A Study on the Emissions of Alternative Aviation Fuels

Abstract

Currently, the aviation sector is seeking for alternatives to kerosene from crude oil, as part of the efforts combating climate change by reducing greenhouse gas (GHG) emissions, in particular carbon dioxide (CO2), and ensuring security of supply at affordable prices. Several synthetic jet fuels have been developed including sustainable biokerosene, a low-carbon fuel. Over the last years, the technical feasibility as well as the compatibility of alternative jet fuels with today's planes has been proven However, when burning a jet fuel, the exhaust gases are a mixture of many species, going beyond CO2 and water (H2O) emissions, with nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC) including aromatic species and further precursors of particles and soot among them. These emissions have an impact on the local air quality as well as on the climate (particles, soot, contrails). Therefore, a detailed knowledge and understanding of the emission patterns when burning synthetic aviation fuels are inevitable. In the present paper, these issues are addressed by studying numerically the combustion of four synthetic jet fuels (Fischer–Tropsch fuels). For reference, two types of crude-oil-based kerosene (Jet A-1 and Jet A) are considered, too. Plug flow calculations were performed by using a detailed chemical-kinetic model validated previously. The composition of the multicomponent jet fuels was imaged by using the surrogate approach. Calculations were done for relevant temperatures, pressures, residence times, and fuel equivalence ratios φ. Results are discussed for NOx, CO as well as for benzene and acetylene as major soot precursors. According to the predictions, the NOx and CO emissions are within about ±10% for all fuels considered, within the parameter range studied: T = 1800 K, T = 2200 K; 0.25 ≤ φ ≤ 1.8; p = 40 bar; t = 3 ms. The aromatics free GtL (gas to liquid) fuel displayed higher NOx values compared to Jet A-1/A. In addition, synthetic fuels show slightly lower (better) CO emission data than Jet A-1/A. The antagonist role of CO and NOx is apparent. Major differences were predicted for benzene emissions, depending strongly on the aromatics content in the specific fuel, with lower levels predicted for the synthetic aviation fuels. Acetylene levels show a similar, but less pronounced, effect.

About the emissions of alternative jet fuels

“…Although these differences are quite low, usually between 1 and 2%, significant lower CO 2 emissions are resulting. This was shown also on engine demonstration flights; see, e.g., [22,72].…”

“…Due to the high activation energy, the thermal-NO x pathway is opening at high temperatures, T [ 1800 K, and becomes dominant at even higher temperatures. Hence, the concentrations of NO x emissions follow the adiabatic flame temperature of the air-fuel mixture as shown by modeling calculations [72].…”

“…An overview of major relevant studies is given in Table 3. The fuels were studied both neat and in blends with petroleum-derived fuels [12, 22-24, 28, 50-71], as a response to the approval protocol; see also [72].…”

“…In addition, it is important to have accurate knowledge on ambient humidity and temperature when evaluating the differences in NO x emissions between different fuels [12,70,72].…”

A review of pollutants emissions in various pressure gain combustors

“…A case in point is the targeted goal of reducing CO 2 by 75% by the year 2050, as put forth by the Advisory Council for Aeronautical Research in Europe. 7 Considering that a significant majority of these emissions are produced by gas-turbines, owing to their providing 10% of all power generation in the US (aviation included), 8 it is essential to understand and treat the production of gaseous emissions as we move forward. Emissions reduction should be construed as a parallel effort that is needed in conjunction with increasing the thermal efficiency of these engines-a 1% increase is tantamount to removing 1,360,000 cars off the roads.…”