Kinetics and mechanism of the oxidation of carbon by NO2 in the presence of water vapor

The kinetics and mechanism of the oxidation of carbon by NO₂ in absence and presence of water vapor were studied in a fixed bed reactor. The rate of carbon oxidation by NO₂ is enhanced in the presence of water vapor in the range of temperature 300–400°C. The benefit effect of water is attributed to the intermediate formation of traces of nitric and nitrous acids, which enhance the rate of the carbon oxidation without modifying the global mechanism reaction. Therefore, water acts as a catalyst for the carbon oxidation by NO₂. A kinetic mechanism derived from this parametric study shows a decrease in the activation energy of carbon oxidation by NO₂ in the presence of water vapor. This result is in agreement with the experimental observation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 236–244, 2009     

Experimental study of a confined premixed metal combustor: Metal flame stabilization dynamics and nitrogen oxides production

This work presents a study of a magnesium/air combustion process in the context of innovative zero carbon dioxide (CO₂) energy carriers for reducing global warming effects. In order to analyze more deeply the confined combustion of magnesium under fluctuating overpressure conditions (0 to 24 hPa) and the generated gaseous by-products, magnesium/air flames have been realized in a combustion chamber with a conical bluff-body as flame holder and different contraction ratios diaphragms at the exit duct. Sieved magnesium samples with two size-fractions were tested: 20–50?µm and 50–70?µm. The gaseous emissions of nitrogen oxides (NO_x) and dioxygen (O₂) were analyzed with on-line infrared, ultraviolet and paramagnetic analyzers. A flame pulsating behavior was clearly observed from light emission intensity (monitored by a photodiode) and pressure fluctuations (monitored by a pressure sensor); the frequencies obtained ranged between 3 and 10?Hz. The frequency of the pulsation exhibited strong dependence on the geometric configuration of the chamber: a contraction diaphragm divided by two the frequency level of the fluctuations in the studied range of maximum overpressure. Such fluctuations may probably be the consequence of periodic perturbations of the recirculation zone behind the bluff-body. These periodic perturbations are themselves caused by strong periodic overpressure variations due to stiff contraction downstream responding to gas velocity fluctuations. This feed-back-loop mechanism was considered in this study. NO_x emissions produced through the thermal pathway were analyzed for equivalence ratios ranging from 0.29 to 1. The representation of NO_x versus equivalence ratio exhibited a parabolic shape with a maximum for an equivalence ratio of 0.4. Moreover, NO_x emissions of this metal combustor have shown a similar order of magnitude than current internal combustion engines.     

Fast pyrolysis of coals under N2 and CO2 atmospheres

Oxyfuel combustion represents one way for cleaner energy production using coal as combustible. The comparison between the oxycombustion and the conventional air combustion process starts with the investigation of the pyrolysis step. The aim of this contribution is to evaluate the impact of N₂ (for conventional air combustion) and CO₂ (for oxy-fuel combustion) atmospheres during pyrolysis of three different coals. The experiments are conducted in a drop tube furnace over a wide temperature range 800–1400 °C and for residence time ranging between 0.2 and 1.2 s. Coal devolatilized in N₂ and CO₂ atmospheres at low temperatures (?1200 °C) and longer residence times (>?0.5 s), the char-CO₂ reaction is clearly observed, whose intensity depends on the nature of the coal. Furthermore, the volatile yields are simulated using Kobayashi’s scheme and kinetic parameters are predicted for each coal. The char gasification under CO₂ is also accounted for by the model.

     

Study of the dynamic and static behavior of de vortex plasma torches: Part II: Well-tye cathode

This work was devoted to the study of the dynamic and static behavior of de vortex plasma torch with a well-type cathode (power level below 100 kW). The dynamic behavior of the torch was characterized by the fulctuations of arc voltage and current, plasma jet radiation, and acoustic pressure. Characteristic frequencies of the arc root movement inside the torch were observed. By numerical simulation (with the numerical codeMelodie, it was shown that the position of the erosion diameter) of the axial velocity along the cathode channel near the wall. The static behavior of the torch was inverstigated for different cathode designs. The variations of voltage U with arc current I, gas flow rate G nature of the gas and cathode design were represented by semiempirical relationships established between dimensionless numbers. By dimensional analysis, the behavior of this torch was compared with that of two powerful torches: the Aerospatiale and the Plasma Energy Corporation torches.     

Simultaneous soot combustion and NOx reduction over a vanadia-based selective catalytic reduction catalyst

Simultaneous NO_x reduction and soot combustion over a commercial vanadia-based selective catalytic reduction (SCR) catalyst were investigated. Carbon black was used as model soot. The impact of the contact intensity between carbon and catalyst was studied. The experiments appeared as promising results for the utilization of vanadia-based SCR catalysts in SCR on filter system as, in the SCR operating temperature range (250–400 °C), no significant impact of the presence of carbon black on NO_x reduction was observed. However, a decrease in the specific carbon oxidation rate was highlighted. This latter increases with the contact between carbon and catalyst and is attributed to a lack of NO₂, consumed by the fast SCR reaction. At temperatures greater than 400 °C, the contact between carbon particles and the SCR catalyst partially inhibits the NO_x reduction, whereas it exhibits a catalytic effect on the carbon oxidation rate. The tighter the contact between the two materials, the more significant is this behavior. A redox mechanism, which competes with the redox cycle of the SCR mechanism, was proposed. The impregnation of a V-based SCR catalyst with 2 wt % of calcium was also performed. A drastic loss of DeNO_x activity was observed, whereas the effect of the contact between carbon and catalyst was reduced.     

Study of experimental and theoretical procedures when using thermogravimetric analysis to determine kinetic parameters of carbon black oxidation

Combustion of carbon black (CB) in the crucible of a thermobalance is controlled by both carbon reactivity and oxygen transport from the oxidizing flux to the surface of the bed and within the porous bed. The kinetic constant of combustion has been determined using a fixed-bed reactor in which CB combustion is mainly under kinetic control. Then, modelling of oxygen transport in the thermobalance allowed determining the oxygen diffusivity within the CB pile. Fickian diffusion is a good approximate value of the diffusion coefficient for modelling of internal oxygen transport. The effects of the initial sample mass and of the sample containment on the initial combustion rate have been investigated. The effectiveness factor of the bed was calculated for different experimental conditions. Advices to correctly extract a kinetic constant from thermogravimetric experiments are given. According to the required precision, an experimental procedure is proposed. Limitations to oxygen transport within the bed may be ignored. They can be minimized by the use of an inert material to remove the stagnant atmosphere between the surface of the bed and the mouth of the crucible. It appears mandatory to account for oxygen transport limitations within the CB pile. It can be assumed that the sample temperature (not known) during reaction is the regulation temperature. Thermal effects are also minimized by use of the inert material. A 30–50 mg sample mass seems to be optimal for determination of the kinetic parameters.     

Non-isothermal oxidation and kinetic analysis of pure magnesium powder

Journal of Thermal Analysis and Calorimetry - With high energy densities, metals appear as innovative energy carriers. In the present study, magnesium powder is considered as fuel for energy...