Appl. Phys. B DOI 10.1007/s00340-015-6139-4 Propene concentration sensing for combustion gases using quantum-cascade laser absorption near 11 μm Robin S. M. Chrystie1 · Ehson F. Nasir1 · Aamir Farooq1 Received: 7 February 2015 / Accepted: 11 May 2015 © Springer-Verlag Berlin Heidelberg 2015 Abstract We report on a strategy to measure, in situ, 1 Introduction the concentration of propene (C3H6) in combustion gases using laser absorption spectroscopy. Pyrolysis of n-butane Laser diagnostics used in combustion applications are was conducted in a shock tube, in which the resultant gases invaluable in addressing issues related to process effi- were probed using an extended cavity quantum-cascade ciency, performance, and pollution. Laser diagnostics laser. A differential absorption approach using online and assist in the development of computational tools, which offline wavelengths near λ 10.9 μm enabled discrimi- ultimately empower engineers to effectively design and = nation of propene, cancelling the effects of spectral inter- develop improved combustion technology. Chemical ference from the simultaneous presence of intermediate kinetic models form a pivotal component of these compu- hydrocarbon species during combustion. Such interfer- tational tools. Detailed models of realistic fuels comprise ence-free measurements were facilitated by exploiting thousands of species and tens of thousands of chemical the C–H bending mode characteristic to alkenes (olefins). reactions. Development of these models requires robust = It was confirmed, for intermediate species present dur- testing against experimental data. Laser-based diagnostics ing pyrolysis of n-butane, that their absorption cross sec- are now typically acknowledged to be capable of provid- tions were the same magnitude for both online and offline ing such robust testing, thus facilitating the development of wavelengths. Hence, this allowed time profiles of propene reliably accurate kinetic models to ultimately improve the concentration to be measured during pyrolysis of n-butane computational design capability towards improved com- in a shock tube. Time profiles of propene subsequent to a bustion technology. passing shock wave exhibit trends similar to that predicted New combustion concepts, such as lean premixed by the well-established JetSurF 1.0 chemical kinetic mech- prevapourized (LPP) gas turbines and reactivity-controlled anism, albeit lower by a factor of two. Such a laser diag- compression ignition (RCCI) engines, tend to be based nostic is a first step to experimentally determining propene on lean, low-temperature, and high-pressure conditions, in real time with sufficient time resolution, thus aiding the chiefly in order to reduce soot and NOx emissions. In the refinement and development of chemical kinetic models for case of LPP technology, temperature was determined to be combustion. one of the main determinants of process behaviour [1, 2]. However, under such homogenously lean conditions the rate of reaction tends to be rate limiting and it is, there- fore, very important to study the role of chemical kinetics in understanding and controlling such process behaviour. * Aamir Farooq This would involve the development, testing, and improve- [email protected] ment of chemical kinetic models for a particular fuel used. One of the traditional methods of testing such models is to 1 Clean Combustion Research Centre, Division of Physical compare the predicted ignition delay time with that from Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955‑6900, shock tube experiments. However, more accurate and com- Saudi Arabia prehensive model testing has been increasingly taking a 1 3 R. S. M. Chrystie et al. multivariable approach through the measurement of species sensors (~100 ms). More recently, there have been develop- concentrations and temperature time profiles [3–7], chiefly ments in optical absorption sensing of small hydrocarbon using laser diagnostics. intermediates in shock tube environments, namely CH4 [23, Detailed investigation of hydrocarbon intermediates, 24], C2H4 [25], C2H2 [26], and i-C4H8 [11]. Hitherto, to the for example, is important for understanding reaction paths best of our knowledge, there is no reported laser absorption controlling overall fuel consumption [8] and pollutant for- diagnostic for the C-3 hydrocarbon propene (C3H6). The mation [9]. In particular, the overall rate of fuel consump- optical approach offers many advantages such as the ability tion can be a strong function of alkene chemistry [10]. It to readily perform in situ measurements with excellent time is, therefore, paramount to gain refined knowledge of the resolution without perturbing the system environment. formation and decay rates of various alkenes [11–17]. The specific objective of the current work is to develop Propene (C3H6) is a relatively abundant alkene inter- a highly time-resolved, non-invasive, and accurate in situ mediate present in combustion gases. Propene is gathering laser diagnostic based on absorption spectroscopy for interest in the kinetics community because of its important measuring the composition of propene (C3H6) in a shock role in combustion chemistry [12–17]. The abundance of tube environment. Such a diagnostic would facilitate the propene as a by-product of combustion is due to its molec- ultimate goal of chemical kinetic model development and ular stability and serves as a reliable marker of combustion validation. inefficiency in internal combustion engines [18]. Propene emission into the atmosphere poses the greatest threat with respect to ground-level photochemical ozone formation [9], 2 Diagnostic scheme and therefore it is of interest to minimize its emission. Soot abatement is also a high priority, with particulate matter 2.1 Likely intermediates originating from the propargyl radical •C3H3. Propene leads a direct pathway to the creation of propargyl, which readily In order to effectively devise a sensing strategy for the pro- forms the benzene molecule during combustion [19]. Ben- pene (C3H6) intermediate during combustion, a prelimi- zene, similar to other aromatics, leads to the formation of nary check of the likely major molecular species formed polycyclic aromatic hydrocarbons and subsequent particu- during pyrolysis and oxidation of n-butane (a simple and late matter. well-studied fuel) was performed using the JetSurF 1.0 The concentration and residence time profile of propene kinetic mechanism [27]. Such identification of intermedi- formed during combustion, and its consequences down- ate species highlights molecules that could potentially lead stream, can be controlled by the chemical profile of the to spectroscopic interference and hence guides the sensing fuel burned and influenced by molecular size, molecular strategy to be invoked. Figure 1a shows dominant species branching, degree of saturation and aromaticity, and degree formed during the pyrolysis of 5 % n-butane at representa- of oxygenation or nitration [20]. Fuel formulation may be tive combustion conditions (1350 K, 3.5 bar). The domi- guided by better insight via quantitative measurements of nant species are primarily stable hydrocarbon molecules. propene formed during combustion, using laser diagnos- At the conditions of Fig. 1a, appreciable quantities of pro- tics, in order to ultimately (1) minimize residual soot from pene are formed (~1 %), equating to about 20 % of the ini- engine exhaust, (2) reduce ground-level ozone formation in tial loading of n-butane here. cities, and (3) control knock in engines. At similar conditions, stoichiometric oxidation of 1 % Direct measurement of propene has traditionally n-butane yields the predicted species time profile shown involved physical sampling using probes [21]. This in Fig. 1b. One per cent loading was chosen as 5 % approach is very effective at detecting many species within would otherwise result in large energy release in shock a single measurement. Unfortunately, it is an invasive pro- tube experiments. Large energy release leads to depar- cedure and is generally less straightforward to implement ture from ideal shock tube behaviour, and thus a smaller compared to laser diagnostic approaches. Most importantly, and more appropriate initial loading was chosen. The physical sampling does not offer a sufficient degree of time simulation predicts about 0.2 % peak formation of pro- resolution compared to laser diagnostics, and therefore is pene along with several other hydrocarbons, radicals, and not ideally suited to rapidly evolving chemical reactions product species. In general, oxidation of fuels results in taking place inside a shock tube. A more recent approach to hydrocarbon intermediate species being formed and con- overcome the limitations of physical sampling is to use an sumed in a relatively short period of time, as exemplified electronic sensor. Samman et al. [22] used a MOSiC (SiC- by the first ~ 500 μs shown in Fig. 1b. In this study, we based metal-oxide semiconductor) sensor which showed have focused on developing a propene diagnostic for fuel signs of species specificity, including that towards propene. pyrolysis to provide an early demonstration of propene However, there is still relatively slow time response of such sensing. 1 3 Propene concentration sensing for combustion gases… Fig. 2 Infrared absorption spectra of C-2 and C-3 alkanes (a, c) and 1 alkenes (b, d). Region of the –C–H stretching mode near 3000 cm− (conventionally used) is compared to