MECH6019 Chap.4 Combustion Pollutant Concentration
Combustion Pollutant Concentration
Combustion concentration calculations build the exhaust denominator from a fuel and air balance. Begin on a stated fuel basis, calculate theoretical oxygen and actual air, and list every product species. The pollutant numerator may be given or derived, but its reported fraction cannot be known until the total product amount is fixed.
Nitrogen and unused oxygen are therefore as important to the denominator as the pollutant is to the numerator.
Wet products include chemically formed water and any moisture entering with fuel or air. Dry products remove all water from the total without changing moles of the pollutant. This necessarily raises the dry fraction. Convert by reconstructing wet and dry totals rather than applying an unexplained factor.
Keeping water on its own line makes the change auditable and prevents accidental removal of hydrogen-containing pollutant species.
Excess air increases total exhaust and can lower a measured concentration without lowering pollutant production. Oxygen-reference correction normalises this dilution by relating measured oxygen to a specified reference.
The measured and reference oxygen must use compatible moisture bases, and the equation's direction should be checked physically: a more diluted stream is corrected upward toward the reference condition.
Mole fraction, ppmv and percent describe mixture ratios. Mass concentration additionally requires molecular weight and molar volume at stated temperature and pressure.
Emission rate requires volumetric flow on that same reference basis. A complete chain may therefore move from balanced product moles to dry ppmv, then to milligrams per cubic metre and finally kilograms per hour. Each transition introduces information that should appear in the working.
A source-control comparison should report more than one number.
Corrected concentration supports comparison with an outlet limit, while mass rate supports a material balance and total-load claim. Pollutant per unit useful energy supports comparison between equipment operating at different outputs. If only raw ppm falls after air flow rises, the evidence supports dilution but not necessarily control.
Use three closing checks. Product fractions must sum to one on a single basis.
Dry concentration should exceed wet concentration when only water is removed. Inlet pollutant mass must equal captured plus emitted mass across a device. These direction and conservation checks expose most denominator errors before they become a final answer.
What this chapter covers
- 01
Exhaust product balance
- 02
Wet concentration
- 03
Dry concentration
- 04
Excess-air dilution
- 05
Reference oxygen
- 06
Mass emission rate
Worked example · free
Wet to dry percentage
- 1Remove water from the total.
- 1Use 80 mol dry total.
- 1Calculate 7.5%.
Key terms
- Dry basis
- Gas composition calculated after excluding water vapour.
- Wet basis
- Gas composition calculated with water vapour included.
- Oxygen correction
- Normalisation of concentration to a stated reference oxygen.
- Dilution
- Reduction in concentration caused by adding carrier gas.
Combustion Pollutant Concentration FAQ
Why is dry concentration usually higher?
Removing water shrinks the denominator while pollutant moles remain unchanged. The dry fraction therefore rises for the same products. Water from fuel moisture and chemically formed water both belong in the wet total, but neither remains when the stated result is converted to dry basis.
Can more excess air lower concentration?
Yes. Extra air increases exhaust volume and can dilute ppm even when pollutant mass is unchanged, which is why oxygen correction and mass flow matter. Compare values on the same oxygen and moisture basis, then use compatible volumetric flow to determine whether the emitted mass actually changed.
What belongs in the final label?
State pollutant, unit, wet or dry basis, reference temperature and pressure, and reference oxygen whenever a correction was applied. If the result becomes a mass emission rate, add compatible gas flow and the time basis so the material balance can be reconstructed.
Exam move
Reconstruct wet and dry totals from species moles rather than memorising a shortcut. Test whether the result moves in the physically correct direction. Start from a complete combustion-product table and calculate total wet moles. Copy the table, cross out only water and recalculate the dry total. Use the unchanged pollutant numerator to obtain both fractions and explain why the dry value is higher.
Add excess air to the same problem and watch nitrogen and oxygen enlarge the denominator. This makes dilution visible before an oxygen correction formula is introduced. Practise oxygen correction only after writing measured and reference oxygen on compatible moisture bases. Predict the correction direction before calculating.
Then convert one mole fraction to ppmv, mass concentration and mass flow, carrying molecular weight, molar volume and volumetric flow with reference conditions. Close a simple device balance by showing inlet mass equals captured plus outlet mass. Keep three coloured annotations in your working: basis changes, unit changes and physical-stream changes.
A result can survive one category while failing another, so do not collapse them into a single check. During final review, choose one calculation and deliberately introduce an error, such as removing water from the numerator, forgetting nitrogen or multiplying normal concentration by actual flow. Diagnose it using sum-to-one, direction and unit checks.
Being able to locate the fault is more valuable than memorising a corrected answer.
