Coal Constituents Significance
Date: 11/10/2008
Moisture 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: 1 Raining 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
Airdried moisture is found by weight loss of coal by heating up to 105oC 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. Airdried 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 (9699% relative humidity) is called near-saturation moisture or capacity moisture or bed moisture
This is a better measure than the airdried moisture for determination of rank of high volatile bituminous, subbituminous and lignitic coals and serves as a parameter in some coal classification systems.
Volatile Matters
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.
VM Contd…
It does not include moisture of coal but it contains water that is formed from the hydrogen and oxygen of coal during composition. Airdried (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
Ash It is the combustion product of mineral matters presents in the coal It comprises mainly of silica (SiO2), alumina (Al2O3) and ferric oxide with varying amounts of other oxides such as CaO, MgO, Na2O 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 Contd…
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.
Ash Problems
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 0.30.4% 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
Total Carbon
It means the fixed carbon plus the carbon present in the volatile matters e.g. in CO, CO2, CH4, hydrocarbons etc.
Total carbon is always more than fixed carbon in any coal.
High total carbon containing coal will have higher calorific value.
Hydrogen
It increases the calorific value of the coal. It is associated with the volatile matter of the coal. Also, the percentage of NH3 in the coke oven gas is more if the coal from which it has come out contains more H2. This NH3 is recovered as (NH4)2 SO4 (by reacting NH3 with H2SO4) 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
Nitrogen
The nitrogen in coal is present up to 13%. 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 N2 and H2 combine thereby producing NH3 which is recovered as NH4)2 SO4 , a valuable fertiliser. It does not bear any relation to the rank of coal
Sulphur
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.
Oxygen
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
Phosphorus
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.
Chlorine
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). Liquid (tar, benzol, naphthalene, pheni anthracene ). Gaseous (coke oven gas) Caking power is: The ability to form coke The ability to form a hard coherent mass on heating It varies from zero (lignite) to maximum with coal having about 88% carbon. It further starts decreasing to zero as the carbon content increases further to 93%. Caking coal should have carbon in the range of 83 – 90% C.
Types of Carbonization
There are mainly two types of carbonisation depending upon the temperature up to which the coal is heated in absence of air namely:
Low temperature carbonisation (LTC)
High temperature carbonisation (HTC).
Low Temperature Carbonization (LTC)
It is carried out at 700°C. It produces semicoke which is used as a smokeless domestic fuel. It can sometimes be used in boiler also to avoid smoke. Yield of coke oven gas is less in LTC. Yield of tar' is high in low temp, carbonisation. It is about 10% of dry coal. Ammonia yield is low. Calorific value of coke oven gas produced in LTC is more due to higher percentage of methane and unsaturated hydrocarbons in it. C.V. is about 6000 6500 kcal/Nm3
LTC Contd…
The tar produced is aliphatic in nature & contains less quantity of aromatic ring. After carbonisation, discharging of coke is difficult as it swells a lot . Free carbon in tar is less . It is about 5 – 10% of tar. Coke produced is weaker (due to less shrinkage), bigger in size and more reactive. Volatile matter content in coke is more (57%) hence it is easier to ignite Ignition temperature of LTC coke is about 425°C. Hydrogen content in coke oven gas is less (3540%). Coke yield is more. It is about 80% of dry coal.
High Temperature Carbonization (HTC)
It is carried out at 1100°C. It produces metallurgical coke for use in blast furnace and cupolas in foundry etc. Yield of coke oven gas is more in HTC due to more cracking of hydrocarbons Tar yield is less here. It is about 3% of dry coal charged. Ammonia yield is more (1015 gm/Nm3 of coke oven gas). C.V. of coke oven gas produced in H.T.C. is less. It is about 42004400 kcal/Nm3
HTC Contd…
Tar produced has more of aromatic ring compounds, (due to crystallisation reaction of straight chain compounds' being favoured at higher temperature). Discharging of coke is easier as it shrinks finally. Free carbon in tar is more (due to more intense cracking of hydrocarbon at higher temperature. It is about 1520% of tar. Coke produced is stronger. V.M. in coke is less (12%), hence its ignition temperature is more. Ignition temperature of high temp, coke is about 605°C. H2 content in coke oven gas is more (5560%). Coke yield is less (about 77% of dry coal).
Process
Coke is produced by the horizontal retort process. The Beehive type ovens which were used until recently are obsolete nowadays. The modern byproduct ovens are made of rectangular chambers, length 10 12 m , height 4 m and width 1 m and closed at both ends. Charging is performed from one end and the coke is discharged from the other. Number of such small units are made and the whole unit is called coke oven battery. Older plants had horizontal passage for flue gases but these have vertical arrangements. It is found that rate of heating is more uniform in this case.
Process Contd…
Heat from outgoing gases is recovered by passing them through regenerators. The chambers are heated by the mixture of blast furnace gas and air, preheated in the regenerators. The chambers are lined with silica bricks Carbonization temperature is about 1200 – 1400 oC. Carbonization processes takes about 72 – 96 hours
Mechanism of Coking
As there is no air present for combustion, the coal will decompose partly. VM in coal will be expelled in the form of various gases like: CH4, C2H4, C6H6, H2, NH3 etc. The remaining residue contains fixed carbon and ash. If some air is supplied, but less than required for complete combustion then, VM will be expelled as above but some combustion of VM and FC will yield carbon monoxide. What ever the amount of N2 will form ammonia.
PhysicoChemical Changes in Coke
A) Raw Coke B) plastic layer C) Semi Coke D) High temperature coke E) Median Crack in Coke