Inorganic Impurities (Mineral Matter):
* Ash-forming Minerals: These are the most abundant impurities and contribute to the ash content of coal. Some common examples include:
* Silica (SiO2): Found in sand and clay, it forms the majority of ash.
* Alumina (Al2O3): Present in clay minerals.
* Iron Oxides (Fe2O3, Fe3O4): Contribute to the color of ash.
* Calcium Oxide (CaO): From calcium carbonate (limestone).
* Magnesium Oxide (MgO): Found in dolomite.
* Sodium Oxide (Na2O): From sodium chloride (salt).
* Potassium Oxide (K2O): From potassium-bearing minerals.
* Sulphur Oxides (SO2, SO3): Often associated with pyrite (iron sulfide).
* Pyrite (FeS2): This iron sulfide mineral is a common impurity in coal and is a significant source of sulfur dioxide (SO2) emissions during combustion.
* Clay Minerals: These are fine-grained minerals like kaolinite, illite, and montmorillonite, which can affect coal's moisture content and burning characteristics.
* Carbonates: Minerals like calcite (CaCO3) and dolomite (CaMg(CO3)2) can contribute to ash formation.
* Sulfates: Minerals like gypsum (CaSO4·2H2O) can also be present in coal.
* Trace Elements: Many trace elements like mercury, arsenic, lead, and selenium are present in coal in varying amounts.
Organic Impurities:
* Moisture: Coal typically contains a certain amount of moisture, which can vary depending on the type of coal and its storage conditions.
* Volatile Matter: This includes organic compounds like methane, ethane, and hydrocarbons that are released when coal is heated.
* Organic Sulfur: Sulfur can also be present in organic compounds within coal, contributing to sulfur dioxide emissions.
* Nitrogen: Coal contains nitrogen, which can be converted to nitrogen oxides (NOx) during combustion, contributing to air pollution.
The specific impurities found in coal vary greatly depending on the geological formation, the type of coal, and the geographic location where it is mined. The presence and concentration of these impurities impact coal's quality, combustion properties, and environmental impact.
