Chemical thermodynamics of the vapor-oxygen gasification of graphite

A thermodynamic analysis of the vapor-oxygen gasification and combustion of graphite with a variation of the oxidizer excess α has been performed. Chemical reactions proceeding in the process of gasification and combustion, as well as components of flammable gases, which are the combustion products, have been determined. The channels of the distributing of the energy of exothermal and endothermal processes have been revealed. The energy of exothermal reactions compensates the endothermal effect of the reaction and increases the physical heat of the system. Expenditures on the endothermal reaction where flammable gases are produced are not irretrievable and transverse to the calorific power of the flammable gases. The effect of the oxidizer concentration on the adiabatic temperature at partial combustion of graphite has been analyzed. The thermal efficiency has been found as the ratio of the total thermal energy in the combustion of flammable gases and their physical heat to the spent energy, which is determined by the calorific power of graphite. The thermal efficiency is about 100%.     

Gas-phase reaction thermodynamics in preparation of pyrolytic carbon by propylene pyrolysis

Density functional theory and accurate model chemistry at G3(MP2) and G3//B3LYP levels of theory have been employed to explore the details of propylene pyrolysis gas-phase reaction thermodynamics. Geometries and vibrational frequencies for all of the interested possible species are calculated by B3PW91/6-31G(d). Standard thermodynamic data are derived by G3(MP2) and G3//B3LYP electronic energies combined with statistical thermodynamics. The data include heat capacities, entropies, enthalpies of formation and Gibbs free energies of formation at 298.15 K. Heat capacities and Gibbs free energies of formation at temperatures in 100–1500 K are calculated and fitted into analytical equations. The theoretical values are in excellent agreements with the available experimental values. Thus the theoretical predictions for all of the other species (stable intermediates in propylene pyrolysis) are believed reliable. Based on the thermo-chemical data, the equilibrium concentration distribution for all of the species in the temperature range of 100–1500 K (i.e. phase diagram) are calculated according to the chemical equilibrium principle. This work has supported the possibility of direct condensation of the high concentration carbon monomers and dimmers produced in the propylene pyrolysis dissociation reactions.     

RF number as a new index for assessing combustion hazard of flammable gases

A new index called RF number has been proposed for assessing the combustion hazard of all sorts of flammable gases and their mixtures. RF number represents the total expectancy of combustion hazard in terms of flammability limits and heat of combustion for each known and unknown compounds. The advantage of RF number over others such as R-index and F-number for classification of combustion hazard has been highlighted.     

Experimental exploration of discrepancies in F-number correlation of flammability limits

Flammability limits measurement has been made by ASHRAE method for some 20 kinds of combustible gases and vapors. These compounds have been selected mainly because the literature values of flammability limits are not consistent with the F-number calculated ones [J. Hazard. Mater. A 82 (2001) 113]. As a result, it has been found that the newly obtained values of flammable range are classified into three groups. For the first group of compounds, the present values agree well to the literature values. For the second group, the present values do not agree to the literature values but agree with the calculated ones. For the third group ones, the present values neither agree to the literature values nor to the calculated ones. There are 4, 13, and 6 compounds in the respective groups.     

Diode laser sensor for measurements of CO, CO(2), and CH(4) in combustion flows

A diode laser sensor has been applied to monitor CO, CO(2), and CH(4) in combustion gases with absorption spectroscopy and fast extraction-sampling techniques. Survey spectra of the CO 3nu band (R branch) and the 2nu(1) + 2nu(2)(0) + nu(3) CO(2) band (R branch) near 6350 cm(-1) and H(2)O lines from the nu(1) + 2nu(2) and 2nu(2) + nu(3) bands in the spectral region from 6345 to 6660 cm(-1) were recorded and compared with calculated spectra (from the HITRAN 96 database) to select optimum transitions for species detection. Species concentrations above a laminar, premixed, methane-air flame were determined from measured absorption in a fast-flow multipass absorption cell containing probe-sampled combustion gases; good agreement was found with calculated chemical equilibrium values.     

Study on the heat transfer of high-vacuum-multilayer-insulation tank after sudden, catastrophic loss of insulating vacuum

One of the worst accidents that may occur in a high-vacuum-multilayer-insulation (HVMLI) cryogenic tank is a sudden, catastrophic loss of insulating vacuum (SCLIV). There is no doubt that the gases leaking into the insulation jacket have some influence on the heat transfer process of it. However, this issue has not been thoroughly studied so far. In this paper, a test rig was built up and experiments were conducted using a SCLIV cryogenic tank and with nitrogen, helium and air as the working medium, respectively. The venting rates of the tank and temperature in the insulation jacket were measured respectively after the three different gases leaking into the jacket. A heat transfer model describing the heat transfer process of a SCLIV tank was also presented. The calculated results using this model were compared against the experimental data. It is found that the heat transfer performance of the HVMLI cryogenic tank after SCLIV is strong relevant to the type of gas leaking into the insulation jacket.     

Nucleate boiling heat transfer coefficients of flammable refrigerants

Nucleate boiling heat transfer coefficients (HTCs) of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature of 7 °C. Test results exhibited a typical trend that HTCs of flammable refrigerants increase with increasing vapor pressure. Existing nucleate boiling heat transfer correlations showed up to 80% deviation as compared to the present data. Hence a new correlation was developed through a regression analysis taking into account dimensionless variables affecting nucleate boiling heat transfer. The new correlation showed a good agreement with data for flammable refrigerants as well as halogenated refrigerants with a deviation of 5.3%.     

Mathematical modeling of the heat treatment and combustion of a coal particle. I. Heating stage

Mathematical modeling of the heat treatment and subsequent combustion of a coal particle as a multistage process has been carried out. The basic parameters of the following sequential stages of this process have been calculated by approximate-analytic dependences: heating of particles; their drying; yield of volatiles, their ignition and combustion; and burning out of the coke residues. A detailed parametric analysis of the influence of the physical and regime characteristics of the process on the burning mechanism of a coal particle (with the example of coal from the Shivee-Ovoo deposit in Mongolia) has been performed. The conditions for effective burning of a single coal particle as the main element of the whole process in the furnace have been determined.     

Highly energetic nitrogen species: reliable energetics via the correlation consistent Composite Approach (ccCA)

Gas-phase enthalpies of formation (ΔH(f(g))°) have been determined for 40 nitrogen-containing compounds at 298 K. Three ab initio composite methods have been compared in their abilities to quantitatively determine ΔH(f(g))°; the G3, G3(MP2), and correlation consistent Composite Approach (ccCA) methodologies. The ccCA method resulted in a mean absolute deviation (MAD) of 1.1 kcal mol(-1) when compared to available experimental values. The comparable G3(MP2) method resulted in a MAD of 1.8 kcal mol(-1), while the G3 method resulted in a MAD of 1.2 kcal mol(-1). As a result of their comparable accuracies, the ccCA and G3 methods have been utilized to predict the ΔH(f(g))° of five energetic but highly endothermic tetrazine-containing compounds with potential applications as insensitive high explosives.     

Ab initio and DFT study on 1,4-dinitroglycoluril configurational isomers: cis-DINGU and trans-DINGU

cis-1,4-Dinitroglycoluril (1,4-dinitrotetrahydroimidazo[4,5-d]imidazole-2,5-(1H,3H)-dione, C(4)H(4)N(6)O(6), known as DINGU in the literature) is an important explosive regarded as one of the potential ingredients for LOVA (low vulnerability ammunition) applications. However, there is no study concerning trans-DINGU, to the best of our knowledge. Thus, in the present study the structural and electronic properties of the configurational isomers of DINGU (cis- and trans-isomers) have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p), UB3LYP/6-31G(d,p) levels and also ab initio calculations at RHF/6-31G(d,p) and UHF/6-31G(d,p) levels. The optimized geometries, vibrational frequencies, electronic structures, and some thermodynamical values for the presently considered DINGU isomers have been obtained in their ground states. Comparing the calculated energy data (total electronic energy, heat of formation values and homolytic bond dissociation values of N-NO(2)) and the calculated bond lengths, cis-DINGU was found to be a more stable molecule than the trans-isomer.     

Special features of the heat capacity of detonation nanocrystalline diamond

The heat capacity of detonation nanocrystalline diamond has been studied by adiabatic calorimetry in the 60–300 K temperature range. It has been found that the heat capacity of commercial samples of nanodiamond exceeds that of bulk diamond by more than 30%. It has been shown that this excess is only partially caused by the impurities and for an impurity-free nanodiamond it is more than 15%. It has been proposed to explain this feature of the heat capacity by a contribution of surface carbon atoms into a low energy density of diamond phonon states. Based on the experimentally obtained temperature dependences of the heat capacity, the standard values of the enthalpy, entropy, and reduced Gibbs free energy of commercial and impurity-free nanodiamond have been calculated.     

A schlieren investigation of ignition downstream of a flat-plate flame trap

High speed Schlieren photographs have been taken downstream of a flat-plate flame trap when the trap successfully arrests a flame and when it fails to do so. In all cases, burnt gases from the trap and fresh flammable mixture form a turbulent mixing region in which chemical reactions can occur. When the effects of these reactions can overcome the cooling effects of entrainment combustion takes place, and products form some distance behind the leading edge of the turbulent zone. However, combustion does not immediately lead to a self-propagating flame. Combustion reactions proceed in an essentially self-contained mixing region at the head of the turbulent plug for a delay time before an abrupt downstream pressure rise indicates that a self-propagating flame is formed.     

Quantum chemical study on 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) and some of its constitutional isomers

Presently, certain isomeric compounds of NTO and their tautomers have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p) and ROB3P86/6-311G(d,p) levels and also ab initio calculations at RHF/6-311G(d,p) level. The optimized geometries, vibrational frequencies, electronic structures and some thermodynamical values for the presently considered NTO isomers have been obtained in their ground states. Also, detonation performances were evaluated by the Kammlet-Jacobs equations, based on the calculated densities and heat of formation values. The homolytic bond dissociation energies (BDEs) (at ROB3P86/6-311G(d,p) level) of NNO(2) and CNO(2) for the molecules were calculated. Moreover, aromatic character of NTO and its isomers and tautomers were investigated by performing NICS calculations using the gauge invariant atomic orbital (GIAO) approach at the B3LYP/6-31G(d,p) and B3LYP/cc-pVDZ levels.     

The critical heat flux under boiling and its dependence on the characteristics of heat-transfer wall

It has been experimentally found to date that boiling heat transfer is a problem with conjugate boundary conditions. Boiling heat transfer and critical heat fluxes depend both on the physical properties of boiling liquid and on a number of characteristics of the heat-transfer wall. In the present paper, experimental data of different authors are analyzed and an attempt is made at understanding the reasons for the significant deviation of the experimentally obtained values of critical heat flux from the values calculated by the available models of origination of crisis.     

GERG Project: Development and Setup of a New Combustion Reference Calorimeter for Natural Gases

Natural gas plays an important role for worldwide energy supply. For billing purposes precise metering of volume and superior calorific value are very important. At present, only a few institutions worldwide are able to determine the superior calorific value (SCV) of gases and their mixtures with an uncertainty of less than 0.2%. Calculations of SCV’s of natural gases using the data of ISO 6976 provides a similar uncertainty as experimental approaches. For this reason a GERG (Groupe Européen de Recherches Gazières) project was initiated to develop a new reference calorimeter for determining the SCV of flammable gases (natural gases), based on the principle of Rossini for a combustion calorimeter. The purpose of such a reference calorimeter is to determine the SCV of pure gases and gas mixtures with an uncertainty of less than 0.05%. The overall uncertainty budget for the SCV is mainly influenced by the mass determination and temperature measurement. An automated weighing and calibration device is used to measure the mass of the combusted gas with an experimental uncertainty of approx. 0.015%. In addition to the experiment, the flow and temperature field in the calorimeter were simulated. These simulations help to reduce each of the combined uncertainties for the combustion and calibration experiment resulting from the temperature measurement. The determination of the adiabatic temperature rise is performed analytically. The assumptions made by early investigators were carefully reconsidered for the first time. The analysis of the temperature–time curves considers (a) the method of evaluation, (b) the interval length of the main period, (c) the location of the heat release during the calibration experiment, and (d) the temperature sensor location.     

Extended Le Chatelier's formula for carbon dioxide dilution effect on flammability limits

Carbon dioxide dilution effect on the flammability limits was measured for various flammable gases. The obtained values were analyzed using the extended Le Chatelier's formula developed in a previous study. As a result, it has been found that the flammability limits of methane, propane, propylene, methyl formate, and 1,1-difluoroethane are adequately explained by the extended Le Chatelier's formula using a common set of parameter values. Ethylene, dimethyl ether, and ammonia behave differently from these compounds. The present result is very consistent with what was obtained in the case of nitrogen dilution.     

Progressive combustion in SI-Engines—Experimental investigation on influence of combustion related parameters

The fuel heat release rate which virtually controls the combustion process is dependent on the ‘Mass-Fraction-Burnt (MFB)’. In the present research work, a ‘logistic model with conditional variability in MFB’, has been developed for precise simulation of combustion in SI engines as the model has built in routines to take into account such factors as location of spark plug, single/dual spark plugs, intake generated swirl, combustion chamber geometry (associated with Bore/Stroke ratio), etc. A major contribution of this paper is that new and improved models for the ‘overall combustion duration’, and ‘ignition delay/flame development angle’, taking into account primarily the influence of compression ratio on the overall combustion process in SI engine have been developed. Taylor’s original equation for estimating the overall combustion duration has been modified by including a logistic equation for the error term and incorporating it in the original equation. Ignition delay as proposed by Keck et al has been modified by incorporating a polynomial of 3^rd order into the original equation. The empirical correlations that have been proposed in this paper may serve to be the starting point for simulation of ‘photodetonation concept’ to simulate HCCI combustion which is presently the hot research work in the area of pre-mixed combustion. A program in Turbo-C++ has been developed for the complete simulation of SI engine combustion, taking into account the conditional variability effect, variable specific heats of burnt gases, dissociation of gases at high temperatures, progressive combustion phenomena, heat transfer (based onWoschni’s equation), gas exchange process based on 1D-steady gas flow equation employing Taylor’s mach index of 0.6 for valve design.     

Influence of cooling conditions on the local parameters of heat and mass transfer in condensation heat-utilization units

Based on the earlier developed model of calculation of the local parameters of heat and mass transfer in deep cooling of flue gases (a vapor-gas mixture) in a bundle of tubes of a condensation heat-utilization unit, the distribution of the parameters of a condensing vapor-gas mixture both along the length of the tubes and in the depth of the tube bundle in a crossflow under various cooling conditions corresponding to the working parameters of heat-utilization units at industrial thermoelectric plants (TEP) and in boiler houses has been obtained. A comparison of the calculated values of the heating efficiency of the indicated heat-utilization unit with the results of tests of the condensation heat-utilization unit at the Ul’yanovsk TEP-3 (Russia) has demonstrated their satisfactory agreement.     

The maximum experimental safe gap: The effects of oxygen enrichment and the influence of reaction kinetics

Measurements of the Maximum Experimental Safe Gaps (MESGs) for several flammable gases with oxygen-enriched air are reported. The MESGs for mixtures in air of carbon monoxide and methane and carbon monoxide and water have been measured also. The 20-cm³ apparatus was used throughout.The results demonstrate that oxygen enrichment has a marked effect on the value of the MESG, which decreases as the oxygen enrichment increases. At high levels of oxygen enrichment very low values of MESG are obtained when the flow of the jet of hot gases through the safe gap becomes sonic.The addition of water to carbon monoxide-air mixtures decreases the MESG rapidly, and the addition of methane to carbon monoxide-air mixtures can give MESGs with values less than those of methane-air and carbon monoxide-air. These results demonstrate the influence of reaction kinetics on the MESG.     

Low temperature solid-phase synthesis of tetragonal BaTiO3 powders and its characterization

We report an alternative synthesis route for tetragonal and submicron barium titanate particles through a low temperature isothermal heat treatment of a precursor powder obtained from the BaO₂-TiO₂-C mixture utilizing combustion synthesis technique. Effort has been concentrated on the investigation and optimization of the synthesis process including combustion temperature and velocity, heat treatment procedure, and the characteristics of the powder obtained. It was shown that precursor powder prepared by the combustion method easily transformed to the tetragonal BaTiO₃ starting from 700 °C. Characterization of the products was performed by powder X-ray diffraction, scanning electron microscopy (SEM) and BET. From the results, it has been found that the formation of tetragonal BaTiO₃ powder at low temperature is conditioned by high chemical activity and specific characteristics of combustion products. The BaTiO₃ specimens were found to possess high dielectric constant and low dissipation factor at room temperature.