Synthesis and dynamics of carbonaceous nanoparticles during enclosed spray combustion

Abstract

Carbonaceous nanoparticles are both pollutants (soot) with negative effects on human health and the environment and valuable nanomaterials (carbon black). Understanding the processes which form such particles is therefore essential for designing combustion engines and reactors that minimize harmful emissions and enhance commercial profit. Aviation is a growing industry and an important source of soot. Soot from aviation tends to have a small size relative to other soot sources (e.g. diesel engines) which may result in greater health risks due to the higher toxicity of small particles. However, questions remain regarding the biological mechanisms tied to the negative health effects of soot. Similarly, aviation is unique in that it emits soot at high altitudes, where the particles may act as ice condensation nuclei forming contrails. These effects, however, are still poorly understood as experiments with real jet engines are costly and difficult to access. Low-cost and high-throughput methods for soot research are needed to better quantify the impact of soot on the climate, human health and to design improved engines. This way, trade-offs that minimize the impact of aviation on human health and the environment can be made. In this thesis, these challenges will be addressed through an overview of existing technologies, development of a laboratory soot generator and the application of this soot generator to better understand soot and NO emissions from jet fuel combustion. Chapter 1 gives an overview of existing strategies to reduce soot emission while considering industry requirements to limit pollutants such as CO2, carbon monoxide (CO), unburned hydrocarbons (UHC), oxides of nitrogen (NOx), and meeting strict safety and performance standards. Computational models are used to aid in aircraft engine design. However, models struggle to accurately capture the soot mobility diameter, dm, and volume fraction, fv, observed experimentally. Part of this discrepancy could be due to models’ oversimplification of the irregular morphology of soot and the current poor understanding of soot formation processes. Even so, aircraft combustors have been reducing soot emissions through extensive oxidation with the Rich-Quench-Lean (RQL) concept or by preventing soot formation with near-premixed, lean combustion such as in Lean Premixed Prevaporized (LPP) combustors. Near-premixed combustion prevents soot from forming in fuel-rich pockets while very lean combustion keeps temperatures low, thus preventing NOx formation. The use of alternative fuels could also reduce soot emissions. Sustainable Aviation Fuels (SAF) tend to have lower aromatic content compared to conventional jet fuels, which reduces the formation of soot particularly at low engine thrusts. The use of SAF is attractive logistically, as it can be a drop-in fuel requiring no new infrastructure or engines; however, the short-term supply is limited. In this regard, it is important that policies promote high blends of SAF which can reduce soot rather than adding a small amount of SAF to all flights, which has little to no impact on soot emissions. Chapter 2 describes the development of a low-cost, laboratory burner to produce aircraft-like soot from real jet fuels with high throughput. Laboratory burners are essential for facilitating research on soot emissions in a lower-cost and more controlled environment compared to a real aircraft engine. However, existing commercial soot generators fail to produce soot similar to that produced by aircrafts at high thrust. High-thrust aircraft soot tends to have Organic Carbon to Total Carbon (OC/TC) ratios < 25 % and small median mobility diameters, dm, in the range of 11 – 61 nm. Here, enclosed spray combustion (ESC) of jet A1 fuel is used to produce soot with similar OC/TC, dm and primary particle diameter, dp, to that observed in soot from real aircraft. Specifically, OC/TC ratios were consistently < 20% while the median dm ranged from 15 – 150 nm depending on the Effective eQuivalence Ratio (EQR) employed. Soot particles produced at the low end of this range (dm < 50 nm) can be considered ‘aircraft-like’. The specific surface area (SSA) was quantified for the first time for aircraft-like particles (160 – 239 m2/g) with mainly small pores (< 2 nm). ESC therefore provides a new, lab-based method to replicate soot produced by aircrafts at high thrust. Chapter 3 explores the formation and growth dynamics of soot produced by the ESC burner developed in Chapter 2 at various EQR. This characterization and modeling of the formation and growth of soot during spray combustion of jet fuel can be used to improve the understanding and modeling of soot from aircraft engines. The centerline flame temperature peaked at Heights Above the Burner (HAB) = 5 – 10 cm then dropped continuously through to the end of the enclosure at HAB = 63 cm. The maximum temperature depended on the EQR with lower EQR (closer to stoichiometric) resulting in higher temperatures than the richer flames. Within a flame, the dm of soot grew continuously from HAB = 5 to 63 cm while the dp was approximately constant at all points along the enclosure. Discrete Element Modeling (DEM) revealed that this behavior is attributable to the leveling off of soot surface growth after short residence times, before HAB = 5 cm and agglomeration then took over as the primary mechanism for particle growth. At low EQR = 1.46, the dp leveled off at approximately 14 nm. At higher EQR (e.g. 1.88) soot surface growth was enhanced leading to larger dp, up to 23 nm. Across the same range, the Raman D/G ratio dropped from 0.9 to 0.8 at EQR = 1.46 and 1.88, respectively, while the crystallite length increased from 1.24 to 1.47 nm. These correlations suggests that high EQR produced larger dp with more graphitic, crystalline particles compared to the smaller more disordered primary particles produced at low EQR. Chapter 4 investigates the trade-off between soot and nitric oxide (NO) during ESC of jet fuel as combustion conditions that reduce soot emissions tend to increase NOx, making it difficult to reduce both pollutants at the same time. Judicious swirl-injection of air downstream of ESC can drastically reduce soot emissions through oxidation. However, this swirl-injection strategy leads to higher temperatures that promote NO. Early injection of air results in the lowest soot emissions, but the highest NO, nearly triple that produced when air is injected far downstream of the burner. Conversely, late injection of air does not reduce soot emissions although NO remained low. Here, a quantitative correlation is found between injection location, temperature, soot and NO. Therefore, the combustion conditions which allowed for a balanced trade-off between NO and soot emissions was found and on par with or lower than the lowest NOx emissions per unit mass of fuel from in-service aircraft engines. Enclosed spray combustion of jet fuel provides a high-throughput method for producing soot with comparable morphology and composition to that from aircraft at high thrust for the first time. The understanding of soot formation and growth during ESC of jet fuel can help to improve modeling and design of aircraft engines through an improved fundamental understanding of the processes involved. Furthermore, quantifying the trade-off between soot and NO emissions is essential for developing engines which minimize both pollutants. In the future, ESC could be used for calibrating regulatory instrumentation, testing novel jet fuels or production of sufficiently large quantities for further research on the biological effects of soot.