Experimental and numerical investigation of supported rhodium catalysts for partial oxidation of methane in exhaust gas diluted reaction mixtures

The partial oxidation of methane/oxygen mixtures with large exhaust gas dilution (46.3 vol% H₂O and 23.1 vol% CO₂) has been investigated experimentally and numerically over Rh/CeO₂-ZrO₂, Rh/ZrO₂ and Rh/α-Al₂O₃ catalysts. Experiments were carried out in a short-contact-time reactor at 5 bar and included exhaust gas analysis, temperature measurements along the reactor, and catalyst characterization. Additional experiments were performed in an optically accessible channel-flow reactor and involved in situ Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and laser-induced fluorescence (LIF) of formaldehyde. A full elliptic two-dimensional numerical code that included elementary hetero-/homogeneous chemical reaction schemes and relevant heat transfer mechanisms in the solid was used in the simulations. The employed heterogeneous reaction mechanism, including only active Rh sites, reproduced the experiments with good accuracy. The ratio of active to geometrical surface area, deduced from hydrogen chemisorption measurements, was the single model parameter needed to account for the effect of different supports. This indicated that water activation occurring on support sites, resulting in inverse OH spillover from the support to the noble metal sites, could be neglected under the present conditions with high water dilution. An evident relationship between noble metal dispersion and catalytic behavior, in terms of methane conversion and synthesis gas yields, could be established. Both measurements and predictions indicated that an increasing Rh dispersion (in the order Rh/α-Al₂O₃, Rh/ZrO₂, and Rh/CeO₂-ZrO₂) resulted in higher methane conversions, lower surface temperatures, and higher synthesis gas yields.     

Low temperature complete combustion of methane over titania-modified alumina-supported palladium

Effects of titania on the catalytic property of Pd/Al₂O₃ towards methane combustion were examined. The results revealed that the catalytic activity of the Pd/Al₂O₃ catalyst was considerably improved by pre-coating the alumina support with titania at low temperature (below 700 °C). Hydrogen chemisorption and BET measurements revealed that the titania-modified alumina supports could modify the support characterization to achieve a high dispersion of palladium. Temperature-programmed reduction and temperature-programmed desorption study further demonstrated that the coating of Pd/Al₂O₃ catalysts with titania can weaken the bond strength of Pd-O and enhance their catalytic activity towards methane combustion at lower temperature.     

Chemical modelling and measurements of the catalytic combustion of CH4/air mixtures on platinum and palladium catalysts

This work reports experimental measurements and a modelling study carried out on palladium and platinum based catalytic monoliths used as methane combustors for heating purposes. It concentrates on the effects of operating conditions on combustion, heat transfer efficiency and pollutant formation. The development of a detailed homogeneous/heterogeneous chemical kinetics model for methane–air combustion over palladium using literature data was undertaken to model the behaviour of one of the experimental catalytic heaters. In addition, a published detailed chemical mechanism for methane combustion over platinum was used in the platinum catalyst model. The fuel–air equivalence ratios ranged from 0.3 to 0.6 and the space velocities used were between 24 000 and 72 000 h⁻¹. Although the model assumed perfectly stirred reactor (PSR) conditions and was applied to localised regions of the monoliths where little radial gradients of temperature and concentrations were measured, it predicted the surface temperature, methane slippage, CO and NO_x at the downstream face of the monolith with reasonable accuracy in some cases, but also highlighted the shortcomings of the PSR assumption in other cases.     

Syngas and a separate nitrogen/argon stream via chemical looping reforming - A 140 kW pilot plant study

A dual circulating fluidized bed pilot plant was operated in chemical looping reforming conditions at a scale of 140 kW fuel power with natural gas as fuel. A nickel-based oxygen carrier was used as bed material. The pilot plant is equipped with an adjustable cooling system. Three experimental campaigns have been carried out at 747 °C (1020 K), 798 °C (1071 K) and 903 °C (1176 K), respectively. In each campaign, the global stoichiometric air/fuel ratio was varied step-wise between 1.1 and the minimum value possible to keep the desired operating temperature when the cooling is finally switched off. The results show that the fuel reactor exhaust gas approaches thermodynamic equilibrium. The residual amount of methane left decreases with increasing fuel reactor temperature. Further, the oxygen in the air reactor can be completely absorbed by the solids as soon as the air reactor operating temperature is higher than 900 °C (1173 K). Even though no steam was added to the natural gas feed no carbon formation was found for global excess air ratios larger than 0.4.     

Kinetics of HCl emission from inorganic chlorides in simulated municipal wastes incineration conditions

Sulfation of inorganic chlorides (KCl, CaCl₂ and NaCl) of mean particles size 75-125 μm was done in a lab-scale tubular reactor. The reactor was supplied with a mixed gas consisting of SO₂:0.3-1.3 vol%, O₂:2.5-15 vol% and H₂O:5-20 vol% in the temperature range 623-1123 K. The rates of HCl emission from the particles samples were measured and compared.It was found that at 1023 K, an HCl of about 8000 ppm was emitted from CaCl₂ and this emission concentration was more than 3 times as high as those from NaCl and KCl. The reaction kinetic parameters such as rate constants and reaction orders with respect to SO₂, O₂ and H₂O partial pressures were determined for the sulfation of NaCl, KCl and CaCl₂. The rate of HCl emission from KCl, CaCl₂ and NaCl became increasingly high when the temperature was raised above 923 K and depend on SO₂, O₂, and H₂O partial pressures. It was considered that an increase in the rate of HCl emission from the inorganic chlorides at temperatures above 923 K was caused by a change in the reaction mode from a gas-solid to a gas-liquid and/or a gas phase reaction, owing to a partial melting and/or volatilization of the inorganic chlorides.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

Burning velocity of methane/diluent mixture with reformer gas addition

The motivation of this study is to explore the feasibility of extending the EGR (exhaust gas recirculation) diluent tolerance for methane/air mixtures with reformer gas (CO and H₂). A preheated cylindrical combustion chamber was used to measure the laminar burning velocity of methane/air mixture with variations of EGR diluent, reformer gas, temperature and pressure. The experiments were carried out at the range of initial temperature from 298 K to 498 K and initial pressure from 1 atm to 5 atm. The maximum EGR fraction is 40%. Reformer gas was introduced to raise the burning velocity of methane/EGR mixture to the undiluted level. Results indicate that the reformer gas has potential to improve the burning velocity while reducing the nitric oxide emission.     

Low temperature gasification using lattice oxygen

Low temperature pyrolysis and gasification has been investigated based on the chemical looping combustion (CLC), where insufficient amount of lattice oxygen was reacted with hydrocarbons. Metal oxides such as nickel oxide, iron oxide and titanium oxide were used as lattice oxygen source and were coated on silica gel or porous aluminum. Single column reactor was used for experiments and 36.1 mmol of polyethylene was dropped to the column whose temperature was ranged from 693 to 1073 K. For the pyrolysis, hydrogen yield was 100% of polyethylene contained hydrogen, while methane, CO and CO₂ were minor products and almost half of the supplied carbon was deposited on the particle surface. On the other hand, for the steam gasification, 2-3 mol of the hydrogen was generated from 1 mol of carbon and almost no carbon deposition was observed. It is found that no wax and heavy tar was observed in the exhaust. Therefore, the lattice oxygen was able to be applied to the low temperature gasification of hydrocarbons.     

Comparison of dual fluidized bed steam gasification of biomass with and without selective transport of CO2

A comparison of dual fluidized bed gasification of biomass with and without selective transport of CO₂ from the gasification to the combustion reactor is presented. The dual fluidized bed technology provides the necessary heat for steam gasification by circulating hot bed material that is heated in a separate fluidized bed reactor by combustion of residual biomass char. The hydrogen content in producer gas of gasifiers based on this concept is about 40 vol% (dry basis). Addition of carbonates to the bed material and adequate adjustment of operation temperatures in the reactors allow selective transport of CO₂ (absorption enhanced reforming—AER concept). Thus, hydrogen contents of up to 75 vol% (dry basis) can be achieved. Experimental data from a 120 kW_{Fuel input} pilot plant as well as thermodynamic data are used to determine the mass- and energy-balances. Carbon, hydrogen, oxygen, and energy balances for both concepts are presented and discussed.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Bench-scale production of liquid fuel from woody biomass via gasification

The bench-scale production of hydrocarbon liquid fuel was achieved from woody biomass via gasification. The daily production capacity of the biomass-to-liquid (BTL) plant used in this study was 7.8 L of hydrocarbon liquid from 48 kg of woody biomass (on a dry basis), corresponding to 0.05 barrels. The BTL process involved the following steps: oxygen-enriched air gasification of the woody biomass, wet and dry gas cleaning, gas compression, carbon dioxide removal, and the Fischer-Tropsch (FT) synthesis reaction. In the gasification step, oxygen-enriched air gasification was carried out using a downdraft fixed-bed gasifier. The content of oxygen, which acts as the gasifying agent, was increased from 21.0 to 56.7 vol%; maximum values of the conversion to gas on a carbon basis and cold gas efficiency-approximately 96 C-mol% and 87.8%, respectively-were obtained at an oxygen content of around 30 vol%. With the increased oxygen content, the concentrations of CO, H₂, and CO₂ increased from 22.8 to 36.5 vol%, from 16.8 to 28.1 vol%, and from 9.8 to 14.8 vol%, respectively, while that of N₂ decreased from 48.8 to 16.0 vol%. The feed gas for the FT synthesis reaction was obtained by passing the product gas from the gasification step through a scrubber, carbon dioxide removal tower, and desulfurization tower; its composition was 30.8 vol% CO, 25.2 vol% H₂, 0.9 vol% CO₂, 2.5 vol% CH₄, 40.6 vol% N₂, < 5 ppb H₂S, and < 5 ppb COS. The hydrocarbon fuel was synthesized in a slurry bed reactor using hexadecane as the solvent and a Co/SiO₂ catalyst. For hydrocarbons with carbon chain lengths of more than 5 carbon atoms (collectively referred to as C₅₊) in the liquid fuel, a selectivity of 87.5% was obtained along with a chain growth probability of 0.84 under the following conditions: 4 MPa, 280 to 340 °C, and a ratio of catalyst weight to feed gas rate (W/F) of 9.3 g·h/mol.     

Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry

The methane hydrate heat of decomposition was directly measured up to 20 MPa and 292 K using a high pressure differential scanning calorimeter (DSC). The methane hydrate sample was formed ex-situ using granular ice particles and subsequently transferred into the DSC cell under liquid nitrogen. The ice and water impurities in the hydrate sample were reduced by converting any dissociated hydrate into methane hydrate inside the DSC cell before performing the thermal properties measurements. The methane hydrate sample was dissociated by raising the temperature (0.5-1.0 K/min) above the hydrate equilibrium temperature at a constant pressure. The measured methane hydrate heat of dissociation (H→W+G), ΔH_d, remained constant at 54.44±1.45 kJ/mol gas (504.07±13.48 J/gm water or 438.54± 13.78 J/gm hydrate) for pressures up to 20 MPa. The measured ΔH_d is in agreement with the Clapeyron equation predictions at high pressures; however, the Clausius-Clapeyron equation predictions do not agree with the heat of dissociation data at high pressures. In conclusion, it is recommended that the Clapeyron equation should be used for hydrate heat of dissociation estimations at high pressures.     

Reprint of: Analysis of radial flow reformer for fuel cell applications

In this paper, the performance of a radial flow reformer is analyzed. Fundamental principles of reaction engineering are utilized to design this reactor where methane is reformed to produce sufficient hydrogen to generate 20 W of power in a fuel cell. It is shown that the radial flow geometry leads to modest pressure drop. The reactor operates at a pressure of 150 kPa, a steam to methane ratio of 3 and an inlet temperature of 848 K and is able to generate sufficient hydrogen for 20 W of power. The heat duty required for the reformer is approximately 43% of the power generated.     

Effect of annular fins on heat transfer of a horizontal immersed tube in bubbling fluidized beds

Experiments were conducted in a bubbling air-fluidized bed to investigate the effect of annular fins of constant thickness on heat transfer. Steady state time averaged local heat transfer coefficient measurements were made by the local thermal simulation technique in a cold bubbling fluidized bed (90 mm ID, 260 mm tall) with horizontally immersed tube initially with no fin and then with three fixed annular fins of constant thickness. Silica sand of mean particle diameter 307 μm and 200 μm were used as the bed materials. The superficial velocity of air was from minimum fluidization conditions, u_mf, to approximately 3 × u_mf. The results indicate that, although the heat transfer coefficient falls with the use of fins, the total heat transfer rises as a result of the greater surface area. Increasing the particle diameter reduces the heat transfer coefficient not only for unfinned horizontal tube but also for annular finned horizontal tube at the same conditions of fluidized bed. Based on the experimental data, correlations are proposed for predicting heat transfer coefficient from fluidized bed to horizontally immersed tubes with and without fins.     

Palladium based sorbents for high temperature arsine removal from fuel gas

The use of Pd-based sorbents for high temperature removal of AsH₃ from gasified coal was investigated using a simulated gas feed. A sorbent consisting of 5 wt% Pd on alumina beads has been tested for AsH₃ removal from synthetic fuel gas (CO, CO₂, H₂) at 204 and 288 °C. Arsenic uptake was found to be essentially linear with exposure time and considerably higher than that for unpromoted alumina beads. Arsenic loadings in excess of 7 wt% were achieved, though the sorbent is unlikely to be saturated at this loading. As the arsenic loading on the sorbent increased a PdAs₂ phase was identified in the XRD pattern. The adsorption of arsine on palladium has potential implications for catalysts, electrodes, and membranes for the separation of hydrogen from fuel gas.     

Sintering,microstrusture and hardness of different alumina–zirconia composites

Two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were used to produce alumina (Al₂O₃)-zirconia (ZrO₂) slip cast composites. The influence of the substitution of Al₂O₃ either by different Al-doped Y-PSZ contents or 50 vol% Y-PSZ on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al₂O₃ and the different composites at temperatures in the range of 1100–1600 °C was studied and related to the sample hardness. An increase in the sintering rate was observed when Al-doped Y-PSZ increased from 22 to 50 vol% or when 50 vol% Y-PSZ was substituted by 50 vol% Al-doped Y-PSZ. 50 vol% ZrO₂ was the most effective concentration to reduce the rate of Al₂O₃ grain growth in the final sintering stage; the Al₂O₃ grain growth began at lower temperatures and became greater with decreasing the Al-doped Y-PSZ content. On the contrary, the ZrO₂ grain growth slightly increased with increasing the Al-doped Y-PSZ concentration. However, for 50 vol% Al-doped Y-PSZ a smaller ZrO₂ grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al₂O₃ grain size of the sintered samples became greater than about 1 µm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400 °C and 1500 °C for Al₂O₃ and the composite with 10.5 vol% Al-doped Y-PSZ, respectively.     

Optimum operating conditions for a two-stage gasification process fueled by polypropylene by means of continuous reactor over ruthenium catalyst

We studied fuel gas production by means of pyrolysis and steam reforming of waste plastics for applications in solid oxide fuel cells. More specifically, we evaluated the effects of pyrolytic gasification temperature, catalyst content, steam reforming temperature, and weight hourly space velocity for a Ru catalyst used in a 60 g h^− 1-scale continuous experimental apparatus, which consisted of a tank reactor for pyrolysis and a packed-bed catalytic reactor for steam reforming. Polypropylene (PP) pellets were used as a model waste plastic. Ru/γ-Al₂O₃ catalysts with two different Ru contents were investigated. To suppress residue formation, the optimum operating temperature of the pyrolyzer was 673 K. To ensure suppressed coke formation, sufficient carbon conversion to gaseous products, and minimized heat loss from the reactor, the optimum operating conditions for the reformer were determined to be 903 K and 0.11 g-sample g-catalyst^− 1 h^− 1 with a 5 wt.% Ru/γ-Al₂O₃ catalyst. The composition of the gas produced with the 5 wt.% catalyst was almost the same as that predicted by chemical equilibrium laws, and it was applicable for a direct hydrocarbon fuel cell.     

Effect of the mode of heat withdrawal on the asymmetry of temperature profiles in reverse-flow reactors. Catalytic combustion of methane as a test case

Results of simulations are presented concerning a reverse-flow reactor for the catalytic combustion of methane that occurs in coal-mine ventilation air. Two variants of heat withdrawal are analysed. The simulations show that a relation exists between the method for heat withdrawal and the asymmetry in the profiles of the catalyst temperature over half-cycles of flow reversal. Too strong asymmetry impairs the efficient utilization of the heat produced. The results reveal that the recovery of heat by hot gas withdrawal from the central part of the reactor, and then, the introduction of the gas into a boiler wherein it is cooled to about 333 K (60 °C) guarantee more favourable symmetry of the half-cycle profiles and much better utilization conditions than the direct withdrawal of heat from the mid-section of the reactor (central cooling).     

Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina supported Pd, Co and Co/Pd catalysts: Part B: On the effect of bimetallic catalyst preparation

The aim of the present work is to study the selective reduction of NO_x from natural gas sources using unburned methane or hydrogen as reducing agents. The results suggest that the NO_x are reduced by H₂ at low temperature, when methane is not activated and at higher temperature the methane is then the main reducing agent. Similar results are obtained for alumina supported palladium and alumina supported cobald-palladium catalysts at low temperature in presence of hydrogen suggesting that the active phase for the reaction NO/H₂ is the palladium. However, at high temperature the higher activity is obtained on bimetallic catalyst. The presence of cobalt enhances the catalytic activity. This result suggests that cobalt and palladium both in cationic form are the active sites when the reducing agent is the methane.     

Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling

Biomass gasification at temperatures below 1273 K produces gas which contains methane and too much tar for Fischer-Tropsch synthesis. The aim of this study is to investigate methane conversion at high temperature. Experimental tests were performed between 1273 and 1773 K, with a mixture of gas representative of wood pyrolysis at 1100 K (main components only: CO, CO₂, CH₄, H₂, H₂O). Two different kinetic schemes were used to predict the gas composition, and PAH molecules formation. For a residence time of 2 s in the reactor, the gas must be heated to at least 1650 K to reach a methane conversion rate of 90%. A parametric study was performed at 1453 K, by varying the initial methane, steam and hydrogen contents, so as to find out which components are the most influent on methane conversion and soot production.