The rapid industrialization brought severe air pollution problems (SOx, NOx) caused by combustion processes from industry and transportation means (car, train, aircraft). Also, domestic energy consumption have increased according to the industrial development. Because of this air pollution problems, air quality became worse and worse for last 20 years so that the air pollution control regulations of Korea became stringent accordingly. Therefore, new technologies should be developed to reduce the pollutants within the regulation limit from combustion processes.
The Selective Non-Catalytic Reduction (SNCR) process is a useful method for NOx reduction by injecting amines or cyanides containing selective reducing agents such as ammonia, urea, cyanuric acid and ammonium sulfate into the flue gases. This process reduces nitric oxides to nitrogen and water rapidly and effectively at rather higher temperatures (1073~1373K). Therefore, it has been studied for improving NO removal efficiency by using additives such as CO, methane, methanol, ethanol, sodium species and surfactants to reduce the optimum NOx reduction temperature.
The maximum NO removal efficiency by $NH_3$ exhibits at 950 ℃ and increases with increasing Normalized Stoichiometric Ratio (NSR) up to 1.8. The optimum reaction temperature is lowered and the reaction temperature window is widened with increasing the concentration of gas additives $(CO, CH_4)$. In the case of $CH_4$ additive, the optimum reduction temperature is about $50^\circ C$ lower than that of CO injection. The optimum reaction temperature is lowered and the maximum NO removal efficiency decreases with increasing the concentration of alcohol additives $(CH_3OH, C_2H_5OH)$. The higher NO removal efficiency can be obtained by the simultaneous injection of gas/liquid additives compared to the injection of single additive at lower temperatures due to the synergic effect. The addition of phenol lowers the optimum reaction temperature similar with that of the toluene addition. Compared to the toluene addition with the same molar ratio, the higher NO removal efficiency can be obtained by the phenol addition since the OH radicals in phenol converts $NH_3$ to $NH_2$ radicals. Therefore, the VOCs pollutant can be utilized in the SNCR process for promoting NO reduction and removing the VOCs at the same time.
An Arrhenius plot of the calculated rate constants $(k_f and k_r)$ was drawn on the basis of the reduction rate and concentrations of NO and $NH_3$ based on the proposed simple kinetic model of Lee and Kim (1996). From the Arrhenius plot, the temperature dependence of reaction rate coefficient can be expressed as:
$k_f = k_{f0} exp [-(E_f-E_{fa})/RT]$
$k_f = k_{r0} exp [-(E_r-E_{ra})/RT]$
The values of $k_{f0}, $E_f. k_{r0}$ and $E_r$ are $6.81x10^{12}$, -270 (kJ/mol), $3.17x10^{12}$, and -234 (kJ/mol), respectively. The lowered activation energies $(E_{fa})$ of CO, $CH_4, CH_3OH, C_2H_5OH$ are found to be 11.04, 22.07, 4.42, 6.62 respectively. The lowered activation energy $(E_{ra})$ of CO, $CH_4, CH_3OH, C_2H_5OH$ are 9.57, 19.13, 3.83, 5.74 respectively. Without the additives, Efa and Era are equal to zero. The proposed simple kinetic model can successfully predict the NO reduction by $NH_3$ and additives
950℃에서 암모니아에 의한 최대 NO 제거 효율을 보였으며 NO에 대한 암모니아의 molar ratio가 1.8까지 증가하다가 그 이상에서는 거의 일정하였다. 기상첨가제인 일산화탄소와 메탄을 사용하였을 시에는 반응온도는 낮아졌고 반응온도창은 넓어졌다. 액상첨가제인 메탄올과 에탄올을 사용하였을 시에는 반응온도는 낮아졌지만 최대 NO 제거효율은 낮아졌다. 기/액상 첨가제의 동시 주입시 Synergic effect에 의하여 첨가제의 개별적인 주입시 보다 더 좋은 효율을 보였다. 고온 영역(750-1000℃)에서 Toluene과 Phenol을 첨가제로 사용했을 경우 최적 반응 온도를 100~150℃정도 낮출 수 있는 것으로 나타났고 이는 Toluene과 Phenol이 배출되는 공정에 SNCR 공정을 적용했을 경우 이러한 휘발성 유기화합물과 NOx를 동시에 제거하는 효과를 얻을 수 있다. Proposed simple kinetic model를 사용하여 속도 상수와 활성화 에너지 값을 얻었으며 이 모델은 SNCR 공정에서 암모니아와 첨가제에 의한 NO 제거효율을 잘 예측할 수 있다.
