Study on combustion mechanism of asphalt binder by using TG-FTIR technique

The combustion mechanism of asphalt binder was investigated by using thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR) in a mixed gas environment of 21% oxygen and 79% nitrogen. The results show that the combustion process of asphalt binder consists of three main consecutive stages at a low heating rate. The combustion reaction becomes more and more intense from the 1st to 3rd stage. The release of volatiles occurs mainly at 300-570 °C, and the gaseous products in each stage are different. The main products in the 1st stage are CO₂, CO, H₂O, hydrocarbons, formaldehyde, tetrahydrofuran, formic acid, aromatic compounds, etc. In the next stage, the combustion products mentioned above keep on increasing, but some new volatiles such as alcohols, phenols, styrene, etc. are present. In the last stage, the CH and CO bonds continue to fracture and aromatization reaction occurs, and the release amount of CO₂, CO, and H₂O reaches the maximum. But the content of other products decreases or even disappears due to burning. Among the above volatiles, CO₂ is the dominant gaseous product in the whole combustion process. The concentration of CO₂ and CO keeps increasing, and reaches the maximum intensity at about 520 °C. The evolution of H₂O, CH₄, and formic acid exhibits the trend of increase first, and then decrease. Over 570 °C, there are few products released at the end of the combustion process. Asphalt binder combustion process includes two modes of complete and incomplete combustion, and the latter may be main combustion mode of asphalt binder.     

On mechanisms of formation of environmentally harmful compounds in homogeneous combustors

A kinetic model is developed for calculating the emission characteristics of homogeneous combustors using methane and synthesis gas (syngas) as a fuel. The model is validated over a large set of experimental data on concentrations of NO, CO, and OH in laminar flames and in the Bunsen burner and on concentrations of OH, NO, and CO in a homogeneous combustor operating on a mixture of syngas with air. At an identical temperature of combustion products, i.e., identical thermodynamic efficiency, the combustor operating on syngas is demonstrated to emit a greater amount of NO, CO, and CO₂, as compared with the combustor operating on methane. Though the use of syngas allows one to organize stable combustion of ultralean mixtures and to obtain extremely low concentrations of NO and CO at the combustor exit (≈1–3 ppm), the amount of CO₂ in the exhaust of even extremely lean mixtures (α ≈ 3) is appreciably greater than that in the case of using methane.     

Measurement and prediction of the emission of pollutants from the combustion of coal and biomass in a fixed bed furnace

The effect of co-combustion of coal and biomass has been studied for a fixed bed appliance originally designed for coal and in widespread use in many parts of the world especially Eastern Europe. Organic, inorganic and gaseous emissions have been measured. Organic compounds have been determined for a range of fuel combinations. These include polycyclic aromatic hydrocarbons PAH, alkyl PAH, a range of oxygenated compounds (including phenols, aldehydes and ketones, oxygenated polycyclic aromatic compounds (O-PAC) and dioxins), polycyclic aromatic sulphur hydrocarbons (PASH), nitrogenated polycyclic aromatic compounds (N-PAC) and common volatile organic compounds (VOC). Inorganic species include trace heavy metals, as well as the gases, CO, CO₂, SO_x and NO_x. The concentration of the trace metals in the ash and fly ash have been compared to equilibrium calculations of the emission profiles during co-combustion.     

Formation Kinetics of Sulfur-Bearing Compounds in Combustion of Hydrocarbon Fuels in Air

Using an extended kinetic model, formation kinetics of compounds of SO_x and HSO_y groups is analyzed in the case of combustion of hydrocarbon sulfur-bearing fuels mixed with air. It is shown that formation mechanisms of S-bearing compounds significantly depend on the air-to-fuel ratio and are closely related to formation kinetics of NO, NO₂, CO, CO₂, H, OH, and HO₂. Both in rich and lean mixtures, there is a rather large period of time after ignition with significantly nonequilibrium variation of concentrations of N- and S-bearing components. Key words: combustion, sulfur-bearing compounds, heavy hydrocarbons, nitrogen-bearing components.     

Studies on the Combustion of Solid Hydrazones with Nitric Acid

The heats of ignition and volumes of the gases released on combustion of various solid hydrazones with nitric acid as oxidizer, have been determined. The experimental values when compared with those calculated indicate incomplete combustion. Both, the heats of ignition and the volumes of gases generated appear to depend upon the substituent in the benzene ring of the hydrazone carbon atom. The électron releasing groups are found to enhance these combustion parameters in the case of phenylhydrazones whereas no such trend was noticed in the case of dimethylhydrazones. The combustion products of these systems have been analyzed by a mass spectrometer. The major products are found to be N₂, CO, NO, H₂O, CO₂ and N₂O along with trace amounts of O₂.     

Estimating Heats of Detonation and Detonation Velocities of Aromatic Energetic Compounds

A new method is introduced to predict reliable estimation of heats of detonation of aromatic energetic compounds. At first step, this procedure assumes that the heat of detonation of an explosive compound of composition C_aH_bN_cO_d can be approximated as the difference between the heat of formation of all H₂O CO₂ arbitrary (H₂O, CO₂, N₂) detonation products and that of the explosive, divided by the formula weight of the explosive. Overestimated results based on (H₂O CO₂ arbitrary) can be corrected in the next step. Predicted heats of detonation of pure energetic compounds with the product H₂O in the liquid state for 31 aromatic energetic compounds have a root mean square (rms) deviation of 2.08 and 0.34 kJ g⁻¹ from experiment for (H₂O CO₂ arbitrary) and new method, respectively. The new method also gives good results as compared to the second sets of decomposition products, which consider H₂, N₂, H₂O, CO, and CO₂ as major gaseous products. It is shown here how the predicted heats of detonation by the new method can be used to obtain reliable estimation of detonation velocity over a wide range of loading densities.     

Experimental study on the effect of different CO2 concentrations on soot and gas products from ethylene thermal decomposition

This research work reports a laboratory study of the influence of environments with different CO₂ levels, representative of conditions in which exhaust gas recirculation is used in combustion systems, on soot and gas products formed in the thermal decomposition of ethylene–CO₂ mixtures. The investigation includes experiments, in a flow reactor, with 30,000 ppm of ethylene at different experimental conditions of temperature (975–1475 K) and CO₂ concentrations (25%, 50% and 78.5%), using nitrogen as bulk gas. The analysis is performed by comparison with the data obtained during the pyrolysis of ethylene in a N₂ atmosphere.The present results highlight the importance of the CO₂ level in the system, since the presence of 25% CO₂ tends to promote the formation of soot, whereas an increased CO₂ addition of 78.5% leads to a diminution in the production of soot, compared to the pyrolysis of pure ethylene in N₂. The different evolution in soot formation tendencies can be attributed to competing reactions that gain importance depending on the different CO₂ levels, boosting or suppressing soot formation as function of the composition of both the O/H radical pool and the reacting species. The outlet concentrations of H₂, CO and C₂H₂, as well as the formation of H₂O, are directly related to the different soot-forming tendency found as function of different CO₂ environments.     

Combustion gas and NO emission characteristics of hazardous waste mixture particles in a fixed bed

Experiments with fixed-bed incinerators were carried out to model the combustion characteristics and gas emission characteristics of hazardous waste mixture particles in a grate furnace. The results indicate that combustion can be divided into three stages: ignition, main combustion and combustion completion stage. According to the various concentrations of O₂, CO₂ and CO, the main combustion stage can be subdivided into pyrolysis gas combustion and char combustion. Primary air rate, moisture and particle size have significant effects on concentrations of combustion gases and NO. Bed height has no effect on CO₂ concentrations but does have an effect on other combustion gases and NO emissions.     

Burning Behaviour of ADN Formulations

The application of ADN for an effective oxidizer of propellants and explosives requires a detailed knowledge of the burning behaviour. The physical and chemical mechanisms of the combustion depend on pressure. Especially profiles of temperature and species in the flame are important to design propellant formulation of high performance and low signature of the rocket plume. In the presented study, pure ADN and ADN/paraffin mixtures were investigated as strands in an optical bomb at pressures of 0.5 MPa to 10 MPa. The application of non-intrusive combustion diagnostics for the investigation of fast burning energetic materials allowed the measurement of burning rates and profiles of temperature and gas components at various distances above the burning propellant surface. The burning rate was determined by using a video system and a special frame analysis. The acquisition and analysis of emission spectra in the UV/VIS allowed the investigation of rotational temperatures, the determination of particle temperatures and the identification of transient flame radicals. The vibrational temperatures of final combustion products resulted from band spectra emitted in the near and mid infrared spectral range. Burning rates of 5 mm/s to 70 mm/s were recorded showing a mesa/plateau-effect in the pressure range of 4 MPa to 7 MPa. The UV/VIS spectra indicated an emission from OH, NH and CN radicals. The strong emission of OH bands of the ADN/paraffin mixture allowed the investigation of rotational temperatures with a mean value of 2700 K which is closely below the adiabatic flame temperature of 2950 K. Additionally, one-dimensional intensity profiles of the flame radicals were measured. As combustion end products H₂O, CO, CO₂ and NO were found. NO could only be detected at a distance up to 2 mm above the propellant surface. The measured CO/CO₂ fraction was higher as 10/1. Water could only be detected far above the propellant surface.     

Molecular beam mass spectrometry investigations of low temperature diamond growth using CO2/CH4 plasmas

Diamond films have been successfully deposited at substrate temperatures as low as 435°C using CO₂/CH₄ gas mixtures in a microwave plasma chemical vapour deposition (CVD) reactor. In order to understand why it is possible to grow diamond at these low temperatures using these gases, we have performed the first in situ molecular beam mass spectrometry studies to measure, simultaneously, the concentrations of the dominant gas phase species present during growth over a wide range of plasma gas mixtures (0–80% CH₄, balance CO₂). Optical emission spectroscopy has also been used to investigate gas phase species present in the microwave plasma. These experimental measurements give further evidence that CH₃ radicals may be the key growth species and suggest that CO may be of greater importance to the plasma chemistry of CO₂/CH₄ gas mixtures than previously thought.     

Generation of H2 gas from polystyrene and poly(vinyl alcohol) by milling and heating with Ni(OH)2 and Ca(OH)2

A two-step process to generate H₂ gas; first by milling polystyrene (PS) or poly(vinyl alcohol) (PVA) with Ni(OH)₂ and Ca(OH)₂, followed by heating of the milled product in the second-step was performed in this work. Polymer and hydroxide mixtures obtained after milling for 60 min and heating to 700 °C showed H₂, CH₄, H₂O, CO, and CO₂ as the main gaseous products with H₂ as the dominant gas generated between 350 and 500 °C. Analysis of the gaseous products by TG-MS and gas-chromatography, and solid products by TG-DTA and XRD shows that CO₂ gas was fixed as CaCO₃ at temperatures between 350 to 600 °C allowing generation of H₂ gas with concentrations over 95% for PS and over 98% for PVA. The results in this study show that milling of solid based hydrocarbon compounds with nickel and calcium hydroxides allows dispersion of nickel to hydrocarbon surfaces and facilitates C-C bond rupture in polymer(s) during heating at temperatures below 500 °C, at the same time calcium adsorbs CO₂. This process could be developed to treat hydrocarbon based wastes such as plastics, biomass or combinations at low temperatures avoiding syngas purification and separation steps.     

Structure of ultrahigh molecular weight polyethylene–air counterflow flame

The combustion of ultrahigh molecular weight polyethylene (UHMWPE) in airflow perpendicular to the polyethylene surface (counterflow flame) was studied in detail. The burning rate of pressed samples of UHMWPE was measured. The structure of the UHMWPE–air counterflow flame was first determined by mass spectrometric sampling taking into account heavy products. The composition of the main pyrolysis products was investigated by mass spectrometry, and the composition of heavy hydrocarbons (C₇—C₂₅) in products sampled from the flame at a distance of 0.8 mm from the UHMWPE surface was analyzed by gas-liquid chromatography mass-spectrometry. The temperature and concentration profiles of eight species (N₂, O₂, CO₂, CO, H₂O, C₃H₆, C₄H₆, and C₆H₆) and a hypothetical species with an average molecular weight of 258.7 g/mol, which simulates more than 50 C₇—C₂₅ hydrocarbons were measured. The structure of the diffusion flame of the model mixture of decomposition products of UHMWPE in air counterflow was simulated using the OPPDIF code from the CHEMKIN II software package. The simulation results are in good agreement with experimental data on combustion of UHMWPE.     

Reduced Kinetic Mechanism of n‐Heptane Oxidation in Modeling Polycyclic Aromatic Hydrocarbon Formation in Diesel Combustion

A reduced chemical mechanism was developed for the chemical kinetics of n‐heptane oxidation in modeling polycyclic aromatic hydrocarbon formation in diesel combustion. The complete kinetic mechanism, which comprises five hundred and seventy‐two reactions and one hundred and eight species, was reduced to a minor mechanism that includes only seventy‐six reactions and forty‐eight species by using net reaction rate analysis and sensitivity analysis, yet the model based on this reduced mechanism predicted the temperature profile and concentrations of C₇H₁₆, O₂, H₂O, CO₂, benzene, naphthalene, phenanthrene, biphenyl, and pyrene that are essentially indistinguishable from those of the complete mechanism under the range of reaction conditions of interest.     

Study of Combustion Properties of a Solid Propellant by Highly Time‐Resolved Passive FTIR

With a time resolution of 0.125 s and a spectral resolution of 4 cm⁻¹, emission spectra of the combustion process of a solid propellant were recorded by highly time‐resolved passive FTIR. Some gaseous combustion products, such as H₂O, CO, CO₂, NO and HCl, were distinguished by the characteristic emission band of each molecule. The equation for flame temperature calculation based on the diatomic molecule emission fine structure theory was improved through judicious utilization of the spectral running number ‘m’ which makes the temperature measurement simpler and faster. Some combustion information of the solid propellant had been given including the characteristic spectral profile, the distribution of the absolute spectral energy, the distribution of the combustion flame temperature, and the concentration distributions of HCl and NO versus burning time. The results will provide theoretical and experimental bases for improving the formula and raising combustion efficiency of solid propellant, and developing the design of rocket motor, infrared guidance and antiguidance systems.     

A THERMODYNAMIC EQUILIBRIUM ANALYSIS OF GLUCOSE CONVERSION TO HYDROCARBONS

Deoxygenation, or removal of oxygen from oxygenates, is an important element in the hydrocarbon fuel production process from biorenewable substrates. A thermodynamic equilibrium analysis gives valuable insights on the theoretical limits of desired products when a substrate is reacted under a given set of conditions. Here we report the equilibrium composition of glucose-to-hydrocarbon system by minimizing the total Gibbs energy of the system. The system was treated as a mixture of 11 components comprised of C₆H₆, C₇H₈, C₈H₁₀ (ethyl benzene), C₈H₁₀ (xylenes), C₆H₅ –OH, CH₄, H₂O, C, CO₂, CO, and H₂. Equilibrium compositions of each species were analyzed between temperatures 300 and 1500 K and pressures 0–15 atm. It was observed that at high temperature, CO and H₂ dominate the equilibrium mixture with mole fractions of 0.597 and 0.587 respectively. At low temperatures the equilibrium mixture is dominated by CH₄, CO₂, H₂O, and carbon. The aromatic hydrocarbon composition observed at thermodynamic equilibrium was extremely small.     

A CuO-CeO2 Mixed-Oxide Catalyst for CO Clean-Up by Selective Oxidation in Hydrogen-Rich Mixtures

A CuO-CeO₂ mixed-oxide catalyst was shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixtures, and an attractive alternative to the noble metal catalysts presently used for CO clean-up in hydrogen mixtures for proton-exchange membrane fuel cells (PEMFC). Although the presence of H₂O and CO₂ in the feed decreased the activity and increased the reaction temperature considerably to achieve a given CO conversion with a reactor, the selectivity profile with respect to the conversion remained virtually the same. The effect of H₂O and CO₂ on the reaction was found to increase the required energy for reduction of the active copper species in the redox cycles undergone during the reaction. The catalyst showed a slow, reversible deactivation, but the activity was restored on heating the catalyst at 300 °C, even under an inert flow. At space velocities above 42 g h m^-3, the catalyst reduced the CO content to less than 10 ppm in the temperature range 166-176 °C for a feed of 1% CO, 1% O₂, 50% H₂, 20% H₂O, 13.5% CO₂ and balance He. Hence, with this catalyst it is feasible to clean up the CO in a single-stage reactor with relatively small excess oxygen, which is in contrast to the typical multistage reactor systems using noble metal catalysts.     

NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

Performance Parameters of Explosives: Equilibrium and Non‐Equilibrium Reactions

For the calculation of the performance parameters of combustion processes, equilibrium thermodynamic processes are taken into account. On the other hand, non‐equilibrium reactions occur, mostly connected with low pressure burning. In this paper, several explosives, explosive mixtures, solid and liquid propellants have been calculated. It is shown how energy output and gas formation depend on the oxygen balance and the enthalpy of formation. It was found that the reason for the higher specific energy of liquid propellants is due to the increased formation of gases consisting of H₂, N₂ and H₂O, compared with conventional solid propellants based on nitrocellulose and nitroglycerin, which produce more CO and CO₂. Non‐equilibrium combustion of solid propellants was found at very low loading densities or pressures lower than 1 to 2 MPa. In this case, the reaction products measured by mass spectrometry, such as NO, N₂O and HCN, are metastable and highly toxic, producing a much lower heat of explosion compared with equilibrium burning measured and calculated.     

Polystyrenes: A review of the literature on the products of thermal decomposition and toxicity

The current English literature through 1984 on the products of pyrolysis and combustion from polystyrenes and the toxicity of those products is reviewed. Among 57 compounds detected by chemical analyses of the thermal decomposition products produced under various atmospheric conditions (vacuum, inert and oxidative), the main volatile component is the styrene monomer, Evidence is provided that the mass fraction of styrene increases with furnace temperatures at least through 500°C. At 800°C and above, the concentration of styrene decreases. In oxidative atmospheres, carbon monoxide (CO), carbon dioxide (CO₂) and oxidative hydrocarbons are formed. The concentrations of CO and CO₂ are a function of temperature and combustion conditions, i.e. greater amounts are produced in the flaming than in the non-flaming mode. Eleven different test procedures were used to evaluate the toxicity of the pyrolysis and combustion atmospheres of polystyrenes. The more toxic environments produced under flaming conditions appear to be mainly attributed to CO and CO₂ but rather to some other toxicant, probably the styrene monomer. When compared with other common materials used in buildings and residences, polystyrenes, in general, are among the least toxic.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.