“In the name of ALLAH, The Most Beneficent

and The Most Merciful”

Subject:

“Ferrous Metallurgy”

Topic:

“Blast Furnace Exhaust Gas Cleaning”

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Objectives:

“The removal of certain elements (impurities) present in the Blast furnace exhaust gas and to prevent plugging and damaging of the checkers or burners and keep dust away to be exposed in combustion chamber”.

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Reason;

“If not removed these impurities would cause problems while burning in boiler-house or burning in blast furnace stoves”.

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General description:

As, the blast furnace gas leaves the top of the furnace, it contains dust particles varying in size from about 6mm(about ¼ inch) to only a few microns(1 micron equals 0.001mm or about 0.00004 inch). The dust is carried out of the top of the blast furnace in the gas stream is generally referred to as flue dust,

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Continued,….

(flue dust) it normally contains dust particles along with fine particles of coke, burden material and chemical compounds that are formed in the blast furnace reactions. Sometimes there may be large lumps of coke, when the burden slips in the blast furnace operation.

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????

Q. How to clean the Blast Furnace Gas?

Ans. Using,

Blast Furnace Gas Cleaning plant;

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Blast Furnace

A Gas Cleaning Plant

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Primary Gas Cleaning

Primary Gas Cleaning consists of Dustcatchers, Cyclones or a combination of both systems(choice of equipment depends upon demand). However, where is a need to remove higher percentages of dust and reduce the water treatment requirements, the dustcatcher has been replaced by a cyclone.

Previously blast furnace gas cleaning systems consisted of a gravity dustcatcher, with variations in the form of secondary cleaning and cooling equipment.

13a dustcatcher

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In this stage ,the blast furnace gas is usually passed through a dustcatcher, where almost all of the particles coarser than 0.8mm (0.03 inch) are removed. The dustcatcher is a large, cylindrical structure and 20 to 30 meters(about 33 to 39 feet) in diameter and 20 to 30 meters (about 66 to 98 feet) high. It is usually lined with brick or gunite to insulate it and prevent condensation of moisture in the gas so that the dust will remain relatively dry and will not ball up but will flow freely into the conically shaped section at the bottom of the dustcatcher.

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The gas is conducted to the dustcatchers by a single downcomer and enters through the top by a vertical pipe that carries the gas downward inside the dustcatcher. These pipes flares outward at its lower extremity like an inverted funnel, so that as the gas passes downward its velocity (and thus its dust carrying potential) decreases, and most of the dust coarses than 0.8mm(0.03 inch) drops out of the gas stream and is deposited in the cone at the bottom of the dustcatcher, because the bottom of the dustcatcher is closed, and the gas outlet is near the top, the direction of the travel of the gas must reverse 180 degrees. This sudden reversal in the direction of flow causes more of the dust to drop out of the gas stream.

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The dust that accumulates in the

dustcatcher is generally removed through a pug mill attached to the bottom cone below shut-off valve. Water is added to the dust as it passes through the pug mill to moisten it and prevent it from blowing into the atmosphere as it is discharged. Each Blast Furnace has set schedule of emptying dustcatcher.

Care is taken to assure that massive amounts of gas are not discharged with the dust.

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Secondary Gas Cleaning

After the gas has passed through the dry dustcatcher, It generally goes to a wet-cleaning system where the very fine particles of dust are literally scrubbed out of the gas with water. In modern Blast Furnace plants high-energy high efficiency scrubbers are used that clean the gas to a dust content less than 12-15 grams per cubic meter (0.005 grain per standard cubic feet), in one operation.

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Conditioning Tower with SingleCone Scrubber

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Conditioning Tower with TripleCone Scrubber

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Primary Cyclone with Conditioning Tower

and Triple Cone Scrubber

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Process

Now-a-days, Siemens VAI gas scrubber has been developed with the option of one or three annular scrubbing units.

The scrubber completes the gas cleaning process to a guaranteed dust content of less than 5 mg/Nm3, whilst controlling the furnace pressure to within 1.5% of its set point.

References:

“The Making, Shaping and Treating of steel” By

“Harold E. Mcganon” (Page # 561-565)

www.siemens-vai.com\blast furnace technology

The End

Prepared & Presented by:

1. Asad Kaleemullah (MME-07-09)2. Zahid Nazir (MME-07-16)3. Muhammad Usman Asghar (MME-

07-17)