Chemical-looping combustion is a novel technique used for CO
2 separation that previously has been demonstrated for gaseous fuel. This work demonstrates the feasibility of using solid fuel (petroleum ...coke) in chemical-looping combustion (CLC). Here, the reaction between the oxygen carrier and solid fuel occurs via the gasification intermediates, primarily CO and H
2. A laboratory fluidized-bed reactor system for solid fuel, simulating a CLC-system by exposing oxygen-carrying particles to alternating reducing and oxidizing conditions, has been developed. In each reducing period, 0.2
g of petroleum coke was added to 20
g of oxygen carrier composed of 60% active material of Fe
2O
3 and 40% inert MgAl
2O
4. The effect of steam and SO
2 concentration in the fluidizing gas was investigated as well as effect of temperature. The rate of reaction was found to be highly dependent on the steam and SO
2 concentration as well as the temperature. Also shown was that the presence of a metal oxide enhances the gasification of petroleum coke. A preliminary estimation of the oxygen carrier inventory needed in a real CLC system showed that it would be below 2000
kg/MW
th.
A screening of Fe- and Mn-based ores and industrial products was made in order to identify suitable low-cost materials that could be used as oxygen carriers in chemical-looping combustion (CLC). A ...laboratory fluidized bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used. Fifteen grams of each material with a particle size of 125−180 μm was exposed to a flow of 450 mLn/min of either methane or syngas (50% CO, 50% H2) during reduction. During the oxidizing phase to a flow of 1000 mLn/min, 5% O2 in nitrogen was used. All materials had a high reactivity with syngas. Some materials such as the Mn-based Colormax and the Fe-based Glödskal had also a high reactivity with methane making them possible candidates for CLC with gaseous fuel. Some of the materials, especially the Mn-based ones, showed poor mechanical stability and poor fluidizing properties. Roughly half of the Fe-based materials, but only one of the Mn-based materials, had properties that could make them suitable as oxygen carriers in a CLC system for solid fuels.
Steel converter slag, also called Linz-Donawitz (LD) slag, has been considered as an oxygen carrier for biofuel chemical looping applications due to its high availability. In addition to its content ...of iron which contributes to its oxygen-carrying capacity, LD slag also contains a significant amount of calcium. Calcium, however, is known to interact with sulfur, which may affect the usability of LD slag. To get a better understanding of the interaction between sulfur and LD slag, batch scale experiments have been performed using solid and gaseous fuel with or without sulfur dioxide, together with LD slag as an oxygen carrier. The reactivity and sulfur interaction were compared to the benchmark oxygen carrier ilmenite. Sulfur increases the gasification rate of biofuel char and the conversion of CO for both LD slag and ilmenite. However, no effect of sulfur could be seen on the conversion of the model tar species benzene. The increased gasification rate of char was suspected to originate from both surface-active sulfur and gaseous sulfur, increasing the reactivity and oxygen transfer of the oxygen carrier. Sulfur was partly absorbed into the LD slag particles with calcium, forming CaS and/or CaSO4. This, in turn, blocks the catalytic effect of CaO towards the water gas shift reaction. When the SO2 vapor pressure was decreased, the absorbed sulfur was released as SO2. This indicates that sulfur may be released in loop-seals or in the air reactor in a continuous process.
A small laboratory fluidized bed reactor is used to test the chemical reactivity of solid particles fluidized with a gas. The novelty of the system is that it can be used for testing any solid ...particles, and, in this work, oxygen carriers with Mn/Si oxide supported on TiO2 are used to present the possibilities and layout of the laboratory system. The system is equipped with automatic valves that make it possible to rapidly change the gas conditions in the reactor. The setup facilitates solid particle testing using a sample of a few grams with gas-solid contact conditions relevant for a full-scale fluidized bed. With this small system, it is possible to mimic a section or a part of a larger system. It is also possible to test extreme conditions that can occur in a bigger unit. The system is designed for determining chemical reactivity in combustion, gasification, and reforming, but it can be used for investigating any type of gas-solid reaction in fluidized bed conditions. The setup presented here is one of the smallest possible devices that can be realized while maintaining fluidized bed conditions.
The feasibility of using ilmenite as oxygen carrier in chemical-looping combustion has been investigated. It was found that ilmenite is an attractive and inexpensive oxygen carrier for ...chemical-looping combustion. A laboratory fluidized-bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used to investigate the reactivity. During the reducing phase, 15
g of ilmenite with a particle size of 125–180
μm was exposed to a flow of 450
mL
n/min of either methane or syngas (50% CO, 50% H
2) and during the oxidizing phase to a flow of 1000
mL
n/min of 5% O
2 in nitrogen. The ilmenite particles showed no decrease in reactivity in the laboratory experiments after 37 cycles of oxidation and reduction. Equilibrium calculations indicate that the reduced ilmenite is in the form FeTiO
3 and the oxidized carrier is in the form Fe
2TiO
5
+
TiO
2. The theoretical oxygen transfer capacity between these oxidation states is 5%. The same oxygen transfer capacity was obtained in the laboratory experiments with syngas. Equilibrium calculations indicate that ilmenite should be able to give high conversion of the gases with the equilibrium ratios CO/(CO
2
+
CO) and H
2/(H
2O
+
H
2) of 0.0006 and 0.0004, respectively. Laboratory experiments suggest a similar ratio for CO. The equilibrium calculations give a reaction enthalpy of the overall oxidation that is 11% higher than for the oxidation of methane per kmol of oxygen. Thus, the reduction from Fe
2TiO
5
+
TiO
2 to FeTiO
3 with methane is endothermic, but less endothermic compared to NiO/Ni and Fe
2O
3/Fe
3O
4, and almost similar to Mn
3O
4/MnO.
Steel converter slag, also called LD slag, is a material that has been suggested for use as a low-cost oxygen carrier for chemical looping applications. Low-cost oxygen carriers are especially ...relevant for the conversion of solid fuels, which may contain large amounts of reactive ashes. Ash may limit the lifetime of the bed material, which is why a high-cost oxygen carrier will likely not be competitive. Applying LD slag on an industrial scale as an oxygen carrier makes the storage properties of the material highly interesting. LD slag has been known to be affected by weathering, thus limiting the possibilities of the material to be used in construction, e.g., as fillers in concrete. In this study, pretreated LD slag for use as an oxygen carrier was weathered outdoors for roughly 1.5 years in southwest Sweden. Afterward, the particles were characterized and used in a laboratory batch fluidized bed reactor system to evaluate the effects of storage on the oxygen carrier properties. It was found that the reactivity with the fuel of the weathered LD slag was similar to that of the original sample when used in a laboratory fluidized bed. However, the physical properties were severely degraded due to weathering. Dissolved CaO formed CaCO3, agglomerating the top layer of the sample. The particles in the bulk of the sample were found to have decreased density and increased attrition rate. This suggests that LD slag particles for use as oxygen carriers should be stored dry to avoid weathering of the particles.
This study describes how potassium salts representative of those in bio ash affect the reactivity of the oxygen carrier ilmenite under moist and dry conditions. Ilmenite is a bench-mark oxygen ...carrier for chemical-looping combustion, a technique that can separate CO2 from flue gases with minimal energy penalty. Different potassium salts were mixed with ilmenite to a concentration of 4 wt % potassium. The salts used were K2CO3, K2SO4, KCl, and KH2PO4. Experiments were performed at 850 °C under alternately oxidizing and reducing conditions in a dry atmosphere or in the presence of steam. Analyses of the oxygen carrier regarding changes in reactivity, structure, and composition followed the exposures. This study showed that salts such as K2CO3, K2SO4, and KCl increase the reactivity of the ilmenite. For the samples mixed with KCl, most of the salt was evaporated. KH2PO4 decomposed into KPO3, forming layers around the ilmenite particles that lead to agglomeration. Additionally, the KPO3 layer was more or less nonpermeable for CO and decreased the reactivity toward H2 significantly in both dry and wet conditions. This decreased reactivity indicates that the concentration of phosphorus in biofuel may have a significant effect on oxygen carrier degradation. It was also observed that the presence of steam changed the chemistry drastically for the nonphosphorus-containing salts. Alkali salts may react with steam, forming volatile KOH that evaporates partly. KOH may also form K-titanates by reaction with the oxygen carrier, leading to segregation of iron and titanium phases in the ilmenite.
Perovskites are well-known oxides for thermochemical energy storage applications (TCES) since they show a great potential for spontaneous O2 release due to their non-stoichiometry. ...Transition-metal-based perovskites are particularly promising candidates for TCES owing to their different oxidation states. It is important to test the thermal behavior of the perovskites for TCES applications; however, the amount of sample that can be used in thermal analyses is limited. The use of redox cycles in fluidized bed tests can offer a more realistic approach, since a larger amount of sample can be used to test the cyclic behavior of the perovskites. In this study, the oxygen release/consumption behavior of Mn- or Cu-substituted SrFeO3 (SrFe0.5M0.5O3; M: Mn or Cu) under redox cycling was investigated via thermal analysis and fluidized bed tests. The reaction enthalpies of the perovskites were also calculated via differential scanning calorimetry (DSC). Cu substitution in SrFeO3 increased the performance significantly for both cyclic stability and oxygen release/uptake capacity. Mn substitution also increased the cyclic stability; however, the presence of Mn as a substitute for Fe did not improve the oxygen release/uptake performance of the perovskite.
A new type of Mn-based oxygen carrier was prepared by impregnating manganese ore with copper nitrate solution. Cyclic reduction and oxidation reactivity of the materials was investigated in a ...fluidized-bed reactor. The potential use of these oxygen carriers for chemical looping combustion (CLC) was examined. The reactivity of the manganese ore can be highly improved by impregnation of copper. The reactivity of the oxygen carrier reduction is higher with a larger amount of copper impregnated. However, the degree of the reactivity enhancement is not proportional to the amount of copper doped on the oxygen carriers. An important finding is that, even with low Cu loading, such as 0.5 wt % copper impregnated on manganese, the period with full CO conversion can be enhanced 6 times. A very interesting phenomenon is that the Cu-modified manganese ore can completely convert CO, even at a low temperature, such as 500 °C. This study proves that the reactivity of the manganese ore could be significantly improved by impregnating copper, even with a small amount. The copper impregnation method could be very promising to improve the reactivity of the manganese ore as oxygen carriers for CLC.
Chemical looping combustion (CLC) is one of the most promising methods for carbon capture and storage (CCS). An oxygen carrier, i.e., a mineral that can be oxidized and reduced, is used to convert ...the fuel in the process. The produced CO2 is inherently separated from the air components that enables easier CCS. The use of biomass-based fuels is desirable since it can lead to negative CO2 emissions. On the other hand, alkali compounds from the biomass may interact with the oxygen carrier causing problems, such as deactivation of the oxygen carrier. The most common oxygen carriers contain iron, since iron-based ores and industrial waste materials are readily available and cost-efficient. Therefore, the interaction between the iron oxygen carriers and the biomass ash-forming compounds needs to be investigated. Since Ca/Mg are abundant in biomass, it is important to clarify how their compounds interact with the oxygen carrier. In this study, the effect of Ca/Mg carbonates, chlorides, nitrates, sulfates, and phosphates along with synthetic biomass-derived ash on iron oxides was investigated. Redox reactions were investigated at 950 °C during 5 h under both oxidizing and reducing atmospheres. The results showed that the effect of Ca/Mg salts on the oxygen carrier varied depending on the anion of the salt. Generally, the nitrate- and phosphate-based salts of both Ca and Mg showed the harshest effect regarding agglomeration of the oxygen carriers. It was shown that the Ca/Mg-based compounds interacted differently with iron oxides, which was an unexpected result.
PDF
