CALORIFIC VALUE OF FUELS Sr Fuel Approx heating value Kcal/Kg Natural State Dry state A BIOMASS 1 Wood 1500 3500 2 Cattle dung 1000 3700 3 Bagasse 2200 4400 4 Wheat and rice straw 2400 2500 5 Cane trash, rice husk, leaves and vegetable wastes 3000 3000 6 Coconut husks, dry grass and crop residues 3500 3500 7 Groundnut shells 4000 4000 8 Coffee and oil palm husks 4200 4200 9 Cotton husks 4400 4400 10 Peat 6500 6500 B FOSSIL FUELS 1 Coal 4000-7000 2 Coke 6500 3 Charcoal 7000 4 Carbon 8000 5 Fuel oil 9800 6 Kerosene and diesel 10000 7 Petrol 10800 8 Paraffin 10500 9 Natural gas 8600 10 Coal gas 4000 11 Electrical (Kcal(KW) 860 12 Bio gas(Kcal/cu mtr) (12 kg of dung  produces 1 cu. Mtr gas) 4700-6000

Calorific Value of Coal

The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight (or volume in the case of a sample of gaseous fuels) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 degree K.

It is usually expressed in Gross Calorific Value (GCV) or Higher Heating Value (HHV) and Net Calorific Value (NCV) or Lower Calorific Value (LHV). The difference being the latent heat of condensation of the water vapor produced during the combustion process. Gross calorific value assumes all vapor produced during the combustion process is fully condensed. Net calorific value assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value. The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal while calorific value of fuel oils are much more consistent.

Energy content of the Indian Coal is expressed in “Useful Heating Value” (UHV) basis. Indian coal (non-coking) is classified by grades (A-G) defined on the basis of Useful Heat Value (UHV). UHV is an expression derived from ash and moisture contents for non-cocking coals as per the Government of India notification. UHV is defined by the formula:

UHV kcal/kg = (8900-138×[percentage of ash content +percentage of moisture content])

In the case of coal having moisture less than 2% and volatile content less than 19%, the UHV shall be the value arrived as above, reduced by 150 kcal/kg for each 1% reduction in volatile content below 19% fraction pro-rata. Both moisture and ash shall be determined after equilibrating at 60% relative humidity and 40°C temperature as per relevant clauses of the Indian Standard Specification No. IS:1350-1959.

The quality of coal depends upon its rank and grade. The coal rank arranged in an ascending order of carbon contents is Lignite → sub-bituminous coal → bituminous coal → anthracite

Indian coal is of mostly sub-bituminous rank, followed by bituminous and lignite (brown coal). The ash content in Indian coal ranges from 35% to 50%.

Chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analysis. The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon. Elemental or Ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen. The calorific value Q, of coal is the heat liberated by its complete combustion with oxygen. Q is a complex function of the elemental composition of the coal. Gross Calorific value Q is mostly determined by experimental measurements. A close estimate can be made with the Dulong formula:

Q = (144.4 %[C])+(610.2 %[H])-(65.9 %[O])+(0.39 %[O]2)
Q is given in kcal/kg or Btu/lb.

Values of the elements C,H, and O, are calculated on a dry ash-free coal basis. Empirical Relationship of GCV, UHV, and NCV UHV:

Useful heat value = 8900 – 138(A+M)
GCV: Gross Calorific Value = (UHV + 3645 -75.4 M)/1.466NCV: Net Calorific Value = GCV – 10.02M

UHV, GCV, NCV in kcal/kg, “A” is %age Ash; “M” is %age Moisture.

Non-coking Coals produced in all states other than Assam, Arunachal Pradesh, Meghalaya, Nagaland are graded on the basis of Useful heat value(UHV) in kcal/kg from Grade:

A (>6200)
B (5600-6200)
C (4940-5600)
D (4200-4940)
E (3360-4200)
F (2400-3360)
G (1300-2400)

Coal from Assam, Arunachal Pradesh, Meghalaya, Nagaland are not graded. Coking coal used for steel industry grade-I (ash<15%) and grade-II (ash 15-18%). Further classification for semi-coking, weakly coking coal is done on ash %age and UHV.

The origin of Indian coal is through drift theory, as a result of which the coal matter is intimately mixed with mineral matter causing deterioration in its quality.

Coal of most of the other coal producing countries originates through insitu theory in which the possibility of deterioration of the quality is far less during its formation stage.

Indian coal due to its origin has some inherent ash content and some extraneous ash content. The inherent ash cannot be taken away because it is embedded in the coal in such a fine manner that you just cannot take it off. Extraneous ash can be taken care of by washing. The wash-ability curve shows that to reduce ash below a certain limit, there is too much of rejects in it and each percentage of ash reduction in the coal will cost lot of money.

It is totally dependent on the economics. Due to this particular fact, the wash-ability characteristics of Indian coal the washing becomes, at times, prohibitive, vis-à-vis cost. Coal is mostly beneficiated before dispatches in most of the countries abroad, which results in consistent quality of the product.

In the GCV system of grading of non-coking coal, it is possible to determine the exact value of non-coking coal grades supplied to consumers whereas in the existing UHV system, the heat value cannot be determined directly but computed by using an empirical formula based on ash and moisture content. The band variation in GCV grades of non-coking coal is narrower than the existing variation of heat value in the UHV system.

The average GCV of total coal supplied to different sectors including power sectors during the past few years has been of the order of 4900 kcal/kg. This is far below the GCV of imported coal which often exceeds 6000 kcal/kg. Existing system of grading of non-coking coal on the basis of Useful Heat Value (UHV) to Gross Calorific Value (GCV) is under review by government.

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Coal GCV Calculation

These are the different formula to calculate GCV, UHV of coals from Proximate analysis.

GCV =8555.555 - [(145.5 x Moist.) + (94.1 x Ash)]
GCV =7200 - (Ash x 82.5)
GCV =(F.C. + 10) x 100

UHV =8900 - 138(A+M)
GCV = (UHV+3645-75.4 X M)/1.466

GCV = 91.67 X F.C. +75.56(VM - 0.1 X ASH) - 60 X M

ASH BY ADB = (ASH/100+IM)/100

GCV BY ADB = [85.56x(100-IM-1.1XAsh)-60xIM]

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WHRB Boiler Steam Generation From 1 X 100 TPD Sponge Iron Plant

Proximate and Ultimate Analysis Of Coal

Ultimate Analysis Of Coal

The "ultimate" analysis" gives the composition of the biomass in wt% of carbon, hydrogen and oxygen (the major components) as well as sulfur and nitrogen (if any). The carbon determination includes that present in the organic coal substance and any originally present as mineral carbonate. The hydrogen determination includes that in the organic materials in coal and in all water associated with the coal. All nitrogen determined is assumed to be part of the organic materials in coal. 

Proximate Analysis Of Coal

The "proximate" analysis gives moisture content, volatile content,consisting of gases and vapors driven off during pyrolysis (when heated to 950 C), the fixed carbon and the ash,the inorganic residue remaining after combustion in the sample and the high heating value (HHV) based on the complete combustion of the sample to carbon dioxide and liquid water. Proximate analysis is the most often used analysis for characterizing coals in connection with their utilization.

Energy Balance in a conventional Rotary KILN Sponge Iron Plant

Direct-reduced iron (DRI) or Sponge Iron

Direct-reduced iron (DRI), also called sponge iron, is produced from direct reduction of iron ore (in the form of lumps, pellets or fines) by a reducing gas produced from natural gas or coal. The reducing gas is a mixture majority of hydrogen (H₂) and carbon monoxide (CO) which acts as reducing agent. This process of directly reducing the iron ore in solid form by reducing gases is called direct reduction.

Direct reduction, an alternative route of iron making, has been developed to overcome some of these difficulties of conventional blast furnaces. DRI is successfully manufactured in various parts of the world through either natural gas or coal-based technology. Iron ore is reduced in solid state at 800 to 1,050 °C (1,472 to 1,922 °F) either by reducing gas (H₂+CO) or coal. The specific investment and operating costs of direct reduction plants are low compared to integrated steel plants and are more suitable for many developing countries where supplies of coking coal are limited.

The most frequently used reduction gases are CO and H2.

Reduction with CO: When the initial state is hematite, and the temperature is over 570°C, reduction of

iron oxide will occur in three steps:

Ι ΙΙ ΙΙΙ

Fe2O3 ⇒Fe3O4 ⇒ FeO ⇒ Fe

3Fe2O3 + CO ⇒ 2Fe3O4 + CO2 ΔH = -7.8 kJ /molFe

Fe3O4 + CO ⇒3FeO + CO ΔH = +11.2 kJ/ molFe

FeO+CO ⇒Fe + CO ΔH = -15.7 kJ /molFe

Summing these reactions gives reduction of hematite

to iron:

Fe2O3 + 3CO ⇒ 2Fe + 3CO2 ΔH = -13.3kJ /molFe

Reduction with H2: This reduction is similar to the

one with CO. It occurs in three steps when temperatures are over 570°C and two steps when the temperature is below 570°C:

3Fe2O3 + H2 ⇒ 2Fe3 O4 + H2O ΔH = -1.0 kJ /molFe

Fe3O4 + H2 ⇒ 3FeO + H2O ΔH = +24.9 kJ molFe

FeO + H2 ⇒ Fe + H2O ΔH = +25.4 kJ molFe

Fe2O3 + 3H 2 ⇒ Fe + 3H2O ΔH =+49.4 kJ molFe

From The above Reactions, we can calculate the carbon/Hydrogen required for reduction or can calculate required C/Fe.

Coal Burning : Effect of excess air or Less air

Coal Burning : Effect of excess air or Less air

Quantity of air required in burning of the cole is most important. By this we can save the energy losses in coal burning.

As in manufacturing of sponge iron coal is used in different size, and it is required that ever part of the coal should come in air contact for proper combustion. (In case of burning of un-burnt coal in ABC.)  

CASE - I (Air is Less than required)

•  In this case some part of coal goes out with out burning which is a loss of potential energy, which is known as un-burnt carbon loss. 
• in the normal combustion reaction Carbon and Oxygen combine to form Carbon dioxide giving out heat. Actually this takes place in two stages , first is the formation of Carbon monoxide and then the Carbon monoxide reacts to form Carbon-dioxide. If the air is less the second stage does not take place, and Carbon monoxide is formed. This means a part of the potential energy of the coal is not released. This is an energy loss.

CASE - II (Air is More than required)

If the air supplied for combustion is more than required, the air that is in excess of the combustion requirement does not contribute to the combustion process , but takes away a part of the combustion heat then goes up the stack as waste heat. This is an energy loss. This is called the Dry Gas loss.

Burning Coal : How much Air is required ?

Written by: johnzactruba • Edited by: Lamar Stonecypher
Updated Oct 20, 2009 • Related Guides: Carbon

The most important requirement to burn coal is the correct quantity of air. This article explains how to calculate the correct quantity of air .

Coal is the most widely used fuel source used to produce Electricity. Air provides the necessary Oxygen for burning coal. It is the chemical reaction between Carbon in coal and the Oxygen in the air that produces the heat energy.

Elements in Coal

There are 3 elements in coal that combine with Oxygen in the air duing the combustion process. The main and important element is Carbon which could be around 30% - 60 % .Then there is Hydrogen in the range of 1 % to 3 % and Sulphur in the range of 0.3 % to 3 % . Apart from this three there is Nitrogen 1% to 2 % and Oxygen from 5 % to 12 % . The Oxygen is also used up in the Combustion process. The exact amounts of each element depends on the type and rank of coal, how the coal was formed millions of years ago and the location or mines from which the coal is sourced.

Stoichiometric or theoretical air quantity.

This is calculated based on the chemical reaction between the elements and oxygen.

• Carbon combines with Oxygen to form Carbon-dioxide and heat.

C+ O₂ > CO₂

1 kg Carbon +2.67 kg Oxygen > 3.67 kg Carbon Dioxide1C+32/12 O > 44/12 CO₂

• Hydrogen Combines with Oxygen to form Water and heat

2 H₂+ O₂> 2H₂O

1 H +32/4 O >36/4 H₂O

1 kg Hydrogen +8 kg Oxygen >9 kg Water

• Sulphur Combines with Oxygen to form Suphur Dioxide

S + O₂ > SO₂

1 S +32 /32 O >64 /32 SO₂

1 kg Sulphur +1 kg Oxygen >2 kg Sulphur Dioxide.

For the purpose of calculation we consider a coal having 57.2 % Carbon, 2.2 % Hydrogen , 0.5 % Sulphur and 6.9 % Oxygen.

• The theoretical Oxygen required to burn this coal is then

2.67 x C % + 8 x H% + 1 x S % - O% = 1.64 kg of Oxygen for 1 kg of Coal.

Air contains 23.2 % by weight of Oxygen.

• The theoretical Air required to burn the coal is

= 1.64 / 23.2% = 7.1 kg of Air for 1 kg of Coal.

This is the theoretical Air required to burn the coal.

Quick Calculation of Theoretical Air.

The Heating value of Coal also depends on the elemental Carbon and Hydrogen. This means that the air required and the heating value have an almost fixed relationship. The theoretical air required for a unit heating value is practically a fixed value. This is around 0.332 kg of air for one MJ of heat input. This is true for a wide range of coals used in power plants.

By knowing the calorific value of coal , the theoretical air quantity can be directly calculated using this factor.

Composition of dry atmosphere, by volume

ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, )

GasVolume
Nitrogen (N₂)                        780,840 ppmv (78.084%)
Oxygen (O₂)                          209,460 ppmv (20.946%)
Argon (Ar)9,                          340 ppmv (0.9340%)
Carbon dioxide (CO₂)           390 ppmv (0.039%)
Neon (Ne)                            18.18 ppmv (0.001818%)
Helium (He)                           5.24 ppmv (0.000524%)
Methane (CH₄)                     1.79 ppmv (0.000179%)
Krypton (Kr)                        1.14 ppmv (0.000114%)
Hydrogen (H₂)                      0.55 ppmv (0.000055%)
Nitrous oxide (N₂O)             0.3 ppmv (0.00003%)
Carbon monoxide (CO)        0.1 ppmv (0.00001%)
Xenon (Xe)                           0.09 ppmv (9×10⁻⁶%) (0.000009%)
Ozone (O₃)                           0.0 to 0.07 ppmv (0 to 7×10⁻⁶%)
Nitrogen dioxide (NO₂)         0.02 ppmv (2×10⁻⁶%) (0.000002%)
Iodine (I₂)                              0.01 ppmv (1×10⁻⁶%) (0.000001%)
Ammonia (NH₃)                     trace


Not included in above dry atmosphere:
Water vapor (H₂O)                ~0.40% over full atmosphere, typically 1%-4% at surface