Gas cleaning for sulphur dioxide (SO2) Which material is used as an adsorbent or chemisorbent to remove SO2 from gas/air streams in control equipment?

Introduction / Context:
Sulphur dioxide control is central to flue-gas cleaning for boilers, furnaces, and process vents. Engineers can choose among dry, semi-dry, and wet scrubbing routes. Some media physically adsorb SO2, while others remove it by chemisorption or neutralisation, which in practice is often grouped under “adsorbent” in exam contexts when a solid bed is used.

Given Data / Assumptions:

• Removal of SO2 from gas or air.
• Focus on common solid media used industrially or in teaching examples.
• Terminology allows chemisorption on solids to be considered within an adsorbent framework.

Concept / Approach:
Alkalised alumina (sodium-impregnated alumina) removes SO2 by chemisorption, forming sulphates/sulphites; it has been used in fixed-bed desulphurisation. Limestone powder (CaCO3) is widely used in dry and wet scrubbing where it neutralises SO2 to calcium sulphite/sulphate; while often described as an absorbent in slurries, dry injection processes effectively utilise the solid surface for reaction. Thus the option that includes alkalised alumina (a canonical SO2 chemisorbent) is the best match among the choices provided.

Step-by-Step Solution:

Verification / Alternative check:
Process control texts describe fixed-bed alkalised alumina systems and limestone-based FGD as standard methods for SO2 removal.

Why Other Options Are Wrong:

• Bog iron: Used historically for H2S removal, not primary SO2 sorbent.
• Silica gel: A desiccant; poor affinity for SO2 removal applications.
• Active carbon: Can adsorb SO2 with impregnation, but the given option expressly names a classic chemisorbent.
• Zeolite 13X: Used for drying and hydrocarbon separation; not typical for SO2 control.

Common Pitfalls:
Confusing wet absorption chemistry with solid-bed chemisorption terminology; exams may treat both under a broad “adsorbent” label depending on context.

Final Answer:
Limestone powder or alkalised alumina