Page 243 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

7.8 Formation of Particulate Matter 219

6
10x10
Bituminous coal
5 (Utah, USA)
8x10
3 )
dV/dlog Dp (µm/cm 4x10 5 Subbituminous coal
3 6x10 5
(Montana, USA)

2x10 5

0
0.01 0.1 1 10 100
Particle diameter, µm
Fig. 7.5 Fly ash particle size distribution (Data source Linak et al. [31])

7.8.1 Ash-Forming Elements in Fuels

Fossil fuels contain highly integrated ash-forming matter in form of discrete
particles, and bounded combustible compounds. A review of the corresponding
particle forming mechanisms can be found in literature [9].
The discrete mineral particles if not removed by fuel cleaning are quickly molten
at high temperatures, followed by condensation while traveling in the ducting
systems as temperature drops before emitting to the atmosphere. The relative
quantities of the bounded minerals increase while hydrocarbon is consumed by
oxygen in combustion. Metal oxides can also contribute to the formation of PM 2.5 .
The elemental metal vapors are also oxidized and consequently form PM 2.5 by
coagulation. Noncombustible solid compounds in the fuel will form the major part
of the fly ash particles, ranging from 10 nm to more than 100 μm.
Most fly ash particles are smaller than 0.1 μm in diameter by number, but larger
than 1 μm in diameter by mass. This is similar to the size distribution shown in
Fig. 7.5, although the exact size distribution depends on the coal and the design and
operation of the boiler. The fly ash size distribution in Fig. 7.5 was measured using
bituminous and subbituminous coals in the USA.
The ash formation is also dependent on the combustion device. For example, the
fate of ash-forming materials in fluidized bed combustion (FBC) is much different
from that of pulverized fuel combustion. In FBC, mechanical abrasion and attrition
play a more important role than the mechanism introduced above, and the ash-
forming materials remain in the fluidized bed. In circulating fluidized bed com-
bustion, more fly ash is formed due to the higher velocities and smaller fuel particle
size.