Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Removal of SO2 by activated carbon fibers in the presence of O2 and H2O

This work describes the potential capability of activated carbon fibers (ACFs) in continuously removing SO₂ from inert atmosphere without requiring further regeneration. A tubular reactor packed with ACF was used to study the conversion of SO₂ into H₂SO₄ in the presence of O₂ and H₂O with varying concentrations of SO₂ (3000-10,000 ppm), O₂ (10-20%), and H₂O (10-70%) and temperatures (313-348 K). The experiments were carried out on two precursors (viscose rayon and phenolic resin) based ACFs. The breakthrough data revealed that the steady-state SO₂ concentration levels at the reactor exit increased with increasing inlet SO₂ concentration and decreased with increasing concentration levels of O₂ as well as H₂O. Increase in the reaction temperature was found to moderately enhance the steady-state exit concentration levels of SO₂. The viscose rayon-based ACF exhibited higher SO₂ removal activity in comparison to the phenolic resin-based ACF. 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 ACF pores, along with the surface reactions on the ACF. The model predictions agreed reasonably well with the data.     

Catalytic properties in deNOx and SO2–SO3 reactions

SCR-deNO_x reaction and SO₂–SO₃ oxidation tests were carried out by different research groups over fresh and used EUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in several laboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis).

Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNO_x reaction and SO₂–SO₃ oxidation than the fresh sample.

An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolith reactor has been performed to evaluate the intrinsic kinetic constant of the deNO_x reaction; a satisfactory comparison has been obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from data collected over powdered catalysts. A good agreement has been found in the experimental results collected in the different labs, both for the deNO_x reaction and SO₂–SO₃ oxidation.     

SO2、CO转化率及SO3、CO2吸收率的计算

推荐硫酸生产中SO     

Impact of SO2 concentration on the corrosion rate of X70 steel and iron in water-saturated supercritical CO2 mixed with SO2

The corrosion behavior of X70 steel and iron in water-saturated supercritical CO₂ mixed with SO₂ was investigated using weight-loss measurements. As a comparison, the instantaneous corrosion rate in the early stages for iron in the same corrosion environment was measured by resistance relaxation method. Surface analyzes using SEM/EDS, XRD and XPS were applied to study the morphology and chemical composition of the corroded sample surface. Weight-loss method results showed that the corrosion rate of X70 steel samples increased with SO₂ concentration, while the corrosion rate increased before decreasing with SO₂ concentration for iron sample. Comparing resistance relaxation method results with weight-loss method results, it is found that the instantaneous corrosion rate of iron is much higher than the uniform corrosion rate of the iron tablet specimens which are covered with thick corrosion product films after a long period of corrosion. The corrosion product films were mainly composed of FeSO₄ and FeSO₃ hydrates. The possible reaction mechanism under such environment was also analyzed, and the electrochemical reaction between the dissolved SO₂ in the condensed water film with iron is the critical reaction step.     

Study of the sulfation kinetics between SO2 and CaO catalyzed by TiO2 nano-particles

The catalytic activity of TiO₂ nano-particles, prepared by a sol-gel method, was studied when added in the reaction between SO₂ and CaO. The reaction products were analyzed by infrared spectrophotometry (IR) and specific surface area analysis and the kinetics and mechanisms of the sulfation catalyzed by the addition of TiO₂ are discussed. The results indicate that nano-TiO₂, which serves as an active catalytic center, enhances O₂ transfer and is helpful in the diffusion of SO₂ from the product layer to the inner unreacted CaO. As a result, the desulfurization efficiency increased. The results also suggest that the SO₂ and NO must both be removed simultaneously in order to keep the sulfation rate. The desulfurization reactions are first order for SO₂ concentration and zero order for O₂ concentration and include two zones, the surface reaction zone and the product layer diffusion zone, with later being the rate limiting step. The apparent activation energy of the desulfurization reaction decreased with the addition nano-TiO₂ as compared to that without. The unreacted shrinking reaction core model was used to investigate the reaction kinetics and was shown to describe the course of desulfurization. Lastly, the results obtained through calculation agree with the empirical data.     

Adsorption of CO2 and SO2 on activated carbons with a wide range of micropore size distribution

The adsorption isotherms of N₂ at 77K, CO₂ at 251, 273 and 298K, and SO₂ at 262 and 273K have been determined on a series of physically activated carbons with a wide range of micropore size distributions. Since the series includes carbons with very high burn-off, it shows the problems involved in the characterization of microporsity in superactivated carbons. On the other hand, the results show that the carbon surface-adsorbate interactions for SO₂ at low relative pressures are weaker than for N₂ and CO₂, as a result of the strong adsorptive-adsorptive interactions in the bulk gas phase.     

凝并和冷凝/蒸发对颗粒尺度分布的影响

利用多重Monte Carlo算法对13种典型工况进行数值模拟,考察不同类型的凝并核和冷凝/蒸发核对多分散性颗粒尺度分布时间演变的影响。发现常凝并核要比线性和二次方凝并核对小颗粒的影响大一些、对大颗粒的影响小一些,线性和二次方凝并核对颗粒尺度分布的时间演变影响则取决于具体情况;常冷凝核要比线性冷凝核对小颗粒的影响大一些、对大颗粒的影响小一些;连续区布朗凝并核类似于常凝并核;扩散冷凝核对颗粒尺度分布的影响界于常冷凝核和线性冷凝核之间。     

Simultaneous SO2/NOx removal by a powder-particle fluidized bed

Simultaneous dry removal of SO₂ and NO_x from flue gas has been investigated using a powder-particle fluidized bed. In a process of flue gas desulfurization by use of solid sorbents such as FeO (dust from a steel plant) and CuO, the smaller the particle size of sorbents, the higher the expected SO₂ conversion. In a powder-particle fluidized bed (PPFB), fine particles less than 40 μm in diameter fed into the bed are fluidized with coarse particles. But only the fine particles are entrained from the bed, and their residence time in the bed is remarkably long.

The reduction of NO_x with NH₃ in the fluidized bed is catalyzed by coarse particles or both coarse and fine particles. In this study, PPFB was applied to simultaneous dry SO₂/NO_x removal process, and several kinds of sorbents or catalysts were evaluated in a PPFB. Using the selected sorbents and catalysts, kinetic measurements were made in the temperature range of 300 to 600°C. SO₂ removal efficiencies were affected by reaction temperature, sorbent/S ratio, and static bed height. NO_x removal efficiencies in excess of 95% were achieved at NH₃/NO_x mole ratio of 1.0. When FeO was used as sorbent, SO₂ conversion increased with increasing temperature and reached 80% at 600°C.     

Effectiveness of SO2 sorbents by thermogravimetry-combustion with coal

A new experimental method is described for non-isothermal thermogravimetry (TG) involving combustion of mixtures of sieved coal with sieved calcium-containing sorbents. This rapid TG method utilizes a baseline for TG combustion of coal alone, derives an equation that gives a semi-quantitative measure (±10% repeatability) of the coal's reactive sulphur retained by the sorbent, the extent of retention of S0₂ generated in situ during combustion varying with different sorbents. The method permits direct variation in separate experiments of the calcium-to-sulphur ratio during combustion and relative ranking of different sorbents by retention of reactive sulphur in combustion. Relative rankings are presented for three pre-calcined natural stones (two limestones and one dolomite); these results correlate with relative rankings from another TG method reported in the literature. It is suggested that this new method is useful for pre-screening the effectiveness of S0₂ sorbents considered for use in fluidizedbed combustion of coal.     

Rate-based modelling of SO2 absorption into aqueous NaHCO3/Na2CO3 solutions accompanied by the desorption of CO2

A rate-based model of a counter-current reactive absorption/desorption process has been developed for the absorption of SO₂ into NaHCO₃/Na₂CO₃ in a packed column. The model adopts the film theory, includes diffusion and reaction processes, and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. Model predictions were compared to experimental data from literature. For the calculation of the absorption rate of SO₂ into NaHCO₃/Na₂CO₃ solutions and concomitant CO₂-desorption, it is important to take into account all reversible reactions simultaneously. It is clear that the approximate analytical based model cannot be expected to predict the absorption rates under practical conditions because of the complicated nature of the reactive absorption processes. The rigorous numerical approach described here only requires definition of the individual reactions in the system, and subsequent solution is independent of specific assumptions made, or operational variables like pH or compound concentrations. As an example of the flexibility of this approach, additional calculations were conducted for SO₂ absorption in a phosphate-based buffer system.     

Pilot plant performance of a SO2 to SO3 oxidation catalyst for flue-gas conditioning

A honeycomb catalyst for the oxidation of endogenous SO₂ from a coal-fired power-station flue-gas has been developed. The catalyst reached a SO₂ to SO₃ conversion of 60 vol.% after 200 h in operation at the pilot plant. When this catalyst is further treated for another 100 h at lab scale to complete its activation, a stable 80 vol.% conversion is obtained. The results have been used to design an industrial unit for flue-gas conditioning to improve the fly ash collection by the electrostatic precipitator in a 220 MW coal-fired power plant.     

Reduction of SO2 on different carbons

The reduction of SO₂ on four carbons (graphite, charcoal, activated carbon and coke) was studied under steady-state conditions and when the kinetics was chemically controlled in a reactor operated under differential conditions. The reaction showed second-order kinetics: first order with respect to carbon and first order with respect to the partial pressure of SO₂. The reactivity of the different carbons, as measured by the second-order rate constants, followed the sequence of decreasing crystallinity: graphite<coke (7.34)<coke (11.73)<charcoal. The difference in reactivity between graphite and charcoal was determined by ΔH^≠, while for cokes it increased with the ash content because of a favorable ΔS^≠. The main reaction products for all carbons were CO₂ and sulfur in the ratio 2:1, considering the sulfur as S₂, which was shown to be formed through the same path. CO, COS, and CS₂ were also detected, and the product distribution depended on the carbon and whether the reaction was diffusion controlled or chemically controlled. Analysis of product ratios strongly suggested that CO, COS and CS₂ were produced from consecutive reactions of the primary products. CO was formed from CO₂ by a slow Boudouard reaction that occurred partially and under conditions of non-equilibrium. Complexed sulfur reacted with CO to form COS and CS₂. There was an interaction between the active site of reduction and the site where sulfur is inserted.     

Modeling of SO2 removal in fabric filter

Fabric filters are involved in most semi-dry flue gas desulfurization process and represent ability of SO₂ removal. SO₂ removal efficiency in fabric filter after a semi-dry scrubber is investigated. Experimental results showed that SO₂ inlet concentration has little effect on SO₂ removal efficiency, SO₂ removal efficiency increases as flue gas inlet temperature increases and relative humidity affects SO₂ removal efficiency significantly. The kinetic model based on shrinking core theory has been presented. It is found that, in the beginning, when calcium hydroxide conversion ratio is less than 0.3, SO₂ removal process is mainly controlled by chemical reaction (Model-2); and when calcium hydroxide conversion ratio is greater than 0.3, SO₂ diffusion through product layer is rate limiting (Model-3). The experimental results in fabric filter are successfully correlated by Model-3.     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Effects of NaCl on the capture of SO2 by CaCO3 during coal combustion

The effects of NaCl on the capture of SO₂ by CaCO₃ during coal combustion were studied using a thermogravimetric analyzer. All experiments were carried out in a flowing air atmosphere at a heating rate of 10, 20 or 30 °C/min. The experimental results showed that more SO₂ reduction was achieved as long as NaCl was added together with CaCO₃ to coal. It indicated that NaCl improved the behavior of CaCO₃ capturing SO₂. NaCl alone reduced SO₂ emission within the range from 300 to 700 °C in the experiments, while the capture of SO₂ by NaCl alone would be reemitted when temperature was raised over 700 °C. Adding sorbents such as CaCO₃ together with NaCl was necessary for a reliable SO₂ reduction.     

Selective photo-oxidation of various hydrocarbons in the liquid phase over V2O5/Al2O3

More than 0.22 mmol of isolated VO₄ species of V₂O₅/Al₂O₃ exhibited the highest evolution of the partial oxidation products (alcohol and ketone) in the oxidation of cyclohexane and cyclopentane. The conversion of cyclohexane and the selectivity of the partial oxidation products were achieved to be 0.49% and 85% over 0.8 g of 3.5 wt.% V₂O₅/Al₂O₃, respectively, where the K/A ratio was 6.2. In addition, V₂O₅/Al₂O₃ can selectively oxidize various hydrocarbons in the liquid phase by the one-step oxygen atom insertion to CH bond. The order of priority was tertiary carbon > secondary carbon > primary carbon > benzene ring.     

Deactivation of copper-ion-exchanged hydrogen-mordenite-type zeolite catalyst by SO2 for no reduction by NH3

Deactivation of copper-ion-exchanged hydrogen-mordenite-type zeolite catalyst by SO₂ for NO reduction by NH₃ was examined in a fixed-bed flow reactor. The deactivation of the catalyst was strongly dependent on reaction temperature. At high reaction temperatures over 300°C, the catalyst did not lose its initial activity up to 50 h of operation, regardless of SO₂ feed concentration from 500 to 20,000 ppm. However, at low reaction temperatures near 250°C, apparent deactivation did occur. Changes in the physicochemical properties such as surface area and sulfur content of deactivated catalyst well correlated with catalyst activity, depending upon reaction temperatures. The deactivation was due to pore blocking and/or filling by deactivating agents, which plugged and/or filled the pores of catalyst. The deactivating agents deposited on the catalyst surface were presumed to be (NH₄)₂SO₄ and/or (NH₄)HSO₄ from the results of TGA and ion-chromatography measurement.