Coal Power Plant

Coal is one of the most abundant fossil fuels on Earth. Coal was formed from the remains of plants that lived and died hundreds of millions of years ago. Coal deposits are found worldwide, but the largest reserves are in the United States, Russia, China, Australia, and India. According to the World Energy Council, global coal reserves in 2020 were estimated to be around one trillion tons, enough to last approximately 150 years at the current rate of global production. Most coal mining activities occur at depths of less than 1,000 meters, with a significant proportion of reserves being found much closer to the surface. However, the actual depth at which coal can be found can greatly exceed this, with coal seams known to exist several kilometers deep in the Earth's crust. However, coal resources at greater depth may not be economical to extract with current technology.

Coal power plants can be categorized into several types based on their design and method of coal combustion. Here are the main types:

Pulverized Coal Power Plant

In a pulverized coal power plant, a process of turning coal into fine powder is used to improve the efficiency of the power plant. The coal is first crushed into small chunks and then fed into a pulverizer, also known as a coal mill. The pulverizer grinds the coal into a fine powder, which is then blown into the boiler using large fans. This process of finely grinding the coal increases the surface area of the coal particles, which enables them to burn more completely and efficiently in the boiler. The high temperatures produced by the burning coal generate steam that powers a turbine to generate electricity.

However, during the coal combustion process, various toxic gases are produced, which can be harmful to human health and the environment. These toxic gases include mercury, heavy metals, hydrogen fluoride (HF), hydrogen chloride (HCl), nitrogen oxides (NOx), and sulfur oxides (SOx). To mitigate the negative impact of these gasses, modern coal-fired power plants have implemented environmental purification equipment, such as scrubbers and electrostatic precipitators, to remove the toxic chemicals from the exhaust gasses before they are released into the atmosphere.

In recent years, there has been a growing focus on capturing and storing carbon dioxide (CO2) emissions from coal-fired power plants. This capture and storage of CO2 add significant costs to the construction and operation of the power plants and reduce their overall efficiency.

Integrated Gasification Combined Cycle (IGCC)

Integrated Gasification Combined Cycle (IGCC) is a newer technology that aims to increase efficiency and reduce the environmental impact of burning coal for power generation. The IGCC process involves two main steps: gasification and power generation in a combined cycle.

Here's a detailed step-by-step breakdown of the IGCC process:

1. Coal Gasification: In the gasification process, coal is converted into a synthetic gas (syngas). The coal is fed into a gasifier where it reacts with steam and oxygen at high temperatures and pressures. Instead of burning, these conditions cause the coal to chemically break down and recombine into syngas, which mainly consists of hydrogen and carbon monoxide. Impurities, such as sulfur and mercury, are also removed from the syngas before it's burned, which helps to reduce harmful emissions.
2. Combustion in a Gas Turbine: The clean syngas is then combusted in a gas turbine. The hot gases from this combustion process spin the turbine, which drives a generator, producing electricity.
3. Heat Recovery Steam Generator (HRSG): The hot exhaust gases from the gas turbine, which would be waste in a traditional plant, are routed through a heat recovery steam generator. In the HRSG, the heat from the exhaust gas is used to generate steam.
4. Steam Turbine Power Generation: The steam generated in the HRSG is used to spin a steam turbine, which is connected to a second generator, creating additional electricity. This part of the process is similar to a conventional steam power plant.

Theoretically, the IGCC process results in an efficient power plant with lower emissions compared to conventional coal-burning technologies. This is because the gasification process allows for easier removal of impurities, and the combined-cycle process (gas and steam turbines) leads to higher efficiency.

However, implementing IGCC faces challenges due to its high capital and operating costs. Notably, pulverized coal plants remain the most cost-effective option. The gasification process is complex and requires advanced, costly technology. Furthermore, operating costs can be higher due to the need for a more sophisticated operation and maintenance routine.

Many IGCC facilities have transitioned to burning natural gas for economic, environmental, and operational reasons. Hydraulic fracturing advances led to cheaper natural gas prices, making it a more cost-effective fuel. Environmentally, burning natural gas produces fewer emissions than coal, helping plants meet regulations. Additionally, natural gas plants offer better operational flexibility, with faster start-up and ramp-up times, and benefit from a more favorable public perception. These combined factors have incentivized many power plants to transition from IGCC coal to natural gas operations.

Circulating Fluidized Bed (CFB) Power Plant

Circulating Fluidized Bed (CFB) power plant is a type of coal power plant known for its ability to burn a wide variety of fuels while producing lower emissions compared to traditional pulverized coal plants. Here's how they work:

1. Fuel Preparation: In CFB plants, coal is typically crushed into small chunks rather than pulverized into a fine powder.
2. Combustion: The coal is then placed into a combustion chamber that contains a bed of hot, flowing particles, typically made up of ash and other particulate matter. Air is blown into the combustion chamber from below, causing the mixture of coal and particles to behave like a fluid. This is the "fluidized bed."
3. Heat Recovery and Power Generation: As the coal burns, it releases heat. This heat is used to boil water in a boiler, creating high-pressure steam. This steam then drives a steam turbine, which is connected to a generator to produce electricity.

The main advantages of CFB power plants are:

• Fuel Flexibility: Because they use a fluidized bed, these plants can burn a wider variety of fuels, including low-grade coal, biomass, and waste fuels. This makes them more versatile than some other types of coal plants.
• Lower Emissions: The combustion temperature in a CFB boiler is lower than in a traditional pulverized coal boiler. This reduces the formation of nitrogen oxides (NOx), a harmful air pollutant. The limestone or dolomite is also often added to the fluidized bed to capture sulfur dioxide (SO2) produced during combustion, reducing SO2 emissions.
• Efficiency: Although CFB power plants are less efficient than pulverized coal plants, they are more efficient than some other types of coal plants, like stoker or cyclone plants.

The main disadvantage of CFB power plants is that they are more complex and costly to build and operate than traditional pulverized coal plants.

Supercritical and Ultra-supercritical Power Plant

Supercritical and ultra-supercritical power plants are advanced thermal power plants operating at extremely high temperatures and pressures, allowing them to achieve higher efficiency than conventional subcritical power plants. They get their names from the "supercritical" state of water that is used in their operation.

Supercritical Power Plants

In a typical thermal power plant, water is heated to produce steam, which drives a turbine to generate electricity. The efficiency of this process is largely determined by the temperature and pressure at which the steam is produced. In a supercritical power plant, the steam is produced at a pressure above the "critical" pressure of water (around 22.1 MPa, or about 3200 psi). At this pressure, water does not undergo a distinct phase transition from liquid to steam but exists in a supercritical state that exhibits properties of both. This results in a more efficient thermodynamic cycle and higher power plant efficiency. Supercritical power plants typically operate at temperatures of 540°C to 570°C.

Ultra-Supercritical Power Plants

Ultra-supercritical power plants take this concept further by operating at even higher temperatures and pressures (above 593°C and 30 MPa). By pushing the temperature and pressure even higher, ultra-supercritical power plants can achieve even greater efficiency—around 45% compared to about 40% for supercritical plants and 35% for subcritical plants.

Advantages and Challenges

The primary advantage of supercritical and ultra-supercritical power plants is their high efficiency, which means they burn less fuel (usually coal) to generate the same amount of electricity compared to subcritical plants. This results in lower operating costs and reduced emissions.

However, operating at such high temperatures and pressures presents a number of challenges. It requires the use of advanced materials that can withstand these conditions without degrading. It also increases the complexity of the power plant and the upfront cost of building the plant. Despite these challenges, supercritical and ultra-supercritical power plants are becoming more common, especially in countries with significant coal resources.

Natural Gas Power Plant

A natural gas power plant is another common type of power plant that uses the combustion of natural gas to generate electricity. The main types of natural gas power plants include:

Combined Cycle Gas Turbine (CCGT) Plants

The process of generating electricity in a CCGT plant begins with the combustion of natural gas and air in a gas turbine. The external air is first filtered and then compressed. The compressed air and natural gas mixture is ignited and burned to produce a high-temperature and high pressure gas, which then drives the gas turbine. The spinning turbine then powers a generator, which converts the rotational energy into electrical energy.

After the gas turbine, the hot exhaust gasses, which still contain significant amounts of energy, are directed through a heat recovery steam generator (HRSG). The HRSG uses the residual heat from the gas turbine to create steam from water. This steam then drives a steam turbine that powers a second generator, which produces additional electricity.

The combined efficiency of the two generators in a natural gas power plant can reach up to 55-64%, making it a highly efficient process. Moreover, the residual heat produced during the steam generation process can be used for other purposes, such as heating buildings. This further improves the overall efficiency of natural gas power plants and makes them a versatile source of energy.

Open Cycle Gas Turbine (OCGT) Plants

OCGT plants are simpler and less expensive than CCGT plants, but they are also less efficient, typically achieving around 30-40% efficiency. In these plants, natural gas is burned in a gas turbine to generate electricity very similar to a CCGT plant, but the exhaust heat from the gas turbine is not used to generate additional electricity. After the gas has passed through the turbine, it's expelled into the atmosphere. This is why it's called an "open cycle" — the exhaust gasses are not reused within the system.

This cycle is an open cycle, meaning that the air/gas does not get reused but is instead expelled and replaced with a new air intake. The efficiency and performance of a gas turbine can vary based on several factors, including the design of the compressor, turbine, and combustor, as well as the operating conditions.

The main advantage of OCGT plants is their operational flexibility. They can be started up and shut down quickly, making them suitable for providing power during peak demand periods or serving as backup power for intermittent renewable energy sources like wind and solar. They also have a smaller physical footprint compared to other types of power plants and require less capital investment.

Steam Turbine Plants

A steam turbine natural gas power plant operates on a similar principle as a coal-fired power plant, but it uses natural gas instead of coal as the fuel source. The process begins with the combustion of natural gas. This combustion happens in a boiler where water is also present. The heat energy from the burning natural gas converts water into high-pressure steam. The flue gasses resulting from the combustion process, which include carbon dioxide and water vapor, are also generated at this stage.

The high-pressure steam, typically generated at several hundred degrees Celsius, is then directed onto the blades of a steam turbine. This steam causes the turbine to rotate. The spinning turbine is connected to an electrical generator, producing electricity. After passing through the turbine, the steam is then condensed back into water within a condenser. This condensed water is returned to the boiler, where it is reheated to repeat the cycle, thus completing the loop. While the power generation process is ongoing, flue gases produced from the combustion process are treated to remove pollutants.

One of the significant benefits of steam turbine natural gas power plants is their ability to generate a large amount of electricity, making them ideal for baseload power generation. Additionally, their emissions are lower compared to coal-fired power plants.

However, they are less efficient than combined cycle gas turbine (CCGT) plants. A steam turbine plant may reach efficiencies of around 35-40%, while a CCGT plant can achieve efficiencies of up to 60% because it utilizes the waste heat from the gas turbine exhaust to generate more electricity.

Combined Heat and Power (CHP) Systems

In addition to large natural gas power plants, there are also many smaller natural gas generators that can provide electricity and useful heat to buildings and facilities from the same energy source, typically natural gas. This dual production improves overall system efficiency and reduces wasted energy. These generators typically range in size from 50 kW to 10 MW and are based on reciprocating engines that operate similarly to car engines.

Reciprocating engine or gas turbine with heat recovery (U.S. Department of Energy)

Unlike traditional power plants, where residual heat is often wasted, this heat is captured and utilized for heating purposes in CHP systems. The residual heat is collected from the exhaust stream using a heat recovery steam generator (HRSG) or a heat exchanger. This recaptured heat can be used for various applications such as space heating, water heating, or industrial processes. This is particularly useful in commercial buildings or university campuses, where a significant amount of energy is needed for heating and cooling in addition to electricity. This process can significantly increase energy use efficiency, resulting in a combined electrical and thermal efficiency of 65% to 80%.

Microturbines

Microturbines are small gas turbines designed for small-scale power applications such as electrical power generation or combined cooling, heating, and power (CCHP) systems. Typically, these Microturbines have capacities ranging from 25 to 500 kilowatts and have evolved from technologies like piston engine turbochargers, aircraft auxiliary power units, or small jet engines. The compact nature of microturbines offers a higher power-to-weight ratio when compared to heavy gas turbines, making them suitable for distributed power applications. The small size of these turbines allows for modular installations, where multiple microturbines can be combined to form larger systems.

Microturbine illustration (U.S. Department of Energy, credited to Capstone Turbine Corporation)

Microturbines can operate on various fuels, including natural gas, propane, diesel, kerosene, and even some renewable fuels such as biogas from landfills or wastewater treatment plants. Typical microturbines electrical efficiency is between 20% - 30% with a recuperator, and combined electrical and thermal efficiency can exceed 80% when used in cogeneration systems.