High Oxygen Transport Capacity Research Articles

Evaluation of low-cost oxygen carriers for biomass chemical looping gasification

Biomass Chemical Looping Gasification (BCLG) is a cost-effective and efficient alternative to conventional gasification. The selection of appropriate oxygen carriers (OCs) is crucial for stable BCLG performance. These OCs need to possess high reactivity, selectivity, material strength, and resistance to sintering. The study investigated various OC materials, including industrial wastes (copper, nickel slag, desulphurization, LD, and ladle slags), residential waste (sewage sludge ash), and natural ore (manganese). The evaluation of OCs focused on reactivity, H2-selectivity, mechanical strength and sintering behaviour. Except for ladle slag, all OC samples exhibited favourable reactivity due to the presence of Fe- and Mn-oxides possessing high oxygen transport capacity (10–17.6%). Nickel slag, manganese ore, and desulphurisation slag displayed notable H2-selectivity (8.7 to 10.4). It can be attributed to the presence of less-active (lattice) oxygen, limiting strong oxygen agents such as Fe2O3, Fe3O4, and Mn2O3. Moreover, desulphurization slag contained highly selective Ca2Fe2O5, which falls within the partial oxidation zone of the Ellingham diagram. Furthermore, all OC samples exhibited desirable material strength (>20 MPa), suitable for fluidised bed reactors. However, nickel, LD, and ladle slags demonstrated limited sintering with sintering onset temperatures exceeding 963 °C. This limited sintering may be attributed to the absence of iron silicates, iron-bearing aluminium silicates, manganese silicates, and potassium that contributed to the low thermal stability observed in the remaining OCs. Altogether, nickel slag calcined at 1100 °C was identified as the most promising OC material with optimal reactivity, selectivity, material strength, and minimal sintering for BCLG. Overall, this study provides a detailed and scientific methodology for OC selection and can aid future OC development.

Investigation on the Redox Properties of a Novel Cu-Based Pr-Modified Oxygen Carrier for Chemical Looping Combustion.

CO2 levels in the atmosphere are growing as a result of the burning of fossil fuels to meet energy demands. The introduction of chemical looping combustion (CLC) as an alternative to traditional combustion by transporting oxygen emphasizes the need to develop greener and more economical energy systems. Metal oxide, also defined as an oxygen carrier (OC), transports oxygen from the air to the fuel. Several attempts are being made to develop an OC with a reasonable material cost for superior fuel conversion and high oxygen transport capacity (OTC). This study aims to synthesize a potential OC using the wet impregnation method for the CLC process. Thermogravimetric analysis (TGA) was used to determine the cyclic redox properties using 5% CH4/N2 and air as reducing and oxidizing gases, respectively. The 10CuPA-based OC retained a high OTC of about 0.0267 mg O2/mg of OC for 10 cycles that was higher than 10CuA-based OC. Furthermore, the oxygen transfer rate for 10CuPA-based OC was relatively higher compared to 10CuA-based OC over 10 cycles. In comparison to 10CuA-based OC, the 10CuPA-based OC presented a steady X-ray diffraction (XRD) pattern after 10 redox cycles, implying that the phase was stably restored due to praseodymium-modified γ alumina support.

Assessment of plants radial oxygen loss for nutrients and organic matter removal in full-scale constructed wetlands treating municipal effluents

Bidirectional cross flow wetlands with different plant species were set to investigate seasonal variation in radial oxygen loss (ROL) and its effects on COD and NH4+-N removal. Findings demonstrated a strong seasonal effect on the rate of ROL, with Arundo donax var.versicolor showing the highest ROL of 2.99 μmol·h−1·g−1. Additionally, ROL showed strong positive correlations with plant total biomass (P < 0.01), aboveground biomass (P < 0.01), height, maximum root length (P < 0.01), root porosity (P < 0.01), and removal efficiency of COD and NH4+-N (P < 0.01). Furthermore, high-throughput sequencing analysis of substrate samples from three wetland units planted with Thalia dealbata, Canna indica and Arundo donax var. versicolor revealedProteobacteria as the predominant rhizospheric phylum. Relative abundance of Alpha- and Gamma-Proteobacteria were higher in the Arundo donax var.versicolor samples due to its higher oxygen transport capacity. These results demonstrate that root properties are important determinants for selecting appropriate plants for constructed wetland.

Hydrogen-rich gas production by catalytic steam gasification of rice husk using CeO2-modified Ni-CaO sorption bifunctional catalysts

Hydrogen-rich gas production from rice husk via steam gasification and catalytic reforming using CeO2-modified Ni-CaO sorption bifunctional catalysts synthesized by sol–gel method in a two-stage system was investigated. The results show that the Ce0.7Ni1Ca5 catalyst achieves maximum H2 concentration (85.81(±0.39) vol.%) and H2 yield (35.82(±0.28) mmol g-1biomass) under a condition of 500 °C, S/C (steam/carbon in biomass) molar ratio of 5, catalyst/biomass mass ratio of 2.5, producing the lowest content of CO2 (3.62(±0.16) vol.%), CO (4.27(±0.11) vol.%), CH4 (4.49(±0.18) vol.%), and C2-C3 (1.81(±0.09) vol.%), correspondingly. Ce0.7Ni1Ca5 also exhibits excellent cyclic stability in H2 production, CO2 sorption, and inhibition of carbon deposition. The H2 concentration and H2 yield remain above 81.88 vol% and 32.11 mmol g-1biomass, respectively, and CO2 emission keeps below 4.85 vol% during 10 cyclic tests. The carbon deposition of Ce0.7Ni1Ca5 is only about 30% of that of Ni1Ca5 after 10 cycles and hardly increased after 5 cycles. It is found that well-dispersed CeO2 can effectively prevent the sintering of NiO and delay agglomeration of CaO species, stabilizing CaO carbonation and CO2 sorption, and the strong Ni-O-Ce interaction induces the creation of oxygen vacancies that facilitate the fracture of O–H bonds of water for the formation of H2. Furthermore, the high oxygen transport capacity of CeO2 not only forms abundant mesopores structure to promote WGS and SMR reactions for enhancing H2 production, but also contributes to reforming the carbon deposited on the catalyst surface by lowering the oxidation temperature of amorphous carbon containing low-molecular aromatic or aliphatic compounds with lower degree of graphitization.

Ca-based sorbents as precursors of oxygen carriers in chemical looping combustion of sulfurous fuels

Limestone Chemical Looping Combustion process (LCL-C™) patented by Alstom Power Inc. proposes the formation of a CaSO4 oxygen carrier via sulfur retention process when supplying a Ca-based sorbent together with sulfurous fuels in the fuel reactor. In the present research work, four Ca-based sorbents, three limestones and one dolomite with different physico-chemical characteristics, have been evaluated as oxygen carrier precursors during consecutive redox cycles by thermogravimetric analysis. 5% H2 and 3000 vppm H2S (N2 to balance) and 10% O2 (N2 to balance) were selected as reducing and oxidizing gases, respectively. Additionally, an anhydrite ore has also been studied for comparison purpose. Results from SEM-EDX characterization of the sorbent particles revealed the existence of three different sulfation patterns: (1) uniform distribution, (2) core–shell structure and (3) high sulfur content with high limitation to be converted into CaSO4. All sorbents exhibited high oxygen transport capacity (Roc = 13.3–20.8%), except for the material showing pattern (3) (Roc = 2.6%). Likewise, the materials presented high reactivity with rate index values ranged between 11.0 and 18.8%/min. Therefore, the Ca-based sorbents herein assessed can be considered suitable precursors for the formation of CaSO4 based oxygen carriers to be used in CLC systems burning sulfurous fuels.

Efficient Metal–Organic Framework-Derived Cu–Zr Oxygen Carriers with an Enhanced Reduction Reaction Rate for Chemical Looping Air Separation

As a key to realize the chemical looping air separation (CLAS) process, CuO is a good oxygen carrier candidate because of its high oxygen transport capacity and excellent thermodynamic characterist...

Biomedical perfluorohexane-loaded nanocapsules prepared by low-energy emulsification and selective solvent diffusion.

Perfluorohexane-loaded nanocapsules are interesting materials for many biomedical applications such as oxygen delivery systems or contrast agents. However, their formulation into stable colloidal systems is challenging because of their hydro- and lipophobicity, high density and high vapour pressure. In this study, perfluorohexane-loaded polymeric nanocapsules are prepared for the first time by low-energy emulsification and selective solvent diffusion. The colloidal stability of the perfluorohexane nano-emulsion templates has been improved by the incorporation of an apolar low-density oil (isopropyl myristate) in the dispersed phase, thus addressing droplet coarsening and migration phenomena. The perfluorohexane-loaded nanocapsules prepared from the nano-emulsions show sizes smaller than the corresponding emulsion templates (below 150 nm by dynamic light scattering) and exhibit good stability under storage conditions. Hyperspectral enhanced dark field microscopy revealed a layered core/shell structure and allowed also to confirm the encapsulation of perfluorohexane which was quantified by elemental microanalysis. Although isopropyl myristate has an unfavourable biocompatibility profile, cell viability is enhanced when perfluorohexane is present in the nanocapsules, which is attributed to its high oxygen transport capacity.

A high-performance oxygen carrier with high oxygen transport capacity and redox stability for chemical looping combustion

Oxygen carrier with high oxygen transport capacity and redox stability can greatly decrease material usage and maintain the stable system operation in chemical looping combustion. However, the trade-off between two aspects must be considered because operating the material at the high oxygen transport capacity often leads to severe sintering. Herein, we reported NiFeAlOx material as an oxygen carrier and investigated its chemical looping combustion performance. The results showed that it had an average oxygen transport capacity of 2.37 mmol/g and good stability during 20 cycles. In contrast, conventional Fe2O3 and FeAlOx material exhibited the transport capacity of 1.07 mmol/g and 0.78 mmol/g respectively. The higher transport capacity and redox stability enable the CO2 yield of 2.37 mmol/g in terms of nearly 100% capture efficiency. Through various characterization techniques, we found the high performance of this material was due to reversible exsolution and dissolution between the active compounds and the parent spinel support.

Development of Suitable CuO-Based Materials Supported on Al2O3, MgAl2O4, and ZrO2 for Ca/Cu H2 Production Process

Functional materials for the sorption enhanced reforming process for H2 production coupled to a Cu/CuO chemical loop have been synthesized. The performance of CuO-based materials supported on Al2O3, MgAl2O4, and ZrO2 and synthesized by different routes has been analyzed. Highly stable materials supported on Al2O3 or MgAl2O4 synthesized by coprecipitation and mechanical mixing with sufficient Cu loads (around 65% wt) have been successfully developed. However, it has been found that coprecipitation under these conditions is not a suitable route for ZrO2. Spray-drying and deposition precipitation did not provide the best chemical features to the materials. As the Ca/Cu process is operated in fixed bed reactors, the best candidates were pelletized and their stability was again assessed. Pellets with high chemical and mechanical stability, high oxygen transport capacity, and good mechanical properties have been finally obtained by coprecipitation. The good homogeneity that provides this route would allow an ea...

Development of (Mn0.77Fe0.23)2O3 particles as an oxygen carrier for coal combustion with CO2 capture via in-situ gasification chemical looping combustion (iG-CLC) aided by oxygen uncoupling (CLOU)

Chemical looping combustion (CLC) involves the use of a solid oxygen carrier to transport the oxygen from the air to a fuel. Attention has recently been focused on oxygen carriers based on Mn-Fe mixed oxides because they are cheap materials that are able to release oxygen at high temperature, the so-called oxygen uncoupling step. The aim of this work was to assess the use of (Mn0.77Fe0.23)2O3 material as an oxygen carrier with the ability to transport oxygen both by reduction with gaseous fuels and by oxygen uncoupling, i.e. typical mechanisms in CLC and in Chemical Looping with Oxygen Uncoupling (CLOU), respectively. The particles prepared by mechanical mixing were screened to obtain particles of sufficient reactivity and mechanical strength for use in a fluidized bed reactor. The preparation methodology and calcining temperature were varied. The reactivity of one selected material was evaluated by performing redox cycles both in a TGA and a batch fluidized-bed reactor. Thus, its behaviour was assessed during both decomposition-regeneration of bixbyite phase [(Mn0.77Fe0.23)2O3] in CLOU and the reduction-oxidation of spinel phase [(Mn0.77Fe0.23)3O4] with gaseous fuels, i.e. H2, CO and CH4. Its oxygen uncoupling and re-oxidation capability was highly influenced by the reaction temperature and oxygen concentration. On the other hand, CLC redox cycles with gaseous fuels showed high reactivity with H2 and CO and high oxygen transport capacity by reduction to mangano-wüstite phase [(Mn0.77Fe0.23)O]. Good fluid-dynamic behaviour was observed for the oxygen carrier particles along the redox cycles without agglomeration problems, regardless of the operating conditions used in the batch fluidized-bed reactor. Thus, the selected material was considered a promising candidate for use in both in CLC with syngas and CLC with coal.

Enhanced performance of ilmenite modified by CeO2, ZrO2, NiO, and Mn2O3 as oxygen carriers in chemical looping combustion

In this paper, Canadian ilmenite ore-based oxygen carriers (OCs) were, for the first time, prepared by coating the ilmenite ore particles with metal oxides of CeO2, ZrO2, NiO, or Mn2O3 by using precursors of Ce(NO3)3, Zr(O-t-Bu)4, Ni(NO3)2 or Mn(NO3)2, respectively. The reduction reactivity of OCs with methane was examined by temperature-programmed methane reduction (CH4-TPR), and their cyclic performance during the chemical looping methane combustion was investigated isothermally in a thermogravimetric analyzer (TGA). It was demonstrated that the addition of all types of these metal oxides had a positive effect, such as an accelerated reactivity towards methane reduction and enhanced resistance to agglomeration. Specifically, the modification using CeO2 and ZrO2 improved the reactivity of ilmenites towards CH4 reduction at a lower temperature of 750°C. However, the promotion effect decreased when the temperature was elevated to 800°C. NiO modification not only enabled the ilmenite to improve reaction with CH4 due to the generation of a more active compound NiFe2O4, but also to result in higher oxygen transport capacity. Similarly, Mn2O3 modification showed a higher reactivity and a fast reaction rate towards the oxidation of CH4 attributed to the generation of ferrimanganic oxides. Furthermore, the overall oxygen transport capacity of Mn2O3 composed OCs was increased caused by the variable valence of Mn. The addition of CeO2, ZrO2, and NiO always retarded the activation process of ilmenite ore, by contrast, Mn2O3 modified samples accelerated the activation due to the irreversible phase transformation from Mn2O3 to Mn3O4 accompanied by a volume shrinking that led to a flawed particle surface reducing the diffusion resistance.

Pressurized chemical looping combustion with CO: Reduction reactivity and oxygen-transport capacity of ilmenite ore

Ilmenite ore has shown promise as an oxygen-carrier in chemical looping combustion operation. Since operating at higher pressures has been proven to hold important benefits for chemical looping combustion, this study aims to investigate the performance of ilmenite ore under pressurized conditions. The reduction reactivity and oxygen-transport capacity of Canadian ilmenite ore in pressurized chemical looping combustion during redox cycles were investigated in a pressurized thermogravimetric analyzer, using carbon monoxide and air as reductant and oxidant respectively. While increasing the CO partial pressure from 0.32 to 0.80MPa (at a constant total pressure) led to higher rates of reaction and higher oxygen-transport capacity, increasing the total pressure up to 2.4MPa (at a constant CO partial pressure while varying the pCO2) had the reverse effect. Furthermore, increasing the temperature from 1123 to 1323K resulted in a pronounced increase in the reduction rate. Although increasing the particle size from 75–125μm to 300–425μm resulted in a slower rate of reduction, the effect was minor. A kinetic grain model with a changing grain size was fitted to reduction rate data obtained at 1.6MPa. The order of reaction with respect to the CO partial pressure was calculated as 0.67, while the activation energy was determined to be 115kJ/mol.

NiO/CaAl2O4 as active oxygen carrier for low temperature chemical looping applications

The implementation of CO2 capture systems in conventional processes has been proposed by the IPCC as an effective way to reduce anthropogenic CO2 emissions. However, these capture systems may represent an important decrease in the global efficiency for conventional processes. Chemical Looping has already been demonstrated as a promising technology for more efficient CO2 capture. Novel reactor concepts have been proposed in the literature, in which the reactions take place at lower temperatures with increased overall energy efficiency. However, few investigations have been carried out regarding the behaviour of oxygen carriers at relatively low operating temperatures. In this work, an active Ni-based oxygen carrier supported on CaAl2O4 inert material has been tested and characterized. The oxygen carrier has shown a promising behaviour for low temperature applications. However, it has been demonstrated that the oxygen carrier has to be pre-treated because of an interesting activation process which takes place only at high reduction temperatures. Oxygen carrier activation is caused by a reorganization of superficial nickel. Fresh oxygen carrier is covered by a layer of nickel with a strong interaction with the support. However, once the sample is reduced at high temperatures Ni is reorganized into small grains with reduced interaction with the support. This results in an enhancement in the reactivity and a higher oxygen transport capacity. After about 200 redox cycles, a small decrease in the solid conversion is observed due to agglomeration of the NiO grains. Nevertheless, the redox kinetics is still sufficiently fast for low temperature applications, provided that the oxygen carrier is pre-activated. The kinetics rates for the gas–solid reactions and gas-phase catalytic reactions have been determined, which can be used to predict the performance of the activated NiO/CaAl2O4 oxygen carrier for low temperature chemical looping applications.

TGA and kinetic modelling of Co, Mn and Cu oxides for chemical looping gasification (CLG)

AbstractOxygen carriers for biomass gasification are capable of absorbing oxygen from air and desorbing it in the gasifier. Based on thermodynamic equilibrium, copper, manganese, and cobalt oxides have the highest oxygen release capacities among the different oxygen carriers. These oxygen carriers were deposited on alumina via incipient wetness impregnation. The weight loss of the CuO–Cu2O carrier, as measured in a thermo‐gravimetric analyzer, was 10 % while it was 7 % for the Co3O4–CoO couple and only 3 % for the Mn2O3–Mn3O4 couple. The optimum operating temperature for the CuO oxygen carrier was 100 °C higher compared to the other two at 950 °C. A modified nuclei growth model (MNG) characterizes the weight loss/gain during the reduction‐oxidation cycles. The reduction rate is 3 times higher at 875 °C compared to 825 °C while the oxidation rate decreases more than 10 times. The CuO carrier surface area decreased by 70 %, while it was 30 % and 60 % in the Co3O4 and Mn2O3 carriers, respectively. Cobalt has a lower tendency to sinter at high temperature compared to either copper or manganese and has a higher oxygen transport capacity and oxidation‐reduction rates. Therefore, despite its higher cost and toxicity, it might be considered as a potential oxygen carrier especially for solid fuel gasification.

Feasibility of a Co Oxygen Carrier for Chemical Looping Air Separation: Thermodynamics and Kinetics

AbstractChemical looping air separation (CLAS) is based on the chemical looping principle: oxygen carriers release oxygen to carrier gas in a reduction reactor and absorb oxygen from air in an oxidation reactor. High oxygen transport capacity, high reactivity in reduction and oxidation reactions, and resistance to attrition and agglomeration are some of the criteria that feasible oxygen carrier materials should fulfill. Thermodynamic analysis was applied to prove the potential of Co3O4 as oxygen carrier. ZrO2 served as binder to improve the anti‐sintering property and reactivity. Kinetic experiments were performed to determine the reaction rate and conversion of the oxygen carrier. Stability and durability of the oxygen carrier were characterized before and after cyclic experiments. The Co/Zr oxygen carrier proved to be a suitable candidate for the CLAS process.

Hydrogen production with CO 2 capture by coupling steam reforming of methane and chemical-looping combustion: Use of an iron-based waste product as oxygen carrier burning a PSA tail gas

In this work it is analyzed the performance of an iron waste material as oxygen carrier for a chemical-looping combustion (CLC) system. CLC is a novel combustion technology with the benefit of inherent CO 2 separation that can be used as a source of energy for the methane steam reforming process (SR). The tail gas from the PSA unit is used as fuel in the CLC system. The oxygen carrier behaviour with respect to gas combustion was evaluated in a continuous 500 W th CLC prototype using a simulated PSA off-gas stream as fuel. Methane or syngas as fuel were also studied for comparison purposes. The oxygen carrier showed enough high oxygen transport capacity and reactivity to fully convert syngas at 880 °C. However, lower conversion of the fuel was observed with methane containing fuels. An estimated solids inventory of 1600 kg MW th −1 would be necessary to fully convert the PSA off-gas to CO 2 and H 2O. An important positive effect of the oxygen carrier-to-fuel ratio up to 1.5 and the reactor temperature on the combustion efficiency was found. A characterization of the calcined and after-used particles was carried out showing that this iron-based material can be used as oxygen carrier in a CLC plant since particles maintain their properties (reactivity, no agglomeration, high durability, etc.) after more than 111 h of continuous operation.