PARTICULATES #1. Introduction

Transcription

1 Introduction PARTICULATES #1 Ash-forming elements in fuels Particulate emission standards Options for particulate control emissions Gravity settlers Gas cyclones Electrostatic precipitators see:

2 Particulate (emissions) control : why? Regulations considering environmental / health hazard Protection of gas turbines / expansion turbines Protection / avoid problems with other gas clean-up equipment The particulate solid or the gas may be a valuable product Dust explosion risks.

3 Ashes and solid residues during typical pulverised coal combustion

4 Typical size distribution for fly ash and bottom ash from pulverised coal combustion

5 Ash-forming elements and ash formation #1 Coalification Mineral impurities during combustion or gasification

6 Ash-forming elements and ash formation #2 Pulverised coal combustion Fluidised bed coal combustion

7 Ash-forming elements and ash formation #3 Biomass fuels

8 Ash content of fuels (dry %-wt)% Fossil fuels Biomasses & waste derived fuels Coal, lignite 5-40 Wood Bark 2-8 Oil < 0.1 Straw 4-8 Natural gas - Light fuel oil < 0.01 Sewage sludge Heavy fuel oil ~0.04 Car tyre scrap 5-8 Munical solid waste (MSW) 5-25 Refuse derived fuel (RDF) Peat 4-10 Packaging derived fuel (PDF) 5-15 Auto shredder residue (ASR) ~25 Petroleum coke, petcoke ~1 Leather waste ~5 Estonian oil shale ~40 Orimulsion ~1.5 Black liquor solids Problem: Ash content of fuel Ash forming elements Fly ash, bottom ash

9 Ash production from western US coal combustion in a 500 MW elec pulverised coal power plant Bituminous Wyoming Powder River Basin Montana Powder River Basin Coal ash content, %-wt Bottom ash, ton/year Fly ash, ton/year Total ash, ton/year

10 Dust emission standards for EU Large combustion plants (LCPs) (directive 2001/80/EC)

11 Dust emission standards for waste (co-) ) firing and cement plants for EU (directive 2000/76/EC) C co-firing = (V( waste. C waste + V process process.c process )/( )/( V waste + V process ), V = exhaust volume

12 Removal of particulates from (flue) gases 1. Methods based on external forces 2. Methods based on barriers Decreasing particle size Gravity settlers Cyclones & centrifuges Electrostatic precipitators Bag filters Ceramic barrier filters Granular bed filters Wet scrubbers

13 Parameters determining particulate control Process Particle Temperature Size distribution Pressure Shape Gas flow Surface properties Concentration Chemical composition : - carbon content - alkali content - tar content - sulphur content Melting point, softening point Chemical stability

14 Particulate removal efficiencies needed for various coal firing methods

15 Efficiencies of several particulate control devices

16 An inertial separator: a settling chamber

17 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 µm Problems above ~ 400 C

18 Cyclones: processes determining separation

19 A standard cyclone (Lapple( Lapple) W D e H S D L b L c High Conventional High efficiency throughput Height of inlet H/D 0.5 ~ ~ 0.8 Width of inlet W/D 0.2 ~ ~ 0.35 Diameter of gas exit De/D 0.4 ~ Length of vortex finder S/D ~ ~0.85 Length of body Lb/D 1.5 ~ ~ ~1.7 Length of cone Lc/D ~2.0 Diameter of dust outlet Dd/D ~ ~ ~ 0.4 D d

20 Removal efficiency for Lapple cyclone Number of gas turns (i.e. revolutions) before entering the vortex finder: N = L b L + 2 H Grade efficiency: Eff ( d ) = p c 1 d + d 1 50 p 2 d 50 Cut size : Typical material properties: dynamic gas viscosity : η gas (T/293) 2/3 Pa.s densities : ρ solid = typically kg/m³ = 1.2 kg/m³ at 20 C, 1 bar (air) ρ gas = 9η W gas 2π NV ( ρ ρ ) in solid gas

21 Forces on particles in cyclones Centrifugal force m p ω² r = m p v t ² / r Force balance gives equilibrium radial position: r Drag force (Stokes) 3 π v r d p η F R 1. m p v t ² / r = 3 π v r d p η F 2. v r v i A/ (2πr r h) (h=length of cylindrical section) 3. v t r n = v i R n, n ~ gives (r/r) n = πhρ s v i d 2 p/(9 AηA F ) into cyclone with velocity v i, inlet area A for capture: large r/r needed

22 Advanced (?) gas cyclone designs Cyclone with vortex collector pockets Aerodyne rotary flow cyclone

23 Typical lay-out of a wire-and and- plate ESP

24 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)

25 ESP : basic principle, efficiency

26 Tubular ESP : basic design features

27 Electric field in ESP, configuration factor equipotential equipotential lines lines field field intensity intensity Electric field, E (V/m), and electric potential, φ (V): E = - φ Electric field as function of distance x from wire : E(x) = U U /x F electric electric field field lines lines wire-in in-tube : F= ln (R/r) U U = voltage difference F= configiration factor of the electrode system

28 ESP configuration factor a. Wire-in in tube b. Wire-plate c. Multiple wire - plate δ=d/r, i.e. relative electrode spacing

29 An ESP at Kotka Finland

30 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, ε r dielectric constant of vacuum, ε 0 = C/(V m) charge q max ~ 12 π E 1 d p ² ε 0 ε r / (ε( r + 2) E-field E 1 in charging zone ~ V/m

31 Classical particle charging theory calculations

32 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 E 2 ~ 10 4 V/m: Coulomb force = Stokes' drag force q p E 2 3π v e η gas d p, with η gas = dynamic gas viscosity (Pa.s) Result 1: diffusion charging : v e 10 8 e E 2 / ( 3 π η gas ) ~ 0.01 m/s Result 2: field charging : v e E 1 E 2 ε 0 d p / (η( gas (ε r + 2)) ~ m/s

33 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 x L = mass removed v e = charged particle electrical mobility u gas D dc/dx 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 Efficiency η ESP = 1 - exp ( - v e A / Q gas ) Deutsch Equation c x L Corrected (Matts( Matts-Öhnfeldt) ) : η ESP = 1 - exp - (v e A / Q gas ) k k =

34 ESP and fly ash resistivity Fly ash sulphur,, temperature Moisture (300 F ~ 150 C, 200 F ~ 95 C, 450 F ~ 220 C)

35 Particle resistivity and electric drift velocity

36 Rebouncing of particles with (too) high conductivity

37 Difficult conditions for ESP ( )( and options for improvement ( )(

38 ESP performance in relation to fuel-sulphur: the effect of fuel switching (PRB is coal from Powder River Basin, Western USA)

39 ESP : flue gas conditioning (EPRICON)

40 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