Presence Of Steam Research Articles

On the zeolitic imidazolate framework-8 (ZIF-8) membrane for hydrogen separation from simulated biomass-derived syngas

Hydrogen separation from biomass-derived syngas is a critical step in the utilization of gasification technology. Compared with the traditional methods, membrane technology provides an effective and low-cost solution for adjusting the gas composition and collecting H2 in syngas environments. In this study, a zeolitic imidazolate framework-8 (ZIF-8)-based membrane, which is a potential candidate for H2 separation from biomass-derived syngas, was successfully fabricated through the seeded (secondary) growth method and the subsequent post-treatments. The prepared ZIF-8 membrane exhibited a modest H2 separation performance for H2/CO2 and H2/CO, with separation factors of 4.95 and 6.08, respectively, and a H2 permeance of 7.81 × 10−8 mol m−2 s−1 Pa−1 at 200 °C in the simulated biomass-derived syngas environments (H2/CO2/CO) with the presence of steam. In particular, the H2/CO2 and H2/CO separation factors were increased by 36% and 97%, respectively, with respect to those obtained through bare supports. Despite the promising H2 perm-selectivity, the H2/CO2 and H2/CO separation factors of ZIF-8 membranes at 200 °C under water-containing syngas environments were maintained up to 10 h but the longer exposure led to the gradual degradation and eventual reduction toward those of bare supports after 15 h seemingly due to the water-involved membrane degradation. This study provides the availability and limitation of ZIF-8 membranes for H2 separations in stimulated biomass-derived syngas environments.

Effect of vanadium contamination on the framework and micropore structure of ultra stable Y-zeolite

Y-zeolites are the main component of fluid catalytic cracking (FCC) catalyst for conversion of crude petroleum to products of high demand including transportation fuel. We investigated effects of vanadium which is present as one of the impurities in FCC feedstock on the framework and micropore structure of ultra-stable (US) Y-zeolite. The zeolite samples were prepared and characterized using standard techniques including: (1) X-ray diffraction, (2) N2 adsorption employing non local density functional theory method, NLDFT, (3) Transmittance and Pyridine FTIR, (4) Transmittance electron microscopy (TEM), and (5) 27Al and 29Si MAS-NMR. Results revealed that in the presence of steam, vanadium caused excessive evolution of non inter-crystalline mesopores and structural damage. The evolved mesopore size averaged about 25.0nm at 0.5wt.% vanadium loading, far larger than mesopore size in zeolitic materials with improved hydrothermal stability and performance for FCC catalyst. A mechanism of mesopore formation based on accelerated dealumination has been proposed and discussed. Vanadium immobilization experiments conducted to mitigate vanadium migration into the framework clearly showed vanadium is mobile at reaction conditions. From the results, interaction of vanadium with the passivator limits and decreases mobility and activity of vanadium into inner cavities of the zeolite capable of causing huge structure breakdown and acid sites destruction. This study therefore deepens insight into the causes of alteration in activity and selectivity of vanadium contaminated catalyst and hints on a possible mechanism of passivation in vanadium passivated FCC catalyst.

Effect of the coupling action between volatiles, char and steam on isothermal combustion of coal char

The effects of the coupling action between volatiles, char and the steam on combustion characteristics of coal char were studied in a customized isothermal thermogravimetric experimental system. The results showed that volatiles released during devolatilization played a key role in the combustion. Heat produced by volatiles combustion accelerated the burning rate of pulverized coal, at the same time, volatiles combustion depleted the oxygen surrounding the char particles. With the decrease of the furnace temperature, the acceleration effect on burning rate due to volatiles combustion improved while its impact to oxygen consuming reduced. The influence of volatiles on burning rate acceleration and oxygen depletion appeared to be inversely affected by the coal rank. When oxygen concentration decreased from 21% to 5% in the reactor, the burning process of volatiles and coal char displayed a tendency of separation. In addition, the presence of steam, which increased the burning rate and reduced the burning time noticeably, had a significant impact on the combustion of coal char.

Steam-assisted catalytic cracking of n-hexane over La-Modified MTT zeolite for selective propylene production

Steam-assisted catalytic cracking is an interesting process to produce petrochemicals. Herein, we report the incorporation of lanthanum (La) into MTT zeolite to improve its stability in the presence of steam. A series of La modified MTT zeolite was prepared in a microwave-assisted ion-exchange. The activity of parent and modified MTT zeolites have been investigated in the steam catalytic cracking of n-hexane. Both parent and La modified zeolites showed excellent selectivity to olefins in the steam-assisted catalytic cracking of n-hexane. The incorporation of La into the cation exchange sites of MTT zeolite enhanced its selectivity to light olefins, especially propylene. La-modified MTT zeolites exhibited better resistance to steam. The modification of MTT with a rare earth metal is a promising approach to improve the stability of aluminosilicate in steam-assisted reactions.

CaO Nanoparticles Coated by ZrO2 Layers for Enhanced CO2 Capture Stability

CaO is an attractive CO2 acceptor, due to its good kinetics and high capture capacity, even under low CO2 concentration conditions. The major problem associated with CaO acceptors is the loss of CO2 capture capacity during cyclic operations. The declining performance of the CaO acceptors is more pronounced in the presence of steam. In this article we propose a method for fabrication of nano-CaO acceptors coated with a ZrO2 layer to improve the stability. The nano-CaO acceptor was prepared by a thermal decomposition method following coating by a sol–gel, incipient wet impregnation, or hydrolysis process. Thermogravimetric analysis of the cyclic process of CO2 carbonation/decarbonation revealed that the nano-CaO acceptors with sol–gel (CaO-SG) and wet impregnation (CaO-IM) coatings have higher CO2 capture capacity and longer lifetime than the uncoated ones. The capture capacity of CaO-IM declined after 14 cycles, whereas CaO-SG remained stable up to 20 cycles. X-ray diffraction, scanning electron microscopy...

Creep in Interlaminar Shear of an Oxide/Oxide Ceramic-Matrix Composite at Elevated Temperature1

Creep behavior in interlaminar shear of an oxide–oxide ceramic composite was evaluated at 1100 °C in laboratory air and in steam environment. The composite (N720/AS) consists of a porous aluminosilicate matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear properties were measured. The interlaminar shear strength (ILSS) was determined as 7.6 MPa. The creep behavior was examined for interlaminar shear stresses in the 2–6 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. Tertiary creep was noted in tests performed at 6 MPa. Creep run-out defined as 100 hrs at creep stress was not achieved in any of the tests. Larger creep strains and higher creep strain rates were produced in steam. However, the presence of steam had a beneficial effect on creep lifetimes. Composite microstructure, as well as damage and failure mechanisms, was investigated.

Mineral sorbents for downstream sodium capture in biomass gasifiers

The concentration of alkali metal vapors (especially sodium and potassium) ordinarily reaches more than 10ppm during agri-based biomass gasification, leading to several problems, as: corrosion of turbine blades, decreased catalytic activity and undesirable deposition on downstream equipment. Adsorption on aluminosilicates is thought to be an interesting option to reduce alkali concentration in syngas thus generated. Therefore, in this work, six samples of mineral sorbents were exposed to sodium vapors at high temperatures and under moisture conditions. Before and after sodium vapor exposure, samples were characterized by various analytical techniques, textural analysis, TGA, XRD and XRF, aimed at better understanding of the binding mechanisms and assessing their ability to remove sodium from the gas phase. The combination of all techniques allowed the confirmation of the sorption mechanisms, which is chemisorption in most cases, with the formation of silicates and aluminosilicate salts. Sorbents with higher Al content, such as green clay and bauxite, showed potential for the application of capturing alkali metal vapors at high temperatures in the presence of steam by fixing sodium as nepheline (NaAlSiO4). Sorbents with higher silica content (e.g., diatomite and kaolin) showed irreversible fixation of Na, which could not be recovered even upon acid leaching. The presence of elements other than Si and Al (e.g. Ca) also plays an important role in Na sorption.

CFD modeling of passive autocatalytic recombiners*

Abstract This study deals with numerical modeling of passive autocatalytic hydrogen recombiners (PARs). Such devices are installed within containments of many nuclear reactors in order to remove hydrogen and convert it to steam. The main purpose of this work is to develop a numerical model of passive autocatalytic recombiner (PAR) using the commercial computational fluid dynamics (CFD) software ANSYS-FLUENT and tuning the model using experimental results. The REKO 3 experiment was used for this purpose. Experiment was made in the Institute for Safety Research and Reactor Technology in Julich (Germany). It has been performed for different hydrogen concentrations, different flow rates, the presence of steam, and different initial temperatures of the inlet mixture. The model of this experimental recombiner was elaborated within the framework of this work. The influence of mesh, gas thermal conductivity coefficient, mass diffusivity coefficients, and turbulence model was investigated. The best results with a good agreement with REKO 3 data were received for k-ɛ model of turbulence, gas thermal conductivity dependent on the temperature and mass diffusivity coefficients taken from CHEMKIN program. The validated model of the PAR was next implemented into simple two-dimensional simulations of hydrogen behavior within a subcompartment of a containment building.

Pyrolysis/reforming of rice husks with a Ni–dolomite catalyst: Influence of process conditions on syngas and hydrogen yield

The influence of process conditions on the production of syngas and H2 from biomass in the form of rice husks was investigated using a two-stage pyrolysis/catalytic reforming reactor. The parameters investigated were, reforming temperature, steam flow rate and biomass particle size and the catalyst used was a 10 wt.% Ni–dolomite catalyst. Biomass was pyrolysed in the first stage, and the product volatiles were reformed in the second stage in the presence of steam and the Ni–dolomite catalyst. Increase in catalyst temperature from 850 °C to 1050 °C marginally improved total syngas yield. However, H2 yield was increased from 20.03 mmol g−1 at 850 °C to 30.62 mmol g−1 at 1050 °C and H2 concentration in the product gas increased from 53.95 vol.% to 65.18 vol.%. Raising the steam flow rate increased the H2 yield and H2 gas concentration. A significant increase in H2:CO ratio along with a decrease in CO:CO2 ratio suggested a change in the equilibrium of the water gas shift reaction towards H2 formation with increased steam flow rate. The influence of particle size on H2 yield was small showing an increase in H2 production when the particle size was reduced from 2.8–3.3 to 0.2–0.5 mm.

Effect of Steam on Partial Decomposition of Dolomite

Partially decomposed dolomite is a promising base material for regenerable MgO-based sorbents to capture CO2 from precombustion syngas at high pressures and temperatures. An important characteristic of the sorbent affecting the economic viability of this class of sorbents is the reactivity and the capacity of the sorbent for CO2 capture. To improve the reactivity of the sorbent, the thermal behavior and the kinetics of partial decomposition of dolomite were studied in the temperature range of 520–610 °C in a dispersed-bed reactor. The microstructure and the nature of the solid products were found to be strongly dependent on the CO2 partial pressure near the reacting interface and on the decomposition temperature. A significant increase in the rate of the dolomite decomposition reaction was found in the presence of steam. Steam improves the kinetics of decomposition, modifies the radial distribution of the pores, and improves the connectivity of the pores inside the dolomite particles, which decreases the diffusion resistance of produced carbon dioxide inside the particle. A shrinking core model with variable product layer diffusivity was used to fit the experimental data and determine the kinetic parameters of the dolomite decomposition reaction. The results indicate that transport of CO2 across the reacting interface in the porous particle was the main limiting factor in the decomposition reaction at the experimental conditions investigated. The model is shown to provide an excellent fit to the experimental data on partial decomposition of dolomite in the temperature range studied.

Thermal decomposition of individual and mixed plastics in the presence of CaO or Ca(OH)2

Individual and mixed plastics consisting of polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET) were decomposed at 600°C in the presence and absence of either calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) under a steam atmosphere. In the presence of CaO, steam cracking was enhanced, increasing gas and liquid yields and decreasing the wax content derived from PE and PP. The production of sublimating substances from PET was also reduced. However, the presence of CaO and steam had a negative influence on PS degradation, reducing the oil yield. In addition, synergistic effects were observed in plastic mixtures. PET enhanced gasification, PS and PET reduced wax production from PE and PP, and mixing all four plastics enhanced oil production in the presence of CaO and steam. Both CaO and Ca(OH)2 enhanced the total gas and oil yield of mixed plastics, achieving a maximum oil yield of 52.2wt% in the presence of steam. Furthermore, solids were almost completely decomposed at 700°C, with only 0.2wt% of residue remaining.

γ-Al<sub>2</sub>O<sub>3</sub> 촉매상에서 열분해와 가수분해에 의한 NF<sub>3</sub> 촉매분해 특성

In this study, the catalytic activity of γ-Al2O3 was investigated for the decomposition of NF3. Reactions for NF3 decom- position were carried out in the range of reaction temperature of 330∼730 ℃ and GHSV of 3,000∼15,000 mL/g-cat⋅h in a fixed-bed catalytic reactor system. Thermal decomposition of NF3 was also performed in order to compare with the cata- lytic decomposition of NF3. The conversion of NF3 by the catalytic decomposition at 400 ℃ was four times higher than that of the thermal decomposition. It was confirmed that the reaction behavior of NF3 over γ-Al2O3 exhibited two reaction pathways in the presence of steam. Fluorine in NF3 over γ-Al2O3 was chemically absorbed to AlF3 by the gas-solid reaction in the absence of steam. The catalytic decomposition of NF3 occurred by hydrolysis with steam. It was also confirmed by FT-IR analysis that NF3 was completely decomposed to NOx and HF above 500 ℃.

Syngas production from coal in presence of steam using filtration combustion

Empirical studies were performed over the effects of volume fractions of coal, steam concentrations, and air flows during filtration combustion of mixtures from an inert solid (alumina spheres) and coal, normally referred as hybrid filtration combustion. Temperature, velocity, and concentration of gaseous products of the combustion wave were reported as function of different operational conditions. The results showed that maximum temperature increases were achieved with a high fraction of coal (<50%), a minor decrease of temperature was observed when increasing the steam concentration of the oxidant gas, and no changes were detected while changing the air flow in the feed stream. Downstream propagation velocities were recorded for each parameter of interest. Experiments showed that a maximum production of syngas was attained under the following conditions: a) 80% of steam concentration with 8 l/min of air flow and 50% volume fraction of coal in the hybrid mixture achieving concentrations of 26.92% of H2 and 17.08% of CO; b) 70% volume fraction of coal with 20% of steam and 8 l/min of air flow reaching a 11.81% and 17.12% of H2 and CO, respectively; c) 10 l/min of air flow with 50% volume fraction of coal and 20% of steam reaching a 10.13% and 11.60%, of H2 and CO respectively. The results show that filtration combustion technology can be used to gasify coal into syngas.

Catalytic partial oxidation of methane over nickel and ruthenium based catalysts under low O 2 /CH 4 ratios and with addition of steam

Abstract Catalytic partial oxidation (CPO) of methane to synthesis gas at low O 2 /CH 4 ratios and in the presence of steam was investigated over nickel and ruthenium catalysts supported on hydrotalcite-derived materials. The influence of catalyst properties and composition on activity, temperature profile and deactivation by carbon formation was examined. All catalyst presented high methane conversions, close to the values predicted by thermodynamic equilibrium and such conversions increased in proportion to the metal surface of the catalyst tested. The temperature profiles at O 2 /CH 4 = 0.2 and H 2 O/CH 4 = 0.3 and a constant exit temperature of 700 °C varied depending on the catalyst type; it was possible to examine catalyst deactivation from the change in the shape of the profile of each catalyst. Since the O 2 /CH 4 and H 2 O/CH 4 ratios were low, the risk or potential for carbon formation was thermodynamically favorable along the entire catalytic bed; however, this potential was qualitatively higher when the temperature profile of the catalyst presented a pronounced maximum peak at the inlet of the reactor. During catalytic reaction tests and methane decomposition experiments, the ruthenium catalyst did not formed appreciable amounts of carbon while a bimetallic catalyst (Ni and Ru) form only small amounts (in comparison with the nickel catalysts). For the ruthenium catalyst, a higher O 2 /CH 4 ratio favored conversions closer to the equilibrium value. The observations presented in this work indicate that during the CPO of methane, at low O 2 /CH 4 ratios and in the presence of steam, the catalyst properties and composition will have a substantial influence on the extent of the combustion and reforming reactions along the catalytic bed. This will in turn define the temperature profile, and therefore the risk or potential for carbon formation; this risk might effectively be overcome by the use of ruthenium-containing catalysts.

Thermodynamic Equilibrium Analysis of Propane Dehydrogenation with Carbon Dioxide and Side Reactions

A thermodynamic analysis of propane dehydrogenation with carbon dioxide was performed using constrained Gibbs free energy minimization method. Different reaction networks corresponding to different catalytic systems, including non-redox and redox oxide catalysts, were simulated. The influences of CO2/C3H8 molar ratio (1–10), temperature (700–1000 K), and pressure (0.5–5 bar) on equilibrium conversion and product composition were studied. In the presence of CO2 with a molar ratio of CO2/C3H8 = 1, the temperature of dehydrogenation can be 30 K lower than that of dehydrogenation in the presence of steam (H2O/C3H8 = 1) and about 50 K lower than that of simple dehydrogenation without dilution to achieve 60% propane conversion. It was found that the occurrence of dry reforming of propane and coke-forming side reactions could strongly impact the equilibrium product composition of the multireaction system and, therefore, these reactions should be kinetically controlled. Comparison of the simulated reactant conversions with those reported in the literatures revealed that the experimental conversion levels of propane are far below the corresponding equilibrium values due to rapid catalyst deactivation by coke, implying that research efforts should be directed toward formulation of more active and selective catalysts.

Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge

Chemical looping combustion (CLC) of sewage sludge is an appropriate solution of how to manage the continuously increasing sewage sludge production and at the same time, how to rationally use renewable resource. This work attempted to investigate the combustion performance of sewage sludge in a continuous CLC unit based hematite oxygen carrier. Besides, the characteristics of hematite and fly ash during chemical looping combustion of sewage sludge were elaborated. Compared to either bituminous coal or anthracite, sewage sludge was a unique solid fuel in the term of high combustion efficiency even at 800°C. Additionally, there were no char particles bypassed to air reactor during CLC of sewage sludge. During 10h continuous operation, hematite shows a slightly decrease reactivity, indicating a good long-term reactivity of the oxygen carrier used. Although some ash particles deposited on the surface of hematite, no reaction between sludge ash and hematite were identified. The only phosphate identified incinerated sludge ash was Ca2P2O7. However, the form of phosphorus in fly ash based on CLC was CaH2P2O7 and CaHPO4. The presence of steam in flue gas and the reduction atmosphere can significantly accelerate the generation of CaH2P2O7 and CaHPO4. Then, possible options of utilizing sludge ash on agriculture were discussed.

Highly stable La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fibre membrane for air separation swept by steam or steam mixture

Perovskite oxide ceramic membranes have the potentials for industrial oxygen production, particularly to replace the conventional expensive air separation units, which is of significance to improve the viability of the clean energy technologies like IGCC and Oxyfuel projects. In most of these applications, the steam presence is unavoidable thus requiring these membranes to be stable in the steam-containing atmosphere under high temperatures. In this work, La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) perovskite hollow fibre membrane was prepared and tested for O2 separation in the presence of steam in the sweep gas. Three membranes were successfully tested at temperature of 900°C using the sweep containing 3%, 4–12% and 100% steam (balanced by helium) for 370, 200 and 500h, respectively. In the long run, the steam can etch the membrane surface, as evidenced by the leaching of iron and the formation of strontium carbonate. This etching process however is too slow to cause substantial damage on the membrane performance. Using pure steam sweep, the LSCF membrane was operated at 900°C for nearly 500h without any sign of O2 flux decay, compared favourably with Ba0.5Sr0.5Co0.8Fe0.2O3−α (BSCF) hollow fibre membranes, which only lasted for 90h. In contrast to the severe poisoning effects of other acid gases like SO2 or NOx on the O2 permeation through similar LSCF membrane, the detrimental effect of steam is negligible. In fact, the presence of minor steam less than 12% in the sweep is kinetically favourable for the surface oxygen exchange reactions, leading to the improvement of the O2 flux.

Evolution of the Surface Area of Limestone during Calcination and Sintering

The calcination reaction of limestone is always companied by sintering of the calcined product. In addition, accelerated sintering rates and a reduced specific surface area are observed in the presence of steam and carbon dioxide. To simulate the change of surface area and the porosity of limestone samples in a simultaneous calcination and sintering process, a combined model based on both a sintering model and a calcination model is established. The calcination model, which predicts calcination conversion as a function of time, is based on the initial properties of the sorbent. The sintering model is according to the German and Munir model in which the main transport mechanism is supposed to be lattice diffusion. In a flow reactor, the surface area value and calcination rate of limestone in the presence of steam and CO2 are also described by the combined model with modified parameters.

Creep behavior in interlaminar shear of a Hi-NicalonTM/ SiC-B4C composite at 1200∘C in air and in steam

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200 ∘ C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-Nicalon TM fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Similar creep strains were accumulated in air and in steam. Furthermore, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. The tested specimens were also examined using electron probe microanalysis (EPMA) with wavelength dispersive spectroscopy (WDS). Analysis of the fracture surfaces revealed significant surface oxidation, but only trace amounts of boron and carbon. Cross sectional analysis showed increasing boron concentration in the specimen interior.

ChemInform Abstract: Strategies for Improving the Performance and Stability of Ni‐Based Catalysts for Reforming Reactions

AbstractReview: [51 refs.