Alexander A. Konnov

Dioxin levels in wood combustion—a review

Dioxins, formed in any combustion process where carbon, oxygen and chlorine are present, are a subject of major interest due to their carcinogenicity. Much research has been carried out to study emissions from hazardous and municipal waste incinerators. Dioxin emissions from wood combustion plants are also of interest, especially those due to the combustion of treated, varnished or PVC-coated wood, which can produce high PCDD/F emissions. This work reviews the available data about the levels of dioxins in gases and ashes produced in wood combustion.

Measurement of adiabatic burning velocity in methane-oxygen-nitrogen mixtures

Experimental measurements of the adiabatic burning velocity in methane-oxygen-nitrogen mixtures are presented. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The oxygen content in the artificial air was varied from 16 percent to 21 percent. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. The overall error of the burning velocities is estimated to be smaller than ± 0.8 cm/s. Experimental results are in very good agreement with recent literature data for methane-air mixtures. They also agree well with detailed chemical model predictions.

Kinetic modeling of the decomposition and flames of hydrazine

Abstract A detailed N/H reaction mechanism has been developed and validated by comparing modeling results with measurements of hydrazine pyrolysis in shock waves, and in hydrazine decomposition flames at low and atmospheric pressures. The mechanism consists of 51 reactions for 11 species. Rate constants for several decomposition reactions have been estimated employing updated thermodynamic data. Analysis of the reactions abstracting an H atom from NH 3 , NH 2 , NH and N 2 H 4 from 1000 to 2000 K demonstrates that the Evans-Polanyi correlation holds for the radicals H, NH, and NH 2 . Probably it is also valid for the radicals N, NNH and N 2 H 3 . Several rate constants were estimated with this assumption. No further adjustment of the mechanism was attempted. The modeling correctly reproduces the experimental rate of decomposition of hydrazine and also the product distribution. The initial decomposition of N 2 H 4 into two NH 2 radicals and the subsequent reaction N 2 H 4 + NH 2 → NH 3 + N 2 H 3 mainly govern the decomposition of hydrazine in dilute mixtures and together with the reaction NH 2 + NH 2 → N 2 H 2 + H 2 control the propagation speed of a hydrazine flame. The computed speeds of such decomposition flames agree well with low-pressure and atmospheric pressure experiments for pure hydrazine and its mixtures with Ar, N 2 , H 2 O and NH 3 . Also the concentration profiles of major and minor species in low-pressure hydrazine flames are well reproduced. A sensitivity analysis identifies the critical reactions in particular experimental conditions. The choice of rate constants for key reactions and further development of the mechanism is discussed.

Probe sampling measurements and modeling of nitric oxide formation in methane-air flames

Probe sampling measurements of the concentrations of O2, CO2, CO, and NO in the postflame zone of the methane-air flames are reported. A heat flux method was used for stabilization of nonstretched flames on a perforated plate burner at 1 atm. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. Radial and axial profiles of the concentrations of stable species and NO in the postflame zone were used to evaluate the influence of the ambient air en-trainment. flame expansion, and downstream heat losses. In the core region of the flames, the radial profiles of the major species and NO are flat from moderately lean to moderately rich mixtures. In the very lean mixtures an ambient air dilutes the burnt gases, while in the very rich mixtures it causes oxidation of the combustion products. The buoyancy and radial flame expansion can significantly modify observed concentration gradients in the postflame region. In the methane-air mixtures, the NO concentrations measured at a fixed distance from the burner as a function of stoichiometric ratio were obtained. These dependencies clearly possess two maxima: in stoichiometric mixtures due to thermal-NO mechanism and in rich mixtures at equivalence ratio around 1·3 due to prompt-NO mechanism. The numerical predictions of the concentrations of O2 CO2, CO, and NO in the postflame zone are in a good agreement with the experiment when downstream heat losses to the environment are taken into account.

A POSSIBLE NEW ROUTE FOR NO FORMATION VIAN2H3

A possible new route for NO formation in hydrogen combustion is explored. The reaction sequence that converts molecular nitrogen into nitrogen oxides involves sequential recombination of N2 with H atoms: N2→NNH→N2H2→N2H3. N-N bond cleavage occurs in the reaction of N2H3 with H2 forming NH3 and NH2; These last species are oxidized mainly in the sequence NH3→NH2→NH→N→NO. Key reactions of the N2H3 formation and consumption as well as other important reactions revealed by sensitivity analysis and reaction path analysis are examined and discussed. Kinetic modeling of hydrogen combustion in stirred reactors demonstrates that this mechanism can be of importance in rich mixtures at relatively low temperatures (below about 1500 K) when other routes of NO formation are suppressed. Available measurements of NO formation in hydrogen combustion in stirred reactors have been modeled and analyzed. They neither confirm nor contradict the proposed route forming NO via N2H3, because these experiments have been conducted outside the range of conditions where this route is manifested.

Measurement of adiabatic burning velocity in ethane-oxygen-nitrogen and in ethane-oxygen-argon mixtures

Abstract Experimental measurements of the adiabatic burning velocity in ethane–oxygen–nitrogen and in ethane–oxygen–argon mixtures are presented. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. Dilution ratio O 2 /(O 2 +N 2 ) was varied from 15% to 21%; dilution ratios O 2 /(O 2 +Ar) were 15% and 16%. A heat flux method was used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. An overall accuracy of the burning velocities was estimated to be better than ±0.8 cm/s. Experimental results are in a good agreement with recent literature data for ethane–air mixtures. New measurements of the adiabatic burning velocity in diluted ethane–oxygen–nitrogen and in ethane–oxygen–argon mixtures extend the basis for validation of detailed reaction schemes. Predictions of the detailed chemical mechanism developed in this laboratory agree well with the measurements. The influence of the inert diluent on the flame burning velocity is discussed.

Temperature-dependent rate constant for the reaction NNH + O → NH + NO

Abstract The rate constant of NNH + O → NH + NO has been derived by comparing experiments in low-pressure (0.05 and 0.103 bar) hydrogen-air flames at ∼1200 K [1] with modeling that uses an updated detailed H/N/O kinetic scheme. To match calculations with experimental [NO] profiles along these flames, the currently adopted rate constant of this reaction has to be reduced significantly (by ∼50%). Combination of the adjusted rate constant with measurements in the range 1800 to 2500 K strongly suggests a non-zero activation energy for the reaction. The rate constant derived from 1200 to 2500 K is k 1 = (2 ± 1)× 10 14 exp (−16.8 ± 4.2 kJ/mol/RT) cm 3 /mol/s. The sum of the rate constants of every channel of the reaction NH + NO → Products, including that of the reverse Reaction −1 proposed in the present work, is in good agreement with available experimental data.

Kinetic Modeling of the Thermal Decomposition of Ammonia

A detailed N/H reaction mechanism has been developed and validated in comparison with experimental data for ammonia pyrolysis in shock waves (Davidson et al., Int. J. Chem. Kinet, 1990, 22:513). It has been shown that incorporation of the reactions with N2H3 and N2H4 into the mechanism significantly influences calculated rise-time and peak concentrations of the NH and NH2 radicals if the currently adopted rate constant of the reaction is employed. A sensitivity analysis reveals which reactions are critical for the quality of the modeling in particular experimental conditions. The choice of the rate constants for these reactions is discussed. It has been found that only significant decrease of the reaction (22) rate constant can improve the agreement between the modeling and experimental data. The best fit in the range 2200 - 2800 K is met with the rate constant k22 = 1.0E+11 T0.5 exp(-21600/RT).

NONCATALYTIC PARTIAL OXIDATION OF METHANE INTO SYNGAS OVER A WIDE TEMPERATURE RANGE

Noncatalytic partial oxidation of methane has been studied over a wide temperature range from 823 to 1531 K using two flow reactors. Highly diluted fuel-rich CH4/O2/N2 mixtures were reacted in uncoated tubular flow reactors at 1.2 atm. Residence time was varied from 1 to 164 s. Major and minor products of the partial oxidation were measured using a gas chromatograph. Kinetic modeling was performed to simulate experiments and key rate-controlling reactions were revealed by sensitivity analysis. It was found that chain-branching reaction H + O2 = OH + O, recombination CH3 + CH3 (+M) = C2H6 (+M), and methyl radical oxidation CH3 + O2 = CH2O + OH govern the overall rate of the process at short residence times. Recent measurements of these rate constants were analyzed and appropriate modifications in the detailed reaction mechanism were proposed. The model was adjusted to reproduce the measurements accurately at short residence times. At longer residence times, a significant impact of the heterogeneous reactions leading to inhibition of the overall process was observed. The model developed in the present study correctly reproduced temporal profiles and final compositions of the products over the entire range of temperatures and the initial mixture compositions.

EXPERIMENTAL STUDY OF ADIABATIC CELLULAR PREMIXED FLAMES OF METHANE (ETHANE, PROPANE) + OXYGEN + CARBON DIOXIDE MIXTURES

Abstract Experimental studies of adiabatic cellular flames of CH4 + O2 + CO2, C2H6 + O2 + CO2, and C3H8 + O2 + CO2 are presented. Visual and photographic observations of the flames were performed to quantify their cellular structure. Non-stretched flames of methane and propane were stabilized at atmospheric pressure on a perforated plate burner of improved design. New measurements are compared with recent results from this group. A Heat Flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Cellularity disappeared when the flames became only slightly sub-adiabatic. Increasing the oxygen content in the artificial air and increasing the temperature of the burner plate led to increase of the number of cells observed. No direct proportionality between the number of cells and propagation speeds in CH4 + O2 + CO2 flames was observed. Dependence of the number of cells as a function of equivalence ratio clearly showed a local minimum in the stoichiometric mixtures.