Development and evaluation of magnetic iron-carbon sorbents for mercury removal in coal combustion flue gas

In order to get a cost effective and recyclable sorbent for mercury removal, a series of magnetic iron-carbon (Fe–C-x) sorbents was developed by co-precipitation. The physical and chemical properties of obtained sorbents were evaluated through various characterization methods. According to the results, Fe₃O₄ precipitate on carbon weakens the surface properties, but mercury removal performance in simulated flue gas is excellent. For flue gas components, HCl promotes mercury oxidation and adsorption on sorbents, O₂ has limited effect on mercury removal and SO₂ plays an inhibitive role. NO could enhance mercury oxidation with O₂ existence because of the generation of NO₂, which could react with Hg⁰ through heterogeneous reaction over iron-carbon surface. Besides, effects of temperature and regeneration performance were further researched under simulated flue gas. Apart from higher temperature will decompose mercury compounds and cause the removal efficiency decrease, Fe–C-3 sorbent shows excellent Hg⁰ removal performance at the temperature window of 100–200 °C. Exceptional regeneration performance on Hg⁰ removal indicates that spent sorbent could be regenerated.     

燃煤烟气脱汞吸附剂研究进展

概述了目前当今世界燃煤烟气中脱汞吸附剂的研究现状,介绍了碳基吸附剂、钙基吸附剂、石油焦、飞灰,以及一些新型吸附剂对汞的脱除效果。指出活性炭对汞的脱除效率高,但成本也高;飞灰吸附剂对汞脱除的成本低,但脱除效率也低。用于燃煤电厂汞吸附的吸附剂研究处于实验室研究阶段,因此开发价廉和高效脱汞吸附剂和吸附剂的再生等是吸附剂发展过程中的一个重要研究课题。     

Stability of lanthanum oxide-based H2S sorbents in realistic fuel processor/fuel cell operation

We report that lanthana-based sulfur sorbents are an excellent choice as once-through chemical filters for the removal of trace amounts of H₂S and COS from any fuel gas at temperatures matching those of solid oxide fuel cells. We have examined sorbents based on lanthana and Pr-doped lanthana with up to 30 at.% praseodymium, having high desulfurization efficiency, as measured by their ability to remove H₂S from simulated reformate gas streams to below 50 ppbv with corresponding sulfur capacity exceeding 50 mg S g_sorbent⁻¹ at 800 °C. Intermittent sorbent operation with air-rich boiler exhaust-type gas mixtures and with frequent shutdowns and restarts is possible without formation of lanthanide oxycarbonate phases. Upon restart, desulfurization continues from where it left at the end of the previous cycle. These findings are important for practical applications of these sorbents as sulfur polishing units of fuel gases in the presence of small or large amounts of water vapor, and with the regular shutdown/start-up operation practiced in fuel processors/fuel cell systems, both stationary and mobile, and of any size/scale.     

W型火焰煤粉炉两段喷钙脱硫技术的试验研究

针对燥粉炉内钙基固硫剂燃绕脱硫率低的难题，研究了W型火焰燥粉炉一次风燃烧区域烟气温度和烟气成分的动态变化规律。在研究添加剂量和添加方式对燃绕脱硫率影响的基础上，提出了分级配风两段喷钙脱硫技术的方案，从而实现一次风燃绕高温区域和中温燃绕区域的同时脱硫，使脱硫剂在高温还原区域生成CaS，在中温氧化区域生成CaSO4，从而避免了CaSO4在高温区域的分解，使Ca／S＝3时燥粉炉的脱硫效率达到71．2％。图2表2参4     

The efficient lignocellulose-based sorbent for oil spill treatment from polyurethane and agricultural residue of Vietnam

In this study, new and efficient sorbent with density 0.2 g/cm³ was fabricated by incorporating rice straw into polyurethane matrix to get an open cell structure material with high oil uptake capacity. The influences of various important factors such as amount of adding rice straw, size of rice straw particles, and adsorption time on oil adsorption capacity of new sorbent material were investigated. The results showed that the oil absorption rate occurred fast in first 15–30 min, then slowed down and reached saturation level after about 2 h of treatment. Oil adsorption capacity of the new sorbent material was relatively high, up to 12.0 g/g. In comparison with pristine polyurethane or lignocellulosic materials, the new sorbents had higher oil adsorption capacity. Some characteristics of the as-obtained sorbent, such as surface shapes and porosity, were also studied by SEM analysis.     

Glass fiber entrapped sorbent for reformates desulfurization for logistic PEM fuel cell power systems

Glass fiber entrapped ZnO/SiO₂ sorbent (GFES) was developed to remove sulfur species (mainly hydrogen sulfide, H₂S) from reformates for logistic PEM fuel cell power systems. Due to the use of microfibrous media and nanosized ZnO grains on highly porous SiO₂ support, GFES demonstrated excellent desulfurization performance and potential to miniaturize the desulfurization reactors. In the thin bed test, GFES (2.5 mm bed thickness) attained a breakthrough time of 540 min with up to 75% ZnO utilization at 1 ppm breakthrough. At equivalent ZnO loading, GFES yielded a breakthrough time twice as long as the ZnO/SiO₂ sorbent; at equivalent bed volume, GFES provided a three times longer breakthrough time (with 67% reduction in ZnO loading) than packed beds of 1–2 mm commercial extrudates. GFES is highly regenerable compared with the commercial extrudates, and can easily be regenerated in situ in air at 500 °C. During 50 regeneration/desulfurization cycles, GFES maintained its desulfurization performance and structural integrity. A composite bed consisting of a packed bed of large extrudates followed by a polishing layer of GFES demonstrated a great extension in gas life and overall bed utilization. This approach synergistically combines the high volume loading of packed beds with the overall contacting efficiency of small particulates.     

Adsorption behavior and kinetics of H2S on a potassium-promoted hydrotalcite

Adsorption of H₂S and the influence of steam on its adsorption capacity and kinetics were studied on a commercial potassium-promoted hydrotalcite. The sorbent shows a very high cyclic working capacity for H₂S compared to CO₂ and H₂O, even at lower partial pressures and at different operating temperatures ranging between 300 and 500 °C. The operating temperature does not significantly influence the cyclic working capacity for half-cycle times of 30 min. The adsorption mechanism, however, changes at higher temperatures. At lower temperatures (300 °C) a fast adsorption with a fast approach to steady state was observed. At higher operating temperatures, H₂S reacts with the hydrotalcite structure, forming strongly bonded sulfuric species on the sorbent. When using dry regeneration conditions, the first cycles in cyclic operation at higher temperatures show a significantly higher adsorption of H₂S (especially the first cycle), which cannot be desorbed during regeneration with N₂. After the first fast initial adsorption rate a continuous slow adsorption of H₂S occurs, probably caused by a surface reaction between H₂S and the hydrotalcite structure. This reaction is, however, reversible if steam is used.The adsorption mechanism for H₂S and H₂O was determined using multiple cyclic experiments comparable to previous studies performed for CO₂ and H₂O adsorption. It is evident that the adsorption mechanism developed for CO₂ on the same sorbents is also valid for H₂S, indicating that the developed mechanism is consistent for sour gas adsorption on this type of sorbents. The cyclic working capacity can be significantly increased if steam is used during the regeneration step of the sorbent. The mechanistic model developed for the adsorption of CO₂ and H₂O was successfully validated with more than 160 different TGA experiments. An operating temperature of 400 °C seems to be optimal to achieve a high cyclic working capacity for H₂S, because at higher temperatures the regeneration of the formed sulfuric species seems to be hindered resulting in a significant decrease in the cyclic working capacity.     

Thermodynamic performance assessment of IGCC power plants with various syngas cleanup processes

The present work explores how much IGCC can benefit from warm gas clean-up(WGCU)in comparison with conventional cold gas clean-up(CGCU) and what are the respective contributions of dry particulates removal and warm gas desulfurization (WGD) in a plant-wide point of view. Influences of key parameters of WGD on ther- modynamic performance of IGCC plant including desulfurization temperature, oxygen concentration in the re- generation stream, and H 2 S removal efficiency are discussed. It is obtained that the net efficiency of IGCC with full WGCU experiences an improvement of 1.77 percentage points compared with IGCC with full CGCU. Of which, dry particulates removal without water scrubber contributes about 1 percentage point. The influence of desulfurization temperature on thermodynamic performance of IGCC with WGD is weak especially when it is higher than about 350℃, which indicates that more focus should be put on investment cost, technical feasibility and sorbent stability for the selection of optimal operation temperature. Generally, 2%~3% of oxygen concentra- tion in the regeneration stream might be reasonable in a thermodynamic performance point of view. In addition, the improvement of 0.31 percentage points can be obtained by removal of H 2 S in the syngas from 27 ppm to 3 ppm.     

A study on the activity of CaO-based sorbents for capturing CO2 in clean energy processes

CaO-based regenerative sorbents for CO₂ capture in power generation and H₂ production are receiving growing attention. A major challenge for this technology is the decay of sorbent activity with increasing number of the sorption/regeneration cycles. Evaluation of long-term sorbent activity currently requires substantial experimental work. In this study, the dependence of the activity on the number of sorption/regeneration cycles is examined, and the apparent dependence on the number of cycles is related to the duration of sorbent regeneration. By relating the decay in activity of the sorbent to its decrease in surface area due to sintering, interesting insights can be drawn. A method for determination of the long-term activity has been proposed, which can greatly reduce the experimental work for sorbent development and process evaluation.     

Development of a Fe/Mg-bearing metallurgical waste stabilized-CaO/NiO hybrid sorbent-catalyst for high purity H2 production through sorption-enhanced glycerol steam reforming

In this work, a Fe/Mg-bearing metallurgical waste (upgraded slag oxide, UGSO) was, for the first time, investigated as a stabilizer for increasing the cyclic stability of CaO-based sorbents. The sorbents were prepared through the wet mixing of the ball-milled UGSO particles with the limestone-derived calcium citrate under sonication. The sorption capacity of samples containing different waste loadings (5, 10, 15, and 25 wt%) was studied for 18 carbonation/regeneration cycles under conditions similar to the sorption-enhanced glycerol steam reforming process. A significant improvement of the cyclic stability was observed for all doped sorbents; however, the sample with 10 wt% UGSO showed the highest sorption capacity among all tested samples. This optimum sorbent was further used to synthesize a UGSO stabilized CaO–NiO hybrid sorbent-catalyst material (20 wt% NiO loading), whose performance was tested in sorption-enhanced steam reforming of glycerol. A H₂ purity of around 95% was obtained in the pre-breakthrough period that lasted for about 30 min. In summary, the results showed a better stability of UGSO stabilized sorbents compared to pure CaO and a good performance of the CaO-UGSO10/NiO sorbent-catalyst hybrid material in the sorption-enhanced reforming process.     

Effects of impregnating variables on dynamic sorption characteristics and storage properties of composite sorbent for solar heat storage

A family of composite sorbents was prepared by impregnating silica gel in the solution of the hygroscopic salt CaCl₂ for solar heat storage. The characteristics of water adsorbed on the composite sorbents prepared under different impregnating conditions were measured by a micromeritics gas adsorption analyzer, a Calvet-type microcalorimeter and an open-type gravimetric method. From the results of these dynamic sorption measurements, the effects of impregnating variables on the characteristics of water adsorbed on the composite sorbents were evaluated. The composite sorbents prepared under different impregnating conditions were also tested on an open-type sorption storage system. The composite sorbent prepared by impregnating in the CaCl₂ solution of 30% showed a high and stable storage capacity of 1020 J g⁻¹ at the charging temperature of about 90 °C. This study demonstrates a great potential in controlling the sorption characteristics as well as the storage properties of the composite sorbents by optimizing the impregnating variables to meet the specific demands of solar heat storage.     

钙基吸收剂脱硫反应特性的研究

本文对两种钙基吸收剂－ＣａＯ和Ｃａ（ＯＨ）２的脱硫反应机理进行了试验研究。对两者的反应活性进行了比较，并就一些因素对脱硫活性和影响作了研究，所得结论对脱硫反应的优化，脱硫剂的选择及干法ＦＧＤ装置的开发和研制有参考意义。     

Electrochemical hydrogenation of organic sulfides

Electrochemical reduction desulfurization is a low cost, environmental friendly technology which is capable of achieving a high degree of automation. Electrochemical coupling reactions of organic sulfur hydrogenation and water electrolysis on the C/Nafion electrodes were performed. The electrochemical desulfurization was determined by cyclic voltammetry (CV), bulk electrolysis with coulometry (BEC) and electrochemical impedance spectroscopy (EIS) techniques. Thiophene and benzyl mercaptan were used as model organic sulfur compounds. The results of cyclic voltammetry showed that the electrochemical hydrogenation reduction reaction of thiophene occurred at about ?0.35 V. The process included proceeding chemical reaction and electrochemical reaction. The currents generated from thiophene hydrogenation reactions increased with the reaction temperature and the H⁺ concentration of the electrolyte acidity of anode. Under the same reaction conditions, the desulfurization efficiency of benzyl mercaptan was significantly higher than that of thiophene. From the products of electrolytic reactions, the mechanisms of electrochemical hydrogenation of thiophene were proposed, consisting of two pathways: ring opening followed by hydrogenation and directly hydrogenation followed by ring opening. The proposed reaction mechanisms were consistent with the EIS results, indicating the predominant reactions were ring opening followed by hydrogenation. The reaction products and EIS results suggested that the reaction mechanisms of electrochemical hydrogenation of benzyl mercaptan were by breaking CS bond to form H₂S and toluene.     

Thermodynamic assessment of IGCC power plants with hot fuel gas desulfurization

In IGCC power plants, hot gas desulfurization (HGD) represents an attractive solution to simplify syngas treatments and to improve the efficiency, potentially reducing the final cost of electricity. In the present study, the various consequences of the introduction of a HGD station in the power plant are discussed and evaluated, in comparison with conventional near-ambient temperature clean-up. Attention is paid to the potential improvements of the overall energy balance of the complete power station, along with the requirements of the sorbent regeneration process, to the influence of the desulfurization temperature and to the different solutions needed to control the NO_x emissions (altered by the presence of HGD).     

Experimental research on two-stage desulfurization technology in traveling grate boilers

In order to promote the desulfurization efficiency of calcium-based sorbents during coal combustion in traveling grate boilers, the influences on sulfur removal of the thermal conditions and the sorbents were discussed in this paper. It was found that the SO₂ concentration first rises, then declines along the traveling grate and reaches the peak near the midpoint of the grate. The fluctuation of the SO₂ concentration over time in the flue gas is mainly affected by the flame temperature. When the particle size of the sorbents decreases from 75 to 0.1 μm, the sulfur removal efficiency will increase slightly. A reasonable Ca/S molar ratio is about 2 when sorbents are blended with the coal on the grate and its further increase has little benefit to desulfurization. A new, so-called two-stage desulfurization process — sulfur capture firstly in the coal bed and secondly in the combustion gas — is suggested as it can greatly promote the sulfur removal efficiency up to 70∼80%. By X-ray powder diffraction analysis, some thermal stable phases were identified in the sulfur retention cinder obtained from the on-grate process.     

Uranium uptake with graphene oxide sponge prepared by facile EDTA‐assisted hydrothermal process

Seeking highly efficient and low‐cost sorbents to removal uranium contaminants is of great concern in demand. In present work, a graphene oxide sponge (GOS) was successfully assembled by a facile Ethylenediaminetetraacetic acid (EDTA)‐assisted hydrothermal method from graphene oxide (GO) sheets. The prepared GOS was characterized, and the influencing factors on uranium sorption were optimized. Results indicate that the GOS shows a spongy porous structure and significantly enhances the U(VI) removal compared with the pristine GO. Langmuir model well describes the equilibrium sorption data. Under experimental conditions, the maximal uranium sorption calculated from Langmuir model was found to be 212.77 mg · L^?1. Sorption kinetics data are matched with pseudo‐second‐order model. Reusability experiments indicate that the prepared GOS can be regenerated using dilute acid treatment and be used repeatedly. The simple and low‐cost preparation of GOS predicts the potential applications of spongy GO as an effective sorbent for U(VI) capture and also expands the possibility of using graphene‐based sorbent in the disposal of liquid radioactive waste. Copyright © 2016 John Wiley & Sons, Ltd.     

Investigation on elemental mercury removal by cerium modified semi-coke

Coal-fired power plant is the largest anthropogenic mercury source. Active carbon injection technique has been widely used to control the mercury emissions. However, high operation cost limits its development and it is necessary to find other potential mercury sorbents. In this study, raw semi-coke and a series of novel cerium (Ce) modified semi-cokes were synthesized and utilized for removing elemental mercury (Hg⁰) from simulated flue gas. It is noteworthy that the efficiencies were tested without hydrogen chloride (HCl) in order to evaluate the sorbents efficacy for low-chlorine (Cl) coal. The results show that the modified sorbents exhibited the best performance at 150 °C. The performance of sorbent could be reinforced due to the existence of oxygen (O₂), nitric oxide (NO) and HCl. The adverse effect caused by sulfur dioxide (SO₂) reduced dramatically after Ce modification. The negative impact of ammonia (NH₃) on Hg⁰ removal in this study could be neglected owing to the tiny concentration of NH₃. Raw semi-coke provided sufficient carbon content, which is favorable to mercury adsorption. As Ce loading increased, the carbon structure changed and the crystal of cerium oxide was formed in the modified semi-coke. The mass fraction of cerium oxide on the sorbent was over 4.4% when the concentration of Ce modification solution was higher than 0.2 mol L⁻¹. The redox reaction activity and the oxygen storage ability of Ce³⁺/Ce⁴⁺ gave a huge boost to the performance of modified semi-coke. The addition of Ce also had an impact on the proportion of oxygen species.     

HCl干式净化过程平衡浓度计算及其应用

利用固体产生生成的能耗分析，在传统的热力学平衡公式中加入入修正项，计算HCl气体在利用石灰类吸收剂对其进行干式净化过程的平衡浓度，首次提出气固反应过程中反应气全的平衡浓度随着固体反应率的增大而上升的概念，计算结果表明：吸收剂在达到一定的反应率以后，HCl的平衡浓度随温度的上升而下降。利用平衡浓度随反应率和温度的变化特征可以解释传统的热力学平衡式所不能解释的一些实验现象，并可以利用平衡浓度来预测利用钙基吸收剂干式净化焚烧炉排的HCl的净化效率和吸收剂用量。预测结果可以判断干式净化的使用范围，局限性和改进方向。     

Experimental investigation of improved calcium-based CO2 sorbent and Co3O4/SiO2 oxygen carrier for clean production of hydrogen in sorption-enhanced chemical looping reforming

In this study, highly pure hydrogen is produced in sorption enhanced chemical looping steam methane reforming (SE-CLSMR) using cobalt-based oxygen carrier (OC) and cerium promoted CaO-based sorbent. In addition, the CO₂ removal from a gas stream at high temperatures is investigated via calcium looping process prior to SE-CLSMR process. The prepared samples are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and energy dispersive X-ray spectroscopy (EDX) techniques. The effect of Ca/Ce molar ratio (100/0.00–0.91/0.09), sorption temperature (550–650 °C) and sorbent lifetime are studied to find the optimal sorbent. The characterization results show the uniform and orderly CeO₂ dispersed sorbent nanoparticles that notably improved the sorbent morphology compared with blank CaO. The sorption results revealed the negative effect of temperature on CO₂ uptake of all the samples. In addition, the CO₂ sorption evaluations indicate that the molar ratio of cerium to calcium plays a significant role in the stability of sorbent and improved the CO₂ sorption capacity significantly. The high CO₂ removal efficiency in the cerium modified sorbents could be due to decrease in diffusion resistance of CO₂ through the sorbent structure during the carbonation reaction. Furthermore, results show that the addition of cerium to the sorbent structure, effectively improves the thermal resistance of synthesis sorbents. The SE-CLSMR results showed that the H₂ purity could be increased up to about 95% considering Co₃O₄/SiO₂ oxygen carrier and cerium promoted calcium-based sorbent at relatively low temperature of 550 °C, which is comparable with 84% in CLR process.     

燃煤烟气中汞吸附的研究综述

概述了燃煤烟气中汞的吸附特性,综述了活性炭、飞灰和其它吸附剂对汞的吸附机理和影响因素,探讨了汞的吸附模型。指出活性炭对汞的脱除效率可达到90%以上,但成本高,飞灰吸附成本虽然低但脱除效率有限,其它吸附剂研究尚处于初级阶段,因此寻找廉价高效的吸附剂十分必要,同时开发烟气中污染物的联合脱除技术是研究方向。