Creating standardized coal classification is a crucial issue for international coal trade, because trading commodities in large quantities requires quick understanding if the certain cargo is appropriate for given coal consumption (electricity production / heating etc.). Despite that fact, unified coal classification, due to the complex structure of coal as a mineral, is very difficult subject and that is why today there is still missing one standardized coal classification and each market or country uses its own categories and terminology. However, all of these countries are creating their own classification basing on the most important quality features, such as calorific value, carbon content, moisture ash and sulphur content, caking and coking properties, volatile matter content and Roga Index.
Historically, coal was formed out of plants remains, accumulated in peat bogs and swamps that changed over time their chemical and physical properties by the high temperature and pressure. Coal deposits explored today were formed in Carboniferous period around 360 to 290 million years ago.
Coal based fuel is formed of flammable substance and ballast. The first one is a mix of hydrocarbons and organic compounds, such as carbon, hydrogen, oxygen, sulphur and nitrogen. Ballast is combined of moisture and other non-flammable components, which after burning leaves ash. Because of the ballast content, coal components are measured and analyzed under different conditions:
- As Received (ar), which includes Total Moisture,
- Air Dried (ad) that includes Inherent Moisture only,
- Dry Basis (db): excludes all Moisture,
- Dry Ash Free (daf): excludes all Moisture and Ash.
Bellow, there are listed all important coal parameters with an explenation of their role in energy production or impact on the power plant machinery (in brackets there are symbols used by Polish standards and units of each parameter):
- calorific value (Q and Qi, kcal/kg or kJ/kg or Btu/lb) expressed as NCV when considered as net calorific value or GCV when it states for heat of combustion (gross calorific value). Determines volume of coal needed to produce the certain volume of energy. It is important for designing boilers, burners, fans performance etc.,
- ash content (A, %) affects coal handling and ash removing systems, mills, furnace, soot blowing system (tendency to slagging, erosion and corrosion) etc.,
- sulphur content (St, %) causes slagging and corrosion,
- moisture content (Wt, %) affects transporting and storage (viscosity and performance in the cold), also size of mills. Higher moisture content causes also the volume of exhaust gases and therefore efficiency of fans, air heaters and size of fumes cleaning systems. Also, it may cause boiler thermal efficiency and construction of burners,
- volatile matter content (V, %) affects coal performance while being stored (oxidation and auto-ignition), also the temperature while exiting the mills and necessary level of fragmentation, placing the burners, furnace construction and coal performance when burned in the boiler.
Other parameters, that might be important for certain consumers (depending on the industry: heating / chemicals / electricity production) are: nitrogen content, Roga Index, dilatation, free swelling index (FSI) and chlorine content.