GAS/SOLID SEPARATIONS Cyclones, Electrostatic precipitators, (Hot) Gas filtration

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1 8 Fluid and Particulate systems /2014 GAS/SOLID SEPARATIONS Cyclones, Electrostatic precipitators, (Hot) Gas filtration Ron Zevenhoven ÅA Thermal and Flow Engineering 8.1 Selection of devices RoNz 2/59

2 Equipment for particulate emissions control - a comparison RoNz 3 Equipment for particulate emissions control - a comparison Source: C06 RoNz 4

3 An inertial separator: a settling chamber d Re d For laminar flow (Re 2300) Efficiency for laminar flow (Re 4000) Efficiency u H pt H u gas gas gas hydraulic diameter eff ( d eff ( d 2HW ugas H W p p u ) u gas ) 1 exp u ptl u H particleterminalsettlingvelocity pt L H gas gas gas Removes particles > 50 µm up to > 1 mm from gas (air) streams RoNz Cyclones RoNz 6

4 A gas cyclone more than a useful pre-separator? Advantages Simple, cheap and compact Large capacity Disadvantages Large pressure drop Low efficiency Catch removal problems No removal below ~5 m Problems above ~ 400 C RoNz 7 Cyclones Utilized for separation of particles (and droplets), with a diameter larger than 5 μm, from gases. The separation is not complete; the incoming gas contains particles of different sizes, and small particles is separated less effectively than bigger ones. RoNz 8

5 Separation efficiency For a certain cyclone and a certain gas, the separation efficiency η c is a function of the particle diameter d p. The particle concentration and size distribution does not normally affect the separation efficiency η c. The particle size for which half of the particles are separated (η c = 50%) is called cut size d pc or d 50 and can be calculated using d pc d w in gas ( p d c gas ) RoNz 9 Removal efficiency Lapple cyclone Number of gas turns, N, (i.e. revolutions, typically 3 ~ 8) before entering the vortex finder: N L b Lc 2 H Grade efficiency: Eff ( d ) p 1 d d 1 50 p 2 d 50 Cut size : Typical material properties: dynamic gas viscosity : gas (T/293) 2/3 Pa.s densities : solid = typically kg/m³ = 1.2 kg/m³ at 20 C, 1 bar (air) gas 9 gas W 2NV ( ) in solid gas RoNz 10

6 H A standard cyclone (Lapple) W S D e D D d L b L c 2 K gas V pressure drop (Pa) : p 2 2 D High Conventional High efficiency throughput Body diameter D/De Height of inlet H/D 0.5 ~0.44 0, ~ 0.8 Width of inlet W/D 0.2 ~ , ~ 0.35 Diameter of gas exit De/D 0.4 ~0.5 0,5 0,75 Length of vortex finder S/D 0, ~0.85 Length of body Lb/D 1.5 ~ ~ ~1.7 Length of cone Lc/D 2, ~2.0 Diameter of dust outlet Dd/D ~ ~ ~ 0.4 inlet exit H W K , use K 16 RoNz 11 Cyclones The velocity of the incoming gas can be calculated using V win 2 d c 8 and the pressure loss in the cyclone as p loss 8 gas w 2 2 in 10.2 RoNz 12

7 Particles in cyclones Centrifugal force m p ² r = m p v t ² / r Drag force (Stokes) 3 v r d p F r R Force balance gives equilibrium radial position: 1. m p v t ² / r = 3 v r d p F 2. v r v i A/ (2r h) (h=length of cylindrical section) 3. v t r n = v i R n, n ~ (conservation of angular momentum) gives into cyclone with velocity v i, inlet area A (r/r) n = h s v i d p2 /(9 A F ) for capture: large r/r needed and: r/r = 1 for cut size particles RoNz 13/59 Cyclones: processes determining separation Dr. Thesis J.G. Bernard (1992) RoNz 14

8 8.3 Exercises 10 RoNz 15 Exercises a) Derive the expressions for the fall velocity w T. w T 2Vp g ( p ) C A D prj d b) What will happen with the expressions when ρ p < ρ? 10.2 Air (80ºC) is containing dust particles that have a compact density ρ p of 2050 kg/m 3 and a particle size d p of 20 μm. A volumetric flow of 2.0 m 3 /s is cleaned in a cyclone, which is designed for an inlet velocity w in of 20 m/s. a) What is the cyclone efficiency? b) How much pressure loss does the cyclone cause? w T 2 p g ) ( p 18 februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 16

9 Exercises Two cyclones, C1 and C2, are used to remove dust from a flue gas. See Figure 1. The efficiency of cyclone C1 = 90%, the efficiency of cyclone C2 = 95%. The dust concentration that leaves the furnace is 5 g/m³ STP, at a gas flow of 50 m³ STP /s. The emission limit for dust is 50 mg/m³ STP. a. What is the dust concentration after the cyclone C2 (mg/m³ STP ) and is this acceptable? How much material (m2) is collected in cyclone C2 (kg/h)? Because there is a lot of carbon in the material collected in cyclone C1 it is decided to put it back into the furnace. The emissions from the furnace increase to 8 g/m³ STP. See Figure 2. b. What will happen to the emissions of the outlet concentration after cyclone C2, is it still acceptable and how much material (m2) is collected now in cyclone C2 (kg/h) februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz Electrostatic precipitators (ESP) RoNz 18

10 ESP: Typical lay-out of a wire-and-plate electrostatic precipitator RoNz 19 Electrostatic precipitators(esps) Where: (conventional pulverised coal combustion) Before (wet) scrubber for SO 2 control Before (hot side) or after (cold side) air preheat Usually before SCR for DeNOx ( hot side, low dust ) Before (hot side) or after (cold side) air preheat Alternative: baghouse filter, because 1) higher efficiency and 2) less effect of particle electric properties 4 process steps: 1. Particle charging 2. Particle movement relative to gas flow 3. Particle collection at deposition surface 4. Particle removal from deposition surface (often discontinuous) RoNz 20

11 ESP : basic principle, efficiency Typical grade efficiency curve Tubular ESP : basic design features RoNz 21 Electric field in ESP, configuration factor equipotential lines field intensity Electric field, E (V/m), and electric potential, (V): E= - Electric field as function of distance x from wire : E(x) = U /x F U = voltage difference electric field lines F = configiration factor of the electrode system wire-in-tube : F= ln (R/r) RoNz 22

12 ESP configuration factor a. Wire-in tube b. Wire-plate c. Multiple wire - plate = d/r, i.e. relative electrode spacing RoNz 23 ESP : particle charging 1, using corona discharge (uni-polar, one direction) Diffusion charging Small particles ( < 1 µm) charge q max ~ 10 8 e d p Note: charge e = C Field charging (Pauthenier (1932) Larger particles ( > 1 µm) relative dielectric constant, r dielectric constant of vacuum, 0 = C/(V m) charge q max ~ 12 E 1 d p ² 0 r / ( r + 2) E-field E 1 in charging zone ~ V/m RoNz 24

13 ESP : particle charging 2, particle drift velocity Alternative methods for charging : 1) Uni-polar (+ or -) : bi-polar corona 2) Uni-directional bi-directional field charging 3) Pulsed corona techniques 4) Impact (contact, tribo,...) charging Electrical mobility, v e of charged particle in E-field E 2 ~10 4 V/m Coulomb force = Stokes' drag force q p E 2 3 v e gas d p, with gas = dyn. gas viscosity (Pa.s) Result 1diffusion charging: v e 10 8 ee 2 /(3 gas ) ~ 0.01 m/s Result 2 field charging: v e E 1 E 2 0 d p / ( gas ( r +2))~ m/s RoNz 25 ESP efficiency : Deutsch equation Set-up: vertical gas flow, velocity u gas, plate height H, spacing D Mass balance for particle concentration, c: u ga D L ( c x -c x+x ) = v e ½ ( c x + c x+ x ) x L = mass removed v e = charged particle electrical mobility u gas Ddc/dx= - v e c Integrate, c = c in at x = 0, to position x : c(x) = c in exp ( - v e x / (u gas D)) c out = c in exp ( - v e A / Q gas x = H for gas flow Q (m³/s) and plate area A = 2 LH (2 sides!!!!!) H x x v e u gas D c x+ x c x L Efficiency ESP = 1 - exp ( - v e A / Q gas ) Deutsch Equation Corrected (Matts-Öhnfeldt) ESP = 1 - exp[-(v e A / Q gas ) k ], k = RoNz 26

14 Particle resistivity and electric drift velocity RoNz 27/59 ESP and (here:) fly ash resistivity Fly ash sulphur, temperature Moisture (300 F ~ 150 C, 200 F ~ 95 C, 450 F ~ 220 C) RoNz 28

15 Typical cold-side ESP for coal fly ash: design data Temperature C Power / collector area Gas flow velocity 1-3 m/s ash resistivity ohm.cm ~ 43 W/m 2 Gas flow / collector area s/m ash resistivity ohm.cm ~ 32 W/m 2 Plate-to-plate distance m ash resistivity ohm.cm ~ 27 W/m 2 Electric drift velocity m/s ash resistivity ~10 11 ohm.cm ~ 22 W/m 2 Corona current / collector area µA/m 2 ash resistivity ~10 12 ohm.cm ~ 16 W/m 2 Corona current / gas flow J/m 3 ash resistivity ~10 13 ohm.cm ~ 11 W/m 2 RoNz 29 Voltage, corona power, efficiency Source: C06 februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 30

16 8.5 Gas filtration RoNz 31 Filters : major types & characteristics Bag filters fiber materials : textile, plastics, ceramic Barrier filters sintered ceramic or metal, powders or fibers Granular bed filters layer of granular solids Factors determining filtration quality : Efficiency Pressure drop, pressure drop increase Filtration velocity = flow / filter area Medium properties : sustain, costs, cleanability Filter clean-up / regeneration RoNz 32

17 Gas flow options in baghouse filters Inside out Outside in RoNz 33 Dust accumulation and formation of cake RoNz 34

18 Dust accumulation on a wire Source: C06 d f 10 μm, d p 1 μm, u 50 cm/sec, ρ p kg/m 3, Stk 3.5, RoNz 35 Three baghouse cleaning methods Pulse-jet Shake / deflate Reversed-air RoNz 36

19 Filters : cleaning methods Type Method Mechanism Bag filter Pulse jet Inertia / drag forces Shaking Inertia Reverse flow Drag forces Granular bed filter fixed bed Reverse flow Elutriation moving bed Media recycle Elutriation Ceramic bag filter Pulse jet Inertia / drag forces Barrier filters Pulse jet Drag forces RoNz 37 Particle capture by a filter fibre RoNz 38

20 Particle capture by a filter fibre Source: C06 RoNz 39 Filtration efficiency of a 5 m fibre dust in ambient air, a = particle size, v = gas velocity Capture mechanism contours Efficiency contours RoNz 40

21 Filter efficiency as function of particle size 100 Removal efficiency (%) 50 Brownian motion electrostatic forces inertia, interception gravity 0 gas velocity Particle size (m) RoNz 41/59 Fabric filter cloth characteristics (Data in brackets) = Registered Trade Names RoNz 42

22 8.6 Hot gas filtration RoNz 43 Hot gas /HTHP clean-up RoNz 44

23 Hot gas filtration concepts and their status at early 2000s RoNz 45 HTHP ceramic filters Ceramic candle filters Ceramic cross-flow filter RoNz 46

24 Hot gas clean-up: ceramic candle filters RoNz 47 Optimised ESP for HTHP operation New concepts (developments 1990s) RoNz 48

25 ESP for high temperatures and pressures Fly ash resistivity (typical) RoNz 49 HTHP granular bed filters + s and - s Principle A stagnant or moving bed of coarse (~3 mm) granular solids In principle chemically inert Filter velocities ~0.1-1 m/s +++ High filter velocity Cheap granular materials Also gases may be removed e.g. HCl, SO 2, alkali Catalytically active medium can be used Continuous operation possible --- Too low efficiency (90-95%) Attrition of the medium Problems with difficult dust e.g. sintering / agglomeration Dust re-entrainment Reliability Medium regeneration problems RoNz 50

26 Moving screenless granular bed filter (Combustion Power Co., USA) RoNz Wet scrubbers RoNz 52

27 Wet scrubbers Source: C06 Liquid droplets (sometimes films) are introduced to the gas flow, these collide with solid particles. Later a liquid (water) / solid separation is needed. See next slide for Performance characteristics. februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 53 Wet scrubbers Source: C06 Separation efficiency where R X = effective collector size, d w = water droplet diameter februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 54

28 8.8 Exercises 14 RoNz 55 Exercises An electrostatical precipitator (ESP) is used to remove particles from a Q = 150 m³/s gas stream at 150 C. K The efficiency of the ESP is given by Efficiency 1 exp( ve A Q with A=collection surface (m²), v=electrical drift velocity of particle (m/s) and K=Matts-Öhnfeldt parameter. For particles with size d p = 1 µm, ESP efficiency = For these particles, K = The electrical mobility v e (m/s) can be related to particle size d p (in µm) as ve 0.2d p a. Calculate the specific collection area A/Q (s/m) for this ESP, and the collection surface A (m²). b. Calculate the efficiency of the ESP for 1) 0.2 µm particles and 2) 5 µm particles c. For another ash, with a different size distribution, K = Calculate the efficiency of the ESP for 1 µm particles of this ash. Is the ESP better or less good for this second ash than for the first ash? d. For the second ash, how can the operation of the ESP be changed so that the efficiency is the same as for the first ash? ) februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 56

29 Exercises A ceramic filter is used to remove fly ash particles from a hot flue gas, at C, 12 bar, at filter inlet dust concentrations of 1-2 g/m³. The pressure drop across the filter, p, and how fast the pressure drop increases during the filtration, are determined by the temperature and the inlet concentration as given by the two Figures below. The filter is cleaned by a reversed gas stream every 30 minutes. The relation between pressure drop, p and time, t, after a cleaning step is given by p filter = p baseline + (dp/dt) * t base line pressure drop (kpa) temperature ( C) pressure drop increase dp/dt (Pa/s) inlet dust concentration (g/m^3) februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 57 Exercises continued Calculate the pressure drop across the filter just before and immediately after a cleaning step, when operating at 800C, 1.5 g/m³ inlet concentration. Give a plot of pressure drop as function of time for this process. Instead of cleaning every 30 minutes, it is decided to clean the filter when the pressure drop is equal to 40 kpa. How long can the filtration time between two cleaning stages be for the two cases 1) 800C, 1 g/m³ inlet dust concentration 2) 800C, 2 g/m³ inlet dust concentration februari 2014 Åbo Akademi University - Värme- och Strömningsteknik RoNz 58

30 Further reading Scarlett, B., Vervoorn, P.M.M. Particle technology I, course notes Delft Univ. of Technol., Delft (1988) Iinoya, K., Gotoh, K., Higashitani, K. (1991) Powder technology handbook, Marcel Dekker, New York Coulson, J.M., Richardson, J.F., Backhurst, J.R., Harker, J.H. Chemical Engineering, Vol. 2 : Unit Operations Pergamon Press, Oxford (1983 Klingspor, J.S., Vernon, J.L. Particulate control for coal combustion report IEACR/03 IEA Coal research, London (1988) Bernard, J.G. Experimental investigation and numerical modelling of cyclones for application at high temperatures, PhD thesis Delft Univ. of Technol., Delft (1992) Mitchell, S.C. Hot gas filtration report IEACR/95 IEA Coal research, London (1997) Carpenter, A.M. Switching to cheaper coals for power generation report CCC/01 IEA Coal research, London (1998) Böhm, J. Electrostatic precipitators, Elsevier Sci. Publ. Co., Amsterdam (1982) Zevenhoven, C.A.P. Particle charging and granular bed filtration for high temperature application PhD thesis Delft Univ. of Technol., Delft (1992 Zevenhoven, C.A.P. Uni-polar charging of particles: effects of particle conductivity and rotation, J. Electrostat. 46 (1999) 1-12 R. Zevenhoven, P. Kilpinen "Control of pollutants in flue gases and fuel gases" Picaset Oy, Espoo (ISBN ) ( 298 pp.) 3rd Ed. February 2004, Chapter 5 online at C06: Crowe, C.T., ed., Multiphase Flow Handbook. CRC Press, Taylor & Francis (2006), Chapter 7 RoNz 59