In general high moisture content in coal is undesirable because
It reduces the calorific value of the fuel
It increases the consumption of coal for heating purposes
It lengthens the time of heating
We pay for it when the coal is purchased and transported.
When Moisture is Required?
Dust nuissance is checked while loading, unloading, transporting and charging coal into the oven when it is wet.
moisture undergoes endothermic decomposition which reduces the temperature of the hot coal bed thereby eliminating its fusion and clinker formation which otherwise would have blocked the passage of air.
Its helps in protecting the methane and other hydrocarbons present in the coke oven gas from cracking in presence of hot coke and hot oven walls
How Moisture Comes in Coal?
Coal/coke absorbs moisture due to:
2- washing of Coal
3- Quenching of coke
The amount of moisture depends upon the
nature, origin, and occurrence. There are two types of moistures present in coals
1- Accidental/free Moisture (handling)
2- Inherent/air dried moisture (equilibrium)
Methods to find Moisture
Air-dried moisture is found by weight loss of coal by heating up to 105 o C
In case of peat and lignite which starts decomposing at low temperature, above method is not used.
For these cases, Dean and Stark method is used. Here the coal is heated in excess of toluene and water distillates is received in separate arrangements. Here it can be weighted.
Air-dried water contents decreased with increasing rank of coal up to 0.5 % for a low volatile bituminous coal.
Near Saturation/ capacity Moisture
If coal is exposed to an atmosphere of increasing humidity at a fixed temperature, it absorbs more and more moisture.
The quantity of moisture held by coal at an approximately saturated atmosphere (96-99% relative humidity) is called near-saturation moisture or capacity moisture or bed moisture
This is a better measure than the air-dried moisture for determination of rank of high volatile bituminous, sub-bituminous and lignitic coals and serves as a parameter in some coal classification systems.
Certain gases like CO, CO2, CH4, H2, N2, O2, hydrocarbons etc
They come out during its heating
The coal with higher volatile matter content:
ignites easily i.e. it has lower ignition temperature
burns with long smoky yellow flame
has lower calorific value
will give more quantity of coke oven gas when it is heated in absence of air
will require larger furnace volume for its combustion
has a higher tendency of catching fire (due to low temperature exothermic oxidation) when stored in open space.
It does not include moisture of coal but it contains water that is formed from the hydrogen and oxygen of coal during composition.
Air-dried (d.a.f) – includes part of mineral matters which escapes into gaseous and vaporous state.
Dried Mineral Matter (D.m.m.f) basis – only include volatile products from organic mass
Higher the VM lower the fixed carbon
With increasing maturity of coal its Volatile matters decreases
It is the combustion product of mineral matters presents in the coal
It comprises mainly of silica (SiO 2 ), alumina (Al 2 O 3 ) and ferric oxide with varying amounts of other oxides such as CaO, MgO, Na 2 O etc .
High ash content in coal is undesirable in general
A coal with high ash content:
is harder and stronger
has lower calorific value
produces more slag (impurities) in the blast furnace when coke made out of it is used therein.
Ash content of the coal is reduced by its washing.
Coal contains inorganic mineral substances which are converted into ash by chemical reactions.
The mineral matter in coals are inherent / extraneous
The inorganic material of the original vegetable substances is responsible for the inherent mineral matter.
The extraneous mineral matter is due to:
1- Substances associated during decaying of vegetable
2- Rocks and dirt mixed during handling (Milling, Mining)
Inherent mineral matters can’t be removed through mechanical methods but extraneous can be.
It can restrict passage of air flow in furnace grates
Lower rate of combustion
High ash leads to large heat losses, carbon losses boiler deposits and clinkering.
1% rise in coal ash is equivalent to 0.3-0.4% fall in boiler efficiency.
High ash is therefore undesirable and harmful.
Clinkering troubles are due to the lower fusion temperatures.
Determination of Ash
Ash of coal is directly determined in the laboratory by completely burning the coal .
mineral matter content from the ash values are determined by Parr formula is :
MM = 1.08 A + 0.55 S
Difference in the weights of mineral matter and ash is due to the water of hydration which is taken to be 10% of the ash value (0.1 A).
Fixed Carbon (FC)
It is the pure carbon present in the coal
Higher the fixed carbon content of the coal, higher will be its calorific value .
In coals fixed carbon is less than total carbon except anthracite because of lower VM.
Carbon is determined by completely burning coal in presence of pure oxygen and finding the amount of resultant carbon dioxide (Liebig's method)
Carbon increases with increasing of rank of coal
It means the fixed carbon plus the carbon present in the volatile matters e.g. in CO, CO 2 , CH 4 , hydrocarbons etc .
Total carbon is always more than fixed carbon in any coal .
High total carbon containing coal will have higher calorific value .
It increases the calorific value of the coal.
It is associated with the volatile matter of the coal. Also, the percentage of NH 3 in the coke oven gas is more if the coal from which it has come out contains more H 2 .
This NH 3 is recovered as (NH 4 ) 2 SO 4 (by reacting NH 3 with H 2 SO 4 ) which is a valuable fertiliser.
Part of the hydrogen of coal remains as such in the coke oven gas thereby increasing its calorific value.
From peat to the bituminous stage it varies between 4.5 and 6.5% and is not related to the rank of coals
The nitrogen in coal is present up to 1-3%.
It comes from the proteinous matter present in vegetable matter.
Presence of inert nitrogen decreases the calorific value of the coal.
However when coal is carbonised (i.e. heated in absence of air), its N 2 and H 2 combine thereby producing NH 3 which is recovered as NH 4 ) 2 SO 4 , a valuable fertiliser.
It does not bear any relation to the rank of coal
Though its presence (mainly as sulphides) increases the calorific value of the coal.
it has several undesirable effects. The oxidation product of sulphur (e.g. SO2, SO3 etc.) especially in presence of moisture cause corrosion damage of the equipment and cause atmospheric pollution.
Sulphur is highly undesirable in metallurgical coal used in iron and steel making.
It causes cracking of steel surface during hot rolling*.
Sulphur is commonly present in coal in three forms, viz. pyritic, organic and sulphate.
The sulphur content of coal has no relation to its rank or composition .
The less the oxygen content, the better is the coal as it reduces its calorific value.
It decreases from lignite to anthracite as the maturity of the coal increases.
As the oxygen content of the coal increases, its moisture holding capacity increases and the caking power decreases.
The oxygen content of coal has a close relation to the rank.
It decreases with the increase in the rank of coal
It is undesirable in metallurgical coal.
It badly affects the properties of the metal and causes cold shortness of steel i.e. cracking of steel surface during cold rolling.
Phosphorus occurs in small quantities in coal. phosphorus which offers no problem for common use in iron and steel production.
It is present mainly as chlorides in the coal.
It reduces the fusion point of the ash of the coal.
It is substantially removed during washing of the coal.
Uses of Coals
Coal is used for :
Directly burning and getting heat
Generating steam for producing electricity (nearly 1 Ib of coal is burnt to generate of 1kWh of electricity)
Driving railway locomotives
Manufacturing coke and coal gas (e.g. coke oven gas, producer gas, water gas etc)
Manufacturing synthetic liquid fuels
Gasification to produce nitrogenous fertiliser from synthesis gas.
Non-fuel uses of bituminous and lower rank coals
Production of activated carbon used for decolouration of glycerine and petroleum products.
Treatment with strong sulphuric acid forms a basic exchange agent used in water treatment process.
Used as pigments for paints.
Powdered coal is used in dynamite.
Used as filter for various purposes due to its lightness, chemical inertness and low cost.
Carbonisation of bituminous coal produces chemicals.
Used for making carbon brushes.
Used for burning in domestic oven.
Uses of peat & Lignite
Peat can be carbonised to produce gas and coke.
It is also briquetted and used as a domestic fuel.
Uses of lignite includes:
For manufacture of producer gas.
For generation of electrical power.
For gasification to produce nitrogenous fertiliser.
Uses of Anthracite Coal
It is used
for recarbonising steel.
for making carbon electrodes, brushes, battery parts, resistors, carbon refractory, corrosion, resisting structural materials.
as filter and paint pigment
for blending with coking coal to check its swelling and improving the coke quality.
Manufacture of Metallurgical Coke
Coke is manufactured by carbonization of coal.
Carbonisation. Heating of coal in absence of air to produce coke is called its carbonisation or destructive distillation . It give a solid residue (Coke).