1. Thermal Power Plant
Thermal power plants are one of the main sources of electricity in both industrialized and developing countries. The variation in the thermal power stations is due to the different fuel sources (coal, natural gas etc.). In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the water into steam. In fact, more than half of the electricity generated in the world is by using coal as the primary fuel.
The function of the coal fired thermal power plant is to convert the energy available in the coal to electricity. Coal power plants work by using several steps to convert stored energy in coal to usable electricity that we find in our home that powers our lights, computers, and sometimes, back into heat for our homes. The working of a coal power plant is explained in brief.
Firstly, water is taken into the boiler from a water source. The boiler is heated with the help of coal. The increase in temperature helps in the transformation of water into steam. The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity. A generator is connected to the steam turbine. When the turbine turns, electricity is generated and given as output by the generator, which is then supplied to the consumers through high-voltage power lines.
1.1 Coal
Coal is classified as fossil fuel, and it is the main energy source for electricity production in the world. This however also means that coal is the main source of carbon emissions in the world meaning that coal significantly contributes to climate change issue. Coal is composed primarily of carbon along with variable quantities of other elements, mostly sulphur, hydrogen, oxygen and nitrogen. (Figure 1)
Figure 1 Composition of Coal
1.2 Coal Type
1.2.1 Lignite: Lignite is a type of coal with little plant remains with dark-brown to black color. The carbon content is rather low, and the humidity is up to 30-70%. Lignite is usually used as fuel and regarded as low quality coal.
1.2.2 Sub-Bituminous Coal: Sub-Bituminous Coal is a dark-brown to black coal. Sub-Bituminous Coal is wax-like soft, not very hard with the carbon content of around 71-77% and the humidity of 10-20%
1.2.3 Bituminous Coal: Bituminous Coal is a dense sedimentary rock and hard, usually black but sometimes dark brown often with well- defined bands of bright material. It contains 80-90% of carbon and 2-7% of humidity.
1.2.4 Anthracite: Anthracite, the highest rank of coal, glossy black, has the highest quality of coal with the carbon content more than 90%, very low humidity, and the most amount of heat. Howerver, it ignites with difficulty and burns with smokeless flame.
2. World Coal Production
World coal production has increased by nearly 60% since 2002, with most of the increase coming from China—up 130%. China accounted for about 50% of coal production in 2011, up from 34% in 2002. The data illustrate that other countries such as Columbia, Indonesia, and India also had significant production increases since 2002. India’s coal production grew by 60% over the past 10 years, while Indonesia’s production more than tripled. Australia increased coal production by 26% over the same time period. Indonesia, a major coal exporter, is likely to use a larger share of its production for domestic consumption. (Figure 2)
Figure 2 World Coal Production 2012
3. Coal-Fired Power Plant
Coal fired power plants are the most common source of power, accounting for 40% of all electricity generated worldwide. However, starting from the mining process in open cast up to pulverized coal combustion, coal has to undergo many size reduction processes before it can be used for power generation. From a coal stockyard, coal will send to mill bowl through the coal bunker before it goes to boiler to heat water to the superheated steam. The superheated steam at the correct temperature and pressure flows into a turbine which spins a generator to produce the electricity before introducing to the transmission line. The died steam is cooled, condensed back into water and it will return to the boiler to start the same process over.(see Figure 3)
Figure 3 Coal-Fired Power Plant basic operation system
4. Coal-Fired Power Plant Main Equipment's
- Coal and Ash Handling
- Pulverizers
- Boiler
- Superheating and Reheater
- Economiser
- Turbine
- Generator
- Condenser
- Cooling Tower
4.1 Pulverizers
The coal is put in the boiler after pulverization. For this pulverization is used. A pulverization is a device for grinding coal for combustion in a furnace in a power plant.
Type of pulverization:
- Ball and Tube Mill
- Ring and Ball
4.2 Ash Handling
Ash is the residue remaining after the coal is incinerated. In Thermal Power Plants coal is generally used as fuel and hence the ash is produced as the byproduct of Combustion. Ash generated in power plant is about 30-40% of total coal consumption and hence the system is required to handle Ash for its proper utilization or disposal.
Composition of ash: SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO
Ash Handling System Types
- Bottom Ash Handling System
- Coarse Ash (Economizer Ash) handling System
- Air Pre Heater ash handling system
- Fly Ash Handling System
- Ash Slurry Disposal System
4.3 Boiler
Coal is burnt to heat up the water. The fuel is burnt inside the boiler, whereas the water which is heated runs in tubes on the surface of the boiler. Thermal Power Plant Boilers are different because of the complexity of the process and different types of system involved in the entire combustion process.
4.4 Superheater and Reheater
Superheater is a component of a steam-generating unit in which steam, after it has left the boiler drum, is heated above its saturation temperature.
Reheater is also steam boiler component in which heat is added to this intermediate-pressure steam, which has given up some of its energy in expansion through the high-pressure turbine.
4.5 Economiser
Flue gases coming out of the boiler carry lot of heat. Function of economizer is to recover some of the heat from the heat carried away in the flue gases up thechimney and utilize for heating the feed water to the boiler.
4.6 Steam Turbine
“A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft.”[1]
4.7 Condenser
The purpose of condenser is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum efficiency and also to get the condensed steam in the form of pure water, otherwise known as condensate, back to steam generator or (boiler) as boiler feed water.
Types of condensers:
- Jet Condensers
- Surface Condensers
5. Clean Coal Technologies
When coal is burned, the carbon in the coal becomes carbon dioxide; the sulfur in the coal becomes sulfur dioxide; and the nitrogen in the coal, and a little of the nitrogen in the air used to burn the coal, become various oxides of nitrogen. Normally, these chemicals are all released into the atmosphere. The solution to this pollution is to capture these undesirable emissions and sequester them. The collection of technologies that do this is called clean coal technologies. Up until now, the approach to solving these problems has been to capture the emissions after they have been generated. Electrostatic precipitators capture the coal ash and soot particles, while scrubbers are used to capture the sulfur and nitrogen compounds. A better approach is to gasify the coal first to generate syngas and then separate out the gaseous components. They can then be used to not only generate electricity, but also to make liquid fuels, petrochemicals and fertilizers, among other things. All clean coal technologies begin with the coal gasification process. In a coal gasified, the coal is reacted with steam and oxygen to produce syngas products as well as small amounts of methane and carbon dioxide.
Categories of Clean Coal Technologies;
- Coal preparation technology
- Coal water mixture technology
- Coal briquetting technology
- Coal gasification technology
- Coal liquefaction technology
- Solid oxide fuel cell technology
- Pressurized fluidized bed combustion (PFBC)
- Atmospheric circulating fluidized bed combustion (ACFBC)
- Integrated gasification combined cycles (IGCC)
- Coal bed methane (CBM) exploitation and utilization
- Flue gas desulfurization (FGD)
- Carbon capture and storage (CCS)
5.1 Coal Preparation Technology
Coal preparation can offer a number of commercial and environmental benefits. These include:
- Increased quality and commercial value of saleable coal
- Potential to exploit coals that would otherwise be classified as unrecoverable because of commercial or environmental constraints
- Increased efficiency and availability and reduced maintenance of coal utilization plant through the supply of lower-ash coal of consistent quality
- Improved environmental performance of coal-utilization plant (this could include reduced SO2, CO2 and particulate emissions, depending on each application)
- Reduced transportation requirement of clean coals compared with raw coals
- Reduced quantities of combustion ash for disposal.
Principle Processes of Coal Preparation;
- Raw coal pre-treatment
- Coal washing
- Coal sizing and classification
- Coal dewatering
- Tailings treatment and water purification.
5.2 Coal gasification technology
Coal gasification processes are divided into several categories, 4 types of coal gasification processes are demonstrated respectively, these are moving bed, fluidized bed, entrained bed, and molten bed. Coal is first crushed and sometimes dried, then fed into gasifier, in which coal reacts with steam and either air or oxygen. The gasification reaction usually occurs at high temperatures from 800 to 1900°C and high pressure up to 10 MPa. When coal is burned with less than a stoichiometric quantity of air, with or without steam, the product is low-heat-content gas, which after purification can be used as fuel gas. Utilizing oxygen in place of air produces medium-heat-content gas. The gas produced is used as synthetic gas; some of CO in gas must be reacted with steam to get additional hydrogen. This step is called shift conversion, which sets up the proper ratio of gaseous components depending on the requirements of different synthetic gases for producing liquid fuels, SNG, ammonia, or methanol.(Figure 4)
Figure 4 Coal Gasification Process
5.3 Coal liquefaction technology
Coal liquefaction is an industrial process in which coal as raw material, through chemical reactions, is converted into liquid hydrocarbon mixture, which, under further processing, becomes desired liquid fuels or chemical feedstock. Therefore, the main purpose of coal liquefaction lies in production of either synthetic oil as a partial substitute resource for crude oil or aromatic hydrocarbons as feedstock in organic chemical industry. Coal liquefaction, in general sense, is classified into two categories, i.e. direct liquefaction and indirect liquefaction. (Table 1)
5.4 Solid Oxide Fuel Cells Technology
Solid oxide fuel cells (SOFCs), are a type of high-temperature fuel cell, which operate between 800 and 1000 °C.
As the electrolyte, they use oxide conductors; in general, zirconia stabilized with yttrium or other elements is used. Stabilized zirconia is conductive through the action of oxygen ions; the following reactions occur at the two electrodes:
In the overall reaction, hydrogen and oxygen combine to generate steam like any other type of fuel cells described in the foregoing sections. The operating principle can be shown in Figure 5.
Figure 5 Principle of Operation of Solid Oxide Fuel Cells
Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.
5.5 Integrated gasification combined cycles (IGCC)
Integrated Gasification Combined Cycle (IGCC) systems, coal is not combusted directly but reacts with oxygen and steam to form a "syngas" (primarily hydrogen). After being cleaned, it is burned in a gas turbine to generate electricity and to produce steam to power a steam turbine. Coal gasification plants are seen as a primary component of a zero-emissions system.(Figure 6)
Figure 6 Integrated gasification combined cycle process
5.6 Flue gas desulfurization
Most flue gas desulfurization systems employ two stages: one for fly ash removal and the other for SO2 removal. Attempts have been made to remove both the fly ash and SO2 in one scrubbing vessel. However, these systems experienced severe maintenance problems and low removal efficiency. In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SO2-absorber. However, in dry injection or spray drying operations, the SO2 is first reacted with the sorbent, and then the flue gas passes through a particulate control device.
Another important design consideration associated with wet FGD systems is that the flue gas exiting the absorber is saturated with water and still contains some SO2. These gases are highly corrosive to any downstream equipment such as fans, ducts, and stacks.
Two methods that may minimize corrosion are: reheating the gases to above their dew point, or using materials of construction and designs that allow equipment to withstand the corrosive conditions. Both alternatives are expensive.
Figure 7 Schematic design of FGD
5.7 Carbon capture and storage (CCS)
5.7.1 Carbon capture process
Current capture technologies can reduce CO2 emissions by up to 90%.They can be incorporated into new projects or retrofitted onto existing plants.
There are two main ways to capture CO2:
- Directly separating CO2 from regular flue gas after the combustion process at the emission source. This is called post-combustion capture.
- By modifying the fossil fuel combustion technology to make the CO2 easier to capture. There are two ways to do this: through processes called gasification and oxy fuel combustion.
Post-combustion carbon capture is the most common and widely used process.CO2 is separated from other gases in the flue stack using a solvent. The most common solvent is chilled ammonia; however, other selective amines are also used. This process is well developed and has existed for several decades. With gasification capture, carbon and hydrogen in the fossil fuel energy source are separated prior to combustion. The fuel source becomes chemically transformed into two gas streams consisting of hydrogen and CO2 that can be easily captured. This is often called pre-combustion capture. Oxy fuel combustion involves the burning of a fossil fuel energy source in the presence of pure oxygen. This removes contaminants, including nitrogen, creating an exhaust stream of pure CO2 that is easier to capture.(Figure 8)
5.7.2 Carbon storage process
Storage as it is sometimes called refers to the processes by which captured CO2 is securely stored in deep geologic formations. It can be stored in deep saline formations, depleted oil and gas reservoirs and unmineable coal seams.
Injection of CO2 down old oil and gas wells into depleted oil and gas reservoirs is a proven and safe process. Depleted oil and gas reservoirs are natural traps for liquids and gas. After all, that is where the oil or natural gas was safely and naturally “stored” for millions of years. The injected CO2 fills the pores in the rock that were previously filled by hydrocarbons, at depths well below local water tables.
Deep saline formations are very large, porous rock formations that typically contain water that is unusable because of its high salt and/or mineral content. Typically this saltwater brine is 10 times saltier than the oceans and has been trapped by impermeable rock, called a “cap rock” for millions of years. Through advanced geologic techniques such as 3D and 4D seismic testing, it is possible to identify these reservoirs and select geologic formations with solid cap rock formations. (Figure 8)
Deep saline formations are very large, porous rock formations that typically contain water that is unusable because of its high salt and/or mineral content. Typically this saltwater brine is 10 times saltier than the oceans and has been trapped by impermeable rock, called a “cap rock” for millions of years. Through advanced geologic techniques such as 3D and 4D seismic testing, it is possible to identify these reservoirs and select geologic formations with solid cap rock formations. (Figure 8)
Figure 8 Carbon capture and storage process
6. Coal Power Plant in the World
6.1 Kemper County
Mississippi Power, a subsidiary of Southern Company, is progressing with plans to build and operate a 582MW lignite-fired IGCC plant at Kemper County in Mississippi. The project will use two transport integrated gasifier trains, and aims to capture 3.5 million tons of CO2 per annum. The project began construction during 2011 and is scheduled to be operational by 2016.
The plant will include carbon capture and sequestration, which aims to reduce 65% of the facility's CO2 emissions. Captured CO2 will then be transported by pipeline for use on EOR projects, and at least one CO2 buyer has already been secured.
6.2 Bełchatów Power Station
The 4,440MW Belchatów power plant is located near Belchatów in the lódz province of Poland. Operated by PGE Elektrownia Belchatow (PGE), it is the biggest lignite-fired power plant in Europe. The plant was commissioned in 1988 and currently accounts for 20% of the Polish power market.
In April 2009, PGE launched a retrofit programmer for unit 6 of the power plant to increase the unit's output to 390 - 400MW.
In January 2011, PGE commissioned the 13th unit with a capacity of 858MW. The new unit has the highest efficiency of 41% and conforms to European standards on greenhouse gas emissions. The generator was commissioned in September 2011.
Retrofit of the existing units is also underway to increase the design life of the plant by 25 years.
The Belchatów plant is considered to be one of the biggest producers of greenhouse gas emissions in the EU, emitting about 30mt/y of CO2. To reduce the emissions from the plant, PGE has launched a two phase programmed to build carbon capture plants (CCP).
In the first phase, a pilot CCP will be built at unit 12. The pilot CCP will use Alstom's Advanced Amine Process with amine solvent supplied by Dow Chemical. It will capture about 100,000t/y of CO2.
In the second phase, a bigger plant will be built to capture carbon emissions from the new 858MW unit. The new plant will capture 1.8mt/y of CO2 and is expected to be operational by 2015. The project will require an investment of $776m and has received a €180m grant from the European Investment Bank in May 2010.
7. Conclusion
Coal is an indispensable energy source in the world. Conclusion is that coal will continue to be used to meet the world’s energy needs in significant quantities. Traditional coal production harms the environment. Coal must be manufactured to a minimum of damage to the environment. Harm to the environment energy produced by giving a lot of problems will occur in the future. So, we have to use clean coal technology. Investors should use clean coal technology, don’t thinking about the extra cost. Clean energy is required for a clean future.
Source: Mustafa Şimşek
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