Particulate formation

In earlier sections of this chapter, the role that particulates play in a given environmental scenario was identified. This section will be devoted exclusively to combustion-generated particulates whose main constituent is carbon. Those carbonaceous particulates that form from gas-phase processes are generally referred to as soot, and those that develop from pyrolysis of liquid hydrocarbon fuels are generally referred to as coke or cenospheres. 

Although various restrictions have been placed on carbon particulate emissions from different types of power plants, these particles can play a beneficial, as well as a detrimental, role in the overall plant process. The detrimental effects are well known. The presence of particulates in gas turbines can severely affect the lifetime of the blades soot particulates in diesel engines absorb carcinogenic materials, thereby posing a health hazard. It 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

The various phenomena involved in carbon particulate formation have been extensively studied. The literature is abundant and some extensive review articles [49-51] are available. Most of the subsequent material in this chapter will deal with soot formation while a brief commentary on the coke-like formation from liquid fuels will be given at the end. 

The characteristics of soot are well described in the article by Palmer and Cullis [49], who provide detailed references on the topic. Aspects of their review are worth summarizing directly. They report the detailed physical characteristics of soot as follows  

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Oravisjarvi, K., Pietikainen, M., and Keiski, R.L. (2006) Particulate Formation in Engines and their Health Effects (Review). Report 324, Department of Process and Environmental Engineering, University of Oulu,... 

The fact that Fischer-Tropsch fuels contain neither sulfur nor aromatics may become a strong selling point for the process. Less sulfur in the fuel has, of course, a direct effect on the sulfur oxides in the emissions, and the newly developed exhaust purification systems for lean burning engines that can be introduced means that all emissions, including GO2 and NOx, will diminish. Aromatics promote particulate formation in the combustion of diesel fuels and are therefore undesirable. We discuss this further in Ghapter 10. 

For premixed fuel-air systems, results are reported in various terms that can be related to a critical equivalence ratio at which the onset of some yellow flame luminosity is observed. Premixed combustion studies have been performed primarily with Bunsen-type flames [52, 53], flat flames [54], and stirred reactors [55, 56], The earliest work [57, 58] on diffusion flames dealt mainly with axisymmetric coflow (coannular) systems in which the smoke height or the volumetric or mass flow rate of the fuel at this height was used as the correlating parameter. The smoke height is considered to be a measure of the fuel s particulate formation and growth rates but is controlled by the soot particle bumup. The specific references to this early work and that mentioned in subsequent paragraphs can be found in Ref. [50],... 

Chi EY, Weickmann J, Carpenter JF, Manning MC, Randolph TW. Heterogeneous nucleation-controlled particulate formation of recombinant human platelet-activating factor acetylhydrolase in pharmaceutical formulation. J Pharm Sci 2005 94(2) 256-274. 

In addition, the hydrodynamics were monitored. The main features were gas bubble formation (HC1) and particulate formation (Et3NHCl) and agglomeration, both due to the reaction. The flow in the glass tube seemed to be rather undisturbed [53] only from time to time, bubble formation due to HC1 gas evolution and passing of EtjNHCl lumps were observed. These bubbles and lumps moved with the liquid mixture and were rinsed out of the tube and hence did not behave as obstacles which could cause a breakdown of the flow. 

In order to provide information which may be useful in the specification of emission control techniques, it is important to understand those parameters which influence particulate formation in practical fuel oil combustion systems. Carbonaceous emissions from liquid fuel spray flames burning synthetic and petroleum... 

Experimental. To further understand the process of droplet combustion and particulate formation, a more fundamental study of the effects of droplet size, local stoichiometry and gas-droplet relative velocity has been carried out. This work made use of a controlled flow variable slip reactor in which the combustion of droplet streams can be examined under well defined conditions. 

Oxidation in the atmosphere may take place homogeneously in the gas-phase, in the aqueous phase, and heterogeneously, and can lead to particulate formation. The latter is poorly understood. 

As indicated before, previous detailed studies of particulate formation and emission processes in automobile engines and exhaust systems have been done using probes which extracted a (hopefully) representative sample of the exhaust gas that was subsequently analyzed for various particulate properties (5). Similar techniques have been used in the most recently developed systems to measure the amounts of particles emitted from auto exhaust. Such sampling techniques, however, suflFer from inherent disadvantages in that they interfere with the hot flow particulate processes and have an unknown effect on the particulates as they flow through the system to an exterior point of analysis. This problem can also be compounded by several phenomena anisokinetic sampling, deposition of particulates in the sampling system, or condensation caused by temperature drops. 

In summary, particle size distributions measured at similar conditions using optical and sampling probe-impactor methods are vastly different. No conclusions can be drawn concerning the relative accuracy of these two techniques because of experimental differences. All indications are that the optical counter is operating properly and is applicable and advantageous for in situ measurements. Further experiments comparing the two techniques directly on a common engine will be performed soon to substantiate the particulate formation hypotheses. 

Ganley, J. T., Springer, G. S., Particulate Formation in Spark Ignition... 

Figure 4 Physical and chemical processes responsible for diesel particulate formation. (Courtesy of Elsevier Science Publishers.)...

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