Coke production for blast furnace ironmaking
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.
a) By-product Coke Production:
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.
Reference :
Website http://www.steel.org
By
Hardarshan S. Valia, Scientist, Ispat Inland Inc
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