A computational fluid dynamics study of flame gas sampling in horizontal dilution tubes

The performance of horizontal dilution tubes is investigated by Reynolds-averaged Navier–Stokes and large-eddy simulations. The flame gas enters the dilution tube through a pinhole. The orifice flow and the dilution process inside the tube are studied. The volume flow through the orifice is shown to be proportional to the square root of the pressure drop. The discharge coefficient is 0.9 ± 0.3 in the cold air (calibration) case and drops to 0.35 under hot (flame) conditions. The resulting dilution ratio is roughly a factor of five below typical literature data. The gas sample remains in the wall boundary layer and the mixing process is not complete at the end of the dilution tube. Turbulence decays rapidly behind the tube inlet, which shifts the flow into the laminar to turbulent transition regime. Turbulence increases significantly in the outlet section which has much smaller pipe cross-sections. Despite its relatively low Reynolds number, the outlet flow to the particle sizer (or to the gas analyzer) is clearly turbulent, and interactions with the wall are probable. The results are in agreement with previous findings from laminar jets in cross-flow. Guidelines for optimization of the sampling conditions are suggested

A CFD Study of the Performance of Horizontal Dilution Tubes

Since about 20 years, horizontal dilution tubes are in use to study soot formation close to the main reaction zone, in order to characterize the properties of nascent soot nanoparticles and to obtain insight into the soot formation process. In this study, the performance of horizontal dilution tubes, both free standing and embedded, is investigated by RANS and LES. The flame gas enters the dilutionctube through a pinhole and, in experimental studies, it is claimed to mix quickly with the cold, inert gas flow in the dilution tube. Previously, the distortion of the flow and temperature profiles around the dilution tube were investigated. Here, the orifice flow as well as the dilution process inside the tube are studied. The volume flow through the orifice is shown to be proportional to the square root of the pressure drop. The discharge coefficient is the range 0.9 0.3 in the cold air (calibration) case and drops to 0.35 under hot (flame) conditions. The resulting dilution ratio is roughly a factor of 5 below typical literature data. The gas sample is found to remain in the wall boundary layer and, the mixing process is not complete at the end of the dilution tube. Turbulence decays rapidly behind the tube inlet and, in the main body of the tube, the flow is in the laminar to turbulent transition regime. Turbulence increases significantly in the outlet section which has much smaller pipe cross sections. Despite its relatively low Reynolds number, the outlet flow to the particle sizer (or to the gas analyzer) is clearly turbulent and, interactions with the wall are probable. The results are in agreement with previouscfindings from laminar jets in crossflow. Guidelines for optimization of the sampling conditions are suggested

Waste incineration of Polytetrafluoroethylene (PTFE) to evaluate potential formation of per- and Poly-Fluorinated Alkyl Substances (PFAS) in flue gas

n recent years, concerns over some per- and polyfluorinated alkyl substances (PFAS) have grown steadily. PFAS are a large group of chemical substances with widely differing properties. While one class of PFAS, fluoropolymers, have been demonstrated to meet the OECD criteria for polymers of low concern during the in use phase of their lifecycle, questions remain regarding waste handling at the end of useful life for products containing fluoropolymers. To show that polytetrafluoroethylene (PTFE) can be almost fully transformed into fluorine (F) (as hydrofluoric acid (HF)) and to study the possible generation of low molecular weight per- and polyfluorinated alkyl substances (PFAS), PTFE combustion under typical waste incineration conditions at the BRENDA (German acronym for “Brennkammer mit Dampfkessel”) pilot plant at Karlsruhe Institute of Technology (KIT) was investigated. Results indicate that, within procedural quantitation limits, no statistically significant evidence was found that the PFAS studied were created during the incineration of PTFE. Therefore, municipal incineration of PTFE using best available technologies (BAT) is not a significant source of the studied PFAS and should be considered an acceptable form of waste treatment

Investigation of the Olive Mill Solid Wastes Pellets Combustion in a Counter-Current Fixed Bed Reactor

Combustion tests and gaseous emissions of olive mill solid wastes pellets (olive pomace (OP), and olive pits (OPi)) were carried out in an updraft counter-current fixed bed reactor. Along the combustion chamber axis and under a constant primary air flow rate, the bed temperatures and the mass loss rate were measured as functions of time. Moreover, the gas mixture components such as O2, organic carbon (Corg), CO, CO2, H2O, H2, SO2, and NOx (NO + NO2) were analyzed and measured. The reaction front positions were determined as well as the ignition rate and the reaction front velocity. We have found that the exhaust gases are emitted in acceptable concentrations compared to the combustion of standard wood pellets reported in the literature (EN 303-5). It is shown that the bed temperature increased from the ambient value to a maximum value ranging from 750 to 1000 °C as previously reported in the literature. The results demonstrate the promise of using olive mill solid waste pellets as an alternative biofuel for heat and/or electricity production

Comparison of wood pyrolysis kinetic data derived from thermogravimetric experiments by model-fitting and model-free methods

The pyrolysis kinetics of beech wood was analyzed using model-free and model-fitting methods. Experimental measurements of the pyrolysis process were conducted in two thermogravimetric analyzers (TGA), a TG 209/2/F from Netzsch and a TGA Q500 from TA Instruments, which were found to have a similar precision in the establishment of the present heating rate. Two experimental procedures were employed: (i) introducing samples which were pre-dried externally before the experiments were executed and (ii) internal (in situ) drying of the samples in the TGA via a special temperature program below 150 degrees C which preceded the pyrolysis process. The kinetic parameters were derived (i) using several model-free methods, namely Kissinger method, iso-conversional methods, a simplified Distributed Activation Energy Model (sDAEM) and, (ii) using a model-fitting method via a five-step reaction model which calculates the differential thermogravimetric (DTG) curves at different heating rates; the calculated DTG curves were further analyzed by Kissinger's method to obtain overall kinetic data. The kinetic parameters were found to be different in the two experimental procedures. Also, they turned out different when the assumed end temperature of the pyrolysis process was varied. This is because the pyrolysis of slowly charring solid residues becomes more important with increasing temperature and finally overruns the release of volatiles from the wood samples. For the same experimental procedure and for sufficiently low end temperatures, corresponding to a degree of conversion less than 85%, model-free and model-fitting methods resulted in similar kinetic parameters.The authors express their gratitude to the BIOLAB experimental facility, to the “Programa de movilidad de investigadores en centros de investigación extranjeros (Modalidad A)” from the Carlos III University of Madrid (Spain) and to the Institute of Combustion Technology at DLR for the financial support conceded to Antonio Soria-Verdugo for a research stay at the German Aerospace Center DLR (Stuttgart, Germany) during the summer of 2018. Funding by the Helmholtz Association of German Research Centers in the research fields energy, fuels and gasification, especially in the Program “Energy Efficiency, Materials and Resources“, is acknowledged by the Institute for Technical Chemistry at KIT, Karlsruhe, and by the Institute of Combustion Technology at DLR Stuttgart.Publicad