| Activated carbons (ACs) are industrially applied in many processes either as catalyst or as adsorbent. Among these processes, carbon-based flue gas desulfurization and denitration attracts worldwide interest, not only due to strict environmental protection regulations, but also it enable sulfur resource to be recovered from gas. Moreover, due to free of water, this process is suitable for power plants located in arid and semi-arid areas. However, the applications in China meet the economic and technical dilemma, so the development of relevant technology suitable for China is essentially necessary. In this dissertation, based on AC, removal of SO2were systemic studied. Bench-scale and pilot-scale tests were carried out. The relevant mathematical model and economic assessments were performed based on the experimental results. Moreover, in order to simutaneously remove NOx, selective catalytic reduction(SCR) of NOx at low temperatures was also investigated. The following are the main conclusion:1. The different properties of ACs are derived from the different precursors. ACs produced from biomass precursors contain more carboxylic, phenol, carbonyl and quinone functional groups, while ACs from coal precursors are mainly ester, phenol, carbonyl and quinone groups. The addition of biomass is helpful to the development of the pore structure. On the other hand, the different properties have an impact on the three key parameters for SO2removal, which are adsorption capacity, inoxidability and mechanical strength. The well-developed texture has a positive effect on adsorption capacity of AC, especially the large specific surface area. In contrast. the CO2-forming groups over the surface may suppress the capacity. The spontaneous ignition temperature (SIT) and the point of initial oxidation (PIO) are of great significance. They are used to evaluate ACs’ inoxidability. Due to the catalytic effect of some metals (Ce, Cu and V), the values of SIT and PIO may reduce over100℃.The mechanical strength is related with the pore structure. Some macropores may result in the notable drop of strength.2. Factors related with the SO2adsorption:space velocity, composition of flue gas and regeneration were investigated. The space velocity is inverse proportion to the breakthrough time due to the prolonged reaction time. The presence of O2and H2O on the AC favors SO2adsorption, this increasing the breakthrough time and the adsorption capacity. While the partial pressure of SO2has a negative influence on the desulfurizing property of AC. Most of the well-established results for SO2adsorption at1000ppm or more, hold for SO2concentrations as high as10000ppm. A significant reduction in the amount of SO2adsorbed during the adsorption-regeneration cycles was observed. The formation of stable C-O complexes, which may serve to occupy active sites, is attributing to the deactivation reason. Besides the change of chemical properties, the regeneration also results in the burn-off of the carbon, and then changes the physical properties.3. Several metal-doped activated carbons (Fe, Co, Ni, V, Mn, Cu and Ce) were prepared and characterized. Activity tests for the removal of SO2and the selective catalytic reduction of NO with ammonia were carried out. Due to different physicochemical properties, different pathways for the SO2removal have been put out, i.e., catalytic oxidation, direct reaction and adsorption. This classification depends on the standard reduction potentials of metal redox pairs. The V, Ce, Fe and Cu-containing carbons show better SO2removal properties due to their catalytic oxidation activity, while samples impregnated with V, Ce and Cu show good activity for NO reduction by NH3, which is also ascribed to the reduction potential values of metal redox pairs. Ce seems to be a promising alternative to V due to the higher activity in NO reduction and the nontoxic property. A metal cation which could easily convert between the two valences seems to be crucial to the good performance of both SO2and NO removal, just like V and Cu.4. Based on V-containing AC, the promoting effect of NO2on NOx removal at low temperatures was found. Firstly, the micropores in AC act as a nano-reactor for the formation of-C(ONO2) complexes, which is composed by NO2adsorption on existing-C(O) complexes. Secondly, selective catalytic reduction (SCR) of NO and NO2with ammonia was investigated over V-containing AC, NOx-conversion to N2increases with increasing NO2/NOx ratio, especially at150℃and the increase vanishes gradually with increasing temperature. Last, to explain the observed behaviors, AC involved NO2-SCR process was proposed, in which NH4NO3is reduced to N2by AC instead of NO. This process shows better reactivity at lower temperatures.5. The pilot scale experimental installation for SO2removal by activated carbons has been built. The effect of molar heat on SO2removal at high concentrations has been investigated. The results show that the adsorption of SO2is the main reason of the temperature rise in the carbon bed. Meanwhile the temperature rise of the hot spot is due to the oxidation of carbon. Thanks to the adoption of the moving bed, the circulation of activated carbon is an effective way to lower temperatures as well as temperature deviations. In more detail, this method has insignificant effects on the pressure drop of the bed. The heat transfer of the hot spot is effectively controlled by the cooling system. However, the ventilation would make situation worse.6. A mathematical model was developed to predict the gas concentration profiles in the reactor, incorporating the mass transfer in the bed as well as within the AC pores, along with the surface reactions on AC. The model predictions agree reasonably well with the data. Based on the discussion of the model parameters. it is found that for particle mass transfer coefficientkm≥0.0215m/s, the external diffusion has little effect on the experiments; while for effective pore diffusivity Deff≥3.84×10-5m2/s, the internal diffusion resistance is insignificant. Based on a certain industrial system, the economic assessment was performed with the comparison with different regeneration methods. The research shows that for regeneration using hot flue gas, the cost is about1478.33CNY/tSO2, while for regeneration using steam, the cost is about2166.12CNY/tSO2, increasing47%. |
PDF Full Text Request