Page 287 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

264 9 In-combustion Air Emission Control

injection engine (IDI). Combustion takes place in the prechamber and the burning
gas enters the diesel engine cylinder through a passageway. Air emissions, espe-
cially fine particulate (soot particles) are reduced from an IDI diesel engine at the
cost of lower engine efficiency.
Diesel is injected into the cylinder by multiple small nozzles. The droplets move
at great initial Reynolds number because of the great relative velocity between
droplets and the surrounding gas. As a result, the droplets may be further broken
down to smaller ones.
Diesel engines are usually run at fuel-lean condition and the corresponding
gaseous air pollutants such as CO and HC are reduced. However, the particulate
matter (soot) emission is much higher than the gasoline engine because of the
slower air-fuel mixing.

9.3 SO 2 Capture by Furnace Sorbent Injection

SO 2 can be captured by injection of proper sorbent such as limestone or lime into
the furnace of a stationary system. The sorbent can be injected into the furnace or
the hot part of the flue gas channel. This works well for older boilers with a
relatively short remaining lifetime. As solid sorbent is injected into the furnace,
more particles will join the fly ash and increase the load of downstream particulate
control devices. Additional soot-blowing device is needed to remove solids accu-
mulated on the inner surfaces of the furnaces, which is not a big technical problem.

9.3.1 SO 2 Capture by FSI in Pulverized Coal Combustion

The principle of furnace sorbent injection (FSI) is shown in Fig. 9.4. The con-
centration of SO 2 in typical flue gases from coal firing can be up to 5,000 ppmv.
The efficiency of SO 2 removal by sorbent injection depends on the temperature
where the sorbent is injected. When the sorbent is injected at a low-temperature
area, the SO 2 removal efficiencies can be in the range of 60–75 % with Ca and S
molar ratios from 2 to 4. This efficiency can be further increased at a relatively
higher cost, by spraying water downstream into the flue gas duct before the par-
ticulate control devices, to reactivate the spent sorbent to capture more SO 2 .
The relation between temperature and the SO 2 removal is shown in Fig. 9.5 for a
sorbent such as limestone (CaCO 3 ) or hydrated lime (Ca(OH) 2 ). With other con-
ditions the same, Ca(OH) 2 gives higher efficiency than CaCO 3 .
The main solid product of the desulfurization reaction is CaSO 4 when temper-
ature is below 1,200 °C. The corresponding mechanism is illustrated in Fig. 9.6.
When exposed to high-temperature gases, the sorbent first decomposes to CaO,
which then reacts with SO 2 and O 2 to form CaSO 4 . A white shell of CaSO 4 formed
by desulfurization reactions surrounds the inner unreacted CaO, which slows down