Environmental Engineering Reference
In-Depth Information
9.3.3
Water Use and Thermal Pollution from Power Plants
It is a consequence of the second law of thermodynamics that heat engines, such as power plants,
must reject some of the fuel energy in the form of heat transfer to a cold reservoir. Roughly
about one-third of the inherent heating value of the fuel is rejected via the steam condenser to the
cold reservoir. Another third is rejected to the atmosphere via the stack gas, and only one-third is
transformed into useful work. The cold reservoir is usually a water body. Some power plants and
industrial boilers are located near surface waters, such as a river, lake, or ocean, which can be used
for absorbing the heat from the condenser. Other facilities need to use a cooling tower or a cooling
pond. The EPA requires that all new power plants use cooling towers for heat rejection, rather
than once-through cooling water from adjacent surface waters. In cooling towers, cold municipal
or well water percolates around the hot condenser tubes, taking up the rejected heat. The water
is chilled in a draft of cold air and then is recycled. Alternatively, spent steam from the steam
turbine is directly condensed in cold air that is drawn through the cooling tower by huge fans. In
both cases a part of the cooling water is evaporated by taking up the heat from the condensing
steam. Evaporated water rises from the cooling tower into the cold ambient air and condenses,
thus forming the visible “steam” plume that emanates from the tower. A 1000-MW electric power
plant working at a thermodynamic efficiency of 33% and ambient temperature of 15
◦
C loses about
1.7 E(7) m
3
/y (about 4.4 billion gallons/y) of cooling water due to evaporation. In the United States,
cooling towers alone consume between 2% and 3% of total withdrawals from surface waters. Some
critics question the wisdom of using precious water resources for evaporative cooling purposes,
rather than surface waters (where available), even though the latter entails (a) some risk of thermal
pollution and surface water contamination by leaching of the heat exchanger components and
(b) some increased evaporation of the surface waters to the atmosphere. It is seen here that there is
no simple or unequivocal solution to environmental and natural resource problems engendered by
fossil fuel usage.
9.3.4
Atmospheric Deposition of Toxic Pollutants onto Surface Waters
In Section 9.2.6 we dealt with one form of water pollution due to combustion of fossil fuels, that of
acid deposition. But in addition to sulfur and nitrogen oxides, there are other combustion products
that escape from smoke stacks and eventually are deposited on land and water, which may cause
deleterious health and environmental effects. Two cases in point are the atmospheric deposition of
toxic metals and polycyclic aromatic hydrocarbons (PAH).
9.3.4.1 Toxic Metals
We noted previously that fly ash particles may contain toxic metals, such as arsenic, cadmium,
mercury, lead, selenium, vanadium, and zinc. These metals are found in small particles, less than
1
µ
m in diameter. The small particles are not efficiently collected by the ESP and thus escape into
the atmosphere. Because of their small size, these particles are little affected by gravity and can be
transported over large distances, hundreds to thousands of kilometers. Eventually, they are deposited
in dry or wet form on land and water. From the land, toxic metals may leach into groundwater, or
run off into streams, lakes, or ocean. Thus, they may enter the food chain, either directly by drinking
water or via aquatic organisms that drink or feed in the water. For example, fish from the Great
