Showing papers on "Partial oxidation published in 2003"

Brief Overview of the Partial Oxidation of Methane to Synthesis Gas

Abstract: A review of the main developments in the partial oxidation of methane to synthesis gas since the first paper in 1929 to the present day is given. The reaction is discussed from the view of the thermodynamics; the main catalysts studied for the reaction are summarised, and the reaction mechanism is discussed. The review is not comprehensive, but it is designed to provide a general background to the most important developments in the field.

Catalytic autothermal reforming of methane and propane over supported metal catalysts

Abstract: Catalytic autothermal reforming of methane and propane over supported metal catalysts has been investigated in the present study. The carbon deposition region and the heat balance of the reaction have been determined from the equilibrium calculations. The sequence of the activities of the 2 wt.% metal on alumina support for autothermal reforming of methane was Rh>Pd>Ni>Pt>Co. The catalytic activity of 10 wt.% Ni/Al 2 O 3 was higher than that of the 2 wt.% Rh/Al 2 O 3 . The activity of Ni was significantly lowered by the preferential oxidation of the catalyst in the reactant gas at low temperatures. Although little carbon deposition was observed for the autothermal reforming of methane in the deposition-free region expected from the equilibrium, a large amount of carbon deposition was observed for the propane autothermal reforming even in the steam-rich conditions. The deposited carbon possessed fibrous morphology. The catalytic autothermal reforming appears to be initiated by decomposition of hydrocarbon at the inlet zone; then the reforming reaction subsequently proceeded in the catalyst bed.

Catalytic decomposition of methane over Ni-Al2O3 coprecipitated catalysts: Reaction and regeneration studies

Abstract: The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al2O3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.

Ultrafast Reaction between LiH and NH3 during H2 Storage in Li3N

Abstract: Li3N is a potential H2 storage material due to its high theoretical H2 capacity (10.4 wt %). A critical potential issue regarding this N-based storage material is the generation of NH3, which consumes some H2 and also constitutes a poison for the downstream processes. In this Letter, by using the temperature-programmed decomposition of a two-layer material (LiNH2 and LiH), we demonstrate that NH3 produced via the decomposition of LiNH2 is completely captured by LiH even at very short contact times (25 ms) with the carrier gas. This ultrafast reaction between NH3 and LiH inhibits NH3 formation during the hydrogenation of Li3N and also prevents the NH3 generated during the dehydrogenation of the hydrogenated Li3N to escape into the H2 stream. However, if the hydrogenated Li3N was previously exposed to the atmosphere, some NH3 could escape into the H2 stream during the H2 desorption, due to the partial oxidation of LiH by the water present in air.

Equilibria in Fuel Cell Gases I. Equilibrium Compositions and Reforming Conditions

Abstract: Using thermochemical data of ca. 300 compounds consisting of carbon, hydrogen, and oxygen, the amounts of equilibrium products have been calculated for various fuel cell fuels including alkanes (CH 4 , C 3 H 8 , C 8 H 1 8 , C 1 2 H 2 6 ), alcohols (CH 3 OH, C 2 H 5 OH, C 3 H 7 OH), alkenes, alicyclic hydrocarbons, and dimethyl ether, as well as for other hydrocarbon-containing fuels such as biogas and coke oven gas, in the temperature range between 100 and 1000°C. It has been resealed that the major constituents in typical fuel cell gases in thermodynamic equilibrium are H 2 (g), H 2 O(g), CO(g), CO 2 (g). CH 4 (g), and C(s). Possible minor constituents are specified while their concentrations were negligibly low. Derived are the minimum amounts of H 2 O and CO 2 for reforming and O 2 for partial oxidation, thermodynamically essential to prevent carbon deposition.

Role of Hydrocarbon Deposits in the Enhanced Performance of Direct-Oxidation SOFCs

Abstract: We have examined the changes that occur in the performance of solid oxide fuel cells (SOFCs) with Cu-ceria-yttria-stabilized zirconia anodes at 973 K following exposure to various hydrocarbon fuels, including methane, propane, n-butane, n-decane, and toluene. For cells with Cu contents of 20 wt % or less, large increases were observed in the power densities for operation in after the anode had been exposed to any of the hydrocarbons except methane. The increased performance is completely reversible upon oxidation of the anode and subsequent reduction in The enhancement decreases with increasing Cu content, implying that the deposits improve the connectivity of the metallic phase in the anode. Impedance spectra taken on cells before and after exposure to hydrocarbon fuels confirm that the conductivity of the anode improves after exposure. Temperature-programmed oxidation and weight changes were used to show that the deposits that enhance performance correspond to of the anode and are probably not graphitic. Measurements of the open-circuit voltages in hydrocarbon fuels suggest that equilibrium is established with partial oxidation products and that the chemical structure of the deposits change upon current flow. Finally, the implications of these results for operation of SOFC on hydrocarbons without added steam and with low copper contents are discussed. © 2003 The Electrochemical Society. All rights reserved.

Catalytic performance of mesoporous VOx/SBA-15 catalysts for the partial oxidation of methane to formaldehyde

Abstract: The ordered hexagonal mesoporous pure silica SBA-15 has been used as a support for preparing highly dispersed V-containing catalysts. VOx/SBA-15 samples with different V loading (1.26–5.54 wt.%) have been prepared by impregnation with an aqueous solution of NH4VO3, characterized by N2 adsorption, UV-Vis spectroscopy, and H2-TPR and then evaluated for the partial oxidation of methane to formaldehyde with oxygen. For vanadium coverages below the theoretical monolayer capacity (corresponding to ca. 4 wt.% V) the SBA-15 surface was predominantly covered by monomeric and low oligomeric vanadium oxide species, while at higher vanadium coverages agglomerated vanadium entities and even some small amounts of microcrystalline V2O5 were also formed. The conversion of methane (at constant temperature and GHSV) passed a maximum at a vanadium loading of 3.85 wt.%, while the maximum of activity typically occurs at lower V contents (1–2 wt.%) in amorphous vanadia-silica catalysts. This feature can be related with the much higher surface area of the mesoporous support, which allows the monolayer capacity to be reached at higher loading than in amorphous silica. The influence of the main reaction parameters (temperature, GHSV, CH4:O2 ratio) on the productivity of formaldehyde was studied. A maximum space–time yield (STY) of formaldehyde of about 2.4 kg kg−1 h−1 was obtained in this work for the most active VOx/SBA-15 catalyst (3.85 wt.% V) at 618 °C, GHSV=417,000 l (N) kg−1 h−1, and CH4:O2 molar ratio of 8:1. This value of STYHCHO is significantly higher than the maximum STY (ca. 1.3 kg kg−1 h−1) reported for VOx/SiO2, and comparable to that recently reported for VOx/MCM-41 (ca. 2.2 kg kg−1 h−1), thus confirming the promising use of high surface area mesoporous supports for the POM reaction on vanadia based catalysts.

Promotion effect of hydrogen on surface steps in SCR of NO by propane over alumina-based silver catalyst as examined by transient FT-IR

Abstract: The mechanistic cause of enhancement of C3H8-SCR activity by addition of H2 over Ag/Al2O3 was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C3H8 + O2, nitrates were mainly formed on Ag/Al2O3. An addition of H2 into a C3H8-SCR atmosphere increased the concentration of surface acetate significantly, but decreased the concentration of surface nitrates. Formation and consumption rates of acetate and nitrates were estimated with transient in-situ IR measurement. By the addition of H2, both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C3H8 + O2 and acetate in a flow of NO + O2 were also increased by the addition of H2. From a comparison between the evolutions of adsorbed species (nitrate and acetate) and gaseous species (NO and C3H8), it was clarified that the NO reduction activity is controlled by partial oxidation of C3H8 to mainly surface acetate. The addition of H2 results in remarkable promotion of partial oxidation of C3H8 to mainly surface acetate, which is the rate-determining step of C3H8-SCR in the absence of H2.

Production of hydrogen from methanol over binary Cu/ZnO catalysts - Part II. Catalytic activity and reaction pathways

Abstract: The activity for conversion of methanol into hydrogen was investigated over binary Cu/ZnO catalysts derived from precursors prepared by two different techniques, viz. oxalates formed in microemulsion and hydroxycarbonates formed in aqueous solution. Some distinct differences in the reaction pathways were observed. During partial oxidation of methanol under a sub-stoichiometric oxygen/methanol ratio, the microemulsion materials exhibited considerably higher combustion activity in the low-temperature region than a catalyst prepared in aqueous solution. Over the former, oxygen was quickly converted by methanol combustion, after which steam reforming was initiated, producing hydrogen at the expense of water and gradually decreasing the net heat of reaction. Hence, a reaction sequence for the partial oxidation reaction over microemulsion catalysts is proposed, consisting of consecutive methanol combustion and steam reforming, followed by decomposition when all oxygen has been consumed. Over the hydroxycarbonate catalyst, the reaction ignited at a higher temperature, directly producing hydrogen by partial oxidation of methanol. When the two types of catalysts were evaluated in the steam reforming reaction, all catalysts displayed the typical S-shaped dependence of methanol conversion on temperature. However, there was a downward shift in the temperature at which methanol reached complete conversion, favouring the hydroxycarbonate material. Hydrogen was produced selectively over all catalysts, but carbon monoxide formation was more pronounced over the microemulsion materials. The differences in catalytic behaviour are discussed in terms of catalyst morphology and the valence state of Cu in the working catalyst.

Autothermal reforming of methane over Ni catalysts supported over CaO–CeO2–ZrO2 solid solution

Abstract: Ni catalysts supported on various solid solutions of ZrO2 with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO2 solid solution showed a high activity. Incorporation of CaO in the ZrO2 matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO2–ZrO2 solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane.

Activation of methane to syngas over a Ni/TiO2 catalyst

Abstract: Partial oxidation of methane (POM) to syngas and CH4/CO2 reforming have been investigated over a Ni/TiO2 catalyst in a fixed-bed reactor. The Ni/TiO2 catalyst has high initial activity but undergoes significant deactivation during the partial oxidation of methane reaction. Deactivation is due largely to the oxidation of Ni(0) to NiTiO3. After the partial oxidation of methane at 700 °C the catalyst was pale yellow. XRD confirmed that Ni(0) had been converted to NiTiO3 and oxygen pulse reactions found only traces of carbon were present after the POM reaction. The Ni/TiO2 catalyst has a high activity and a long term stability in the CO2 reforming reaction. XRD found Ni(0) was present after the reforming reaction but NiO and NiTiO3 were absent. Activation of methane over Ni/TiO2 was also investigated using pulse reaction techniques in the absence of gas phase oxygen. Methane pulse reactions demonstrated that the mechanism of methane oxidation changes as the oxidation state of nickel changes. CH4 may have been oxidized by oxygen from solid NiO or by active oxygen within the TiO2 support via the non-selective Rideal–Eley mechanism over the oxidized Ni/TiO2 catalyst surface. In contrast, CH4 is efficiently converted to CO and H2 via a direct oxidation mechanism when Ni/TiO2 is reduced. Pulse reaction studies provide evidence that the oxidation state of nickel controls the methane activation mechanism and the product distribution.

A comparative study of Pt/CeO2 catalysts for catalytic partial oxidation of methane to syngas for application in fuel cell electric vehicles

Abstract: In the framework of a project aimed at developing a reliable hydrogen generator for mobile polymer electrolyte fuel cells (PEFCs), particular emphasis has been addressed to the analysis of catalysts able to assure high activity and stability in transient operations (frequent start-up and shut-down cycles). In this paper, the catalytic performance of 1 at.% Pt/ceria samples prepared by coprecipitation, impregnation and combustion, has been evaluated in the partial oxidation of methane. Methane conversion and hydrogen selectivity of 96 and 99%, respectively, associated with high stability during 100 h of reaction under operative conditions (start-up and shut-down cycles), have been obtained.

Gas turbine power plant and method of operating the same

Abstract: It is disclosed a method of operating a gas turbine power plant and gas turbine power plant wherein hydrogen (6) for the combusting process is produced by feeding natural gas (14) mixed with steam (15) through a membrane/partial oxidation reactor (4) and converting the natural gas (14) at least to H2 and CO Thereby oxygen is transferred from the compressed air (3) through the membrane (18) of the membrane/partial oxidation reactor (4) and the oxygen is used for the partial oxidation process of the natural gas (14) The process is followed by converting the syngas in a CO shift reactor (22) and a CO shift reactor (22) to a C02 removal equipment (25) to mainly hydrogen (6)

Production of hydrogen by partial oxidation of methanol over ZnO-supported palladium catalysts prepared by microemulsion technique

Abstract: Selective production of hydrogen by partial oxidation of methanol, using air as oxidant, was studied over a series of ZnO-supported Pd catalysts. Microemulsion-assisted synthesis and conventional impregnation techniques were used for preparation of catalysts containing Pd particles of different sizes. Catalyst characterisation included BET, XRD and TEM analyses. The influence of Pd particle size on catalytic activity and product distribution was studied by carrying out activity measurements at temperatures between 230 and 300 ◦ C using a stoichiometric feed composition. All catalysts performed well with respect to methanol conversion and hydrogen yield. Both methanol conversion and hydrogen selectivity increased with increasing reaction temperature, the latter at the expense of water formation. Oxygen conversion was complete throughout the examined temperature range. These selectivity trends, with a strong dependence of hydrogen and carbon monoxide selectivities on methanol conversion and reaction temperature, support a reaction scheme consisting of consecutive methanol combustion, steam reforming and decomposition. More importantly, a correlation between Pd particle size and carbon monoxide selectivity was found. When the microemulsion catalysts are compared, carbon monoxide formation increases with increasing particle size. This was not observed over the impregnated reference catalysts, which exhibited high carbon monoxide-levels throughout the examined temperature range. Bimetallic PdZn particles were detected in spent catalysts by means of XRD and it is suggested that the catalytic activity is dependent on the formation of PdZn, the catalytic function being different from that of Pd 0 . © 2002 Elsevier Science B.V. All rights reserved.