Article pubs.acs.org/est
High-Efficiency Removal of NOx Using a Combined AdsorptionDischarge Plasma Catalytic Process Qinqin Yu,⊥ Hui Wang,⊥ Tong Liu, Liping Xiao, Xiaoyuan Jiang, and Xiaoming Zheng* Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (XiXi Campus), Hangzhou 310028, China S Supporting Information *
ABSTRACT: A combined adsorption-discharge plasma catalytic process was used for the removal of NOx using zeolites as catalysts without external heating. It was found that the types of plasma carrier gases exert great effect on the conversion of adsorbed NOx. The conversion of adsorbed NOx is much lower in N2 plasma than in Ar plasma, which is attributed to the reverse reaction, NOx formation reaction. The momentary increase of oxygen species derived from the decomposition of adsorbed NOx is considered to be the main cause as their collisions with nitrogen species can generate NOx again. Thus, solid carbon was added to the catalyst to act as a scavenger for active oxygen species to improve the conversion of adsorbed NOx in N2 plasma. A NOx removal rate of 97.8% was obtained on 8.5wt.% carbon mixed H-ZSM-5 at an energy efficiency of 0.758 mmol NOx/W·h.
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INTRODUCTION Nitrogen oxides (NOx), emitted from mobile and stationary sources, are considered to be one of the most dangerous air pollutants for their devastating effect on atmosphere, ecosystem and human health. Several NOx emission control technologies such as three-way catalysts for gasoline-fueled vehicles,1 NOx storage-reduction systems for lean-burn engines,2 and selective catalytic reduction (SCR) for large-scale combustion facilities3 have been developed. Among various de-NOx strategies, direct decomposition of NO to N2 and O2 is a desirable process since it does not need any gaseous reducing agents. However, NO decomposition process is a high temperature phenomenon, and the coexistence of O2 inhibits the conversion of NO.4 Thus, there is still a great challenge for the development of novel and more efficient de-NOx strategies. An alternative method for activating and dissociating reactant molecules is using nonthermal plasma (NTP) at ambient temperature.5−7 Plasma derived NO decomposition process8−11 and plasma-assisted SCR of NO12,13 are often used for NOx removal. The decomposition of NO using plasma is attractive, due to the avoidance of high temperatures required in the case of thermal decomposition. However, an effective reduction of NOx is possible only in an oxygen-free atmosphere,8−11 which is not the case in real combustion gases. For the plasma-assisted SCR of NO, NTP was usually combined with a postplasma © 2012 American Chemical Society
thermal catalytic process for the removal of NOx. The electric energy cost is high in these processes since the plasma should be always on performing. An improved process for removing low-concentration gasphase pollutants has been proposed by several groups,14−19 which operates in a cycled system consisted of an adsorption stage and a discharge stage. In the present work, we investigated the removal of NOx on various catalysts through the combined adsorption-discharge process without external heating. NOx in an oxygen-rich gas stream is first adsorbed by the catalysts in the reactor in the adsorption stage (plasma off). Then in the discharge stage, Ar or N2 is switched into the reactor as plasma carrier gas to avoid the negative effect of O2 for the decomposition of adsorbed NOx on the catalysts (plasma on). The characteristics of the decomposition of adsorbed NOx on catalysts in different plasma carrier gases as well as the chemistry of plasma reactions have been investigated. Moreover, solid carbon as a scavenger for oxygen species was mechanically added to the catalyst to improve the decomposition of adsorbed NOx in N2 plasma. A greatly improved decomposition Received: Revised: Accepted: Published: 2337
September 27, 2011 January 15, 2012 January 18, 2012 January 18, 2012 dx.doi.org/10.1021/es203405c | Environ. Sci. Technol. 2012, 46, 2337−2344
Environmental Science & Technology
Article
as catalysts in the present study. Carbon/H-ZSM-5 was prepared by mechanically mixing activated carbon (AC) powder with H-ZSM-5, grounded, pressed and sieved to be 20−40 mesh size pellets. Chemical Analysis. The concentrations of NO and NO2 were analyzed online by a chemiluminescent NO/NO2/NOx analyzer (Eco Physics). N2O was analyzed using a Fuli-9750 gas chromatograph (GC) with TCD and a Porapak Q column. A capillary-sampled quadrupole mass spectrometer (MS, Dymaxion Mass Spectrometer, AMETEK) was used to quantitatively and/or qualitatively analyze the gas mixtures. The definitions of the conversion of NOx to N2 and NaOb, nitrogen balance (BN), and energy efficiency were defined as follows:
efficiency of adsorbed NOx was obtained, the carbon was progressively burned off and the energy efficiency for NOx removal is rather high, providing an interesting possibility for the simultaneous and centralized removal of NOx and carbon particulates at rather low energy cost.
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EXPERIMENTAL SECTION The Combined Adsorption-Discharge System. The combined adsorption-discharge system is operated at atmospheric pressure and without external heating (Figure 1). NTP
conv. NOx → N2 (%) =
adsorbed emitted residual − a × n Nproduced − n NO − n NO n NO x x x aOb adsorbed n NO x
× 100%
conv. NOx → NaOb (%) =
B N (%) =
a × n Nproduced aOb adsorbed n NO x
× 100%
emitted residual + a × n Nproduced + n NO + n NO 2 × n Nproduced x x 2 aOb adsorbed n NO x
× 100% Figure 1. Schematic diagram of the combined adsorption-discharge plasma catalytic system.
energy efficiency(mmol NOx /W ·h)
was generated in a cylinder-type dielectric barrier discharge (DBD) reactor with sinusoidal high voltage power input. The waveforms of the applied voltage and current were monitored with a digital oscilloscope (Tektronix, TDS 1002B-SC). The typical waveform pattern of voltage and current was shown in Figure S1 in the Supporting Information (SI). Discharge power was measured by the V-Q Lissajous program. The catalyst pellets (20−40 mesh, 1.0 g) were filled in the center of the discharge zone, occupying 1/6 of the discharge length (24.0 cm). Before performing the experiment, the catalysts were pretreated at 500 °C for 60 min in dry N2, then cooled down to room temperature. In the adsorption stage (plasma off), a gas stream containing 1800 ppm NO, 10% O2 balanced He was introduced into the catalysts-packed reactor at a total flow rate of 66 mL/min. In the discharge stage (plasma on), Ar or N2 was switched into the reactor. The temperature of the reactor can increase to a certain extent due to the collision of energy transferred from the high energy species to stable molecules. In the discharge power range investigated, the temperature of the reactor is always lower than 80 °C during the discharge stage. Catalysts. The catalysts included H-ZSM-5 (Si/Al = 25), Htype synthesized mordenite (H-SMOR, Si/Al = 6.8), H-type natural mordenites (H-NMOR, Si/Al = 5), NaA (Si/Al = 1), NaY (Si/Al = 2.8), and Fe−Mn−Zr mixed oxide (FeMnZrOx, Fe/Mn/Zr = 1/1/1). FeMnZrOx was synthesized according to ref 20. H-ZSM-5, H-SMOR, NaA, and NaY were used as received from Nankai Catal. Corp.. As there is an abundant source of natural mordenites in nature, natural mordenites from Jinyun Mountain, Zhejiang Province of China were also investigated
=
adsorbed n NO × conv.NOx → N2 x
P×t
adsorbed where nNO is the amount of NOx adsorbed on the catalyst; x emitted nNOx is the amount of NOx emitted to the exhaust during the residual plasma reaction; nNO is the amount of NOx species remained on x the catalyst after the plasma reaction, determined by a temperatureprogrammed desorption (TPD) process; nN2 produced and nproduced NaOb are the amounts of N2 and NaOb produced; P is the discharge power (W); t is the discharge time in the discharge stage. Catalyst Characterization. TPD experiments were performed to investigate the desorption characteristics of NOx on the catalysts. After NOx saturation, and purged with He to remove gas-phase NOx, the catalyst was heated from room temperature to 900 °C at a heating rate of 10 °C/min in He atmosphere. MS was used to monitor the effluent gas, which may contain N2 (m/e = 28), NOx (m/e = 30), O2 (m/e = 32), and N2O (m/e = 44). The reason using m/e = 30 for NOx (NO + NO2) is that in our MS, even injecting dilute NO2, only a mass peak at m/e = 30 and no peak at m/e = 46 can be detected. TPD was also used to determine the amount of NOx species that remained on the catalyst after the plasma reaction. The catalysts were swept by an Ar stream for more than 1 h at room temperature. Then, TPD was performed as the above. The effluent gas was qualitatively and quantitatively monitored by MS and NOx analyzer. X-ray diffraction (XRD) was taken by using a Rigaku power diffractometer (D/MAX-RB) using Cu Kα radiation ((λ =
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Thus, a synergistic function occurred between the plasma and the catalyst. The interaction of NTP with the catalyst through surface discharge as well as the direct interaction of gas-phase radicals with the catalyst surface are expected to play important roles in the synergistic effects.21−23 In N2 plasma (Figure 2(b)), the phenomenon is similar to that in Ar plasma, but the efficiency is much lower than the latter, which can be explained by that Ar plasmas present higher electron temperatures and densities than nitrogen plasmas at identical discharge conditions.24 It is considered that in Ar plasma, the decomposition of dilute NO follows the following reactions:
0.15418 nm) in the 2θ range of 4- 40° at a scanning rate of 4°/min. The amount of carbon in AC/HZSM-5 was determined using thermogravimetric analysis (TG, PE-TGA). The catalysts were pretreated in N2 at 300 °C for 60 min and, after reached 50 °C, they were submitted to an effluent of 20% O2/N2. TG was performed at a heating rate of 10 °C/min from 50 to 800 °C.
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RESULTS AND DISCUSSION
Direct DBD Plasma Processing of Dilute NO in Gas Streams. The decomposition of dilute NO (980 ppm) was first investigated in the absence and presence of catalysts including several zeolites and FeMnZrOx. The selection of the catalysts is based on their high adsorption capacity for NOx so as to facilitate the combined adsorption-discharge process as discussed in the following sections. Thermal catalytic investigation showed that all catalysts exhibited no catalytic effect in NO decomposition below 500 °C. Figure 2(a) and (b) show the decomposition of NO in Ar and N2 plasmas, respectively. In Ar plasma, N2 and O2 with nN2/nO2 = 1 were detected as the products, confirming the
e + Ar → Ar* + e NO + Ar* → NO* + Ar NO* → N + O
N + N + Ar → N2 + Ar O + O + Ar → O2 + Ar In N2 plasma, electron collision reactions result in the formation of nitrogen active species:
e + N2 → N(4 S) + N(4 S) + e e + N2 → N(2 D) + N(2 D) + e
e + N2 → N2(A) + e where Ar*, NO*, and N2(A) are excited molecules of Ar, NO, and N2, respectively. The ground-state N(4S) species can effectively lead to the decomposition of the dilute NO in N2. The excited-state N(2D) species might also contribute to the decomposition of NO, although it is considered that they are rapidly converted to NO in the presence of oxygen (at 300 K). From ref 25:
N( 4S) + NO → N2 + O(3P) k1 = 1.87 × 1013cm 3·mol−1· s−1 From ref 26:
N(2 D) + NO → N2 + O(3P) k 2 = 3.61 × 1013cm 3·mol−1· s−1 From ref 27:
O(3P) + O(3P) + N2 → O2 + N2 k3 = 1.12 × 1015cm 3·mol−1·s−1[N2] Decomposition of Adsorbed NOx on Catalysts in Ar Plasma. High energy cost is a main problem restricting the practical application of the NTP decomposition of NOx. Moreover, the presence of O2 in real combustion gas greatly inhibits the decomposition of NO in plasma.8−11 A combined adsorption-discharge process is an alternative approach for the treatment of diluted NO in exhaust or flue gases. NOx Desorption Characteristics of the Catalysts. All catalysts exhibited high adsorption capacity for NO in the presence of O2 (Figure S2 in SI). It has been reported that NOx is mainly stored as −NO, −(NO)2, −NOy (y = 2 or 3) species.28−31 TPD was conducted to investigate the characteristics of NOx desorption on the catalysts (Figure 3). All catalysts showed broad temperature ranges for NOx desorption,
Figure 2. Decomposition of diluted NO (980 ppm) in (a) Ar plasma and (b) N2 plasma. Conditions: 50 mL/min flow rate of the plasma carrier gas.
decomposition of NO. The conversion of NO is similar on H-ZSM-5, H-SMOR, NaA and NaY with the discharge power in the range of 0.03−0.24 W, higher than FeMnZrOx and H-NMOR. The conversion of NO is the lowest in the absence of catalyst. When the discharge power is higher than 0.24 W, NO is decomposed nearly completely in all cases. It is clear that all catalysts exhibited positive effects in promoting the NO decomposition. Notably, the temperature of the reactor is close to ambient temperature in the studied discharge power range. 2339
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increasing. There are also small amount of NOx emitted to the exhaust during the reaction (
