Language
Country/Area
No result found
The Mystery of Moisture: Why is the moisture in coal so important to its quality?
With the increasing demand for energy today, coal is an important fuel, and its quality assessment is receiving more and more attention. Moisture in coal, whether it is detailed analysis or the key factors affecting coal quality, has increasingly become an issue that the industry and academia pay attention to.
Coal analysis techniques are specifically designed to measure specific physical and chemical properties of coal. These methods are primarily used to evaluate the suitability of coal for coking, power generation, or smelting.
Chemical properties of coal
Coal is divided into four main types or grades: lignite, bituminous coal, anthracite and graphite. Each type of coal has different physical parameters, which are mainly controlled by moisture, volatile matter and carbon content.
Moisture
Moisture is an important characteristic of coal, and all coals contain moisture when mined. Groundwater and other external moisture is called additional moisture and can easily evaporate. The moisture contained within the coal itself is called intrinsic moisture and usually requires quantitative analysis.
Moisture can exist in coal in four forms: surface water, hygroscopic water, decomposed water and mineral water.
Analysis of bulk moisture is usually performed by comparing the mass loss of untreated and analyzed samples. Scientists can effectively measure moisture content by heating coal samples and drying them in a nitrogen atmosphere at 150°C or in air at 100 to 105°C.
Volatile substances
Volatile substances refer to coal components released under air-free conditions at high temperatures, usually including short- and long-chain hydrocarbons, aromatic hydrocarbons and certain sulfides. The importance of measuring volatiles lies in its ability to evaluate the energy potential of coal and its use in activated carbon.
Ash
Ash is the non-combustible residue left after burning coal and represents the total amount of minerals after combustion. The ash content analysis of coal is relatively simple. After the coal is completely burned, the ash content produced is expressed as a percentage relative to the original weight, which is also one of the important indicators of coal quality.
Fixed carbon
Fixed carbon refers to the carbon that remains in coal after being driven by volatile substances. Fixed carbon content is often used to estimate the amount of coke-producing material from a coal sample and is measured by removing the volatile mass from the total mass of the coal sample.
Physical and mechanical properties
Relative density
The relative density of coal depends on the grade of coal and the degree of mineral impurities, and knowledge of the density of each coal is necessary to determine the properties of complexes and mixtures. Density is generally measured by the weight loss of a sample in water.
Particle size distribution
The particle size distribution of coal depends on the grade of the coal, which affects the brittleness of the coal, as well as the handling, crushing and grinding processes it undergoes. This information must be known before mining in order to utilize the coal at the appropriate size in furnaces and cokers.
Flotation-Settlement Test
The flotation-settlement test is an analysis of coal particles and coal seams. It uses liquid to soak high-ash particles to increase the selling price and energy content of coal. This test can help determine the optimal particle size for washing and the required wash liquor density.
Special combustion test
Specific energy
Specific energy can be determined by a bomb calorimeter, which measures the energy output of complete combustion of coal, especially coal used for steam generation.
Ash melting point test
The melting behavior of coal ash is a key factor in coal selection, especially in steam power generation applications. An excessively high melting point will cause coke and ash to agglomerate in the furnace, which requires regular cleaning. The melting temperature is usually tested by observing the shape changes of the molten ash in a high-temperature furnace.
Free expansion index
The free expansion index test is the simplest test to assess the suitability of coal for coke production. After being added to a standardized crucible for combustion, the cross-sectional shape of the obtained coke cake is judged by comparing it with the standard cross-section.
Grade classification of coal
International standards classify coal by grade. As the grade increases, the carbon content of coal becomes higher. The ASTM system classifies coal with more than 69% fixed carbon based on the content of carbon and volatile matter.
The moisture content of coal not only affects the energy output of coal, but also affects its competitiveness in the market.
In the face of the challenges of global energy transition and environmental protection, how should the management of moisture in coal be combined with new technologies to achieve the best environmental and economic benefits?
