Question

The most efficient equipment for removal of sub-micron dust particles from blast furnace gas is

• A. Venture atomiser
• B. Gravity settling chamber
• C. Electro-static precipitator
• D. Cyclone separator

Hint:

Particulate Collection Equipment Selection Guide: - Large coarse dust (\(\gt 50\,\mu\text{m}\)) \(\implies\) Gravity Settling Chamber. - Medium particles (\(\gt 10\,\mu\text{m}\)) \(\implies\) Cyclone Separator. - Sub-micron fine dust (\(\lt 1\,\mu\text{m}\)) \(\implies\) Electrostatic Precipitator (ESP) or Fabric Baghouse.

Answer 1

The Correct Option is C

Concept: Industrial exhaust streams, such as blast furnace gas, contain particulate matter with wide variations in particle size. Separating sub-micron particles (diameters less than \(1\,\mu\text{m}\)) is challenging because their mass is too small for standard mechanical separation methods like gravity or centrifugal force to be effective. An Electrostatic Precipitator (ESP) is an industrial gas cleaning device that uses electrical energy to charge and remove fine dust particles. It can collect sub-micron particles with efficiencies often exceeding \(99\%\) while imposing minimal pressure drop on the gas stream.

Step 1: Analyzing the collection limits of mechanical separation equipment.
Let us evaluate why standard mechanical options fail to capture sub-micron dust effectively:

• Gravity Settling Chambers: Rely purely on gravitational settling velocity (\(v_t \propto D_p^2\)). For sub-micron particles, the settling velocity is nearly zero, meaning they remain suspended and pass straight through the chamber.

• Cyclone Separators: Use centrifugal force to throw particles outward against a wall. Their collection efficiency drops sharply for particles smaller than \(5\,\mu\text{m}\) due to fluid drag limitations.

Step 2: Evaluating the operating mechanism of an ESP.
Inside an electrostatic precipitator, the dirty gas stream passes through an intense electrical field generated between high-voltage discharge electrodes and grounded collecting plates. This process involves:

• High voltage ionizes the surrounding gas, producing electrons via corona discharge.

• The moving dust particles collide with these ions and acquire a net negative electrical charge.

• The charged sub-micron particles are pulled toward the positively grounded collecting plates by electrostatic forces, regardless of their small mass.
This allows ESPs to easily capture ultra-fine sub-micron dust particles from blast furnace gas.