Details

Understanding kinetics of chemical reactions occurring in combustion systems requires experimental data on spatial and temporal species profiles. This includes concentrations of reactants, products, stable and reactive intermediate species. Research at the CPC lab endeavors to study the combustion chemistry of gaseous and liquid fuels, through well-defined experiments (0D-1D) and advanced diagnostics (e.g., mass spectrometry and optical).

0D-JSR pyrolysis and oxidation

Jet Stirred Reactors (JSR) are Continuous Stirred Tank Reactors (CSTR) in which the reactants and products are homogenously mixed under well-controlled temperature and pressure conditions. Jets are formed as gas phase reactants flow through nozzles inside the reactor and create turbulent mixing (Fig. 1). This apparatus has been used for studying speciation during gas and liquid fuel pyrolysis and oxidation under various temperatures and pressures relevant to combustion systems. The reactor is kept in an oven that allows for controlling the reactor temperature from room temperature to 1200 K. The system is presently operated at 1 atm with plans to build a pressure chamber to reach up to 10 atm. Probe sampling and non-intrusive diagnostics are adopted at CPC lab to elucidate the formation of stable products and reactive intermediates, which are valuable to explore the reaction mechanism and develop detailed kinetic models of fuels and their surrogates.

Figure 1: Picture of JSR at KAUST

1D-low pressure premixed flame

Ions in flames have been studied for decades in an effort to understand the underlying chemistry. Recent studies indicate the use of external electric fields on engine combustion can reduce emissions and increase efficiency. In our laboratory, we study ions in low-pressure premixed flames, as these permit high spatial resolution. In this research, we identified and quantified ions in low-pressure flames, providing experiment data to help with developing ion chemistry mechanisms.

The scheme of experiment set up is in Figure 3. The MBMS system is coupled to a combustion chamber capable of sustaining flames at low pressures and a quadrupole mass spectrometer is used to separate the products. A chiller was applied on the McKenna burner to establish premixed flat flames at low pressure with Argon as the diluent gas. The McKenna burner was mounted on a motorized translation stage. Ions in the centerline of the flame were sampled by a detector, and the signals were then transformed to mole fraction by using mass discrimination calibration and FKT calibration. Flame temperature was measured using a thermocouple. Neutrals were also measured by electron ionization in MBMS to validate the flame structure.

Figure 3. Scheme of low pressure flame coupled with a quadruple MBMS

In addition, a thorough and detailed investigation on the neural species in flames can be achieved using Photon Ionisation–Molecular Beam Mass Spectrometry (PI-MBMS) at the Advance Light Source of Lawrence Berkeley National Laboratory. The utilization of a single-photon synchrotron radiation allows for soft ionization of the molecules slightly above their ionization threshold. Such techniques minimize the possibility of fragmentation of molecules sampled from the flame zone, while the high resolution of the photons energy, 0.05 eV, allows for the differentiation of isomers.

 

1D-Counterflow diffusion flames

A better knowledge of high-temperature combustion processes in transport affected environments would facilitate the understanding of ignition events in practical combustors. Our counterflow facility is an atmospheric pressure setup designed to understand the interaction between flow and chemistry in diffusion flames. Our main focus is in measurements of autoignition temperature, extinction limits, and flame structure of various liquid and gaseous fuels. The experimental data generated is used to validate high temperature kinetics and transport in reacting flows. The current fuels under study include blends of natural gas mixtures with DME and H2, light & heavy naphtha and gasoline fuels.

Soot formation within combustion applications has substantial malignant impact on the environment and humans. The formation of soot from fuels combustion is a complicated process, starting with the formation of the PAHs molecule precursors, followed by first ring formation, PAHs growth, Particle inception, particle growth, and oxidation.

In this research we investigate the effect of molecular fuel structure, dopants addition, and the characteristic flow time on the formation of PAHs and their precursors in a counterflow diffusion flame burner. This research is being conducted in the Advanced Light Source at Lawrence Berkeley National Laboratory using synchrotron vacuum-ultraviolet molecular beam mass spectrometry.

Diagnostics

Gas Chromatography

In combustion and pyrolysis systems, species produced by chemical reactions should be analyzed with the appropriate analytical device. The most common analytical technique which is extensively used for the identification and the quantification of species in gas-phase is Gas Chromatography (GC). This system is well adapted to the analysis of most stable molecules such as permanent gases and hydrocarbons. However, this doesn’t include unstable molecules and radical intermediates species.

At the CPC lab, the GC employs an advanced electronic pneumatic control (EPC) modules and a high performance GC oven temperature control. These features are keys of the GC’s superior performance in all applications needed. This system is also equipped with three different detectors (one FID and two TCD) allowing the detection of several products depending on their properties and on the calibration. Moreover, this kind of GC may be combined with mass spectrometry to improve identification of measured species.

Molecular Beam Mass Spectrometry

The system at CPC lab presently is connected to an Electron Impact Ionisation–Molecular Beam Mass Spectrometry (EI-MBMS) sampling system for identification and quantification of reactants, intermediates, and products. The sampling of the reactor contents is done by a quartz nozzle and a skimmer, which allows for the formation of a molecular beam. The beam prevents further reaction by rapidly expanding the gas stream through which intermolecular distances become greater than collision distances. This preserves the integrity of the molecules that are sampled and is especially required for short-lived species (e.g., radicals).

The sampled molecules are ionised by electrons generated with an electron gun whose energy and flux can be controlled. The ions are directed by optics to a Time of Flight (TOF) tube where they are separated by their mass to charge ratios. The resolution of the TOF in our setup is 3000-5000.

This research in KAUST runs complimentary with our collaboration with researchers from Sandia National Laboratory, UC Berkeley, Bielefeld University, CNRS Orleans, and Princeton University at the Advanced Light Source (ALS) in Lawrence Berkeley National Lab (LBNL). We also have collaborations with Shanghai Jiao Tong University with experiments conducted at the Hefei Light Source (HLS). The experiments at the ALS and HLS are coupled to Photon Ionisation–Molecular Beam Mass Spectrometry (PI-MBMS). The synchrotron generates the vacuum ultra-violet photons used for ionisation. The tunability of the photon energy and high photon flux aids in separation of isomers, and avoids fragmentation via near threshold ionisation, which is not possible using EI-MBMS. This greatly helps in the detection and identification of crucial intermediates in low temperature oxidation.

 

Laser diagnostics

Laser diagnostics are widely used to study combustion kinetics, due to its high resolution and high accuracy. In the CPC lab, we use a Ti:S laser absorption technique that provides non-intrusive measurements of radicals in JSR oxidation. Experimental data are used for validating combustion kinetic mechanisms.

A schematic of the experiment setup is shown in Figure 2. A pump laser at 532 nm is transformed into 760-1000 nm laser light by a Ti:S crystal, and then an optical grid is used to select the desired wavelength. The light can be doubled and/or quadrupled later by a second and/or fourth harmonic generator box. The light is separated into two beams, one as reference beam and another one as signal beam. A silicon detector detects both beams and the data is collected with an oscilloscope. The absorbance was determined by taking ratio of two signals, and by knowing absorption cross-section, the species mole fraction can be calculated using the Beer-Lambert Law.

Figure 2. Scheme of laser diagnostic for JSR oxidation

In addition, our research is facilitated by the various diagnostics in the core labs at KAUST, like GC-FID/TCD/MS, APCI/ESI-orbitrap mass spectrometry, and NMR, etc.