Activated carbon has a highly developed pore structure and a huge specific surface area, so it has a strong adsorption, coupled with the surface of activated carbon contains multiple oxygen-containing functional groups, so it is an excellent adsorbent, catalyst and catalyst carrier.
1. Principle of activated carbon desulfurization
The adsorption of activated carbon for SO2 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, activated carbon flue gas desulfurization is a process of both chemical adsorption and physical adsorption. Physical adsorption first occurs, and then catalytic oxidation of SO2 adsorbed on the surface of activated carbon to H2 SO4 in the presence of water and oxygen.
2. Activated carbon desulfurization reaction process in the presence of H2O
Activated carbon flue gas desulphurization is different from other flue gas desulphurization technology. It is a technology based on traditional microporous adsorption theory. However, this adsorption process is very different from the common industrial adsorption water purification technology, because it involves multi-component material adsorption mass transfer, so its adsorption process is very complex. In the presence of water, in the near the surface of the activated carbon, surface, hole, large pore and micro hole, all can form water, steam, SO2, SO2-3, SO2-4, and other components of the complex mixture, the presence of these molecules, or ions and its quantity, or can promote the improvement of the performance of the adsorption, or can be activated carbon adsorption ability. The participation of H2O fundamentally changes the reaction mechanism of SO2 on the carbon surface, and there are many hypotheses about the reaction process. Lizzio, Mochida, Cazorla-Amoros et al. believed that SO2 and O2 compete for active sites. Among the three possible oxidation reactions, only the following formula can be carried out smoothly: C -- SO2 +O2 +C -- SO3 +C -- O, that is, only gaseous oxygen can react with adsorbed SO2.
Tamura believed that H2O, SO2 and O2 molecules could be absorbed by activated carbon. As long as there was a close enough distance between them and a certain spatial configuration, they could react directly with each other and eventually generate H2 SO4. In this theoretical model, the oxidation equation is C -- SO2 +C -- O C -- SO3 +C.
Zawadzki et al. believed that the participation of H2O changed the reaction mechanism of SO2 on the carbon surface, and the oxidation reaction could not be carried out in the absence of H2O. In the presence of H2O, the pyranone functional groups and delocalized π electrons on the surface of activated carbon react with H2O molecules to generate H2O2, which can oxidize THE H2 SO3 formed after SO2 is dissolved in water to H2 SO4.
We believe that in the presence of water, the number of effective adsorption sites is not determined by the volume and number of micropores, and the micropore filling theory is not suitable for activated carbon desulfurization with water elution, Tamura mechanism and Lizzio theory are not suitable for this technology. Zawadzki's theoretical analysis is a reasonable explanation. The surface of activated carbon should follow the following equation: SO2 ·H2O +H2O2 2H+ -- SO2-4 +H2O.
3. Principle of nitrogen removal by activated carbon
Activated carbon denitrification technology can be divided into adsorption method, NH3 selective catalytic reduction method and hot carbon reduction method. The adsorption method uses the microporous structure and functional groups of activated carbon to adsorb NOx, and oxidizes NO with low reactivity to NO2 with high reactivity. On the mechanism of activated carbon adsorption NOx, there are still large differences between researchers. NH3 selective catalytic reduction method uses activated carbon to adsorb NOx to reduce the activation energy of reaction between NOx and NH3 and improve the utilization rate of NH3. Hot carbon reduction method is the use of carbon and NOx reaction at high temperature to generate CO2 and N2, the advantage is that there is no need for catalyst, solid carbon is cheap, wide source, the heat generated by the reaction can be recycled. However, kinetic studies show that the reaction between O2 and carbon is earlier than that between NOx and carbon, so the presence of O2 in flue gas increases the consumption of carbon.
The results show that the adsorption of SO2 on activated carbon is mainly chemical adsorption, and the desulfurization efficiency is more than 96% with the mixture of high-purity SO2, air and water vapor to simulate the actual industrial flue gas. The mixture of high purity NOx, air and water vapor is used to simulate the actual industrial flue gas, and the adsorption of NOx by activated carbon includes physical adsorption and chemical adsorption. Under the condition of no SO2 gas in the gas flow, the nitrogen removal efficiency of activated carbon is higher than 75% when the activated carbon reaches the dynamic adsorption equilibrium. A mixture of high-purity SO2, NOx, air and water vapor was used to simulate the actual industrial flue gas. When SO2 and NOx were present in the gas flow, the adsorption capacity and adsorption saturation time of activated carbon increased, while the desulfurization efficiency, adsorption speed and adsorption band length changed little. Due to the replacement of NO by SO2, the NOx adsorption capacity and dynamic adsorption equilibrium time of activated carbon decrease sharply, the nitrogen removal efficiency is very low, the length of NOx adsorption band increases, and the adsorption speed decreases. Neither SO2 nor NOx occupy the active adsorption center alone, but co-exist in the active adsorption center. Activated carbon preferentially adsorbs SO2 selectively, and the NOx of physical adsorption is replaced and resolved by SO2. Chemisorbed NOx can promote the adsorption of activated carbon on SO2. At the same time, SO2 can also promote the adsorption of NOx by activated carbon.
