Low-temperature combustion

Control of oxides of nitrogen can be accomplished by catalysts or ab-sorbants, but most control systems have concentrated on changing the combustion process to reduce the formation of NOj. Improved burners, change in burner location, staged combustion, and low-temperature combustion utilizing fluidized-bed systems are all currently in use. These combustion improvement systems do not generate waste products, so no disposal problems exist. 

Typical re-entrant piston-bowl design for a small, high-speed direct-injection Cl engine. (From Kook, S., Bae, C., Miles, P.C., Choi, D., Bergin, M., and Reitz, R.D., The Effect of Swirl Ratio and Fuel Injection Parameters on CO emission and Fuel Conversion Efficiency for High-Dilution, Low-Temperature Combustion in an Automotive Diesel Engine, SAE, 2006-01-019 2006. With permission.)... 

R. G. Compton and G. Hancock, Comprehensive Chemical Kinetics, Low-Temperature Combustion and Autoignition, Vol. 35 (Ed. M. J. Pilling), Elsevier, Amsterdam, 1997. 

In addition, another computational study in the frame of DFT, using the hybrid functional MPW1K,53 had suggested that o-QM may be an intermediate in the reaction of the peroxy radical (HO2") with the benzyl radical at the ortho-position (Scheme 2.16),54 which should be significant in atmospheric processes and low-temperature combustion systems (T < 1500 K). 

The Effect of Mineral Matters on the Decomposition Ethers. Recently, the effect of mineral matters of coal on the coal liquefaction has received much attention. It was shown that small amounts of FeS or pyrite are responsible for the hydro-genative liquefaction of coal. Therefore, it is interesting to elucidate the effect of mineral matters of coal on the decomposition rate and products of aromatic ethers, and so three diaryl ethers were thermally treated in the presence of coal ash obtained by low temperature combustion of Illinois No.6 coal at about 200°C with ozone containing oxygen. 

Each of these dissociation reactions also specifies a definite equilibrium concentration of each product at a given temperature consequently, the reactions are written as equilibrium reactions. In the calculation of the heat of reaction of low-temperature combustion experiments the products could be specified from the chemical stoichiometry but with dissociation, the specification of the product concentrations becomes much more complex and the s in the flame temperature equation [Eq. (1.11)] are as unknown as the flame temperature itself. In order to solve the equation for the n s and T2, it is apparent that one needs more than mass balance equations. The necessary equations are found in the equilibrium relationships that exist among the product composition in the equilibrium system. 

As before, reaction (3.71) is slow. Reactions (3.72) and (3.73) are faster since they involve a radical and one of the initial reactants. The same is true for reactions (3.75M3.77). Reaction (3.75) represents the necessary chain branching step. Reactions (3.74) and (3.78) introduce the formyl radical known to exist in the low-temperature combustion scheme. Carbon monoxide is formed by reaction (3.76), and water by reaction (3.73) and the subsequent decay of the peroxides formed. A conversion step of CO to C02 is not considered because the rate of conversion by reaction (3.44) is too slow at the temperatures of concern here. 

Calculated lifetimes of N20 in combustion products indicate that for temperatures above 1500K, the lifetime of N20 typically is less than 10ms, suggesting that except for low-temperature combustion, as found in fluidized bed combustors and in some postcombustion NO removal systems, N20 emissions should not be significant, a conclusion that is in agreement with the most recent measurements of N20 emissions from combustion devices. 

More generally, low-temperature combustion relies heavily on the tendency of radical propagation to yield chain-branching reactions, a phenomenon first explored... 

As shown, peroxy radical chemistry plays a substantial role in low-temperature combustion as opposed to the alkoxy radical chemistry of high-temperature combustion. Thus, the peroxy radicals constitute an important class of reactive intermediates with significant implications for low temperature combustion and atmospheric reactions. 

In terms of temperature regions, low-temperature combustion occurs over the range 298-550 K, whereas high-temperature combustion mechanisms dominate at temperatures over 1000 K. Intermediate temperatures, from 550 to 700 K, demonstrate an unusual phenomenon called the negative temperature coefficient (NTQ, which is observed for methane and larger hydrocarbon fuels. As shown in Fig. 3, when the correct alkylperoxy radical chemistry is included in a fuel s combustion mechanism, a NTC range exists (Fig. 3, plot C) where an increase in temperature causes a decrease... 

The chemistry of the troposphere (the layer of the atmosphere closest to earth s surface) overlaps with low-temperature combustion, as one would expect for an oxidative environment. Consequently, the concerns of atmospheric chemistry overlap with those of combustion chemistry. Monks recently published a tutorial review of radical chemistry in the troposphere. Atkinson and Arey have compiled a thorough database of atmospheric degradation reactions of volatile organic compounds (VOCs), while Atkinson et al. have generated a database of reactions for several reactive species with atmospheric implications. Also, Sandler et al. have contributed to the Jet Propulsion Laboratory s extensive database of chemical kinetic and photochemical data. These reviews address reactions with atmospheric implications in far greater detail than is possible for the scope of this review. For our purposes, we can extend the low-temperature combustion reactions [Equations (4) and (5)], whereby peroxy radicals would have the capacity to react with prevalent atmospheric radicals, such as HO2, NO, NO2, and NO3 (the latter three of which are collectively known as NOy) ... 

For pentyl radical, internal H-atom transfers can occur regardless of whether further oxidation occurs. These unimolecular reactions can directly compete with oxidation steps and so have implications for low-temperature combustion. For instance, n-pentyl radical can quickly isomerize to iso-pentyl radical via 1,4-H atom transfer each of these radicals can undergo p-scission reactions to yield a new alkyl radical + alkene ... 

Phenylperoxy radical, originally assumed to be a factor in low-temperature combustion only, has actually been shown to play a substantial role in dictating the overall combustion trends of benzene. Just as the isomerizations and eliminations of the alkylperoxy radicals significantly affected their overall combustion pathways, rearrangements and other intramolecular pathways available to phenylperoxy radical similarly impact the overall progress of benzene combustion. This knowledge can be extrapolated to more complex aromatic species. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

There is further evidence for the role played by methyl hydroperoxide in the low temperature combustion of acetone. Knox (23) showed that if one assumes a simple basic chain mechanism for oxidation, then the acceleration constant, < , which characterizes the exponential acceleration to maximum rate, is given by... 

Figure 12 Theoretical and experimental NO emissions at coal combustion that were calculated by model (90)—(95) (curve 7) and presented in the work by Gorban (2001,2006) equilibrium (1), maximum (2) actual (3-6) fluidized bed combustion (3), low-temperature combustion of brown coals (4), high-temperature combustion of hard coals (5), averaged for coal-fired boilers (6) A— prompt NO, B— fuel NO, C— thermal NO.

In general, tropospheric chemistry is analogous to a low-temperature combustion system, the overall reaction is given by... 

Koreberg, J, and Scaroni, A. W. Low Temperature Combustion of Alternate Fuels in Circulating Fluidized Beds, in Circulating Fluidized Bed Technology (Basu, P., ed.), p. 273. Pergamon Press, Canada (1986). 

Ga.s S5mthesis and application of carbon monoliths Environmental, VOC Oxidation of xylenes Low-temperature combustion Carbon-coated monolith (165,166)... 

Modem Aspects of Diffusion-Controlled Reactions Low-temperature Combustion and Autoignition Photokinetics Theoretical Fundamentals and Applications Applications of Kinetic Modelling Kinetics of Homogeneous Multistep Reactions Unimolecular Kinetics, Part 1. The Reaction Step Kinetics of Multistep Reactions, 2nd Edition... 

The basis of the hydrogen enrichment concept to provide low-temperature combustion stems from experimental observations that admixtures of hydrogen with HC/air mixtures so depress the lean flammability limit that burning can take place at ultralean combined fuel-to-air ratios. The potential reduction in lean limits provided by such tertiary mixtures can be illustrated by the use of Le Chatelier s formula which predicts the lean limit of any mixture of fuel gases from a knowledge of the lean limits for the individual fuel gases. This formula is (17) ... 

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