Thermal Power Plants/Fossil Fuels Flashcards
List some of the challenges faced with regards to conventional fossil fuel reserves (coal/oil/gas).
- Refine exploration techniques
- Make less ‘polluting’ (decarbonise)
- Enchanced extraction (sequestrate carbon)
- New resources (coal bed methane, oil shale, tar sand)
- New uses (methanol production)
List some of the challenges faced with regards to unconventional reserves (shale/fracking).
- Refine exploration techniques
- Make less polluting (decarbonise, reduce water pollution)
- Enchanced extraction (sequestrate carbon)
- Possible increased sesimic activity
List the five GENERAL stages in power plant operation.
- Heat from fossil fuel combustion
- Boil water
- Generate steam
- Drive a multi-stage turbine
- Drive a generator
NOTE: Generator is connected to the grid so its speed must be carefully controlled.
What is the maximum efficiency of a heat engine?
Carnot efficiency = 1 - T2/T1 (Kelvin)
Where T1 is the supplied heat temperature and T2 is the rejected heat temperature.
What is the realistic efficiency of a heat engine?
Endo-reversible = 1 - root(T2/T1) (Kelvin)
Where T1 is the supplied heat temperature and T2 is the rejected heat temperature.
With the aid of a diagram, explain the operation of a combined cycle gas turbine power plant,
commenting on its thermal efficiency and how this might be improved.
CCGT power stations have a secondary steam raising circuit using the heat from the gas turbine exhaust.
● Conventional gas turbine systems (25-30% efficiency) with massive heat rejection from exhaust.
● CCGT systems (55-60% efficiency)
● Nat Gas used to fuel gas turbine - turbine exhaust temperature is high and contains a great deal of energy; gases pass through heat exchanger to boil water and produce steam, which then passes through the turbine that drives a second generator.
With the aid of a diagram, explain why a combined cycle gas-fired power plant is more efficient than a conventional coal-fired power plant.
A thermal power plant uses heat of combustion to boil water to generate steam, drive a multi-task turbine and finally a generator. It has an efficiency of about 35%. A gas fired plant uses natural gas pumped into a gas turbine, where it is mixed with air and burned, converting its chemical energy into heat energy.
Alone, this plant has an efficiency of between 25 and 30%
● The two plants can be combined by having a secondary steam raising circuit using the heat from the gas turbine exhaust.
● Overall, this raises the thermal efficiency to between 55/60%. Primarily due to the relevant temperature differences within each cycle and the carnot
efficiency.
With the aid of a diagram, explain why a combined cycle gas fired power plant is more efficient than a nuclear power plant.
- A nuclear power plant relies on a standard steam cycle where the boiler produces steam through heat provided by the nuclear reactor. The efficiency of such a cycle is constrained by the laws of thermodynamics for a single heat engine process, along with the further losses in the heat exchanger and condenser, this system’s efficiency is around 35%.
- In a CCGT power plant, the exhaust fumes are still high temperature, as a single gas turbine
generator is not very efficient (around 25%). However, the heat from these is exhaust fumes is
utilised in order to heat a secondary boiler, which produces steam to be driven through a turbine
in a standard steam cycle. This secondary circuit raises the overall efficiency to around 55%.
Large coal power plants operate with an efficiency of below 40%. Using a diagram if required, explain why the efficiency is low and state two ways in which this could be improved.
Not all heat is utilized in power production, heat is rejected through exhaust fumes and
also the condenser.
The use of Integrated Gasification Combined Cycle (IGCC) can turn coal into a high pressurised gas which can then be used in a combined cycle, where the gas is
combusted through a turbine and its exhaust fumes are used in a steam cycle.
● Efficiency could be increased by used the rejected heat as low-grade heat, supplied to nearby buildings/structures.
● Use of heat exchangers with exhaust fumes and the input working fluid, water, supply, leading to a decreased heat input required in the boiler.
With reference to the process of nuclear fission, explain the function of the graphite in the core of an advanced gas-cooled reactor.
If U235 nucleus absorbs a neutron, it may split (nuclear fission) with the release of energy in the form of heat. The process emits further neutrons, ⅔ average, which are then available to collide with further U235 nuclei (Chain Reaction).
● Difficult to sustain a chain reaction in natural circumstances - Graphite is used as a moderator.
● Moderators are used to slow down the neutrons within the reaction vessel, which are released at high velocity as a result of previous fission reactions.
● Slowing down the neutrons will insure that the neutrons are more readily absorbed by the fissile U235 nuclei.
NOTE: Neutrons produced by fission have high energies and move extremely quickly. These so-called fast neutrons do not cause fission as efficiently as slower-moving ones so they are slowed down in most reactors by the process of moderation.
With reference to the process of nuclear fission, explain the roles of the water in the core of a
PWR.
In a Pressurised Water Reactor, the water serves as a moderator and a coolant. The function of the moderator in a nuclear fission reaction chamber is to slow down neutrons, which are released at high velocity following a fission reaction. The neutrons are slowed down by collisions with the hydrogen atoms in water, which are of a similar size. With slower neutrons,
they are more readily absorbed by the Uranium nuclei leading to instability and further fission
reactions.
In conventional nuclear reactors briefly explain the function of the moderator and the control rods.
The moderator’s function is to slow the neutrons within the reaction chamber. Slower neutrons are more likely to be absorbed by the uranium nuclei, which results in nucleus instability and fission, releasing further high speed neutrons.
The function of the control rods is to actively adjust the reactivity, in the reaction chamber, through the absorption of neutrons. With fewer free neutrons, fewer fission neutrons will be absorbed by the uranium nuclei, resulting in less fission reactions. Control rods are made from materials that can stably absorb neutrons and are lowered or retracted to slow or increase reaction rate, respectively.
Identify the principal emissions from fossil fuel combustion and state how the impact might
be mitigated.
- Carbon Dioxide (CO2)
Mitigate Impact: Carbon Capture and storage. - Sulphur Dioxide (SO2) and Nitrous Oxide (NOx)
Mitigate Impact: Careful control of combustion process and treatment of exhaust gases using chemical action or filtration. - Particles (soot)
Mitigate Impact: Fuel cleaning and using cleaner fuels (gas and oil produce less particulates than coal, low-ash coal).
Identify three emissions associated with the burning of fossil fuels and briefly describe the potential environmental problems arising from each one.
Sulphur dioxide
○ Concentration influences habitat stability for plant and animal communities
○ Can dissolve in water, leading to acid rain
Oxides of nitrogen
○ React to form smog and acid rain. Central to the formation of fine particles (PM) and ground level ozone.
Soot/Particulates
○ Primary cause of haze, which severely decreases visibility in cities and national parks. Also lowers life expectancy of presumably all organic life. Respiratory problems for humans and wildlife.
Carbon Dioxide
○ Causing more heat to be trapped in the earth and worldwide temperatures to rise (global warming).
Explain how a breeder reactor works.
In contrast to most normal nuclear reactors a fast reactor uses a coolant that is not an efficient moderator, such as liquid sodium, so its neutrons remain high-energy. Although these fast neutrons are not as good at causing fission, they are readily captured by an isotope of uranium (U238), which then becomes plutonium (Pu239). This plutonium isotope can be reprocessed and used as more reactor fuel. Reactors can be designed to maximize plutonium production, and in some cases they actually produce more fuel than they consume. These reactors are called breeder reactors.
Some argue that without breeder reactors supplying fuel for more numerous thermal power plants, nuclear power will remain relatively a relatively small contributor to our energy needs.
