J.C. Abanades

The maximum capture efficiency of CO2 using a carbonation/calcination cycle of CaO/CaCO3

Abstract The use of natural calcium carbonates as regenerable CO2 sorbents in industrial processes is limited by the rapid decay of the carbonation conversion with the number of cycles carbonation/calcination. However, new processes are emerging to capture CO2 using these cycles, that can take advantage of the intrinsic benefits of high temperature separations in energy systems. This work presents an analysis of a general carbonation/calcination cycle to capture CO2, incorporating a fresh feed of sorbent to compensate for the decay in activity during sorbent re-cycling. A general design equation for the maximum CO2 capture efficiency is obtained by incorporating to the cycle mass balances a simple but realistic equation to estimate the decay in sorbent activity with the number of cycles.

Minimum fluidization velocities of fluidized-bed coal-combustion solids

Abstract The minimum fluidization velocities of coal, char, limestone, lime and partially sulphated lime under typical AFBC conditions and at room temperature have been determined experimentally. On fitting the experimental data to a Wen and Yu type equation, the two fitted parameters are found to be highly sensitive to the fitting method used to obtain them. The same occurs with the results of other authors, and different pairs of parameter values fit the experimental data reasonably well. After a statistical analysis based on Montecarlo simulations, the most suitable method for fitting the experimental data from these studies has been selected. With this method, parameters for use in the Wen and Yu equation for umf under AFBC conditions have been obtained.

A calibration procedure to obtain solid concentrations from digital images of bulk powders

Abstract In this work, different approaches to obtain calibration curves of powder concentration from digital images of bulk materials are reviewed and compared. It is shown here that a logarithmic relation between the grey scale values of the bed images and the powder volume fraction produces a high quality calibration curve without the need of fitting constants. This conclusion is valid both for our experimental data and other previously published that have used polynomial equations to define calibration curves.

Methods for characterization of sorbents used in fluidized bed boilers

Abstract Limestone addition to fluidized bed coal combustors permits the reduction of SO 2 levels emitted into the atmosphere. However, for its industrial use it is necessary to develop simple laboratory methods for the determination of parameters that, inserted in a model, permit the reliable prediction of the behaviour of the sorbent in the boiler. In this paper, the principal methods used for the characterization and determination of these parameters, thermogravimetric and batch fluidized bed, are analysed. Three limestones with different pore size distributions and hardness have been used for this analysis. The effect of the principal process variables affecting sulfur retention, i.e. calcination conditions, sorbent particle size, temperature and SO 2 concentration, has been analysed and compared using the two above mentioned methods. The very different sorbent behaviour, depending on particle size, requires the use of different particle size intervals for the characterization. Moreover, different effects of CO 2 and SO 2 concentration, over the sorbent sulfation behaviour in each method, have been found. The characterization parameters obtained depend on the method and operation conditions used. Therefore, different predictions of limestone requirements in the boiler are obtained. Depending on the type of pore size distribution in the limestones, thermogravimetric analysis cannot be used for sorbent characterization. Batch fluidized bed characterization must be made at an average CO 2 and SO 2 partial pressure, similar to those present during coal combustion.

The use of two different models to describe the axial mixing of solids in fluidised beds

Two widely used models to describe axial solid mixing in fluidised beds (the dispersion model and the countercurrent backmixing (CCBM) model) are evaluated against identical sets of experimental data. Experimental work has been obtained at different conditions (gas velocity, particle properties and two column diameters) using an image analysis technique. Previously published data by other authors are also compiled to enlarge the experimental database for model development and validation. It is shown that both models are capable to fit the majority of experiments well, in agreement with a well-known relation between the models in some extreme conditions. This relation is further explored by incorporating independent measurements of the tracer rise velocities during the mixing experiments. It is concluded that, although a simple correlation for the solid dispersion coefficients compiled in this work is useful, the CCBM model is a much more reliable idealisation in describing and scaling up axial solid mixing in fluidised beds.

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.

Carbon efficiency in atmospheric fluidized bed combustion of lignites

Abstract The effect of operating conditions (coal particle size, temperature, percentage of excess air and linear gas velocity) on carbon combustion efficiencies obtained in an atmospheric fluidized bed combustor with four lignites from the Teruel basin was studied. The working conditions affect the combustion efficiency of each coal differently. To explain this, a simplified model has been developed. The model uses an adjustable parameter to take into account the differences between some factors which affect the combustion efficiency: intrinsic reactivity and modification of the physical size of the coal. A comparison between experimental and theoretical results gave an acceptable fit.

Effect of formulation of steady-state heat balance for char particles on AFBC modelling

Abstract For the solution of population balances of char particles burning in the bed in modelling of an atmospheric fluidized-bed combustor (AFBC), a submodel for calculating the surface temperature of reacting char particles is included by solving the corresponding heat balance. This particle heat balance is strongly interrelated with other submodels through the kinetic term of heat generation at the reaction surface. Three possible formulations for calculating the particle temperature are analysed: 1. (a) without particle heat balance; 2. (b) heat balance around the particle with a log mean of oxygen concentration; 3. (c) heat balance around the particle at each height in the bed. These assumptions are introduced into a model for an AFBC without a plume of volatiles, with slow bubble regime in the bed and with plug flow of gas, using the shrinking unreacted-core model and considering char losses by carryover and overflow. The effect of incorporating these hypotheses in the model is analysed through the solution of the population balance. The results obtained indicate that the effect is appreciable on the carbon combustion efficiency and carbon loading in the bed.

Calcium looping CO2 capture system for back-up power plants

This paper analyses a CO2 capture system based on calcium looping, designed for power plants that operate with very low capacity factors and large load fluctuations, including shut-down and start-up periods. This can be achieved by decoupling the operation of the carbonator and calciner reactors and connecting them to piles filled with CaO or CaCO3. When the power plant enters into operation, calcined solids are fed into the carbonator to provide the necessary flow of CaO for capturing CO2 and storing the carbonated solids. An oxy-CFB calciner designed to have a modest thermal capacity and operate continuously refills the CaO pile. Mass and energy balances of the entire system, combined with state-of-the-art performance criteria for reactor design, have been solved to identify suitable operating windows. An analysis of the effect of the CaO reactivity of the material stored in the piles indicates that temperatures of around 500–600 °C in the carbonator are compatible with the storage of solids at low temperature (<250 °C). This, together with the low inherent cost of the material, allows large piles of stored material. Electricity costs between 0.13–0.15

Calcium looping reactor design for fluidized-bed systems

per kWhe are possible for the system proposed in contrast to standard CaL systems where the cost would increase to above 0.19