Uncertainties in the Emission Estimates

A major uncertainty in the emission estimates is the quantity of emissions that are not captured and

escape as fugitive emissions with the ventilation air. The plants reported measured emissions from point

sources, which were the stacks from which emissions from the control device were discharged. However, the

capture efficiency of hoods used on several emission points associated with the discharge end was reported as

about 95 percent,7 _{which means that the quantity that was not captured was far more than the quantity emitted}

from control devices that were generally rated as 99 to 99.9 percent efficient in the control of PM. Some of the

larger particles may settle out in the building, and other PM that escapes capture is emitted with the ventilation

air to the atmosphere.

Uncertainty is also introduced by differences in the composition of the feed materials used by the plants.

The percent of Pb and Mn in the dust may be directly related to the amount of these metals in the feed

materials. In addition, some of the more volatile metal compounds may be more concentrated in fine particles

(i.e., the concentration of HAP metals may vary as a function of particle size). The quantity and type of

organics in the feed material (such as oily scale), may also affect the type and quantity of organic compounds

3-12 3.2 BLAST FURNACES

The blast furnace converts iron oxide into molten iron for subsequent refining in the BOPF shop to

produce steel. A typical burden (feed) may consist of iron ore, pellets, sinter, limestone, coke, mill scale,

BOPF slag, and other iron bearing materials. The burden material is charged into the top of the furnace and

slowly descends through the furnace. The coke provides the thermal energy required for the process and

provides carbon to reduce the iron oxide and to remove oxygen in the form of CO.

The blast furnace is a vertical shaft furnace. Raw materials are charged into the top of the furnace and

fall to the top of the burden of raw materials already in the furnace. As they descend in the furnace, they are

heated by a countercurrent flow of gas. Heated air is injected through the tuyeres, located near the bottom of

the furnace just above the hearth. The air moves countercurrent to the burden, consuming the coke (carbon).

Raw materials are introduced at the top of the blast furnace; the hottest temperature zone in the furnace is at the

hearth level, where the burden is molten.

The furnace filling is controlled by the level of burden in the furnace. When the level is below a preset

point, the stockhouse functions continuously, filling the skips with predetermined weights of materials in the

ordered sequence. The top of the blast furnace is enclosed so that blast furnace gas can be drawn off above

the stock level and a bell and hopper arrangement can be used for charging the furnace. Most installations use

a combination of two bells so that a gas tight space can be provided between the two bells to prevent gas from

escaping while the lower bell is opened. Raw materials are taken to the furnace top by a skip hoist or a

conveyor belt and dropped into the upper hopper. With the large bell closed, the small bell is lowered and the

charge material is dropped into the large-bell hopper. When the large-bell hopper is full, the small bell is held

closed, the large bell is lowered, and the material is dumped into the blast furnace without allowing any of the

gas to escape.

A more recent innovation, used on several blast furnaces in the industry, is the Paul Wurth bell-less

top, in which the charge materials are deposited into hoppers located at the top of the furnace. The hoppers

can be depressurized for loading and repressurized for discharging the material into the furnace. There are at

least two hoppers so that while one is being loaded, the other can be discharged into the furnace. As the

With this design, the furnace burns fuel more efficiently, leaks less, and can hold pressure. There is also not a

problem with wearing a hole in the bell or sealing bell rods.1

In the blast furnace process, the heated raw materials react chemically with one another. The principal

set of reactions are the complex ones between coke, air, and iron ore. Part of the coke is consumed by the

oxygen in the air to produce heat for the process. Another part of the coke combines with the oxygen in the

iron ore and releases free iron, which melts, drips to the bottom of the furnace, and collects in the hearth. A

final portion of the carbon dissolves in the iron. The heat of the blast furnace serves to calcine the limestone.

The resulting calcium oxide reacts with the impurities in the ore, principally sulfur, and, in molten form, descends

to the hearth. The slag, being about one-third the density of the iron, floats in a separate layer on the iron bath.

Ironmaking is a continuous process within the blast furnace; however, it is a semi-continuous process

with respect to periodic charging of materials into the top of the furnace and periodic tapping of molten iron and

slag from the bottom of the furnace. Periodically, the hearth becomes full of molten iron and slag. Because

there is a limit to the amount that can be tolerated before it interferes with the furnace operation, they must be

removed from the furnace at regular intervals. The iron notch, which is used for tapping the hot metal, is

located just above the floor of the hearth; each furnace has one or more iron notches When the furnace is in

operation, the iron notch is completely filled with a refractory material, called taphole clay. To cast the hot

metal from the furnace, a tapping hole is drilled through this material.

The hot metal flows through this hole and is discharged into a trough, which is a long narrow basin

typically 3 to 5 feet wide and 26 to 40 feet long; the trough generally has a slightly sloping bottom away from

the furnace. At the far end of the trough, there is a dam to hold back the hot metal until the depth of the metal

in the trough is sufficient to contact the bottom of a refractory skimmer block. The skimmer holds back the slag

and diverts it into the slag runners. The hot metal flows over the dam and down the iron runner, where it is

directed in sequence to a train of ladles positioned under stationary spouts along the runner. At several large

blast furnaces, a tilting spout is used, positioned between two hot metal tracks. The spout is first tilted to fill the

ladle on one track and then to fill the one on the other track. While the second ladle is being filled, the first one

can be replaced with an empty one so that the cast can be continued uninterrupted while several ladles are

3-14

After the flows of iron and slag cease, the tap hole is plugged with fresh clay by a device called a "mud

gun", and the ironmaking process resumes. The hot metal is transported from the blast furnace to the BOPF

shop in refractory-lined ladles that have a course of insulating material between the lining and the steel shell.23

Blast furnace gas (primarily CO) is collected from offtakes at the top of the furnace; this gas is cleaned

of PM and is used to fire the blast furnace stoves that heat the furnace air. Excess blast furnace gas is used as a

fuel in other processes at the plant.

There are currently a total of 39 blast furnaces at 20 plants that are owned by 14 companies in the U.S.

The plants are located in 10 different States, with the largest number in Ohio and Indiana. Each furnace has the

capacity to produce 700,000 to 3,440,000 tpy of hot metal.