ENE HELSINKI UNIVERSITY OF TECHNOLOGY PARTICULATES #2

Transcription

1 PARTICULATES #2 Filter systems Wet scrubbers Cost comparison cyclone / ESP / filter High temperature high pressure particulate control Particulate emission control for vehicles see:

2 Principle of filtration Feed Cake Medium Filtrate

3 Filters : major types & characteristics Bag filters Barrier filters Granular bed filters fiber materials : textile, plastics, ceramic sintered ceramic or metal, powders or fibers 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

4 Gas flow options in baghouse filters

5 Dust accumulation and formation of cake

6 Three baghouse cleaning methods Pulse-jet Shake / / deflate Reversed-air

7 A shake/deflate-cleaned baghouse filter

8 Filters : cleaning methods Type Bag filter Granular bed filter fixed bed moving bed Ceramic bag filter Barrier filters Method Pulse jet Shaking Reverse flow Reverse flow Media recycle Pulse jet Pulse jet Mechanism Inertia / drag forces Inertia Drag forces Elutriation Elutriation Inertia / drag forces Drag forces

9 Particle capture by a filter fiber

10 Filtration efficiency of a 5 µm fiber dust in ambient air, a = particle size, v = gas velocity Capture mechanism contours Efficiency contours

11 Filter efficiency as function of particle size 100 Removal efficiency (%) 50 Brownian motion inertia, interception gravity electrostatic forces 0 gas velocity Particle size (µm)

12 Filtration theory : #1 of 4 1. Flow through a packing (Re < 0.2) : Darcy s Law p u = K with permeability, K Lη velocity, u layer thickness, L pressure drop, p dynamic viscosity, η fluid cake solids mass/m 2, w fluid 2. Specific cake resistance, α : 1 α = with cake porosity, K( 1 ερ ) solid ε 3. Ruth equation u 1 p p = = α( 1 ε) ρ Lη ( αw+ R) η solid fluid fluid using dw = (1-ε)ρ solid dx (per m 2 ) with medium resistance, R

13 Solids fraction S s (kg/kg) Solids fraction S c (kg/kg) Volume V (m 3 ) Filtration theory : #2 of 4 Cake Medium 4. Mass balance (w in kg/m² filter surface) w w Ss = ρ fluidv + ( 1 Sc) Sc 1 S = Vf( ρ, S, S) f ( ρ, S, S ) fluid c s fluid c s 1 Ss 1 S S S Note : u and V per m² surface, A! Throughput Q (m³/s) = ua = s ρ s fluid = c c

14 Filtration theory : #3 of 4 u 5. Constant pressure drop filtration t / V R η fluid / p dv η fluidα f dt p V η fluid R 2 = + p V = 2 * * * V * * * * θ tan θ = η fluid α ƒ/2 p t

15 Filtration theory : #4 of 4 u R η fluid u 6. Constant velocity (or: flow) filtration dv 2 = = constant p= η fluid ( αf u t + ur) dt p * * * * t * * * θ tan θ = η fluid α ƒu 2

16 Fabric filter cloth characteristics (1996) (Data in brackets) = Registered Trade Names

17 Properties of fiber materials for high temperature filters

18 Granular bed filters Long experience (~80 yrs) at temperatures up to ~ 450 C Relatively small filter unit size due to relatively high filter velocity Relatively large influence of dust particle properties : Dust particle shape Dust particle to bed particle adhesion Types : Packed bed filters Efficiencies ~ 99 % Moving bed filters Efficiencies ~ 95 % Fluidised bed filters Efficiencies ~80 %

19 Wet particulate collectors

20 Wet scrubbers : some characteristics Collects still very fine particles, and also gases & alkali Low capital costs compared to ESP and baghouse filters High pressure drop, operation costs, up to % s of a power plant net output Gaseous waste stream liquid waste stream Typical data : Gas inlet velocities ~ 100 m/s Collection efficiencies ~ 99 % Pressure drop up to 1 bar (!) Operation problems : corrosion, abrasion, solids build-up, rotating parts failure re-start after a down-period

21 Wet scrubbers : operating characteristics

22 Cost comparison cyclone, ESP, baghouse Cost comparison for particulate control equipment at 10 MWthermal Efficiency % Capital cost US$ 1982 Operation cost US$/ton removed High efficiency cyclone ESP Reverse air baghouse Assumptions: coal ash, electricity costs US$/kWh, 8000 h/year, filter bags lifetime 2 years

23 Hot gas /HTHP clean-up

24 Hot gas /HTHP clean-up : options

25 Hot gas filtration concepts and their status

26 Hot gas clean-up: gas turbine specifications

27 HTHP ceramic filters (Siemens Westinghouse) Ceramic candle filters Ceramic cross-flow filter

28 Hot gas clean-up: ceramic candle filters

29 Hot gas clean-up: ceramic tube filters (Asahi Glass Co.)

30 Hot gas filter candle degradation problems

31 HTHP ceramic filter : ash deposit chemistry Foster Wheeler (Livingstone, NJ) Siemens Westinghouse candle

32 Optimised ESP for HTHP operation New concepts

33 ESP for high temperatures and pressures Fly ash resistivity

34 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 High filter filter velocity Cheap granular materials Also Also gases gases may may be be removed e.g. e.g. HCl, HCl, SO SO 2, 2, alkali alkali Catalytically active active medium can can be be used used Continuous operation possible --- Too Too low low efficiency (90-95%) Attrition of of the the medium Problems with difficult dust e.g. e.g. sintering // agglomeration Dust Dust re-entrainment Reliability Medium regeneration problems

35 Moving screenless granular bed filter (Combustion Power Co., USA)

36 HTHP particulate removal options

37 Particulates from vehicles Petrol driven cars : Leaded gasoline, Pb(Et) g / km Size distribution 80% < 2µm Lead compounds % of the total unleaded fuel ~10x cleaner Petrol and diesel driven cars : High boiling-point C x H y fractions of the fuel Fuel rich conditions give smoke / soot Sulphates Exhaust limit for diesel driven cars (EU) : 1997 : 0.08 g/km 2000 : 0.05 g/km 2005 : g/km

38 Particle trap for diesel engine exhaust