An Investigation of Precursors of Combustion Generated Soot Particles in Premixed Ethylene Flames Based on Laser-Induced Fluorescence

Transcription

1 7 th Annual CE-CERT-SJTU Student Symposium An Investigation of Precursors of Combustion Generated Soot Particles in Premixed Ethylene Flames Based on Laser-Induced Fluorescence Chen Gu

2 Problems of Fossil Fuel Combustion As everyone knows, there are some problems of the fossil fuel combustion: Finite limited reserves of fossil fuels Global climate change due to green house effect Air pollution

3 Primary Sources of Air Pollutants As you can see from this table, soot is considered as primary sources of air pollutants.

4 What is the origin of Soot? Origin of Soot Derives from incomplete combustion processes in trucks planes ships power stations, and so on.

5 Effects of Soot Soot can causes Health effects: Cancer Respiratory diseases Increased mortality rate

6 Effects of Soot Radiative forcing Climate effect Increased cloud density Soot deposition is responsible for polar ice melting Brown clouds causes regional warming

7 Research Objective To reduce or eliminate soot generated from incomplete combustion of fossil fuels. Fundamental understanding of soot formation mechanisms are needed to achieve this goal.

8 Kinetic Mechanism of Soot Formation Nucleation Mass Growth Over the last twenty years, fundamental theory system of soot formation had been built.however, there are still critical gaps in many areas of our knowledge, especially about: quantitatively description of the precursors and soot formation, accurate chemical structure of precursors and soot. Gas-Phase Chemistry PAH Chemistry Courtesy of D Anna Generally, soot formation process is divided into four steps: Step 1: Gas-phase Chemistry Step 2: PAH(polycyclic aromatic hydrocarbon) Chemistry Step 3: Nucleation Step 4: Mass Growth

9 Step 1 Gas-Phase Chemistry Gas-phase chemistry describes the formation process of the first benzene ring(phenyl radical and benzene) Two reaction pathways: 1. Combination of acetylene(c 2 H 2 ) Gas-Phase Chemistry Courtesy of D Anna 2. Self-combination of propargyl(c 3 H 3 ) radicals

10 PAH Chemistry Courtesy of D Anna Step 2 PAH Chemistry (1) PAH chemistry describes the formation process of soot precursors - so called PAH (Polycyclic Aromatic Hydrocarbon) species, which have high stabilities to survive from fragmentation at high temperatures. The important precursors are the Stein s Stabilomers PAH: Naphthalene Anthracene Phenanthrene Pyrene Coronene Two reaction pathways: 1.HACA mechanism(h-abstractionacetylene-addition): Take naphthalene for example (the first compound in PAH series): it is formed by addition of acetylene to phenyl radical. larger PAHs: each acetylene addition sequence forms a closed aromatic ring and hence it leads to the formation of larger PAHs called the peri-condensed aromatic hydrocarbons (PCAH).

11 Step 2 PAH Chemistry (2) 2.Combination of stabilized radicals: Take naphthalene for example: it is formed by combination of two cyclopentadienyl radicals or combination of benzyl and propargyl radicals. PAH Chemistry larger compounds: aromatic molecule addition to aromatic radicals leads to the formation of larger compounds called aromatic-aliphatic-linked hydrocarbons (AALH) Courtesy of D Anna

12 Step 3 Nucleation Nucleation Courtesy of D Anna PCAHs and AALHs grow up to form agglomerates of the molecules,forming compounds a few nanometers in diameter held together,by the weak van der Waals-interactions. As the molecular mass of the molecular compounds increases, van der Waals-interactions become stronger, leading to the formation of stable clusters of PCAHs or AALHs.

13 Step 4 Mass Growth (1) Mass Growth Mass growth process of soot is promoted by two procedures : 1.Coagulation of soot nuclei 2.Surface growth of several chemical reactions taking place on the surface of soot : the addition of mass on the surface of a soot particle proceeds by addition of acetylene and aromatics; OH radical O2 remove mass from a soot particle through the formation of HCO and CO. Courtesy of D Anna

14 Step 4 Mass Growth (2) Mass Growth The quantitative speciation of chemical compositions, especially those on the surface of soot particles, is critical to the advancement of soot models that can accurately predict the size and chemical composition. Previous researches assumed that the species on the surface of the soot particles are aromatic. Courtesy of D Anna

15 Step 4 Mass Growth (3) Recently some researchers find aliphatic on the surface of soo particles. Sunny side up morphology (TEM, Transmission Electron Microscopy,left picture & AFM, Atomic Force Microscopy, middle picture) suggests an aromatic core aliphatic shell structure. Aromatic core is carbonized and rigid due to high C/H ratio, and aliphatic shell is liquid-like due to low C/H ratio. FTIR ( Fourier Transform Infrared Spectroscopy, right picture) measurements again show aliphatic dominance, compared with aromatic at different height of the flame. So the presence of aliphatic components affect intrinsic particle properties: such as mass density reactive surface site density, surface growth rates and coagulation behavior, due to the presence of non-aromatic functionalities and reactive sites. Abid et al. 2008; Cain et al. 2010

16 Step 4 Mass Growth (4) Probe sampling, induces probe perturbation of flames Only functional groups(aliphatic C-H) are found However, they do not find which kind of specific aliphatic species dominate? Abid et al. 2008; Cain et al. 2010

17 Experimental Scheme Laser-induced fluorescence method to detect the Aliphatic, in suit. Most possible Aliphatic may be Alkylated-Stabilomers Alkylated-naphthalene: Methyl-naphthalene Alkylated-anthracene: Methyl-anthracene Alkylated-phenanthrene : Methyl-phenanthrene Alkylated-pyrene: Methyl-pyrene Alkylated-Stabilomers Aromatic-Stabilomers at different heights of the flames

18 Experimental System Made up of three parts: Combustion System: Flat Flame Burner Laser System: Dye Laser Detecting System: Spectrograph ICCD(Intensified-Charge-Couple-Device)

19 Combustion System (1) Burner Water C 2 H 4 +O 2 +Ar N 2 Flat flame burner. Premixed fuel: ethylene/oxygen/argon. Shielding Nitrogen: to prevent flames from the surrounding air. Cooling water. Flow rate: standardized by Mini-Buck calibrator. Flat stainless plate: to stabilize flames. Flame temperature: controlled by fuel flow rate, detected by thermocouple.

20 Combustion System (2) Premixed flame 16.3% Ethylene 27.3% Oxygen 60% Argon Equivalence Ratio : 2.07 Maximum flame temperature 1736K

21 Laser System Laser source: Dye Pulse Laser Excitation wavelength: 266nm and 281 nm Concerned emission wavelength: 300~500nm Cylindrical Lens Convex Lens Pass through the flame parallelly to the burner surface at different height of the flame.

22 Detecting System-Spectral Analysis Spectrograph: Princeton Instrument-Acton SP-2556 Spectrograph (On order) Ruled grating,1800groove/millimeter,500nm blaze, High resolution 0.075nm. - to split the continuous emission fluorescence beam to obtain emission fluorescence at different wavelength. ICCD(Intensified-Charge-Couple-Device): Princeton Instrument-PI-MAX 3, 1024*1024 pixel - to detect the fluorescence relative intensity with different wavelength. Emission Fluorescence: Peak Value Wavelength Relative intensity demonstrate Precursor : Species Relative Concentration

23 Apology Because the spectrograph is still on order, so I am sorry I can not show you any experimental results at present! And it is my future work! If you were interested in my research, I m glad to show you the detailed results as soon as possible in the future! diegoguchen@sjtu.edu.cn

24 Thank You!