Sulfation diffusion model for SO2 capture on the T-T sorbent at moderate temperatures

A sulfation model was developed for dry flue gas desulfurization (FGD) at moderate temperatures to describe the reaction characteristics of the T-T sorbent clusters and the fine CaO particles that fall off the sorbent grains in a circulating fluidized bed (CFB) reactor. The cluster model describes the calcium conversion and reaction rate for various size sorbent clusters. The sulfation reaction is first order with respect to the SO₂ concentration above 973 K. The calcium conversion and reaction rate for the CaO particles were obtained by extrapolation. In the model for CaO particle, the reaction rate is linearly related to the calcium conversion and the SO₂ concentration in the rapid reaction stage and linearly related only with the calcium conversion after the product layer forms. The sulfation model accurately describes the sulfation of the T-T sorbent flowing through a CFB reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Kinetic behaviour of iron oxide sorbent in hot gas desulfurization

Although a number of reports on sorbents containing ZnO for H₂S removal from coal-derived gases can be found in the literature, it is shown in our study that a special sorbent containing Fe₂O₃·FeO (SFO) with minor promoters (Al₂O₃, K₂O, and CaO) as the main active species is more attractive for both sulfidation and regeneration stages, also under economic considerations. This paper presents the kinetic behaviour of SFO in a hot gas desulfurization process using a thermogravimetric analysis under isothermal condition in the operating range between 500 and 800 °C. The gas stream was N₂ with a 2% wt of H₂S. Experiences carried out on sorbent sulfidation with SFO (particle sizes in the range of 0.042-0.12 mm) indicate that the sorbent sulfidation capacity sharply increases with temperature in the range of 500-600 °C. It is also shown that the sample weight reaches its maximum absorption capacity, near saturation, at 600 °C so that it makes no sense to increase the sulfidation temperature from this point. To make a comparison between SFO and a zinc titanate based sorbent, a set of sulfidation tests was carried out at 600 °C during 7200 s using the same sieve range for both sorbents between 42 and 90 μm. Results show that the sulfidation capacity of SFO is 1.9 times higher than that of zinc titanate.     

甲基丙烯酸甲酯(MMA)吸附剂失活的探析

通过对吸附剂差热-热失重分析(TG-DTA)、红外线光谱(IR)、扫描电镜(SEM)、X射线衍射(XRD)、比表面积和孔结构表征研究,得出结论:甲基丙烯酸甲酯(MMA)中甲酸与吸附剂的碱性中心CaO进行物理-化学吸附,形成甲酸钙堵塞了吸附剂的孔道,致使MMA吸附剂失活。     

Kinetic study of H-terminated silicon nanowires oxidation in very first stages

Oxidation of silicon nanowires (Si NWs) is an undesirable phenomenon that has a detrimental effect on their electronic properties. To prevent oxidation of Si NWs, a deeper understanding of the oxidation reaction kinetics is necessary. In the current work, we study the oxidation kinetics of hydrogen-terminated Si NWs (H-Si NWs) as the starting surfaces for molecular functionalization of Si surfaces. H-Si NWs of 85-nm average diameter were annealed at various temperatures from 50°C to 400°C, in short-time spans ranging from 5 to 60 min. At high temperatures (T ≥ 200°C), oxidation was found to be dominated by the oxide growth site formation (made up of silicon suboxides) and subsequent silicon oxide self-limitation. Si-Si backbond oxidation and Si-H surface bond propagation dominated the process at lower temperatures (T < 200°C).     

Kinetic study of graphite oxidation along two lattice directions

A systematic kinetic study of the oxidative etching of graphite basal planes along the a and c directions at temperatures up to 950 °C was undertaken. It was found that at temperatures above 875 °C, oxidation of graphite surfaces is initiated from basal plane carbon atoms as well as from point defects. This oxidation process gives rise to monolayer-depth pits with round shapes, as observed by scanning tunneling microscopy. The distribution of the diameters of the pits formed at high temperatures was broad. At lower temperatures, however, where pit formation is initiated solely at point defects, a narrow pit diameter distribution was observed. By analyzing the etching rates as a function of oxidation temperature, kinetic parameters such as etch rates and activation energies for the oxidation reactions along the a and c crystallographic directions of the graphite lattice were obtained.     

Kinetic study of the electrochemical mineralization of phenols in thin-layer condition

Electrochemical oxidation of p-nitrophenol (PNP) and p-methoxyphenol (PMP) at Bi-doped PbO₂ electrodes under thin-layer condition was studied by Fourier transform infrared spectroelectrochemistry (FTIR) as a function of electrode potential and the initial concentration of organic compound. The results show that complete mineralization of the organic species was achieved at potentials more positive than 1.4 V vs. SCE and the mineralization process was not determined by the concentration of phenols. From the electric charge transferred during thin-layer controlled potential electrolysis, we found that PNP and PMP electrooxidation follow Langmuir-Hinshelwood kinetics.     

Kinetic study of three-way catalyst of automotive exhaust gas: Modeling and application

The kinetics of carbon monoxide and propylene oxidation as well as nitrous oxide reduction on a Pd–Rh catalyst were determined with synthetic gas mixtures between 500 and 700 K. The overall reaction rate expression was constructed from elementary reaction steps and the kinetic parameters were determined from the experiments. A transient mathematical model has been developed to simulate automobile exhaust gas conversion based on the kinetics model equations. A comparison of simulated results with engine experiments proved to be successful. Results indicate that the reaction mechanisms proposed in this study are capable to describe the behavior of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary kinetic steps.     

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.     

A study of desulfurization ability and activation energy for CuO-AgO sorbents

In this study, we investigated desulfurization abilities and activation energy using TGA for CuO-AgO sorbents calcined at 700 °C. CuO was used as a main active material and AgO was used as an additive material and 25 wt% SiO₂ was used as a support material. The desulfurization reaction temperatures were 450 °C, 500 °C, and 550 °C and the regeneration reaction temperature was 700 °C. From the TGA experiments, the best sulfur loading of CAS1 sorbent containing 1 wt% AgO was about 14.95 g sulfur/100 g sorbent at 550 °C. The activation energy was calculated by the Chatterjee-Conrad method based on the TGA experiment. Desulfurization ability and activation energy of sorbent were decreased as the content of AgO increased.     

STAGED CASCADED FLUIDIZED BED COMBUSTOR (SCFBC) EVALUATION

The SCFBC is basically a countercurrent contacting unit of downward moving solids with the hot combustion products moving up the stages. Each contacting stage is a shallow fluidized bed providing intimate contact between gas and solids. The design of SCFBC's requires rigorous determination of temperatures, stream flow rates, and heat transfer rates at each stage. The determination is made by solving material balance, energy balance, and appropriate rate equations for each stage.

In this work a technical and economic evaluation of the SCFBC is presented. Analysis of an SCFBC producing 75,000 Ib/hr steam at 200psig is presented with respect to its thermal efficiency, carbon combustion efficiency, sulfur capture efficiency, and sorbent utilization. Effects of the Ca/S ratio on the percent sulfur retention, and carbon burnout efficiency, and effects of stack gas temperature on plant thermal efficiency, total capital costs, and unit cost of steam, and effects of boiler size on the total capital cost and on the unit cost of steam are presented. Effects of variations in cost of operating parameters, e.g., water, sorbent, fuel, etc., on the steam cost are discussed.     

Preparation of diatomite/Ca(OH)2 sorbents and modelling their sulphation reaction

Mixtures of Ca(OH)₂ and diatomite were hydrated at different conditions to produce reactive SO₂ sorbents. Two different hydration techniques were used; namely, atmospheric and pressure hydration. The effect of the hydration temperature, time and diatomite/Ca(OH)₂ weight ratio on the physical properties of the activated sorbents were investigated. In atmospheric hydration, it was found that increasing the temperature and hydration time caused an increase in the total surface area of the sorbents. However, surface area values of the sorbents prepared from mixtures which have different diatomite/Ca(OH)₂ weight ratio were generally not changed significantly. In pressure hydration, the surface area of the activated sorbents was positively affected from the hydration temperature and pressure. Finally, Ca(OH)₂ and two diatomite/Ca(OH)₂ sorbents were sulphated at constant temperature using a synthetic gaseous mixture consisting of 5% O₂, 10% CO₂, and the balance of nitrogen with a 55% relative humidity. The sulphation reaction of these sorbents were investigated and modelled. The unreacted shrinking core model was chosen to describe this non-catalytic solid/gas (hydrated sorbent/SO₂) reaction mechanism. The experimental results were found to be correlated successfully by this model.     

Oxidation kinetics of Fe(II)NTA to Fe(III)NTA by oxygen

The oxidation of aqueous solutions of Fe(II)NTA was investigated in a bubble column reactor. The rate of the oxidation reaction was found to be proportional to the partial pressure of oxygen and to have an order of 0.5 with respect to the Fe(II)NTA concentration. The overall transfer of oxygen appears to be predominantly reaction controlled for Fe(II)NTA concentrations in the order of 1 mmol l⁻¹, whereas for high concentrations (100 mmol l⁻¹) mass transfer becomes the controlling factor. Overall mass transfer coefficients and reaction rate constants were determined in the range between 25 and 60°C and for a wide range of Fe(II)NTA concentrations (5-250 mmol l⁻¹).     

Hydration study of limestone blended cement in the presence of hazardous wastes containing Cr(VI)

Considering the increasing use of limestone cement manufacture, the present paper tends to characterize limestone behavior in the presence of Cr(VI). The research reported herein provides information regarding the effect of Cr(VI) from industrial wastes in the limestone cement hydration.The cementitious materials were ordinary Portland cement, as reference, and limestone blended cement.The hydration and physicomechanical properties of cementitious materials and the influence of chromium at an early age were studied with X-ray diffraction (XRD), infrared spectroscopy (FTIR), conductimetric and mechanical tests. Portland cement pastes with the addition of Cr(VI) were examined and leaching behavior with respect to water and acid solution were investigated.This study indicates that Cr(VI) modifies the rate and the components obtained during the cement hydration.     

Kinetic study of calcite particle (powder) thermal decomposition: Part I

In the first step of the firing cycle used to manufacture white-firing wall tile bodies, the calcium carbonate that is added, as a source of CaO, to the clay raw materials mixture used to form these bodies needs to decompose. In order to study the kinetics of this process, a representative rate equation of the chemical reaction step of calcium carbonate thermal decomposition is needed.There is a disparity of opinions regarding the reaction order of this chemical reaction step. For this reason, it was decided to obtain a rate equation for this chemical decomposition step, using the same type and size calcite particles that are usually added to these clay mixtures in industrial practice.The present paper proposes a rate equation for this chemical reaction step. With this rate equation and the application of the “Uniform Conversion Model”, an expression has been derived that relates the fractional calcium carbonate conversion, in the studied calcite particles, to residence time and operating temperature. The equation satisfactorily fits the experimental results obtained under isothermal conditions in the studied temperature range for particles smaller than 100 μm.The calcite particles studied are highly porous aggregates (smaller than 1.1 mm) of very small single crystals (smaller than 20 μm). The experiments were conducted in the 850–950 °C temperature range.     

Kinetic study of pyrrole polymerization with iron (III) chloride in water

The chemical synthesis of polypyrrole in water, with FeCl₃ as an oxidant of pyrrole, was investigated. During these experiments in ambient air, a large evaporation of pyrrole was observed. Moreover, a kinetic study of pyrrole disappearance by high-performance liquid chromatography (HPLC), with various FeCl₃ concentrations, proved that the polymerization is first order with respect to the pyrrole and second order with respect to the FeCl₃. These kinetics can be explained by the formation of an intermediary FeCl₃–pyrrole complex as the first step of the polymerization. The second step, which is rate-determining, is the pyrrole oxidation by FeCl₃, © 1994 John Wiley & Sons, Inc.     

Sulfidation of an alumina supported CoMo hydrotreating catalyst: variation of the S/(Co+Mo) ratio with the sulfiding temperature

An alumina supported CoMo catalyst was activated in the temperature range 295–773 K with H₂/H₂S. At several temperatures, elemental analysis, EDX, XPS and TPO techniques were used to measure the degree of sulfidation, expressed as the S/(Co+Mo) atomic ratio. A particular attention was paid to EDX analysis. A remarkable agreement was obtained between elemental analysis, EDX and XPS. No quantitative information but qualitative ones could be obtained from TPO. An attempt was made to relate catalyst sulfidation with the catalytic activity in thiophene and tetralin model reactions of hydrotreatment.     

Kinetic and reaction mechanisms of calcium bisulfite catalytic oxidation

In this paper, the oxidation of calcium bisulfite in the presence of ferrous ions as a catalyst is studied in a laboratory scale apparatus at a fixed oxygen partial pressure (21.3 kPa) and at a temperature of 45 °C. The analysis of the experimental results, carried out by using the theory of mass transfer with chemical reaction, indicates that the slow reaction regime has been explored, and the transition from the kinetic to the diffusional subregime identified. A kinetic analysis allows to develop a reaction rate equation, assuming that the bisulfite oxidation in the presence of ferrous ions follows a parallel reaction mechanism; the resulting overall reaction rate has been found to be of first order with respect to ferrous ion concentration and of three halves with respect to bisulfite concentration. Moreover, the catalytic effect of ferrous ions has been compared with that of manganese ions previously studied, showing that ferrous is a catalyst more active than manganese.     

Thermal degradation of poly(ethylene oxide-propylene oxide-ethylene oxide) triblock copolymer: comparative study by SEC/NMR, SEC/MALDI-TOF-MS and SPME/GC-MS

By comparing size exclusion chromatography/matrix assisted laser desorption ionisation (SEC/MALDI) and SEC/NMR spectra from virgin poly(ethylene oxide-propylene oxide-ethylene oxide) triblock copolymer, we were able to understand the bimodal distribution observed in poloxamer 407. Propylene oxide, isomerised to allyl alcohol during polymerisation, eventually forms a Poly(ethylene oxide-propylene oxide) diblock copolymer when EO is added to the feed. The oxidative thermal degradation of poloxamer 407 at 80°C in air was studied. We found by MALDI that degradation starts after 21 days in the PPO block of the copolymer. This result was confirmed by solid phase microextraction/gas chromatography-mass spectrometry (SPME/GC-MS): The first volatile degradation product to appear is 1,2-propanediol,1-acetate,2-formate. The structure of this molecule suggests that a six-ring intramolecular decomposition reaction of the PPO chain occurs at the very beginning of the polymer breakdown. Thus, the secondary hydroperoxide formed on the PPO chain plays a major role on the thermoxidation of poloxamer materials.     

Sonochemical treatment of FBC ash: A study of the reaction mechanism and performance of synthetic sorbents

This work explores the reaction mechanisms for the sonochemical-enhanced carbonation of fluidized bed combustion (FBC) ash. Ashes from Nova Scotia Power’s 165 MWe circulating fluidized bed combustor (CFBC) as well as synthetic ash prepared directly from limestone have been used. Acetone tests were carried out using pure acetone as well as acetone/water mixtures (4:1 ratio). Tests with acetone demonstrated that, without previous hydration of the ash, significant carbonation is not achieved. Experiments were also conducted to determine the role of hydration temperature on the carbonation of FBC ash. X-ray diffraction (XRD) analysis of synthetic ash after sonication has also been carried out. Analysis of the data obtained revealed that the process is well described by a series reaction mechanism. Initial hydration temperature does not appear to significantly impact the carbonation of FBC ash. Synthetic ash does not behave like FBC ash due apparently to its extreme susceptibility to the size reduction capability of ultrasonics.