CEE 452/652. Week 13, Lecture 1 Electrostatic Precipitators. Dr. Dave DuBois Division of Atmospheric Sciences, Desert Research Institute

Transcription

1 CEE 452/652 Week 13, Lecture 1 Electrostatic Precipitators Dr. Dave DuBois Division of Atmospheric Sciences, Desert Research Institute

2 Today s topics Today s topic: Electrostatic Precipitators Read chapter 5 No class Thursday Presentations on Nov 29 and Dec 4 schedule Noe Steven Heston Sarah Casey Kyra James 29-Nov 29-Nov 29-Nov 4-Dec 4-Dec 4-Dec 4-Dec 2

3 Electrostatic Precipitation Around since 1915 Techniques of collection Apply force to move particles from gas stream Trap moved particles and prevent re-entrainment Forces Electrostatic Used for powerplant fly ash, refinery mists (sulfuric acid) & other fine particulate emissions 3

4 Pros and Cons Advantages Low pressure drop (Cyclones have high pressure drop) High collection efficiency for small particles Disadvantages Operational efficiency depends on particle resistivity They consume significant electrical power Not suitable for flammable (explosion hazard) or sticky particles 4

5 Operating principles 5

6 Simplified schematic 6

7 7

8 8

9 Airflow Between 2 Plates Operating principle, charged particle drifts towards collecting plate 9

10 What collecting plates look like 10

11 What rappers look like 11

12 Key empirical design relationship Deutsch equation C L /C O = e (-wa/q) w = particle migration velocity, m/sec A = two-sided collector area, m 2 Q = volumetric gas flow m 3 /sec Many factors affect particle migration velocity 12

13 What affects particle migration velocity Terminal drift velocity, w w = w = τ F E Characteristic time of the particle in the gas where τ = Cρ p d p2 / 18μ = time to come to rest Stokesian, (eqn 3.20) and F E = q E co / M p particle acceleration X Electrostatic force per unit mass where Mass M p = ρ p V p ; V p = πd p3 / 6 E co = collecting field strength (Volts/m) and charge q = πd p2 ε 0 KE ch (saturation charge on sphere) 13

14 What affects particle migration velocity Combine all above equations to obtain: w = [Cd p /3μ]ε 0 KE ch E co C = Cunningham correction factor µ = gas viscosity (kg/m-s) ε 0 = permittivity of free space (C/V-m) d p = diameter of particle (m) K = 3ε/(ε + 2); ε = dielectric constant E ch =charging field strength (V/m) E co =collecting field strength (V/m) 14

15 What affects w Precipitator design/operations set E ch and E co Upstream particle generation processes affect d p Air temperature affects, gas viscosity, μ Temperature, relative humidity, chemical composition affect K = 3ε / (ε+2) 15

16 Real life drift velocity However, real life is not so convenient: particles are randomly shaped and in various sizes Electric fields are not constant Gas flows are not uniform Particles are re-entrained from the walls and during rapping back into the flow In practice, w is estimated from pilot studies or based on previous designs In that case use an effective drift velocity, w e 16

17 Plate Sizing Plates are usually taller than long Plates placed in parallel in several sections For ESP with N s sections in the direction of flow, Total collection area = Total number of active plates X Doublesided area per plate A total = A p (N - N s ) A p =two-sided plate area (=2HL p ); L p =plate length N=number of plates in ESP N s =number of sections in the direction of flow 17

18 Corona Corona = the ionization of gas molecules by high energy electrons in the region of a strong electric field Negative ions produced are adsorbed onto particles that migrate to the grounded plates Negatively charged discharge wire Typical voltages around 20,000 volts on a 0.1 inch wire 18

19 Particle Layer Resistivity Property of particles, resistivity Once collected on the plates, the particles can loose their charge If P too low (good conductor), charge drained off, particles fall back into flow If P too big (good insulator), charge doesn t drain off and difficult to rap off Resistivity depends on temperature and chemical composition (e.g. fly ash) Optimum gas temperature, F 19

20 Design considerations Input gas flow rate, Q, affects size of precipitator (#plates) and plate area, A p Plate separation and # of plates, across flow & along flow, affect Operating flow velocity, u Particle migration velocity, w, affected by Particle resistivity affected by temp and relative humidity Corona (charging field) and collecting plate (collecting field) strengths, alignment of discharge wires 20

21 Key design aspects Guidelines for redundancy based on 90 years experience in operations and maintenance, see text Chapter 5 tables Redundancy Multiple plate sets along gas flow path Parallel plate sets across gas flow path Allow for space to expand the gas flow and recapture it 21

22 Design, estimating # plates Example 5.1, equations 5.7 and 5.13 interplay. Given Q and target efficiency and w Rearrange 5.7 to solve for Area 6,520 m 2 Given plate dimensions A p and N s Rearrange 5.13 to calculate number of plates = Use N s (=2) to adjust N (from 183 to 184) to a reasonable whole number multiple that can be split into 2 sections = 184 / 2 = 92 22

23 Explaining equation 5.15, N d D D D D D D H Shaded zone is Available area for flow (less plate thickness, which is neglected) If Q = 60 m 3 /min N d = 6, D = 0.20 m, H = 2.5m N d DH = cross sectional area, A c in m 2 Velocity, u = Q / A c = Q / (N d DH) u = 60 / (6 x.20 x 2.5) = 20 meter/min 23

24 Using equation 5.15 (N d ) in design Workable flow velocity, u, and plate separation D, are known from experience So can get number of Ducts, N d, from u, D and plate height, H See Example 5.3, page 162 N d = 67 ducts 24

25 Explaining Equation 5.16 (L o ) Case for N s = 2 L o = N s L p + (N s -1)L s + L en + L ex Lp H L en + L p + L s + L p + L ex L o = 2L p + L s + L en + L ex L o = 2x = 17 m 25

26 Explaining Equation 5.16 (L o ) Case for Ns = 3 L o = N s L p + (N s -1)L s + L en + L ex L en + L p + L s + L p + L s + L p + L ex L o = 3L p + 2L s + L en + L ex L o = 3x5 + 2x = 23 m 26

27 Using 5.16, (L o ) in design Typical best dimensions obtained operating experience Page 160 Table 5.1 has typical values Page 161 discusses values L s = meters L p = meters H = 6-12 meters Use best dimensions and flow rate to estimate number of sections, N s 27

28 Explaining equation 5.17, N s R, aspect ratio = ratio of total plate length / plate height R = N s L p / H H Lp Lp Lp R = N s x L p / H For N s = 3, R = 3L p / H L p = 5 m, H = 7.5 m R = 15/7.5 = 2 28

29 Explaining equation 5.18 (A a ) with an example N d = (#plates 1) 4 plates, 3 ducts Colorful interior plates collect on both sides Grey Exterior plates collect only on interior duct side 29

30 Explaining equation 5.18, A a (1) Total collecting plate sides per section = 2 interior x 2 sides + 2 exterior x 1 side = 6 = (# plates - 1) x 2 = N d x 2 (2) Individual Plate area = H x L p (3) Number of sections = N s Total plate area = (1) x (2) x (3) = 2 N d H L p N s 30

31 Example calculation, for N s =2 H = 8 m, L p = 5m, N d = 3 Collecting Area = 2 N d H L p N s Collecting Area = 2 x 3 x 8 x 5 x 2 = 480 m 2 31

32 You can now follow example 5.3, p Given Q, minimum efficiency, specified collection area based on w 2. Assume H, D, u, R from Table Calculate N d = Calculates N s = 4 then 5.18 A a (19,296m 2 ), check against Aspecified (14,000m 2 ) too high 5. Back to step 2, new values 6. Repeat N d, N s, A a, until get A a ~= Aspecified 7. Calculate specific area and area per set & compare to Table 5.1 for reasonableness 32

33 Now verify that it will work!! (not in book!) Know your drift velocity, w for the target particle size, d p your flow rate, Q Your N d (5.15), N s (5.17), A a (5.18) Estimate overall removal for smallest target particle with Deustch equation using A a, Q and w Does it meet the minimum design efficiency? (larger particles migrate faster and will be removed at higher efficiency) 33

34 What about electrical costs? Corona sets up current of charged gas molecules and charged particles that migrate from corona grid or wire to collecting plates, giving a corona current, I c Corona voltage is V avg Corona Power P c = I c x V avg Eqn A - Corona operating cost = $/(kw-hr) x P c (kilowatts) x #hours operating 34

35 Power vs efficiency of collection Eqn 5.20, drift velocity w e = kp c /A 0.5 < k < 0.7 feet 3 /(sec-watt) Substitute into Deutsch equation obtain Eqn 5.21 Use Eqn 5.21 or a data plot like Fig 5.9 To get P c /Q for required collection efficiency Then use Q x (P c /Q) to calculate P c 35

36 Trade off, cost vs efficiency of collection!! Obtain Pc Then calculate cost using electricity rates and Eqn A See Example 5.4 triple Pc to go from 98% collection efficiency to 99.8% collection efficiency! You could also build a larger precipitator and reduce operating power costs because have higher plate area and can use lower migration velocity, we, for the same efficiency, therefore reducing corona power, Pc (i.e your value of Pc/A is lower, you operate at a lower power density (sounds very cool, like science fiction and the movies!) But, at what point does higher capital cost no longer make sense? 36

37 Costs Capital costs ( C ) Operating costs (O&M) Depreciation Maintenance Operations - Energy to power corona, rappers and run fans Corona energy cost = Voltage x field current, watts Fan energy cost = ΔP x Q = Power in watts Total energy, kwhr = (fan power, kw + corona power, kw) x run time, hours Electricity sold in units of $ per kilowatt-hr. typically $/kwhr 37