Page 195 - New Trends In Coal Conversion

158 New Trends in Coal Conversion

produces road dust and air turbulence that can stir up road dust. Near coastal areas, the
evaporation of sea spray can produce large particles. Pollen grains, mold spores, and
plant and insect parts are all in this larger size range. The amount of energy required to
break these particles into smaller sizes increases as the size decreases, which effec-
tively establishes a lower limit for the production of these coarse particles of approx-
imately 1 mm.
Smaller particles, called the fine fraction or mode, are largely formed from gases.
The smallest particles, less than 0.1 mm, are formed by nucleation, which is the
condensation of low-vapor-pressure substances formed by high-temperature vaporiza-
tion or by chemical reactions in the atmosphere to form new particles (nuclei). Four
major classes of sources with equilibrium pressures low enough to form nuclei
mode particles can yield particulate matter: heavy metals (vaporized during combus-
tion), elemental carbon (from short C molecules generated by combustion), organic
carbon, and sulfates and nitrates.
Coagulation occurs more efficiently when a large numbers of particles interact, and
condensation is most efficient for large surface areas. Therefore, the efficiency of both
coagulation and condensation decreases as particle size increases, which effectively
produces an upper limit such that particles do not grow by these processes beyond
approximately 1 mm. Thus, particles tend to “accumulate” between 0.1 and 1 mm.
Submicrometer-sized particles can be produced by the condensation of metals or
organic compounds that are vaporized in high-temperature combustion processes.
They can also be produced by the condensation of gases that have been converted
in atmospheric reactions to low-vapor-pressure substances. For example, sulfur diox-
ide is oxidized in the atmosphere to form sulfuric acid (H 2 SO 4 ), which can be neutral-
ized by NH 3 to form ammonium sulfate. Nitrogen dioxide (NO 2 ) is oxidized to nitric
acid (HNO 3 ), which in turn can react with ammonia (NH 3 ) to form ammonium nitrate
(NH 4 NO 3 ). The particles produced by the intermediate reactions of gases in the atmo-
sphere are called secondary particles. Secondary sulfate and nitrate particles are usu-
ally the dominant components of fine particles. The combustion of fossil fuels such
as coal, oil, and petrol can produce coarse particles from the release of noncombustible
materials, i.e., fly ash, fine particles from the condensation of materials vaporized dur-
ing combustion, and secondary particles through the atmospheric reactions of sulfur
oxides and nitrogen oxides initially released as gases (European Commission, 2006).

6.2.3.3 General techniques for PM control and removal
Generally, the technology used to control primary PM emissions from coal-fired power
plants is classified under the following categories (Zhang, 2016):
• Precombustion control
• Coal-type selection
• Coal preparation
• In-combustion control
• Optimization of combustion processes
• Injection of sorbents into flame zone
• Postcombustion control