At present, the flue gas removal NOx
technology that has been studied and applied can be roughly classified into two types: dry flue gas denitration, and wet flue gas denitration. The main methods of dry flue gas denitration are solid adsorption, plasma activation, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR) [1
]. Wet flue gas denitration mainly includes the acid absorption method, alkali neutralization absorption method, and the selective catalytic oxidation absorption method (SCO). At present, the most commonly used SCR method to remove NOx
from the flue gas is to reduce NO to N2
as the reducing agent, but the requirement for catalyst is very high. The catalyst in SCR requires a high temperature to produce a good catalytic effect. Therefore, in order to meet the temperature demand of the catalyst, the SCR reactor must be placed in a high temperature area. This makes the catalyst more susceptible to erosion and poisoning from the corrosive substances in high-concentration soot, thereby reducing the service life of the catalyst, and increasing the operating cost of the industry. Additionally, gas treated by the SCR method contains an excessive amount of NH3
. In view of these defects in the SCR method, this paper mainly studies the selective catalytic oxidation (SCO) method for removing NO. This method can be carried out in a low temperature environment, which greatly reduces energy consumption and costs [3
Selective Catalytic Oxidation (SCO) can, not only simultaneously desulfurize and denitrify, but also produce cost-effective by-products. It has great competitiveness in industrial applications. In the SCO denitration process, NO in the flue gas is oxidized to NO2
by the oxygen in the flue gas, under the action of the catalyst, so that the degree of oxidation (NO2
) reaches 50–60% (at this time the absorption efficiency is the highest). Then, the wet desulfurization absorption process is used for spraying to simultaneously achieve wet desulfurization and denitration. The SCO method can achieve simultaneous desulfurization and denitration with a wet spray absorption process, and this method utilizes the auto-oxidation of NOx
to generate valuable by-products, such as ammonium sulfate. This process has lower investment and operating costs, and when combined with downstream wet spray absorption, the treatment efficiency can reach over 99% [5
NO can be oxidized to NO2
by oxygen in the air without a catalyst, but the NO concentration in coal-fired flue gas is low. As the concentration of NO increases, the rate of NO oxidation to NO2
also increases [6
]. Therefore, it is very difficult to oxidize NO to NO2
by spontaneous reaction alone. The SCO absorption method has a catalytic oxidation process, and this process greatly increases the denitration rate. If the catalyst used in the SCO process has good performance, the conversion ratio of NO will be high [9
]. As a result, the subsequent wet spray absorption process will be more effective, and the NOx
removal rate can also reach higher standards. Therefore, for the SCO denitration method, it is very important to select the appropriate catalyst [10
The most widely used carriers for denitration catalysts are molecular sieves and activated carbon. These two carriers have a highly efficient catalytic effect after loading with active metal components, but they have high temperature requirements. In this paper, the dried sludge produced in the sewage treatment process of soybean oil plants (referred to as oil sludge for short) is used as a catalyst carrier. The C/N ratio of the oil sludge is 9.64, and the C/N ratio of urban sludge is 10–20. According to previous research, the smaller the carbon–nitrogen ratio, the better the denitration efficiency of the catalyst. According to Inductive Coupled Plasma Emission Spectrometer (ICP) heavy metal detection, the oil sludge contains more than 50 kinds of metal components, and these metal oxides can very well improve the catalytic activity of the catalyst [12
The dried sludge contains a large amount of organic matter, and its surface contains a large number of pore-like structures. However, if it is prepared as a denitration catalyst carrier, the organic matter is easily generated in a heated environment, thereby affecting the removal of NO by the catalyst. Therefore, before preparation of the catalyst, the dried oil sludge is alkali activated. This process will corrode a large amount of the organic matter in the oil sludge, and etch more pore-like structures on the surface. Moreover, after being adsorbed on the surface of the alkali activated dry sludge, NO reacts with O2
to form NO2
]. The generated NO2
is easily adsorbed by a basic functional group. This will result in a good desorption of NO2
on the surface of the sludge, eventually releasing the active sites used to adsorb NO. This promotes the continuous adsorption of NO on the surface of the sludge, so that the reaction continues, and the denitration rate is significantly improved [17
Numerous studies have shown that catalysts loaded with metal oxides have better stability in the reaction. Moreover, these catalysts have high catalytic activity. The traditional catalysts are mostly made of TiO2
as a catalyst carrier, and loaded with active components such as vanadium and tungsten. However, the adaptation temperature of this catalyst is around 400 °C, and it cannot meet the requirement of low temperature denitration. At 150 °C, only Mn showed significant activity. The high catalytic activity of manganese oxide is related to the valence electron structure of manganese. The electronic structure of manganese is 3d5
, and there are 7 valence electrons. Manganese has more variable valence states than other transition metals. Moreover, it is easy to switch between oxidation states under low temperature conditions. This will lead to better low temperature catalytic activity of manganese in the SCO reaction [20
The conventional preparation method of the catalyst is the impregnation method. This method must adopt high temperature roasting, where the roasting temperature is generally above 400 °C. The dried sludge is often ignited during this high-temperature roasting process, so the dried sludge must be roasted in the absence of oxygen. In addition, the roasting process is often difficult to control. This makes the prepared catalyst particularly easy to agglomerate, and makes the distribution of active components in the catalyst non-uniform. Based on the above problems, a low-temperature plasma roasting method was used to prepare the sludge catalyst. A large number of studies have shown that after plasma treatment, the surface of the catalyst becomes rough due to etching. In addition, the distribution of the surface active components of the catalyst have an important influence on the catalytic activity. Many researchers have found that plasma affects certain specific structures of the catalyst [24
]. For example, plasma roasting increases the number of active sites of the catalyst. In addition, plasma roasting enhances the strong interaction between the supported catalyst metal and the catalyst support, forming a special metal-support interface. This will increase the electron transfer efficiency between the semiconductor and the metal, and will give the catalyst a very high catalytic activity. Finally, plasma roasting produces new functional groups on the catalyst surface [26
In this paper, low temperature plasma roasting is used instead of muffle furnace roasting. The energy of the particles in the low-temperature plasma is generally several to several tens of electron volts, and after the reaction of the material, the chemical bonds of the molecules on the surface of the material can be broken to form a new bond. This will increase the chemical reactivity of the particles, and combine them with free radicals, such as oxygen and nitrogen, in the discharge space. This process creates oxygen and nitrogen containing functional groups on the surface of the material. Studies have shown that the following physicochemical changes may occur after the plasma acts on the surface of the material. Firstly, the active particles strike the surface of the material. This process causes the chemical bonds between the surface molecules to open, generating macromolecular radicals, which cause activity on the surface of the material. Secondly, the plasma etches the surface of the material. The high-energy particles strike the surface of the material to cause physical etching, and the active particles chemically react with the surface of the material to generate chemical erosion.
Therefore, by combining the advantages of the above plasma treatment methods, the physical and chemical properties of the catalyst are greatly optimized, after being treated in low-temperature plasma. Carriers carrying active ingredients, such as metals, are directly placed in a plasma reactor for reduction or oxidation. This treatment method can not only maintain the catalyst skeleton, remove organic impurities such as template, and prevent the sintering of metal clusters from becoming large, but also has a relatively short processing time compared to conventional roasting. Many experiments have shown that this plasma roasting can replace conventional high-temperature roasting [27
2. Materials and Methods
The experimental raw material for this study was dried oil sludge from the Xi’an Bangqi Oil Technology Company Wastewater Treatment Station (Xi’an, China), and the results of elemental analysis, and ICP analysis, of the raw materials are shown in Table 1
and Table 2
2.2. Catalyst Pretreatment
The surface of strong alkali-activated sludge still contains a large amount of organic matter. When plasma is used to roast the catalyst, the organic matter on the surface of the sludge is consumed first. This makes it impossible for the manganese salt loaded on the surface of the sludge catalyst, to be completely oxidized to manganese oxide. Therefore, before the manganese salt is loaded, the raw sludge is first roasted in a muffle furnace so as to consume most of the organic matter on the surface of the sludge. Studies have shown that the sludge catalyst has better denitration performance after being roasted at 450 °C for 1 h in a muffle furnace. Therefore, before the sludge catalyst is loaded with manganese salt, the muffle furnace is used to roast the raw sludge at 450 °C for 1 h, to consume most of the organic matter on the sludge surface.
2.3. Catalyst Preparation
The sludge is air-dried first and then crushed into fine particles. A certain quantity of sludge particles (particle size of 2 mm) were selected for use, placed in a muffle furnace (Shanghai Shiyan Electric Furnace Factory, Shanghai, China), and roasted at a temperature of 450 °C for 1 h. Then different concentrations of LiOH activator (5%, 10%, 15%, 20%) were used, and different activation times (2 h, 3 h, 4 h, 5 h) chosen, to prepare modified sludge catalyst. The optimum activation conditions are selected by the catalyst evaluation device.
The sludge obtained from roasting at 450 °C for 1 h in the muffle furnace was selected. After that, the roasted sludge is subjected to LiOH activation under the above selected optimum activation conditions. Subsequently, the manganese salt is loaded on the catalyst using the equal volume impregnation method. Finally, the catalyst loaded with manganese salt was placed in plasma (Suman Plasma Co., Ltd., Nanjing, China) and roasted. The roasting power was 90 watts, and the roasting time was 9 min. A molded sludge denitration catalyst that loads 2% manganese oxide was prepared (The AP catalyst was used to replace the catalyst prepared by plasma roasting after LiOH activation).
The sludge catalyst obtained from roasting at 450 °C for 1 h in the muffle furnace is selected. After that, manganese salt was loaded onto the catalyst using the equal volume impregnation method. Subsequently, the catalyst loaded with manganese salt was placed in plasma and roasted. The roasting power was 90 watts, and the roasting time was 9 min. Finally, the sludge catalyst is subjected to LiOH activation under the above selected optimum activation conditions. A molded sludge denitration catalyst that loads 2% manganese oxide was prepared (The PA catalyst was used to replace the catalyst prepared by LIOH activation after plasma roasting).
2.4. Catalyst Evaluation Device
is a process diagram of a catalyst activity evaluation experiment.
Catalyst activity evaluation means that the catalysts obtained by different preparation methods are evaluated through a flue gas simulation device. First, the total gas flow rate into the device was set to 1000 mL/min, the NO flow rate was set to 20 mL/min, the O2 flow rate was set to 60 mL/min, and the N2 flow rate was set to 920 mL/min. The three gases are then passed into a mixing tank for mixing. Subsequently, the mixed gas is passed to a reaction tower equipped with catalyst to carry out the denitration reaction. Finally, the catalyzed gas is passed into the gas cylinder. The gas is detected using the flue gas analyzer, and the remaining gas is discharged. In this experiment, the NO concentration was measured using the Flue gas analyzer (Testo340, Mingle Instrument, Guangzhou, China). The Flue gas analyzer is a custom instrument with NO and O2 modules. It can detect NO and O2. The amount of catalyst in the reaction is 3 g, the catalyst can build up a 2 cm high reaction layer, and it can make a good contact with simulated flue gas and catalyst. The Set temperature of the reaction tower is 150 °C.
The NO conversion ratio is calculated as follows: The initial NO concentration of the flue gas (concentration A) passed into the reaction system is obtained using the flow meter, and then the NO concentration of the flue gas (concentration B) after the catalytic reaction, is obtained using the flue gas analyzer. A-B/A is the NO conversion ratio.
The sludge was activated for 2 h using 5% LiOH, 10% LiOH, 15% LiOH, and 20% LiOH, and then the denitration experiment was performed on the modified sludge. The conversion ratio of NO is shown in Figure 2
. The sludge was activated for 2 h, 4 h, and 6 h, using a concentration of 15% LiOH, and then the denitration experiment was performed on the modified sludge. The conversion ratio of NO is shown in Figure 3