MECH6019 Chap.3 Chemical Kinetics and Pollutant Formation
Chemical Kinetics and Pollutant Formation
Kinetics explains how quickly pollutant-forming reactions proceed, while equilibrium describes the composition approached if a reversible system has enough time. A reaction may be thermodynamically favoured but too slow during the available residence time. Conversely, a rapid high-temperature route may create species that remain as the gas cools because reverse reactions become slow.
Air-pollution analysis therefore needs both the equilibrium direction and the rate at operating conditions.
A rate law converts concentration into formation speed. For an elementary bimolecular reaction, a form such as r = k[A][B] means the rate is first order in each reactant. Doubling A at fixed B and temperature doubles the rate; doubling both multiplies it by four.
Reaction order for an overall process is not safely inferred from the overall stoichiometric equation. It comes from the mechanism or measured rate relationship. Units of the rate constant provide a useful consistency check.
The rate constant changes strongly with temperature through Arrhenius behaviour. Higher temperature makes the negative activation-energy exponent less restrictive, raising k.
Peak temperature alone is not the full history: formation also depends on how long gas remains hot and whether mixing brings reactants together. A control that shortens residence time can reduce total formation even if the local peak changes little.
Nitrogen oxides illustrate the need for pathway diagnosis.
Thermal NOx begins mainly with atmospheric nitrogen in a hot flame and is sensitive to temperature and hot residence time. Prompt NOx is associated with rapid radical chemistry near the flame front. Fuel NOx begins with nitrogen bound in the fuel and depends on its conversion pathway.
These routes can coexist, but a fuel switch, flame modification or downstream treatment will not affect them equally.
Other combustion pollutants also reveal mechanism. Carbon monoxide and unburned hydrocarbons indicate incomplete oxidation, mixing limitations or insufficient time. Sulphur oxide formation follows sulphur entering with fuel, while particulate formation depends on fuel properties and combustion conditions.
A response should identify the precursor, reaction zone and operating variable before recommending a control.
Measured outlet concentration is a net result of formation, destruction, dilution and flow. To attribute a trend, hold important variables fixed or explicitly normalise them. A higher rate constant does not prove a higher final amount, and a lower outlet concentration does not prove slower formation.
The strongest analysis states what changed, which rate term it affects and what evidence would distinguish that explanation from a competing one.
What this chapter covers
- 01
Reaction rate
- 02
Reaction order
- 03
Rate constant
- 04
Arrhenius behaviour
- 05
Chemical equilibrium
- 06
NOx formation routes
Worked example · free
Read a bimolecular rate law
- 1Hold k and B fixed.
- 1Conclude that rate doubles.
Key terms
- Reaction order
- The exponent describing rate sensitivity to concentration.
- Rate constant
- The proportionality factor in a rate law at stated conditions.
- Thermal NOx
- NOx formed mainly from atmospheric nitrogen at high temperature.
- Fuel NOx
- NOx derived from nitrogen bound in fuel.
Chemical Kinetics and Pollutant Formation FAQ
Are kinetics and equilibrium interchangeable?
No. Kinetics addresses speed, while equilibrium addresses the limiting composition. A favoured reaction may still be too slow in the available residence time. Rapid cooling can also preserve a composition formed upstream because reverse reactions become slow before equilibrium is restored.
Why does temperature matter strongly?
The Arrhenius relation makes the rate constant increase rapidly with temperature, which is central to high-temperature pollutant formation. Peak temperature, hot residence time and reactant mixing should still be separated because each can change total formation through a different part of the mechanism.
Why identify the NOx route?
Thermal, prompt and fuel pathways respond to different levers. A fuel change may affect fuel NOx without eliminating thermal formation from air nitrogen. Temperature, residence time, flame-front chemistry and fuel-bound nitrogen should therefore be diagnosed before a control is selected.
Exam move
Translate each rate law into a verbal sensitivity statement, then connect each NOx pathway to its nitrogen source and reaction zone. Begin with proportional drills before inserting numbers. For each rate expression, double one concentration, halve another and state the change while temperature remains fixed. Check the units of the rate constant against the overall order.
Next sketch an Arrhenius curve and explain why the rate constant changes rather than the reactant concentration term. Add residence time as a separate control variable so rate is not confused with total formation. Build a three-column NOx table for thermal, prompt and fuel pathways. Record the nitrogen pool, reaction region, sensitive operating variable and first plausible control for each.
Test the table with mixed cases: a hot low-nitrogen gas flame, a nitrogen-bearing solid fuel and a rapidly mixed flame front. Require yourself to justify the dominant pathway rather than merely name it. Compare equilibrium and kinetics using one sentence each: equilibrium describes the limiting composition, while kinetics describes approach speed.
Then analyse an outlet trend by listing four possible causes: changed formation rate, changed destruction rate, changed dilution and changed flow. Identify which measurements would distinguish them. In the final week, practise short mechanism paragraphs that move from precursor to reaction condition to control and verification.
This structure earns more than a technology list because it shows why the intervention reaches the observed pollutant.
