How Bag Filters Can Remove Both SO₂ and Dust in One System

In industries like steel, power, and sintering plants, flue gas usually contains two major pollutants:

• Sulphur dioxide (SO₂)
• Fine dust particles

Traditionally, these are treated using separate systems. But this study explored something smarter — combining dry desulfurization and dust collection into one integrated bag filter system.

Let’s break it down in simple terms.

What Was the Idea?

Instead of installing a separate desulfurization tower and then a bag filter, the researchers injected sodium bicarbonate (NaHCO₃) directly into the duct before a bag filter.

This created a two-stage reaction:

1. Injection Section (Duct Reaction)

• Sodium bicarbonate particles mix with hot flue gas.
• It reacts with SO₂ and starts neutralizing it.

2. Bag Filter Section

• The remaining particles form a thin layer (reaction bed) on the filter bags.
• SO₂ continues reacting inside this dust layer.
• Meanwhile, dust is captured with >99.9% efficiency.

So essentially, the filter cake itself becomes a chemical reactor.

Why Sodium Bicarbonate?

Sodium bicarbonate works well in dry systems because:

• It decomposes when heated.
• It forms sodium carbonate.
• It reacts with SO₂ to form harmless sodium salts.
• It creates micro-pores after heating, which improve gas absorption.

This makes it suitable for compact, dry desulfurization systems, especially where water-based systems are not preferred.

Inside vs Outside Filtration – Which Is Better?

The study compared:

• Inside filtration bags
• Outside filtration bags

Result:

• Inside filtration performed about 6–7% better in SO₂ removal.
• Dust removal efficiency stayed above 99.9% in both cases.

For practical applications, this means bag design and airflow direction matter if you want better gas removal.

Key Factors That Affect Performance

1. Temperature

Best performance was observed around 150°C.

• From 130°C to 150°C → Efficiency increased
• Above 150°C → Efficiency decreased

Why? Because sodium bicarbonate decomposes too much at higher temperatures, reducing its reactive ability.

👉 For industrial applications, temperature control is critical.

2. Sodium-to-Sulphur Ratio (NSR)

This is the ratio of chemical injected compared to SO₂ present.

• Increasing NSR improves removal.
• But after 1.5 ratio, improvement becomes small.
• At NSR = 1.5, efficiency reached around 91% (inside filtration).

👉 Adding excess chemical increases cost without much benefit.

3. Particle Size of Sodium Bicarbonate

Smaller particles performed much better. Why?

• More surface area
• Faster reaction
• Better pore formation
• Improved gas-solid contact

If you are planning a system, fine milling of sodium bicarbonate is important.

4. Inlet SO₂ Concentration

Interestingly:

• Increasing SO₂ concentration had only minor impact.
• Slight drop in efficiency in inside filtration.
• Almost no change in outside filtration.

This suggests the system is stable even with fluctuating SO₂ loads.

5. Effect of Dust Presence

This is very interesting.

When artificial dust (1000 mg/m³) was added:

• SO₂ removal increased by around 10%.
• Reaction became more effective.

Why?

• Dust created turbulence in the duct.
• Filter cake became thicker.
• Gas stayed longer inside the system.
• More contact time = better reaction.

This means in real industrial conditions (where dust already exists), performance may actually be better than lab conditions.

Overall Performance Achieved

Under optimized conditions:

• SO₂ removal up to 96%
• Dust removal above 99.9%

That’s quite impressive for a dry compact system.