Juan Adánez

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

The grace project: Development of oxygen carrier particles for chemical-looping combustion. Design and operation of a 10 kW chemical-looping combustor

Publisher Summary A comprehensive research program was undertaken to develop chemical-looping combustion technology. First, a large number of possible oxygen-carrier materials were produced and tested with the best ones selected for further testing and development. Second, fluidization conditions and recirculation flows were studied in cold-flow models indicating the feasibility of both the full-scale design and a small 10 kW prototype unit. Thirdly, a 10 kW prototype unit was constructed and operated, demonstrating both this new combustion process as well as the durability of the oxygen-carrier particles under realistic process conditions. Lastly, a preliminary costing of a 200 MWth chemical-looping boiler unit for refinery gas combustion has been performed, indicating that chemical-looping combustion (CLC), should feature strongly among the best options for reducing the cost of CO2 capture.

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.

Kinetics of a lignite-char gasification by CO2

Abstract The gasification reaction C CO 2 has been studied using a char produced from an Utrillas lignite (Spain). The kinetics were investigated under conditions in which the chemical reaction was the controlling step, using 2.5 MPa total pressure, temperatures between 800 and 1000 °C and a fluidized-bed reactor. Constants for a kinetic equation and activation energies have been determined.

Study of modified calcium hydroxides for enhancing SO2 removal during sorbent injection in pulverized coal boilers

Several modified calcium hydroxides were prepared by lime hydration using NaCl, KCl, calcium lignosulfonate, ethanol-water solutions and combinations of them, to obtain sorbents with enhanced capacity to remove SO2. The sorbents were characterized in a drop tube reactor in similar conditions to those existing during sorbent injection in pulverized coal boilers. The greatest increases in SO2 retention were obtained with calcium lignosulfonate as additive or using ethanol-water solutions in the CaO hydration process. The alkali promoters (NaCl and KCl) acted mainly on the product layer diffusivity. Calcium lignosulfonate modified the pore size distribution of the calcined hydrate and decreased the mean particle size of the modified calcium hydroxide by up to five times. The use of ethanol-water solutions during the hydration process decreased the mean particle size of the modified hydrate and improved the porous structure of the calcined hydrate, mainly by increasing the porosity. To combine the favourable effects of these additives, several sorbents were prepared by combinations of them. The best sulfur retentions were obtained with sorbents modified by a combination of calcium lignosulfonate and hydration with ethanol-water solutions. The effect of the combination of additives on the sorbent properties was not the same as that of their action separately, and their effect on sulfur retention was not cumulative.

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.