Nonthermal Plasma Chemical Processing of Bromomethane

ABSTRACT

Nonthermal plasma chemical decomposition of bromomethane (CH₃Br) was investigated with a coaxial type packed-bed plasma reactor. It has been demonstrated that plasma chemical processing is an effective approach to decompose CH₃Br in a wide concentration range. It has been shown that CH₃Br decomposition reactivity depends on reactor operating factors such as background gas, O₂ concentration, and humidification. Higher decomposition efficiencies can be obtained in dry N₂. However, organic byproducts such as BrCN are concurrently produced under deaerated conditions. Water suppresses CH₃Br decomposition and also affects the yields of CO_x (CO and CO₂) and organic byproducts due to the involvement of some active species generated from water. The presence of O₂ retards CH₃Br decomposition by quenching high-energy electrons, while it suppresses organic byproducts and improves CO_x yield. The reacted carbons in CH₃Br are recovered as CO_x almost quantitatively in air. Higher CO₂ selectivities cannot be achieved by increasing O₂ concentration. NO_x formation, which is accompanied by CH₃Br decomposition, can be effectively suppressed by decreasing O₂ concentration down to 2%. Furthermore, reaction mechanisms are discussed by comparing the reactivities of CH₃Br and its congeners.     

Measurement of methyl halides in the marine atmosphere

An analytical method for measuring atmospheric methyl halides was established based on canister sampling and capillary GC/MS. Stability tests for air samples collected in two kinds of canisters (electro-chemical buffing and fused-silica lined) with smooth inner surfaces, showed that both provided stable storage for CH₃Cl, CH₃Br, and CH₃I. The method was applied to the measurement of methyl halides at a remote island (Okinawa, Japan) in August 1996, where nocturnal ozone depletion had been observed in summer. We found that atmospheric CH₃Cl increased during stable nights and was negatively correlated with surface ozone concentration. The highest CH₃Cl concentration amounted to as high as 1400 pptv and indicated that CH₃Cl emitted from the surrounding coastal areas had accumulated in the boundary layer under a stable atmosphere at that time. A positive correlation was observed between CH₃Br and CH₃I, suggesting a common source in the area.     

活性炭纤维吸附含溴甲烷气体的性能

采用动态吸附法在25℃下,测定了3种活性炭纤维(ACF-1、ACF-2和ACF-3)对含溴甲烷气体的吸附性能和回收效果,并对活性炭纤维的孔结构进行表征.探讨了孔结构、溴甲烷浓度、气体流量、循环使用次数等因素对活性炭纤维吸附溴甲烷性能的影响.结果表明,活性炭纤维比表面积大小及0.4～0.8 nm左右的微孔数量决定了其对溴甲烷吸附性能的优劣;气体中溴甲烷的浓度的提高使活性炭纤维对溴甲烷的穿透和饱和吸附量增加,而气体流量的增加则使活性炭纤维对溴甲烷的穿透和饱和吸附量降低,但两者均使穿透和饱和吸附时间缩短;活性炭纤维多次循环使用后,对溴甲烷的吸附容量明显地降低,循环12次后达到稳定吸附,其稳定吸附值为133.5 mg/g.     

Biogenic fluxes of halomethanes from Irish peatland ecosystems

Irish peatland ecosystems have been shown to be important sources of low molecular weight halocarbons. Emission of CH₃Br, CH₃Cl, CH₃I and CHCl₃ was recorded from all peatland sites monitored, with minor flux of other halocarbons at certain sites. Fluxes were found to be highly linked to incident light, with strong diurnal cycles recorded at all open peatland sites. Estimates of halomethane emissions, particularly from coastal peatland and conifer plantation forest floor sites, suggests that these ecosystems may make a significant contribution to the global budgets of several important halocarbons. Global annual fluxes of 4.7 (0.1–151.9), 0.9 (0.1–3.3), 5.5 (0.9–43.4), and 1.4 (0.1–12.8) Gg yr⁻¹ for CHCl₃, CH₃Br, CH₃Cl, and CH₃I, respectively, were determined for peatland ecosystems.     

Identification and quantification of methyl halide sources in a lowland tropical rainforest

In conjunction with the OP3 campaign in Danum Valley, Malaysian Borneo, flux measurements of methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) were performed from both tropical plant branches and leaf litter in June and July 2008. Live plants were mainly from the Dipterocarpaceae family whilst leaf litter samples were representative mixtures of different plant species. Environmental parameters, including photosynthetically-active radiation, total solar radiation and air temperature, were also recorded. The dominant factor determining magnitude of methyl halide fluxes from living plants was plant species, with specimens of the genus Shorea showing persistent high emissions of both gases, e.g. Shorea pilosa: 65 ± 17 ng CH₃Cl h^?1 g^?1 (dry weight foliage) and 2.7 ± 0.6 ng CH₃Br h^?1 g^?1 (dry weight foliage). Mean CH₃Cl and CH₃Br emissions across 18 species of plant were 19 (range, <LOD ?76) and 0.4 (<LOD ?2.9) ng h^?1 g^?1 respectively; fluxes from leaf litter were 1–2 orders of magnitude smaller per dry mass. CH₃Cl and CH₃Br fluxes were weakly correlated. Overall, the findings suggest that tropical rainforests make an important contribution to global terrestrial emissions of CH₃Cl, but less so for CH₃Br.     

Distribution,Sources and Sinks of Atmospheric Halogenated Compounds

Based on two comprehensive field studies conducted in California, background concentration (parts per trillion) of N₂O (296.0 X 10³), SF₆ (0.16), CCI₂F₂ (180.8), CCI₃F (103.8), CCI₂FCCIF₂ (16.3), CCI₄ (114.2), CH₃CI (952.9), CHCI3 (23.4), CH₃I (2.4), CH3CCI3 (84.0), CCI₂CCI₂ (43.1), CHCICCI2 (14.5) and CH₃Br (—) have been reported. These trace constituents were identified using retention data on eight GC columns, their electron attachment properties, and their EC thermal response. All but CHCICCI₂ and CH₃Br were measurable 100% of the time at both sites. Cryogenic procedures for SF₆ ambient measurement were developed and successfully used. By an analysis of worldwide emissions of these trace constituents, their ambient levels, and their atmospheric lifetimes, it was possible to determine their origin (natural or anthropogenic). Our results indicate that 27% of organic chlorine contribution to the troposphere comes from fluorocarbons as opposed to a 73% contribution from the chloro-carbons. Further, the anthropogenic organic content in the troposphere was found to be about twice the natural content. Very high CHCI3 concentrations in onshore ocean waters were measured. Ambient data supporting the anthropogenic origin of CCI₄ have been presented.     

Tropospheric halocompounds and nitrous oxide monitored at a remote site in the Mediterranean

Analysis of time series and trends of nitrous oxide (N₂O) and halocompounds weekly monitored at the Mediterranean island of Lampedusa are discussed. Atmospheric N₂O levels showed a linear upward growth rate of 0.78 ppb yr^?1 and mixing ratios comparable with Northern Hemisphere global stations. CFC-11 and CFC-12 time series displayed a decline consistent with their phase-out. Chlorofluorocarbons (CFCs) replacing compounds and SF₆ exhibited an increasing temporal behaviour. The most rapid growth rate was recorded for HFC-134a with a value of 9.6% yr^?1. The industrial solvents CCl₄ and CH₃CCl₃, banned by the Montreal Protocol, showed opposite trends. While CH₃CCl₃ reported an expected decay of ?1.8 ppt yr^?1, an increasing rate of 5.7 ppt yr^?1 was recorded for CCl₄ and it is probably related to its relatively long lifetime and persisting emissions. Chlorinated halomethanes showed seasonality with a maximum in early April and a minimum at the end of September. Halon-1301 and Halon-1211 displayed a decreasing trend consistent with industry emission estimates.An interspecies correlation analysis gave positive high correlations between HCFC-22 and HFC-134a (+0.84) highlighting the common extensive employment as refrigerants. Sharing sources inferred the high coupling between CH₃Cl and CH₃Br (+0.73) and between CHCl₃ and CH₂Cl₂ (+0.77). A singular strong relationship (+0.55) between HFC-134a and CH₃I suggested the influence of an unknown anthropogenic source of CH₃I.Constraining of source and sink distribution was carried out by transport studies. Results were compared with the European Environment Agency (EEA) emission database. In contrast with the emission database results, our back trajectory analysis highlighted the release of large amounts of HFC-134a and SF₆ from Eastern Europe. Observations also showed that African SF₆ emissions may be considerable. Leakages from SF₆ insulated electrical equipments located in the industrialized Northern African areas justify our observations.     

Quantification of the inhibitory effect of methyl fluoride on methanogenesis in mesophilic anaerobic granular systems

The inhibitory effect of CH₃F on methanogenesis in mesophilic anaerobic granules was tested at different concentrations (0-10% v/v, in the gas phase) and verified by the stable carbon isotopic signatures of CH₄ and CO₂. The results showed that the inhibitory effect increased with the initial CH₃F concentration up to 5%. The CH₃F concentration causing 50% metabolic inhibition was 0.32%. Complete inhibition of acetoclastic methanogenesis with a 91% reduction in total methanogenic activity was achieved when 5% CH₃F was initially added to the headspace, which resulted in 870 μM dissolved CH₃F in the liquid. It was much higher than that applied in other natural anoxic non-granular systems, indicating that the layered granular structure influenced the inhibitory effect. The obvious increase in hydrogen content indicated that high concentrations of CH₃F (?5%) suppressed hydrogenotrophic methanogenesis as well. The stable inhibition lasted for at least 6 d as the CH₃F concentration decreased slowly with incubation time. These results suggested that CH₃F could be used for investigating methanogenic processes in anaerobic granular systems after the CH₃F concentration and incubation time for specific inhibition of acetoclastic methanogenesis were carefully determined. In the present system, CH₃F concentration of 5% was suggested to be optimal.     

Concentration characteristics of bromine and iodine in aerosols in Shanghai,China

Aerosol samples (TSP and PM₁₀) during each season were collected at a national monitoring point in Shanghai in 2008. Halogens (Br, I) were determined in samples along with sodium (Na) by ICP-MS and ICP-OES after microwave digestion. In this report we focused on the concentration characteristics of halogen elements Br and I and their seasonal distributions. The mean annual concentrations of total Br and I were 24 ng m^?3 and 12 ng m^?3 for TSP, 21 ng m^?3 and 9 ng m^?3 for PM₁₀, respectively. Concentrations of Br and I in TSP and PM₁₀ were lowest in summer but an increase occurred in autumn and winter. Water-soluble Br and I accounted for about 32% of the total Br and I in aerosols, and about 68% of Br and I was non soluble which may be non-soluble organic species. These non-soluble organic species are present in aerosols in the possible binding forms as mineral dust, natural organic matter, and adsorption to black carbon or mineral material such as iron oxides. Soluble Br and I in PM₁₀ extracted by a dilute acid solution (HNO₃ + H₂SO₄) increased by 22% and 18%, respectively, compared with water-soluble Br and I. A positive correlation with Na and sea water enrichment factors for Br and I indicated that bromine and iodine in aerosols originated mostly from marine sources in Shanghai.     

Effects of ammonia on methane oxidation in landfill cover materials

The effects of ammonia (NH₃) on CH₄ attenuation in landfill cover materials consisting of landfill cover soil (LCS) and aged municipal solid waste (AMSW), at different CH₄ concentrations, were investigated. The CH₄ oxidation capacities of LCS and AMSW were found to be significantly affected by the CH₄ concentration. The maximum oxidation rates for LCS and AMSW were obtained at CH₄ concentrations of 5 % and 20 %(v/v), respectively, within 20 days. CH₄ biological oxidation in AMSW was significantly inhibited by NH₃ at low CH₄ concentrations (5 %, v/v) but highly stimulated at high levels (20 % and 50 %, v/v). Oxidation in LCS was stimulated by NH₃ at all CH₄ concentrations due to the higher conversion of the nitrogen in NH₃ in AMSW than in LCS. NH₃ increases CH₄ oxidation in landfill cover materials.     

Sensitivity of tropospheric oxidants to global chemical and climate change

A photochemical model has been used to quantify the sensitivity of the tropospheric oxidants O₃ and OH to changes in CH₄, CO and NO emissions and to perturbations in climate and stratospheric chemistry. Coefficients of the form ∂1n[O₃]/∂1n[X] and ∂1n[OH]/∂1n[X], where [X] = flux of CH₄, CO, NO; stratospheric O₃ and H₂O have been calculated for a number of “chemically coherent” regions (e.g. nonpolluted continental, nonpolluted marine, urban) at low and middle latitudes. Sensitivities in O₃ and OH vary with regional emissions patterns and are nonlinear within a given region as [X] changes. In most cases increasing CH₄ and CO emissions will suppress OH (negative coefficients) and increase O₃ (positive coefficients) except in areas where NO and O₃ influenced by pollution are sufficient to increase OH. Stratospheric O₃ depletion will tend to decrease O₃ (except in high NO_x areas) and increase OH through enhanced u.v. photolysis. Increased levels of water vapor (one possible outcome of a global warming) will also decrease O₃ and increase OH. We conclude that in most regions, NO, CO and CH₄ emission increases will suppress OH and increase O₃, but these trends may be opposed by stratospheric O₃ depletion and climate change. A regional survey of OH and O₃ levels suggests that the tropics have a pivotal role in determining the earth's future oxidizing capacity.     

Methylchloroform: Global distribution,seasonal cycles,and anthropogenic chlorine

It is evident that the global concentrations of methylchloroform (CH₃CCl₃) are increasing although at much lower rates than in the past. The ratio of concentrations in the two hemispheres has varied and is now declining, which reflects the constancy of the industrial emissions over the past 5 years. Observations show that the mid-latitude concentrations in both hemispheres are slightly lower during the summer than at other times, probably reflecting the greater removal of CH₃CCl₃ by OH radicals during summer. Calculations show that the lifetime of CH₃CCl₃ is about 6 (±1.5) years, which is considerably shorter than many previous estimates. It implies that there are probably 8 × 10⁵ molecules of OH/cm₃ of air, although this estimate may be uncertain by ±75%. The shorter lifetime is partly due to a revision of the estimated absolute concentration of CH₃CCl₃ in the atmosphere, which was found to be about 20% less than estimated previously. The relatively short lifetime suggests that in the future CH₃CCl₃ will contribute <15% of the anthropogenic chlorine in the troposphere, which is an approximate measure of its relative contribution to the depletion of the stratospheric ozone layer.     

Performance of an Innovative Two-Stage Process Converting Food Waste to Hydrogen and Methane

Abstract

This study was conducted to evaluate the performance of an innovative two-stage process, BIOCELL, that was developed to produce hydrogen (H₂) and methane (CH₄) from food waste on the basis of phase separation, reactor rotation mode, and sequential batch technique. The BIOCELL process consisted of four leaching-bed reactors for H₂ recovery and post-treatment and a UASB reactor for CH₄ recovery. The leaching-bed reactors were operated in a rotation mode with a 2-day interval between degradation stages. The sequential batch technique was useful to optimize environmental conditions during H₂ fermentation. The BIOCELL process demonstrated that, at the high volatile solids (VS) loading rate of 11.9 kg/m³-day, it could remove 72.5% of VS and convert VS_removed to H₂ (28.2%) and CH₄ (69.9%) on a chemical oxygen demand (COD) basis in 8 days. H₂ gas production rate was 3.63 m³/m³ ·day, while CH₄ gas production rate was 1.75 m³/m³ ·day. The yield values of H₂ and CH₄ were 0.31 and 0.21 m³/kg VS_added, respectively. Moreover, the output from the post-treatment could be used as a soil amendment. The BIOCELL process proved to be stable, reliable, and effective in resource recovery as well as waste stabilization.     

Atmospheric methylchloride (CH3Cl)

A global time series of atmospheric methylchloride (CH₃Cl) concentrations is reported showing the variability with latitude and season. Springtime concentrations of CH₃Cl at 45°N latitude are 7.5%±3.5% higher than during other seasons. Lowest concentrations were generally observed during fall. CH₃Cl was found to be more abundant by about 6% (±5% NH, ±3% SH) in the tropical regions of both hemispheres. No significant difference in the burdens of CH₃Cl was observed between the two hemispheres. Based on these data the ratio of the average hydroxyl radical (HO) concentration over the southern hemisphere to that over the northern hemisphere was estimated to be 1.3 or less.     

Woody stem methane emission in mature wetland alder trees

Methane (CH₄) is an important greenhouse gas that is predominantly emitted to the atmosphere from anoxic wetland ecosystems. Understanding the sources and emissions of CH₄ is crucially important for climate change predictions; however, there are significant discrepancies between CH₄ source estimates derived via so-called bottom-up and top-down methods. Here we report CH₄ emission from the stems of mature wetland alder (Alnus glutinosa) trees in the UK, a common tree of northern hemisphere floodplains and wetlands. The alder stems most likely behave as conduits for soil-produced CH₄ either in the gaseous or aqueous phase, and may, therefore, help to reconcile methodological differences in the way the wetland CH₄ source is estimated.Alder tree stems emitted average peak CH₄ fluxes of 101 μg CH₄ m^?2 h^?1 (on a stem area basis) in early October, a rate that is similar to that obtained from mature Japanese ash (Fraxinus mandshurica var. japonica) in Japan and amounting to approximately 20% of the measured CH₄ flux from the soil surface. The finding suggests that trees, which occupy 60% of Earth's wetlands and are normally excluded from the measurement programmes that form the basis for bottom-up estimates of the global wetland source, could be important contributors to overall terrestrial ecosystem CH₄ flux.     

Catalytic Destruction of Dichloromethane Using Perovskite-Type Oxide Catalysts

Abstract

Dichloromethane (DCM, also known as methylene chloride [CH₂Cl₂]) is often present in industrial waste gas and is a valuable chemical product in the chemical industry. This study addresses the oxidation of airstreams that contain CH₂Cl₂ by catalytic oxidation in a tubular fixed-bed reactor over perovskite-type oxide catalysts. This work also considers how the concentration of influent CH₂Cl₂ (C_o = 500-1000 ppm), the space velocity (GHSV = 5000-48,000 1/hr), the relative humidity (RH = 10-70%) and the concentration of oxygen (O₂ = 5-21%) influence the operational stability and capacity for the removal of CH₂Cl_2.

The surface area of lanthanum (La)-cobalt (Co) composite catalyst was the greatest of the five perovskite-type catalysts prepared in various composites of La, strontium, and Co metal oxides. Approximately 99.5% CH₂Cl₂ reduction was achieved by the catalytic oxidation over La-CoO₃-based perovskite catalyst at 600 °C. Furthermore, the effect of the initial concentration and reaction temperature on the removal of CH₂Cl₂ in the gaseous phase was also monitored. This study also provides information that a higher humidity corresponds to a lower conversion. Carbon dioxide and hydrogen chloride were the two main products of the oxidation process at a relative humidity of 70%.     

Methane in the Atmosphere

Methane is present in the troposphere with a volume concentration of about 1.5 ppm. Estimates of Koyama (1963) indicate a predominantly biological origin with a total production rate of about 2.7 × 10¹⁴ g CH₄/yr. From that he estimated the atmospheric lifetime of methane to be around 20 years. Measurements of the C-l4 in methane by Libby and later by Bainbridge, et al. (1961 ) gave a C-l4 content of 75% of recent wood and, therefore, confirm the predominant biological origin, the addition of inactive CH; from industrial sources being only about 25%. Much less is known about sinks of CH₄. Cadle (1966) reported fairly high destruction rates by atomic O, a reaction which should be important at high altitude. Bainbridge (1966) indeed reports a decrease in the measured methane concentration above the tropopause. He, however, considers this decrease too small to account for the destruction rate of 20 years estimated by Koyama. Our measurements on air samples collected on aircraft flights at various altitudes show a high variability of the CH₄ content both with time and altitude.     

The Statistical Form for the Ambient Participate Standard Annual Arithmetic Mean vs Annual Geometric Mean

Abstract

The long-term stability of a biofilter loaded with waste gases containing NH₃ concentrations larger than 100 ppmv was studied in a laboratory-scale compost reactor. At an empty bed residence time (τ) of 21 sec, elimination capacities of more than 300 g NH₃/m³/day were obtained at elimination efficiencies up to 87%. Because of absorption and nitrification, almost 80% of the NH₃-N eliminated from the waste gas could be recovered in the compost as NH₄+-N or NO₂ ^?/NO₃ ^?-N. The high elimination capacities could be maintained as long as the NH₄+/NO_x concentration in the carrier material was less than 4 g NH₄+/NO_x ^?-N/kg wet compost. Above this critical value, osmotic effects inhibited the nitrifying activity, and the elimination capacity for NH₃ decreased. To restore the biofilter performance, a carbon source (methanol) was added to reduce NH₄+/NO_x ^? accumulated in the compost. Results indicate that methylotrophic microorganisms did convert NH₄+/NO_x ^? into biomass, as long as the NO₃ ^? content in the compost was larger than 0.1 g NO₃ ^?-N/kg compost. Removal efficiencies of CH₃OH of more than 90% were obtained at volumetric loads up to 11,000 g CH₃OH/m³/day. It is shown that addition of CH₃OH is a suitable technique for regenerating the compost material from osmotic inhibition as a result of high NH₃ loading. The biofilter was operated for 4 months with alternating loading of NH₃ and CH₃OH.     

Assessing the uncertainty associated with national greenhouse gas emission inventories:: a case study for Austria

The uncertainty associated with the Austrian Greenhouse Gas emission inventory has been determined for the gases CO₂, CH₄ and N₂O and for the overall greenhouse potential. Expert interviews were conducted to obtain uncertainties in inventory input data. Based on these interviews, error distributions were developed and combined using Monte-Carlo analysis. Results for all sources and gases combined indicate an overall uncertainty between 10.5% and 12% depending on the base year considered. Excluding emissions and the uncertainty associated with forest sinks and natural sources, overall uncertainty decreased by 2% points. The mere ‘random error’, which is considered the level of uncertainty to be achieved with the current methodology (excluding all systematic errors) is 5% points lower. Detailed evaluation shows that much of the overall uncertainty derives from a lack of understanding the processes associated with N₂O emissions from soils. Other important contributors to GHG emission uncertainties are CH₄ from landfills and forests as CO₂ sinks. The uncertainty of the trend has been determined at near 5% points, with solid waste production (landfills) having the strongest contribution. Theoretical considerations do not permit a decrease of the trend uncertainty—even when forest sinks are not considered—below 3% points.     

Snowpack processing of acetaldehyde and acetone in the Arctic atmospheric boundary layer

Acetaldehyde (CH₃CHO) and acetone (CH₃C(O)CH₃) concentrations in ambient air, in snowpack air, and bulk snow were determined at Alert, Nunavut, Canada, as a part of the Polar Sunrise Experiment (PSE): ALERT 2000. During the period of continuous sunlight, vertical profiles of ambient and snowpack air exhibited large concentration gradients through the top ∼10 cm of the snowpack, implying a flux of carbonyl compounds from the surface to the atmosphere. From vertical profile and eddy diffusivity measurements made simultaneously on 22 April, acetaldehyde and acetone fluxes of 4.2(±2.1)×10⁸ and 6.2(±4.2)×10⁸ molecules cm⁻² s⁻¹ were derived, respectively. For this day, the sources and sinks of CH₃CHO from gas phase chemistry were estimated. The result showed that the snowpack flux of CH₃CHO to the atmosphere was as large as the calculated CH₃CHO loss rate from known atmospheric gas phase reactions, and at least 40 times larger (in the surface layer) than the volumetric rate of acetaldehyde produced from the assumed main atmospheric gas phase reaction, i.e. reaction of ethane with hydroxyl radicals. In addition, acetaldehyde bulk snow phase measurements showed that acetaldehyde was produced in or on the snow phase, likely from a photochemical origin. The time series for the observed CH₃C(O)CH₃, ozone (O₃), and propane during PSE 1995, PSE 1998, and ALERT 2000 showed a consistent anti-correlation between acetone and O₃ and between acetone and propane. However, our data and model simulations showed that the acetone increase during ozone depletion events cannot be explained by gas phase chemistry involving propane oxidation. These results suggest that the snowpack is a significant source of acetaldehyde and acetone to the Arctic boundary layer.