Adiabatic flame temperature | EPFL Graph Search

In the study of combustion, the adiabatic flame temperature is the temperature reached by a flame under ideal conditions. It is an upper bound of the temperature that is reached in actual processes. There are two types adiabatic flame temperature: constant volume and constant pressure, depending on how the process is completed. The constant volume adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any work, heat transfer or changes in kinetic or potential energy. Its temperature is higher than in the constant pressure process because no energy is utilized to change the volume of the system (i.e., generate work). Common flames In daily life, the vast majority of flames one encounters are those caused by rapid oxidation of hydrocarbons in materials such as wood, wax, fat, plastics, propane, and gasoline. The constant-pressure adiabatic flame temperature of such substances in air is in a relatively narrow range

Effect of hot spots in microstructured reactors on product distribution during quasi-instantaneous exothermic reactions

Navid Borhani, Julien Haber, Bo Jiang, Lioubov Kiwi, Thomas Maeder, Albert Renken, John Richard Thome

Background. The small characteristic sizes (in the range of 102 - 103 μm) of Microstructured reactors (MSR) lead to very high specific interfacial area which, in turn, increases heat- and mass transfer efficiency. However, for quasi instantaneous exothermic reactions, the overall reaction kinetics in MSR is strongly influenced by mixing. The characteristic time of the latter phenomenon varies from millisecond to seconds depending on the operating conditions used. If mixing is the rate determining step for heat release, fast homogenization of reactants may lead to an unwanted hot spot that even in MSR cannot be avoided completely (1). Aims. To study the product distribution of a model reaction carried out in a cooled MSR as a function of mixing quality and temperature profile for the optimization of performance towards the target product. Methods. As model reaction, the quasi instantaneous cyclisation of Pseudoionone to beta-Ionone was chosen. This reaction, which is commercially used in the synthesis of vitamin A and in perfumery, exhibits a reaction enthalpy of -120 kJ/mol. The product distribution can be monitored via gas chromatography and displays sensitivity towards several parameters, including the quality of mixing and the reaction temperature. The MSR used to carry out the reaction was fabricated by Low Temperature Co-Fired Ceramic Technology, which allows the integration of a cooling unit. A newly developed method based on infrared thermometry (2) was applied to investigate the temperature profile inside MSR. It allows quantitative measurements of the axial temperature profile with a precision of 1°C. The adiabatic boundary conditions required for better reproducibility and precision is created by placing the reactor inside a vacuum chamber. Results. By monitoring the axial temperature profile inside the uncooled reactor (quasi adiabatic case), the mixing profile could be deduced. It was demonstrated that the onset of mixing in this MSR lies in the range of Re = 60 leading to a temperature rise close to adiabatic temperature. Subsequently, the coolant temperature was varied between 0 - 40 °C for Reynolds numbers between 30 – 120 and samples were taken at the outlet. As expected, small conversions were observed for the lower range of Re. Best performance was found around Re = 70. A further increase of the latter lead to loss of selectivity due to higher hot spot temperatures. Summary/conclusions. For the first time, axial temperature profiles of a quasi-instantaneous and highly exothermic reaction were quantitatively measured in a cooled MSR. Using this non-intrusive technique, the influence of several parameters, such as the flow rate and cooling temperature, on mixing quality and hot spot temperature was monitored. The low performance at small Re could be attributed to slow mixing, whereas working at too high throughput lead to a loss of selectivity ascribed to limited temperature control. (1) Haber, J.; Kashid, M. N.; Renken, A.; Kiwi-Minsker, L. Industrial and Engineering Chemistry Research 2012, 51, 1474-1489 (2) Haber, J.; Kashid, M. N.; Borhani, N.; Thome, J.; Krtschil, U.; Renken, A.; Kiwi-Minsker, L. Chemical Engineering Journal 2013, 214, 97-105.

2013

Technologies for gas turbine power generation with CO2 mitigation

François Maréchal

A review of the state of the art gas turbine power plants designs with CO2 capture was performed to set up a list of technologies to be considered in a conceptual process superstructure. It has been found that the efficiency and economics of carbon dioxide capture in gas turbine combined cycle power plants can be remarkably improved by introducing Flue Gas Recirculation (FGR) so as to increase the CO2 concentration in the flue gas and to reduce the volume of the flue gas treated in a CO2 capture plant. Thus, this process was chosen as the main focus of the thermo-economic modeling and the combustion system related experimental testing. Combustion studies were performed to quantify the impact of reduced oxygen content (caused by flue gas recirculation) on combustion stability and emission characteristics (NO x, CO). Tests were performed with methane, methane/ethane (simulated natural gas), methane/hydrogen and natural gas (as distributed in Switzerland) at different FGR ratios. For all conditions the addition of ethane or hydrogen shows beneficial effects (restoration of flame reactivity) on flame stability and CO emission. Using PSI's high pressure test rig, it has been shown, that by redirecting up to 30% of exhaust gas volume back to the combustor, the CO 2 concentration in the exhaust can be increased to 8% vol. while keeping the flame temperature constant (Tad = 1750 K) and meeting the emission requirements (CO, NOx < 25ppm @ 15% O2). Benchmark tests of catalytic partial oxidation reactors were performed to determine whether sufficient H2 could be produced in situ by reaction of a slip stream of natural gas with extracted compressor oxidant. The work confirmed that FGR has the potential to significantly reduce the energy penalty connected with post-combustion carbon capture techniques and still meet the requirements on NOx and CO emissions.

Elsevier2011

Effect of hot spots in microstructured reactors on product distribution during quasi-instantaneous exothermic reactions

Navid Borhani, Julien Haber, Bo Jiang, Lioubov Kiwi, Thomas Maeder, Albert Renken, John Richard Thome

2013