Coal-fired power plants continue to be an important part of the power generation mix around the world. Despite a slow-down in new construction, the number of plants in operation means that there remain significant maintenance requirements.
This page introduces the specific equipment and operating conditions of a coal-fired power plant, and then discusses what makes this type of plant unique.
This lesson includes the following topics:
A thermal plant analysis framework.
Some interesting facts about coal.
Coal management at the plant.
The four processing stages in a coal-fired power plant.
A word about power plant controls.
Three primary coal-fired plant classifications.
Pulverized coal combustion.
Environmental challenges, including:
Fly ash,
Emissions, and
Carbon Dioxide.
Finally discussed is the conversion of coal plants.
Thermal Plant Analysis Framework
Let’s first introduce an analysis framework to help evaluate any thermal power plant. The essence of the power generation process is fuel conversion to energy using two types of systems.
The primary system is often called the fuel-air system or the fireside of the plant.
It is fuel-specific and for a coal-fired plant includes:
Transportation,
Containment at the site,
Pulverizing the coal, and
Fuel combustion in the boiler.
The secondary system is the water-steam system – often called the waterside of the plant.
Here, steam from the boiler is expanded through the turbine – which drives a generator – creating power to the grid, as described in the ‘Thermal Power Plant Fundamentals module.
The waterside system is common when there is a boiler – such as in coal or oil-fired power plants. The waterside for gas-fired power plants may differ slightly.
Coal Facts
Note that there are two commonly used terms for coal:
- Steaming coal or soft coal – used in power plants, versus
- Coking coal or hard coal – used in steelmaking and a variety of manufacturing processes.
Variation in fuel quality is one of the key problems affecting coal-fired power plants.
This variability affects the safety, stability and efficiency of the combustion process. It can also affect the maintenance cost of the plant.
Coal Management
As you can imagine, the fuel management system for coal is much more complicated than for oil or natural gas.
Ideally, these power plants are located as close to an economic source of coal as possible.
Coal travels from the mine primarily by rail because of the sheer volumes needed.
Approximately 2 million tons of coal per year are needed to produce a continuous 500 megawatts of power. That’s roughly 5,400 tons of coal managed at the plant every day!
From the mine, power plant coal is moved in specialty railroad cars. Most are designed with drop-away bottoms to ease the unloading process.
Others are designed to be rotated to dump the coal.
A common term for coal trains is that they are blocked, meaning only used to move coal. The 500 megawatt plant example would need over 50 coal cars every day.
At the plant, coal is first stored above ground or in underground containment tanks.
Coal from the receiving area is moved by numerous conveyer belts and a system of hoppers.
Finally, it gets sent to the grinder or mill adjacent to the boiler to get pulverized into a powder-like consistency.
Coal-fired Power Plant Process
As you learned in the ‘’module, more than half the world’s electricity is generated by coal-fired power plants.
Once the fuel is on-site, the conversion of coal into electricity happens in four stages.
- Coal is pulverized before being combusted in the boiler.
- Heat from combustion boils water to produce high temperature and high pressure steam.
- This steam is then expanded in a turbine, rotating to a given speed – often 3,000 to 3,600 RPM.
- As the turbine rotates, it drives the generator to produce electricity.
Finally, a boiler feedwater and cooling water system is essential to safely operate the plant.
Coal-Fired Power Plant Controls
Now, a quick word about plant operation controls.
There are three major control measurements in a coal-fired power plant:
Temperature,
Pressure, and
Fluid flow.
The main task of the control system is to regulate the plant’s electrical power output to meet a variable demand in the grid, while stabilizing key operating conditions in the plant such as the:
Flow rate of the coal fuel feed to the boilers,
Combustion temperature in the boiler and throughout the steam system,
Pressure in the condenser and other equipment, and the
Level and flow rate of boiler feedwater.
Power plants have increased in size to gain efficiencies as they match the growth in electric power demand. Size alone increases the magnitude of the grid load power cycles.
Addition of renewable sources requires that the grid power load be regulated even more frequently – also impacting the power plant output.
A sophisticated control system is in every power plant to quickly and safely respond to power demand variations – and stabilize the plant within an acceptable operating range.
Primary Plant Classifications
Through technology developments, coal-fired power plants can now operate at a wide variety of pressure and temperature conditions. The common industry terminology is:
• A subcritical plant,
• Supercritical plant, and
• Ultra-Supercritical plant.
Each type of plant has a different set of design criteria for piping, exchanger, drum materials and insulation.
Let’s go into more detail on how plant operating conditions can vary…
In a subcritical plant, the steam pressure is below 3,200 psi and the temperature is below 374° Celsius – what is called the critical point.
At these conditions both steam and water exist, and large insulated drums are needed to separate the steam as it leaves the boiler.
This plant design is the most common; and it is expected to remain the main choice in some countries because of its simplicity in operation, somewhat higher reliability and lower technical risk.
In a supercritical plant, steam operates at pressures slightly above 3,200 psi and temperatures are slightly higher than 374 °Celsius.
In this plant, all liquid water immediately becomes steam and the drums needed in a subcritical plant can be eliminated.
In an ultra-supercritical plant, the steam pressure can be as high as 5,600 psi and the temperature can reach, or even exceed, 600° Celsius.
However, with their higher efficiency, these plants can also have lower reliability and higher maintenance costs, because of thermal stresses and risk of fatigue-cracking in critical sections of the plant.
Operating the plant at a higher temperature and pressure increases the plant efficiency, lowers fuel consumption and reduces emissions.
Subcritical plant efficiency is in the range of 33%,
A supercritical plant can achieve greater than 42%, and
An ultra-supercritical plant could achieve efficiency to more than 50%.
According to the International Energy Agency, in 2017 global coal-fired power capacity was 2,087 Gigawatts.
Subcritical units had the majority with 45% of global capacity,
Supercritical units accounted for 19%, and
Ultra-supercritical units accounted for 13%.
The remaining 23% of capacity is from combined heat and power plants.
Pulverized Coal Combustion
Moving on to Pulverized Coal Combustion…. It is by far the most prolific type of boiler design currently in use.
A pulverizer is used to grind the coal into a fine, almost talcum powder consistency.
A pulverizer is a very complex piece of equipment with high maintenance needs.
Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or “races”.
The fuel-feed process starts when stored raw coal is fed into the pulverizer along with hot boiler recycle air at 340° Celsius.
As the coal gets crushed by the rolling action, it is directly blown to a burner in the boiler.
Once the coal is pulverized, it will burn almost as easily and efficiently as a gas.
The feed rate of coal varies according to the boiler demand and the amount of hot air available for drying and transporting the pulverized coal.
The burner mixes the powdered coal with additional hot combustion air and forces it out a series of nozzles – similar in action to the fuel injectors in modern cars.
Under controlled operating conditions, there is enough heat in the boiler combustion zone to spontaneously ignite all the incoming fuel.
Environmental Challenges – Coal Ash
One environmental challenge in coal-fired power plants is managing coal combustion by-products.
Fly ash is composed of fine particles of burned fuel or particulates mixed within the flue gas.
Note that the flue gas in power plants can reach temperatures above 1,000° Celsius.
In modern coal-fired power plants, fly ash is captured by electrostatic precipitators or other filtration equipment located between the boiler and the stack.
Ash also falls to the bottom of the boiler and it is called bottom ash.
Captured ash is periodically removed from collection hoppers below the boilers, precipitators or filtering equipment. It is pneumatically transported to storage silos.
From there, the ash is sent by truck or rail to become a component in road building or agriculture – because of its high concentration of fertilizer-like elements.
If fly ash is stored in the open for extended periods of time, efforts must be undertaken to mitigate groundwater pollution as the coal ash pile gets rained on.
Environmental Challenges – Emissions
The second set of environmental challenges is that fossil fueled power plants also generate harmful emissions in the flue gas that include sulfur dioxide, nitrogen oxide, particulate matter, and other hazardous air pollutants.
A common environmental control system used in coal-fired facilities is called Flue Gas Desulfurization, or FGD.
There is another training module in this series which focuses solely on FGD. In summary, an FGD unit removes the sulfur dioxide using large limestone or salt-water scrubbers.
Environmental Challenges – CO2
Another emissions challenge is that coal-fired power plants are the world’s largest emitter of carbon dioxide.
Reduction of carbon dioxide is a major issue not only for coal plants but also for other major manufacturing processes.
A technique to reduce carbon dioxide under evaluation around the world is called Carbon Capture and Storage, or CCS.
In the CCS technology, the carbon dioxide is separated from flue gas using sophisticated equipment, heat, pressure and chemical processes.
It is then stored safely underground, or even used in mature oilfields to improve production, called Enhanced Oil Recovery.
Currently CCS projects are in their infancy, but they are expected to become more frequent in coming years as technologies improve.
Conversion of Coal-Fired Plants
That’s all on emissions… Now let’s turn briefly to conversion of coal–fired power plants…
In North America there has been a tendency to convert existing plants to run on natural gas. This is very capital intensive due to new equipment required.
In other parts of the world, new, smaller gas-fired power plants are being constructed, instead of using coal.
Some developed countries are now converting coal-fired power plants to run on biomass fuel.
Summary
You should now have a good idea of what it takes to operate a coal-fired power plant.
There are many coal-fired power plants operating around the world.
Even with the challenge of managing carbon dioxide emissions, there are still some new coal-fired projects underway – due to increasing global power demand.
New technologies, like Carbon Capture and Storage, are being developed to mitigate carbon dioxide emissions.
From our discussion today, you should now understand that:
- More than half the world’s electricity is generated by coal-fired power plants.
- It takes an enormous amount of coal to produce electricity.
- The power plant control system manages: temperature, pressure and fluid flow.
- There are three classifications of coal-fired power plants: Subcritical, Supercritical and Ultra-Supercritical.
- Today the main coal-fired boiler technology is called pulverized combustion.
- Pollution controls are put into place to reduce coal plant toxic emissions, and
- Carbon Capture and Storage technologies are being developed to reduce plant carbon dioxide emissions.
- Though it is expensive, coal-fired plants continue to be converted to natural gas and biomass fuels.