Characteristics and reactivities of Ca(OH)2/silica fume sorbents for low-temperature flue gas desulfurization

Ca(OH)₂/silica fume sorbents were prepared with various Ca(OH)₂/silica fume weight ratios and slurrying times at 65°C and a water/solid ratio of 10/1. Dry sorbents prepared were characterized, and their reactivities toward SO₂ were measured in a differential fixed-bed reactor at the conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization (FGD) processes. The reaction between Ca(OH)₂ and silica fume in the slurry was very fast. The formation of calcium silicate hydrates, which were mainly C-S-H(I), resulted in sorbent particles with a highly porous structure that seemed compressible under high pressures. The sorbents were mesoporous, and their specific surface areas and pore volumes were much larger than those of Ca(OH)₂ alone. The utilization of Ca of sorbent increased with increasing silica fume content mainly due to the increase in the specific surface area of sorbent. The sorbent with Ca(OH)₂ had the maximum SO₂ capture. Sorbents with Ca(OH)₂ contents less than and greater than would have a SO₂ capture greater than that of Ca(OH)₂ alone. Both the utilization of Ca and SO₂ capture per unit specific surface area of sorbent decreased in general with increasing specific surface area. At the same Ca(OH)₂ content, the utilization of Ca or SO₂ capture of the Ca(OH)₂/silica fume sorbent was greater than that of the Ca(OH)₂/fly ash sorbent; however, the amount of SO₂ captured per unit surface area of the former sorbent was smaller than that of the latter sorbent. The results of this study are useful to the preparation of silica-enhanced sorbents for use in the dry and semidry FGD processes.     

A rate equation for Ca(OH)2 and CO2 reaction in a spouted bed reactor at low gas concentrations

In the present study, the carbonation reaction of hydrated lime in semi-dry condition is investigated experimentally in a laboratory-scale spouted bed reactor. Results show that for operating conditions where the concentration of CO₂ is low, the capture efficiency is raised by increasing the inlet CO₂ concentration. Additionally, because of the inconsistency between the experimental reaction rate and the calculated values based on the previous proposed equations, a new rate equation is introduced that considers the dependency of CO₂ concentration too. To validate the proposed equation, its predictions were compared with another set of experimental data.     

Carbonation reaction of lime, kinetics at ambient temperature

The uptake of carbon dioxide due to the carbonation reaction of Ca(OH)₂ in ambient temperature of approximately 20 °C has been studied. Different types of lime have been used and the CO₂ concentration has been varied to identify the influence of different variables on the kinetics of the reaction. A closed loop system has been developed and validated that allows measurement of the carbonation progress directly from monitoring CO₂ uptake. Thermal analysis (TA) was used to verify the degree of carbonation that reached up to 83%. Factor analysis on the data set has demonstrated that reaction speed is not dependent upon the CO₂ concentration within the limits tested. Carbonation speed depends on the specific surface of the lime. The results of this study contribute to research carried out on lime mortar carbonation models and on the carbonation process in general.     

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.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

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.     

Development of kinetic model for the reaction between SO2/NO and coal fly ash/CaO/CaSO4 sorbent

Sorbents for semidry-type flue gas desulfurization (FGD) process can be synthesized by mixing coal fly ash, calcium oxide, and calcium sulfate in a hydration process. As sorbent reactivity is directly correlated with the specific surface area of the sorbent, reacting temperature, concentration of the reacting gas species and relative humidity, two major aim in the development of a kinetic model for the FGD process are to obtain an accurate model and at the same time, incorporating all the parameters above. Thus, the objective of this work is to achieve these two aims. The kinetic model proposed is based on the material balance for the gaseous and solid phase using partial differential equations incorporating a modified surface coverage model which assumes that the reaction is controlled by chemical reaction on sorbent grain surface. The kinetic parameters of the mathematical model were obtained from a series of experimental desulfurization reactions carried out under isothermal conditions at various operating parameters; inlet concentration of SO₂ (500 ppm  C_0,SO2  2000 ppm), inlet concentration of NO (250 ppm  C_O,NO  750 ppm), reaction temperature (60 °C  T  80 °C) and relative humidity (50%  RH  70%). For a variety of initial operating conditions, the mathematical model is shown to give comparable predictive capability when used for interpolation and extrapolation with error less than 7%. The model was found useful to predict the daily operation of flue gas desulfurization processes by using CaO/CaSO₄/coal fly ash sorbent to remove SO₂ from flue gas.     

Removal of SO2 at low temperature using dead Bacillus licheniformis

In this paper we studied the adsorption and desorption behavior of SO₂ by the dead Bacillus licheniformis R08 biomass. The effects of water vapor, temperature and O₂ on the removal of SO₂ by the biomass were studied. FTIR and XPS were used to characterize the mechanism of the SO₂ adsorption on the biomass. The experimental results showed that water vapor and temperature deeply influenced the adsorption of SO₂ by the biomass. However, O₂ cannot oxidize SO₂ to SO₃ on the biomass. FTIR and XPS results showed that oxygenous and nitrogenous functional groups on the cell walls of biomass may be related to the SO₂ adsorption and three sulfur species were formed on the biomass in adsorption process. In the desorption process, weakly adsorbed SO₂ could be desorbed by increasing temperature and the biomass can be reused for 10 cycles.     

Pozzolanic reactions of common natural zeolites with lime and parameters affecting their reactivity

The pozzolanic reaction between portlandite and different types of nearly pure natural zeolites was studied. Analcime, phillipsite, chabazite, erionite, mordenite and clinoptilolite-rich tuffs were mixed with portlandite and water (1:1:2 by weight), and the progress of the pozzolanic reaction was quantitatively determined by thermogravimetrical analyses from 3 to 180 days.A thorough characterization of the raw materials was performed by quantitative XRD, XRF, SEM-EDX, BET specific surface area measurements, grain-size analyses, FTIR and Cation Exchange Capacity measurements. The difference in reactivity of the samples containing zeolites with varying Si/Al ratios, as well as between clinoptilolite-rich samples exchanged with different cations or ground to different grain sizes was assessed.The results indicate that the external surface area only influences the short-term reactivity, whereas the cation content has an effect on both the long- and short-term reactivity. The early reactivity of the unexchanged samples can be explained by these two parameters, but their long-term reactivity is mainly related to the Si/Al ratio of the zeolites. Samples with zeolites rich in Si react faster then their Al-rich counterparts.     

The preparation of Mg3Si2O5(OH)4 nanotubes under solvothermal conditions

With magnesium carbonate hydroxide and nanoporous silica as the starting materials, chrysotile (Mg₃Si₂O₅(OH)₄) nanotubes were prepared by using a solvothermal method at 400^∘C within four hours. This new method needs no strong alkali medium and the reaction time is very short. EDX analysis showed a molar ratio of 3Mg:2Si:9O of the product. Selected Area Electron Diffraction (SAED) pattern indicated that the tube axis is along [100] direction. HRTEM image showed the nanotubes were multi-walled and the distance between the two close layers was about 0.75 nm, which is very near to the distance of {001} planes. Thus, combining the results of SAED and HRTEM, we can conclude that the {001} planes of serpentine roll up along the [100] direction to form the tubular structure. The effects of various reaction conditions and the formation mechanism were also discussed.     

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.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

Computational modeling of furnace sorbent injection for SO2 removal from coal-fired utility boilers

Furnace sorbent injection (FSI) is used to remove SO₂ formed during coal combustion by injecting sorbent into the high temperature zone of a furnace above the fireball. FSI is cost effective for older coal-fired boilers, especially when space or capital budgets are limited. To optimize the design and performance of FSI, an SO₂/sorbent modeling scheme that simultaneously considers calcination (or dehydration), sintering, and sulfation has been developed and implemented. It is coupled with a three-dimensional combustion model based on computational fluid dynamics to determine the most desirable locations for sorbent injection and to optimize the amount of sorbent needed to achieve a targeted SO₂ removal efficiency. A sensitivity analysis was conducted to determine the effect of flue gas temperature, particle diameter, and SO₂ concentration on the extent of sulfation. This SO₂/sorbent sub-model was applied to a 126-MW front-wall fired boiler firing eastern bituminous coal. The SO₂ removal efficiencies predicted by the model agreed well with those measured in the field. The modeling results indicated that sorbent injected directly into the furnace through boosted over-fired air ports is more effective at removing SO₂, due to longer residence time and better mixing, relative to ports higher in the furnace with poor mixing. This modeling approach is optimized for full-furnace application to facilitate the design process.     

Influence of SiO2 modification on hydrogarnets formation during hydrothermal synthesis

Formation and stability of hydrogarnet and Al-substituted tobermorite were examined at 175 °C temperature in saturated steam environment processing CaO-quartz and CaO-amorphous SiO₂ suspensions. A large quantity of Al₂O₃ was added to the starting mixtures [molar ratio A/(S+A)=0.10, duration of hydrothermal synthesis—from 0 to 24 h]. It was determined that hydrogarnets always tend to form more rapidly than 1.13 nm tobermorite. However, later, with extension of synthesis duration, they start to fracture and their quantity reduces almost in half during 24 h. CaO is present in the further reaction with SiO₂ forming hydrated calcium silicates, and released Al³⁺ ions are inserted into Al-substituted tobermorite crystal lattice. Using amorphous SiO₂·nH₂O as SiO₂ component, starting raw materials react considerably quicker—the total Ca(OH)₂ is joined already while increasing the temperature up to 175 °C. Meanwhile, in the mixtures with quartz when their composition is described by the molar ratio C/(S+A)=1.0, traces of Ca(OH)₂ are found even after 24-h isothermal treatment at 175 °C temperature. Moreover, it depends on SiO₂ modification the hydrogarnets of what type are to be formed. Si-free hydrogrossular forms in the mixtures with quartz and katoite in the mixtures with SiO₂·nH₂O. Si⁴⁺ ions are inserted into the crystal lattice of the latter compound while the first one remains undisturbed. This is presumably related to the lower solubility of the quartz. It was also noticed that an isomorphic Si⁴⁺ ions substitution with Al³⁺ ions in the hydrated calcium silicate lattice is considerably quicker when an amorphous SiO₂ is used as SiO₂ component instead of quartz.     

Effects of grinding and firing conditions on CaAl2Si2O8 phase formation by solid-state reaction of kaolinite with CaCO3

The effects of grinding and firing conditions on CaAl₂Si₂O₈ phase formation by solid-state reaction of kaolinite with CaCO₃ were investigated by differential thermal analysis (DTA)–thermogravimetry (TG), X-ray powder diffraction (XRD) and ²⁹Si and ²⁷Al MAS NMR. Unground and ground samples showed similar crystallization behavior at about 850 °C, and the crystallizing temperature was relatively unaffected by grinding. On the other hand, the crystalline products were strongly influenced by the grinding. Gehlenite (Ca₂Al₂SiO₇) was the dominant phase in the unground samples but layer-structured CaAl₂Si₂O₈ was dominant in the ground samples, together with a small amount of anorthite, which is the stable phase. The amount of anorthite gradually increased with higher firing temperature, the sample fired at 1000 °C being almost completely anorthite. Grinding treatment before firing was effective in accelerating the decomposition of CaCO₃ and extending the temperature range for the formation of CaAl₂Si₂O₈, a phase with local structure similar to that of layered CaAl₂Si₂O₈.     

Advanced experimental analysis of the reaction of Ca(OH)2 with HCl and SO2 during the spray dry scrubbing process

This work evaluates both the removal efficiencies of HCl and SO₂ at different points in a spray dryer using Ca(OH)₂ as the absorbent. The operating conditions were specified in terms of the temperature of the flue gas (200-300 °C), the HCl concentration (120-1000 ppm), the SO₂ concentration (150-500 ppm) and the amount of CaCl₂ added (10-30 wt.%).The experimental results showed that the SO₂ removal efficiencies were higher in the presence of HCl (120-500 ppm) than in the absence of HCl at 250 °C and 20% relative humidity (RH). However, the removal efficiency of SO₂ decreased as the HCl concentration increased. The removal efficiency of SO₂ also increased with the amount of CaCl₂ in the spray dryer.     

Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2

Water permeability resistant behavior and microstructure of concrete with nano-SiO₂ were experimentally studied. A water permeability test shows that, for concretes of similar 28-day strength, the incorporation of nano-SiO₂ can improve the resistance of water penetration of concrete. An ESEM test reveals that the microstructure of concrete with nano-SiO₂ is more uniform and compact than that of normal concrete. Mechanism about the effect of nano-SiO₂ on concrete is described.