大气污染控制工程.ppt

1、1,大气污染控制工程Air Pollution Control Engineering,2,1 Introduction to Air Pollution control 2 Air Pollution effects 3 Air Pollution control philosophies 4 Air Pollution Measurement, Emission Estimates 5 Meteorology for Air Pollution control Engineers 6 Air Pollutant Concentration Models 7 General Ideas in

2、 Air Pollution control 8 Nature of Particulate Pollutants 9 Control of Primary Particulates 10 Control of Volatile Organic Compounds 11 Control of Sulfur Oxides 12 Control of Nitrogen Oxides 13 Other topics,Contents,3,1 Introduction to Air Pollution control,1.1 Definition of Air Pollution 1.2 Source

3、 of Air Pollution 1.3 Results of Air Pollution,4,1.1 Definition of Air Pollution,Air pollution is the presence of undesirable material in air, in quantities large enough to produce harmful effects. It causes harmful effects on human health, property, aesthetics, and the global climate. Key point: (1

4、) it is harmful to vegetation, biotic or abiotic. (2) The quantities are too great to be accepted (The pollutant concentration exceeds the acceptable lever),6,1 .2 Source of Air Pollution,Structure of the Electricity generation in China,9,Polluted Air,10,1.3 Results of Air Pollution,A. Global Warmin

5、gcaused by Carbon Dioxide, Other Greenhouse Gases, Aerosols B. Ozone Depletioncaused by （CFCS） Chlorofluorocarbons 氯氟碳化合物，氟里昂 C. Acid Rain,11,Global Warming,improve,12,A. Global Warming,Carbon Dioxide Other Greenhouse Gases Aerosols,13,14,15,Caused by（CFCS）-Chlorofluorocarbons 氯氟碳化合物，氟里昂 ozone is ab

6、out 1040km in height from the surface of the earth Use the thickness of the ozone to measure it： 1 Dobson unit=110-3cm If the ozone is less than 220 DU, Ozone hollow turns up. The cavity in the Antarctic pole (the south pole) is becoming larger and larger.,Ozone( O3) Depletion,consumption,16,Ozone D

7、epletion,18,C. Acid Rain -pH5.6,20,21,Acid Rain Distribution In China,22,Acid Rain Effects,Roman cathedral,Roman arena,23,2 Air Pollution Effects, Effects of Air Pollution on Human Health Air Pollution Effects on Property Air Pollution Effects on Visibility,24,A. Effects of Air Pollution on Human He

8、alth,Sulfur oxides Fine particulate matter Nitrogen oxides Carbon monoxide Ozone Benzene Inorganic arsenic (砷，砒霜） Vinyl chloride （氯乙烯） All above air pollutants are harmful to human health,25,But Whether a pollutant is “harmful” or not depends on the concentration and the breathing time. The concept

9、of “threshold limit values”:If the dosage is less than this value, we can handle the pollutant into our bodies through excretory system, e.g. ,urination or sweating. If not, it will lead to damage onto our bodies.,Dct is the pollutant quantity in our body (harmful dosage) Cin is the pollutant quanti

10、ty of breathing per time Cout is the elimination quantity of human,26,Notice:,1. If more than two different pollutants exist, Dct =k(Dct1 + Dct2), K1 2. some pollutant has no threshold limit values For instance: arsenic; isocyanate (异氰酸盐); carcinogens(致癌物),27,B. Air pollution Effects on Property,28,

11、C. Air Pollution Effects on Visibility,3 Air Pollution Control Philosophies,Emission Standard* Air Quality Standard * Emission Tax (税）* Cost Benefit,What is Emission Standard? Answer: It is the regulation of the upper limit concentration values of the pollutants emitted from the smoke stack. These m

12、aximum values are determined by the acceptable pollutant concentration value on the ground according to the diffusion theory calculation. What is Air Quality Standard? Answer: It is the regulation of the maximum (threshold) value of the pollutant concentration in air. These (threshold) values will n

13、ot lead to air pollution damage on human and property. What is Emission Tax？ Answer: It is an air pollution law about that the money must be paid by the pollution emitters according to the emission rate, e.g., dollars/(kg pollutant/h).,A. Emission Standard,Different country has different emission st

14、andard, Why? It depends on the economy, The technology, the living condition , the knowledge and the requirements of people. So it is a Philosophy. For Example:,Emission Standard of cement factory (kiln窑炉 ),New Emission Standard for the Main Pollutants in China(GB16297-1996),B. Air Quality Standard

15、GB3095-1996,C. Emission Tax,The polluters must pay the emission tax if they do not want to install the pollution control equipment. In this case, the pollutants discharged could possibly cleaned by the natural removal ability. Otherwise the pollution control equipment should be put in to use except

16、bearing the emission tax. However, the disadvantages of the emission tax (fee) is obvious. Why? Since this may lead to the managers to have less consideration about the effect on the environment due to the pollutant discharge.,D. Cost-Benefit Philosophy,The air pollution control philosophy is not to

17、 eliminate the pollution totally, but to minimize the costs of both control and damage in the condition of the economy and technology at present.,Problem (Classwork),1. Explanation (1) Air pollution (2) Dobson unit (3)（CFCs） (4) Threshold limit values of the pollutant breathing 2. composition Advant

18、ages and Disadvantages of Emission Forfeit or Penalty in China (more than 150 wards),4 Air Pollution Measurement, Emission Estimates,4.1 A representative sample 4.2 Concentration determination 4.3 Averaging 4.4 Determining pollutant flow rate 4.5 Isokinetic sampling,38,4.1 A representative sample,Tw

19、o main tasks for any air pollution measurement are involved: One is to obtain a suitable representative sample. The other is to determine the concentration of the pollutant of interest in it correctly.,Air pollution control system and Measuring system,39,4.2 Concentration determination,Measuring sys

20、tem,Example: particle concentration determination,steps,Weigh the original filter m1 and put the filter into the holder Turn on the fan Use the thermo-ball meter to measure the velocity in the duct Calculate the flow rate Q according to the velocity since the diameter of the sampling tube is known T

21、urn on the air pump and adjust the rotameter（转子流量计） at the calculated flow rate Generate the particulate Take out the filter from the holder after 3 or 5 minutes Weigh the filter m2 because the particles are collected on the filter The concentration C is:,41,4.3 Averaging,Almost all the experimental

22、 results have certain fluctuation due to the measurement error (deviation偏差) at different time. the average value is, therefore, much more reliable (reasonable) in scientific research. There are two cases: (1) An experiment is repeated several times Average value: (2) An experiment has been done ove

23、r some period of time Average value,4.4 Determining pollutant flow rate,There are two flow rates, mass flow rate and volume flow rate. Qm=vA, or where, v-gas velocity, A-area of the duct, and -the density of the polluted air QV=vA, or,Example If the velocity distribution is,and the radius of tube is

24、 R=0.1m.,Please find the volume flow rate in the tube.,m/s,43,4.5 Isokinetic sampling-equivalent velocity sampling,The gas velocity inside the sampling probe is equal to the gas velocity in the duct. Why? (Page 73) If the gas velocity is too great in the probe, less particles get into the probe due

25、to the inertial （惯性）effect. If the gas velocity in the probe is slower than the gas velocity in the duct, more particles get into the probe. Therefore, isokinetic sampling is necessary to reduce the measuring deviation in the experiment.,44,5 Meteorology for Air Pollution Control Engineers,5.1 The a

26、tmosphere 5.2 Horizontal atmosphere motion 5.3 vertical motion in the atmosphere 5.4 Winds 5.5 Temperature inversion 5.6 Fumigations, stagnation,45,5.1 The atmosphere,The composition of the atmosphere: 78% nitrogen, 21% oxygen, 1% argon (氩）and other trace(微量) gases. The humidity of the atmosphere: c

27、hanging significantly with place and time, and increasing rapidly with increasing temperature. of the mass of the atmosphere is within 6 km above the earth surface. Almost all the mass of the atmosphere and the moisture are within 20 km above the surface. The thickness of the atmosphere: Its upper b

28、oundary is not well defined. Fortunately, the air pollution problems we discussed are often within 2km.,46,5.2 Horizontal atmosphere motion,1. Atmosphere motion caused by temperature difference Key point: hot air goes up, cold air goes down.,47,2. Atmosphere motion caused by Coriolis force -the forc

29、e acts at right angles to the motion of the body,cannon,bomb,48,5.3 vertical motion in the atmosphere,In the atmosphere any parcel of air that is less dense than the air that surrounds it will rise by buoyancy. Otherwise, any parcel of air more dense than the surrounding air will sink by negative bu

30、oyancy. Most vertical motions in the atmosphere are caused by changes in air density.,5.31 Air density changes with temperature and humidity The density of gas is given by perfect gas law,Where, Mmolecular weight, Ppressure, Rgas constant, Ttemperature,(1),49,Example Estimate the change in air densi

31、ty due to 1 increase in temperature for dry air 1percent increase in relative humidity (both at 20) Solution a. Differentiating Eq. (1),If M and P are constant, we have,(2),we find,50,b. The molecular weight of dry air is 29. The mol fraction of water vapor at 20 is 0.023RH. And the molecular weight

32、 of water is 18.,Thus,The molecular weight of the air with water vapor is,The variation of temperature has much more effect than that of the humidity. Therefore, most of the vertical motion of the atmosphere is driven by changes in temperature rather than by changes in humidity.,relative humidity,51

33、,5.3.2 Air density change with pressure,The basic equation is,The negative refers to the pressure in the opposite direction of the coordinate z .,Substitute Eq. (1) into Eq. (3), yields,(3),The changes of M can be negligible, but the changes of temperature are not .,(4),52,Suppose a parcel of air in

34、 a invisible flexible balloon moving upward in the atmosphere. The air in the balloon does not work on its surroundings as it expands. It undergoes a reversible, adiabatic（绝热） process. According to thermodynamics, we have,Where CP is the heat capacity of gas at constant pressure. Divided Eq. (4) by

35、Eq. (5) on both sides, we find,(5),(6),Here, g=9.81m/s2, M=29g/mol, Where CP =3.5R=3.58.314m3Pa/molKs. So that,balloon,/km,This is called dry adiabatic lapse rate（干绝热直减率）,53,5.3.3 Atmosphere stability,54,Vertical temperature distribution on a cloudless day with low wind,5.3.4 Mixing Height (混合层高度）,S

36、ince the temperature reduces 10 when the height increases 1km, if the ground temperature is 15. The temperature will become 0 at 1.5km in height. The humid air must be condensed. The cloud can forms above 0 （05 ).,15,0 ,0 ,cloud,Mixing height,z,T,The mixing height the height where the cloud can get

37、to. The pollutant diffusion happens within the mixing height.,lapse rate line,56,5.4 Winds,Seacontinent wind,At day time,4 A.M.,mountainvalley wind,57,5. 5 Temperature inversions warm air layer on the top of the cold layer,5.6 Fumigations(熏烟）, stagnations（滞流）,T,5.7 City island effect,Plume（烟羽）,stagn

38、ation,Fumigation,6 Air Pollutant Concentration Models 6.1 Introduction,The purpose of establishing the concentration models To meet the requirements of the air pollution law, we should predict the concentrations of contaminants. To plan air pollution control programs or to build up some new projects

39、, we should predict the emission concentrations of the pollutant from the sources. The basis of the concentration models: principle of mass balance.,6.2 Fixed-box models (箱式模型）,Assumptions: 1. The pollution area is rectangular with W and L. 2. Pollutants are complete mixed up to the mixing height H（

40、混合层）. 3. The concentration is uniform in the box due to the turbulence. 4. The wind velocity u is constant and uniform . 5. No pollutant leaves or enters through the top of the box, nor through the sides that are parallel to the wind direction.,60,Let: the concentration of pollutant in the air enter

41、ing pollution area is C0 (g/m3), the emission rate per unit area is q (g/sm2).The mass flow rate(质量流量）is:,The creation mass flow rate in the pollution area is:,The mass flow rate out of the pollution area is:,According to the material (mass) balance theory, we have,61,6.3 diffusion models,6.3.1 The

42、Gaussian plume idea The pollutant gas comes out as a point source. The coordinates of the point source is 0, 0, H, where H is the effective stack height. Where, h is the stack height, h is the plume rising height. The plume appears to have a twisting, snake like shape. However, time averages out the

43、 variations of the plume, the shape of the plume appears uniform and symmetrical. The concentration distribution follows the Normal distribution function.,62,6.3.2 Gaussian plume derivation,1. Basic Gaussian plume equation,Where, are called horizontal and vertical dispersion coefficients (弥散参数， m）.

44、If are determined, the concentrations of the plume at any point can be calculated .,2. How to determine the,are proportional to the square root of the downwind distance. At the different point (x, y, z), are the function of wind speed and the degree of solar heating. These can be related with the at

45、mosphere stability.,(1),63,Surface wind speed(10meters 6CDDDD,Stability categories,64,Example: estimate the valu of at a point 0.5km downwind from a point source,on a bright summer day. wind speed greater than 6m/s. From the above table, atmosphere stability C should be selected. Using the fig. 6.7

46、and fig. 6.8, we find the horizontal dispersion coefficient and vertical dispersion coefficient at a point 0.5km downwind respectively: Then we can use the equation (1) to calculate the concentration at any point of x, y, z.,3. The effect of the ground Equation (1) is a diffusion model in unlimited

47、space for the point source, which can be used to predict the concentration approximately in the fume considerably far from the ground. It means that the effect of the ground can be negligible.However, we are often interested in concentrations at the ground level since people and property are exposed

48、 there (living on the ground). But near the ground, because the boundary condition is changed, the concentration distribution will not follow the Equation (1).,(1) “reflection wall hypothesis”(反射壁假设） When the plume goes down and touches to the ground surface, the ground looks like a mirror, the poll

49、utant is reflected upward to the atmosphere again. (2) principle of superposition（叠加原理） The pollutant concentrations from different sources can be added directly.,66,In order to know the concentration in the reflection area, suppose there is an image point source symmetrically under the ground. The

50、original plume concentration distribution is,The image plume concentration distribution, obviously, is,67,According to superposition principle, the concentration in the reflection area is,4. the concentration on the ground Let z=0, the concentration on the ground can be found directly from Equation

51、(3),5. the concentration on the ground under the centerline of the plume For this condition, y=0, z=0, therefore, the concentration on the ground under the centerline of the plume is,6.4 plume rise,We have known that , Where, h is the stack height, h is the plume rising height. The h is determined b

52、y : smoke exit velocity from the stack vs, stack diameter D wind speed u, the pressure around the stack P, smoke temperature Ts and the atmosphere temperature Ta. There are many formulae for calculating the plume rising height h. Now we just give the Hollands formula,Example (page 143),7 General Ide

53、as in Air Pollution control,7.1 Alternatives Three control options are available ( three methods can be made use of to control the air pollution). 1. Improve dispersion first option (1) Tall stack From Eq.(6), when the stack height h is determined, to increase the smoke exit velocity vs, the differe

54、nce of the smoke temperature Ts and the atmosphere temperature Ta is the main ways to increase the plume rising height h. The smoke dispersion can be improved. (2) Intermittent control schemes (the plan to control the emission quantity at different time and deferent place) For instance, in the after

55、noon, the diffusion ability is stronger, the amount of pollutant discharged could be higher. But in the morning, the amount of pollutant discharge should be reduced due to the temperature inversion. Similarly, Seacontinent wind and mountainvalley wind, and others factors should also be considered.,2

56、. Reduce emissions by process change, pollution prevention - second option,The idea of reduce emissions by process change means that using the less pollution production techniques to substitute the heavy pollution processes. In this book (page 165), many examples were given: Painting: water-based pa

57、int replaces the oil-based paint to reduce the hydrocarbon emission; Burning: Open burning is substituted by the close burning in incinerator; Traveling: get people from home to workcarpool. Taking buses with others, instate of driving a car individually. Lighting: replacing low efficiency incandesc

58、ent lights with higher-efficiency fluorescent lights to save energy (less fuel is used, less air pollutants are emitted). And so on. The key point is: preventing pollution is much better than control it.,3. Use a downstream pollution control device-third option,a downstream pollution control device

59、is often called tailpipe control device (末端控制). Even though downstream pollution control device, that is, the air pollution control equipment, is the main content of this book, this is the last option. A prudent engineer will always consider the previous two options to see if they are more practical and economical than a downstream pollution control device.,7.2 Resource recovery-recycling For the gaseous pollutant, gener