Treatment of reduced sulphur compounds and SO2 by Gas Phase Advanced Oxidation

Accepted Manuscript Treatment of Reduced Sulphur Compounds and SO2 by Gas Phase Advanced Oxidation Carl Meusinger, Anders B. Bluhme, Jonas L. Ingemar, Anders Feilberg, Sigurd Christiansen, Christina Andersen, Matthew S. Johnson PII: DOI: Reference:

S1385-8947(16)31174-3 http://dx.doi.org/10.1016/j.cej.2016.08.092 CEJ 15658

To appear in:

Chemical Engineering Journal

Received Date: Accepted Date:

9 June 2016 20 August 2016

Please cite this article as: C. Meusinger, A.B. Bluhme, J.L. Ingemar, A. Feilberg, S. Christiansen, C. Andersen, M.S. Johnson, Treatment of Reduced Sulphur Compounds and SO2 by Gas Phase Advanced Oxidation, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.08.092

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Treatment of Reduced Sulphur Compounds and SO2 by Gas Phase Advanced Oxidation Carl Meusingera,∗, Anders B. Bluhmea,1 , Jonas L. Ingemara,2 , Anders Feilbergb , Sigurd Christiansena,3 , Christina Andersena,4 , Matthew S. Johnsona a b

Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark. Department of Engineering - Air Quality Engineering, Aarhus University, Hangvej 2, 8200 Aarhus N, Denmark.

Abstract Reduced sulphur compounds (RSCs) emitted from pig farms are a major problem for agriculture, due to their health and environmental impacts and foul odour. This study investigates the removal of RSCs, including H2 S, and their oxidation product SO2 using Gas Phase Advanced Oxidation (GPAO). GPAO is a novel air cleaning technique which utilises accelerated atmospheric chemistry to oxidise pollutants before removing their oxidation products as particles. Removal efficiencies of 24.5 and 3.9 % were found for 461 ppb of H2 S and 714 ppb of SO2 in a laboratory system (volumetric flow Q = 75 m3 /h). A numerical model of the reactor system was developed to explore the basic ∗

Corresponding author Email address: [email protected] (Carl Meusinger) 1 Now at: DTU Nanotech, Department of Micro- and Nanotechnology, Technical University of Denmark, 2800 Kongens Lyngby, Denmark. 2 Now at: Infuser ApS, Ole Maaløe’s Vej 5, 2200 Copenhagen N, Denmark. 3 Now at: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark. 4 Now at: Ergonomics and Aerosol Technology, Lund University, Box 118, Lund SE22100, Sweden.

Preprint submitted to Chemical Engineering Journal

August 23, 2016

features of the system; its output was in fair agreement with the experiment. The model verified the role of OH radicals in initiating the oxidation chemistry. All sulphur removed from the gas phase was detected as particulate matter, assuming the observed particles were made of sulphuric acid. In a second set of experiments a range of RSCs at mixing ratios typically found in pig farms were treated using a larger industry-scale system (Q = 600 − 1200 m3 /h) that included a wet scrubber. Removal efficiencies > 90 % were found for all compounds. The study demonstrates the ability of GPAO to control RSC emissions with a low energy input relative to many currently available techniques. Keywords: Pollution control, Smell abatement, Reduced sulfur compounds, Advanced oxidation, Gas-phase treatment 1. Introduction Over the course of a generation, pig production has evolved from many small farms to fewer, much larger facilities [1, 2] intensifying the emitted pollution into fewer and larger sources. The increasing odour emissions have proven to be a large problem for the industry, for example limiting expansion near residential areas, since the odour impacts quality of life [3, 4]. The intense odour originates from a suite of compounds including ammonia, reduced sulphur compounds (RSCs), carbonyl compounds, and indoles [5, 6, 7]. The RSCs are key components of the offensive odour. They are powerful odourants with a very high correlation with smell [7] and low odour thresholds [8]. A number of methods are currently used to reduce odour emissions from livestock facilities including acidic wet scrubbers [9], animal diet

2

manipulation [10], slurry treatment [11, 12], and several biofiltration techniques [13]. Wet acid scrubbers are by far the most common technique in operation [14, 15] due to a focus on ammonia control. Water-based scrubbers are not well-suited for RSCs [16, 17] due to RSCs low water solubility. Basic scrubbers can increase alkalinity and help to favour partitioning from the gas phase into the aqueous phase, but this also adds to the complexity and the use of hazardous chemicals. Other techniques designed for RSC removal include bio-filters, however these systems may suffer from a high pressure drop, sensitivity to changing temperatures and pollutant concentrations and potential clogging due to their design [18]. In this study we use Gas Phase Advanced Oxidation (GPAO) to reduce odour emissions [19, 20, 21]. GPAO utilises UV-C light and ozone (O3 ) to produce highly reactive radicals including O(3 P), O(1 D), HO2 , and OH. The OH radical is the main initiator of oxidation in GPAO for a wide range of compounds, leading to oxidation products with lower vapour pressures than their reduced counterparts. This favours condensation onto pre-existing particles or participation in formation of new ones, effectively moving compounds from the gas phase into the particle phase. Particles are removed at the end of treatment using an electro static precipitator (ESP), while excess ozone is removed by a MnO2 catalyst. RSCs emitted from pig farms include (in order of decreasing concentration): H2 S (hydrogen sulphide), CH3 SCH3 (dimethyl sulphide, DMS), and CH3 SH (methanethiol or methyl mercaptan) [5]. In both the atmosphere and GPAO, the oxidation of all three RSCs is initiated by reaction with OH radicals and results in the formation of SO2 (sulphur dioxide) [22, 23]. SO2

3

is subsequently oxidised to H2 SO4 (sulphuric acid) which readily participates in particle formation [24]. In GPAO, H2 SO4 is a key driver of particle nucleation, growth, and acidity, and is therefore an important factor allowing the GPAO system to collect gaseous pollutants in the particle phase, including the three key RSCs. Table 1 lists some chemical properties of these RSCs and SO2 , including their reaction rate constants with the OH radical. This study investigates the mechanism and efficiency of GPAO treatment for the three RSCs, H2 S, CH3 SCH3 , and CH3 SH and of the oxidation product SO2 , while monitoring particle formation. Two series of experiments were performed using GPAO setups of different size to document the applicability and scalability of this novel air-cleaning approach. 2. Experimental Two series of experiments were performed to determine the removal efficiency, RE, of GPAO under different conditions. RE is defined as RE = χ◦ −χ , χ◦

where χ◦ is the pollutant inlet concentration and χ its outlet concentra-

tion. The first set of experiments used the laboratory-scale (LS) GPAO system shown in Figure S1 in the supplementary material (SM), which was described in detail elsewhere [20]. The second set of experiments used a larger, industrial-scale (IS) prototype shown schematically in Figure 1. GPAO treatment can be tested on scales ca. 10-times larger (in terms of flow, volume, and UV power) using the IS prototype rather than the LS system. In addition to the different size, a water scrubber is included in the industrial-scale prototype. Details of both setups are given in Table 2. Experimental details and results of all experiments are summarised in Table S1. 4

Experiments were performed by changing parameters including volumetric flow and UV power. A convenient measure for comparing these conditions is given by the Volumetric Energy Input, Σ, defined as Σ =

Ptotal . Q

Here, Ptotal

is the total power consumption of the air cleaner and Q the volumetric flow. Σ is usually reported in units of kJ/m3 . In the two GPAO setups presented here the UV lamps are the main energy consumer and for the rest of the study it is assumed that

Ptotal Q

≈

PUV Q

= ΣUV . Table 2 lists typical ranges of

these quantities for the two setups. 2.1. Laboratory-scale (LS) experiments Two sets of experiments were conducted using the LS setup. The first studied removal of H2 S, and the second, SO2 . H2 S was supplied from a 50 L steel cylinder (99.5 %, Air Liquide). A 5 µm critical orifice (Lenox Laser, USA) was used to ensure a stable, low flow. This airstream was diluted with laboratory technical air to obtain the desired inlet molar fraction at ppb level. SO2 (507 ppm ±5 % in technical air, Yara Praxair) was injected directly into the airstream, controlled by a needle valve. Ozone was produced from oxygen in technical air using a corona discharge ozone generator (Infuser ApS, Denmark). Ozone and either H2 S or SO2 were injected into the system after the fan. The tower below the fan was installed to ensure all gases were mixed homogeneously before UV exposure. After the mixing tower, air flowed past four 55 W UV-C lamps mounted in the centre of the air stream before passing two sampling ports. The ports were located before the ESP and ozone catalyst. This study does not consider the removal of particles and ozone by the LS-GPAO, which has been described previously [20]. Concentrations of both H2 S and SO2 were monitored using a Thermo 5

Scientific 450i pulsed fluorescence sulphur monitor [25], while ozone concentrations were monitored with an Eco Sensor UV-100 Ozone monitor. Particle measurements were conducted simultaneously using a TSI Scanning Mobility Particle Sizer (SMPS) consisting of a Series 3080 Electrostatic Classifier with Differential Mobility Analyzer (DMA) and a model 3772 Condensation Particle Counter (CPC) with a 0.0508 cm impactor. A dilution was made between the DMA and CPC, allowing accurate measurements above particle concentrations of 106 particles/cm3 , up to approximately 107 particles/cm3 . The sulphur and ozone monitors were connected to the sampling port using Teflon tubing, which ensured an inert surface, while the SMPS was connected using rubber tubing to minimise particle loss. Temperature, relative humidity and pressure were monitored continuously at the inlet and outlet of the system, and the flow rate was measured at the outlet. In both experiments a flow rate of 75 m3 /h was used, corresponding to treatment times, τ , of 9.8 and 11.4 s for ports 1 and 2 respectively. The UV exposure time was 4.3 s independent of the sampling port. Temperature and relative humidity were 298.3±0.3 K and 20.6±0.3 % respectively. 2.2. Kinetic model A kinetic model was written in Python 2.7 using the open source package Cantera, version 2.1.1 [26]. Cantera is an ordinary differential equation generator and solver. A number of parameters including the physical dimensions of the system, chemical reactions, and their rates are specified. Details for the model including a table of reactions are given in the SM and the input file is available from the authors. Cantera was used to model the flux of species through the LS prototype, as well as the mass fractions, temperatures and 6

HV

O3 cat

Room Air

MFC

Fan H2O

Clean Air

O3

UV Inlet

Growth PTR-MS

UV

Growth

ESP

Outlet

Pollutant

Figure 1: Schematic of the industrial-scale GPAO setup. The fan on the left pushes air through the system. A mass flow controller (MFC) was used to introduce different pollutants to the airstream. The setup consists of six modules: a water scrubber where ozone is added, two UV modules followed by one ’growth’ module each, and a module with the ESP and ozone catalyst. The ESP uses high voltage (HV) to charge particles. The ’growth’ modules increase the residence time in the system and allow for longer treatment time and particle growth.

pressures. A total of five elements, 39 species and 150 reactions were included in the model. From the logged values it was possible to estimate the steady state concentration of molecules in the reactor including radical species OH, O(1 D), and HO2 , which are difficult to measure in situ. It was also possible to investigate the concentrations of reactants and their oxidation products after treatment, allowing insight regarding the main oxidation routes. From the results a theoretical removal efficiency was determined for the H2 S and SO2 experiments performed on the LS setup. 2.3. Industry-scale (IS) tests Known amounts of pollutants were added to a stream of room air blown through the IS prototype. Briefly, the setup consisted of six modules measuring 60 cm × 60 cm × 60 cm each. The modules had different functions as

7

shown in Figure 1. The first module was a water scrubber where ozone was added, enabling removal of pollutants by dissolution and aqueous reaction with ozone. The scrubber was filled with fresh tap water at the beginning of experiments and normally was at least half full during the experiments. In the scrubber, a recirculation pump generated a continuous flow of water over a bed of plastic beads. Ozone was added to the scrubber water using a venturi valve. Next there are two UV stages each followed by an empty reactor module. These modules increased the treatment time in the system allowing for exposure of pollution to radicals and particle growth. Each UV module contained five 220 W UV lamps. In contrast to the LS setup, tubes of the UV lamps allowed emission by the strong mercury line at 184 nm. As a consequence these lamps generated a significant amount of ozone. The final module contained an ESP and ozone catalyst. A Proton Transfer Reaction - Mass Spectrometer (PTR-MS, Ionicon Analytik, Innsbruck, Austria) measured the removal of selected RSCs by the IS prototype. PTR-MS is based on chemical ionisation of compounds by proton transfer from hydronium (H3 O+ ) in a drift tube, and subsequent detection of ionised compounds using a quadrupole mass spectrometer. H2 S, CH3 SH and CH3 SCH3 were detected by the protonated molecular ions, m/z 35, 49 and 63. Standard drift tube conditions (600 V, T = 60 ◦ C, p = 2.2 mbar) corresponding to an E/N-number of ca. 135 Townsend were used. Because H3 O+ is used for protonation, only compounds with a proton affinity higher than that of water (691 kJ/mol) can be measured. The main constituents of air (O2 , N2 , CO2 , CH4 ) as well as small alkanes all have proton affinities lower than water, whereas organic compounds containing double bonds

8

and/or heteroatoms (N, S, O) have higher proton affinities and can be detected. SO2 has a proton affinity of 672.3 kJ/mol [27] and is therefore not detected by PTR-MS. H2 S is potentially a difficult case as it has a proton affinity just above water (705 kJ/mol). As a consequence, backwards proton transfer from H3 S+ to H2 O occurs in the drift chamber, which causes the sensitivity towards H2 S to be humidity dependent [5]. Therefore, a humidity dependent calibration with respect to H2 S was carried out as described previously [5, 7]. Calibration of the PTR-MS with respect to CH3 SCH3 and CH3 SH was done using a certified gas cylinder with nominal concentrations of 5 ppm (±10 %). The PTR-MS was configured to sample at the inlet, after mixing distributed the pollutant evenly in the air stream, and at the outlet after the ESP. Sampling the outlet concentration before the ESP did not significantly change the removal efficiencies reported here. Ozone concentrations inside the setup were monitored with an Eco Sensor UV-100 Ozone monitor with an error of ±0.1 ppm. Temperature and relative humidity inside the setup were between 287.7 and 290.6 K and 77.1 and 80.1 %, respectively, during the experiments. The flow rate throughout the prototype was set using a variable fan and confirmed by PTR-MS measurements using a known flow of a standard dilution of CH3 SCH3 from a certified gas cylinder as a an internal standard. In addition to the flow, the following parameters were varied: UV intensity, compound, pollutant concentration, and ozone concentration. Pollutant concentrations were based on typical upper end-of-range values reported for pig farms [17]. Due to a malfunction the scrubber pump did not work during the CH3 SH experiments which may have compromised the efficiency of the

9

a

b

Figure 2: Measured and modelled mole fractions, χ, of H2 S and SO2 obtained both before and after GPAO treatment of H2 S (a) and SO2 (b), using 6.4 ± 0.1 ppm of O3 and a flow rate of 75 m3 /h, equivalent to ΣUV ≈ 11 kJ/m3 .

scrubber. However, the solubility of RSC’s is typically low, cf. Table 1. The kinetic model does not incorporate aqueous-phase chemistry at this point and could therefore not be used for modelling the IS results. 3. Results and Discussion 3.1. Results of LS experiments Mole fractions of H2 S and SO2 measured before and after GPAO treatment at the first sampling port are presented in Figure 2. In the first experiment a clear decrease of H2 S from 461±4 to 348±4 ppb is observed at port 1, corresponding to a removal efficiency of approximately 24.5 %. The results at sampling port 2 are nearly identical (not shown). Furthermore 10

Figure 3: Particle size distributions resulting from GPAO treatment of 461±4 ppb of H2 S and 714±3 ppb of SO2 , using 6.4±0.1 ppm of O3 and a flow rate of 75 m3 /h (ΣUV ≈ 11 kJ/m3 ). Mean modes of the size distributions are indicated by vertical dotted lines and Dp denotes the particle diameter.

a clear production of SO2 is observed, which followed the removal of H2 S, meaning that the Combined Sulphur (CS) present in the gas phase remained constant to within the uncertainty throughout the treatment. Conversion of H2 S to SO2 is expected based on the short UV exposure times of the setup. No removal is observed between the two sampling ports, indicating that little or no radicals were left once the air reached the first sampling port. A multi-stage UV treatment could increase removal of both H2 S and CS. The second experiment showed that GPAO treatment of SO2 is much less efficient than for H2 S (panel b in Figure 2), as is expected based on the rate constants for their reactions with OH, cf. Table 1. Here a reduction from 714±3 to 686±3 ppb is observed at both sampling ports (only the first one is shown), corresponding to removal efficiencies of approximately 3.9 %. Decreased CS concentrations indicate removal of sulphur from the gas phase, as expected from the oxidation to H2 SO4 and subsequent particle formation. 11

Size distributions obtained from the SMPS measurements during both experiments are shown in Figure 3. While little or no particles are present before treatment, significant formation of ultra-fine particles was observed after treatment. Particles produced when treating H2 S show log-normal distributions with mean modes at 13.1 and 13.6 nm for ports 1 and 2 respectively. SO2 treatment yields both more and larger particles, as expected from the larger decrease in CS observed in the gas phase. Here the lognormal distributions showed mean modes at 31.1 and 32.3 nm for ports 1 and 2. Furthermore a slight increase in both particle number and their size is seen at port 2 compared to port 1. The measured differences are just outside the standard deviations of one another. Uncertainties in flow, ozone concentration and pollutant concentrations are not accounted for. Coagulation of particles seems too slow to have an effect on particle size distributions for 1.6 s of additional residence time between the two ports. Since no removal of gaseous species is observed between the two ports, the observed shifts are assumed to be random fluctuations. A sulphur mass balance was calculated based on the assumption that particles are made of pure sulphuric acid with a density of 1.84 g/cm3 . Measured particle masses in the H2 S experiment are too low to be relevant for the mass-balance, in line with the observed constant CS levels. In the SO2 experiment large particle masses are observed. At port 1 the removed sulphur from the gas phase amounts to 36.95 ± 5.45 µgS/m3 which is very close to the total sulphur mass in the particles, 33.24 ± 1.83 µgS/m3 . Data for port 2 shows very similar results. The assumption that particles have the density of pure sulphuric acid constitutes an upper limit of the particle mass

12

as hydration is very likely given the ambient relative humidity of the system [28]. For densities suggested in literature (around 1.25 g/cm3 ) [29] the particle sulphur mass and removed gaseous sulphur mass would be out of balance by about 1/3. In conclusion the mass balance is in approximate agreement as for example wall losses have also not been taken into account. 3.2. Model results Model results are plotted next to the experimental ones in Figure 2. In the H2 S experiment, 132 ppb of H2 S are predicted to be removed (RE = 29 %) while 74 ppb of SO2 are predicted to be produced. Compared to the measured removal efficiency of 25 % the model slightly overestimated H2 S oxidation. For the SO2 experiments the model predicted removal of 104 ppb of sulphur dioxide (RE = 15 %), compared to a measured removal of 28 ppb (RE = 4 %). In this case, the model also over-predicted removal. The model is not intended to give a comprehensive description of all processes in the reactor, but rather to serve as a guide to determine radical concentrations and reaction pathways. Figure 4 shows the elemental sulphur reaction diagram generated by Cantera at equilibrium conditions based on the kinetic model described in the SM. The diagram shows fluxes of elemental sulphur as arrows (equivalent to reaction rates). The wider and darker the arrow the larger the flux of sulphur associated with the specific reaction. All fluxes are given relative to the largest flux, in this case H2 S+OH → SH+H2 O. Details of Figure 4 are described in the SM. In the sulphur reaction diagram in Figure 4 the dominant reaction route is indicated by the darkest arrows or the largest numbers. Figure 4 shows that the full oxidation of hydrogen sulphide to sulphuric acid involves a series 13

Figure 4: Elemental sulphur reaction diagram for H2 S treatment in LS GPAO. Modelled conditions: χ◦ (H2 S) = 0.5 ppm, RH = 25 %, T = 298 K, χ◦ (O3 ) = 6 ppm, Q = 75 m3 hr−1 . Arrow width and colour intensity indicate flux strength relative to the largest flux (H2 S + OH → SH + H2 O). More detail is given in the text and the SM.

14

of oxidation steps: OH·

O +M

O

OH·

O

H O

2 2 2 H2 S −−→ SH· −−2−−→ HSO2 · −→ SO2 −−→ HSO3 · −→ SO3 −− → H2 SO4 (1)

As expected from observations the flux going to sulphur dioxide is much larger than the flux leaving it, resulting in the observed production of sulphur dioxide and low removal of sulphur from the gas phase. Hydroxyl radicals were the main initiators of oxidation for both hydrogen sulphide and sulphur dioxide. Other reactions, including direct photolysis are of minor importance. The path also showed that oxygen molecules play a vital role in the oxidation process, by reacting with SH·, HSO2 ·, and HSO3 ·, cf. reaction scheme (1). The model reproduces removal of sulphur species sufficiently based on a set of important reactions and their reaction coefficients. The deviation is due to several factors including errors in the rate coefficients reported in the literature, assumptions about homogeneity in the reactors (see SM), the lack of wall interactions in the model and uncertainties in the input values, e.g. flow, temperature, and ozone photolysis rate (see also SM). The model is helpful in testing strategies for improving performance. 3.3. Results of IS tests H2 S removal efficiencies are plotted in Figure 5 and the removal of CH3 SH is depicted in Figure 6. In general, all pollutants were removed with a RE > 90 % when the system is fully switched on, i.e. all lights and the scrubber running, corresponding to ΣUV ≤ 13.2 kJ/m3 . The results show that a higher UV intensity gives higher removal efficiencies. In the case of H2 S (Figure 5) the removal scaled with the applied UV power for all tested flow rates. Higher UV input power results in larger OH 15

Removal Efficiency /%

100 80 60 40 3

1200 m /h 3

900 m /h

20 0

3

600 m /h

0.0

0.5

1.0

1.5

2.0

2.5

PUV /kW

Figure 5: H2 S removal by the industrial-scale GPAO prototype. The full symbol denotes that only the water scrubber was operating. Open symbols denote presence of ozone and lights turned on.

Removal Efficiency /%

100 80 60 40 3

1200 m /h

20 0

3

600 m /h

0

2

4

6

8

10 12 14 -3

Volumetric energy input, ΣUV /kJ m

Figure 6: CH3 SH removal by the industrial-scale GPAO prototype. The scrubber pump failed during these experiments and water is not recirculated, potentially limiting the scrubber efficiency. The full symbol denotes that no ozone is added and no lights are turned on.

16

radical concentrations inside the setup, which promotes removal, in line with what has been reported earlier on GPAO [20, 21]. CH3 SH showed a similar picture, but the RE quickly reaches values above 95 % for ΣUV ≥ 2 kJ/m3 . To within the uncertainties, it is not possible to detect higher removals of CH3 SH. Very high removal efficiencies of CH3 SCH3 were achieved at full UV power at 600 m3 /h (ΣUV = 13.2 kJ/m3 ), cf. Table S1. The removal efficiencies scale inversely with volumetric flow. This is in line with the gas-phase removal approach of GPAO and the reported higher removal efficiencies at longer residence times [20]. CH3 SCH3 removal is tested for different initial CH3 SCH3 concentrations, χ◦ (CH3 SCH3 ), under otherwise similar conditions. The results in Table S1 show that RE was independent of χ◦ (CH3 SCH3 ). This is also in line with the general understanding of GPAO since the removal follows a typical second-order reaction rate. The corresponding lifetime, τOH , of a pollutant towards the OH radical at mixing ratio χOH is then τOH = 1/kOH χOH . Under similar flow conditions the residence time in the system, τres , stays the same and the removal (in percent of χ◦ ) is independent of the pollutant concentration assuming constant OH concentration. Two experiments show removal with neither ozone nor UV light present (full symbols in Figures 5 and 6). 16 % of H2 S is removed at 1200 m3 /h, and 11 % of CH3 SH is removed at 600 m3 /h without oxidants added to the airstream. The maximum background ozone concentration is around 0.1 ppm and cannot account for these observations. In the case of H2 S the main reason could be aqueous oxidation by oxygen involving homogeneous catalysis (e.g. dissolved iron impurity in water) or heterogeneous catalysis (reactions

17

on reactor surfaces). In the case of CH3 SH the scrubber pump is not working, so partitioning into the water phase could only play a minor role due to greatly reduced contact time in the scrubber. Instead, chemisorption on the walls constitutes another possible removal mechanism. Observed removal efficiencies are small when ozone and light are not present compared to removal efficiencies under conditions where the air cleaner is turned on. All tested pollutants are partly removed by reaction with ozone. Around 40 % of H2 S is removed at a flow of 1200 m3 /h, at different ozone mixing ratios. At a lower flow of 600 m3 /h, 55 and 66 % of H2 S are removed with no clear reason for the spread. In the presence of ozone 55 and 58 % of CH3 SH and 27 % of CH3 SCH3 are removed. The removal efficiency from the ozone reaction scales inversely with the flow, but does not scale with the ozone mixing ratio. This indicates that ozone is always in excess. Even though the RE scales with flow, it is unlikely that gas-phase ozone reactions play a role given the very low reaction rate constants, cf. Table 1. For some compounds including H2 S aqueous phase reactions with ozone are known and are more likely to play a role [30]. Higher flows lead to lower removal efficiencies by aqueous phase reactions because the contact time in the scrubber is decreased. Adding additional ozone beyond that produced by the UV lamps does not show any effect on removal efficiency. This strengthens the Σ = ΣUV approximation made above, since no additional energy is required for ozone generation. An exception is CH3 SCH3 removal at 1200 m3 /h where an additional 4.5 ppm of ozone increased RE from 63 to 73 %. It is unlikely that more OH radicals were produced in this case, as higher OH concentrations

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would have also lead to higher removal of the other pollutants. A possible explanation is that higher ozone concentrations lead to an increased concentration of ozone in the aqueous phase, which might lead to higher CH3 SCH3 removal in the scrubber. However, this was not verified experimentally and is not observed for other RSCs like H2 S, leaving room for further studies. The SM details how PTR-MS can be used to detect products other than SO2 . Figure S2 suggests for example the production of CH3 SCHO during CH3 SCH3 treatment in GPAO. PTR-MS is a useful tool to assess production of a broad range of oxidation products that may be produced when using GPAO. Particle measurements are performed with the same SMPS system as is used in the LS experiments, but without dilution of the CPC. The measured particle size distributions are not quantitatively useful as the produced particle numbers exceed the CPC detection limit of 106 particles/cm3 per DMA size bin at diameters in the lower tens of nanometers. The large amount of particles indicates sulphuric acid production and therefore full oxidation of sulphur compounds with subsequent gas-particle partitioning. 4. Conclusion and outlook Relative to the LS system, the multistage UV treatment in the IS setup is preferable in applications outside the laboratory, for several reasons. First, the removal efficiency is increased due to longer exposure to UV light and OH radicals. Second, potentially unwanted products can be further oxidised, e.g. transforming SO2 to H2 SO4 and particles. The water scrubber used here did not involve the addition of strong acids or bases. It has a powerful effect 19

and complements GPAO in removing water soluble pollutants. In the future, an improved model that incorporates aqueous-phase chemistry will allow to model integrated GPAO systems. Such a model will help to optimise energy efficiency e.g. by pointing out ways to improve ozone management. Complex agricultural exhaust streams like those from pig farms consist of many pollutants, at different concentrations and with very different physical and chemical properties. Such cases pose a challenge for any single air-cleaning technology and multi-pollutant cleaning solutions consisting of several different methods may be required [14]. In order to remove different combinations of pollutants by GPAO, the order and size of modules needs to be adapted, as presented for a range of applications previously [21]. A preliminary test on manure emissions performed with the IS setup showed promising simultaneous removal efficiencies for compounds like H2 S, phenol, 4-methylphenol, and 4-ethylphenol. The effect of changes in temperature and relative humidity on the GPAO performance have to be investigated in the future to guarantee high removal rates independent of weather and season. The direct photolysis of sulphur compounds is not expected to be an important removal pathway due to the low concentrations of these compounds. This study demonstrated the applicability and scalability of GPAO for treating H2 S and other RSCs from pig farms using known gas-phase chemistry. Similar mixtures of pollutants are also found in water treatment plants and biogas plants, with similar consequences and need for mitigation as in the case of pig farms. Such efforts will likely intensify in the future as global animal husbandry and associated pollution is predicted to increase with increasing global population and wealth [31]. Since different technologies are

20

better than others at removing certain pollutants, integrated solutions pose a promising way of solving complex emission problems. The industrial-scale setup presented here included a scrubber and some RSCs showed removal in the aqueous phase. The study demonstrated that GPAO is a strong standalone air cleaning technology for RSCs and can be a powerful part of future multi-pollutant concepts.

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Table 1: Chemical properties of RSCs under consideration. The odour threshold value, Ω, is given in ppb [8], reaction rate constants [27] for reactions with the OH radical and ozone are given as kOH and kO3 at 298 K. Henry’s law constants are listed as kH .

Compound

Formula

Hydrogen sulphide

H2 S

Methanethiol

CH3 SH

Dimethyl sulphide

CH3 SCH3

Sulphur dioxide

SO2

a

kH increases strongly at higher pH.

b

kH increases strongly with falling pH.

Ω / ppb kOH / cm3 s−1

kO3 / cm3 s−1

kH / mol L−1 atm−1

0.4

4.70 × 10−12

2.00 × 10−20

0.1 (pH < 7)a

0.07

3.30 × 10−11

–

0.4

3

5.38 × 10−12

1.50 × 10−19

0.6

870

1.30 × 10−12

1.89 × 10−22

≈ 105 (pH = 5)b

Table 2: Physical parameters of the two setups used on laboratory scale (LS) and on industrial scale (IS).

Property

Units

LS

IS

Reaction volume, V

m3

0.18

1

Air linear velocity, v

m s−1

0.05 - 0.5

0.5 - 0.9

Volumetric flow, Q

m3 h−1

15 - 150

600 - 1200

Treatment time, τres

s

4.3 - 45

3-6

UV Power, PUV

W

4 × 55

10 × 220

Volumetric Energy Input, ΣUV

kJ m−3

1.0 - 5.3

1.32 - 13.2

Scrubber volume, Vscrub

m3

–

0.2

22

5. Acknowledgement The authors would like to thank Kristoffer Nannerup, Julien Rzepkowski, and Infuser ApS, Denmark, for help in performing a part of the experiments. The authors would like to acknowledge financial support from Innovation Fund Denmark under grant agreement no.: 156-2013-6, Infuser ApS, Denmark, and the Department of Chemistry at the University of Copenhagen, Denmark. Appendix A. Supplementary material Supplementary material containing details on experimental procedure, a table with all experimental details and results, a Figure showing detailed PTR-MS data and details on the model can be found in the online version of this article.

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Highlights, C. Meusinger, et al., Treatment of Reduced Sulphur Compounds and SO2 by Gas Phase Advanced Oxidation. • • • •

Reduced sulphur compounds (RSCs) threaten quality of life due to strong smell Gas-phase advanced oxidation (GPAO) is a novel air cleaning technology GPAO removes RSC on laboratory and industrial scales Kinetic model verifies OH radical chemistry