Alternative Method for the Formulation of Surrogate Liquid Fuels

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Alternative method for the formulation of surrogate liquid fuels based on evaporative and sooting behaviors. Alvaro Muelas, Diego Aranda, and Javier Ballester Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00737 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Energy & Fuels

Alternative method for the formulation of surrogate liquid fuels based on evaporative and sooting behaviors. Álvaro Muelas1, Diego Aranda1, Javier Ballester2* 1

Laboratory of Research on Fluid Dynamics and Combustion Technologies (LIFTEC), CSIC –

University of Zaragoza, Spain. 2

Fluid Mechanics Group / LIFTEC, CSIC-University of University of Zaragoza, Spain.

* CORRESPONDING AUTHOR: Javier Ballester Fluid Mechanics Group School of Engineering and Architecture María de Luna, 3, 50018-Zaragoza - Spain Phone: +34 976 762 153; Fax: +34 976 761 882 [email protected]

KEYWORDS: surrogate, single droplet, evaporation, soot yield, heating oil

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ABSTRACT

This study proposes a novel approach to design and evaluate surrogates for liquid fuels, aimed at replicating their evaporative and sooting behaviors. The method was demonstrated for a commercial heating oil. The lack of surrogates found in the literature for this relevant fuel, in addition to its physicochemical complexity, were the primary reasons for its choice to test the proposed method. A first surrogate aiming to emulate the evaporative behavior of the target fuel was designed through the combination of a theoretical evaporation model and experimental tests. The second surrogate was formulated in order to replicate the sooting behavior of heating oil, whereas a third surrogate aimed to match physicochemical properties relevant for both processes. The so designed surrogates were validated afterwards by means of single droplet evaporation tests under high temperature conditions. The obtained evaporation curves served as a benchmark for evaluating the evaporative characteristic, whereas an aspirating probe collecting all the soot produced at a high-temperature and reducing atmosphere was used for the validation of the sooting tendency. It was found that surrogates specifically designed to match the evaporative and sooting behaviors of the target fuel displayed a remarkably good agreement when validated against experimental data for heating oil. Overall, the obtained results confirmed the validity of the methodologies proposed for surrogate formulation, combining predictive methods and droplet evaporation tests at high temperatures.


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1. INTRODUCTION

The optimization and efficient use of the energy contained in fuels is a major challenge in combustion science, as it can significantly mitigate its impact in environmental aspects such as pollutant emissions or climate change. A substantial complication in the design of combustion optimization strategies arises due to the physicochemical complexity of most conventional petroleum-derived fuels, which usually display a marked multicomponent character with hundreds of different constituents. It is nearly impossible to identify all the individual molecules and compositions and, even if it could be attainable, the combustion modeling of such a mixture would be unfeasible due to the lack of detailed data and huge computation costs [1]. In addition to this, real petroleum-based fuels typically display an extensive variability depending on the nature of the crude, the manufacturer or even the season [2]. In light of these limitations, the most followed approach to simulate the combustion performance of real fuels is through the use of surrogates i.e., mixtures of a few well-characterized pure compounds of known chemical species and mixture fractions which mimic certain physical and chemical properties of the target fuel [2, 3]. These simpler blends not only ease computational studies, but also provide timeinvariant reference fuels for experimental studies [4] and facilitate insight into the underlying combustion-related processes [2, 4]. With appropriate blending strategies, a surrogate might match a set of desired design properties, and therefore its behavior in certain related combustion processes can emulate those of the target fuel. By increasing the number of constituents in the surrogate, it is possible to match a greater number of design properties, although at the cost of increasing its complexity [2]. Much work has been done regarding the design and testing of surrogates for transportation fuels and, particularly, for diesel and gasoline (e.g. see reviews [5] and [6] respectively). It can


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be seen that most of the effort has been focused on emulating gas-phase chemistry properties (ignition, extinction, sooting propensity, etc.), with only a few studies trying to match physical behaviors [4]. The cited gas-phase features are undoubtedly of critical importance for most combustion-related applications, particularly for those in which the fuel is completely vaporized prior to its intake in the combustion chamber. However, for applications where the fuel enters the combustion chamber in liquid state (typically in the form of a spray), matching also certain liquid physical properties such as volatility, density or viscosity as well as its evaporative behavior can be of critical relevance for a complete description of the target fuel [4, 6]. This would be particularly true for the case of the combustion of heating oil in boilers, but also for diesel in CI engines [4] or even for gasoline in GDI engines [6]. In spite of the wide range of applications where the fuel is injected in liquid form into the combustion chamber, the use of configurations taking into account the phase change process are a minority within the surrogate literature, and most work has been performed at configurations with completely pre-vaporized fuel [7, 8]. Due to the multicomponent character of real liquid fuels, there are certain behaviors which are intrinsic and most relevant to spray burning in real applications but cannot be accounted for in tests with pre-vaporized fuel [7]. For instance, it is generally accepted that the light-end components of the fuel preferentially evaporate near the injection point, whereas the heavier, less volatile compounds become predominant downstream. These differences among liquid and vapor compositions throughout the combustion chamber can have a significant impact in relevant application-related aspects such as emissions or burner flame-stability. The use of combustion configurations which incorporate the phase change of the fuel is therefore an appropriate approach for a complete description of real, multicomponent liquid fuels [7]. For this purpose, the simplified single droplet configuration has been found in


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Energy & Fuels

several recent works to be particularly useful in the design and evaluation of a wide range of surrogates for liquid fuels, both by conducting experimental tests [7-9] or through the use of multicomponent droplet evaporation models [10-14]. This configuration has the advantage of incorporating the aforementioned particularities associated with liquid fuels while also keeping a much simpler analysis and modeling when compared with the stochastic environment of real spray flames. At this point it seems important to distinguish between surrogate design and its subsequent evaluation. Different methodologies have been used for both processes in the literature and, although it is not the objective here to exhaustively review all of them, a brief description of the approaches employed so far for some relevant design properties seems appropriate to place the current work in context. When it comes to surrogate design, a well-established approach consists in matching a set of properties which are relevant for certain combustion aspects (e.g. cetane number for chemical kinetics, molecular weight for diffusive properties or H/C for flame temperature in [15]). By matching the former set of properties, the surrogates obtained through this methodology are thought to be able to emulate the latter features, and therefore the desired complex combustion behaviors (e.g. the gas phase combustion kinetic phenomena in [15]). A comparatively small amount of studies have formulated surrogate fuels aiming to match the evaporative behavior of the target fuel, even though (as discussed above) it is a most relevant design property for applications where the fuel enters the combustion chamber in liquid state. Among them, the most followed approach is to use the distillation curve as design property representative of the evaporative behavior (e.g. in [4, 10, 14, 16]). Even though the distillation curve certainly provides valuable information regarding the evaporative characteristics of multicomponent fuels,


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droplets evaporating in real combustion environments may exhibit different behaviors from those observed in a batch distillation process because of the slow mass diffusion rate of species within the liquid [17, 18]. An alternative and application-oriented description of the evaporation behavior of a multicomponent fuel could be achieved by means of the simplified single droplet configuration. It is noteworthy that any surrogate formulation process always needs a method for estimating the design property value in terms of the mixture composition. This would lead to the use of single droplet multicomponent models as a predictive tool for surrogate formulation, as it has been done in some recent works such as [12, 13]. The target fuels in those works were, however, mixtures of discrete and well-identified chemical components (a FACE A gasoline comprising 66 species in [12], and a light naphtha containing 15 constituents in [13]). This allows for the simulation of the target fuel's evaporative behavior, providing therefore a target value in order to formulate surrogates. However, to the authors’ knowledge, this approach has not been applied to more conventional petro-fuels such as regular gasoline or diesel, comprising hundreds of unknown species. Besides the evaporative behavior, the propensity to soot of the target fuel is also found to be a relevant feature in many liquid combustion applications. In this regard, and taking into account the inherent complexity of soot chemistry, the most followed approach has been to use experimentally obtained indices such as the TSI (Threshold Sooting Index) [19, 20] or the YSI (Yield Sooting Index) [21, 22]. Once a surrogate has been formulated, different methodologies can be applied for evaluating its adequacy. A good example of straightforward surrogate evaluation would be the case of a surrogate designed for matching a certain property such as the derived cetane number, where a simple cetane number test would reveal its suitability. As numerous studies on surrogates seek to match gas-phase chemistry characteristics of the target fuel, common evaluation methods


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Energy & Fuels

comprise gas-phase tests such as shock tubes, rapid compression machines or reactors designed for flame speed measurements. Regarding physical surrogates, and more particularly surrogates formulated in order to match the evaporative behavior, the most followed validation method involves the distillation curve extraction, with comparatively fewer studies testing the surrogate's adequacy through the use of the single droplet evaporation/combustion approach. The already cited works by Elwardany et al. [11-13] evaluated the ability of several physical surrogates to emulate different target fuels' evaporative behaviors by means of a single droplet evaporation model. When it comes to experimental validations, Liu et al. used a single droplet apparatus to evaluate the combustion characteristics of surrogates designed in order to match certain gas phase combustion properties of Jet-A [7], and also to compare some standard reference fuels for gasoline (such as indolene or heptane-isooctane mixtures) with a commercial gasoline [8, 9]. The use of models for the design and validation of surrogates of practical fuels entails some difficulties, since the accurate modeling of multicomponent fuels is still a challenging objective and also requires a detailed characterization of their composition as well as the physicochemical properties of their components. In those cases, experimental characterizations by means of evaporation/combustion tests offer some advantages, providing reliable data without requiring a comprehensive description of the fuel properties. A recent work by Chen et al. [14] combines both approaches for validating the evaporative behavior of a Jet A surrogate, comparing the droplet vaporization curves of the target fuel (experimentally obtained) and that of a 4component surrogate (estimated through a multicomponent evaporation model). Regarding the sooting tendency, a vast majority of studies evaluate the surrogate's soot yield in a gaseous phase configuration (e.g., in [21]), with no reported work validating a surrogate's soot yield through the


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use of the single droplet configuration, where the reducing conditions in the droplet's proximity and concentration effects may play a significant role. In summary, the single droplet configuration clearly offers a suitable environment and some interesting and unexplored opportunities of study towards the development of surrogates for real liquid fuels. The main objective of this work consists therefore in using this approach in order to formulate and subsequently validate new surrogates which match the evaporative and sooting behaviors of a light heating oil. This approach combines modeling and experimental work on a single droplet evaporating under conditions representative of real flames. Both design properties are considered to be critical for the target fuel's main application, i.e. its combustion in boilers, and therefore different surrogates were designed with the aim of emulating them. A first surrogate matching the heating oil's evaporative characteristics was formulated based on a combination of single droplet evaporation tests at high temperature (required to obtain the behavior of the complex target fuel) and a multicomponent evaporation model (needed to estimate the characteristics of the different surrogate mixtures). The subsequent evaluation of surrogate blends was also experimentally obtained through droplet evaporation tests. A second surrogate was designed to emulate the heating oil's soot tendency. Its formulation was based on the well-known YSI indicator, whereas its validation was experimentally gained by means of soot sampling at the single droplet evaporation tests. The third and last surrogate was designed following a well-established and common methodology, as it is the matching of a group of rather simple physicochemical properties which are related with the more complex evaporation and sooting behaviors. The so formulated surrogate can thus be considered to be a reference case, and it was also validated through the aforementioned single droplet tests at the Droplet Combustion Facility (DCF). When compared with prior works, the current study proposes novel


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Energy & Fuels

approaches both on the formulation and on the evaluation sides. The use of DCF tests to provide the target behavior (required in the formulation phase) extends the use of theoretical droplet evaporation models as a tool for the design of surrogates for target fuels with unknown composition. Furthermore, there is no precedent of this kind of experimental validation applied for surrogates designed ad-hoc in order to precisely match the target fuel's single droplet evaporation behavior. The validation method proposed for estimating the sooting tendency would not only serve as a novel approach to evaluate the adequacy of liquid surrogate fuels, but it would also provide insight into the possibility of using the YSI as a soot predictor for the single droplet configuration (a case of study significantly different to that used in YSI tests [21, 22]). As it has been introduced, the main objective of this work is to develop and test novel approaches which can contribute to the recent surrogate formulation and validation developments based on the use of the isolated single droplet configuration. By applying these methodologies with the aim of matching the evaporative and sooting behaviors of heating oil, a quite unexplored fuel within the surrogate literature, the second main objective of the current study would be attained. In spite of the significant advance of natural gas during the last decades, the share of light heating oil still accounts for roughly a 17% of the EU domestic heating market, reaching more than 40% of all households in certain countries such as Switzerland or Ireland [23]. Due to its geographical reach, this liquid fuel can be readily transported to off-grid regions, and therefore its use in residential and rural areas is expected to remain predominant in the near future. In view of its magnitude, the development of surrogate fuels matching heating oil's relevant behaviors seems most desirable for the design of combustion optimization strategies. However, to the authors' knowledge, no previous study has addressed this issue, and therefore


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the surrogates obtained here aiming to match the evaporative and sooting behaviors of heating oil would contribute to fill this gap.

2. EXPERIMENTAL AND PREDICTIVE METHODS

2.1 FUELS INVESTIGATED A commercial Spanish heating oil (fuel-oil No. 2) was used as target fuel. A complete characterization was performed at the Instituto de Carboquímica (ICB-CSIC) in order to determine its most significant properties, which are listed in Table 1. The distribution by families is presented in the Appendix A of the Supplementary materials for the sake of completeness. Table 1. Main properties of the studied heating oil Molecular formula

C13.21H24.63

LHV (MJ/kg)

41.92

𝑴𝑾 (g/mol)

184.5

Density at 20 °C (kg/m3)

861

C/H (-)

0.54

Viscosity at 40 °C (cP)

3.43

In addition to the presented properties, the distillation curve of heating oil was also measured by means of a distillation apparatus similar to that featured in [16], that is, following the Advanced Distillation Curve (ADC) methodology developed by Bruno [24] rather than using the more common ASTM D86 technique [25]. The ADC method was chosen because by measuring kettle temperatures inside the liquid, the provided points are thermodynamically consistent and representative of the liquid-vapor equilibrium. The classical D86 apparatus, on the other hand, measures the vapor temperature in the distillation head, providing therefore somewhat lower temperatures than the actual thermodynamic state points [4, 6, 16, 24]. Two sequential ADC atmospheric distillation curves were acquired for heating oil, both displayed in Fig.1 along with their least squares fitting.


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Energy & Fuels

Fig.1. Experimental distillation curves extracted for the target heating oil. Two pure fuels were used in this work as surrogate palette compounds: n-Eicosane (C20H42, >99% purity) and 1-Methylnaphtalene (C11H10, >95% purity). The criteria for choosing these two compounds will be addressed below.

2.2 DROPLET COMBUSTION FACILITY As the formulation and subsequent evaluation of surrogates is intended to be based on the evolution of fuel droplets in a high temperature environment, a set of experiments were performed at LIFTEC's Droplet Combustion Facility (DCF). This facility has already been described in detail in previous works [26-28], and therefore only a brief exposition will be provided here. A schematic showing the main parts of the DCF is displayed in Fig.2 along with representative pictures of the droplet evaporating and burning processes.


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Fig.2. Schematic of the droplet combustion facility (a) and representative pictures captured with both cameras for the combustion of heating oil droplets showing the droplet surrounded by the soot shell and the envelope flame (b).

A piezoelectric device at the top of the facility generated a monosized stream of free-falling droplets, with a nominal diameter of 150 micrometers. This is considered to be a good compromise between satisfactory experimental accuracies and real sizes found in sprays. The interdroplet space was always over 100 diameters, and thus interaction between droplets can be considered to be negligible [29]. The droplet generator device possesses a heating system which allows for the dosing of fuels with high pour point or which are too viscous. The monosized, isolated droplets were introduced into the exhaust gases produced by a flat-flame burner (McKenna) fed with methane and air. This allows for the vaporization and (if oxygen is available) burning of the small droplets in an atmosphere representative of those found in real flames. As this work intends to study the vaporization process of the droplets rather than their


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Energy & Fuels

combustion, the feed flows of methane and air were adjusted stoichiometrically, and therefore the flat-flame combustion products contained no oxygen. As the experimental conditions of this gaseous coflow are critical for the validity of the experimental droplet evaporation results and their subsequent modeling, both the coflow temperatures and velocities were thoroughly measured along the tube centerline, as detailed in [28]. The droplet vaporization process of different fuels was characterized through the images acquired with a CCD camera (QImaging Retiga SRV, 'Camera 1' in Fig.2a). This camera was synchronized with the droplet generator and with a LED strobe, allowing the use of the doubleshot technique. The LED strobe emitted very short (

Alternative Method for the Formulation of Surrogate Liquid Fuels

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