Sodium Bicarbonate Powder Air Classifier Mill Low Emissions

Sodium bicarbonate can be used an adsorbent for acid gases,such as increasing the relative humidity in the exhaust gas. The deposition occurs as a gas-solid reaction, usually at temperatures of 180C to 200C and residence times of less than 2 seconds.

The sodium bicarbonate is first ground to increase its active surface area and injected directly after the mill into the hot exhaust stream. There the thermal decomposition, also called activation, takes place:

2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

This reaction can be observed, to a smaller extent, beginning with temperatures of around 60C. A highly porous structure, which provides a large surface area and thus ensures a high level of activity for further reactions of the produced Na2CO3 arises through the degassing of CO2 and water. Figure 1 shows scanning electron microscope recordings of surfaces of bicarbonate particles before activation, and Na2CO3, which occurred during activation.

Before and after. Scanning electron microscope pictures of a NaHCO₃ particle before (left) and a Na₂CO₃ particle after (right) thermal activation. 

Even if a certain proportion of the acidic components directly react with the bicarbonate, about 85% of the adsorbent is decomposed. Therefore the actual reaction partner is soda-ash:

Na₂CO₃ + 2 HCl → 2 NaCl + CO₂ + H₂O

Na₂CO₃ + 2 HF→ 2 NaF + CO₂ + H₂O

The reaction pathways for the reduction of sulfur oxides are more complex and are visualized in Figure 2.

2. Reaction pathways of sulfur oxides with sodium bicarbonate for flue gas cleaning. The byproducts CO₂ and H₂O are indicated only for the thermal activation reaction. Source: Dr. Peter Quicker et al.

The reaction of Na₂CO₃ with SO₂ is highly temperature-dependent and can be described using the Arrhenius equation. Therefore, the reaction rate can be almost tripled by increasing the temperature from 160C to 220C. However, this behavior at temperatures above 400C is limited by sintering processes within the grain (the surface structure of the particles). This sintering affects both the sodium carbonate and its reaction products.

The reaction is limited with larger particle diameters, above 30µm, by pore diffusion.

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