Electricity from Fossil Fuels without CO2 Emissions: Assessing the Costs of Carbon Dioxide Capture and Sequestration in U.S. Electricity Markets

ABSTRACT

The decoupling of fossil-fueled electricity production from atmospheric CO₂ emissions via CO₂ capture and sequestration (CCS) is increasingly regarded as an important means of mitigating climate change at a reasonable cost. Engineering analyses of CO₂ mitigation typically compare the cost of electricity for a base generation technology to that for a similar plant with CO₂ capture and then compute the carbon emissions mitigated per unit of cost. It can be hard to interpret mitigation cost estimates from this plant-level approach when a consistent base technology cannot be identified. In addition, neither engineering analyses nor general equilibrium models can capture the economics of plant dispatch. A realistic assessment of the costs of carbon sequestration as an emissions abatement strategy in the electric sector therefore requires a systems-level analysis. We discuss various frameworks for computing mitigation costs and introduce a simplified model of electric sector planning. Results from a “bottom-up” engineering-economic analysis for a representative U.S. North American Electric Reliability Council (NERC) region illustrate how the penetration of CCS technologies and the dispatch of generating units vary with the price of carbon emissions and thereby determine the relationship between mitigation cost and emissions reduction.     

Concentrations of carbon gases and oxygen and their emission ratios from the combustion of rice hulls in a wind tunnel

Rice hulls are widely burnt in agricultural fields in Asia because it is difficult to find other uses for them. Farmers burn rice hulls usually under incomplete combustion conditions to avoid accidental fires. In this study we investigated carbon gas emissions from rice hull fires at controlled wind speeds in a wind tunnel to clarify the effect of wind on such fires. Burning of the rice hulls resulted in relatively incomplete combustion: the ratio of [CO] to [CO₂] was high, >0.2, except when burning occurred at high wind speeds. Distinct differences in the carbon ratios of emitted carbon gases (CO₂, CO, CH₄, and nonmethane volatile organic compounds [NMVOC]) were found between high and low wind speeds: at high wind speeds, flames were usually present, and the CO₂ contribution to total carbon gases was higher; at low wind speeds, the NMVOC and CH₄ contributions to total carbon gases were greater. Therefore, a compensatory relationship existed between NMVOC and CH₄ and CO₂. Additionally, the ratio of [consumed O₂] to [CO₂] was <1 during the smoldering phase of combustion and >1 during the charcoal phase, synchronous with changes in [CH₄] and [NMVOC].     

Emissions of Sulfur Trioxide from Coal-Fired Power Plants

Abstract

Emissions of sulfur trioxide (SO₃) are a key component of plume opacity and acid deposition. Consequently, these emissions need to be low enough to not cause opacity violations and acid deposition. Generally, a small fraction of sulfur (S) in coal is converted to SO₃ in coal-fired combustion devices such as electric utility boilers. The emissions of SO₃ from such a boiler depend on coal S content, combustion conditions, flue gas characteristics, and air pollution devices being used. It is well known that the catalyst used in the selective catalytic reduction (SCR) technology for nitrogen oxides control oxidizes a small fraction of sulfur dioxide in the flue gas to SO₃. The extent of this oxidation depends on the catalyst formulation and SCR operating conditions. Gas-phase SO₃ and sulfuric acid, on being quenched in plant equipment (e.g., air preheater and wet scrubber), result in fine acidic mist, which can cause increased plume opacity and undesirable emissions. Recently, such effects have been observed at plants firing high-S coal and equipped with SCR systems and wet scrubbers. This paper investigates the factors that affect acidic mist production in coal-fired electric utility boilers and discusses approaches for mitigating emission of this mist.     

The stable carbon isotope composition of PM2.5 and PM10 in Mexico City Metropolitan Area air

The sources and distribution of carbon in ambient suspended particles (PM_2.5 and PM₁₀) of Mexico City Metropolitan Area (MCMA) air were traced using stable carbon isotopes (¹³C/¹²C). Tested potential sources included rural and agricultural soils, gasoline and diesel, liquefied-petroleum gas, volcanic ash, and street dust. The complete combustion of LP gas, diesel and gasoline yielded the lightest δ¹³C values (?27 to ?29‰ vs. PDB), while street dust (PM₁₀) represented the isotopically heaviest endmember (?17‰). The δ¹³C values of rural soils from four geographically separated sites were similar (?20.7 ± 1.5‰). δ¹³C values of particles and soot from diesel and gasoline vehicle emissions and agricultural soils varied between ?23 and ?26‰. Ambient PM samples collected in November of 2000, and March and December of 2001 at three representative receptor sites of industrial, commercial and residential activities had a δ¹³C value centered around ?25.1‰ in both fractions, resulting from common carbon sources. The predominant carbon sources to MCMA atmospheric particles were hydrocarbon combustion (diesel and/or gasoline) and particles of geological origin. The significantly depleted δ¹³C values from the industrial site reflect the input of diesel combustion by mobile and point source emissions. Based on stable carbon isotope mass balance, the carbon contribution of geological sources at the commercial and residential sites was approximately 73% for the PM₁₀ fraction and 54% for PM_2.5. Although not measured in this study, biomass-burning emissions from nearby forests are an important carbon source characterized by isotopically lighter values (?29‰), and can become a significant contributor (67%) of particulate carbon to MCMA air under the prevalence of southwesterly winds. Alternative sources of these ¹³C-depleted particles, such as cooking fires and municipal waste incineration, need to be assessed. Results show that stable carbon isotope measurements are useful for distinguishing between some carbon sources in suspended particles to MCMA air, and that wind direction has an impact on the distribution of carbon sources in this basin.     

Adsorption of elemental mercury vapors from synthetic exhaust combustion gas onto HGR carbon

An activated carbon commercially available named HGR, produced by Calgon-Carbon Group, was used to adsorbe metallic mercury. The work is part of a wider research activity by the same group focused on the removal of metallic and divalent mercury from combustion flue gas. With respect to previously published papers, this one is aimed at studying in depth thermodynamic equilibria of metallic mercury adsorption onto a commercial activated carbon. The innovativeness lies in the wider operative conditions explored (temperature and mercury concentrations) and in the evaluation of kinetic and thermodynamic data for a commercially available adsorbing material. In detail, experimental runs were carried out on a laboratory-scale plant, in which Hg° vapors were supplied in a nitrogen gas stream at different temperature and mercury concentration. The gas phase was flowed through a fixed bed of adsorbent material. Adsorbate loading curves for different Hg° concentrations together with adsorption isotherms were achieved as a function of temperature (120, 150, 200°C) and Hg° concentrations (1.0?7.0 mg/m³). Experimental runs demonstrated satisfying results of the adsorption process, while Langmuir parameters were evaluated with gas–solid equilibrium data. Especially, they confirmed that adsorption capacity is a favored process in case of lower temperature and they showed that the adsorption heat was –20 kJ/mol. Furthermore, a numerical integration of differential equations that model the adsorption process was proposed. Scanning electron microscopy (SEM) investigation was an useful tool to investigate about fresh and saturated carbon areas. The comparison between them allowed identification of surface sites where mercury is adsorbed; these spots correspond to carbon areas where sulfur concentration is greater.

Implications: Mercury compounds can cause severe harm to human health and to the ecosystem. There are a lot of sources that emit mercury species to the atmosphere; the main ones are exhaust gases from coal combustion and municipal solid waste incineration. Furthermore, certain CO₂ capture processes, particularly oxyfuel combustion in a pulverized fuel coal-fired power station, produce a raw CO₂ product containing several contaminants, mainly water vapor, oxygen, and nitrogen but also mercury, that have to be almost completely removed; otherwise these would represent a strong drawback to the success of the process.     

Rates of Conversion of Sulfur Dioxide to Sulfate in a Scrubbed Power Plant Plume

ABSTRACT

The rate of conversion of SO₂ to SO₄ ^2- was re-estimated from measurements made in the plume of the Cumberland power plant, located on the Cumberland River in north-central Tennessee, after installation of flue gas desulfurization (FGD) scrubbers for SO₂ removal in 1994. The ratio of SO₂ to NO_y emissions into the plume has been reduced to ~0.1, compared with a prescrubber value of ~2. To determine whether the SO₂ emissions reduction has correspondingly reduced plume-generated particulate SO₄ ^2- production, we have compared the rates of conversion before and after scrubber installation. The prescrubber estimates were developed from measurements made during the Tennessee Plume Study conducted in the late 1970s. The post-scrubber estimates are based upon two series of research flights in the summers of 1998 and 1999. During two of these flights, the Cumberland plume did not mix with adjacent power plant plumes, enabling rate constants for conversion to be estimated from samples taken in the plume at three downwind distances. Dry deposition losses and the fact the fact that SO₂ is no longer in large excess compared with SO₄ ^2- have been taken into account, and an upper limit for the conversion rate constant was re-estimated based on plume excess aerosol volume. The estimated upper limit values are 0.069 hr^-1 and 0.034 hr^-1 for the 1998 and 1999 data, respectively. The 1999 rate is comparable with earlier values for nonscrubbed plumes, and although the 1998 upper limit value is higher than expected, these estimates do not provide strong evidence for deviation from a linear relationship between SO₂ emissions and SO₄ ^2- formation.     

Stable carbon isotope ratio in atmospheric CO2 collected by new diffusive devices

In this paper, stable carbon isotope ratios (δ ¹³C) were determined in the atmosphere by using a Ca-based sorbent, CaO/Ca₁₂Al₁₄O₃₃ 75:25 w/w, for passively collecting atmospheric CO₂, in both field and laboratory experiments. Field measurements were conducted in three environments characterized by different carbon dioxide sources. In particular, the environments under consideration were a rather heavily trafficked road, where the source of CO₂ is mostly vehicle exhaust, a rural unpolluted area, and a private kitchen where the major source of CO₂ was gas combustion. Samplers were exposed to the free atmosphere for 3 days in order to allow collection of sufficient CO₂ for δ¹³C analysis, then the collected CO₂ was desorbed from the adsorbent with acid treatment, and directly analyzed by nondispersive infrared (NDIR) instrument. δ ¹³C results confirmed that the samplers collected representative CO₂ samples and no fractionation occurred during passive trapping, as also confirmed by an appositely designed experiment conducted in the laboratory. Passive sampling using CaO/Ca₁₂Al₁₄O₃₃ 75:25 w/w proved to be an easy and reliable method to collect atmospheric carbon dioxide for δ ¹³C analysis in both indoor and outdoor places.     

Emissions and risks associated with oxyfuel combustion: State of the science and critical data gaps

Oxyfuel combustion is a promising technology that may greatly facilitate carbon capture and sequestration by increasing the relative CO₂ content of the combustion emission stream. However, the potential effect of enhanced oxygen combustion conditions on emissions of criteria and hazardous air pollutants (e.g., acid gases, particulates, metals and organics) is not well studied. It is possible that combustion under oxyfuel conditions could produce emissions posing different risks than those currently being managed by the power industry (e.g., by changing the valence state of metals). The data available for addressing these concerns are quite limited and are typically derived from laboratory-scale or pilot-scale tests. A review of the available data does suggest that oxyfuel combustion may decrease the air emissions of some pollutants (e.g., SO₂, NOx, particulates) whereas data for other pollutants are too limited to draw any conclusions. The oxy-combustion systems that have been proposed to date do not have a conventional “stack” and combustion flue gas is treated in such a way that solid or liquid waste streams are the major outputs. Use of this technology will therefore shift emissions from air to solid or liquid waste streams, but the risk management implications of this potential change have yet to be assessed. Truly useful studies of the potential effects of oxyfuel combustion on power plant emissions will require construction of integrated systems containing a combustion system coupled to a CO₂ processing unit. Sampling and analysis to assess potential emission effects should be an essential part of integrated system tests.

Implications: Oxyfuel combustion may facilitate carbon capture and sequestration by increasing the relative CO₂ content of the combustion emission stream. However, the potential effect of enhanced oxygen combustion conditions on emissions of criteria and hazardous air pollutants has not been well studied. Combustion under oxyfuel conditions could produce emissions posing different risks than those currently being managed by the power industry. Therefore, before moving further with oxyfuel combustion as a new technology, it is appropriate to summarize the current understanding of potential emissions risk and to identify data gaps as priorities for future research.     

活性炭吸附和脱附-等离子体氧化净化有机废气的研究

根据滑动弧放电等离子体适于降解高浓度有机物废气的特性,结合活性炭吸附法,提出了吸附器的吸附浓缩和热脱附-等离子体氧化净化有机废气的方法。在活性炭吸附过程中,最初2 h内甲苯净化率达到100%,随着时间的增加净化率下降;在热脱附滑动弧放电等离子体净化过程中,甲苯降解效率最高为97.3%。将滑动弧放电等离子体反应器出口气相产物收集进行FT-IR检测,发现放电后有CO2、CO、H2O和NO2产生,并分析了甲苯的降解机理。     

Smoke emissions from biomass burning in a Mediterranean shrubland

Gaseous and particulate samples from the smoke from prescribed burnings of a shrub-dominated forest with some pine trees in Lousã Mountain, Portugal, in May 2008, have been collected. From the gas phase Fourier transform infrared (FTIR) measurements, an average modified combustion efficiency of 0.99 was obtained, suggesting a very strong predominance of flaming combustion. Gaseous compounds whose emissions are promoted in fresh plumes and during the flaming burning phase, such as CO₂, acetylene and propene, produced emission factors higher than those proposed for savannah and tropical forest fires. Emission factors of species that are favoured by the smouldering phase (e.g. CO and CH₄) were below the values reported in the literature for biomass burning in other ecosystems. The chemical composition of fine (PM_2.5) and coarse (PM_2.5–10) particles was achieved using ion chromatography (water-soluble ions), instrumental neutron activation analysis (trace elements) and a thermal–optical transmission technique (organic carbon and elemental carbon). Approximately 50% of the particulate mass was carbonaceous in nature with a clear dominance of organic carbon. The organic carbon-to-elemental carbon ratios up to 300, or even higher, measured in the present study largely exceeded those reported for fires in savannah and tropical forests. More than 30 trace elements and ions have been determined in smoke aerosols, representing in total an average contribution of about 7% to the PM₁₀ mass.     

Nitrogen oxides emissions from the MILD combustion with the conditions of recirculation gas

The nitrogen oxides (NO_x) reduction technology by combustion modification which has economic benefits as a method of controlling NO_x emitted in the combustion process, has recently been receiving a lot of attention. Especially, the moderate or intense low oxygen dilution (MILD) combustion which applied high temperature flue gas recirculation has been confirmed for its effectiveness with regard to solid fuel as well. MILD combustion is affected by the flue gas recirculation ratio and the composition of recirculation gas, so its NO_x reduction efficiency is determined by them. In order to investigate the influence of factors which determine the reduction efficiency of NO_x in MILD coal combustion, this study changed the flow rate and concentration of nitrogen (N₂), carbon dioxide (CO₂) and steam (H₂O) which simulate the recirculation gas during the MILD coal combustion using our lab-scale drop tube furnace and performed the combustion experiment. As a result, its influence by the composition of recirculation gas was insignificant and it was shown that flue gas recirculation ratio influences the change of NO_x concentration greatly. Implications: We investigated the influence of factors determining the nitrogen oxides (NO_x) reduction efficiency in MILD coal combustion, which applied high-temperature flue gas recirculation. Using a lab-scale drop tube furnace and simulated recirculation gas, we conducted combustion testing changing the recirculation gas conditions. We found that the flue gas recirculation ratio influences the reduction of NO_x emissions the most.     

Effects of biodiesel made from swine and chicken fat residues on carbon monoxide,carbon dioxide,and nitrogen oxide emissions

The effects of two alternative sources of animal fat-derived biodiesel feedstock on CO₂, CO, NO_x tailpipe emissions as well as fuel consumption were investigated. Biodiesel blends were produced from chicken and swine fat waste (FW-1) or floating fat (FW-2) collected from slaughterhouse wastewater treatment processes. Tests were conducted in an unmodified stationary diesel engine operating under idling conditions in attempt to simulate slow traffic in urban areas. Significant reductions in CO (up to 47% for B100; FW-2) and NO_x (up to 20% for B5; FW-2 or B100; FW-1) were attained when using biodiesel fuels at the expense of 5% increase in fuel consumption. Principal component analysis (PCA) was performed to elucidate possible associations among gas (CO₂, CO, and NO_x) emissions, cetane number and iodine index with different sources of feedstock typically employed in the biodiesel industry. NO_x, cetane number and iodine index were inversely proportional to CO₂ and biodiesel concentration. High NO_x emissions were reported from high iodine index biodiesel derived especially from forestry, fishery and some agriculture feedstocks, while the biodiesel derived from animal sources consistently presented lower iodine index mitigating NO_x emissions. The obtained results point out the applicability of biodiesel fuels derived from fat-rich residues originated from animal production on mitigation of greenhouse gas emissions. The information may encourage practitioners from biodiesel industry whilst contributing towards development of sustainable animal production.

Implications: Emissions from motor vehicles can contribute considerably to the levels of greenhouse gases in the atmosphere. The use of biodiesel to replace or augment diesel can not only decrease our dependency on fossil fuels but also help decrease air pollution. Thus, different sources of feedstocks are constantly being explored for affordable biodiesel production. However, the amount of carbon monoxide (CO), carbon dioxide (CO₂), and/or nitrogen oxide (NO_x) emissions can vary largely depending on type of feedstock used to produce biodiesel. In this work, the authors demonstrated animal fat feasibility in replacing petrodiesel with less impact regarding greenhouse gas emissions than other sources.     

Plume analysis from field evaluations of a portable air quality monitoring system

ABSTRACT

Near-road measurements in Rochester, NY with a Portable Air Quality Monitoring System indicate a significant plume control of PM_2.5 black carbon (BC) concentrations. This study evaluates the performance of two portable air quality enclosures deployed at collocated research sites to determine their accuracy and usefulness in field deployments, and specifically in pollution plume analysis. One system deployed collocated sensors for measurement of particulate matter mass concentration (Thermo pDR 1500 against Tapered Element Oscillating Microbalance (TEOM) measurement) and the second system deployed sensors for measurement of black carbon (Magee AE33 aethalometer and Brechtel Tricolor Absorption Photometer) in ambient and near-road locations in Rochester, New York, respectively. While the optical PM_2.5 sensors tended to be biased in their determination of concentration by ~15%, they followed changes and trends in concentration very well. The black carbon sensors in the portable systems agreed very well with each other and with the collocated sensor. As a case study to determine the contribution from statistically significant short-lived excursions of pollutant concentration, Morlet wavelet analysis was performed on data from the portable system sensors. Black carbon was found to be strongly influenced by plume behavior with significant plume excursions representing just over 12% of all data points and contributing on average 1 µg/m³ of black carbon above ambient concentrations.

Implications: This paper first evaluates two air pollutant monitoring enclosures with wide applicability including near-road detection of pollutants. Then, we present a novel method to designate isolate statistically significant excursions in air pollution concentration which can be used to determine the impact of pollutant plumes as observed in PM and black carbon behavior near road.     

The change of gaseous carbon fluxes following the switch of dominant producers from macrophytes to algae in a shallow subtropical lake of China

Successions of lake ecosystems from clear-water, macrophyte-rich conditions into turbid states with abundant phytoplankton have taken place in many shallow lakes in China. However, little is know about the change of carbon fluxes in lakes during such processes. We conducted a case study in Lake Biandantang to investigate the change of carbon fluxes during such a regime shift. Dissolved aquatic carbon and gaseous carbon (methane (CH₄) and carbon dioxide (CO₂)) across air–water interface in three sites with different vegetation covers and compositions were studied and compared. CH₄ emissions from three sites were 0.62±0.36, 0.70±0.36, and 1.31±0.57 mg m⁻² h⁻¹, respectively. Correlation analysis showed that macrophytes, rather than phytoplankton, directly positively affected CH₄ emission. CO₂ fluxes of three sites in Lake Biandantang were significantly different, and the average values were 77.8±20.4, 52.2±14.1 and 3.6±26.8 mg m⁻² h⁻¹, respectively. There were an evident trend that the larger macrophyte biomass, the lower CO₂ emissions. Correlation analysis showed that in different sites, dominant plant controlled CO₂ flux across air–water interface. In a year cycle, the percents of gaseous carbon release from lake accounting for net primary production were significantly different (from 39.3% to 2.8%), indicating that with the decline of macrophytes and regime shift, the lake will be a larger carbon source to the atmosphere.     

Remote monitoring of air pollutant emissions from point sources by a mobile lidar/sodar system

This paper describes remote monitoring of air pollutant emissions by a mobile lidar (light detection and ranging)/ sodar (sound detection and ranging) system. First, measurements are carried out in the flue gas plume of a public power plant. The investigations focus mainly on quantifying SO2 emissions, but the uncertainties of such measurements are also emphasized. Furthermore, an example providing valuable data sets for the development and validation of plume dispersion models is outlined with measurements of the dilution of SO2 along the plume axis. Series of repeated determinations of SO2 emissions show a large variation in the obtained flux values, with moderate margins of error. Incomplete recording of the plume within the individual lidar scans, induced by strong looping movements of the flue gas plume, predominantly causes the variations of flux values. Therefore, the highest flux values determined are considered to be the most exact. This is verified by a comparison of measured fluxes with in situ measurements made by the plant operators. The results further indicate that lidar measurements illustrate the location and dimension of aerosol plumes better than the location and dimension of the plumes of gaseous compounds. The wind direction affecting the plume at any moment can be determined faster by lidar than by sodar because the latter requires much longer time intervals of signal averaging. Measurements show higher concentrations of SO2 compared with results from a Gaussian plume model for periods of less than 5 min after dispersion. The findings emphasize the suitability of remote sensing for detecting emissions and for investigating the propagation and dilution of air pollutant plumes.     

Effects of natural organic matter on the microporous sorption sites of black carbon in a Yangtze River sediment

Black carbon (BC), characterized by high microporosity and high specific surface area (SSA), has been demonstrated to have substantial contributions to the sorption of hydrophobic organic chemicals in soils and sediments. Other naturally occurring organic matters provide soft and penetrable sorption domains while may cling to BC and affect its original surface properties. In this work, we studied the sorption sites of a Yangtze River sediment sample with organic carbon (OC) content of 3.3 % and the preheated sediment (combusted at 375 °C) with reduced OC content (defined as BC) of 0.4 % by gas and pyrene sorption. The SSA and microporosity of the pristine and preheated sediments were characterized by N₂ and CO₂ adsorption. The results suggest that the adsorption of N₂ was hindered by amorphous organic carbon (AOC) in the pristine sediment but CO₂ was not. Instead, the uptake of CO₂ was higher in the presence of AOC, likely due to the partition of CO₂ molecules into the organic matter. The pyrene adsorptions to BC in pristine and preheated sediments show a similar adsorption capacity at high concentration, suggesting that AOC of ca. 2.9 % in the pristine sediment does not reduce the accessibility to the sorption sites on BC for pyrene.     

Pollution influences on atmospheric composition and chemistry at high northern latitudes: Boreal and California forest fire emissions

We analyze detailed atmospheric gas/aerosol composition data acquired during the 2008 NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) airborne campaign performed at high northern latitudes in spring (ARCTAS-A) and summer (ARCTAS-B) and in California in summer (ARCTAS-CARB). Biomass burning influences were widespread throughout the ARCTAS campaign. MODIS data from 2000 to 2009 indicated that 2008 had the second largest fire counts over Siberia and a more normal Canadian boreal forest fire season. Near surface arctic air in spring contained strong anthropogenic signatures indicated by high sulfate. In both spring and summer most of the pollution plumes transported to the Arctic region were from Europe and Asia and were present in the mid to upper troposphere and contained a mix of forest fire and urban influences. The gas/aerosol composition of the high latitude troposphere was strongly perturbed at all altitudes in both spring and summer. The reactive nitrogen budget was balanced with PAN as the dominant component. Mean ozone concentrations in the high latitude troposphere were only minimally perturbed (<5 ppb), although many individual pollution plumes sampled in the mid to upper troposphere, and mixed with urban influences, contained elevated ozone (ΔO₃/ΔCO = 0.11 ± 0.09 v/v). Emission and optical characteristics of boreal and California wild fires were quantified and found to be broadly comparable. Greenhouse gas emission estimates derived from ARCTAS-CARB data for the South Coast Air Basin of California show good agreement with state inventories for CO₂ and N₂O but indicate substantially larger emissions of CH₄. Simulations by multiple models of transport and chemistry were found to be broadly consistent with observations with a tendency towards under prediction at high latitudes.     

Alternative carbon dioxide modelling approaches accounting for high residual gases in LandGEM

High Canadian waste disposal rates necessitate landfill gas monitoring and accurate forecasting. CO₂ estimates in LandGEM version 3.02 currently rest on the assumptions that CO₂ is a function of CH₄, where the two gases make up nearly 100% of landfill gas content, leading to overestimated CO₂ collection estimates. A total of 25 cases (five formulas, five approaches) compared annual CO₂ collection at four western Canadian landfills. Despite common use in literature, the 1:1 ratio of CH₄ to CO₂ was not recommended to forecast landfill gas collection in cold climates. The existing modelling approach significantly overestimated CO₂ production in three of four sites, resulting in the highest residual sum of squares. Optimization resulted in the most accurate results for all formulas and approaches, which had the greatest reduction in residual sums of squares (RSS) over the default approach (60.1 to 97.7%). The 1.4 Ratio approach for L _o:L _o-CO2 yielded the second most accurate results for CO₂ flow (mean RSS reduction of 50.2% for all sites and subsection models). The annual k-modified LandGEM calculated k’s via two empirical formulas (based on precipitation) and yielded the lowest accuracy in 12 of 20 approaches. Unlike other studies, strong relationships between optimized annual k’s and precipitation were not observed.

     

Factors influencing ozone chemistry in subsonic aircraft plumes

This study investigates several factors that could influence ozone chemistry occurring in subsonic aircraft plumes in the upper troposphere. The study focuses on uncertainties in gas-phase rate parameters, but also examines the influence of selected heterogeneous reactions, the rate of expansion of the plume, ambient and initial plume concentrations, and the time of emissions. Monte Carlo analysis with Latin hypercube sampling was applied to an expanding box model of an aircraft plume, in order to estimate the sensitivities of O₃ perturbations (ΔO₃) to uncertainties in rate constants in the RADM2 chemical mechanism. The resulting coefficient of variation in ΔO₃ at the end of a 36 h simulation was about 50%. Influential uncertainties in gas-phase rate parameters include those for photolysis of NO₂ and HCHO, O₃+NO, HO₂+NO, and formation of PAN and HNO₃. With high background concentrations of non-methane hydrocarbons, uncertainties in rate parameters of reactions involving peroxy radicals from ethene and propene oxidation were also influential. The coefficient of variation for ΔO₃ due to uncertainties in emission indices of NO_x, CO, and organic compounds was less than 15%. The effects of the heterogeneous reaction of N₂O₅ leading to HNO₃ formation, and hypothesized reactions of HNO₃ and NO₂ on soot, were also investigated. The results suggest that the latter two reactions could be influential for ΔO₃ if published estimates of reaction probabilities and high estimates of soot concentrations in plumes are realistic.     

Application of gas diffusion biocathode in microbial electrosynthesis from carbon dioxide

Microbial catalysis of carbon dioxide (CO₂) reduction to multi-carbon compounds at the cathode is a highly attractive application of microbial electrosynthesis (MES). The microbes reduce CO₂ by either taking the electrons or reducing the equivalents produced at the cathode. While using gaseous CO₂ as the carbon source, the biological reduction process depends on the dissolution and mass transfer of CO₂ in the electrolyte. In order to deal with this issue, a gas diffusion electrode (GDE) was investigated by feeding CO₂ through the GDE into the MES reactor for its reduction at the biocathode. A combination of the catalyst layer (porous activated carbon and Teflon binder) and the hydrophobic gas diffusion layer (GDL) creates a three-phase interface at the electrode. So, CO₂ and reducing equivalents will be available to the biocatalyst on the cathode surface. An enriched inoculum consisting of acetogenic bacteria, prepared from an anaerobic sludge, was used as a biocatalyst. The cathode potential was maintained at ?1.1 V vs Ag/AgCl to facilitate direct and/or hydrogen-mediated CO₂ reduction. Bioelectrochemical CO₂ reduction mainly produced acetate but also extended the products to ethanol and butyrate. Average acetate production rates of 32 and 61 mg/L/day, respectively, with 20 and 80 % CO₂ gas mixture feed were achieved with 10 cm² of GDE. The maximum acetate production rate remained 238 mg/L/day for 20 % CO₂ gas mixture. In conclusion, a gas diffusion biocathode supported bioelectrochemical CO₂ reduction with enhanced mass transfer rate at continuous supply of gaseous CO₂.

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