Environ. Sci. Technol. 2005, 39, 1835-1842
Catalytic Ozonation of Gaseous Reduced Sulfur Compounds Using Wood Fly Ash JAMES R. KASTNER,* QUENTIN BUQUOI, RANGAN GANAGAVARAM, AND K. C. DAS Department of Biological and Agricultural Engineering, Driftmier Engineering Center, The University of Georgia, Athens, Georgia 30602
The feasibility of reusing wood ash as an inexpensive catalyst in a catalytic ozonation process has been demonstrated. Catalytic ozonation was demonstrated to oxidize H2S, methanethiol (MT), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) at low temperatures (23-25 °C). The process oxidized 25-50% of an inlet MT stream at 70 ppmv without the formation of DMDS (contrary to ash plus oxygen in air), oxidized 90-95% of an 85 ppmv stream of DMS, and oxidized 50% of a 100 ppmv DMDS stream using 2 g of wood ash at a space velocity of 720 h-1 using ozone concentrations ranging from 100 to 300 ppmv. Similarly, 60-70% conversion of a 70 ppmv H2S stream was achieved with 2 g of ash in 1.1 s without catalytic deactivation (∼44 h). The overall oxidation rate of H2S, DMS, and DMDS increased with increasing ozone concentration contrary to the oxidation rate of MT, which was independent of ozone concentration. Dimethyl sulfoxide and dimethyl sulfone were identified as the primary end products of DMS oxidation, and SO2 was the end product of H2S and MT oxidation.
Introduction High volume low concentration emissions (HVLC) from many industries (e.g., pulp and paper and wastewater treatment facilities) contain a range of reduced sulfur compounds, such as H2S, methanethiol, and dimethyl disulfide, which are odorous, toxic at high concentrations, and when oxidized (in air pollution control devices or the atmosphere) can lead to SOx generation and subsequent acid aerosol formation (1-3). However, oxidation of inorganic and organic sulfur compounds to oxides, sulfoxides, or sulfones can lead to a significant reduction in the odor threshold (e.g., the threshold for H2S [rotten egg] is 4.7 × 10-3 ppm vs 4.7 × 10-1 ppm for SO2 [pungent], 3). Regenerative thermal oxidation (RTO) and wet scrubbers are two primary air pollution control technologies used to treat the reduced sulfur compounds (2, 4). RTO’s have high operating costs, because oxidation occurs at high temperatures (760-870 °C), and produce greenhouse gases (NOx, CO2) due to combustion of an external carbon source at high temperatures. Wet scrubbers require costly oxidizing chemicals, such as ClO2 or NaOCl, large amounts of water, and can produce chlorinated hydrocarbons if not properly controlled (4). A cost-effective air pollution control technology for treatment of reduced sulfur compounds that reduces energy costs, water consumption, and greenhouse gas production is required for many industries. * Corresponding author phone: (706)583-0155; fax: (706)542-8806; e-mail: [email protected]
r.uga.edu. 10.1021/es0499492 CCC: $30.25 Published on Web 01/26/2005
2005 American Chemical Society
Coal and wood ash are produced in large volumes in the USA (75 and 5.5 million tons/yr, respectively) with a majority landfilled (5, 6). It has been suggested that coal ash can act as an adsorbent for H2S (7), but little research has been performed to determine if ash can be reused as a catalyst. Recent research has demonstrated that coal and wood fly ash can catalytically oxidize H2S, ethanethiol, and methanethiol (8); however, several limitations were reported in the process. The reaction capacity of wood ash for H2S, although lower than activated carbon (an expensive technology), was significantly higher than coal fly ash and within an acceptable range for low concentration applications (94% for 4 days using wood ash at inlet H2S concentrations of 60 ppmv. However, research indicated that wood fly ash oxidized methanethiol (MT) to dimethyl disulfide (DMDS) and did not react with DMDS or dimethyl sulfide (i.e., reduced sulfur compounds present in a wide range of industrial emissions). Clearly, the capacity of ash to oxidize organic sulfur compounds using molecular oxygen is limited at ambient temperatures. Alternatively, we theorize that wood ash (or other inexpensive waste materials) given the high metal content and the presence of carbon could act as an inexpensive catalytic oxidizer of reduced sulfur compounds (“odor”) and volatile organic compound (VOC) removal when coupled with ozone (O3). It is theorized that the ash or selective crystalline phases in the ash may act to catalyze the formation of free radicals from ozone and subsequently catalyze the oxidation of a wide range of odor-causing compounds and VOCs or provide a catalytic site for direct oxidation via O3. Recent research has demonstrated the use of transition metals in catalytic ozonation, primarily for wastewater treatment (10, 11). Cobalt(II) catalyzed the oxidation of oxalic acid using O3 (12), and activated carbon coupled with ozone was also demonstrated to generate •OH free radicals and oxidize oxalic acid (13, 14). Multiple metals (Fe, Mn, Ni, Co, Zn, Cr) in different forms ranging from salts, solid oxides, and deposited metals on supports have been used in catalytic ozonation (10, 15). Moreover, catalytic ozonation using noble metal-supported catalysts has been shown to significantly reduce operating temperatures required for deep oxidation of VOCs such as toluene and styrene (16). Unfortunately, there has been limited research on the gas-phase catalytic ozonation of air pollutants (11) and odor-causing compounds such as sulfides, mercaptans, and aldehydes (8, 9). Thus, the objectives of this research were to determine if wood ash could be used as a catalyst in the ozonation of gaseous reduced sulfur compounds at low temperatures and determine the kinetics of the reaction.
Experimental Methods Ash Characterization. Wood ash from a pulp mill was used in this study. The physical and chemical characteristics of the fly ash, including pH, surface area, bulk density, and the elemental composition were previously determined (8) and are reported in Table 1. XRD Methods. X-ray diffraction (XRD) analyses were performed with a Scintag XDS 2000 diffractometer equipped with a cobalt X-ray tube. Powdered samples were mounted on quartz plates and stepped scanned over the angular range 15-50° 2θ (9). Continuous Flow Studies. The extent of H2S, MT, DMS, DMDS, and propanal conversion in a catalytic ozonation system was measured in a continuous flow, packed bed reactor (Figure 1, Supporting Information), and all experiVOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
TABLE 1. Physical and Chemical Characteristics of Wood Fly Ash (WFA) Used in Catalytic Ozonation (O3) Studies and a Description of Experiments Conducted with the Ash at 297-298 K in This Studya
VOSC/VISC/VOC inlet (ppmv)
propanal H2S methanethiol dimethyl sulfide dimethyl disulfide
80 125, 250, 410 125, 410 60, 250 125, 250
selected elements (ppm or mg/kg)
Co Cu Mn Mo Ni V Fe
4.5-5.2 32.0-39.0 500.0-584 2.2-2.7 18-19 ND 6600-8300
4.94 34.57 542.74 2.43 18.27 ND 7470.54
14-20 64, 72 54, 66 125, 300 105, 139
WFA (mean ( SD)
surface area, m2/g pH bulk density, g/cm3 carbon, % (dry basis)
44.89 ( 8.34 12.13 ( 0.17 0.54 18.75 ( 1.87
ND: not determined.
mental conditions are described in Table 1. Compressed air or N2 was first passed through a bubble column to humidify the gas (>90% relative humidity within 20 min after start-up, 23-25 °C), mixed with H2S (5% in N2), MT (0.5% in N2), DMS (0.5% in N2), or DMDS (0.5% in N2) using a mass flow controller, passed through a static mixer, and then transported downward through the reactive column (Kimax, 2.5 cm i.d., 30.0 cm length). The compressed air or N2 rate was also controlled using a mass flow controller (UNIT UFC8100). An ozone generator (OL100H/DS, Yanco Industries Ltd., B. C., Canada), utilizing a high frequency corona discharge, was used with a medium grade tank of oxygen (99.9%, National Wielders, NC) to generate the ozone required for these experiments. Ozone was added after the static bed mixer, approximately 2 in. from the reactor inlet. In one case, propanal, a representative aldehyde and VOC, which reacts slowly with O3, was tested in the continuous flow, packed-bed reactor system. Using the apparatus described above, propanal (97% Sigma-Aldrich) was added via a syringe pump (Cole-Parmer, model 74900-30) downstream from the humidifier and upstream from the static bed mixer (Supporting Information, Figure 1-SI). The propanal flow rates ranged from 100 to 250 µL/min, and the total volume of propanal injected for each continuous flow study ranged from 75 to 80 mL. For the experimental results reported here, the inlet propanal concentration ranged between 14 and 20 ppmv (at 24 °C and 1 atm). The syringe used in this process was a Becton Dickinson syringe (plastipak 10 cm3, 14.48 mm i.d.), and propanal was injected via a tee (stainless steel, Swage-Lock) into the main airflow upstream from the bubble column into the static mixer. Wood fly ash (2 g of ash, 25% H2O (g/g), wet basis) was distributed in glass wool (8 µm, Corning Glass, NY) and packed over a defined height in the reactor (∼9-10 cm). Glass wool was also distributed above and below the distributed ash to promote plug flow (over the entire reactor height). Residence times based on gas flow rates and packing height of the ash ranged from 1.0 to 1.25 s, and inlet reactant concentrations ranged from 60 to 300 ppmv (0.0035-0.065% (v/v)). The reactor was covered in black felt to prevent artifacts due to photochemical oxidation. All kinetic studies were carried out at 23-25 °C and atmospheric pressure and demonstrated to be isothermal and constant pressure (102.4 kPa or 1.01 atm, 9). Ozone Decomposition. The continuous flow reactor system was also used to measure ozone decomposition in an empty bed, in the presence of glass wool only, and glass 1836
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wool plus wood ash (2 g). In this case, the wood ash was distributed throughout the entire column (30 cm), and the residence time, based on the entire column, ranged from 2.3 to 2.8 s for all experiments. Ozone Analysis. The concentration of ozone in the inlet and outlet of the decomposition experiments was measured using an IN-USA ozone analyzer, using UV absorption (model IN-2000-L2-LC, 0-10 000 ppmv, 0.01 ppmv resolution in the 0-100 ppmv range). The inlet ozone concentration ranged from 86 to 95 ppmv in the decomposition experiments and propanal studies. The ozone analyzer was also used to calibrate the ozone generator and confirm the generators capability of producing a defined ozone concentration. Ozone concentrations above 100 ppmv were calculated on the basis of the manufacturer’s calibration data and dilution of the O3 gas with the main air flow. Analytical Methods. Gas Analysis. Portable GC/MS units were used to measure H2S, MT, DMS, DMDS, and propanal concentrations at the inlet and outlet of the packed-bed reactor (HAPSITE Inficon, East Syracuse NY (8)). Gas samples were analyzed under isothermal conditions (60 or 80 °C) using a selective ion monitoring (SIM) method and in some cases full scan (45-250 amu). In the SIM mode, mass/charge ratios (m/z) of 34 (H2S), 47 (methanethiol), 79 (dimethyl disulfide), 62 (dimethyl sulfide), 58 (propanal), and 69 (internal standard; 1,3,5-trifluoromethylbenzene) were selectively scanned. In addition to these reactants, SO2 (full scan and SIM at 64), CS2 (full scan and SIM at 76), DMSO (full scan, 63, 78), and DMSO2 (full scan, 79, 94) were monitored using SIM as potential reaction products of methanethiol, H2S, DMDS, and DMS oxidation. Standard curves and continuous flow studies were based on the peak area ratio of the component peak (e.g., 34 for H2S) to the internal standard. Kinetics. The overall rate of oxidation was calculated from the measured fractional conversion, mass of the ash, volumetric gas flow rate, inlet mole fraction, pressure, and temperature using the following equation.
P X -r ) Q y MW RT W
where, -r is the reaction rate (mg/g/min), Q is the flow rate (L/min), y is the mole fraction in the inlet, X is the fractional conversion, W is the mass of ash (g), P is pressure (atm), R is the ideal gas constant, T is the temperature (K), and MW is the molecular weight of the reactant (e.g., H2S). Fractional
TABLE 2. Comparison of Gas-Phase Catalytic Oxidations Using Different Catalysts with and without Ozone (Reaction Rates Were Calculated Using Eq 1, and Variables Were Estimated from the Literature, Where Noted) substrate ethanol propanal H2S
ethanethiol dimethyl sulfide
catalyst Al2O3, SiO2 (11) wood ash (this work) wood ash (this work) wood ash (this work) wood ash (9) act. charcoal (9) wood ash (this work) wood ash (this work) wood ash (9) Pt/Al2O3d (27, 28) wood ash (this work)
wood ash (this work) Pt/Al2O3e (27, 28) dimethyl disulfide wood ash (this work)
temp (K) 293-363 297 297-298 297-298 297-298 297-298 297-298 297-298 297-298 488-573 297-298
products aldehydes ? SO2 SO2 S0 S0 SO2 SO2 DMDS SO2, CO2 DMSO, DMSO2a
297-298 400-586 SO2, CO2 297-298 ?
CuO(5)-MoO3/Al2O3f (26) 493-513 SO2, CO2
rate,b O2 (mol/m2‚s)
0.6 × 10-9,c 21% O2 0.05 × 10-9,c 21% O2 0.5 × 10-9, 21% O2 0.3-0.8 × 10-9, 21% O2
rate, O3 (mol/m2‚s) 1.7 × 10-9 0.08 ( 0.01 × 10-9 0.6 ( 0.05 × 10-9 1.3 ( 0.01 × 10-9 0.24 ( 0.01 × 10-9 0.3 ( 0.004 × 10-9 0.8 ( 0.11 × 10-9
O3, ppmv (mol/m3)
VOSC/VISC/ VOC inlet, ppmv
(0.05) 80 (328) 15 ( 0.6 125 (513) 64 ( 3.8 410 (1681) 72 ( 0.43 50 50 125 (513) 68 ( 1.7 410 (1681) 54 ( 1.0 85 100 125 (513) 55.0 ( 3.0
2.8 ( 0.015 × 10-9 250 (1025) 270 ( 1.3 110 0.38 ( 0.09 × 10-9 125 (513) 110 ( 11 -9 0.95 ( 0.45 × 10 250 (1025) 100 ( 24 0.06-0.2 × 10-9, 21% O2 80 0.2-0.9 × 10-9,d 21% O2
DMSO, dimethyl sulfoxide; DMSO2, dimethyl sulfoxone. b The reaction rate for catalytic ozonation using wood ash was based on a surface area of 45 m2/g (9). c Initial reaction rate; the surface area of the activated charcoal was 700 m2/g (9). d,e Estimated initial catalytic oxidation rate using PtAl2O3 of 158 m2/g (27, 28). f Estimated initial catalytic oxidation rate using CuO(5)-MoO3/Al2O3 at 124 m2/g (26).
conversion (X) was determined as the difference between the inlet and outlet concentration divided by the inlet concentration. Inlet and outlet concentrations at each time point were measured in triplicate (the coefficient of variation ranged from 0.3% to 8.0%). Equation 1 was also used to estimate catalytic incineration reaction rates of gaseous reduced sulfur compound using literature data (when rates were not reported), and they were compared to the catalytic ozonation rates on a surface area basis (Table 2). Reaction Byproduct Analysis. In some cases (e.g., DMS), although significant fractional conversion was measured, gaseous byproducts were not measured in the outlet gas (e.g., SO2). These data suggested potential byproducts were bound to the ash. Thus, reactions were carried out with MT, DMS, DMDS, and ozone over wood fly ash (WFA) for 3 days. The reaction was terminated, and 2-3 g of the ash (as well as residual glass wool) was placed in a 250 mL beaker and extracted with 40 mL of methanol via stirring rapidly for 30 min with a glass rod. The solution was gravity filtered and the extract was collected. One µL of the extract was injected and analyzed via GC/MS (HP model 6890, 5973 MS detector). Both DMSO (dimethyl sulfoxide) and DMSO2 (dimethyl sulfoxone) were confirmed using NIST mass spectral search software (NIST/EPA/NIH Version 1.7, 1999) as byproducts of DMS oxidation with match factors of 95% and 91%, respectively. Confirmation of DMSO2 formation was verified using a standard (DMSO2, over the counter dietary supplement, Rexall Sundown, Inc.). The DMSO2 source contained 1000 mg of DMSO2, which constituted approximately 7080% of each tablet; a single table of DMSO2 was dissolved in 100 mL of methanol and mixed for about 30 min. Similar to the extract analysis, 1 µL was injected and analyzed via GC/ MS and NIST analysis. A Supelco SPB-1 Sulfur column was used (30.0 m × 230 µm × 4.00 µm) to separate the compounds in question using a temperature program (60 °C for 1 min, 15 °C/min temperature ramp until 240 °C, 1 min at 240 °C) at a pressure of 3.24 psig and helium flow of 1.5 mL/min. It should be noted that this technique (methanol extraction and GC/MS method) would not have identified potential oxidation end products, such as sulfates, that would remain bound to the reacted wood fly ash.
Results and Discussion Our particular target stream in this research was a high volume, low concentration emission from the pulp and paper
FIGURE 1. Continuous catalytic ozonation removal of H2S (A, transient; B, steady-state fractional removal and reaction rate; error bars are 3σ) using a reactor packed with 2 g of wood ash (25% moisture) and ozone at 23-25 °C in an approximate 1.125 s residence time. Plots: 125 ppmv O3, 66.3 ( 3.6 ppmv H2S - 0 inlet, 9 outlet; 250 ppmv O3, 72.3 ( 0.55 ppmv H2S - O inlet, b outlet; 410 ppmv O3, 72.0 ( 0.43 ppmv H2S - 3 inlet, 1 outlet (error for inlet H2S is 1σ). industry predominantly consisting of H2S, methanethiol (MT), dimethyl disulfide (DMDS), dimethyl sulfide (DMS), and methanol. With such a complex mixture, our approach was to first study the oxidation of the individual reduced sulfur compounds followed by future work on mixtures and the effect of methanol. Wood ash in the presence of O2 has been shown to catalytically oxidize H2S to elemental sulfur at high rates (3.15 mg/g‚h or 0.6 × 10-9 mol/m2‚s, 21% O2) and fractional conversions (94% for 4 days at 60 ppmv); however, catalytic deactivation limits its use for continuous removal processes (without developing a low cost regeneration method and subsequent market for the spent catalyst, 9). Prolonged catalytic ozonation of H2S did not appear to lead to catalytic deactivation (Figure 1, ∼44 h), and SO2 appeared to be the primary oxidation product as identified by GC/MS analysis (SO2 concentrations were not determined and thus selectivity was not reported). In addition, the overall rate of H2S oxidation increased with ozone concentrations (Figure 1) and was higher than previously reported with O2 only (0.6 × 10-9 mol/m2‚s with O2 vs 1.3 × 10-9 mol/m2‚s at 410 ppmv O3). Thus, the presence of ozone potentially altered the catalytic mechanism as compared to O2 only, potentially VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
FIGURE 2. Continuous catalytic ozonation removal of a gaseous DMS stream (A, transient response to O3 - 117.9 ( 11.6 ppmv DMS inlet; B, approach to steady-state - 86.1 ( 1.5 ppmv DMS) using a reactor packed with 2 g of wood ash (25% moisture) and ozone (120 ppmv) at 23-25 °C in an approximate 1.125 s residence time. ---- Inlet mean DMS, b outlet DMS, and 3 fractional conversion of DMS (error for inlet DMS is 1σ). eliminating or significantly reducing the formation of elemental sulfur, which leads to catalytic deactivation, and increased the H2S oxidation rate above 125 ppmv ozone, relative to O2 only (9). The next research step was to determine if this process would also oxidize and remove the organic sulfur compounds. Catalytic oxidation of volatile organic sulfur (VOSC) emissions using wood ash and O2 would not be feasible at low temperatures (25 °C (18, 19). Magnetite and hematite have been shown to catalyze the formation of •OH free radicals from H2O2 and the subsequent oxidation of organic compounds by the hydroxyl free radical (20). Moreover, activated carbon supported with Fe2O3 and MnO2Fe2O3 was shown to catalytically decompose O3; however, a mechanism was not proposed (21). Finally, metal oxides (e.g., Fe2O3) in soil and on supported catalysts have been suggested to generate •OH or other surface radicals and subsequently oxidize adsorbed organic compounds (10, 11, 22). Combined with results reported in the literature, there are several lines of evidence that crystalline phases (mullite, magnetite, and hematite) within the ash contributed to the catalytic ozonation of the reduced sulfur compounds. In the presence of O2 only, ash was required to catalyze the oxidation of H2S and MT (i.e., glass wool alone with O2 did not initiate a reaction, data not shown; see refs 8 and 9). Additionally, oxidation of DMS and DMDS was only possible with ozone addition in the presence of the wood ash/glass wool matrix and DMDS was not formed from MT, indicating that O3 was required for oxidation to occur at low temperatures and altered the catalytic mechanism of MT oxidation relative to the MT/O2/ash system (9). However, we cannot rule out the possibility that carbon (∼19% dry basis, Table 1) in wood ash (activated or as carbon black) contributed to the catalytic activity, potentially by increasing the adsorptive capacity of the ash for the organic sulfur compounds. Activated carbon impregnated with platinum (Pt) was shown to significantly increase the catalytic ozonation of p-chlorobenzoic acid
FIGURE 3. Identification of dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO2) in the extract obtained from wood ash reacted with DMS and O3: GC/MS analysis of the DMSO2 standard (top, inset - scan of the 7.0 min DMSO2 peak) and analysis of the reacted catalyst (bottom, inset - scan of the 7.0 min DMSO2 peak). (pCBA) relative to platinum dispersed on aluminum in liquidphase reactions (15). In addition, it has been reported that activated carbon and carbon black catalyze the formation of •OH from O (13). 3 The transient kinetics of ozone decomposition on wood ash (ozone alone) indicated a high initial reaction rate (0.27 mg/g/min or 2.1 × 10-9 mol/m2‚s) and subsequent decline (Figure 8). As compared to controls (empty reactor and glass
wool only), the initial ozone decomposition rate was significantly higher and comparable to initial rates measured using activated carbon (0.25 mg/g/min or 1.93 × 10-9 mol/ m2‚s, 23). However, within 3 h, the fractional conversion declined from 0.96 to 0.15 and the O3 reaction rate declined from 0.27 (2.1 × 10-9 mol/m2‚s) to 0.03 mg/g/min (2.3 × 10-10 mol/m2‚s). These data suggest that a stable surface oxide may be formed that saturates the surface and prevents VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
FIGURE 4. Continuous catalytic oxidation of a gaseous DMDS stream using a reactor packed with 2 g of wood ash (25% moisture) and ozone (120 ppmv) at 23-25 °C in an approximate 1.125 s residence time. 9 Inlet DMDS (112.2 ( 1.4 ppmv), 0 outlet DMDS (101.1 ( 2.0 ppmv), and 3 fractional conversion (0.1 ( 0.02) (error at 1σ).
FIGURE 5. Continuous catalytic oxidation of a gaseous methanethiol (MT) stream (with and without ozone) using a reactor packed with 2 g of wood ash (25% moisture) and ozone (120 ppmv) at 23-25 °C in an approximate 1.125 s residence time. 0 Inlet MT, b outlet MT, and 3 fractional conversion of MT, 4 continuous and transient formation of DMDS. The inlet concentration of MT in the system without O3 was 86.5 ( 4.1 ppmv as compared to 68.0 ( 2.3 ppmv with O3 (error at 1σ). further decomposition of O3 without the presence of a VOSC or VOC capable of being oxidized and subsequently regenerating the surface. Similar results were obtained with metal oxide, impregnated activated carbon, yet the decay rate was lower (e.g., ozone conversion declined from 98% to 55% using Co3O4-NiO/AC catalyst) potentially due to the significantly higher surface area (45 vs 1600-3100 m2/g, 21). Our results can rule out the possibility that the glass wool or SiO2 phases (at least the structural form in glass wool) contributed to the catalytic ozonation process. Experiments performed with an empty bed and glass wool only (in the presence of O3) did not result in measurable loss of reduced sulfur compounds. For example, using a reactor packed with glass wool, over a 28 h period a fraction removal of 0.0 ( 0.07 (6 points) conversion of DMS was measured in a 3.75 s residence-time (based on the entire column) with an inlet stream composed of 70 ppmv DMS and 120 ppmv O3. In addition, it is highly unlikely that gas-phase reactions between O3 and DMS were responsible for DMS removal (or the other organic sulfur compounds tested), because sulfur compounds are unreactive with O3 in the gas phase (11); the empty bed reactor results also confirm this conclusion. There are three general mechanisms proposed for the catalytic ozonation process. The volatile organic sulfur compound/volatile inorganic sulfur compound (VOSC/VISC) of interest is adsorbed (physically or via chemisorption) on the catalyst surface and O3 directly oxidizes the compound on the surface (after diffusion from the bulk gas phase and through the liquid film to the surface - mechanism 1). If activation of the compound is involved, contrary to physical adsorption, then the process is considered catalytic. This type of interaction was proposed for the oxidation of lower 1840
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FIGURE 6. Effect of ozone concentration on the oxidation of MT, DMS, and DMDS (top) using a reactor packed with 2 g of wood ash (25% moisture) at 23-25 °C in an approximate 1.125 s residence time (steady-state results). b, 60 ppmv DMS; 0, 110 ppmv DMS; and 9, 300 ppmv DMS; O, 75-140 ppmv DMDS; 1, 53-70 ppmv MT, and the effect of ozone concentration on rate of the oxidation for DMS (0, 60-300 ppmv), DMDS (9, 75-140 ppmv) and MT (b, 50-70 ppmv) using the identical reactor (bottom) (error bars are 1σ).
FIGURE 7. X-ray diffraction patterns of wood fly ash identifying crystalline phases, M - mullite (Al2O3); Mg - magnetite (Fe3O4); Ht - hematite (Fe2O3); Q - quartz (SiO2). molecular weight alcohols using O3 and SiO2 (11). In another scenario, O3 is converted to •OH on the catalyst surface, which subsequently oxidizes the physically adsorbed VOC/VIC (mechanism 2). Finally, both the VOSC/VISC and the O3 could be activated on the catalyst surface promoting a surface reaction between the two chemisorbed substrates (mechanism 3). The fact that propanal, which reacts very slowly with O3 in the aqueous phase (e.g., k2,O3 ) 2.5 L/mol/s for propanal reacting with O3 (24), as compared to k2,OH• ) 6 × 108 L/mol/s for OH• reacting with propionic acid (25)), was oxidized in the catalytic reactor using wood ash (Figure 9) suggests either O3 activation and/or radical formation are involved in the catalytic process (mechanisms 2 and 3), but not direct oxidation of propanal via dissolved aqueous ozone (mechanism 1). As noted previously, most research on catalytic ozonation has centered on liquid phase systems (10, 11); this work
catalytic role of the different phases in the ash, to determine catalytic ozonation kinetics in anticipated industrial gas mixtures, and to develop methods to enhance catalytic activity, potentially by manipulating the ash composition and/or increasing reaction temperatures (within such limits that minimize ozone decomposition).
Acknowledgments This research was supported by the State of Georgia through the Traditional Industries Program (TIP3) and a grant (DEFC26-98FT40028) from the Combustion Byproducts Recycling Consortium (Program of the National Mine Land Reclamation Center, WVU, in cooperation with DOE National Energy Lab). FIGURE 8. Ozone decomposition in an empty reactor (b), the presence of glass wool only (0), and wood ash dispersed in glass wool (9) using a reactor packed with 2 g of wood ash (25% moisture) at 23-25 °C in an approximate 3.75 s residence time.
Supporting Information Available Schematic drawing of the apparatus used to study the catalytic ozonation of propanal using wood ash. This material is available free of charge via the Internet at http://pubs.acs.org.
FIGURE 9. Continuous catalytic oxidation of a gaseous propanal stream using a reactor packed with 3.5 g of wood ash (25% moisture) and ozone (80 ppmv) at 23-25 °C in an approximate 4 s residence time. 9 inlet propanal, b outlet, and 3 fractional conversion. Steadystate (70-200 min) inlet and outlet concentrations and fractional removal error was 34.3 ( 1.5, 20.9 ( 1.8, and 0.39 ( 0.06 ppmv, respectively (1σ). represents new and novel results in gas-phase catalytic ozonation, especially for VOSCs and H2S. Most gas-phase catalytic ozonation research has focused on hydrocarbons, such as ethanol, heptane, or benzene (11). It is also clear that wood ash acted as a catalyst to promote the O3 oxidation reaction at 25 °C, a significantly lower temperature than without O3. For example, a temperature of 250 °C is required to oxidize dimethyl sulfide (100% conversion) using a cobaltmolybdenum catalyst at a residence time of 0.23 s (26). Similarly, platinum- and copper-based oxides required temperatures ranging from 135 to 313 °C, to achieve reaction rates comparable to catalytic ozonation (Table 2). An additional benefit of catalytic ozonation was the fact that catalytic deactivation was not measured over the period of testing, suggesting that elemental sulfur did not form and deposit on the surface, contrary to previous catalytic oxidation experiments using wood ash to oxidize H2S in air only (9) and other reduced sulfur compounds using metal catalysts (27, 28). Significant catalytic deactivation reportedly occurs during the catalytic oxidation of ethanethiol and dimethyl disulfide at temperatures below 250 and 300 °C, respectively (28, 29). It is also clear that ozone altered the catalytic mechanism because the products of oxidation were significantly different from O2 only; SO2 was formed from H2S, DMSO and DMSO2 were formed from DMS, and DMDS was not formed during the catalytic ozonation of methanethiol (Table 2). Further research is required to determine longterm catalyst activity toward all VISCs/VOSCs, to determine the reaction order with respect to the VOSC and ozone, to determine the catalytic mechanism for each VOSC and the
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Received for review January 9, 2004. Revised manuscript received December 8, 2004. Accepted December 9, 2004. ES0499492