Emissions

Emissions is where every design decision made in the previous eight chapters comes back and presents its bill. The aerodynamics set the mixing pattern. The injector determines the initial fuel distribution. The cooling design allocates air. The stability requirements constrain how lean the primary zone can operate. All of these, together, determine what comes out of the combustor exit — and how it compares to the NOₓ, CO, smoke, and UHC limits set by ICAO CAEP regulations.

This is one of the most actively researched areas in propulsion engineering, and it is the chapter of Lefebvre's book that most directly connects fundamental combustion physics to regulatory compliance.

The three enemies

1. Oxides of nitrogen (NOₓ)

NOₓ (primarily NO, with some NO₂) is formed by three mechanisms in combustion:

Thermal NOₓ (Zeldovich mechanism): The dominant pathway in gas turbines. At temperatures above ~1700 K, molecular nitrogen and oxygen dissociate and recombine to form NO through the chain:

$\text{O} + \text{N}_2 \rightleftharpoons \text{NO} + \text{N}$ $\text{N} + \text{O}_2 \rightleftharpoons \text{NO} + \text{O}$ $\text{N} + \text{OH} \rightleftharpoons \text{NO} + \text{H}$

The rate-limiting step is the first reaction, which has a high activation energy. The formation rate is thus extremely sensitive to temperature:

$\frac{d[\text{NO}]}{dt} \propto [\text{O}_2]^{0.5} [\text{N}_2] \exp\!\left(-\frac{69090}{T}\right)$

Prompt NOₓ (Fenimore mechanism): Formed in fuel-rich zones through reactions involving CH radicals attacking N₂. Important in rich-burning stages of staged combustors.

Fuel NOₓ: When the fuel itself contains nitrogen-bearing compounds. Jet-A has very low fuel-bound nitrogen, so this pathway is minor for conventional aviation fuels. It becomes important for some alternative fuel blends.

The practical implication of thermal NOₓ dominance: NOₓ is almost entirely a temperature problem. High temperatures, long residence times at high temperature, and near-stoichiometric combustion are the three conditions that produce NOₓ. Reduce any of them and NOₓ falls.

The pressure scaling adds urgency at higher engine pressure ratios:

$\text{EI}_{NO_x} \propto P^n \cdot \exp\!\left(\frac{T_3}{T_\text{ref}}\right)$

where EI is the emissions index (g NOₓ per kg fuel), $P$ is the combustor inlet pressure, and $T_3$ is the compressor exit temperature. The exponent $n \approx 0.5$. This means that a doubling of engine pressure ratio (driven by thermodynamic efficiency improvements) would increase NOₓ by ~40% for identical combustor technology. Every engine generation requires a more aggressive combustor to hold NOₓ flat or reduce it.

2. Carbon monoxide and unburned hydrocarbons

CO and UHC are products of incomplete combustion — chemistry that started but did not finish. The conditions that produce them are the opposite of those that produce NOₓ:

• Low temperature (cold zones near the liner walls due to film cooling, or cold-soak fuel evaporation)
• Short residence time (gas quenched below ~1400 K before CO can oxidize to CO₂)
• Fuel-rich zones (insufficient oxygen for complete oxidation)
• Large fuel droplets at idle (slow evaporation, insufficient time for complete burnout)

The NOₓ–CO trade-off is the central constraint of combustor design. Lowering the primary zone temperature to suppress NOₓ increases CO and UHC. The cross-over point — the temperature at which both NOₓ and CO emissions are simultaneously minimized — is typically around $\phi = 0.5$–$0.6$ (lean), at temperatures of 1600–1800 K. Modern lean combustors try to operate as close to this optimum as stability allows.

3. Soot and particulate matter

Soot forms in fuel-rich, high-temperature regions where hydrocarbon pyrolysis occurs faster than oxidation. The formation pathway involves:

1. Pyrolysis of large fuel molecules into smaller unsaturated species
2. Nucleation of polycyclic aromatic hydrocarbons (PAHs)
3. Growth of soot particles by surface reactions and coagulation
4. Oxidation of soot by OH and O₂ (competing with growth)

The net soot emission depends on the balance between formation (in rich zones) and oxidation (if the rich zone products subsequently pass through oxygen-rich, high-temperature regions). This is why the intermediate zone in a conventional combustor — where secondary air oxidizes rich combustion products — is soot-critical.

Three strategies for low-emissions design

Lean Premixed (LPM) combustion

The philosophically direct solution: if NOₓ comes from high temperatures, eliminate high temperatures by burning lean everywhere. Pre-mix fuel and air to $\phi \approx 0.5$–$0.6$ before the flame zone, so the entire flame front is lean and the adiabatic flame temperature is well below the NOₓ formation threshold.

Results: NOₓ reductions of 60–90% relative to conventional diffusion flames. Essentially zero soot.

Challenges:

• Flashback and autoignition in the premixer at high pressure
• Combustion instability (Lesson 7): lean premixed flames are highly susceptible
• Poor stability and ignition performance at low power
• Requires pilot flame (a small diffusion flame) for stability at idle — which is a NOₓ source at high power if not staged off

LPM is the dominant approach for stationary industrial gas turbines (GE DLN, Siemens E3.P, etc.) where operating conditions are relatively constant. For aero applications, the wide power range makes pure LPM extremely challenging.

Rich-Burn, Quick-Quench, Lean-Burn (RQL)

RQL avoids the problematic stoichiometric region by moving through it quickly:

1. Rich stage ($\phi \approx 1.5$–$3.0$): Primary combustion is fuel-rich. NOₓ formation is suppressed because, paradoxically, too much fuel consumes available oxygen-containing species. Soot forms here, but it will be oxidized in the next stage.

2. Quick quench: A large number of dilution air jets very rapidly oxidize the rich products and drive the mixture lean. The jets must move the equivalence ratio from $\phi > 1$ to $\phi < 0.7$ in a very short axial distance — ideally faster than the NOₓ formation time scale at stoichiometric conditions.

3. Lean stage ($\phi \approx 0.5$): Final burnout and heat release. Temperature is below the NOₓ threshold. CO and soot from the rich stage are oxidized.

The critical engineering challenge: The "quick" in quick-quench is non-trivial. The jets must penetrate completely across the combustor and mix the rich gas to lean in milliseconds. If they do not mix fast enough, the gas spends finite time near stoichiometric conditions — exactly where NOₓ forms most rapidly. The quench mixing efficiency is the dominant lever in RQL performance.

RQL is used in the GE TAPS combustor (used in GEnx, CFM LEAP) and the Pratt & Whitney TALON series.

Staged combustion

Many production aero engines use a staging approach: a lean main stage with a rich pilot flame that can be independently controlled:

• At low power (idle, approach): Only the pilot flame burns, providing stability at conditions where lean premixed combustion would blow out.
• At high power (climb, cruise): The main lean stage is activated, taking over the majority of heat release. The pilot is staged to minimum or off.

This gives the combustor a "dual personality" — stable over the wide power range of an aircraft engine while achieving near-LPM NOₓ performance at high power where it matters most for cruise emissions.

Putting it together: the emissions design space

The emissions design problem can be visualized in a two-dimensional space: primary zone equivalence ratio vs. residence time. NOₓ is high in the upper-right (hot, long residence time). CO and UHC are high in the lower-left (cold and short). The design target is a narrow region where both are simultaneously low — which corresponds to moderate temperatures (~1600–1800 K) and moderate residence times.

The entire aerodynamic, injection, and staging design of the combustor is, in the end, aimed at hitting that narrow window reliably across the entire flight envelope.

Lesson 10 asks: what happens when the fuel itself changes? The rise of sustainable aviation fuels and hydrogen forces a re-examination of every concept in this course.