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.     

Simultaneous absorption of mercury and chlorine in sulfite solutions

Simultaneous absorption of mercury (Hg) and chlorine (Cl₂) into aqueous sulfite/bisulfite (0 to S(IV)) at pH 4.7 and 5.7 was measured in a wetted wall column. Experiments were performed at ambient temperature and pressure using 5- Cl₂ and 46 ppb Hg. Absorption was modeled using the theory of mass transfer with chemical reaction. At the gas/liquid interface, chlorine oxidizes the elemental Hg to a more soluble form. The rate constant for the reaction of mercury and chlorine was determined to be . Mercuric chloride and sodium hypochlorite also enhanced Hg absorption. The addition of sodium chloride did not affect the rate of Hg/Cl₂ absorption with S(IV). When no S(IV) was present, the chloride significantly enhanced Hg absorption. Possible reaction pathways are discussed. These results are relevant in the simultaneous removal of chlorine, sulfur dioxide, and elemental mercury from flue gas. A model was developed to predict the expected Hg removal in a limestone slurry scrubber. Mercury removal decreases as the S(IV) concentration increases. The process feasibility will depend on the SO₂/S(IV) concentration of the scrubber, the desired Hg removal, and the amount of Cl₂ which can be tolerated.     

Desulfurization in the gas-continuous impinging stream gas-liquid reactor

An investigation is made to evaluate the flue gas desulfurization (FGD) by absorption in a gas-continuous impinging stream gas-liquid reactor recently developed for systems involving fast reaction(s) in liquid. The mixture of air and SO₂ was used as the pseudo-flue gas and Ca(OH)₂-water suspension as the absorbent. By employing horizontal two-impinging streams, the reactor is simple in structure with few internal parts, while exhibits satisfied overall performance for FGD. Under moderate conditions, the content of SO₂ in the cleaned gas can achieve a level much lower than that permitted, while the pressure drop across the device is about 400 Pa only. The influences of some operating and structural parameters, such as V_L/V_G, Ca/S mole ratio, SO₂ concentration in flue gas, impinging distance S, and nozzle location, etc., are examined. The gas-film mass transfer coefficient, k_G, is determined based on Sauter mean diameter of spray droplets. The results show that k_G is essentially independent of concentration of SO₂ in flue gas, implying the process can be considered to be controlled by diffusion through gas film. The relationship between k_G and impinging velocity, u₀, is fitted to be with the standard deviation of , suggesting u₀ is a strong effecting variable on mass transfer, and, consequentially, important operating variable. In the range of u₀ from 5.53 to , the values determined for the volumetric mass transfer coefficient, k_Ga, are 0.577-, and those for k_G are ranged from 0.00641 to .     

Removal of sulfur dioxide using absorbent synthesized from coal fly ash: Role of oxygen and nitrogen oxide in the desulfurization reaction

Removal of SO₂ from flue gas by the absorbent synthesized from coal fly ash and calcium oxide was studied under different reaction conditions to elucidate the effect of the coexistence of NO and O₂ in the flue gas. The presence of O₂ and NO in the flue gas was found to be necessary to produce sulfate salts instead of sulfite salts as the final product of the desulfurization reaction. The roles of O₂ and NO were postulated as an oxidizing agent to oxidize SO₂ to SO₃, which then reacts with the absorbent. NO itself is not an oxidizing agent, but with the presence of O₂, it can be oxidized to NO₂ which acts as an oxidizing agent. It was also found that NO₂ (from NO) is a better oxidizing agent compared to O₂ in oxidizing SO₂ to SO₃.     

Kinetics of the reactive absorption of hydrogen sulfide into aqueous ferric sulfate solutions

The reactive absorption of H₂S into aqueous Fe₂(SO₄)₃ solutions, was studied in a stirred cell reactor operated batchwise with and without a flat interface. The temperature was varied from 25°C to 65°C and the concentrations of aqueous Fe₂(SO₄)₃ solutions ranged from 0.025 to . The corresponding initial pH values ranged from 2 to 0.8, respectively. Additional measurements were conducted at other pH values by addition of NaOH. The H₂S partial pressure was varied between 0 and . The rate of H₂S absorption was measured by recording the pressure drop as a function of time during batch absorption experiments. In this system the absorbed H₂S reacts with ferric iron and is oxidized to elemental sulfur. The kinetic results are in agreement with enhanced absorption due to a fast chemical reaction according to the film theory. The reaction of ferric sulfate and H₂S appears to proceed irreversibly and is first order in both the total concentrations of ferric iron and H₂S. The activation energy for the reaction was calculated to be .     

Removal of Hg from flue gases in wet FGD by catalytic oxidation with air - An experimental study

About 46% of global mercury emissions are due to fossil fuel combustion for electrical and thermal energy production. Since more stringent emission standards are expected, important research efforts are being focused on the development of mercury removal technologies, mainly directed to two alternative approaches: (i) the enhancement of homogeneous oxidation in the flue gases of Hg⁰ to water soluble Hg²⁺ by the addition of chlorides or bromides to the boiler or; (ii) the adsorption of Hg²⁺ and Hg⁰ on impregnated activated carbon (AC). The latter may require the treatment of the entire gas volume of the thermal power plant and constantly consumes relatively large quantities of AC.A third option gaining more attention lately is based on the oxidation and retention of dissolved Hg⁰ in the wet flue gas desulphurisation (FGD) system. A series of chemical oxidants, such as halogens, hydrogen peroxide, sulphur and oxygen, are theoretically able to oxidize Hg⁰ in the wet FGD system. Most chemical oxidants when applied in the FGD, however, are non-selective and are largely consumed by SO₂ absorbed from the flue gas. The less expensive oxidant, non-selective as well, is oxygen (as air) which is already being dispersed into FGD absorbing suspension for the conversion of into .The experimental evidence of the present work showed that Hg⁰ present in the gaseous phase can be dissolved and oxidized to a high degree (70-90%) by air together with in wet FGD solutions. Transition metals such as Fe²⁺ and Mn²⁺ act as catalysts, chloride enhances the reaction, while some oxosulphur compounds, e.g. tetrathionate, inhibit the oxidation. A combination of several catalysts at a concentration of sulphite () below 100 mg L⁻¹ and an adequate redox potential of the solution can assure reasonable mercury removal even in the presence of oxidation inhibiting compounds.The main competitive reactions that govern final Hg⁰ removal in the FGD are as follows: (1) oxidation of Hg⁰ together with SO₂ with air, enhanced by catalysts; (2) removal of catalysts by precipitation in the form of Fe(OH)₃ and eventually as MnO₂ (to overcome this problem continuous addition of catalysts to the solution is required); (3) reduction of Fe³⁺ by tetrathionate to Fe²⁺ which (4) may reduce Hg²⁺ to Hg⁰ and probably (5) the complexation of Hg²⁺ by anions present which may play an important role in the mechanism by complexing the product(s) of the Hg⁰ oxidation reaction.     

Carbon dioxide absorption with aqueous potassium carbonate promoted by piperazine

Many commercial processes for the removal of carbon dioxide from high-pressure gases use aqueous potassium carbonate systems promoted by secondary amines. This paper presents thermodynamic and kinetic data for aqueous potassium carbonate promoted by piperazine. Research has been performed at typical absorber conditions for the removal of CO₂ from flue gas.Piperazine, used as an additive in 20- potassium carbonate, was investigated in a wetted-wall column using a concentration of at 40-80°C. The addition of piperazine to a potassium carbonate system decreases the CO₂ equilibrium partial pressure by approximately 85% at intermediate CO₂ loading. The distribution of piperazine species in the solution was determined by proton NMR. Using the speciation data and relevant equilibrium constants, a model was developed to predict system speciation and equilibrium.The addition of piperazine to potassium carbonate increases the rate of CO₂ absorption by an order of magnitude at 60°C. The rate of CO₂ absorption in the promoted solution compares favorably to that of MEA. The addition of piperazine to potassium carbonate increases the heat of absorption from 3.7 to . The capacity ranges from 0.4 to for PZ/K₂CO₃ solutions, comparing favorably with other amines.     

Oxidation of FGD-CaSO3 and effect on soil chemical properties when applied to the soil surface

Use of high-sulfur coal for power generation in the United States requires the removal of sulfur dioxide (SO₂) produced during burning in order to meet clean air regulations. If SO₂ is removed from the flue gas using a wet scrubber without forced air oxidation, much of the S product created will be sulfite (). Plants take up S in the form of sulfate (). Sulfite may cause damage to plant roots, especially in acid soils. For agricultural uses, it is thought that in flue gas desulfurization (FGD) products must first oxidize to in soils before crops are planted. However, there is little information about the oxidation of in FGD product to under field conditions. An FGD-CaSO₃ was applied at rates of 0, 1.12, and 3.36 Mg ha⁻¹ to the surface of an agricultural soil (Wooster silt loam, Oxyaquic Fragiudalf). The in the surface soil (0-10 cm) was analyzed on days 3, 7, 17, 45, and 61. The distribution of and Ca in the 0-90 cm soil layer was also determined on day 61. Results indicated that in the FGD-CaSO₃ rapidly oxidized to on the field surface during the first week and much of the and Ca moved downward into the 0-50 cm soil layer during the experimental period of two months. It is safe to grow plants in soil treated with FGD-CaSO₃ if the application is made at least three days to several weeks before planting.     

Chemical forms of ash-forming elements in woody biomass fuels

Advanced fuel characterization helps to predict ash fouling and slagging. Chemical fractionation analysis, i.e. sequential leaching in H₂O, NH₄Ac(aq), and HCl(aq), was applied to the biomass of spruce, pine, birch, and aspen. All of the Cl in the samples and most of the K, Na, and P were water-soluble; most of the Mg and Mn, and some of the Ca were leached in NH₄Ac; most of the Ca was leached in HCl; and most of the Si and S remained insoluble in the biomass. Ion Chromatography found the water-soluble Cl, P, and S present as Cl⁻, , and , respectively, and equimolar concentrations of as leached Ca in the acid fraction. The biomass solids were determined for anionic groups by methylene blue sorption. The contents were lowest in the wood samples (22-118 mmol/kg_D.S.) and highest in the bark samples (130-453 mmol/kg_D.S.). The closing of the ion charge balance led to a quantitative model for the ash-forming matter: water-soluble salts (KCl, K₂HPO₄, and K₂SO₄), acid-soluble minerals (CaC₂O₄), non-soluble minerals (SiO₂), and organically associated ash-forming elements (ionically bonded Ca²⁺, Mg²⁺, Mn²⁺, and K⁺, and covalently bonded P and S).     

Oxidation kinetics of Iron(II) complexes of trans-1,2-diaminocyclohexanetetraacetate (cdta) with dissolved oxygen: Reaction mechanism, parameters of activation and kinetic salt effects

The present investigation takes concern about a spiny environmental problem afflicting the pulp mill industry exploiting the Kraft sulfate-pulp process where dilute total reduced sulfur contaminants are co-mixed with oxygen in large-volume gas effluents. A potential Redox process for removing the total reduced sulfurs consists in oxidizing them by means of iron(III) organometallic complexes while the co-mixed oxygen mediates the oxidative regeneration of iron(II) into iron(III) complexes. In this work, the oxidation kinetics of iron(II) trans-1,2,-diaminocyclohexanetetraacetate (cdta) complexes with molecular oxygen (O₂) as the source oxidant was investigated for a wide pH range (1.75<pH<12) in a 3.2 dm³ single-phase stirred cell reactor within the [281-323 K] temperature range. Simultaneous measurements of iron(II)-cdta (50-) and O₂ (0.5-) were used to clarify the reaction mechanism which has been interpreted differently in previous works. The observed kinetic data in alkaline solutions could be accounted for in terms of three forward [Fe²⁺cdta^4-+O₂ (rate-limiting, k_1,app), , 2Fe²⁺cdta^4-+H₂O₂] and one reverse [ (k_-1,app,n=0 or 1)] elementary steps. Assessment of the rate-limiting apparent rate constant led to the following results ( at and , , ). Fe³⁺OH^-cdta^4-, being the dominating iron(III) product at pH>10, was found to be less reactive than Fe³⁺cdta^4- with the superoxide intermediate , thus reducing the effect of the reverse step at higher pH. A study on the effect of electrolytes on the reaction rate led to the conclusion that salts increase the rate constant k_1,app. Finally, kinetic results in acidic conditions leading to the formation of other iron(II)-cdta complexes (i.e., Fe²⁺cdta^4-H⁺) and another superoxide intermediates are reported and discussed.     

CFD simulation of dilute phase gas-solid riser reactors: part II—simultaneous adsorption of SO2-NOx from flue gases

Simultaneous adsorption of SO₂-NO_x in a riser configuration is a novel route for flue gas cleaning. The riser operates at a low flux of small diameter Na-γ-Al₂O₃ sorbent particles. The reaction scheme is adopted from previous work (Ind. Eng. Chem. Res. 40 (2001) 119), without adjusting any of the kinetic parameters. The significant concentration gradient between the gas and solid phase mainly arises from the low solid fraction (typically 5×10⁻⁴) in the riser. Enhancing the fluctuating kinetic motion of gas and solid phase increases the SO₂ adsorption, whereas the NO adsorption is decreased marginally. The solid recirculation in the top section of the riser, induced by the abrupt T outlets, significantly decreases the NO and NO₂ removal, while the SO₂ removal remains mostly unaffected. Therefore, it is desirable to avoid recirculation for a maximum NO_x removal. A comparison of the 3D and a 1D model shows that higher SO₂ and NO removal efficiencies are predicted by the 3D model in the major part of the riser. However, these positive effects are largely neutralized by the negative effects of the outlet-induced recirculation, resulting in similar overall removal efficiencies calculated by the two models. Unlike the 1D model, the 3D simulation shows a considerable axial variation in the solid fraction and slip velocity. The 3D simulation also allows to calculate the effects of outlet geometry on the flow and reaction fields. The reactor efficiency can be improved by modifying the outlet configuration to minimize the recirculation.     

Reduction and oxidation kinetics of Mn3O4/Mg-ZrO2 oxygen carrier particles for chemical-looping combustion

The kinetics of reduction with methane and oxidation with oxygen of Mn₃O₄ supported on Mg-ZrO₂ prepared by freeze granulation has been investigated. The reactivity experiments were performed in a thermogravimetric analyzer (TGA) using different reacting gas concentrations and temperatures in the range of 1073-1223 K. The oxygen carrier particles showed high reactivity during both reduction and oxidation at all investigated temperatures. An empirical reaction model, which assumes a linear relation between time and conversion, was used to determine the kinetic parameters for reduction and oxidation, with chemical reaction being the main resistance to the reaction. The order of reaction found was 1 with respect to CH₄ and 0.65 with respect to O₂. The activation energy for the reduction reaction was 119 and for the oxidation reaction. The reactivity data and kinetic parameters were used to estimate the solid inventory in the air and fuel reactor of a CLC system. The optimum solid inventory obtained was at a value of ΔX_s=0.4. At these conditions, the recirculation rate of oxygen carrier between air and fuel reactor was per MW of fuel, which could be accomplished in an industrial reactor. The high reactivity of the Mn₃O₄/Mg-ZrO₂ with both methane and oxygen showed that this is a very promising oxygen carrier for CLC.