News

Each type of boiler is designed to be compatible with certain types of fuel. For solid-fuel boiler systems, coal is considered an ideal fuel due to its strong combustion capability and long burning time. Although coal is a common fuel in the market and is widely used in many manufacturing sectors, not every type of coal is suitable for use in boiler equipment.

1. Characteristics of coal for boiler combustion

- Chemical composition

• Carbon: Carbon is the primary combustible component in coal. The longer the formation age of coal, the higher the carbon content and the greater the bond strength, making it more difficult to burn.
• Hydrogen: Hydrogen is an essential combustible component of coal, with a heat value of approximately 144,500 kJ/kg. The hydro content in liquid fuel is higher than in solid fuel.
• Sulfur: Sulfur exists in coal combustion in three forms: organic sulfur (OS), mineral sulfur (MS), and sulfate sulfur (SS). Organic and mineral sulfur can participate in the combustion process, while sulfate sulfur usually exists as CaSO4, MgSO4, FeSO4, etc. These bonds do not participate in the combustion process but instead transform into slag. The heat value of sulfur is approximately one-third of carbon. When sulfur burns, it produces SO2 or SO3 gases, which, when combined with water vapor in the combustion chamber, can form sulfuric acid (H2SO4), corroding metals, and damaging boiler systems. SO2 emitted into the environment is a hazardous toxic gas.
• Oxygen and Nitrogen: Oxygen and nitrogen are inert substances in coal. Their presence reduces the combustible content of coal, resulting in lower heat value. Nitrogen does not participate in the combustion process but is converted into free form in the smoke. The younger the coal, the more oxygen it contains.

- Physical properties

• Heat value: The heat value of coal is the amount of heat released when one kilogram of coal burns completely. Coal is classified based on its heat value, ranging from high to low. Coal is primarily carbon-based, with a heat value of approximately 34,150 kJ/kg of carbon. Therefore, the higher the carbon content, the higher the heat value of coal.
• Moisture content: Moisture content is the amount of water contained in coal. The total moisture content of coal is determined by drying the fuel in an oven at 1050°C until the fuel weight no longer changes. The weight loss is called the moisture content. In reality, at this temperature, not all moisture is completely removed from coal; some types of moisture, such as crystal moisture, usually require temperatures of 500-800°C for complete evaporation.
• Ash content: Ash content is the residue remaining after coal combustion. Mineral substances in coal transform into ash when burned. The presence of ash reduces the combustible content of coal, affecting its combustion. The ash ratio in coal greatly affects its combustion ability, reducing heat value, causing surface erosion of heat-absorbing tubes, accumulating dirt, and reducing the heat transfer coefficient across tube walls. Additionally, an important characteristic of ash significantly affecting the operation of combustion equipment is the ash fusion temperature.
• Volatile matter: When fuel is burned under conditions where there is no oxygen in the environment, the bonds of organic molecules are broken. This process, known as thermal decomposition, produces gaseous substances, including hydrogen, hydrocarbons, carbon monoxide, and carbon dioxide. The products of thermal decomposition are called "volatile matter." Bonds with more oxygen are less stable and break more easily at high temperatures. The amount of volatile matter produced depends on the formation age of the coal; the younger the coal, the lower the temperature at which volatile matter begins to form. The amount of volatile matter produced also depends on the duration of thermal decomposition; the younger the coal, the shorter the duration of thermal decomposition, resulting in more volatile matter produced, leading to greater fuel consumption during combustion.

3. Classification of coal for boiler combustion

- Synthetic Natural Gas (SNG): SNG is a process of using gasification agents (oxygen, air, oxygen-enriched air, steam, hydrocarbons) to react with coal at high temperatures, converting the fuel from solid to gas form with primarily CO, H2, CH4...

- Lignite: This is the lowest-grade coal, with carbon content ranging from 25-35%, high moisture content of approximately 66%, and ash content ranging from 6-19%. Lignite is usually in granular form and is primarily used for power generation. It is the first type of coal to be gasified.

- Sub-bituminous: Geologically speaking, sub-bituminous coal is considered young coal, formed over 200 million years ago. In a dry and ash-free state, sub-bituminous contains 35% – 45% carbon. This type of coal typically has a wood-like appearance rather than a solid structure, with colors ranging from dark brown to black. Sub-bituminous coal has a lower moisture content than non-caking coal, making it slightly harder and easier to transport and store. Many power plants have switched to using sub-bituminous coal because of its low sulfur content (less than 1%), reducing environmental pollution.

- Bituminous coal: Bituminous coal contains many volatile substances, burns with a long flame, and can create binding substances when heated in a reducing atmosphere. Bituminous coal has a moisture content of approximately 17%, nitrogen content ranging from 0.5-2%, fixed carbon content of 85%, and ash content of up to 12% by weight. There are two types of bituminous coal: thermal coal and coking coal.

• Thermal coal: Used to provide energy for industrial manufacturing plants.
• Metallurgical coal: Used in the steel refining process for steel production. Metallurgical coal is a dense carbonaceous rock produced by heating bituminous coal to extremely high temperatures without oxygen. The melting process of metallurgical coal in the absence of oxygen to remove impurities is called pyrolysis.

- Anthracite coal: Anthracite coal has the highest heat value of all coal grades, containing 86% - 97% carbon, with few impurities and a metallic luster. Anthracite coal is used in power generation and steelmaking industries. Additionally, due to its high hardness and strong structure, anthracite coal is resistant to chemicals, and stable even in acidic and alkaline environments, making it widely used in large-capacity water treatment stations, industrial and domestic water purification plants to treat heavy metals, decolorize, eliminate unpleasant odors, dissolve organic matter, and remove trace pollutants.

3. Choosing coal for different types of boilers

- Fluidized bed boiler: Fluidized bed boilers use a fluidized bed combustion chamber to burn fuel. The fluidized bed creates excellent mixing conditions throughout the chamber, allowing the fuel to burn completely while maintaining a relatively low combustion chamber temperature, reducing the emission of harmful gases. With its unique structure, the type of coal used for fluidized bed boilers typically has small particle sizes ranging from 0-10 mm, low volatile matter, and moisture content (ARB) < 25, with corresponding heat values (ADB) as follows:

• Bottom ash-fired (non-circulating): 3800 – 4100
• Bottom ash-fired (circulating): 5200 – 5700
• Sand-fired fluidized bed: 3800 – 4100

- Chain grate boiler: Chain grate boilers are devices that use heat from burning fuel to heat water into steam for production or power generation needs. The principle of operation involves feeding fuel into the furnace using a grate, with air supplied underneath to support combustion. The energy released from the combustion process is absorbed by water in the heat-absorbing tube bundles to convert it into steam. The ash produced after combustion is discharged at the end of the combustion chamber and removed by a scraper or conveyor. The combustion chamber of chain grate boilers has the highest level of mechanical engineering among fuel-fired combustion chambers. Chain grate boilers allow the burning of fuels with large particle sizes, uneven particle sizes, and multiple fuel types simultaneously, or individually. Chain grate boilers are suitable for burning coal with large particle sizes ranging from 0-50 mm, moisture content (ARB) < 40, with corresponding heat values (ADB) as follows:

• Below 50% boiler load: 3800 – 4100
• Between 50% and 75% boiler load: 4800 – 5000
• Above 75% boiler load: 5200 – 5400

- Step grate boiler: Step grate combustion technology is widely used in various heat applications such as saturated boilers, superheated boilers, heat transfer oil boilers, hot water boilers, and hot gas boilers. In particular, step grate boilers are compatible with various solid fuels (coal, biomass, waste). Therefore, when there is a need to burn coal of various sizes without installing additional complex fuel processing equipment, step grate boilers are a suitable choice. This type of boiler has excellent combustion capabilities, even for difficult-to-burn fuels with high moisture content, low heat values, inconsistent particle sizes, and ash-containing alkali metals that are prone to melting and agglomeration.

- Pulverized coal boiler: Among the solid fuel-fired boiler types currently in use, pulverized coal boiler have the highest efficiency. However, the design of these boiler systems is complex, requiring significant investment costs and highly skilled engineering teams to operate, limiting their practical application to large-scale power plants. The operating principle of a pulverized coal boiler involves grinding coal into fine powder with very small particle sizes and injecting it directly into the combustion chamber using high-pressure compressed air. Here, the coal is ignited and burns similar to gas or oil burner nozzles. Coal injection boilers can only use high-quality coal with uniform particle sizes. Coals with high ash content, sulfur content, and moisture content cannot be used. Furthermore, this type of boiler is designed specifically for coal and cannot be modified to burn other solid fuels.

Conclusion

Selecting coal for boiler combustion requires choosing a type with low volatile matter and high flame resistance to save fuel. Additionally, coal for boiler combustion needs to have a high slagging ability and produce minimal smoke to improve boiler efficiency and reduce harmful emissions into the environment.

The process of selecting coal types should focus on matching particle sizes and heat values for each type of boiler. Matching heat values not only enhances combustion capability but also ensures efficiency and energy savings during production. Furthermore, coal particle size is a crucial factor for efficient and stable combustion, optimizing heat and boiler productivity.

With the industrial sector's development, the demand for using coal as boiler fuel will continue to increase. However, burning coal poses many challenges to the environment. Therefore, businesses should prioritize using high-quality coal that has undergone refinement and apply advanced exhaust gas treatment technologies in operating coal-fired boilers, specifically, and fossil fuels in general, to minimize negative environmental impacts.