Combustion soot formation

The diesel engine operates, inherently by its concept, at variable fuel-air ratio. One easily sees that it is not possible to attain the stoichiometric ratio because the fuel never diffuses in an ideal manner into the air for an average equivalence ratio of 1.00, the combustion chamber will contain zones that are too rich leading to incomplete combustion accompanied by smoke and soot formation. Finally, at full load, the overall equivalence ratio... 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Magnussen, B. F., and B. H. Hjertager. 1976. On the mathematical modelling of turbulent combustion with special emphasis on soot formation and combustion. 16th Symp. (Int.) on Combustion, pp. 719-729. The Combustion Institute, Pittsburgh, PA. 

Schocker, A., Kohse-Hoinghaus, K., and Brockhinke, A., Quantitative determination of combustion intermediates with cavity ring-down spectroscopy Systematic study in propene flames near the soot-formation limit, Appl. Opt., 44, 6660,2005. 

Bockhom, H. (Ed.), Soot Formation in Combustion, Springer-Verlag, Berlin, 1994. 

Generation of Pollutants from Combustion 6.3.1 PAH and Soot Formation... 

Effer- vescent Atomization 20-340 [87] Combustion Simple, reliable, very good atomization. Low risk of plugging due to large holes. Easy maintenance. Beneficial effects on reducing soot formation exhaust smoke. Cheap. Need for separate supply of atomizing air... 

This approach, however, requires the absence of ill-defined carbon deposits originating from defect-induced soot formation on the surface of nanocarbons during their synthesis. Pyrolytic structures often counteract the control over activity and selectivity in catalytic applications of well-defined nanocarbons by offering an abundance of highly reactive sites, however, in maximum structural diversity. Although some nanocarbons are equipped with a superior oxidation stability over disordered carbons [25], such amorphous structures can further induce the combustion of the well-ordered sp2 domains by creating local hotspots. Thermal or mild oxidative treatment,... 

This chapter seeks not only to provide better understanding of the oxidation processes of nitrogen and sulfur and the processes leading to particulate (soot) formation, but also to consider appropriate combustion chemistry techniques for regulating the emissions related to these compounds. The combustion— or, more precisely, the oxidation—of CO and aromatic compounds has been discussed in earlier chapters. This information and that to be developed will be used to examine the emission of other combustion-generated compounds thought to have detrimental effects on the environment and on human health. How emissions affect the atmosphere is treated first. 

Flame turbulence should not affect soot formation processes under premixed combustion conditions, and the near correspondence of the results from Bunsen flames [52] and stirred reactors [55] tends to support this contention. 

SOOT FORMATION IN COMBUSTION OF HIGH-ENERGY FUELS... 

The fuel-to-air ratio was varied from 0.8 to 1.4 to simulate differing soot formation conditions. The fuels used were propane, ethylene, or ethylene with gaseous benzene (the fuel was bubbled through liquid room-temperature benzene to provide up to 20% of the combustible content or the entrainment air flow was bubbled through benzene to provide up to 66%). The centerline velocity at the nozzle exit was 3 m/s, the unforced (natural) RMS was 4.5%, and the forced RMS was 30%. The Reynolds number based on the exit diameter was 3700. 

The control system was also demonstrated in open and enclosed systems with highly sooting fuels including benzene. When the proper phase angle of fuel injection was used, soot formation could be prevented, and an entirely blue flame realized, even when gaseous benzene constituted 66% of the combustible content. The combustion efficiency of the benzene was beyond 99.999% even at an overall equivalence ratio of 1.0. 

Future combustion devices may burn alternative fuels with higher carbon-to-hydrogen ratios and operate at higher pressures. The combustion of such fuels under these conditions will result in more intense turbulence, higher levels of soot formation, and the associated increase in radiative heat loss compared to more traditional fuels burned at lower pressures. Depending upon the design objectives, it may be desirable to control soot levels using predictive capabilities. 

---