Compressed natural gas | EPFL Graph Search

Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in cylindrical or spherical shapes. CNG is used in traditional petrol/internal combustion engine vehicles that have been modified, or in vehicles specifically manufactured for CNG use: either alone (dedicated), with a segregated liquid fuel system to extend range (dual fuel), or in conjunction with another fuel (bi-fuel). It can be used in place of petrol (gasoline), diesel fuel, and liquefied petroleum gas (LPG). CNG combustion produces fewer undesirable gases than the aforementioned fuels. In comparison to other fuels, natural gas poses less of a threat in the event of a spill, because it is lighter than air and disperses quickly when released. Biomethane – refined biogas from anaerobic digestion or landfills – can be used. In response t

A New Pulsed Multipoint Injection System to Reduce Natural Gas Vehicle Emissions.

Daniel Favrat

Nowadays, most of the natural gas conversion systems available on the market for bifuel vehicles are based on the venturi principle or on the continuous injection principle for the carburation of the engine. Even if the driveability of these systems is satisfactory, their emission performance does often not reflect the full environmental potential offered by natural gas. This is mainly due to the low response of these systems to transient conditions. During the past years, EPFL has been involved in a co-operative program with the industry to develop a new natural gas injection system intended for multipoint sequenced and phased indirect gaseous fuel injection. Mounted on a four cylinder 1.4-liter engine, the system includes specifically developed injectors and a PC based electronic control unit. Emission results on the engine test vench are shown for typical transient sequences as well as steady state conditions. Results on the chassis dynamometer are given for the New European Driving Cycle (94/12/EC directive). Keeping the standard vehicle components (like catalytic converter and lambda sensor), the current emissions results from our novel injection system are well below the legal emissions limits. Those results are compared with the performance of a state of the art multipoint continuous injection system from the market, measured on the same engine. In order to control accurately the narrower lambda window when working with natural gas, the system, among others, accounts for the sensitivity of the lambda sensor to the composition of the exhaust gases. While maintaining a good driveability, this system has shown significant emissions improvements compared to actual natural gas systems. New engine control strategies currently under evaluations are likely to still further reduce emissions.

1998

Aliphatic Carboxylic Acids as Coupling Partners in Carbon-Heteroatom Bond-Forming Reactions

Runze Mao

Carboxylic acids are one of the most suitable starting materials for synthesis. They are readily available and therefore inexpensive, stable and non-toxic. Many carboxylic acids can be obtained directly from natural resources, thus avoiding the extraction of fossil resources, such as oil or natural gas, in the production of chemicals. Furthermore, renewable carboxylic acids derived from biomass have a wide structural diversity (e.g., amino acids, fatty acids and sugar acids), which not only holds greater potential for the synthesis of complex molecules, but also advances the development of green chemistry. In this context, this thesis aims at establishing new carbon-heteroatom coupling reactions with carboxylic acids as versatile coupling partners. These reactions are hitherto unknown, which not only fill some of the gaps in synthetic organic chemistry, but also provide solutions for the rapid assembly of bioactive molecules. In order to overcome the challenge of forming C(sp3)-heteroatom bonds, Chapters 4, 5 and 6 detail the development of photoredox/copper bi-catalytic systems and their applications in decarboxylative C(sp3)-N and C(sp3)-O coupling reactions. These methods are not only compatible with numerous functional groups, but also allow for rapid late-stage functionalization. More importantly, these methods can simplify the pathway for building drug core skeletons, highlighting the potential of these methods to expedite drug discovery. To bridge the disconnection in the synthesis of trifluoromethylthioesters from carboxylic acids, Chapter 7 describes the establishment of a "umpolung" strategy and its application in the conversion of carboxylic acids to trifluoromethylthioesters. This approach provides the most concise synthetic pathway to date for the synthesis of trifluoromethylthioesters, which not only accommodates a variety of functional groups, but also allows for the rapid functionalization of carboxylic acid-containing natural products and drug molecules. In summary, this thesis illustrates how novel catalytic methods can be used to convert inexpensive, readily available and environmentally benign aliphatic carboxylic acids into high value-added compounds.

EPFL2020

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

The modelling of natural gas combustion with detailed as well as reduced kinetic mechanisms has been subject to many publications already. Most of the mechanisms are validated at higher temperatures and ambient pressures. Only recently - in parallel to a gain of interest in the homogeneous charge compression ignition (HCCI) engine concept - there are more efforts trying to reproduce the low temperature chemistry, and in particular autoignition of natural gas under engine-like conditions. This work is part of a project on stationary natural gas engines equipped with unscavanged prechambers. It has been demonstrated that it is possible to reduce the emissions below the Swiss limits both with natural gas and biogas with the spark ignition prechamber, while keeping high efficiencies of around 38 %. We are now aiming at converting the prechamber from spark ignition to autoignition leading to longer maintenance intervals in the case of stationary natural gas engines and to a probable cleaner combustion - HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front - compared to prechamber spark ignition. In order to be able to model the fluid dynamics and the chemistry inside the cylinder, a good chemical model is one of the most important aspects. To obtain an appropriate mechanism - by validating and/or optimising existing mechanisms - experiments have been conducted in the rapid compression machine facility at the University of Lille with different mixtures of methane, ethane and propane. The relative air-to-fuel ratio λ varied from 1 to 1.6, the temperature and pressure at the end of compression varied from 850 to 925 K and 13 to 21 bar respectively. The compression ratio was constantly kept at 9.2 for all experiments and the final temperature and pressure were defined by varying the charge and the composition of the inert gases (Ar/N2). The compression time was between 50 and 60 ms for all experiments. For modelling the ignition delay a zero-dimensional model assuming homogeneity was used, based on the measured pressure and calculated core temperature at the end of compression of each experiment. The chemical solver package used for the simulations is CHEMKIN. Different mechanisms (PRF, GRI3.0, Konnov, …) have been tested. Most of the mechanisms are mainly validated at higher temperature ranges and therefore their applicability to ignition delay prediction in our conditions is questionable. Several attempts to adjust the GRI mechanism (version 1.2) to the lower temperature regime exist, whereof we chose the UBCMech and Petersen mechanism (GRI+RAMEC) to test on our simulations. Both mechanisms initially were developed based on pure methane mixtures while the UBCMech recently was extended to model natural gas as well. The experimentally obtained ignition delays vary from 40 ms at high pressures and temperatures to more than 300 ms for the lower regime of the experimental space. For the ignition delays up to 60 ms the best fit was obtained by a reduced version of PRF accounting only for the C1 to C3 compounds. Konnov’s mechanism and UBCMech are under predicting the experimental ignition delays while the GRI3.0 mechanism (with and without RAMEC sub module) over predicts the ignition delay for the natural gas mixtures. It has to be mentioned that the GRI3.0+RAMEC mechanism as well as the initial mechanism of Petersen (GRI1.2+RAMEC) on the other hand under predict the experimental ignition delay for pure methane mixtures. This indicates that the low temperature chemistry for methane that has been added in the RAMEC sub module is enhancing ignition too strong. Replacing part of methane in pure methane mixtures by equivalent amounts of ethane and propane - know as ignition promoters - lead to longer ignition delays in the simulations. But as the GRI3.0+RAMEC mechanism (without nitrogen chemistry) is the smallest of all tested mechanisms considering number of species and reactions, it still is an interesting option for coupling it to a computational fluid dynamics code as we are planning to do. We though performed a sensitivity analysis to obtain a set of the most dominating reactions and used a multi-objective optimisation algorithm to optimize the reaction rates within their limits of uncertainty. The results show a good fit for the short ignition delays of natural gas mixtures but still an under prediction of the methane delays. For the longer ignition delays the assumption of a constant core temperature during the post compression phase becomes questionable and the under prediction of all mechanisms of the experimental delays is an obvious consequence. A heat loss model predicting the temperature evolution has to be implemented in the simulations to properly simulate the ignition delays.

2006