Experimental Study on the Effect of Particulate and NH 3 on NOx Removal Efficiency by Non‐thermal P

International Journal of Energy and Power (IJEP) Volume 4, 2015 www.ijep.org doi: 10.14355/ijep.2015.04.011

Experimental Study on the Effect of Particulate and NH3 on NOx Removal Efficiency by Non‐thermal Plasma Technology Jian Zhang*1, Jianxing Ren2, Shan Lv3, Tianyu Sun4, Qinyang Wang5 School of Energy and Mechanical Engineering, Shanghai University of Electric Power, 2588 Changyang Raod, China *1

jinhanlin6330@126.com; 2ren608@163.com; 3lv_s9983@163.com; 4suntianyu588@126.com; 5qingyang0716@163.com

Abstract The experiment of NOx removal by non‐thermal plasma based on dielectric barrier discharge (DBD) was conducted and mainly focused on the influence of particle and NH3 on NOx removal efficiency. The results indicated the particle has negative effect on the de‐NOx process. However, the presence of NH3 has the ability to enhance the NOx removal since firstly, it can generate more radicals facilitating NO transforming to NO2; secondly, the product of the reaction, NH4NO3, can completely consolidate NO which prevents NO2 from reconverting to NO in high temperature circumstance.

Keywords DBD; Plasma; NO Removal

Introduction The non‐thermal plasma technology has been increasingly applied to hazardous gases disposal such as the VOCs and flue gas from internal combustion engines. Lv etc. [1] invested the mechanism of NOx removal process in diesel based on plasma oxidation. He found the absence of oxygen make NO mostly convert to N2 but the presence of oxygen would mainly convert NO to NO2. Li Kai etc. [2] theoretically analysed the influence of vapour, oxygen and voltage on the NOx removal efficiency. Yu Gang etc. [3] found the dielectric property affect de‐NOx efficiency.

The bigger the dielectric constant, the stronger the electric field, thus the higher the de‐NOx efficiency. Sun etc. [4‐6] invested the effects of flue gas composition, such as vapour, CO2, O2, NO and SO2 on the de‐NOx efficiency by experiments. All the mentioned experiments and accumulated experiences were the starting point for this research of possible application of DBD plasma for treatment of NOx. And yet, less research explored the effects of particle and NH3 on NOx removal efficiency. Therefore, two main problems are about to be resolved in this study, namely, I) what is the effect of diameter on the NOx removal efficiency when the particulate and NO coexist? II) what is the effect of NH3 on the de‐NOx efficiency?

Experiment Apparatus and Methodology This simulation experiment facility was projected for gas flow of 6 m3/h. The layout of the facility is shown in Fig. 1.

It mainly consists of a power supply (0‐30 KV, 5‐10 KHz), DBD reactor, flue gas analyser, digital oscilloscope (RIGOL, DS1104Z). DBD reactor is co‐axial structure. The outer electrode is made from ceramic tube, 20mm in diameter, 5mm in thickness covered with stainless steel net. The inner electrode is made from stainless steel tube with fins, 14mm in diameter. The gap between electrode and barrier is 3mm. The simulating working flue gas was blended firstly to the designed concentration in the gas mixer. Then the gas was tested by the flue gas analyser before it was pumped into DBD reactor. After that, the disposed flue gas would be tested again by the gas analyser to get the de‐NOx efficiency based on following equation,

η

100%

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is the inlet concentration of NOx,

is the outlet concentration of NOx and η is the NOx removal efficiency.

FIG. 1 SCHEME OF EXPERIMENTAL FACILITY

Results and Discussion The Influence of Particle on NOx Removal Efficiency To investigate the NOx removal efficiency dependence on particle exists, dust collected from a 600 Mw coal fired power plant was mixed with N2 and NO blending gas (Group 1). Another group of gas without particles was also set as comparison (Group 2). The frequency of power supplier was set to 8 KHz and the initial NO concentration was 550 ppm, tested by gas analyser. Changing the input voltage, here FIG. 2 shows the corresponding trend.

NO Removal Efficiency

0.8

Without Particles With Particles

0.6

0.4

0.2 6

8

10

12

14

16

Voltage (KV)

FIG. 2 THE INFLUENCE OF PARTICLE ON NOx REMOVAL EFFICIENCY

The existing of particle decreases the NOx removal efficiency. The particles in the gap would be charged in the DBD reactor which consumed mass of high energy electrons and ions. Compared to the condition containing no particles, less energy would be applied to generate radicals such as N* and O* which can excite the NOx removal process. At the meantime, the mass of particle is heavier than air. Thus, the particles would get low velocity and more residence time in the electric field. The particles would get more charged and then generate an intrinsic electric field which was resistant to the electric field provided by power supplier. All mentioned above contributed to the NOx removal efficiency reduction.

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Attention should be paid to the circular zone in FIG. 2 as well. When the applied voltage was over 10 KV, the NOx removal efficiency in the condition containing particles is higher than that without particles. This is because when the Group 2 was applied to high voltage, plenty of high energy electrons were generated. The provided high energy electrons were much more than consumed. As a result, the left injected energy was converted to heat and released to increase the ambient temperature which facilitates the conversion from NO2 to NO. However, there were many particles in Group 2, which consumed lots of high energy electrons and ions. So in Group 2, there was no friendly environment for NO2 conversion to NO. The Influence of Diameter of Particle on NOx Removal Efficiency In order to understand how the diameter of particle influences NOx removal efficiency, three groups of experiments were conducted. The test results of dust collected from three 600MW coal fired power plant were shown in FIG. 3.

(a)

(b)

(c)

FIG. 3 DISTRIBUTION OF DUST PARTICLE DIAMETER

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600

NO Concentration (ppm)

D50=10.37 D50=11.2 D50=14.82 400

200

0 0

5

10

15

Voltage (KV)

FIG. 4 THE INFLUENCE OF PARTICLE SIZE ON NOx REMOVAL EFFICIENCY

Three groups of dusts (1. Average diameter D50=14.82 , 2. D50=11.20 , 3. D50=10.37 ) were introduced into the DBD reactor respectively. The results are shown in FIG. 4. The NOx removal efficiency was the highest in Group 3 and this is because the particles with smaller diameter consumed less high energy electrons and ions compared to other two groups. The smaller particles can be accelerated to high speed and have less residence time in the electric field. Most of applied energy would be utilized for radical generation which progresses NOx removal.

NH3 Addition Ammonia is widely used in Selective Catalyst Reduction (SCR) unit which is well known NOx removal technique.

It is interesting to do some research about the ammonia behaviour during the NOx removal process in DBD plasma reactor. The dominating chemical reactions are as follows: ∗

→ → →

∗

→ →

→

→

∗

→

→2 → →

→

When the ammonia is injected into the DBD reactor, the ammonia is ionized to reduction radical such as NH, NH2, O, OH and so forth. Then the NO is reduced to N2 or oxidized to NO2 and HNO3. The controlled experiment was conducted. The flow rate, NO concentration, temperature and pressure in both experiments were set as 1.1 L/min, 662ppm, 25℃ and 1Atm, respectively. The power frequency was set to 8 KHz and then changed the applied voltage.

0.75 0.70

NO Removal Efficiency

0.65 0.60

With NH3 Without NH3

0.55 0.50 0.45 0.40 0.35 0.30 0.25 4

6

8

Voltage (KV)

10

FIG. 5 THE INFLUENCE OF NH3 ON NOx REMOVAL EFFICIENCY

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FIG. 5 shows the corresponding trend. In the start stage, the NH3 addition can enhance the NOx removal efficiency sharply with the applied voltage increase and finally reach a stable status. This is because in the start stage, the NH3 is ionized to some radicals, for instance, H, OH, NH2 and so forth, which can greatly progress the NOx removal. Nevertheless, the group without NH3 presented a very different way. When the DBD plasma reactor was supplied with high voltage (over 9 KV), the de‐NOx efficiency went down. It can be explained that by the identical reason in the section “The Influence of Particle on NOx Removal Efficiency” relating to the NOx removal efficiency declines in high voltage zone.

Conclusions By comparing the NOx removal performance in different gas condition, the effect of particles existing, diameter and ammonia addition on NOx removal efficiency have been demonstrated clearly. The experiments provide the following conclusions, 1.

The existing of particle makes the NOx removal efficiency decrease and the influence becomes more severe with the increase of particle size. But when the dust goes into the discharge gap, the coagulation is inevitable for the charged particles. It is still an issue to balance the NOx removal efficiency and the particulate filter.

2.

The ammonia addition can promote the NOx removal efficiency. The presence of ammonia can highly consolidate the NO and avoid the transformation of NO2 to NO. This method can ensure a relatively high NOx removal efficiency.

ACKNOWLEDGMENT

The authors gratefully acknowledge Professor Ren for all kinds of support. This work is financially supported by Shanghai Engineering Research Centre of Power Generation Environment Protection (11dz2281700) and the Key Fund of Shanghai Science Technology Committee (13160501000).

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Jian Zhang was born in Jiangsu in 1989. Zhang is a master student of Shanghai University of Electric Power since the year of 2012 whose research interests are NOx, CO2 mitigation, biomass and biogas Cogeneration.

He studied Power Plant Technology in BTU Cottbus, Germany from 2013‐2014 and now he comes back for completing his master program in Shanghai University of Electric Power. He has several published articles, for instance, “CO2 Capture with MEA Absorption”, “The mechanism of flue gas de‐NOx based on plasma ionization technology and the influence of DBD electrode configuration parameter on the de‐NOx efficiency” and so forth. His current research focuses on NOx abatement and biomass and biogas Cogeneration.

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