Reduction in Powder Wall Friction by an a-C:H:Si Film.

The wall friction angle is an important parameter in powder flow. In a recent study for various powders, a reduction in the wall friction angle for steel was demonstrated by the application of an a-C:H:Si film on the steel surface. This work presents the results of a study of this effect in more detail regarding the influence of the powder material, the wall normal stress and the particle size of the powder for mass median diameters from 4 µm to approximately 150 µm. The wall friction angles were measured using a Schulze ring shear tester for three different powder materials: aluminum oxide, calcium carbonate and silicon carbide. The results showed little difference with respect to powder chemistry. For the coarser powders, the reduction in the wall friction angle due to the a-C:H:Si coating was highest (10° to 12°) and rather stress-independent, while for the fine and medium-size powders the reduction was lower and stress-dependent. With increasing wall normal stress, the reduction in the wall friction angle increased. These results can be explained by the friction reduction mechanism of a-C:H:Si, which requires a certain contact pressure for superficial graphitization.

Variation in the Concentration of Metals in Road Dust Size Fractions Between 2 µm and 2 mm: Results from Three Metallurgical Centres in Poland.

The composition of road dust is influenced by emissions from local industry as well as by traffic emissions. Thus, the composition of urban road dust can be used as an indicator for environmental pollution. Pollutants contained in road dust also are transferred into the atmosphere by resuspension and into the aquatic system by wash-off. In this transfer, the particle size of the road dust particles is of extreme importance. Therefore, information about the composition of road dust in dependence of the particle size is crucial. In this study, road dust samples were separated by air classification into size fractions down to 2 µm. The chemical analysis of the size fractions also revealed a significant size dependence of the metal concentrations in the finest size fractions. The least polluted size fraction was generally the fraction 200-500 µm, whereas the highest concentrations were measured in the finest size fraction < 2 µm. These results are important for the assessment of the mass fraction of the various pollutants in the mobile size fractions in re-entrainment as well as in run-off during rainfall.

The influence of air-dispersed essential oils from lemon (Citrus limon) and silver fir (Abies alba) on airborne bacteria and fungi in hospital rooms.

Airborne bacteria and fungi are an ongoing problem in hospitals. Because of the antimicrobial activities of essential oils (EOs) dispersion of EOs into the air may help to reduce this contamination. The aim of this study was to evaluate the efficacy of the dispersion of selected EOs in reducing the microbial contamination in two hospital wards. The study was carried out at two wards of a 1,227-bed acute-care hospital in Austria. The concentration of airborne bacteria and fungi was measured in patient rooms before and after dispersion of a mixture of Citrus limon EO and Abies alba EO. Before dispersion of the EOs in both wards the mean concentration of bacteria was in a typical range (123 colony forming units (CFU) m-3 and 104 CFU m-3) while the mean concentration of fungi differed substantially (155 CFU m-3 and 28 CFU m-3). After dispersion of the EOs, a reduction in both bacterial and fungal contamination was observed. In the first two hours the mean concentration of airborne bacteria and fungi was reduced by approximately 40% and 30%-60% respectively. The selected EO mixture is effective in reducing the microbial contamination of the indoor air.

The upper size limit of the dust samples in road dust heavy metal studies: Benefits of a combined sieving and air classification sample preparation procedure.

The concentrations of heavy metals and other components in road dust from a Polish coal mining district were investigated. The sample was split into eight size fractions ranging from <5 µm to 1-2 mm applying the combined classification procedure of dry sieving and air classification. For most elements, significantly higher concentrations were found in the finest size fractions. In combination with the mass fractions of the various size fractions the concentrations of the components were calculated for this road dust assuming different upper size limits. The results demonstrate the important influence of the upper size limit applied on the composition of road dust. An analysis of the composition of road dust by particle size, instead of analysing whole road dust samples, proves to have several advantages. On the one hand, a reliable comparison of the results with the concentration data available in the literature is possible and on the other hand, the results can be used in assessments, for example resuspension or wash-off by rainfall, which require different upper size limits. Upper size limit applied in road dust sampling significantly influences the results. Combined sieving and air classification sample preparation procedure allows flexible use of the results.

Heavy metals in the finest size fractions of road-deposited sediments.

The concentration of heavy metals in urban road-deposited sediments (RDS) can be used as an indicator for environmental pollution. Thus, their occurrence has been studied in whole road dust samples as well as in size fractions obtained by sieving. Because of the limitations of size separation by sieving little information is available about heavy metal concentrations in the road dust size fractions <20 µm. In this study air classification was applied for separation of dust size fractions smaller than 20 µm from RDS collected at different times during the year. The results showed only small seasonal variations in the heavy metals concentrations and size distribution. According to the Geoaccumulation Index the pollution of the road dust samples deceased in the following order: Sb ¼â€¯As > Cu ≈ Zn > Cr > Cd ≈ Pb ≈ Mn > Ni > Co ≈ V. For all heavy metals the concentration was higher in the fine size fractions compared to the coarse size fractions, while the concentration of Sr was size-independent. The enrichment of the heavy metals in the finest size fraction compared to the whole RDS <200 µm was up to 4.5-fold. The size dependence of the concentration decreased in the following order: Co ≈ Cd > Sb > (Cu) ≈ Zn ≈ Pb > As ≈ V ¼â€¯Mn. The approximation of the size dependence of the concentration as a function of the particle size by power functions worked very well. The correlation between particle size and concentration was high for all heavy metals. The increased heavy metals concentrations in the finest size fractions should be considered in the evaluation of the contribution of road dust re-suspension to the heavy metal contamination of atmospheric dust. Thereby, power functions can be used to describe the size dependence of the concentration.

Chemical composition and properties of ashes from combustion plants using Miscanthus as fuel.

Miscanthus giganteus is one of the energy crops considered to show potential for a substantial contribution to sustainable energy production. In the literature there is little data available about the chemical composition of ashes from the combustion of Miscanthus and practically no data about their physical properties. However, for handling, treatment and utilization of the ashes this information is important. In this study ashes from two biomass combustion plants using Miscanthus as fuel were investigated. The density of the ashes was 2230±35kg/m3, which was similar to the density of ashes from straw combustion. Also the bulk densities were close to those reported for straw ashes. The flowability of the ashes was a little worse than the flowability of ashes from wood combustion. The measured heavy metal concentrations were below the usual limits for utilization of the ashes as soil conditioner. The concentrations in the bottom ash were similar to those reported for ash from forest residue combustion plants. In comparison with cyclone fly ashes from forest residue combustion the measured heavy metal concentrations in the cyclone fly ash were considerably lower. Cl-, S and Zn were enriched in the cyclone fly ash which is also known for ashes from wood combustion. In comparison with literature data obtained from Miscanthus plant material the concentrations of K, Cl- and S were lower. This can be attributed to the fact that the finest fly ash is not collected by the cyclone de-dusting system of the Miscanthus combustion plants.

Variations in the composition of house dust by particle size.

In this study, the distribution of heavy metals and other components in the various size fractions of house dust is investigated. A house dust sample collected from a vacuum cleaner was separated into size fractions by sieving and air classification. The analysis of the size fractions showed that the heavy metals and other components are not uniformly distributed in the various size fractions. The highest total carbon concentrations were found in the size fractions with a mass median diameter of 18-95 µm, while in the coarser size fractions and in the finest size fraction, the total carbon concentration was lower. In contrast, for many heavy metals and other metals (Al, Fe, Ca, S, Mn, Ti, Ba, Sr, As, Co, and V), the maximum concentrations were found in the finest size fraction. With increasing size of the dust fractions, the concentrations decreased. For several of these components, the dependence of the concentration on the particle size can be approximately assessed well using a power function. The distribution of Zn, Cu, Mg and Na was different. While the concentration of Na and Mg was higher in the coarser size fractions, no distinct trend was found for the concentrations of Cu and Zn.

Characterization of dust from blast furnace cast house de-dusting.

During casting of liquid iron and slag, a considerable amount of dust is emitted into the cast house of a blast furnace (BF). Usually, this dust is extracted via exhaust hoods and subsequently separated from the ventilation air. In most BFs the cast house dust is recycled. In this study a sample of cast house dust was split by air classification into five size fractions, which were then analysed. Micrographs showed that the dominating particle type in all size fractions is that of single spherical-shaped particles. However, some irregular-shaped particles were also found and in the finest size fraction also some agglomerates were present. Almost spherical particles consisted of Fe and O, while highly irregular-shaped particles consisted of C. The most abundant element was Fe, followed by Ca and C. These elements were distributed relatively uniformly in the size fractions. As, Cd, Cu, K, Pb, S, Sb and Zn were enriched significantly in the fine size fractions. Thus, air classification would be an effective method for improved recycling. By separating a small fraction of fines (about 10-20%), a reduction of the mass of Zn in the coarse dust recycled in the range of 40-55% would be possible.

Grate-Fired Biomass Combustion Plants Using Forest Residues as Fuel: Enrichment Factors for Components in the Fly Ash.

The aim of this work is to investigate the enrichment factors for various nutrients and heavy metals in the fly ash from grate-fired combustion plants using forest residues as fuels. Sustainable energy production requires recycling of the ash on the soil to close the nutrient cycles. The coarser bottom ash which is discharged from the boiler usually contains lower amounts of heavy metals compared to the fly ash which is separated from the off-gas. The discharge of the finest fly ash to landfill sites serves to remove the unwanted heavy metals from the cycle. For this purpose, the enrichment of these components in the finest fly ash should be maximized. At the same time the enrichment of the nutrients in the fine fly ash should be minimal. The enrichment of the components in the electrostatic precipitator fly ash of three biomass combustion plants was determined. The enrichment of the critical heavy metals Cd, Pb and Zn in the fly ash was higher than the values reported in one study but less than values calculated from the data presented in another study. Further investigations would be required to clarify this deviation. Thereby additional data on the furnace operation conditions e.g. combustion temperature should be included. The enrichment of most nutrients (Ca, Mg and PO4 3-) in the fly ash was low. For K the enrichment in the fly ash was higher resulting in notable K losses to the fly ash.

Characterization of sintering dust collected in the various fields of the electrostatic precipitator.

Sinter plant off-gas is usually de-dusted by electrostatic precipitators. Compliance with the dust emission limits is often difficult because of the high specific resistivity of the emitted dust. Mechanical properties of the dust are also relevant for the electrostatic precipitator design. Dust samples from the four consecutive electrostatic precipitator fields were characterized in this study. Most measured parameters showed a considerable variation in the various dust samples. The particle size of the dust as well as its bulk density continuously decreased from the first field to the fourth field. The flowability of the dusts was generally bad and decreased from the first to the last field. In contrast, the wall friction angles with structural steel were quite constant at approximately 30°. The Fe content was lower in the dust from the last two fields while the concentration of K, Na, Cl(-) and [Formula: see text] was significantly higher. At the same time the particle density was lower. The maximum specific dust resistivity for the first field and second field dust was approximately 3 × 10(11) Ω cm and no signs for the occurrence of back corona were detected. For the dusts from the last two fields the maximum value was approximately 2 × 10(12) Ω cm. Back corona was observed in the temperature range from 120°C to 210°C. In this area the dust resistivity values were higher than 4 × 10(11) Ω cm.

Air classification: Potential treatment method for optimized recycling or utilization of fine-grained air pollution control residues obtained from dry off-gas cleaning high-temperature processing systems.

In the dust collected from the off-gas of high-temperature processes, usually components that are volatile at the process temperature are enriched. In the recycling of the dust, the concentration of these volatile components is frequently limited to avoid operation problems. Also, for external utilization the concentration of such volatile components, especially heavy metals, is often restricted. The concentration of the volatile components is usually higher in the fine fractions of the collected dust. Therefore, air classification is a potential treatment method to deplete the coarse material from these volatile components by splitting off a fines fraction with an increased concentration of those volatile components. In this work, the procedure of a sequential classification using a laboratory air classifier and the calculations required for the evaluation of air classification for a certain application were demonstrated by taking the example of a fly ash sample from a biomass combustion plant. In the investigated example, the Pb content in the coarse fraction could be reduced to 60% by separation of 20% fines. For the non-volatile Mg the content was almost constant. It can be concluded that air classification is an appropriate method for the treatment of off-gas cleaning residues.

Mechanical properties of dust collected by dust separators in iron ore sinter plants.

The flow-related mechanical properties of dusts from the de-dusting systems of several sinter plants were investigated. The mass median diameters of the dusts were in the range from approximately 3 to 100 µm. Also, the bulk density of the dusts varied in a wide range (approximately 400 to 2300 kg/m³). A good correlation between the bulk density and the mass median diameter for most of the dusts was found. In contrast, the angles of repose did not vary very much, only for the coarsest dust a significantly lower value was measured. The angles of internal friction as well as the wall friction angles were lower for coarse dust and higher for fine dust. The shear tests showed that both angles depend considerably on the stress level. At low stress, the angles decreased significantly with increasing values of stress, whereas at higher stress, the dependence was small or even disappeared. The only exception to this behaviour was shown by the finest dust. The flowability decreased with the particle size. The flowability categories suggested by the three flowability indicators were passable only for the coarser dusts. For the finer dusts, the flowability was overestimated by all flowability indicators.

Chemical composition and physical properties of filter fly ashes from eight grate-fired biomass combustion plants.

For the handling, treatment and utilization of fly ash from biomass combustion its chemical composition and physical properties are important. In this study eight filter fly ashes from different grate-fired biomass combustion plants were investigated. In fly ash from straw combustion high concentrations of (K) were found, whereas in the fly ash from wood combustion the concentrations of Ca and Mg were higher. The average concentration of PO4(3-) was similar in both types of fly ashes. In all wood fly ashes some measured heavy metal concentrations were above the limits for utilization. The straw fly ashes were much less contaminated and can be utilized. For wood fly ash most parameters showed little variation, except from one fly ash where the dust pre-separator is in poor condition. The average values were: mass median diameter 4.3±0.8 µm, spread of particle size distribution 19±11, particle density 2620±80 kg/m3 and angle of repose 50°±1°. The density of the straw fly ashes is lower (2260±80 kg/m3) and the spread of the size distribution is higher (72±24). For one straw combustion fly ash the values of the mass median diameter and the angle of repose were similar to the values of wood combustion fly ash, for the other straw fly ash the values differed considerably. While the particle size of this fly ash was much smaller, surprisingly the angle of repose was also lower. This can be attributed to the formation of small agglomerates in this fly ash, which were not disintegrated without a certain stress.