Comprehensive sulfation model verified for T-T sorbent clusters during flue gas desulfurization at moderate temperatures

An empirical sulfation model for T-T sorbent clusters was developed based on amassed experimental results under moderate temperatures (300-800 °C). In the model, the reaction rate is a function of clusters mass, SO₂ concentration, CO₂ concentration, calcium conversion and temperature. The smaller pore volume partly results in a lower reaction rate at lower temperatures. The exponent on SO₂ concentration is 0.88 in the rapid reaction stage and then decreases gradually as reaction progresses. The exponent on the fraction of the unreacted calcium is 1/3 in the first stage and then increases significantly in the second stage. The CO₂ concentration has a negative influence on SO₂ removal, especially for the temperature range of 400-650 °C, which should be avoided to achieve a high effective calcium conversion. The sulfation model has been verified for the T-T sorbent clusters and has also been applied to CaO particles. Over extensive reaction conditions, the predictions agree well with experimental data.     

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.     

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.     

Efficient MgO-doped CaO sorbent pellets for high temperature CO2 capture

Novel MgO-doped CaO sorbent pellets were prepared by gel-casting and wet impregnation. The effect of Na⁺ and MgO on the structure and CO₂ adsorption performance of CaO sorbent pellets was elucidated. MgO-doped CaO sorbent pellets with the diameter range of 0.5-1.5 mm exhibited an excellent capacity for CO₂ adsorption and adsorption rate due to the homogeneous dispersion of MgO in the sorbent pellets and its effects on the physical structure of sorbents. The results show that MgO can effectively inhibit the sintering of CaO and retain the adsorption capacity of sorbents during multiple adsorption-desorption cycles. The presence of mesopores and macropores resulted in appreciable change of volume from CaO (16.7 cm³∙mol⁻¹) to CaCO₃ (36.9 cm³∙mol⁻¹) over repeated operation cycles. Ca2Mg1 sorbent pellets exhibited favorable CO₂ capture capacity (9.49 mmol∙g⁻¹), average adsorption rate (0.32 mmol∙g⁻¹∙min⁻¹) and conversion rate of CaO (74.83%) after 30 cycles.     

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.     

Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds

To demonstrate process feasibility of in situ CO₂ capture from combustion of fossil fuels using Ca-based sorbent looping technology, a flexible atmospheric dual fluidized bed combustion system has been constructed. Both reactors have an ID of 100 mm and can be operated at up to 1000 °C at atmospheric pressure. This paper presents preliminary results for a variety of operating conditions, including sorbent looping rate, flue gas stream volume, CaO/CO₂ ratio and combustion mode for supplying heat to the sorbent regenerator, including oxy-fuel combustion of biomass and coal with flue gas recirculation to achieve high-concentration CO₂ in the off-gas. It is the authors' belief that this study is the first demonstration of this technology using a pilot-scale dual fluidized bed system, with continuous sorbent looping for in situ CO₂ capture, albeit at atmospheric pressure. A multi-cycle test was conducted and a high CO₂ capture efficiency (> 90%) was achieved for the first several cycles, which decreased to a still acceptable level (> 75%) even after more than 25 cycles. The cyclic sorbent was sampled on-line and showed general agreement with the features observed using a lab-scale thermogravimetric analysis (TGA) apparatus. CO₂ capture efficiency decreased with increasing number of sorbent looping cycles as expected, and sorbent attrition was found to be another significant factor to be limiting sorbent performance.     

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.     

A regenerable Fe/AC desulfurizer for SO2 adsorption at low temperatures

A novel regenerable Fe/activated coke (AC) desulfurizer prepared by impregnation of Fe(NO₃)₃ on an activated coke was investigated. Experiment results showed that at 200 °C the SO₂ adsorption capacity of the Fe/AC was higher than that of AC or Fe₂O₃. Temperature-programmed desorption (TPD) revealed that H₂SO₄ and Fe₂(SO₄)₃ were generated on the desulfurizer upon adsorption of SO₂. Effect of desulfurization temperature was also investigated which revealed that with increasing temperature from 150 to 250 °C, the SO₂ removal ability gradually increases. The used Fe/AC can be regenerated by NH₃ at 350 °C to directly form solid ammonium-sulfate salts.     

Molecular dynamics simulation on diffusion of lithium atom pair in C150H30 cluster model for glassy carbon at very low temperatures

As the preliminary step to elucidate the ion transfer mechanism of lithium (Li) species in the cathode electrode of glassy carbon material, molecular dynamics (MD) simulation was taken place describing the initial thermal movement of two Li atoms in the simulation time range of 10 ps maximally in the hydrogen terminated cluster model, C₁₅₀H₃₀, at molecular mechanics 2 (MM2) level. The formation of Li₂ atom pair is specified in the central area of the cluster model as the result of the structural optimization. In the low temperature range from 4 to 10 K, accompanied by the rotational and stretching vibrational motions, it goes around the central area of the cluster model. Diffusion process of the atom pair is simulated dynamically for the first time in the present study. Decomposition of the atom pair occurred at 50 K and it produces two Li atoms which go and back independently from the center to the edge of the cluster model crossing the CC bonds orthogonally. Formation and diffusion processes of Li₂ atom pair may be responsible for the charge-discharge cycles in the glassy carbon electrode in the lithium secondary battery.     

Economical assessment of competitive enhanced limestones for CO2 capture cycles in power plants

CO₂ capture systems based on the carbonation/calcination loop have gained rapid interest due to promising carbonator CO₂ capture efficiency, low sorbent cost and no flue gases treatment is required before entering the system. These features together result in a competitively low cost CO₂ capture system. Among the key variables that influence the performance of these systems and their integration with power plants, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by CaO/CO₂ ratio and make-up flow of solids. New sorbents are under development to reduce the decay of their carbonation conversion with cycles. The aim of this study is to assess the competitiveness of new limestones with enhanced sorption behaviour applied to carbonation/calcination cycle integrated with a power plant, compared to raw limestone. The existence of an upper limit for the maximum average capture capacity of CaO has been considered. Above this limit, improving sorbent capture capacity does not lead to the corresponding increase in capture efficiency and, thus, reduction of CO₂ avoided cost is not observed. Simulations calculate the maximum price for enhanced sorbents to achieve a reduction in CO₂ removal cost under different process conditions (solid circulation and make-up flow). The present study may be used as an assessment tool of new sorbents to understand what prices would be competitive compare with raw limestone in the CO₂ looping capture systems.     

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.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.     

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V₂O₅/TiO₂ catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V₂O₅/TiO₂ catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V₂O₅/TiO₂ catalysts showed high catalytic activity for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V₂O₅/TiO₂ > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V₂O₅/TiO₂ for maximum NO_x conversion, which also showed high resistance to SO₂ in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V₂O₅/TiO₂ and compared with commercial W(10%)V₂O₅/TiO₂ catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V₂O₅/TiO₂ catalyst with temperature was observed. SO₂ deactivation experiments were carried out for Sb(2%)/V₂O₅/TiO₂ and W(10%)/V₂O₅/TiO₂ at a temperature of 230 °C for 90 h, resulted Sb(2%)/V₂O₅/TiO₂ was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V₂O₅/TiO₂ than Sb(2%)/V₂O₅/TiO₂ catalyst.     

等速升温流态化下CaO/生物质焦的SO2/NO联合脱除特性

燃煤锅炉污染物超低排放标准对电厂脱硫和脱硝系统提出了更高的要求。CaO作为脱硫剂可以实现循环流化床锅炉烟气中SO₂的高效脱除,焦炭作为还原剂直接还原NO,同时CaO的存在对焦炭还原NO起催化作用,可以实现燃煤烟气中SO₂/NO的联合脱除。为了探究连续温度变化对CaO/生物质焦联合脱硫脱硝性能的影响,在钙循环捕集CO₂技术背景下,研究了等速升温流态化下CaO/生物质焦的SO₂/NO联合脱除特性。探究了烟气中O₂和CO₂对CaO/椰壳焦脱除SO₂/NO的影响。结果表明,O₂通过对椰壳焦表面碳原子的活化作用降低了异相还原NO温度,在300~950℃等速升温过程中CaO/椰壳焦的NO脱除效率逐渐增加,780℃以上能实现100%脱硝。O₂也提高了CaO/椰壳焦的脱硫效率。CO₂与CaO的碳酸化反应以及与椰壳焦的气化反应对同时脱除SO₂/NO有明显抑制作用。O₂和CO₂共同作用下,在500~800℃内CaO/椰壳焦的脱硝效率随温度升高而增加,脱硫效率先降低后升高。NO促进了CaO/椰壳焦脱除SO₂,而SO₂对脱硝有抑制作用。800℃时CaO/椰壳焦同时脱除SO₂和NO的效率分别为97.7%和93.9%。     

Cyclic CO2 capture behavior of KMnO4-doped CaO-based sorbent

This study examines the CO₂ capture behavior of KMnO₄-doped CaO-based sorbent during the multiple calcination/carbonation cycles. The cyclic carbonation behavior of CaCO₃ doped with KMnO₄ and the untreated CaCO₃ was investigated. The addition of KMnO₄ improves the cyclic carbonation rate of the sorbent above carbonation time of 257 s at each carbonation cycle. When the mass ratio of KMnO₄/CaCO₃ is about 0.5-0.8 wt.%, the sorbent can achieve an optimum carbonation conversion during the long-term cycles. The carbonation temperature of 660-710 °C is beneficial to cyclic carbonation of KMnO₄-doped CaCO₃. The addition of KMnO₄ improves the long-term performance of CaCO₃, resulting in directly measured conversion as high as 0.35 after 100 cycles, while the untreated CaCO₃ retains conversion less than 0.16 at the same reaction conditions. The addition of KMnO₄ decreases the surface area and pore volume of CaCO₃ after 1 cycle, but it maintains the surface area and pores between 26 nm and 175 nm of the sorbent during the multiple cycles. Calculation reveals that the addition of KMnO₄ improves the CO₂ capture efficiency significantly using a CaCO₃ calcination/carbonation cycle and decreases the amount of the fresh sorbent.     

Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures

Combined effect of H₂O and SO₂ on V₂O₅/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH₃ at lower temperatures was studied. In the absence of SO₂, H₂O inhibits the catalytic activity, which may be attributed to competitive adsorption of H₂O and reactants (NO and/or NH₃). Although SO₂ promotes the SCR activity of the V₂O₅/AC catalyst in the absence of H₂O, it speeds the deactivation of the catalyst in the presence of H₂O. The dual effect of SO₂ is attributed to the SO₄²⁻ formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H₂O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H₂O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V₂O₅ loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h⁻¹ and temperature of 250 °C.     

MDEA-二元羧酸离子液体溶液吸收/解吸SO2性能

设计合成了三类N-甲基二乙醇胺（MDEA）-二元羧酸离子液体水溶液,研究了它们的物理性质和SO₂吸收容量。结果表明：MDEA/有机酸的摩尔比及阴离子种类是影响吸收容量的主要原因。碱酸摩尔比越高,离子液体吸收能力越强,解吸能力越差且解吸时间也越长。此外,不同阴离子的离子液体溶液吸收SO₂的容量有如下关系：MDEA-丁二酸>MDEA-戊二酸>MDEA-苹果酸。对于MDEA-戊二酸和MDEA-苹果酸离子液体溶液,最具工业化应用前景的碱酸摩尔比分别为1.2：1和1.4：1。     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

Kinetics of the reaction of iron blast furance slag/hydrated lime sorbents with SO2 at low temperatures: effects of sorbent preparation conditions

Sorbents highly reactive towards SO₂ have been prepared from iron blast furnace slag and hydrated lime under different hydration conditions. The reaction of the dry sorbents with SO₂ has been studied under the conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction was well described by a modified surface coverage model which assumes the reaction rate being controlled by chemical reaction on sorbent grain surface and takes into account the effect of sorbent Ca molar content and the surface coverage by product. The effects of sorbent preparation conditions on sorbent reactivity were entirely represented by the effects of the initial specific surface area (S_g0) and the Ca molar content (M⁻¹) of sorbent. The initial conversion rate of sorbent increased linearly with increasing S_g0, and the ultimate conversion increased linearly with increasing S_g0M⁻¹. The initial conversion rate and ultimate conversion of sorbent increased significantly with increasing relative humidity of the gas. Temperature and SO₂ concentration had mild effects on the initial conversion rate and negligible effects on the ultimate conversion.