Coal-fired power plants are a major source of energy production around the world. In a coal-fired plant, coal is burned in a boiler to heat water into steam, which then spins a turbine to generate electricity. Coal generates around 40% of the world’s electricity today. Understanding how coal-fired plants work provides insight into an important energy source.
Coal Delivery
Coal is delivered to coal-fired plants in bulk by train or barge. A typical plant may receive 1-2 trains of coal per day, with each train having over 100 cars carrying 100 tons of coal each. Upon arrival, bulldozers are used to push the coal off the train cars into large pits. These coal piles can be many stories tall and take up acres of land.
Big mechanical arms then scoop up the coal and place it onto conveyor belts, which bring the coal into the plant itself. The coal moves through crushers which break it into smaller pieces before it enters the plant for combustion. Having a uniform and appropriate size of the coal pieces is important for efficient burning.
Boiler
The boiler is a massive structure where the coal is burned and the steam is generated. Within the boiler, there are several key components:
Firebox – This is where the coal is fed into the boiler and burned within a raging fireball. Temperatures can reach over 1,700°F here.
Water tubes – While the firebox contains the flame, the adjacent water tubes are filled with water which gets heated into steam. Miles of these tubes absorb the heat.
Air intake – Fans pull air into the firebox to provide oxygen for the coal fire. The ratio of air to coal is carefully controlled.
Ash collection – Ash leftover from burning coal falls to the bottom of the firebox and is removed by ash hoppers.
Here is a diagram of a boiler showing these components:
|
The boiler operates at pressures over 3,000 psi and temperatures reaching 1,000°F. Careful monitoring and safety systems are critical to avoid explosions which could destroy the plant.
Steam Turbine
The superheated high-pressure steam generated in the boiler is piped into a turbine. The turbine has thousands of blades which are spun by the force of the steam. The turbine is connected to a generator shaft, so as the turbine spins, so does the generator.
There are generally 3 stages of turbines:
High pressure turbine – The first stage which uses the full pressure steam from the boiler. Approximately 50% of the turbine power is generated here.
Intermediate pressure turbine – After exiting the high pressure turbine, the steam flows here at lower pressure for more spinning.
Low pressure turbine – The last stage which extracts residual energy before the steam condenses back to water.
This 3-stage extraction allows maximum energy to be derived from the steam. The stages have to be carefully controlled to maintain optimal spinning conditions.
Generator
The turbine is coupled to a massive generator which produces the electricity. The generator works based on electromagnetic induction. As the turbine causes the generator rotor to spin, it rotates coils of wire within a magnetic field. This then induces an electric voltage and current based on Faraday’s Law of Induction.
The electricity generated is at around 25,000 volts. Transformers located nearby then step up the voltage for efficient transmission on the electric grid. A single generator at a large coal plant may be rated for over 600 megawatts of output.
Cooling System
After passing through the turbines, the steam must be cooled back into water before being reused in the boiler. This cooling process happens in the plant’s condenser.
The condenser contains tubes with cool circulating water. As the exhaust steam passes over the tubes, it condenses back to water which gets collected at the bottom. This condensate is then pumped back to the boiler to repeat the steam generation cycle.
Cooling water for the condenser generally comes from a nearby river, lake or ocean. The water intake and discharge structures are large civil engineering projects themselves. Pumping the cooling water consumes a significant fraction of a plant’s energy.
Pollution Controls
Burning coal produces sulfur dioxide, nitrogen oxides and particulate matter that must be controlled. Coal-fired plants utilize several pollution control systems:
Electrostatic precipitators – These use large electrodes to apply an electric charge to particle pollutants in the flue gas stream, causing them to stick to metal plates that can be periodically cleaned. This removes over 99% of particulate emissions.
Flue gas desulfurization – Also known as scrubbers, these mix the flue gas with limestone slurry which chemically absorbs sulfur dioxide. The sulfur is recovered for industrial uses.
Selective catalytic reduction – Ammonia is injected into the flue gas and passed over a catalyst, converting nitrogen oxides into harmless nitrogen and water vapor.
Baghouses – Filters containing thousands of fabric bags collect any remaining particles before the cleaned flue gas is released from the plant’s smokestack.
Proper operation and maintenance of these pollution systems is essential for clean power generation. Advanced technologies continue to be developed to reduce coal’s environmental impact.
Coal Ash Management
In addition to flue gas pollutants, coal-fired plants generate large amounts of solid waste known as coal ash. Coal ash includes fly ash, bottom ash, and boiler slag. Fly ash represents the majority, while bottom ash and slag make up lower quantities.
U.S. coal plants generate over 100 million tons of coal ash per year. Approximately 43% of this ash gets recycled into products like cement and wallboard. The remainder must be disposed of into landfills or surface impoundments. Proper handling of the ash is crucial to avoid environmental contamination.
| Coal Ash Type | Description |
| Fly Ash | Fine powder constituent captured in exhaust gas by electrostatic precipitators and bag houses. Often recycled into concrete manufacturing. |
| Bottom Ash | Coarser ash particles that collect at the bottom of the furnace. Generally mixed with water and sluiced to ash ponds. |
| Boiler Slag | Molten bottom ash cooled by water quenching. Has a coarse, glassy, granular appearance. Recycled into blasting grit. |
Plant Efficiency
The efficiency of the plant indicates how much useful energy is produced versus total energy input from the fuel. A typical coal plant efficiency ranges from 33-35%.
Factors affecting efficiency include:
– Coal quality – Higher heat content and low moisture are desired
– Boiler design – Newer circulating fluidized bed boilers can reach 42% efficiency
– Steam parameters – Higher temperature and pressure improve efficiency
– Turbine performance – Erosion and fouling reduce efficiency over time
– Pollution controls – Treating flue gas requires auxiliary power
Regular maintenance and optimizing operational parameters help maximize efficiency. But ultimately coal plant efficiency is constrained by the fundamental Carnot limit where significant heat is unavoidably lost.
Carbon Capture Options
Coal combustion emits approximately 0.9 metric tons of CO2 per megawatt-hour generated, contributing greatly to climate change. Installing carbon capture systems at coal plants is one option being researched to reduce these emissions:
Post-combustion Capture – The most widely deployed approach where the CO2 is removed by chemicals after combustion occurs. Reduces efficiency significantly.
Pre-combustion Capture – The coal is first gasified into “syngas” and the CO2 stripped out before combustion. Complicated process with high capital costs.
Oxy-Fuel Combustion – Coal is burned in pure oxygen to produce higher CO2 concentrations for easier capture. Requires an air separation unit on-site.
These technologies are still under development and not yet economically feasible for widescale deployment. Alternatively, coal plants can co-fire with natural gas or biomass to reduce the net emissions.
Conclusion
Modern coal plants utilize complex processes and systems to efficiently convert the energy in coal into reliable electricity. Understanding the plant layout – from coal pile to smokestack – provides insight into this important electricity generation method. However, alternatives like natural gas and renewables are displacing coal due to lower costs and environmental impacts. The future role of coal plants will depend on enhanced efficiency and emission control technologies.
