Pilar Gayán

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.

Syngas combustion in a chemical-looping combustion system using an impregnated Ni-based oxygen carrier

Greenhouse gas emissions, especially CO2, formed by combustion of fossil fuels, highly contribute to the global warming problem. Chemical-Looping Combustion (CLC) has emerged as a promising option for CO2 capture because this gas is inherently separated from the other flue gas components and thus no energy is expended for the separation. This technology would have some advantages if it could be adapted for its use with coal as fuel. In this sense, a process integrated by coal gasification and CLC could be used in power plants with low energy penalty for CO2 capture. This work presents the combustion results obtained with a Ni-based oxygen carrier prepared by impregnation in a CLC plant under continuous operation using syngas as fuel. The effect on the Manuscript Click here to view linked References

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.

Modelling of sulfur retention in circulating fluidized bed combustors

The effects of operating conditions (linear gas velocity, CaS molar ratio, type of coal and limestone, etc.) on sulfur retention efficiency in a circulating fluidized bed combustor were studied. The operating conditions affect the sulfur retention differently. To explain this, a mathematical model was developed. To determine axial voidage profiles, the model uses a modified exponential decay hydrodynamic model, which divides the bed into a dense region at the bottom of the bed and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The SO2 retention rate is considered to depend on the bed height through the SO2 profiles generated by the coal devolatilization and char combustion rates, which change with the bed height. The model gives good predictions of the effect of the operating variables on sulfur retention in a CFB pilot plant of 20 cm i.d. and 6.5 m height, burning two different lignites and an anthracite with four types of limestone.

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.

Development of Oxygen Carriers for Chemical-Looping Combustion

This chapter discusses the development of oxygen carriers with enough reduction and oxidation rates, resistant to the attrition and with high durability, maintaining the chemical, structural and mechanical properties in a high number of reduction-oxidation cycles, to be used in a chemical-looping combustion (CLC) system. A significant number of oxygen carriers, composed up to 80% of Cu, Fe, Mn or Ni oxides on Al 2 O 3 , sepiolite, SiO 2 , TiO 2 or ZrO 2 , are prepared by different methods, and tested in a thermogravimetric analyzer (TGA) and in a fluidized bed. Based on data of crushing strength, reactivity, attrition, and agglomeration of the carriers and its variation during successive reduction-oxidation cycles, the three most promising oxygen carriers based on Cu, Fe, and Ni are selected and prepared to be tested in a pilot plant. The effect of the main operating variables, such as temperature, gas composition, and gas concentration on the reduction and oxidation reaction rates are analyzed in a TGA to determine the kinetic parameters of the selected carriers.

Optimizing the Fuel Reactor for Chemical Looping Combustion

A mathematical model for a bubbling fluidized bed has been developed to optimize the performance of the fuel reactor in chemical looping combustion systems. This model considers both the hydrodynamic of the fluidized bed (dense bed and freeboard) and the kinetics of the oxygen carrier reduction. Although the model is valid for any of the possible oxygen carriers and fuels, the present work has been focused in the use of a carrier, CuO-SiO2 , and CH4 as fuel. The shrinking core model has been used to define the particle behavior during their reduction. The simulation of the fuel reactor under different operating conditions was carried out to set the operating conditions and optimize the process. The effect of different design or operating variables as the bed height, the oxygen carrier/fuel ratio, and the gas throughput was analyzed. Finally, a sensitivity analysis to the solid reactivity, the bubble diameter, and to the gas/solid contact efficiency in the freeboard was done. At vigorous fluidization, solid present in the freeboard can strongly contribute to the gas conversion in the fuel reactor. However, the gas/solid contact efficiency in this zone must be determined for each particular case.Copyright