Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectrometer

The overall goal of this experiment is to get an overview of reactive chemical species in an combustion process and investigate the combustion chemistry of technical fuels and fuel components. This method can help to answer questions in the field of combustion chemistry and pollutant formation, such as soot formation. One of the main advantages of this technique is to get an overview of the chemical species and detect even highly reactive radical species without prior knowledge.

This flexible tool offers us an observation of the chemical gas-phase kinetics under well-controlled conditions. The data can be used for kinetic model validation and fuel assessment strategies. The vast range of operating conditions available for such a laminar flow reactor enables access to combustion applications that are typically not achievable by flame experiments.

The schematic of the flow reactor system shows all major components. The oven is coupled to the MBMS setup with the time-of-flight, or TOF Detection System, mount the to the sampling direction and to a gas supply system. First, heat the oven to the designated start temperature, which is the highest temperature in the designated measurement series.

Prepare the TOF spectrometer for intermediate species detection. Now prepare the quadrupole spectrometer for major species detection by placing it in the ionization chamber of the MBMS system and starting the software. To prepare the fuel supply system, first prepare a metal syringe for fuel supply.

Then fill the metal syringe with 30 milliliters of the fuel sample. Following this, pressurize the metal syringe up to five bar by opening the valve and adding pressurized air to the system. Then heat up the vaporizer and fuel supply lines.

For this experimental design, set the water cooling system to 80 degrees Celsius so that the diluted fuel cannot recondense at the coldest spot in the system, which is the temperate in that flange to the oven. Place the oven to the sampling position which is close to the plateau value of the spatial temperature profile of the oven. Next, start the diluent of choice by adding gas to the Coriolis mass flow meter.

Start continuous data recording by clicking on the start buttons in the TOF and quadrupole software. Add oxygen as an oxidizer by setting the appropriate flow condition of the Coriolis mass flow meter software. Observe the incoming oxidizer as a new peak in the mass spectrum.

Next, add fuel by setting the appropriate flow condition of the Coriolis mass flow meter. Check the spectre to confirm if complete oxidation is achieved and a stable carbon dioxide mass signal is observed. After the stabilization period, apply a continuous temperature decay ramp of 200 Kelvin per hour to the oven, which leads to typical measurement times of two hours per run.

At a specific oven temperature during the ramp, observe a rapid change of the mass spectre with sole combustion products disappearing and small combustion intermediates appearing. With further decreasing temperature, visible intermediates become larger and larger. At cold oven temperatures, only the signal of fuel compounds and oxygen can be observed.

When the final temperature is stabilized, switch off the oxidizer. Continue recording measurements and obtain fuel characterization measurements at conditions without oxidizer. Following this, switch off the fuel in the Coriolis mass flow meter software by setting the value to zero.

Then stop the data recording by clicking the stop buttons in the software. For calibration issues, mount a closed chamber in front of the sampling cone. Then open the valve to the pump to evacuate the chamber.

Apply binary mixtures or commercial calibration gasses for calibration. Next, start TOF software again without data recording. Adjust the pressure in the calibration chamber by a needle valve to obtain a signal intensity above the signal-to-noise ratio and below the saturation limit.

Following this, start the calibration measurements and enable data recording. At each recorded temperature for each chosen species, calculate its mole fraction from the corresponding signal. Then plug the mole fraction profiles versus the oven temperature.

A typical mass spectrum of the sampled gas composition is shown here. The peaks are integrated for each mass-to-charge ratio for evaluating not-fully resolved signals. Signals are plotted against the average temperature of the 2.5 Kelvin interval, resulting in a typical mole fraction versus oven temperature plot.

The spatial mole fraction profiles of formaldehyde and acetylene obtained from a stoichiometric methane measurement shows agreement between the measured data and kinetic model values for the main components and intermediate species. The potential jet fuel compound, p-menthane, featuring major species profiles, is depicted here. The stoichiometry dependence of ethylene and formaldehyde, and the selected intermediate species for stoichiometric conditions are obtained.

In the flow reactor setup, the oxygen and fuel profile start at a maximum at low temperatures and are consumed as the temperature increases. In depth analysis shows a similar decay for the hydrocarbon species while aromatic species show a distinct plateau region Higher mole fraction for soot precursors, propargyl radical and benzene, are measured for p-methane compared to Jet A-1 and farnesane, indicating a higher tendency to form pollutants. For farnesane, lower mole fractions for both species are measured compared to p-methane and Jet A-1 fuel.

After its development, this technique paved a way for researchers in the field of future fuel design strategies to explore combustion kinetics and pollutant formation for conventional and alternative fuels and components.