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Fossil Technology Top
Fossil facts and figures This is a collection of information from different resources and represents the situation in 2005 / 2006. Sources are the International Energy Agency (France), BWK (Germany) and Australian sources. The problem is mainly CO2 emission from fossil fuels. That adds to the problem that nearly all of this fuel comes from non-renewable sources.
Levelised Cost of Different Power Generation Technologies
Fossil fuelled power stations using boilers and steam turbines This technology is the provider for most of the world’s electricity. The principle is simple: Boil water to make steam, then use the steam to drive a turbine, which is connected to an electricity generator. The fuels for boiling water is mostly coal (black or brown, also known as lignite), oil, gas, and to a lesser extent, wood and waste. Reducing the amount of fuel for the same amount of electricity is and has been the reason why more and more complex power stations have been developed. There are many methods of generating electricity from coal, gas and oil and practically all are being used in modern power stations. One method is to increase the steam pressure and temperature, in order to increase the amount of energy the turbine can convert into power. Today in the most advanced standard boilers for power stations the typical pressure of steam is between 25 and 30 MPa (30,000 kPa or 120 times car tyre pressure) and steam temperatures of up to 600°C. These steam conditions require very clean water, therefore the water and steam flowing through a boiler and turbine are in a closed cycle. The boiler needs water to make steam, the turbine needs steam to turn and generate power for the generator, but the steam leaving the turbine has to be converted back to water. This happens in the condenser, a heat exchanger (like the car radiator, only bigger) under the turbine. This condenser is cooled by water from a lake or cooling tower, or it can be cooled by air if water is a rare commodity. Other processes in the generation of electricity are the handling of the fuel; and the handling of it the fuel’s waste products, such as ash, sulphur from flue gas, and many other smaller waste products. Converting the energy contained in the fuel for a power station and comparing it to the electricity leaving the power station measures the efficiency of these stations. Typically one tonne (metric) of black coal contains around 8,000 MJ or 2.22 MWh of energy (MJ = Mega Joule, MWh = Megawatt hour). One tonne of this coal typically generates only 30 to 40% of electricity; the rest is wasted during the conversion of steam into water. It leaves as heat in the cooling water or cooling air. It takes 2.25 MJ of energy to convert water into steam (boiler) and steam into water (condenser). [?]The turbine can only convert steam into power. If it would turn the steam into water, then it would need to compress the steam to become water (and heat, because compression generates heat and also only, if this would be possible) and that would use the same amount of energy again, therefore no gain at all.[?] This is a wasteful process with limitations by the laws of physics and by inefficiencies. Since the first power stations were built, development has been driving efficiencies higher and higher. One common method is using higher pressures and temperatures. For example typical steam condition for turbines 20 years ago were 12 to 18 MPa pressure and 520 to 540 degrees C. Now there are new power stations using 27 MPa and 600 to 700 degrees C. There are many alternative ways of improving efficiency in electricity generation in power stations, such as using the waste heat from the steam cooling process for industry and domestic heating, in particular in (the cooler climates of) northern countries; using Ash from coal burning as a supplement in road surface and concrete mixtures; and the removal of sulphur from the flue gas cleaning (mostly in Europe, Asian and some US power stations) where lime slurry is used rather than seawater, giving an end product of gypsum which can be used in the building industry. The complexity of these stations (boiler, turbine, generator, coal handling, coal mills, feedwater system and supply, water treatment, condensate system, combustion air and flue gas removal system, cooling water system, ash and dust removal system, flue gas washer or scrubber and associated system, automation and control system and electrical system) requires an increase in the size of these stations, in order to increase efficiency – the ratio of support systems and internal power consumption to power generated is favourable for large power stations. Typical unit size ranges from 200 MW to 1200 MW. In particular lignite (brown coal) as found in Australia (Victoria) and Germany has special challenges because of the high water content of the lignite (around 65%). Efficiency can be improved by drying this lignite before using it in the boilers. Drying requires energy, therefore the efficiency is reduced. Using a combination of highly efficient boilers and turbines at supercritical steam conditions with fuel cells increases overall efficiency. This reduces CO2 emission in relation to electricity generated and is therefore a temporary solution only until clean technologies have been developed. An example of a German 500 MW power plant being commissioned by Vattenfall is shown below. The German government supports this technology.
CO2 Separation Concepts: CO2 separation after combustion (flue gas decarbonising post combustion) currently used in some chemical industry and typically uses amines – washing of flue gases similar to desulphurisation. So far not applied or developed for use in power stations because of the very much larger volume of gas to be treated. CO2 separation using oxyfuel process burns carbon with pure oxygen and separates the CO2 and H2O generated by this combustion by condensation. So far only laboratory processes have worked. One process challenge is the very highly combustible process (risk of explosion) and the control of the very high temperatures during combustion. The graph below indicates the status of development in 2005/2006, pilot plant are under construction indicating that some of the technological challenges appear to be past the early development stage.
CO2 separation before combustion (pre-combustion decarbonising) uses pressure and generates a hydrogen rich gas and practically pure CO2. Applied in some gas and steam driven power plants with integrated coal gasification (IGCC) and natural gas driven combined cycle power plants (NGCC). Until now the CO2 has not been stored away. Research shows that if in a typical black coal fired power station a CO2 separation grade of 88% would lead to actual reduction of CO2 by 76% and only a reduction of 65% greenhouse gas emissions. This is under best conditions with no leakages and other “normal” real plant issues. The introduction of any of these processes uses additional energy, in average 34% more. This reduces the efficiency of a modern power station from 49% by around 9%. Using the same technologies in older power stations with typical efficiency levels of 32% the reduction of efficiency would be around 15%. Using these figures the use of renewable energy sources like wind, solar and bio fuels becomes much more competitive.
Removing CO˛ from smokestacks by Dan Stojanovich A novel approach to removing CO˛ from power station smokestacks is about to be trialled for six months at International Power’s Hazelwood power station in Victoria’s Latrobe Valley. Coal-fired Hazelwood with a capacity of some 1635MW has the reputation of being one of the most greenhouse polluting power stations in Australia. It produces some 17Mt of CO˛ per year. The new process uses micro algae and photosynthesis in a bioreactor to take the CO˛ out of the flue gases and turn it into biofuels and biomass. Energetix, a division of the Australian Victor Smorgon Group (VSG), has secured the rights to the technology for Australia and New Zealand from Greenfuel Technologies of the US. The technology was developed by NASA and the Massachusetts Institute of Technology. This is the first such trial in Australia. In the technology, draft fans are used to remove the gas which is then introduced into the bioreactor. The reactor is effectively an extensive network of plastic pipes in which micro algae and nutrients are suspended in a water-based medium. The nutrients optimise the growth rate of the algae. Exposure to sunlight results in photosynthesis, whereby the algae “devour” the CO˛. A portion of the medium is removed on a regular basis to harvest the growing algae. The medium is usually dewatered in a two-stage process to provide algal solids for further processing. At the end of the process, what remains are basically oils, proteins and carbohydrates in approximately equal proportions. These three compounds can then be turned into bio-diesel, stockfeed and ethanol respectively. Furthermore, the stockfeed component can also be dried and turned into fuel. The technology is expected to be able to process some 700t of CO˛ per hectare of bioreactor. The conversion rate of CO˛ can be site specific, depending upon factors such as amount and intensity of sunlight, water quality, flue gas composition, nutrients, algae composition, pipe network layout and harvesting management procedures. The test plant at Hazelwood will be 10m x 60m. Engineers Australia magazine, December 2006, p19
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Last modified: 04/05/08 |
