4.4. Particulate emissions

1. Material arising from the organic content of the fuel and its failure to complete the burn-out process:
   1. Unburned hydrocarbons (smoke)
   2. Particulates formed via gas phase combustion or pyrolysis (soot)
   3. Cenospheres produced from cracked fuel or carbon along with ash (coke).
   
   
2. Material from the inorganic content of the fuel:
   1. Ash.

European legislation imposes the maximum limit for particulate emissions at 50 mg/nm³ from sources burning liquid fuels (EEC: Council Directive of 24 November 1988) .

4.4.1. Smoke

4.4.1.a. Smoke emissions

Smoke emissions in gas turbines occur when the combustion gases reach the relatively cool secondary zone. Considering the primary zone of the combustor as a stirred reactor, the composition of the gases leaving determines the smoke emissions from the secondary zone, where reactions are "frozen" and do not proceed further (Sawyer (1969)) . Emissions of unburned hydrocarbons from gas turbines are maximum at idle conditions and low at any other conditions (Sawyer (1969)) .

4.4.2. Soot

4.4.2.a. Soot emissions

Soot is most likely to be formed in fuel-rich conditions, and is normally fully burned as it mixes with air at very high temperature in highly oxidising zones, eg, as secondary air is injected in the combustion chamber of a gas turbine.

Experiments involving soot formation in premixed conditions with different fuels showed similar sooting tendency (Glassman (1988)) . It was suggested that all fuels break down into the same species, which leads to soot. Such a species has been postulated to be acetylene, which builds up into ring structures to give a soot particle by polymerisation and cyclisation processes (Glassman (1988), Tesner et al. (1971)) aided by H and vinyl radicals (Glassman (1988)) . Assuming that a minimum soot particle is formed by 25 acetylene molecules, the time necessary for the formation of such a soot precursor has been calculated to be 10^-5 seconds (Tesner et al. (1971)) . The activity of the ring structure decreases as the size of the structure grows by the successive addition of radicals (C₄, C₆ ...). Eventually a constant value is reached which indicates the formation of a physical surface, ie, the smallest soot particle (Tesner et al. (1971)) . In fact, the final particle size is determined by a decrease in the reactivity of the soot particles as they grow (Glassman (1988)) .

4.4.2.b. Factors that influence soot formation

1. The flame temperature, due to the high activation energy of the pyrolysis processes (Glassman (1988), Najjar (1982)) . Several researchers have reported the existence of a threshold temperature (between 1,000 and 1,300 °C) above which soot formation occurs in flames. Soot is eventually emitted when burnout ceases at temperatures below 1,000 °C (Glassman (1988), Kesten et al. (1980)) .

2. Local oxygen concentration (Kesten et al. (1980), Najjar (1982)) : Single droplet experiments with furfuryl alcohol showed that incipient soot formation commences as the space between consecutive droplets is diminished below a distance equivalent to 10 diameters. Network flames formed in such conditions are thought to cause a depletion in local oxygen concentration. Soot particles can also be eliminated by reducing the overall oxygen concentration, although this may be an indirect effect by the lower flame temperatures associated with low oxygen concentrations (Kesten et al. (1980)) .

3. Pressure, provided it affects temperature, and then only to a small extent. Pressure increases cracking to a greater extent than evaporation, thus promoting soot formation (Najjar (1982)) . Najjar observed a two-fold increase of soot formation when pressure was increased from 0.3 to 1 MPa in gas turbines (Najjar (1982), Najjar (1986)) . Improved atomisation caused by higher pressure led to lower penetration of the droplets into the combustion chamber. Thus, the high concentration of fuel in the high temperature, low oxygen region establishes the conditions for soot formation.

4. Chemical additives may enhance pyrolysis reactions (Glassman (1986)) . Halogens cause increase of hydrocarbon radicals by scavenging H radicals. On the other hand, emulsification with water may deplete soot formation (Gollahalli and Salek (1983)) . Metals such as nickel or manganese, charge the soot particles electrostatically and reduce agglomeration (Glassman (1986)) . The smaller particles thus formed burn more easily.

5. The following characteristics of the fuel also affect soot occurrence (Najjar (1982)) :
   
   1. Viscosity, as it determines atomisation characteristics.
   2. Volatility, particularly the final boiling point, as it determines evaporation.
   3. Thermal stability, as it determines the tendency for the fuel to crack.
   4. Higher aromatic content and higher C/H ratio give more soot. Under premixed conditions hydrocarbons show the following smoking tendency:
      aromatics > alkanes > alkenes > alkynes

      However soot formation correlates to H content rather than to global aromatics as different aromatics may have different H content (Najjar (1982)) .

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