Chemical-looping Combustion Experiments Research Articles

Chemical-looping combustion and chemical-looping reforming of kerosene in a circulating fluidized-bed 300 W laboratory reactor

The reaction between a nickel-based oxygen carrier and a liquid fuel has been demonstrated in a chemical-looping reactor with continuous particle circulating. An injection system was constructed, in which sulfur-free kerosene was evaporated, mixed with superheated steam and fed directly into the lab scale chemical-looping reactor. A nickel-based oxygen carrier composed of 40wt% NiO and 60wt% MgO–ZrO2 was used for both chemical-looping combustion (CLC) and chemical-looping reforming (CLR) experiments, which were performed for about 34h and 20h, respectively. For the CLC experiments, 95–99% of the fuel carbon was converted to CO2 and only a minute amount of hydrocarbons was detected in the off-gas. For the CLR experiments, synthesis gas was produced with concentrations of hydrocarbons as low as 0.01%. The particles were analyzed before and after the experiments using XRD, SEM, BET surface area and particle size distribution.It was shown that it is possible to use liquid fuel in a continuous chemical-looping process and also achieve nearly complete fuel conversion. With a nickel-based oxygen carrier virtually all hydrocarbon could be fully oxidized.

Chemical Looping Combustion (CLC) of two Victorian brown coals – Part 2: Assessment of interaction between CuO and minerals inherent in coals during multi cycle experiments

This paper is part two in a series of two papers on chemical looping combustion using Victorian brown coal. Part one described the assessment of interaction between CuO and minerals inherent in two Victorian brown coals during one cycle CLC experiments with CuO. This second paper describes the assessment of performance of CuO with the same two coals but in multicycle operations. The chemical and physical properties of CuO, such as reactivity and mass loss rate, have been investigated in five consecutive reduction and oxidation cycles with two brown coals. The experimental results are compared with thermodynamic simulation results using FACTSAGE. The percentage of combustion achieved using CuO and CuO conversion during multicycle operation are reported. Also investigated is the carbon deposition on CuO during CLC with both the coals during five cycles operation.The extent of Loy Yang coal combustion and reoxidation of CuO resulted in less than 0.3% weight loss per cycle. However, this value was around 2% per cycle for CuO–Morwell coal mixture reduction and re-oxidation of CuO in air. The high reactivity and conversion rate with high conversion of CuO was observed during cyclic operation. The percentage of combustion at the end of the 5th cycle with CuO was more than 97% with Loy Yang coal, as compared to 91% with Morwell coal. Fresh oxide particles and solid residues were characterized using SEM to illustrate surface morphological changes due to combustion.

Simulations of a Circulating Fluidized Bed Chemical Looping Combustion System Utilizing Gaseous Fuel

Numerical studies using Computational Fluid Dynamics (CFD) have been carried out for a complete circulating fluidized bed chemical looping combustor described in the literature (Abad et al., 2006 Fuel 85, 1174-1185). There have been extensive experimental studies in Chemical Looping Combustion (CLC), however CFD simulations of this concept are quite limited. The CLC experiments that were simulated used methane as fuel. A 2-D continuum model was used to describe both the gas and solid phases. Detailed sub-models to account for fluid-particle and particleparticle interaction forces were included. Global models of fuel and carrier chemistry were utilized. The results obtained from CFD were compared with experimental outlet species concentrations, solid circulation rates, solid mass distribution in the reactors, and leakage and dilution rates. The transient CFD simulations provided a reasonable match with the reported experimental data.

Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system

Two kinds of particles consisting of mainly manganese and iron oxides have been examined as oxygen carrier for chemical-looping combustion with O2 uncoupling in a circulating fluidized-bed reactor. The first was produced by spray drying and consisted of 66.8 wt% iron oxide and 33.2 wt% manganese oxide. The second was a manganese ore which also contained iron oxide and silica oxide. During O2 uncoupling experiments, both materials were found to release O2 in gas phase at temperatures above 850 °C, when fluidized with CO2. 7–8 h of continuously operating experiments were recorded for each oxygen carrier, and it was found that the O2 release increased with increased reactor temperature. At 1000 °C, the O2 concentration in the outlet from the fuel reactor was in the order of 7.5 vol% for the synthetic particles. For the ore, the O2 concentration was roughly 0.7 vol% at 990 °C. Further, chemical-looping combustion experiments with natural gas as fuel were carried out. While the conversion of fuel to CO2 and H2O initially was very high (96%) for the synthetic particle and decent (75%) for the ore, both oxygen carriers were found to erode into dust during combustion experiments. Some of the ore particles also swelled greatly. The solids circulation stopped abruptly after 4 h of combustion experiments for the synthetic particle, and after 2 h for the ore. In both cases, the stoppage was likely associated with the physical breakdown of the particles. It is concluded that combined oxides of manganese and iron have very interesting thermodynamical properties and could potentially be suitable for chemical-looping applications. The physical and chemical stability of such materials will have to be further studied and improved though.

CaMn 0.875Ti 0.125O 3 as oxygen carrier for chemical-looping combustion with oxygen uncoupling (CLOU)—Experiments in a continuously operating fluidized-bed reactor system

Particles of the perovskite material CaMn 0.875Ti 0.125O 3 has been examined as oxygen carrier for chemical-looping with oxygen uncoupling, and for chemical-looping combustion of natural gas, by 70 h of experiments in a circulating fluidized-bed reactor system. For the oxygen uncoupling experiments, it was found that the particles released O 2 in gas phase at temperatures above 720 °C when the fuel reactor was fluidized with CO 2. The effect increased with increased temperature, and with the O 2 partial pressure in the air reactor. At 950 °C, the O 2 concentration in the outlet from the fuel reactor was in the order of 4.0 vol%, if the particles were oxidized in air. For the chemical-looping combustion experiments the combustion efficiency with standard process parameters was in the order of 95% at 950 °C, using 1000 kg oxygen carrier per MW natural gas, of which about 30% was located in the fuel reactor. Reducing the fuel flow so that 1900 kg oxygen carrier per MW natural gas was used improved the combustion efficiency to roughly 99.8%. The particles retained their physical properties, reactivity with CH 4 and ability to release gas-phase O 2 reasonably well throughout the testing period and there were no problems with the fluidization or formation of solid carbon in the reactor. X-ray diffraction showed that the particles underwent changes in their phase composition though.

Effect of fuel particle size on reaction rate in chemical looping combustion

Chemical looping combustion (CLC) uses an oxygen carrier circulating between an air and a fuel reactor to replace direct burning of fuels in air. The very low energy penalty for CO 2 separation in CLC gives it the potential to become an important technology on the way to a CO 2 neutral energy supply. In this work, the influence of the particle size of coal on the rate of reaction of the coal was investigated in a bed of oxygen carrier. In order to do this, a method to quench the reaction of coal with oxygen carriers at a specified time and measure the particle size distribution of the remaining coal was developed. Three size fractions of coal were used in the experiments: 90–125, 180–212 and 250–355 μm. Particle size distributions of the fuel show a decrease in particle size with time. The influence of devolatilisation of the coal on the coal particle size was measured, showing that coal particles do not break in the fluidized bed reactor used for the experiments. Reaction rates based on measurements of gas phase concentrations of CO 2, CO and CH 4 showed that the reaction rate is independent of the particle size. These results are in line with literature findings, as studies have shown that carbon gasification is size-independent at conditions similar to those in the performed CLC experiments.

Ilmenite with addition of NiO as oxygen carrier for chemical-looping combustion

The naturally occurring mineral ilmenite, FeTiO 3, has been examined as oxygen carrier for chemical-looping combustion. NiO-based particles have been used as an additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H 2. Firstly, ilmenite was examined by oxidation and reduction experiments in a batch fluidized-bed reactor. These experiments indicated moderate reactivity between ilmenite and CH 4, which was used as reducing gas. However, adding 5 wt.% of NiO-based particles to the ilmenite improved the conversion of CH 4 greatly, resulting in an increase in combustion efficiency with a factor of 3. Secondly, 83 h of chemical-looping combustion experiments were conducted in a small circulating fluidized-bed reactor, using ilmenite as oxygen carrier and natural gas as fuel. A wide range of process parameters and different levels of NiO addition were examined. Occasionally, there were problems with the circulation of solids between the air reactor and fuel reactor, but most of the time the experiments worked well. The products were mostly CO 2, H 2O and unconverted CH 4. Adding small amounts of NiO-based particles to the reactor increased the conversion of the fuel considerably. For the base case conducted at 900°, the combustion efficiency was 76% for pure ilmenite and 90% for the corresponding experiments with 1 wt.% NiO-based particles added to the reactor. The properties of ilmenite were found to change considerably during operation. Used particles had lower density, were more reactive and more porous than fresh particles. These changes appear to have been physical, and no unexpected chemical phases could be identified.

Gasification and Chemical-Looping Combustion of a Lignite Char in a Fluidized Bed of Iron Oxide

Gasification and chemical-looping combustion experiments with a lignite (Hambach) char are reported, using an electrically heated fluidized bed in a 25 mm diameter tube at about 1073 K. The fluidiz...

Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier

Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO2. Most of the previous investigations on CLC of solid fuels were conducted under atmospheric pressure. A pressurized CLC combined cycle (PCLC-CC) system is proposed as a promising coal combustion technology with potential higher system efficiency, higher fuel conversion, and lower cost for CO2 sequestration. In this study pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor. CVRD iron ore particles were exposed alternately to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87.2% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C. The operating pressure was varied between 0.1 MPa and 0.6 MPa. First, control experiments of steam coal gasification over quartz sand were performed. H2 and CO2 are the major components of the gasification products, and the operating pressure influences the gas composition. Higher concentrations of CO2 and lower fractions of CO, CH4, and H2 during the reduction process with CVRD iron ore was achieved under higher pressures. The effects of pressure on the coal gasification rate in the presence of the oxygen carrier were different for pyrolysis and char gasification. The pressurized condition suppresses the initial coal pyrolysis process while it also enhances coal char gasification and reduction with iron ore in steam, and thus improves the overall reaction rate of CLC. The oxidation rates and variation of oxygen carrier conversion are higher at elevated pressures reflecting higher reduction level in the previous reduction period. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM–EDX) analyses show that particles become porous after experiments but maintain structure and size after several cycles. Agglomeration was not observed in this study. An EDX analysis demonstrates that there is very little coal ash deposited on the oxygen carrier particles but no appreciable crystalline phases change as verified by X-ray diffraction (XRD) analysis. Overall, the limited pressurized CLC experiments carried out in the present work suggest that PCLC of coal is promising and further investigations are necessary.

Diet and Its Relation to Children

The physio-chemical stability of the oxygen carrier material during chemical looping combustion (CLC) operation is crucial. In the present paper we discuss the challenges connected to operating a metal oxide base material in a cyclic manner between oxidizing and reducing atmospheres. Especially, focus has been put on the phase changes occurring within the oxygen carrier particles leading to changes in particle volume during operation and consequently, with time, also particle disintegration. Particle sintering may also occur for some oxygen carrier materials in their reduced form. These challenges have been exemplified through lab-scale CLC experiments carried out both in fixed bed and fluidized bed reactors.