Mechanism and kinetics of Al2O3 nanoparticles formation by reaction of bulk Al with H2O and CO2 at sub- and supercritical conditions

Oxidation of bulk samples of 〈Al〉 by water and H₂O/CO₂ mixture at sub- and supercritical conditions for uniform temperature increase and at the injection of H₂O (665 K, 23.1 MPa) and H₂O/CO₂ (723 K, 38.0 MPa) fluids into the reactor has been studied. Transition of 〈Al〉 into AlOOH and Al₂O₃ nanoparticles has been found out. Aluminum samples oxidized by H₂O and H₂O/CO₂ fluids at the injection mostly consist of large particles (300-500 nm) of α-Al₂O₃. Those oxidized for uniform temperature increase contain smaller particles (20-70 nm) of γ-Al₂O₃ as well. Mechanism of this phenomenon is explained by orientation of oxygen in H₂O polar molecules to the metal in the electric field of contact voltage at Al/AlOOH and Al/Al₂O₃ boundary. Addition of CO₂ to water resulted in CO, CH₄, CH₃OH and condensed carbon, increase in size of Al₂O₃ nanoparticles and significant decrease in time delay. In pure CO₂ 〈Al〉 oxidation resulted in oxide film. Using temperature and time dependences of gaseous reactant pressure and Redlich-Kwong state equation, kinetics of H₂ formation has been described and oxidation regularities determined. At aluminum oxidation by H₂O and H₂O/CO₂ fluids, self-heating of the samples followed by oxidation rate increase has been registered. The samples of oxidized aluminum have been studied with a transmission electronic microscope, a thermal analyzer and a device for specific surface measurement. The effect of oxidation conditions on the characteristics of synthesized nanoparticles has been found out.     

CO2 Adsorption Kinetics of K2CO3/Activated Carbon for Low‐Concentration CO2 Removal from Confined Spaces

K₂CO₃ supported on activated carbon (K₂CO₃/AC) is a promising means to remove low‐concentration CO₂ from confined spaces. In this removal process, physical adsorption plays an important role but it is difficult to quantify the amount of CO₂ adsorbed when both H₂O and CO₂ are present. The linear driving force mass transfer model is adopted to study the CO₂ adsorption kinetic characteristics of K₂CO₃/AC by analyzing the experimental data. The effect of K₂CO₃ and H₂O on the adsorption of CO₂ in K₂CO₃/AC was also evaluated. K₂CO₃ loaded on the support is found to increase the mass transfer resistance but decrease the activation energy required for the physical adsorption process. The presence of water vapor is disadvantageous to achieve high physical adsorption capacity since it enhances the chemical sorption in the competitive dynamic sorption process.     

The Effects of CO2 and H2O on the NO x Destruction Performance of a Model NO x Storage/Reduction Catalyst

The effects of CO₂ and H₂O on the NO _x storage and reduction characteristics of a Pt/Ba/Al₂O₃ catalyst were investigated. The presence of CO₂ and H₂O, individually or together, affect the performance and therefore the chemistry that occurs at the catalyst surface. The effects of CO₂ were observed in both the trapping and reduction phases of the experiments, whereas the effect of H₂O seems limited to the trapping phase. The data also indicate that multiple types of sorption sites (or mechanisms for sorption) exist on the catalyst. One mechanism is characterized by a rapid and complete uptake of NO _x . A second mechanism is characterized by a slower rate of NO _x uptake, but this mechanism is active for a longer time period. As the temperature is increased, the effect of H₂O decreases compared to that of CO₂. At the highest temperatures examined, the elimination of H₂O when CO₂ is present did not affect the performance.     

Effects of diluents on NO x formation in coflow CH4/air diffusion flames

The effect of diluent addition on NO _x formation in a laminar CH₄/air coflow diffusion flame was investigated by numerical simulation with experimental verification. The hydrocarbon fuel stream was diluted with N₂, CO₂, and Ar. The volume fraction of diluents systematically changed from 0.0 to 0.5. The simulation data agree well with the experimental one. The computational results indicate that overall the three diluents reduce the formation of NO and the effects vary from weak to strong in the order: N₂, Ar and CO₂. Differences between the influences of the various diluents are discussed in terms of the thermal, the dilution and the direct chemical effects, respectively. Further, the addition of CO₂ reduces the formation of NO₂, while the addition of N₂ or Ar has little effect on it. However, the formation rate of N₂O increases by each of the added diluents.     

Effects of water vapor on gas permeation and separation properties of MFI zeolite membranes at high temperatures

Understanding the effects of water vapor on gas permeation and separation properties of MFI zeolite membranes, especially at high temperatures, is important to the applications of these zeolite membranes for chemical reactions and separation involving water vapor. The effects of water vapor on H₂ and CO₂ permeation and separation properties of ZSM‐5 (Si/Al ～ 80) zeolite and aluminum‐free silicalite membranes were studied by comparing permeation properties of H₂ and CO₂ with the feed of equimolar H₂/CO₂ binary and H₂/CO₂/H₂O ternary mixtures in 300–550°C. For both membranes, the presence of water vapor lowers H₂ and CO₂ permeance to the same extent, resulting in negligible effect on the H₂/CO₂ separation factor. The suppression effect of water vapor on H₂ and CO₂ permeation is larger for the less hydrophobic ZSM‐5 zeolite membrane than for the hydrophobic silicalite membrane, and, for both membranes, is stronger at lower temperatures and higher water vapor partial pressures. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures

Hydrogenation of CO₂ was studied on 1% Rh/TiO₂ reduced at different temperatures. The interaction of CO₂ with the catalyst and that of the CO₂+H₂ mixture was also studied. FTIR and TPD measurements revealed that CO₂ dissociation depends on the reduction temperature of the catalyst. In the surface reaction, besides Rh carbonyl hydride, formate groups and different carbonates and surface formyl species were also formed. The surface concentration of the formyl group depended on the reduction temperature. The initial rate of CO₂ hydrogenation significantly increased with increasing reduction temperature but after some time it drastically decreased. The promotion effect of the reduction temperature was explained by the formation of oxygen vacancies on the perimeter of the Rh/TiO₂ interface, which can be re-oxidized by the adsorption of CO₂ and H₂O.     

Effects of water vapor, CO2 and SO2 on the NO reduction by NH3 over sulfated CaO

Gas effects on NO reduction by NH₃ over sulfated CaO have been investigated in the presence of O₂ at 700–850 °C. CO₂ and SO₂ have reversible negative effects on the catalytic activity of sulfated CaO. Although H₂O alone has no obvious effect, it can depress the negative effects of CO₂ and SO₂. In the flue gas with CO₂, SO₂ and H₂O co-existing, the sulfated CaO still catalyzed the NO reduction by NH₃. The in situ DRTFTS of H₂O adsorption over sulfated CaO indicated that H₂O generated Br?nsted acid sites at high temperature, suggesting that CO₂ and SO₂ competed for only the molecularly adsorbed NH₃ over Lewis acid sites with NO, without influencing the ammonia ions adsorbed over Br?nsted acid sites. Lewis acid sites shifting to Br?nsted acid sites by H₂O adsorption at high temperature may explain the depression of the negative effect on NO reduction by CO₂ and SO₂.     

Effects of space velocity on methanol synthesis from Co2/Co/H2 over Cu/ZnO/Al2O3 catalyst

The space velocity had profound and complicated effects on methanol synthesis from CO₂/CO/H₂ over Cu/ZnO/Al₂O₃ at 523 K and 3.0MPa. At high space velocities, methanol yields as well as the rate of methanol production increased continuously with increasing CO₂ concentration in the feed. Below a certain space velocity, methanol yields and reaction rates showed a maximum at CO₂ concentration of 5–10%. Different coverages of surface reaction intermediates on copper appeared to be responsible for this phenomenon. The space velocity that gave the maximal rate of methanol production also depended on the feed composition. Higher space velocity yielded higher rates for CO₂/ H₂ and the opposite effect was observed for the CO/H₂ feed. For CO₂/CO/H₂ feed, an optimal space velocity existed for obtaining the maximal rate.     

NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

Preferential CO oxidation over Pt–SnO2/Al2O3 in hydrogen rich streams containing CO2 and H2O (CO removal from H2 with PROX)

The effects of reaction gases including CO₂ and H₂O and temperature on the selective low-temperature oxidation of CO were studied in hydrogen rich streams using a flow micro-reactor packed with a Pt–SnO₂/Al₂O₃ sol–gel catalyst that was initially designed and optimized for operation in the absence of CO₂ and H₂O. 100% CO conversion was achieved over the 1 wt% Pt–3 wt% SnO₂/Al₂O₃ catalyst at 110 °C using a feed composition of 1.0% CO, 1.5% O₂, 25% CO₂, 10% H₂O, 58% H₂ and He as balance at a space velocity of 24,000 cm³/(g h). CO₂ in the feed was found to decrease CO conversion significantly while the presence of H₂O in the feed increased CO conversion, balancing the effect of CO₂.     

Investigation of the Phase Equilibria of CO2/CH3OH/H2O and CO2/CH3OH/H2O/H2 Mixtures

The storage of excess electricity from renewable energy sources is nowadays a crucial topic. One promising technology is the methanol (CH₃OH) synthesis from H₂/CO₂ mixtures. The achievable one‐pass conversion is limited within this exothermic equilibrium reaction. A possibility to overcome this limitation would be withdrawing CH₃OH and H₂O from the gas phase through in situ condensation under reaction conditions. In this work, the phase equilibrium for mixtures representative for different degrees of conversion was studied. A view cell was employed to determine systematically the single‐ and two‐phase regimes and obtain phase envelopes for mixtures of H₂, CO₂, CH₃OH, and H₂O from 66 to 305 °C and 61 to 233 bar. Furthermore, the densities in the single‐phase area were determined and quantified by an empirical model.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

The combined effect of H2O and SO2 on the simultaneous calcination/sulfation reaction in CFBs

The combined effect of H₂O and SO₂ on the reaction kinetics and pore structure of limestone during simultaneous calcination/sulfation reactions under circulating fluidized bed (CFB) conditions was first studied in a constant-temperature reactor. H₂O can accelerate the sulfation reaction rate in the slow-sulfation stage significantly but has a smaller effect in the fast-sulfation stage. H₂O can also accelerate the calcination of CaCO₃, and should be considered as a catalyst, as the activation energy for the calcination reaction was lower in the presence of H₂O. When the limestone particles are calcining, SO₂ in the flue gas can react with CaO on the outer particle layer and the resulting CaSO₄ blocks the CaO pores, increases the diffusion resistance of CO₂, and, in consequence, decreases the calcination rate of CaCO₃. Here, gases containing 15% H₂O and 0.3% SO₂ are shown to increase the calcination rate. This means that the accelerating effect of 15% H₂O on CaCO₃ decomposition is stronger than the impeding effect caused by 0.3% SO₂. The calcination rate of limestone particles was controlled by both the intrinsic reaction and the CO₂ diffusion rate in the pores, but the intrinsic reaction rate played a major role as indicated by the effectiveness factors determined in this work. This may explain the synergic effect of H₂O and SO₂ on CaCO₃ decomposition observed here. Finally, the effect of H₂O and SO₂ on sulfur capture in a 600 MWe CFB boiler burning petroleum coke is also analyzed. The sulfation performance of limestone evaluated by simultaneous calcination/sulfation is shown to be much higher than that by sulfation of CaO. Based on our calculations, a novel use of the wet flue gas recycle method was put forward to improve the sulfur capture performance for high-sulfur low-moisture fuels such as petroleum coke. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1256–1268, 2019     

EFFICIENT DESULFURIZATION IN O2/CO2 COMBUSTION: DEPENDENCE ON COMBUSTION CONDITIONS AND SORBENT PROPERTIES

Based on experiments on desulfurization, CaSO₄ decomposition, and a system approach using theoretical analysis, efficient in-furnace desulfurization in O₂/CO₂ combustion was investigated. The influence of combustion conditions and sorbent properties on system desulfurization efficiency was clarified. The global desulfurization efficiency was found to increase with O₂ purity. The global desulfurization efficiency in a dry recycle was higher than that in a wet recycle. The global efficiency of in-furnace desulfurization decreased with initial O₂ concentration. As the temperature increased, the global desulfurization efficiency increased first and then decreased due to the decomposition of CaSO₄. In the temperature range investigated, the global desulfurization efficiency in O₂/CO₂ coal combustion was much higher than that of conventional coal combustion in air. The global desulfurization efficiency decreased with sorbent size. When the particle radius decreased to one quarter, the global desulfurization efficiency doubled, becoming as high as 80%. The global desulfurization efficiency was very different among the three sorbents investigated, whether in O₂/CO₂ combustion or in conventional air combustion. The global desulfurization efficiency increased in the order of Ca(OH)₂, scallop, and limestone in O₂/CO₂ combustion, but in the order of scallop, Ca(OH)₂, and limestone in conventional air combustion. Nevertheless, all three sorbents demonstrated much higher desulfurization efficiency in O₂/CO₂ combustion than in conventional air combustion.     

Autothermal Reforming of Methane with Integrated CO2 Capture in a Novel Fluidized Bed Membrane Reactor. Part 2 Comparison of Reactor Configurations

The reactor performance of two novel fluidized bed membrane reactor configurations for hydrogen production with integrated CO₂ capture by autothermal reforming of methane (experimentally investigated in Part 1) have been compared using a phenomenological reactor model over a wide range of operating conditions (temperature, pressure, H₂O/CH₄ ratio and membrane area). It was found that the methane combustion configuration (where part of the CH₄ is combusted in situ with pure O₂) largely outperforms the hydrogen combustion concept (oxidative sweeping combusting part of the permeated H₂) at low H₂O/CH₄ ratios (<2) due to in situ steam production, but gives a slightly lower hydrogen production rate at higher H₂O/CH₄ ratios due to dilution with combustion products. The CO selectivity was always much lower with the methane combustion configuration. Whether the methane combustion or hydrogen combustion configuration is preferred depends strongly on the economics associated with the H₂O/CH₄ ratio.     

Hydrogen production via chemical looping dry reforming of methane: Process modeling and systems analysis

The present study presents and analyses a family of “chemical looping dry reforming” (CLDR) processes that produce inherently separated syngas (H₂ and CO) streams via a combination of methane cracking in a “cracker reactor” and the Boudouard reaction (i.e., conversion of the formed carbon with CO₂ to CO) in a “CO₂ reactor,” and then further maximize the H₂ yield via conversion of the produced CO via water-gas-shift. Remaining CO₂ emissions are minimized via CO₂ capture and sequestration. Four different configurations are evaluated which differ in how the heat required for the highly endothermic dry reforming reaction is supplied: (i) combustion of additional CH₄ feed (CLDR-CH₄); (ii) combustion of some of the CO produced in the CO₂ reactor (CLDR-CO); and combustion of some of the carbon produced in the cracker reactor with (iii) pure oxygen (CLDR-C-oxy); or (iv) with air (CLDR-C-air). Process models are developed to comparatively analyze the mass and energy balances of these configurations, and benchmark them against H₂-production via conventional dry reforming and steam reforming of methane. Our results show that CLDR-C-oxy is the most promising H₂-production pathway among the chemical looping and conventional technologies both in terms of chemical energy efficiency and in terms CO₂ emissions. Thus, the unique flexibility offered by the production of inherently separated syngas streams in CLDR enables overcoming the disadvantage of the strongly endothermic dry reforming reaction by combusting carbon internally in the reactor and thus achieving highly effective heat integration. Overall, the results support the technical viability and demonstrate the promise for strong process intensification of CLDR compared to conventional dry reforming and even steam reforming, the most widely used H₂-production pathway to-date.     

Pulse study of methane partial oxidation to syngas over SiO2-supported nickel catalysts

Methane was pulsed over pure CuO and NiO as well as Cu/La₂O₃ and Ni/La₂O₃ catalysts at 600° C. Results indicate that the mechanisms for methane activation over copper and nickel are quite different. Over CuO, methane is converted to CO₂ and H₂O, most likely via the combustion mechanism; whereas metallic copper does not activate methane. Over NiO in the presence of metallic nickel sites, methane activation follows the pyrolysis mechanism to give CO, CO₂, H₂ and H₂O. Similar results were obtained over the Cu/La₂O₃ and Ni/La₂O₃ catalysts. XRD investigations indicate that copper and nickel existed as CuLa₂O₄ and LaNiO₃ respectively in the La₂O₃-supported catalysts. The effect of La₂O₃ on the activation of methane is discussed.     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

Conversions of fuel-N,volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N₂O during combustion of a single coal particle in O₂/N₂ and O₂/H₂O were studied in a tube reactor at low temperature. In O₂/N₂, NO reaches the maximum value in the devolatilization stage and N₂O reaches the maximum value in the char combustion stage. In O₂/H₂O, both NO and N₂O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N₂O in O₂/N₂ are obviously higher than those in O₂/H₂O, due to the reduction of H₂O on NO and N₂O. Temperature changes the trade-off between NO and N₂O. In O₂/N₂ and O₂/H₂O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N₂O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at < O₂ > = 30 vol% in O₂/N₂. In O₂/H₂O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at < O₂ > = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N₂O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N₂O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N₂O in both atmospheres. This work is to develop efficient ways to understand and control NO and N₂O emissions for a clean and sustainable atmosphere.