Electrical Power Generation Systems
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
Main components of a coal fired electrical generating power station
Grid of ehv transmission distributes energy.
The electrical power generated is transformed to an extra high voltage (ehv) for example 132 kV or 400 kV (1 kV is 1000 volts) for efficient travel via transmission lines. It is transformed down to lower voltages for use in towns, homes and factories etc. A 'Grid' of interconnected transmission lines enables stability of the whole system and diversity by sharing the constantly varying 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 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.
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.
and/or 2) Increase top (steam or gas) temperature with 'Advanced Supercritical ' or 'CCGT' or 'IGCC' systems. In an 'Advanced Supercritical Station,' steam is raised up to top pressures of 250 to 350 bar, and top temperature of 800 - 700 deg. Efficiencies of up to 50/55% are possible (ref 115).
3) Metal Oxide fuel cells may be practical for converting gas energy to electrical energy at 80% efficiency. The cells operate at 800-900 degC which is self sustaining once up to temperature. Local installations distributing to housing estates or towns could save on transmission equipment. See hydrogen - fuel cell page.
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, and a550 MW station at Baglan Bay, Wales which also includes district heating so overall efficiency could rise towards 85% in the latter case.
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, from coal or gas power stations, can be made by adding the 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 reservoirs (ref 126)(ref141). Up to 90% of CO2 emitted from a power station can be captured in this way from a coal or gas power station (ref135).
The process would cost an extra 20-30% probably bringing it in line with renewable energy sources while extra generation capacity is required to make up for lost efficiency.
The Global Geological site potential for CO2 storage is between 1000 and 10,000 Giga tonnes (Gt) of CO2. The potential for CO2 capture is around 2.6 to 4.9 Gt CO2 (0.7 to 1.3 Gt of carbon) per year (ref 160). The total of all global emissions in 2001 was 24 Gt of CO2 (6.5 Gt of carbon) which is estimated to rise to 38 Gt by 2030 so this process could help for many years to reduce CO2 in the air.
Potential in UK North Sea areas is for 40 giga tonnes of CO2 to be liquefied and stored over 200+years in sandstone (porous) capped with mudstone at 3 mile depth. (ref ST p6 news 16 08 09)
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).
An alternative is to inject the CO2 underground, at least 800m (2600feet). CO2 takes on a liquid form under 800 metres underground so is unlikely to escape. (Ref 1 p292). It dissolves into saline water and may eventually become solid mineral carbonates.
IGCC plants with carbon capture and hydrogen production.
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, some Integrated Gasification Combined Cycle (IGCC) (ref 120) plants have been built with several 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 process lends itself to CO2 capture with near zero emissions and storage. 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 for an IGCC plant in the future is to obtain gas from underground coal gasification.
RWE (owner of N Power) plans an underground gasification trial plant in Germany. Oxygen or enriched air 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.
Most projects are at 500 to 800 m depth, but future projects may be at 1200m . At 1600m CO2 could probably be stored in liquid form.
Cooling towers and chimney at a coal fired power station.
Transformers and grid connections
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