L.F. de Diego

Calcination of calcium -based sorbents at pressure in a broad range of CO2 concentrations

The calcination reaction of two limestones and a dolomite with different porous structures was studied by thermogravimetric analysis. The effects of calcination temperature (1048-1173 K), particle size (0.4-2.0 mm), CO2 concentration (0-80 %) and total pressure (0.1-1.5 MPa) were investigated. SEM analysis indicated the existence of two different particle calcination models depending on the sorbent type: a shrinking core model with a sharp limit between the uncalcined and calcined parts of the particle and a grain model with changing calcination conversion at the particle radial position. The appropriate reaction model was used to determine the calcination kinetic parameters of each sorbent. Chemical reaction and mass transport in the particle porous system were the main limiting factors of the calcination reaction at the experimental conditions. A Langmuir-Hinshelwood type kinetic model using the Freundlich isotherm was proposed to account the effect of the CO2 during sorbent calcination. This allowed us to predict the calcination conversion of very different sorbents in a broad range of CO2 partial pressures. Total pressure also inhibited the sorbent calcination. This fact was accounted by an additional decrease in the molecular diffusion coefficient with increasing total pressure with respect to the indicated by the Fuller’s equation.

Circulating fluidized bed combustion in the turbulent regime: modelling of carbon combustion efficiency and sulphur retention

A model has been developed considering the hydrodynamic behaviour of a turbulent circulating fluidized bed, the kinetic of coal combustion and sulphur retention in the riser. The hydrodynamic characteristics of the turbulent fluidization regime were integrated together with the kinetic submodels of char combustion and sulphur retention by limestone. From the combustion of a lignite and an anthracite with limestone addition in a hot CBF pilot plant of 20 cm internal diameter and 6.5 m high, the effect of operating conditions such as temperature, excess air, air velocity, Ca/S molar ratio, coal and limestone particle size distributions on carbon combustion efficiency and sulphur retention were studied. The experimental results were compared with those predicted by the model and a good correlation was found for all the conditions used.

Axial voidage profiles in fast fluidized beds

Abstract The axial solid distributions in a circulating fluidized bed of 10 cm i.d. and 4 m height have been studied. These distributions are measured at room temperature with two solids (sand and coal) from group B of the Geldart classification, as a function of air velocity and solids flux. These profiles are S-shaped or non-S-shaped depending on the operating conditions, and are similar to those reported in the literature with other types and sizes of solids. The Kunii and Levenspiel exponential-decay model for the axial voidage profiles has been modified to consider a freeboard height lower than the transport disengaging height (TDH), developing some equations to obtain the axial voidage profiles at heights lower than the TDH. Two equations have been proposed to calculate the K constant and the decay constant a as a function of the operating conditions. The modified model gives a good fitting of the experimental results obtained in this work and those obtained by other authors in different conditions and experimental set-ups.

Transport velocities of coal and sand particles

Abstract Transport velocities of narrow cut sizes of coarse particles of sand and coal were determined at room temperature and atmospheric pressure. These velocities were obtained by four different methods previously utilized by other authors with fine particles. The four methods tested gave good predictions of the transport velocities. The method based on the measurement of the time required for all the solids to leave the bed without feeding in any fresh solid is specially interesting because of its rapidity and simplicity. The determined transport velocities were strongly dependent on the solid particle size and density. The experimental values were fitted to an equation which fitted both the experimental results obtained in this work and other published results obtained with fine particles.

Radial gas mixing in a fast fluidized bed

Abstract A steady-state dispersion model was used to determine the radial gas mixing coefficients Dr in the dilute region of a cold fast fluidized bed, 0.1 m i.d. and 4 m high. CO2 used as tracer was injected into the centre of the bed from a point source. Radial concentration profiles of tracer gas were measured in two planes downstream of the injection point for a broad range of air velocities and solid fluxes, using two sizes of sand particles of 710 and 380 μm. A systematic experiment was carried out to find out the main variables acting on the radial gas mixing in the dilute region of a fast fluidized bed. An analytical solution derived by Towle and Sherwood for the description of radial gas mixing in turbulent single-phase flow was applied to determine the radial dispersion coefficients, which were found to be dependent on the superficial gas velocity and solid circulating flux. The Dr values were well correlated with an apparent suspension Reynolds number (Re) by an equation of the type Dr = aReb. The proposed equation allows the radial gas coefficient to be predicted as a function of the air velocity and external solid flux present in the riser. This equation, with its corresponding parameters, is applied to the results of other authors and an acceptable fit was found. The high Pe numbers (500–2000) obtained in the dilute region of the fast fluidized bed indicate that the flow of the gas in the dilute region approximates to plug flow.

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

Abstract The effects of operating conditions (coal particle size, temperature, excess air and linear gas velocity) on carbon combustion efficiency in a circulating fluidized bed combustor were studied. The operating conditions affected the combustion efficiency of each coal differently. To explain this, a mathematical model was developed. To determine axial voidage profiles the model uses a modified version of the hydrodynamic model of Kunii and Levenspiel, which divides the bed into a dense region at the bottom and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The model gives good predictions of the effect of the operating variables on carbon combustion efficiencies obtained in a CFB pilot plant burning two different lignites and an anthracite.

Modelling of the flow structure in circulating fluidized beds

Abstract Radial solids flux profiles, at different axial positions, gas velocities, particle sizes and solids circulating fluxes, in a cold circulating fluidized bed were measured with two solids (sand and coal), type B of the Geldart classification. These profiles showed a core-annulus flow structure with upward solids flux in the core in a dilute suspension and downward solids flux in the annulus in a denser suspension. The upward and downward solids fluxes decreased with the height in the dilute region. This demonstrated there was a net solid transfer from the core to the annulus. A mathematical model for prediction the solids flow structure in the circulating fluidized bed was developed. The proposed model is based on a core-annulus flow structure, with solids rising in the core in a dilute suspension and solids descending in the annulus in a denser suspension with a constant solid velocity and voidage changing with the height. The model allows us to predict the external solid circulation flux in a riser as a function of the operating conditions. Moreover, it gives predictions on the internal flow structure at different riser heights. To solve the model it was necessary to know the solids flux rising from the dense region and the constant for the solid dispersion from the core to the annulus. Two equations are proposed to determine them as a function of operating conditions. The developed model together with the equations proposed in this work gave a good fit of experimental results.

Determination of coal combustion reactivities by burnout time measurements in a batch fluidized bed

Abstract A rapid procedure to obtain combustion rate equations useful in the modelling of AFBC boilers is proposed, based on the technique of burnout time measurements in batch fluidized beds. To improve the reliability of the apparent kinetic parameters, the variation of reaction temperature and Sherwood number as the particle shrinks is considered. To avoid phase-exchange resistance, experimental burnout times are obtained under differential conditions. The surface temperature of the particles is considered in the data analysis as a function of the char particle size and the kinetic rate constant. Valid results can be obtained by a simple experimental technique and assumptions in deriving the kinetics and analysing the data. Some uncertainty in the individual values of the activation energy and the pre-exponential factor (especially for lignites) is observed in the fitting process. Nevertheless, the apparent kinetic parameters obtained for five lignites and two anthracites are satisfactory, and ready to use in a global model of an AFBC boiler, in which overall kinetic information is used to define the rate of shrinkage of the fuel particles.

The effect of the porous structure on sorbent sulfation under coal-fired boiler conditions

Abstract A study of the behavior of different sorbents (3 limestones, a commercial calcium carbonate and a commercial calcium hydroxide) in the sulfation process has been carried out in a drop-tube, using similar conditions to those existing in sorbent injection in pulverized-coal boilers. The main variables affecting the sulfation process have been analyzed, namely, Ca S molar ratio, SO2 concentration, temperature, sorbent particle size and reaction time. The work concentrates on the evolution of the porous system in the sorbents and its influence on their sulfation capacity. A direct relation between chemical structure (calcium hydroxide-calcium carbonate) and sorbent reactivity was not found. Sorbents with a widespread pore size distribution with pores above 100 A showed the highest reactivity and sulfation capacity, maintaining pores of small size (under 80 A) at all times and operating conditions. In the less reactive sorbents, pores less than 80 A became blocked during sulfation, being more sensitive to the effect of the different operating variables affecting the sulfation process.

Modelling and simulation of the sulphur retention in circulating fluidized bed combustors

To optimise the circulating fluidized bed coal combustion (CFBC) with sorbent addition and make predictions about the combustor behaviour in a broad range of operating conditions it is necessary to make a mathematical model of the process. This model was developed integrating hydrodynamic and sulphation kinetic submodels. A simulation, from the point of view of sulphur retention, was made. The variables most affecting the sulphur retention were the pressure drop, the gas velocity, the Ca/S molar ratio, the efficiency of solids recovery by the cyclone and specially the sorbent reactivity and its particle size distribution through the amount of small particle sizes fed in the combustor.