To date, numerous studies have investigated the aftertreatment of exhaust gases from fossil-fueled combustors and combustion engines by plasmas as an environmental plasma application. Owing to the ...high power requirements of environmental plasma, it is difficult to use the plasma alone for aftertreatment; hence, a hybrid process that combines plasma processing with other techniques is required to reduce the overall power consumption. In developing countries, low-cost plasma hybrid processing has attracted considerable attention as an alternative to the selective catalytic reduction NO
x
decomposition (De-NO
x
) method and wet lime–gypsum SO
x
decomposition method. Moreover, reduced catalytic activity can be enhanced by the plasma because of the decreased exhaust gas temperature, owing to the increased combustion efficiency. This paper reviews studies on successful air pollutant decomposition processes using the plasma chemical process with scale-up reactors. First, experimental techniques and block diagrams of various environmental plasma systems are presented. Subsequently, real-world systems of scale-up plasma reactors are described in detail. Several experimental results suggest that the hybrid treatment of particulate matter and dry De-NO
x
is very promising from the viewpoint of energy consumption and material recycling. CO
2
treatment is a very important direction for future work in environmental plasma.
In recent years, global warming has become a worldwide problem. Carbon dioxide (CO 2 ) is one of the primary substances causing global warming, and its emission should be reduced. A promising ...technology is the conversion of CO 2 to carbon monoxide (CO) using plasma treatment, so that the CO itself or its conversion to methane can be used as fuel for combustion engines. In this article, a CO 2 dissociation or reduction treatment system using an adsorbent and nonthermal plasma (NTP) flow is investigated. To improve the conversion efficiency, the conversion efficiency for Ar or He plasma is evaluated. In the physical adsorption process, ∼10% CO 2 gas with a flow rate of 10 L/min is prepared, and then, it is introduced into the flow channel; CO 2 is adsorbed by the adsorbent. After the adsorption process, the circulation flow channel is set, and then, the Ar or He NTP flow is generated with a blower. As a result, the adsorbed CO 2 is desorbed with CO 2 concentrations of 20% and 23% by the Ar NTP flow and the He NTP flow, respectively. The CO 2 conversion efficiency reaches 21%, and the energy efficiencies reach 13% and 10% with plasma in the cases using the Ar and He NTP flows, respectively. The energy efficiency with regard to CO is better with the Ar plasma, but more atomic carbon can be generated using the He plasma.
As NO x and SO x have significant environmental impacts, advanced treatments are required to remove them from the exhaust gas of a glass melting furnace. Here, we investigate a plasma-chemical hybrid ...process (PCHP) for this purpose. A pilot-scale experiment of the simultaneous removal of NO x and SO x using a PCHP combined with the existing semi-dry type desulfurization reactor is conducted on actual high-temperature exhaust gas from a glass melting furnace. NO (the majority of NO x exist as NO) in the exhaust gas is oxidized to NO 2 using active oxygen (ozone: O 3 ) generated by a plasma ozonizer. The exhaust gas must be cooled to less than 150 °C in order to suppress the thermal decomposition of O 3 , while the gas temperature at the outlet of the semi-dry reactor must be kept at 200-;250 °C to protect the dry-type electrostatic precipitator. Therefore, it is important to form a local cooling area for NO oxidation in the reactor. In this article, we use the three-fluid nozzles of O 3 , water, and air to form the local cooling area and effectively oxidize NO to NO 2 . In addition, we spray NaOH aqueous solution for SO 2 absorption downstream of the NO oxidation area to allow sufficient time for NO oxidation. The SO 2 reacts with NaOH to produce Na 2 SO 3 , a powerful reducing agent. Subsequently, NO 2 reacts with Na 2 SO 3 and is reduced to N 2 , and the Na 2 SO 4 generated in this reaction is reused as a clarifier of the raw materials for glass manufacturing. As a result, the ratio of the amount of removed NO and NO x to the amount of injected O 3 (de-NO/O 3 and de-NO x /O 3 ) is 64% and 78%, respectively; therefore, high efficiency is obtained. This article includes actual examples of the treatment of exhaust gas in a glass melting furnace, using PCHP de-SO x and de-NO x technologies along with results from pilot-scale experiments.
In this study, nitrogen oxides (NO x ) and sulfur oxides (SO x ) are removed using a wet-type plasma reactor without chemical agents. An NO and SO 2 mixed gas is prepared and sent to the reactor. In ...the reactor, NO is oxidized to NO 2 by air plasma first, and then NO 2 and SO 2 are absorbed into a water film on the inner wall of the reactor. The wastewater from the reactor contains nitrate ions (<inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula>), nitrite ions (<inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^{\rm{-}}</tex-math></inline-formula>), sulfite ions (<inline-formula><tex-math notation="LaTeX">{\rm{SO}}_{3}^{{\rm{2-}}}</tex-math></inline-formula>), and sulfate ions (<inline-formula><tex-math notation="LaTeX">{\rm{SO}}_{4}^{{\rm{2-}}}</tex-math></inline-formula>). The nitrogen compounds such as <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^-</tex-math></inline-formula> are harmful to human health and the environment; therefore, they must be treated before disposal. Accordingly, we treat the wastewater using argon (Ar) plasma to remove <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^{\rm{-}}</tex-math></inline-formula> after the NO x and SO x removal. The results show that more than 98% of the NO x and SO 2 are removed, and the <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^-</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^-</tex-math></inline-formula> ions in the wastewater are reduced by the Ar plasma; 25.6 mg of <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula> and 2.06 mg of <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^-</tex-math></inline-formula> are removed for 50 min. However, the pH does not increase with the reduction of these ions. In other words, <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^{\rm{-}}</tex-math></inline-formula> could not be reduced to N 2 but be converted to other ions. From these results, it is confirmed that the wet-type discharge plasma reactor is useful for the simultaneous removal of NO x and SO x . Further investigations are required for the treatment of wastewater containing <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{3}^{\rm{-}}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{\rm{NO}}_{2}^{\rm{-}}</tex-math></inline-formula>.
During pulsed corona discharges, both negative and positive ions react with moisture to generate ion clusters. These ion clusters are useful for cleaning environmentally hazardous materials such as ...hazardous chemicals, pollutants, and allergens. In this study, numerical simulations are carried out on the ion cluster formation of such ion-generation devices. The behavior of streamer evolution and cluster formations is simulated, and we show that the clustering progresses toward H 3 O + (H 2 O) 4 and O 2 − (H 2 O) 4 when the chemical species of H 3 O + (H 2 O) n and O 2 − (H 2 O) n ( n = 0−5) are considered. Ion number densities measured in the study are smaller than those calculated because of known differences between the numerical model and the experimental apparatus (in particular, the difference due to the loss of ions in the path of the chamber). The evaluation of the lifetime and diffusion coefficients of the ion clusters could be important.
Particulate matter (PM) and NO x emitted from diesel engines is simultaneously reduced by a barrier-type catalyst-packed nonthermal plasma (NTP) application, driven by a pulse high-voltage power ...supply under oxygen-lean conditions. Catalyst particles of γAl 2 O 3 and Ag/γAl 2 O 3 with a diameter of 2-4 mm are used as packed pellets and carbon PM is loaded among the pellets. The reduction results are compared with those in a previous study using a noncatalytic BaTiO 2 pellet packed bed reactor. NO x is reduced by N radicals, and PM is incinerated by oxygen radicals induced either by NO x or ozone (O 3 ) reduction at elevated local temperatures among the pellets. Since CO and CO 2 are generated, the carbon PM is actually combusted under oxygen-lean conditions, resulting in the simultaneous removal of PM and NO x with NTP-assisted catalysts. It is found that in the presence of the catalysts, PM removal calculated from CO and CO 2 generation increases with an increase in the oxygen concentration. The maximum PM removal energy efficiency of 0.92 g(C) /kWh is observed for O 2 = 2%. Furthermore, NO x removal decreases with an increase in the oxygen concentration. The maximum NO x removal energy efficiency of 14.2 g(NO 2 ) /kWh is observed for O 2 = 0%. Compared with the experimental results obtained without the catalyst pellets, the NO x removal energy efficiency increases by a maximum of 47% for O 2 = 2%. When comparing the present results with those in the previous study using BaTiO 3 pellets, NO x removal energy efficiency is larger, but the PM removal is lower in the case of catalyst packed-bed reactor.
To use the continuous combustion of fossil fuels as a sustainable energy source, a low-cost and energy-saving exhaust gas treatment for NO x , SO x , and suspended particulate matter is required. The ...plasma hybrid exhaust gas treatment technology proposed by the authors is not only a method of plasma treatment, but also a method that combines it with a chemical process, such as chemical solution stabilizer, to clean exhaust gas. This study reports the simulation and application of glass melting furnace exhaust gas treatment as a practical result of the plasma chemical hybrid process (PCHP). In the experiment, higher removal efficiencies of NO, NO x , and SO x (33, 16, and 55%, respectively) were obtained. The energy efficiency of NO x removal by evaluating the electrical power using the cost of NaOH and the power of ozonizers was measured to be 23 g(NO 2 )/kWh. From comparison, the simulated and experimental temperature distributions and NO x concentration were found to be in good agreements. This indicates that numerical design simulation produces a satisfactory result. PCHP can potentially be the most suitable method for exhaust gas treatment in glass melting furnaces.
We present a highly efficient carbon dioxide (CO 2 ) reduction treatment technique using an adsorbent and nonthermal plasma flow at the laboratory scale. The plasma reactor consists of twelve surface ...discharge elements, which are energized by a 10-kHz bipolar pulsed high-voltage power supply. As an adsorbent, the molecular sieve zeolite 13X is used, and it is placed downstream of the plasma reactor. In the physical adsorption process, approximately 10% CO 2 gas with flow rate 10 L/min is prepared by 99.5% CO 2 /N 2 cylinder and a nitrogen cylinder. It is then introduced into the flow channel, and CO 2 is adsorbed by the adsorbent. After the adsorption process, the circulation flow channel is set and the N 2 plasma flow is generated with a blower. Consequently, CO 2 adsorbed by the adsorbent is desorbed with higher concentration (currently ∼20%) and reduced to CO with high efficiency. The generated CO can be reused as a fuel. We evaluate the experimental results by calculating the conversion efficiency of CO 2 to CO and the energy efficiency of the plasma. When the number of repetitions of experiments is 5, a maximum CO concentration of 5% and a maximum CO 2 concentration of 20% are reached. The energy efficiency associated with the conversion of desorbed CO 2 to CO reaches 14% with the plasma power of 300 W. Discussions are carried out by comparing the value with those obtained in other studies on the plasma reduction of CO 2 .
An effective NO x -reduction aftertreatment system for a marine diesel engine that employs combined nonthermal plasma (NTP) and adsorption is investigated. The system can also treat particulate ...matter (PM) that employs a diesel particulate filter (DPF) and its regeneration by NTP-induced ozone. The effect of the combined reduction of accumulated PM and NO x is studied. The marine diesel engine of interest provides an output power of 1 MW at 100% of the engine load. The NO x reduction is composed of repeated adsorption and desorption flow processes using NTP combined with NO x adsorbents. With a view toward practical use, experiments on a higher number of cycles are carried out. The amount of adsorbents is 105 kg, which is greater than that in the authors' previous study. As a result, NO x reduction efficiency by NTP is higher than in the previous study and is greater at higher amounts of adsorbents owing to higher NO x desorption. Furthermore, DPF regeneration with plasma-induced O 3 injection is achieved simultaneously. This aftertreatment system demonstrates an excellent energy efficiency of 114 g(NO 2 )/kWh. The present aftertreatment can generate a synergistic effect of PM and NO x reduction.
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