Page 281 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

258 9 In-combustion Air Emission Control

ash particles formed in a pulverized coal combustion process is very fine and
approximately 65–85 % of the ash is fly ash.
Pulverized coal combustion technology is used for the majority of fossil-fuel
fired electricity generation. As to be introduced in the coming chapter, flue gas
cleaning typically involves the emission controls of particulate (i.e., fly ash), oxides
of sulfur and nitrogen, and others. It is characterized with high-combustion effi-
ciency and high-combustion temperature.

9.1.2 Fluidized Bed Combustion

Advanced coal technologies have been developed to improve cost-effectiveness and
environmental protection. These emerging technologies differ significantly from the
existing conventional combustion technologies such as pulverized coal combustion,
cyclone firing, and stoker firing. They are briefly introduced as follows.
Fluidized bed combustion (FBC) is primarily for solid fuel combustion because
there is no need to fluidize liquid or gaseous fuels. In FBC, the combustion takes
place in a bed where materials are fluidized by air blown from beneath of the layer
of particles. The bed materials are fuel particles, sand, ash, char residue, and other
solids.
There are two typical FBC systems, bubbling fluidized bed and circulating
fluidized bed, which are shown in (Fig. 9.1). In bubbling fluidized bed combustion
(BFBC) the bed particles behave like a boiling fluid but remain in the bed, when the
gas flows upward at the velocity of 1–3 m/s. In circulating fluidized bed combustion

Gas to
boiler
Gas to
boiler Cyclone

Fuel
Fuel
Fuel

Secondary
Secondary air
air
Primary air
Fig. 9.1 Fluidized bed combustion chambers (Left Bubbling fluidized bed, Right Circulating
fluidized bed)