Environmental Engineering Reference
In-Depth Information
Fabric filters with a high collection efficiency for small particles are expected to have a higher
capital and operating cost than ESPs.
5.2.9.3 Sulfur Control
As living organisms contain sulfur in their cellular make-up, this sulfur is mostly retained in the
fossilized remnants of these organisms. Coal can contain up to 6% by weight of sulfur, and oil up
to 3%. However, most coals and crude oils contain a much lower percentage of sulfur. Generally,
bituminous, subbituminous, and lignite coals are used in power plants. They contain 0.7-3% by
weight of sulfur. Residual oil used in power plants contains 0.7-2% sulfur. Without sulfur emission
control devices, the oxidized sulfur, mainly sulfur dioxide SO
2
, minor quantities of SO
3
, and
sulfuric acid H
2
SO
4
, would be emitted through the smoke stack into the environment. The oxides
of sulfur are precursors to acid deposition and visibility impairing haze (see Sections 9.2.3 and
9.2.7). Because coal-fired power plants emit the majority of all sulfur oxide emissions worldwide
(industrial boilers, nonferrous smelters, diesel and home heating oil make up the rest of sulfur
emissions), operators of these plants are required to limit the emissions by either switching to
low-sulfur containing fuels or installation of sulfur emission control devices.
There are basically three approaches to reducing sulfur emissions: before, during, and after
combustion of the fossil fuel.
Before Combustion
Coal Washing.
When coal is removed from the coal seams in underground or surface mines, there
is always some mineral matter included in the coal. The majority of mineral matter is composed
of silicates, oxides, and carbonates of common crustal elements, such as calcium, magnesium,
aluminum, and iron, but some of it contains pyrites, which are sulfides of iron, nickel, copper,
zinc, lead, and other metals. Because the specific gravity of mineral matter, including the pyrites,
is greater than that of the carbonaceous coal, a part of the mineral matter can be removed by
“washing” the coal. Coal washing not only reduces the sulfur content of coal, but also reduces its
ash content, thereby increasing its heating value (Btu/lb or J/kg) and putting a lesser load on the
particle-removing systems.
Coal washing is usually performed at the mine mouth. Typically, the crushed raw coal is floated
in a stream of water. The lighter coal particles float on top, and the heavier minerals sink to the
bottom. The wet coal particles are transferred to a dewatering device, generally a vacuum filter,
centrifuge, or a cyclone. The coal can be further dried in a hot air stream.
One problem with coal washing is that the stream containing mineral matter may contain
dissolved toxic metals and it can be acidic. Strict regulations have been introduced in the United
States and elsewhere to prevent dumping of these toxic acidic streams into the environment without
prior treatment.
In the United States, about 50% of all coals delivered to power plants are washed. These are
mainly coals from eastern and midwestern shaft and strip mines. Western coals generally have
a low mineral and sulfur content, so washing is not necessary. Coal washing can remove up to
50% of the pyritic sulfur, which is equivalent to 10-25% removal of the total sulfur content of the
coal.
Coal Gasification.
Coal can be converted by a chemical process into a gas, called synthesis gas,
or syngas for short. In the process of coal gasification, most of the sulfur can be eliminated before
