Activated carbon desulfurization is a desulfurization technology developed in the 20th century that uses activated carbon to remove sulfur from exhaust gas. Compared with the traditional desulfurization technology, the carbon-based flue gas desulfurization technology has many advantages: (1) The desulfurization agent consumes less and can be reused , which is conducive to saving raw materials and reducing operating costs; (2) desulfurization products can be recycled; (3) the process is relatively simple and easy to operate; (4) there is no secondary pollution problem.
Activated carbon has a large specific surface area and a well-developed pore structure, and contains abundant functional groups on its surface, which can be used as both an adsorbent and a catalyst carrier.
Studies have shown that when the temperature is 20-100°C, SO2 is adsorbed on the surface of activated carbon, which is mainly physical adsorption at this time; as the temperature increases, the adsorption process also changes. When the temperature is 100- At 160°C, SO2 adsorbed on the surface of activated carbon is further catalyzed and oxidized to SO3, which is mainly chemisorption at this time; when the temperature is greater than 250°C, it is completely chemisorption, and H2SO4 generated by the reaction of SO3 and H2O is stored in the inside the pores.
The main use of activated carbon desulfurization is the adsorption and catalytic properties of activated carbon. We know that activated carbon has a developed pore structure and a strong specific surface area, so the adsorption capacity is very strong, but in fact activated carbon is also very catalytic. Under the action, the elemental sulfur generated by the oxidation reaction between the sulfur dioxide in the gas and a small amount of oxygen is adsorbed on the surface of the activated carbon. The desulfurization efficiency will decrease when the adsorption of activated carbon reaches saturation, and the saturated activated carbon must be regenerated at this time. According to the specific substances adsorbed by the desulfurization activated carbon, it is determined under what conditions it needs to be regenerated. Generally, the activated carbon desulfurizer is regenerated by steam at about 450~500°C. When the temperature is raised to a certain level, the elemental sulfur will be released It is precipitated from the saturated activated carbon and flows into the sulfur recovery tank, and becomes solid sulfur after cooling.
There are many mechanisms for the desulfurization of activated carbon materials, and the following mechanisms are generally supported. They believe that activated carbon not only acts as an adsorbent in the desulfurization process, but also acts as a catalyst and its carrier.
C+O2→C-
SO2+C→C-SO2
C-SO2+C-O→C-SO3
C-SO3+C-H2O→C-H2SO4+C
(C represents the active site on the surface of the activated carbon material)
The mechanism shows that the amount of O2 and water vapor will affect the efficiency of desulfurization. Moreover, if the flue gas contains nitrogen oxides, it will also affect the desulfurization efficiency. When activated carbon fibers are used for simultaneous desulfurization and denitrification, SO2 and NO will compete with each other for active centers on the carbon surface. Therefore, in order not to waste resources and improve adsorption efficiency, modified activated carbon came into being.
If metal ions are loaded on the surface of activated carbon, the metal ions are first adsorbed on the surface of activated carbon, and then undergo a redox reaction with the adsorbate to reduce the metal ions to elemental or lower-state ions, thereby accelerating the oxidation of SO2 to SO3.
Among various flue gas treatment methods, activated carbon adsorption is the only method that can remove every impurity in flue gas, including SO2, nitrogen oxides, soot particles, mercury, dioxins, furans, heavy metals, Volatile organic compounds and other trace elements. The development of this kind of flue gas desulfurization and denitrification technology is of great significance to the sustainable development of economy.
1. The principle of activated carbon desulfurization and denitrification
1.1 The principle of activated carbon desulfurization
The adsorption of SO2 by activated carbon includes physical adsorption and chemical adsorption. When there is no water vapor and oxygen in the flue gas, physical adsorption mainly occurs, and the adsorption amount is small. When the flue gas contains sufficient water vapor and oxygen, the activated carbon flue gas desulfurization and dust collector is a process of chemical adsorption and physical adsorption at the same time. SO2 adsorbed on the surface of activated carbon is catalytically oxidized to H2SO4, and the adsorption amount of sulfur dioxide increases.
1.2 Principle of activated carbon denitrification
The technology of using activated carbon to remove nitrogen can be divided into adsorption method, NH3 selective catalytic reduction method and hot carbon reduction method. The adsorption method is to use the microporous structure and functional groups of activated carbon to adsorb NOx, and oxidize the lower reactive NO to the higher reactive NO2. Regarding the mechanism of activated carbon adsorption of NOx, researchers still have great differences. NH3 selective catalytic reduction method is to use activated carbon to adsorb NOx, reduce the reaction activation energy of NOx and NH3, and improve the utilization rate of NH3. The hot charcoal reduction method uses charcoal to react with NOx to generate CO2 and N3 at high temperature. Its advantages are that no catalyst is needed, the solid carbon is cheap, has a wide range of sources, and the heat generated by the reaction can be recycled. However, the kinetic studies show that the reaction between O2 and carbon is prior to the reaction between NOx and carbon, so the presence of O2 in the flue gas increases the consumption of carbon.
2. Mechanism study
2.1 Activated carbon desulfurization reaction process in the presence of H2O Activated carbon flue gas desulfurization is different from other flue gas desulfurization technologies. It is a technology based on the traditional microporous adsorption principle. However, this kind of adsorption is very different from the commonly used industrial adsorption and purification technology. Because it involves the adsorption and mass transfer of multi-component substances, the adsorption is very complicated. In the presence of water, complex mixtures of water, water vapor, SO2, SO32-, SO42- and other components may be formed near the surface of activated carbon, on the surface, in the mesopores and macropores, and in the micropores. The existence and quantity of these molecules or ions may promote the improvement of adsorption performance, and may also restrict the adsorption capacity of activated carbon. The participation of H2O fundamentally changes the reaction mechanism of SO2 on the carbon surface, and there are different hypotheses about the specific reaction process.
Some people think that the participation of H2O changes the reaction mechanism of SO2 on the carbon surface, and the oxidation reaction cannot proceed without H2O. In the presence of water, the pyrone functional groups and delocalized π electrons on the surface of activated carbon will react with H2O molecules to generate H2O2, and H2O2 can oxidize H2SO2 formed by dissolving SO2 in water into H2SO4. The activated carbon surface should follow the following reaction formula:
SO2·H2O+H2O2→2H++SO2-4+H2O
2.2 Selective adsorption of SO2 and NOx by activated carbon
Some people have conducted in-depth research on the performance and mechanism of desulfurization and denitrification of activated carbon and the mechanism of competitive adsorption of SO2 and NOx on activated carbon. The actual industrial flue gas is simulated by a mixture of high-purity SO2, air and water vapor. The research shows that the adsorption of activated carbon on SO2 is mainly chemical adsorption, and activated carbon has a high desulfurization efficiency, which is greater than 96%. The actual industrial flue gas is simulated by a mixture of high-purity NOx, air and water vapor. Studies have shown that the adsorption of activated carbon on NOx includes physical adsorption and chemical adsorption. When the activated carbon reaches the dynamic adsorption equilibrium, the nitrogen removal efficiency is greater than 75%.
The actual industrial flue gas is simulated by a mixture of high-purity SO2, NOx, air and water vapor. The research shows that when SO2 and NOx exist in the gas flow at the same time, the capacity and adsorption saturation time of activated carbon to adsorb SO2 increase, while the desorption capacity increases. The sulfur efficiency, adsorption rate, and adsorption band length varied little. Because the physically adsorbed NOx is decomposed by SO2, the NOx adsorption capacity and dynamic adsorption equilibrium time of activated carbon decrease sharply, the denitrification efficiency is very low, the NOx adsorption band length increases, and the adsorption speed decreases. Both SO2 and NOx will not occupy the active adsorption centers alone, but co-exist in the active adsorption centers. Activated carbon preferentially and selectively adsorbs SO2, and physically adsorbed NOx is replaced by SO2=. Chemisorbed NOx can promote the adsorption of SO2 by activated carbon. SO2 can also promote the adsorption of NOx by activated carbon.