Roda Bounaceur

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

In the context of limiting the environmental impact of transportation, this critical review discusses new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions (212 references).

Ethanol as an Alternative Fuel in Gas Turbines: Combustion and Oxidation Kinetics

Some research is currently carried out in order to limit CO2 emissions in power generation. Among alternative fuels to natural gas and gasoil in gas turbines, ethanol offers some advantages. However, while the studies dealing with the combustion of methanol are numerous, the research devoted to ethanol flames is rather scarce, in particular with regard to the use in gas turbines. The combustion of ethanol has been theoretically studied by means of a detailed kinetic model well validated in flame conditions. Thanks to quantum chemistry calculations, the reactions necessary to represent low temperature oxidation have been identified and incorporated in the mechanism and their rate parameters have been determined. Several key parameters, such as auto-ignition temperature (AIT), ignition delay times, laminar burning velocities of premixed flames, adiabatic flame temperatures, and formation of pollutants such as CO and NOx have been investigated in an effort to covers gas turbine applications. One has also explored conditions close to ambient in order to address the related safety aspects (leakages of ethanol). To take into account the potential presence of water in ethanol based fuels, similar studies have been performed for ethanol-water-air mixtures. At last, the data have been compared with those calculated for methane combustion. In the low pressure range, the calculated minimum ignition temperatures have been found to be very sensitive to the pressure and the equivalence ratio for lean mixtures. For pressures above 5 bar and moderately lean or rich mixtures, AITs tend to remain close to 440K. Ignition delay times have been calculated in adiabatic conditions at constant pressure. Surprisingly the addition of limited water contents has a very low influence on these results. The addition of water in the ethanol-air mixture decreases slightly the flame temperatures. In the low temperature range, water increases slightly the auto ignition delay times whereas an opposite effect is observed at high temperature. Calculated flame speed has been compared to that deduced from empirical relations found in the literature and the agreement is satisfactory. The formation of CO in pure ethanol flame was always higher than in methane flame while NO formation showed no difference between the amount calculated in ethanol flame and in methane flame. This result is consistent with the slight difference observed between the adiabatic flame temperatures for the two fuels. When increasing the water content up to 10% in ethanol, the laminar velocities become close to those calculated for methane.Copyright

DME as a Potential Alternative Fuel for Gas Turbines: A Numerical Approach to Combustion and Oxidation Kinetics

Many investigations are currently carried out in order to reduce CO2 emissions in power generation. Among alternative fuels to natural gas and gasoil in gas turbine applications, dimethyl ether (DME; formula: CH3 -O-CH3 ) represents a possible candidate in the next years. This chemical compound can be produced from natural gas or coal/biomass gasification. DME is a good substitute for gasoil in diesel engine. Its Lower Heating Value is close to that of ethanol but it offers some advantages compared to alcohols in terms of stability and miscibility with hydrocarbons. While numerous studies have been devoted to the combustion of DME in diesel engines, results are scarce as far as boilers and gas turbines are concerned. Some safety aspects must be addressed before feeding a combustion device with DME because of its low flash point (as low as −83°C), its low auto-ignition temperature and large domain of explosivity in air. As far as emissions are concerned, the existing literature shows that in non premixed flames, DME produces less NOx than ethane taken as parent molecular structure, based on an equivalent heat input to the burner. During a field test performed in a gas turbine, a change-over from methane to DME led to a higher fuel nozzle temperature but to a lower exhaust gas temperature. NOx emissions decreased over the whole range of heat input studied but a dramatic increase of CO emissions was observed. This work aims to study the combustion behavior of DME in gas turbine conditions with the help of a detailed kinetic modeling. Several important combustion parameters, such as the auto-ignition temperature (AIT), ignition delay times, laminar burning velocities of premixed flames, adiabatic flame temperatures, and the formation of pollutants like CO and NOx have been investigated. These data have been compared with those calculated in the case of methane combustion. The model was built starting from a well validated mechanism taken from the literature and already used to predict the behavior of other alternative fuels. In flame conditions, DME forms formaldehyde as the major intermediate, the consumption of which leads in few steps to CO then CO2 . The lower amount of CH2 radicals in comparison with methane flames seems to decrease the possibility of prompt-NO formation. This paper covers the low temperature oxidation chemistry of DME which is necessary to properly predict ignition temperatures and auto-ignition delay times that are important parameters for safety.Copyright

Modelling of Weak Acid Conversion in an EDl Cell

The modelling of weak acid conversion in an EDl cell is described. The model takes into account the solution and resin ion transport numbers, acid dissociation, and ion exchange equilibrium. Simulations, which provide results that agree well with experiments, enable us to evaluate concentrations and conductivities in each position of the ion exchange packed bed, leading to 2D representations. A reduced flow rate F red , defined as the ratio of the experimental flow rate over the maximum flow rate which would enable 100% conversion, in the case of 100% current efficiency, has been introduced. Steady-state conversion rate appears to have a strong nonlinear dependency on this dimensionless parameter.

Gas Turbines and Biodiesel: A Clarification of the Relative NOx Indices of FAME, Gasoil and Natural Gas

There is currently a sustained interest in biofuels as they represent a potential alternative to petroleum derived fuels. Biofuels are likely to help decrease greenhouse gases emissions and the dependence on oil resources. Biodiesels are Fatty Acid Methyl Esters (FAMEs) that are mainly derived from vegetable oils; their compositions depend from the parent vegetables: rapeseed (“RME”), soybean (“SME”), sunflower, palm etc. A fraction of biodiesel has also an animal origin (“tallow”). A key factor for the use of biofuels in gas turbines is their Emissions Indices (NOx, CO, VOC, PM) in comparison with those of conventional “petroleum gasoils”. While biodiesels reduce carbon-containing pollutants, experimental data from diesel engines show a slight increase in NOx. The literature relating to gas turbines is very scarce. Two recent, independent field tests carried out in Europe (RME) and in the USA (SME) showed slightly lower NOx while a lab test on a microturbine showed the opposite effect. To clarify the NOx index of biodiesels in gas turbines, a study has been undertaken, taking gasoil and natural gas (NG) as reference fuels. In this study, a calculation of the flame temperature developed by the 3 classes of fuels has been performed and the effect of their respective compositions has been investigated. The five FAMEs studied were RME, SME and methyl esters of sunflower, palm and tallow; these are representative of most widespread vegetable and animal oil bases worldwide. The software THERGAS has been used to calculate the enthalpy and free energy properties of the fuels and GASEQ for the flame temperature (Tf ), acknowledging the fact that “thermal NOx” represents the predominant form of NOx in gas turbines. To complete the approach to structural effects, we have modeled two NG compositions (rich and weak gas) and three types of gasoil using variable blends of eleven linear/branched/cyclic molecules. The results are consistent with the two recent field tests and show that the FAMEs lie close to petroleum gasoils and higher than NG in terms of NOx emission. The composition of the biodiesel and regular diesel fuel influences their combustion heat: methyl esters with double bonds see a slight increase of their Tf and their NOx index while that of gasoil is sensitive to the aromatic content.Copyright