Combustion - Caltech Authors

6 Particle Formation in

Combustion

Combustion processes emit large quantities of particles to the atmosphere. Particles fonned in combustion systems fall roughly into two categories. The first category, referred to as ash, comprises particles derived from noncombustible constituents (primarily mineral inclusions) in the fuel and from atoms other than carbon and hydrogen (so-called heteroatoms) in the organic structure of the fuel. The second category consists of carbonaceous particles that are fonned by pyrolysis of the fuel molecules. Particles produced by combustion sources are generally complex chemical mixtures that often are not easily characterized in tenns of composition. The particle sizes vary widely, and the composition may be a strong function of particle size. This chapter has a twofold objective-to present some typical data on the size and chemical composition of particulate emissions from combustion processes and to discuss the fundamental mechanisms by which the particles are fonned.

6.1 ASH Ash is derived from noncombustible material introduced in the combustor along with the fuel and from inorganic constituents in the fuel itself. The ash produced in coal combustion, for example, arises from mineral inclusions in the coal as well as from heteroatoms, which are present in the coal molecules. Heavy fuel oils produce much less ash than coals since noncombustible material such as mineral inclusions is virtually absent in such oils and heteroatoms are the only source of ash. Fuel additives ranging from lead used to control "knock" in gasoline engines, to barium for soot control in

358

Sec. 6.1

Ash

359

diesel engines, to sulfur capture agents also contribute to ash emissions. We focus our

discussion of ash on the particles produced in coal combustion. 6.1.1 Ash Formation from Coal

Ash particles produced in coal combustion have long been controlled by cleaning the flue gases with electrostatic precipitators (see Chapter 7). Most ofthe mass of particulate matter is removed by such devices, so ash received relatively little attention as an air pollutant until Davison et al. (1974) showed that the concentrations of many toxic species in the ash particles increased with decreasing particle size, as illustrated in Figure 6.1. Particle removal techniques, as we shall see in Chapter 7, are less effective for small (i.e., submicron) particles than for larger particles. Thus, even though the total

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Sec. 6.3

379

Soot

(6.17 ) Growth of a species smaller than size 1 is inception. In summary, according to the model above, soot particles form by a poorly understood inception process and then grow by coagulation and the impingement of vapor species on their surface. They shrink by oxidation, which is also represented as a surface reaction process, presumably involving the collision of oxidants such as O 2 and OH with the soot particles. Harris et al. (1986) estimated the soot inception rate in a premixed ethylene-argon-oxygen flame by solving the set of ordinary differential equations for the N i for diameters between 1.5 nm (i = 1) and 22 nm (i = 3000),0.022 /-tm, to conform with optical data on soot particles obtained in a premixed ethylene flame. * Figure 6.13 shows the soot particle total number density, mean particle diameter, and inception rate, J, as functions of time over positions corresponding to t = 1.1 to 9.6 ms beyond the estimated reaction zone of the flame. Figure 6.13 shows the calculated rate of soot particle inception. The data, which start 1.1 ms beyond the reaction zone, missed the rise in the inception rate. Using the curve, the authors estimate that a total of about 10 19 incipient particles m -3 were created in this flame, with a peak new particle formation rate of 10 23 m- 3 S-I. This quantity corresponded to 50% of the soot volume present at 2.4 ms, 3% of the soot volume present at 9 ms, and less than 1 % of the soot volume ultimately formed.

6.3.2 Soot Oxidation Soot particles formed in the fuel-rich regions of diffusion flames are eventually mixed with gases sufficiently fuel-lean that oxidation becomes possible. As a result, the soot actually emitted from a combustor may represent only a small fraction of the soot formed in the combustion zone. Since the conditions leading to soot formation in diffusion flames cannot be entirely eliminated, soot oxidation downstream of the flame is very important to the control of soot emissions. We desire to develop a useful model for soot oxidation. Park and Appleton (1973) observed that the rate of oxidation of carbon black (a commercially produced soot-like material) by oxygen could be described by a semiempirical model originally developed by Nagle and Strickland-Constable (1962) to describe the oxidation of pyrolytic graphite (2.156). There are other potential oxidants in flames besides 02' As we have seen, in fuelrich flames, OH may be more abundant than 02, and the and O 2 concentrations may be comparable. Let us estimate the rate of oxidation of a soot particle by reaction with OH. The flux of molecules at a number concentration N impinging on a unit area of surface in a gas is Nc /4, where c is the mean molecular speed of the gas molecules, c

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*The authors discuss those physical phenomena that influence the coagulation rate (e.g., van der Waals forces, charging, etc.) together with a variety of assumptions that must be made.

380

Particle Formation in Combustion

Chap. 6

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primary automobile exhaust aerosol produced under cruise conditions during combustion of unleaded gasoline as rep0l1ed by Miller et al. (1976). (Reprinted by permission of Air Pollution Control Association).

shows the aerosol volume distributions from an identical vehicle equipped to run on unleaded fuel. Unlike the case with leaded fuel, the mode in the volume distribution remains between 0.01 and 0.03 p,m for all cruise speeds investigated. The shift to slightly larger mean sizes between 20 and 35 mph was attributed to increase gas-to-particle conversion. We note that the conditions leading to the increase in aerosol volume at high speeds with leaded fuel appear to be absent in the case of unleaded fuel combustion. Because the data shown in Figures 6.16 and 6.17 represent a very limited sample, they should not be viewed as indicative of all motor vehicle exhaust aerosols. Nevertheless, they do exhibit the general features with respect to size distribution of automobile exhaust aerosols. Pierson and Brachaczek (1983) reported the composition and emission rates of airborne particulate matter from on-road vehicles in the Tuscarora Mountain Tunnel of the Pennsylvania Turnpike in 1977. Particulate loading in the tunnel was found to be dominated by diesel vehicles, even though on the average they constituted only about 10% of the traffic. Diesel emission rates were of the order 0.87 g km -I, the most abundant component of which is carbon, elemental and organic. Thirty-four elements were measured, in descending order of mass, C, Pb, H, SO~-, and Br together accounting for over 90%. Size distributions of motor vehicle particulate matter exhibited a mass median diameter of 0.15 p,m. Diesel particulate matter consists primarily of combustion-generated carbonaceous soot with which some unburned hydrocarbons have become associated (Amann and Siegla, 1982). Photomicrographs of particles collected from the exhaust of a passenger car diesel indicate that the particles consist of cluster and chain agglomerates of single spherical particles, similar to the other soot particles and the fly ash fume described previously. The single spherical particles vary in diameter between 0.0 I and 0.08 /-L m , with the most usually lying in the range 0.015 to 0.03 /-Lm. Volume mean diameters of the particles (aggregates) tend to range from 0.05 to 0.25 tim. The diesel particulate matter is nonnally dominated by carbonaceous soot generated during combustion. In addition, 10 to 30% of the particulate mass is comprised of solvent-extractable hydro-

Chap. 6

387

Problems

carbons that adsorb or condense on the surface of the soot particles and that consist of high-boiling-point fractions and lubricating oil.

PROBLEMS 6.1. The amount of ash vaporized during coal combustion is typically about I % of the mineral matter in the coal. Reduced vapor species are oxidized as they diffuse from the surface of the burning char particle leading to rapid homogeneous nucleation and the formation of large numbers of very small particles. Assuming that the initial nuclei are 0.001 lim in diameter and the ash density is 2300 kg m - 3, compute and plot the particle number concentration as a function of residence time in the furnace, assuming the gas temperature is 1700 K for 1 second and then decreases to 400 K at a rate of 500 K s -I. The gas viscosity may be taken as Ii = 3.4 X 10- 7 y"J7 kg m- I S""I. The aerosol may be assumed to be monodisperse. 6.2. A 50-lim-diameter char particle is burned in air in a furnace that is heated to 1800 K. Examine the volatilization of silica (Si0 2 ) from this particlc as it bums. Silica may vaporize directly

or by means of reduction to the monoxide 2

Si0 2(,) + CO ( • SiO(v) + CO 2 The oxidation rate expression of the char is that for the Whitwick coal in Table 2.10. (a) Calculate the equilibrium partial pressures for Si0 2 and SiO at the surface of the char particle. (b) What vaporization rate corresponds to this partial pressure? (c) Compare the vaporization rate with that predicted for a pure silica particle of the same size. Assume the silica particle temperature is the same as that of the gas. 6.3. For the conditions of Problem 6.2, compute the vapor concentration profile as a function of distance from the surface of the particle, assuming that no condensation takes place. The binary diffusivity may be calculated from cD = 9.1 x 10 7 1' + 7.4 X 10- 4 mole m -I s -I, where Tis the mean of gas and particle temperatures. Neglecting vapor loss, calculate and plot the supersaturation ratio and homogeneous nucleation rate. How far from the surface will nucleation occur? Use the following properties: Surface tension: Density: Molecular Weight:

a = 0.30 J m- 2 = 2200 kg m"' M = 60.9 g mole- I Pc

6.4. Harris et al. (1986) observed that no growth of the soot nuclei occurred in the region of the flame where particles are forn1ed and that the oxygen mole fraction in this region is about 0.01. Assuming that the Nagle and Strickland-Constable kinetics describe the oxidation of the soot nuclei and that the growth species is acetylene, estimate the minimum acetylene concentration that would be required to maintain the observed soot nucleus size of 1.8 nm, for temperatures ranging from 1200 K to 1500 K. The flux of acetylene reaching the particle surface may be taken as the effusion flux, and the reaction probability is unity for this estimate. The soot particle density is 1800 kg m- 3

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Chap. 6

References

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