Enhancing properties of iron and manganese ores as oxygen carriers for chemical looping processes by dry impregnation

The use of naturally occurring ores as oxygen carriers in CLC processes is attractive because of their relative abundance and low cost. Unfortunately, they typically exhibit lower reactivity and lack the mechanical robustness required, when compared to synthetically produced carriers. Impregnation is a suitable method for enhancing both the reactivity and durability of natural ores when used as oxygen carriers for CLC systems. This investigation uses impregnation to improve the chemical and mechanical properties of a Brazilian manganese ore and a Canadian iron ore. The manganese ore was impregnated with Fe2O3 and the iron ore was impregnated with Mn2O3 with the goal of forming a combined Fe/Mn oxygen carrier. The impregnated ore's physical characteristics were assessed by SEM, BET and XRD analysis. Measurements of the attrition resistance and crushing strength were used to investigate the mechanical robustness of the oxygen carriers. The impregnated ore's mechanical and physical properties were clearly enhanced by the impregnation method, with boosts in crushing strength of 11-26% and attrition resistance of 37-31% for the impregnated iron and manganese ores, respectively. Both the unmodified and impregnated ore's reactivity, for the conversion of gaseous fuel (CH4 and syngas) and gaseous oxygen release (CLOU potential) were investigated using a bench-scale quartz fluidised-bed reactor. The impregnated iron ore exhibited a greater degree of syngas conversion compared to the other samples examined. Iron ore based oxygen carrier's syngas conversion increases with the number of oxidation and reduction cycles performed. The impregnated iron ore exhibited gaseous oxygen release over extended periods in an inert atmosphere and remained at a constant 0.2% O2 concentration by volume at the end of this inert period. This oxygen release would help ensure the efficient use of solid fuels. The impregnated iron ore's reactivity for CH4 conversion was similar to the reactivity of its unmodified counterpart. The unmodified manganese ore converted CH4 to the greatest extent of all the samples tested here, while the impregnated manganese ore exhibited a decrease in reactivity with respect to syngas and CH4 conversion

Combined Iron-Manganese Oxides for Chemical-Looping with Oxygen Uncoupling

AbstractThe most important factor affecting global warming is the increased concentrations of greenhouse gases in the atmosphere. Carbon dioxide is considered as the most important anthropogenic greenhouse gas. An option which can be employed to reduce the CO2 emissions from combustion is to capture the CO2 and store it in deep geological formations. One innovative technology that can be used for CO2 capture is Chemical-Looping Combustion (CLC). The CLC system is composed of two interconnected fluidized bed reactors. In the fuel reactor the added fuel reacts with an oxygen carrier, usually a metal oxide, to produce CO2 and H2O. The reduced metal oxide is then transported to the air reactor, where it is oxidized back to its original form, and the exit stream from this reactor will contain only nitrogen and some unused oxygen. The advantage of this technology is that carbon dioxide from the combustion is inherently obtained separate from the rest of the flue gases. Chemical-looping with oxygen uncoupling (CLOU) is very similar to CLC, but uses oxygen carriers with the ability to release gas phase oxygen, which can react directly with the fuel, hence avoiding the direct reaction between fuel and oxygen carrier. In this work, CLOU has been studied with gaseous and solid fuels in a small fluidized bed batch reactor, using new Fe-Mn-based oxygen carriers. Particles with different molar ratios of Mn/Fe produced by spray-drying were investigated. They were examined by decomposition in N2 and by reaction with methane and syngas (50/50% CO/H2) at 850˚C, 900˚C and 950˚C. At the higher reaction temperature, 950˚C, the oxygen carriers with a manganese content in the range of 25% to 33%, show both the highest gas conversion of methane as well as the highest concentration of released oxygen. At 850˚C, on the other hand, the best methane conversion and oxygen release was seen for particles with a high manganese content. In fact the oxygen carriers with a manganese content of 67%, 75% and 80% calcined at 950˚C had almost full conversion of methane to CO2 and H2O at 850˚C using an oxygen carrier mass in the batch reactor corresponding to 70 kg/MW. The release pattern of oxygen seen as a function of the Fe/Mn ratio and temperature was explained using the phase diagram of the Fe-Mn-O system.An oxygen carrier with a manganese content of 33% was also tested with solid fuel using inert fluidization gas, N2, at 950˚C. Further, oxygen carriers with a manganese content of 67%, 75% and 80% were investigated at 850˚C. The char originating from the fuel particles effectively removed the oxygen released from the oxygen carrier particles, producing CO2. The tests show that the oxygen carrier with a manganese content of 33% releases oxygen corresponding to approximately 0.5% of its mass and oxygen carriers with a manganese content of 67%, 75% and 80% release oxygen corresponding to approximately 2.7% of their mass. Thus, applying these materials in Chemical-Looping of solid fuels, could contribute both to faster fuel conversion and to higher conversion of gas, as compared to a normal oxygen carrier that does not release oxygen. Due to the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical-looping combustion with oxygen uncoupling

Combined Iron-Manganese Oxides for Chemical-Looping with Oxygen Uncoupling

AbstractThe most important factor affecting global warming is the increased concentrations of greenhouse gases in the atmosphere. Carbon dioxide is considered as the most important anthropogenic greenhouse gas. An option which can be employed to reduce the CO2 emissions from combustion is to capture the CO2 and store it in deep geological formations. One innovative technology that can be used for CO2 capture is Chemical-Looping Combustion (CLC). The CLC system is composed of two interconnected fluidized bed reactors. In the fuel reactor the added fuel reacts with an oxygen carrier, usually a metal oxide, to produce CO2 and H2O. The reduced metal oxide is then transported to the air reactor, where it is oxidized back to its original form, and the exit stream from this reactor will contain only nitrogen and some unused oxygen. The advantage of this technology is that carbon dioxide from the combustion is inherently obtained separate from the rest of the flue gases. Chemical-looping with oxygen uncoupling (CLOU) is very similar to CLC, but uses oxygen carriers with the ability to release gas phase oxygen, which can react directly with the fuel, hence avoiding the direct reaction between fuel and oxygen carrier. In this work, CLOU has been studied with gaseous and solid fuels in a small fluidized bed batch reactor, using new Fe-Mn-based oxygen carriers. Particles with different molar ratios of Mn/Fe produced by spray-drying were investigated. They were examined by decomposition in N2 and by reaction with methane and syngas (50/50% CO/H2) at 850˚C, 900˚C and 950˚C. At the higher reaction temperature, 950˚C, the oxygen carriers with a manganese content in the range of 25% to 33%, show both the highest gas conversion of methane as well as the highest concentration of released oxygen. At 850˚C, on the other hand, the best methane conversion and oxygen release was seen for particles with a high manganese content. In fact the oxygen carriers with a manganese content of 67%, 75% and 80% calcined at 950˚C had almost full conversion of methane to CO2 and H2O at 850˚C using an oxygen carrier mass in the batch reactor corresponding to 70 kg/MW. The release pattern of oxygen seen as a function of the Fe/Mn ratio and temperature was explained using the phase diagram of the Fe-Mn-O system.An oxygen carrier with a manganese content of 33% was also tested with solid fuel using inert fluidization gas, N2, at 950˚C. Further, oxygen carriers with a manganese content of 67%, 75% and 80% were investigated at 850˚C. The char originating from the fuel particles effectively removed the oxygen released from the oxygen carrier particles, producing CO2. The tests show that the oxygen carrier with a manganese content of 33% releases oxygen corresponding to approximately 0.5% of its mass and oxygen carriers with a manganese content of 67%, 75% and 80% release oxygen corresponding to approximately 2.7% of their mass. Thus, applying these materials in Chemical-Looping of solid fuels, could contribute both to faster fuel conversion and to higher conversion of gas, as compared to a normal oxygen carrier that does not release oxygen. Due to the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical-looping combustion with oxygen uncoupling

Experimental evaluation and modeling of steam gasification and hydrogen inhibition in Chemical-Looping Combustion with solid fuel

A Chemical-Looping Combustion (CLC) system is composed of two fluidized bed reactors, an air reactor and a fuel reactor. Oxygen is transferred from the air to the fuel by solid oxygen carrier particles circulating between these two reactors. By this arrangement, the N2 from the air is kept separated from the fuel gases as a part of the process and an almost pure stream of CO2 is obtained from the fuel reactor. </br></br> This work investigates and models the influence of the steam and hydrogen concentration in the fuel reactor on the rate of solid fuel conversion in Chemical-Looping Combustion. Two kinds of fuel were examined, Swedish wood char and El Cerrejon bituminous coal (Colombian coal). Four different bed materials have been used in the reactor, ilmenite, nickel and oxide scales as an oxygen carrier and quartz sand for gasification experiments. The temperature was 970 °C for all experiments. Different fractions of steam and hydrogen were added to the fluidizing stream. Additionally, gasification experiments of fuel particles pretreated in mixtures of H2 and N2 were performed in order to determine the reversibility of the observed hydrogen inhibition. </br></br> The results show that the best models for describing the behavior of steam gasification and fuel conversion in Chemical-Looping Combustion for a Swedish wood char and the El Cerrejon coal is the oxygen exchange model. For both fuels, it can be seen that higher steam concentration increases the rate of char conversion and, higher hydrogen concentration decreases the rate as a result of hydrogen inhibition. No irreversible hydrogen inhibition could be observed

Chemical-looping with oxygen uncoupling using combined Mn-Fe oxides, testing in batch fluidized bed

Chemical-looping with oxygen uncoupling (CLOU) has been studied with gaseous and solid fuel in a small fluidized bed batch reactor, using new Fe-Mn-based oxygen carriers. CLOU is a development of chemical-looping combustion, using oxygen carriers with the ability to release oxygen, which can react directly with the fuel. The carbon dioxide from the combustion is inherently obtained as separated from the rest of the flue gases. In this work manganese is combined with iron oxides, giving new bimetallic oxide compounds with different thermodynamic properties compared to pure manganese oxides. Four different combinations of iron manganese oxide have been examined by decomposition in N2 and also reaction with methane and syn-gas. F3, a material with a molar ratio of Fe:Mn of 2:1, showed the best behaviour in terms of its release of oxygen, take up of oxygen, fluidizability and methane conversion. Therefore, F3 was also tested with solid fuel using inert fludization gas, N2. The char particles effectively remove oxygen released as it is converted to CO2. Thus, CO2 will represent the oxygen release. The tests show that the particles release oxygen corresponding to approximately 0.5% of their mass. Moreover, a test where steam was added in the fluidization gas showed high gas conversion, with essentially no unconverted gas. Thus, the tests indicate that the F3 particles, if used in chemical-looping of solid fuels, could contribute both to faster fuel conversion and to higher conversion of gas, as compared to a normal oxygen carrier that does not release oxygen

Investigation of Different Mn–Fe Oxides as Oxygen Carrier for Chemical-Looping with Oxygen Uncoupling (CLOU)

The appropriate oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) should be thermodynamically capable of being oxidized in the air reactor and also release gaseous O2 in the fuel reactor at appropriate temperatures and oxygen partial pressures. It should also be mechanically durable, cheap, and environmentally friendly. Iron–manganese oxides appear to be especially promising due to favorable thermodynamics. In this work, combined metal oxides of iron and manganese were investigated for the CLOU process. Particles with different ratios of Mn/Fe were produced using spray drying. The particles were calcined at 950 and 1100 °C for 4 h and then tested with respect to parameters important for CLOU. The crushing strength for these materials was between 0.1 to 1.7 N, depending on their composition and sintering temperature. The ability of the iron–manganese oxide particles to release oxygen in the gas phase was examined by decomposition of the material in a stream of N2. Moreover, the reaction with both methane and synthesis gas (50/50% CO/H2) was examined in a batch fluidized bed reactor. Here, the particles were alternately oxidized with 5% O2 and reduced in N2 or with fuel at 850 °C, 900 and 950 °C. From the results, it can be concluded that during the nitrogen period, the oxygen carriers with Mn3O4 content in the range from 20 wt % to 40 wt % release oxygen at 900 °C, whereas the materials with higher manganese content show no oxygen release. This is because they could not be oxidized to bixbyite. By decreasing the temperature from 900 to 850 °C, it was possible to oxidize oxygen carriers with manganese oxide content of 50 wt % and higher, and consequently, oxygen release during the nitrogen period was seen for these materials. This is in agreement with the phase diagram for this system. The reaction rate with methane follows the oxygen release trend very well. At the higher reaction temperature, 950 °C, oxygen carriers with manganese content in the range from 25% to 33% show the best gas conversion of methane. At 850 °C, on the other hand, high methane conversion is seen for particles with high manganese content. In fact, several particles had almost full conversion of methane to CO2 and H2O at 850 °C using a bed mass in the batch reactor corresponding to 70 kg oxygen carrier/MW

(MnzFe1—z)yOx combined oxides as oxygen carrier for chemical-looping with oxygen uncoupling

Oxygen carrier particles with the composition (Mn0.8,Fe0.2)2O3 were found to readily release gas phase oxygen at 850\ub0C, and were capable to oxidize CH4 completely and convert wood char rapidly to CO2 during experiments in a batch fluidized bed reactor. The particles were able to release oxygen corresponding to more than 3% of their mass in less than 40 s. Because of the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical-looping combustion with oxygen uncouplin