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Qwestrum Engineering360 · Environmental Engineering · Air Pollution Control

Gaseous Pollutant Control

Gaseous-pollutant control targets acid gases, VOCs, and NOx through transfer, reaction, or catalytic conversion mechanisms. Proper technology choice depends on pollutant chemistry and concentration range.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Adsorption activated carbon VOC
  • Combustion thermal catalytic oxidiser
  • Catalytic converter automobile three-way

Topic details

Introduction

Unlike particulate removal, gas control frequently requires phase transfer and chemical transformation. Metcalf & Eddy and standard air-pollution texts discuss absorption, adsorption, and catalytic routes as distinct design families.

Scope in B.Tech and GATE syllabus

Indian regulatory control programs, especially for thermal and process industries, emphasize SO₂ and NOx abatement using FGD and SCR systems. Exam questions usually test mechanism and stoichiometric understanding together.

Key relations & formulas

Formulas (Indian textbook notation)

  • absorptionHenry′slawCl=H×Cgabsorption Henry's law C_{l} = H \times C_{g}

Formulas (Indian textbook notation)

  • SCRNOx+NH3→N2+H2OwithcatalystSCR NOx + NH_{3} → N_{2} + H_{2}O with catalyst

Formulas (Indian textbook notation)

  • FGDSO2+limestone→gypsumFGD SO_{2} + limestone → gypsum

Notation and sign conventions

Relation 1 —
absorptionHenry′slawCl=H×Cgabsorption Henry's law C_{l} = H \times C_{g}

Formulas (Indian textbook notation)

  • absorptionHenry′slawCl=H×Cgabsorption Henry's law C_{l} = H \times C_{g}
Write this relation with symbols exactly as in Air Pollution Control — CP Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
SCRNOx+NH3→N2+H2OwithcatalystSCR NOx + NH_{3} → N_{2} + H_{2}O with catalyst

Formulas (Indian textbook notation)

  • SCRNOx+NH3→N2+H2OwithcatalystSCR NOx + NH_{3} → N_{2} + H_{2}O with catalyst
Write this relation with symbols exactly as in Air Pollution Control — CP Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
FGDSO2+limestone→gypsumFGD SO_{2} + limestone → gypsum

Formulas (Indian textbook notation)

  • FGDSO2+limestone→gypsumFGD SO_{2} + limestone → gypsum
Write this relation with symbols exactly as in Air Pollution Control — CP Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Absorption transfers pollutants from gas to liquid phase based on solubility and mass-transfer driving force, with Henry's law offering equilibrium insight. Packed-tower design then depends on transfer units and solvent regeneration strategy.

Governing relations in practice

SCR reduces NOx using ammonia over catalysts under suitable temperature windows, producing nitrogen and water. Reaction efficiency and ammonia slip management are key operational concerns.

Design and analysis considerations

FGD removes SO₂ by reacting flue gas with alkaline slurry, commonly limestone, producing gypsum byproduct. Adsorption and thermal oxidation are preferred for VOC streams where wet-scrubbing is less effective or economically unsuitable.

Assumptions and validity limits

State assumptions explicitly before using any relation for gaseous pollutant control — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Air Pollution Control viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Air Pollution Control papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to gaseous pollutant control.
4. Use equation 1:
absorptionHenry′slawCl=H×Cgabsorption Henry's law C_{l} = H \times C_{g}
.
5. Use equation 2:
SCRNOx+NH3→N2+H2OwithcatalystSCR NOx + NH_{3} → N_{2} + H_{2}O with catalyst
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Gaseous Pollutant Control appears in industry stack emissions. In Indian environmental curricula this topic is tested because it connects theory to particulate and gaseous control.
GATE and semester exams often combine gaseous pollutant control with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use gaseous pollutant control?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Applying Henry's law without consistent phase concentration units
• Ignoring required temperature window in SCR discussions
• Writing incomplete stoichiometry for SO₂ removal reactions
• Confusing adsorption with absorption in descriptive answers

Quick revision checklist

Before attempting gaseous pollutant control problems, confirm you can:
1. Adsorption activated carbon VOC
2. Combustion thermal catalytic oxidiser
3. Catalytic converter automobile three-way
Revise the solved examples in Air Pollution Control — CP Rao and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

If Henry constant H

Problem

If Henry constant H = 1.8 and gas concentration C_g = 0.05 mol/m³ (equivalent basis), equilibrium liquid concentration C_l = H×C_g = 0.09...

Solution

If Henry constant H = 1.8 and gas concentration C_g = 0.05 mol/m³ (equivalent basis), equilibrium liquid concentration C_l = H×C_g = 0.09 mol/m³.

Conceptual check — Gaseous Pollutant Control

Problem

In a Air Pollution Control semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of gaseous pollutant control." What should a complete answer include?

📖 Standard books (India)

  • Air Pollution Control — CP Rao

    Read: Syllabus unit

    Indian environmental engineering reference

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