Environmental Engineering Reference
In-Depth Information
major public electricity producer in the
province, has a well-established practice of
blending low-sulphur, western Canadian coal,
with the high-sulphur product from the eastern
United States. As a result of this plus the use of
washed coal, the utility's SO
2
output per unit of
electricity has been declining with some
regularity since the early 1970s (Ontario:
Ministry of the Environment 1980).
The amount of SO
2
released during
combustion can be reduced if the coal or oil is
treated beforehand to remove some of the
included sulphur, in a process called fuel
desulphurization. The methods can be quite
simple and quite cost effective. Crushing and
washing the coal, for example, can reduce
subsequent SO
2
emissions by 8 to 15 per cent
(Park 1987), which represents a reduction of 1.5
to 2 million tonnes of SO
2
per year in the eastern
United States alone (Cortese 1986). More
complex chemical cleaning methods involving
the gasification or liquefaction of the coal are
also possible, but at considerable cost (Ramage
1983).
If it is not possible to reduce sulphur levels
significantly prior to combustion, there are
techniques which allow reduction during the
combustion process itself. Basically, they involve
the burning of coal in the presence of lime.
Although the technology has been studied since
the 1950s, it has yet to be adopted on a large
scale (Ramage 1983). There are two promising
developments, however, which are expected to
be available in the early 1990s. These are, lime
injection multi-stage burning (LIMB) and
fluidized bed combustion (FBC). LIMB involves
the injection of fine lime into the combustion
chamber, where it fixes the sulphur released from
the burning coal to produce a sulphate-rich lime
ash. This process can reduce SO
2
emissions by
35-50 per cent (Burdett
et al.
1985). In the FBC
system, air under pressure is injected into a
mixture of coal, limestone, and sand, until the
whole mass begins to act like a boiling fluid
(Ramage 1983). The continual mixing of the
materials under such conditions ensures that
combustion is very efficient, and that up to 90
per cent of the sulphur in the fuel is removed
(Kyte 1986b). It has the added advantage that,
since furnace temperatures are relatively low, it
reduces NO
X
emissions also. In the United States,
four thermal electric generating stations utilizing
FBC technology came on stream in the late 1980s.
Together they produce only 400 MW, but it has
been estimated that a further 150 stations—
producing 20,000 MW—could be retrofitted
(Ellis
et al.
1990).
Flue gas desulphurization (FGD) is the name
given to a group of processes which remove SO
2
from the gases given off during combustion. The
devices involved are called scrubbers, and may
be either dry or wet operations. The simplest dry
scrubbers act much like filters, removing the gas
on contact by chemical or physical means.
Sulphur dioxide passing through a dry pulverized
limestone filter, for example, will react chemically
with the calcium carbonate to leave the sulphur
behind in calcium sulphate (Williamson 1973).
Other filters—such as activated charcoal—work
by adsorbing the gas on to the filter (Turk and
Turk 1988).
Wet scrubbers are more common than the dry
variety. The flue gases may be bubbled through
an alkaline liquid reagent, which neutralizes the
SO
2
and produces calcium sulphate in the process
(Kyte 1981). A variation on this approach
involves the use of a lime slurry through which
the flue gases are passed, and in more modern
systems the combustion gases are bombarded by
jets of lime (LaBastille 1981) or pass through
spray systems (Ramage 1983). The system
described by LaBastille removes 92 per cent of
the SO
2
from the exhaust gases and many
scrubbers achieve a reduction of between 80 and
95 per cent (Cortese 1986). By 1978, Japanese
industry had installed scrubbers on more than
500 plants (Howard and Perley 1991). In the
United States, some 70,000 MW of electricity is
currently being produced from plants employing
FGD technology, and in Canada, a FGD retrofit
programme is in place with the goal of reducing
SO
2
emissions by about 50 per cent by 1994 (Ellis
et al.
1990). The European Community directive
of 1988—requiring all EC nations to reduce SO
2
