Generic term of all solid fossile fuels. Coal is a product of thermal and pressure treatment of of biomass in the earth crust. Different chemical and physical transformations lead to the end product coal. Coal is rich of carbon, but also contains hydrogen, oxygen, sulphur and other elements in variing fractions.
Mining of Coals
Depending on the coal deposit structure and depth of the coal deposits below ground there exist two types of coal mining:
- Strip mining (or surface mining) of near-surface coal deposits and
- Deep mining (or underground mining) of deeply located coal deposits
Strip mining is technically much easier than deep mining. On the other hand it means large surface alterations from natural/agriculturural landscapes into (temporarily) plantless areas. In many cases people have to move away from their previous dwelling location (e.g. North Rhine-Westphalia). Natural landscapes are devastated (e.g. Apalache Mountain region).
Deep mining reduces the surface effects drastically but it is much more expensive due to the difficult mining conditions deep under the earths surface: Stabilization of the rock structures after coal removal, air conditioning and water removal are some aspects of the additional work compared to strip mining. Another type of side effect is the destabilization of the ground in many mining areas leading to sinking ground levels which destructs whole houses.
Energetic and Non-Energetic Coal Use
Energetic: Coal is mainly used for the production of electricity in coal fired power plants. Another energetic use of coal is resident heating and cooking. Coal can be converted into liquid fuels by steam refining and in a second step by the fisher tropsch synthesis. These liquid fuels can substitute oil products like kerosene, diesel fuel, gas(oline), etc. Another strategy is coal gasification by steam reforming followed by the shift reaction to produce hydrogen which can be used energetically in gas turbines or - after purification - in fuel cells.
Mixed: Steel/iron production uses preprocessed coal for 1. reduction and 2. heating of the iron ore. Coal is used to release the element iron from its bound state (e.g. iron oxides) by a chemical reaction called reduction. Coal also produces the heat to separate the liquid iron from other components.
Non-Energetic: Coal as mixture of chemical compounds can deliver different compound mixtures for the chemical industry. In that case coal is a material ressource.
Side Effects of Coal Use
Coal is a composition of organic compounds. Burning that composition of organic compounds leads to different compounds:
- Carbon dioxide (CO2) - major reaction product
- Water vapour - non-problematic reaction product
- Sulphur dioxide (SO2) - depending on sulphur content of the coal
- Different nitrogen oxides (NOx) - caused by the nitrogen in the air which is used for combustion
- Soot, soot particles - depending of coal type, coal treatment, combustion conditions
The latter reaction products are effectively removed by flue gas processing at the outlet of the coal oven of a typical coal fired powerplant:
- Sulphur dioxide: Conversion to CaSO4 falling out of the flue gas stream. Can be used for building materials.
- Nitrogen oxides: Conversion to NH4+NO3- and removal from flue gas stream.
- Soot, soot particles: Removal by electrostatic fields.
The reaction products can be reused, in the case of soot and soot particles they have to be disposed as hazardous waste.
Carbon dioxide release from coal contributes to the enrichment of carbon dioxide in the earths atmosphere. Carbon dioxide transmitts the visible sunlight but absorbs/reemitts the infrared radioation emitted by the earths surface. Hence it leads to an increased energy content of the atmosphere because the infrared radiation travels longer through the atmosphere before it is radiated into space. This process leads to higher temperatures (global warming) of the earths surface/atmosphere.
Removal of carbon dioxide from the flue gas ist discussed now (2010) and first demonstration plants emerge. Major problems of carbon dioxide removal are the large amount of flue gas which has to be treated and the method to deposit the carbon dioxide gas securely.