Coal & Grading
Coal is the primary fuel for the Thermal power plants, the reason coal is preferred more is that it is available in a larger quantity as compared to the other Non-renewable resources And apart from that, it is also cheaper and having a good calorific value. The coal is also categorized into different types as per their CVs (Calorific Value).There are mainly 4 types of coal and below mentioned are the coal types as per their CVs (Calorific Value)in an ascending order:
Peat
Lignite
Bituminous
Anthracite
In most of the thermal power plants we use Bituminous coal, because it is having a good calorific value and it is present in a larger amount in India as compared to the other coal types. In Bituminous also we are having different grades such as A Grade, B Grade and so on up to F Grade. Here is Gradation table for the Bituminous.This is the coal gradation in India by the Ministry of Coal.
(Coal Gradation ,Courtesy:Ministry of Coal)
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.
Coal Sampling & Testing
For checking the quality of the coal, we do sampling for sample preparation and after the sample is prepared we go for testing. In the CHP, we are dealing with tons of coal but out of them, for testing purpose we only go for a smaller quantity which will be around 3 to 4 Kgs, but the main problem is we are getting coal of different qualities so the sampling should be done in such a way that the final sample prepared should be having the property of the total coal mixture or we can say the average property of the total coal.
In India, all the coal mines are operated by the Ministry of Coal (By Govt. of India) and from each different mine we are getting the coal of a certain GCV (Gross Calorific Value), but this GCV may vary .So before feeding the coal to boiler it has to be tested well for ensuring the desired output of the boiler.
Sampling Process: In the plant, the coal is coming by the help of Trains and Trucks. As the coal arrives at the plant, a person takes the coal samples by the help of a shovel from the 4 corners & the centre of a railway wagon or the truck.
(Conning & Quartering Process)
When the coal arrives by the train, it consists of around 60 wagons containing coal.so it is not possible to take sample from all the wagons. So we divide the total rack into Sub-Lots, Let us take for example, 6 sub-lots. Then from each sub-lot we take samples from first, last and middle wagons. The sample taking process from the wagon is same as that of trucks; the sample is taken by the help of shovels and from 5 points.
After the samples are taken then our next job is to reduce the quantity of the coal as I already said for testing we need only 3 to 4 Kg of coal, but the samples that we have received are of much more quantity. So for reducing the quantity we go for a process called Conning & Quartering.
For example, if we are having a sample of 450Kg from a rack (Railway), then we pass the sample through a 50mm mesh and from the filtered coal we form a cone type structure (coal heap), then through a Quarterer device, we divide the cone into 4 sections and out of those 4 sections we only take coal samples from 2 opposite sides. So we are ultimately reducing the quantity into half. This process is carried out for 3 times so finally we are having a uniform sample of 55 Kg.Then this 50mm sized coal sample is taken for further crushing as the coal testing will be done with the coal samples of 12.5mm and 220 microns size. In the sample preparation room you will be having Jaw Crushers and small Pulverisers (Pulverising means reducing the coal size into some microns).
Apart from manual sampling, we are also having automatic samplers. The Auto-samplers take the sample from the running conveyor belt or from the trucks and wagons automatically and they are also having a crusher for preparing sample of a certain size. There are various auto-sampling mechanisms, but the Screw type sampler and Scraper type samplers are most widely used.
(Auto-Sampler with crusher & collecting plate)
Example: if a boiler is designed for combusting a coal of GCV of 4200 Kcal/Kg, then the coal that we are feeding should be having a GCV of near to 4200 Kcal/Kg, else it will affect the performance of the boiler.The coal consists of various components and those are Carbon, Ash, Moisture, Volatile matter, Nitrogen and Sulphur. In the testing process, we mainly go for these following tests;
Proximate Analysis: In proximate analysis we mainly find the Ash content, Moisture Content and the Volatile Matter percentage. After the percentages of these 3 constituents are found then we find the carbon percentage by subtracting the summation of the other three components from 100.
[Carbon Percentage = 100 – (Ash % + Moisture % + Volatile Matter %)]
For finding Inherent Moisture in coal, we heat 1 gram of pulverized coal sample after air drying for about 2 hours at 108°C.Then after heating we measure the weight by subtracting from 1 gram of coal and we get the Moisture content and similarly for finding Ash content, we heat 1 gram of pulverized coal sample for about 2 hours at 815°C in the oven. For finding the Volatile matter, we heat the 1 gram sample for about 7 minutes at 900°C and the difference in weight is the Volatile matter.
Ultimate Analysis: This is a bit lengthy process and here we do a detailed analysis and find all the constituents such as Carbon Percentage, Nitrogen Percentage and Sulphur Percentage.
GCV is the Gross Calorific Value of the coal. It is denoted as Kcal per Kg.For example if the GCV of a coal is 4200 Kcal/Kg.That means if we are burning 1 kg of coal then the energy liberated is of 4200 Kcal(Kilo Calorie).So for measuring the GCV of the coal, a special equipment is used called Bomb Calorimeter. Now we are having automatic Bomb calorimeters, in which we are directly adding pulverised coal sample of 1 gram in a pot and when we start the machine it shows the final GCV after a certain time.
