Can the coal ever run out Why
coal-fired power station
Lexicon> Letter K> Coal Power Plant
Definition: a coal-fired thermal power plant
More general term: power plant
More specific terms: hard coal power plant, lignite power plant
English: coal power station
Categories: electrical energy, prime movers and power plants
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: June 21, 2010; last change: 02/21/2021
A coal-fired power station is a thermal power plant (thermal power plant), which is fired with coal. It essentially consists of the following components:
- A Coal mill crushes the coal into fine coal dust, which is much easier to burn than solid pieces. Before that is v. a. lignite requires drying, which consumes considerable amounts of heat.
- In the coal furnace, the coal dust is blown in together with preheated combustion air and burned. Usually hundreds of tons are burned every hour.
- The hot flue gas gives off heat to a steam generator (a heat exchanger built into the boiler consisting of many pipes). In modern systems, the steam reaches a temperature of z. B. 600 ° C and a pressure of just under 300 bar.
- The cooled flue gas passes through a flue gas cleaning system (see below) before it is distributed over a large chimney as flue gas.
- The high-pressure steam generated drives a multi-stage steam turbine, which in turn drives a generator to generate electricity. Transformers increase the electrical voltage to hundreds of kilovolts, as required for long-distance transport with high-voltage lines.
- The expanded steam is condensed in a condenser downstream of the turbine. River water cooling and an additional cooling tower are usually used for this purpose. The condensate is then fed back to the steam generator as feed water via several preheaters. A powerful feed water pump moves the water in this circuit.
Various systems such as coal mill, feed water pump and exhaust gas purification are large consumers of electricity; the resulting own demand is typically in the order of magnitude of 10% of the gross electricity generation, so it is an essential factor for the effective efficiency of the power plant.
Some future coal-fired power plants could also have devices for separating and liquefying carbon dioxide (CO2) in order to prevent the underground storage of the otherwise climate-damaging CO2 to enable (see below, CCS). In this case, a significant change in the system concept may be necessary, e.g. B. for combustion with pure oxygen (Oxyfuel process) instead of air.
Performance, efficiency and application
Typical coal-fired power plant blocks achieve outputs between 100 MW (megawatts) and 1 GW (gigawatts). For higher outputs (in some cases several gigawatts), several power plant blocks are used at the same location. The electrical efficiency of the most modern and largest coal-fired power plants is a little over 45%, i.e. H. a good half of the heat generated by coal firing is lost as waste heat. By using new materials in steam turbines, the steam temperatures can probably be increased even further (e.g. to 700 ° C), which would enable efficiencies of approx. 50%. Lignite power plants generally achieve lower values. It should also be noted that the extraction of lignite in particular requires a high level of energy input, which can destroy around 10% of the electrical energy generated, and is of course not yet taken into account in the efficiency levels mentioned. Only the power plant's own internal requirements (e.g. for coal mills and flue gas cleaning) are included in this.
All over the world, coal-fired power plants that are decades old are still in operation, which have far lower efficiencies of often well below 35%. In Germany, the current power plant population is a little less than 40%.Future coal-fired power plants could be even more efficient than gas and steam combined cycle power plants. But there are still hardly any concrete projects for this.
One possibility for more efficient power plants would be to implement combined cycle power plants that combine gas and steam turbines. Initially, coal is not suitable as a fuel for gas turbines, but coal gasification could be used here, or combustible carbon monoxide (CO) by burning coal in a lack of air as Generator gas win. Another possibility is the use of a gas turbine fired with natural gas, the exhaust gas of which is still used for coal firing. Such Upstream gas turbines can not only increase the overall efficiency, but also generate additional control energy, because their output can be quickly adapted to demand. Such concepts are still in development and have not been implemented very often; In the future, they could enable efficiencies of over 50% even when operated with coal alone. Since 2010, however, a number of these development projects have been endangered or have already been discontinued because uncertainties regarding climate protection policy make the profitability of such measures appear questionable.
Some power plants work with combined heat and power, i. H. with an at least partial use of the waste heat z. B. for heating purposes (district heating). The overall efficiency can then be significantly higher, even if the electrical efficiency is somewhat lower. However, the use of waste heat is difficult, especially for large power plant units, because enough consumers for huge amounts of low-temperature heat have to be found in a sufficiently small distance.
The construction and mode of operation of coal-fired power plants depends on the type of coal used:Hard coal power plants mainly work in the medium load range.
- Hard coal power plants achieve somewhat higher degrees of efficiency. They are often used for medium loads, i.e. preferably in winter when the demand for electricity is higher, but also for additional electricity demand during the day, i.e. during the day. H. with strong fluctuations in the generated power within 24 hours. Since coal is easy to transport, it can, for. B. be delivered by ships over long distances. (Imported coal is practically always hard coal.)
- Lignite power plants work with the cheaper lignite. Since this is less easily transportable, it is often converted into electricity near the opencast mine (→Mine-mouth power station). Lignite power plants then mostly work all year round to generate the base load. Their efficiency is typically a few percentage points lower than that of hard coal-fired power plants. This is related to the lower fuel quality.
To a limited extent, the electrical power generated in a coal-fired power station can be adapted to demand. However, this requires periods of hours and generally leads to a reduced degree of efficiency, and possibly also to increased pollutant emissions. In this respect, coal-fired power plants, like nuclear power plants, are not very suitable for z. B. to be combined with wind energy.
Flue gas cleaning
In large coal-fired power plants, the flue gas coming from the boiler typically goes through several cleaning stages:
- First there is a denitrification system (DENOX system) to break down nitrogen oxides with the help of a catalytic converter.
- Dust is then removed in an electrostatic precipitator, i.e. the removal of soot and dust particles.
- Finally, a desulphurisation system (FGD) mainly removes sulfur dioxide (SO2). Gypsum (CaSO4), which chemically binds the sulfur removed from the flue gas and can mostly be used in construction.
In the case of smaller furnaces, the method of Fluidized bed combustion applied. Here the combustion temperatures are significantly lower (e.g. 800 ° C), so that far fewer nitrogen oxides are generated and a denitrification system is often dispensed with. In addition, sulfur dioxide can be bound by adding lime dust, so that a separate desulphurisation system can be omitted and only dedusting with an electrostatic precipitator is necessary.
Although a considerable effort is made for modern flue gas cleaning systems, there are still considerable environmental pollution from various pollutants (see below).
Harmfulness to the climate of coal power plants; Plans for CCS
Coal-fired power plants lead to the highest carbon dioxide emissions per generated kilowatt hour of all common types of power plant. In modern coal-fired power plants, values of well over 1100 g / kWh with lignite and 950 g / kWh with hard coal are typical. B. around 400 g / kWh for modern gas-fired combined cycle power plants. Older power plants are more inefficient and accordingly emit even more CO2. Since carbon dioxide (CO2) is the most important man-made greenhouse gas, coal-fired power plants are particularly harmful to the climate. In the EU, they cause around a quarter of all CO2Emissions. In 2013, 41% of the world's electrical energy was generated in coal-fired power plants.
At present, methods of separating the CO2 developed from the flue gas, which enable underground storage (sequestration) of this gas (e.g. in exploited oil deposits) (CCS = carbon capture and sequestration = CO2Separation and storage). When the CO2 largely separated and permanently kept away from the atmosphere, this would defuse the climate problems of coal-fired power plants. However, this CCS technology (which is being developed in different variants) has massive disadvantages:Will China ever use CCS if it massively increases coal consumption?
- The efficiency of the power plant drops considerably (e.g. from 45% to 30 to 35%), so that far more coal is required for the same amount of electrical energy. This fact alone should z. B. prevent the widespread introduction of CCS in countries like China with chronic coal shortages.
- The installation costs are quite high.
- There are underground storage facilities for CO2 with enormous recording capacities and a high level of security that cannot be found everywhere. A transport z. B. liquefied carbon dioxide over long distances would make the method even more expensive.
For these reasons, CCS can realistically promise only very small contributions to defusing the climate problem over the next few decades. So far, only individual prototypes of such CCS power plants have been built; Most of the coal-fired power plants built today (even in the industrialized countries) are hardly suitable even for a later conversion to CCS. Therefore, the reference to CCS cannot possibly serve as a justification for building new coal-fired power plants today. The article on CCS discusses this approach in more detail.
Cooling effect of aerosols
Coal-fired power plants, which, unlike in the more advanced industrialized countries, are operated without effective exhaust gas cleaning (which is often the case in China or India, for example), emit large amounts of sulfur dioxide (see below), from which aerosols are formed. These aerosols have a cooling effect on the earth's climate, which has so far been a significant part of the effects of CO2 compensate. However, the aerosols only work for a few weeks, during the CO2-Effect persists for several centuries. If the majority of coal-fired power plants operated worldwide are equipped with effective exhaust gas cleaning systems in the future (which is essential for reasons of health protection and to avoid crop failures), global warming will advance even faster than before.
Other environmental pressures
Old coal-fired power plants without flue gas cleaning emit large amounts of pollutants, especially fine dust, sulfur dioxide (SO2) and nitrogen oxides (NOx). This problem has been significantly alleviated by the development of effective flue gas cleaning systems, especially for SO2. For this reason, coal-fired power plants, at least in the industrialized countries, contribute significantly less than in the past to the formation of acid rain and smog and thus to the damage to forests and human health. Nevertheless, the remaining environmental pollution is by no means insignificant - it can be assumed that z. For example, German coal-fired power plants are also responsible for massive damage to health and the loss of many years of life .Did you know that coal-fired power plants in Germany cause over two thirds of all mercury emissions? And that even with power plants built today, the mercury problem has by no means been solved?
Emissions of toxic heavy metals such as mercury (Hg), cadmium, lead and arsenic, as well as radioactive substances such as uranium, thorium and radium are also of concern. Because of the huge amounts of coal burned (several million tons per year in a larger power plant), even low percentages of coal with such substances lead to considerable environmental pollution. For example, coal-fired power plants are among the largest emitters of mercury and radioactive substances. In Germany, coal-fired power plants cause a good 70% of all mercury emissions despite modern filter systems (as of 2009). In the USA, much lower emission limits for mercury are to apply from 2016; In Germany, however, there is still effective resistance to such tightening.
The separation of mercury during flue gas cleaning can be improved considerably in the future, e.g. B. by adding bromide salts to the coal. (These lead to a stronger oxidation of the mercury in the exhaust gas and consequently to a stronger separation in the exhaust gas scrubbing.) The mercury emissions can also be reduced by blowing activated carbon into the exhaust gas. Unfortunately, there are many power plants in operation around the world that emit far more mercury than is technically possible today. Even with modern filter systems, a typical coal-fired power station unit emits several hundred kilograms of mercury per year in the exhaust gas and other quantities in the wastewater. In addition, highly toxic solid residues arise from the flue gas cleaning system, which z. B. sometimes have a higher concentration of uranium than some uranium ores. That is why coal ash has already been partially used for uranium extraction.Did you know that coal ash can have a higher uranium content than uranium ores?
In the case of flue gas scrubbing, a considerable part of these pollutants remains in the ash or other residues, which then have to be disposed of accordingly. (Gypsum from the desulphurisation plant can mostly still be used as a building material if it is not too heavily contaminated.) Some of the uranium that can still be used is extracted from coal ash, as the uranium concentration in the ash can be even higher than in typical uranium ores.
Another problem is the warming of rivers. In particular, where no cooling tower gives off most of the heat into the air, large amounts of heat are usually given off into a river. In summer, the warming can e.g. B. be critical for fish, as it reduces the oxygen content of the water. Occasionally, such power plants have to be taken out of service in summer.
Coal mining also creates long-term environmental pollution, for example through major interventions in the groundwater balance. These also cause considerable perpetual costs.
In some coal-fired power plants, it is possible to add other fuels to the burned coal, such as B. wood, add other biomass (including organic waste); this is called Co-incineration (Co-firing). In this way, the consumption of coal can be reduced to a certain extent, and the corresponding part of the electrical energy generated can be CO2-be neutral.
The additional fuel does not have to have a very high quality and a high calorific value, as long as its proportion should not be very high. However, various aspects have to be considered, e.g. B. possible effects on the flue gas scrubbing.
New power plants with coal gasification
Nowadays, larger gas-fired power plants are often designed as so-called gas-and-steam combined-cycle power plants, which can achieve a particularly high level of efficiency. This principle cannot be used directly with coal, since coal is not suitable as a fuel for a gas turbine. However, coal can first be gasified to run a gas turbine. This takes place in so-called IGCC power plants with the Integrated Gasification Combined Cycle. Coal can also be burned in a lack of oxygen, so that carbon monoxide (CO) is produced, which is then burned in a gas turbine. With these methods, a higher overall efficiency (possibly over 55%) can be achieved than with conventional coal-fired power plants, and at the same time the CO2-Separation and storage facilitated.However, such types of coal-fired power plants have so far only been implemented in the form of individual prototypes, since the high construction costs endanger economic viability; most coal-fired power plants built today are based on conventional approaches.
Evaluation of new power plants
When it comes to the construction of new coal-fired power plants, it is usually argued that the most modern power plants not only have greatly improved flue gas cleaning compared to old ones, but also achieve a significantly higher level of efficiency. This actually has the consequence that the amounts of pollutants and also the amount of climate-damaging carbon dioxide are significantly reduced. Of course, a net reduction in emissions presupposes that old power plants with a similar output are switched off and that new power plants do not emit additional emissions.
Even if old power plants are actually replaced by better new ones, the intensive use of coal (and practically always without CO2-Deposition) continued for a long time. The CO2Emissions and the perpetual costs of coal mining remain well above those of z. B. from gas power plants. Mainly because of this and because of the health hazards, the construction of new coal-fired power plants is very controversial in many places. Climate protection measures such as CO2-Taxes torpedoed in order not to make it impossible to build new coal-fired power plants.In Germany, the capacity utilization of coal-fired power plants and thus their profitability is significantly reduced due to the increasing feed-in of solar and wind power.
One advantage of coal over natural gas is, of course, that fuel costs are lower and global coal reserves are still quite large, while natural gas may become scarce and expensive in the next few decades. Because of the impending introduction of CO2-Taxes in many countries, however, endanger the profitability of coal-fired power plants. The increasing yield from photovoltaic systems also contributes to this in Germany, which regularly largely covers the midday peak in consumption on nice days, which was previously often covered by hard coal-fired power plants. These developments are increasingly preventing the construction of new power plants. Conversely, however, the feared economic effects of CO mobilize2- Control many lobbyists around the world in the fight against climate protection measures.
Influences on coal power production in Germany
In Germany, lignite-fired power plants and hard coal-fired power plants each produce more electrical energy than any other type of power plant. In 2013 it was approx. 149 TWh from lignite and 114 TWh from hard coal, followed by nuclear energy with 92 TWh. In terms of installed capacity, on the other hand, photovoltaics is now in first place, closely followed by wind energy. Such power plants still generate less electricity per year because they have significantly fewer full-load hours than coal-fired power plants.
The use of coal in power plants is subject to fluctuations, which can have different causes. In Germany, significantly more electricity from coal was generated in 2012 than before, which means that the German CO2Emissions have risen again for the first time since 1990. The reason for this is not that the energy transition led to the replacement of nuclear power with coal power, but that natural gas power plants were pushed back at the expense of coal. This in turn is related to the price development for natural gas and emissions trading: natural gas has become more expensive, while at the same time the prices for CO2-Emissions certificates, which otherwise place the greatest burden on coal-fired electricity, have become very low: European emissions trading has become practically ineffective due to a politically induced oversupply of certificates.
Coal electricity also makes a significant contribution to Germany's electricity export surplus. For example, large amounts of coal-fired electricity are exported to France in winter because the electrical heating systems there create a huge demand that, despite the large number of nuclear power plants, cannot be met by French power plants alone.
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See also: coal, power plant, thermal power plant, fossil fuels, climate hazards, CO2-Separation and storage, climate protection, incineration, co-incineration
and other items in the electrical power, prime movers, and power plant categories
Question: Which of the following components does a typical modern coal-fired power plant contain?
Correct answers: (b), (c), (d), (e), (g)
Question: Why are hard coal-fired power plants more often than lignite-fired power plants near large ports?
Correct answer: (b)
To (a): Hard coal-fired power plants usually need Less Cooling water than lignite power plants, as they are more efficient.
Question: Which harmful substances do coal-fired power plants emit in significant quantities?
Correct answers: (a), (b), (c), (e)
Question: How much CO2-Emissions arise approximately if a household consumes 4000 kWh of electricity in one year, which is produced entirely in coal-fired power plants?
Correct answer: (c)
It is around 1 kg of CO per kilowatt hour2, so 4000 kg = 4 tons for 4000 kWh. For comparison: In the long term, we have to reduce our CO2-Limit emissions per capita and year to approx. 1 ton - for heating, traffic, electricity, industrial production, etc. all together!
See also our energy quiz!
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