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Iron / Steel (Ferrous): Blast Furnaces

A blast furnace is a large refractory-lined chamber or shaft, generally about 30 feet in diameter and 100 feet tall. Some are larger. Many are smaller. Raw materials are fed at the top of the furnace and heated air (at about 1,800°F or more) from the stones is blown or "blasted" into the furnace. The majority of the raw materials loaded into the blast furnace consist of iron ore, coke, and limestone. These materials are commonly referred to as the burden.

Iron ore is used in the form of raw ore, sintered pellets (made from iron ore fines and blast furnace dust) or HBI (i.e., Midrex DRI).

Coke (produced from coal in coke ovens) is used for its carbon content, to form carbon monoxide for the ore reduction process (oxygen removal), and to provide mechanical strength to support the unmelted burden above the melt zone in a blast furnace.

Limestone binds the materials in liquid slag that are not wanted in the liquid iron.

The heated air (or hot blast) first melts iron ore or the iron containing pellets at the tuyere level. As these gases move up the furnace shaft, they "reduce" the iron above the melted zone by removing oxygen from the ore. The oxygen reacts with the impurities in the iron and other burden material to form slag. Slag contains the majority of the waste products (silica, manganese, alumina, calcium, sulfur, etc.) that were in the iron ore, coke and limestone.

The hot blast is provided from a unit called a stove. Hot blast serves as the combustion air required to burn the carbon in the coke in order to attain the 2,900°F plus molten metal temperature required for downstream refining steps. Generally three or four stoves per blast furnace are used. Stoves are preheated using blast furnace gas (gasses exiting the top of the blast furnace) to about 2,300°F in order to produce the 1,800°F wind temperatures. Stoves can also use a blend of coke oven gas and even natural gas to heat the hot blast.

Natural Gas Use:
When natural gas is injected into a blast furnace it reduces coke consumption. It also allows additional iron ore to be added in the burden (instead of coke) and provides operators with the ability to increase production rates.

Natural gas use has other intangible values. In addition to containing low- or no-sulfur, it is simple to use (involves no storage space or inventory cost) and has "extra" hydrogen effect. Other hydrocarbon injection systems (pulverized coal or oil) compete with natural gas. The table below provides an understanding of the natural gas volume involved with blast furnace injection.

All U.S. operating blast furnaces are injecting or coinjecting supplemental fuels at the tuyeres.

Supplemental fuel is used by blast furnace operators to obtain:

  • Reduced coke consumption
  • Increased process control and productivity
  • Improved quality of hot metal (i.e., decreased sulfur)
  • Lower costs

Natural gas injection allows all these objectives to be met. The chart below illustrates the reduction in coke rates for three conditions:

1) Base-line without natural gas injection.
2) At 150 pounds per ton hot metal (lb/THM).
3) At 200 lb/THM.

The natural gas injection work completed in the 1990s shows that injection rates resulting in up to a 40% reduction in coke consumption is possible.

One pound of natural gas injected into a blast furnace for each ton of hot metal produced equates to 22.5 Standard Cubic Foot per Ton Hot Metal (SCF/THM). Injection rates at 100 lbs per THM would result in 2,250 SCF/THM. For a fairly common 4,000 ton per day furnace, the volume per pound of natural gas injection would be:

4,000 x 22.5 cf / THM = 90,000 cf (or 90 Mcf per day per pound of natural gas injected)

Most blast furnaces injecting natural gas do so at flow rates between 75 to 150 lbs of natural gas per THM. Therefore, the natural gas consumption for a 4,000 ton per day blast furnace at these two levels would be:

75 lb./THM = 90 Mcf x 75 = 6,750 Mcf per day (281,250 SCFH) or 2.36 BCF per year

150 lb./THM = 90 MCF x 150 = 13,500 Mcf per day (562,500 SCFH) or 4.73 BCF per year

GTI sponsored several high-injection rate papers (with Charles River Associates - CRA) in the mid-1990's. Cost savings results for tests injecting up to 300 lb / THM are shown in the graph. A report outlining a 300 Lb / THM test at Acme Steel is attached. It provides an excellent overview of BF injection practice and blast furnace operators will find it an excellent technical reference.

Total Value & Natural Gas Sales versus Production Rate Increase
Source: Charles River Associates, 1997,
Presentation: "High Rates of Gas Injection in North American Blast Furnaces - Update, Results, and Implications for the Gas Industry"

Comments on Hydrogen Effect:
a) Composition of some injectable fuels:

  • Natural Gas: CH4
  • Oil:CH2
  • Coal: CH

Note: Natural gas has the highest hydrogen level and, as indicated on the table on the next page, can substitute for more coke than any other fuel.

b) One pound of carbon from an injectable fuel will substitute for about one pound of carbon from coke. However, one pound of hydrogen from injectable fuel substitutes for about three pounds of carbon from coke. Hydrogen is beneficial as it "eases" furnace operation by providing:

  • Fewer furnace "slips" - the burden moving down suddenly.
  • Improves chemical reduction of "acid pellets" (and may be more effective than converting to "Fluxed" pellets).
  • Reduces pressure drop through the furnace.

Use of each supplemental fuel brings some consideration, beyond price and availability, which affects the economics of coke replacement in the blast furnace. The following table summarizes some of the major differences between injectants:

Competitive Coal Injection Systems / Possibility of Co-injecting Coal and Natural Gas
High-rate coal injection is the major competitor to natural gas injection in the blast furnace. Coal injection is strong in Japan and is used widely in Europe. A number of systems have been installed in the U.S as well. However, in addition to the hydrogen effect, a key benefit for gas injection systems is the relatively lower capital cost for installation compared with a coal injection method.