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The process using coal oil and gas as fuel.
Water is pressurized, heated in boilers to superheated steam which is then passed through steam turbines to drive (ac) electrical generators. Coal was the first fuel to be widely used in boilers followed by oil; and nuclear reactors in place of boilers
Simplified diagram of coal fired electrical generating power station
Grid of ehv transmission distributes energy.
The electrical power generated is then transformed to an extra high voltage (ehv) (for example 132 kV up to 400 kV (1 kV is 1000 volts) for efficient transfer via transmission lines and then transformed down to lower voltages where required for use in towns, homes and factories etc. A 'Grid' of interconnected transmission lines enables stability of the whole system and diversity by the sharing of variable loads or large changes in generation.
Inherent heat loss limits efficiency up to 38%
Much heat is lost inherently in the conversion of heat to mechanical energy (and thus to electrical energy) in the water/steam 'cycle' in power stations limiting efficiency to 38% or less. Once the steam has passed through the turbine it is then condensed in a condenser still at quite a high temperature before being pressurized and reused in the boiler again. The condenser uses external water to cool the steam and this heat is lost to the sea, if taken from the sea, or river, lake, or via cooling towers to the air, depending on location.
Ways to increase efficiency and therefore reduce CO2 emission from power stations in the future.
1) If the waste heat (otherwise lost via the condenser) can be used for heating buildings in a local town or in an industrial process it is not wasted and therefore the overall efficiency is considerably increased or doubled (an old technology.)
and/or 2) Increase top (steam or gas) temperature with 'Supercritical ' or CCGT or IGCC systems.
Efficiency could be improved up to 55% with CCGT systems
In a combined cycle gas turbine station (CCGT) gas is burnt in gas turbines to drive alternators and the hot exhaust gas is used to raise steam to drive a turbo alternator.
Examples are a 600 MW station at Baja Mexico, 530 MW Irsching in Bavaria where 60% efficiency is projected, and 550 MW station at Baglan Bay, Wales which also includes district heating so overall efficiency could rise towards 85% in the latter case.
Supercritical steam cycle
In an 'Advanced Supercritical Station,' steam is raised to a top pressure of 350 bar, and top temperature of 700 deg. Efficiencies of up to 50/55% are possible (ref 115).
Carbon capture and storage (sequestration) would enable coal and gas fired power stations to be mostly green (90 % effective probably).
A significant reduction in CO2 emissions can be made by adding capture of the carbon dioxide (in addition to dust and SO2 collection), compress it, liquefy it and store it permanently underground in deep saline aquifers or depleted oil reserves (ref 126)(ref141). Perhaps 90% can be captured in this way from a coal or gas power station (ref135).
The process would cost an extra 20% probably bringing it in line with renewable sources. The extra power required can reduce efficiency.
The Global Geological site potential for CO2 storage is between 1000 and 10,000 Gtonnes of CO2. The potential for CO2 capture is around 2.6 to 4.9 Gt CO2 (0.7 to 1.3 Gtc) per year (ref 160). The Global emissions in 2001 totaled 24 Gtonnes of CO2 (6.5 Gtc) which is estimated to rise to 38 Gt by 2030 so this process could be a help for many years to reduce CO2 in the air.
The brine filled aquifer (brine in pore spaces) UTSIRA is solid rock 500 km X 50 km and 200 meters thick, 1000 meters beneath the sea bed under the Sleipner Norwegian oil field. 0.8 million tonnes of CO2 per year has been pumped into the pore spaces for ten years. (One power station would provide 4 million tonnes of CO2 per year) ( ref 127).
CO2 takes on a liquid form 800 metres underground so is unlikely to escape. (Ref 1 p292). It dissolves into saline water and may eventually become solid mineral carbonates.
More on Sequestration of CO2 | More on Carbon Capture and storage
IGCC plants with carbon capture and hydrogen production.
Extracting CO2 from exhaust gasses in a conventional power station adds an appreciable cost and loss of output power. An Integrated Gasification Combined Cycle plant (IGCC) lends itself to CO2 separation, with hydrogen as a by product, and efficiencies of power production up to 55%.
Syngas is mainly H2 and CO. Recently, four Integrated Gasification Combined Cycle (IGCC) (ref 120) plants have been built with 6 more to come. These produce synthetic gas (syngas) from coal or gas which drive a gas turbine/generator acting in a combined cycle with a steam turbine/generator. A by product is hydrogen, a potential fuel, and CO2. The cost of electricity is expensive but it lends itself to CO2 capture with near zero emissions and storage. If this is taken into account it could be a preferred option for future coal or gas use as a fuel. Efficiencies of 41 to 44% have been achieved and 55% is expected in future (ref 128). See world coal.
Underground Coal Gasification
Mining coal is expensive or in some locations not possible. An alternative IGCC plant in the future is underground coal gasification.
RWE (owner of N Power) plans an underground gasification trial plant in Germany. Oxygen and steam is pumped into the coal seam, ignited, raw syngas extracted. This would require precision drilling, skills similar to that required for offshore drilling.
Cooling towers and chimney at a coal fired power station.
Transformers and grid connections
