Environmental Engineering class

D. Chemisana and Chr. Lamnatou

Thermal Pollution

28/10/2013

Thermal Pollution

Source: http://www.eoearth.org/article/Thermal_pollution?topic=49471#endnote_1; Goudie A. (2006), The Human Impact on the Natural Environment, 6th Edition, Blackwell publishing, UK

THERMAL POLLUTION is the act of altering the temperature of a natural water body (river, lake, etc)

Reasons such as: waste heat generated by an industrial process e.g. power plants

Thermal pollution is one parameter of the broader subject of water pollution

Rainbow trout, a species sensitive to water temperature change Source: Ken Hammond, USDA

ECOLOGICAL IMPACTS:Waste heat depresses the dissolved oxygen affects aquatic species: fishes, etc original species may migrate away; alien species may enter a local aquatic system loss of biodiversity

Thermal Pollution

Source: http://www.eoearth.org/article/Thermal_pollution?topic=49471#endnote_1

ECOLOGICAL IMPACTS: Many aquatic organisms are very sensitive to small

temperature changes (e.g. one degree Celsius)

The temperature change negative effects such as: alters metabolic rates reproductive success and mortality

A large increase in temperature decreased enzyme activity in aquatic organisms problems such as the inability to break down lipids, which leads to malnutrition of the organisms

Thermal Pollution

Source: http://www.eoearth.org/article/Thermal_pollution?topic=49471#endnote_1

ECOLOGICAL IMPACTS:

Primary producers are affected by thermal pollution since elevated water temperature increases aquatic plant growth rates algae bloom reduction of oxygen content in the water

The higher aquatic vegetative density leads to an increased plant respiration rate and also to a reduced underwater light intensity

The outcome is similar to the eutrophication that occurs when watercourses are polluted with leached agricultural inorganic fertilizers

Algal bloom in Orielton Lagoon, Australia, 1994(Photo by Geoff Prestedge) Source: Cloern et al. (2010), Eutrophication, Encyclopedia of Earth

Thermal Pollution

Source: http://www.eoearth.org/article/Thermal_pollution?topic=49471#endnote_1

ECOLOGICAL IMPACTS:

In the case of injection of cooler water from a reservoir into a warmer stream or river below, there can also be significant impacts upon: fish, especially in the egg and larval stage total aquatic productivity in the receiving river, etc

These cold water forms of thermal pollution can also create a modified aquatic environment such that certain alien species may have a competitive advantage over native species

Power plantsPower plant: is an assembly of systems/subsystems which generates electricity, i.e. power

Thermal Pollution

Source: Raja et al. (2006), Power Plant Engineering, New Age International Publishers, New Delhi

A thermal power plantSource: www.bikyamasr.com/21072/egypt-erecting-first-thermal-power-station

Classification of the power plants (based on the type of the fuel):1) ConventionalFossil fuels (Coal, etc)2) Non-ConventionalPV, solar thermal, biomass, geothermal, wind, etc

Thermal power plants: convert thermal energy to electric power (Mulvaney D. and Robbins P. (2011), Green Energy: An A-to-Z Guide, SAGE public., USA)

Power plantsUnits: boiler, turbine, generator, feed water (pumps, etc), cooling water, fuel handling, ash handling

Thermal Power Plants:-can have capacity: few 1000 MW

-60% of the global power generation is from Steam Thermal Power Plants (steam is the working fluid and steam turbines are used) WATER SUPPLY is important: e.g. for the cooling of the condensers

-generate electricity using heat energy of steam generated in the boiler

-the heat energy required can be supplied e.g. from fossil fuels (coal, etc)

-the fuels are burned in the boiler combustion chamber

-the hot flue gas generated exchanges heat with the feed water to generate superheated steam

-the superheated steam at high pressure is expanded in a steam turbine to make it rotate

-the electrical generator coupled with the turbine rotates and generates electricity

Thermal Pollution

Sources: Krishnaswamy K. and Bala M. P. (2011), Power Plant Instrumentation, PHI, New Delhi; Raja et al. (2006), Power Plant Engineering, New Age International Publishers, New Delhi

Steam Thermal Power Plants:

-steam generatora boiler in which steam is generated from water by application of heat (water receives heat from the hot gases though the heating surfaces of the boiler; the hot gases are formed e.g. by burning fuel e.g. coal, etc)

-steam prime moverssteam turbine its output is utilized to run the generator

-steam condensersthe steam from the prime mover is exhausted into a vessel (condenser) there the pressure is maintained below the atmospheric by continuously condensing the steam by means of circulating cold water at atmospheric temperature

Thermal Pollution

Sources: Krishnaswamy K. and Bala M. P. (2011), Power Plant Instrumentation, PHI, New Delhi; Raja et al. (2006), Power Plant Engineering, New Age International Publishers, New Delhi

Thermal Pollution

Source: http://processengineers.info/steam_power_plant.html

A simple steam power plant

Cooling of the power plantsThis cooling function (to condense the steam) may be done:

-Direct (once-trough) cooling: if the power plant is next to sea, lake, etc running a large amount of water though the condensers and discharging it back into the sea, lake, etc warmer (the water may be salt or fresh)

-Indirect (recirculating) cooling: using a cooling tower

-Dry cooling: cooling simply by air (may involve cooling towers); few plants use this

Thermal Pollution

Source: http://www.world-nuclear.org/info/cooling_power_plants_inf121.html

Cooling towers evaporating water at Ratcliffe-on-Soar Power Station, United Kingdom Source: wikipedia

Cooling of the power plants

Power plants and steam cycle heat transferwater is circulated continuously in a closed loop steam cycle is turned to steam in order to drive the turbine then is condensed and returned to the heat source

Cooling to condense the steam and discharge surplus heatcooling of the system to condense the low-pressure steam and recycle it as the steam condenses back to water, the surplus (waste) heat which is removed from it, needs to be discharged to the air or a body of water

Thermal Pollution

Source: http://www.world-nuclear.org/info/cooling_power_plants_inf121.html

Schematic diagram of a thermal power plantSource: Gotto et al. (2002), Control Engineering Practice 10(1), p. 15-22

Thermal Pollution Additional information about the power plants

Cogeneration power plants (CHP: Combined Heat Power):Cogeneration refers to the simultaneous production of both useful steam and electricity. By combining these processes into one power plant, the overall fuel efficiency increases significantly, when compared to a thermal-electric power plant without cogeneration

Fuel is burned boilers: production of steam (high pressure) the steam is fed into turbines that generate electricity the steam leaving the turbines is at a much lower pressure. This low pressure "discharge" steam can be e.g. fed into steam pipes for distribution to buildings

The process of cogeneration contrasts dramatically to the practice of the plants without cogeneration which "throw away" the exhaust steam from the turbines. By using this exhaust steam, the fuel efficiency can be e.g. 78%, compared to a fuel efficiency of approximately 40% for plants without cogeneration

Source: http://www.p2000.umich.edu/energy_conservation/ec4.htm

Thermal Pollution Additional information about the power plants

Cogeneration power plants: impact of cogeneration on environmental emissions

-energy losses which are related with environmental impacts such as thermal pollution are significantly reduced with cogeneration (e.g. 80 or 95%)

-significant reduction in fossil fuel consumption elimination of fossil fuel emissions

Source: Dincer I. and Rosen M. A. (2013), EXERGY: Energy, Environment and Sustainable Development, 2nd ed., Elsevier, USA.