AIR POLLUTION – MONITORING MODELLING AND HEALTH

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216 Air Pollution <strong>–</strong> Monitoring, Modelling and Health Two-dimensional wind simulation of the set-up does not take into account surface roughness effects. This does not seriously affect the wind field except close to the surface, in the so-called surface layer. The CFD simulation by Fluent 6.2 shows, that because of the pile, the wind velocity at 0.20 m above the pile centre has increased to 13 m/s compared to the value of 10 m/s at the same height above the flat terrain before the pile. While both profiles in the boundary layer: the "pile" and the "flat", fit well the functional form of Eq. (2), the flat profile surpasses the logarithmic extrapolation outside the boundary layer and the "pile" profile undershoots the logarithmic extrapolation. The extrapolation of fit from the boundary layer suggests the value of the wind velocity at 0.20 m above the base in the absence of geometrical and wind source constraints. According to this procedure, a measured 13 m/s wind velocity at 0.20 m above the pile is equivalent to a velocity of 14.6 m/s in a flat terrain situation. This translation is important since formulas quoted previously apply for the flat terrain situation. This simulation suggests that the measured u .20 should be increased by a factor of 1.12 and this corrected value then used to determine the wind friction velocity. In conclusion, a particular geometry of the set-up does not affect more than 10% the threshold velocity values in the sense that these can be related to a wind velocity u .20, measured at 0.20 m above the centre of the test surface using just the flat terrain formulae. So, when u .20 velocity distributions for any pile, modelled smoothly at a 0.5 m length are given, the experimental u .20 data presented in Figure 4 can be used to estimate the total mass outflow and the corresponding PM10 and BC concentrations. Table 3 summarises our results for dry materials. d 50 (mm) z 0 (mm) u t.20 (m/s) u * t (m/s) u + t10 (m/s) FR2 coal 10 0.33 14.8 0.93 23.8 FR3 coal 3 0.10 13.0 0.68 19.7 FR4 coal 0.04 0.0014 5.6 0.18 7.4 FR1234 coal 0.1 5.6, 13.0 7.6, 19.7 FR2 iron >0.3 >18 FR3 iron 0.05 14.0 0.67 20.6 Alumina 0.03 2.5 0.11 3.6 Table 3. Threshold wind velocities u t.20 for different fractions of coal, iron ore and alumina as determined by our measurements. Also reported are u * t and u + t10 threshold velocities which are calculated from measured u t.20 by using the estimates for d 50 (z 0 ). 4.7 FR4 coal watering Dusting from the coal stockpile can be significantly reduced by spraying the surface with water. The water effectively increases the mass of the particles by binding the particles to the surface layer of the coal. However, the protection afforded by a single spraying is not permanent because the water evaporates. We measured that watering of FR4 coal by 0.5 l/m 2 every two hours is sufficient to suppress dusting at u .20 = 8.3 m/s and at 95% air humidity. Compared to a dry FR4, after 364 seconds more than 100 times less mass was removed from the test vessel (1.16 kg versus 0.01 kg). To achieve the same effect at u .20 = 11.3 m/s, 2 l/m 2 would be required. The estimates of a water quantity needed to suppress the dusting from FR4 surface should be considered as upper estimates when applied to realistic surface FR1234. The results are consistent with the
Fugitive Dust Emissions from a Coal-, Iron Ore- and Hydrated Alumina Stockpile 217 belly scraper case at a landfill described by Fitz & Bumiller (2000) and with the unpaved road surface watering efficiency determined by Cowherd et al. (1988). On the other hand, it is difficult to compare this study’s results with those of Nicol & Smitham (1990), because neither exposure time nor wind velocity is reported. While they made wind tunnel tests with uniformly humid and conditioned coal, in our case the water was sprayed directly onto a dry coal surface just prior to switching on the air stream. Measurements show that mass loss is not a steady function of time, except when the watering is sufficient to establish a lasting linear dependence of mass loss with time. For the two linear cases with u .20 equal to 8.3 m/s and 11.3 m/s the mass losses for the coal (+ water) system were 480 g/m 2 /h and 1400 g/m 2 /h, respectively. In both cases water evaporation contributed mostly to the mass loss, as shown by a separate wind track experiment with water sample only. 4.8 Iron ore watering Concerning iron ore, the following phenomenon was observed: after watering, the surface of the iron ore (FR4, FR3) developed a firm crust upon drying. The formation of the crust strongly reduces the dusting potential and a much lower mass loss per unit time is observed for crusted material than for freshly sieved iron ore under the same experimental conditions. Only 0.06 kg loss in 1548 seconds was observed for the crusted FR3 iron ore at u .20 =18 m/s, which is 9-times less than for sieved FR3. In view of this finding, it is highly desirable to crust the iron ore, which is stored for a significant period. Different to the coal, watering the iron ore only once is sufficient to immobilize it, provided the crust remains intact and is not disrupted by subsequent movement of heavy machinery. The mechanism behind the hardening of iron ore is attributed to agglomeration of iron ore particles. PIXE analysis, as well as x-ray diffraction of an iron ore sample shows that the ore consists of 98% pure hematite (Fe 2 O 3 ) with small impurities of K, Ca, Cl, Ti, Mn and Sr. According to specifications, other impurities may be mixed with the ore in small quantities (SiO 2 < 3%, Al 2 O 3 ≈ 1%), but in this authors opinion it is the content of water which is responsible for binding. When hematite is watered and left to dry a uniform crust develops over the sample. The crust can be destroyed by mechanical action or by re-watering the iron ore. As found out previously, the first two water layers near the hematite surface are highly ordered, and effectively are adsorbed to the surface through hydrogen bonding and further from the surface, water still displays some ordering in the form of layering (Catalano et al., 2009). Different hematite particles are bound between themselves by the interaction of these water layers, which are not removed in the process of drying. Upon heating an iron ore sample to 600 o C the water is completely removed and the mass percentage of this “residual” water is about 1%. No such effect is observed for coal. Upon drying, the chemical structure of iron ore is not changed, as shown by x-ray diffraction measurements. 5. Conclusions Time dependence of mass loss from the test surface was measured together with concentrations of inhalable particulate matter (PM10) and black carbon (BC) in the vicinity of the test surface for wind velocities ranging from 1 - 18 m/s at 0.20 m above the centre of the test surface. Threshold wind velocities u t.20 were measured for four different size fractions of coal and for three different fractions of iron ore stored at the terminal, and for hydrated alumina - a strong dust emitter. The same set up was employed to study the
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AIR POLLUTION - MONITORING, MODELLI
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Contents Preface IX Chapter 1 Urban
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Preface This book provides a compre
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2 Air Pollution - Monitoring, Model
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10 Air Pollution - Monitoring, Mode
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AIR POLLUTION – MONITORING MODELLING AND HEALTH

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