Structured catalytic cartridges for SO2 oxidation in flue gases of coal-fired powerplants

Chemical Engineering Journal 378 (2019) 122194

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Structured catalytic cartridges for SO2 oxidation in flue gases of coal-fired powerplants

T

Kseniya Golyashovaa, Pavel Mikenina, Andrey Elyshevb, Andrey Bobylevb, Alexey Matigorovb, ⁎ Eugene Paukshtisa, Sergey Lopatina, Andrey Zagoruikoa, a b

Boreskov Institute of Catalysis, Novosibirsk, Russia Tyumen State University, Tyumen, Russia

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

Oxidation of SO in the media of flue • gases from coal-fired powerplants 2

considered.

at glass-fiber by spraying • PtPt supported precursor shows best performance. catalytic cartridges provide • Structured appropriate activity and pressure drop.

performance shown by multi• Best layered cartridges with flat structuring elements.

A R T I C LE I N FO

A B S T R A C T

Keywords: Oxidation Glass-fiber catalyst Platinum Sulfur dioxide Conditioning Ash particles

Structured glass-fiber catalysts (GFC) are considered for application in the process of oxidation of endogenous SO2 directly in the media of flue gases from coal-fired powerplants. Such oxidation may be used for conditioning of flue gases to improve the efficiency of ash particulates in electrostatic precipitators. Comparison of Pt-based and vanadia-based GFC showed the much better performance by Pt/GFCs, having much higher activity and significantly lower ignition temperature (~300 °C). The best performance is demonstrated by Pt/GFC synthesized by high-temperature synthesis method in combination with spraying of active component precursor solution on the glass-fiber support surface instead of conventional impregnation. Application of structured GFC cartridges makes possible to apply them directly inside the flue gas duct with appropriate pressure drop. High permeability of such cartridges makes possible to apply them in the gas fluid contaminated with ash particulates with minimized risk of clogging. Beyond the mentioned advantages, application of GFC may be useful for resolution of related environmental problems: incineration of polyaromatic compounds and mercury-containing organic substances in flue gases.

1. Introduction Despite the active development of the new approaches in the area of

electricity production, coal-fired powerplants still remain the major source of electricity in a global scale. Most probably, the active use of coal as the combustible fuel will last for at least a few next decades.

Abbreviations: DSC, differential scanning calorimetry; DTA, differential thermal analysis; GFC, glass-fiber catalyst; GFF, glass-fiber fabric; SEM, scanning electron microscopy; STA, synchronous thermal analysis; STS, surface high-temperature synthesis; TEM, transmission electron microscopy; TGA, thermogravimetric analysis; WDXRF, Wavelength Dispersive X-ray Fluorescence; XRD, X-ray diffraction ⁎ Corresponding author. E-mail address: [email protected] (A. Zagoruiko). https://doi.org/10.1016/j.cej.2019.122194

Available online 09 July 2019 1385-8947/ © 2019 Published by Elsevier B.V.

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any size and shape, which are characterized with high mass transfer efficiency, extra-low pressure drop and applicability in dusty fluids [14,15], making them especially attractive for application in flue gas conditioning purposes. At the same time, the mentioned GFCs used thermo-stable zirconiapromoted glass-fiber supports, which are quite expensive and have very limited availability at the open market, so the development of the new GFC using cheaper and more available support is an important research task. Basic engineering of the conditioning process applying oxidation of SO2 directly in the flue gas media is also an important problem which was never successfully resolved earlier. The aim of the current study was development of the appropriate catalytic system and process basements for the process of flue gas conditioning.

Coal itself is cheap and efficient fuel, its natural reserves are estimated as sufficient for centuries, but its use is connected with significant environmental problems. Among these problems, the most dangerous is the pollution of atmosphere with fly ash and dust particulates, which is associated with higher morbidity and mortality from respiratory, cardiovascular and cerebrovascular diseases, and lung cancer in the regions located near the powerplants [1]. The most efficient tool for abatement of fly ash are the electrostatic precipitators, but their efficiency may be insufficient in case of high electric resistivity of ash, produced in result of combustion of some types of coal, characterized with high content of non-polar components like silica. This problem may be resolved by application of flue gas conditioning technologies based on introduction of various substances which may adsorb at ash particles and modify their electrophysical properties [2,3]. This effect may be achieved by use of water vapor or ammonia, but the most strong and universal action is provided by sulfur trioxide (SO3) [4–6]: it forms the microdroplets of sulfuric acid, which are adsorbed at the surface of ash particles thus decreasing their electric resistivity and improving the operation of precipitators even under quite low SO3 content in flue gases (e.g. few tens of ppm). Sulfur trioxide is formed during oxidation of sulfur-containing components of coal in a furnace, but its content in flue gases is too small for conditioning purposes. Application of pure SO3 as an external conditioning agent is complicated due to problems with its transportation, storage and injection into the flow of flue gases. Therefore, the most attractive method of SO3 supply to flue gases is on-site oxidation of sulfur dioxide. This may be realized in two ways: by oxidation of endogenous SO2 present in flue gases as a product of coal combustion or by oxidation of synthetic sulfur dioxide produced locally from elemental sulfur. The first way does not need to use any additional consumable reagents, while the second one gives the way to create relatively small oxidation units, located separately outside the flue gas duct and being independent from the operation of the main coal furnace. Anyway, SO2 oxidation occurs only in presence of appropriate catalyst. The most widely used catalyst for this reaction is V2O5 supported at silica support, which is resistant to sulfation [7]. It is characterized with good oxidation activity and extra-long period of its successful practical application, currently encountering more than a century [8]. At the same time, the conditioning process specifics include very strong limitations on the pressure drop, very high fluid velocity and significant content of fly ash in the flue gases. To fit the process requirements the catalyst should be structured in a form of honeycomb monolith, characterized with low hydraulic resistance, applicability in dusty flows and high resistance to mechanical and thermal shocks. All this makes impossible to apply conventional fixed beds of granulated vanadia SO2 oxidation catalysts due to their very high pressure drop and high risk of clogging by particulates. Structuring of vanadia catalysts in form of monoliths is complicated by insufficient mechanical strength of silica support, while more strong alumina is hardly applicable here due to possible formation of sulfates, leading to support degradation and destruction. Though some attempts to create such monoliths were reported [9], they have not resulted in successful commercial applications yet. The breakthrough in this area may be provided by application of glass-fiber catalysts (GFCs) [10]. It was demonstrated that Pt-containing GFCs show high activity in SO2 oxidation reaction and low ignition temperature even at platinum content as low as ~0.02% mass [11]. Our previous research with this catalyst [12] resulted in basic engineering of the SO3 manufacturing process. The pilot unit [13] with a capacity of ~10 st.m3 SO3 per hour (which is sufficient for conditioning of flue gases from 300 MW coal burner) showed easy start-up, stable operation and practical absence of catalyst deactivation after more than 900 h on-stream in the reaction media, containing up to 10% vol. SO2. Very important, GFCs may be arranged into structured cartridges of

2. Experimental 2.1. Synthesis of catalyst samples The surface high-temperature synthesis (STS) approach was used for synthesis of GFC samples [16–21]. Formation of the catalytically active component in this method occurs under thermal treatment of the combined precursor, containing the active metal and the combustible additive, directly at the support surface. Interaction between air and combustible additive at elevated temperature leads to the formation of highly dispersed particles of active component, potentially having high catalytic activity. Accounting for specific process requirements on catalyst thermal stability, we used the thermostable glass-fiber fabric (GFF) KT-11-TO (manufactured by Sudogda Stekloplastik Co., Sudogda town, Vladimir region, Russia) as a support. This GFF, containing 94.5–96.0% SiO2, up to 3.5% Al2O3, up to 1% CoO and up to 1% SO3, is thermally stable up to 1100 °C. Before catalyst synthesis the fabric was incinerated at 450 °C in air for 2 h to remove the traces of organic impurities and oils used in GFF manufacturing. Basing on the earlier successful experience, Pt was selected as the active component for synthesis of GFC samples. Besides, the attempt to create the vanadia-based GFC was made, as soon as vanadia is known as active component in conventional SO2 oxidation catalysts, it is interesting due to its much lower price compared to platinum. Though, the specific activity of vanadia was known to be much lower than that for platinum, some optimism was based on the possibility to support much more vanadia at GFF than Pt. The Pt/GFC synthesis method uses the combined precursor, consisting of platinum tetraammonium dichloride with organic fuel additive in form of water solution [22]. The precursor was supported by both the conventional impregnation method [22] and by spraying the precursor solution on the surface of glass-fiber fabric. Spraying was expected to be more efficient supporting way as soon as it may provide preferential distribution of active component at the external surface of glass fibers, while in the impregnated sample the active component is distributed more uniformly across the whole fiber volume, which is less accessible to reactants. To evaluate the influence of Pt content on its dispersion and activity two different Pt/GFC were prepared by impregnation, one containing ~0.06% weight Pt [22] and another one with decreased (~0.02–0.03% w) Pt content, using more diluted precursor solution. The sample of the Pt/GFC of the previous generation [10–13], using the glass fiber support, modified with ZrO2, was also selected for comparative studies. Vanadia-based GFCs were synthesized earlier [23,24] for H2S oxidation, but in the given study it was necessary to create the different catalyst as soon as SO2 oxidation reaction requires additional presence of potassium pyrosulfate, needed for decrease of vanadia melting temperature. The initial glass-fiber fabric was treated with 20% silica sol to create the secondary porous silica layer at the surface of glass fibers. Then the treated support was impregnated by water solution of vanadyl oxalate 2

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the studied objects, published in X-ray Powder Diffraction File JCPDSICDD and ICSD-for-WWW (Fachinformationszentrum (FIZ) database, Karlsruhe, Germany, 2003–2010). Size of active particles in prepared GFCs was measured by raster graphics editor GIMP (GNU AGPLv3, Free Software Foundation, USA, 2007). Synchronous thermal analysis (STA) method was performed at Netzsch STA 449 F5 Jupiter device with data processing software. The method unites the differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) [26,27]. Synthetic air (20.9% O2 + 79.1% N2) with the flow rate 50 ml/ min was used a main gas media, argon with flow rate of 20 ml/min as a protective media. Corundum was used as a crucible material to exclude any interaction with GFCs, crucible volume was 0.3 ml. The temperature rise ramp of 10 K/min was set for the range from 21 to 1000 °C. The heat effects of reactions were determined by means of Proteus 6 2012 software. Identification of surface phases in vanadia-based GFCs was performed by XRD analysis [29] at «General-Purpose X-ray diffractometer DRON-7» (Bourevestnik JSC, Russia, St.Petersburg), using the “powder” method [30]. Studies were made in copper and cobalt filtered emission (Cu Kα – emission, Ni – filter; Co Kα – emission, Fe – filter). Calibration was made on the base of α-quartz powder, dispersed monocrystal silicon was used as a standard. Processing of obtained data was performed by TOPAS software (Bruker) connected to ICSD and PDF databases. Parameters of electronic phase cell were defined in the angle range 10° < 2θ < 75° with accuracy ± 0,001 nm. Pt content in GFCs was measured by atomic absorption spectroscopy method using the Hitachi Z8000 spectrophotometer with mean relative error equal to 0.15. Analysis of V/GFCs composition was performed by means of Wavelength Dispersive X-ray Fluorescence (WDXRF) method at the ARL Optim’X device with OXSAS software.

Table 1 List of catalyst samples. Notation

Synthesis method

Active metal content, % mass

Pt/GFC-I Pt/GFC-S Pt/GFC-I min Pt/Zr/GFC 1V2O5/GFC 2V2O5/GFC 4V2O5/GFC 8V2O5/GFC

impregnation + STS spraying + STS impregnation + STS described in [10] impregnation + STS impregnation + STS impregnation + STS impregnation + STS

0.058 (Pt) 0.025 (Pt) 0.028 (Pt) 0.023 (Pt) 0.98 (V) 2.32 (V) 4.96 (V) 10.3 (V)

VO2C2O4 (which simultaneously served as the V precursor and combustible additive) and potassium pyrosulfate К2S2O7. Varying the dilution of vanadia precursor solution, four different V/GFC samples were synthesized with calculated vanadium content from 1% to 8% mass. Impregnated samples were then dried at ambient temperature for 6–10 h and thermally treated at 300 °C for 3 h. The description of all synthesized samples is given in Table 1.

2.2. Catalyst characterization techniques SEM was used for study of glass fiber surfaces with high spatial resolution. Scanning of GFC samples was performed at scanning electron microscope JSM-6510LV (JEOL, Japan) [25] under accelerating potential of 20 – 30 kV, with the SpotSize parameter equal to 30 – 50 relative units. Electron images in secondary electrons were used to reflect the properties of GFC surfaces, such images are mostly influenced by the sample surface topology and its conductivity [26]. TEM studies were performed at electron microscope JEM-2010 (JEOL, Japan) under accelerating potential 200 kV and resolution 1.4 Å. The digital processing of obtained images was performed by means of Gatan Digital Micrograph software. It provides the Fourier analysis of imaging area, its filtration and creation of diffraction image from selected area, used for revealing of periodical motive of the crystal structure image area and calculation of the inter-planar distances. The obtained data were compared with crystallographic characteristics of

2.3. Catalyst testing technique Testing of synthesized GFCs was performed at the lab-scale experimental installation, its flow-sheet is shown in Fig. 1. Reaction mixture is obtained by mixing of sulfur dioxide from vessel

Fig. 1. Flow-sheet of the experimental setup. 1 – SO2 vessel, 2 – air source, 3 – reactor, 4 – vessel with sulfuric acid, 5 – gas analyzer, 6 – flow-mass controllers, 7 – thermocouples. 3

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Fig. 3. Electron diffusion reflectance spectra of the initial glass-fiber fabric with supported precursor (curve 1), Pt/GFC after thermal treatment at 400 °C (curve 2) and 700 °C (curve 3). Fig. 2. GFC experimental cartridge.

3.1. GFC characterization results

Three Pt types are seen in the GFC after thermal treatment: ionic platinum Pt2+ with maximum intensity in the region of 20 000 cm−1, charged platinum clusters Ptδ+ (~38 000 cm−1) and surface particles of metal Pt with the size of 5 nm and more (in the range above 50 000 cm−1). Similar Pt forms are seen in the sample after high-temperature treatment, though amount of charged clusters Ptδ+ decreases and the rise of amount of ionic Pt and large surface particles is observed. However, these changes are not dramatic, this confirms the potentially high catalyst thermal stability. Similar Pt states were observed earlier for the Pt/Zr/GFC of previous generation [10]. Notably, in the mentioned work the bands 38 900 cm−1 were attributed to highly dispersed (up to 1 nm) “subsurface” Pt particles. Most probably, this attribution was incorrect, as soon as the similar bands are seen in the described GFC, where formation of any subsurface Pt forms is excluded by the nature of the used glass-fiber support and, besides, is directly disproved by electron microscopy data, which showed complete absence of subsurface Pt particles both in Pt/ GFC-I and Pt/GFC-S. The difference in GFCs produced by impregnation and by spraying of precursor solution is seen in Figs. 4 and 5. The SEM data (Fig. 4) shows definite non-uniformities in size and disposition of active component in impregnated sample and more uniform distribution of precursor supported by spraying. According to statistical analysis of SEM images (Fig. 5), Pt is present in the impregnated sample Pt/GFC-I in form of relatively large particles (10 nm and larger). Despite expectations, Pt particles in impregnated sample with lower platinum content Pt/GFC-I min appeared to be even larger. In the sprayed sample Pt/GFC-S, the picture is opposite – the main part of Pt is present here in form of highly dispersed (up to 5–6 nm) and dispersed (up to 10–15 nm) particles. Pt/Zr/GFC is in intermediate position – it contains both the highly dispersed Pt (up to 15 nm) and medium size (20–90 nm) particles. STA of Pt/GFCs showed practically no loss of mass sample and no phase transfer peaks. In particular, it means that neither the GFF support, nor the synthesized GFCs undergo any serious changes even at quite high temperatures (up to 1000 °C), thus again confirming the potentially high thermal stability of catalyst proposed in this work.

3.1.1. Pt/GFCs Fig. 3 shows the electron diffusion reflectance spectra of the initial GFF with precursor supported by impregnation (curve 1) and Pt/GFC-I after thermal treatment at 400 °C (curve 2) and 700 °C (curve 3). The spectrum of initial sample contains bands typical for precursor molecules (d–d-transition ~36 000 cm−1 and band of metal–ligand charge transfer 46 000 cм–1 in platinum tetraammonium dichloride).

3.1.2. V/GfcS According to XRD data, the synthesized vanadia-based GFCs shows the reflexes of VO(OH)2, V2O5 and K2S2O7 phases. Fig. 6 shows the SEM images of vanadia GFCs. It is seen that surface of all fibers is covered with highly dispersed vanadia particles of oval and needle shape with the size from 0.02 to 0.7 μm. The number of

1 and air 2 and then it is fed to reactor 3. The flow rates of SO2 and air are controlled by flow-mass controllers FM1 and FM2, respectively. Reactor represents itself the stainless-steel parallelepiped with internal square sequence of 44x44 mm. Reactor is placed inside the protecting cover, including two zones of electric heating. Uniformity of gas distribution across the reactor sequence is provided by a bed of steel 3 mm balls. As a result, the maximum value of temperature nonuniformity inside the GFC cartridge in course of experiments does not exceed 1 °C. GFCs were charged into reactor in a form of structured cartridges with alternating corrugated and plain metal meshes (Fig. 2). Such cartridge has a shape of cube with the size 44x44x44 mm, the height of channels in the cartridge was 3 mm. The mass of GFC in each cartridge was equal to 7.2 g, except the Pt/Zr/GFC sample which mass was 11.3 g due to its higher density per unit surface area because of significantly higher Zr-GFF support thickness. Outlet gas from reactor is passed through the vessel with sulfuric acid 4 to remove SO3. Afterwards, the SO2 concentration is measured by MRU Vario Plus Industrial gas analyzer. Control of temperature, flow rate and gas composition analysis are completely automated. Experiments were performed with a mixture of 200 ppmv SO2 in air, reasonably reproducing typical sulfur dioxide content in flue gases. Air flow rate was equal to 18 L/min, corresponding to gas hourly space velocity of 12,678 h−1. Experiments included temperature variation in the range from ambient temperature to 500 °C. Blank experiment with the cartridge charged with “empty” GFF without active component showed zero SO2 conversion in the major part of temperature region with negligible conversion (max 3–5%) only at 500 °C, thus confirming practical absence of homogeneous non-catalytic reaction. 3. Experimental results

4

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Fig. 4. SEM images of Pt/GFCs produced by impregnation (a) and spraying (b) methods.

high activity. Another important advantage of all Pt/GFCs is low ignition temperate – the reaction starts at ~290–300 °C. Pt/GFC-I shows a little bit higher activity, this agrees with highest content of Pt in this sample. Difference between activities of Pt/GFC-S and Pt/Zr/GFC is negligible, while the mass Pt content is almost equal, but Pt/Zr/GFC loading in the reactor is 1.5 times higher due to its higher density. Fig. 9 shows the specific catalytic activity of Pt/GFCs expressed in form of apparent rate constant related to unit mass of Pt, calculated as follows:

kmPt = −

Q ln (1 − x ), (std. lg −1sec −1 ) mCPt

(1)

where Q – reaction mixture flow rate (std.l/sec), m – GFC mass in the catalytic cartridge (g), CPt – Pt content in GFC (mass fraction), x – SO2 conversion. It is seen that specific activity of Pt/GFC produced by spraying method is twice higher than that for impregnated similar sample and by 1.5 times higher than that for conventional Pt/Zr/GFC prepared on the base of zirconia modified GFF. Pt/GFC-I min with the worst Pt dispersion demonstrates the lowest activity. This difference correlates quite well with the data on Pt size distribution (see Fig. 5), it is possible to conclude that the specific activity is directly connected with fraction of highly dispersed Pt particles at the GFC surface. The vanadia samples are much less active. The ignition temperature here is ~380–400°, this is rather typical for conventional V2O5 + K2S2O7 catalysts [7]. It is interesting that the observed activity of all V2O5/GFCs is approximately equal, despite very different content of active vanadia. As soon as the best results were shown by Pt/GFC-S, this catalyst was selected for further process development studies.

Fig. 5. Size distribution of Pt particles in the Pt/GFCs.

needle-shaped 0.4–0.7 μm particles rises in a sequence from 1V2O5/GFC to 8V2O5/GFC samples. Intra-fiber space in all samples is filled with secondary support (silica sol), the coverage is strongly nonuniform. The thermogravimetric curves (Fig. 7) obtained by STA method in the temperature range from 21 to 1000 °C in air, show the change of samples mass up to 5% (green curve), connected with the loss of volatile compounds, adsorbed by support in course of GFC synthesis. All samples shows the heat peak at 540 °C, related to oxidation of VO(OH)2 in air to V2O5 (blue curve) [28]. 3.2. Activity tests The dependence of SO2 conversion upon temperature at different GFC samples is shown in Fig. 8. It is seen that all Pt/GFCs, except Pt/GFC-I min shows relatively

Fig. 6. SEM images of 1V2O5/GFC (A), 2V2O5/GFC (B) and 8V2O5/GFC (C). 5

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Fig. 7. Thermograms of 1V2O5/GFC(a), 2V2O5/GFC(b), 4V2O5/GFC(c) and 8V2O5/GFC(d).

4. Process modelling 4.1. Process concept and model Oxidation of endogenous SO2 directly in the media of flue gases was considered. The bed of GFC cartridges is installed directly in the flue gas duct part, where the temperature is sufficient for sulfur dioxide oxidation. In the standard coal-fired powerplants such high-temperature zone with temperatures 420–450 °C is located between economizer and air-heater. The initial SO2 content may be taken at the level of 200 ppmv, quite typical for many coal types. The required concentration of SO3 in flue gases is ~10 ppm, therefore, the necessary conversion of sulfur dioxide is ~5%, higher content of sulfur trioxide is undesired due to potential problems with corrosion of duct and downstream equipment. In case of such low concentrations and conversions the heat effect of oxidation reaction will be negligible, so we may assume the isothermal

Fig. 9. Apparent SO2 oxidation rate constant related to unit mass of Pt for different Pt/GFCs at 450 °C.

Fig. 8. SO2 conversion vs temperature in oxidation of SO2. Inlet SO2 concentration – 200 ppm. 6

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conditions in the GFC bed. It is also possible to neglect the theoretical reversibility of SO2 oxidation, this relates to high oxygen excess and to interaction between formed SO3 and water vapors in the flue gas, completely shifting the equilibrium towards SO3 formation. Assuming the plug flow regime in the GFC cartridge, which is typical for streams with high velocities, the mass balance equations may be formulated as follows:

u

∂C = −W ∂l

(2)

u

∂C = βSsp (Ccat − C ) ∂l

(3)

kapp =

kapp L

x = 1 − e− u (10) Calculations of geometrical properties of GFC cartridges (specific external surface area, porosity, equivalent pass diameter) were described in detail earlier [14]. Values of mass transfer coefficient were calculated using the criterial equations [15]: Sh = ARe n Sc 0.33 (11)where Sh, Re and Sc – Sherwood, Reynolds and Schmidt numbers with empiric coefficients A and n. The values of these coefficients for GFC cartridges of various geometry are described in [15].

(4)

where C and Ccat – SO2 concentrations (molar fractions) in the gas flow and on the GFC surface, respectively, u – linear superficial velocity of the flue gas (m/sec), l – coordinate along the GFC bed axis (m), W – reaction rate (sec-1), β – mass transfer coefficient (m/sec), Ssp – specific external area of GFC in cartridges (m−1). 4.1.1. Kinetic model Accounting for very low SO2 concentration and significant oxygen excess, the reaction rate may be described by the first order rate equation in respect to SO2. Assuming the plug flow reactor model, it is possible to calculate the volumetric values of rate constant using the equation

kv = −

Q ln (1 − x ), (sec −1 ) v

4.1.3. Pressure drop Pressure drop in GFC cartridges was calculated using equation [32] ρ u2 Ssp

ΔP = ζ 2 3 L (Pa) (12)where ρ – flue gas density at reaction conε ditions (kg/m3), ε – GFC cartridge void fraction, L – catalyst bed length in axial direction (m), ζ – hydraulic resistance coefficient calculated from equation [14]: ς = 0.418Re−0.153 (13)

(5)

where v – experimental catalytic cartridge volume (m ). Representation of calculated values in Arrhenius coordinates (Fig. 10) shows that at moderate temperatures (below 470 °C) the dependence of ln k upon 1/T is almost linear, this corresponding to kinetic region, while at higher temperatures the influence of diffusion limitations becomes visible. Approximation of experimental data gives the values of kinetic parameters: pre-exponent k0 = 1.473*105 s−1, activation energy E = 65.9 kJ/mole. Accounting for apparent GFC density in the experimental cartridge ρGFC = 71.5 kg/m3, the final kinetic equation may be presented in a form making possible to calculate the reaction rate constant for cartridges of various structure with different GFC density: 3

k v = 2.06*103e−

7933 T ρGFC

(7)

where η – efficiency factor, calculated from equation for one-dimensional catalyst layer [31] tanhφ η = φ (8) φ – Thiele modulus: φ = d kv (9) D d – GFC fabric thickness (m), D – diffusion coefficient (m2/sec). The internal porosity of the fabric is high and typical size of internal passages has a magnitude of micrometers, therefore we may use the conventional equation for calculation of D assuming molecular diffusion of SO2 in air. SO2 conversion x may be then calculated as

with boundary condition

l = 0 → C = Cin

1 1/ βSsp + 1/ ηk v

4.2. Process simulation results Three basic types of GFC structures were considered in the process simulation study: cartridges with and without corrugated structured elements and lemniscate beds [15] (Fig. 11). Lemniscate beds [33] consist of the layers (Fig. 11c), each layer including two plain GFC surfaces, parallel to the gas flow direction, with the space between them filled with regularly positioned GFC threads in shape of closed loops (lemniscates). The loop threads, weaved from GFC microfibers, have quite high external area, which may additionally increase under influence of the moving flow, thus providing uniquely high intensity of external mass transfer in combination with low pressure drop [15]. Another important advantage of lemniscate systems is mechanical selfsufficiency of the lemniscates, making possible to create the 3D catalyst beds without use of additional metal-consuming structuring elements. Process simulation included variation of passage height h under constant values of other parameters: inlet SO2 content – 200 ppmv, gas linear velocity (calculated for standard conditions and full sequence of gas duct) – 10 m/sec, temperature − 450 °C. As stated above, the SO2 conversion equal to 5% is sufficient for conditioning purposes. Therefore, the process simulation study was oriented towards the calculation of the GFC bed length L5%, required for achieving such conversion at fixed process conditions (temperature, gas linear velocity) under variation of GFC cartridge parameters. Reformulating the equation (10), this length may be calculated as follows: u L5% = − k ln(1 − 0.05) (14)

(6)

4.1.2. Mass transfer limitations Taking into account the formulated process model (2–4) the apparent reaction rate constant kapp for the plug flow isothermal reactor with the first-order kinetic equation may be formulated as follows:

app

Fig. 12 shows the dependence of the required bed length upon the height of the cartridge passage. In cartridges with and without structuring corrugated meshes, the rise of the passage height leads to the decrease of GFC loading per unit volume of cartridge, resulting in decrease of both intrinsic rate constant kv (due to lower ρGFC in Eq. (6)) and Ssp in equation (7). Moreover, it also leads to decrease of mass

Fig. 10. ln k – 1/T dependence for Pt/GFC-S catalyst. 7

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Fig. 11. GFC structures considered in process simulation studies: cartridge with (a) and without (b) corrugated wire mesh elements, lemniscate layer (c). Height of the gas passage h is shown by arrows.

Fig. 13. Dependence of the GFC bed pressure drop upon the height of the cartridge passage. Gas linear velocity 10 st.m/sec, temperature 450 °C.

Fig. 12. Dependence of the GFC bed length, required for 5% conversion of SO2 upon the height of the cartridge passage. Gas linear velocity 10 st.m/sec, temperature 450 °C.

Summarizing, the best performance is provided by multilayered cartridges with flat structuring elements and by lemniscate structures, showing the minimum required bed length and the lowest pressure drop. Lemniscate system demonstrates better apparent activity and, thus, less bed length, but its mechanical stability under long-term influence of high-speed gas flow is not currently evident, so the final selection of optimal GFC structure will require additional study.

transfer coefficient β, also causing the general decrease of kapp in Eq. (7) and increase of L5%. In lemniscate structure the geometrical parameters of the GFC layers depend mostly upon the parameters of the lemniscate loops and are much less dependent upon the formal value of interplanar distance, therefore, the connection between equivalent pass diameter and Ssp with gas passage height h is essentially indirect. As a result, the rise of L5% with the rise of h here is less expressed. In general, the lemniscate structure shows best performance due to more intensive mass transfer. Cartridges with structuring meshes demonstrate approximately equal activity with slightly better results for the corrugation-free structure. The calculated bed length varies from 0.2 to 0.4–0.5 m, these values look acceptable from the technical point of view. Pressure drop calculations are presented in Fig. 13. In cartridges with structuring meshes, the rise of passage height leads to lower values of hydraulic resistance coefficient in equation (13), as well as to decrease of Ssp and increase of ε in equation (12). All in all, it results in the decrease of pressure drop of the GFC bed despite the rise of its length. As described earlier, the indirect influence of h on structural parameters in lemniscates (with equivalent pass diameter even slightly decreasing with the rise of interplanar height in considered lemniscate structures) leads to almost constant ΔP for these systems. As expected, among cartridges with structuring meshes the lower pressure drop is observed for corrugation-free system due to higher ε. This system also shows lower ΔP than lemniscate structure at passage height above 8 mm. The absolute value of pressure drop (below 300 Pa) may be considered as acceptable from the technological point of view.

5. Conclusions The performed study has demonstrated that structured glass-fiber catalysts may be potentially applied in the oxidation of endogenous SO2 in the media of flue gases from coal-fired powerplants. Such oxidation may be used for conditioning of flue gases to improve the efficiency of ash particulates in electrostatic precipitators. This optimism is based on both the high observed efficiency of GFCs in oxidation of sulfur dioxide and possibility to use structured GFC-based cartridges, characterized with highly intensive mass transfer, extra-low pressure drop and increased permeability. Comparison of Pt-based and vanadia-based GFC showed the much better performance by Pt/GFCs, having much higher activity and significantly lower ignition temperature. Notably, the best performance was demonstrated by Pt/GFC synthesized by high-temperature synthesis method in combination with spraying of active component precursor solution on the GFF support surface instead of conventional impregnation. This method provides deposition of active component at the surface of glass-fiber threads and minimizes Pt amount inside the threads, where they are less accessible for reactants. Moreover, it provides more uniform Pt distribution in form of highly dispersed surface 8

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K. Golyashova, et al.

particles. Finally, Pt/GFC-S sample showed higher specific activity than the conventional Pt/Zr/GFC synthesized by means of complicated multistage technology on the base of expensive and rare GFF modified by zirconia. Application of structured GFC cartridges makes possible to apply them directly inside the flue gas duct. As shown by process simulation studies, they may provide necessary conversion of SO2 using the reasonable amount of catalyst – the calculated bed length is within the range 0.5 m, it is possible to find enough space for it directly in the flue gas duct, not constructing any reactors or special shells for catalyst. Even more important, such bed will have appropriate pressure drop (below 300 Pa). High permeability of such cartridges makes possible to apply them in the gas fluid contaminated with ash particulates with minimized risk of clogging. As soon as Pt/GFC also have high activity in oxidation of organic compounds [10,22], their application may be useful for resolution of two related environmental problems, which currently attracts a lot of attention in respect to flue gases from coal-fired powerplants: incineration of traces of polyaromatic compounds, formed during coal combustion and oxidation of mercury-containing organic substances.

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Acknowledgements The work on GFC synthesis, testing and process simulation was conducted within the framework of Russian budget project No.03032016-0017 for Boreskov Institute of Catalysis. The work on GFC characterization was conducted within the framework of the project № 1873-00285 (Russian Presidential program of research projects).

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