Emissions and Fuels

CO₂, water, CO, unburned hydrocarbons, NOx and soot, thermal NO and the Zeldovich mechanism, the NOx–CO trade, the ICAO LTO cycle, SAF and hydrogen, and the fuel system

Moscow, 1946

Nitrogen makes up four-fifths of the air, and it is famously inert — we breathe it in and out unchanged. Yet flames make nitrogen oxides. In 1946 the young Soviet physicist Yakov Zeldovich worked out the mechanism: at flame temperatures, oxygen atoms are energetic enough to break the triple bond of N₂, starting a short chain of reactions that turns nitrogen into nitric oxide (the Zeldovich mechanism). It is the reason a combustor's hottest zone is also its dirtiest, and the reason modern combustors are designed around one temperature band. This lesson covers what an engine emits, how the emissions are measured and regulated, and what may replace kerosene.

Burning a kilogram of Jet A produces about 3.16 kg of CO₂ and 1.24 kg of water — unavoidable products of burning a hydrocarbon. The pollutants are the small, avoidable remainder:

Carbon monoxide and unburned hydrocarbons: fuel that did not finish burning, mostly at low power, when the flame is cool and the pressure low.
Nitrogen oxides (NOx): formed in the hottest regions, mostly at high power.
Soot (non-volatile particulate matter): formed in fuel-rich pockets, then mostly burned out.

\mathrm{N_2 + O \;\rightleftharpoons\; NO + N}, \qquad \mathrm{N + O_2 \;\rightleftharpoons\; NO + O}, \qquad \mathrm{N + OH \;\rightleftharpoons\; NO + H}

The extended Zeldovich mechanism of thermal NO. The first reaction, with an activation temperature of 38,370 K, is the slow step that sets the rate.

Derivation

Why thermal NO depends so strongly on temperature

The first step breaks the N≡N bond, one of the strongest in chemistry, so its rate constant has a huge activation temperature:

k_1 \propto e^{-38\,370/T}

. Once an N atom is made it reacts quickly in the second or third step, making a second NO. So the formation rate is about

\frac{d[\mathrm{NO}]}{dt} \approx 2\,k_1\,[\mathrm{N_2}]\,[\mathrm{O}],

and the oxygen-atom concentration itself rises steeply with temperature through dissociation. Lesson 7-2 found the rate of the first step doubling every 41 K at 1500 K; with the O atoms, a flame 200 K cooler makes several times less NO.

📐Thermal NO in a few milliseconds

Hot flame gas left to react, with its NO removed at the start (Cantera, GRI-Mech 3.0, methane as a stand-in; Lab 1):

$\phi = 1.0$ , 2613 K: about 3400 ppm after 2 ms, near equilibrium already.
$\phi = 0.8$ , 2403 K: 2400 ppm after 2 ms, still climbing to 6900 ppm at 20 ms — more oxygen, more NO in the end.
$\phi = 0.6$ , 2097 K: only 81 ppm after 2 ms.

At a combustor's residence time of a few milliseconds, the lean flame makes 42 times less NO.

💡The NOx–CO trade and how combustors escape it

Make the flame hot and NOx rises; make it cool and CO and unburned fuel survive. The cure is to keep the burning zone in a narrow band — roughly 1700–1900 K — at every power setting:

Lean premixed burning (the TAPS family): fuel and air mixed before the flame, so no hot stoichiometric pockets.
Staging: a pilot flame for low power, main stages switched in as power rises, so each stays in the band.
Rich-quench-lean: burn rich (little NO, no free oxygen), then quench with air so fast that the gas skips the hottest mixtures.

📐The ICAO landing and take-off cycle

Engine emissions are certified over a standard cycle below 3000 feet: take-off at 100 % thrust for 0.7 min, climb-out at 85 % for 2.2 min, approach at 30 % for 4 min and taxi/idle at 7 % for 26 min. For an illustrative 120 kN engine (Lab 2):

431 kg of fuel over the cycle; 6.4 kg of NOx, most of it at take-off and climb-out.
The regulated figure is the mass per unit rated thrust: $D_p/F_{00}$ = 53 g/kN of NOx.
Idle burns 40 % of the cycle's fuel and makes 95 % of its CO.

Fuels. Sustainable aviation fuels (SAF) — kerosene made from waste oils, biomass, or hydrogen and captured CO₂ — are close enough to Jet A to be blended into it and burned in today's engines; their benefit is in the carbon they did not take from the ground. Hydrogen burns with no CO₂ at all and very cleanly, but it is bulky: liquid hydrogen carries 120 MJ/kg against 43, yet only 8.5 MJ per litre against 34.4 (Lab 3). A mission that would burn 20 t of Jet A needs 7.2 t of hydrogen — and 101 m³ of insulated tank instead of 25 m³ of fuel in the wings.

The fuel system must deliver clean fuel without ice, from the cold of a long cruise — Jet A freezes at −40 °C — to the engine's hot fuel nozzles. On 17 January 2008 British Airways Flight 38, a Boeing 777 arriving from Beijing, came down short of the runway at Heathrow when its engines did not respond on final approach. Ice had formed from water in the fuel during the long, cold flight, built up in the fuel lines, and broke loose on the approach to clog the fuel/oil heat exchangers. All 152 on board survived, and Rolls-Royce redesigned the heat exchanger (British Airways Flight 38).

I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable.

– Jules Verne, The Mysterious Island (1874) (Wikiquote)

⚠Not only the ground

The landing and take-off cycle covers only what happens near airports. At cruise altitude, engine NOx, water vapour and soot also affect ozone, contrails and cirrus clouds — climate effects comparable to those of the CO₂ by some estimates, and an active field of research.

💡Where you will use this

Lesson 10-2 returns to hydrogen and electric propulsion. The same emissions chemistry governs industrial gas turbines, which meet stricter NOx limits with lean premixed burners. Unit 8 puts the combustor back into the whole engine.

📚Further reading

Arthur H. Lefebvre and Dilip R. Ballal, Gas Turbine Combustion (3rd ed., CRC Press, 2010), on emissions.
Stephen R. Turns, An Introduction to Combustion (3rd ed., McGraw-Hill, 2012), on pollutant formation.
The Zeldovich mechanism and British Airways Flight 38 on Wikipedia.

↓But there is a problem

Seven units have taken an engine apart: inlet, compressor, combustor, turbine, nozzle, each with its own physics and its own limits. But an engine is not a set of parts. Its compressor can only pump what its turbine can swallow, and its turbine only gets the flow its nozzle lets through. The next unit puts them back together.