFB boiler, showing good agreement

Modeling of the solids inventory in a CFB boiler

This work presents a model for prediction of the solids inventory in CFB boilers. The model has as input the operational strategy undertaken to control the solids inventory and accounts for the internal solids size segregation in the circulating loop and solids attrition. The model is transient in that it resolves changes in the size distribution of the bed inventory over time. Model results are compared to experimental data from the Chalmers 12 MWth CFB boiler, showing good agreement

Modeling of the heat transfer in large-scale fluidized bed furnaces

A 3-dimensional model for the heat transfer in the furnace of a fluidized bed boiler is presented. The model, which is part of a comprehensive modeling work for large-scale CFB boilers, describes separately the convective and radiative heat transfer mechanisms at different heat extraction surfaces in the furnace (waterwalls, wing walls and division walls). The focus of the paper is on the heat transfer in the furnace, but the heat balance closure at a unit level including the return leg is also treated. Modeled data are compared to measurements from CFB boilers at two different scales: in the Chalmers 12 MWth research boiler and in a large scale boiler of about 300 MWth. Modeled and measured data generally show a good agreement. Since convective heat extraction depends strongly on the local properties of the solids flow, 3D modeling of the convective heat transfer requires other expressions than those found in literature (which are typically. based on cross-sectional averaged solids concentration). In regions with low solids concentration (upper part of furnace), the radiative heat transfer is significantly influenced from regions several meters away from the heat extraction surface. Thus, in the description of the radiative heat transfer, optical factors accounting for the absorption in the gas-solids suspension are used. The model results reveal the importance of the exchange of radiative heat between the upflowing core and the downflowing wall layers. In addition, the importance of the fluid dynamics (wall layer flow properties, local solids flow properties such as the backflow effect and corner effects,) on the heat transfer is discussed with help of the model presented

Modeling of the heat transfer in large-scale fluidized bed furnaces

A 3-dimensional model for the heat transfer in the furnace of a fluidized bed boiler is presented. The model, which is part of a comprehensive modeling work for large-scale CFB boilers, describes separately the convective and radiative heat transfer mechanisms at different heat extraction surfaces in the furnace (waterwalls, wing walls and division walls). The focus of the paper is on the heat transfer in the furnace, but the heat balance closure at a unit level including the return leg is also treated.Modeled data are compared to measurements from CFB boilers at two different scales: in the Chalmers 12 MWth research boiler and in a large scale boiler of about 300 MWth. Modeled and measured data generally show a good agreement.Since convective heat extraction depends strongly on the local properties of the solids flow, 3D modeling of the convective heat transfer requires other expressions than those found in literature (which are typically. based on cross-sectional averaged solids concentration). In regions with low solids concentration (upper part of furnace), the radiative heat transfer is significantly influenced from regions several meters away from the heat extraction surface. Thus, in the description of the radiative heat transfer, optical factors accounting for the absorption in the gas-solids suspension are used. The model results reveal the importance of the exchange of radiative heat between the upflowing core and the downflowing wall layers. In addition, the importance of the fluid dynamics (wall layer flow properties, local solids flow properties such as the backflow effect and corner effects,) on the heat transfer is discussed with help of the model presented