Modes of Coal Transportation
Ship: Generally the coal that is imported from the foreign countries is coming by the Ship. The Coal from the foreign countries comes to a port and at the port the coal is stacked for storage. From the Port by the help of Trucks or Trains, the coal is transferred to various plant locations. Some plants which are located on the coastal areas are having their own port facility for receiving Coal such as TATA Power, Mumbai. The reason we are importing coal from the countries like Indonesia, South Africa and Australia is that these imported coals are having a very good calorific value and they are very rich in terms of carbon percentage and while feeding the coal in the boiler, in order to increase the GCV (Gross Calorific Value) of the Coal, we mix the Indian coal with the Imported coal.
Train: It is one of the major modes of transportation for supplying coal to various plant locations. All the thermal power plants are having the facility for receiving coal by Trains. It is very cost effective as compared to the Trucks and one rack of wagons can carry coal up to 4000 Tons.
Truck: Trucks are also another medium for surface transportation. If the company is bringing coal by trucks from a nearby location then it will be cost effective. Now-a-days we don’t even need those truck tipplers for unloading coal as the new generation trucks are having a tipper-body and hydraulics.
Rope-way: Some plants are located near to the Coal Mines or some sort of Stacking yard. As the distance is less so for reclaiming the coal instead of any vehicles they use rope-ways as they are very cost effective.
Conveyors convey coal from one point to another with the help of Belts, Pulleys and idlers. Conveyor belts are made of Polyester or Nylon. In some industries we use conveyor belts of code EP200 & EP300.The width of belts are around 1200mm and capacity of the belts are around 750T/Hr. to 1500T/Hr. (Tons/Hour).Each conveyor belt system is having a Head-end pulley and a Tail-end pulley. The head end pulley is also known as the driving pulley as it drives the conveyor belt being attached to a driving motor.
Idlers: These are nothing but rollers, which support to the conveyor belt and these, can rotate in any direction. The idlers are also divided into various types depending upon their requirement, Such as;
- Impact idler: It is placed at those locations where the coal is falling directly from a higher point through the hopper. It absorbs the Impact.
- Self-Cleaning Idler: It is designed in a screw format so that it can automatically clean the belt and it is placed in the return path of the belt.
- Adjusting Idler: It is designed in such a way that it can adjust the belt sway while running.
- Flat Idler: It is a normal idler which only supports the belt while running.
- Snub Idler: It is located at those locations where the conveyor belt changes its direction. Where ever the conveyor belt changes its direction, snub idlers are placed.
DES: DES stands for Dust Extraction System. The objective of the DES is to extract the coal dust in the CHP area. It is present throughout the conveyor belt area and it sucks the coal dust and separates the clean air from the coal dust. The DE System sucks the coal dust from the conveyor belt, wherever there is coal dust. Generally it is located at the tail end of each conveyor belt as at the tail end, the coal falls from a higher point and creates coal dust.
Working of DES: The DE system sucks the coal dust from the belt and inside the DE system many bags are provided which separate the coal dust and passes the clean air to the atmosphere.as the coals get deposited on the bags then after a certain interval we provide compressed air from the other side of the bag for purging. This purging air removes the coal from the bags and drops it on the conveyor belt. For the purging a separate compressor is provided.
Crusher & Rollers: As the name signifies the work of Crusher is to crush the coal up to a certain size and this crushing size depends upon the requirement. The raw coal that is coming to the plant is sent to the coal bunkers by the help of conveyor belts but before feeding it into the bunker, the size of the coal is reduced by crushing. Generally the coal size after crushing is around 30mm, which is directly fed to the bunker. The most commonly used crusher is of Ring-Hammer Type.
Before passing the coal through the crusher, we pass it through a roller screen. Roller screen consists of a set of rollers in a series in such a manner that if the incoming coal is of size more than 30mm then it will pass over the rollers to the crusher else it will fall below the rollers through the gaps.
(Roller Screen Arrangement before Crusher)
Movable Telescopic Chute: It is also known as a movable chute. This is provided at those locations where there is provision for transforming coal from one side of the belt to the other side of the belt. For example two conveyor belts are there A & B, one is called 2A and the other one is called 2B, similarly 3A & 3B and 4A & 4B and so on. Here A & B are two different coal conveyor belts. If due to some emergencies you want to transfer coal from 2A belt to 3B belt, which is a different coal path then you have to use a telescopic chute/movable chute for transferring.it will be clarified from the diagram below.
(In the Movable chute the whole assembly moves front & back for coal feeding)
Pit-Pump: Generally some part of the coal conveying system is located below the ground level. So there is always a chance of water getting stored at the belt area due to rain. So in order to remove the water from the pit a separate pump is provided and the water is dumped outside.
Auto-sampler: It is an automatic sampler which is present in the conveyor belt area for taking coal samples at certain intervals. If the sampling time is set at 120 seconds that means after every 120 seconds interval it will take coal samples. In the assembly has a Scrapper for taking the coal samples from the conveyor belt and after taking the sample the coal goes to a crusher and then the coal size reduced to around 12 mm for sample preparation. Auto-sampler consists of these components,
- Filter Screens:
- Sample Box
- Excess Return Conveyor
V-Ploughers: At the Coal Bunker level, the coal is to be transferred from the conveyor belt to the Bunker and for that V-Ploughers are used and this is latest technology that is used. The assembly consists of a V shaped Scrapper and during feeding the V-Plougher is operated and from the middle of the conveyor belt and it drops the coal on the two sides of the Bunker.
Buffer Drum: It is equipment, which acts as a buffer for the coal flow, while the coal is dropped from a height and it is passed through the telescopic chute. It is provided for the safety purpose of the belt as the coal lumps are dropped from a higher point so there is chance that the belt may get damaged. It consists of a roller which bears the impact of the coal lumps and the roller is rotated by the help of a motor.
Vibrofeeders: It is used for feeding coal onto the conveyor belts and the main reason we are providing Vibrofeeders is to control or regulate the coal flow onto the conveyor belts.it also acts as a buffer for the coal flow as direct coal feeding onto the belts may damage the conveyor belt. In the Vibrofeeders, a variable speed motor-shaft arrangement is provided. Shaft is not uniform in its design so that the coal will be evenly distributed throughout the width of the belt. So basically it does two works:
- Feed Regulation for the Coal onto the conveyor belt.
- Impact absorber for the Coal flow.
Bunker & Feeder: Bunker is used for storing coal for a longer time in order to supply coal on a continuous basis to feeder. As we are running a boiler on a continuous basis for 24*7, so it is always needed to be ensured that the coal supply is continuous and for that reason only we are using coal bunker for storing coal. The bunker is having a capacity in such a way that it can hold coal for 24 Hours of use.
Feeder: As the name signifies it is used for feeding coal into the Coal Mills, but mainly it controls the coal feeding rate or how much coal is to be fed into the coal mill depending upon the boiler requirement. There are two types of feeders:
- Volumetric Feeder
- Gravimetric Feeder
The volumetric feeder decides the amount of coal to be fed in terms of volume of the coal and after multiplying Volume to the Bulk density of coal we get the mass of the coal that is being fed, but this type of feeder is having one demerit, that the density of coal that we are feeding is varying in nature as we getting coal of different densities so while calculating the mass of coal we get an error in mass .So it is not successful.
Now-a-days most of the feeders that we are having are of Gravimetric type. In this type the amount of coal to be fed is decide in terms of weight.so we get the accurate reading for the amount of coal to be fed.
Motors: In the CHP, we are having both HT (High Tension) & LT (Low Tension) motors. The motors are categorised into HT & LT motors depending upon their voltage levels. The motors operating at Higher Voltage levels are known as HT motors and the motors operating lower voltages are known as LT motors. Some HT motors are operated at 11Kv and 6.6Kv whereas LT motors are operated at 415 volts and all are generally 3-phase induction motors.
Magnetic Separator: while bringing coal from mines, sometimes along with the coal many metal particles come and they may damage the crusher or any other equipment.so for removing those metal debris we use electrically energised magnets nearer to the conveyor belt. During running condition of the belt it attracts the metal objects from the coal.
Stacker-reclaimer: It is used for stacking(storing) and reclaiming coal from the coal yard.it is one of the most commonly used equipment in any CHP.it can cover a wide area in the coal yard as it can travel along the distance and also can rotate about its own axis. It consists of a long arm which can move sidewise as well as up & down. The sidewise (Left-Right) movement is known as Swaying and the up & down movement is known as Loafing.
- Bucket Wheel: This wheel consists of around 9 buckets and by the help of those buckets stacking and reclaiming is done from the coal hips in the coal yard. This wheel can rotate in both directions depending upon the process weather its stacking or reclaiming.
- Boom conveyor or Arm: It is just like an arm which consists of a conveyor belt and it can move in all three dimensions. It collects the coal from the bucket wheel and transfers to the main conveyor.
- Hydraulic Power pack: The Boom of the stacker reclaimer is lifted up and down by the help of this hydraulic unit and for the operation of the hydraulic unit a pump with a motor is provided and for the sidewise movement of the boom/arm, a motor drive is used.