Coal has been one of the most widely used sources of energy for centuries. It is burned to produce heat, which is then used to generate electricity, power industries, and fuel transportation. In this article, we will explore in detail how coal is burned, the processes involved, and the technologies that make coal combustion efficient and effective.
Understanding Coal and Its Composition
Before diving into the burning process itself, it’s important to understand what coal is and its composition. Coal is a black or brownish-black sedimentary rock primarily composed of carbon, hydrogen, oxygen, nitrogen, and sulfur. The carbon content of coal varies depending on its type, which can range from peat to anthracite.
Types of Coal
Peat: This is the earliest stage of coal formation. It has a high moisture content and low carbon content, making it less efficient for burning.
Lignite: Known as brown coal, it has a higher carbon content than peat but is still low in energy compared to other types of coal.
Bituminous Coal: This is the most commonly used type of coal in power plants and industry. It has a high carbon content and provides a significant amount of energy when burned.
Anthracite: This is the highest grade of coal with the highest carbon content. It burns cleaner and hotter than other types but is less common.
Coal’s composition and grade affect its burning efficiency and the pollutants it releases during combustion.
The Process of Coal Burning
Burning coal is a complex process involving several stages. The energy stored in coal is released through combustion, where coal reacts with oxygen to produce heat, carbon dioxide (CO2), and other by-products.
Step 1: Preparation and Pulverization
In most industrial settings, coal must first be pulverized into a fine powder before it can be burned effectively.
Coal Sizing: Coal is often too large to be burned directly, so it is crushed into smaller pieces.
Pulverization: The coal is then ground into a fine powder, typically through a coal mill. The powder is much more reactive and burns more efficiently than large chunks of coal.
This pulverized coal is then mixed with air to form a combustible mixture that can easily burn in the furnace.
Step 2: Combustion in the Furnace
Once pulverized, the coal is fed into a furnace where it will be burned. Combustion is a chemical reaction in which coal reacts with oxygen (from the air) to produce heat, carbon dioxide, and water vapor.
The Role of the Furnace
Furnace Design: Coal-burning furnaces vary in design, but all are designed to provide a controlled environment for combustion. The furnace is typically a large chamber where coal and air are mixed and ignited.
Ignition: Once the pulverized coal enters the furnace, it is ignited using a spark or an initial flame. The temperature inside the furnace rises significantly, often reaching temperatures as high as 2,000°F (1,093°C).
Burning Process: During combustion, carbon in the coal reacts with oxygen in the air to form carbon dioxide, releasing a significant amount of heat in the process.
Stages of Combustion
Volatile Matter Release: Coal contains volatile matter, such as water vapor and gases like methane. These volatile substances evaporate at the start of combustion and react with oxygen to form carbon dioxide and other gases.
Char Formation: After volatile matter is released, the remaining carbon (known as char) continues to burn, producing heat and carbon dioxide.
Ash Production: The inorganic minerals in coal do not burn and are left behind as ash. This ash is removed from the furnace and disposed of properly.
Step 3: Heat Transfer
The heat generated by coal combustion is the energy that powers various systems. In power plants, the heat is transferred to water to generate steam, which drives turbines to produce electricity.
Heat Exchangers
In power plants, the heat generated from burning coal is transferred to water in a heat exchanger. This process is efficient because water has a high heat capacity, which means it can absorb a lot of heat without a large temperature increase.
Steam Production: The heat from the coal combustion turns water into steam. The steam is then sent to a turbine.
Turbine Movement: The steam passes through turbines, causing them to spin. This mechanical energy is then converted into electrical energy via a generator.
Step 4: Exhaust Gases and Emissions
Once coal has been burned and the heat has been extracted, the exhaust gases—mainly carbon dioxide, sulfur dioxide, nitrogen oxides, and other particulates—must be dealt with.
Air Pollution Control
Flue Gas Desulfurization: To control sulfur emissions, many coal-burning plants use scrubbers. These devices remove sulfur dioxide from the exhaust gases by reacting with a chemical reagent, such as limestone, which forms a solid waste that can be safely disposed of.
Nitrogen Oxide Control: Nitrogen oxides, which contribute to acid rain and smog, are reduced using technologies like low-NOx burners, selective catalytic reduction (SCR), and selective non-catalytic reduction (SNCR).
Particulate Matter Control: Ash and soot particles produced during coal combustion are captured by electrostatic precipitators or fabric filters before being released into the atmosphere.
Step 5: Ash Disposal
After the coal has been burned, it leaves behind ash. The ash consists of both fly ash (the fine particles suspended in exhaust gases) and bottom ash (the heavier, coarser particles that fall to the bottom of the furnace).
Fly Ash: This ash is captured by air pollution control devices and can be stored in landfills or reused in construction materials such as concrete.
Bottom Ash: The heavier bottom ash is removed from the furnace, cooled, and often disposed of in landfills or reused.
Coal Combustion Technologies
Over time, several technologies have been developed to burn coal more efficiently and reduce environmental impacts.
Fluidized Bed Combustion
Fluidized bed combustion (FBC) is a technology that uses a bed of granular material, such as sand, which is fluidized by upward-flowing air. This process allows coal to burn more efficiently and at lower temperatures, reducing the formation of nitrogen oxides and sulfur emissions.
Types of Fluidized Bed Combustion:
Bubbling Fluidized Bed: Used for smaller-scale combustion.
Circulating Fluidized Bed: Common in larger, industrial power plants.
Integrated Gasification Combined Cycle (IGCC)
In an integrated gasification combined cycle system, coal is gasified into a mixture of carbon monoxide, hydrogen, and carbon dioxide. This gas is then burned in a gas turbine to produce electricity. The heat from the gas turbine is used to produce steam, which drives a steam turbine to generate more electricity.
Advantages of IGCC: IGCC allows for better efficiency in power generation and can reduce carbon emissions because it captures CO2 before combustion, which can be stored underground.
Supercritical and Ultra-Supercritical Steam
Supercritical steam conditions involve using water under extreme pressure and temperature, above the critical point (about 3,200 psi and 705°F). This allows power plants to generate electricity more efficiently by extracting more energy from the same amount of coal.
Benefits of Supercritical Technology: More energy is produced with less coal, and emissions are reduced due to improved efficiency.
Conclusion
Burning coal is a highly efficient process when done properly. It involves pulverizing the coal, igniting it in a furnace, transferring the heat, and dealing with the exhaust gases through various technologies. While coal combustion produces energy, it also results in emissions, which must be managed using advanced pollution control systems. Despite the environmental concerns surrounding coal, it remains a critical energy source worldwide, particularly in electricity generation. Through ongoing advancements in combustion technologies, the efficiency of coal burning continues to improve, helping to balance the demand for energy with environmental stewardship.
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