Environmental Engineering Reference
In-Depth Information
the final stage of gasification. The clean, desulfurized syngas can be used to fuel a gas turbine or
combined cycle power plant. The process of coal gasification will be described in Section 5.3.2.
Oil Desulfurization.
Refineries can reduce the sulfur content of crude oil almost to any desired
degree. This is usually done in a catalytic reduction-oxidation process called the Claus process.
First, sulfur compounds in the oil are reduced to hydrogen sulfide by blowing gaseous hydrogen
through the crude oil in presence of a catalyst:
RS
+
H
2
→
H
2
S
+
R
(5.2)
where R is an organic radical. Next, H
2
S is oxidized to SO
2
by atmospheric oxygen, and simulta-
neously SO
2
is reduced by H
2
S to elemental sulfur, also in presence of a catalyst:
3
2
O
2
→
H
2
S
+
H
2
O
+
SO
2
(5.3)
2H
2
S
+
SO
2
→
2H
2
O
+
3S
(5.4)
The elemental sulfur is an important byproduct of oil refining and can be often seen as a yellow
mound within the refinery complex. It is a major raw material for sulfuric acid production.
Even though the sulfur in crude oil is a salable byproduct of oil refining, refineries charge more
for low sulfur oil products. Thus, utilities pay a premium price for low sulfur oil, and we pay higher
rates for electricity generated by the oil burning power plant. However, it is usually cheaper for a
utility to buy low sulfur oil than to remove SO
2
from the stack gas.
During Combustion
Fluidized Bed Combustion.
Fluidized bed combustion (FBC) is the burning of coal (or any other
solid fuel) imbedded in a granular material, usually limestone, riding on a stream of air. The primary
aim of the development of FBC was not specifically to reduce SO
2
emissions, but rather to enable
the combustion of all sorts of fuel, including nonpulverizable coal, municipal solid waste, industrial
and medical waste, wood, tar, and asphaltene (residue of oil refining). The admixed limestone acts
as a sorbent, extracting sulfur and other impurities from the fuel.
A schematic of a FBC with simultaneous coal and sorbent injection is shown in Figure 5.10.
The FBC is a cylindrical retort with a grate in the bottom. Crushed coal, 6-20 mm in size, together
with limestone (CaCO
3
), is blown pneumatically over the grate. Combustion air is blown from
beneath the grate. After ignition, the burning coal-limestone mixture floats over the grate, riding
on an air cushion. The burning mixture acts dynamically like a fluid, hence the name fluidized bed
combustion. Boiler tubes are immersed into the fluidized bed where there is direct heat transfer
from the bed to the tubes. Another bank of boiler tubes is mounted above the bed, to which the heat
transfer occurs by convection and radiation from the flue gases.
The sulfur is captured by the sorbent to form a mixture of calcium sulfite (CaSO
3
) and calcium
sulfate (CaSO
4
) particles. These particles, together with unreacted CaCO
3
and unburnt coal parti-
cles, are carried with the flue gas into a cyclone. The larger particles are separated in the cyclone
and are recycled into the fluidized bed for reburning. Smaller particles exit the cyclone and are
removed in an electrostatic precipitator or baghouse.
