Environmental Engineering Reference
In-Depth Information
SNCR can be used in conjunction with low-NO
x
burners. The final reduction of NO
x
is in the
75-90% range, and the incremental cost of electricity production is 3-4.5%.
5.2.9.5 Toxic Emissions
Coal and (to a lesser extent) oil contain mineral matter that during the combustion process may
produce toxic vapors and particles. Particles larger than 1-2 microns in diameter are almost all
captured by the particle removal system, either by an electrostatic precipitator or a fabric filter.
Smaller particles and vapors may escape through the smoke stack and pollute the environment.
Mercury, selenium, cadmium, and arsenic are semivolatile toxic metals that in part may escape
through the smoke stack as vapors. In particular, mercury emissions are causing concern because
excessive mercury concentrations have been found in some lakes and coastal waters. Fish and other
aquatic organisms may bioaccumulate mercury and pass it on through the food chain to humans.
Currently, in the United States, Europe, and Japan, studies are being conducted on the extent of
the mercury problem and on the technologies that could be employed in order to reduce mercury
emissions from power plants.
9
Another problem associated with coal-fired power plants is radon emissions. Radon is a disin-
tegration product of uranium, the minerals of which may cling to the coal. Radon is a radioactive
gas that emits alpha particles. A part of the omnipresent radon in the atmosphere may be traced to
coal-fired power plants.
5.2.10
Waste Disposal
Coal-fired power plants produce a significant amount of solid waste. Oil-fired plants produce much
less waste, and gas-fired plants produce practically none. We calculated before that a 1000-MW
plant fired with coal that contains 10% by weight mineral matter produces about 3.2E(5) t y
−
1
of fly ash. If the coal contains 2% by weight of sulfur, and if that sulfur is removed by flue gas
desulfurization using a wet limestone scrubber, another 3-4E(5) t y
−
1
of wet sludge is created
containing hydrated calcium sulfite, calcium sulfate, and unreacted limestone. While some plants
succeed in selling, or at least giving away, the fly ash for possible use as aggregate in concrete and
asphalt or as road fill material, the scrubber sludge has practically no use.
10
For some coals the
fly ash and the scrubber sludge may contain toxic organic and inorganic compounds. In that case,
the waste needs to be disposed of in a secure landfill. The landfill must be lined with impenetrable
material, so that leaching into the soil and groundwater is to be prevented. Typical liner materials
are natural clays or synthetic fabrics. Because transport costs can be substantial, the landfill area
should be in the vicinity of the power plant. The mixed fly ash and sludge from a 1000-MW
coal-fired power plant would fill about 10-20 acres (4-8 hectares) about 1 foot (0.305 m) deep
every year. Thus, a new coal-fired power plant must be situated where a suitable landfill area
is available.
9
Municipal solid waste incinerators are the principal sources of mercury and other toxic metals emissions
because electrical switches, batteries, and fluorescent light bulbs are often thrown into the trash that is being
incinerated.
10
It is cheaper to produce gypsum wall boards from mined gypsum than from scrubber sludge.
