Development of a compact MnO₂ filter for removal of SO₂ from diesel vehicle emissions

Abstract

Increasing concern about sulfur dioxide (SO₂) from diesel vehicle exhausts causing detrimental effects on NO_x removal catalysts has resulted in the development of dry desulfurization filters for complete removal of SO₂. In this study, a compact MnO₂ filter was developed for diesel emission control. The SO₂-capture behavior of the compact MnO₂ filter was investigated by using a volumetric device in a low temperature range (200–400 °C) and low SO₂ pressure conditions. The maximal capacity of the MnO₂ filter was 304.1 mg_SO2 per g_MnO2 at 400 °C. Based on the experimental results, the required volume of the MnO₂ filter was estimated as only 0.6 L for a diesel car with 30 000 km distance traveled per year. The thickness of the MnO₂ filter had significant influence on its SO₂-capture performance. The sulfate reaction mechanism was also discussed by using a grain model under four reaction temperature conditions for improving the efficiency of the design of the desulfurization filter. The sulfate process can be divided into two control stages (the chemical reaction control stage and the solid diffusion control stage) and the prediction models fit the experimental data well for both control stages, indicating that the two-stage grain model is suitable for the sulfate reaction between the MnO₂ filter and SO₂. The calculated apparent activation energy of 18.8 kJ mol⁻¹ indicates that the MnO₂ filter exhibits high activity for SO₂ adsorption in a pure SO₂ atmosphere.

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Introduction

The removal of sulfur dioxide (SO₂) from vehicle exhausts is of great interest for the reason that SO₂ has detrimental effects on human health and the environment.^1,2 Besides, SO₂ from diesel vehicle exhaust gas greatly deactivates the NO_x removal catalysts because sulfates are more stable than nitrates.³ Compared with the flue gas from large power plants, diesel exhaust gas is quite different because of the low emission amounts, wide temperature range, high space velocity and low sulfur concentration (the sulfur content of diesel varying from tens of ppm to hundreds of ppm). Therefore the traditional wet flue gas desulfurization technologies for large power plants are not suitable for diesel exhaust systems. To solve the issues caused by SO₂ emissions from diesel exhausts, compact SO₂ traps of dry desulfurization technologies have been proposed for complete capture of SO₂ from diesel vehicle exhausts.^4–7

The traditional dry desulfurization materials, such as calcined limestone⁸ and CaCO₃ materials,⁹ have good sulfate reactivity over 650 °C with high SO₂ concentration over 1000 ppm. However, SO₂ capture capacity of these materials declines sharply below 400 °C because the sulfate reaction activity is limited by the decarbonation.⁹ With continuous development of the diesel engine technology, the maximum temperature of diesel exhaust will be decreased to 400 °C. Therefore the development of low-temperature activation and downsizing of the compact SO₂ trap is needed for mounting on mobile diesel vehicle.

To enhance the low-temperature SO₂-capture performance of desulfurization materials, Rubio and Izquierdo¹⁰ studied the SO₂ removal performance of coal fly ash based carbons at flue gas conditions and the amount of SO₂ removed is 13 mg g⁻¹ by the Lada activated sample at 100 °C and 1000 ppmv SO₂ gas conditions. The SO₂ oxidation activity of copper oxide supported on the activated carbon could be improved at low temperature at 250 °C.^11,12 The SO₂ capture capacity of CeO₂ based materials was investigated at low temperature conditions.^13–15 Manganese dioxide with a simple sulfate reaction path (MnO₂ + SO₂ → MnSO₄) showed high SO₂-capture capacity at low temperature conditions.¹⁶ The MnO₂ with high specific surface area had the potential to be more compact for disposable SO₂ traps on account of high desulfurization SO₂-capture performance at low temperature conditions. Li et al.¹⁷ investigated the SO₂ capture capacity of manganese oxide at 325 °C and found that cryptomelane could be application as a replaceable absorbent for the removal of SO₂ from diesel exhaust.

Traditionally, the materials were wash-coated on ceramic carrier (Fig. 1a) by using the binder for final application in the diesel vehicle exhaust system.^18–20 The SO₂ capture capacity per unit volume of desulfurization filter greatly decreased because the ceramic carrier with low SO₂-capture performance occupied too much volume of the filter. Moreover, as the sulfur concentration of diesel is very small (a few hundred ppm), the mutual diffusion resistance could be large, which will greatly lower the diffusion rate of SO₂ reaction gas from the external air to the sites of the reactant.²¹ Therefore, in this study, the compact dry desulfurization filter without ceramic carrier (MnO₂ filter) was developed to downsize the desulfurization filter, as shown in Fig. 1b, and the volumetric method was proposed to exclude the mutual diffusion resistance to obtain the real SO₂-capture behavior of compact MnO₂ filter. The effects of the reaction temperature and thickness of MnO₂ filter were investigated at low SO₂ pressure condition. The sulfate reaction mechanism between the MnO₂ filter and pure SO₂ was also discussed under four reaction temperature conditions for improving the efficiency of designing MnO₂ filter.

Fig. 1 Photograph of traditional filter (a) and MnO₂ filter (b).

Experimental

MnO₂ filter preparation

50 mg MnO₂ was blended with 5 mg Carboxymethyl Cellulose (CMC) in the beaker. Exactly 2 mL of silica-sol (3.4 w% in H₂O) was added dropwise and mixed the blender by glass rod. Then, the blender was extruded through the metal mold and calcined at 400 °C to form a rounded compact MnO₂ filter. The MnO₂ used for experiments was supplied by Japan Material and Chemical Co., Ltd. These materials were obtained by the acid treatment of raw material. The chemicals, CMC and silica sol (34 w% suspension in H₂O), were purchased from Aladdin Co., Ltd., People's Republic of China and were of analytical reagent grade.

Characterization

Morphology of the sample was analyzed by using transmission electron microscope (TEM). In this study, specific surface area, average pore diameter and pore volume were conducted to assess the physical characteristics of the target sample. The specific surface area was measured by the Brunauer–Emmett–Teller with the nitrogen adsorption uptake at the boiling point of nitrogen of 77 K using a capacitive measurement method. The average pore diameter and pore volume were measured by nitrogen physisorption under normal relative pressure of 0.1–1.0 using the Barrett–Joyner–Halenda (BJH) method. The particle size distribution of the sample was analyzed by Malvern laser particle size analyzer.

Volumetric measurements

The volumetric test-rig consists mainly of two compartments, as depicted in Fig. 2. The first is sample cell, in which a sample of 0.1 g rounded absorbent is placed. The reaction temperature of sample cell can be adjusted and controlled by using the heater cover. The second compartment is a large constant volume tank. The temperature of this tank is controlled by using a thermostatic chamber.

Fig. 2 Schematic diagram of the volumetric test-rig.

The volumetric test-rig was used in this study to measure the SO₂ capture capacity of the prepared absorbent. Table 1 presents the experimental conditions used in the present study. The MnO₂ filter was placed into the sample cell and slowly heated (5 K min⁻¹) to the target reaction temperature. At the same time, the tank and sample was evacuated for 1 h by using the vacuum 1 with the valves V1–V3 opened. After the pressure was below 10⁻² Pa, the valves V1–V3 were closed. The valves V4 & V2 were slowly opened to put the SO₂ gas into the tank and then closed when the pressure of the SO₂ gas was up to 40 Pa (equivalent to the partial pressure of 400 ppm SO₂ in the exhaust). The temperature of thermostatic chamber was kept at 35 °C. The sulfate reaction process started once the valve V1 was opened, leading to decreasing the pressure in the tank. The pressure variation was measured to determine the amount of the SO₂ gas on the sorbent sample by using the high resolution ceramic capacitance manometer P2. The SO₂ capture capacity per unit mass of absorbent C and the conversion of MnO₂ X are expressed by the following equations:

C is the SO₂ capture performance per unit mass [g_SO2 per g_material], R is the ideal gas constant, V is the volume of the SO₂ gas, P₀ is the initial pressure of the absorbent [Pa], P_t is the pressure after t seconds [Pa], s₀ is the initial weight of the absorbent [mg]; X is the conversion of MnO₂, ω is the mass fraction of MnO₂ in the absorbent, M_MnO2 is the molar mass of MnO₂ [g mol⁻¹], M_SO2 is the molar mass of SO₂ [g mol⁻¹].

Table 1 Experimental conditions

Variables	Conditions
Temperature of the thermostatic chamber (K)	308
Temperature of the sample (K)	473–673
Pressure of tank (Pa)	40
Mass of the sample (g)	0.05–0.15
Thickness of the absorbent sample (mm)	0.5–2

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Results and discussion

Material characterization

The distribution of pore diameter of the prepared sample is displayed in Fig. 3. From the results of Fig. 3, the relatively large pore diameter of the sample is about 7 nm. The specific surface area, average pore diameter and pore volume of the prepared sample are 275 m² g⁻¹, 7.5 nm and 0.5 cm³ g⁻¹, respectively. The relatively large particle diameter of the sample is about 1.6 μm. XRD analysis (Fig. 4) is carried out to study the crystal structure of MnO₂ filter before and after desulfurization. From the results of Fig. 4, the diffraction peaks of MnO₂ filter are similar to those of tetragonal phase MnO₂ (JCPDS 00-044-0141), and after desulfurization the diffraction peaks of the sample are changed to the orthorhombic phase MnSO₄ (JCPDS 00-011-0088).

Fig. 3 Distribution of pore diameter of the prepared sample.

Fig. 4 XRD patterns of MnO₂ filter (a) before and (b) after desulfurization.

Basic SO₂-capture performance of MnO₂ filter by using volumetric device

Fig. 5 shows the effect of desulfurization temperature on SO₂ capture capacity of MnO₂ filter. The SO₂ capture capacity of MnO₂ filter was investigated at 200 °C, 300 °C, 350 °C and 400 °C. From the results shown in Fig. 5, the SO₂ capture capacity of MnO₂ filter increases with the experimental temperature. At these experimental conditions, the trap achieves the highest SO₂ capture capacity and obtained about 76.3 mg_SO2 per g_MnO2 after 1 h at 400 °C. At 200 °C, the SO₂ trap capacity decreased to about 20% of its capacity at 400 °C and obtained 15.0 mg_SO2 per g_MnO2. SO₂ capture capacity (mg_SO2 per g_MnO2) of MnO₂ filter at 40 Pa pure SO₂ under different sulfate reaction temperature conditions has been listed in Table 2. As seen in the results of Table 2, the maximal capacity of MnO₂ filter was 304.1 mg_SO2 per g_MnO2 at 400 °C, significantly higher than the traditional desulfurization materials,^14,17,22 as shown in Table 3.

Fig. 5 Influence of temperature on SO₂ capture performance of MnO₂ filter at 200 °C, 300 °C, 350 °C and 400 °C.

Table 2 SO₂ capture capacity (mg_SO2 per g_MnO2) of MnO₂ filter at 200–400 °C, 40 Pa pure SO₂

Reaction temperature	200 °C	300 °C	350 °C	400 °C
After adsorption 1 h	15.0	25.7	52.7	76.3
Maximal capacity	15.0	25.7	71.7	304.1

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Table 3 SO₂ capture capacity (mg_SO2 per g_material) of some investigated materials

Materials	SO2 capture capacity	Reaction conditions	References
MnO2 filter	304.1	400 °C, 40 Pa pure SO2	In present work
CuO–CeO2	24	400 °C, 3600 ppm SO2	Rodas-Grapaín et al.^14
TO-AC	161	400 °C, 2000 ppm SO2	Guo et al.^22
Pt–Pd–Al2O3	11	600 °C, 30 ppm SO2	Limousy et al.^4
CaO	36	325 °C, 250 ppm SO2	Li et al.^17
Ca(OH)2	32	325 °C, 250 ppm SO2	Li et al.^17
MgO	20	325 °C, 250 ppm SO2	Li et al.^17
ZrO2	16	325 °C, 250 ppm SO2	Li et al.^17
CuO-AC	30	250 °C, 200 ppm SO2	Tseng et al.^12
Coal fly ash	13	100 °C, 1000 ppm SO2	Rubio and Izquierdo^10

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Based on the experimental results, the optimum volume of the MnO₂ filter required for a diesel engine was estimated. A diesel car (traveled distance: about 30 000 km of one year) is considered in which the concentrations of sulfur in the diesel oil and in lubricating oil are both 30 ppm. It is assumed that the SO₂-capture capacity is not affected by the initial sulfur concentration in this evaluation. Fuel consumption of the diesel car is 7 liter per hundred kilometers. The specific gravity of diesel oil is 0.83 kg L⁻¹. The amount of discharged SO₂ for one year is 104.6 g. Based on the evaluation that the SO₂ capture capacity at a reaction temperature of 400 °C is 182.5 g_SO2 per L_{MnO2 filter} (SO₂ capture performance is 304.1 mg_SO2 per g_MnO2 at 400 °C and the density of MnO₂ filter is 0.6 g cm⁻³), this means that a 1.0 L MnO₂ filter can capture 182.5 g of SO₂. Therefore, the required volume of the MnO₂ filter was calculated as only 0.6 L for a diesel vehicle with 30 000 km traveled distance per one year, remarkably smaller than the traditional wash-coated desulfurization filter.⁷ Morever, as a part of the storage sites on MnO₂ after regeneration are non-renewable¹⁶ and the desulfurization product (MnSO₄) is always in great demand for the industrial and agricultural raw material, MnO₂ filter could be suitable for the disposable desulfurization filter without being recycled every year for economy.²³

The effect on SO₂ capture capacity of MnO₂ filter with different thickness

In the traditional SO₂ traps,⁴ the amount of SO₂ capture was dependent on the quantity of sorbents in the desulfurization trap. In this study, similar experimental results were obtained. The amount of SO₂ adsorption was proportional to the thickness of MnO₂ filter, as shown in Fig. 6. The thickness of materials had much influence on the removal performance of diesel exhaust purifier.²⁴ The dimensionless SO₂ capture performance per unit mass (mg_SO2 per g_MnO2)–time (t) curve of MnO₂ filter with different thickness was obtained by using the volumetric test-rig. As shown in Fig. 7, the shape of the curve is different when the packing height is changed. The SO₂ capture rate of MnO₂ filter decreased with the thickness. The 0.5 mm-thick MnO₂ filter showed the highest SO₂ capture rate than 2 mm-thick MnO₂ filter. The reason is that the mass transfer resistance of gas diffusion in the thicker layer is larger than that in the thinner layer in the adsorbent.⁷

Fig. 6 Effect of the thickness of the MnO₂ filter on the amount of SO₂ adsorption.

Fig. 7 Effect of the thickness of the MnO₂ filter on the SO₂ adsorption performance.

Sulfate reaction mechanism

The sulfate reaction mechanism is very important for improving the efficiency of designing the desulfurization filter. In this study, in order to investigate the sulfate reaction mechanism between MnO₂ filter and sulfur dioxide, the MnO₂ filter with thin layer and low density was prepared to minimize the gas diffusion resistance in the traps. Moreover, a series of experiments were carried out at four reaction temperature conditions (200 °C, 300 °C, 350 °C and 400 °C) with low SO₂ pressure of 40 Pa. Fig. 8 shows the conversion-time profiles of MnO₂ filter for different temperature at low SO₂ pressure of 40 Pa. As this figure shows, the reaction rate increases considerably at higher temperatures. The sulfate degree in the experimental conditions increased steeply at the beginning of the reaction, and then the slopes of sulfate degree curves become gradual after ∼5 min.

Fig. 8 Effect of temperature on solid conversion of MnO₂ filter at 40 Pa of SO₂ gas pressure.

The sulfate reaction between manganese dioxide and sulfur dioxide with a simple reaction path (MnO₂ + SO₂ → MnSO₄) is a solid–gas non-catalytic reaction. Various models had been discussed to describe the mechanism of the solid–gas non-catalytic reaction.²⁵ Among these models, the grain model was a suitable mathematical representation of gas–solid reactions in chemical and metallurgical industries consist of solid pellets produced from small particles or grains, which assumes that the reaction was accomplished on the surface of the fine and non-porous grains.²⁶ TEM were employed to obtain surface features of the samples before and after desulfurization, the results as shown in the Fig. 9. Based on the TEM images in Fig. 9, the prepared MnO₂ filter was formed with a series of smooth spherical grains and these grains are non-porous. Therefore, the grain model should be suitable for the solid–gas reaction between MnO₂ filter and SO₂.

Fig. 9 TEM images of MnO₂ filter before (a, b) and after desulfurization (c, d).

In the grain model, the sulfate rate is controlled by four steps: external diffusion, gas reactant diffusion to the external surfaces of the solid particles from the bulk gas phase; surface reaction, gas reactant being adsorbed on the solid reactant surface; solid diffusion, gas reactant diffusion through the product layers; internal diffusion, gas reactant intra-particle transport through pores. For the whole systems, the controlling regime changes with the reaction time and was continuously consumed because the solid is a reactant.²⁷ Most solid–gas non-catalytic reactions always presented two consecutive stages: the initial stage with the fast sulfate rate was controlled by the surface chemical reaction; the second stage with the slower sulfate rate was controlled by the product layer diffusion.^28,29 According to the grain model, the relationship between the solid conversion equation X and the reaction time t at the chemical reaction control stage for the spherical particle and grain can be expressed by the following equation:

where ρ_s is the solid reactant concentration, k_s is the surface chemical reaction rate constant, C_A is the concentration of reactant gas, b is the stoichiometric coefficient of the reaction, r_g is the initial radius of the spherical grain, X the conversion of solid reactant. The relationship between the solid conversion equation X and the reaction time t at the solid diffusion control stage for the spherical particle and grain can be expressed by the following equation:where D_e is the effective diffusivity, t₀ is the revision factor of the delay time for sulfate rate from the chemical reaction control stage to solid diffusion control stage.

The grain model prediction of chemical reaction control, expressed in eqn (3), was applied to curve-fitting for the beginning of the sulfate process (the initial stage); and the grain model prediction of solid diffusion control, expressed in eqn (4), was applied to fit experimental data for the increase reaction time of sulfate process (the second stage). Fig. 10 shows the lines of the model predictions of chemical reaction control and solid diffusion control under the different reaction temperature conditions. From the results shown in Fig. 10, it can be seen that the prediction models fit the experimental data well for both control stages at 200 °C and 300 °C. At 350 °C and 400 °C, the prediction model of chemical reaction control fit the experimental data well at the initial stage of sulfate process. At higher temperature, the sulfate rate was controlled the zone of sulfate reaction between porous solid and gas became a relative narrow with the sulfate reaction proceeding and the surface reaction rate and diffusion rate were of the same order of magnitude.³⁰ Therefore, the sulfate rate could be controlled by the chemical reaction and solid diffusion at the second stage at 350 °C and 400 °C. Generally, it can be safely concluded that the grain model is suitable for the solid–gas reaction between MnO₂ filter and SO₂ and the sulfate process could be divided into two control stages (the chemical reaction control stage and the solid diffusion control stage) under the experimental conditions.

Fig. 10 Experimental data and model predictions of MnO₂ conversion under four reaction temperature conditions.

In order to determine the activation energy of the sulfate reaction between the MnO₂ filter and pure SO₂, the sulfate reaction rate constant of the chemical reaction control stage was measured at 200 °C, 300 °C, 350 °C and 400 °C. The corresponding Arrhenius plot is shown in Fig. 11. The apparent activation energy of 18.8 kJ mol⁻¹ was calculated by using Arrhenius equation under the experimental conditions. Compared with the value of sulfate reaction of manganese oxide (33.5 kJ mol⁻¹) reported by Tikhomirov,³¹ it can be concluded that in the pure SO₂ atmosphere the MnO₂ filter exhibits high activity for SO₂ adsorption.

Fig. 11 Arrhenius plot of sulfate rate of MnO₂ filter with chemical reaction control.

Conclusions

MnO₂ filter is used for compact SO₂ trap to protect the NO_x removal catalyst against SO₂ poisoning in diesel exhaust system. The SO₂-capture performance of MnO₂ filter without mutual diffusion resistance was measured by volumetric method under low temperature and low SO₂ pressure conditions. MnO₂ filters with different thickness and packing density were prepared, and the influences of reaction temperature, thickness of MnO₂ filter and recycling characteristic of MnO₂ filter on SO₂ capture capacity were investigated under low temperature range from 200 °C to 400 °C and low SO₂ pressure of 40 Pa conditions. The results of the present work clearly reveal that:

• The maximal capacity of MnO₂ filter was 304.1 mg_SO2 per g_MnO2 at 400 °C. The required volume of the MnO₂ filter was estimated as only 0.6 L for a diesel car with 30 000 km traveled distance per one year and MnO₂ filter could be suitable for the disposable desulfurization filter.

• The 0.5 mm-thick MnO₂ filter showed the higher SO₂ capture rate than 2 mm-thick MnO₂ traps because the mass transfer resistance in the thicker layer was larger than that in the thinner layer.

• The grain model is suitable for the sulfate reaction, and the sulfate process can be divided into two control stages (the chemical reaction control stage and the solid diffusion control stage). The prediction models fit the experimental data well for both control stages. The apparent activation energy of 18.8 kJ mol⁻¹ was calculated by using Arrhenius equation under the experimental conditions, which exhibits high sulfate activity between the MnO₂ filter and pure SO₂.

Acknowledgements

This research was supported by National Natural Science Foundation of China (NSFC) through International (Regional) Cooperation and Exchange Projects (Grant No. 21550110494) and Chinese Academy of Sciences President's International Fellowship Initiative (Grant No. 2016VTC068).

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