The coal preparation includes sizing, removal of rock originating from mine roof, removal of ash forming and sulphur bearing minerals, drying to remove excessive surface moisture and blending of different coals to achieve desired physical and chemical properties. The fig. shows the flow diagram of coal preparation plants. The coal cleaning equipment is relied on the relative differences in specific gravity. The specific gravities of coal and impurities are given below.
The coal preparation plant includes crushers, sizers, dryers and magnetic separators. Coal crushers are used to size the coal before feeding to the combustion chamber. Sizers are used for the separation of coal to the required size. Sized coal passed over the drier is to remove moisture from the coal. For that, hot flue gases are used. Before coal goes to storage hopper, the iron scrap and particles are removed. Then the coal goes to the magnetic pulley.
- Magnetite in water
Coal Cleaning Equipment
The equipments required for coal cleaning are coal sizing equipment, size classification equipments, coal drying equipments. concentrating equipment and de-watering systems.
Removal of dirt
It is done by passing the coal into primary screening to separate the large coal cleaning equipment above 8cm in size from the small coal. Then, the large coal is cleaned by hand and small size coal (less than 8cm size) is cleaned by washing.
It is essential before the coal transportation takes place. The common methods used for drying are
(i) Steam drying
(ii) Oil dehydration
(iii) Fuel gas drying.
About 10% of the moisture in the coal is reduced by steam heating for lump sizes and by flue gas heating.
The uniforming of the coal size is essential for better utilization of coal. Before recommending the coal in the grates, the consideration must be given for installation of both stokers fired and pulverized coal plant.
The use of coal containing more than 2.5% of sulphur is restricted due to fouling and corrosion action. Normally, the sulphur may occur in any of the following forms
- The phyritic contains 50 to 80%
- The organic upto 40%
The washing is done by the following ways.
(i) Water Jigs
(ii) Concentrating table
(iv) Heavy medium
It is the oldest method for cleaning the coal and jig is operated by means of the stratification that takes place as fluid pulsates through a bed of coal particles. The particles with specific gravity higher than predetermined sizes are continuously removed from the bed and carried by bucket elevators.
This method is widely used for cleaning fine coal. It consists of a flat, wooden deck, and tapered riffles. It is reciprocated horizontally and the feed particles spread out across the surface and they become arranged.
The hydro cyclone consists of a cylindrical and a conical section. The feed slurry is flow tangentially under pressure into the cylindrical section and moves into the conical section.
The high-density particles move downwards and are discharged from the cyclone apex. The low specific gravity particles (coal) are carried out in the cyclone overflow as shown in fig.
It is used to clean coal in heavy medium vessels and heavy medium cyclones. The heavy medium vessels are used to clean coarse coal (about 6mm). The raw coal is continuously introduced at one end of the vessel and the high specific gravity materials are removed as refuse. The low gravity particles float on the surface and are discharged from the vessel.
Advantages of coal preparation
- It improves coal quality.
- Reduced transportation (about 15-20%) of waste is eliminated.
- Reduced pulveriser operation and maintenance cost are less.
- Boiler performance is improved.
- Sulphur removal is easier.
- Ash handling is easy.
Coal Blending or Mixing
The parameters of coal like heating value, moisture, sulphur content, ash and its composition vary from region to region. So. mixing of coal is very important.
There are three types of blending.
1. Bed blending
2. Belt blending
3. Automatic blending
- In the bed blending, bulk material is layered in long narrow piles to form layer cake with 100 to 400 layers.
- In belt blending, the coal is reclaimed from several piles at once via belt feeders.
- In automatic blending, the desired coals and blend ratios are fixed by the operator and the mixing is done automatically.
The chemical separation of sulphur from the coal is known as coal desulphurization, for that the method used is chemical TRW Meyers process.
Working of TRW Meyers process
- First the coal is crushed and sent to mixing tank.
- It is mixed with a Fez (SO4)3 (Ferric sulphate) solution.
- The resulting slurry is heated to 100-130°C.
- This coal solution is pumped to a filter.
- It is neutralized by the addition of coal.
- In the extractor, the coal is mixed with warm acetone and water which dissolves the elemental sulphur left in the coal.
- Next, the acetone water and coal slurry go to a filter where the acetone is removed.
- This filtered acetone is evaporated by leaving the elements of sulphur.
- The sulphur is removed and the acetone is condensed and sent back to the extractor.