Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

The overall goal of this procedure is to analyze the chemical composition of laboratory scale hydrocarbon flames, to develop a detailed understanding of such formation in combustion environments. This is accomplished by first establishing burner stabilized premixed flames under a reduced pressure of about 20 to 80 millibars. The second step is to withdraw gases from these flames and to then determine the chemical identity of the sampled species by using a custom-built time of flight mass spectrometer with vacuum ultraviolet photo ionization.

Next, the concentration of these species is determined as a function of distance from the burner. The final step is to use a similar approach to study the chemical composition of combustion generated, so particles, which are the results of the gas phase formation processes. Ultimately, flame sampling mass spectrometry with vacuum ultraviolet.

Photo ionization is used to show that resonance stabilize radicals are important intermediates, and that one simple mechanism cannot explain all observed soot components. While this method can provide insight into combustion processes, it can also be used to study atmospheric chemistry, molecular dynamics, and kinetics among other things. First, establish a one liter per minute flow of Argonne and a 1.5 liter per minute flow of oxygen through the burner surface, and maintain a pressure of 80 millibars in the flame chamber.

Position the hot wire igniter over the burner surface. Then set the hydrogen flow to 0.4 liters per minute and quickly activate the igniter after ignition. Turn off the hot wire igniter and reposition it away from the burner.

Next, establish the desired flows of Argonne oxygen, hydrogen, and fuel. Adjust the pressure to match the conditions of the target flame, which is typically between 20 and 40 millibars when the pressure in the ionization chamber is less than or equal to 10 to the minus six millibar. Apply the voltages to the ion optics of the time of flight mass spectrometer and microchannel plate detector, and open the beamline valves.

Start the lab view data acquisition program general interface vi and move the burner to the desired position using the motor tab in the software. Following this, use the general tab to define the scan parameters or the number of steps per electron volt of photon energy. Following this, use the A LS tab to set the photon energy to the desired start value and define the a LS energy to be active on the P 78 86 tab.

Use the set parameters button to set the number of sweeps, the number of bins and the bin width. Then enter a valid file path and name and click start to start the computer control data acquisition process. For the acquisition of the burner scans, apply the voltages as for the energy scans to the ion optics of the time of flight mass spectrometer and microchannel plate detector.

Open the beamline valves to allow the photon beam in the chamber. After opening the LabVIEW data acquisition program general interface vi, use the jogger under the motor tab to move the burner surface as close as possible to the sampling cone and define that position as the origin. Then define the motor to be active.

Use the general tab to define the scan parameters or the number of steps per millimeter of burner movement. Next, use the a LS tab to set the photon energy to the desired value, which is typically between eight and 16.65 electron bolts on the P 78 86 tab. Use the set parameters button to start a sub virtual instrument to set the number of sweeps or the number of mass spectra added on top of each other at every burner position the number of bins and the bin width.

Following this, provide a valid file path and name and click start to begin the automated data acquisition process RAAs for the aerosol experiment opposed flow flames are used. To begin fill the flame chamber with argon and bring the pressure up to about 860 millibars. Play igniter coil approximately in the center of the two burner outlets.

Set the oxidizer stream to 0.3 liters per minute for oxygen and 1.6 liters per minute for argon and the oxidizer stream co flow to 2.5 liters per minute for argon. Then set the fuel stream to 0.3 liters per minute for hydrogen and 2.5 liters per minute for argon and the fuel stream co flow to 2.5 liters per minute. For argon, open the hydrogen and oxygen valves and immediately switch on the igniter coil.

Once the flame is ignited, turn off the igniter coil and retract it. At this point, establish the desired flows of oxygen, argon, and fuel. Turn off the flow of hydrogen and set the pressure and reactant outlet separation to the desired values.

For the target flame. Apply the appropriate voltages to the ion optics and detector of the aerosol mass spectrometer. Then open the lab view data acquisition program general interface opposed flow vi.

Use the jogger application on the motor tab to translate the opposed flow burner so that the quartz microbe is at the position nearest to the fuel stream outlet. While at this position, reset the motor step position to zero. Following this.

Slowly open the quarter turn ball valve allowing flow from the flame sampling line into the aerodynamic lens or a DL system. Confirm that the pressure at the outlet of the A DL is near one times 10 to the negative two millibars. Use the general tab to define the scan parameters or the number of steps per millimeter of burner movement or photon energy.

Use the a LS tab to set the desired photon energy and use the motor tab to move the burner to the desired burner position. Next, use the P 78 86 tab and the set parameters button therein. To set the acquisition parameters, define motor or a LS energy to be active.

Insert a valid file path and name in the appropriate fields and click start to acquire the aerosol mass spectra. A typical mass spectrum of flame sampled gases from the low pressure pre-mixed burner is shown here. The identities of the species contributing to the signal are revealed by the flame sampled photo ionization efficiency or PIE curves for each master charge ratio and their comparison to known isomer specific ionization energies and PIE curves.

Typical examples of flame sampled PIE curves are shown here. For master charge ratio equals 39 and 41. The data are taken from a stoia metric propane flame.

The signal is unambiguously identified by their characteristic ionization thresholds to originate from the ly stabilized arial and allele radicals. For many master charge ratio values, multiple isomers are routinely identified by observing multiple thresholds. Many examples have been discussed extensively in the literature.

For example, master charge ratio equals 40 for alene and propane, 44 for a ethanol and acetaldehyde 54 for one, three butadiene, one butane, and two butane or 78 for fuline and benzene. Once isomer composition is known, mass spectra are taken at various photon energies and from within different positions in the flame to allow for the determination of isomer specific mole fraction profiles of the individual species. As a function of distance from the burner surface representative mole fraction profiles of fuline and benzene in a STO geometric propane flame from the low pressure premixed burner are shown here.

For each flame, typically a total of 40 to 50 individual mole fraction profiles are determined for species ranging from master charge ratio equals one to master charge ratio equals 78 or even higher depending on the scientific goals. These mole fraction profiles are then used to assess the predictive capabilities of combustion chemistry models and to validate them. A typical aerosol mass spectrum is shown here, it taken from within a propane opposed flow diffusion flame ion signal was observed for species with master charge ratios ranging from 150 to 600 with a peak around 202.

It is beyond the current experimental capabilities to identify all of the species observed in the mass spectrum or to unravel their possible formation Pathways taking such mass spectra as a function of the distance from the fuel outlet as described provides spatially resolved profiles. A representative example is shown here for the mass charge ratio equals 256. Similar profiles can be obtained for any of the other species as well, which consequently could be used as validation targets for any combustion chemistry model.

Once mastered, a proficient operator can use this technique to record a complete set of flame data within just a few hours.