Kuliah Pencemaran Udara MODEL GAUSS UNTUK DISPERSI PENCEMAR UDARA

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Dennis Haynes
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1 Kuliah Pencemaran Udara MODEL GAUSS UNTUK DISPERSI PENCEMAR UDARA

2 ADVANTAGES OF EMPLOYING ATMOSPHERIC DISPERSION Dispersion of the waste gases leads to the dilution of the pollutants in the atmosphere. Self-purification mechanisms of atmospheric air also assists the process. Tall stacks emit gas into the upper layer of the atmosphere and lower the ground concentration of the pollutants. The method is commonly used, cheap and easily applicable. By selecting the proper location of stacks through the use of different models for dispersion, it is possible to significantly reduce the concentration of waste gases in the atmosphere.

3 DISADVANTAGES OF EMPLOYING ATMOSPHERIC DISPERSION Any particulate matter contained in the dispersed gases have a tendency to settle down to the ground level. The location of the industrial source may prohibit dispersion as an option. Plume rise can significantly vary with ambient temperature, stability conditions, molecular weight, and exit velocity of the stack gases. The models of atmospheric dispersion are rarely accurate. They should only be used for estimation and comparative analysis.

4 SISTEM KOORDINAT DISTRIBUSI GAUSS ARAH HORIZONTAL DAN VERTIKAL

5 PLUME RISE Several plume rise equations are available. Briggs used the following equations to calculate the plume rise:

6 Where Δh = plume rise, m F = buoyancy flux, m 4 /s 3 = 3.7 x 10-5 Q H u = wind speed, m/s x* = downward distance, m X f = distance of transition from first stage of rise to the second stage of rise, m Q H = heat emission rate, kcal/s

7 If the term Q H is not available, the term F may be estimated by F = (g/π)q(ts - T)/T s where g = gravity term 9.8 m/s 2 q= stack gas volumetric flowrate, m 3 /s (actual conditions) T s,t = stack gas and ambient air temperature, K, respectively

8 Many more plume rise equations may be found in the literature. The Environmental Protection Agency (EPA) is mandated to use Brigg's equations to calculate plume rise. In past years, industry has often chosen to use the Holland or Davidson-Bryant equation. The Holland equation is :

9 where d= inside stack diameter, m vs = stack exit velocity, m/s u = wind speed, m/s P = atmospheric pressure, mbar T s,t = stack gas and ambient temperature, respectively, K ΔT=T s - T Δh = plume rise, m

10 The Davidson-Bryant equation is

11 THE GAUSSIAN EQUATION The short term model for stacks uses the steady-state Gaussian plume equation for a continuous elevated source. For each source and each hour, the origin of the source's coordinate system is placed at the ground surface at the base of the stack. The x axis is positive in the downwind direction, the y axis is crosswind (normal) to the x axis and the z axis extends vertically. The fixed receptor locations are converted to each source's coordinate system for each hourly concentration calculation. The hourly concentrations calculated for each source at each receptor are summed to obtain the total concentration produced at each receptor by the combined source emissions.

12 For a steady-state Gaussian plume, the hourly concentration at downwind distance x (meters) and crosswind distance y (meters) is given by: where: Q = pollutant emission rate (mass per unit time) K = a scaling coefficient to convert calculated concentrations to desired units (default value of 1 x 106 for Q in g/s and concentration in μg/m3) V = vertical term (See Section 1.1.6) D = decay term (See Section 1.1.7) σ y, σz = standard deviation of lateral and vertical concentration distribution (m) (See Section 1.1.5) u s = mean wind speed (m/s) at release height

13 The origin is at ground level or beneath the point of emission, with the x axis extending horizontally in the direction of the mean wind. The y axis is in the horizontal plane perpendicular to the x axis, and the z axis extends vertically. The plume travels along or parallel to the x axis (in the mean wind direction). The concentration, C, of gas or aerosol at (x,y, z) from a continuous source with an effective height, He, is given by:

MODELING Untuk memprediksi pencemaran udara Model Gauss distribusi konsentrasi Rumus menghitung C gas atau aerosol (<20 u) pada permukaan tanah arah downwind (x): Di mana: C = konsentrasi polutan,

14 MODELING Untuk memprediksi pencemaran udara Model Gauss distribusi konsentrasi Rumus menghitung C gas atau aerosol (<20 u) pada permukaan tanah arah downwind (x): Di mana: C = konsentrasi polutan, g/m3 m = laju emisi polutan, g/s = kecepatan angin rata-rata, m/s z = standar deviasi konsentrasi flume arah horizontal y = standar deviasi konsentrasi flume arah vertikal H = tinggi efektif cerobonhg, m X = jarak downwind sepanjang centerline flume dari titik sumber, m Y = jarak crosswind dari centerline flume, m

15 The assumptions made in the development of the above equation are: the plume spread has a Gaussian (normal) distribution in both the horizontal and vertical planes, with standard deviations of plume concentration distribution in the horizontal and vertical directions of av, and oz, respectively; uniform emission rate of pollutants, m; total reflection of the plume at ground z = 0 conditions; and the plume moves downstream (horizontally in the x direction) with mean wind spead, u. Although any consistent set of units may be used, the cgs system is preferred.

16 For concentrations calculated at ground level (z = 0), the equation simplifies to If the concentration is to be calculated along the centerline of the plume (y = 0), further simplification gives

17 The plume rise model examines a range of stability classes and wind speeds to identify the "worst case" meteorological conditions Table. Stability Categories

18 Note that A, B, C refer to daytime with unstable conditions; D refers to overcast or neutral conditions at night or during the day; E and F refer to night time stable conditions and are based on the amount of cloud cover.

19 TABLE. PARAMETERS USED TO CALCULATE PASQUILL-GIFFORD FY

20 TABLE 3.3 PARAMETERS USED TO CALCULATE PASQUILL-GIFFORD FZ

22 Figure Dispersion coefficients, y direction

23 Figure. Dispersion Coefficient, z direction

24 Wind speed at elevation from known wind speed and elevation where u = wind speed at height h, (m/s) u 0 = wind speed at anemometer level h 0, (m/s) n = coefficient, approximately 1/7

25 TECHNICAL DATA AND COMPUTATION RESULTS FOR EFFECTIVE STACK HEIGHT AND ATMOSPHERIC STABILITY Parameter Case 1 Case 2 Case 3 Case 4 site condition emission velocity rate m/s inside diameter stack m wind speed m/s 31.10; 10; ; 10; ; 10; ; 10; 3 atmospheric pressure mbar stack gas temperature K air temperature K stack height m plume rise Δh pada u = 31.2 m/s m Δh pada u = 10 m/s m Δh pada u = 3 m/s m efective stack height m 45 + Δh 45 + Δh 45 + Δh 45 + Δh atmospheric stability neutral type D or B D or B D or B D or B

26 EMISSION LOAD FROM DATA ANALYSIS RESULT FOR DISPERSION GAUSSIAN MODEL INPUT Parameter Unit Case 1 Case 2 Case 3 Case 4 SO2 in exhaust g/s Carbon monoxide (CO) g/s Nitrogen Dioxide (NOx) g/s

27 case 1 Plant Operating in combined cycle full load (Fuel Gass) case 2 Plant Operating in simple cycle GT full load (Fuel Gass) case 3 Plant Operating in combined cycle full load (Fuel Oil) case 4 Plant Operating in simple cycle GT full load (Fuel Oil)

28 TABLE. QUALITY OF GROUND QUALITY FROM GAS EMISSION Parameter unit Case 1 Case 2 Case 3 Case 4 Stand ard Total Particle mg/m Dioxide (SO 2 ) mg/m Nitrogen Oxide (NOx) Carbon Monoxide (CO) mg/m mg/m

29 x (km) CO (μg/nm3) SO2 (μg/nm3) NOx (μg/nm3) x (km) CO (μg/nm3) SO2 (μg/nm3) NOx (μg/nm3) E E

30 Dispersion model CO and NOx on centerline, u = 3 m/s

32 LINE SOURCE APPLICATION A six-story hospital building is located 300 m east and downwind from an expressway. The expressway runs northsouth and the wind is from the west at 4 m/s. It is 5:30 in the afternoon on an overcast day. The measured traffic flow is 8000 vehicles per hour during this rush hour and the average vehicle, traveling at an average speed of 40mph, is expected to emit 0.02 g/s of total hydrocarbons. Concentrations at the hospital are required as part of a risk assessment study. How much lower, in percent, will the hydrocarbon concentration be on top of the building (where the elderly patients are housed) as compared with the concentration estimated at ground level? Assume a standard floor to be 3.5 m in height

33 q = source strength per unit distance, g/(s m) HQ = effective stack or discharge height, m u =wind speed, m/s oz = vertical dispersion coefficients, m

35 Pada Ground Level Pada Gedung Lantai 6

36 LINE SOURCE APPLICATION Concentrations from infinite line sources, when the wind is not perpendicular to the line, can also be approximated. If the angle between the wind direction and the line source is Φ. This equation should not be used when Φ is less than 45.

37 A power plant burns 12 tons of 2.5% sulfur content coal per hour. The effective stack height is 120 m and the wind speed is 2m/s. At one hour before sunrise, the sky is clear. A dispersion study requires information on the approximate distance of the maximum concentration under these conditions. {Hint: Calculate concentrations for downward distances of 0.1, 1.0, 5, 10, 20, 25, 30, 50 and 70 km.)

38 Model Line Source, 2 way Kelompok I : 2 way sejajar, angin tegak lurus Kelompok II: 2 way tdk sejajar, angin tegak lurus salah satu jalan Kelompok III : sama dengan kelompok I, angin tidak tegak lurus jalan Kelompok IV : sama dengan kelompok II, angin tidak tegak lurus jalan

39 Inputan (variabel) : data angin, data lalu lintas(kepadatan kendaraan dan beban emisi), stabilitas atmosfer Hasil (output) : Konsentrasi polutan di ambien tiap titik yang dihitung Grafik dispersi polutan mulai dari sumber

39th International Physics Olympiad - Hanoi - Vietnam - 2008. Theoretical Problem No. 3

39th International Physics Olympiad - Hanoi - Vietnam - 2008. Theoretical Problem No. 3 CHANGE OF AIR TEMPERATURE WITH ALTITUDE, ATMOSPHERIC STABILITY AND AIR POLLUTION Vertical motion of air governs many atmospheric processes, such as the formation of clouds and precipitation and the dispersal