Page 197 - New Trends In Coal Conversion

160 New Trends in Coal Conversion

temperature causes a greater possibility of coalescence between small particles into
agglomerated ash, which will obviously reduce the fine particle amount (Lu and
Ren, 2014).
The effect of burning time on PM formation is very similar to the temperature one.
The longer the burning time, the finer the PM produced. Also, longer burning time
makes fly ash particles have an irregular shape, whereas shorter burning time makes
them have a normal shape and small deformation of coal. This can be explained as
longer burning time could cause not only the severe fragmentation of coke and min-
erals but also a more thorough vaporization of volatile matter and refractory oxides,
which results in the formation of more fine PM (Zhang, 2016; Lu and Ren, 2014).
As the boiler load drops, the air supply and flue gas volume decline and the velocity
of flue gas decreases. The large ash particles in the flue gas become more likely to de-
posit or adhere to the furnace surface (Zhang, 2016).
A lower load rates give the result of a higher quantity fine PM to total particles. The
smaller the particles, the higher the relative enrichment of trace elements, which tend to
perch on the surface area. That makes one think that with deficient oxygen and short
residence time, the carbon cannot be burned in a low load rate boiler, resulting in a
higher proportion of fine PM enriched with trace elements after combustion. Thus,
high load factors can help with PM 2.5 emission reduction.
Injection of sorbents into the flame zone. One of the most promising techniques to
reduce the formation of fine PM is the injection of sorbents into the flame zone, which
has been proven by some researchers to increase the emission of coarse PM but
decrease the emission of fine PM. The increased emission of coarse PM is attributed
to a combined increase in fine particle scavenging by coarse fly ash and sorbent par-
ticles, and the decreased fine PM emission is considered a result of reduced sulfur di-
oxide concentration. Of course, the election of the sorbent plays a critical role in the
control of fine PM.
There are two kinds of situations: under high temperature conditions, an agglomer-
ation between fine coal particles and sorbent particles occurs, whereas under normal
combustion conditions, only a small percentage of particles contact with other particles
suspended in the flue gas. The sorbents with high porosity provide large surface areas
for condensation and further chemical reactions of trace element vapors into large ash
particles. It has been determined that the injection of sorbents into the furnace zone
leads to an increasing emission of coarse PM, whereas the emissions of fine PM
and trace elements are reduced. This change in the characteristics of PM emissions
absolutely favors the latter particle removal process (Lu and Ren, 2014).

6.2.3.3.3 Postcombustion control techniques
Electrostatic precipitator. An ESP uses an electrical charge to separate the particles in
the flue gas stream under the influence of an electric field. More than 70% of existing
coal-fired power plants are reported to have installed ESPs.
In brief, an ESP works by imparting a positive or negative charge to particles in the
flue gas stream. The particles are then attracted to an oppositely charged plate or tube
and removed from the collection surface to a hopper by vibrating or rapping the