Alkali Sorption Research Articles

Influence of additive surface area degradation on fine particle formation during biomass pulverised-fuel combustion

This study examined the surface area degradation of kaolin, coal fly ash and the resulting impact on the alkali capture efficiency in a pilot-scale real flame combustion test rig using beech wood as fuel. Experiments in an electrically heated furnace revealed the thermal surface area evolution of kaolin with residence times between 30and 3600 s and temperatures between 790 and 1400 °C under calcination conditions. Subsequent measurements of the resulting specific BET (Brunauer-Emmet-Teller method) surface area of the kaolin samples were carried out. The temperature profile in the top-down fired biomass combustion test rig (BoCTeR) was measured with a suction pyrometer when combusting natural gas and beech wood powder. Additional experiments were performed by injecting kaolin and coal fly ash particles directly into a natural gas flame at different reactor heights. Those kaolin particles exposed to high temperatures in the natural gas flame were collected and subsequently analysed for the specific BET area to evaluate the corresponding degree of surface loss. The main test series comprises a setup combusting beech wood with kaolin or coal fly ash injected at different positions of the combustion chamber. The alkali sorption capacity of the kaolin and coal fly ash used is indirectly measured by the combustion aerosol reduction performance determined with an electrical low-pressure impactor (ELPI). The goal of this work is to show the impact of surface area degradation phenomena of additives and the resulting optimisation potential when using Al/Si-based in-furnace additive during the pulverised-fuel combustion of biomass. The surface area development of the additives at high temperatures affects the most suitable temperature window for the additive injection. The results show an optimum for the kaolin injection. The coal fly ash reduced PM best when injected as early possible into the combustion chamber.

Alkali removal with mineral sorbents – Part I: Sorption capacity and reaction kinetics

Aluminosilicate sorbents show the potential to reduce alkali-related problems during solid fuel conversion such as deposit build-up and corrosion damages. The alkali capacity, the reaction kinetics and the surface area development of the sorbents under operating conditions are essential parameters for the mathematical description of the alkali sorption. In this work, three sorbents, kaolin, bauxite and bentonite, are characterized in this regard by thermogravimetric analysis. The sorption behavior of kaolin surpasses the other sorbents due to the highest loading capacity and the temperature-stability of the porous structure. However, the suitability as sorbent in solid fuel conversion processes is given for all materials. A mathematical sorption model is applied to the experimental data in order to derive kinetic parameters of the sodium and potassium sorption under syngas and flue gas atmosphere. Thus, this work provides the necessary dataset for the design and optimization of the sorbent use in industrial processes.

Alkali removal with mineral sorbents – Part II: Fixed-bed experiments and model validation

In this work, alkali capture experiments with three sorbents, kaolin, bauxite and bentonite, are performed in a fixed-bed reactor under industry-oriented conditions. The influence of temperature and sorption duration on the loading curve and the sorption efficiency as well as the influence of the sample preparation and the operating conditions on structural properties of the sorbents are investigated. Regarding the alkali reduction in the gas phase, bauxite and bentonite show higher efficiencies compared to kaolin, although sintering leads to a significant decline of the porous structure of both sorbents. In addition to the experimental studies, a 1-D simulation model of the fixed-bed reactor including kinetic data of the sorbents is developed. The simulated loading curves show a good agreement with the experimental results. This validation confirms the suitability of the kinetic model and the model parameters for the mathematical description of the alkali sorption in industrial processes.

Atomistic and mesoscale simulation of sodium and potassium adsorption in cement paste.

An atomistic and mesoscopic assessment of the effect of alkali uptake in cement paste is performed. Semi-grand canonical Monte Carlo simulations indicate that Na and K not only adsorb at the pore surface of calcium silicate hydrates (C-S-H) but also adsorb in the C-S-H hydrated interlayer up to concentrations of the order of 0.05 and 0.1 mol/kg, respectively. Sorption of alkali is favored as the Ca/Si ratio of C-S-H is reduced. Long timescale simulations using the Activation Relaxation Technique indicate that characteristic diffusion times of Na and K in the C-S-H interlayer are of the order of a few hours. At the level of individual grains, Na and K adsorption leads to a reduction of roughly 5% of the elastic moduli and to volume expansion of about 0.25%. Simulations using the so-called primitive model indicate that adsorption of alkali ions at the pore surface can reduce the binding between C-S-H grains by up to 6%. Using a mesoscopic model of cement paste, the combination of individual grain swelling and changes in inter-granular cohesion was estimated to lead to overall expansive pressures of up to 4 MPa-and typically of less than 1 MPa-for typical alkali concentrations observed at the proximity of gel veins caused by the alkali-silica reaction.

The Reaction Kinetics of Gaseous Alkali Capture by Kaolin in Syngas Atmosphere

AbstractAlumosilicate minerals are suitable sorbents at high temperatures for removing alkalis in syngas. Thermogravimetric investigations of these sorbents usually focus on the reaction mechanisms. Herein, a crucible arrangement is described that allows evaporation of the alkali source and sorption of gaseous alkali components in one temperature zone. With this experimental setup, kinetic investigations of alkali sorption are possible in a conventional thermobalance. Experiments were performed with sodium chloride as the alkali source and kaolin as the getter material. The reaction rate increased with alkali concentration and showed an exponential dependence on temperature. Thus, the Arrhenius model approach and power law model were selected for mathematical description.

Determining alkali leaching during accelerated ASR performance testing and in field exposed cubes using cold water extraction (CWE) and µXRF

Considerable leaching of alkali metals was documented, both in concrete samples exposed to accelerated laboratory testing and field samples. CWE allowed to quantitatively determine the free alkali metal profiles as a function of the depth. However, CWE cannot account for the changes in the paste content towards a cast surface leading to a seemingly increase in alkali metals. The μXRF allowed to distinguish paste and aggregates. It allowed thereby to determine qualitative Na and K profiles in the cement paste phase of the concrete samples. The laboratory exposed samples showed a clear leaching profile into a depth of about 15 mm after 21 weeks of exposure at 60°C. Corresponding numbers for the 12 years field exposed cube were 50-60 mm. Alkali sorption by alkali silica gel was detected using the μXRF. For the laboratory exposed samples, the prisms prepared with Portland fly ash cement leached less alkali compared to the prisms prepared with ordinary Portland cement, as expected. The leaching in the middle of the prisms estimated based on the μXRF profiles agreed rather well with the level of alkali leaching determined based on the analysis of the leachate (i.e. the water below the samples during exposure).

A sorption model for alkalis in cement-based materials – Correlations with solubility and electrokinetic properties

In cement-based materials, radionuclide uptake is mainly controlled by calcium silicate hydrates (C-S-H). This work presents an approach for defining a unique set of parameters of a surface complexation model describing the sorption behavior of alkali ions on the C-S-H surface. Alkali sorption processes are modeled using the CD-MUSIC function integrated in the Phreeqc V.3.0.6 geochemical code. Parameterization of the model was performed based on (1) retention, (2) zeta potential, and (3) solubility experimental data from the literature. This paper shows an application of this model to sodium ions. It was shown that retention, i.e. surface interactions, and solubility are closely related, and a consistent sorption model for radionuclides in cement-based materials requires a coupled surface interaction/chemical equilibrium model. In case of C-S-H with low calcium-to-silicon ratios, sorption of sodium ions on the C-S-H surface strongly influences the chemical equilibrium of the C-S-H + NaCl system by significantly increasing the aqueous calcium concentration. The close relationship between sorption and chemical equilibrium was successfully illustrated by modeling the effect of the solid-to-liquid ratio on the calcium content in solution in the case of C-S-H + NaCl systems.

Investigation of the Ability of Alkali Binding by C-S-H and C-A-S-H Gels

The binding of sodium and potassium into cement paste influences the performance of concrete: for example, alkali balances between solid and paste constituents and pore fluid affect the potential for alkali aggregates reaction. However, quantification of the binding potential into paste solids has proven to be difficult, although much empirical data are available from pore fluid analyses. In this study, single-phase homogeneous C-S-H phases have been prepared at Ca: Si molar ratios (C/S) of 1.8, 1.6, 1.3, 1.0 and 0.8 and reacted with four alkali hydroxide concentrations, both NaOH and KOH, between 10 and 300 mM. A steady-state alkali partition is attained in less than 4d at 20°C by flame emission spectroscopy and the ξ potential of C-S-H and C-A-S-H gel unreacted and reacted with alkali hydroxide are measured. The results indicate：alkali binding into the C-S-H and C-A-S-H gel improves as its C/S decreases；Al3+ has been introduced to form C-A-S-H gels, and the influence of Al3+ on alkali sorption properties was determined: the replacement of Si4+ by Al3+ and Na+ or k+ markedly increases alkali binding and decreases the ξ potential of gel；the more Ca:Si ratio，the less alkali binding, the more ξ potential of gel.

Prediction of chloride ingress into saturated concrete on the basis of a multi-species model by numerical calculations

A multi-species model based on the Nernst-Planck equation has been developed by using a finite volume method. The model makes it possible to simulate transport due to an electrical field or by diffusion and to predict chloride penetration through water saturated concrete. The model is used in this paper to assess and analyse chloride diffusion coefficients and chloride binding isotherms. The experimental assessment of the effective chloride diffusion coefficient consists in measuring the chloride penetration depth by using a colorimetric method. The effective diffusion coefficient determined numerically allows to correctly reproduce the chloride penetration depth measured experimentally. Then, a new approach for the determination of chloride binding, based on non-steady state diffusion tests, is proposed. The binding isotherm is identified by a numerical inverse method from a single experimental total chloride concentration profile obtained at a given exposure time and from Freundlich's formula. In order to determine the initial pore solution composition (required as initial conditions for the model), the method of Taylor that describes the release of alkalis from cement and alkali sorption by the hydration products is used here. Finally, with these input data, prediction of total and water-soluble chloride concentration profiles has been performed. The method is validated by comparing the results of numerical simulations to experimental results obtained on various types of concretes and under different exposure conditions.

Alkali Sorption in Second-Generation Pressurized Fluidized-Bed Combustion

Modern second-generation pressurized fluidized-bed combustion (2nd-Gen. PFBC) combined cycle power systems are developed to increase the efficiency of coal combustion. However, combined cycle power systems require reliable hot gas cleaning. The alkali metals sodium and potassium can lead to severe damage of the gas turbine blades, because of hot corrosion. This investigation was conducted to assess the potential for the reduction of alkali metals from hot gas by different aluminosilicate sorbents, such as silica, bauxite, bentonite, and mullite, under reducing atmospheres at a temperature of 750 °C. Using a flow channel reactor, an alkali chloride-laden gas stream was passed through a bed of aluminosilicate sorbents. Qualitative and quantitative analysis of the hot gas downstream of the sorbent bed was performed using high-pressure mass spectrometry (HPMS). Thus, the influence of different gas atmospheres on the alkali sorption was determined and conclusions were drawn concerning the chemical reactions that were occurring inside the sorbent bed. Moreover, annealing experiments were performed to determine the sorption capacity of the sorbent. Subsequent scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray diffraction (XRD) analyses showed that the phases formed inside the tested sorbent materials. Sorbent capacities for sodium and potassium of up to 110 mg/g have been achieved after 192 h for bentonite. Finally, thermodynamic computations were performed to scale the results of the laboratory experiments up to the conditions prevalent in 2nd-Gen. PFBC systems. The investigations revealed the possibility of reducing the overall alkali concentration in the hot gas under 2nd-Gen. PFBC conditions to values of <50 ppbv through the use of bentonite and activated bauxite. The amount of sorbent needed for a 500 MW el power plant was estimated to be ∼100 kg/h at a hot gas temperature of 700 °C.

Alkali sorption by C-S-H and C-A-S-H gels: Part II. Role of alumina

In Part I, it was shown that alkali partition between C-S-H gel and an aqueous phase can be represented by a partition function, R d, the numerical value of which, at constant temperature, is defined by the Ca/Si ratio. This R d value is constant or nearly so over wide ranges of NaOH and KOH concentrations up to ∼0.3 M. In the present paper, Al has been introduced to form C-A-S-H gels, and the influence of Al on alkali sorption properties was determined: Approximately 6–7% replacement of Si by Al was used. Microprobe evidence is presented to show that the Al is actually in solid solution. Introduction of Al into C-S-H markedly increases R d, indicating enhancement of alkali binding. The results underpin and quantify the beneficial effects of alkali binding arising from the introduction of aluminous supplementary cementing materials, such as fly ash, into cement pastes.

Ion exchange with silica-titania gels

Silica-titania gels containing 25, 50 and 75 mol% TiO2 were prepared and the sorption of alkali and alkaline earth metal ions by these materials has been studied. Distribution coefficient values for Cs+ and Sr2+ ions were found to pass through broad maxima as a function of TiO2 content except in the case of Sr under alkaline conditions where there was a continuous increase. Capacity values also increased with TiO2 content and samples containing 50% (for K+ and Cs+), 75% (for Li+ and Na+) and 25%/50% (for Ca2+, Sr2+ and Ba2+) TiO2 exhibited maximum capacities. However, unlike with alkali metals, capacities of a given sorbent for the three alkaline earth ions were almost the same. Large capacities obtained for the latter ions seem to indicate a mineral-forming reaction with 25% and 50% materials. On the other hand, the 25% TiO2 gel seems to sorb Sr at trace level by an ion exchange mechanism. Coupled with its Cs sorption capability, this material may find potential use in large scale decontamination of low level waste solutions.

Influence of medium polarity upon selectivity and efficiency of alkali metal cation sorption by polyether carboxylic acid resins

The influence of medium polarity upon competitive sorption of alkali—metal cations from aqueous and aqueous methanolic solutions by the polymethacrylic acid resin Amberlite CG-50, two acyclic polyether carboxylic acid resins, and six sym-(alkyl)dibenzo-16-crown-5-oxyacetic acid resins has been investigated. Addition of methanol was found to strongly depress the selective Li + sorption of CG-50 and one of the acyclic polyether carboxylic acid resins. Enhancement of metal ion-crown ether interactions as the percentage of methanol in the medium was increased accentuates the Na + sorption selectivity for the lariat ether carboxylic acid resins. The highest Na + sorption selectivity was obtained for sym-(alkyl)-dibenzo-16-crown-5-oxyacetic acid resins with ethyl and propyl groups in 80% methanol-20% water.

Ion exchange properties of tin(IV) materials—III: Anion exchange and further cation exchange studies on hydrous tin(IV) oxide

The sorption of alkali and alkaline earth cations on hydrous tin(IV) oxide has been investigated, in particular on the exchanger in the NH 4 + form, and some chromatographic separations among these ions are described. The use of the exchanger in the H + form to remove UO 2(II) and transition metal ion contamination from solutions containing large amounts of the above ions has also been investigated. Some anion-exchange properties of this material are reported, and the marked selectivity series established predicts the possibility of facile separations of certain anionic species on columns of hydrous SnO 2.

Some Observations on the Absorption of Alkali by Wool Keratin from Aqueous Solutions of Sodium Carbonate and Bicarbonate

Sorption of alkali by wool from aqueous solutions of sodium carbonate and bicarbonate has been examined by measurement of hydrogen‐ion concentrations. The effects of concentration of alkaline salt, time of immersion, liquor ratio, and temperature of equilibration have been studied in relation to subsequent measurement of the pH of an aqueous extract of a treated sample. A qualitative interpretation of results is given, and a quantitative treatment employing the Donnan theory of membrane equilibria has been attempted.

THE SORPTION OF SOAP BY TEXTILE FIBERS

The sorption of the sodium soaps of lauric, myristic, palmitic, stearic, and oleic acids from aqueous solutions by various textile fibers has been measured. The sorption of both the fatty acid and alkali components of the soaps by dull acetate rayon and dull nylon fibers was essentially the same as that shown by the corresponding bright (undelustered) fibers, while dull viscose rayon sorbed considerably more fatty acid than the bright fiber. In general, the order of increasing sorption was: cotton, nylon, acetate, bright viscose, dull viscose, and wool. Of the saturated soaps, the maximum sorption of fatty acid by all fibers was obtained with sodium myristate, while the alkali sorptions were approximately the same for myristate, palmitate, and stearate, all of which were higher than for laurate. The sorption from sodium oleate solutions corresponded approximately to that from the C14–C16 saturated soaps. Preferential sorption of alkali by cotton and viscose rayon was observed for all soaps, while acetate rayon, nylon, and wool showed preferential sorption of fatty acid with the lower molecular weight soaps and preferential sorption of alkali with the higher soaps. Suppression of hydrolysis by the addition of excess free alkali resulted in a reduction in fatty acid sorption in every case, and shifted the maximum from the C14 to the C16 soap. It is concluded that the sorption of soap by textile fibers is a complex process involving the more or less independent sorption of neutral soap, hydrolytic fatty acid (or acid soap), and hydrolytic alkali.