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Qwestrum Engineering360 · Chemical Engineering · Process Calculations

Combustion Calculations

Combustion calculations balance a fuel against air, compute the stoichiometric oxygen from the C, H and S content, add a stated percentage of excess air, and then report flue-gas composition on either a wet or a dry (Orsat) basis.
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.

  • ExcessairExcess air % = \frac{(O_{2} supplied - O_{2} stoichiometric)}{O_{2}} stoich \times 100
  • Orsat analysis gives dry basis composition
  • Lower heating value excludes latent heat of water vapour

Topic details

Introduction

Bhatt & Vora treat combustion as a reactive material balance with air as the oxidant. You take a convenient basis (100 mol fuel or 100 kg fuel), write the complete-combustion reactions, sum the theoretical oxygen, scale up by the excess-air factor, and carry the accompanying nitrogen (79/21 per mole of O₂) through unreacted. The flue-gas analysis then follows from a species tally.

Key relations & formulas

C_{xH}_y + (x + \frac{y}{4}) O_{2} → x CO_{2} + (\frac{y}{2}) H_{2}O
(complete combustion)
O2required=32(x+y4)gpermolfuelO_{2} required = 32(x + \frac{y}{4}) g per mol fuel
(air: 21% O₂ by volume)

Formulas (Indian textbook notation)

  • Wetanddryfluegas:subtractH2OfromwetbasisfordryanalysisWet and dry flue gas: subtract H_{2}O from wet basis for dry analysis

Notation and sign conventions

Relation 1 —
C_{xH}_y +
C_{xH}_y + (x + \frac{y}{4}) O_{2} → x CO_{2} + (\frac{y}{2}) H_{2}O
(complete combustion)
Write this relation with symbols exactly as in Stoichiometry — Bhatt & Vora before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
O2required=32O_{2} required = 32
O2required=32(x+y4)gpermolfuelO_{2} required = 32(x + \frac{y}{4}) g per mol fuel
(air: 21% O₂ by volume)
Write this relation with symbols exactly as in Stoichiometry — Bhatt & Vora before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
Wetanddryfluegas:subtractH2OfromwetbasisfordryanalysisWet and dry flue gas: subtract H_{2}O from wet basis for dry analysis

Formulas (Indian textbook notation)

  • Wetanddryfluegas:subtractH2OfromwetbasisfordryanalysisWet and dry flue gas: subtract H_{2}O from wet basis for dry analysis
Write this relation with symbols exactly as in Stoichiometry — Bhatt & Vora before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

The heart of the topic is the excess-air concept. Theoretical air supplies exactly the oxygen for complete combustion; real furnaces use excess air to ensure burnout, so unused O₂ and the full nitrogen appear in the stack gas. Orsat apparatus absorbs CO₂, O₂ and CO and reports composition on a dry basis, so the water of combustion must be removed before comparing with an Orsat reading. Heating value distinguishes higher (water condensed) from lower (water as vapour) — the latent heat of the product water is the whole difference.

Assumptions and validity limits

State assumptions explicitly before using any relation for combustion calculations — 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 Process Calculations 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 Process Calculations 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 combustion calculations.
4. Use equation 1:
C_{xH}_y +
.
5. Use equation 2:
O2required=32O_{2} required = 32
.
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

Combustion Calculations appears in every chemical process design. In Indian chemical curricula this topic is tested because it connects theory to material and energy balances.
GATE and semester exams often combine combustion calculations with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use combustion calculations?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

The most frequent trap is reporting wet-basis composition when the question (or Orsat data) is dry basis, and vice versa. Others are forgetting nitrogen carried in with excess air, computing excess air on a mass basis when it should be molar, and omitting the sulphur-to-SO₂ oxygen demand.

Quick revision checklist

Before attempting combustion calculations problems, confirm you can:
1.
ExcessairExcess air % = \frac{(O_{2} supplied - O_{2} stoichiometric)}{O_{2}} stoich \times 100

2. Orsat analysis gives dry basis composition
3. Lower heating value excludes latent heat of water vapour
Revise the solved examples in Stoichiometry — Bhatt & Vora 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.

Excess air for methane

Problem

Methane is burned with 20% excess air. How many moles of air are supplied per mole of CH₄?

Solution

CH₄ + 2O₂ → CO₂ + 2H₂O, so theoretical O₂ = 2 mol. With 20% excess, O₂ = 2.4 mol. Air = O₂/0.21 = 2.4/0.21 = 11.43 mol air per mol methane.

Conceptual check — Combustion Calculations

Problem

In a Process Calculations semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of combustion calculations." What should a complete answer include?

Exams & GATE

Bhatt & Vora combustion examples — watch mole vs mass basis.

📖 Standard books (India)

  • Stoichiometry — Bhatt & Vora

    Read: Syllabus unit

    Process calculations for chemical engineering

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