Classification of coal:
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According to |
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|
1 |
Purpose |
Coking coal, Smithy
coal, steam coal, gas coal |
|
2 |
Fixed carbon
volatile matter and oxygen content |
Wood, Peat,
lignite, bituminous, anthracite. |
|
3 |
Pear's method |
On the basis of
calorific value of sulphur free coal and volatile matter |
|
4 |
Ralson’s
method |
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5 |
White |
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6 |
Freezer |
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7 |
Gurner |
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Selection of coal for thermal power plant:
Firing quality of coal are the prime importance and
influence the design of combustion chamber, type of combustion equipment and
layout of heat transfer part of thermal power plant. Coal with low volatile
content have slower burning characteristics but generate high fuel bed temperature,
hence forced draft is required. Therefore grate should be so design and fuel
feed so controlled that the grate protected by adequate ash.
Cold with high volatile content have high rate of burning and required a large combustion chamber for combustion of volatile, such coal is very useful in thermal power plant to meet the sudden increase of load. Also important factor of consideration are sizing, cooking characteristic of coal, grindability, resistance to degradation, Ash fusion temperature and sulphur content of coal.
Coal Sampling:
IS: 436-1-2 (1976) : Method of sampling of coal and coke.
At a time amount of coal collected is called increment. The standard practice is to collect 350 Kg sample from 250 Ton of coal received in increments of 7 Kg.
Samples are well mixed and reduced to 50 Kg by mechanical sample devider or by hand method using shovel by coneing. The 50 Kg mass is crushed to 1/8" size and again reduced to 2 Kg by coneing and quartering method.
1 Kg sample - below 1/2" size - for gross moisture content.
1/2 Kg sample - 1/8" size - preserved
1/2 Kg sample - crushed in raymond mill to powder (72 mesh) - for analysis
ARB - As Received Basis
ADB - Air Dry Basis (at a temperature of 40 C and 60% relative humidity, keeping in a humidifier chamber for 48 hours)
Coal
Analysis:
Types of tests:
1. Proximate Analysis
Proximate Analysis enables us to understand the percentage by weight of Fixed Carbon, Volatile Matter, Ash and Inherent Moisture content of a sample of coal.
Procedure-of-proximate-analysis-of-coal.html
2. Ultimate Analysis
Ultimate Analysis Test is done to determine the element composition of coal which includes carbon, hydrogen, nitrogen, sulphur and oxygen. With the help of Ultimate analysis and GCV we can determine the NCV or Net Calorific Value of Coal.
Conversion-from-proximate-analysis-to-ultimate-analysis
SOME IMPORTANT CHARACTERISTICS FOR COAL AND COKE AND THEIR SIGNIFICANCE:
1.
Moisture
Analysis
It is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture are known as adventitious moisture and are readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed quantitatively. Moisture may occur in four possible forms within coal:
· Surface moisture: water held on the surface of coal particles or macerals
· Hygroscopic moisture: water held by capillary action within the micro fractures of the coal
· Decomposition moisture: water held within the coal's decomposed organic compounds
· Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays
2.
Volatile
Matter:
Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. It is volatile substance present in coal and the gaseous product of thermal decomposition of coal. It first catches fire, burn with flame and give support in burning. The volatile matter of coal is determined under rigidly controlled standards.
Normal value - 20 to 30 % (fines 70% pass through 200 mesh)
Low VM - less than 19% (fines 80% pass through 200 mesh)
- Give oil support to sustain combustion.
- High flame length.
- Longer time for combustion.
- Higher fineness required for proper combustion.
Any coal which has high volatile matter content:
- Can ignite easily.
- Burns with long smoky yellow flame.
- Has low calorific value.
- Needs large furnace volume for combustion.
- High tendency of catching fire.
There are three basic component of volatile matter.
Gases - containing CO, H2, CH4, C2H6 and H2S.
Tar - a complex mixture of hydrocarbon such as benzene, toluene etc.
Ammoniacal liquor - Aqueous condensate of N2 and S compounds + cyanide.
It effects the design of furnace volume and arrangement of heating surface. Heat liberated due to combustion of volatiles helps in heating of Coke particles. If the volatile yield of coal is high, the liberated heat of volatile combustion will ignite the coke particles. If the yield of volatile is low, the coke particles must be heated up externally to ignition temperature.
3.
Ash:
It is a inorganic mineral substance. Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion. Identification of Ash content is necessary because it directly affects the combustion and boiler efficiency.
Coal containing high ash:
- Is Harder and Stronger.
- Has Low calorific value.
- Produces slag.
- Causes clinker formation.
4. Mineral Matter: There are two types:
1) Inherent - Inherent comes from inorganic constituent of plant materials (organic vegetables substance), but the amount is less. It can not be removed.
2) Extraneous - Extraneous comes from the decaying vegetable matter, rocks & dirt. It can be removed by washing.
Mineral matter = 1.1 x Ash%
5.
Fixed
carbon:
It is the main heat generator during the process of burning. The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal.
FC = 100 - ( A% + M% + VM%)
6.
Fuel
Ratio:
Fuel ratio is the ratio of FC to VM i.e. FC / VM. According to their fuel ratios, coals have been classified
|
at least 10 |
Anthracite |
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6 to 10 |
Semi Anthracite |
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3 to 6 |
Semi Bituminous |
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3 or less |
Bituminous |
7.
Gross
Calorific Value:
GCV is determined to measures the heat value that coal can generate.
|
Grade |
Calorific Value Range (
in kCal/Kg) |
|
A |
Exceeding 6200 |
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B |
5600 – 6200 |
|
C |
4940 – 5600 |
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D |
4200 – 4940 |
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E |
3360 – 4200 |
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F |
2400 – 3360 |
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G |
1300 – 2400 |
The amount of heat liberated by the complete combustion, under specified conditions, by a unit volume of a gas or of a unit mass of a solid or liquid fuel, in the determination of which the water produced by combustion of the fuel is assumed to be completely condensed and its latent and sensible heat made available.
GCV = 8080.C + 34500(H-1/8.O) + 2220.S kcal
Net calorific value (NCV): The amount of heat generated by the complete combustion, under specified conditions, by a unit volume of a gas or of a unit mass of a solid or liquid fuel, in the determination of which the water produced by the combustion of the fuel is assumed to remain as vapor.
NCV = GCV – Heat of steam formed during combustion.
Ash
Fusion Test:
This is determined to understand the coals suitability to the boiler and maintain the efficiency of the boiler. At high temperatures, the heat from burning coal is often sufficient to melt its own ash which, when cooled solidifies into clinker. In industrial coal burning application, clinkers can be extremely large and can cause both disposal problem and operation downtime. Coal Ash Fusion testing is therefore required to determine the ash fusion ability characteristics of the fuel.
The temperature at which the Ash starting to melt and forms clinker is called Ash fusion temperature.
Ignition temperature: The temperature, above which a self sustained oxidation reaction between fuel and oxident is possible, is called ignition temperature
Auto ignition temperature: it is the temperature at which coal spontaneously ignites in air.
Low ranking coal - 430 to 475 K
Higher ranking coal - 475 to 500 K
Ash softening temperature: it is the temperature at which the Ash softens and become plastic. This is somewhat below the melting point of Ash. If the Furnace temperature is higher than Ash softening temperature, all the ash will melt and would come out of the furnace bottom continuously as a molten slag.
9.
Hardgrave
Grindability Index:
It indicates its easiness towards pulverization or power consumption for pulverization of coal. This was developed as an empirical test to indicate how difficult it would be to grind a specific coal to the particle size necessary for effective combustion in a pulverized coal fired boiler or blast furnace. In case of coking coal we analyze the sample of Crucible Swelling Number (CSN).
G = 13 + 6.93 W
W = Grams of coal passing through 200 mesh after 50 gm of coal size 16-30 mesh are ground in standard mill for 60 revolution.
Normal value - 45 to 55
Average value of Indian coal - 50 to 60
HGI = 105 - R(1.16 + 0.002R) - 0.04A
R = VM% + M% and A = Ash% (from proximate analysis of coal)
Low HGI => Difficult in coal pulverization.
High HGI => High fines, difficult in handling in rainy season, high VM, high risk of fire during storage.
10.
Plastic
properties:
When coal is heated, it passes through a transient stage which is called as plastic state (caking). If a particular coal does not pass through a plastic state, it is called sintered mass (non-coking). Plastic properties of coal are determined by caking index test, free swelling test, GKLT, Plastometer etc.
11.
Caking
Index:
It is the measure of binding or agglutinating property of coking coal. Coal with ash higher than 17% must be washed before testing.
12.
Crucible
Swelling Number:
One of the most common, simple caking tests finely crushed coal is heated rapidly in a crucible and the coke button obtained is compared with a series of standard profiles to give a number, which is the crucible swelling number (CSN).
Values range from
0 to 9 - no caking characteristics at all
9+ - superior coking properties
Coal having free swelling index between 4 - 5 are taken for coke making.
Grindability of coal: coefficient of grindability is the ratio of unit energy consumption of a standard laboratory mill in grinding reference solid fuel and the fuel under consideration, provide that both have same initial particle size and same ground dust characteristics.
Kgr = Er/E
Low grindability of fuel means it consume more energy to grind.
Fineness of coal: fineness increases the surface area per unit mass of solid fuel. Since combustion is a surface reaction, greater the extent of PF surface availability, higher will be the rate of combustion.
| Impact of Coal quality variation on Boiler | ||
| Coal Property | New Boiler Design and selection | Existing Boiler |
1 | High Moisture in coal | Increase in furnace size and volume, | Difficult to maintain steam and reheat temp. |
Large Pulveriser size | High desuperheating flow | ||
High power availability | Low Boiler efficiency | ||
Low Boiler efficiency | | ||
Higher fuel requirement per MW of power generation | | ||
2 | High Ash content in fuel | Increase in furnace size and volume, | Increase in erosion of pressure parts |
Large Pulveriser size | Slagging and fouling of pressure parts | ||
High power availability | Poor ESP performance | ||
Higher fuel requirement per MW of power generation | Overloading of draft fans | ||
3 | Low Volatile Matter | Special type down-shot burners are required. | Issues in maintaining flame stability. |
Boiler turn down ratio | | ||
Fuel residence time in furnace. | | ||
4 | Low Calorific value | Size and design of boiler, mills, ESP, draft fans. Air and flue gas ducting size. | Additional coal consumption |
Fuel handling and storage system. | Higher heat transfer in convective zone. | ||
Ash handling and storage system. | High desuperheating flow | ||
Aux. power consumption. | Low efficiency of boiler, mills and ESP. | ||
| Erosion of pressure parts and draft fans. | ||
5 | Low Hard Grove Index | High equipment cost | Frequent mill maintenance and downtime. |
| Impact on Boiler efficiency and performance. | ||
6 | Fuel constituents - C, H, S & N | It helps in calculating stoichiometric air and excess air requirement for efficient combustion. | Effects combustion process |
7 | Sulphur Content | | Affects clinkering and slagging tendencies |
| | | Corrodes chimney and other equipment such as air heaters and economizers |
| | | Limits exit flue gas temperature |
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