Forms of Coal Used in Boilers and Their Characteristics

Coal is a widely used fuel in industrial and commercial applications, particularly in boilers for heating and power generation. The form in which coal is utilized can greatly impact its efficiency and the design of the boiler itself. This article will explore the various forms of coal used in boilers, their characteristics, and the factors that determine their selection.

The Forms of Coal Used in a Boiler

Pulverized Coal

The most common form of coal used in large-scale power plants is pulverized coal. The coal is ground into a fine powder, allowing for more efficient combustion. This process significantly increases the surface area of coal particles, which enhances the burning efficiency and reduces emissions. Pulverized coal is blown into the boiler furnace, where it mixes with hot air, and burns at high temperatures. This method is highly efficient and has become the standard in modern large-scale power generation.

Stoker Coal

Stoker coal is primarily used in smaller and less industrialized boilers. This form consists of larger pieces of coal that are fed into the boiler using a stoker mechanism. There are two main types of stoker systems: chain grate and underfeed. Chain grate systems use a chain mechanism to move the coal across a grate, while underfeed systems use a belt or scraper to move the coal underneath a grate. Each system has its advantages and disadvantages in terms of efficiency and maintenance.

Briquettes

Briquettes are compacted blocks of coal dust mixed with a binder. This form of coal is less common but can be used in certain types of boilers. Briquettes are advantageous because they are easier to handle and transport compared to large chunks of coal. However, they are less efficient than pulverized coal due to their lower surface area, which can affect the combustion process.

Lump Coal

Lump coal refers to whole, unprocessed blocks of coal that can be used in traditional and smaller-scale boilers. While lump coal is still used, it is generally less efficient than pulverized coal due to its larger particle size, which leads to a slower and less uniform combustion process. However, it is still a viable option for certain applications.

The Geological Order of Formation of Coal

Coal is formed from plant material in a process that can take millions of years. The geological order of formation from lowest to highest carbon content is as follows:

Peat

Peat is the least mature form of coal and is characterized by its low carbon content and high moisture. It is the earliest stage of coal formation and has not undergone significant transformation.

Lignite

Lignite, also known as brown coal, has a higher carbon content than peat and a lower content of fixed carbon. It is highly combustible but also has a high moisture content.

Sub-bituminous

Sub-bituminous coal has a higher carbon content and lower volatility than lignite. It is more energy-dense and is used in larger-scale industrial applications.

Bituminous

Bituminous coal is the largest group of coal, characterized by a higher fixed carbon content (46-86%) and lower volatile matter (20-40%). It is subdivided into low-volatile, medium-volatile, and high-volatile types, each with varying heating values. The lower the volatility, the higher the heating value, making bituminous coal a popular choice for larger-scale applications.

Sub- Anthracite and Anthracite

Anthracite contains the highest amount of fixed carbon (more than 86%) in amorphous form and has the least amount of volatile matter. While anthracite is highly efficient in terms of heat output, it is more challenging to ignite due to its low volatile matter content, making it less commonly used in modern applications.

Coal Combustion for Boilers

In a typical boiler setup, coal is first pulverized into a fine powder before being sent to the burners via hot air and ignited, releasing a tremendous amount of heat. Some boilers may also have a moving grate at the bottom on which chunks of coal burn and release their heat. The grate system is more efficient for certain applications, especially in smaller-scale operations, where the heat generated is more manageable.

Generally speaking, the choice of coal form and the boiler design are interdependent. Larger-scale operations tend to use pulverized coal for its efficiency, while smaller-scale and traditional boilers may use stoker or lump coal. The specific requirements of the combustion process, such as heat output and emissions, are also critical factors in determining the coal form and boiler design.

Choosing the right form of coal and boiler design is crucial for maximizing efficiency, reducing emissions, and ensuring reliable operation in industrial and commercial applications.