Study On The Characteristics Of Non-pitch Coal-based Activated Carbon/Coke And Its Application Of NO_x Removal At Low Temperatures

Activated carbon/coke has proven to be the most efficient catalysts for the selective catalytic reduction（SCR） of NO_x with NH₃. Activated carbon/coke with cheap raw materials has good adsorption capacity and acid and alkali corrosion advantages. Activated carbon/coke with cheap raw materials contains elaborate pore structures and abundant surface functional groups. These surface chemical characteristics are altered by oxidation and reduction processes in either the gas phase or the liquid phase. These performances of activated carbon/coke provide very good conditions for its application of denitrification in flue gas. The unit of activated carbon/coke can be placed after the units of precipitators and desulfurization devices. So it can realize the removal of SO₂, dioxins and other toxic substances under the low temperature, which avoids reheating the flue gas and thus to reduce energy consumption. Activated carbon/coke denitrification is considered to be a kind of advanced, economic and effective deeply purification technology. But the most object of study is activated carbon/coke made of pitch binder, it is rare of non-pitch activated carbon/coke made of novel non-pitch binder. The tar and pitch were used currently in the preparation of AC, which increased its cost and limited its wider application. Bes ides, the denitrification mechanism of activated carbon/coke is determined by its physical and chemical properties, and its change with the preparation conditions, active components. Combined with the nature of the activated carbon/coke itself is extremely complex, therefore, there is no unified understanding that the mechanism of NO_x removal on the activated carbon/coke at low temperature, and remains to be further enrich and perfect.Therefore, independent development non-pitch columnar carbonized material, using a novel non-pitch binder, was studied by systematically. In this work, research was conducted on the operational factors in the process of activation, oxidation and modification, the changes in acidic/basic surface functional groups and pore structure, the denitrification performance. The reaction mechanism and reaction kinetics of NO_x removal are discussed on the non-pitch coal-based activated coke（NPAC）. The main conclusions are as follows.（1） In the activation process of NPAC, water vapor reacts with the carbon skeleton and its surface groups leading to the formation of the micropores distributed in 0.7 1.2 nm and 1.5 1.6 nm and basic/acidic functional groups. the surface of NPAC contains basic functional groups and acidic functional groups, whose contents are 0.15 mmol·g-1 and 0.05 mmol·g-1, respectively. The main basic functional groups are nitrogen-containing species such as pyridine and acidic functional groups are lactone, carbonyl and carboxylic acid derivatives. The basic functional groups on the NPAC are benefit to the removal of NO at low temperature 30 ℃, and the removal rate of NO_x can be high as 86.35% at 30℃. Mixing with SO₂ of low concentration in the flue gas, the SO₂ was removed completely and the denitrification rate is at lower level but has no obvious decrease. Low-cost NPAC is a promising catalyst of desulfurization and denitrification, which achieved removal of SO₂ and NO_x in low concentration.（2） In the oxidation of catalyst NPAC, oxygen reacted with microcrystalline edges, defect parts and specific chemical functional groups adsorbed on the surface of NPAC to build and ream pores, such that the total pore volume increased. Additionally, the basic functional group content decreased by 36.42%, which was ascribed to a reduction in the pyrrolic-like and-CH₂, whereas the acidic functional group content increased by 128.7%, which was a result of the introduction of C=O groups. The polar functional groups such as the large π bonding systems in aromatic rings and the protonated pyridine and/or pyrrole moieties improve the adsorption of O₂ and NO in the denitrification process. The acidic groups absorb NH₃, and oxygen assists in H-abstraction of NH₃ to formation NH₂, which then reacts with adsorbed NO₂ or NO to generate N₂ and H2 O in the presence of oxygen. After denitrification, the contents of –COOR, the large π bonding systems in aromatic rings, N-6 and N-Q decreased. Additionally, at 30℃, the NPAC-O0 sample can directly adsorbs NO_x in the absence of ammonia. When the ratio of v（NH₃） / v（NO） is greater than 0.8, NH₃ existing in the system also promotes removal of NO_x. Oxidized activated coke may be a kind of promising absorbent or catalyst of NO removal at room temperature（3） The high activity and low cost modified activated coke were prepared by MnO_x and CeO_x co-impregnation. The denitrification rate of pretreatment sample MnO_x-CeO_x-7.40 at 140°C was high than 95% within 80 min. For co-impregnation modification of MnO_x and CeO_x, the manganese species and cerium species incorporated activated coke to formed Cerium Manganese Carbide, and the species of Ce³⁺、Ce⁴⁺、Mn⁴⁺and Mn³⁺ were formed on its surface. In the process of modification, it was developed that the pore volume of mesopores（2⁷nm） and macropores. The increase in macropores could be attributed to the mesopores developed into macropores or collapsed. The presence of CeO_x species promote NO adsorption by absorbed NO₂ and NO₃, the MnO_x species improved NH₃ adsorption on the catalyst surface.（4） Employing NO adsorption-desorption experiments and XPS, FTIR, DRIFTS techniques, the reaction mechanism of NO_x removal was put forward on the modified activated coke. NO strongly adsorbs on modified activated coke at 140 ℃. The groups of C-O, cycloalkane and aliphatic hydrocarbon on the support surface partic ipate in the adsorption and oxidation of NO. In the process of adsorption,the adsorption of NO generates adsorbed NO₂, and the adsorbed NO₃ is produced by the disproportionation of chemisorbed NO₂. The existence form of adsorbed NO on the activated coke surface are adsorbed NO₂ and adsorbed NO₃ in the presence of oxygen. The surface functional groups of =C-H、-COO-、C=C and C=O transformed and vanished in the NH₃ adsorption. NH₃ adsorbed on the acid sites leading to the formation of coordinated NH₃, and adsorbed on the Bronted acid active sites converted to NH⁴⁺ species. Adsorbed NH₃ species translate into adsorbed NH₂ and intermediate from oxidation of NH₃ under the oxidation effect of adsorbed oxygen. In SCR reaction, adsorbed NH₃ species can react with adsorbed NO species following Langmuir–Hinshelwood mechanism or adsorbed NH₃ species can react with the gaseous NO following Eley-Rideal mechanism. Langmuir–Hinshelwood mechanism is the main reaction mechanism of NO_x removal, and NH₃ adsorption is the rate-limiting step on the surface of modified activated coke.（4） Kinetic experiments of modified activated coke were studied under the condition of eliminating internal and external diffusion to determine the reaction orders of O₂, the NO and NH₃. When O₂ concentrations is less than or equal to7.2%, the mean reaction order for O₂ is 0.31 order; when O₂ concentration from 7.2 to 14%, the reaction order for O₂ is close to zero. When NH₃ concentrations is below 500 ppm, the reaction order for NH₃ is approximately half order; when NH₃ concentrations is over 500 ppm, the reaction order for NH₃ is zero. The mean reaction order for NO is 0.69 order. The kinetic equation of denitrification reaction over the modified activated coke has been built under the condition: O₂≤7.2%, NH₃ <500ppm, at the temperature from 120 to 160℃。The rate constants of reaction . The kinetics of surface reactions for NO are calculated using Langmuir–Hinshelwood models. It is suggested that NO removal with NH₃ over modified activated coke proceeds via a process similar to the Langmuir–Hinshelwood mechanism.