5. Coal Chemicals
(Various processes for obtaining coal chemicals, coal tar distillation, F-T and
Bergious processes for hydrocarbon production)
Unit 1
Chemical Technology (BCT-26)
6. industries. 24
Coal
✓ Coal is a organic and combustible black or brownish-black sedimentary rock, formed as rock strata
called coal seams. Coal is the largest source of energy all over the world.
✓ Coals are typical heavy carbon resources with much higher contents of carbon, oxygen, sulfur, and
nitrogen than other heavy carbon resources such as heavy petroleum, natural pitch, oil shale, and oil
sand.
✓ Coal can serve a potential source of synthetic fuel, a source of power production, coke production
and large number of chemicals which are now being derived from petroleum and natural gas.
✓ Coal is composed of chiefly carbon and other like hydrogen, sulphur, oxygen, nitrogen, moisture and
noncombustible inorganic matter containing, silica, iron, calcium, magnesium, mercury etc.
✓ Some of the recent development in utilization of coal are coal gasification, coal to synthesis gas, coal
o oil through FT synthesis, coal to methanol, coal to plastic alternate feed stock for chemical
7. Coal: Environmental perspective
25
✓ At least because of their higher contents of carbon, oxygen, sulfur, and nitrogen, coals should not
mainly be used as energy.
✓ Clean-coal technology (CCT) was invented; that have led to more efficient combustion of coal with
reduced emissions of sulfur dioxide and nitrogen oxide.
✓ CCT based Power plants being built today emit 90 percent less pollutants (SO2, NOx, particulates and
mercury) than the plants they replace from the 1970s, according the National Energy Technology
Laboratory, USA (NETL). Regulated emissions from coal-based electricity generation have
decreased overall by over 40 percent since the 1970s, while coal use has tripled.
✓ More or less nitrogen and sulfur can be removed before/during/after coal combustion using CCT,
economical removal of CO2 emitted from coal combustion is not feasible.
✓ Thus, the so-called CCT does not facilitate low-carbon utilization of coals.
8. Coal: Origin and presence
26
✓ Coal is formed when dead plant matter (deeply buried for over millions of years) decays under the
conditions of anaerobic decay, in the so-called biochemical stage of coal formation, a carbon-rich
material called peat was formed.
✓ In the subsequent geochemical stage, the different time-temperature histories led to the formation of
coals of widely differing properties.
✓ Vast deposits of coal originate in former wetlands—called coal forests—that covered much of the
Earth's tropical land areas during the late Carboniferous (Pennsylvanian) and Permian times.
✓ Most of the world's coal exists in the northern hemisphere. The United States, former Soviet Union
and China together possess more than 80% of the ultimately recoverable resources.
✓ Though, the exact quantity of coal resources is not known.
9. 27
Coal: Presence
✓ COAL RESOURCES: Category of coal resources are based on degree of assurance i.e
(i) Coal reserves- proved, indicated, inferred or (ii) Depth range
Proved reserve are those resources which has been reliably estimated and can be recovered
economically. Indicated reserves is the coal resource which is based on combination of direct
measurement and reasonable geological assumptions. Inferred coal resource is based on the assumed
continuity of coal beds. Depth range determines the economy of extraction and a cope of exploration.
✓ Worlds proven coal reserves are estimated at about 860 billion tones which is expected to last up to
120 years at the current level of production.
✓ Largest coal reserves of coal are in China, USA, Russia, Australia and India.
✓ Till 2018, As a result of exploration carried out up to the maximum depth of 1200 m, a cumulative
total of 319.02 Billion tonnes of Geological Resources of Coal have so far been estimated in India.
10. Coal: Presence in India
28
✓ Till 2018, As a result of exploration carried out up to the maximum depth of 1200m, a cumulative
total of 319.02 Billion tonnes of Geological Resources of Coal have so far been estimated in India.
✓ Coal deposits are chiefly located in Jharkhand, Orissa, Chhattisgarh, West Bengal, Madhya Pradesh,
Andhra Pradesh and Maharashtra.
✓ The Coal resources of India are available in older Gondwana Formations of peninsular India and
younger Tertiary formations of north-eastern region.
13. 31
Coal: Types
✓ Coal are classified into various grades based on the composition, calorific value and degree of
coalification that has occurred during its formation.
14. 32
(1 BTU/lb = 2.326 kJ/kg = 0.556 kC/kg)
Coal: Types and properties
▪ The global coal reserves consistof
53% anthracite and bituminous
coals, 30% sub-bituminous and
17% lignite [BP statistics 2011,
EIA, US Departments of Energy].
✓ Further, Coal may also classified
as hard or soft coal, low sulphur or
high sulphur coal.
✓ Coal may be also classified in rock
types based on petro logical
components known as maceral.
✓ Based on maceral content coal
may be classified as clarain,
durain, fusain and vitrain.
15. 33
Coal: Analysis
➢ Coal Analysis techniques are specific analytical methods designed to measure the particular physical
and chemical properties of coal.
➢ There are two methods to analyze coal i.e, the proximate analysis and the ultimate analysis.
➢ Carbon and hydrogen are the principal combustible elements in coal. On a weight basis, carbon is the
predominant one. It constitutes about 60% to about 95% of the total.
➢ For most coals of 90% or less carbon, hydrogen content is generally in the range of 5%; it drops to
about 2% for coals having 95% carbon.
➢ Nitrogen content of almost all coals is in the range of 1-2%.
➢ Sulfur content of coals is seen to be quite variable.
➢ Oxygen content is nearly inversely proportional to Carbon content of coal. The more oxygen a coal
contains, the easier it is to start to burn it, or to achieve its ignition.
16. Coal: Proximate Analysis
34
➢ The proximate analysis determines only the fixed carbon, volatile matter, moisture and ash
percentages. Can be done with simple analysis equipment.
➢ Proximate analysis involves the following determinations in terms of percentage by weight:
1. Moisture means the water expelled from the fuel, lesser the moisture content better is the quality
of fuel;
1 g of finely powdered coal, taken in a crucible, is heated in an electric oven at 105-110 0C for
l hour, cooled in a dessicator and weighed Percentage moisture content can be calculated from the
loss of weight
17. Coal: Proximate Analysis
35
➢ Proximate analysis involves the following determinations in terms of percentage by weight:
2. Volatile matter: A high volatile matter containing coal burns with a long flame, high smoke
and has low calorific value.
The dried sample of coal left in the crucible is then covered with a lid and is placed in a muffle
furnace at ~925oC for exactly seven minutes The crucible is cooled first in air, then in the
dessicator and weighed The loss in weight is due to volatile matter which is calculated as
18. Coal: Proximate Analysis
36
➢ Proximate analysis involves the following determinations in terms of percentage by weight:
3. Ash is the inorganic residue left when the fuel is completely burnt in air under specified
conditions.
It is the residue obtained after burning of the coal in a muffle furnace at 700-7500C for half an
hour till a constant weight is obtained.
4. Fixed carbon: It is essentially carbon containing minor amounts of nitrogen, sulphur, oxygen
and hydrogen.
It is determined indirectly by deducting the sum of total moisture, volatile matter and ash
content from 100.
19. Coal: Ultimate Analysis
37
➢ Ultimate analysis is also known as elemental analysis, it is the method to determine the Carbon,
Hydrogen, Nitrogen, Sulphur and Oxygen content present in the solid fuel.
➢ It is a chemical approach to analyze coals to determine the amounts of the principal chemical elements
in them.
1. Determination of % of C & H: Accurately weighed coal sample is burnt in a combustion apparatus
Carbon and hydrogen of coal are converted into carbon dioxide and water vapour respectively. The
products of combustion are absorbed respectively in KOH and CaCl2 tubes respectively of known
weights. After complete absorption of CO2 and H2O the tubes are again weighed.
20. Coal: Ultimate Analysis
38
2. Determination of Nitrogen: Nitrogen is calculated by Kjeldahl’s Method. The nitrogen is
converted to NH3 and passed through a known volume of standard acid. On neutralization, the excess
acid is back titrated with a base.
3. Determination of Sulphur: during this determination S is converted into sulphate. The washings
are treated with barium chloride solution and gets converted to barium sulphate precipitate. The
precipitate is filtered, washed and heated to constant weight.
21. Coal: Ultimate Analysis
4. Determination of Ash: Same as in proximate analysis.
It is the residue obtained after burning of the coal in a muffle furnace at 700-7500C for half an
hour till a constant weight is obtained.
5. Determination of Oxygen: The oxygen is determined by calculation as following:
✓ The proximate analysis involves the determination of moisture, volatile matter, ash, and fixed carbon.
This gives quick and valuable information regarding commercial classification and determination of
suitability for a particular industrial use.
✓ The ultimate analysis involves the determination of carbon, hydrogen, sulphur, nitrogen, oxygen and
ash. The ultimate analysis is essential for calculating heat balances in any process for which coal is
employed as a fuel. 39
22. 40
Coal: Analysis of Indian Coal
Average Composition of Indian Coal and Other Countries
Type of Coal Proved Indicated Inferred Total
Coking 19082 13368 2073 34522
Non-coking 129112 125697 28102 282910
Tertiary Coal 594 99 895 1588
Total 148787 139164 31069 319020
(kC/kg)
23. Coal: as a Fuel
➢ Coal accounts for 53% of the commercial energy sources in India which is high compared to the
world average of 30 %.
➢ The 11th plan projected India’s coal demand to grow at 975 MT per annum against 5.7% during 10th
plan almost two-fold increase.
➢ The commercial coal consumed by India 72% for power, 14% for steel, 9% for cement and 9% for
others.
41
24. Coal: as chemical feed stock for Coal Chemicals
➢ With starting of coke oven plants, Coal became source of organic and some inorganic chemicals.
➢ Coal tar from coke-oven plants continues to be a source of aromatics, naphthalene and other
valuable aromatics like pyridine, picoline, quinolene.
➢ Before the coming of petrochemical production a large number of organic chemicals was produced
from acetylene produced from calcium carbide route in which coal was a important feed.
➢ With the rising cost of crude oil and dwindling crude oil reserves, coal has again received attention
all over the world to utilize coal as an alternative source of chemical feedstock.
➢ Various routes for production of organic and inorganic chemicals from coal are:
Coal carbonization and coal tar distillation Coal gasification and use of synthesis gas as feed stock
for ammonia production
Coal liquefaction by hydrogenation Coal to methanol technology
Coal to olefin technology Coal to plastic technology
Acetylene from calcium carbide made from lime and coal 42
26. Coal: Coking of Coal
44
✓ This step is also known as Carbonization of Coals.
✓ Coal carbonization involves heating of coal in the absence of air in a plant called Coke oven plant.
✓ Coke making process is multistep complex process and variety of solid liquids and gaseous products
are produced which contain many valuable products such as coke oven gases. coal tar, light oil, and
aqueous solution of ammonia and ammonia salt.
✓ With the development of steel industry there has been continuous development in coke oven plant
since latter half of nineteenth century. to improve the process conditions, recovery of chemicals and
environmental pollution control strategies and energy consumption measures.
✓ Carbonization can be carried out at low temperature or high temperature.
✓ Low temperature carbonization (450-750oC) is used to produce liquid fuels while high temperature
carbonization (above 900oC) is used to produce gaseous products.
27. Coal: Coking of Coal
45
✓ Coking coal is the type of coal which on heating in the absence of air undergoes a transformation
into a plastic state, swells, and then solidifies to form coke.
✓ Coking coals begin to soften at about 300oC.
✓ Coking coals can bee of three types on the basis of their coking properties: prime coking coal,
medium coking coal, semi coking coal.
✓ Low moisture, ash, sulphur and phosphorous content in the coal are desirable for production of good
quality coke.
✓ The desired analysis of typical coal charge to coke oven is→→→→
29. Coking of Coal: Process Description
47
1. Coke oven plant consists of Coke oven batteries containing number of ovens (around 65 ovens in
each battery).
2. Coke oven are used to convert coal into coke by carbonizing coal in absence of air and there by
distilling the volatile matter out of coal. Coke is taken as product which is used as fuel and as a
reducing agent in smelting iron ore in a blast furnace and coke oven gas as byproduct is treated
for recovery of coal chemicals.
3. The coal is charged to the coke oven through charging holes.
4. The coal is then carbonized for 17-18 hours, during which volatile matter of coal distills out as
coke oven gas and is sent to the recovery section for recovery of valuable chemicals.
5. The ovens are maintained under positive pressure by maintaining high hydraulic main pressure
of 7 mm water column in batteries.
30. Coking of Coal: Process Description
48
6. The coking is complete when the central temperature in the oven is around 950-1000 oC.
7. At this point the oven is isolated from hydraulic mains and after proper venting of residual
gases, the doors are opened for coke pushing.
8. At the end of coking period the coke mass has a high volume shrinkage which leads to
detachment of mass from the walls ensuring easy pushing.
9. The coke is then quenched and transferred to coke sorting plant.
31. Coking of Coal: By product recovery
ammonia and other chemicals. 49
➢ Coke oven gas produced during the process of coking of coal are used in coke oven gas plant for the
recovery of various valuable chemicals like tar, ammonia and benzoyl.
➢ Typical yield of some important byproduct are: Tar 3.2%,
ammonium sulphate 1.1%, crude benzoyl 0.9%.
➢ Coke oven gas containing water vapours and chemical
products of coking (tar, ammonia, benzoyl etc.) at
temperature about 750-800oC from the coke oven plant is
cooled to temperature of 80-82oC.
➢ During gas cooling 65-70% of the tar is condensed. Further
cooling of gas, the water vapors and theremaining part of
the tar get condensed along with some
32. Coking of Coal: By product recovery
50
➢ The gases from exhaust goes stripping section where tar is separated and the tar free gases goes
bubbled through dil. solution of sulphuric acid in saturators.
➢ Ammonia is absorbed by sulphuric acid and Ammonium Sulphate is formed.
➢ The gases from saturator goes to series of coolers and then to benzoyl scrubbers where benzoyl is
scrubbed with wash oil.
➢ Benzoyl crude oil goes to benzoyl recovery section where benzoyl is removed and the wash oil after
treatment is sent to the scrubbers.
➢ Crude Benzoyl thus recovered goes to benzoyl rectification plant. Light crude benzoyl contains low
boiling sulphur compound, BTX, solvents, still bottom residue.
➢ Benzoyl after washing and neutralization with caustic soda is send to benzoyl column for
fractionating into different fraction.
33. Coking of Coal: Coal Tar distillation
51
➢ Coal tar is produced as result of high temperature carbonization and is a viscous dark brown product
with characteristic odour and consists of about 300 different products.
➢ Some of the major constituents are the aromatics and heterocyclic compounds; benzene, toluene,
xylene, phenol cresol, naphthalene, anthracene, phenanthrene, pyridine, carbazole, coumarone etc..
34. Coking of Coal: Coal Tar distillation
52
➢ Tar containing around 5% moisture is first dehydrated before distillation.
➢ The dehydrated tar is heated to 375-400oC using superheated steam to drive out the flashed vapour
and the residue is taken as pitch.
➢ The oil vapour is sent to anthracite column for anthracite recovery while the vapour is sent to other
column for recovery of various fraction light oil, phenol, naphthalene and heavy oil fraction.
Naphthalene fraction is sent to crystalliser to separate naphthalene.
➢ Phenol is recovered from various fractions by treating with a sodium hydroxide to form sodium
phenolate which is reacted with CO2 to release phenol.
➢ Pyridine is recovered by washing different fraction with sulphuric acid.
36. 54
Coal Liquefaction
➢ Liquefaction of coal of coal is also called Hydrogenation of coal.
➢ Hydrogenation of coal involves raising the atomic hydrogen to carbon ratio.
37. Bergius process
oil for use in the next liquefaction run. 55
➢ It is a method of coal liquefaction. Liquefaction is the process of converting solid coal into liquid
fuels.
➢ The Bergius process is a method of production of liquid hydrocarbons for use as synthetic fuel by
hydrogenation of high-volatile bituminous coal at high temperature and pressure.
➢ It was first developed by Friedrich Bergius in 1913. In 1931 Bergius was awarded the Nobel Prize
in Chemistry for his development of high-pressure chemistry.
➢ This involves mixing coal in an oil recycled from a previous liquefaction run and then reacting the
mixture with hydrogen under high pressures ranging from 200 to 700 atmospheres.
➢ An iron oxide catalyst employed.
➢ Temperatures in the reactor are in the range of 425–480 °C (800–900 °F).
➢ Light and heavy liquid fractions are separated from the ash to produce, respectively, gasoline and
38. Bergius process
56
➢ In general, one ton of coal produces about 150 to 170 litres (40 to 44 gallons) of gasoline, 190
litres of diesel fuel, and 130 litres of fuel oil.
➢ The separation of ash and heavy liquids, along with erosion from cyclic pressurization, pose
difficulties that have caused this process to be kept out of use since World War II.
39. 57
Coal Gasification
➢ Gasification is the process of converting organic part of solid fuel to combustible gases of high
heat value by interaction with steam and oxygen.
➢ Gasification converts the low value fuel to high heat value gas.
40. Fischer-Tropsch synthesis process
58
➢ Liquid transportation hydrocarbon fuels and various other chemical products can be produced from
syngas via the well-known and established catalytic chemical process called Fischer-Tropsch (FT)
synthesis, named after the original German inventors, Franz Fischer and Hans Tropsch in the
1920s.
➢ Depending on the source of the syngas, the technology is often referred to as coal-to-liquids (CTL)
and/or gas-to-liquids (GTL).
➢ The Fischer-Tropsch process is a catalytic chemical reaction in which carbon monoxide (CO) and
hydrogen (H2) in the syngas are converted into hydrocarbons of various molecular weights
according to the following equation:
➢ Where n is an integer. Thus, for n=1, the reaction represents the formation of methane, which in
most CTL or GTL applications is considered an undesirable byproduct.
41. Fischer-Tropsch synthesis process
59
➢ The gasification section consists of all the supporting process technologies of coal handling & feed
preparation, heat recovery, syngas cleanup and conditioning, water-gas-shift, sulfur recovery, etc.
42. Fischer-Tropsch synthesis process
60
➢ The clean syngas leaving the gasification section is sent onto the FT synthesis section, where the
clean shifted syngas is converted into primary products of wax, hydrocarbon condensate, tail gas,
and reaction water.
➢ The wax is sent on to an upgrading unit for hydrocracking in the presence of hydrogen, where it
is chemically split into smaller molecular weight hydrocarbon liquids.
➢ A hydrogen recovery unit is used to extract the required quantity of hydrogen from the tail gas as
shown, or alternatively from the feed syngas stream.
➢ The reaction products, along with that from the upgrading section, are fractionated into the final
products of diesel, naphtha, and other light ends, depending on the desired product mix.
➢ The Fischer-Tropsch process conditions are usually chosen to maximize the formation of higher
molecular weight hydrocarbon liquid fuels which are higher value products.
43. FT synthesis process: Products
61
➢ There are other side reactions taking place in the process, among which the water-gas-shift
reaction is predominant:
➢ Depending on the catalyst, temperature, and type of process employed, hydrocarbons ranging from
methane to higher molecular paraffins and olefins can be obtained.
➢ Small amounts of low molecular weight oxygenates (e.g., alcohol and organic acids) are also
formed.
➢ The Fischer-Tropsch synthesis reaction, in theory, is a condensation polymerization reaction of
CO.
➢ Its products obey a well-defined molecular weight distribution according to a relationship known
as Shultz-Flory distribution.
44. FT synthesis process: Catalyst and Reaction conditions
❖ In comparison to iron, Co has much less water-gas-shift activity, and is much more costly
62
.
➢ Catalysts considered for Fischer-Tropsch synthesis are based on transition metals of iron, cobalt,
nickel and ruthenium. FT catalyst development has largely been focused on the preference for high
molecular weight linear alkanes and diesel fuels production.
➢ Among these catalysts, it is generally known that:
❖ Nickel (Ni) tends to promote methane formation, as in a methanation process; thus generally it
is not desirable
❖ Iron (Fe) is relatively low cost and has a higher water-gas-shift activity, and is therefore more
suitable for a lower hydrogen/carbon monoxide ratio (H2/CO) syngas such as those derived
from coal gasification
❖ Cobalt (Co) is more active, and generally preferred over ruthenium (Ru) because of the
prohibitively high cost of Ru
45. FT synthesis process: Catalyst and Reaction conditions
have longer lifetimes. 63
➢ Only iron-based FT catalysts are currently used commercially for converting coal-derived syngas
into FT liquids, given Fe catalyst's inherent water gas shift capability to increase the H2/CO ratio
of coal-derived syngas, thereby improving hydrocarbon product yields in the FT synthesis.
➢ Fe catalysts may be operated in both high-temperature regime (300-350°C) and low-temperature
regime (220-270°C), whereas Co catalysts are only used in the low-temperature range.
➢ This is a consequence of higher temperatures causing more methane formation, which is worse for
Co compared to Fe.
➢ Co catalysts are 230 times more expensive than Fe but are a useful alternative to Fe catalysts for FT
synthesis because they demonstrate activity at lower synthesis pressures, so higher catalyst costs
can be offset by lower operating costs.
➢ Also, coke deposition rate is higher for Fe catalyst than Co catalyst; consequently, Co catalysts
46. FT synthesis process: Catalyst and Reaction conditions
➢ It is observed that low temperatures yield high molecular mass linear waxes while high
temperatures produce gasoline and low molecular weight olefins.
➢ If maximizing the gasoline product fraction, it is best to use an iron catalyst at a high temperature
in a fixed fluid bed reactor.
➢ If maximizing the diesel product fraction, a slurry reactor with a cobalt catalyst is the best choice.
➢ Both Fe and Co FT catalysts are sensitive to the presence of sulfur compounds in the syngas and
can be poisoned by them.
➢ However, the sensitivity of the catalyst to sulfur is higher for Co-based catalysts than for their iron
counterparts.
➢ This is one reason why Co catalysts are preferred for FT synthesis with natural gas derived syngas
(GTL), where the syngas has a higher H2:CO ratio and is relatively lower in sulfur content; Fe
catalysts are preferred for lower quality feedstocks such as coal (CTL). 64
47. FT synthesis process: Reactors
➢ The Fischer-Tropsch reaction is highly exothermic; therefore heat removal is an important factor
in the design of a commercial reactor.
➢ In general, three different types of reactor design might be used for FT synthesis:
❖ Fixed bed reactor
❖ Fluidized bed reactor
❖ Slurry bed reactor
➢ The multitubular fixed-bed reactors, known as Arge reactors, were developed jointly by Lurgi and
Ruhrchemie and commissioned in the 1955.
➢ Most of Arge reactors are now be replaced by slurry-bed reactors, an state-of-the-art technology
for low temperature FT synthesis which offer better temperature control and higher conversion.
➢ Fluidized-bed FT reactors were developed for high temperature FT synthesis to produce low
molecular gaseous hydrocarbons and gasoline. 65
