Utilization of Coking Coal in Metallurgical Process

8/13/2019 Utilization of Coking Coal in Metallurgical Process

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Assignment No. 1

Utilization of coking coal in Metallurgical process

Subject: Fuel technology

Submitted by: Submitted to:

Vinay C. Mathad Prof. M.G.Dastidar

2012jen2111


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Introduction:

Coal is a combustible, sedimentary, organic rock; it is a complex mixture of organic

chemical substances containing carbon, hydrogen and oxygen in chemical combination,

together with smaller amounts of nitrogen and sulfur. It is formed from vegetation, which

has been consolidated between other rock strata and altered by the combined effects of

bacterial action, pressure and heat over millions of years to form coal seams. The build-up

of silt and other sediments, together with movements in the earths crust (known as

tectonic movements) buried these swamps and peat bogs, often to great depths. With

burial, the plant material was subjected to high temperatures and pressures. This caused

physical and chemical changes in the vegetation, transforming it into peat and then into

coal [1].

Fig 1:Formation of coal [2]

Coal is an essential input in the production of steel. Steel is a man-made alloy of iron and

carbon and that carbon usually comes from coal. Almost 70% of the steel produced today

relies directly on metallurgical coal, also referred to as coking coal. The remainder is

produced by recycling scrap steel (itself originally produced directly using coal) using

electricityoften generated using affordable and reliable steam coal.[3]

Fig 2:Use of different coals [3]


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Over the last 35 years steel production worldwide has almost doubled, from less than

600 million tonnes (Mt) in 1970 to around 1.2 billion tonnes in 2006. The period 2000-

2006 has seen unprecedented growth, with global figures rising over 47%. Coal will

continue to play a major part in the manufacture of the worlds steel for the foreseeable

future. The well-supplied world market means that metallurgical coal can be delivered

worldwide, facilitating the manufacture of steels which will ultimately deliver the goods

and services that growing economies demand. [3]

Fig 3: World Coking Coal and steel production [3]

Coking coal

Coke was first produced commercially in England in the early eighteenth century. By the

early to mid-1800s, coke was being widely produced in Europe and the United States of

America as the major fuel for blast furnaces. Coal carbonization is a process that yields

metallurgical coke for use in iron-making blast furnaces and other metal-smelting

processes. Coke is a solid carbonaceous residue derived from low-ash, low-sulfur

bituminous coal from which the volatile constituents are driven off by baking in an oven

without oxygen at temperatures as high as 1,000 C (1,832 F) so that the fixed carbon


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and residual ash are fused together. Coke is used as a fuel and as a reducing agent in

smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a

heating value of 24.8 million Btu/ton (29.6 MJ/kg) [ 2 World Coal Institute:

www.worldcoal.org]. Carbonization entails heating the coal to temperatures as high as

1300 C in the absence of oxygen to distill out tars and light oils. A gaseous by-product,

referred to as coke-oven gas, together with ammonia, water and sulfur compounds are

also removed thermally from the coal. The coke that remains after this distillation largely

consists of carbon in various crystallographic forms, but also contains the thermally

modified remains of various minerals that were in the original coal. These mineral

residues, commonly referred to as coke ash are not combustible and are left after the

coke is burned. Coke also contains part of the sulfur from the coal. Coke is principally

used as a fuel, as a reducing agent and support for other raw materials in iron-making

blast furnaces. A much smaller amount of coke is used similarly in cupola furnaces in the

foundry industry. The carbonization by-products are usually refined within the coke plant

to commodity chemicals such as elemental sulfur, ammonium sulfate, benzene, toluene,

xylene and naphthalene. Subsequent processing of these chemicals produces a large

number of other chemicals and materials. Cokeoven gas is a valuable heating fuel that is

used mainly within steel plants, for example, to fire blast-furnace stoves, to soak

furnaces for semifinished steel, to anneal furnaces and lime kilns as well as to heat the

coke ovens themselves.

Properties of coking coal in metallurgical processes:

Over the last few decades, due to iron making trends towards increased size and

throughput of blast furnace performance together with a simultaneous reduction in coke

rate by high levels of injection of carbon via the tuyeres, in particular

pulverized coal injection (PCI), the role of coke as a permeable support became greater

in importance and so further improvements in coke quality are required. In such

conditions, a decrease in coke rate produces a decrease in the thickness of the coke

layer in the stack and cohesive zone together with an increased residence time of coke

in the lower part of the blast furnace.

In modern blast furnace practice, the trend is toward use of iron-bearing burden materials

of controlled size such as sinter and pellets; thus, the size of the coke used in the burden


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assumes more importance then in the past when only crude ore was used. The size of

coke produced in byproduct ovens is somewhat dependent upon the type of coal,

heating rate, width of the ovens, and the bulk density of the coal charge, greater

amounts of low-volatile coal, wider ovens and greater bulk density of the coal charge

generally tend to produce larger coke while faster heating rates tend to produce smaller

coke. Because relatively uniform size is desired, crushing and screening of the coke

must be resorted to when controlled size is desired. Most blast furnace operators prefer

coke sized between about 18.5 and 76 rnm ( 3 /4 in. and 3 in.) for optimum furnace

performance. Other physical properties of the coke such as porosity, density and

combustibility arc controllable only to a small extent, and their importance in affecting

blast furnace operation has not been definitely established.

Coke residence times increase by more than 30% with variations in coal rate from 100 to

200 kg t1hot metal [4]. This means that coke is subjected to longer periods under

mechanical, thermal, and chemical stresses that further increase coke degradation in the

blast furnace.

To ensure good blast furnace performance, coke should be moderately large, with a

narrow size range, and have a high mechanical strength in order to withstand the

weakening reactions with carbon dioxide and alkali, abrasion, and thermal shock in the

blast furnace. Because of the many unknown factors, it is not achievable to establish

universal quality indices common to all blast furnaces, although typical specifications

for metallurgical coke quality are available.

Impurities present in coke affect its performance in the blast furnace by decreasing its

role as a fuel in terms of amounts of carbon available for direct and indirect reduction

roles and also its role as a permeable support. Such impurities are moisture, volatile

matter, ash, sulphur, phosphorous, and alkali contents. Their levels are kept as low as

possible.

Moisture content is a direct consequence of the coke-quenching process with some

dependence on size. High and variable moisture contents affect both the coke rate and

the balances within the blast furnace, while high volatile matter contents cause

operational problems in the cleaning of blast furnace gas. Coke moisture content ranges

from 1 to 6 wt.% maximum and common values are in the range 34 wt.%.


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Of other chemical properties, sulphur and ash (content and chemistry) are of particular

importance because as they increase, coke productivity in the blast furnace decreases.

The coke ash is a non-productive part of coke which influences slag volume and

composition. Industrial experience indicates that a 1 wt.% increase of ash in the coke

reduces metal production by 2 or 3 wt.%. The yield of ash from a coal should be

minimized, but practically speaking should be less than 10%. Ash contributes non-

combustible components to the blast furnace burden, has been shown to lower flame

temperature and makes grinding difficult. Along with this goes the need to select a coal

that contributes a minimal amount of sulfur and alkali content. Volatile matter should be

maximized to the extent possible as it has been found that low volatile content coals do

not combust completely in the raceway. This is advantageous to reduce possible

carryover of particulate materials in the off gas of the blast furnace. However, high

volatile content coals generally have higher moisture contents. Coal hardness, as

measured by Hardgrove grindability, should be minimized for the efficient operation of

the grinding mills.

Types of Coking Coal

Metallurgical coke is a macroporous carbon material of high strength produced by

carbonization of coals of specific rank or of coal blends at temperatures up to 1400 K.

About 90% of the coke produced from blends of coking coals is used to maintain

the process of iron production in the blast furnace where it has three major roles:

1. as a fuel, it provides heat for the endothermic requirements of chemical reactions and

the melting of slag and metal;

2. as a chemical reducing agent, it produces gases for the reduction of iron oxides; and

3. as a permeable support, it acts as the only solid material in the furnace that supports

the iron-bearing burden and provides a permeable matrix necessary for slag and metal to

pass down into the hearth and for hot gases to pass upwards into the stack.

Of these three roles, the first two can be substituted by oil, gas, plastics, and coal. These

are injected at the tuyeres as generating energy and a carbon source. Such a

substitution brings about a reduction in coke rates for the blast furnace (coke rate is the


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weight of coke required to produce 1 t of iron). However, there is no other satisfactory

material available, which can replace, fully or partially, metallurgical coke as a permeable

support of blast furnace charge.

There are three principal kinds of coke, classified according to the methods by which

they are manufactured: low-, medium- and high temperature coke. Coke used for

metallurgical purposes must be carbonized in the higher ranges of temperature (between

900C and 1095C) (1650F and 2000F) if the product is to have satisfactory physical

properties. Even with good coking coal, the product obtained by low temperature

carbonization between 480C and 760C (900F and 1400F) is unacceptable for good

blast furnace operation.

Methods of Manufacturing Metallurgical Coke

There are two proven processes for manufacturing metallurgical coke, known as the

beehive process and the byproduct process.

In the beehive process, air is admitted to the coking chamber in controlled amounts for

the purpose of burning therein the volatile products distilled from coal to generate heat

for further distillation. A beehive coke oven was a circular, domed firebrick structure with

a flat floor that sloped toward the door through which coke was removed when the coking

process was completed. An opening called the tunnel head in the center of the domed

roof permitted coal lo be charged into the oven and allowed the gases generated during

the coking process (called foul gas) to escape into the atmosphere. A typical oven was

about 3.7 m(l2 ft) in diameter and processed about 4.5 - 6.3 tonnes of coal per charge.

From 48 to 72 hours were required to complete a coking cycle. Beehive ovens were

charged as soon as practicable after drawing the coke from the oven at the end of theprevious cycle, in order that stored heat from the previous charge would be sufficient to

start the coking process. New ovens had to be heated gradually to the coking

temperature by wood and coal fires, after which small charges of coal were coked until

the ovens reached normal working conditions. In order to secure uniform coking of the

coal, this pile had to be leveled so that the coal would lie in a bed of uniform depth of

about 46- 610 mm ( 1-24 in.) over the entire floor of the oven. This leveling was done by

machine or by hand. In works not equipped with n machine, the Ieveling was

accomplished with a long-handled scraper, operated through the door of the oven, which


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was purposely bricked up to only two-thirds of its height at the lime of charging. Thc

coking process began very soon after leveling was completed, as the oven retained

enough heat in the brick of the walls and the earth fill to start liberation of volatile matter

from the coal, As the temperature of the coal charge increased, the temperature of the

combustible volatile gases soon reached the kindling (ignition) point and, in presence of

the air admitted to the oven, they ignited with a slight explosion at first and then

continued to burn quietly in the crown of the oven or as small candle-like flames at the

surface of the coking mass. thus supplying heat to continue the process. Coking

proceeded from the top of the coal downward. so that the coking time depended upon

the depth of the coal. The generation of gas from the coal rapidly approached a

maximum which was maintained for a period and then declined to practically nothing The

burning of the volatile matter during this period was regulated by gradually closing up the

opening at the top of toe door for the admission of air flux regulation was necessary to

maintain the temperature at a maximum and conserve coke, as an excess of air at the

beginning of the coking period tended to cool the oven and later consumed some of the

carbon of the coke. The yield was also reduced by improper leveling. If the coal was not

of uniform depth, the thin portions coked through before the thick, and some of the thin

sections was consumed while the coking of the thick sections was being completed. On

the other hand, if the process was stopped when the thin areas had coked through, there

was a loss due to uncoked butts on the thick areas. Coking proceeded downward from

the top of the charge, in which the coal at increasing depths passed through a plastic

stage as the temperature rose. This produced expansion and contraction of thc charge

with the result that the coke was ramified by a great number of irregular vertical fissures

that divided the coke mass into very irregular columnar pieces that extended from the top

to the bottom of the mass. At the end of the coking period, the brickwork closing the door

was tom out and the coke was wata out by spraying with ~ stream of water directed

through the door of the oven or, in later ovens, by ~ use of a spraying device inserted

through the door. Aflcr watering, the coke was removed ffom I oven by hand or by

machine.


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Fig 4: Beehive process for coal carbonizing [5]

In the byproduct method, air is excluded from the coking chambers, and the necessary

heat for distillation is supplied from external combustion of some of the gas recovered

from the coking process (or, in some instances, cleaned blast furnace gas or a mixture of

Coke oven anal blast furnace gas) With modem byproduct ovens, properly operated all

the volatile products liberated during coking are recovered as gas and coal chemicals,

and, when coke oven gas alone is used as fuel, about 41)% of the gas produced isreturned to the ovens for heating purposes. The byproduct coking process, being a true

distillation process, involves the use of retort ovens. While there are many modifications,

these ovens consist essentially of three main parts, namely: the coking chambers, the

heating flues and the regenerative chambersall constructed of refractory brick. The

following discussion of components applies generally to all types of ovens, but is related

principally to those used in the United States. Ovens are constructed in batteries that

have contained from as few as ten to over 100 ovens. In the United States, large

batteries of 45 or more ovens generally have been preferred, while batteries of fewer

ovens have been more common elsewhere. Coking chambers in a battery alternate with

heating chambers so that, in effect, there is a heating chamber on each side of a coking

chamber. The regenerative chambers are underneath the beating and coking chambers.

Separating walls between regenerators also serve as foundation walls for the heating

and coking chambers. The entire structure is supported either from the ground or by

columns under a reinforced concrete or structural steel base. The coal is charged

through openings in the top of the oven and, after the coal has become cot the coke is


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pushed out from one end by a power driven ram, or pusher, During the coking period, the

ends of the coking chamber are closed by refractory lined doors, which are constructed

to completely seal the ends of the ovens The ovens first constructed in the industry

provided a space between the door and the jamb which was filled with a special luting

mixture seal the oven prior to charging Later, several types of self-sealing doors were

developed, which seal the opening when put in place and require no luting.

To permit the escape of volatile matter driven from the coal during coking, an opening is

provided at the top of the oven at either one or both ends of the coking chamber Each

such opening is f ted with an outtake pipe, which connects the oven with the gas

collecting main for the battery.The combustion chambers consist of a large number of

flues which permit uniform heating of t entire length of the coking chamber. Ovens have

been built with either horizontal heating flues vertical heating flues, but vertical flues are

used almost exclusively in present installations. Some of the older ovens employed the

recuperative principle for preheating combustion air Modern practice utilizes the

regenerative principle to achieve higher thermal efficiency whereby less gas required to

heat the ovens In all modern oven batteries, individual regenerators arc provided t each

heating wall and arc located under each oven. This permits separate control of the flow

of pre heated air for combustion to individual vertical heating flue walls and allows close

control of heating. An advantage of individual regenerators is that the control of heating

for each oven is relative independent of the operation of the remainder of the battery.

While the beehive process was the leading method for manufacture of coke up to 1918,

largely the byproduct process as discussed later in this chapter now has replaced it

There is a difference of temperature of coking in the two processes, that of the byproduct

being somewhat lower than the beehive. Beehive coke is usually larger, though not as

uniform in size. In general, properly carbonized beehive coke and byproduct cokc both

are silvery gray in appearance. A modification of the beehive technology is also known

as non-recovery ovens.


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Fig 5:By-Product process [5]

Other processes for producing metallurgical coke are known as continuous processes;

many variations have been proposed but none has been adopted on a commercial scale.

In one continuous process, finely pulverized coking or non-coking coal is dried and

partially oxidized with steam or air in fluidized bed reactors to prevent agglomeration

when coking coal is used. Thc reactor product is carbonized in two stages a successively

higher temperatures to obtain a char. Using a binder produced from tar obtained in the

carbonization stages, the char is briquetted in roll presses. The "green" briquettes are

cured at low temperatures, carbonized at high temperatures, and finally cooled in an inert

atmosphere to produce a metallurgical coke of low volatile content. This type of coke is

often referred to as form coke.


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Utilization of Coke in metallurgical processes

As iron occurs only as iron oxides in the earths crust, the ores must be converted, or

reduced, using carbon. Although the oxides of iron may be reduced to metallic iron by

many agents, carbon (directly or indirectly) is the reducing agent found to be best suited

for the economical production of iron. Carbon of suitable reactivity and physical strength

was at one time produced from wood by distillation, yielding wood charcoal; but for the

operation of a modern large blast furnace the carbon required for the smelting of iron is

obtained from the destructive distillation of selected coking coals at temperatures in the

range from 900C to 1095C. Coal is a key raw material in steel production. Coke, made

by carburising coal (i.e. heating in the absence of oxygen at high temperatures), is the

primary reducing agent of iron ore. Coke reduces iron ore to molten iron saturated with

carbon, called hot metal. Global steel production is dependent on coal around 68% of

total production relies directly on the input of coal. 761Mt of coking coal and Pulverised

Coal Injection (PCI) coals are used in global steel production, which is around 12% of

total hard coal consumption worldwide. [Coal and Steel Statistics, World Coal Institute,

worldcoal.org]

In the integrated route, raw materials comprise iron ore in various forms and a reductant,

coke and various fluxing minerals, such as limestone and dolomite. A particular type of

coal coking or metallurgical coal is used to prepare the coke. The coking coal is

first crushed and then heated up in a coke oven without oxygen over several hours. This

drives off volatiles and some of the impurities, leaving a solid sponge-like mass of

carbon-rich material. The iron ore is also prepared prior to use and is converted into

sinter and pellets. The coke, together with iron ore and limestone, is then charged into

the blast furnace and heated so that the coke becomes gasified, producing a

combination of carbon monoxide and carbon dioxide. The carbon monoxide reacts with

the iron ore to form a high quality molten iron known as hot metal. The hot metal is

collected from the base of the furnace and transferred to the BOF. During the iron

making process, a blast furnace is fed with the iron ore, coke and small quantities of

fluxes. Air which is heated to about 1200C is blown into the furnace through nozzles

called tuyeres in the lower section. The air causes the coke to gasify, producing carbon

monoxide/carbon dioxide which reacts with the iron ore, as well as heat to melt the iron.

Blast furnaces have two or three tap holes and the hot metal and slag are tapped off


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regularly as they accumulate in the base or hearth of the BF. The hot metal drains into a

vessel known as a ladle car, which is used to transport it to the BOF plant.

Fig 6:Feeding of coking coal in blast furnance [3]

Other elements, such as nickel, chromium and molybdenum may be added in the BOF to

give particular properties to the final steel. Oxygen is blown through the molten metal

bath inside the BOF, causing excess carbon to be oxidised and emitted, leaving liquid

steel with low carbon contents.

Some steel plants use electric arc furnaces to generate steel, where recycled material orsteel scrap is melted and then reformed for further use. The recycled steel is loaded into

the furnace with some iron ore, often in a partially reduced form, and high intensity

electrical power is supplied to electrodes forming an arc of electricity to raise the internal

temperature and melt the scrap. In both cases some additional refining may be carried

out to achieve the required steel specification, but the integrated route offers the most

capability for achieving the highest quality steels, whereas the EAF route is limited in

what it can produce by the quality of the scrap.

The liquid steel, whether it is produced in BOF or EAF, is then processed via rolling mills

to form a variety of products from rails to bars, wires to pipes which are then further

transformed for their end-use.


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Fig 7:Coking coal utilization in metallurgical processes [3]

References:

1. Prof. Dr. E. Stach: Stachs Textbook of Coal Petrology, Third revised and enlarged

edition. Gebruder Borntraeger. Berlin. Stuttgart 1982

2. World Coal Institute:www.worldcoal.org

3. world coal report, World Coal Institute:www.worldcoal.org

4. ( Negro et al., 1996)

5. Chapter 7, Manufacturing of coking coke, AISE steel foundation pittsburg
http://www.worldcoal.org/http://www.worldcoal.org/http://www.sciencedirect.com/science/article/pii/S0166516202001234#BIB85http://www.sciencedirect.com/science/article/pii/S0166516202001234#BIB85http://www.worldcoal.org/http://www.worldcoal.org/

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Utilization of Coking Coal in Metallurgical Process