Evaluation of the use of different hydrocarbon fuels for gas reburning

Gas reburning is a NO_x reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.     

Methanol interaction with NO2: An attempt to identify intermediate compounds in CH4-SCR of NO with Co/Pd-HFER catalyst

The objective of this work is the study of fundamental common aspects of NO_x catalytic reduction over a Co/Pd-HFER zeolite catalyst, using methanol or methane as reducing agent. Temperature Programmed Surface Reaction (TPSR) studies were performed with reactant mixtures comprising NO₂ and one of the reducing agents.The formation of formaldehyde was detected in both studied reactions (NO₂–CH₄ and NO₂–CH₃OH) in the temperature range between 100 and 220 °C. At higher temperature, when the NO_x reduction process effectively begins, formaldehyde starts to be consumed.Using methanol as reducing agent, nitromethane and nitrosomethane, are detected. At 300 °C these species are consumed and cyanides and iso-cyanides formation occurs. On the contrary, with methane, these last species were not detected; however, there are strong evidences for CH₃NO and CH₃NO₂ formation.Thus, using methanol or methane, similar phenomena were detected. In both cases, common intermediary species seem to play an important role in the NO_x reduction process to N₂.These results suggest that methanol can be considered as a reaction intermediate species in the mechanism of the reduction of NO₂ with methane, over cobalt/palladium-based ferrierite catalysts.     

Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina-supported Pd, Co and Co/Pd catalysts: Part A. On the effect of palladium precursors and catalyst pre-treatment

The aim of the present work is to study the selective reduction of NO_x from natural gas sources. The unburned methane can be used as reductant. Another reductant such as hydrogen can be created in situ, using a microreformer. 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 reductant. However, the catalytic behaviour depends on the metal precursor and the catalyst treatment. The most prominent result is obtained on the palladium catalyst prepared from Pd(NH₃)₄(NO₃)₂ precursor. Comparing the reduction and the calcination step in the course of catalyst preparation, one can conclude that calcination lead to the higher activity in deNO_x, since reduced catalysts are oxidized during the deNO_x process.     

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.     

NO2 Reduction with Nitromethane over Ag/Y: A Catalyst with High Activity over a Wide Temperature Range

Nitromethane (NM) is a very efficient reductant for converting NO₂ to N₂ over Ag/Y: Between 140 °C and 400 °C, the N₂ yield is close to 100%. This high N₂ yield results from the ability of Ag/Y to effectively catalyze the reaction between NM and NO₂. This high catalytic activity of Ag/Y is minimally affected by surface bound CN⁻, NC⁻, or acetate, all of which are stable at temperatures below ∼300 °C. At T ≥ 400 °C, there is a reaction path that yields N₂ from NM even in the absence of NO₂. However even at 400 °C, under typical deNO _x conditions, most N₂ molecules are formed as a result of the reaction of NM and NO₂.     

Effect of Iron Addition on the Durability of Pd–Pt-Loaded Sulfated Zirconia for the Selective Reduction of Nitrogen Oxides by Methane

The effects of adding iron to Pd–Pt/sulfated zirconia (SZ) on the selective NO _x reduction by methane were examined based on durability tests under conditions simulating natural gas combustion exhaust. While Pd–Pt/SZ was severely deactivated at 500 °C, Pd–Pt/Fe-SZ maintained a NO _x conversion higher than 70% for over 2400 h under the same conditions. Methane conversion on Pd–Pt/Fe-SZ was significantly lower than that on Pd–Pt/SZ. XRD analysis of fresh and used catalysts showed that a part of the SZ had transformed to monoclinic ZrO₂ and that adding Fe suppressed the transformation. These results suggested that the improvement in NO _x conversion by adding Fe was due to the suppression of methane combustion and the stabilization of SZ against transformation to ZrO₂.     

Ultra-rapid synthesis of syngas by the catalytic reforming of methane enhanced byin-situ heat supply through combustion

Development in highly active catalysts for the reforming of methane with CO₂ and partial oxidation of methane was conducted to produce hydrogen and carbon monoxide with high reaction rates. An Ni-based four-components catalyst, Ni-Ce₂O₃-Pt-Rh, supported on an alumina wash-coated ceramic fiber in a plate shape was suitable for the objective reaction. By combining the catalytic combustion of ethane or propane, methane conversion was markedly enhanced, and a high space-time yield of syngas, 25,000 mol/l·h was obtained at a catalyst temperature of 700 ‡C or furnace temperature of 500 ‡C. The extraordinary high space-time yield of syngas was also confirmed even under the very rapid flow rate conditions as a contact time of 3 m-sec by using a monolithic shape of catalyst bed without back pressure.     

Aromatization of n-Butane Over Supported Mo2C Catalysts

Similarly to the case of methane, ethane and propane, Mo₂C deposited on ZSM-5 significantly enhanced the aromatization of n-butane observed on ZSM-5 (SiO₂/Al₂O₃ ratio of 80) alone. The catalytic performance of Mo₂C/ZSM-5 sensitively depended on its preparation and pretreatment. The selectivity of aromatics measured for pure ZSM-5 increased from 11-13% to 28-34% at the conversion level of 60-65%. The formation of aromatics was also observed over Mo₂C/SiO₂.     

Promotional effect of Ce on the activity of In/W–ZrO2 for selective reduction of NO x with methane

The Ce modified In/W–ZrO₂ catalysts were prepared by impregnation and mechanical mix method. Their activities for SCR of NO _x with methane were investigated. The activity of the In/W–ZrO₂ catalyst was enhanced by addition of Ce with both methods, while the promotional effect was more pronounced for catalyst prepared by mechanical mix method compared to impregnation method. The function of Ce was to improve the oxidation of NO to NO₂. The maximum NO _x conversion over the mechanical mixed catalyst can be stabilized at 74% at 450 °C in a dry gas flow and 37% at 500 °C in wet flow (24,000 h⁻¹). For the impregnated catalysts, Ce was found to compete with In to adsorb on strong acid site over W–ZrO₂ support and inhibited the formation of InO⁺, which resulted in the lower activity of these catalysts than mechanical mixed catalysts.     

NO x -Catalyzed Gas-Phase Activation of Methane: A Reaction Study

The catalytic effect of NO _x on methane oxidation in the absence of any solid catalyst has been investigated. The experimental results show that NO _x has very good catalytic activity in the partial oxidation of methane. The predominant products for reactions in a CH₄-O₂-NO _x co-feed mode are CO, CO₂, H₂O and H₂, CH₃OH, HCHO, and C₂H₄. Aromatics are also observed.     

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

The interaction of methane at various temperatures with NO _x species formed by room temperature adsorption of NO + O₂ mixture on tungstated zirconia (18.6 wt.% WO₃) and palladium(II)-promoted tungstated zirconia (0.1 wt.% Pd) has been investigated using in situ FT-IR spectroscopy. A mechanism for the reduction of NO over the Pd-promoted tungstated zirconia is proposed, which involves a step consisting of thermal decomposition of the nitromethane to adsorbed NO and formates through the intermediacy of cis-methyl nitrite. The HCOO⁻ formed acts as a reductant of the adsorbed NO producing nitrogen.     

NOx Abatement by Catalytic Traps: On the Mechanism of NOx Trapping Under Automotive Conditions

NO _x adsorption was measured with a barium based NO_x storage catalyst at an engine bench equipped with a lean burn gasoline direct injection engine (GDI). In order to study the influence of gas phase NO₂ on the NO_x storage efficiency two different pre-catalysts were used: One with excellent NO oxidation activity to produce a high NO₂ concentration and another pre-catalyst without NO oxidation activity and therefore high NO concentration at the NO _x storage catalyst inlet. Both pre-catalyst had excellent HC and CO conversion efficiency and therefore the CO and HC concentration at the NO _x storage catalyst inlet was practically zero. No lean NO _x reduction was observed. Under that conditions, experiments with NO _x storage catalysts of different length show that a high NO₂ inlet concentration did not enhance the NO _x storage efficiency. Moreover, we observed reduction of NO₂ to NO over the NO_x storage catalyst. However, in presence of a high NO inlet concentration NO₂ formation was observed which may proceed parallel to NO _x storage.     

Osmium‐Catalyzed Selective Oxidations of Methane and Ethane with Hydrogen Peroxide in Aqueous Medium

Various transition metal chlorides including FeCl₃, CoCl₂, RuCl₃, RhCl₃, PdCl₂, OsCl₃, IrCl₃, H₂PtCl₆, CuCl₂ and HAuCl₄ were studied for the selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium. Among the metal chlorides investigated, osmium(III) chloride (OsCl₃) exhibited the highest turnover frequency (TOF) for the formation of organic oxygenates (mainly alcohols and aldehydes) from both methane and ethane. For the OsCl₃‐catalyzed oxidation of methane with hydrgen peroxide, methyl hydroperoxide was also formed together with methanol and formaldehyde. The effects of various kinetic factors on the catalytic behavior of the OsCl₃‐H₂O₂ system were investigated, and TOF values of 12 and 41 h⁻¹ could be obtained for oxygenate formation during the oxidations of methane and ethane, respectively. In the presence of OsCl₃, NaClO, NaClO₄ or NaIO₄ as oxidant was incapable of oxidizing methane and ethane to the corresponding oxygenates, and the use of tert‐butyl hydroperoxide (TBHP) instead of H₂O₂ provided remarkably lower rates of formation of oxygenates. UV‐Vis spectroscopic measurements suggested that OsCl₃ was probably oxidized into an Os(IV) species by H₂O₂ in aqueous medium, and the Os(IV) species might be involved in the oxygenation of methane or ethane. The result that the conversions of both methane and ethane to oxygenates were suppressed by the addition of a radical scavenger suggested that the reactions proceeded via a radical pathway.     

The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NO _x with propene or propane in the presence of 1 or 4% O₂ was studied at low conversions over a 7.4 wt% Cu-ZrO₂ and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N₂ production were compared in experiments using only NO or a mixture of NO and NO₂ in the feed. They were also compared with the rates of NO₂ reduction to NO under the same conditions, and of NO oxidation to NO₂ in the absence of hydrocarbon. It was found that the reduction of NO₂ to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO₂ to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N₂ under lean NO _x conditions.On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.     

Steam effect on NO x reduction over lean NO x trap Pt–BaO/Al2O3 model catalyst

The effect of steam on NO _x reduction over lean NO _x trap (LNT) Pt–Ba/Al₂O₃ and Pt/Al₂O₃ model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C₃H₈ as reductants, respectively. Compared to dry atmosphere, steam promoted NO _x reduction; however, under rich conditions the primary reduction product was NH₃. The results of NO _x reduction and NH₃ selectivity versus temperature, combined with temperature programmed reduction of stored NO _x over Pt–BaO/Al₂O₃ suggest that steam causes NH₃ formation over Pt sites via reduction of NO _x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C₃H₈/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO _x contributed to NH₃ formation. The NH₃ formed under these conditions could be effectively trapped by a downstream bed of Co²⁺ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO _x reduction, leading to improved NO _x storage efficiency.     

Exceptional Activity for NOx Reduction at Low Temperatures Using Combinations of Hydrogen and Higher Hydrocarbons on Ag/Al2O3 Catalysts

The effect of the addition of hydrogen on the SCR of NO _x with a hydrocarbon reaction was investigated. It was found that hydrogen had a remarkable effect on the temperature range over which NO _x could be reduced during the SCR reaction with octane. Reduction of NO _x was initiated at as low a temperature as 100 °C and >95% NO _x conversion was achieved over a temperature range of 200–450 °C. Hydrogen has the effect of activating octane at lower temperatures and also promotes the oxidation of NO to NO₂ in the absence of hydrocarbon. Transient kinetic and in situ DRIFTS measurements indicated that hydrogen has a direct role in the reaction mechanism by either promoting the formation and storage of an organic C = N species which can then readily reduce NO _x and/or removing a species which acts as a poison to the SCR reaction at low temperatures.     

Correlations for dependence of NOx emissions on heat loss in premixed CH4/air combustion

The present study represents an effort to correlate the dependence of NO_x emissions on heat losses to the atmospheric environment in a CH₄/air fueled combustor. To this end, the numerical analysis was performed over a wide range of residence times, equivalence ratios and heat losses using a perfectly stirred reactor (PSR) code. The numerical results showed that the calculated NO_x concentration initially increased, reached a maximum value and then decreased with increasing residence time when the heat loss was present. The similar variation was observed in changes in the thermal NO concentration that was evaluated by only considering the reactions associated with the thermal (Zeldovich) NO mechanism. With the heat loss increased, the calculated NO_x concentration was substantially reduced for all equivalence ratios investigated. In addition, the reductions in the NO_x concentration with respect to residence time became faster with increasing the equivalence ratio particularly for fuel rich conditions. The observed variations in the calculated NO_x concentration over the residence time (NO_x/τ) were found to fit well to the following correlation:ln(NO_x/τ)=a(HLI)+b. In the correlation, HLI is the dimensionless heat loss parameter and coefficients a and b are constants expressed as a function of adiabatic flame temperature (for a given equivalence ratio) and equivalence ratio, respectively.     

Methane activation by NO2 on Co loaded SBA-15 catalysts: The effect of mesopores (length, diameter) on the catalytic activity

In a general model of “three-function deNO_x” catalyst, the partial oxidation of methane by NO₂ is an important step (CH₄ + NO₂ → C_xH_yO_z + NO). To study the effect of the length and diameter, in the mesopores of SBA-15, we have synthesized catalysts with 3 wt.% cobalt supported on SBA-15, with differences in length and diameter of channels. Three different cobalt species were detected on all catalysts. We demonstrated by TPSR experiments that the activity of cobalt/SBA-15 catalysts is affected by the length, the diameter and connections between mesopores of the SBA-15 supports. We show that by changing textural properties of silica support the temperature of 100% conversion of NO₂ into NO can decrease by more than 100 °C.     

Low Activation Energy Pathway for the Catalyzed Reduction of Nitrogen Oxides to N<Subscript>2</Subscript> by Ammonia

A low activation energy pathway for the catalytic reduction of nitrogen oxides to N₂, with reductants other than ammonia, consists of two sets of reaction steps. In the first set, part of the NO _x is reduced to NH₃; in the second set ammonium nitrite, NH₄NO₂ is formed from this NH₃ and NO + NO₂. The NH₄NO₂ thus formed decomposes at ~100 °C to N₂ + H₂O, even on an inert support, whereas ammonium nitrate, NH₄NO₃, which is also formed from NH₃ and NO₂ + O₂, (or HNO₃), decomposes only at 312 °C yielding mainly N₂O. Upon applying Redhead's equations for a first order desorption to the decomposition of ammonium nitrite, an activation energiy of 22.4 is calculated which is consistent with literature data. For the reaction path via ammonium nitrite a consumption ratio of 1/1 for NO and NO₂ is predicted and confirmed experimentally by injecting NO into a mixture of NH₃ + NO₂ flowing over a BaNa/Y catalyst. This leads to a yield increase of one N₂ molecule per added molecule of NO. Little N₂ is produced from NH₃ + NO in the absence of NO₂.     

Effects of Lean/Rich Timing and Nature of Reductant on the Performance of a NOx Trap Catalyst

A NO _x trap catalyst was studied in a laboratory reactor under simulated diesel passenger car conditions. The effects of lean/rich duration and the nature of reductant are investigated. At 300°C, the average NO _x conversion decreases with increasing lean duration; conversely the NO _x conversion increases with increasing rich duration. The NO _x conversion at this temperature was found to be a direct function of reaction stoichiometry. That is, the quantity of trapped NO _x under lean conditions must be balanced by the quantity of reductant during the rich trap regeneration step. At extreme temperatures, other factors, reaction kinetics (at lower temperatures) and NO _x storage capacity (at higher temperatures), dominate the NO _x conversion process. Overall, carbon monoxide was found to be the most effective reductant. Hydrocarbon, e.g., C₃H₆, is effective at higher temperatures (T>350°C), while H₂ is more efficient than other reductants at low temperatures (T<200°C). The individual steps of the NO _x conversion process are discussed.