Page 196 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 159

• Conventional particle emission control devices (PECDs)
• Innovative PECD technology
• Agglomeration
• Hybrid systems
• Multipollutant control systems
The most commonly used of these methods are going to be described in the
following section.

6.2.3.3.1 Precombustion control techniques
Coal-type selection. The combustion of a lower ash-containing coal would result in
less fly ash, hence less PM (White Pine Energy Station Project, 2008). There are dif-
ferences in the selection of coal for combustion in power plants due to the properties of
coal (for example, pore structures or modes of occurrence of elements). These charac-
teristics greatly affect the performance obtained in the furnace zone. Different coal
pore structures could have an effect on the breakage degree of coal particles, different
size distribution of primary PM, and different diffusion resistance. Those factors deter-
mine the amount of volatile elements in the vapor phase, which participates in the
condensation process (Staudt, 2011).
Coal preparation. When finer carbon is present, the formation of fine PM is higher
due to the direct transferring of different minerals. For pulverized coal, depending on
the particle size, a different size distribution is obtained. For example, for particles
sizes lower than 63 mm, a bimodal size distribution of 0.5 mm was obtained, whereas
for coals between 125 and 250 mm particles size, PM emission forms a single-mode
distribution at 4 mm. Moreover, finer coal combustion tends to emit PM containing
more volatile trace elements, which makes it very important to prepare the carbon
with the fineness desired before the combustion process. R90 is an engineering param-
eter used to define the coal fineness that affects the stability of combustion in the
furnace.
The finer the pulverized coal, the finer the fly ash and the larger the amount of fine
particles in the emitted fly ash. Moreover, combustion of finer coal tends to emit PM
containing more volatile trace elements. Therefore, it is important to prepare the coal
with appropriate fineness before combustion (Staudt, 2011).

6.2.3.3.2 In-combustion control techniques
Combustion optimization. Important factors that affect PM formation and PM size dis-
tribution in the combustion process by optimizing the combustion of coal are mainly
combustion temperature, burning time, and boiler load (Zhang, 2016; Lu and Ren,
2014).
Higher combustion temperature increases the temperature gradient inside the coal
particles and results in more fragmentation of coke and excluded minerals into fine
PM. However, the combustion temperature affects the formation of PM in two oppo-
site ways. First, using a high combustion temperature, the coal particles swell exten-
sively and bring in severe fragmentation and high vaporization rate of elements,
which favors the formation of more fine particles. On the contrary, a higher