COKE PRODUCTION FOR BLAST FURNACE IRONMAKING
By Hardarshan S. Valia, Scientist, Ispat Inland Inc
INTRODUCTION
A world class blast furnace operation demands the highest quality of raw
materials, operation, and operators. Coke is the most important raw
material fed into the blast furnace in terms of its effect on blast
furnace operation and hot metal quality. A high quality coke should be
able to support a smooth descent of the blast furnace burden with as
little degradation as possible while providing the lowest amount of
impurities, highest thermal energy, highest metal reduction, and optimum
permeability for the flow of gaseous and molten products. Introduction
of high quality coke to a blast furnace will result in lower coke rate,
higher productivity and lower hot metal cost.
COKE PRODUCTION
The cokemaking process involves carbonization of coal to high
temperatures (1100°C) in an oxygen deficient atmosphere in order to
concentrate the carbon. The commercial cokemaking process can be broken
down into two categories: a) By-product Cokemaking and b)
Non-Recovery/Heat Recovery Cokemaking. A brief description of each
coking process is presented here.
The majority of coke produced in the United States comes from
wet-charge, by-product coke oven batteries (Figure 1). The entire
cokemaking operation is comprised of the following steps: Before
carbonization, the selected coals from specific mines are blended,
pulverized, and oiled for proper bulk density control. The blended coal
is charged into a number of slot type ovens wherein each oven shares a
common heating flue with the adjacent oven. Coal is carbonized in a
reducing atmosphere and the off-gas is collected and sent to the
by-product plant where various by-products are recovered. Hence, this
process is called by-product cokemaking.
Figure 1: "Coke Side" of a By-Product Coke Oven Battery. The oven has
just been "pushed" and railroad car is full of incandescent coke that
will now be taken to the "quench station". |
Figure 2: Incandescent coke in the oven waiting to be "pushed". |
The coal-to-coke transformation takes place as follows:
The heat is transferred from the heated brick walls into the coal
charge. From about 375°C to 475°C, the coal decomposes to form
plastic layers near each wall. At about 475°C to 600°C, there
is a marked evolution of tar, and aromatic hydrocarbon compounds,
followed by resolidification of the plastic mass into semi-coke. At
600°C to 1100°C, the coke stabilization phase begins. This is
characterized by contraction of coke mass, structural development of
coke and final hydrogen evolution. During the plastic stage, the plastic
layers move from each wall towards the center of the oven trapping the
liberated gas and creating in gas pressure build up which is transferred
to the heating wall. Once, the plastic layers have met at the center of
the oven, the entire mass has been carbonized (Figure 2). The
incandescent coke mass is pushed from the oven and is wet or dry
quenched prior to its shipment to the blast furnace. |
b) Non-Recovery/Heat Recovery Coke Production:
In Non-Recovery coke plants, originally referred to as beehive ovens,
the coal is carbonized in large oven chambers (Figure 3). The
carbonization process takes place from the top by radiant heat transfer
and from the bottom by conduction of heat through the sole floor.
Primary air for combustion is introduced into the oven chamber through
several ports located above the charge level in both pusher and coke
side doors of the oven. Partially combusted gases exit the top chamber
through "down comer" passages in the oven wall and enter the sole flue,
thereby heating the sole of the oven. Combusted gases collect in a
common tunnel and exit via a stack which creates a natural draft in the
oven. Since the by-products are not recovered, the process is called
Non-Recovery cokemaking. In one case, the waste gas exits into a waste
heat recovery boiler (Figure 3) which converts the excess heat into
steam for power generation; hence, the process is called Heat Recovery
cokemaking.
Figure 3: Heat Recovery Coke Plant. |
COKE PROPERTIES
High quality coke is characterized by a definite set of physical and
chemical properties that can vary within narrow limits. The coke
properties can be grouped into following two groups: a) Physical
properties and b) Chemical properties.
a) Physical Properties:
Measurement of physical properties aid in determining coke behavior both
inside and outside the blast furnace (Figure 4). In terms of coke
strength, the coke stability and Coke Strength After Reaction with
CO2 (CSR) are the most important parameters. The stability
measures the ability of coke to withstand breakage at room temperature
and reflects coke behavior outside the blast furnace and in the upper
part of the blast furnace. CSR measures the potential of the coke to
break into smaller size under a high temperature CO/CO2
environment that exists throughout the lower two-thirds of the blast
furnace. A large mean size with narrow size variations helps maintain a
stable void fraction in the blast furnace permitting the upward flow of
gases and downward of molten iron and slag thus improving blast furnace
productivity.
Blast Furnace Operating Zones and Coke Behavior. |
b) Chemical Properties:
The most important chemical properties are moisture, fixed carbon, ash,
sulfur, phosphorus, and alkalies. Fixed carbon is the fuel portion of
the coke; the higher the fixed carbon, the higher the thermal value of
coke. The other components such as moisture, ash, sulfur, phosphorus,
and alkalies are undesirable as they have adverse effects on energy
requirements, blast furnace operation, hot metal quality, and/or
refractory lining. Coke quality specifications for one large blast
furnace in North America are shown in Table I.
Table I. Coke Quality Specifications:
| Physical: (measured at the blast furnace) |
Mean |
Range |
| Average Coke Size (mm) |
52 |
45-60 |
| Plus 4" (% by weight) |
1 |
4 max |
| Minus 1"(% by weight) |
8 |
11 max |
| Stability |
60 |
58 min |
| CSR |
65 |
61 min |
| Physical: (% by weight) |
| Ash |
8.0 |
9.0 max |
| Moisture |
2.5 |
5.0 max |
| Sulfur |
0.65 |
0.82 max |
| Volatile Matter |
0.5 |
1.5 max |
| Alkali (K2O+Na2O) |
0.25 |
0.40 max |
| Phosphorus |
0.02 |
0.33 max |
FACTORS AFFECTING COKE QUALITY
A good quality coke is generally made from carbonization of good quality
coking coals. Coking coals are defined as those coals that on
carbonization pass through softening, swelling, and resolidification to
coke. One important consideration in selecting a coal blend is that it
should not exert a high coke oven wall pressure and should contract
sufficiently to allow the coke to be pushed from the oven. The
properties of coke and coke oven pushing performance are influenced by
following coal quality and battery operating variables: rank of coal,
petrographic, chemical and rheologic characteristics of coal, particle
size, moisture content, bulk density, weathering of coal, coking
temperature and coking rate, soaking time, quenching practice, and coke
handling. Coke quality variability is low if all these factors are
controlled. Coke producers use widely differing coals and employ many
procedures to enhance the quality of the coke and to enhance the coke
oven productivity and battery life.
| Valia, coke production, coke oven, battery, non-recovery, coke properties, coke quality |
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