Oxygen Carrier For Chemical Looping Combustion Research Articles

Reactivity and stability of Ni/Al 2O 3 oxygen carrier for chemical-looping combustion (CLC)

Chemical-looping combustion (CLC) is a technology that reduces the carbon dioxide emissions from fossil fuel power stations. A nickel supported on α -alumina oxygen carrier is investigated in this study, for use in a CLC process. Oxygen carriers with various nickel loadings on alumina are prepared according to the incipient wetness technique. The reactivity and stability of the prepared oxygen carrier samples, during repeated reduction–oxidation cycles, is demonstrated using temperature programmed reduction and oxidation. Pulse chemisorption results show that the dispersion and active crystallite diameter of the nickel particles remain constant over multiple reduction–oxidation cycles, indicating that no agglomeration occurs up to a nickel loading of 20 wt% supported on alumina. The stability and reactivity of the oxygen carriers, under industrial relevant conditions, are also investigated using the CREC fluidized bed riser simulator. It is observed that a 20 wt% nickel supported on alumina oxygen carrier is stable under industrial relevant fluidized bed reaction conditions, converting 76% of methane to carbon dioxide and water vapor, the combustion products. The metal support interaction is assessed by H 2 temperature programmed desorption, which shows that the metal-support interaction decreases as more nickel is loaded onto the alumina support.

Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion

Chemical-looping combustion (CLC) is a two-step combustion process that produces a pure CO 2 stream, ready for compression and sequestration. A CLC system is composed by two reactors, an air and a fuel reactor, and an oxygen carrier (OC) circulating between the reactors, which transfers the oxygen necessary for the fuel combustion from the air to the fuel. This system can be designed similar to a circulating fluidised bed, but with the addition of a bubbling fluidised bed on the return side. A mapping of the range of operational conditions, design values, and OC characteristics is presented for the most usual metal oxides (CuO, Fe 2O 3, and NiO) and different fuel gases (CH 4, H 2, and CO). The pressure operation of a CLC system is also considered. Moreover, a comparison of the possible use of three high reactive OCs (Cu10Al-I, Fe45Al-FG, Ni40Al-FG) previously characterised is carried out. It was found that the circulation rates and the solids inventories are linked, and the possible operating conditions are closely dependent on the reactivity of the OCs. The operational limits of the solids circulation rates, given by the mass and heat balances in the system, were defined for the different type of OCs. Moreover, a plot to calculate the solids inventories in a CLC system, valid for any type of OC and fuel gas, is proposed. The minimum solids inventories depended on the fuel gas used, and followed the order CH 4 > CO > H 2 . Values of minimum solids inventories in a range from 40 to 133 kg / MW f were found for the OCs used in this work, excepting for the reaction of Fe45Al-FG with CH 4 , which needs a higher amount of solids because of its low reactivity. From the economic analysis carried out it was found the cost of the OC particles does not represent any limitation to the development of the CLC technology.

Titania-supported iron oxide as oxygen carrier for chemical-looping combustion of methane

Chemical-looping combustion is a two-stage process proposed as an alternative for the combustion of carbonaceous materials, such as natural gas or coal gas, for almost complete CO 2 capture. In the reduction stage, the structural oxygen contained in the lattice of a reducible inorganic oxide, is used for combustion of the carbonaceous material. In the regeneration stage the oxygen carrier, found in a reduced state after the reduction stage, is regenerated with pure air to recover the physical and chemical properties of the carrier, ready to reinitiate a new cycle reduction-regeneration. In a typical multicycle reactor test, the carriers are subjected to accumulative chemical and thermal stresses and the performance will, probably, decay progressively with the number of cycles. The occurrence of some side reactions may limit the efficiency of the overall process in CO 2 capture. In this paper, titania-supported iron oxides with different iron loadings have been tested in multicycle tests in a fixed-bed reactor at 900 °C and atmospheric pressure, as oxygen carriers for the chemical-looping combustion of methane. The study shows that the available oxygen for methane combustion in the reduction stage is lower than expected since the active phase interacts with the support forming FeTiO 3 ilmenite. The reactivity of these iron based carriers in the reduction stage is independent on the iron oxide content but lower than that exhibited by other tested carriers, such as CuO or NiO. However, iron carriers are cheaper no showing any tendency to carbon deposition.

Comparison of oxygen carriers for chemical-looping combustion

Chemical-looping combustion is a combustion technology with inherent separation of the greenhouse gas CO2. This technique involves combustion of fossil fuels by means of an oxygen carrier which transfers oxygen from the air to the fuel. In this manner a decrease in efficiency is avoided for the energy demanding separation of CO2 from the rest of the flue gases. Results from fifty oxygen carriers based on iron-, manganese- and nickel oxides on different inert materials are compared. The particles were prepared using freeze granulation, sintered at different temperatures and sieved to a size 125-180 mm. To simulate the environment the particles would be exposed to in a chemical-looping combustor, reactivity tests under alternating oxidizing and reducing conditions were performed in a laboratory fluidized bed-reactor of quartz. Reduction was performed in 50% CH4/50% H2O while the oxidation was carried out in 5% O2 in nitrogen. In general nickel particles are the most reactive, followed by manganese. Iron particles are harder but have a lower reactivity. An increase in sintering temperatures normally leads to an increase in strength and decrease in reactivity. Several particles investigated display a combination of high reactivity and strength as well as good fluidization behavior, and are feasible for use as oxygen carriers in chemical-looping combustion.

Characterization and Performance in a Multicycle Test in a Fixed-Bed Reactor of Silica-Supported Copper Oxide as Oxygen Carrier for Chemical-Looping Combustion of Methane

Chemical-looping combustion of carbonaceous compounds is a proposed two-step process for complete CO2 capture and substantial reduction of NOx emissions. In the first stage, the reduction stage, the framework oxygen of a reducible inorganic oxide is used for the combustion of the carbonaceous material. In the second stage, the regeneration stage, the carrier in a reduced state is regenerated with air to recover the properties of the fresh carrier, ready to reinitiate a new cycle. This article provides results for the performance of a copper oxide silica-supported oxygen carrier in a 20-cycle test of chemical-looping of methane in a fixed-bed reactor at 800 °C and atmospheric pressure. The mesoporous nature of silica provided a good dispersion of the active phase imparting a high mechanical strength to the overall carrier. Additionally, silica is stable under highly reducing agents and inert in the two involved processes. The respective CH4, CO2, and CO breakthrough curves in the reduction stage show that ...

The use of NiO as an oxygen carrier in chemical-looping combustion

The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH 4 as fuel showed that the yield of CH 4 to CO 2 and H 2O was between 97.7 and 99.8% in the temperature range 700–1200 °C, with the yield decreasing as the temperature increases. Carbon deposition is not expected as long as sufficient metal oxide is supplied to the fuel reactor. Hydrogen sulfide, H 2S, in the fuel gas will be converted partially to SO 2 in the gas phase, with the degree of conversion increasing with temperature, but decreasing as a function of pressure. There is the possibility of sulfide formation as Ni 3S 2 at higher partial pressures of H 2S+SO 2 in the reactor. The reactivity of freeze granulated particles of NiO with NiAl 2O 4, MgAl 2O 4, TiO 2 and ZrO 2 sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH 4/50% H 2O and 5% O 2 at 950 °C. During the reducing period, the NiO initially reacted with the CH 4 to form CO 2 and H 2O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO 2, which was due to the thermodynamic limitations. Here, the ratio CO/(CO 2+CO+CH 4) was between 1.5 and 2.5% at 950 °C for the oxygen carriers with alumina based inert. A small amount of CH 4 was released from the reactor at high degrees of oxidation of the NiAl 2O 4 and MgAl 2O 4-based carriers. As the time under reducing conditions increased, steam reforming of CH 4 to CO and H 2 became considerable, with Ni catalyzing this reaction. Whereas the ZrO 2 particles showed similar behavior as the alumina-based carriers, the TiO 2-based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO 2.

Characterization Study and Five-Cycle Tests in a Fixed-Bed Reactor of Titania-Supported Nickel Oxide as Oxygen Carriers for the Chemical-Looping Combustion of Methane

Recent investigations have shown that in the combustion of carbonaceous compounds CO2 and NOx emissions to the atmosphere can be substantially reduced by using a two stage chemical-looping process. In this process, the reduction stage is undertaken in a first reactor in which the framework oxygen of a reducible inorganic oxide is used, instead of the usual atmospheric oxygen, for the combustion of a carbonaceous compound, for instance, methane. The outlet gas from this reactor is mostly composed of CO2 and steam as reaction products and further separation of these two components can be carried out easily by simple condensation of steam. Then, the oxygen carrier found in a reduced state is transported to a second reactor in which carrier regeneration with air takes place at relatively low temperatures, consequently preventing the formation of thermal NOx. Afterward, the regenerated carrier is carried to the first reactor to reinitiate a new cycle and so on for a number of repetitive cycles, while the carrier is able to withstand the severe chemical and thermal stresses involved in every cycle. In this paper, the performance of titania-supported nickel oxides has been investigated in a fixed-bed reactor as oxygen carriers for chemical-looping combustion of methane. Samples with different nickel oxide contents were prepared by successive incipient wet impregnations, and their performance as oxygen carriers was investigated at 900 degrees C and atmospheric pressure in five-cycle fixed-bed reactor tests using pure methane and pure air for the respective reduction and regeneration stages. The evolution of the outlet gas composition in each stage was followed by gas chromatography, and the involved chemical, structural, and textural changes of the carrier in the reactor bed were studied by using different characterization techniques. From the study, it is deduced that the reactivity of these nickel-based oxygen carriers is in the two involved stages and almost independent of the nickel loading. However, in the reduction stage, carbon deposition, from the thermal decomposition of methane, and CO emissions, mainly derived from the partial reduction of titania as support acting as an additional oxygen source, may impose some constraints to the efficiency of the overall chemical-looping combustion process in CO2 capture.

Carbon Formation on Nickel and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion

For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g., metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. Carbon deposition on oxygen-carrier particles was investigated to assess whether it could have adverse effects on the process. The oxygen-carrier particles used were based on oxides of nickel and iron and produced by freeze granulation. They were sintered at 1300 degreesC for 4 h and sieved to a size range of 125-180 mum. The study of carbon deposition was performed in a laboratory fluidized-bed reactor, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. The particles with nickel oxide were tested at 750, 850, and 950 degreesC, and the particles with iron oxide at 950 degreesC. On the oxygen carrier with nickel oxide, only minor amounts of carbon formed during most of the reduction. However, when more than 80% of the oxygen available was consumed, significant carbon formation started. The formation of carbon was also clearly correlated to low conversion of the fuel. No carbon was formed on the oxygen carrier based on iron oxide. The interpretation for the actual application of this process is that carbon formation should not be a problem, because the process should be run under conditions of high conversions of the fuel.

Development of Cu-based oxygen carriers for chemical-looping combustion

In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc.) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO 2 and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO 2 can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO 2 emissions to the atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use. In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser. The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation–reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for CLC process.

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO 2 capture, chemical-looping combustion (CLC) with inherent separation of CO 2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125–180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80–330 kg/MW th and a needed recirculation flow of oxygen carrier of 4–8 kg/s, MW th.

The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO 2

Chemical-looping combustion (CLC) has been suggested as an energy efficient method for capture of carbon dioxide from combustion. The technique involves the use of a metal oxide as an oxygen carrier which transfers oxygen from the combustion air to the fuel, and the direct contact between fuel and combustion air is avoided. Thus, the products of combustion, i.e. carbon dioxide and water, are kept separate from the rest of the flue gas. After condensation of the water almost pure CO 2 is obtained, without any energy lost for the separation. In this paper, the feasibility of using Fe 2O 3 as an oxygen carrier has been investigated in a fixed bed quartz reactor. Iron oxide was exposed to repeated cycles of air and methane at 950 °C, with the outlet gas concentrations measured. The time under reducing conditions and the amount of bed material were varied in a wide range. The reduction rate of Fe 2O 3 to Fe (d X/d t) with 100% methane was between 1–8%/min and was a function of both the conversion range of the solid material, Δ X, as well as the yield of methane to carbon dioxide, γ red. The rate of oxidation was also a function of Δ X and the gas conversion, γ ox, and was considerably faster than the reduction, with rates up to 90%/min. The parameters d X/d t, Δ X, and γ are closely related and can be used to establish design criteria of a CLC system based on interconnected fluidized beds. The rates of both reduction and oxidation found should be sufficient to be employed in a CLC system based on two interconnected fluidized beds.