Anthropogenic emissions of CO2 and CH4 in an urban environment

Regular observations of atmospheric mixing-ratios of carbon dioxide and methane in the urban atmosphere, combined with analyses of their carbon-isotope composition (δ¹³C, δ¹⁴C), provide a powerful tool for assessing both the source strength and source partitioning of those gases, as well as their changes with respect to time. Intense surface fluxes of CO₂ and CH₄, associated with anthropogenic activities result in elevated levels of these gases in the local atmosphere as well as in modifications of their carbon-isotope compositions. Regular measurements of concentration and carbon-isotope composition of atmospheric CO₂, carried out in Krakow over the past two decades, were extended to the period 1995–2000 and also to atmospheric mixing-ratios of CH₄ and its carbon-isotope composition. Radiocarbon concentrations (δ¹⁴C) in atmospheric CO₂ recorded at Krakow are systematically lower than the regional background levels. This effect stems from the addition of ¹⁴C-free CO₂ into the local atmosphere, originating from the burning of fossil fuels. The fossil-fuel component in the local budget of atmospheric carbon calculated using a three-component mixing model decreased from ca. 27.5 ppm in 1989 to ca. 10 ppm in 1994. The seasonal fluctuations of this component (winter–summer) are of similar magnitude. A gradually decreasing difference between the ¹⁴CO₂ content in the local atmosphere and the regional background observed after 1991 is attributed to the reduced consumption of ¹⁴C-free fuels, mostly coal, in southern Poland and the Krakow municipal area. The linear regression of δ¹³C values of methane plotted versus reciprocal concentration, performed for the data available for Krakow sampling site, yields the average δ¹³C signature of the local source of methane as being equal to −54.2‰. This value agrees very well with the measured isotope signature of natural gas being used in Krakow (−54.4±0.6‰) and points to leakages in the distribution network of this gas as the main anthropogenic source of CH₄ in the local atmosphere.     

Postflame reaction chemistry of dichloromethane: variations in equivalence ratio and temperature

We report on the destruction pathways and byproduct formation of dichloromethane (CH₂Cl₂) in conditions typical of incinerator postflame regions (injection temperature = 900–1200 K; equivalence ratio = 0.6, 0.9, 1.0, 1.1; residence time = 0.28–0.35 s). This is the first study to independently vary equivalence ratio and temperature, and evaluate their impacts on byproduct yield and destruction efficiency. We inject 750 ppm CH₂Cl₂ into postflame combustion products and measure byproducts with extractive FTIR spectroscopy. We use a detailed chemical kinetic mechanism and reaction rate analysis to predict the changes in reaction pathways as a function of equivalence ratio. The predictions for major products and several intermediate species compare well with experiments; the largest disparities are an underprediction of phosgene (CCl₂O) and an overprediction of methyl chloride (CH₃Cl). Both the experiment and the numerical predictions show increased destruction at lower equivalence ratios. However, the experiments reveal increased levels of stable chlorinated organics at lower equivalence ratios, opposite to the numerical prediction. We discuss reasons for this discrepancy and implications of these results for designing control strategies to promote full conversion to HCl and to reduce chlorinated byproduct emissions.     

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.     

Ni-Co双金属催化剂上沼气重整制氢宏观动力学

用传统湿式浸渍法制备La2O3掺杂的商业γ-Al2O3负载的沼气重整催化剂Ni-Co/La2O3-γ-Al2O3,通过对NiCo双金属催化剂上沼气重整制氢在常压下的宏观动力学分析,得出该催化剂上CH4与CO2消耗、H2与CO生成时的表观反应速率方程.通过改变进料中CH4与CO2的分压,求出各物质的反应分级数,确定总反应...     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Thermo-neutral production of metals and hydrogen or methanol by the combined reduction of the oxides of zinc or iron with partial oxidation of hydrocarbons

Stoichiometry and temperature requirements are determined for combining the endothermic reduction of metal oxides (ZnO, Fe₂O₃, and MgO) with the exothermic partial oxidation of hydrocarbons (CH₄, n-butane, n-octane, and n-dodecane) in order to co-produce simultaneously metals and syngas in thermo-neutral reactions. Thermogravimetric and GC measurements on the combined reduction of ZnO and Fe₂O₃ with the partial oxidation of CH₄ were conducted at 1400 K to experimentally verify the products predicted by equilibrium computations, and resulted in the complete reduction to Zn and Fe, respectively, while producing high quality syngas. A preliminary economic assessment that assumes a natural gas price of 11.9 US$/MWh and credit for zinc sale at 750 US$/metric ton, indicates a competitive cost of hydrogen production at 6.0 US$/MWh, based on its high heating value. The proposed combined process offers the possibility of co-producing metals and syngas in autothermal non-catalytic reactors, with significant avoidance of CO₂ emission.     

Mechanism of soot initiation in methane systems

Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C₂H₂ produced in a chlorine catalyzed polymerization of CH₄ are presented. An earlier examination of the data and mechanism leading to C₂H₂ supports a methyl and chloromethyl recombination path to C₂ hydrocarbons. The relative yield of CH₃ and CH₂Cl depends on the excess of methane.

In the CH₄, system consumption of C₂ species to ultimately form benzene is shown to proceed by a stepwise addition of CH₃ radicals to C_nH_m species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly.

A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1–3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300 K are estimated to be log [k(sec⁻¹)] = 13.7 − 37/θ = 13.7 - 37/0 where 0 = 2.303 RT in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH₃/CH₂Cl radical, sequential addition mechanism.

Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics.     

Integration of acidogenic and methanogenic processes for simultaneous production of biohydrogen and methane from wastewater treatment

Feasibility of integrating acidogenic and methanogenic processes for simultaneous production of biohydrogen (H₂) and methane (CH₄) was studied in two separate biofilm reactors from wastewater treatment. Acidogenic bioreactor (acidogenic sequencing batch biofilm reactor, AcSBBR) was operated with designed synthetic wastewater [organic loading rate (OLR) 4.75 kg COD/m³-day] under acidophilic conditions (pH 6.0) using selectively enriched acidogenic mixed consortia. The resultant outlet from AcSBBR composed of fermentative soluble intermediates (with residual carbon source), was used as feed for subsequent methanogenic bioreactor (methanogenic/anaerobic sequencing batch biofilm reactor, AnSBBR, pH 7.0) to generate additional biogas (CH₄) utilizing residual organic composition employing anaerobic mixed consortia. During the stabilized phase of operation (after 60 days) AcSBBR showed H₂ production of 16.91 mmol/day in association with COD removal efficiency of 36.56% (SDR_A—1.736 kg COD/m³-day). AnSBBR showed additional COD removal efficiency of 54.44% (SDR_M—1.071 kg COD/m³-day) along with CH₄ generation. Integration of the acidogenic and methanogenic processes enhanced substrate degradation efficiency (SDR_T—4.01 kg COD/m³-day) along with generation of both H₂ and CH₄ indicating sustainability of the process.     

Production of hydrogen gas from novel chemical hydrides

Six ligand-stabilized complexes have been synthesized and tested for use as hydrogen storage media for portable fuel cell applications. The new hydrides are: [HC(3,5-Me₂pz)₃]LiBH₄ (1), {[H₂C(3,5-Me₂pz)₂]Li(BH₄)}₂ (2) (pz = pyrazolyl), [(TMEDA)Li(BH₄)]₂ (3) (TMEDA = (CH₃)₂NCH₂CH₂N(CH₃)₂), [HC(pz)₃]LiBH₄ (4), {[H₂C(pz)₂]Li(BH₄)}₂ (5) and Mg(BH₄)^.₂ 3THF (6) (THF = tetrahydrofuran). Hydrolysis reactions of the compounds liberate hydrogen in quantities which range from 56 to 104 (± 5%) percent of the theoretical yield. Gas chromatographic analysis of the product gases from these reactions indicate that hydrogen is the only gas produced. Thermally initiated reactions of the novel compounds with NH₄Cl were unsuccessful. Although the amount of hydrogen energy which can be theoretically obtained per unit weight is lower than that of the classical hydrides such as LiBH₄ and NaBH₄, the reactions are less violent and hydrolysis of compounds 1, 2, 4, 5 and 6 releases less heat per mole of hydrogen generated.     

Kinetic study on fermentation from CO2 and H2 using the acclimated-methanogen in batch culture

Methane was produced from H₂ and CO₂ using the acclimated-mixed methanogens in a 3.71 fermentor in batch culture at pH 7.2 and 37°C. The Fermentation kinetics parameter for the growth of methanogens, overall mass transfer coefficient of the reactor, and the conversion rate of H₂ and CO₂ to CH₄ by the acclimated-mixed culture were determined using the technique of Vega et al. The maximum specific growth rate (μ_max) and H₂ specific consumption rate (q_max) were found to be 0.064(h⁻¹) and 104.8 (mmol h⁻¹ g⁻¹) respectively. Monod saturation constants for growth (Kp) and for inhibition (K′p) were found to be 3.54 (kPa) and 0.57 (kPa), respectively. These findings indicate that without very low dissolved H₂ levels, the fermentations are carried out under μ_max, and the specific uptake rate (q) was almost not affected at any dissolved H₂ level in the range studied. The yield of CH₄ (Yp/s) was calculated to be 0.245 (mol CH₄ mol⁻¹ H₂), which is near the stoichiometric value of 0.25. DH₂ was also measured using the Teflon tubing method and was in good agreement with those estimated by kinetic calculations.     

A new method for estimating solar radiation from bright sunshine data

Daily values of H/H₀, the ratio of total horizontal radiation to that outside the atmosphere has been correlated with s/S, bright sunshine as a fraction of daylength for 3 yr measurements in Adana and Ankara, Turkey. Using a maximum-likelihood quadratic fit, we show that monthly averages s/S and its standard deviation σ_s/S can be used to estimate the monthly average H/H₀ as

H/H₀ = 0.204 + 0.758s/S − 0.250[s/S² + σ²_s/S.

Comparison of the estimations of the above equation with measurements from different regions of Turkey indicate that less than 5 per cent relative error is possible. A further correlation σ²_s/S with s/S makes it possible to estimate H/H₀ with just the knowledge of s/S.     

纤维素温和条件下一步水热法制备甲烷的研究

生物质发酵法制备甲烷存在甲烷收率低、CO₂含量高等问题。本研究以纤维素为原料,在温和条件下采用水热催化转化的方法制备甲烷。对一系列催化剂进行了考察,发现Ru/C对该反应的催化活性最高。采用Ru/C催化剂进一步考察了一系列反应条件,结果表明,升高反应温度、延长反应时间、增加催化剂用量以及提高氢气初始压力对甲烷的生成具有促进作用。在1 MPa H₂、220℃、12 h反应条件下,甲烷碳摩尔收率最高,达88%,反应过程中无CO₂产生。采用TEM、BET、XRD和FT-IR等对催化剂进行了表征,结果表明,Ru/C催化剂的高催化活性可能与催化剂本身比表面积大、钌粒子颗粒小且分散均匀的特性有关。本研究采用的催化转化方法具有甲烷收率高、CO₂排放量小（<5%）、反应条件更为温和等特点。     

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.     

微量元素对蔬菜废弃物厌氧消化的促进效果

以蔬菜废弃物为原料的厌氧消化系统,由于原料的易酸化特性,在高负荷条件下易失稳,而低负荷的运行会导致较低的池容产气率。本研究采用自行设计的70-L厌氧发酵罐,在中温35℃条件下进行蔬菜废弃物厌氧消化的连续冲击负荷试验,根据气体成分（CH₄）的变化规律,添加微量元素（Fe, Co, Ni）以调控消化过程,使其由失稳状态恢复至稳定状态,旨在提高高负荷厌氧发酵的稳定性。研究结果表明,蔬菜废弃物中温厌氧消化系统的有机负荷率增大至2.0 g VS/(L•d)时,CH₄含量由50%降至40%,从第103天开始连续添加5天微量元素（Fe, Co, Ni）后,CH₄含量迅速恢复至50% ~ 55%的稳定状态,池容产甲烷率由0.38 L/(L•d) 增大至0.6 L/(L•d)左右并保持稳定。停止添加微量元素后,继续增大有机负荷率,厌氧消化系统稳定运行83天。当运行至第195天时（3.0 g VS/(L•d)）,CH₄含量再次出现下降趋势,由58.9%降至53.4%,添加3天微量元素后,CH₄含量再次恢复到55%以上的稳定状态。微量元素的添加可有效提高蔬菜废弃物厌氧消化的稳定性,能够快速恢复失稳的系统。     

Singlet methylene removal by saturated and unsaturated hydrocarbons

The technique of laser flash phyotolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, ¹CH₂ (ã¹A₁), with various saturated and unsaturated hydrocarbons. The removal rate constants for CH₄, C₂H₆, C₃H₈, C₂H₄, C₃H₆, C₂H₂, CH₂CCH₂, and C₆H₆ were found to be in excellent agreement with previously reported results. Removal rate constants were also measured for n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂, c-C₃H₆, c-C₆H₁₂, 1-C₄H₈, cis-2-C₄H₈, trans-2-C₄H₈, and 1-C₄H₆, and determined to be (3.17 ± 0.15), (2.53 ± 0.11), (3.35 ± 0.24), (1.63 ± 0.08), (3.77 ± 0.21), (3.80 ± 0.20), (3.67 ± 0.16), (3.43 ± 0.16) and (4.05 ± 0.18) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. This series of hydrocarbons forms the basis of a larger series of compounds containing a wide variety of organic functional groups. The removal rate constants are reported here, both as a series within its own right, and as a reference point for future work.     

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.     

The effect of doping rare-earth chloride dopant on the dehydrogenation properties of NaAlH4 and its catalytic mechanism

A series of rare-earth chlorides has been adopted to catalyze dehydrogenation reaction of NaAlH₄. X-ray diffraction analysis and isothermal dehydrogenation measurement have proved that these chlorides enhance the dehydrogenation kinetics and lower the decomposition temperature of NaAlH₄. The catalytic effect from high to low is in following order: SmCl₃>CeCl₃>TiCl₃>NdCl₃>GdCl₃>LaCl₃>ErCl₃. In order to reveal the catalytic mechanism of the rare-earth chlorides on dehydrogenation reactions, systems doped with LaCl₃ were investigated under different milling and dehydrogenation conditions. It has been proposed that the La cation reacts with hydrogen, which was released from NaAlH₄, and then forms some sort of La–Al alloys. This process improves the performance of NaAlH₄ dehydrogenation at a relatively low temperature. In the present research, the catalytic effect of La₂O₃ has also been investigated. Results show that La₂O₃ also have a catalytic effect on the dehydrogenation of NaAlH₄, but the effect is less obvious than that of the rare-earth chlorides.     

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.     

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.     

Preparation and properties of transparent conducting zinc oxide and aluminium-doped zinc oxide films prepared by evaporating method

Undoped and aluminium-doped zinc oxide films have been prepared by thermal evaporation of zinc acetate [Zn(CH₃COO)₂ 2H₂O] and aluminium chloride [AlCl₃] onto a heated glass substrate. The structural and optoelectrical properties of the films have been studied. The effects of heat treatment for the as-deposited films in air and vaccum are investigated. Highly transparent films with conductivity as low as 2×10⁻³ Ω cm can be produced by controlling the deposition parameters. The electron carrier densities are in the range 0.2–7×10¹⁹ cm⁻³ with mobilities of 22–58 cm² V⁻¹ s⁻¹.