Effect of steam and CO2 activation on characteristics and desulfurization performance of pyrolusite modified activated carbon

Adsorption characteristics of SO2 onto novel activated carbon fixed bed: kinetics, isotherms, thermodynamics and washing regeneration

The problem of SO2 pollution in industrial flue gas has brought great pressure to environmental governance. In this study, a new type of activated carbon fixed bed device was designed and built for flue gas desulfurization. The results showed that activated carbons (AC1-AC5) were microporous activated carbons with abundant functional groups on the surface, and the desulfurization performance was ranked as AC1 > AC2 > AC3 > AC4 > AC5. The specific surface area of AC1 was as high as 624.98 m2/g, and the maximum adsorption capacity was 29.03 mg·g-1 under the optimum reaction conditions. The Freundlich adsorption isotherm model and Bangham pore diffusion model are more suitable for describing the dynamic adsorption process of SO2 on AC1. Combined with thermodynamic research, it is shown that the adsorption process of SO2 is a spontaneous, exothermic, and chaotic reduction process, which is mainly a physical adsorption between single-layer adsorption and multi-layer adsorption. Finally, the desulfurization-washing regeneration cycle experiment results showed that the regeneration rate of AC1 increases with the washing time and washing temperature, up to 95%, which provides data reference for industrial application.

Carbon consumption and regeneration of oxygen-containing functional groups on activated carbon for flue gas purification

High carbon consumption is an important factor restricting the wide application of activated carbon technology for flue gas purification. A fixed-bed reactor combined with a Fourier transform infrared (FTIR) spectrometer was used to explore the source of carbon consumption at various SO₂ concentrations and cyclic adsorption-regeneration times. The results demonstrate that carbon consumption originates from two sources and is mainly determined by the reaction of H₂SO₄ and C at high SO₂ concentrations and by the thermal decomposition of oxygen-containing functional groups at low SO₂ concentrations. An interesting observed phenomenon is that carbon consumption does not increase as the SO₂ concentration increases. The conversion mechanism reveals that carboxylic and anhydride groups are converted to phenol and quinone groups, which do not easily decompose with increasing SO₂ concentration. In the process of cyclic adsorption-regeneration, it is discovered that the carbon consumption in the first cycle is several times higher than that in the following cycles due to the decomposition of functional groups from the activated carbon itself. The regeneration mechanism of functional groups has been elucidated. The carboxylic acid and the phenolic hydroxyl on the surface of activated carbon are consumed in the regeneration process and formed again from the conversion of carbonyl groups in the next adsorption process under the roles of O₂ and H₂O. It is proposed that the functional groups are regenerated in the adsorption process rather than in the regeneration process.

Preparation of Manganese Blending-Modified Activated Coke for Flue Gas Desulfurization

In this study, the preparation and desulfurization application of MnO₂ and pyrolusite blending-modified activated cokes (ACM and ACP) were studied. Thermodynamic calculation shows that the blended metal oxides could be reacted with the solid carbon and gaseous products H₂, CO, and CO₂ for activation. The physicochemical properties of the blending-modified ACP and ACM responded considerably differently to preparation conditions. The blended metal oxide significantly improved the mesoporous structure of the modified activated cokes, as well as the surface acidic and basic functional groups. Different metal oxides played different roles in the pore structure and surface functional group evolution, and the current investigation indicates that MnO₂ is more favorable than pyrolusite. The enhanced acidic and basic functional groups, coupled with the catalysis of metal oxides, improved the desulfurization performance of the modified activated cokes. The sulfur capacities of the prepared ACP and ACM were 47.9-208.9 and 119.4-205.9 mg/g, respectively, which were much greater than the sulfur capacity of the fresh activated coke.

Evolution of physico-chemical properties of Dicranopteris linearis-derived activated carbon under various physical activation atmospheres

This work emphasizes the effect of the physical activation using CO2 and steam agents on the physicochemical properties of activated carbon produced from Dicranopteris linearis (D. linearis), a fern species widely distributed across tropic and subtropic ecoregions. The D. linearis-derived chars produced under pyrolysis at 400 °C for 1 h were activated in various CO2-steam proportions. As revealed by the IR and Raman spectra, the structure of the activated chars was heavily dependent on the relative proportion of CO2 and steam. The total specific surface area (SSA) of the activated chars proportionally increased with the increase in steam proportion and was comparable to the values of commercial activated char products. Specifically, the activation under CO2- and steam-saturated conditions has correspondingly resulted in SSA increasing from 89 to 653 m2g-1 and from 89 to 1015 m2g-1. Steam also enhanced the development of mesoporous structures of the D. linearis-derived char products, thereby extending their potential applications, particularly for industries that require high rigidity in the product such as pharmaceutical and cosmetic sectors.

Effect of Physical and Mechanical Activation on the Physicochemical Structure of Coal-Based Activated Carbons for SO2 Adsorption

The SO2 adsorption efficiency of activated carbons (ACs) is clearly dependent on its physicochemical structure. Related to this, the effect of physical and mechanical activation on the physicochemical structure of coal-based ACs has been investigated in this work. In the stage of CO2 activation, the rapid decrease of the defective structure and the growth of aromatic layers accompanied by the dehydrogenation of aromatic rings result in the ordered conversion of the microstructure and severe carbon losses on the surfaces of Char-PA, while the oxygen content of Char-PA, including C=O (39.6%), C–O (27.3%), O–C=O (18.4%) and chemisorbed O (or H2O) (14.7%), is increased to 4.03%. Char-PA presents a relatively low SBET value (414.78 m2/g) owing to the high value of Non-Vmic (58.33%). In the subsequent mechanical activation from 12 to 48 h under N2 and dry ice, the strong mechanical collision caused by ball-milling can destroy the closely arranged crystalline layers and the collapse of mesopores and macropores, resulting in the disordered conversion of the microstructure and the formation of a defective structure, and a sustained increase in the SBET value from 715.89 to 1259.74 m2/g can be found with the prolonging of the ball-milling time. There is a gradual increase in the oxygen content from 6.79 to 9.48% for Char-PA-CO2-12/48 obtained by ball-milling under CO2. Remarkably, the varieties of physicochemical parameters of Char-PA-CO2-12/48 are more obvious than those of Char-PA-N2-12/48 under the same ball-milling time, which is related to the stronger solid-gas reactions caused by the mechanical collision under dry ice. Finally, the results of the SO2 adsorption test of typical samples indicate that Char-PA-CO2-48 with a desirable physicochemical structure can maintain 100% efficiency within 30 min and that its SO2 adsorption capacity can reach 138.5 mg/g at the end of the experiment. After the 10th cycle of thermal regeneration, Char-PA-CO2-48 still has a strong adsorptive capacity (81.2 mg/g).

Comparison on surface properties and desulfurization of MnO2 and pyrolusite blended activated carbon by steam activation

In this study, MnO₂ and pyrolusite were used as the catalysts to prepare modified activated carbon, that is, AC-Mn and AC-P, respectively, from coals by blending method and steam activation. The Brunauer-Emmett-Teller (BET) results indicated that the AC-P had higher surface areas and micropore volumes than the AC-Mn with the same blending ratio. The relative contents of basic functional groups (i.e., C = O, π-π*) on AC-P were slightly lower than those on AC-Mn, while both contained the same main metal species, namely, MnO. The desulfurization results showed that with 3 wt% of blending ratio, AC-Mn3 and AC-P3 had higher sulfur capacities at 220 and 205 mg/g, respectively, which were much higher than for the blank one (149.6 mg/g). Moreover, the AC-P had relatively higher sulfur capacity than the AC-Mn with the same contents of Mn, which might be attributed to the existence of other metals in pyrolusite. After the desulfurization process, MnO were gradually transferred into MnSO₄, and the relative contents of basic functional groups decreased evidently for both AC-Mn3 and AC-P3. The results demonstrated that pyrolusite could be one good alternative to MnO₂ to prepare modified activated carbon for desulfurization.

IMPLICATIONS

MnO₂ and pyrolusite were used as the additives to prepare modified activated carbon from coals by a blending method and by steam activation, that is, AC-Mn and AC-P, respectively. The AC-P had higher surface areas and micropore volumes than the AC-Mn with the same blending ratio. The AC-Mn and AC-P had higher sulfur capacities than a blank one. Moreover, the AC-P had relatively higher sulfur capacity than the AC-Mn with the same contents of Mn. The results demonstrated that pyrolusite could be one good alternative to MnO₂ to prepare modified activated carbon for desulfurizatio.