Modelling Fine Particle Formation and Alkali Metal Deposition in BFB Combustion

Transcription

1 Modelling Fine Particle Foration and Alkali Metal Deposition in BFB Cobustion Jora Jokiniei and Olli Sippula University of Kuopio and VTT, Finland e-ail: Flae Days, Naantali

2 Outline of the Presentation 1. Introduction. Experiental data for the odelling 3. KCAR odel Aerosol dynaics Deposition odelling 3. Results Aerosol foration in the boiler Deposit foration Effect of fuel on the deposits 4. Conclusions

3 University of Kuopio VTT TECHNICAL RESEARCH CENTRE OF FINLAND 3 VTT TECHNICAL RE SEARCH CENTRE OF FINLAN D Introduction The goals of this research are to odel: deposit foration in bioass cobustion echaniss & process conditions controlling hard deposit foration factors controlling corrosive deposit foration possible easures to avoid above entioned deposit foration

4 4 Deposition of ash on heat exchangers Therophoresis and diffusion of fine particles (< 1 u) Condensation and reactions of vapours Ipaction of large particles (> 10 u) Sintered layer Solid layer Solid porous deposit can be reoved by soot blowing Sintered deposit is difficult to reove Sintering caused by partial elting of alkali species deposited ainly as fine particles and vapours Also cheical reactions ay induce Sintering, for exaple sulphation of alkali chlorides Chlorides are corrosive Tube surface

5 5

6 6 KCAR Model Solves Nuerically The behaviour of alkali vapours and particles gas phase species cheistry fine particle foration and growth - particle size and coposition Deposit foration fine particle deposition echaniss coarse ash deposition echaniss vapour deposition deposit layer growth rate and cheical coposition

7 7 INPUT DATA in the BFB -case study: Fine particle foring eleents: all eleents are volatilised during cobustion and are in the vapour phase at axiu teperature 900 C) in the freeboard - (KOH, KCl, K, KO, NaOH, NaCl, volatilisation of ash eleents is taken fro boiler easureents before ESP by iteration Large ash particles: aount, size distribution and coposition is given as input Boiler geoetry, flow rates, heat exchanger tubes, teperatures

8 66 MW BFB -boiler 8 [9] 700 C; 9. in 480 C out [8] 750 C; C [7] 750 C; 7.9 in 481 C in [6] 870 C; C [5] 880 C; 4 SH SH1 [4] 900 C; [10] 700 C; 1.3 SH3 [11] 500 C; 14.9 [1] 500 C; 15.8 [13] 440 C; 17.7 out 469 C [15] 40 C; 5. in 74 C out 75 in C 76 [14] 430 C; 1.7 C Evaporator [16] 415 C; 7. out 76 C 1 3 calculated cases: - wood residues (saw ill, forest) clean superheaters - wood residue with 5 % chipboard clean superheaters dirty superheaters (higher superheater surface teperatures) [3] 900 C; 1.5 [] 950 C; 1 [1] 900 C; 0.5 [0] 950 C; 0 KCAR Nodalization of the Forssa BFB Boiler in 5 C Stea tep. Econoizer [17] 15 C; 38. Measureents before ESP

9 9 Aerosol dynaics odelling The code used, solves the GDE using a sectional ethod in 1-D dn k dx 1 * dnk Jk( k k ) u dx coag dnk dx grow v d A d uv n k Particle concentration (in size class dx) Hoogeneous nucleation coagulation Growth by condensation and cheical reactions Deposition by: condensation therophoresis diffusion ipaction (or evaporation)

10 10 Cheistry in foration of fine particles Based on therodynaic equilibriu Kinetics and ass transfer can be considered Following global reactions are iportant: KOH(g,c) +SO (g)+½o (g) K SO 4 (g,c) + H O(g) KCl(g,c) + H O(g)+SO (g)+½o (g) K SO 4 (g,c) + HCl(g) KOH(g,c) + CO K CO 3 (c)+so (g)+½o (g)+½o (g) K (g) K CO SO 4 3 (c)+ H O(g) (c)+ HCl(g) Sae for Na

11 11 Deposition Chloride vapours Alkali aerosol particles & coarse ode particles Vapour condensation cheical reactions Fine particles therophoresis Brownian diffusion Condensation on aerosol particles Therophoresis Condensation on deposit layer Sulphation SO HCl Turbulent flow in staggered tube array Boundary layer Large ash particles direct ipaction (cross flow - windward side) turbulent deposition (surface parallel to flow - platens) Coarse particle sticking Sintering & reovability by sootblowing Corrosion Heat transfer Diffusion in porous deposits

12 1 Dependence of the deposition rates on flow variables: Vapour condensation Sh(Re,Sc) * pv Sc = rate of oentu transport / rate of ass transport Re = U * dc / u Fine particle therophoretic deposition Nu(Re,Pr)*T/(T*d) Pr = rate of oentu transport / rate of energy transport Large ash particles - cross flow f(re * ) * Up * D p / d c - parallel flow U p * fr(nu,re,pr) 3/ *Re*4 / d x

13 13 Results Fine particle coposition and size distributions: Alkali sulphates fored fine particles prior to superheaters Alkali chlorides condensed on the sulphate particles after superheater section the siulated particle size distribution in agreeent with the easureents upstrea of the ESP d/dlogdp [g/n3] Alkali species in particles inside Particle Mass Size Distributions 1.0 superheaters 1.80E E E MODEL 1.60E-0 KSO4 1.40E E KSO4 MEASURED 1.0E-0 1.0E E E-0 NaSO4 8.00E E E E-03 NaSO4 KCl 4.00E E-03 NaCl.00E-03.00E E E Dp [µ] Particle size Dp [µ] Dp [µ] dm/dlogdp [g/n**3] dm/dlogdp [g/n3] Particle size distribution of condensed alkali species after superheaters

14 Potassiu concentration [g(k)/n3] VTT University TECHNICAL of Kuopio RESEARCH CENTRE OF FINLAND 8.0E E E E E E-06.0E E E Potassiu speciation and particle size distributions, fuel 1 KOH (g) KCl (g) KSO4 (g) Location [] KSO4 (p) KCl (p) Superheaters 1- Superheater 3 Particle nuber concentration (#c3) 3.E+07.E+07.E+07 1.E+07 5.E+06 0.E+00 Particle nuber concentration Mean particle size Mean Particle size (n) Location []

15 15 Particle Deposition velocities at different locations Deposition velocity Vd[/s] 1.0E E E E-0 SH1-inlet SH1-inside Evaporator-inlet Eko-inside turbulent & inertial ipaction 1.0E Therophoresis & diffusion Therophoresis Particle size d pa [µ]

16 16 Deposition growth (alkali species only): clean superheater tubes FUEL: Wood residue+chipboard Deposition growth rate Vd [/day] Evaporator Econoizer K NA O S CL C Location [] Superheaters 1-3

17 17 Deposition growth (alkali species only): dirty superheater tubes FUEL: Wood residue+chipboard Deposition growth rate Vd [/day] Evaporator Econoizer K NA O S CL C Location [] Superheaters 1-3

18 18 Results Effects of fuel Chipboard containing chlorine-rich fuel released ore condensible alkali species increased the alkali deposit growth approxiately 3 fold when copared to pure wood residue wood residue Deposition growth rate in the superheater section condensible species only wood residue with 5% chipboard Deposition growth rates in the superheater section condensible species only Deposition growth rate Vd[/day] K NA O S CL C Deposition growth rate Vd[/day] X[] X[]

19 19 Results Deposition Large ash deposition doinates (if the tube surface is sticky) Deposition of chlorine 3-6 % (clean tubes) Deposition of alkali etals 5 % (clean tubes) In the superheaters alkali chlorides are ainly deposited by direct condensation

20 Conclusions on the BFB case study 0 Deposit cheical coposition is different to that of fly ash due to different deposition echaniss and gas phase reactions taking place total deposition rates: /day in cross flow (windward) - 3 /day in flow parallel to surfaces (platens) alkali copound deposition rates: - up to 0,3 /day at the superheater inlets large particles concentration and size iportant paraeters for deposition increasing fue particle size decreases deposition

21 Conclusions () 1 The odel provides inforation on the ash behaviour and deposition characteristics based on particle easureents prior to filters and teperature easureents in the boiler. good agreeent between calculated and easured deposits (Mikkanen et al., 000) experiental heat and ass transfer correlations for fue and vapour deposition ipleented (correct boiler geoetry) large particle deposition odel iproved into the KCAR code In future? verifying results by easureents large particle deposition growth estiates require data on the stickiness of the surfaces.

22 University of Kuopio VTT TECHNICAL RESEARCH CENTRE OF FINLAND VTT TECHNICAL RE SEARCH CENTRE OF FINLAN D Acknowledgeents The author acknowledges the Finnish funding agency for Technology and Innovation (TEKES), VTT, Univ. of Kuopio, Forssan Energia Oy, Metso and Foster Wheeler for funding this research.