SO3 Concentrations Research Articles

New dynamic models for simulation of industrial SO2 oxidation reactors and wet gas sulfuric acid plants

Simulation of plant performance during start-up, shutdown, and changes in the feed for industrial sulfuric acid plants requires dynamic modeling of the complete plant. A dynamic tanks-in-series model has been set up for the SO2 oxidation reactor with accumulation terms accounting for absorption and desorption of SO3 in the catalyst melt, which can have significant effect on operation due to the large absorption capacity of the SO2 oxidation catalyst. Simulations are compared with pilot and industrial scale measurements, and good agreement is found for temperatures, SO2, and SO3 concentrations when flow rate, temperature, and composition of the feed are changed. A corresponding model for the transient operation of a sulfuric acid condenser has also been developed. Implementation of the models in a commercial dynamic simulator has enabled full closed-loop simulation of industrial SO2 oxidation reactors and complete WSA plants.

Mercury and SO3 Emissions in Oxy-fuel Combustion

This paper presents results on experiments carried out at a 20 kW combustion rig simulating different extents of oxy-fuel recycle gas cleaning by impurities injection to the oxidant gas of the once-through combustion reactor. A comprehensive set of total (Hgtot), elemental (Hg0) and oxidized (Hg2+) mercury as well as SO3 concentrations was obtained before and after the combustion rig's baghouse filter for in total 14 air and oxy-fuel experiments with 3 Australian coals. Based on this data, an assessment in respect to Hg oxidation, SO2/SO3 conversion and Hg and SO3 capture on the test rig's filter was performed. The air and the oxy-fuel experiments with different extents of recycle gas cleaning, revealed differences in the Hg and SO3 formation and capture behavior: The Hg2+/Hgtot ratios in the flue gas are higher during oxy-fuel combustion compared to air-firing. This effect is even more pronounced at the filter outlet, after flue gas has passed through the filter ash. In some experiments, even a net oxidation of Hg0 entering the filter to Hg2+ was observed. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. However, the Hg capture was altered by the different oxy-fuel recycle configurations, leading to decreased Hg capture efficiencies on the filter for one of the coals. A coal-specific trend of increased SO2/SO3 conversion ratios with increased flue gas SO2 levels was observed that could be related to the ash composition of the three different coals. This and the higher SO2 concentrations in the flue gas lead to considerably higher SO3 levels in oxy-fuel combustion with SO2 recycling. During the experiments, also a considerable capture of SO3 in the baghouse filter was observed (up to 80% under air- and up to 66% under oxy-fired conditions). A reduction of the SO3 capture on the filter under oxy-fuel conditions may be related to the higher SO3 levels in this process.

Sulfur recirculation for increased electricity production in Waste-to-Energy plants

Sulfur recirculation is a new technology for reducing boiler corrosion and dioxin formation. It was demonstrated in full-scale tests at a Waste to Energy plant in Göteborg (Sweden) during nearly two months of operation. Sulfur was recirculated as sulfuric acid from the flue gas cleaning back to the boiler, thus creating a sulfur loop.The new technology was evaluated by extensive measurement campaigns during operation under normal conditions (reference case) and operation with sulfur recirculation. The chlorine content of both fly ash and boiler ash decreased and the sulfur content increased during the sulfur recirculation tests. The deposit growth and the particle concentration decreased with sulfur recirculation and the dioxin concentration (I-TEQ) of the flue gas was reduced by approximately 25%. Sulfuric acid dew point measurements showed that the sulfuric acid dosage did not lead to elevated SO3 concentrations, which may otherwise induce low temperature corrosion.In the sulfur recirculation corrosion probe exposures, the corrosion rate decreased for all tested materials (16Mo3, Sanicro 28 and Inconel 625) and material temperatures (450°C and 525°C) compared to the reference exposure. The corrosion rates were reduced by 60–90%. Sulfur recirculation prevented the formation of transition metal chlorides at the metal/oxide interface, formation of chromate and reduced the presence of zinc in the corrosion products. Furthermore, measured corrosion rates at 525°C with sulfur recirculation in operation were similar or lower compared to those measured at 450°C material temperature in reference conditions, which corresponds to normal operation at normal steam temperatures. This implies that sulfur recirculation allows for higher steam data and electricity production without increasing corrosion.

A comparison of sulfur and chlorine gas species in pulverized-coal, air- and oxy-combustion

Gas phase measurements of sulfur species (SO2, H2S, SO3, and COS) and HCl were collected in the staged near-burner region and the post tertiary injection oxidizing region of a pulverized-coal flame under air- and oxy-fired conditions. The near burner region was characterized by a high CO region near the center of the reactor directly below the burner. This reducing region as indicated by high CO concentrations produced high H2S and COS and lower SO2 concentrations in comparison to the post tertiary injection oxidizing region for both air and oxy-fired conditions. The concentrations of HCl were in most cases insensitive to radial location. Concentrations of all measured sulfur gases were 2.4–2.9 times higher in oxy-fired conditions than air-fired conditions consistent with the increase expected by the removal of N2 from the oxidizer. The concentration of H2S were on average 5.8 times higher in oxy-fired reducing conditions than air-fired reducing conditions indicating that the oxy-fired near burner regions were higher in H2S and lower in SO2 than would be predicted by removal of N2 alone from an air-fired system. The concentrations of SO3 were less than 2% of the total sulfur for both air and oxy-fired conditions but the uncertainty of the measurement did not allow for a comparison between the two firing modes. The concentrations of HCl were 2.4–2.8 times higher in oxy-fired conditions consistent with the removal of N2 from the oxidizer. The total measured sulfur, HCl, SO3, and reducing zone H2S were all found to increase linearly with increasing sulfur and chlorine concentrations in the coal.

Measurement and modeling of sulfur trioxide formation in a flow reactor under post-flame conditions

The present work focuses on the impacts of different combustion parameters on the formation of sulfur trioxide (SO3). The outlet SO3 concentrations from a quartz reactor operated within the temperature range of 800K to 1673K were measured using the controlled condensation method. Post-flame conditions were examined with and without combustibles, and the effects of SO2, O2, NO, H2O, and CO2 on SO3 formation were investigated. The formation of SO3 along the quartz reactor was modeled with a detailed chemical reaction mechanism by assuming plug-flow and using measured temperature profiles. Only reactions that occurred in the gas phase were considered.In the absence of combustibles, the outlet SO3 concentration increased as the experimental temperature and O2 concentration increased. When reactive gases (e.g., NO, CO, and CH4) were introduced, the formation of SO3 was increased, mainly as the result of increased concentrations of radicals. In addition, the combustion atmosphere (comprising N2, CO2, and H2O) influenced the amount of SO3 formed. A higher concentration of H2O clearly increased SO3 formation in the absence of combustibles.

Evaluation of SO3 Measurement Techniques in Air and Oxy-Fuel Combustion

SO2 is enriched in oxy-fuel combustion due to flue-gas recycle, and a significant higher SO3 concentration can be expected compared to air-firing. Since SO3 can cause high and low temperature corrosion, it is important to measure the SO3 concentration under oxy-fuel fired conditions. However, measurement of SO3 is not straightforward, since SO3 is a highly reactive gas. This paper presents an experimental study in the Chalmers oxy-fuel test unit, comparing different SO3 measurement techniques applied during oxy-fuel and air combustion. Propane (60 kWth) was used as fuel and SO2 was injected in the oxidizer to generate a controllable amount of SO3. The SO3 concentration was measured with four techniques: the controlled condensation method, the salt method, the isopropanol absorption bottle method, and with the Pentol SO3 monitor (previously: Severn Science analyzer). The controlled condensation method was used as the standard for comparison. Additionally, the acid dew-point temperature was measured with a dew-point meter. The controlled condensation and the salt method gave comparable results, and the repeatability with these methods was good. The SO3 concentrations measured with the Pentol SO3 monitor differed in average less than 20% from the SO3 concentrations obtained with the controlled condensation method. With the isopropanol absorption bottle method, a large amount of the SO2 was absorbed in the isopropanol solution, which gives a positive bias if the SO2 is oxidized to sulfate in the isopropanol solution. This was minimized by reducing the measurement time, bubbling argon through the absorption bottles after the measurement to force the SO2 out, and analyzing the solution immediately after the measurement. No principal differences between measuring the SO3 concentration during oxy-fuel combustion and air-firing were obtained. However, a correction factor for the mass flow meter of the Pentol SO3 monitor has to be used because of the high CO2 concentration during oxy-fuel operation.

Effect of SCR operation variables on mercury speciation

It is important to understand how existing air pollution control devices can remove mercury as co-benefit. This paper presents the results of a test programme at a 20kWth PF pilot scale unit and a lab scale micro-reactor to investigate the catalyst performance on the concentration and speciation of mercury in coal combustion flue gas. The SCR temperature, the ammonia dosing rate and the SO2 concentration were varied; increasing the temperature the mercury oxidation rate decreased, being the optimal temperature for a high mercury oxidation rate between 300 and 350°C. The oxidation of SO2, as undesired side reaction, is strongly dependent on temperature. The optimal temperature range for low SO3 concentrations downstream of the SCR is below 360°C, while the reduction of NOx by NH3 is faster for higher temperatures. A higher ammonia dosing rate leads to a high loading of the catalysts active sites and reduced mercury oxidation rate. Increasing the SO2 concentrations only slightly enhanced the mercury oxidation rate due to the higher acid loading of the catalyst, which promoted the DeNOx-reaction and made more active sites of the catalyst available for the mercury oxidation. Increasing SO2 concentration at the inlet of the SCR leads to higher concentrations of SO3 downstream of the SCR, while the relative oxidation rate of SO2 is lower at higher SO2 concentrations. Ashes were characterized by X-ray Fluorescence and Thermal Decomposition–Atomic Fluorescence Spectrometry to investigate mercury content and occurrence, respectively. Measurable mercury release began at approximately 200°C. It was also found that the ash samples showed the occurrence of only insoluble/partially soluble mercury.

Utilization of the Alkaline White Mud as Cement-Based Materials for the Production of Cement

The effects of using alkaline white mud as substitutes for cement property in the process of clinker burning were investigated in this paper. Samples were prepared with adding 0.5-5.0% of alkaline white mud as substitutes for unit raw meal and then these mixtures were burned at 1450°C for 2 hrs. The results showed that the compressive strength and flexibility strength of the pastes were a little better than the plain sample. The concentrations of MgO, SO3 and chloride ions in clinkers were far below the limits in the standards when the addition of alkaline white mud was below the content of 5.0%. The Results also showed that the concentrations of free lime were in a growing tendency with the increasing of alkaline white mud. Moreover, the addition of AWM in raw meal increased the content of C3S phase in the clinker, something leading the increase of the setting times and enhances the strengths of the cement pastes in the early curing ages.

Beckmann rearrangement of cyclohexanone oxime in a microchemical system: The role of SO3 and product inhibition

AbstractThe mechanism of Beckmann rearrangement of cyclohexanone oxime in oleum is investigated with a multiphase microchemical system, which is designed to give good control of the reaction temperature and residence time. The influences of SO3 and caprolactam concentrations in oleum on the conversion are investigated. The results indicate that SO3 acts as the catalyst and caprolactam plays a product inhibition role. Based on these results and previous mechanism analysis, an equilibrium relation between protonated caprolactam and caprolactam hydrogen sulfate is proposed and the equilibrium constants at 70, 80, and 90°C are gained. According to the equilibrium, the lowest acid/oxime molar‐ratio of 0.5 for sufficient conversion (>99%) at 100°C has been provided. This quantitative mechanism analysis gives the reason for the negative influences of higher caprolactam concentration and lower temperature on the reaction conversion, which is very useful for the optimization and reliable design of Beckmann rearrangement processes. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3156–3160, 2012

ПРОЦЕС КИСЛОТНОГО РОЗКЛАДУ СіРіЙСЬКОГО ФОСФОРИТУ

The article touches upon the results of researches of sulfuric- phosphoric acid decomposition of Syrah phosphourites, which are the raw material of the mineral fertilizers. Optimal concentrations of SO3 in liquid phase of decomposition’s slurry and functional dependence of degree of decomposition from basic technologies variable are determined.

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO3, Ba and Sr. In addition to the presence of SO3, calculated fO2 for magnetites indicate a high oxygen fugacity and that Fe+3>Fe+2 in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO3-rich and characterized by high oxygen fugacity.

Impact of Sulfur Oxides on Mercury Capture by Activated Carbon

Recent field tests of mercury removal with activated carbon injection (ACI) have revealed that mercury capture is limited in flue gases containing high concentrations of sulfur oxides (SOx). In order to gain a more complete understanding of the impact of SOx on ACl, mercury capture was tested under varying conditions of SO2 and SO3 concentrations using a packed bed reactor and simulated flue gas (SFG). The final mercury content of the activated carbons is independent of the SO2 concentration in the SFG, but the presence of SO3 inhibits mercury capture even at the lowest concentration tested (20 ppm). The mercury removal capacity decreases as the sulfur content of the used activated carbons increases from 1 to 10%. In one extreme case, an activated carbon with 10% sulfur, prepared by H2SO4 impregnation, shows almost no mercury capacity. The results suggest that mercury and sulfur oxides are in competition for the same binding sites on the carbon surface.

Effect of O-superfamily conotoxin SO3 on synchronized spontaneous calcium spikes in cultured hippocampal networks

SO3 belongs to the O-superfamily of conotoxins and is known to have analgesic effects in experimental animals. In order to explore the mechanism of its potential pharmacological actions, the effect of SO3 on synchronized spontaneous calcium spikes was examined in cultured hippocampal networks by calcium imaging. Spontaneous oscillations of intracellular concentrations of calcium (Ca(2+)) in the form of waves and spikes are found in cultured hippocampal networks. Exposure to increasing concentrations of SO3 resulted in a progressive decrease in synchronized spontaneous calcium spikes. The higher concentrations (0.1 micromol/L and 1 micromol/L) of SO3 showed the strongest inhibition. The rank order of inhibition was 1 micromol/L > 0.1 micromol/L > 10 micromol/L > 0.01 micromol/L. This action of SO3 in reducing synchronized calcium spikes suggests a possible application for therapeutic treatment of epilepsy.

Guano-derived deposits within the sandy cave fills of Naracoorte, South Australia

Geochemical and mineralogical data suggest that bat guano has made a major contribution to the development of several separate sedimentary layers, in the predominantly clastic sand and coarse silt deposits of the Naracoorte Caves, South Australia. A 20 cm thick group of reddish fine silt sediments located in Robertson Cave contains the phosphate minerals whitlockite and apatite, significant concentrations of SO3, P2O5, and CaO, and organic matter with low C/N ratios and highly enriched δN values. These data suggest that accumulations of bat guano and weathered cave limestone contributed significantly to the red silts formation. Sedimentary deposits located in Wet and Blanche Caves also contain horizons that exhibit increased concentrations of SO3, P2O5 and CaO, relatively lower C/N ratios and enriched δN values, suggesting that bat guano may have also contributed to their formation. Physical attributes evident for both these horizons further support this interpretation. A milky-white material that appears...

Guano-derived deposits within the sandy cave fills of Naracoorte, South Australia

Geochemical and mineralogical data suggest that bat guano has made a major contribution to the development of several separate sedimentary layers, in the predominantly clastic sand and coarse silt deposits of the Naracoorte Caves, South Australia. A 20 cm thick group of reddish fine silt sediments located in Robertson Cave contains the phosphate minerals whitlockite and apatite, significant concentrations of SO3, P2O5, and CaO, and organic matter with low C/N ratios and highly enriched δN values. These data suggest that accumulations of bat guano and weathered cave limestone contributed significantly to the red silts formation. Sedimentary deposits located in Wet and Blanche Caves also contain horizons that exhibit increased concentrations of SO3, P2O5 and CaO, relatively lower C/N ratios and enriched δN values, suggesting that bat guano may have also contributed to their formation. Physical attributes evident for both these horizons further support this interpretation. A milky-white material that appears to be whitlockite is present in the Wet Cave horizon, while similarities are identified by thin section petrography, between the Blanche Cave horizon and the guano layer in Robertson Cave. The development of such horizons relies on minimal sediment input from outside the cave system; hence their presence may serve as an indicator of periods of landscape stability in the palaeoenvironmental record.

Effects of Solvent Acidity on the Free-Radical-Initiated Synthesis of Methanesulfonic Acid from CH4 and SO3

The effects of solvent acidity on K2S2O8-initiated synthesis of methanesulfonic acid (MSA) from CH4 and SO3 have been investigated. Dimethyl sulfoxide, sulfuric acid, and trifluromethanesulfonic acid were used as the solvents. The initial rate of MSA synthesis increased with solvent acidity, being lowest in DMSO and highest in triflic acid. This effect is attributed to the influence of solvent acidity on the extent of decompostion of the S2O82- anion, which dissociates to form SO4-•. The latter species is responsible for activation of CH4 to form CH3•. The maximum conversion of SO3 to MSA also depends on the solvent, increasing in the order DMSO < TFMSA < H2SO4. The highest conversion of SO3 to MSA observed for the conditions of these studies was 87% when H2SO4 containing 0.2 M TFMSA was used as the solvent. The limit on the maximum conversion of SO3 to MSA is set by the reaction CH3SO3H + SO3 ⇌ CH3(SO3)2H, which removes unconverted SO3 from solution as MSA is formed. Although the extent to which this reaction proceeds decreases with increasing solvent acidity, the use of TFMSA as the solvent is detrimental because it appears to stabilize the formation of CH3(SO3)2H, possibly in the form of CH3(SO3)2H·(CF3SO3H)2. The influences of the initial concentrations of SO3 and K2S2O8 and of the reaction temperature on the maximum conversion of SO3 to MSA are discussed in terms of a proposed mechanism for MSA synthesis.

Utilization of Sulfur Oxides for the Production of Sodium Sulfate

The use of coal and other fossil fuels generates large quantities of sulfurous byproducts which could find an outlet in the manufacture of Na2SO4. In this work, the feasibility of converting SO2 and SO3 into Na2SO4 by employing modern technology, i.e., a fluidized-bed reactor, was investigated. Bench-scale and larger fluidized-bed reactors were used to study the reaction of SO2 or SO3 with NaCl, steam, and air at temperatures ranging from 400 to 600 °C. Because the rate of reaction of SO2 with salt as in the Hargreaves process proved too low to be practical, only the results of reacting SO3 are reported. Reasonable rates of reaction and trouble-free operation were generally achieved by employing SO3 concentrations in the range of 1−3 vol % and temperatures in the range of 500−600 °C. With higher SO3 concentrations and lower temperatures, particle agglomeration and bed caking took place as a result of the formation of lower melting point byproducts believed to be NaHSO4 and Na2S2O7. This problem was overcome in some cases by increasing the fluidizing gas velocity and/or bed turbulence. Therefore, better results were achieved with gas distributors which created more turbulence. Through selection of appropriate reaction conditions and careful design of the gas distributor, the technical feasibility of the process seems assured. Furthermore, because similar results were achieved with reactors which differed greatly in size, the scale-up to larger reactors should be straightforward.

An Alternative to Additional SO3 Injection for Fly Ash Conditioning

ABSTRACT Small concentrations, approximately 2-10 parts per million (ppm), of injected sulfur trioxide (SO3) have improved particulate collection efficiencies of electrostatic precipita-tors burning lower-sulfur coal. However, the addition of extra SO3 not only incurs costs but also presents negative environmental effects. This work explored a method that could be applied to existing coal-fired power plants to convert the sulfur dioxide (SO2) already present in the flue gas to sufficient levels of SO3 for fly ash conditioning as an alternative to adding SO3 by burning elemental sulfur. During this research, a pre-mixed natural gas flame was used to promote the conversion of SO2 to SO3 in a drop-tube furnace with average non-flame, free stream gas temperatures of 450 and 1000 K. SO3 concentrations measured by wet chemistry and confirmed using elemental balances of other sulfur species measured by gas chromatography revealed that as much as 7% of SO2 was homogeneously transformed to SO3. The results also showed that at low temperatures, the rate at which SO3 is converted back to SO2 decreased, thus extending the time period during which SO3 concentrations would be sufficient for ash conditioning. An additional benefit of this technique is speculated to result from increased flue gas humidity.

Mineralogy, composition, and alteration of Mars Pathfinder rocks and soils: Evidence from multispectral, elemental, and magnetic data on terrestrial analogue, SNC meteorite, and Pathfinder samples

Major element, multispectral, and magnetic properties data were obtained at Ares Vallis during the Mars Pathfinder mission. To understand the compositional, mineralogical, and process implications of these data, we obtained major element, mineralogical, and magnetic data for well‐crystalline and nanophase ferric minerals, terrestrial analogue samples with known geologic context, and SNC meteorites. Analogue samples include unaltered, palagonitic, and sulfatetic tephra from Mauna Kea Volcano (hydrolytic and acid‐sulfate alteration), steam vent material from Kilauea Volcano (hydrolytic alteration), and impactites from Meteor Crater (relithification). Salient results for Mars Pathfinder include: (1) Band depths BD530b and BD600 and the reflectivity ratio R800/R750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of H2O and occurring in composite particles along with subordinate amounts of other ferric minerals. Hematite and hematite plus nanophase goethite are most consistent with the data, but maghemite, akaganeite, schwertmannite, and nanophase lepidocrocite are also possible interpretations. Ferric oxides that are consistently not favored by the data as sole alteration products are jarosites and well‐crystalline goethite and lepidocrocite. (2) The strength of the ferric adsorption edge (R750/R445) implies the Fe3+/Fe2+ values for Pathfinder rock and soil are within the ranges 0.7–3 and 3–20, respectively. (3) Ferrous silicates are indicated for subsets of Pathfinder rocks and soils. One subset has a band minimum near 930 nm that can attributed to low‐Ca pyroxene. Alternatively, the band could be a second manifestation of certain ferric oxides, including nanophase goethite, maghemite, akaganeite, and schwertmannite. Another subset has a negative spectral slope from ∼800 to 1005 nm which could result from the high‐energy wing of a high‐Ca pyroxene and/or olivine band, a mixture of bright and dark materials, and, for rocks, thin coatings of bright dust on dark rocks. (4) Chemical data on Pathfinder rocks and soils are consistent with two‐component mixtures between an “andesitic” rock with low MgO and SO3 concentrations (soil‐free rock) and a global, basaltic soil with high MgO and SO3 concentrations (rock‐free soil). Pathfinder rock‐free soil can be modeled as a chemical mixture of SNC meteorites and the Pathfinder soil‐free rock. (5) Pathfinder soil cannot be obtained by chemical alteration of Pathfinder rocks by any of the hydrolytic and acid‐sulfate alteration processes we studied. Presumably, global mixing has obscured and possibly erased the elemental signatures of chemical alteration. (6) The strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite (Fe‐Ti spinels). The saturation magnetization of the tephra samples (0.5–2.0 Am2/kg) is at or below the low end of the range inferred for Martian dust (4±2 Am2/kg), implying that lithogenic Fe‐Ti spinels are a possible candidate for the Martian strongly magnetic phase. (7) The predominantly palagonitic spectral signature and magnetic nature of Martian soil and dust are consistent with glassy precursors with imbedded Fe‐Ti spinel particles. Comparison with lunar glass production rates suggests that production of sufficient quantities of glassy materials on Mars by volcanic and impact processes is sufficient to account for these observations.

Pressure and Temperature Dependence of the Gas-Phase Reaction of SO3 with H2O and the Heterogeneous Reaction of SO3 with H2O/H2SO4 Surfaces

The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (<1012 molecule cm-3) the rate coefficient is independent of the initial SO3 level, as expected for a gas-phase reaction mechanism involving one SO3 and two H2O molecules. However, at higher concentrations and lower temperatures, increased rate coefficients were observed, indicating a fast heterogeneous reaction after the onset of binary homogeneous nucleation of acid hydrate clusters leading to particle formation, which was verified by light-scattering experiments. The heterogeneous loss of SO3 to the reactor walls has also been investigated under low pressure (1.1−12.5 Torr) laminar flow conditions. The loss rate is highly dependent on the humidity of the surface. In the presence of excess water the reactive sticking coefficient approaches unity and the wall loss rate is gas diffusion limited; under dry conditions it approaches zero, as expected. The atmospheric implications of the homogeneous and heterogeneous SO3−water reaction are discussed.