Abstract
Emissions of nitrogen oxides such as NO and NO2, which are commonly known as NOx, are threats to human existence and cause environmental problems. Mainly, two techniques have been developed to drastically reduce these emissions, which are dry and wet processes. The wet process has several advantages, major identifiable advantages are the adaptability to the flue gas, low operating temperatures and no poisoning and inactivation catalyst. Also, a mixture of hydrogen peroxide and nitric acid are used as absorbents solution for NOx reduction in the wet process. The advantages of using this mixture include the ability to reduce the negative effect of NOx and does not contaminate the scrubbing solution. In addition, nitric acid has an economical advantage in the process considering the fact that it is produced in the process. Finally, it can be conducted at ambient temperature. This study furthermore used a mixture of hydrogen peroxide and nitric acid solutions as an absorbent to reduce NOx in hollow fiber membrane modules. The hydrogen peroxide oxidized HNO2 to nitric acid, while enhances the oxidation through an autocatalytic reaction. The effects of the feed gas flow rate, hydrogen peroxide concentrations and number of fibers on the NOx reduction, absorbed NOx and flux were varied to study. The experimental results showed that the increase in the feed gas flow rate from 100 to 200 mL/min decreased NOx reduction from about 98 to 94% but increased the absorbed NOx and flux from about 0.13 to 0.255 mmol/h and 0.85–1.63 mmol/m2.h, respectively The increase in proportion of NOx in the feed gas effect was dominant than the increase in absorbed NOx. An increase in hydrogen peroxide concentration from 0.5 to 10 wt.% in the absorbent solutions increased NOx reduction, absorbed NOx and flux from about 94 to 98%, 0.257–0.267 mmol/h and 1.09–1.13 mmol/m2.h, respectively. Additionally, the H2O2 plays an important role in enhancing HNO2 oxidation to HNO3. Furthermore, an increase in the number of fibers from 50 to 150 in the membrane module increased NOx reduction and absorbed NOx from 86 to 97% and 0.23–0.27 mmol/h. Flux decreased from 2.98 to 1.13 mmol/m2.h due to increment in the gas-liquid contact surface area.
Highlights
Nitrogen oxides, such as NO and NO2, commonly known as NOx are usually emitted from the consumption of fossil fuel in power generation and industrial production, endanger life and pose a threat to the environment [1, 2]
Increasing the feed gas flow rate will increase the amount of NOx in the feed gas, which is distributed in the lumen fibers, thereby increasing the number of moles of NOx gas, which spreads out from the lumen fibers to the pores of the membrane fibers toward the shell side containing the absorbent solutions where the reactions (Eqs. (5), (6), (7), and (8)) occurred
The increase in the feed gas flow rate increased the amount of NOx in the feed gas, it was disadvantageous for the NOx reduction, as expressed in Eq (11)
Summary
Nitrogen oxides, such as NO and NO2, commonly known as NOx are usually emitted from the consumption of fossil fuel in power generation and industrial production, endanger life and pose a threat to the environment [1, 2]. The emissions of NOx mainly depend on the combustion temperature, time, and air-fuel ratio [10]. Several technological techniques have been developed to reduce NOx emissions in order to meet regulations and mainly based on two methods: dry and wet processes [11]. SCR using NH3 has the reducing reagent over catalysts based on V2O5-WO3/TiO2 or Cu- and Fe-zeolites [14, 15]. This has been proven highly efficient for NOx removal, involving flue gas temperatures typically ranging from 300 to 400 C [16]. The commercial V2O5-WO3/TiO2 catalyst shows high NOx removal efficiency (>90%) at 350–400 C [17]
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