Catalytic decomposition of ammonia over fly ash supported Ru catalysts

Fly ash (FA), an industrial waste material, has been treated by physical and chemical methods. These materials were then employed as supports for preparation of Ru-based catalysts for H₂ generation from ammonia decomposition. The physicochemical properties of the supports and Ru-based catalysts were characterised using several techniques. The results revealed that the surface area of FA could be enhanced and thus improved the dispersion of Ru particles, resulting in higher catalytic activity. Ru/FA-800 exhibits the highest conversion due to higher surface loading of Ru, stronger NH₃ adsorption and least acid sites.     

分光光度法快速测定粉煤灰水泥中粉煤灰含量

通过分光光度法,对粉煤灰水泥中Fe2O3和SO3总含量进行检测,后分别检测粉煤灰、熟料、脱硫石膏中的Fe2O3和SO3含量,最后通过混合材掺量计算公式,计算粉煤灰水泥中的粉煤灰含量。该检测方法准确性较高,并且检测快捷方便,检测结果绝对误差范围小于5%。     

The effect of BaCO3 addition on the sintering behavior of lignite coal fly ash

The effect of BaCO₃ (witherite) addition on the sintering behavior of lignite coal fly ash taken from the Seyitömer power plant of Kütahya/Turkey was examined at temperatures of 1100, 1150 and 1200 °C in air atmosphere. Bloating of the fly ash samples sintered at 1150 °C was prevented, that is, the decomposition temperature of CaSO₄ in the fly ash is shifted to a higher temperature, and their physico-mechanical properties (porosity, water absorption, bulk density and bending strength) were improved with BaCO₃ addition. Positive effects of BaCO₃, however, were not seen on the fly ash samples sintered at 1100 °C. All the fly ash samples sintered at 1200 °C were bloated due to the gas evolving and also they melted. During the thermal treatment at 1150 °C a phase transformation from CaSO₄ (anhydrite) to BaSO₄ (Barite) occurred in the fly ash with BaCO₃ addition as seen from the X-ray diffraction (XRD) patterns and the bar shaped fly ash samples with BaCO₃ saved their structural integrity up to 1150 °C.     

A new way to increase the long-term bond strength of new-to-old concrete by the use of fly ash

The short-term and long-term bond strengths of new-to-old concrete were experimentally investigated with an emphasis on the influence of new concretes and binders. These new concretes included ordinary Portland cement concrete, expansive concrete and high-volume fly ash concrete, while the binders included pure cement paste (C-binder), expansive binder (E-binder) and fly ash mortar (F-binder). The results showed that the short-term bond strength of all specimens with fly ash concrete was lower than that with ordinary Portland cement concrete, which in turn was lower than that with expansive concrete. The bond strength of the specimens with F-binder was the lowest at the age of 7 days. However, the long-term bond strength of all specimens with added fly ash was the highest and strength losses were observed in the specimens repaired with expansive concrete or E-binder at the age of 3 years. The microstructure of the transition zone with F-binder was also studied by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) at the ages of 28 days and 1 year, respectively.     

Influence of steam curing on the compressive strength of concrete containing supplementary cementing materials

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.     

Geochemistry of fly ash from desulphurisation process performed by sodium bicarbonate

The application of NEUTREC^® technology - desulphurisation by means of sodium bicarbonate - has been tested at the Trebovice coal-fired power plant (Ostrava, Czech Republic). This technology significantly influences the chemical composition of fly ash and the leachability of total dissolved substances (TDS), e.g., sulphates, fluorides and oxyanions (Se, Sb, Cr, As), which are monitored according to the Council of the European Union Decision 2003/33/EC. An increase of TDS in the water leachate from the fly ash obtained at 60% desulphurisation was influenced by sodium content, which is present in the form of Na⁺ ions (85-90%). The percentages of sodium sulphate and sodium carbonate were between 5 and 10% of the total sodium content. In order to decrease the leachability of TDS, sodium, sulphates and oxyanion mixtures were prepared containing a sorbent (60% bentonite) and mixed with desulphurised and non-desulphurised fly ash in various ratios. The addition of CaO resulted in the formation of a new mineral phase, burkeite. None of the applied technologies tested for the processed fly ash resulted in the preparation of a water leachate which complied in all monitored parameters to the requirements of Council Decision 2003/33 EC for nonhazardous wastes.     

Design of an optical instrument to measure the carbon content of fly ash

The polarisation ratio resulting from the backscattering of linearly polarised incident light by fly ash particles is indicative of the carbon content. Determination of this parameter is useful for characterising the efficiency of coal burning furnaces. This paper describes the development of an instrument to make these measurements, which is designed to be attached furnace ducts. The performance of the instrument is tested in laboratory measurements on a range of ashes from different coals. It is concluded that if the mineral content of the ashes is not known then the carbon mass fraction can be determined to within ±1%. If the mineral content is known then the use of neural network analysis can reduce this to ±0.5%.     

Improve the strength of concrete-filled steel tubular columns by the use of fly ash

Concrete-filled steel tubular columns (CFTs) are becoming widely used in engineering. In the present paper, the addition of fly ash and an expansive agent to the concrete of CFTs or a thin layer of fly ash to the interface between steel tube and concrete (CFTFCs) to improve the compressive strength and the bond strength of CFTs was experimentally investigated. The results show that the expansive concrete-filled steel tubular columns (CFETs) have the highest bond strength and compressive strength at the age of 7 days, and CFTFCs have higher bond strength and compressive strength than fly ash concrete-filled steel tubular columns (CFFTs), which in turn are higher than CFTs. However, both bond strength and compressive strength of CFTFCs become the highest at the age of 28 days. The morphology (size and shape) of mineralogy and microstructure of the interface at the age of 28 days were also investigated by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is shown that the strength improvement of CFTFCs mainly depends on the content of SiO₂ and CaO in the interface, and higher content of SiO₂ and/or lower content of CaO are preferred.     

Pozzolanic reactivity of fly ash - API method and K-value

The pozzolanic reaction of fly ash in mortar was primarily examined using a K-value from Ferét’s law. As a result, the pozzolanic reaction would be mainly controlled by the diffusion kinetic, and the diffusion constant would increase with water content in the mortar. The K-value is useful to estimate the degree of the pozzolanic reaction of mortar. Secondly, the relation between K-value and assessed pozzolanic activity index derived from accelerated chemical test (API method) was inspected. It is concluded that the API is one of the useful acceleration methods to evaluate the degree of the pozzolanic activity, because API has a good relation with K-value.     

Carbonation of fly ash in oxy-fuel CFB combustion

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO₂ ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO₂ partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO₂ concentrations of 80-85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO₂ concentration set at 80% and H₂O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H₂O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ? 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H₂O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H₂O. For T ? 400 °C, no fly ash carbonation occurred without the presence of H₂O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ? 300 °C, small amounts of Ca(OH)₂ were found in the final product alongside CaCO₃. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.     

Influence of the co-firing on the leaching of trace pollutants from coal fly ash

The (co)-firing of low-cost alternative fuels is expected to increase in the forthcoming years in the EU because of the economic and environmental benefits provided by this technology. This study deals with the impact of the different coal/waste fuel ratio of the feed blend on the mineralogy, the chemical composition and especially on the leaching properties of fly ash. Different blends of coal, petroleum coke, sewage sludge, wood pellets, coal tailings and other minor biomass fuels were tested in PCC (pulverised coal combustion) and FBC (fluidized bed combustion) power plants. The co-firing of the studied blends did not drastically modify the mineralogy, bulk composition or the overall leaching of the fly ash obtained. This suggests that the co-firing process using the alternative fuels studied does not entail significant limitations in the re-use or management strategies of fly ash.     

Experimental study of chemical treatment of coal fly ash to reduce the mobility of priority trace elements

Coal fired electric power plants produce large volumes of fly ash and other coal combustion by-products (CCBs) every year. Although almost 50% of the fly ash produced in the US is recycled for beneficial use, most of the ash material is disposed in dry landfills and ash lagoon impoundments. Fly ash may contain hazardous leachable trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V which have a negative impact on the environment due to potential leaching by acid rain and groundwater with time. Many of the older CCB disposal facilities are unlined and unmonitored and as a result the EPA is currently developing national standards for monitoring CCB disposal sites. The cost to the US electric power industry could exceed one billion dollars if existing and closed CCB disposal facilities come under regulation. Thus simple, low-cost and effective in situ chemical treatment techniques are needed to stabilize hazardous leachable trace elements in the coal combustion by-product (CCB) materials. This paper reports the results of experiments designed to chemically treat fly ash with ferrous sulfate solutions to immobilize hazardous leachable trace elements after disposal.The current study is focused on three acidic and one alkaline fly ash samples collected from electric power plants located in the southeastern United States that were treated with two ferrous sulfate treatment solutions. The first treatment solution contained ferrous sulfate (FS) to give 322 mg/L of dissolved iron, while the second treatment solution contained the same concentration of ferrous sulfate along with excess calcium carbonate (FS + CC) to buffer the pH. Fly ash treatment experiments were carried out at solid:liquid (S:L) ratios of 1:3 and 1:30. The effectiveness of the treatment methods was evaluated by sequentially leaching the treated and the untreated fly ash samples using a synthetic acid rain (SAR) solution (US EPA Method 1312B SPLP fluid) as the leachate. The best overall treatment result was shown by the unbuffered ferrous sulfate solution at the 1:30 S:L ratio, which substantially reduced the mobility of the oxyanion trace elements. The overall mobility reduction achieved for As was 23-72%, B mobility was reduced by 43-80%, Cr by 45-77%, Mo by 21-90%, Se by 41-85% and V by 41-53% The unbuffered ferrous sulfate treatment was not effective for immobilization of the cationic trace elements Ni and Sr.     

粉煤灰对混凝土拌合物泵送性能的影响

介绍了粉煤灰对掺有M17塑化剂的混凝土拌合物泵送性能和混凝土强度的影响，通过扫描电镜对粉煤灰颗粒形貌进行了观察，初步探讨了粉煤灰的作用机理。试验说明在接有M17塑化剂的泵送混凝土中加入粉煤灰，可增加坍落度，降低泌水率，泵逆性能可得到改善。     

Anomalous fluorescence in near-infrared Raman spectroscopy of cementitious materials

The results of near-infrared Fourier transform Raman spectral examination of cement minerals, several commercial Portland cements, and hydrating Portland cements are critically examined. It is shown that structured fluorescent effects dominate the observed spectra for C₃S, C₂S, and cement, and none of the bands generated by this mode of examination is a true Raman band. The apparent bands for the Portland cements are in different positions to those for the individual cement minerals. The fluorescence-derived spectra for different cements are similar to each other but vary enormously in intensity for different cements. Hydration progressively reduces the intensity of the bands, but does not generate bands at new locations. It is tentatively suggested that the fluorescence effect may be somehow associated with the status of the cement components as orthosilicates (i.e., composed of isolated silica tetrahedra. Samples of high-purity C₂S exhibit the fluorescence effect, but samples of CS (wollastonite) of similar purity do not. The latter are metasilicates (i.e., composed of linked chains of silica tetrahedra). They exhibit normal Raman Stokes and anti-Stokes bands.     

A new technique to reduce the radioactivity of fly ash utilized in the construction industry

High volume utilization of industrial wastes and by products is the solution for high disposal costs. Acceptable radioactivity levels in addition to other environmental factors is a key factor for safe utilization of wastes and byproducts of coal burning power plants. In general the radioactivity levels of most fly ashes are similar to natural materials. For higher radioactivity fly ash the radioactivity values must be reduced to acceptable limits. This can be done by mixing the fly ash with less radioactive natural materials. In this study a new technique involving the use of snow as an additive to the compaction water of fly ash is presented. Fly ash at optimum water content, and fly ash with additional 10% by weight snow are compacted, hermetically sealed to allow for equilibrium of ²²⁶Ra and ²³²Th with their decay products and cured for 28 days at the curing room. Radioisotope activity analysis are conducted with a gamma analyst integrated gamma spectrometer. The activities of ²³⁵U, ²²⁶Ra, ²³⁸U, and ²³²Th of the fly ash and snow-added fly ash samples compacted at optimum moisture content are determined. The control samples revealed radioactivity values above UNIPEDE maximum allowable limits. Addition of snow caused a decrease of 31-42% in the radioisotope activity levels to that of control samples in Bq kg⁻¹.The decrease in radioactivity is linked to increased void ratio after melting of ice, increased densification of matrix around the pores due to higher level of cementitious mineral formation. The decrease in the radioisotope activity levels will allow utilization of fly ash in highway embankment construction where large surface area exposure and large volume usage makes it more critical for human health. Another advantage of the developed technology is the reduction of transportation costs by more than ten per cent by using less material for construction.     

Microanalysis of alkali-activated fly ash-CH pastes

Samples of a Class F fly ash and calcium hydroxide (CH) hydrated in pH 13.2 sodium hydroxide solution were analyzed using backscattered electron, scanning Auger, and X-ray microanalysis. The Class F fly ash, composed mainly of aluminosilicate glass and silica, was reacted for 8, 14, and 78 days at various temperatures. These samples represent both long-term and early-age stages of hydration. Results show that a hydrate product with calcium to silica ratio near 1.4 and katoite are formed. X-ray and scanning Auger microanalysis show evidence of the formation of hydrate product on the surface of both fly ash and CH particles at early ages. This finding suggests a new mechanism to explain prior data that shows that the hydration rates increase with increasing CH-ash content in the starting mixture.     

The role of sulfide in the immobilization of Cr(VI) in fly ash geopolymers

The use of fly ash-based geopolymer binders to immobilize chromium is investigated in detail, with particular regard to the role of the sulfide ion as a reductant for Cr(VI) treatment. In the absence of sulfide, Cr added as Cr(VI) is highly leachable. However, addition of a small quantity of Na₂S reduces the Cr to Cr(III), and enables leaching efficiencies in excess of 99.9% to be reached after 90 days' exposure to deionized water, Na₂CO₃ or MgSO₄ solutions. Leaching in H₂SO₄ is somewhat greater than this, due most probably to the oxidation of the Cr(III) present. Addition of the Cr(VI) as a highly soluble salt is preferable to its addition as a sparingly soluble salt, because a higher salt solubility means the Cr(VI) is more available for reduction prior to geopolymeric setting. The potential value of geopolymer technology as an immobilization process for problematic heavy metal waste streams is highlighted by these results, and the need for a full understanding of binder chemistry in any immobilization system outlined.     

Effect of vermicomposting on the transformation of some trace elements in fly ash

One major constraint of the agricultural uses of fly ash (FA) is the low availability of different plant nutrients despite their high occurrence in the total amount. However, degrading FA through increased microbial activity can improve the availability of these nutrients substantially. It has been found that intestines of epigeic earthworms contain a high concentration of different microorganisms. Therefore, in the present study we addressed the effects of vermicomposting technology on the solubility of some micronutrient cations (Fe, Mn, Cu, and Zn) and some heavy metals (Pb, Cd, and Cr) in different combinations of fly ash and organic matter, applied in the form of cow dung (CD). Various combinations of FA and CD were treated with and without an epigeic earthworm (Eisenia fetida) and the solubility of different trace elements in the treatments were estimated periodically. The results revealed that the inclusion of epigeic earthworm Eisenia fetida in different combinations of fly ash and cow dung converted a considerable amount of the micronutrients into bio-available forms. On the other hand, the solubility of heavy metals tended to be reduced by the microorganisms, presumably by formation of some organo-metallic complex. Application of these vermicomposted FA and CD combinations to a red lateritic soil was found to improve the soluble Fe, Mn, Cu, and Zn status of the soil. Furthermore, the use of vermicomposted FA and CD (1:1) in potato cultivation demonstrated that use of this mixture at 10 ton per hectare (t ha⁻¹; fresh weight) was able to compensate 80% of the recommended NPK fertiliser, along with farm yard manure application, without compromising the crop yield.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

Pozzolanic activity of clinoptilolite: A comparative study with silica fume, fly ash and a non-zeolitic natural pozzolan

Pozzolanic activity of clinoptilolite, the most common natural zeolite mineral, was studied in comparison to silica fume, fly ash and a non-zeolitic natural pozzolan. Chemical, mineralogical and physical characterizations of the materials were considered in comparative evaluations. Pozzolanic activity of the natural zeolite was evaluated with various test methods including electrical conductivity of lime-pozzolan suspensions; and free lime content, compressive strength and pore size distribution of hardened lime-pozzolan pastes. The results showed that the clinoptilolite possessed a high lime-pozzolan reactivity that was comparable to silica fume and was higher than fly ash and a non-zeolitic natural pozzolan. The high reactivity of the clinoptilolite is attributable to its specific surface area and reactive SiO₂ content. Relatively poor strength contribution of clinoptilolite in spite of high pozzolanic activity can be attributable to larger pore size distribution of the hardened zeolite-lime product compared to the lime-fly ash system.