Francisco García-Labiano

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

Determination of sulfur release and its kinetics in rapid pyrolysis of coal

Abstract Four coals of different rank (one anthracite, one bituminous and two lignites) were flash-pyrolysed at temperatures between 700 and 1200°C in a heated wire mesh reactor at atmospheric pressure. The yields of volatile sulfur-containing products (H2S, COS, CS2 and thiophenic compounds) were determined as a function of pyrolysis temperature by gas chromatography with flame photometric detection. A single reaction model was used to describe the sulfur release and pre-exponential factors and activation energies were determined for each coal. The results showed a relation between coal rank and the kinetic parameters. The activation energy varied between 20 and 100 kJ mol−1, the lowest values corresponding to the lignites.

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.

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.

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

Sulfur release during the devolatilization of large coal particles

Experimental data are reported on sulfur release rates during the flash pyrolysis of coal particles sized up to 2 mm, at heating rates and temperatures found typically in fluidized bed reactors. Predicted sulfur yields are also reported for pyrolysis conditions studied using an existing model for devolatilization of large coal particles, coupled with kinetics from an earlier study. The predictions of the experimental data were found to be sensitive to the value of the thermal diffusion coefficient used in the model, with each coal and pyrolysis temperature requiring a specific value in order to accurately predict the experimental results. The rate of heat transfer is also shown to be an important rate-controlling mechanism and the transition from reaction-controlled to heat-transfer-controlled pyrolysis is determined as a function of thermal diffusivity, particle size and sulfur release kinetics.

Characterization of oxygen carriers for chemical-looping combustion

Publisher Summary This chapter analyzes the behavior of the different oxygen carriers with respect to selectivity towards complete oxidation products, durability in the cyclic reactions, and attrition and agglomeration during fluidized bed cyclic reactions. In the multicycle tests in thermogravimetric analyzer (TGA), it was observed that most of the oxygen carriers exhibited high reactivity and excellent chemical stability but the Cu and Ni based oxygen carriers prepared by mechanical mixing underwent a rapid degradation of their mechanical properties as the number of cycles increased. The Ni- and the Fe based oxygen carriers did not agglomerate. The attrition rates of the carriers were usually high in the first cycles due to the rounding effects on the particles and because of the fines sticked to the particles during preparation. Later, the attrition rates due to the internal changes produced in the particles by the successive reduction and oxidation processes decreased, and all carriers showed low attrition rates. The product distribution during the oxidation of the fuel depended on the metal oxide used in the oxygen carrier. Complete conversion of CH 4 to CO 2 and H 2 O was obtained with the oxygen carrier Cu-AI-I.

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.