Reactivity of a NiO/Al2O3 oxygen carrier prepared by impregnation for chemical-looping combustion

Available online April 8,2010The reactivity of a Ni-based oxygen carried prepared by hot incipient wetness impregnation (HIWI) on -Al2O3 with a NiO content of wt% was studied in this work. Pulse experiments with the reduction period divided into 4-second pulses were performed in a fluidized bed reactor at 1223 K using CH4 as fuel. The number of pulses was between 2 and 12. Information about the gaseous product distribution and secondary reactions during the reduction was obtained. In addition to the direct reaction of the combustible gas with the oxygen carrier, CH4 steam reforming also had a significant role in the process, forming H2 and CO. This reaction was catalyzed by metallic Ni in the oxygen carrier and H2 and CO acted as intermediate products of the combustion. No evidence of carbon deposition was found in any case. Redox cycles were also carried out in a thermogravimetric analyzer (TGA) with H2 as fuel. Both tests showed that there was a relation between the solid conversion reached during the reduction and the relative amount of NiO and NiAl2O4 in the oxygen carrier. When solid conversion increased, the NiO content also increased, and consequently NiAl2O4 decreased. Approximately 20 % of the reduced nickel was oxidized to NiAl2O4, regardless Xs. NiAl2O4 was also an active compound for the combustion reaction, but with lower reactivity than NiO. Further, the consequences of these results with respect to the design of a CLC system were investigated. When formation of NiAl2O4 occurred, the average reactivity in the fuel reactor decreased. Therefore, the presence of both NiO and NiAl2O4 phases must be considered for the design of a CLC facility.This research was conducted with financial support from the Spanish Ministry of Science and Technology (Project No. CTQ2007-64400). C. Dueso thanks MICIN for a FPI fellowship. C. Dueso thanks Erik Jerndal his valuable help with the experimental work during her stay at Chalmers University of Technology, Göteborg, Sweden.Peer reviewe

Evaluation of (MnxFe1-x)2TiyOz Particles as Oxygen Carrier for Chemical Looping Combustion

The present work accomplishes a screening of the performance of Mn-Fe-Ti based oxygen carriers, prepared with different Mn/(Mn+Fe) molar ratios in the general formula (MnyFe1-y)Ti0.15Ox. The oxygen carriers were prepared by physical mixing followed by pelletizing under pressure, calcining, crushing and sieving in the 100-300 µm particle size interval. The characterization of the carriers is based on the evaluation of their crushing strength, magnetic properties and reduction and oxidation behavior through TGA experiments at temperatures suitable for the CLC process (i.e. 850-950 °C). In addition, the main chemical structures of the Mn-Fe-Ti system were identified as a function of the Mn/(Mn+Fe) molar ratio. Oxygen uncoupling property was analyzed by reduction under a N2 atmosphere and the capability to interact with fuel gases was analyzed by using CH4, H2 and CO. Results indicate that the (MnyFe1-y)Ti0.15Ox oxygen carriers with Mn/(Mn+Fe) molar ratios of 0.55-0.87 have very promising properties for the CLC process with solid fuels

Effect of pressure on the behavior of copper-, iron-, and nickel-based oxygen carriers for chemical-looping combustion

8 pages, 8 figures, 2 tables.The combustion process integrated by coal gasification and chemical-looping combustion (CLC) could be used in power plants with a low energy penalty for CO2 capture. This work analyzes the main characteristics related to the CLC process necessary to use the syngas obtained in an integrated gasification combined cycle (IGCC) power plant. The kinetics of reduction with H2 and CO and oxidation with O2 of three high-reactivity oxygen carriers used in the CLC system have been determined in a thermogravimetric analyzer at atmospheric pressure. The iron- and nickel-based oxygen carriers were prepared by freeze-granulation, and the copper-based oxygen carrier was prepared by impregnation. The changing grain size model (CGSM) was used for the kinetic determination, assuming spherical grains for the freeze-granulated particles containing iron and nickel and a platelike geometry for the reacting surface of the copper-based impregnated particles. The dependence of the reaction rates on temperature was low, with the activation energy values varying from 14 to 33 kJ mol-1 for the reduction and 7 to 15 kJ mol-1 for the oxidation. The reaction order depended on the reacting gas and oxygen carrier, with values ranging from 0.25 to 1. However, an increase in the operating pressure for the IGCC + CLC system increases the thermal efficiency of the process, and the CO2 is recovered as a high pressure gas, decreasing the energy demand for further compression. The effect of pressure on the behavior of the oxygen carriers has been analyzed in a pressurized thermogravimetric analyzer at 1073 K and pressures up to 30 atm. It has been found that an increase in total pressure has a negative effect on the reaction rates of all the oxygen carriers. Moreover, the use of the CGSM with the kinetic parameters obtained at atmospheric pressure predicted higher reaction rates than those experimentally obtained at higher pressures, and therefore, the kinetic parameters necessary to design pressurized CLC plants must be determined at the operating pressure. © 2006 American Chemical Society.This work was carried out with financial support from the European Coal and Steel Community Project (7220-PR125) and the Spanish Ministry of Education and Science (Project CTQ 2004-04034). The authors thank Dr. Anders Lyngfelt and Dr. Tobias Mattisson for the preparation of the freeze-granulated particles.Peer Reviewe

Calcination of calcium-based sorbents at pressure in a broad range of CO2 concentrations

11 figures, 3 tablesThe 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 for 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 for by an additional decrease in the molecular diffusion coefficient with increasing total pressure with respect to that indicated by Fuller's equation.This research was carried out with the financial support from the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Project No. AMB 98-0883). The authors thank Dr. Diego Alvárez for his assistance with the SEM technique.Peer Reviewe

Promising Impregnated Mn-based Oxygen Carriers for Chemical Looping Combustion of Gaseous Fuels

Promising impregnated oxygen carriers, based on copper and iron, have been previously developed for CLC with gaseous fuels (CH4, syngas, LHC). Recently, because of its low cost and environmental compatibility, Mn-based oxygen carriers are now being considered as an attractive option for chemical-looping combustion (CLC) applications. In this work, a screening of different commercial supports in fluidizable particle size for impregnated Mn-based materials has been carried out. Different oxygen carriers have been prepared by incipient impregnation on ZrO2, and CaAl2O4, and evaluated with respect to their mechanical resistance, fuel gas reactivity and fluidization properties such as agglomeration and attrition rate. In a first step, particles showing high enough crushing strength values were selected for the reactivity investigation. The redox reactivity was evaluated through TGA experiments at suitable temperatures for the CLC process (i.e. 850-950 °C) using H2, CO and CH4. Multi cycle redox analysis and full physical and chemical characterization was also performed. In a second step, materials with high enough reactivity were prepared for fluidized bed evaluation. A batch fluidized bed installation with continuous gaseous fuel feed was used to analyze the product gas distribution during reduction and oxidation reactions at different operation temperatures, and agglomeration and attrition behavior of the selected materials. Results showed that an oxygen carrier impregnated using ZrO2 as support, had high enough reactivity and low attrition rate. Therefore, this material can be selected as a candidate for the development of CLC with syngas with promising results

Titanium substituted manganese-ferrite as an oxygen carrier with permanent magnetic properties for chemical looping combustion of solid fuels

Mixed oxides of Mn-Fe have been identified as suitable materials for Chemical Looping Combustion (CLC) with solid fuels both via in-situ Gasification Chemical Looping Combustion (iG-CLC) and Chemical Looping with Oxygen Uncoupling (CLOU) processes. These materials show the property of react with gaseous fuels as well as release oxygen under given conditions, while cheap metals are used. In addition, these materials can show magnetic properties that can be used for an easy separation from ash in CLC with solid fuels. Thus, losses of oxygen carrier material in the ash drain stream would be reduced. Different cations have been proposed for improving the magnetic properties of manganese ferrites, including Ti4+. In this context, the present work accomplishes a screening of (MnxFe1-x)2O3 doped with 7 wt.% TiO2, with x ranging from 0 to 1. The influence of Mn:Fe ratio on their physical and chemical properties was evaluated. In general, particles with high crushing strength values (>4 N) were obtained, and magnetic characteristics were highlighted when x ¿ 0.66. The oxygen uncoupling capability depended on the Mn:Fe ratio and the oxidation conditions, i.e. temperature and oxygen partial pressure. Broader oxidation conditions to take advantage of the oxygen uncoupling capability were found for materials with low Mn content. On contrary, the reactivity with fuel gases (CH4, H2 and CO) increased with the Mn content. Thus, oxygen carriers with Mn/(Mn + Fe) molar ratio in the 0.5–0.9 interval showed interesting properties at suitable temperatures for the iG-CLC and CLOU processes (i.e. 850–980 °C). The material with Mn/(Mn + Fe) = 0.55 was preferred considering a trade-off between reactivity and magnetic properties

The use of ilmenite as oxygen-carrier in a 500Wth Chemical-Looping Coal Combustion unit

12 pages, 13 figures, 6 tablesChemical-Looping Combustion, CLC, is a promising technology to capture CO2 at low cost in fossil-fuelled power plants. In CLC the oxygen from air is transferred to the fuel by a solid oxygen-carrier that circulates between two interconnected fluidized-bed reactors: the fuel- and the air-reactor. This work studies the CLC technology in a 500Wth facility fuelled with bituminous coal with ilmenite as oxygen-carrier. The effect of temperature and coal particle size on coal conversion and combustion efficiency was assessed. Char gasification and combustion of both gasification products and volatile matter were evaluated. At higher temperatures, gasification and combustion reactions are promoted. Carbon capture and combustion efficiencies grow with the temperature, with faster increase at temperatures higher than 910°C. The outgoing unburnt gases come from volatile matter that was not fully oxidized by ilmenite. Little CH4 was measured and there were neither hydrocarbons heavier than CH4 nor tars in the fuel-reactor outlet. At 870°C the char conversion was 15% and reached 82% at 950°C. The combustion efficiency in the fuel-reactor increased from 70% at 870°C to 95% at 950°C. The results show that ilmenite has good behavior as oxygen-carrier and that optimizing CLC with coal can lead to energy production with high CO2 capture.This work was partially supported by the European Commission, under the RFCS program (ECLAIR Project, Contract RFCP-CT-2008-0008), from Alstom Power Boilers and by the Spanish Ministry of Science and Innovation (Project ENE2010-19550). A. Cuadrat thanks CSIC for the JAE Pre. fellowship. Alberto Abad thanks to the Ministerio de Ciencia e Innovación for the financial support in the course of the I3 Program.Peer Reviewe

Scale-up of CLC oxygen carriers for gaseous fuels

Chemical Looping Combustion, CLC, is one of the most promising processes to capture CO2 at a low cost. It is based on the transfer of the oxygen from air to the fuel by using a solid oxygen carrier that circulates in dual fluidized bed systems. The CO2 capture is inherent to this process, as the air does not get mixed with the fuel. However, the CLC process is still under development waiting for a large scale demonstration experience. The key issue in the system performance is the oxygen carrier material. The oxygen carrier must fulfil several characteristics such as high reactivity and good fluidization properties, that will rely on their redox system and the support. Therefore, the identification of raw materials, available at multi-tonn scale at a competitive price, is one of requirements for the success of the technology. Promising impregnated oxygen carriers, based on copper and iron, have been developed to perform well for gaseous fuels (CH4, syngas , LHC..), although they were prepared from not commercially scalable production supports. In this work, the performance of different impregnated materials, prepared with commercial-scale supports, was analyzed during methane combustion in a continuous 500 Wth CLC unit to identify the best material based on reactivity, attrition resistance and sulfur tolerance. A copper-based material with improved performance than the reference material was identified and therefore proposed as the best oxygen carrier for scale-up CLC technology for gaseous fuels

Prompt considerations on the design of chemical-looping combustion of coal from experimental tests

13 Figures, 5 TablesThe Chemical-Looping Combustion of coal in the reactor system has been proposed as an interesting option to process a solid fuel in a CLC system. In this process, a solid fuel is directly fed to the fuel reactor in a CLC system. Solid fuel pyrolysis, char gasification and oxidation of gaseous products by reaction with the oxygen-carrier are the main chemical processes happening in the fuel reactor. The aim of this study is to analyze the performance of ilmenite as oxygen-carrier for CLC of coal regarding to the conversion of gaseous products from char gasification. Successive reduction-oxidation cycles were carried out in a fluidized bed using bituminous coal char as reducing agent. The changes on chemical and physical properties of ilmenite particles were determined. An activation process of ilmenite through the redox cycles was evidenced which was justified by an increase of porosity. The results showed that the activation for ilmenite reduction reaction was completed after 7 redox cycles. However, the oxidation reaction rate was increasing still after 16 redox cycles because the porosity was not fully developed. The gasification reaction rate and the ilmenite reactivity were analyzed. The effect of ilmenite itself and the influence of the gasification agent, i.e. H2O, CO2 or H2O/CO2 mixtures, and temperature on the gasification rate were evaluated. Limited use of CO2 in the fluidizing gas was identified in order to maintain high gasification rates. Higher temperature improved the char gasification rate, mainly using steam as gasification agent, and the combustion efficiency of the gasification products. Nevertheless, the effect of temperature on the combustion efficiency was of lower relevance than that on the gasification rate. Finally, a theoretical approach was developed to easily evaluate the conversion of char in the fuel-reactor by gasification.This work was partially supported by the European Commission, under the RFCS program (ECLAIR Project, Contract RFCP-CT-2008-0008), from Alstom Power Boilers and by the Spanish Ministry of Science and Innovation (Project ENE2010- 19550). A. Cuadrat thanks CSIC for the JAE Pre. fellowship. Alberto Abad thanks to the Ministerio de Ciencia e Innovación for the financial support in the course of the I3 Program.Peer reviewe

Characterization for disposal of Fe-based oxygen carriers from a CLC unit burning coal

Chemical Looping Combustion (CLC) is an emerging low cost CO2 capture technology for large scale power units. The oxygen needed for combustion is supplied by a solid oxygen carrier circulating between two reactors. Fe-based oxygen carriers have been proposed for CLC of coal due to their low cost. Some of them are minerals or industrial residues which can contain toxic trace elements. After its use, the oxygen carrier should be disposed in a landfill and therefore, the presence of soluble toxic elements in the oxygen carrier should be analyzed. In this study, lixiviation tests were carried out with three different Fe-based oxygen carriers used in coal CLC experiments in a continuous unit: ilmenite, a bauxite waste and an iron ore. All the spent oxygen carriers, except the bauxite waste, can be classified as a stable non-reactive hazardous waste and therefore can be disposed in a landfill for non-hazardous residues. An estimation of the amount of solid waste generated in the process based on the fly ash content of the coal and the oxygen carrier particle lifetime was made.The authors thank the Spanish Ministry for Science and Innovation (MICCIN) for the financial support via the ENE2011-26354 project. This work was also partially supported by FEDER funds. T. Mendiara thanks for the “Ramón y Cajal” post-doctoral contract awarded by the Ministry of Economy and Competitiveness.Peer reviewe