Study of Low- and Intermediate-Temperature Oxidation Kinetics of Diethyl Ether in a Supercritical Pressure Jet-Stirred Reactor

Growing demand for low-emission and high-efficiency propulsion systems spurs interest in understanding low-temperature and ultra-high-pressure combustion of alternative biofuels like diethyl ether (DEE). In this study, DEE oxidation experiments are performed at 10 and 100 atm, over a temperature range of 400-900 K, at fuel-lean, stoichiometric, and fuel-rich conditions by using a supercritical pressure jet-stirred reactor (SP-JSR). The experimental data show that DEE is very reactive and exhibits an uncommon low-temperature oxidation behavior with two negative temperature coefficient (NTC) zones. The first NTC zone is mainly governed by the competition reactions of QOOH + O₂ = O₂QOOH and QOOH = 2CH₃CHO + OH, while the second one is mainly governed by the competition reactions of R + O₂ = RO₂ and the β-scission reaction of fuel radical R. It is shown that the increase of pressure stabilizes RO₂ and promotes HO₂ chemistry. Moreover, the branching ratios of β-scission reactions of R and QOOH decrease. As a result, it is shown that, with the increase of pressure, both NTC zones become weaker at 100 atm. In addition, the intermediate-temperature oxidation is shifted considerably to lower temperature at 100 atm. The existing DEE model in the literature well predicts the experimental data at low temperature; however, it underpredicts the fuel consumptions at intermediate temperature. The H₂/O₂ subset in the existing DEE model is updated in this study based on the Princeton updated HP-Mech, including the singlet/triplet competing channels of HO₂ related reactions. The updated model improves the overall predictability of key species, especially at intermediate temperature.

Heterogeneous nanocatalyst for biodiesel fuel production: bench scale from waste oil sources

Biodiesel is a promising clean energy supply that can be made from sustainable and low-grade fuels using a variety of methods. Transesterification is one of the processes that can occur in the manifestation of an effective catalyst. The catalyst may be homogeneous or heterogeneous in nature. This article reviews on the formation of biodiesel from various sources of waste oils using heterogeneous nanocatalysts. The manufacture of biodiesel using homogeneous and heterogeneous catalysis had been extensively studied, and new heterogeneous catalysts are constantly being examined. In general, homogeneous catalysts are effective at remodeling biodiesel with low free fatty acid (FFA) and single-origin feedstock having water. Heterogeneous catalysts, instead have higher interest, a wider scope of selectivity, better FFA, and better water adaptability. These properties are regulated by the number and intensity of active basic or acid sites. In order to achieve a viable alternative to conventional homogeneous catalysts for biodiesel processing, heterogeneous catalysts made from waste and biocatalysts are needed. Nanocatalysts have recently attracted interest due to their high catalytic performance under favorable operating conditions. This review evaluates the usage of heterogeneous nanocatalysts for the production of biodiesel from different sources of waste oil and the factors effecting the process of biodiesel production.

Diastereomers and Low-Temperature Oxidation

Diastereomers have historically been ignored when building kinetic mechanisms for combustion. Low-temperature oxidation kinetics, which continues to gain interest in both combustion and atmospheric communities, may be affected by the inclusion of diastereomers in radical chain-branching pathways. In this work, key intermediates and transition states lacking stereochemical specification in an existing diethyl ether low-temperature oxidation mechanism were replaced with their diastereomeric counterparts. Rate coefficients for reactions involving diastereomers were computed with ab initio transition state theory master equation calculations. The presence of diastereomers increased rate coefficients by factors of 1.2-1.6 across various temperatures and pressures. Ignition delay simulations incorporating these revised rate coefficients indicate that the diastereomers enhanced the overall reactivity of the mechanism by almost 15% and increased the peak ketohydroperoxide concentration by 30% in the negative temperature coefficient region at combustion-relevant pressures. These results provide an illustrative indication of the important role of stereomeric effects in oxidation kinetics.

Combustion in the future: The importance of chemistry

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.

Elevated pressure low-temperature oxidation of linear five-heavy-atom fuels: diethyl ether, n-pentane, and their mixture

Diethyl ether (DEE) has been proposed as a biofuel additive for compression-ignition engines, as an ignition improver for homogeneous charge compression ignition (HCCI) engines, and as a suitable component for dual-fuel mixtures in reactivity-controlled compression ignition (RCCI) engines. The combustion in these engines is significantly controlled by low-temperature (LT) chemistry. Fundamental studies of DEE LT oxidation chemistry and of its influence in fuel-mixture oxidation are thus highly important, especially at elevated pressures. Elevated pressure speciation data were measured for the LT oxidation of DEE, of its similarly-structured linear five-heavy-atom hydrocarbon fuel (n-pentane), and of a mixture of the two fuels in a jet-stirred reactor (JSR) in the temperature range of 400–1100 K and at various pressures up to 10 bar. The pressure influence on the LT oxidation chemistry of DEE was investigated by a comparison of the measured profiles of oxidation products. The results for DEE and n-pentane were then inspected with regard to fuel structure influences on the LT oxidation behavior. The new speciation data were used to test recent kinetic models for these fuels [Tran et al., Proc. Combust. Inst. 37 (2019) 511 and Bugler et al., Proc. Combust. Inst. 36 (2017) 441]. The models predict the major features of the LT chemistry of these fuels well and could thus subsequently assist in the data interpretation. Finally, the LT oxidation behavior of an equimolar mixture of the two fuels was explored. The interaction between the two fuels and the effects of the pressure on the fuel mixture oxidation were examined. In addition to reactions within the combined model for the two fuels, about 80 cross-reactions between primary reactive species generated from these two fuel molecules were added to explore their potential influences.

Kinetics of autoignition: a simple intuitive interpretation and its relation to the Livengood-Wu integral

It is well known that the gas-phase autoignition phenomenon often involves branched chain reactions as well as the acceleration of reactions by thermal feedback. Despite the huge combustion kinetic mechanisms of large hydrocarbons found in practical fuels, chain reactions in the early stages of alkane autoignition exhibit simple kinetics since the pseudo-first-order assumption and the linear approximation are valid. In this study, this simple picture of autoignition will be presented starting from the H2-O2 system and then extending to practical fuel-air mixtures. The present interpretation gives the theoretical rationale for the Livengood-Wu integral which is known as an empirical method to predict the timing of knock in spark-ignition engines.