Mixed Oxygen Carriers Research Articles

Enhanced performance of red mud oxygen carrier for chemical looping combustion via tandem reactions

Biomethane power generation, particularly chemical looping combustion technology, has garnered significant attention recently due to its ability to capture CO2 in situ. In this study, dual bed layers oxygen carriers (DBL) for chemical looping combustion of methane were designed. The LaFe0.93Ni0.07O3 oxygen carrier with excellent CH4 partial oxidation was applied in upper bed layer and low-cost red mud was located into lower bed layer. This tandem reaction showed much higher methane conversion (87.1 %) than that over the single oxygen carrier of red mud (33.9 %) and the mechanically mixed oxygen carrier of LaFe0.93Ni0.07O3 and red mud (77.7 %). In addition, the mechanically mixed sample showed a quick deactivation, with the methane conversion dropping 54.2 % after 40 redox cycles due to the encapsulation of the active LaFe0.93Ni0.07O3 by red mud, while the double bed system showed high stability in the long-term redox cycle. The limitedly increased crystallite size of LaFeO3, as well as completely replenished lattice oxygen, were responsible for superior activity and stability of DBL. This study provides a new strategy to improve the efficiency of chemical looping combustion of methane in fixed bed reactor by tandem reactions over different oxygen carriers with complementary functions in terms of activity and cost.

Performance of fuel reactor in chemical looping combustion system with mixed metal oxides

The Chemical Looping Combustion (CLC) technique shows great potential as an innovative technology to enhance thermal efficiency of system. It has a capability to achieve complete carbon capture (up to 100 %) and prevent the formation of nitrogen oxides (NOx). The current study focuses on the computational analysis of bubble behavior within fuel reactor of CH4-fueled Chemical Looping Combustion (CLC) setup. During numerical examination, a user-defined function (UDF) was utilized to integrate a reaction kinetics into active environment of fuel reactor. In the current research, CuO and NiO are employed as mixed oxygen carrier substances, while CH4 is utilized as fuel within the combustion procedures. The preparation, growing, rising, and bursting of bubbles' hydrodynamics has been observed; additionally, it has been evaluated in addition to examining the solidus-gaseous molar fraction within a fuel reactor. The current simulation opted for a gaseous fuel composed of natural gas (CH4), a hydrocarbon, alongside mixed metal oxides as CuO and NiO in varying ratios. A mixed metal oxide has been chosen in the proportion of CuO as 0.3 and NiO as 0.7, CuO as 0.5 and NiO as 0.5, and CuO as 0.7 and NiO as 0.3 by weightage. The fuel conversion rate has been observed 20 %, 25 %, and 30 % respectively for the different metal oxide mixture proportions. It has been clearly noticed that in the mixture of NiO and CuO; if the CuO amount is more, then the fuel conversion rate is high due to highly reactivity of CuO at high temperatures.

Enhanced chemical looping gasification of biomass coupled with CO2 splitting based on carbon negative emission

Under the background of fossil fuel consumption and global climate change, an enhanced chemical looping gasification of biomass coupled with CO2 splitting (CLGCS) was proposed in this work to produce high-quality syngas from biomass and split CO2 into CO by using Fe-based mixed oxygen carriers (OCs) without extra catalyst and excessive energy consumption, which provided a carbon negative emission technology for the cascade conversion of biomass and resource utilization of greenhouse gas. The reaction equilibrium and regulation mechanism of CLGCS were analyzed in Ellingham diagrams via theoretical calculation. Variable conditions were designed and performed to investigate the reaction performance of OCs. The intermetallic synergy was analyzed, and the improvement order of heterologous metal on lattice oxygen transfer of OCs was Co2O3 > NiO > CeO2 > CaO. The NiO modified sample exhibited excellent reaction behavior in CLGCS, which provided active sites for catalytic cracking of tar in CLG stage and activating CO2 molecules in splitting stage. Syngas with heating value 13.32 MJ/m3 and carbon conversion 74.18% were achieved in CLG stage, corresponding to CO2 conversion 64.09% and CO yield 23.59 mmol/g in splitting stage. High temperature enhanced the endothermic reactions in CLGCS process, and the biomass occurrence ratio indicated a positive correlation with CO2 splitting. Although some OC particles agglomerated in reaction process, the pore structure and reactivity remained stable. After 20 cyclic reaction tests, the carbon conversion in CLG stage and CO2 conversion in splitting stage were kept at 67.52% and 73.5%, respectively. The reaction path of CLGCS with NiO modified OC was summarized as: Fe2O3/NiFe2O4 → Fe/FeO/Ni → Fe3O4/Ni → Fe2O3/NiFe2O4.

Reactivity investigation on chemical looping gasification of coal with Iron-Manganese based oxygen carrier

Chemical looping gasification (CLG) of Yunnan lignite with Fe-Mn mixed oxygen carriers (OCs) was performed to achieve high purity synthesis gas and reduce pollutant emissions. The Fe-Mn composite OCs presented the characteristics of oxygen decoupling, and there is a synergistic effect between the active components using various analytical methods. The maximum syngas yield (0.054 mol/g) and the maximum carbon conversion rate (62.9%) were achieved under the following conditions; temperature at 900 °C, steam flow rate at 0.045 ml/min and O/C ratio equal to 1. The OC acts as both oxygen source and catalyst in gasification reaction process, after studying the gasification reaction characteristics between the OC and the model compound. The oxygen transport mechanism and phase evolution law of the OC was revealed by conducting XRD and XPS tests. The migration of lattice oxygen of the OC in CLG transferred from bulk phase to surface phase with the migration pathway of oxygen species OI → OII → OIII, and the OC phase underwent the reduction process of (Mn, Fe)2O3 → (FeO)0.664(MnO)0.336 → Fe/MnO. The reactivity performance of OCs remained stable after 20-cycle redox experiments, indicating that the Fe-Mn OC was a good candidate OC for the CLG process.

Multifunctional Ni-based oxygen carrier for H2 production by sorption enhanced chemical looping reforming of ethanol

The sorption enhanced chemical looping reforming is a prospective technology for high-quality hydrogen production. In this technology, the full benefits of catalysis, oxygen transfer, and CO2 adsorption are achieved by the utilization of oxygen carriers and sorbents. However, most sorbents that are mixed physically into the reforming bed are liable to encountering ash fouling and porosity reduction, preventing CO2 adsorption and sorbent regeneration. In this work, we develop a multifunctional composite oxygen carrier, which combines the NiO/NiAl2O4 catalyst with the CaO sorbent into a one-body composite pellet. Compared with the conventional physically mixed oxygen carrier, the composite oxygen carrier shows uniform element distribution, superior reducibility, and shortened dead time, resulting in enhanced CO2 adsorption capacity, high H2 production efficiency, and excellent ethanol conversion. Furthermore, during 20 cycle tests, the stable CO2 adsorption capacity and H2 concentration of composite oxygen carrier were maintained, while the H2 concentration of mixed oxygen carrier sharply decreased because of deteriorated adsorption performance. The results confirm that the multifunctional composite oxygen carrier is more suitable than the mixed oxygen carrier for CO2 adsorption and high purity hydrogen production. This work opens a new avenue toward the performance improvement of oxygen carriers.

Synergistic reaction investigation of the NiO modified CaSO4 oxygen carrier with lignite for simultaneous CO2 capture and SO2 removal

Natural gypsum ore, being dominated with CaSO4, is quite applicable to be used as an alternative oxygen carrier (OC) in the chemical looping combustion (CLC). However, such overwhelming problems of CaSO4 as its inferior reactivity and potential sulfur release ought to be properly solved. In this research, the NiO modified CaSO4 OC of desired “core-shell” structure was first prepared using the combined template method. The synergistic effect between the CaSO4 substrate and the doped NiO in the mixed OC during its reaction with a typical lignite was ascertained by comprehensive experimental means combined with thermodynamic simulation. The improved reactivity as obtained for the NiO modified CaSO4 ore was found to mainly ascribe to activation of the reduced Ni on the intractable carbon groups present in the residual char as well as in situ oxidization of the reduced Ni to active NiO again via transfer of the oxygen involved in the unconverted CaSO4. Meanwhile, the gaseous sulfur released from the accompanied side reactions of CaSO4 was directionally fixed. In addition, both strong resistance to sintering and good regeneration of the prepared NiO modified CaSO4 OC were promising to apply in the real CLC system for simultaneous CO2 control and SO2 removal.

Techno-economic evaluation of electronic waste based oxygen carriers for co-chemical looping combustion of coal and biomass integrated combined cycle power generating systems

Chemical looping combustion (CLC) is an emerging technology to capture CO2 in a cost-effective way. Metal particles such as Fe, Ni, and Cu can separate oxygen from air by forming their oxides and they could thereby act as oxygen carriers for the combustion of fuels producing nitrogen-free CO2 enriched flue gas. To commercialize the CLC based carbon capture technology, low cost oxygen carriers are essential for economical operation. Electronic waste contains enormous metal components, which can be a potential source for the production of oxygen carriers. In the present study, mixed oxygen carrier particles obtained from a printed circuit board (PCB) based electronic wastes are proposed to use in the CLC process. A combustion experiment is conducted to extract metals from the PCB and their composition is identified using an XRF analyzer. These metals are found with 21% Fe2O3, 22.8% CuO and 3% NiO, 9.6% CaO and 10% Al2O3. Using these oxidized printed circuit board metals (OPCB) as oxygen carriers, Aspen plus simulations are performed by integrating a CLC system with a combined cycle power plant having a net capacity of 150 MW for electricity generation. High ash coal (33% ash) and low ash coal (2.1% ash) and rice straw are used in a co-combustion mode for the CLC operation. The effect of co-firing on the net thermal efficiency of the power plants is analyzed. Further economic analyses are carried out for the proposed system and the levelized cost of electricity (LCOE) is estimated for the power plants using various fuels. It is found that the use of electronic waste based oxygen carriers achieved almost an equivalent net thermal efficiency of 42.4% based on the combined cycle power plants, compared to pure Fe2O3 based power plants. Further the economic analysis has shown that the levelized cost of electricity (LCOE) of the CLC integrated combined cycle power system employed with electronic waste based oxygen carriers is 85.9 $/MWh, which is the lowest among other power systems. Metal oxide interaction with fuel ash is investigated experimentally using a thermogravimetric analyzer (TGA). The X-ray diffraction (XRD) analyses of the residue confirmed that there is no formation of complex components due to the ash interaction with OPCB.

Reaction characteristics investigation of CeO2-enhanced CaSO4 oxygen carrier with lignite

Calcium sulfate (CaSO4) has been verified as a promising oxygen carrier (OC) in the chemical looping combustion (CLC) for its high oxygen capacity, abundant reserve and low cost, but its low reactivity and deleterious sulfur species emission from the side reactions of CaSO4 should be well considered for its wide application in CLC. In order to promote the reactivity of CaSO4 and increase its potential to inhibit the gaseous sulfur emission, a CeO2-enhanced CaSO4 OC mixed OC of core–shell structure was prepared using the combined template synthesis method. Reaction characteristics of the prepared CaSO4-CeO2 mixed OC with a typical lignite was first conducted and systematically investigated, and an improved reactivity of the prepared CaSO4-CeO2 mixed OC was demonstrated than its single component CaSO4 or CeO2 due to the fast transfer and exchange of oxygen from the CaSO4 substrate to coal via the doped CeO2. Furthermore, the solid products formed from the mixed CaSO4-CeO2 OC with the selected coal were collected and analyzed. Especially, evolution and redistribution of the sulfur species of different forms were focused. At the latter reaction stage of YN reaction with the CaSO4-CeO2 mixed OC, the SO2 emitted from the side reactions of CaSO4 was greatly diminished and the doped CeO2 was proven effective to directionally fix the SO2 released to turn into different solid sulfur compounds, which were determined as Ce2O2S, Ce2S3 and Ce2(SO4)3·5H2O and formed through the different pathways. In addition, good regeneration of the reduced CaSO4-CeO2 mixed OC could be reached in spite of the unavoidable interaction between the included minerals in coal and the reduced mixed OC. Overall, the combined template method-made CaSO4-CeO2 mixed OC reported herein was not only endowed with enhanced reactivity for coal conversion, but also owned the potential to directionally fix the gaseous sulfur emission, which is quite applicable as OC for simultaneous decarbonatization and desulfurization in the real CLC process.

Liquid chemical looping gasification of biomass: Thermodynamic analysis on cellulose

Liquid chemical looping technology is an innovation of chemical looping conversion technology. Using liquid metal oxide as the oxygen carrier during gasification process could prolong the service life of oxygen carrier and improve the process efficiency. In this paper, based on Gibbs minimum free energy method, the thermodynamic characteristics of biomass liquid chemical looping gasification were studied. Cellulose and lignin, the main components of biomass, were taken as the research objects. Bismuth oxide and antimony oxide were selected as liquid oxygen carriers. The results showed that when the temperature increased from 600 °C to 900 °C, the output of H 2 and CO in the products of cellulose gasification increased from 0.5 and 0.3 kmol to 1.3 and 2.6 kmol respectively. Different ratios of oxygen carriers to gasification raw materials had the best molar ratio. The addition of steam in the system was beneficial to the increase of H 2 content and the increase of H 2 /CO molar ratio. Bi 2 O 3 and Sb 2 O 3 with different mass ratios were used as mixed oxygen carriers. The simulation results showed that the gasification temperature of biomass with different mixed oxygen carriers had the same equilibrium trend products. It could be seen from the results of product distribution that the influence of the mixing ratio of Bi 2 O 3 and Sb 2 O 3 on gas product distribution could be neglected. These results could provide simulation reference and data basis for subsequent research on liquid chemical looping gasification.

Reaction performance of Ce-enhanced hematite oxygen carrier in chemical looping reforming of biomass pyrolyzed gas coupled with CO2 splitting

Hematite enhanced by CeO2 were investigated in chemical looping reforming (CLR) coupled with CO2 splitting to achieve high quality synthesis gas and recycle CO2 into CO. CeO2, Fe2O3 and CeFeO3 solid solution generated from integration of Fe3+ and CeO2 lattice were detected in mixed oxygen carriers as major components. Owing to the charge compensation and oxygen defects formation in lattice rearrangement, CeFeO3 solid solution contributed to promoting oxygen mobility and creating oxygen vacancies. Fe2O3 particles helped provide oxygen spillover pathway from subsurface to surface and improve lattice oxygen transfer through direct contact with the surface of CeFeO3 solid solution. CeO2 enhanced metallic interaction and catalytic oxidation reactivity. The highest gas product amount and H2/CO in CLR process increased by 43.46% and 51.31%, respectively. Also, the instantaneous CO productivity of 2.47 mmol/min/g with 86.13% CO2 conversion was realized in CO2 splitting. Reaction parameters including temperature, CeO2 content and MCO2/mOC indicated positive correlation with CO productivity. The mixed oxygen carrier maintained its reactivity and stability after 15 cycles reaction following the pathway of Ce2O3/Fe2O3/CeFeO3→Ce2O3/Fe→CeFeO3/CeO2/Fe3O4, though sintering behavior was observed in reacted sample. Fe–Ce interactions and oxygen mobility were improved by successive redox reactions, which can offset the negative effects of sintering.

Enhanced performance of copper ore oxygen carrier by red mud modification for chemical looping combustion

Copper ore has been considered as a promising low-cost oxygen carrier candidate for chemical looping combustion (CLC) due to its high reactivity, but pure Cu ore suffers from serious deactivation during long term redox cycling. The present work proposes to combine Cu ore with red mud, an abundant polluting waste of aluminum industry, to improve the reactivity and stability of Cu ore for CLC of methane. Characterizations, redox experiment and kinetic analysis are performed on the mixed oxygen carriers to study the relationship between the physiochemical properties and the redox activity. The results show that the addition of a suitable proportion of red mud (30 wt%) significantly improves the reactivity of Cu ore. Specifically, the average CH4 conversion increases from 67% to 86%, and the actual oxygen carrying capacity rises from 8.2% to 13.8%. In addition, the activation energy for the reduction of Cu ore oxygen carrier by methane decreases from 76.6 kJ/mol to 58.5 kJ/mol after modification. The synergy among CuO in copper ore, Fe2O3 and other components (e.g., Na+, Ca2+, SiO2 and Al2O3) in red mud play an important role in this enhancement. The interaction between CuO and Fe2O3 promotes the reducibility of oxygen carrier via forming a more active oxide (CuFe2O4), and the presence of foreign ions (Na+ and Ca2+) and inert oxides (Al2O3 and SiO2) contributes to the enhanced redox stability of oxygen carrier via acting as supports or additives.

Chemical looping combustion of lignite with the CaSO4–CoO mixed oxygen carrier

Chemical looping combustion (CLC) has well developed as a novel combustion technology for simultaneous completion of the coal combustion and CO2 capture with a low energy penalty. Among all the oxygen carriers available, CaSO4 has gained great attention as a promising oxygen carrier (OC) in CLC due to its high oxygen capacity and low price. But further application of CaSO4 OC also suffers the problems of low reactivity and even deactivation due to the sulfur loss via the side reactions of CaSO4, which should be well addressed. In this research, the CaSO4–CoO mixed OC was prepared firstly, and experiments based on thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) were conducted to evaluate the reaction characteristics and evolution of the gaseous products during the reaction of the prepared CaSO4–CoO mixed OC with lignite (abbreviated as YN). Both the higher reaction rate of the prepared mixed OC with YN coal and the elevated CO2 concentration fully reflected the enhanced reactivity of the prepared mixed OC for YN coal conversion. Furthermore, the micromorphology of the solid reaction products was analyzed by the field emission scanning electron microscopy spectrometry (FESEM). Good sintering resistance of the prepared CaSO4–CoO mixed OC during its reaction with YN was verified, which was ascribed to the temporary inert support role played by the CaSO4 substrate. In order to further study the desulfurization ability of CoO in the mixed OC, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermodynamic simulation were used for in-depth analysis. The gaseous sulfur species released by the CaSO4 side reactions were mainly fixed as solid CoS, Co9S8 and CaS, with the total content higher than 99.8%. And the sulfided OC could be completely regenerated to its original state at the oxidation stage according to the X-ray diffraction (XRD) result. Overall, the prepared CaSO4–CoO mixed OC not only has the enhanced reactivity and good sintering resistance, but also owns the potential to control sulfur released from the CaSO4 side reactions, which has broad application prospect to simultaneously achieve decarbonization and desulfurization in the CLC process.

Chemical looping reforming of biomass based pyrolysis gas coupling with chemical looping hydrogen by using Fe/Ni/Al oxygen carriers derived from LDH precursors

Chemical looping reforming (CLR) coupling with chemical looping hydrogen (CLH) was proposed to reform biomass based pyrolysis gas and achieve high purity H2 by using Fe/Ni/Al oxygen carriers derived from layered double hydroxide (LDH) precursors. The crystal form, morphology and reactivity of oxygen carriers were characterized by various analysis methods. The chemical looping reaction process was investigated in a fixed bed reactor. The experimental results exhibited that Fe0.99Ni0.6Al1.1O4 high dispersed oxygen carrier was synthesized. Moderate lattice oxygen releasing rate 0.00175 wt%/s and escape ratio 95.14% were achieved by the mixed oxygen carrier with Fe/Ni ratio of 3/1. Ni had positive effect on lattice oxygen escape ratio of oxygen carrier while Fe contributed to improve the instantaneous lattice oxygen releasing rate. Furthermore, the CH4/CO2 in biomass based pyrolysis gas was eliminated via CLR process based on the metal catalytic effect of oxygen carrier. The highest CH4 conversion 98.26% and CO2 conversion 71.92% were obtained in CLR process accompanying with the reduction of Fe0.99Ni0.6Al1.1O4 to Fe/Ni alloy. Additionally, H2 with 96.56% purity was achieved in CLH process with the conversion of Fe/Ni alloy to Fe3O4. The H2 instantaneous generation rate increased sharply to the maximum value 34.70 ml/min and then declined gradually. Ni0.6Fe2.4Al1.1O4 new high dispersed compound was generated in the regeneration stage with similar structure to fresh sample and more active Fe species content. Though the particle size of oxygen carrier tended to decline after whole reaction process, the porous structure was still reserved.

Effect of the Addition of CeO2 or MgO on the Oxygen Carrier Capacity and Rate of Redox Reactions of NiO/Fe2O3/Al2O3 Oxygen Carriers.

Two compositions of mixed oxygen carriers (OCs), N1F1A1 (mole ratio of NiO to Fe2O3 is 1:1) and N1F2 (mole ratio of NiO to Fe2O3 is 1:2), were fabricated by mechanical mixing, and the effects of CeO2 and MgO additives on the oxygen-transfer capacity (OTC) and crystal structure were analyzed. In addition, X-ray diffractometry after the oxidation and the reduction steps was carried out to determine the reaction mechanism. Potential mixed OCs of the candidates of various OCs, such as NiO/Fe2O3, are proposed based on the OTC and oxygen-transfer rate evaluated via X-ray diffraction (XRD) phase analysis and thermogravimetric analysis. The chemical reaction equation for the oxidation and reduction was suggested based on the XRD characterization of the phases including NiFe2O4 and MgAl2O4 in N1F1A1 (①) and N1F1A1-Mg5 (④). Addition of MgO is confirmed to enhance the OTC and oxygen-transfer rate. The mechanism of the oxidation and reduction of metal oxides and the role of MgO were suggested. Even though the addition of MgO does not transfer oxygen to the fuel, it does accelerate the oxidation and reduction reactions of NiFe2O4.

The reactivity of CuO oxygen carrier and coal in Chemical-Looping with Oxygen Uncoupled (CLOU) and In-situ Gasification Chemical-Looping Combustion (iG-CLC)

Chemical-looping combustion (CLC) has the primary advantage of the generation of a CO2 enriched flue gas, which greatly benefits CO2 capture. Many researchers have been particularly interested in developing combined or mixed oxygen carriers (OC) to enhance and improve single metal OC properties. To advance the development of mixed OC, this study first focused on the conditions that impact the chemical reactions taking place in both In-situ Gasification Chemical-Looping Combustion (iG-CLC) and Chemical-Looping with Oxygen Uncoupled (CLOU) for single metal Cu based OC with coal and the gas product distributions. The Cu compounds present during CuO reduction (CuO→Cu2O→Cu) were also investigated using X-ray diffraction (XRD) as a function of carbon conversion during the reaction. In this study, the reactivity of char with CuO was tested at two different ratios of CuO to coal char (ϕ = 26 and 8) at 850 °C and 950 °C in Ar and Ar + H2O. Char conversion rates were greater at higher reaction temperatures (950 °C vs 850 °C) for all rests. At a high ratio of oxygen carrier to char (ϕ = 26), char was fully converted by reacting with O2 released from CuO at both 950 °C and 850 °C in Ar. CuO was reduced to Cu2O resulting from the thermal release of gaseous oxygen. At a low ratio of oxygen carrier to char under Ar + H2O (ϕ = 8), char was fully converted at both 950 °C and 850 °C by char combustion with O2 from the OC and syngas CLC from char-H2O gasification. Additionally, CuO was reduced to metallic Cu. IG-CLC had a lower CO2 capture efficiency with CO in the product gas and a lower carbon conversion rate compared to CLOU. From a reactivity view point, CLOU is a promising process for coal chemical-looping combustion.

Chemical Looping Combustion of a Typical Lignite with a CaSO4–CuO Mixed Oxygen Carrier

Calcium sulfate (CaSO4) has attracted a great amount of attention as a potential oxygen carrier (OC) to be applied in chemical looping combustion (CLC) due to its high oxygen transfer capacity, wide distribution, and easy accessibility, but its low reactivity and sulfur emission from side reactions of CaSO4 should be well resolved. In this research, the CaSO4–CuO mixed OC was prepared using the template method combined with the sol–gel combustion synthesis (SGCS). Its reaction characteristics with a selected lignite (designated as YN) were investigated and the greatly enhanced reactivity of this mixed OC was confirmed relative to the single CaSO4 and CuO. Meanwhile, the comprehensive heat effect showed the desired exothermic characteristics for this mixed OC reaction with YN when the mass ratio of CaSO4 to CuO was fixed as 6:4. Furthermore, morphological analysis indicated that the solid products from YN reaction with the CaSO4–CuO mixed OC were porous without discernible sintering, mainly because the CaS...

In Situ Gasification Chemical Looping Combustion of Coal Using the Mixed Oxygen Carrier of Natural Anhydrite Ore and Calcined Limestone

Abstract The utilization of CaSO4-based oxygen carrier suffers the deactivation problem caused by sulphur loss. To capture the gas sulphides and to improve the stability of CaSO4 oxygen carrier, calcined limestone was introduced into the fuel reactor of Chemical Looping Combustion (CLC). Kinetic behaviors and thermodynamics of the combined process of coal gasification and oxygen carrier reduction using the mixed oxygen carrier CaSO4-CaO under different atmospheres were investigated. The effects of reaction temperature, gasification intermediate, and molar ratio of CaO to CaSO4 on gas sulphide emissions, CO2 generation, and distribution of other gas emissions and characterization of solid products are taken into account. It is found the CaO-based additive evidently suppressed the sulphur emissions, and improved both chemical reaction rate and CO2 generation efficiency. The sulfation products, both CaS and CaSO4, can be used as oxygen carrier later. The optimum reaction parameters are evaluated and obtained in terms of gas sulphide emissions, CO2 capture, other gas releases and maintenance of oxygen transfer capability.

Reduction kinetics of iron-based oxygen carriers using methane for chemical-looping combustion

The performance of three iron-based oxygen carriers (pure Fe2O3, synthetic Fe2O3/MgAl2O4 and iron ore) in reduction process using methane as fuel is investigated in thermo-gravimetric analyzer (TGA). The reaction rate and mechanism between three oxygen carriers and methane are investigated. On the basis of reactivity in reduction process, it may be concluded that Fe2O3/MgAl2O4 has the best reactivity with methane. The reaction rate constant is found to be in the following order: Fe2O3/MgAl2O4 > pure Fe2O3 > iron ore and the activation energy varies between 49 and 184 kJ mol−1. Reduction reactions for the pure Fe2O3 and synthetic Fe2O3/MgAl2O4 are well represented by the reaction controlling mechanism, and for the iron ore the phase-boundary controlled (contracting cylinder) model dominates. The particles of iron ore and synthetic Fe2O3/MgAl2O4 have better stability than that of pure Fe2O3 when the reaction temperature is limited to lower than 1223 K. These preliminary results suggest that iron-based mixed oxygen carrier particles are potential to be used in methane chemical looping process, but the reactivity of the iron ore needs to be increased.

New fabrication of mixed oxygen carrier for CLC: Sludge and scale from a power plant

After successful operation of a 200kW gas circulating combustion system for over 100h with NiO/Al2O3 oxygen carrier (OC) particles, we are ready to scale-up the process up to a 1–10MW system. To supply large quantity of OC particles for the scaled-up system at a lower cost, iron oxides are considered as a cheaper source material of OC. Mixed NiO/Fe2O3/Al2O3 OC particles were fabricated with commercial iron oxide reagents by a spray-dryer and tested for oxygen transfer capacity (OTC) and physical/chemical properties to investigate the feasibility of mass production. While looking for cheap iron oxides from wastes or industrial products, considerable amounts of scale and sludge were found from power plants. The main component of the sludge and scale collected from power plants was more than 95% of iron oxides and the residue was also oxides, such as MnO, SiO2, Cr2O3 and CaO, which can be an active material or a binder for OC. The particle size of the sludge and scale is also suitable for the OC fabrication, eliminating the need of pre-treatment. Four different OC particles were fabricated by a spray-dryer, calcined at 1100, 1200 and 1300°C, and their physical/chemical properties were measured. It was found that mixed NiO/Fe2O3/Al2O3 oxide carriers are good candidates for a 1–10MW class circulation combustion of fluidized bed. The contribution of this research is that the sludge and scale collected from power plants can be used to capture CO2 emitted from power plants.

Analysis of Reactivity of a CuO-Based Oxygen Carrier for Chemical Looping Combustion of Coal

Chemical looping combustion (CLC) of coal was conducted with 12 CuO-based oxygen carriers supported on SiO2, Al2O3, TiO2, and ZrO2 prepared by mechanical mixing. The influence of an inert support and the effect of particle size on the reactivity of CuO-based oxygen carrier particles were studied. Five mixed oxide oxygen carriers composed of different amounts of CuO and Fe2O3 on TiO2 were prepared to study the synergistic effect between two active metallic oxides. It was found that CuO-based oxygen carriers showed high reactivity in redox reactions. The onset temperature of the reduction reaction decreased when TiO2 was added as an inert support. The particles of CuXTi950 showed the highest reactivity to convert the coal. Although the smaller particle size of the oxygen carrier can enhance the coal conversion, it also makes it more difficult in the separation of the oxygen carrier from fly ash and unburned carbon. For mixed oxygen carriers, the synergistic effect between different active oxides was observed. From a reactivity and an energy-balance perspective, particles of CuO–Fe2O3/TiO2 mixed oxide oxygen carriers are suitable for use in CLC of coal. The X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used for the characterization of the fresh and used oxygen carriers. Results support that direct CLC of coal is promising when CuO-based oxygen carriers are used at low temperatures.