Kinetics of redox reactions of ilmenite for chemical-looping combustion

The objective of this study was to establish the kinetic of both reduction and oxidation reactions taking place in the chemical-looping combustion (CLC) process using ilmenite as an oxygen carrier. Because of the benefits of using of pre-oxidized ilmenite and the activation of the ilmenite during the redox cycles, the reactivity of both the pre-oxidized and activated ilmenite was analyzed. The experimental tests were carried out in a thermogravimetric analyzer (TGA), using H₂, CO or CH₄ as reducing gases, and O₂ for the oxidation step. Thus, the reactivity with the main reacting gases was analyzed when natural gas, syngas or coal are used as fuels in a CLC system. The changing grain size model (CGSM) was used to predict the evolution with time of the solid conversion and to determine the kinetic parameters. In most cases, the reaction was controlled by chemical reaction in the grain boundary. In addition, to predict the behaviour of the oxidation during the first redox cycle of pre-oxidized ilmenite, a mixed resistance between chemical reaction and diffusion in the solid product was needed. The kinetic parameters of both reduction and oxidation reactions of the pre-oxidized and activated ilmenite were established. The reaction order for the main part of the reduction reactions of pre-oxidized and activated ilmenite with H₂, CO, CH₄ and O₂ was n=1, being different (n=0.8) for the reaction of activated ilmenite with CO. Activation energies from 109 to 165 kJ mol⁻¹ for pre-oxidized ilmenite and from 65 to 135 kJ mol⁻¹ for activated ilmenite were found for the different reactions with H₂, CO and CH₄. For the oxidation reaction activation energies found were lower, 11 kJ mol⁻¹ for pre-oxidized and 25 kJ mol⁻¹ for activated ilmenite.Finally, simplified models of the fuel and air reactors were used to do an assessment of the use of ilmenite as an oxygen carrier in a CLC system. The reactor models use the reaction model in the particle and the kinetic parameters obtained in this work. Taking into account for its oxygen transport capacity, the moderated solids inventory and the low cost of the material, ilmenite presents a competitive performance against synthetic oxygen carriers when coal or syngas are used as fuel.     

Catalytic combustion of methane on palladium single crystals

Catalytic methane combustion was studied over the palladium single crystals Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0). Under lean reaction conditions at 600 K (O₂:CH₄ = 10:1), stoichiometric palladium oxide was formed with an increase in surface area by a factor of approximately two. The oxide phase formed a “cauliflower-like” surface structure composed of approximately 4 nm sized semispherical oxide agglomerates. This oxide structure was independent of the original metal single crystal orientation. The turnover rates over the oxide structure starting with metal single crystals were 0.7 s⁻¹ on Pd(1 1 1), 0.9 s⁻¹ on Pd(1 0 0) and 0.9 s⁻¹ on Pd(1 1 0) at 600 K, 160 Torr O₂, 16 Torr CH₄, 1 Torr H₂O and N₂ balance to 800 Torr, suggesting that the methane combustion reaction is independent of the initial structure of the catalyst. Methane combustion on palladium single crystals experienced an activation period in which the initial turnover rates based on the initial Pd surface area were about 1/8–1/4 of the steady-state rates determined based on the oxide surface area. This activation period was caused by the slow oxidation of palladium single crystals and concomitant surface area increase during reaction. The increase in surface area happened mostly in the first 10 min of reaction. Carbon dissolution into the crystal was not found during methane combustion under reaction conditions in excess oxygen.     

Preparation and characterisation of SrTi1−x−yZrxMnyO3 solid solution powders in relation to their use in combustion catalysis

Mixed oxides with compositions SrTi_1−x−yZr_xMn_yO₃, with 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.2 have been prepared with a conventional coprecipitation method. Some of them are constituted by very pure perovskite-type solid solution phases, with tetravalent Zr and Mn substituting for Ti in the B site. The addition of Zr to SrTiO₃ tends to increase the surface areas, while the insertion of Mn tends to decrease it. Mn-containing materials are active in the catalytic combustion of 1% methane in air at temperatures higher than 700 K and can be competitive with pure manganite perovskites like LaMnO₃ in spite of the lower Mn content. Pyridine adsorption experiments show that medium strength Lewis acid sites are located at the surface of these materials, and could be involved in the hydrocarbon CH bond activation.     

Catalytic combustion technology to achieve ultra low NOx, emissions: Catalyst design and performance characteristics

A catalytic combustion system has been developed which feeds full fuel and air to the catalyst but avoids exposure of the catalyst to the high temperatures responsible for deactivation and thermal shock fracture of the supporting substrate. The combustion process is initiated by the catalyst and is completed by homogeneous combustion in the post catalyst region where the highest temperatures are obtained. Catalysts have been demonstrated that operate at inlet temperatures as low as 320°C at 11 atm total pressure and conditions typical of high performance industrial gas turbines. The ignition temperature is shown to correlate with the specific catalytic activity of the washcoat layer over a rather broad range of activities. A reaction model has been developed that can predict ignition behavior from the measured catalytic activity.     

The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane

Supported palladium catalysts are very active in the combustion of methane, but still little is known about the kinetic parameters. In this paper a rate expression is presented for an alumina-supported palladium oxide catalyst in the temperature range 180–515°C. Special care was taken to ensure differential conditions during the experiments. In this way, an apparent activation energy of 151±15 kJ/mol was found. The orders in methane, oxygen and water were 1.0±0.1, 0.1±0.1 and −0.8±0.2, respectively. For carbon dioxide a zero order was observed under all conditions. Inhibition by water produced during the reaction was demonstrated to cause non-differential conditions, when a dry feed was used. The rate constant that corrects for this effect could be derived.     

Membrane catalysis

The main events in the development of membrane catalysis are presented in this paper. The pioneering studies of Graham performed in London were continued by Zelinskii in Moscow and by Snelling in the USA. The finding by Frost in the USSR that hydrogen evolved from Pd film is much more active in hydrogentation than feeded as gas with the hydrogenable substance stimulated the researchers of several countries, including the author of this paper, who proposed the method for carrying out simultaneously dehydrogenation on one surface of membrane catalyst and hydrogenation by the diffused hydrogen on the other surface. Similar reactions couplings were carried out later but independently performed by Pfefferle in the USA. The types of hydrogen porous membrane catalysts for selective hydrogenation and dehydrogenation elaborated in Russia were described in next sections. The final section deals with the examples of coupling of hydrogen evolution and consumption reactions on membrane catalyst as well as on the systems of membrane and granular catalysts.     

Evidence of PAH production under lean combustion conditions

While it is common knowledge that PAH formation can be expected at rich combustion conditions, this paper presents experimental and computational evidence that PAH formation can also be expected under lean, or cool, combustion conditions. Experimental evidence for diesel engine operation at lean conditions, as well as the literature references related to waste tire and coal combustion, are cited as supportive evidence of conclusions drawn here. A computational model based on reaction mechanisms for three surrogate fuels: C₂H₂, C₇H₁₆ and C₁₄H₂₈, was validated against the available diesel engine data, and projected to both lean and rich combustion conditions. The finding was that for the adiabatic case, minimum production of PAHs occurs at equivalence of unity and increases as equivalence becomes either lean or rich; the latter observation being consistent with the common knowledge.     

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 °C and completely convert methane to CO₂ below 550 °C where the conversion rate reaches 35 μmol (CH₄)/g_cat s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.     

Study on catalytically ignited premixed combustion

This paper describes some of the major issues related to catalytically ignited premixed combustion and the effect of pressure and velocity on the mass transfer of the catalytic reaction. The first issue is concerned with partial burning of the fuel air mixture on the catalyst surface prompting gas phase combustion to take place downstream of the catalyst layer. The latter part of this paper deals with the potential for reduced combustion efficiency from a mass transfer perspective, when going from atmospheric testing conditions to actual high pressure gas turbine operating conditions.     

Complete methane oxidation over Pd catalyst supported on α-alumina. Influence of temperature and oxygen pressure on the catalyst activity

The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally.Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters.Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample.The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed.     

Origin of transient species present on the surface of a PdO/γ-Al2O3 catalyst during the methane combustion reaction

Operando DRIFTS was applied to the study of the evolution of surface species formed on a Pd (2 wt.%)/γ-Al₂O₃ catalyst in various conditions. No differences were observed as a function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (<400 °C) and disappeared when CO₂ production started. These species come from the Pd-catalyzed interaction of CO with the alumina support, while CO₂ induces hydrogenocarbonates formation at low temperature (<300 °C). Their presence does not explain the inhibiting effect of CO₂ observed in CCM on Pd/γ-Al₂O₃ catalysts.     

Kinetics of mineral matter transformation during coal combustion

The transformation of individual minerals was investigated based on TG and DTG analysis at temperature up to 1700 K in inert and oxidizing atmospheres. The decomposition of minerals in inert atmosphere and the reaction with gaseous atmosphere was described by first order reactions for which the kinetic data were found. The evaluated kinetic parameters were then tested on a complex mineral matter of coals. It has been demonstrated on example of two different compositions that the mass loss during the transformation of coal mineral matter during combustion can be modelled as a mixture of individual minerals.     

Catalytic combustion concept for gas turbines

Catalytic combustion for gas turbines was investigated, based on a partial catalytic combustion section followed by a homogeneous combustion zone. A pressurized test rig (<25 bar) was built to test the influence of various parameters on this concept using Pd and Pt catalysts.

The pressure influence on the apparent catalytic reaction rate was of the order 0.4, assuming that the reaction kinetics could be described by a power rate function which was of first order with respect to methane. Pd catalysts showed a pressure-dependent temperature for the transition of the active PdO to the much less active Pd. Combining Pd and Pt within one catalyst resulted in a considerably lower transition temperature.

Homogeneous combustion reactions set on from 650°C, depending on the methane concentration, pressure and flow. With inlet temperatures above 800°C the homogeneous combustion always started. At outlet temperatures below 1050°C high CO concentrations could be measured. At higher temperatures the CO, CH₄ and NO_x concentrations were lower than 5 ppm. During several experiments total conversion of CH₄ and CO was observed.     

Laser technique for determining solid propellant transient burning rates during oscillatory combustion

The laser technique was developed for measurement of transient burning rates of solid propellant during oscillatory combustion. the design and operation of the system are discussed. A high pressure window bomb was used for the combustion chamber. A modulating disk was used to induce the pressure oscillation above the combustion bomb. The oscillatory frequency was controlled easily with selecting the rotating velocity of the disk. Test results with AP/PS propellants, were in agreement with the results of previous techniques.     

Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO_x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation or coking.

Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h⁻¹ in lab test and 500 000 h⁻¹ in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS.

In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another.

Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium.     

Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Catalytic activity of ZrO₂ supported PdO catalysts for methane combustion has been investigated in comparison with Al₂O₃ supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO₂ support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al₂O₃ catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO₂ catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO₂ catalyst. In order to confirm the formation of the solid solution of PdO and ZrO₂, X-ray diffraction (XRD) analyses of the mixtures of ZrO₂ and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO₂ in the mixture was larger than that of ZrO₂. Furthermore, the Pd thin film on ZrO₂ substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO₂ support might be stabilized by the formation of the solid solution of PdO and ZrO₂.     

乙烯装置废热锅炉焦炭催化燃烧过程的研究

考察了各种金属离子型烧焦催化剂对乙烯装置废热锅炉焦炭燃烧过程的影响 ,讨论了其催化燃烧机理。研究结果表明 :第四周期过渡金属离子型催化剂在焦炭烧焦前期催化效果好 ,后期催化效果不明显 ,其催化机理可用氧传递过程来解释 ;IA族金属离子型烧焦催化剂的催化效果显著 ,能大大提高烧焦速度 ,并促进焦炭完全燃烧 ;IIA族金属离子型催化剂的催化烧焦效果较IA族金属离子型催化剂差。IA族与IIA族金属离子的催化作用效果与金属元素的第一热电离电势成反比     

Toluene combustion over palladium supported on various metal oxide supports

Metal–support interaction in the catalytic combustion of toluene was studied using metal oxides with different acid–base properties as supports for Pd. The catalytic performance was correlated with XPS data and the reaction order for oxygen. These studies revealed that the affinity for oxygen of Pd surface changed according to the acid–base character of metal oxide over MgO, Al₂O₃, SiO₂, SnO₂, Nb₂O₅, and WO₃. However, ZrO₂ exhibited exceptional character in that metal Pd was unusually stabilized, which was derived from the weak interaction between Pd and support surface. The reason for high toluene combustion activity of Pd/ZrO₂ was ascribed to the stabilization of metal Pd on ZrO₂.     

Development of a high-temperature combustion catalyst system and prototype catalytic combustor turbine test results

A new combustion catalyst system for gas turbines composed of high-temperature durable ceramic catalysts made of fine manganese-substituted hexaaluminate particles and noble metal-carrying cordierite honeycomb catalysts has been developed. A 160 kW prototype catalytic combustor turbine rigged with a 220 mm in diameter catalyst system was constructed and tested including a continuous 215-hour operation and repeated sudden stops. The results showed practical combustion performance with ultra-low NO_x emissions less than 40 ppm (converted to 0% oxygen) and the feasibility of long term catalyst durability.     

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.