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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "I2: Energy and Combustion Science".

Deadline for manuscript submissions: 30 September 2024 | Viewed by 6595

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Special Issue Editors

Dr. Marina Braun-Unkhoff

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Guest Editor

Institute of Combustion Technology, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
Interests: high-temperature combustion of gaseous and liquid fuels; jet fuels; bio-fuels; alternative fuels for energy, aviation, and transport; chemical kinetic modeling; flame chemistry; reduction of pollutants (NOx, PAH, soot); advancements of combustion modeling, including alternative fuels

Dr. Sandra Richter

E-Mail Website
Guest Editor

Institute of Combustion Technology, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
Interests: alternative fuels for transport; E-fuels; jet fuels; sustainable aviation fuels (SAFs); combustion properties and emissions

Special Issue Information

Dear Colleagues,

We are pleased to announce the upcoming Special Issue, “Advances in Fuels and Combustion”, of Energies. Currently, many global efforts and initiatives are motivated by the need to identify effective pathways for providing and using energy in a sustainable way to combat the effects of global warming. In our daily life, transportation is omnipresent and is expected to grow due to an increasing population and further globalization. Today’s automotive transportation, and likely for several decades, heavily relies on internal combustion engines, which run on hydrocarbon fuels stemming mostly from fossil feedstocks, with an increased share of renewable feedstocks.

However, we need to drastically reduce the use of fossil fuels and limit their harmful emissions, their contribution to greenhouse warming and, ultimately, to climate change. While this change will ideally be realized in the very near future, there is no doubt that the transition will only be possible if we can manage the challenges associated with implementation, reliance, acceptance, and cost.

Current research demands are two-fold: increase the availability of appropriate sustainable fuels for mobility and energy and provide a profound and comprehensive understanding of their use in terms of reliability and environmental amenity in terms of today’s engines and using today’s structures.

As a consequence, comprehensive studies should focus on emission patterns, thermophysical and thermochemical properties, and fundamental combustion properties, such as ignition, flame speeds, and speciation data for neat novel fuels but also for their blends with today’s fuels, thus fostering quick market penetration.

These areas include but are not limited to the following: novel and sustainable fuels; multi-component fuel blends; variation in oxidizer composition also related to the use of exhaust gas recirculation; homogeneous and inhomogeneous ignition; high-temperature regime as well as lower temperatures (<~1000 K); elevated pressures (>1 bar); fuel-lean to stoichiometric and fuel-rich premixed mixtures; reaction models, detailed and reduced; soot precursor chemistry; design of novel fuels in terms of thermochemical and thermophysical properties and emission; production pathways; regulations; socio-economic analysis.

The papers in this Special Issue are expected to advance our understanding of novel fuels (including conventional, alternative, and surrogate fuels) through new experimental, theoretical, and/or kinetic modeling studies, which include but are not limited to the following:

Experimental studies related to facilities such as burners, jet-stirred reactors, flow reactors, shock tubes, rapid compression machines, and engines;
Chemical kinetic modeling, including development and validation of reaction mechanisms;
Studies of oxidative and pyrolytic reaction pathways, including species thermodynamic, transport properties, and rate expressions;
CFD simulations of experiments, engines, and other facilities;
Production pathways of renewable fuels, including techno-economic analysis and life-cyle analysis;
Application of synthetic fuels in road transport, shipping, aviation, and power generation;
Regulations and market launch;

Dr. Marina Braun-Unkhoff
Dr. Sandra Richter
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Energies is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

synthetic sustainable fuels, including e-fuels and biofuels
aviation fuels
shipping fuels
fuels for road transport
combustion (ignition, flame speed, speciation)
experiments (flames, jet stirred reactor, flow reactor, shock tube, rapid compression machine RCM)
emissions and their impact on the environment (e.g., global warming potential GWP, non-CO₂-effects, human health)
chemical kinetic modeling

Published Papers (9 papers)

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Research

16 pages, 4682 KiB

Open AccessArticle

Experimental Investigation of Thermoacoustics and High-Frequency Combustion Dynamics with Band Stop Characteristics in a Pressurized Combustor

by Mehmet Kapucu, Jim B. W. Kok and Artur K. Pozarlik

Energies 2024, 17(7), 1680; https://doi.org/10.3390/en17071680 - 01 Apr 2024

Viewed by 415

Abstract

In combustor systems, thermoacoustic instabilities may occur and must be avoided for reliable operation. An acoustic network model can be used to predict the eigenfrequencies of the instabilities and the growth rate by incorporating the combustion dynamics with a flame transfer function (FTF). The FTF defines the interconnection between burner aerodynamics and the rate of combustion. In the current study, the method to measure the FTF in a pressurized combustor is explored. A siren unit, mounted in the fuel line, induced a fuel flow excitation of variable amplitude and high maximum frequency. This was performed here for pressurized conditions at 1.5 bar and 3 bar and at a thermal power of 125 kW and 250 kW. In addition to the experimental investigation, a 1-D acoustic network model approach is used. In the model, thermoviscous damping effects and reflection coefficients are incorporated. The model results compare well with experimental data, indicating that the proposed method to determine the FTF is reliable. In the approach, a combination of an FTF with a band stop approach and a network modeling approach was applied. The method provides a good match between experimentally observed behavior and an analytical approach and can be used for instability analysis. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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26 pages, 6462 KiB

Open AccessArticle

Thermodynamic Reactivity Study during Deflagration of Light Alcohol Fuel-Air Mixtures with Water

by Rafał Porowski, Arief Dahoe, Robert Kowalik, Joanna Sosnowa and Katarzyna Zielinska

Energies 2024, 17(6), 1466; https://doi.org/10.3390/en17061466 - 19 Mar 2024

Viewed by 980

Abstract

In this paper, a thermodynamic and reactivity study of light alcohol fuels was performed, based on experimental and numerical results. We also tested the influence of water addition on fundamental properties of the combustion reactivity dynamics in closed vessels, like the maximum explosion pressure, maximum rate of pressure rise and the explosion delay time of alcohol–air mixtures. The substances that we investigated were as follows: methanol, ethanol, n-propanol and iso-propanol. All experiments were conducted at initial conditions of 323.15 K and 1 bar in a 20 dm³ closed testing vessel. We investigated the reactivity and thermodynamic properties during the combustion of liquid fuel–air mixtures with equivalence ratios between 0.3 and 0.7 as well as some admixtures with water, to observe water mitigation effects. All light alcohol samples were prepared at the same initial conditions on a volumetric basis by mixing the pure components. The volumetric water content of the admixtures varied from 10 to 60 vol%. The aim of water addition was to investigate the influence of thermodynamic properties of light alcohols and to discover to which extent a water addition may accomplish mitigation of combustion dynamics and thermodynamic reactivity. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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16 pages, 2667 KiB

Open AccessArticle

Techno-Economic Comparison of Low-Carbon Energy Carriers Based on Electricity for Air Mobility

by Jean-Baptiste Jarin, Stéphane Beddok and Carole Haritchabalet

Energies 2024, 17(5), 1151; https://doi.org/10.3390/en17051151 - 28 Feb 2024

Viewed by 643

Abstract

The decarbonization of air mobility requires the decarbonization of its energy. While biofuels will play an important role, other low-carbon energy carriers based on electricity are considered, such as battery electrification and liquid hydrogen (LH₂) or eFuel, a hydrogen-based energy carrier. Each energy carrier has its own conversion steps and losses and its own integration effects with aircraft. These combinations lead to different energy requirements and must be understood in order to compare their cost and CO₂ emissions. Since they are all electricity-based, this study compares these energy carriers using the well-to-rotor methodology when applied to a standard vertical take-off and landing (VTOL) air mobility mission. This novel approach allows one to understand that the choice of energy carrier dictates the propulsive system architecture, leading to integration effects with aircraft, which can significantly change the energy required for the same mission, increasing it from 400 to 2665 kWh. These deviations led to significant differences in CO₂ emissions and costs. Battery electrification is impacted by battery manufacturing but has the lowest electricity consumption. This is an optimum solution, but only until the battery weight can be lifted. In all scenarios, eFuel is more efficient than LH₂. We conclude that using the most efficient molecule in an aircraft can compensate for the extra energy cost spent on the ground. Finally, we found that, for each of these energy carriers, it is the electricity carbon intensity and price which will dictate the cost and CO₂ emissions of an air mobility mission. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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15 pages, 6909 KiB

Open AccessArticle

Automatic Extension of a Semi-Detailed Synthetic Fuel Reaction Mechanism

by Marleen Schmidt, Celina Anne Kathrin Eberl, Sascha Jacobs, Torsten Methling, Andreas Huber and Markus Köhler

Energies 2024, 17(5), 999; https://doi.org/10.3390/en17050999 - 20 Feb 2024

Viewed by 436

Abstract

To identify promising sustainable fuels, e.g., to select novel synthetic fuels with the greatest impact on minimizing global warming, new methods for rapid and economical technical fuel assessment are urgently needed. Here, numerical models that are capable of predicting technical key data quickly and without experimental setup are necessary. One method is the use of chemical kinetic models, which are able to predict the technical key parameters related to combustion behavior. For a rapid technical fuel assessment, these chemical kinetic models need to be validated for new fuel components and for different temperature and pressure ranges. This work presents a new approach to extend the existing semi-detailed chemical kinetic models. For the application of the approach, the semi-detailed reaction mechanism DLR Concise was selected and extended for the low temperature combustion modeling of n-heptane and isooctane. The open-source software reaction mechanism generator (RMG) was used for this extension. Furthermore, an optimization of the merged chemical kinetic model with the linear transformation model (linTM) was conducted in order to improve the reproducibility of ignition delay times. The improvement of the predictive performance of ignition delay times at low temperatures for both species was successfully demonstrated. Therefore, this approach can be used to quickly add new species or reaction pathways to an existing semi-detailed reaction mechanism to enable a model-based technical fuel assessment for the early identification of promising fuels. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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21 pages, 676 KiB

Open AccessArticle

Evaluation of the Applicability of Synthetic Fuels and Their Life Cycle Analyses

by Sandra Richter, Marina Braun-Unkhoff, Samuel Hasselwander and Sofia Haas

Energies 2024, 17(5), 981; https://doi.org/10.3390/en17050981 - 20 Feb 2024

Viewed by 622

Abstract

This paper summarizes the findings of a detailed assessment of synthetic, electricity-based fuels for use in aviation, shipping, and road transport. The fuels considered correspond to the most promising alternatives that were analyzed as part of the German research project BEniVer (Begleitforschung Energiewende im Verkehr—Accompanying Research for the Energy Transition in Transport) initiated by the German Federal Ministry for Economic Affairs and Climate Action (BMWK). Focusing on usage, infrastructure, and ecological analyses, several e-fuels were evaluated and compared to fossil fuels according to the specific sector. It turns out that for all sectors evaluated, the existing sustainable synthetic fuels are already compatible with current technology and regulations. In shipping and road transport, the use of advanced, sustainable fuels will allow for a more distinct reduction in emissions once technology and regulations are adopted. However, standard-compliant synthetic gasoline and diesel are considered the most promising fuels for use in road transport if the transition to electricity is not realized as quickly as planned. For the aviation sector, the number of sustainable aviation fuels (SAFs) is limited. Here, the current aim is the introduction of a 100% SAF as soon as possible to also tackle non-CO₂ emissions. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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20 pages, 7383 KiB

Open AccessArticle

Pyrolysis of Cyclohexane and 1-Hexene at High Temperatures and Pressures—A Photoionization Mass Spectrometry Study

by Robert S. Tranter, Colin Banyon, Ryan E. Hawtof and Keunsoo Kim

Energies 2023, 16(24), 7929; https://doi.org/10.3390/en16247929 - 06 Dec 2023

Viewed by 721

Abstract

Cycloalkanes are important components of a wide range of fuels. However, there are few experimental data at simultaneously high temperatures and pressures similar to those found in practical systems. Such data are necessary for developing and testing chemical kinetic models. In this study, data relevant to cycloalkane pyrolysis were obtained from high repetition rate shock tube experiments coupled with synchrotron-based photoionization mass spectrometry diagnostics. The pyrolysis of cyclohexane was studied over 1270–1550 K and ~9 bar, while the more reactive primary decomposition product, 1-hexene, was studied at 1160–1470 K and ~5 bar. Insights into the decomposition of the parent molecules, the formation of primary products and the production of aromatic species were gained. Simulations were performed with models for cyclohexane and 1-hexene that were based on literature models. The results indicate that over several hundred microseconds reaction time at high pressures and temperatures the pyrolysis of cyclohexane is largely dominated by reactions initiated by cyclohexyl radicals. Furthermore, good agreement between the simulations and the experiments were observed for cyclohexane and 1-hexene with a modified version of the cyclohexane model. Conversely, the 1-hexene model did not reproduce the experimental observations. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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17 pages, 4419 KiB

Open AccessArticle

Validation of a Generic Non-Swirled Multi-Fuel Burner for the Measurement of Flame Stability Limits for Research of Advanced Sustainable Aviation Fuels

by Paul Zimmermann, Julian Bajrami and Friedrich Dinkelacker

Energies 2023, 16(22), 7480; https://doi.org/10.3390/en16227480 - 07 Nov 2023

Viewed by 626

Abstract

Future aviation concepts should be both CO₂-neutral and without other emissions. One approach to reaching both targets is based on sustainably produced synthetic liquid fuels, which may allow very clean, lean premixed prevaporized (LPP) combustion. For that, fuels are needed with much longer ignition delay times and a lower flashback propensity than current jet fuels. We describe an experimental setup to investigate the flashback stability of liquid fuels in a multi-fuel burner. In this work, the measurement procedure and the determination of the experimentally obtained accuracy are in focus with regard to prevaporized and preheated iso-propanol/air flames in an equivalence ratio range of 0.85 to 1.05 involving three preheating levels (573, 673, and 773 K). As the determination of the accurate unburnt gas temperature just ahead of the flame is of strong importance for flashback but not directly possible, a model is implemented to determine it from the measurable quantities. Even with this indirect method, and also regarding the hysteresis of the experimental preheating temperature, it is found that the relevant quantities, namely, measured temperatures, mass flows, and values derived from them, can be determined with accuracy in the range below 1.7%. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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22 pages, 12874 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

An Experimental and Detailed Kinetics Modeling Study of Norbornadiene in Hydrogen and Methane Mixtures: Ignition Delay Time and Spectroscopic CO Measurements

by Matthew G. Sandberg, Claire M. Grégoire, Darryl J. Mohr, Olivier Mathieu and Eric L. Petersen

Energies 2023, 16(21), 7278; https://doi.org/10.3390/en16217278 - 26 Oct 2023

Viewed by 742

Abstract

High-energy-density compounds such as norbornadiene (NBD) are being considered as potential cost-effective fuel additives, or partial replacements, for high-speed propulsion applications. To assess the ability of NBD to influence basic fuel reactivity enhancement and to build a database for developing future NBD kinetics models, ignition delay times were measured in two shock-tube facilities at Texas A&M University for H₂/O₂, CH₄/O₂, H₂/NBD/O₂, and CH₄/NBD/O₂ mixtures (ϕ = 1) that were highly diluted in argon. The reflected-shock temperatures ranged from 1014 to 2227 K, and the reflected-shock pressures remained near 1 atm for all of the experiments, apart from the hydrogen mixtures, which were also tested near 7 atm, targeting the second-explosion limit. The molar concentrations of NBD were supplemented to the baseline mixtures representing 1–2% of the fuel by volume. A chemiluminescence diagnostic was used to track the time history of excited hydroxyl radical (OH*) emission, which was used to define the ignition delay time at the sidewall location. Spectroscopic CO data were also obtained using a tunable quantum cascade laser to complement both the ignition and the chemiluminescence data. The CH₄/O₂ mixtures containing NBD demonstrated reduced ignition delay times, with a pronounced effect at lower temperatures. Conversely, this additive increased the ignition delay time dramatically in the H₂/O₂ mixture, which was attributed to changes in the fundamental chemistry with the introduction of molecules containing carbon bonds, which require stronger activation energies for ignition. Correlations were developed to predict the ignition delay time, which depends on species concentration, temperature, and pressure. Additionally, one tentative mechanism was tested, combining base chemistry from NUIGMech 1.1 with pyrolysis and oxidation reactions for NBD using the recent efforts from experimental and theoretical literature studies. The numerical predictions show that the rapid decomposition of NBD provides a pool of active H-radicals, significantly increasing the reactivity of methane. This study represents the first set of gas-phase ignition and CO time-history data measured in a shock tube for hydrogen and methane mixtures containing the additive NBD. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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15 pages, 7827 KiB

Open AccessArticle

Experimental Characterization of a Novel Foam Burner Design for the Low-Excess-Enthalpy Combustion of Very Lean Syngas Mixtures

by Kyriakos Fotiadis, Akrivi Asimakopoulou, Penelope Baltzopoulou, Georgia Kastrinaki, Dimitrios Koutsonikolas, George Karagiannakis, George Skevis, Jana Richter and Fabian Mauss

Energies 2023, 16(19), 7014; https://doi.org/10.3390/en16197014 - 09 Oct 2023

Viewed by 820

Abstract

In the present work, a novel foam burner design is proposed and experimentally evaluated for operation with highly diluted syngas mixtures. The lab-scale burner consists of a purpose-built, square-shaped, high-temperature-grade stainless steel tubular reactor filled with square-sectioned siliconized silico carbide (SiSiC) foams. The assembly was installed in an electrical furnace. Spatially resolved temperature measurements were obtained along the reactor axis, while simultaneous measurements of CO, CO₂, H₂, O₂, and N₂ were taken at the burner exit and the water levels were recorded upstream and downstream of the reactor. The results clearly show that flames can be stabilized along the reactor for a range of foam characteristics and operating conditions. Hydrogen conversion efficiencies in excess of 98%, and overall thermal efficiencies close to 95% were achieved for the selected operating conditions. Overall, the denser 10 ppi foam demonstrated superior combustion characteristics in terms of stability, lower enthalpy rises, and a wider operating range at the expense of a very modest pressure drop penalty. Finally, scanning electron microscopy, coupled with energy dispersion spectroscopy (SEM/EDS) and Raman spectroscopy analyses, was used to determine the morphological and compositional characteristics of the pristine and aged foams. After more than 100 h of operation, no significant performance degradation was observed, even though the burner design was subjected to considerable thermal stress. Full article

(This article belongs to the Special Issue Advances in Fuels and Combustion)

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