Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

Experimental Studies on Hydration of Partially Sulphated CFBC Ash

The hydration of partially‐sulphated fluidized bed combustor (FBC) ash with water was carried out at laboratory scale. The bottom ash fractions and the as‐received fly ash were hydrated for different lengths of time, at different temperatures between 5°C and 80°C. The free lime and calcium hydroxide content in the samples were analyzed before and after the hydration process. Scanning electron microscopy (SEM) with an energy dispersive X‐ray system (EDX) was employed to investigate the physical characteristics of the samples. X‐ray diffractograms (XRD) were used to obtain information on the phase composition. The current results show that, during the hydration treatment, the unreacted CaO in the partially‐sulphated material can be almost quantitatively converted to Ca(OH)₂ but that the free lime content is not constant. It is also clear that effectiveness of the hydration depends on hydration time and temperature. In addition, the behaviour of different particle size fractions is different and there is evidence that the hydration of CaO is not the only reaction occurring in this system.     

Influence of triethanolamine on the hydration characteristics of tricalcium silicate

The hydration characteristics of 3CaO.SiO₂ or β2CaO.SiO₂ are studied by an addition of 0.0, 0.1, 0.5 or 1.0% triethanolamine. The amount of Ca(OH)₂ found at 1, 3, 7 or 28 days was in the order C₃S + 0% TEA > C₃S +0.1% TEA > C₃S + 0.5% TEA > C₃S+1.0% TEA, irrespective of whether lime was estimated by X-ray, DTA, TGA or chemical analysis. The rate of hydration, in terms of the disappearance of 3CaO.SiO₂, showed that hydration proceeded faster in the presence of TEA after 1 day. Additions of TEA increase the induction period, promote the formation of a C-S-H with higher CaO/SiO₂ ratio, increase the formation of non-crystalline Ca(OH)₂ and enhance the surface area of the hydrated silicate product.     

Steam hydration-reactivation of FBC ashes for enhanced in situ desulphurization

Bed and fly ashes originating from industrial-scale fluidized bed combustors (FBCs) were steam hydrated to produce sorbents suitable for further in situ desulphurization. Samples of the hydrated ash were characterized by X-ray diffraction analysis, scanning electron microscopy and porosimetry. Bed ashes were hydrated in a pressure bomb for 30 and 60 min at 200 °C and 250 °C. Fly ash was hydrated in an electrically heated tubular reactor for 10 and 60 min at 200 °C and 300 °C. The results were interpreted by considering the hydration process and the related development of accessible porosity suitable for resulphation. The performance of the reactivated bed ash as sulphur sorbent improved with a decrease of both the hydration temperature and time. For reactivated fly ash, more favourable porosimetric features were observed at longer treatment times and lower hydration temperatures. Finally, it was shown that an ashing treatment (at 850 °C for 20 min) promoted a speeding up of the hydration process and an increase in the accessible porosity.     

Steam hydration of CFBC ash and the effect of hydration conditions on reactivation

A detailed study has been carried out on how hydration methods and conditions influence the sulphur capture potential of ash from a 165 MWe circulating fluidized bed combustion boiler firing a petroleum coke and coal blend. Both bed ash and fly ash were hydrated with saturated steam at various saturation conditions for different periods of time. Samples of the hydrated residues were then analyzed for free lime and calcium hydroxide content after the hydration process. Some size fractions of the steam-hydrated samples and those hydrated with liquid water in previous work were re-sulphated for 90 min using synthetic flue gas in a thermogravimetric analyzer at 850 °C to investigate how reactivation conditions affect the final sulphur capture behaviour of the ash. This work confirms that either hydration method is effective for reactivating the bed ash fractions tested but not fly ash, which should either be re-injected directly or reactivated in some other manner to improve its sulphur capture potential.     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

Mineralogical and engineering characteristics of dry flue gas desulfurization products

Fifty-nine coal combustion products were collected from coal-fired power plants using various dry flue gas desulfurization (FGD) processes to remove SO₂. X-ray diffraction analyses revealed duct injection and spray dryer processes created products that primarily contained Ca(OH)₂ (portlandite) and CaSO₃·0.5H₂O (hannebachite). Most samples from the lime injection multistage burners process contained significant amounts of CaO (lime), CaSO₄ (anhydrite), and CaCO₃ (calcite). Bed ashes from the fluidized bed process were often dominated by CaSO₄ but also contained CaCO₃, CaO (lime), and MgO (periclase). Cyclone ashes were similar in composition to the bed ashes but contained more unspent sorbent and CaSO₄ and less MgO. Fly ash in all samples ranged from 10 to 79 wt%. Samples usually exhibited two distinct swelling episodes. One occurred immediately after water was applied due to hydration reactions, especially the conversion of CaO to Ca(OH)₂ and CaSO₄ to CaSO₄·2H₂O (gypsum). The second began between 10 and 50 d later and involved formation of the mineral ettringite (Ca₆[Al (OH)₆](SO₄)₃·26H₂O). The final pH after 112 d ranged from 10.0 to 12.1. If samples are incubated under ‘closed’ (i.e. incomplete recarbonation with atmospheric CO₂) and alkaline weathering conditions, gypsum and portlandite are initially formed followed by the conversion of the gypsum to ettringite. Closed, alkaline conditions typically can occur when FGD products are placed in confined settings such as a road embankment or buried as a discrete layer as occurs in some surface mine reclamation projects.     

Alkali release characteristics of blended cements

This paper presents results of an experimental program conducted to investigate the capacity of hydration products of different cementing materials to retain “bound” alkalis when the alkalinity of the surrounding solution drops. The study covered paste samples containing high-alkali Portland cement and various levels of silica fume and/or fly ash. The results showed that the ability of the hydration products of cement-fly ash systems to bind alkalis is a function of the CaO content of the fly ash, the binding increasing as the calcium content decreases. High-alkali fly ashes (Na₂O_e > 5.0% and CaO in the range of 15% to 20%) showed considerable amounts of alkali contributed to the test solutions. Silica fume does not have a high capacity to retain alkalis in its hydration products; however, ternary blends containing silica fume and fly ash have excellent capacity to bind and retain alkalis.     

HYDROTHERMAL REACTION OF FLY ASH/HYDRATED LIME: CHARACTERIZATION OF THE REACTION PRODUCTS

Class F coal fly ash was slurried with hydrated lime at 90°C in 1/3, 5/3, 9/3, and 15/3 weight ratios and for 3, 5, 7, and 9 hours of hydration, in a process to prepare sorbents for SO₂ removal. The amounts of aluminum, silicon, and calcium in the product of the pozzolanic reaction were determined in order to study the evolution of product composition with the initial raw materials ratio and hydration time and to relate this composition to the desulfurization capability of the material. Al, Si, and Ca were present in the solid product for any raw materials ratio and hydration time, showing that calcium silicates, calcium aluminates, and/or calcium aluminum silicates were obtained simultaneously. The products formed show a nearly constant molar ratio of Al₂O₃/SiO₂ independent of the experimental conditions tested and similar to the Al₂O₃/SiO₂ ratio in the fly ash. The SiO₂/CaO molar ratio in the products decreased as the initial fly ash/Ca(OH)₂ ratio decreased, being approximately constant for each ratio with respect to hydration time after 5 hours of hydration. The maximum moles of CaO, SiO₂, and Al₂O₃ per gram of sorbent in the reaction product were found for any hydration time for the 5/3 sorbents, meaning that at this initial ratio the pozzolanic reaction takes place at the highest rate. The capacity of the sorbent for SO₂ removal depends not only on the amount of products produced by the pozzolanic reaction but also on the specific surface area of the sorbent.     

Mineralogical and elemental composition of fly ash from pilot scale fluidised bed combustion of lignite, bituminous coal, wood chips and their blends

The chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO₂ is the dominant oxide in fly ash with Al₂O₃ and Fe₂O₃ found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidised bed combustion in general.     

Preparation of environmental-friendly lime paints from hydrated lime and hydrated light burned dolomite

Environment-friendly lime paints were prepared from 38 wt% slurries of hydrated lime (Ca(OH)₂) and hydrated light burned dolomite (Ca(OH)₂·Mg(OH)₂). These materials and the resulting paints?? antibacterial and antimold properties were investigated. The hydrated lime used in this study contained 97 wt% Ca(OH)₂, which implies a 71.85 wt% equivalence of CaO. This 71.85 wt% CaO content was responsible for the antibacterial and anti-mold characteristics. The antibacterial and anti-mold characteristics of the hydrated light burned dolomite were attributed to its 56 wt% CaO and 44 wt% MgO contents. The antibacterial-reducing activities of 38 wt% hydrated lime and hydrated light burned dolomite slurries were found to be 99%. Their anti-mold activities against mixed strains were outstanding. Lime paints produced from the slurries and various additives also showed 99% antibacterial activity and outstanding anti-mold activity. The paints?? low total volatile organic compounds (TVOCs) releases were graded as excellent. Their formaldehyde (HCHO) releases were classed as best through excellent, indicating their suitability as environment-friendly building materials.     

A Trend to the Production of Calcium Hydroxide and Precipitated Calcium Carbonate with Defined Properties

In this paper, production of precipitated calcium carbonate (PCC) with required particle size and morphological structure, along with its dependence on technological parameters and the properties of Ca(OH)₂, is discussed. The effect of the reaction environment on the kinetics of CaO hydration and the formation of crystals in water suspension was established. A remarkable difference in the system's restoration ability after stirring was observed. The hydration process is initially controlled by a kinetic mechanism, followed by a diffusion‐controlled process. The dissolution speed of lime hydrated to suspension is eight times higher than that of lime hydrated to powder. Particles of hydrated lime appeared in various forms.     

Simultaneous SO2 and NO removal using sorbents derived from rice husks: An optimisation study

In this study, rice husk-derived ash (RHA) was hydrated with CaO and then impregnated with copper to synthesize a sorbent that was subsequently tested for its capacity in simultaneous removal of SO₂ and NO from a simulated flue gas. The effect of various sorbent preparation parameters, including copper loading, RHA/CaO ratio, hydration period and NaOH concentration, on the desulphurisation/denitrification capacity of the sorbents was studied using Design-Expert Version 6.0.6 software. Specifically, central composite design (CCD) coupled with response surface method (RSM) was used. The individual parameters that were found to significantly affect the sorbent capacity were RHA/CaO ratio and NaOH concentration. In addition, the interactive effect between RHA/CaO ratio, hydration period and NaOH concentration was also found to have a significant effect on the sorbent activity. The preparation condition for optimal sorbent activity was found to be CuO loading of 3.0%, RHA/CaO ratio of 1.4, hydration period of 20.0 h and NaOH concentration of 0.2 M. Characterisation of the sorbent was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and nitrogen adsorption-desorption method to describe the effect of the sorbent preparation parameters on its desulphurisation/denitrification activity.     

Mineralogy of western fly ash

Twenty-six fly ash specimens from North Dakota, Wyoming and Montana lignite and sub-bituminous source coals have been studied in detail by X-ray diffraction. Chemically, these western fly ashes are characterized by higher CaO+MgO+SO₃ contents and lower Al₂O₃+SiO₂ contents than eastern bituminous fly ashes. These western fly ashes have greater proportions of crystalline material. The characteristic phases are quartz, lime, periclase, anhydrite, ferrite spinel, tricalcium aluminate, merwinite and melilite. Alkali sulfates, a sodalite structure phase and hematite also occur in some fly ashes.     

Self-cementitious properties of fly ashes from CFBC boilers co-firing coal and high-sulphur petroleum coke

Self-cementitious properties of fly ash from circulating fluidized bed combustion boiler co-firing coal and high-sulphur petroleum coke (CPFA) were investigated. CPFA was self-cementitious which was affected by its fineness and chemical compositions, especially the contents of SO₃ and free lime (f-CaO). Higher contents of SO₃ and f-CaO were beneficial to self-cementitious strength; the self-cementitious strength increases with a decrease of its 45 μm sieve residue. The expansive ratio of CPFA hardened paste was high because of generation of ettringite (AFt), which was influenced by its water to binder ratio (W/A), curing style and grinding of the ash. The paste cured in water had the highest expansive ratio, and grinding of CPFA was beneficial to its volume stability. The hydration products of CPFA detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) were portlandite, gypsum, AFt and hydrated calcium silicate (C-S-H).     

Studies on blended cement with a large amount of fly ash

The influence of the contents of the clinker, activators and fly ash on the properties of blended cement with high fly ash content was studied. Experimental data from X-ray diffraction and pore size distribution indicated that the main hydration product of the fly ash blended cement was C-S-H gel, ettringite and a small amount of Ca(OH)₂. The volume porosity of the pores with diameter bigger than 0.1 μm was lower than that of the micro pores and gel pores with diameter lower than 0.05 μm. The amount of chemical combined water has increased with the curing age duration, while the content of Ca(OH)₂ has reduced after 7 days.     

Effects of flue gas desulphurization sludge on the pozzolanic reaction of reject-fly-ash-blended cement pastes

Reject fly ash (rFA), a coarse portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, has remained unused due to its high carbon content and large particle size (>45 μm). However, the reject ash may have potential uses in chemical stabilization/solidification (S/S) processes that require relatively low strength and low chemical reactivity. Flue gas desulphurization (FGD) sludge is a by-product of the air pollution control process in coal-fired power plants. Its chemical composition is mainly gypsum. As there is no effective usage of both of these materials, it was of interest to conduct research on the possible activation of rFA using FGD. This paper presents experimental results of the effect of FGD on the pozzolanic reaction of rFA-blended cement pastes with or without Ca(OH)₂ and chemical activators. The results show that FGD take effect as an activator only at late curing ages. Adding Ca(OH)₂ activates the hydration of rFA. Chemical activator, such as alkali sulphate, is more effective in enhancing the strength development and degree of hydration of rFA than CaCl₂ in the rFA-Ca(OH)₂-cement system. But CaCl₂ is more effective in the rFA-Ca(OH)₂-FGD-cement system. The chemical activators speed up the reaction of the rFA through the formation new hydration products and elevating the pH value.     

Continuous experiment regarding hydrogen production by Coal/CaO reaction with steam (II) solid formation

Integration of coal gasification and CO₂ separation reactions in one reactor may produce a high concentration of hydrogen. To design a reactor for this new reaction system, it is necessary to know all reaction behaviors in the integrated reaction system. In our previous study, we performed a continuous reaction experiment of coal/CaO under high steam pressure and confirmed that H₂ concentration higher than 80 vol% with little CH₄ was produced; and that almost all CO₂ was fixed by adding CaO. In this study, the behaviors of solid products during the continuous experiment were investigated. It was found that, CaO first reacted with high-pressure steam to form Ca(OH)₂ (hydration), then the Ca(OH)₂ absorbed the CO₂ generated by coal gasification to form CaCO₃. The hydration of CaO restored sorbent reactivity. Eutectic melting of Ca(OH)₂/CaCO₃ was found to occur in the experiment at 973 K, and this eutectic melting led to the growth of large particles of solid materials. However, at the relatively low temperature of 923 K, eutectic melting could be avoided. Carbon conversion of the coal in the continuous reaction of the coal/CaO mixture with steam was high as 60–80%, even at the lower temperature of 923 K.     

Effect of CaO content on hydration rates of Ca-based sorbents at high temperature

The CaO hydration behavior of four Ca-based sorbents (Deyang, Yangguan, Dawa, and Kuzuu) with different CaO contents was examined. A high-pressure thermogravimetric apparatus was used to measure the hydration rates at high temperatures (773–923 K) and high steam pressures (1.3–2.3 MPa). CaO hydration occurred at high temperatures up to 923 K at high steam pressure. The hydration rates and the final CaO conversion values varied substantially among the different sorbents. Hydration rate and CaO conversion decreased with increasing CaO content. The order of CaO hydration rate with respect to the difference between the reactant steam pressure and the equilibrium steam pressure (P_H2O − P^?_H2O) varied with CaO content. The apparent activation energy for rate constant k also varied with CaO content.     

Differential thermal method of estimating calcium hydroxide in calcium silicate and cement pastes

DTA is applied to estimate Ca(OH)₂ in cementitious phases by determining the peak areas caused by the decomposition of Ca(OH)₂ to CaO+H₂O. In the hydration of C₃S generally, the chemical method yields slightly higher values. DTA is also used as a monitoring technique in preparing a practically Ca(OH)₂-free product from hydrated portland cement or hydrated C₃S. Hydrated portland cement or C₃S has now been exposed to an unsaturated Ca(OH)₂ solution and extraction continued until the sample indicate no endothermal peak for Ca(OH)₂. The thermal method permits determination of the rate of formation of Ca(OH)₂ in portland cement hydrated in the presence of 0, 1, 2 and $\text{3}\text{1}\text{2}$ per cent CaCl₂.