Molecular Beam Mass Spectrometer System Research Articles

Numerical and experimental study of the combustion of acetic acid in laminar premixed flames

Experimental mole fraction profiles of chemical species (stable, radical and intermediates) have been measured in three CH 3COOH/O 2/Ar flat premixed flames burning at low pressure (50 mbar) and with equivalence ratios equal to 0.77, 0.9 and 1.05, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionisation (EI). Experimental results have highlighted an important role played by ketene (CH 2CO) as an intermediate during the combustion of acetic acid. In order to simulate the experimental results, a sub-mechanism concerning the combustion of CH 3COOH and CH 2CO has been built and added to a global mechanism recently proposed. This ensures a reasonably good modelling for the structures of acetic acid flames.

Numerical and experimental study of ethanol combustion and oxidation in laminar premixed flames and in jet-stirred reactor

The main objectives of this research consist in achieving both experimental and numerical studies of the combustion and oxidation of ethanol. Experimental mole fraction profiles of chemical species (stable, radical, and intermediates) were measured in three C 2H 5OH/O 2/Ar flat premixed flames stabilized at low pressure (50 mbar) and with equivalence ratios equal to 0.75, 1, and 1.25, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionization (EI). The oxidation of ethanol was also experimentally studied using a fused silica jet-stirred reactor (JSR). Experiments were performed in the temperature range 890–1250 K, at 1 atm, at four equivalence ratios equal to 0.25, 0.5, 1, and 2 and with an initial fuel concentration of 2000 ppm. A kinetic study was conducted in order to simulate all experimental data measured. It enabled building a kinetic mechanism by thoroughly reviewing the available literature and by taking into account specificities of the two kinds of experiments performed. Validity of the mechanism was also checked against experimental results previously published (ethanol oxidation in a JSR at 10 atm, ignition in a shock tube, combustion in premixed, partially-premixed, and non-premixed flames). This mechanism ensures a reasonably good modelling of the combustion and oxidation of ethanol over the wide range of experimental conditions investigated.

Experimental Investigation and Numerical Simulation of the Structure of CH3CHO/O2/Ar Flames at Different Equivalence Ratios

The main objectives of this research consist in achieving both experimental and numerical studies of the combustion of several flames using acetaldehyde as a fuel. Experimental mole fraction profiles of chemical species (stable, radical, and intermediates) have been measured in three CH3CHO/O2/Ar flat premixed flames stabilized at low pressure (50 mbar) and with equivalence ratios equal to 0.75, 1, and 1.25, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionization (EI). The reaction mechanisms proposed by Yasunaga et al. (2007) and by Marinov (1999) are tested by comparison of model predictions with experimental results. The results show that modeling predicts reactants and products mole fraction profiles reasonably well, but significant differences for many intermediate species remain. In order to improve the predictions for these intermediate species, several improvements on the Marinov's mechanism are suggested. They ensure a reasonably good modeling of the acetaldehyde flame structures.

Atmospheric pressure plasma analysis by modulated molecular beam mass spectrometry

Fractional number density measurements for a rf plasma “needle” operating at atmospheric pressure have been obtained using a molecular beam mass spectrometer (MBMS) system designed for diagnostics of atmospheric plasmas. The MBMS system comprises three differentially pumped stages and a mass/energy analyzer and includes an automated beam-to-background measurement facility in the form of a software-controlled chopper mechanism. The automation of the beam modulation allows the neutral components in the plasma to be rapidly and accurately measured using the mass spectrometer by threshold ionization techniques. Data are reported for plasma generated by a needle plasma source operated using a helium/air mixture. In particular, data for the conversion of atmospheric oxygen and nitrogen into nitric oxide are discussed with reference to its significance for medical applications such as disinfecting wounds and dental cavities and for microsurgery.

A Study of Cellulose Pyrolysis Chemistry and Global Kinetics at High Heating Rates

Cellulose pyrolysis has been studied in a laminar entrained flow reactor (LEFR). As described in the previous companion paper, the reactor is capable of high heating rates (∼104 K/s) and has been characterized in detail to ensure that pyrolysis of lignocellulosic materials occurs under kinetic control at the conditions of the reactor. The extent of cellulose pyrolysis in the LEFR was monitored by sampling the gas phase products with a molecular beam mass spectrometer system, and independently by sampling and weighing residues on a filter paper. Varying the reactor furnace temperature controls the pyrolysis severity. Quantitative cellulose pyrolysis data have been compared to several published reaction rates. Published models that involve low activation energy rates from other high temperature experiments described in the literature best approximate the results obtained in this study. Factor analysis of the mass spectral data requires two principal components to interpret the gas phase product composition. This suggests that the primary cellulose pyrolysis products were involved in subsequent secondary reactions that directly compete with the primary release of products at the conditions in this reactor. A rate is presented that describes the observed thermal destruction of primary pyrolysis products.

Release of Inorganic Constituents from Leached Biomass during Thermal Conversion

Leaching of inorganic materials has recently been shown to substantially improve the combustion properties of biomass fuels, especially straw but including other herbaceous and woody fuels. Leaching with water removes large fractions of alkali metals (typically >80% of potassium and sodium) and chlorine (>90%). Smaller fractions of sulfur and phosphorus are also removed. Alkali metals are heavily involved in ash fouling and slagging in combustion and thermal gasification systems. Chlorine is a facilitator of alkali volatilization, and contributes to corrosion and air pollution. The presence of these elements has reduced or eliminated the use of certain biomass fuels in many combustion applications, even where such use might provide significant environmental benefits. Leaching could mitigate the undesirable effects of biomass ash in thermal systems. Reported here for the first time are comparative studies of volatile inorganic species evolving from leached and unleached biomass fuels during thermal conversion. Leached and unleached samples of rice straw, wheat straw, switchgrass, commercial power plant wood fuel, and banagrass (Pennisetum purpureum) were tested in bench-scale combustion studies using an alumina-tube flow reactor housed in a variable temperature furnace and coupled to a molecular beam mass spectrometer (MBMS) system. Sugarcane bagasse, as the leached byproduct of sugar production, was also tested. The MBMS system was used to monitor the combustion products, including inorganic vapors, directly and in real time during each batch combustion event. Total relative amounts of HCl(g), SO2(g), NaCl(g), KCl(g), and other species were compared for leached and unleached samples. The MBMS results were consistent with the levels of alkali metals and chlorine in the samples as determined from the proximate, ultimate, and ash analyses of the samples. The more alkali and chlorine in a given sample, the more gas-phase HCl, KCl, and NaCl detected with the MBMS during combustion of that particular sample. The MBMS results clearly support earlier results, which indicated that leaching biomass effectively reduces or eliminates the release of alkali metal vapors during combustion.

Decomposition of allylselenium sources in the metalorganic chemical vapor deposition of zinc selenide

The gas-phase decomposition of methylallyl selenide (MASe) and diallyl selenide (DASe) was studied in a molecular beam mass spectrometric system under conditions used for the metalorganic chemical vapor deposition of zinc selenide (ZnSe). Consistent with reported growth results, the decomposition temperature of DASe was determined to be about 60[degree]C lower than that of MASe. Pyrolysis experiments carried out in H[sub 2] and He carrier gases showed that allyl selenides decompose by a combination of homolysis and an intremolecular rearrangement pathway, with 1,5-hexadiene and propene being the major hydrocarbon products. By comparison, methylallyl telluride (MATe) was found to decompose almost exclusively by homolysis. Selenoaldehyde products generated by the rearrangement mechanism are concluded to be the source of carbon incorporation into ZnSe films grown using allyl Se sources. 19 refs., 11 figs., 1 tab.

New Organometallic Selenium Reagents for Low Temperature Omcvd of Znse

AbstractZnSe epitaxial films have been grown on (100) GaAs by reduced pressure organometallic chemical vapor deposition (OMCVD) from tertiary-butyl(allyl)selenium (tBASe) and dimethylzinc triethylamine adduct (DMZnNEt3) at temperatures of 325-450ΰC. Good surface morphology, film crystallinity and interface quality have been found with scanning electron microscopy (SEM), double crystal X-ray diffraction (DCD) and Rutherford back scattering spectroscopy (RBS). Secondary ion mass spectrometry (SIMS) shows negligible carbon concentration (below 5x1017atoms/cm3). Low temperature photoluminescence (PL) exhibits a strong near band-edge emission with a dominant donor-bound peak. Gas-phase pyrolysis of tBASe has been probed at reduced pressure in a molecular beam mass spectrometric system in hydrogen and deuterium carrier gases. The precursor decomposes above 200ΰC by β-hydrogen elimination and by homolysis of the Se-C bonds. High isobutene vs. isobutane ratios (50-100) indicate a predominance of β-hydrogen elimination over homolysis at temperatures below 400ΰC. Diallylselenium is present in the gasphase in low concentrations at temperatures of 200-350ΰC. Diallylselenium, methylallylselenium and dimethyl-selenium have been observed as minor by-products during pyrolysis of co-dosed tBASe and DMZnNEt3. The effect of the retro-ene decomposition pathway of allylselenium reagents on carbon incorporation into ZnSe films is further probed by growth experiments with in situ generated 2-methylpropaneselenal.

Application of molecular beam mass spectrometry to chemical vapor deposition studies

A molecular beam mass spectrometer system has been designed and constructed for the specific purpose of measuring the gaseous composition of the vapor environment during chemical vapor deposition of diamond. By the intrinsic nature of mass analysis, this type of design is adaptable to a broad range of other applications that rely either on thermal- or plasma-induced chemical kinetics. When gas is sampled at a relatively high process pressure (∼2700 Pa for our case), supersonic gas expansion at the sampling orifice can cause the detected signals to have a complicated dependence on the operating conditions. A comprehensive discussion is given on the effect of gas expansion on mass discrimination and signal scaling with sampling pressure and temperature, and how these obstacles can be overcome. This paper demonstrates that radical species can be detected with a sensitivity better than 10 ppm by the use of threshold ionization. A detailed procedure is described whereby one can achieve quantitative analysis of the detected species with an accuracy of ±20%. This paper ends with an example on the detection of H, H2, CH3, CH4, and C2H2 during diamond growth.

Probe-induced concentration distortions in molecular-beam mass-spectrometer sampling

Undistorted one-dimensional concentration profiles in a binary one-dimensional stream (no sampling probe present) are generated, in a numerical analysis, using a source, a sink, and differing combinations of massdiffusion coefficient and chemical-relaxation time. Corresponding to each of these undistorted concentration profiles, several distorted concentration distributions are generated using the same combination of massdiffusion coefficient and chemical-relaxation time but inserting an infinitely long conical sampling probe at differing axial locations. The velocity and concentration distributions are calculated numerically taking into account axial symmetry, compressibility, and the nonzero sampling-orifice diameter d. A large radial concentration gradient is found at the orifice. Each distorted concentration profile is compared with the corresponding undistorted profile in order to determine the apparent axial shift of the centerline concentration due to the sampling. (The centerline concentration is the important concentration when analyzing the composition using a molecular-beam mass spectrometer system.) If, at the sampling orifice, the chemical relaxation time is much larger than the characteristic flow time and Re Sc ⪢ 1, then the apparent axial shift δ of the sampling location is found to be δ d=0.19 ( Re Sc) 1 2 where Re Sc ≡ flow velocity times orifice diameter divided by mass-diffusion coefficient at the orifice. The results are relevant to molecular-beam sampling from flames.

An experimental investigation of fuel sulfur-fuel nitrogen interactions in low-pressure premixed flames

In this paper we report the results of an experimental investigation of the structure of six premixed, burner stabilized, low pressure H 2 /O 2 /Ar flames, each of which was seeded with C 2 N 2 or C 2 N 2 /SO 2 mixtures. These flames range in equivalence ratio from 1.27–1.77 and in pressure from 35–88 torr (4.7–12.×10 4 dynes cm −2 ). A molecular beam-mass spectrometer system was used to determine the structure of each flame. Stable reactants, products and intermediates (H 2 , O 2 , H 2 O, C 2 N 2 , HCN, CO, CO 2 , NO and N 2 ) were monitored quantitatively along with the major radicals (H, O, OH). The addition of small amounts of SO 2 to the flames was found to affect the H, O and OH concentrations. The concentration of each was lowered, with the depletion of atomic hydrogen being especially large. This effect was most noticeable in the high pressure (88 torr) flame closest to stoichiometric (=1.29). NO was formed rapidly in the reaction zone of each flame. In the absence of sulfur, it was consumed further downstream at a rate which increased with both pressure and equivalence ratio. For the sulfur seeded flames, essentially no consumption was observed in the past reaction zone, leading to increased NO emissions. The structure of these flames was found to be qualitatively consistent with NO consumption via H+NO+M=HNO+M HNO+H=NH+OH HNO+H=NO+H 2 NH+NO=N 2 +OH and sulfur catalyzed radical recombination via H+SO 2 +M=HSO 2 +M H+HSO 2 =H 2 +SO 2 OH+HSO 2 =H 2 O+SO 2

Composition profiles and reaction mechanisms in a near-sooting premixed benzene/oxygen/argon flame

The structure of a near-sooting flat low-pressure laminar benzene-oxygen-argon flame was studied using a molecular beam mass spectrometer system. Mole fraction profiles of fifty-one species are presented and their mechanistic implications are discussed with regard to the decay of benzene and the formation of polycyclic aromatic hydrocarbons and soot. A mechanism is outlined for production of C 2 , C 3 , C 4 and C 5 species from initial attack of O atom on benzene. Trends observed as the fuel equivalence ratio is increased past the sooting limit suggest a sequential growth from polycyclic aromatic hydrocarbons of 120–210 amu to heavier species and then to soot. A critical step in mechanisms for rapid growth of aromatic structures by free radical addition reactions is the stabilization of the adduct by forming six-membered rings. A mechanism requiring methyl substitution on the aromatic ring is discussed.