Co-Combustion of Biomass and Coal in Circulating Fluidized Bed: Modeling and Validation

2003 ◽  
Author(s):  
Juan Ada´nez ◽  
Luis F. de Diego ◽  
Pilar Gaya´n ◽  
Francisco Garci´a-Labiano ◽  
Andre´s Cabanillas ◽  
...  
Keyword(s):  
Fluidized Bed ◽  
Circulating Fluidized Bed ◽  
Plug Flow ◽  
Weighted Average ◽  
Combustion Efficiency ◽  
Operating Conditions ◽  
Experimental Results ◽  
Hydrodynamic Characteristics ◽  
Particle Model ◽  
Carbon Combustion

In this work carbon combustion efficiencies in circulating fluidized bed combustion (CFBC) when co-firing biomass and coal mixtures were studied. Experimental results were obtained from the combustion of two kind of coals with a forest residue (Pine bark) in a CBF pilot plant (0.3MWth) with 20-cm i.d. and 6.5-m height. The effect of operating conditions such as percentage of biomass in the feed, temperature, excess air, air velocity and percentage of secondary air on carbon combustion efficiency was studied. A mathematical model for the co-combustion of coal and biomass in a circulating fluidized bed boiler has been developed. The riser is divided in three zones with different hydrodynamic characteristics: bottom, splash and freeboard. The bottom bed has a constant voidage, determined by a modified two-phase theory. The solids are considered in perfect mixing and the gas in plug flow. The voidage in the splash region follows an exponential decay model. In the freeboard region, the solids and the gas are in plug flow, and a core-annulus structure is considered. Devolatilization of solid fuels is modeled with a particle reaction model which allows to determine the volatiles generation rate as a function of time and operating conditions. Kinetics of char combustion is modeled with the shrinking particle model with mixed control by chemical reaction and gas film diffusion, assuming that the ashes separate once formed. To consider that the char particles are a mixture of coal and biomass char particles, a weighted average combustion rate is defined taking into account the individual combustion rates. Population balances of char particles in the different regions were developed to calculate carbon concentrations. The developed model can predict the different gas concentrations along the riser, such as oxygen, SO2, CO, CH4, etc..., and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies and gas emissions were compared with those predicted by the model and a good correlation was found for all the conditions used.

A model for prediction of carbon combustion efficiency in circulating fluidized bed combustors

Fuel ◽  
1995 ◽  
Vol 74 (7) ◽  
pp. 1049-1056 ◽  
Author(s):  
J. Adánez ◽  
L.F. de Diego ◽  
P. Gayán ◽  
L. Armesto ◽  
A. Cabanillas
Keyword(s):  
Fluidized Bed ◽  
Circulating Fluidized Bed ◽  
Combustion Efficiency ◽  
Carbon Combustion

scholarly journals Biological treatment of water contaminated by hydrocarbons in three-phase gas-liquid-solid fluidized bed

2013 ◽  
Vol 8 (1) ◽  
pp. 9-15
Keyword(s):  
Mass Transfer ◽  
Fluidized Bed ◽  
Biological Treatment ◽  
Plug Flow ◽  
Point Of View ◽  
Operating Conditions ◽  
Solid Particles ◽  
Hydrodynamic Characteristics ◽  
Intimate Contact ◽  
Three Phase

Biological treatment has been carried out in two different systems: aerated closed and threephase fluidized bed reactors for hydrocarbons removal from refinery wastewaters. For the two systems, hydrodynamic study allowed the determination of operating conditions before treatment experiments. Then, in a second time, biological treatments have been conducted in the same operating conditions. The obtained results showed that in the three-phase fluidized bed we can degrade hydrocarbons more rapidly than in a closed aerated bioreactor. Among the different appropriate techniques available to create efficient contacts between phases, the three-phase fluidization G/L/S where carrier particles are moving inside the reactor seems very interesting. It allows an intimate contact between phases and present many advantages concerning hydrodynamic and mass transfer phenomena. In fact, depending on operating conditions and the bubble flow behaviour, the three-phase fluidized bed could display different flow regimes In these systems called bioreactors the solid particles covered with a biofilm are fluidized by two ascending flows of air and contaminated water. With favourable operating conditions, from a hydrodynamic and mass transfer point of view, the pollutant can be biologically degraded up to 90%. Until this date, the three-phase bioreactors modelling remains very complex because it required taking into account several factors: the pollutant biodegradation rate in the biofilm, the bioreactor hydrodynamic characteristics, and the reactant interfacial gas-liquid and liquidsolid mass transfer. Thus the essential purpose of modelling is to integrate the microbial kinetics with the reactor hydrodynamics. We can notice that a few models have incorporated both bioreactor hydrodynamics and microbial kinetics. For the steady state bioreactor model, we generally assume that the particles are uniform in size, the biofilm is uniform in thickness, and the biofilm can be considered as homogeneous matrix through which oxygen and substrate diffuse and are consumed by the microbes. The liquid phase in the bioreactor substrate is considered to be axially dispersed while the gas phase is assumed to be in plug flow [2]. Rittmann (1997) proposed a model based on wake theory for predicting bed expansion and phase hold-ups for three-phase fluidized bed bioreactors. In this model he modified the correlation for the computation of the bioparticles drag coefficient CD [3]. He also attempted to explain the biofilm detachment which can occur with three broad patterns: erosion, sloughing and scouring and assumed that the factors affecting detachment rates can be grouped into two categories (physical forces and microorganisms physiology in the biofilm).

Experimental and Theoretical Information on Solid Flow in a Circulating Fluidized Bed

1995 ◽  
Vol 117 (2) ◽  
pp. 133-141
Author(s):  
F. R. Steward ◽  
M. F. Couturier ◽  
S. Poolpol ◽  
S. Wang
Keyword(s):  
Fluidized Bed ◽  
Circulating Fluidized Bed ◽  
Equations Of Motion ◽  
Flow Patterns ◽  
Operating Conditions ◽  
Laboratory Scale ◽  
Experimental Results ◽  
Solid Flow ◽  
Model Based ◽  
Fundamental Equations

Data are presented on a laboratory-scale circulating fluidized bed for various column geometries, solid fluxes, and gas velocities. Particular attention is paid to the solid flux profiles under various operating conditions. A model based on the fundamental equations of motion has been developed to describe the flow patterns of the solids particles and gas within a circulating fluidized bed. The predictions of the model are compared with experimental results. The model predicts that instabilities at the base of the bed propagate up the column for significant distances before uniform flows of solids and gas are established.

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