Combustion Technology vs. Fuel: A Dual Approach to Cutting Aviation Emissions

This article was published on LinkedIn in July 2024.

Feature photo by Jeff Gritchen, Orange County Register/SCNG)

A recent study commissioned by Transport & Environment (T&E) and conducted by CE Delft attempted to quantify health effects of ultrafine particles (UFP, particles < 100 nm) from airport operations in Europe. This is no easy task since we have no measurement data of direct exposure effects and UFP concentrations (and composition) around airports decoupled from other sources. This study involved numerous assumptions, some more reasonable than others, which the authors acknowledge. As stated in the report, it is a “a crude first-order estimation of what the health effects caused by aviation-relation UFP around major European airports could be”.

One key conclusion is that cleaner jet fuel, with lower aromatics (compounds responsible for the sooting tendency) achieved through fuel hydrotreatment or blending with sustainable aviation fuel (SAF), will reduce the UFP health effects.  While this is likely true, the significance of the potential reductions and whether focusing solely on fuel is the way to go needs to be questioned.

T&E’s website states: “The amount of UFPs emitted from planes depends largely on the composition of the fuel. The cleaner the aviation fuel, the less pollution it generates when burnt.” However, this statement overlooks the fact that emissions, especially those of non-volatile particles (soot), also depend significantly on the combustor design and operating conditions. The same fuel can produce vastly different emissions when burnt in different engines, as demonstrated here.

Old vs new

The graphs show a compilation of recent Swiss studies and certification data for non-volatile PM (nvPM) mass (left) and number (right) for the standardized landing and takeoff cycle (LTO), on which engines are emissions-tested and compared.

SAF blends can reduce particle emissions of legacy aircraft. However, this effect is no match for the latest combustion technology optimized for low particle emissions. EIS: entry into service (year).

• The engine in [1] was a good old 737 engine. This particular engine was a “high mileage engine”, nearing the end of its overhaul cycle. A 32% SAF blend (~11% aromatics) reduced nvPM emissions by ~20%. But here is the interesting bit: the engines used in [2] are the same engine model but with a much lower “mileage”. They emitted one-third of the soot mass of the older engine, which burned clean fuel. As engines age, inefficiencies creep in, and they tend to be more smoky. There is no publicly available data on these effects as emission certification is usually only done for hand-picked newly made engines.

• Engines in [3] and [4] are smaller business jet engines, but they can pollute as much as or more than their larger counterparts. Here again, cleaner fuel (~12% aromatics) reduced the nvPM emissions by ~20%, but they remained quite high in absolute terms.

• Note that the SAF blends used in engines [1] and [3] are very clean fuels by today’s standards and were specially procured for the tests (commercial blends today contain ~1% SAF). The lower limit of aromatics allowed for these engines is 8%. Conventional Jet A-1 in Europe has around 18% aromatics.

Engines [1] to [4] represent the 1980s – 1990s technology, and they were never optimized for nvPM emissions, especially not for particle number. Now, engines [5] and [6] are recent developments and already in service. They are equipped with staged lean-burn and advanced rich-quench-lean (RQL) combustors (evolution of conventional combustors). The mass concentrations are barely quantifiable and the number concentrations measured at some operating conditions are at the level of urban background – and with regular jet fuel.

Even the cleanest fuel does not cure high-emitting engines

A top experimental study done on an Airbus A350 at cruising speeds and altitudes found ~35% reduction of nvPM number with 100% SAF (zero aromatics), which translates to a similar order of magnitude reduction in contrail ice particles. The fuel composition benefits diminish with increasing engine operating pressures and temperatures. Therefore, modern high-pressure ratio engines may see only moderate reductions in flight even with the cleanest fuels. In absolute terms, the A350 emissions with 100% SAF are still higher than what could be achieved with the cleanest combustion technology, whether advanced RQL or lean burn. Retrofitting in-service engines with the latest combustion technology could be an option, but surely not an economical one. This idea is not that far fetched – combustor redesigns after entry into service to improve particulate emissions have happened. The A350’s engine is one of them.

The way forward is the best of both worlds

Clean combustion wins over clean fuel when it comes to nvPM, but clean fuel (low aromatics and sulfur content) will still be needed to reduce toxic gaseous hydrocarbons and sulfur-induced particles (sulfur oxide emissions are mostly independent of engine technology). The combination of fleet renewal and clean fuels will be vital for cutting air pollution around airports and non-CO2 climate effects.