Emissions of nitrous oxide from combustion sources

Nitrous oxide (N₂0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N₂O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N₂0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO₂, H₂O and NO in combustion gases react in a sampling vessel to produce N₂O, has revealed that, in fact, less than 5 ppm of N₂0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N₂O levels of ca. 50ppm in their off-gases.

The gas-phase reactions of N₂0 in flames are reviewed first. It is clear that in most cases N₂0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N₂0 are thermal decomposition to N₂ and O₂ and also radical attack in e.g. N₂O + H → N₂ + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH₃; the reactions are NCO + NO → N₂0 + CO and NH + NO → N₂0 + H. The levels of N₂O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH₃ at producing N₂0. The removal of N₂O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts.

When coal burns in a fluidized bed, most of the N₂O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH₃ are produced from nitrogenous material released during devolatilization; these two compounds give N₂0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O₂ on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N₂0, in addition to CO, CO₂ and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N₂O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N₂O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N₂O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N₂O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N₂O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N₂O emissions from fluidized bed combustors are identified.     

Comparison of species profiles between O2 and NO2 oxidizers in premixed methane flames

The profiles of the species H, OH, CH, NH, CN, NCO, NO₂, and CH₃O are compared in a series of five premixed stoichiometric 15-torr CH₄/O₂/NO₂/N₂ flames with NO₂ comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO₂; addition of several reactions involving NO₂ and HONO produces excellent agreement with experiment for most species. The reaction CH₃ + NO₂ → CH₃O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH₃O show this species to be present in far larger quantities in the NO₂ supported flames than in the CH₄/O₂ system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO₂ flame relative to the 10% NO₂ flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO₂ flame, the dominant pathway to N₂ formation is through N₂O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO₂* for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO₂ supported flames is caused by thermal excitation of NO₂, not by a chemiluminescence mechanism.     

Decomposition of nitrous oxide at medium temperatures

Flow reactor experiments were done to study the decomposition of N₂O at atmospheric pressure and in a temperature range of 600–1000°C. Dilute mixtures of N₂O with H₂, CH₄, CO with and without oxygen with N₂ as carrier gas were studied. To see directly the relative importance of the thermal decomposition versus the destruction by free radicals (i.e.: H, O, OH) iodine was added to the reactant mixture suppressing the radicals’ concentrations towards their equilibrium concentrations. The experimental results were discussed using a detailed chemistry model. This work shows that there are still some uncertainties regarding the kinetics of the thermal decomposition and the reaction between N₂O and the O radical. Using the recommendations applied in this work for the reaction N₂O + M ↔ N₂ + O + M and for N₂O + O ↔ products, a good agreement with the experimental data can be obtained over a wide range of experimental conditions. The reaction between N₂O and OH is of minor importance under present conditions as stated in latest literature. The results show that N₂O + H ↔ N₂ + OH is the most important reaction in the destruction of N₂O. In the presence of oxygen it competes with H + O₂ + M ↔ HO₂ + M and H + O₂ ↔ O + OH, respectively. The importance of the thermal decomposition (N₂O + M ↔ N₂ + O + M) increases with residence time. Reducing conditions and a long residence time lead to a high potential in N₂O reduction. Especially mixtures of H₂/N₂O and CO/H₂O/N₂O in nitrogen lead to a chain reaction mechanism causing a strong N₂O reduction.     

Nitrous oxide formation and destruction in lean, premixed combustion

Concentraion profiles of N₂O, NO, and N₂ from atmospheric pressure, flat-flame burner experiments are presented. Axial profiles of species and temperature are described for CH₄-air and H₂---O₂---Ar flames doped with either NH₃, NO, or N₂O.

Species concentrations were determined by microprobe sampling and direct analysis by gas chromatography or chemiluminescence analysis, for flames ranging in temperature from 1300 to 2000 K. Burner surface temperatures were also estimated for these flames, using heat transfer analysis and an optical method. Nitrogen-atom balances were achieved in each of the H₂---O₂---Ar flames to within experimental error and better than 6%.

Axial profiles of N₂O were similar for all flames. At sampling locations nearest to the burner (0.5–1.0 mm), N₂O concentrations were highest. Concentrations decreased monotonically in the downstream direction, with N₂O destruction essentially complete in the postflame region (height greater than 3 mm) for all flames except the one at the lowest temperature (1300 K). With NH₃ as dopant, early-flame and postflame N₂O concentration varied inversely with temperature. The lowest early-flame N₂O concentration was observed with NO as dopant, and the highest was observed with direct N₂O addition. Increased initial concentrations of NH₃ led to higher early-flame N₂O concentrations. With N₂O as dopant, product branching to NO and N₂ in the postflame region is approximately 8% and 92%, respectively. An NO removal process involving NH_i is active in lean, NO-doped flames.     

Defect reduction in electrochemically deposited CdS thin films by annealing in 02

Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdS0₄ and Na₂S₂0₃ on indium-oxide coated glass substrates. These films were annealed in N₂, air, or O₂ atmosphere at 200–500°C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N₂, the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300°C. However, for the films annealed in air or O₂, the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O₂ annealing is useful for the defects reduction.     

铁基载氧体的污泥化学链气化过程中氮迁移热力学模拟与实验研究

基于吉布斯自由能最小化原理,采用HSC Chemistry 6.0软件,对污泥化学链气化过程中NO_x前驱物（NH₃和HCN）与Fe₂O₃载氧体的氧化还原行为进行了热力学模拟。基于污泥热解实验中NO_x前驱物的含量,计算载氧体与污泥的摩尔比（OC/SS）对NH₃、HCN以及NH₃和HCN混合气氧化过程的影响。热力学模拟结果表明：Fe₂O₃能显著促进NO_x前驱物的氧化和裂解,主要生成N₂,几乎无NO_x生成;当NH₃、HCN以及混合气（NH₃和HCN）分别作为还原剂时,其最优OC/SS分别为0.02、0.04和0.05;由于HCN还原性强于NH₃,其氧化速率较快。基于Fe₂O₃/Al₂O₃混合物（FeAl）载氧体,实验对比了污泥化学链气化与污泥热解过程中NO_x前驱物的释放特性,发现Fe₂O₃能显著降低烟气中NO_x前驱物的产率,NH₃和HCN产率分别下降32%和62%。实验结果与热力学模拟结果一致。     

Synthesis and characterization of osmium carbonyl cluster compounds with molecular oxygen electroreduction capacity

A transition metal cluster electrocatalyst based on Os_x(CO)_n was synthesized by pyrolysis of Os₃(CO)₁₂ in 1,2-Dichlorobenzene (b.p.≈180°C) under inert atmosphere (N₂). The electrocatalytic parameters of the oxygen reduction reaction (ORR) for an Os_x(CO)_n catalyst were studied with a rotating disk electrode in 0.5 MH₂SO₄ electrolyte. The diffusion coefficient and solubility of O₂ in 0.5 MH₂SO₄ were calculated. Koutecky–Levich analysis of the linear voltamperometry data showed that the reaction follows first-order kinetics and the value of the Koutecky–Levich slope indicates a multielectron charge transfer during the ORR. The value of the Tafel slope obtained from the mass transfer corrected Tafel plots is 131 mV/decade. The performance of the catalyst in a H₂/O₂ PEM fuel cell cathode was evaluated and found to be nearly as good as that of Pt.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Mechanism of decay of ammonia in flame gases from an NH3/O2 flame

Measurements are reported on the decay of NH₃, NH, NO, and OH in the flame gases from an NH₃/O₂ flame burning on a flat flame burner at atmospheric pressure. Cold gas velocity was 7.8 cm/sec and the measured temperature was 2010°K; concentrations were determined by optical absorption. The data are interpreted in terms of a relatively complete mechanism containing 12 species by simplifying the mechanism into a set of 6 pseudo-first-order reactions. Analysis was carried out with an electrical analog of the system. It is concluded that the decay occurs through the combination of one set of reversible reactions that convert the various NH_i(NH_i --- NH₃, NH₂, NH, or N) into each other, and another set of irreversible reactions that remove one or more of the NH_i permanently. The behavior of NO is accounted for. It is proposed that the observation that [OH] is below equilibrium can be accounted for by the fact that some dissociation process, which produces radicals, cannot keep up with the consumption of radicals required by the mechanism.     

NiO–ScSZ and Ni0.9Mg0.1O–ScSZ-based anodes under internal dry reforming of simulated biogas mixtures

Solid oxide fuel cells (SOFCs) with NiO–ScSZ and Ni_0.9Mg_0.1O–ScSZ-based anodes were operated by directly feeding a fuel mixture of CH₄, CO₂ and N₂ (CH₄ to CO₂ ratio of 3:2). Stable operation under constant current load (200 mA cm⁻²) was achieved with a NiO–ScSZ type anode during 200 h operating hours at 900 °C. Less stable operation occurred with a Ni_0.9Mg_0.1O–ScSZ type anode. In the case of SOFC with Ni_0.9Mg_0.1O–ScSZ as the anode, the methane reforming activity was higher than that with NiO–ScSZ. This was explained by change in the microstructure promoting reforming reactions. However, the addition of MgO resulted in degradation of electrochemical performance due to increase in ohmic resistance of the anode material during operation.     

Probing the reaction pathway of dehydrogenation of the LiNH2 + LiH mixture using in situ H NMR spectroscopy

Using variable temperature in situ ¹H NMR spectroscopy on a mixture of LiNH₂ + LiH that was mechanically activated using high-energy ball milling, the dehydrogenation of the LiNH₂ + LiH to Li₂NH + H₂ was investigated. The analysis indicates NH₃ release at a temperature as low as 30 °C and rapid reaction between NH₃ and LiH at 150 °C. The transition from NH₃ release to H₂ appearance accompanied by disappearance of NH₃ confirms unambiguously the two-step elementary reaction pathway proposed by other workers.     

Comparisons of the structure of stoichiometric CH4---N2O---Ar and CH4---O2---Ar flames by molecular beam sampling and mass spectrometric analysis

Measurements are reported of the profiles of composition and temperature in laminar premixed flat flames of CH₄---N₂O---Ar and CH₄---O₂---Ar. Measurements were made in near stoichiometric mixtures at 30 torr by molecular beam sampling and mass spectrometric sample analysis. All major stable species and many important unstable species were measured by this technique, many species being identified in the flame with N₂O as oxidizer for the first time. Calibration of the concentration profiles was accomplished by the use of calibration gases for stable species and by comparison of the mass spectrometer signal in the well-characterized CH₄---O₂---Ar flame with signals in the CH₄---N₂O---Ar flame and by partial equilibrium for the hydrogen---oxygen system. The measurements have identified the presence of NCO, HCN, and HNCO as reaction intermediates and the importance of these species in the reaction mechanism is discussed.     

Reactive and selective redox system of Ni(II)-ferrite for a two-step CO and H2 production cycle from carbon and water

A number of redox systems composed of carbon and bivalent metal-ferrites, MFe₂O₄(M = bivalent metal ions), were studied to find the most reactive and selective working materials for a thermochemical water-decomposition cycle combined with CO production from a carbon compound. Magnetite and Mg(II)-, Mn(II)-, Co(II)-, Ni(II)-, and Zn(II)-ferrites mixed with carbon powder have been screened for reactivity and selectivity in the CO-production step (first step) and subsequent water-decomposition step (second step). The Ni(II)-ferrite showed the most reactivity and selectivity for CO formation from carbon in the first step at temperatures above 700°C. The Ni(II)-ferrite could be completely reduced with carbon to the metallic phase of the Ni---Fe alloy and Ni in the first step although only a small portion of magnetite was reduced to wustite under similar conditions. The ferrites reduced in the first step were oxidized with water vapor to generate H₂ in the second step. The highest conversion of H₂O to H₂ was obtained using the Ni(II)-ferrite. The total amount of evolved H₂ using Ni(II)-ferrite was 10 times larger than for the magnetite at 500°C. The processes could be repeated using the phase transition between Ni(II)-ferrite and Ni---Fe alloy in the temperature range 700–800°C, with the highly efficient net reaction H₂O + C → H₂ + CO.     

Preparation and properties of LiCoyMnxNi1−x−yO2 as a cathode for lithium ion batteries

The preparation of LiCo_yMn_xNi_1−x−yO₂ from LiOH·H₂O, Ni(OH)₂ and γ-MnOOH in air was studied in detail. Single-phase LiCo_yMn_xNi_1−x−yO₂ (0y0.3 and x=0.2) is obtained by heating at 830–900°C. The optimum heating temperatures are 850°C for y=0–0.1 and 900°C for y=0.2–0.3. Excess lithium (1z1.11 for y=0.2) and the Co doping level (0.05y0.2) do not significantly affect the discharge capacity of Li_zCo_yMn_0.2Ni_0.8−yO₂. The doping of Co into LiMn_0.2Ni_0.8O₂ accelerates the oxidation of the transition metal ion, and suppresses partial cation mixing. Since the valence of the manganese ion in LiMn_0.2Ni_0.8O₂ is determined to be 4, the formation of a solid solution between LiCo_yNi_1−yO₂ and Li₂MnO₃ is confirmed.     

Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders

Thermogravimetry was used to study the oxidation of aluminum powders at elevated temperatures. Aluminum powders of various particle sizes and surface morphologies were heated in oxygen up to 1500 °C at different heating rates. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by X-ray diffraction. The experimental data were related to current information on stabilities and phase changes of Al₂O₃ polymorphs. Aluminum powders were observed to oxidize in four distinct stages in the temperature range from 300 to 1500 °C. During stage I, from 300 to about 550 °C, the thickness of the natural amorphous alumina layer on the particle surface increases. The rate of this process is controlled by the outward diffusion of Al cations. At about 550 °C, when the oxide layer thickness exceeds the critical thickness of amorphous alumina of about 4 nm, the oxide transforms into γ-Al₂O₃. The specific volume of γ-Al₂O₃ is less than that of amorphous alumina; therefore, the newly formed γ-Al₂O₃ only partially covers the aluminum surface. The oxidation rate increases rapidly at the onset of stage II, but it decreases when the γ-Al₂O₃ layer becomes continuous. During stage III oxidation, the γ-Al₂O₃ layer grows and partially transforms into the structurally similar θ-Al₂O₃ polymorph. Finally, oxidation stage IV is observed after the transition to stable -Al₂O₃ results in an abrupt reduction of oxidation rate. Qualitative analysis of the rates of oxidation at the different stages enables one to understand the wide range of aluminum ignition temperatures observed for particles of different sizes.     

Active MgH2---Mg-systems for hydrogen storage

Novel, highly active MgH₂-Mg-systems, which can be used both in synthetic chemistry and as high temperature hydrogen storage materials, have been studied at the Max-Planck-Institut für Kohlenforschung in Mülheim-Ruhr, West Germany, since 1978. The original preparative procedure was based on the hydrogenation of magnesium under mild conditions (20–60°C. 1–80 bar) in an organic solvent and in the presence of a soluble organo transition metal catalyst. This process yields a MgH₂ storage material with outstanding kinetic properties (w.r.t. the release and take-up of H₂ at normal pressure and 230–350°C), a high specific surface area (100–130 m²g⁻¹) and a high storage capacity for hydrogen (7 wt%). It suffers, however, from the disadvantage of being pyrophoric. Subsequently, MgH₂-Mg---H₂-storage materials have been developed using magnesium powder doped with small quantities of transition metal complexes or organotransition metal compounds and these can be safely handled in air. The new MgH₂---Mg-systems can be used for hydrogen storage, hydrogen purification and separation and heat storage, provided a 300–350°C heat source is available to release the hydrogen. Their main features include low material and production costs, highly satisfactory kinetics, high hydrogen and heat capacity, relative insensitivity toward impurities in hydrogen (H₂O, O₂, CO, etc.) and stability toward air.     

Potential GHG mitigation options for agriculture in China

China's agriculture accounts for about 5–15% of total national emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Land-use changes related to agriculture are not major contributors of greenhouse gas emissions in China.

Mitigation options are available that could result in significant decrease in CH₄ and N₂O emissions from agricultural systems, and are likely to increase crop and animal productivity. Implementation has the potential to decrease CH₄ emissions from rice paddies, ruminants, and animal waste by 4–40%. Improving the efficiency of plant utilization of fertilizer N could decrease N₂O emissions from agriculture by almost 20%. Analyses of several of the proposed options show positive economic as well as environmental benefits.     

Method of gasification in IGCC system

A coal gasifier is designed to operate at the temperature range of 1200–1300 °C. The 1200 °C sets the lower limit to the carbon reforming efficiency of the high temperature reformer, and the 1300 °C is the lower limit of the fluid temperature of coal slags, below which they may be collected as non-fluid slag. The gasifier is connected to two syngas burners where a portion of product syngas is combusted with O₂ gas and produce ultra hot H₂O and CO₂ gases, these two gases enter into the gasifier and maintain the gasifier temperature at above 1200 °C and reform carbon into syngas. The temperature of the gasifier is controlled by the flow of O₂ gas into the syngas burner, where O₂ gas is completely consumed and none left to enter into the gasifier. This removes any possibility of forming oxidated products, and compressed CO₂ gas spray coal powder into the gasifier column and non-fluid slag is collected at the bottom. A higher level integration of oxidation–reduction cycle is shown for a IGCC system, wherein the exhaust gas of syngas turbine drives the reduction reaction of coal gasification.

A smooth and uniform temperature control within the gasifier assures high efficiency of carbon reforming and quality of product syngas. Conventional Lurgi gasifier relies on its large heat capacity and accumulating coal slag along the inner walls of the gasifier has made the gasifier bigger, lately as large as a three story building. The gasifier of the present design is constructed much smaller in its size, but with greater reforming efficiency.     

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H₂/O₂/N₂ mixtures at values of flame stretch up to 7600 s⁻¹, and existing measurements for C₃H₈/O₂/N₂ mixtures at values of flame stretch up to 900 s⁻¹. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H₂/O₂/N₂ mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.     

Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures

Iron offers the possibility of transformation of a syngas or gaseous hydrocarbons into hydrogen by a cycling process of iron oxide reduction (e.g. by hydrocarbons) and release of hydrogen by steam oxidation. From the thermodynamic and chemical equilibrium point of view, the reduction of magnetite by hydrogen, CO, CH₄ and a model syngas (mixtures CO + H₂ or H₂ + CO + CO₂) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe₃O₄ to iron at temperatures 400–800 K but also on the possible formation of undesired soot, Fe₃C and iron carbonate as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of magnetite at low temperatures requires a relatively high H₂/H₂O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe₃C formation. At lower temperatures and higher CO₂ concentrations in the reducing gas, the possibility of FeCO₃ formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe₃C and FeCO₃ in the iron after the reduction step. Magnetite reduction is the more difficult stage in the looping process. Pressurized conditions during the reduction step will enhance formation of soot and carbon containing iron compounds.