Fuel-rich conditions

Prompt NO Hydrocarbon fragments (such as C, CH, CH9) may react with atmospheric nitrogen under fuel-rich conditions to yield fixed nitrogen species such as NH, HCN, H9CN, and CN. These, in turn, can be oxidized to NO in the lean zone of the flame. In most flames, especially those from nitrogen-containing fuels, the prompt... 

The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at temperatures above 2732 °F (1500 °C). Due to the onset of the thermal mechanism the formation of NOx in the combustion of fuel/air mixtures increases... 

Typical examples of electrophilic reactions are the reduction of NO by ethylene on Pt32 and the CO oxidation on Pt under fuel-rich conditions.51,62... 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

Figure 11.5. Concentration profile for N02 and CO during a switch from fuel-lean to fuel-rich conditions for a typical NSR catalyst. N02 decreases and CO increases with time [61].

SR greater than 1 refers to fuel lean conditions, while SR less than 1 refers to fuel rich conditions. To convert from fuel rich to fuel lean experimentally, a portion of the helium in the reactant gases was replaced with an equal volume of 02 using a 4-way switching valve. The pressures of the switching valve outlets were balanced such that only the reactant concentration is changed while keeping the flow rates and the pressure constant. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

Skreiberg, O., Kilpinen, P., Glarborg, P. Ammonia chemistry below 1400 K under fuel-rich conditions in a flow reactor, Combustion Flame, 136, 501-518, (2004). 

This reaction may account for as much as 20% of the methanol disappearance under fuel-rich conditions [49], The chain branching system originates from the reactions... 

Figure 2.3 The sensor signal (the voltage at a constant current) in a cylinder-specific measurement, when one cylinder is driven under excess fuel (rich) conditions. When cylinder 1 ignites, the sensor signal changes to a lower voltage.

Polycyclic aromatic hydrocarbons (PAH) are produced by the combustion, under fuel rich conditions, of almost any fuel. Although a few PAH with vinylic bridges (such as acenaphthylene) are lost, most PAH are quite stable in the atmosphere and eventually accumulate in environmental sinks such as marine sediments. Spatial and historical measurements of PAH in sediments Indicate that these compounds are stable, conservative markers of man s energy producing activities. 

The nitrous oxide-acetylene flame is both hot and reducing. A characteristic red, interconal zone is obtained under slightly fuel-rich conditions. This red feather is due to emission by the cyanogen radical. This radical is a very efficient scavenger for oxygen, thus pulling equilibria such as... 

All molar fractions are considered locally in catalyst pores, the superscript s is omitted for brevity. If the DOC is operated temporarily also under fuel-rich conditions (e.g. during regeneration of the NSRC or DPF in a combined system) the reactions R6-R7, R8-R9 and R11-R14 in Table III (Section VI) should also be considered. 

Even comprehensive mechanisms, however, must be utilized with caution. The GRI-Mech fails, for instance, under pyrolysis or very fuel-rich conditions, because it does not include formation of higher hydrocarbons or aromatic species. Its predictive capabilities are also limited under conditions where the presence of nitrogen oxides enhances the fuel oxidation rate (NO f sensitized oxidation), a reaction that may affect unbumed hydrocarbon emissions from some gas-fired systems, for example, internal combustion engines. 

The reaction diagram of Fig. 14.1 applies to methane oxidation under both flame [423] and flow reactor [146] conditions. At high temperatures and fuel-lean to stoichiometric conditions, the conversion of methane proceeds primarily through the sequence CH4 -> CH3 -> CH2O -> HCO -> CO -> CO2. At lower temperatures or under fuel-rich conditions the reactions of CH3 with O or O2 are less competitive. Under these conditions two CH3 radicals may recombine and feed into the C2 hydrocarbon pool,... 

At higher temperatures the C2 hydrocarbons (ethane, ethylene, acetylene) are oxidized along the same pathways as outlined in Section 14.3.1. The C2 radicals C2H5 and C2H3 are much more reactive than CH3, and consequently C2 hydrocarbons are more easily oxidized than methane. This is illustrated in Fig. 14.4, which shows data for oxidation of selected hydrocarbons in a flow reactor. Measurements of the outlet CO concentration, obtained in the 800 to 1500 K range under slightly fuel-rich conditions, are compared with modeling predictions [148]. 

Fig. 14.4 Comparison between experimented data (points) and modeling predictions (curves) for methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) oxidation in a flow reactor under very dilute, slightly fuel-rich conditions [148]. The excess air ratio X is about 0.9, and the residence time is of the order of 100 ms.

Prompt NO Formation A second source of NO in gas firing is prompt NO. This formation pathway can be the dominating source of NO under conditions characterized by lower temperatures, fuel-rich conditions, and short residence times. This route, which is also called Fenimore NO, was first proposed by C. P. Fenimore [125]. Prompt NO formation is initiated by attack of CH, radicals on N2, forming cyanide species. The most important initiation step is the reaction... 

The similarities between N-oxidation pathways shown in Fig. 14.10 break down at lower temperatures or under very fuel-rich conditions. Here a number of alternative reaction pathways for NCO and NH2 become competitive, and both the overall reaction rate and product-N speciation may vary significantly among HCN, NH3, and HNCO. The oxi-... 

The gas-phase sulfur chemistry occurring in the front-end furnace of the Claus process is presumably similar to reactions occurring under fuel-rich conditions in combustion. However, in both systems the chemistry is quite complex and involves a number of unresolved issues. 

The disulfur reaction subset, which is important mainly under pyrolysis or very fuel-rich conditions, is also currently under investigation. 

The reaction takes place under fuel-rich conditions to maintain a nonflammable feed mixture. Typical feed composition is 13% to 15% ammonia, 11% to 13% methane and 72% to 76% air on a volumetric basis. Control of feed composition is essential to guard against deflagrations as well as to maximize the yield. The yield from methane is approximately 60% of theoretical. Conversion, yields, and productivity of the HCN synthesis are influenced by the extent of feed gas preheat, purity of the feeds, reactor geometry, feed gas composition, contact time, catalyst composition and purity, converter gas pressure, quench time and materials of construction. 

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