Catalytic filter for the removal of dust and NOx at low temperature

The catalytic filter was fabricated by supporting selective catalytic reduction (SCR) catalyst on the low-density ceramic (LDC) for the removal of dust and nitrogen oxides (NOx) in the flue gases at relative low temperature. MnOx–ZrO2/TiO2 catalyst was selected as SCR catalyst. The NOx and dust removal efficiency, filter resistance, regeneration performance and anti-sulfur performance were investigated. The result showed that the NOx removal efficiency at 180°C reached 98.4% (1 m/min filtration velocity) for 6 wt% MnOx–ZrO2/TiO2 catalytic filter with Mn/Zr molar ratio of 2. Furthermore, MnOx–ZrO2/TiO2 catalytic filter performed good anti-sulfur performance. In the presence of 10 vol% water vapor and 100 ppm SO2 at 180 °C, the NOx removal efficiency for MnOx–ZrO2/TiO2 catalytic filter could retain up to 83.2% and it could recover to 91.8% when the water vapor and SO2 were cut off. MnOx–ZrO2/TiO2 catalytic filter showed the high dust removal efficiency of 99.99% and the low filter resistance of less than 200 Pa. The filter resistance of MnOx–ZrO2 /TiO2 catalytic filter could maintain 235.7 Pa after 200 times pulse blowback. The result illustrated that MnOx–ZrO2/TiO2 catalytic filter showed good regeneration performance.


Introduction
Air pollution has been regarded as one of the most important health issues in recent years. The emission of dust and nitrogen oxides gives rise to human's focus. Dust (especially particulate matter 10 μm) could float in the air and fine dust comprises toxic components to human health [1,2]. In industry plants, the electrostatic precipitators and fabric filters are the main facilities to control the particulate matter [3,4]. Moreover, the application of LDC filter fabricated by ceramic fibers has proved the improvement in the gas-solid separation in recent years [5,6].
Nitrogen oxides are one of the main causes of haze, acid rain and greenhouse. Removal of NO x is essential to accord with the environmental emission requirement. Selective catalytic reduction (SCR) process performs high efficiency and reliability, and it becomes the most common technology to eliminate NO x in industry plants [7]. The common commercial catalyst for SCR process is V 2 O 5 -WO 3 (MoO 3 )-TiO 2 , which is highly active in the temperature range of 300°C-400°C [8,9]. The constitution of flue gas in industry plants is more complex and the fine particles in flue gas can produce physical or chemical poisoning effects on SCR catalysts. The dust removal apparatus is usually installed with the denitrification unit in the industrial application. But these two units occupy a lot of space and consume enormous amounts of energy. A new technique to remove dust and NO x simultaneously has been developed. Kim developed V 2 O 5 -WO 3 /TiO 2 supported-SiC catalytic filters by using rotational coating. [10]. The catalytic filter element showed high SCR activity leading to 99% NO x conversion efficiency at 300°C. But the filter resistance of V 2 O 5 -WO 3 /TiO 2 supported-SiC catalytic filters was very high (over 1000 Pa). On the other hand, the temperature of exhaust gas emitted from some industry boilers is lower than 300°C. The catalytic activity of V 2 O 5 -WO 3 (MoO 3 )-TiO 2 reduced at relative low temperature and cannot meet the requirement for exhaust gas treatment. Abubakar et al reported a catalytic bag filter using V 2 O 5 -MoO 3 -TiO 2 as catalyst [11]. The catalytic bag filter showed only 60% NO x conversion efficiency at 200°C. Furthermore, vanadium pentoxide is toxic to environment and human health. Thus, it is necessary to develop a new catalytic filter for the application at lower temperature. Mn-based complex oxides have been exhibited an Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. excellent performance for the NO x removal at low temperature [12][13][14][15][16]. Kang et al prepared a catalytic filter with MnO x catalyst coating on P84 bag filter [17]. The NO x conversion efficiency of catalytic filter was over 90% at 150°C. A catalytic filter with Mn-Ce-Nb-O x catalyst coated on P84 felts was also studied for the removal of particulates and NO x [18]. The NO x removal efficiency and PM2.5 removal efficiency at 200°C reached 95.3% and 99.98%, respectively. For the above studies, the bag filter or P84 felts used as filter materials and catalysts support is nonrigid, which deformed during the pulse blowback operation. The most staple problem for these catalytic filters is the peeling of catalysts during the operation of gas-solid separation and NO x removal, which caused the decline of NO x conversion efficiency. Polytetrafluoroethylene (PTFE) was used as binder in the catalyst coating emulsion for fixing the catalysts on the bag filter or P84 filter. But it could increase the filter resistance and decrease the NO x removal efficiency. The solution to solve these problems is using the rigid ceramic filter as the catalysts support for avoiding the degradation of catalytic performance caused by catalyst falling off. The low-density ceramic (LDC) filter composed of short ceramic fibers (mullite fiber, Al 2 O 3 fiber and aluminum silicate fiber) possesses the high porosity, good thermal shock resistance and low filter resistance. Furthermore, LDC displays better resistance to acids and alkalis [19]. Therefore, LDC filter was used as the catalyst support in this work.
The objective of this study is to develop an integration technology for the removal of dust and NO x in the flue gases at relative low temperature and improve the service life of catalytic filter. The MnO x -ZrO 2 /TiO 2 catalyst was loaded on the pore surface of LDC filter and dispersed throughout the filter by introducing TiO 2 using homogeneous precipitation method. The effect of Mn/Zr molar ratio, catalyst loading, filtration velocity and anti-poison ability on the NO x removal was discussed. The dust removal efficiency, filter resistance and regeneration performance of the MnO x -ZrO 2 /TiO 2 catalytic filter were also studied to provide a reference for the industrial applications of catalytic filters.

Preparation of LDC catalytic filter element
The LDC filter element for gas-solid separation was prepared by following process. Firstly, the mullite fiber (Zhejiang Hongda Crystal Fiber Co., Ltd, China), glass powder (Nanjing Sanle Co., Ltd, China), activated carbon powder (200 mesh, Shanghai Xitan Environmental Protection Technology Co., Ltd, China) and carboxymethyl cellulose (Sinopharm Chemical Group Co., Ltd) was mixed in a certain mass ratio. Then, the appropriate amount of water was added to green mud with stirring. The mixture was aged at room temperature for 24 h and pressed into a circular pellet (d=20 mm, h=10 mm). The specimens were dried at 60°C for 12 h and sintered at 1100°C for 1 h to obtain the LDC filter element. The TiO 2 was loaded on the LDC filter element by the homogeneous precipitation method [20]. Then, TiO 2 -loaded filter element was immersed in the mixed solution of Mn(NO) 2 (50 wt%, Sinopharm Chemical Group Co., Ltd) and ZrO(NO 3 ) 2 (AR, Aladdin Reagent Co., Ltd, China). The LDC filter element with catalyst solution was dried at room temperature for 24 h, followed by calcining at 500°C for 2 h. The catalyst loading was controlled by changing the concentration of solution or immersion times.

Catalyst characterization
The micromorphology of LDC catalytic filter element was analyzed using field-emission scanning electron microscopy (FESEM, ZEISS Ultra 55) with energy dispersive x-ray spectrometry (EDX). The pore size distribution was measured using Mercury Porosimeter (GT-60, Quantachrome, America).

Catalytic activity and dust removal efficiency
SCR catalytic activity was determined on a fix-bed flow reactor. The reactor was composed of two quartz tubes and a tubular programmable temperature control furnace ( figure 1). The basic composition of flow gas was composed of 600 ppm NO x , 600 ppm NH 3 , 6 vol% O 2 , with balanced N 2 . A mass flow controller has been used to control the total gas flow rate. The total flow rate of flue gas was 314 ml min −1 (corresponding to the filtration velocity of 1 m min −1 ). The NO x concentration was detected by flow gas analyzer (ECOM-D, RBR, Germany). The NO x removal efficiency was calculated as follows: x in x out The dust removal test was performed with an air filter efficiency instrument (CW-HAT2100). The filter resistance was characterized by the Chinese standard 'test method of the performance of high efficiency particulate air filter' (GB/T 6165-2008). Song et al [21] presented the details on how to test the filter resistance. The fly-ash was used as pollution dust (Φ=2-3 μm). The procedures of regeneration performance measurement were mentioned in our previous work [19].

Result and discussion
3.1. Micromorphology and pore size distribution of catalytic filter The micromorphology of LDC catalytic filter element was showed in figure 2. It could be observed that the mullite fibers were interconnected with each other to build three dimensional connected channels, which separate the gases and particulates effectively ( figure 2(a)). The catalysts showed ball-like and distributed on the fibers ( figure 2(b)). Figure 3 showed the EDX mapping images of catalyst distribution for MnO x -ZrO 2 /TiO 2 catalytic filter. Mn and Zr were coated on the surface of TiO 2 and mullite fibers. No accumulation of catalysts was founded at the intersection of the fibers. The catalysts highly dispersed and adhered firmly on the fibers throughout the filter. The pore size distribution for LDC filter element with and without MnO x -ZrO 2 /TiO 2 catalyst was also studied (figure 4). Both filter elements have a narrow pore size distribution. The predominant pore size was distributed in the range of 15-60 μm for LDC filter element and 10-50 μm for MnO x -ZrO 2 /TiO 2 catalytic filter element. The change of pore size distribution was contributed from the catalyst loading on the fiber.

Effect of molar ratio and catalyst loading on NO x removal
The effect of Mn/Zr molar ratio on the NO x removal efficiency was studied to obtain the optimum catalytic activity for SCR. The results showed that the highest catalytic activities were obtained at Mn/Zr molar ratio of 2 ( figure 5(a)). The NO x removal efficiency was more than 90% in the temperature range of 140°C-240°C and reached 98.4% at 180°C especially. The MnO x -ZrO 2 /TiO 2 catalyst showed excellent NO x removal efficiency  and it could be attributed to an intimated interaction between Mn, Ti and Zr, which allowed more electron transfer between Mn, Ti and Zr. Figure 5(b) showed the redox mechanism of NO x reduction via the Eley-Rideal mechanism over the MnO x -ZrO 2 /TiO 2 catalyst. Ti 4+ and Zr 4+ could restore the Mn 3+ to Mn 4+ . The intimate interaction facilitated the electron transfer and accelerated the circulation of Mn 4+ and Mn 3+ redox couple, which promotes the NH 3 -SCR process [22][23][24][25][26] The filtration and NO x removal principle of LDC catalytic filter were illustrated in figure 6. The fine particles in the flue gas could be intercepted by the filter and accumulated on the surface of LDC catalytic filter element, while the gases could infiltrate into the pores of LDC catalytic filter. The NH 3 , NO x in the flue gas were sufficiently absorbed by the SCR catalyst dispersed throughout the filter and converted into N 2 and H 2 O in the presence of O 2 .
The catalyst loading could influence the performance of NO x removal. As can be seen in figure 7, the NO x removal efficiency exhibited a growing trend with the catalyst loading increasing within 6 wt%. The higher catalyst loading means the more acid active sites for the adsorption of NH 3 , which makes the improvement of NO x removal efficiency. The NO x removal efficiency for MnO x -ZrO 2 loading of 6 wt% was over 90% in the temperature range of 140-200°C. When the MnO x -ZrO 2 catalyst loading increased to 8 wt%, the NO x removal  efficiency remained flat. Considering the cost and the NO x removal efficiency, the MnO x -ZrO 2 catalyst loading of 6 wt% is selected for later experiment.

Effect of the filtration velocity, SO 2 and H 2 O on NO x removal efficiency
The filtration velocity is an important factor for the filter. The filtration velocity is inversely proportion to the reaction time. The higher filtration velocity would result in the decline of NO x removal efficiency.   Usually, the industrial flue gas contains 3-10 vol% water vapor and small amount SO 2 , which makes catalysts poisoning. The NO x removal efficiency declines with the deterioration of catalyst. Figure 9 illustrated the effect of 10 vol% water vapor and 100 ppm SO 2 on the NO x removal efficiency. The NO x removal efficiency for MnO x -ZrO 2 /TiO 2 catalytic filter decreased significantly in first two hours when the water vapor and SO 2 were injected into the flue gas. There is the competitive adsorption of water vapor, SO 2 and NH 3 on the acid active sites, which had an inhibiting effect on the NH 3 -SCR process [27,28]. Moreover, water vapor and SO 2 reacts with NH 3 to produce sulfur ammonium salt (NH 4 HSO 4 , (NH 4 ) 2 SO 4 ), which occupied the acid active sites. Thus, the NO x removal efficiency decreases with the reduction of acid active sites. The consumption of NH 3 would weaken the SCR process too. The NO x removal efficiency for MnO x -ZrO 2 /TiO 2 catalytic filter could retain up to 83.2% in the presence of 10 vol% water vapor and 100 ppm SO 2 and it returned to 91.8% when the water vapor and SO 2 were cut off.

Dust removal efficiency and regeneration performance
During the filtration process, the filter resistance increases with the dust entering the pores of filter elements or with the cake accumulating on the surface of filter elements. The pulse blowback was used to clean the filter elements, which resulted in an instant decrease of the filter resistance. The filter resistance observed after pulse blowback is named residual filter resistance. The residual filter resistance increases continuously and eventually stabilize to a value, defined as baseline filter resistance. The baseline filter resistance is one of the important criteria for evaluating regeneration performance of filters [19].
The filter resistance for MnO x -ZrO 2 /TiO 2 catalytic filter with catalyst loading of 6 wt% was measured as 198 Pa. As shown in figure 10, the residual filter resistance of LDC filter and MnO x -ZrO 2 /TiO 2 catalytic filter increased at first, and then reached a balance with the increase of cycle times. The baseline filter resistance of MnO x -ZrO 2 /TiO 2 catalytic filter was 236 Pa. The filter resistance of MnO x -ZrO 2 /TiO 2 catalytic filter just increased 38 Pa after 200 times pulse blowback compared with the initial filter resistance. The result indicated that the MnO x -ZrO 2 /TiO 2 catalytic filter had good regeneration performance. The dust removal efficiency for the MnO x -ZrO 2 /TiO 2 catalyst filter reached 99.99%.

Conclusion
The MnO x -ZrO 2 /TiO 2 catalytic filter with the higher NO x conversion efficiency and the lower filter resistance was successfully fabricated for the removal of dust and nitrogen oxide simultaneously. The NO x removal efficiency (1 m min −1 filtration velocity) at 180°C with Mn/Zr molar ratio of 2 and catalyst loading of 6 wt% reached 98.4%. Furthermore, MnO x -ZrO 2 /TiO 2 catalytic filter exhibited good anti-sulfur performance and regeneration performance. In addition, the dust (PM2.5) removal efficiency for MnO x -ZrO 2 /TiO 2 catalytic filter reached 99.99% and the filter resistance was less than 200 Pa. The filter resistance of MnO x -ZrO 2 /TiO 2 catalytic filters just increased 38 Pa after 200 times pulse blowback, which implied that the service life of catalytic filter could be improved.