저온 플라즈마와 촉매 복합 공정을 이용한 질소산화물 조감기술 = Removal of Nitrogen Oxides by Using Non-Thermal Plasma Process Combined with Selective Catalytic Reduction

다국어 초록 (Multilingual Abstract)

Removal of nitrogen oxides(NO_(x)) using non-thermal plasma-assisted selective catalytic reduction(SCR) was experimentally investigated. A coaxial dielectric-packed bed reactor was used as the plasma reactor, which was combined with catalyst. Two different commercial monolithic catalysts such as V_(2)O_(5)/TiO_(2) and Cr_(2)O_(3)/TiO_(2) were compared with respect to the removal characteristic of NO_(x). It is well known that the performance of SCR largely depends on the ratio of NO_(2) to NO, and thus, the oxidation of NO to NO_(2) in the plasma reactor was separately studied first before combining it with catalyst.
The plasma reactor was able to readily oxidize NO to NO_(2) at room temperature. As the reaction temperature increased, however, the rate of the NO oxidation greatly decreased. Although the presence of water vapor somewhat improved the oxidation of NO at high temperatures in the range of 100 to 200℃, the use of a chemical additive such as ethylene was necessary for the effective oxidation at this temperature range. In the presence of small amount of ethylene, NO was found to be easily oxidized to NO_(2) while the sum of NO and NO_(2) was almost kept constant. The plasma reactor can be operated by either AC or pulse voltage. Comparison of AC with pulse voltage in terms of the energy efficiency for the NO oxidation was made, and almost no difference between the two voltage types was observed.
In the combined system, the main role of the plasma reactor is to simply oxidize NO to NO_(2), but it should be noted that the increase in the fraction of NO_(2) leads to an enhancement in NO_(x) removal. Without plasma discharge, the NO_(x) removal efficiency obtained with V_(2)O_(5)/TiO_(2) catalyst was around 50% at 150℃ and that with Cr_(2)O_(3)/TiO_(2) was around 10%. However, more than 80% of NO_(x) with V_(2)O_(5)/TiO_(2) and 40% of NO_(x) with Cr_(2)O_(3)/TiO_(2) were removed when plasma was generated. Changes in the oxygen content from 5 to 20%(v/v) did not significantly affect the results either from the plasma or the catalytic reactor. High concentration of NO_(x) up to 400 ppm was successfully treated in the present plasma-catalytic system. The change in the content of water vapor from 3 to 5%(v/v) had little influence on the removal of NO_(x). At an identical discharge power, higher flow rate resulted in lower NO_(x) removal efficiency because the oxidation of NO to NO_(2) in the plasma reactor decreased and the residence time in the catalytic reactor decreased. However, though a higher flow rate required more discharge power, about 60% of NO_(x) removal efficiency was achieved at a space velocity as large as 40,000/h, because the catalytic activity was largely improved by the plasma discharge.
The present combined process allowed high NO_(x) removal efficiency, but the formation of harmful byproducts due to the use of ethylene should be importantly considered. The ethylene-related chemical reactions suggest that the major byproducts are formaldehyde and carbon monoxide. Besides, according to the gas chromatogram obtained by using the flame ionization detector, no noticeable peaks other than ethylene were observed, implying that the formation of any other organic species from ethylene is negligible. While the emission of formaldehyde from the plasma reactor was significant, it was completely removed on the catalyst surface, i.e., the concentration of formaldehyde at the outlet of the combined process was always zero. But, the concentration of CO at the outlet of the combined process was much higher than that at the outlet of the plasma reactor. Such increase in the concentration of CO at the outlet of the combined process is understood to have arisen from the decomposition of formaldehyde on the catalyst. In real situations, similar problem is anticipated because real exhaust gases contain some amount of unburned hydrocarbons, and further studies are required to bring down the CO level to allowable limit for practical application of this system.

목차 (Table of Contents)

• 목차 = i
• LIST OF FIGURES = iii
• LIST OF TABLES = vi
• SUMMARY = vii
• I. 서론 = 1
• II. 이론적 배경 = 5
• 2.1. 저온 플라즈마 기술 = 5
• 2.1.1. 기술의 원리 = 5
• 2.1.2. 화학 반응기구 = 15
• 2.2. SCR 촉매 탈질 기술 = 16
• 2.3. 저온 플라즈마/SCR 복합공정 = 22
• III. 실험 = 26
• 3.1. 실험장치 및 방법 = 26
• 3.2. 방전전력 측정방법 = 32
• IV. 실험 결과 및 고찰 = 37
• 4.1. 플라즈마 반응기의 특성 = 37
• 4.1.1. 방전전력 = 37
• 4.1.2. NO 산화반응 = 39
• 4.1.3. 부산물 발생 = 44
• 4.1.4. 가스조성의 영향 = 48
• 4.1.5. 고전압 형태의 영향 = 51
• 4.2. 저온 플라즈마/SCR 공정 = 53
• 4.2.1. 반응온도의 영향 = 53
• 4.2.2. 기체유량(공간속도)의 영향 = 62
• 4.2.3. 초기 NO_(x) 농도의 영향 = 65
• 4.2.4. 수분함량의 영향 = 68
• 4.2.5. 촉매 특성 비교 = 68
• 4.2.6. 부산물 분석 = 74
• V. 결론 = 77
• 참고 문헌 = 79
• 감사의 글