Sulfation Of Limestone Research Articles

Regeneration of Sulfated Dolomite and Limestone by Reductive Decomposition

Regeneration properties of sulfated dolomite and limestone samples were investi gated. Natural stones were first fully calcined at 1223 K in a gaseous atmosphere consisting of CO215 vol.% and dry air 85 vol.%; second, sulfation of the calcines was achieved by reacting them with a gaseous mixture consisting of CO215 vol.%, SO20.35 vol.%, and a balance of dry air at 1223 K; last, sulfated calcines were regenerated at 1373 K by a reductive decomposition process. During regeneration a 3:1 volumetric ratio of CO2/CO was maintained in the reducing gaseous atmosphere to minimize CaS formation. It has been found that for the five sulfation generation cycles the reactivity of the limestone and dolomite samples remained at acceptable levels. Since the repeated sulfation-regeneration steps caused an important change on the crystal lattice, as compared to the fresh stones, sorbent reactivity was also changed.

Sedimentology of the lowest Windsor carbonate rocks; base metal hosts in the Maritimes Basin of Eastern Canada

The Mississippian (Visean) Macumber and Gays River Formations of the Windsor Group in Nova Scotia are host to Pb-Zn-Ba (Cu-Ag) deposits. Transport and deposition of base metals were possible because of an efficient porosity-permeability system developed in both units. Depositional facies is a key parameter controlling the potential openness of carbonate units.The Macumber facies were deposited after the collapse of the Acadian orogen and ensuing tectonically driven sea-level rise. The Macumber Formation is divided into two regional lithosomes: a lower thin unit (<2 m) of fine- and coarse-grained limestones deposited below the fairweather wave base positioned on an outer shelf with local chemosynthetic mounds, and an upper thicker unit (avg 10 m) of planar-bedded micritic limestones, most likely representing upper slope microbial mats interbedded with deep marine sulfates. The permeability system was created by various brecciation events affecting the upper part of the formation.The Gays River Formation, deposited upslope of the Macumber facies, is dominated by various organic mounds with intervening facies. The boundstone facies are indicative of below fairweather wave base deposition, roughly at the limit of the photic zone. Primary growth porosity of the mounds provided the plumbing system later used by mineralizing fluids. These mounds also record the effects of a sea-level rise culminating in regional deposition of deep marine sulfates.Outside the eastern margin of the Laurentia craton, a similar Visean-aged mixed limestone sulfate succession is known from the Franco-Belgian basin in Baltica. Distribution of shelf and slope facies within the Visean mid-European ocean separating Laurentia, Baltica, and Gondwana suggests that restricted oceanic circulation in a rapidly closing sea was instrumental in the local occurrence of a salinity-stratified water mass and, hence, responsible for the presence of below wave base marine limestone and sulfate facies on the Laurentia and Baltica continental margins.

5714132 Production of hydrogen and carbon monoxide from oxyfuel furnace off-gas

Coal combustion with O2/CO2 is promising because of its easy CO2 recovery, extremely low NOx emission and high sulfation efficiency. The mechanism of direct sulfation of limestone under high CO2 concentration, different from that of CaO–SO2 sulfation, is one of the factors to account for its high sulfation efficiency. However, many unknowns about the mechanism of direct sulfation exist. The mechanisms and kinetics of direct sulfation were examined. Both experiment and modeling suggested that sintering is much mitigated during direct sulfation of limestone. The rate of direct sulfation does not decrease so much with sulfation degree as CaO–SO2 sulfation. Direct sulfation is favorable for high sulfation degree. Furthermore, the diffusivity in the product layer demonstrates high temperature dependence and hardly changes with sulfation degree. This result is associated with the fact that the calcium sulfate layer produced is porous owing to CO2 formation.

Catalytic and gas–solid reactions involving HCN over limestone

AbstractIn coal‐fired combustion systems solid calcium species may be present as ash components or limestone added to the combustion chamber. In this study heterogeneous reactions involving HCN over seven different limestones were investigated in a laboratory fixed‐bed quartz reactor at 873–1,173 K. Calcined limestone is an effective catalyst for oxidation of HCN. Under conditions with complete conversion of HCN at O2 concentrations above about 5,000 ppmv the selectivity for formation of NO and N2O is 50–70% and below 5%, respectively. Nitric oxide can be reduced by HCN to N2 in the absence of O2 and to N2 and N2O in the presence of O2. At low O2 concentrations or low temperatures, HCN may react with CaO, forming calcium cyanamide, CaCN2. The selectivities for formation of NO and N2O from oxidation of CaCN2 is 20–25% for both species. The catalytic activity of limestone for oxidation of HCN decreases with increasing degree of sulfation. Simultaneously the selectivity for formation of NO decreases while tht for N2O increases. The catalytic activity of sulfated limestone in‐creases with decreasing SO2 concentration, indicating a competition between SO2 and HCN for sites on the surface. The results indicate that heterogeneous oxidation of HCN is important in calciners and fluidized‐bed combustors with limestone addition or when burning coals with an ash with a high catalytic activity.

Effective diffusivity changes during calcination, carbonation, recalcination, and sulfation of limestones

This study investigates the changes occurring in the intraparticle mass transport resistance of gases in limestone particles used to control SO 2 emissions from coal-fired power plants. The evolution of intraparticle mass transfer resistance in calcined limestones undergoing carbonation and sulfation and carbonated calcines undergoing recalcination is investigated experimentally using the pulse chromatographic technique. The large quantities of calcined, carbonated, recalcined, or sulfated samples needed to pack the chromatographic column are produced in a fluidized-bed reactor and two types of limestones containing more than 95% CaCO 3 are employed. Effective diffusivities are estimated by matching the experimental and theoretical chromatographic response curves in the time domain. The analysis of the experimental data show that the intraparticle diffusion resistance in calcined limestone particles (both reacted and unreacted) depends not only on the pore size distribution but on the connectivity of the pore structure as well. This conclusion is in agreement with a result reached in past studies from the analysis of sulfation data for the same solids.

A variable diffusivity shrinking-core model and its application to the direct sulfation of limestone

A variable diffusivity shrinking-core model that allows for temporal and spatial variation of the effective diffusivity in the solid product layer was developed in this study. The model was used to analyze a set of experimental data for the direct sulfation of three limestones (that is, their reaction with SO2 under noncalcining conditions), and a parameter estimation procedure was formulated for extracting the effective diffusivity as a distributed parameter from the experimental conversion vs. time curves. Very strong effects of temperature, concentration, and depth in the product layer on the effective diffusivity were revealed by the analysis. On the basis of the obtained results, it was concluded that despite being essentially nonporous, uncalcined limestones can react with SO, under noncalcining conditions at rates comparable to those of calcined samples because they present much lower resistance for diffusion of SO2 through the product layer than the latter. On a mis au point dans cette ètude un modèle de retrècissement du noyau à diffusivitè variable qui permet une variation temporelle et spatiale de la diffusivit.5 effective dans la couche des solides. On a utilisè ce modèle pour analyser une sèrie de donnèes exprimentales pour la sulfatation directe de trois calcaires (c'est-à-dire leur rèaction avec le SO, dans des conditions de non-calcination), et une procèdure d'estimation des paramètres a ètè formulèe pour extraire la diffusivitè effective comme paramètre distribuè de la conversion expèrimentale en fonction des courbes de temps. L'analyse montre que la tempèrature, la concentration et la profondeur de la couche de produits influent fortement sur la diffusivitè effective. À partir des résultats obtenus, on a conch que bien qu'ils soient essentiellement non poreux, les calcaires non calcinés peuvent réagir au SO2 dans des conditions de non-calcination à des vitesses comparables & celles des échantillons calcinés, car ils présentent une résistance beaucoup plus faible à la diffusion du SO, dans la couche de produits que les échantillons calcinés.

Influence of SO 2 on the [formula omitted] chemistry in fluidized bed combustion : 2. Interpretation of full-scale observations based on laboratory experiments

In circulating fluidized bed combustion of coal, significant interactions between desulfurization by limestone and emissions of nitrogen oxides (NO, N 2O) are generally observed. In order to facilitate interpretation of experimental results obtained in a 12 MW circulating fluidized bed boiler, a series of laboratory tests have been carried out. The tests include homogeneous CO and HCN oxidation in the presence of different concentrations of SO 2, CO oxidation catalysed by calcined and partly sulfated limestone particles, and formation and reduction of NO and N 2O over bed material containing char. The presence of SO 2 in the gas is shown to decrease the rate of homogeneous CO and HCN oxidation and thereby change the product distribution of the nitrogen-containing species. Unlike sulfated limestone, calcined limestone (CaO) is shown to be a good catalyst for oxidation of CO. A lower mean CO concentration is therefore expected in fluidized bed combustors during injection of limestone. This indirectly influences the nitrogen chemistry. Finally, the influence of NO and O 2 on the formation of N 2O from char was studied.

An Investigation of Fluidized Bed Combustion By-Products from Four Iowa Power Plants

ABSTRACTFluidized bed combustion (FBC) residues from four Iowa power plants were characterized to provide background with respect to chemical, physical, mineralogical and microstructural parameters that might affect their utilization potential. Methodologies used include: bulk chemical analysis; grain size analysis; specific gravity determination; mineralogical analysis by x-ray diffraction; scanning electron microscopy; and thermal analysis.Chemical analysis of the major components showed the separation of the constituents during combustion. The bottom ash is derived from the calcined and partly sulfated limestone, thus the concentration of calcium and sulfur compounds is greater. The fly ash however is derived mostly from the coal and therefore exhibits higher concentrations of coal ash components such as silica (SiO2), alumina (Al2O3), and ferric oxide (Fe2O3). The alkalis appear to be more concentrated in the fly ash.The measured particle size distributions of the bulk samples varied greatly, from extremely coarse (mean particle size ˜2000 μm) to fine (mean particle size ˜25 μm). The size distributions also varied in breadth. Specific gravities values recorded ranged from about 2.65 to about 3.05.X-ray diffraction analyses showed that most of the FBC by-products contain, as easily detectable crystalline components, only quartz, anhydrite and lime, and as is to be expected due to the low combustion temperatures, do not show broad peaks characteristic of glass in conventional fly ashes. Scanning electron microscopy revealed the FBC by-products to be extremely intricate mixtures of particles of complex and variable composition and internal structure.FBC by-products are shown to be extremely complex mixtures of particles of variable composition and internal structure. However, despite their local variation, useful results can be obtained by overall chemical and physico-chemical analyses of their composition. The exploitable properties of FBC residue include the somewhat cementitious nature and the granularity. However, their heterogeneous nature, lack of pozzolanic properties, and elevated sulfate content may limit their uses, especially in construction.

Decomposition of ammonia over calcined and sulfated limestone at 725-950.degree.C

Decomposition of ammonia over calcined and sulfated limestone at 725-950.degree.C

Diffusional study of the effect of sodium chloride on limestone sulfation in fluidized-bed combustion. part 2

The effective diffusivity of argon through calcined Greer limestone is given during sulfation, salt addition, and additional sulfation. The corresponding extents of sulfation are also presented. The effective diffusivity of SO 2 decreases as sulfation takes place. Introduction of sodium chloride vapor to the system rearranges the sulfated product layer structure, thus increasing the effective diffusivity of SO 2. This increases the SO 2 capture capacity of Greer limestone by more than 200% at sulfation temperatures above 840°C.

Spectral Properties of Sulfated Limestone and Marble: Implications for In Situ Assessment of Atmospheric Pollution Damage to Carbonate Rock Building Materials

Infrared spectral reflectance of Salem Limestone and Shelborne Marble exposed to simplified artificial weathering by dilute sulfuric acid solutions is studied as a function of the concentration of surficial gypsum formed. Data presented indicate that calibration curves relating gypsum concentration to reflectance band strengths at 1.93, 6.5, and 8.5 μm can be constructed for both limestone and marble and that nondestructive quantitative compositional analysis is possible using either near-infrared (NIR) or mid-infrared (MIR) measurements. The preferred spectral region depends on the nature of the carbonate material under attack and the amount of gypsum formed. Detection limits for gypsum established for both the limestone and the marble indicate these spectroscopic techniques could provide a viable tool for early monitoring of the effects of certain atmospheric pollutants on carbonate rock building materials.

Small pore closure and the deactivation of the limestone sulfation reaction

AbstractThe sulfation model of Simons and Rawlins (1980) is extended to include the effect of product deposits. The model includes: 1. the plugging of the smallest pores and the subsequent loss in the internal surface area, 2. the diffusion of the SO2 through the product deposits, and 3. the loss of intraparticle diffusion due to the complete plugging of the largest pores. It is shown that the plugging of the smallest pores is generally rate‐controlling.

An analytical model for freeboard and in-bed limestone sulfation in fluidized-bed coal combustors

A new model, which combines in-bed and freeboard sulfation, significantly improves the ability to predict sulfur capture by limestone sorbents in fluidized-bed coal combustors. In this model, the in-bed hydrodynamics are described in terms of a bubble phase and an emulsion phase while the freeboard region has only a diluted emulsion phase. The solids, which are in the emulsion phases, are considered to be completely back-mixed; the gaseous bubble phase travels in plug-flow but exchanges with the emulsion phase. The sulfation reaction occurs principally in the emulsion phase and the reaction rate is a direct function of the sulfur dioxide concentration, the extent of the calcium oxide conversion (as measured by a thermogravimetric analyzer), and the amount of limestone present in the bed and in the freeboard. The amount present, or holdup, in the free-board is calculated from empirical correlations for elutriation and from particle-time trajectories as predicted from equations of motion. The model indicates that a significant amount of sulfur capture can occur in the freeboard region, especially with high superficial gas velocities and small particle sizes, both of which lead to increased freeboard holdup. For an overall sulfur retention of 90%, approximately 9% of the sulfur is predicted to be captured within the freeboard above a combustor bed 1.8 m by 1.8 m by 1.2 m high of 1200 μm diameter limestone particles that are fluidized at 2.4 m/s. The model also predicts that the sulfur captured by the entrained particles is negligible. These predictions compare favorably with the actual sulfur retentions experienced in similar sized pilot-scale combustors.

Distributed pore‐size model for sulfation of limestone

AbstractA model is proposed to study the reaction between a porous solid and a reactant gas resulting in solid and gaseous products. The model describes the reacting solid as a sphere made up of a distribution of randomly oriented open pores. The evolution of the pore‐size distribution is followed by use of a population balance, using a combination of the independent variables of time and location first presented by Dudukovic (1976). The macroscopic properties of the solid are obtained by integrating over the pore size distribution. The results of the population balance are used with a mass balance on the reacting gas to obtain rate and conversion data.The results of the model are compared to experimental data obtained for the sulfation of limestone (Ulerich et al., 1977). Both rate vs. conversion as well as conversion vs. time plots are presented, which serve as a more strenuous test of the model than either type of plot alone.

Comparison of structural models for gas-solid reactions in porous solids undergoing structural changes

In many gas-solid reactions the porous solid undergoes structural changes due to differences in molar volume between solid reactants and solid products. In this paper some new models are proposed which take into account the effects of structural changes on the reaction rate. The models differ in their geometric descriptions of the initial structure of the porous solid but they all take into consideration the decreasing diffusion rate in the growing ash layer and the decreasing gas-solid interfacial area at swelling of the solid structure during reaction. The different models are compared with each other and with previously suggested models. A model that regards the initial solid structure as an aggregate of truncated spheres in contact with each other as in the initial stage of sintering seems to be the most realistic and flexible model of those investigated. This model has been used to predict conversion-time curves for the sulfation of limestone. The predicted curves are in good agreement with experimental results reported in literature. It is also shown that a semi-empirical approach can give essentially the same results with less numerical effort.

Hydration Enhanced Sulfation of Limestone and Dolomite in the Fluidized-Bed Combustion of Coal

Hydration Enhanced Sulfation of Limestone and Dolomite in the Fluidized-Bed Combustion of Coal

A thermogravimetric study of the sulfation of limestone and dolomite — prediction of pressurized and atmospheric fluidized-bed desulfurization

A pressurized thermogravimetric (TG) analysis system was used to study the isothermal reaction of sulfur dioxide with limestone and dolomite at 1 and 10 atm pressure, and temperatures from 750 to 1050°C. Sorbents of 44–4000 μm particle size were sulfated in gases simulating a 10–300% excess air level in a coal combustion atmosphere. The TG rate data were used to predict the desulfurization performance of limestone and dolomite sorbents in atmospheric and pressurized fluidized-bed combustion systems. The projections are compared to available pilot plant data (i.e., the pressurized Exxon miniplant), and their limitations are discussed.

The effects of CaCl 2 on limestone sulfation in fluidized-bed coal combustion

In an investigation into the effects of CaCl 2 on the sulfation of limestones in a laboratory furnace simulating fluidized-bed coal combustion conditions, small additions of CaCl 2 (<1 mole%) to the limestone prior to calcination were found to increase the extent of sulfation by changing the limestone pore structure. These changes were effected by trace amounts of liquid in the system in a synthetic SO 2/O 2 flue gas at 850°C. At much higher concentrations of CaCl 2, large amounts of a liquid phase are produced, containing a substantial quantity of dissolved CaO and leading to greatly enhanced sulfation when exposed to SO 2/O 2. The use of CaCl 2 additive in fluidized-bed combustion would reduce the quantity of limestones required to meet air pollution standards for SO 2 and also reduce the quantity of solid waste generated.

Developing regenerable SO 2 sorbents for fluidized bed coal combustion using thermogravimetric analysis

Exxon Research and Engineering Company, funded by the National Science Foundation RANN Program, is using thermogravimetric analysis as the major research tool in a program to develop for fluidized bed coal combustion (FBCC) regenerable SO 2 sorbents that are superior to limestone or dolomite. In FBCC, powdered coal is burned in a fluidized bed of limestone and the SO 2 produced during combustion reacts with the CaO to form CaSO 4. However, only a fraction of the limestone or dolomite reacts (low utilization) and the sulfated limestone is difficult to regenerate because high temperatures (≈ 1100°C) are required and deactivation occurs after repeated cycles of SO 2 sorption and regeneration. A DuPont TGA was modified to handle reactive gases. Over 100 tests were made in order to compare the relative abilities of different materials to sorb SO 2 and be regenerated. The sample, either in the form of a fine powder or small pellet, was maintained at constant temperature as it reacted in a gas stream of nearly constant composition. Reaction conditions for sulfation were 900°C, one atmosphere pressure and a gas composition of 0.1–0.25%SO 2, 5% O 2, balance N 2. Reaction conditions for regeneration were usually 1100°C, one atmosphere, and 5% CO in nitrogen. Tests were made in which the sample was cycled as many as 50 times between sulfation and regeneration conditions. Conversion of the sample (to sulfate) was determined as a function of time and number of cycles. Candidate sorbents included simple metal oxides, mixed metal oxides (mostly aluminates and titanates), carbonates, composites containing CaO and promoters, and combinations of CaO and cement. All sorbents regenerated much faster than they sulfated. Deactivation was manifested by a decline in the sulfation rate and utilization of the sorbent. Of all materials tested, barium and calcium titanates were unique in that no measurable loss of activity occurred. All other sorbents deactivated to some extent, although many were superior to limestone. Calcium aluminate cement was also promising because it had good activity maintenance and could be made into hard pellets that would resist attrition in a fluidized bed.

A thermogravimetric study of the sulfation of limestone and dolomite—the effect of calcination conditions

A pressurized thermogravimetric analysis system, constructed to operate with corrosive gases and connected to a programmable calculator used as a data acquisition system, has been used to study the isothermal reaction of sulfur dioxide with limestones and dolomites at pressures from 1.01 x 10 5 Pa to 1.01 x 10 6 Pa (1 to 10 atm), and temperatures from 750 to 1050°C. The sulfation reactions have been studied (a) under conditions where calcium carbonate is the stable form of the reacting solid, and (b) where calcium oxide is the stable form of the sorbent. The data show that both sample size and gas flow-rate strongly influence the observed kinetics. However, the dominant influence on the sulfation reaction of calcined sorbents is the carbon dioxide pressure in which the stone is originally calcined. The results are discussed in the light of previous studies on the sulfation of lime, and the application of this reaction to pollution abatement by removal of sulfur dioxide from hot gases.