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

Summary

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.

Abstract

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.

Introduction

Understanding combustion processes in the wake of modern, low-emission fuels from renewable resources is a challenge for today's societies' 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 net....

Protocol

1. Setup of the molecular beam mass spectrometer (MBMS) and flow reactor system

1. Heat oven to designated start temperature, which is the highest temperature in designated measurement series. For typical conditions of Jet A-1 with Φ=1, total oxidation is observed below 850 °C (~1,100 K). The choice of proper starting temperatures depends on the chemical nature of the investigated fuel and the stoichiometry (Φ).
2. Prepare Time-of-Flight (TOF) spectrometer for intermediate species detection. The TOF spectrometer is aligned to the molecular beam and thus provides reliable detection of labile species.
   NOTE: The mass resolution is su....

Results

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.......

Discussion

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 studies^21,22,23,27 demonstrated the flexibility of the experiment starting from rich methane conditions relevant for partial oxidation phenomena (φ = 2.5), to investigating the co.......

Materials

List of materials used in this article

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