(6hd) Dynamics of Carbonaceous Nanoparticles: Climate Impact & Fire Detection

Incomplete combustion of fossil or biofuels and open fires release about 8 Tg of soot per year, worldwide. This is in the same order with the annual production (12 Tg) of nanostructured carbon black across the globe and motivates the multiscale design of carbonaceous nanoparticle formation, interfacing quantum mechanics, atomistic, mesoscale and continuum models [1]. Quantum mechanics and atomistic models are used to describe the electronic structure and composition of carbonaceous nanoparticles, respectively [1]. Continuum models interface Navier-Stokes equations with chemical kinetics and particle dynamics to describe the effect of process variables (e.g., temperature, residence time, fuel composition) on particle size, structure and optical properties [1].

Mesoscale discrete element modeling (DEM) for the dynamics of flame-made carbonaceous nanoparticles by surface growth, aggregation [2] and agglomeration [3] bridged quantum, atomistic and continuum models, providing a realistic pathway for carbon black formation. So, right after inception by reactive dimerization [4], small agglomerates of physically-bonded, monodisperse carbonaceous primary particles are formed by agglomeration. Then, surface growth by acetylene pyrolysis chemically bonds carbon black primary particles into compact aggregates [2]. After the complete conversion of gaseous hydrocarbons, ramified carbon black agglomerates of physically-bonded aggregates and single primary particles are formed by agglomeration alone [3]. A DEM-derived power law for the fractal-like morphology of carbon black agglomerates [3] is coupled with a continuum moving sectional model and particle size measurements to derive accurate nucleation [4], internal and surface oxidation kinetics [5]. Similar power laws can be derived for flame-made inorganic nanomaterials accounting for the agglomerate compaction during atmospheric aging in the presence of humidity [6]. The DEM-derived carbon black size distribution and morphology found excellent agreement with data from different combustion groups, explaining several experimental observations made in the last 15 years. This novel understanding of flame-made nanoparticle dynamics during surface growth, aggregation and agglomeration is essential for the accurate estimation of their environmental impact, characterization by optical diagnostics and selective sensing by fire detectors.

For example, the climate models of the Intergovernmental Panel on Climate Change (IPCC) list CO₂, CH₄ and carbon black, as the most potent contributors to global warming based on their radiative forcing (RF) impact. Among them, carbon black exhibits the highest uncertainty (~ 90 %). Examining closely these models, it becomes apparent that they could significantly under-predict the direct RF for carbon black, largely due to their assumption of spherical carbon black morphology and use of the corresponding Mie theory for light absorption. Here it is shown that carbon black rather CH₄ is the second largest contributor to global warming after CO₂ due to the reduced light scattering and enhanced light absorption by its filamentary structure [3]. The available data on agglomerate morphology of carbon black derived from various carbon sources, optical band gaps, mobility size, and mass absorption cross-section, and the analysis of the light absorption from the Rayleigh-Debye-Gans theory for carbon black are provided to affirm this discovery [7].

Optical diagnostics are widely used to monitor and mitigate carbon black emissions. For instance, early and reliable fire detection is essential to prevent thousands of human deaths and injuries every year in the US alone and protect the environment. Modern fire (smoke) sensors detect the light scattered from carbon black emitted during open fires. The optical properties of ramified carbon black agglomerates are calculated typically by Rayleigh or Mie theories for spheres impeding their selective sensing. So, fire detectors are not selective enough to light scattering from carbon black and thus produce false alarms costing up to 1 billion £/y in United Kingdom alone. Here, the morphology and light scattering from carbonaceous nanoparticle agglomerates are measured in premixed ethylene flames [8] and simulated by coupling DEM for surface growth, aggregation and agglomeration with the Discrete Dipole Approximation (DDA) [9]. Using the Mie theory for spheres overestimates the measured carbon black mass up to 10 times. This results in 60 % larger light scattering than that measured in premixed ethylene flames. In contrast, the DEM-derived carbon black structure and light scattering is in excellent agreement with those measured here in premixed flames. Thus, the DEM-DDA will be used to design and optimize the selective sensing of fire detectors to carbon black.

In sum, the development of detailed models for nanoparticle dynamics and their thorough validation with experiments provides insight into the morphology and optical properties of carbon black during flame synthesis. This facilitates the mitigation of particulate emissions, control of their environmental impact and detection by robust fire sensors.

References

[1] Buesser, B., and Pratsinis, S.E. “Design of Nanomaterial Synthesis by Aerosol Processes” (2012) Annu Rev Chem Biomol 3, 103-127.

[2] Kelesidis, G.A., Goudeli, E., and Pratsinis, S.E. “Flame synthesis of functional nanostructured materials and devices: Surface growth and aggregation” (2017) Proc Combust Inst 36, 29-50.

[3] Kelesidis, G.A., Goudeli, E., and Pratsinis, S.E. “Morphology and mobility diameter of carbonaceous aerosols during agglomeration and surface growth” (2017) Carbon 121, 527-535.

[4] Kholghy, M.R., Kelesidis, G.A., and Pratsinis, S.E. “Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation” (2018) Phys Chem Chem Phys 20, 10926-10938.

[5] Kelesidis, G.A., and Pratsinis, S.E. “Estimating the internal and surface oxidation of soot agglomerates” (2019) Combust Flame, under review.

[6] Kelesidis, G.A., Furrer, F.M., Wegner, K., and Pratsinis, S.E. “Impact of humidity on silica nanoparticle agglomerate morphology and size distribution” (2018) Langmuir 34, 8532-8541.

[7] Kelesidis, G.A., Kholghy, M.R., Fan, L.-S., and Pratsinis, S.E. “Radiative Forcing in Climate Change: the Critical Role of Soot” (2019) in preparation.

[8] Kelesidis, G.A., Kholghy, M.R., Zurcher, J., Robertz, J., Allemann, M., Duric, A., and Pratsinis, S.E. “Light scattering from nanoparticle agglomerates” (2019) Powder Technol, doi.org/10.1016/j.powtec.2019.02.003.

[9] Kelesidis, G.A., and Pratsinis, S.E. “Soot light absorption during agglomeration and surface growth” (2019) Proc Combust Inst 37, 1177-1184.

Teaching Interests:

One of my main motivations for pursuing an academic career is the joy and fulfillment I gain by teaching, interacting and mentoring students.

During my PhD studies I had the chance to be a teaching assistant at the Department of Mechanical and Process Engineering of ETH Zurich, Switzerland. I also supervised the research of four BSc students (three months long), one MSc student (three months long), as well as five shorter projects for BSc level class. Furthermore, I was responsible for the organization (scheduling, exercise and project preparation and evaluation) of the Introduction to Nanoscale Engineering (BSc level) and Micro- and Nanoparticle Technology classes (MSc level). Finally, I assisted the exercise and exam sessions of the Mass Transfer class (BSc level) and conducted experimental and numerical exercises on Brownian motion for the Laboratory Practicum in Process Engineering (MSc level).

In the future, I would be interested in teaching core BSc level courses, including “Heat and Mass Transfer”, “Thermodynamics” and “Numerical Analysis”, that are indispensable for every chemical engineer. Furthermore, I would also like to give MSc level lectures based on my research interests, such as “Nanoparticle Science and Engineering” and “Computational Chemistry and Physics”. These classes will introduce the students to the current frontiers of academic and industrial research, addressing large challenges of our society, such as global warming, and providing valuable skills for their potential future careers, e.g. in fire detection and chemical industries.