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1.
The measurement of hydrochloric acid (HCl) on a continuous basis in coal-fired plants is expected to become more important if HCl standards become implemented as part of the Federal Mercury and Air Toxics Standards (MATS) standards that are under consideration. For this study, the operational performance of three methods/instruments, including tunable diode laser absorption spectroscopy (TDLAS), cavity ring down spectroscopy (CRDS), and Fourier transform infrared (FTIR) spectroscopy, were evaluated over a range of real-world operating environments. Evaluations were done over an HCl concentration range of 0–25 ppmv and temperatures of 25, 100, and 185 °C. The average differences with respect to temperature were 3.0% for the TDL for values over 2.0 ppmv and 6.9% of all concentrations, 3.3% for the CRDS, and 4.5% for the FTIR. Interference tests for H 2O, SO 2, and CO, CO 2, and NO for a range of concentrations typical of flue gases from coal-fired power plants did not show any strong interferences. The possible exception was an interference from H 2O with the FTIR. The instrument average precision over the entire range was 4.4% for the TDL with better precision seen for concentrations levels of 2.0 ppmv and above, 2.5% for the CRDS, and 3.5% for the FTIR. The minimum detection limits were all on the order of 0.25 ppmv, or less, utilizing the TDL values with a 5-m path. Zero drift was found to be 1.48% for the TDL, 0.88% for the CRDS, and 1.28% for the FTIR. Implications: This study provides an evaluation of the operational performance of three methods/instruments, including TDL absorption spectroscopy (TDLAS), cavity ring down spectroscopy (CRDS), and FTIR spectroscopy, for the measurement of hydrochloric acid (HCl) over a range of real-world operating environments. The results showed good instrument accuracy as a function of temperature and no strong interferences for flue gases typical to coal-fired power plants. The results show that these instruments would be viable for the measurement of HCl in coal-fired plants if HCl standards become implemented as part of the Federal Mercury and Air Toxics Standards (MATS) standards that are under consideration. 相似文献
2.
The primary goal of this paper is to reveal the reaction behavior of SO 2 in the sinter zone, combustion zone, drying–preheating zone, and over-wet zone during flue gas recirculation (FGR) technique. The results showed that SO 2 retention in the sinter zone was associated with free-CaO in the form of CaSO 3/CaSO 4, and the SO 2 adsorption reached a maximum under 900ºC. SO 2 in the flue gas came almost from the combustion zone. One reaction behavior was the oxidation of sulfur in the sintering mix when the temperature was between 800 and 1000ºC; the other behavior was the decomposition of sulfite/sulfate when the temperature was over 1000ºC. However, the SO 2 adsorption in the sintering bed mainly occurred in the drying–preheating zone, adsorbed by CaCO 3, Ca(OH) 2, and CaO. When the SO 2 adsorption reaction in the drying–preheating zone reached equilibrium, the excess SO 2 gas continued to migrate to the over-wet zone and was then absorbed by Ca(OH) 2 and H 2O. The emission rising point of SO 2 moved forward in combustion zone, and the concentration of SO 2 emissions significantly increased in the case of flue gas recirculation (FGR) technique. Implications: Aiming for the reuse of the sensible heat and a reduction in exhaust gas emission, the FGR technique is proposed in the iron ore sintering process. When using the FGR technique, SO2 emission in exhaust gas gets changed. In practice, the application of the FGR technique in a sinter plant should be cooperative with the flue gas desulfurization (FGD) technique. Thus, it is necessary to study the influence of the FGR technique on SO2 emissions because it will directly influence the demand and design of the FGD system. 相似文献
3.
A series of porous γ-Al 2O 3 materials was prepared by solution-combustion and ball-milling processes. The as-prepared powders were physicochemically characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and N 2 physisorption measurements and their performances in CO 2 adsorption at different pressures (0.5 to 1.5 MPa) and temperatures (40 to 60ºC) were investigated. It was found that γ-Al 2O 3 synthesized by the solution-combustion process and ball milled at 10 hr exhibited the best CO 2 adsorption performance at 60ºC and 1.5 MPa, achieving a maximum of 1.94 mmol/g compared to the four studied materials, as a result of their interesting microstructure and surface properties (i.e., nanocrystallinity, specific surface area, narrow pore size distribution, and large total pore volume). Our study shows that the γ-Al 2O 3 prepared by solution combustion followed by ball milling presents a fairly good potential adsorbent for efficient CO 2 capture. Implications: In this work, γ-Al2O3 materials were successfully obtained by solution combustion and modified via ball milling. These improved materials were systematically investigated as solid adsorbents of accessible surface areas, large pore volumes, and narrow pore size distribution for the CO2 capture. These studied solid adsorbents can provide an additional contribution and effort to develop an efficient CO2 capture method as means of alleviating the serious global warning problem. 相似文献
4.
ABSTRACT U.S. Environmental Protection Agency (EPA) Method 26A is the recommended procedure for capturing and speci-ating halogen (X 2) and hydrogen halide (HX) stack emissions from combustion sources. Previous evaluation studies of Method 26A have focused primarily on hydrogen chloride (HCl) speciation. Capture efficiency, bias, and the potential interference of Cl 2 at high levels (>20 ppm [u,g/m 3]) and NH 4Cl in the flue gas stream have been investigated. It has been suggested that precise Cl 2 measurement and accuracy in quantifying HX or X 2 using Method 26A are difficult to achieve at Cl 2 concentrations <5 ppm; however, no performance data exist to support this. Coal contains low levels of Cl, in the range of 5-2000 ppmw, which results in the presence of HCl and Cl 2 in the products of combustion. HCl is the predominant Cl compound formed in the high-temperature combustion process, and it persists in the gas as the products of combustion cool. Concentrations of Cl 2 in coal combustion flue gas at stack temperatures typically do not exceed 5 ppm. For this research, bench-scale experiments using simulated combustion flue gas were designed to validate the ability of Method 26A to speci-ate low levels of Cl 2 accurately. This paper presents the results of the bench-scale tests. The effect of various flue gas components is discussed. The results indicate that SO 2 is the only component in coal combustion flue gas that has an appreciable effect on Cl 2 distribution in Method 26A impingers, and that Method 26A cannot accurately speciate HCl and Cl 2 in coal combustion flue gas without modification. 相似文献
5.
A previously proposed technology incorporating TiO 2 into common household fluorescent lighting was further tested for its Hg 0 removal capability in a simulated flue-gas system. The flue gas is simulated by the addition of O 2, SO 2, HCl, NO, H 2O, and Hg 0, which are frequently found in combustion facilities such as waste incinerators and coal-fired power plants. In the O 2 + N 2 + Hg 0 environment, a Hg 0 removal efficiency (η Hg) greater than 95% was achieved. Despite the tendency for η Hg to decrease with increasing SO 2 and HCl, no significant drop was observed at the tested level (SO 2: 5–300 ppm v, HCl: 30–120 ppm v). In terms of NO and moisture, a significant negative effect on η Hg was observed for both factors. NO eliminated the OH radical on the TiO 2 surface, whereas water vapor caused either the occupation of active sites available to Hg 0 or the reduction of Hg 0 by free electron. However, the negative effect of NO was minimized (η Hg > 90%) by increasing the residence time in the photochemical reactor. The moisture effect can be avoided by installing a water trap before the flue gas enters the Hg 0 removal system. Implications: This paper reports a novel technology for a removal of gas-phase elemental mercury (Hg 0) from a simulated flue gas using TiO 2-coated glass beads under a low-cost, easily maintainable household fluorescent light instead of ultraviolet (UV) light. In this study, the effects of individual chemical species (O 2, SO 2, HCl, NO, and water vapor) on the performance of the proposed technology for Hg 0 removal are investigated. The result suggests that the proposed technology can be highly effective, even in real combustion environments such as waste incinerators and coal-fired power plants. 相似文献
6.
The research team analyzed the emission characteristics of gaseous pollutants, including volatile organic compounds (VOCs), from biomass combustion in improved stoves in rural China. The research included measurements from five biofuels and two stove types in the months of January, April, and September. The measurements were conducted according to U.S. EPA Method 25 using a collection system with a cooling device and two-level filters. CO, CO 2, NO x, CH 4 and THC analyzers were used for in-field, real-time emission measurements. The emission data indicate that gaseous pollutants were emitted at higher concentrations in the early combustion stage and lower concentrations in the later stage. CH 4 and THC, as well as CO and CO 2, presented positive relationships during the whole entire combustion process for all tests. The chemical profiles of flue gas samples were analyzed by GC/MS and GC/FID/ECD. Aromatics, carbonyls, and alkenes & alkynes dominated the VOC emissions, respectively accounting for 37%, 33%, and 23% of total VOC emissions by volume. Benzene was the most abundant VOC species, consisting of 17.3 ± 8.1% of VOCs, followed by propylene (11.3 ± 3.5%), acetone (10.8 ± 8.2%), toluene (7.3 ± 5.7%) and acetaldehyde (6.5 ± 7.3%). Carbon mass balance approach was applied to calculate CO, CO 2, CH 4, NO x, and VOC species emission factors. This analysis includes a discussion of the differences among VOC emission factors of different biofuel-stove combinations. 相似文献
7.
The following report discusses current bench- and pilot-plant advances in preparation of ADVAnced siliCATE (ADVACATE) calcium silicate sorbents for flue gas desulfurization. It also discusses current bench- and pilot-plant advances in sorbent preparation. Fly ash was ground in a laboratory scale grinder prior to slurring in order to decrease the slurring time needed for the sorbent to be reactive with SO 2. Reactivity of ADVACATE sorbents with SO 2 in the bench-scale reactor correlated with their surface area. ADVACATE sorbents produced with ground fly ash were evaluated in the 50 cfm (85 m 3/h) pilot plant providing 2 s duct residence time. ADVACATE sorbent was produced by slurrying ground fly ash (median particle size of 4.3 µm) with Ca(OH) 2 at the weight ratio of 3:1 at 90°C (194°F) for 3hto yield solids with 30 weight percent of initial free moisture. When this sorbent was injected into the duct with 1500 ppm SO 2 and at 11°C (20°F) approach to saturation, the measured SO 2 removal was approximately 60percent at a Ca/S stoichiometric ratio of 2. Previously, when ADVACATE sorbent was produced at 90°C (194°F) and at the same fly-ash-to-Ca(OH) 2 weight ratio using unground fly ash, removal under the same conditions in the duct was approximately 50 percent following 12 h slurring. The report presents the results of pilot-scale recycle tests at the recycle ratio of 2. Finally, the report discusses future U.S. Environmental Protection Agency plans for commercialization of ADVACATE. 相似文献
8.
A procedure was developed for the 24-h determination of SO 2 and CO 2 in effluent gas from fossil fuel combustion sources. Laboratory experiments were conducted to test absorption of SO 2 in hydrogen peroxide solution and absorption of CO 2 by sodium hydroxide on an inert substrate at expected ambient temperatures of 15 to 45°C. Isopropyl alcohol cannot be used to trap sulfuric acid and particulates because it permeates the sampling train and prevents complete absorption of CO 2. Elemental analysis of stack particulates revealed that at least 31 elements were present. Iron and other elements interfered with SO 2 analysis. These particulates were completely removed by a heated borosilicate glass filter. Both laboratory and field experiments showed that molecular sieves are a promising alternative for CO 2 absorption. Statistical evaluation of data collected at three units equipped with flue gas desulfurization scrubbers proved that the new procedure is accurate and precise. 相似文献
9.
Potassium-fly ash (K-FA) sorbents were investigated for high-temperature CO 2 sorption. K-FAs were synthesised using coal fly ash as source of silica and aluminium. The synthesised materials were also mixed with Li 2CO 3 and Ca(OH) 2 to evaluate their effect on CO 2 capture. Temperature strongly affected the performance of the K-FA sorbents, resulting in a CO 2 uptake of 1.45 mmol CO 2/g sorbent for K-FA 1:1 at 700 °C. The CO 2 sorption was enhanced by the presence of Li 2CO 3 (10 wt%), with the K-FA 1:1 capturing 2.38 mmol CO 2/g sorbent at 700 °C in 5 min. This sorption was found to be similar to previously developed Li-Na-FA (2.54 mmol/g) and Li-FA (2.4 mmol/g) sorbents. The presence of 10 % Li 2CO 3 also accelerated sorption and desorption. The results suggest that the increased uptake of CO 2 and faster reaction rates in presence of K-FA can be ascribed to the formation of K-Li eutectic phase, which favours the diffusion of potassium and CO 2 in the material matrix. The cyclic experiments showed that the K-FA materials maintained stable CO 2 uptake and reaction rates over 10 cycles. 相似文献
10.
1,2,3,4-T 4BrDD was pyrolyzed with PVC, HCl and NaCl at 900°C. In all reactions bromine was exchanged with chlorine. During the pyrolysis with PVC at T = 900°C, 69 % of the starting material reacted to give BrCl-dioxins (34.3 %) and 1,2,3,4-T 4CDD (34.7 %). In the reaction with HCl (T = 900°C), 49.2 % BrCl-dioxins and 1,2,3,4-T 4CDD formed. The pyrolysis with NaCl (T = 900°C) yielded 10.5 % BrCl-dioxins and 1,2,3,4-T 4CDD. 相似文献
11.
The interaction of a typical flue gas with active charcoal and bituminous coal char at temperatures between 600 and 800°C and atmospheric pressure has been studied. The SO 2 in the flue gas interacts with the carbon to form primarily H 2S, COS, and a carbon-sulfur surface complex. H 2S and COS break through the carbon bed much in advance of SO 2. At 800°C, sulfur retention on the bed exceeds at least 11% before SO 2 breakthrough occurs. The reaction of H 2S and COS with O 2 over active charcoal at 100–140°C to produce sulfur, which deposits on the carbon, has also been studied and found to be feasible. As a result of this study, a new process is outlined for the removal of SO 2 from flue gas, with the ultimate conversion 相似文献
12.
Abstract Combustion flue gases of three different industrial boilers firing miscellaneous fuels were monitored for a twoweek period. Nitric oxide (NO), sulfur dioxide (SO 2), carbon monoxide (CO), carbon dioxide (CO 2), and total hydrocarbons (CxHy) were continuously measured using single-component gas analyzers in parallel with a lowresolution Fourier Transform Infrared (FTIR) gas analyzer. Hydrogen chloride (HCl) was measured continuously using the FTIR analyzer and semi-continuously using a traditional liquid-absorption technique. Nitrous oxide (N 2O), nitrogen dioxide (NO 2), and water vapor (H 2O) were continuously measured using the FTIR analyzer only. Laboratory tests were conducted prior to the field measurements to assess the detection limits of the different measurement methods for each gas component. No significant differences were found between the results of the low-resolution FTIR analyzer and the single-component analyzers or the liquid absorption method. 相似文献
13.
In the present study, an attempt has been made to grow microalgae Scenedesmus quadricauda, Chlorella vulgaris and Botryococcus braunii in mixotropic cultivation mode using two different substrates, i.e. sewage and glucose as organic carbon sources along with flue gas inputs as inorganic carbon source. The experiments were carried out in 500 ml flasks with sewage and glucose-enriched media along with flue gas inputs. The composition of the flue gas was 7 % CO 2, 210 ppm of NO x and 120 ppm of SO x . The results showed that S. quadricauda grown in glucose-enriched medium yielded higher biomass, lipid and fatty acid methyl esters (FAME) (biodiesel) yields of 2.6, 0.63 and 0.3 g/L, respectively. Whereas with sewage, the biomass, lipid and FAME yields of S. quadricauda were 1.9, 0.46, and 0.21 g/L, respectively. The other two species showed closer results as well. The glucose utilization was measured in terms of Chemical Oxygen Demand (COD) reduction, which was up to 93.75 % by S. quadricauda in the glucose-flue gas medium. In the sewage-flue gas medium, the COD removal was achieved up to 92 % by S. quadricauda. The other nutrients and pollutants from the sewage were removed up to 75 % on an average by the same. Concerning the flue gas treatment studies, S. quadricauda could remove CO 2 up to 85 % from the flue gas when grown in glucose medium and 81 % when grown in sewage. The SO x and NO x concentrations were reduced up to 50 and 62 %, respectively, by S. quadricauda in glucose-flue gas medium. Whereas, in the sewage-flue gas medium, the SO x and NO x concentrations were reduced up to 45 and 50 %, respectively, by the same. The other two species were equally efficient however with little less significant yields and removal percentages. This study laid emphasis on comparing the feasibility in utilization of readily available carbon sources like glucose and inexpensive leftover carbon sources like sewage by microalgae to generate energy coupled with economical remediation of waste. Therefore on an industrial scale, the sewage is more preferable. Because the results obtained in the laboratory demonstrated both sewage and glucose-enriched nutrient medium are equally efficient for algae cultivation with just a slight difference. Essentially, the sewage is cost effective and easily available in large quantities compared to glucose. 相似文献
14.
Abstract The removal system for the absorption of CO 2 with amines and NH 3 is an advanced air pollution control device to reduce greenhouse gas emissions. Absorption of CO 2 by amines and NH 3 solutions was performed in this study to derive the reaction kinetics. The absorption of CO 2 as encountered in flue gases into aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), and NH 3 was carried out using a stirred vessel with a plane gas-liquid interface at 50 °C. Various operating parameters were tested to determine the effect of these variables on the absorption kinetics of the reactants in both gas and liquid phases and the effect of competitions between various reactants on the mass-transfer rate. The observed absorption rate increases with increasing gas-liquid concentration, solvent concentration, temperature, and gas flow rate, but changes with the O 2 concentration and pH value. The absorption efficiency of MEA is better than that of NH 3 and DEA, but the absorption capacity of NH 3 is the best. The active energies of the MEA and NH 3 with CO 2 are 33.19 and 40.09 kJ/mol, respectively. 相似文献
15.
Annual CO 2 emission tallies for 210 coal-fired power plants during 2009 were more accurately calculated from fuel consumption records reported by the U.S. Energy Information Administration (EIA) than measurements from Continuous Emissions Monitoring Systems (CEMS) reported by the U.S. Environmental Protection Agency. Results from these accounting methods for individual plants vary by ± 10.8%. Although the differences systematically vary with the method used to certify flue-gas flow instruments in CEMS, additional sources of CEMS measurement error remain to be identified. Limitations of the EIA fuel consumption data are also discussed. Consideration of weighing, sample collection, laboratory analysis, emission factor, and stock adjustment errors showed that the minimum error for CO 2 emissions calculated from the fuel consumption data ranged from ± 1.3% to ± 7.2% with a plant average of ± 1.6%. This error might be reduced by 50% if the carbon content of coal delivered to U.S. power plants were reported. Implications: Potentially, this study might inform efforts to regulate CO2 emissions (such as CO2 performance standards or taxes) and more immediately, the U.S. Greenhouse Gas Reporting Rule where large coal-fired power plants currently use CEMS to measure CO2 emissions. Moreover, if, as suggested here, the flue-gas flow measurement limits the accuracy of CO2 emission tallies from CEMS, then the accuracy of other emission tallies from CEMS (such as SO2, NOx, and Hg) would be similarly affected. Consequently, improved flue gas flow measurements are needed to increase the reliability of emission measurements from CEMS. 相似文献
16.
Soil moisture and organic matter level affects soil respiration and microbial activities, which in turn impact greenhouse gas (GHG) emissions. This study was conducted to evaluate the effect of irrigation levels (75% [deficit], 100% [full], and 125% [excess] of reference crop evapotranspiration requirements), and organic amendments (OA) type (chicken manure [CM] and bone meal [BM]) and OA application rates (0,168, 336 and 672 kg total N ha ?1) on (i) soil physical properties (bulk density, organic matter content and soil moisture content) and (ii) soil carbon dioxide (CO 2) emissions from a highly weathered tropical Hawai'ian soil. Carbon dioxide readings were consistently taken once or twice a week for the duration of the cropping season. A drip irrigation system was used to apply the appropriate amount of irrigation water to the treatment plots. Treatments were randomly selected and corresponding organic amendments were manually incorporated into the soil. Plots were cultivated with sweet corn (Zea mays ‘SS-16’). Soil moisture content within and below the rootzone was monitored using a TDR 300 soil moisture sensor (Spectrum Technologies, Inc., Plainfield, IL, USA) connected with 12 cm long prongs. Soil bulk density and organic matter content were determined at the end of the cropping season. Analysis of variance results revealed that OA type, rate, and their interaction had significant effect on soil CO 2 flux ( P < 0.05). Among the OA rates, all CM mostly resulted in significantly higher soil CO 2 fluxes compared to BM and control treatment ( p < 0.05). The two highest rates of BM treatment were not significantly different from the control with regard to soil CO 2 flux. In addition, organic amendments affected soil moisture dynamics during the crop growing season and organic matter content measured after the crop harvest. While additional studies are needed to further investigate the effect of irrigation levels on soil CO 2 flux, it is recommended that in order to minimize soil CO 2 emissions, BM soil amendments could be a potential option to reduce soil CO 2 fluxes from agricultural fields similar to the one used in this study. 相似文献
17.
A unique dataset of airborne in situ observations of HCl, O 3, HNO 3, H 2O, CO, CO 2 and CH 3Cl has been made in and near the tropical tropopause layer (TTL). A total of 16 profiles across the tropopause were obtained at latitudes between 10°N and 3°S from the NASA WB-57F high-altitude aircraft flying from Costa Rica. Few in situ measurements of these gases, particularly HCl and HNO 3, have been reported for the TTL. The general features of the trace gas vertical profiles are consistent with the concept of the TTL as distinct from the lower troposphere and lower stratosphere. A combination of the tracer profiles and correlations with O 3 is used to show that a measurable amount of stratospheric air is mixed into this region. The HCl measurements offer an important constraint on stratospheric mixing into the TTL because once the contribution from halocarbon decomposition is quantified, the remaining HCl (>60% in this study) must have a stratospheric source. Stratospheric HCl in the TTL brings with it a proportional amount of stratospheric O 3. Quantifying the sources of O 3 in the TTL is important because O 3 is particularly effective as a greenhouse gas in the tropopause region. 相似文献
18.
Abstract The proposed mercury (Hg) oxidation mechanism consists of a 168-step gas phase mechanism that accounts for interaction among all important flue gas species and a heterogeneous oxidation mechanism on unburned carbon (UBC) particles, similar to established chemistry for dioxin production under comparable conditions. The mechanism was incorporated into a gas cleaning system simulator to predict the proportions of elemental and oxidized Hg species in the flue gases, given relevant coal properties (C/H/O/N/S/Cl/Hg), flue gas composition (O 2, H 2O, HCl), emissions (NO X, SO X, CO), the recovery of fly ash, fly ash loss-on-ignition (LOI), and a thermal history. Predictions are validated without parameter adjustments against datasets from lab-scale and from pilot-scale coal furnaces at 1 and 29 MW t. Collectively, the evaluations cover 16 coals representing ranks from sub-bituminous through high-volatile bituminous, including cases with Cl 2 and CaCl 2 injection. The predictions are, therefore, validated over virtually the entire domain of Cl-species concentrations and UBC levels of commercial interest. Additional predictions identify the most important operating conditions in the furnace and gas cleaning system, including stoichiometric ratio, NO X, LOI, and residence time, as well as the most important coal properties, including coal-Cl. 相似文献
19.
In this paper, stable carbon isotope ratios ( δ 13C) were determined in the atmosphere by using a Ca-based sorbent, CaO/Ca 12Al 14O 33 75:25 w/w, for passively collecting atmospheric CO 2, 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 CO 2 is mostly vehicle exhaust, a rural unpolluted area, and a private kitchen where the major source of CO 2 was gas combustion. Samplers were exposed to the free atmosphere for 3 days in order to allow collection of sufficient CO 2 for δ 13C analysis, then the collected CO 2 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 CO 2 samples and no fractionation occurred during passive trapping, as also confirmed by an appositely designed experiment conducted in the laboratory. Passive sampling using CaO/Ca 12Al 14O 33 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. 相似文献
20.
Chamber techniques can easily be applied to field trials with multiple small plots measuring carbon- and nitrogen-trace gas fluxes. Nevertheless, such chamber measurements are usually made weekly and rarely more frequently than once daily. However, automatic chambers do allow flux measurements on sub-daily time scales. It has been hypothesized that sub-daily measurements provide more reliable results, as diurnal variations are captured better compared to manual measurements. To test this hypothesis we compared automatic and manual measurements of N 2O, CO 2 and CH 4 fluxes from tilled and non-tilled plots of a rice–wheat rotation ecosystem over a non-waterlogged period. Our results suggest that both techniques, i.e., either manual or automatic chambers of N 2O and CO 2 emissions resulted in biased fluxes. The manual measurements were adequate to capture either day-to-day or seasonal dynamics of N 2O, CO 2 and CH 4 exchanges, but overestimated the cumulative N 2O and CO 2 emissions by 18% and 31%, respectively. This was due to neglecting temperature-dependent diurnal variations of C and N trace gas fluxes. However, the automatic measurements underestimated the cumulative emissions of N 2O and CO 2 by 22% and 17%, respectively. This underestimation resulted from chamber effects upon soil moisture during rainfall processes. No significant difference was detected between the two methods in CH 4 exchanges over the non-waterlogged soils. The bias of manual chambers may be significant when pronounced diurnal variations occur. The bias of automatic measurements can only be avoided/minimized if chamber positions are frequently changed and/or if chambers are automatically opened during rainfall events. We therefore recommend using automatic chambers together with continuous measurements of soil chamber moisture to allow for soil moisture correction of fluxes or to correct flux estimates as derived by manual chambers for possible diurnal variations. 相似文献
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