Role of active sites in the steam activation of high unburned carbon fly ashes

Previous studies on carbon gasification have not included high unburned carbon content fly ashes, and therefore it remains unclear why not all fly ash carbon samples are equally suitable for activation. The concentration of active sites is well known to influence carbon gasification reactions. Therefore, the objective of this work was to investigate the effect of the concentration of active sites on the behavior of fly ash carbon samples upon steam activation. Six fly ash carbons were selected to produce activated carbons using steam at 850 °C. The concentration of active sites was determined by non-dispersive infrared analysis (NDIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD analyses were also conducted to determine the crystallite size. It was observed that the concentration of active sites played a more significant effect on the surface areas of activated carbons in the carbon burn-off zone of >60%. Statistical analysis was used to relate the surface areas of activated carbon variances with carbon burn-off levels.     

Influence of iron species present in fly ashes on mercury retention and oxidation

The aim of this study was to assess the effect of iron species present in fly ashes produced from coal combustion on mercury retention and oxidation. To achieve this objective the work was divided into two parts. In the first part the relationship between the mercury and iron content in fly ashes of different origin and characteristics was evaluated. In the second, a series of fractions enriched in iron oxides were separated from the fly ashes to determine the effect of increasing iron content on mercury retention and oxidation. Special attention was paid to the influence of iron on mercury behavior in enriched carbon particles in fly ashes. From the results obtained it can be inferred that, in the range of fly ashes studied, iron species do not affect the retention of mercury and do not play any role in heterogeneous mercury oxidation.     

Mercury and selenium retention in fly ashes: Influence of unburned particle content

Mercury and selenium are present as trace elements in coal and may be emitted to the environment in gas phase during coal conversion processes or be partially retained on the fly ashes. The present work explores the possibility that selenium may contribute to mercury capture in fly ashes in two different situations: firstly the power station itself, in order to evaluate the influence of typical working conditions, and secondly in a fixed bed of fly ashes enriched with Se, in order to study the capture of mercury in more severe conditions. It was found that the presence of selenium in fly ashes may improve their capacity to capture mercury. However, in the four fly ashes of different origin studied, selenium is not the most important component for mercury retention. In fact, the presence of selenium in fly ash samples enriched in unburned carbon does not have any significant effect on mercury retention.     

Classification of carbon in Canadian fly ashes and their implications in the capture of mercury

Fly ashes produced from Canadian power plants using pulverized coal and fluidized bed combustors were examined for their carbon content to determine their ability to capture mercury. The feed coal used in these power plants were lignite, subbituminous, high and medium volatile bituminous, their blends, and also blends of coal with petroleum coke (Petcoke). The carbon and mercury content of the coals and fly ashes were determined using the ASTM standard method and by the cold vapour atomic absorption spectrometry method. The carbon content of the fly ash was concentrated by strong acid digestion using HCl and HF. The quantitative and qualitative analyses of the carbon concentrate were made by using a reflected light microscope. The results show that the carbon content of fly ash appears to be partially related to depositional environment during coalification and to the rank of the coal. The Hg captured by the fly ash depends on the rank and blend of the feed coals and the type of carbon in the fly ash. The isotropic vitrinitic char is mostly responsible for the capture of most Hg in fly ash. The inadvertent increase in carbon content due to the blending of coal with petroleum coke did not increase the amount Hg captured by the fly ash. The fly ash collected by the hot side electrostatic precipitator has a low Hg content and no relation between the Hg and carbon content of the ash was observed. These results indicate that the quantity of carbon in the fly ash alone does not determine the amount Hg captured. The types of carbon present (isotropic and anisotropic vitrinitic, isotropic inertinitic and anisotropic Petcoke), the halogen content, the types of fly ash control devices, and the temperatures of the fly ash control devices all play major roles in the capture of Hg.     

Activated carbon for mercury control: Implications for fly ash management

As more utilities begin to use activated carbon injection (ACI) for mercury control, the potential for the presence of elevated concentrations of mercury, other air toxic elements, and activated carbon to impact fly ash management needs to be evaluated. Several Energy & Environmental Research Center (EERC) projects have allowed the collection of comparative baseline fly ash samples and associated fly ash–activated carbon (AC) samples from full-scale demonstrations of ACI for mercury emission control. These samples were evaluated for mercury and air toxic element content and mobility and for performance criteria to facilitate a better understanding of the impact of these components to specific utilization applications, including use as a mineral admixture in concrete. These data are compared with published data from samples collected at similar large-scale mercury emission control tests.     

Speciation of mercury in fly ashes by temperature programmed decomposition

Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen to identify the Hg species present in fly ashes obtained from two Pulverized Coal Combustion (PCC) plants and a Fluidized Bed Combustion (FBC) plant. The fly ashes were then used as Hg sorbents in a simulated flue gas of coal combustion and gasification. The Hg compounds found in the fly ash from the FBC plant after elemental mercury retention were mainly HgCl₂ and HgSO₄. The Hg species present in the two fly ashes from the two PCC plants were HgCl₂ and Hg⁰. The Hg species formed in the coal gasification atmosphere was HgS for all three fly ashes. The only Hg compound identified in the fly ashes after the retention of mercury chloride was HgCl₂.     

Effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample

The effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample has been investigated. The samples were tested for mercury adsorption using a fixed‐bed with a simulated flue gas. The activated fly ash carbon sample has lower mercury capacity than its precursor fly ash carbon (0.23 vs. 1.85 mg/g), although its surface area is around 15 times larger, 863 vs. 53 m²/g. It was found that oxygen functionality and the presence of halogen species on the surface of fly ash carbons may promote mercury adsorption, while the surface area does not seem to have a significant effect on their mercury capacity.     

Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content

The influence of selected properties of fly ash on the measurements of an on-line analyser was described. Fly ash studied in the research originated from the pulverized coal fired boiler. The samples were taken using an inspection method within a period of 3 months. Systematic observation of the properties of the ash allowed monitoring of the work of the industrial analyzer during a relatively long period of the power plant work. Samples of coal fly ash were examined for their chemical and physical properties. Morphology was analysed by scanning electron microscopy. Unburned carbon content in fly ash was determined by using loss-on-ignition (LOI) and thermogravimetry analysis (TGA). The particle size distribution of fly ash was examined. Correlation between laboratory and on-line industrial measurements of the unburned carbon content of ash was discussed.     

Microwave attenuation characteristics of unburned carbon in fly ash

Experimental research was conducted on the relationship between the unburned carbon content in fly ash and the microwave power attenuation at frequencies ranging from 1.0 to 4.0 GHz. The experimental results show that the unburned carbon in fly ash exhibits the frequency characteristics of microwave power attenuation. At some frequencies, the microwave power attenuation increases with the carbon content. At other frequencies, the relationship between them changes. There is also a range of frequencies where the loss decreases while the unburned carbon content increases. A detailed analysis on the mechanisms of this phenomenon is made based on the conductivity loss, absorption and dispersion theories.     

Thermal stability of mercury captured by ash

The thermal stability of mercury captured by ash was studied by sampling ash throughout the collection train of two Kentucky power plants. Sampling occurred over multiple years and involved both fresh and archived samples. During one ash collection episode, sampling was from the combustion of a single pulverized coal feed. The other collections involved ash from blended feeds. Ash was collected from economizer, mechanical and electrostatic precipitator hoppers. Feed coals, rejects and bottom ash were also sampled. Fractions of all the samples were heated in a thermal analyzer to maximum temperatures increased sequentially from 100 to 500 °C in 100 °C increments. The mercury content of the spent material was then determined by analysis of the solids for Hg. From this data the thermal decomposition temperature of the captured mercury was determined. The total mercury captured by each sample, thermal stability of the mercury in relation to collection site, and correlations between mercury capture and chemical composition of the sample were also determined. The data showed that mercury was released between 300–400 °C for all ash samples. The thermal release of Hg between 300–400 °C was studied in greater detail by following the Hg release in several samples at 25 °C intervals from 300–400 °C. The concentration of mercury captured in the ESPs hoppers was greater than in the ash collected from the economizer or mechanical separators.     

粉煤灰合成SiAlON粉体研究

以粉煤灰、石墨粉、氧化铝、二氧化硅等为主要原料,采用碳热还原氮化法合成了SiAlON粉体。研究了Fe2O3含量不同的粉煤灰,即未除铁粉煤灰(Fe2O3的质量分数为4.16%)、酸洗除铁后的粉煤灰(Fe2O3的质量分数几乎为0)和磁选除铁后的粉煤灰(Fe2O3的质量分数为3.46%),配料的m(SiO2)∶m(Al2O3)(分别为1.176、1.55和2.35),碳加入量(分别为理论加入量、过量10%、过量100%和过量150%)以及反应温度(分别为1350℃、1400℃、1420℃、1430℃和1460℃)对合成产物相组成的影响。研究表明:以经过磁选除铁后的粉煤灰(Fe2O3的质量分数为3.46%)为原料,当配料的m(SiO2)∶m(Al2O3)为1.4,碳粉加入量为理论值的2倍(即过量100%)时,于1420℃保温20h合成的产物的主要物相为βSiAlON。     

Experimental and theoretical investigation on unburned coal char burnout in a pilot-scale rotary kiln

Oxidation reactivity studies are imperative for improving carbon re-burn technologies and valuing the heat content of unburned carbon within coal combustion ashes. Non-isothermal, thermal gravimetric analysis (TGA) was used to examine the oxidation kinetics of unburned carbon in coal combustion fly ashes having different particle size distributions; TGA results were related to combustion efficiencies as measured in a bench-scale rotary kiln. The activation energy and pre-exponential factor were determined for the chemically-controlled reaction regime; the transition temperatures between chemically-controlled and partially diffusion-controlled combustion regimes were obtained for unburned carbon particles of different sizes. After the oxidation reaction rates were evaluated, the residence time distribution (RTD) of fly ashes in the rotary kiln were experimentally measured and the mean residence times related to process parameters, including the rotating velocity and kiln inclination. By comparing these results with an advective-dispersive model, the axial dispersion coefficient of fly ashes was determined. The reaction rates obtained by thermal analyses and the RTDs were used to predict combustion efficiencies within the kiln and oxidation conditions of unburned carbon using various processing options.     

Characterisation of fly ashes. Potential reactivity as alkaline cements

A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and ²⁹Si MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution.     

Removal of mercury in aqueous solution by fluidized bed plant fly ash

Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg²⁺) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-aluminous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes.     

Size distribution of unburned carbon in coal fly ash and its implications

A set of nine coal fly ashes, obtained from various US utilities, were fractionated by standard dry-sieving techniques. The carbon contents of the different size fractions were measured, and the nature of the carbon particles was microscopically examined. Significant differences were found in the distribution of carbon in class F and class C ashes. The ‘foam index’ test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. This test measures adsorption of air entraining admixtures (AEAs) by the carbon in the fly ash. Application of this test to the different ash fractions confirmed that the smallest particle size fractions of ash make the major contribution to AEA adsorption. The carbon from class F ash has a comparable capacity for AEA adsorption as carbon from class C ash, when compared on a surface area basis. What makes the class C carbons apparently ‘worse’ is the fact that they have much higher surface areas than class F carbons (and it is only by virtue of the low carbon mass in most class C ashes that problems with these ashes are not more common). The importance of accessibility of the surface is also clearly seen from these results.     

Physico-chemical characteristics of European pulverized coal combustion fly ashes

Fly ashes sourced from European pulverised coal burning power plants (from Spain, The Netherlands, Italy and Greece) were characterised in terms of their chemical composition, mineralogy and physical properties. The amount and composition of the glass present in the ashes were also determined. The materials analysed have very different compositions and were selected with a view to determining their suitability for different applications and for further studies on applications. The results were compared to the literature to determine their similarities to UK coal fly ashes. Chemical analysis has enabled the categorisation of the ashes based on their oxide contents. Devitrification of the glass phase has been effected using suitable heat treatments and crystal phases formed are used as an indicator of glass reactivity. Based on leaching tests, certain ashes were identified as having limitations for some further uses due to the relatively high levels of leachable trace elements. A wide range of physical properties such as density were observed and these are related to factors such as mineralogical content and particle morphology.     

Role of activated carbon structural properties and surface chemistry in mercury adsorption

In this work the performance of activated carbons prepared from raw and demineralised lignite for gas-phase Hg° removal was evaluated. A two-stage activation procedure was used for the production of the activated samples. In order to study the effect of mineral matter on pore structure development and surface functionality of the activated carbons, a demineralisation procedure involving a three-stage acid treatment of coals, was used, prior to activation. Hg° adsorption tests were realized in laboratory-scale unit consisted of a fixed-bed reactor charged with the tested activated samples. The examined adsorbent properties that may affect removal capacity were the pore structure, the surface chemistry and the presence of sulphur on the surface of activated carbons. The obtained results revealed that activated carbons produced from demineralised lignite posses a high-developed micropore structure with increased total pore volume and BET surface area. These samples exhibit enhanced Hg° adsorptive capacity. In all cases, mercury removal efficiency increased by sulphur addition. Finally, the starting material properties and activation conditions affect the concentration and the type of the oxygen groups on activated carbon surface, that have been determined with TPD-MS experiments.     

The impact of fly ash resistivity and carbon content on electrostatic precipitator performance

Despite the wide use of electrostatic precipitators for particulate control in pulverised coal combustion, there are still some aspects surrounding the performance of these devices which are not fully understood, particularly with respect to high carbon ash. The introduction of low NO_x burners to meet more stringent emissions regulations has also resulted in an increase in the carbon present in fly ash. An investigation was carried to assess how the operating conditions of a pilot-scale single-stage electrostatic precipitator test facility (ESPTF) affected the collection efficiency of two high carbon fly ashes. Whilst our findings demonstrated the effectiveness of the ESPTF in collecting both fly ash and carbon particles, the performance of the high-resistivity fly ash was significantly better than expected, particularly at higher temperatures. It was concluded that ESP performance of any particular fly ash and the separation of the carbon fraction may not necessarily be predicted by its resistivity alone.     

Role of unburnt carbon in adsorption of dyes on fly ash

Various fly ash samples with different unburnt carbon contents were collected, characterised and tested for adsorption of basic dyes, Methylene Blue and Crystal Violet, in aqueous solution. It was found that unburnt carbon plays a major role in dye adsorption. The mineral matter of fly ash has little adsorption capacity and most of the adsorption capacity of fly ash can be attributed to the unburnt carbon. The fly ash with higher unburnt carbon content will have higher adsorption capacity. For the carbon‐free fly ash, adsorption capacities for Methylene Blue and Crystal Violet are only 2 × 10^?6 mol g^?1 and 1.0 × 10^?6 mol g^?1, respectively, while the adsorption capacities for Methylene Blue and Crystal Violet on carbon‐enriched fly ash are 1.2 × 10^?4 mol g^?1 and 1.0 × 10^?4 mol g^?1, respectively. A two‐site Langmuir adsorption model best describes the adsorption isotherm. Copyright © 2005 Society of Chemical Industry     

Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization. 1. Characterization of feed coals and fly ashes

The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide-sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite-muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite-metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.