María U. Alzueta

Inhibition and sensitization of fuel oxidation by SO2

Abstract An experimental and theoretical study of the interaction of SO 2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz flow reactor at 1 atm; temperature ranged from 800 to 1,500 K and stoichiometries from fuel-rich to very lean. In addition, literature data on sulfur species concentration profiles and H atom decay in fuel-rich H 2 /O 2 flames doped with SO 2 were analyzed. The results show that under flow-reactor conditions SO 2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO 2 promotes oxidation through the sequence: SO 2 + H ⇌ SO + OH, SO + O 2 ⇌ SO 2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO 2 +O+M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO 2 addition under reducing conditions an upper limit of 3.0 × 10 14 cm 6 mol −2 s −1 at 1,060 K can be estimated for the rate constant of H + SO 2 + N 2 ⇌ HOSO + N 2 . This value is consistent with a significant barrier to reaction as proposed theoretically, but an order of magnitude lower than indicated by both ab initio calculations (Marshall and co-workers) and reaction rates derived from flames. However, we find that data on H atom decay in flames doped with SO 2 are not suitable for deriving rate constants because of uncertainty in important side reactions involving SO. Furthermore, we propose that the enhanced H atom decay observed in these flames may be attributed to recombination of H atoms with SO and S 2 species, rather than to a mechanism initiated by the H + SO 2 + M reaction.

Evaluation of the use of different hydrocarbon fuels for gas reburning

Abstract Gas reburning is a NOx reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.

Parabenzoquinone pyrolysis and oxidation in a flow reactor

An experimental and theoretical study of the pyrolysis and oxidation of parabenzoquinone has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 600–1500 K. The main variables considered are temperature, oxygen concentration, and presence of CO. A detailed reaction mechanism for the pyrolysis and oxidation chemistry of parabenzoquinone is proposed, which provides a good description of the experimental results. Both the experimental work and the kinetic mechanism proposed for the pyrolysis and oxidation of parabenzoquinone represent the first systematic study carried out for this important aromatic compound. Our pyrolysis results confirm that the primary dissociation channel for p-benzoquinone leads to CO and a C5H4O isomer, presumably cyclopentadienone. However, significant formation of CO2 during the pyrolysis may indicate the existence of a secondary dissociation channel leading to CO2 and a C5H4 isomer. Under oxidizing conditions, consumption of p-benzoquinone occurs mainly by dissociation at lower temperatures. As the temperature increases interaction of OC6H4O with the radical pool becomes more significant, occurring primarily through hydrogen abstraction reactions followed by ring opening reactions of the OC6H3O radical.

The recombination of hydrogen atoms with nitric oxide at high temperatures

The rate constant for the H+NO+N 2 reaction (R1,N 2 ) has been determined in the temperature range 1000–1170K from flow-reactor experiments on the CO/O 2 /H 2 O/N 2 system perturbed with different amounts of NO. The initiation temperature of this system is highly sensitive to reaction R1, which is the rate-controlling step in the nitric oxide catalyzed removal of hydrogen atoms. Based on the flow-reactor results and the limited amount of data reported in literature, a rate constant for the H+NO+N 2 reaction of 4.0×10 20 T −1.75 cm 6 /(mol 2 s ) was determined. This value is in good agreement with the recent result of Allen and Dryer at 1000K but significantly lower at high temperatures than the recommendation of Tsang and Herron. With the recently determined value of ΔH f,298 (HNO) of 26.0 kcal/mol, which is 2 kcal/mol higher than previous estimates, our results correspond to a rate constant of 1.7×10 19 T −1.5 exp(−23,400/ T ) cm 3 /(mol s) for the HNO+N 2 dissociation reaction in the 1000–2500 K range. The sharp drop-off in the rate constant for H+NO+M at high temperatures suggested by the flow-reactor results are supported by reinterpretation of data reported in literature on H 2 /O 2 /N 2 flames doped with NO. Theoretical considerations suggest that the effect can be attributed to weak-collision effects.

Characterization of Soot

The characterization of physical and chemical properties of the carbon particulate matter commonly named soot is relevant in the research on pollutants emitted in the atmosphere from combustion and industrial plants. The selection and the standardization of advanced analytical methods are necessary to provide reliable and reproducible results on the characteristics of carbon material. This chapter reports an overview of the main off-line techniques available to characterize carbon materials as: elemental analysis, physical adsorption with the determination of the specific surface area, electronic microscopy techniques, X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, infrared spectroscopy, soot reactivity toward O2 and NO, UV–Visible spectroscopy, size exclusion chromatography and fluorescence spectroscopy. The results of the implementation of these techniques on a commercial standard carbon material (Printex-U carbon black), considered as analog of soot, are reported as case study.

Experimental study and modelling of the burnout zone in the natural gas reburning process

Abstract The reburning process with natural gas is an effective technique which can be applied in existing coal combustors in order to reduce the NO x emissions. The importance of the burnout zone in this process is great, because significant amounts of CO, unburned fuel and reactive nitrogen can reach this zone. The influence of the temperature and the air excess introduced in the burnout zone on the concentration of the nitrogenous species has been studied. Experimental results have been determined of the concentrations of NO, HCN and NH 3 in the exit gas for temperatures ranging between 900 and 1100°C and air excesses between 5 and 15%. An empirical correlation which fits the output NO concentration has been obtained. A simplified model has been developed to predict the output concentrations of the most interesting nitrogenous species.

Influence of the Temperature and 2,5-Dimethylfuran Concentration on Its Sooting Tendency

ABSTRACT The sooting tendency of 2,5-dimethylfuran (2,5-DMF), as a proposed fuel or fuel additive, has been studied in a flow reactor at different reaction temperatures (975, 1075, 1175, 1275, 1375, and 1475 K) and inlet 2,5-DMF concentrations (5000, 7500, and 15,000 ppm) under pyrolytic conditions. The quantification of soot and light gases has been done. Additionally, the experimental results of the light gases have been simulated with a detailed gas-phase chemical kinetic model. The experimental results indicate that the temperature has a great influence on both the soot and gas yields, as well as on the concentration of the light gases of pyrolysis. The inlet 2,5-DMF concentration influences the soot yield, whereas no significant effect is observed on the gas yield.

Dimethoxymethane Oxidation in a Flow Reactor

ABSTRACT The simultaneous reduction of NOx and soot emissions from diesel engines is a major research subject and a challenge in today’s world. One prospective solution involves diesel fuel reformulation by addition of oxygenated compounds, such as dimethoxymethane (DMM). In this context, different DMM oxidation experiments have been carried out in an atmospheric pressure gas-phase installation, in the 573–1373 K temperature range, from pyrolysis to fuel-lean conditions. The results obtained have been interpreted by means of a detailed gas-phase chemical kinetic mechanism. Results indicate that the initial oxygen concentration slightly influences the consumption of DMM. However, certain effects can be observed in the profiles of the main products (CH4, CH3OH, CH3OCHO, CO, CO2, C2H2, C2H4, and C2H6). Acetylene, an important soot precursor, is only formed under pyrolysis and reducing conditions. In general, a good agreement between experimental and modeling data was observed.

Pyridine conversion in a flow reactor and its interaction with nitric oxide

An experimental study of the oxidation of pyridine both in the absence and in the presence of NO has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 700-1500 K. The influence of temperature, air excess ratio (corresponding to fuel-rich, stoichiometric, and lean conditions), and NO on the concentrations of CO, CO 2 , HCN, and NO has been analyzed. The current work contributes to a series of works on the conversion of volatile nitrogen compounds with experimental results related to pyridine. Also, a reaction mechanism based on various literature mechanisms has been used for simulating the experimental results.

A Comparison of Acetylene Soot and Two Different Carbon Blacks: Reactivity to Oxygen and NO

There is not much known about the reactivity of hydrocarbon soot to oxygen and NO. According to the chemical and structural properties of one soot obtained from acetylene pyrolysis, SEVACARB MT and CABOT carbon blacks were selected to perform a reactivity study. The properties of the materials were determined through ultimate and proximate analysis as well as SEM, TEM and XRD techniques. A study of the oxidation of the three materials is performed for different oxidation conditions of temperature (1123-1273 K) and oxygen concentration (100-1000 ppmv). The reaction of acetylene soot with NO and SEVACARB MT carbon black is carried out at 1373 K with 1800-3000 ppmv NO. Results are compared to analyze their reactivity similarities and the possibility of using carbon blacks as acetylene soot substitutes in laboratory work.