Name: combustion chemistry of fuels: quantitative speciation data obtained from an atmospheric high-temperature flow reactor with coupled molecular-beam mass spectrometer
Uploaded: 2018-02-19T14:00:00
Description: Watch this Scientific Journal Video about Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectr

The experiments were carried out in the mass spectrometry department at the Institute of Combustion Technology, Deutsches Zentrum f&#252;r Luft- und Raumfahrt (DLR) in Stuttgart, Germany. The work was also supported by the Helmholtz Energy-Alliance &#34;Synthetic Liquid Hydrocarbons&#34;, the Center-of-Excellence &#34;Alternative Fuels&#34; and the DLR project &#34;Future Fuels&#34;. The authors wish to thank Patrick Le Clercq and Uwe Riedel for fruitful discussions on jet fuels.

1. Setup of the molecular beam mass spectrometer (MBMS) and flow reactor system
<ol>
	<li>Heat oven to designated start temperature, which is the highest temperature in designated measurement series. For typical conditions of Jet A-1 with &#934;=1, total oxidation is observed below 850 &#176;C (~1,100 K). The choice of proper starting temperatures depends on the chemical nature of the investigated fuel and the stoichiometry (&#934;).</li>
	<li>Prepare Time-of-Flight (TOF) spectrometer for intermediate species detection...

<ol>
	<li>Moore, R. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biofuel+blending+reduces+particle+emissions+from+aircraft+engines+at+cruise+conditions.">Biofuel blending reduces particle emissions from aircraft engines at cruise conditions.</a> Nature. 543 (7645), 411-415 (2017).</li><li>Braun-Unkhoff, M., Kathrotia, T., Rauch, B., Riedel, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=About+the+interaction+between+composition+and+performance+of+alternative+jet+fuels.">About the interaction between composition and performance of alternative jet fuels.</a> CEAS Aeronautical Journal. 7 (1), 83-94 (2016).</li><li>Egolfopoulos, F. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+and+challenges+in+laminar+flame+experiments+and+implications+for+combustion+chemistry.">Advances and challenges in laminar flame experiments and implications for combustion chemistry.</a> Prog Energ Combust. 43, 36-67 (2014).</li><li>Lynch, P. T., Troy, T. P., Ahmed, M., Tranter, R. 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The presented combination of an atmospheric high-temperature flow reactor with a molecular-beam mass spectrometry detection system enables quantitative speciation data for a range of operating conditions. Several studies21,22,23,27 demonstrated the flexibility of the experiment starting from rich methane conditions relevant for partial oxidation phenomena (&#966; = 2.5), to investigating the co...

Understanding combustion processes in the wake of modern, low-emission fuels from renewable resources is a challenge for today&#39;s societies&#39; ecological and economic topics. They have the potential to reduce our dependence on fossil fuels, offset CO2 emissions, and have a positive impact on harmful pollutant emissions such as soot and its toxic precursors1. Combining this fast growing field with their utilization in modern combustor systems, the demand on a fundamental understanding of the governing chemical and physical processes has increased dramatically2. Even today, the complex chemical reaction networks resulting from the radical chain reactions are still not fully understood. To analyze or even control phenomena like pollutant formation or (auto) ignition processes, the detailed knowledge of chemical reaction networks is a crucial piece of the puzzle3.
To investigate and understand those chemical reaction networks, experimental and numerical approaches are mandatory. Experimentally, the combustion chemistry is typically studied by applying experiments with simplified and well controlled flow environments to target specific questions. The high complexity and dynamics of individual sub processes prevent exact reproduction of the conditions of technical combustors by the fundamental experiments, while allowing the tracking of the designated key features such as temperature, pressure, heat release, or chemical species. Early on, the need for different experimental approaches became apparent, each tackling a specific question and providing a subsequent set of information contributing to the overall global picture of the combustion chemistry. To cover the full range of conditions and gather those subsequent information sets to describe complex conditions occurring in technical systems various approaches were successfully developed. Well established techniques include:
<ul>
	<li>Shock tubes4,5,6 and rapid compression machines7. These devices provide high control of pressure and temperature over a wide range. However, the accessible reaction time and suitable analytical techniques are limited.</li>
	<li>Laminar premixed flames3,8,9,10,11 are ideal to gain high-temperature conditions in combination with a simple flow field. Since the spatial dimension of the reaction zone decreases with increasing pressure, premixed flames are typically investigated at low-pressure conditions for speciation purposes.</li>
	<li>Counterflow diffusion flames12,13,14,15 are ideal for investigating the flamelet regime in turbulent combustion. They mimic the strain due to inhomogeneities in a real turbulent flow, but are, again, highly limited in analytical speciation techniques.</li>
	<li>Various reactor experiments16,17,18 (static, stirred and plug-flow) provide access to high-pressure environments, while temperatures are typically lower compared to flame environments. Common approaches are:
	<ul>
		<li>Static reactors are widely used for e.g. pulse photolysis experiments, but are in general limited by long residence times and low temperatures.</li>
		<li>Jet-stirred reactors, i.e. gas version of a perfectly stirred reactor (PSR), rely on the efficient mixing of the gas phase and can be operated at steady state with constant residence time, temperature and pressure, making it easy to model. However, molecules have time to migrate to the hot surfaces and undergo heterogeneous reactions.</li>
		<li>Numerous flow reactor approaches are known, with the plug flow reactor (PFR) as one of the most popular approaches for describing chemical reactions in continuous, flowing systems of cylindrical geometry. Plug flow conditions at steady state are assumed with fixed residence time of the plug as a function of its position for ideal PFRs.</li>
	</ul>
	</li>
</ul>
Complementary to those valuable techniques in the field of experimental combustion kinetics, a high-temperature laminar flow reactor experiment19,20 employing the molecular beam mass spectrometry (MBMS) technique for tracing species development in detail is presented21,22 herein. Laminar flow conditions, working at atmospheric pressure and accessible temperatures up to 1,800 K are the main characteristics of the flow reactor, while the sensitive MBMS technique allows the detection of almost all chemical species present in the combustion process. This includes highly reactive species such as radicals that are not or hardly traceable with other detection methods. The MBMS technique is widely used for the detailed investigation of reaction networks in flames of conventional and modern alternative fuels, such as alcohols or ethers23,24,25 and has demonstrated to be of great value for modern kinetic model development.
Figure 1 shows the schematic of the high-temperature flow reactor with a zoomed frame of the sampling probe (A) and two pictures highlighting the overall experiment (B) and the probe setup (C). The system can be divided in two segments: first, the high-temperature flow reactor with gas supplies and vaporizer system and second, the MBMS time-of-flight detection system. In operation, the exit of the flow tube is mounted directly to the sampling nozzle of the MBMS system. The gas is sampled directly from the reactor outlet and transferred to the high-vacuum detection system. Here, ionization is performed by electron ionization with subsequent time-of-flight detection.
The reactor has a 40-mm inner diameter ceramic (Al2O3) pipe of 1,497 mm length placed in a high temperature oven (e.g., Gero, Type HTRH 40-1000). The total heated section is 1,000 mm in length. Gases are fed premixed and pre-vaporized into the reactor by a tempered flange (typically tempered to ~80 &#176;C). The highly diluted (ca. 99 vol% in Ar), laminar flowing reactant mixture passes through a known temperature profile (details on temperature characterization will be given below). Detection of the gas composition takes place at the reactor outlet as a function of the oven temperature. Measurements are performed at constant inlet mass flow, while a monotonically decreasing temperature ramp (-200 K/h) is applied to the oven in the range of 1,800 K to 600 K. Note that similar results may be obtained when distinct temperatures are measured at isothermal oven temperatures and thermal inertia is considered properly. The thermal stabilization of the system still takes some time and the temperature ramp is selected as a compromise of averaging time for a (negligible) small temperature increment and total measurement time per series. The averaging time (45 s) of the MBMS corresponds to 2.5 K. The resulting residence times are around 2 s (at 1,000 K) for the given conditions. Finally, due to the temperature reproducibility, a relative precision of the measured temperatures of &#177;5 K or better can be stated for the present reactor experiment.
Figure 2 shows the schematic of the vaporizing system, optimized to investigate even complex hydrocarbon mixtures such as technical jet fuels. All input streams are metered in high precision (accuracy &#177;0.5 %) by Coriolis mass flow meters. Vaporization of the fuel is realized by a commercial vaporizer system at temperatures up to 200 &#176;C. All supply lines with pre-vaporized fuels are preheated with temperatures of typically 150 &#176;C to prevent condensation of the liquid fuels, while avoiding thermal degradation at the same time. Complete and stable vaporization is routinely checked and may even occur at temperatures below the normal boiling point of the respective fuels. Complete evaporation was ensured by the small fuel fraction and the low partial pressure (typically below 100 Pa) needed.
The gases are sampled by a quartz cone at the centerline of the reactor exit at ambient pressures (around 960 hPa) as seen in more detail in the zoomed frame of Figure 1. The nozzle tip has a 50 &#956;m orifice, which is located roughly 30 mm inside the ceramic tube at the end of the reaction zone. Note, that the sampling location is fixed with respect to the inlet. Thermal expansion of the oven tube only takes place at the outlet, which is not mechanically connected to the sampling system resulting in a temperature independent length of the reaction segment. All reactions are immediately quenched due to the formation of a molecular beam, when gasses are expanded into high vacuum (two differential pumping stages; 10-2 and 10-4 Pa)25,26. The sample is guided to the ion source of an electron impact (EI) time-of-flight (TOF) mass spectrometer (mass resolution R = 3,000) capable of determining the exact mass of the present species in suitable precision to determine the elemental composition within a C/H/O system. The electron energy is set to low values (typically 9.5-10.5 eV) in order to minimize fragmentation due to the ionization process. Note that the diluent and reference species argon is still detectable due to the broad energy distribution of the ionizing electrons (1.4 eV FWHM). While Ar can be measured with good S/N, the low electron energy does not allow for sufficient determination of the major species (H2O, CO2, CO, H2, O2, and fuel) profiles, which are present in significant lower concentrations.
In addition to the detection by TOF, a residual gas analyzer (RGA), i.e. a quadrupole mass spectrometer, is placed in the ionization chamber to monitor the six species above with a higher electron energy (70 eV) simultaneously to the MBMS-TOF measurements.

<table>
	<tbody>
		<tr>
			<td>Time-Of-Flight MBMS</td>
			<td>Kaesdorf</td>
			<td>n.a.</td>
			<td>custom design</td>
		</tr>
		<tr>
			<td>Molecular Beam Samling Interface</td>
			<td>self made</td>
			<td>n.a.</td>
			<td>custom design</td>
		</tr>
		<tr>
			<td>Laminar Flow Reactor</td>
			<td>Gero</td>
			<td>Type HTRH 40-1000</td>
			<td>custom design</td>
		</tr>
		<tr>
			<td>Quadrupole MS</td>
			<td>Hiden</td>
			<td>HAL/3F 301</td>
			<td>adapted to ionization chamber</td>
		</tr>
		<tr>
			<td>Vaporizer</td>
			<td>Bronkhorst</td>
			<td>CEM</td>
			<td>Vaporizer</td>
		</tr>
		<tr>
			<td>Mass Flow Meter</td>
			<td>Bronkhorst</td>
			<td>Mini Cori-Flow M12, M13, M14</td>
			<td>Flow Controller</td>
		</tr>
		<tr>
			<td>Jet A-1</td>
			<td>n.a.</td>
			<td>n.a.</td>
			<td>Standard Jet fuel of interest</td>
		</tr>
		<tr>
			<td>Metal syringe</td>
			<td>Hugo Sachs</td>
			<td>70-2252</td>
			<td>Fuel Supply</td>
		</tr>
		<tr>
			<td>Heating Hoses</td>
			<td>Hillesheim</td>
			<td>HMI series</td>
			<td>Gas Preheating</td>
		</tr>
		<tr>
			<td>Gas</td>
			<td>Linde</td>
			<td>Ar, O2</td>
			<td>Diluent, Oxidizer</td>
		</tr>
	</tbody>
</table>

combustion chemistry of fuels: quantitative speciation data obtained from an atmospheric high-temperature flow reactor with coupled molecular-beam mass spectrometer

This manuscript describes a high-temperature flow reactor experiment coupled to the powerful molecular beam mass spectrometry (MBMS) technique. This flexible tool offers a detailed observation of chemical gas-phase kinetics in reacting flows under well-controlled conditions. The vast range of operating conditions available in a laminar flow reactor enables access to extraordinary combustion applications that are typically not achievable by flame experiments. These include rich conditions at high temperatures relevant for gasification processes, the peroxy chemistry governing the low temperature oxidation regime or investigations of complex technical fuels. The presented setup allows measurements of quantitative speciation data for reaction model validation of combustion, gasification and pyrolysis processes, while enabling a systematic general understanding of the reaction chemistry. Validation of kinetic reaction models is generally performed by investigating combustion processes of pure compounds. The flow reactor has been enhanced to be suitable for technical fuels (e.g. multi-component mixtures like Jet A-1) to allow for phenomenological analysis of occurring combustion intermediates like soot precursors or pollutants. The controlled and comparable boundary conditions provided by the experimental design allow for predictions of pollutant formation tendencies. Cold reactants are fed premixed into the reactor that are highly diluted (in around 99 vol% in Ar) in order to suppress self-sustaining combustion reactions. The laminar flowing reactant mixture passes through a known temperature field, while the gas composition is determined at the reactors exhaust as a function of the oven temperature. The flow reactor is operated at atmospheric pressures with temperatures up to 1,800 K. The measurements themselves are performed by decreasing the temperature monotonically at a rate of -200 K/h. With the sensitive MBMS technique, detailed speciation data is acquired and quantified for almost all chemical species in the reactive process, including radical species.

A typical mass spectrum of the sampled gas composition is shown in Figure 3. With the given setup of a mass resolution of approx. 3,000, species up to m/z = 260 u can be detected within the C/H/O system. After a mass calibration procedure, the peaks are integrated for each mass-to-charge (m/z) ratio with deconvolution algorithms for evaluating under-resolved signals. After background and fragmentation corrections, the signal can be quantified using the approp...

Watch this Scientific Journal Video about Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectr

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

An investigation of the oxidative combustion chemistry of novel biofuels, fuel components, or jet fuels by comparison of quantitative speciation data is presented. The data can be used for kinetic model validation and enables fuel assessment strategies.This manuscript describes the atmospheric high-temperature flow reactor and demonstrates its capabilities.

This manuscript describes a high-temperature flow reactor experiment coupled to the powerful molecular beam mass spectrometry (MBMS) technique. This ...

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