Luis F. de Diego

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

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.

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.

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.

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

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.

Mineral matter effects on the reactivity of chars during gasification

Abstract The gasification reaction C + CO 2 was studied using chars obtained from three lignites from the Teruel basin. The kinetic model which best describes the progress of the reaction, and the influence of the mineral matter, was found. Working conditions were: 900 to 1000°C, atmospheric pressure, in a fluidized bed differential reactor.

Co-Combustion of Biomass and Coal in Circulating Fluidized Bed: Modeling and Validation

In this work carbon combustion efficiencies in circulating fluidized bed combustion (CFBC) when co-firing biomass and coal mixtures were studied. Experimental results were obtained from the combustion of two kind of coals with a forest residue (Pine bark) in a CBF pilot plant (0.3MWth ) with 20-cm i.d. and 6.5-m height. The effect of operating conditions such as percentage of biomass in the feed, temperature, excess air, air velocity and percentage of secondary air on carbon combustion efficiency was studied. A mathematical model for the co-combustion of coal and biomass in a circulating fluidized bed boiler has been developed. The riser is divided in three zones with different hydrodynamic characteristics: bottom, splash and freeboard. The bottom bed has a constant voidage, determined by a modified two-phase theory. The solids are considered in perfect mixing and the gas in plug flow. The voidage in the splash region follows an exponential decay model. In the freeboard region, the solids and the gas are in plug flow, and a core-annulus structure is considered. Devolatilization of solid fuels is modeled with a particle reaction model which allows to determine the volatiles generation rate as a function of time and operating conditions. Kinetics of char combustion is modeled with the shrinking particle model with mixed control by chemical reaction and gas film diffusion, assuming that the ashes separate once formed. To consider that the char particles are a mixture of coal and biomass char particles, a weighted average combustion rate is defined taking into account the individual combustion rates. Population balances of char particles in the different regions were developed to calculate carbon concentrations. The developed model can predict the different gas concentrations along the riser, such as oxygen, SO2 , CO, CH4 , etc..., and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies and gas emissions were compared with those predicted by the model and a good correlation was found for all the conditions used.Copyright

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.