A Three-Dimensional Model Frame for Modelling Combustion and Gasification in Circulating Fluidized Bed Furnaces

2011 ◽  
Vol 9 (1) ◽  
Author(s):  
Kari Myöhänen ◽  
Timo Hyppänen
Keyword(s):  
Heat Transfer ◽  
Fluidized Bed ◽  
Large Scale ◽  
Circulating Fluidized Bed ◽  
Three Dimensional ◽  
Process Conditions ◽  
Three Dimensional Model ◽  
Macro Scale ◽  
Semi Empirical ◽  
Meso Scale

In a large-scale circulating fluidized bed furnace, the local feeding of fuel, air, and other input materials, and the limited mixing rate of different reactants produce spatially non-uniform process conditions. To simulate the real conditions, the furnace should be modelled three-dimensionally or the three-dimensional effects should be accounted for. The fluidized beds can be studied by different model approaches, ranging from micro-scale particle models and meso-scale two-fluid models to macro-scale engineering models. The fundamentals-oriented micro- and meso-scale models are not yet capable for practical comprehensive calculations of industrial scale circulating fluidized bed units, including modelling of reactions, attrition of particles, and heat transfer. The following paper introduces a three-dimensional semi-empirical steady state model for modelling combustion and gasification in circulating fluidized bed processes. The incorporated submodels include fluid dynamics of solids and gases, fuel combustion and limestone reactions, comminution of solid materials, homogeneous reactions, heat transfer within suspension and to surfaces, models for separators and external heat exchangers, and a model for nitrogen oxide chemistry. The model structure and the main features together with a sample calculation are described. A review of the currently used model approaches for fluidized bed systems at different scales is included to relate the presented model to other modelling field and to justify the need for semi-empirical modelling approach.

Three-Dimensional Modeling and Model Validation of Circulating Fluidized Bed Combustion

2003 ◽  
Author(s):  
Kari Myo¨ha¨nen ◽  
Timo Hyppa¨nen ◽  
Jouni Miettinen ◽  
Riku Parkkonen
Keyword(s):  
Heat Flux ◽  
Fluidized Bed ◽  
Large Scale ◽  
Circulating Fluidized Bed ◽  
Combustion Process ◽  
Three Dimensional ◽  
Hot Water ◽  
Combustion Model ◽  
Process Conditions ◽  
Model Parameters

This paper presents a three-dimensional, steady state combustion model for a circulating fluidized bed (CFB) furnace and several calculation cases which have been used for the validation of the model. The model includes essential submodels to describe the complex combustion process in a circulating fluidized bed boiler. These include the hydrodynamics of the bed, devolatilization of fuel, combustion of char, combustion of hydrocarbons, carbon monoxide and hydrogen, calcination and sulfation, fragmentation and attrition of solids, heat transfer, overall mass balance of the furnace, and three-dimensional balance equations based on the finite volume method. The code was initially developed in 1989, and it has been updated and improved over the years as new methods and new information have become available. The model is used for increasing process knowledge and for studying such phenomena inside the furnace which are often difficult or impossible to study by direct measurements. The knowledge obtained is then applied to optimize boiler design and process performance in terms of efficiency, economy and environmental issues. Reliable experiments and measurements in commercial boilers are used for the validation of the model and for tuning the model parameters. For the validation of a three-dimensional model, extensive profile measurements of the various parts of the furnace are required. This paper presents validation studies for an 80 MWth hot water boiler burning bituminous coal and for a 235 MWe subcritical boiler burning lignite. The measurements with these units included profile measurements of heat flux, pressure, temperature and gas composition under different process conditions. The model was tuned according to the measurements and used for the prediction of the heat flux profile of a large scale supercritical CFB boiler.

A 3D Model of Combustion in Large-Scale Circulating Fluidized Bed Boilers

2004 ◽  
Vol 2 (1) ◽  
Author(s):  
Karsten Luecke ◽  
Ernst-Ulrich Hartge ◽  
Joachim Werther
Keyword(s):  
Combustion Chamber ◽  
Fluidized Bed ◽  
Large Scale ◽  
Gas Flow ◽  
Circulating Fluidized Bed ◽  
Combustion Process ◽  
Three Dimensional ◽  
Three Dimensional Model ◽  
Cross Sectional ◽  
Secondary Air

In a circulating fluidized bed (CFB) combustor the reacting solids are locally fed into the combustion chamber. These reactants have to be dispersed across the reactor's cross-sectional area. Since the rate of mixing is limited this leads to a mal-distribution of the reactants and to locally varying reaction conditions. In order to describe the influence of mixing a three-dimensional model of the combustion chamber is suggested. The model is divided into three sub-topics. First, the flow structure in terms of local gas and solids velocities and solids volume concentrations is described. Second, mixing of the solids and the gas phase is quantified by defining dispersion coefficients, and finally the combustion process itself, i.e. the reaction kinetics, is modelled. The model was validated against data from measurements in the large-scale combustor of Chalmers University of Technology in Göteborg/Sweden. Insufficient fuel mixing generated mal-distributions of locally released volatiles, which were the basis for the uneven reactants distribution at steady-state. In the case of two-stage operation, the injected secondary air did not reach immediately the reactor's center but was slowly mixed with the main gas flow. The concentration gradients hardly vanish before the exit of the combustion chamber.

3-D Modeling of Liquid Injection into Fluidized Beds

2004 ◽  
Vol 2 (1) ◽  
Author(s):  
Joachim Werther ◽  
Stefan Bruhns
Keyword(s):  
Fluidized Bed ◽  
Flow Structure ◽  
Three Dimensional ◽  
Fluidized Bed Reactors ◽  
Three Dimensional Model ◽  
Liquid Injection ◽  
Bubbling Fluidized Bed ◽  
Solids Mixing ◽  
Semi Empirical ◽  
Bubble Splitting

A three-dimensional model has been developed to describe the injection of liquid reactants into fluidized bed reactors operating in the bubbling fluidized bed regime. The model considers the processes of liquid transport and evaporation in the vicinity of the point of injection. The underlying idea, which is supported by previous measurements, is that the particles in the dense suspension phase are wetted by the liquid or gas-liquid spray. The wetted particles are subsequently dried while they are following the gross solids circulation within the bed. The model considers the flow structure of the bubbling fluidized bed and the solids mixing with the aid of a hybrid model which combines semi-empirical models for bubble growth by coalescence and for bubble splitting with a CFD approach for the continuous emulsion phase surrounding the bubbles. Submodels for heat and mass transfer are used to describe the temperature and concentration fields in the vicinity of the injection nozzle and the drying process of the wetted particles with the resulting release of the vaporized injection liquid. The model was validated separately against flow structure measurements, solids tracer measurements and experiments with the injection of water and ethanol, respectively, into beds of FCC particles.

Heat transfer in a large-scale circulating fluidized bed boiler

2007 ◽  
Vol 1 (4) ◽  
pp. 477-482 ◽  
Author(s):  
Leming Cheng ◽  
Qinhui Wang ◽  
Zhenglun Shi ◽  
Zhongyang Luo ◽  
Mingjiang Ni ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Fluidized Bed ◽  
Large Scale ◽  
Circulating Fluidized Bed ◽  
Circulating Fluidized Bed Boiler

Investigation on Heat Transfer Performance of External Heat Exchanger for Circulating Fluidized Bed Boiler

2013 ◽  
Vol 732-733 ◽  
pp. 11-17
Author(s):  
Da Long Zhang ◽  
Ding Hua Yang ◽  
Gen Sheng Yang ◽  
Zhong Li ◽  
Jun Fu Lv
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Fluidized Bed ◽  
Large Scale ◽  
Heat Transfer Performance ◽  
Circulating Fluidized Bed ◽  
External Heat ◽  
Transfer Performance ◽  
Cfb Boiler ◽  
External Heat Exchanger

A heat transfer model of external heat exchanger (EHE) in large-scale circulating fluidized bed (CFB) boiler was suggested based on experiment data and theoretical analysis. And the model parameters were also provided. With this model, the heat transfer performance of the EHE in an actual operating 300MW CFB boiler was predicted. The comparison between the model predictive results and the actual results showed that the relative error is less than 7%. This indicated that the model established in this paper is reliable and can provide a significant reference in EHE of large-scale CFB boiler design.

Sign in / Sign up

Export Citation Format

Share Document