Alkali Sulfate Research Articles

Measures to reduce chlorine in deposits: Application in a large-scale circulating fluidised bed boiler firing biomass

Combustion of biomass with a high content of alkali (mainly potassium, K) and chlorine (Cl) can result in operational problems including deposit formation and superheater corrosion. Among the measures applied to decrease such problems are co-combustion and the use of additives. The positive effects of these measures are to a large extent either sulphation of the alkali chlorides (KCl) to less corrosive alkali sulphates or capture of alkali from KCl during release of HCl. A test campaign was carried out in a large-scale circulating fluidised boiler fired with biomass where the measures applied were sulphation by ammonium sulphate and co-combustion with peat. Their performance was evaluated by means of several advanced measurement tools including: IACM (on-line measurements of gaseous KCl); a low-pressure impactor (size distribution and chemical composition of extracted fly ash particles) and deposit measurements (chemical composition in collected deposits). The overall performance was better for ammonium sulphate, which significantly lowered KCl in the flue gas. Meanwhile no chlorine was found in the deposits. Only a minor reduction of gaseous KCl was obtained during co-combustion with peat although the chlorine content in the deposits was greatly reduced. These findings were supported by the results from the impactor measurements.

Influence of the polyethyleneimine grafting on the adsorption capacity of chitosan beads for Reactive Black 5 from aqueous solutions

The adsorption capacity of chitosan hydrogel beads generated by alkali (CB) and sodium dodecyl sulphate (CSB) gelation was investigated after polyethyleneimine (PEI) grafting for adsorption of Reactive Black 5 (RB5) from aqueous solutions. The adsorption capacities of PEI-grafted CB (PEI-CB) and CSB (PEI-CSB) were varied with the amount of PEI used during grafting. The maximum adsorption capacity values of PEI-CB (709.27 mg/g) and PEI-CSB (413.23 mg/g) obtained from the Langmuir isotherm model were higher than those of CB (201.90 mg/g) and CSB (168.07 mg/g), indicating that the adsorption performance of CB and CSB could be highly enhanced by PEI grafting. All of the adsorption systems showed better fits to the Langmuir isotherm model than the Freundlich isotherm model, except PEI-CSB. The kinetic data of the adsorption systems showed better fits to a pseudo-first-order rate model than a pseudo-second-order model.

Effects of Supplementary Cementitious Materials on Apparent Activation Energy

Abstract The application of the Arrhenius equation to characterize the progress of cement hydration requires the use of an apparent activation energy (Ea) value to describe the temperature sensitivity of the combined hydration reactions. Supplementary cementitious materials (SCMs) such as fly ash, ground-granulated blast-furnace slag, and silica fume are an integral part of most concrete mixtures, but their impact on Ea has not been fully explored for a wide range of cement and SCM compositions and replacement levels. This information is important in order to predict heat from the hydration of concrete or strength development using the “maturity method.” This paper examines the effects of SCMs on the apparent activation energy (Ea). It was observed that increasing amounts of Class F fly ash and silica fume generally lowered Ea, but the effects of slag and Class C fly ash were dependent on the composition of the cement, particularly the aluminate, alkali, and soluble sulfate contents.

High Efficiency Waste-to-Energy Plants − Effect of Ash Deposit Chemistry on Corrosion at Increased Superheater Temperatures

Thermodynamic modeling has been used to study the ash deposit chemistry in waste-fired power plants and the implications for corrosion for increased superheater temperatures. Melting of ash particles and deposits can increase the corrosion of the superheater tubes due to molten salt corrosion. Predictions were made for various waste fuel mixtures with varying contents of Na, K, Pb, Zn, Cl, and S at superheater temperatures (350−550 °C). The results show that the formation of a low-melting, chloride-rich melts containing lead and zinc is expected for all of the waste fuel mixtures in the present study at 350−450 °C, which are typical superheater temperatures for waste-fired power plants. A molten phase in deposits can increase molten salt corrosion of superheaters. Predictions show that addition of calcium or firing fuel mixtures with a molar ratio of (Na + K)/(Cl + 2S) > 1 may decrease the tendency of the ash to form the corrosive melts in superheater deposits below 500 °C. However, if the superheater temperature are increased to around 500 °C, these measures will not be as effective due to the formation of molten alkali sulfate−chloride mixtures at temperatures around 500 °C and higher. A comparison of predicted values with experimental data for Na, K, Pb, and Zn in the gas phase in combustion of different waste mixtures at different temperatures showed that for several cases the general trends were satisfactorily predicted taking into account experimental and modeling uncertainties. However, in some cases the differences between the measured and predicted values were considerable.

Cement-Dispersant Incompatibility due to Ettringite Bridging

Sulfonated naphthalene formaldehyde, a common dispersant for portland cement, was found to be incompatible with some cements, the incompatible combinations producing gelation rather than dispersion of cement–water mixtures. The gelation was found to be prevented by the addition of alkali hydroxide or alkali sulfate. The objective of this study was to explore the mechanism behind these incompatibilities. Three hypotheses were tested: ettringite bridging, superplasticizer consumption, and polymer bridging. Results showed that the ettringite formation was responsible for the gelation and that alkali addition inhibited the ettringite formation.

Caratterizzazione idrogeochimica delle acque sotterranee abruzzesi e relative anomalie

The geological and stratigraphical sequence of the Abruzzo region (Central Italy, fig. 1) is dominated by the Jurassic to Oligocene succession, constituted mainly by limestone, marley limestones, calcareous sandstones, calcareous marls, marls and clayey marls along the Apennines, hilly areas and shoreline. The Miocene sequence, outcropping along the Apennines front, is constituted of limestones, marley limestones, calcareous marls and marls. The top of the succession, in the central and northern areas, is constituted by Messinian deposits of the marley-arenaceous turbidite formation, while, in the southern area delimited by Pescara river at north, by the evaporitic terrigeneous succession. The Pliocene-Pleistocene sequence represents the greater part of the hill domain, mainly constituted by marley clays and clayey marls surrounding arenaceous and conglomerate bodies. A gravity flow (Oligocene-Miocene Molisan sequence) originated from the Apennines is located at south of Maiella within the Pliocene-Pleistocene sequence; it is mainly constituted by clays and marls, with smaller proportions of limestones, calcareous sandstones, calcareous marls, Cretaceous and Miocene reworked marls and chalks, with maximum thicknesses up to approximately 2000 m. Lacustrian deposits are found in the intra-mountain basins, while Quaternary alluvial deposits lay on top of the marine deposits along main rivers and streams. The alluvial domain is mainly constituted by continental terraced deposits in the fluvial plains, while intramountain basins by fluvial-lacustrian deposits, continental head deposits and in a lesser extent by Pliocene-Pleistocene sandstones and conglomerates (fig. 1). Aquifers, mainly recharged by rainfall river contribution; groundwater flow is significantly influenced by paleo-river bodies and bedrock geometries (Desiderio et alii , 1999). These mostly non-confined aquifers, mainly constituted by gravelly-sandy deposits, are locally multilayered and semi-confined, especially in the intra-mountain and coastal zones. Exchange between surface waters and groundwater can be significant. The Abruzzo main aquifers are located in the Appennine carbonate domain (Celico, 1983; Boni et alii , 1986; Conese et alii , 2001; Petitta & Tallini, 2002; Nanni & Rusi, 2003; Desiderio et alii , 2002). The major springs (Q > 50 l/s) are located at the border of the carbonate hydro-structures along the contact with less permeable deposits that constitute the aquiclude (Miocene, Pliocene and Pleistocene terrigenous deposits) or aquitards (continental Pliocene and Pleistocene deposits). Minor springs (Q < 20 l/s) are controlled by local factors as head deposits, impermeable layers within the lithological sequence and tectonic discontinuities. Perennial springs are seldom found in the Appennines fore-areas and hilly areas, constituted by calcareous sandstones, marls, clays, sandstones and chalks originated from the alloctonous units, Miocene turbidite and evaporitic successions, including clayey marls and clays from Pliocene and Pleistocene pelagic successions. The few springs are related to aquifers within the more permeable deposits, mainly recharged by rainfall. Surface and buried bituminous and evaporitic sediments sometimes increase the spring waters salinity (Desiderio & Rusi, 2004). Groundwater classification of the four hydrogeological domains has been accomplished within this study on the basis of 690 chemical analysies of 325 monitoring points. Piper diagram heighlights that spring waters from the Apennine carbonate domain are classified as calcium bicarbonate facies (fig. 2); groundwaters from the intra-mountain domain are slightly shifted (fig. 5), while those from springs and wells in the hilly domain (fig. 6), thus originating from terrigenous deposits, show a greater dispersion with a slight prevalence of alkaline earth-bicarbonate and sulphate-chloride-alkaline earth waters. Alluvial domain groundwaters (fig. 7) are even more disperse with a prevalence of calcium bicarbonate facies. The mother groundwaters in the Apennine carbonate domain undergo a first mixing within the intra-mountain basins with waters flowing in the alkali rich mainly lacustrian continental deposits. More significant mixing occurs when the Apennine groundwater flow into the alluvial domain. Greater residence times and alluvial lithological heterogeneity imply greater groundwater salinity with a significant increase in alkali elements, sulphates and chlorides. Saline intrusion in coastal areas are related to groundwater withdrawals causing significant variations of original facies. Groundwater flowing within the terrigenous domain do not undergo mixing due to aquifer geometries and small discharges. The hydrochemical facies dispersion is related to the extreme lithological heterogeneity of this domain. A modest circulation of mineralised waters originates from groundwater flow within the Adriatic Pliocene-Pleistocene foredeep, allochthonous deposits and Miocene evaporites (figs. 9, 10 and 12). The hydrochemical facies are sodium-chloride, sodium-chloride-sulphate, sodium-bicarbonate, sodium-sulphate and without dominant ions; these groundwaters undergo different degrees of mixing with those of the main domains, thus modifying their characteristics (fig. 13).

The importance of SO 2 and SO 3 for sulphation of gaseous KCl – An experimental investigation in a biomass fired CFB boiler

This paper is based on results obtained during co-combustion of wood pellets and straw in a 12 MW circulating fluidised bed (CFB) boiler. Elemental sulphur (S) and ammonium sulphate ((NH 4) 2SO 4) were used as additives to convert the alkali chlorides (mainly KCl) to less corrosive alkali sulphates. Their performance was then evaluated using several measurement tools including, IACM (on-line measurements of gaseous alkali chlorides), a low-pressure impactor (particle size distribution and chemical composition of extracted fly ash particles), and deposit probes (chemical composition in deposits collected). The importance of the presence of either SO 2 or SO 3 for gas phase sulphation of KCl is also discussed. Ammonium sulphate performed significantly better than elemental sulphur. A more efficient sulphation of gaseous KCl was achieved with (NH 4) 2SO 4 even when the S/Cl molar ratio was less than half compared to sulphur. Thus the presence of gaseous SO 3 is of greater importance than that of SO 2 for the sulphation of gaseous KCl.

Reduction of superheater corrosion by co-firing risky biomass with sewage sludge

Iron and aluminium sulphates are added to processes producing sewage sludge (SWS), and SWS also contains aluminium silicates. These compounds can destroy alkali chlorides and prevent Cl deposition on superheaters. Sulphation of chlorides requires formation of SO 3 in the furnace. Raw and digested SWSs were mixed with a fuel containing bark and recycled fuel (REF). Combustion experiments were conducted with a pilot-scale bubbling bed reactor, and concentrations and deposition of critical elements (Cl, Na, K, S…) in the furnace were measured. Change of the iron and aluminium sulphates added to the SWS to phosphates and hydroxides evidently reduced the power of the sulphur to form SO 3. Nevertheless, addition of SWS to the biomass feedstock effectively decreased alkali chloride concentration and Cl deposition, and the alkali sulphates found in the fine fly ash confirmed the contribution of sulphation to alkali capture. The importance of aluminium silicate reaction in alkali capture was indicated.

Fundamental mechanisms for polycarboxylate intercalation into C 3A hydrate phases and the role of sulfate present in cement

The fundamental reactions leading to the intercalation of polycarboxylate (PC) superplasticizers into calcium aluminum hydrates were studied by hydration of pure C 3A in the presence of PC at 75 °C. It was found that the amount of dissolved sulfate present in cement pore solution determines whether organo-mineral phases are formed or not. In the absence of sulfate, PCs easily intercalate during C 3A hydration in alkaline solution. Under these conditions, only excessive steric size of the PC will prevent intercalation. At low sulfate concentrations (SO 4 2−/C 3A molar ratios of 0.1–0.35), PC intercalates with intersalated alkali sulfate, are formed. At high sulfate concentrations (SO 4 2−/C 3A molar ratios of 0.7–2), PC can no longer intercalate. Instead, sulfate, because of its higher negative charge density, fills the interlayer space and monosulfoaluminates with different water contents are formed. Anion exchange experiments confirm that from the initially formed C 4AH 13, PC will exchange the interlayer OH − anion whereas with monosulfoaluminates, no replacement of sulfate by PC was found. Consequently, in alkaline solution, PC intercalates will not exchange their PC against OH − anions whereas sulfate will gradually replace the PC. Generally, intercalation of PC is an unwanted process because it consumes superplasticizer which is effective only when it adsorbs onto the cationic surfaces of AF m and AF t phases. Our experiments demonstrate that intercalation can be avoided by using PCs with long side chains or highly sulfated cements (SO 4 2−/C 3A molar ratio ≥ 0.75) containing alkali or calcium sulfates which dissolve fast. In undersulfated cements, however, PC intercalates can be formed, either directly during the stacking process of the [Ca 2Al(OH) 6] + main layer, with PC acting as the template which determines the interlayer distance, or by anion exchange between initially formed aluminate hydrates (e.g. C 4AH 13 or C 2AH 8) and the PC anion.

Hot corrosion of two nickel-base superalloys under alkali sulfate deposition conditions

The single crystal MD2 (Ni – 8Cr – 5Al –5Co– 2Mo– 8W–6Ta – 1Ti) and the polycrystalline Inconel 792 (Ni –13Cr – 3Al –9Co– 2Mo– 4W–4Ta– 4Ti) were exposed at 850 and 900°C in an severely sulfidising environment comprising synthetic air with 8% water vapour and 2000 ppm SO2, and a deposit of 20 mol% Na2SO4+80 mol% K2SO4 applied every 160 hours.Results were evaluated in terms of mass change, which indicated the existence of an incubation time before the onset of extensive attack, and post-test metallography to evaluate metal loss. Metallographic studies are presented showing the complex external oxide scale formation and internal oxidation and sulfidation.

Chlorine from the mantle: Magmatic halides in the Udachnaya-East kimberlite, Siberia

The Devonian Udachnaya-East pipe (Siberia) presents a rare example of exceptionally fresh kimberlite rocks, containing a rich assemblage of alkali carbonate, chloride and sulphate minerals in its groundmass. Accordingly, bulk groundmass compositions have unusually high concentrations of sodium and chlorine (up to 6 wt.% each), while water contents are very low. High Na 2O and low H 2O are inconsistent with currently accepted model kimberlite magma compositions; and high Na and Cl contents in the Udachnaya-East pipe have been ignored by the geoscience community, because of possibility of contamination from sedimentary crustal rocks. New textural evidence suggests that the chloride and alkali carbonate minerals in this kimberlite are co-magmatic with perovskite. Radiogenic isotope ratios for the perovskite ( 87Sr/ 86Sr ~ 0.7031, ε Nd + 5, ε Hf + 5.3, obtained by solution-mode and laser-ablation methods) are very primitive and provide no support for a crustal contaminant. The new perovskite data confirm previously published Sr–Nd–Pb isotopic evidence for the chlorides and alkali carbonates themselves, but are considered more reliable because of possible open system behaviour and analytical problems with unstable halide and carbonate minerals. Similar isotope ratios ( 87Sr/ 86Sr = 0.70292, ε Nd = + 5, ε Hf = + 6.5) are recorded in a clinopyroxene macrocryst, which contains chlorides and alkali carbonates within melt inclusions, implying a similarly primitive isotopic signature for the host kimberlite. We infer that assimilation of evaporitic sediments is unlikely and that abundant chlorine and sodium in the kimberlite originated in the upper mantle. Our results and published experimental data on mantle chloride melts and presence of a chloride component in diamond-hosted fluid inclusions suggest a previously unrecognised role for mantle-derived halide melts in the genesis, composition and rheological properties of kimberlite primary melts.

Electrokinetic potentials of Al 2O 3 in concentrated solutions of metal sulfates

The zeta potentials of alumina (10% by mass dispersions) in 0.1–1 M solutions of metal sulfates were determined by means of the electroacoustic method. The background correction was used to account for the electroacoustic signal of the electrolyte. The zeta potentials in the solutions of alkali sulfates were in the range from −8 to −19 mV. They were rather insensitive to the pH (4–9.5), and to the nature and concentration of the salt. In MgSO 4 solutions (0.05–0.5 M, pH 4–8), the zeta potentials were also negative (−3 to −9 mV) showing the preference of alumina to adsorb sulfate anions rather than magnesium cations.

Raman spectroscopic study of aqueous alkali sulfate solutions at high temperature and pressure to yield precipitation

The shift of the Raman band of ν 1(SO 4 2−) in aqueous alkali (Li, Na and K) and ammonium sulfate solutions was studied over a wide temperature range up to 350 °C under 25 MPa. The temperature shift for Li sulfate that was in the direction to low wavenumber was smaller and its dependence on the molality was larger than for the other two alkali sulfates. These trends were discussed based on the solution structure controlled by the cation. The discontinuous change of the spectral position from the solution phase to the solid phase at 390 °C was observed during the precipitation progress of Li and Na sulfates. Based on the present experimental results, K + cations are suggested to be most adequate among the investigated alkali cations as the neutralizing reagent in supercritical water oxidation (SCWO) process.

Hot corrosion of two nickel-base superalloys under alkali sulfate deposition conditions

Hot corrosion of two nickel-base superalloys under alkali sulfate deposition conditions

Corrosion of Lead Glasses by SO&lt;sub&gt;2&lt;/sub&gt; Gas

This paper presents the corrosion of potassic glasses by SO2 gas at high-temperature. The corrosion phenomenon of potassic glass is well-known for medieval stained-glass windows under natural conditions of weathering [1]. Nevertheless the corrosion is also possible without humidity at high temperature in presence of polluting gas such as SO2 gas in the furnace. All source of SO2 gas can thermodynamically involve the main formation of K2SO4 and other alkaline salts such as Na2SO4. This corrosion manifests itself by the presence of “white flakes” in the bulk glass and this defect leads to discard of the piece for a crystal-maker like Daum. Alkali sulphate formation was determined by using scanning electron microscopy and presents a very particular morphology. This study allowed underlining on, one hand the influence of the temperature and on the other hand the role of gas concentration on the formation of the potassium sulphate. Thermal analysis technique (TGA) was carried out in order to understand the corrosion chemical kinetic of potassic glasses by SO2 gas.

Influence of hydration on the fluidity of normal Portland cement pastes

The rheological properties of cement paste strongly influence the workability of concrete. It is known that early hydration processes alter phase composition and microstructure of cement pastes. These processes affect fluidity and setting behaviour of cement paste. While many studies tried to measure and model rheological properties of cement pastes, only a few studies assessed the influence of the hydrate morphology on the fluidity of cement pastes. Results of the present study compare the influence of long prismatic hydrates (i.e. syngenite, secondary gypsum) on the fluidity of cement pastes with the effect of other hydrates (AFm). To induce the formation of certain hydration products the cement composition was modified by addition of set regulators and alkali sulphates. Furthermore a combination of various analytical methods such as fluidity (viscometric) testing and microstructural analysis (phase quantification by XRD-Rietveld analysis, investigation by Environmental SEM, BET analysis etc.) was performed. Results are implemented into a fundamental discussion on the influence of various hydration products on the fluidity of the paste.

A New Approach to Synthesis of Layered Fluorites Containing Molecular Anions: Synthesis of Ln2O2CO3, K(LnO)CO3, and Ln2O2CrO4 via Metathesis Reactions

A new synthetic approach is suggested for preparation of layered rare-earth oxide compounds containing [Ln2O2] slices and molecular anion sheets (CO32-, SO42-, and CrO42-). It is based on exchange reactions of rare-earth oxychlorides, [Ln2O2]Cl2, and alkali carbonates, sulfates, or chromates. Five new rare-earth oxychromates [Ln2O2]CrO4 (Ln=Pr-Gd) have been prepared which adopt a new, probably layered, structure type. In addition, significantly easier and more efficient synthetic pathways were found to the known compounds [Ln2O2]K2(CO3)2 and [La2O2]CrO4. The structure of the latter compound has been determined from neutron powder diffraction data. Factors affecting reaction pathways and products are discussed, as well as prospects for applying the approach to more complex layered compounds.

The Effects of Alkali Sulfate on the Hydration of a C3A−CaSO4⋅2H2O System

The hydration mechanism of the 3CaOㆍAl₂O₃-CaSO₄ㆍ2H₂O system in the presence of alkali sulfates has been investigated. The early hydration rate of 3CaOㆍAl₂O₃ was accelerated by the addition of Na₂SO₄ and K₂SO₄. This is closely related to the formation of syngenite (CaSO₄ㆍK₂SO₄ㆍH₂O), and the U-phase added K₂SO₄ and Na₂SO₄ in the 3CaOㆍAl₂O₃-CaSO₄ㆍ2H₂O system, respectively. The formation of the rigid syngenite and U-phase structure led to rapid setting and decreases the sulfate content in the liquid phase of the hydrating cement to the extent that it cannot adequately retard the hydration of 3CaOㆍAl₂O₃. In case of the alkali sulfate not added to the 3CaOㆍAl₂O₃-CaSO₄ㆍ2H₂O system, the ettringite was transformed to monosulfoaluminate immediately after the consumption of gypsum. However, when the alkali sulfates were added to this system, the ettringite did not transform to monosulfoaluminate immediately even though the gypsum was completely consumed. There was a stagnation period to transform to the monosufoaluminate after the consumption of gypsum because the syngenite and U-phase remained as the sulfate source.

An exploratory study of alkali sulfate aerosol formation during biomass combustion

It is still in discussion to what extent alkali sulfate aerosols in biomass combustion are formed in the gas phase by a homogeneous mechanism or involve heterogeneous or catalyzed reactions. The present study investigates sulfate aerosol formation based on calculations with a detailed gas phase mechanism. The modeling predictions are compared to data from laboratory experiments and entrained flow reactor experiments available in the literature. The analysis support that alkali sulfate aerosols are formed from homogeneous nucleation following a series of steps occurring in the gas phase. The rate-limiting step may be the oxidation of sulfite to sulfate, rather than the oxidation of SO 2 to SO 3 proposed previously. Even though the proposed model is consistent with experimental observations, experiments in a rigorously homogeneous system are called for to test its validity.

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe, Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.