Ash and ash deposition for solid fuels Bengt-Johan Skrifvars FPK II, ÅA, PDF Free Download

Ash and ash deposition for solid fuels Bengt-Johan Skrifvars FPK II, ÅA, 2018

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Why? Ash related problems Slagging, fouling and corrosion most important single reason for unscheduled shut downs of boilers Fireside deposits on heat exchanger tubes - decreased heat transfer to steam/water side - increased pressure drop in fluegas channel - corrosion of heat exchanger tubes Emission problem Trace elements, health risk

Amager Power Station: Pendant SH after 1 week of Coal-Firing /F.Frandsen DTU Ash Chemistry Course, October 1998/

Ash related problems Depend on - fuel ash content and type - boiler type and operation Fireside deposits and corrosion - all types of boilers Bed agglomeration - fluidized bed boilers Trace emissions - all types of boilers

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Slagging, fouling Fly ash separation Transportation, transformation, reactions Fuel Air Additives Release of ash forming elements Bottom ash Pathways of ash forming elements entering a boiler

Formation of a troublesome deposit: Fuel Formation of ash particles Transportation of ash particles to a surface Adhesion of ash particles to a surface Densification of ash particles on a surface

Ash-forming elements in a fuel Ash = incombustible rest Quality & quantity depends on fuel Major elements: Si, Al, Fe, Ti, Ca, Mg, Mn, P, Na, K, S, Cl Minor elements (trace elements, EU heavy metals): As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, V

Ash-forming elements in a fuel Expressed often as weight-% oxides in ash Si SiO 2 Mn MnO Al Al 2 O 3 P P 2 O 5 Fe Fe 2 O 3 Na Na 2 O Ti TiO 2 K K 2 O Ca CaO S SO 3 Mg MgO Cl Cl Elements as oxides in ash is an assumption everything in the fuel has oxidized If all elements have been analyzed oxide sum = 100 %

Ash-forming elements in a fuel Minerals: - included minerals - excluded minerals - Si, Al, Fe, Ca, Mg, Na, K, S Included mineals Excluded minerals Organically associated: - Ca, Mg, K, Na, S Na + O - O Org S Water soluble: - Na, K, S, Cl H 2 O-soluble salts

Summary Ash-forming elements in a fuel FUEL ORGANICALLY MINERAL ASSOCIATED PART Brown coal 30 % 70 % Bit. coal 15 % 85 % Antrasite 5 % 95 % Wood 100 % 0 % Bark 98 % 2 % Annual biom. 98 % 2 % Oil 100 % 0 % Waste derive 2-100 % 100-2 %

Formation of a troublesome deposit: Fuel Formation of ash particles Transportation of ash particles to a surface Adhesion of ash particles to a surface Densification of ash particles on a surface

Homogeneous nucleation Coagulation Heterogeneous condensation Fly ash 0.1-1 mm Mineral inclusions Convective transport Vaporization Excluded minerals Pyrolysis Char burning and fragmentation Formation of fly ash from coal /Flagan & Seinfield 1988/ Mineral coalescence and fragmentation Fly ash 1-100 mm

Mass concentration frequency dm/d(logdp) g m 3 2.4 2.0 1.6 Minerals 1.2 0.8 Particles condensed from gas phase (aerosols) 0.4 0.01 0.1 1 10 100 mm Particle size dp Mass-size distribution of fly ash from PC coal combustion /Flagan & Seinfield 1988/

Mass concentration frequency, dm/d(logdp)?? g/m 3???

Number Number frequency frequency Let s consider: 1000 particles 0 50 mm size nbr 0-4 mm 104 4-6 mm 160 6-8 mm 161 8-9 mm 75 9-10 mm 67 10-14 mm 186 14-16 mm 61 16-20 mm 79 20-35 mm 90 35-50 mm 18 S 1000 200 160 120 80 40 10 20 30 40 50 /Hinds, W: Aerosol technology 2 nd Ed. Wiley & Sons, 1999/ Particle size, mm

Number frequency/size 1000 particles 0 50 mm nbr size nbr size 0-4 mm 104 26 4-6 mm 160 80 6-8 mm 161 80.5 8-9 mm 75 75 9-10 mm 67 67 10-14 mm 186 46.5 14-16 mm 61 30.5 16-20 mm 79 19.7 20-35 mm 90 6 35-50 mm 18 1.1 S 1000 mm -1 100 80 60 40 20 10 20 30 40 50 Particle size, mm

Number fraction/size 1000 particles 0 50 mm nbr nbr fract size nbr size size 0-4 mm 104 26 0.026 4-6 mm 160 80 0.080 6-8 mm 161 80.5 0.0805 8-9 mm 75 75 0.075 9-10 mm 67 67 0.067 10-14 mm 186 46.5 0.0465 14-16 mm 61 30.5 0.0305 16-20 mm 79 19.7 0.0197 20-35 mm 90 6 0.006 35-50 mm 18 1.1 0.0011 S 1000 mm -1 0.1 0.08 0.06 0.04 0.02 dn/d(dp) = f(dp) 10 20 30 40 50 Particle size, mm

Mass fraction/size 1/mm 0.1 1000 mg of particles 0 50 mm mass mass mass fract size (mg) size size 0-4 mm 104 26 0.026 4-6 mm 160 80 0.080 6-8 mm 161 80.5 0.0805 8-9 mm 75 75 0.075 9-10 mm 67 67 0.067 10-14 mm 186 46.5 0.0465 14-16 mm 61 30.5 0.0305 16-20 mm 79 19.7 0.0197 20-35 mm 90 6 0.006 35-50 mm 18 1.1 0.0011 S 1000 0.08 0.06 0.04 0.02 dm/d(dp) = f(dp) 10 20 30 40 50 Particle size, mm

Concentration frequency/size 1000 mg/nm 3 of particles 0 50 mm conc conc/size size (mg/nm 3 ) (mg/nm 3 /mm) 0-4 mm 104 26 4-6 mm 160 80 6-8 mm 161 80.5 8-9 mm 75 75 9-10 mm 67 67 10-14 mm 186 46.5 14-16 mm 61 30.5 16-20 mm 79 19.7 20-35 mm 90 6 35-50 mm 18 1.1 S 1000 mg/(nm 3 mm) 100 80 60 40 20 dc/d(dp) = f(dp) 10 20 30 40 50 Particle size, mm

Mass concentration frequency dm/d(logdp) mg Nm 3 6000 5000 4000 3000 2000 1000 0.01 0.1 1 10 100 mm Particle size dp Mass-size distribution of fly ash from PC coal combustion /Jokiniemi & Kauppinen, 1995/

Mass concentration frequency, dm/d(logdp) = - average mass of particles within a certain particle size range - particle size range expressed on a log-scale - may be treated mathematically as a frequency function for ex. total mass concentration = total area under the curve mass concentration within a certain range log(dp) 0 f(log(dp)d(log(dp))) g/m 3 = - should actually be g/m 3 /1 since the term log(dp) is in the denominator

~10 µm r p : particle density C(dp): Cunningham slip factor function v: jet velocity dp: particle diameter m: gas viscosity W: jet diameter Stk = r p C(dp) v dp 2 9 m W ~10 nm Particle size-distribution measurements with a low-pressure cascade impactor /Berner 1972/

Number concentration frequency dn/d(logdp) 1 cm 3 1E+08 1E+07 1E+06 1E+05 1E+04 0.01 0.1 1 10 Particle size dp mg Nm 3 100 6000 5000 4000 3000 2000 1000 mm Mass concentration frequency dm/d(logdp) Number-size and mass-size distribution of fly ash from PC coal combustion /Jokiniemi & Kauppinen, 1995/

Particle size plots - number-size distribution, mass-size distribution, or concentration-size distribution - expressed often as a frequency-per-size function - x-axis particle size range often expressed on a log-scale - y-axis numbers do not express directly number-, mass-, or concentration values. Particle size measurements - Low pressure cascade impactor useful - Gives mass vs size or number vs size information ~10 nm 10 mm

Formation of ash particles Ash formation from other fuels, indications from coal: High amount of organically associated minerals a lot of sub-micron sized fly ash particles High amount of excluded minerals a lot of larger fly ash particles

Formation of a troublesome deposit: Fuel Formation of ash particles Transportation of ash particles to a surface Adhesion of ash particles to a surface Densification of ash particles on a surface

Transport of ash particles to a surface Diffusion small particles (< 0.5-5 mm) Impaction large particles (> 0.5-5 mm) /Hedley et al., Samms et al. 1966/

Transport of ash particles to a surface Diffusion - small particles (< 0.1 mm) - diffusion in direction of concentration gradient (Fick s law) Thermophoresis - small particles (< 5 mm) - diffusion in direction of temperature gradient Inertial impaction - large particles (> 5 mm) - dependent on gas velocity - angle of impact

Temperature Particle net movement gas molecule movements small ash particle gas molecule movements The physics of thermophoresis

Transport of ash particles in a boiler Large particles Small particles

Transport of ash particles to a surface Summary Diffusion - small particles (< 0.1 mm) - diffusion down the concentration gradient (Fick s law) - Termophoresis one diffusion mechanism in heat gradient direction Impaction - large particles (> 5 mm) - dependent of gas velocity & particle mass

Formation of a troublesome deposit: Fuel Formation of ash particles Transportation of ash particles to a surface Adhesion of ash particles to a surface Densification of ash particles on a surface

2-component phase diagram o C 900 800 Liquid T 100 700 NaCl(s)+ L Na 2 SO 4 (s)+l 600 NaCl(s)+Na 2 SO 4 (s) T 0 500 0 50 100 NaCl mol-% Na 2 SO 4

Amount of melt = Amount of solid = o C 900 800 Lever-rule A - B C - B A - C C - B x 100 % x 100 % Liquid Bulk composition = A Liquid composition = C Solid composition = B T 100 700 600 Na 2 SO 4 (s)+l NaCl(s)+ L C A B NaCl(s)+Na 2 SO 4 (s) T 0 500 0 50 100 NaCl mol-% Na 2 SO 4

Amount of melt, w-% Amount of melt vs temperature 100 80 85 mol-% Na 2 SO 4 15 mol-% NaCl 60 40 20 0 500 600 700 800 900 Temperature, ( C)

Deposit at its initial growth T fluegas 1000 o C T sticky, sticky temperature silicates, glas, slag : viscosity < 10 5 dpa s /Walsh et al 1990/ 450 o C T steam T sticky low-viscous melt: melt amount > 15 % /Backman et al 1987/

Deposit at its equilibrium thickness T flow T fluegas 1000 o C T flow, flow temperature silicates, glas, slag : viscosity < 10 5 dpa s /Walsh et al 1990/ 450 o C T steam low-viscous melt: melt amount > 70 % /Backman et al 1987/

Amount of melt, w-% Melting behavior of different alkali salts 100 90 80 70 60 50 40 30 20 10 0 T 70 T 15 500 550 600 650 700 750 800 850 900 950 1000 Temperature, o C

Deposit thickness, mm 10 Deposit equilibrium thickness 8 6 4 2 T 15 T 70 0 600 700 800 900 1000 Temperature, o C

Deposit thickness, mm Deposit equilibrium thicknesses for various compositions 10 8 T 15 = 850 o C, T 70 = 900 o C T 15 = 710 o C, T 70 = 830 o C T 15 = 530 o C, T 70 = 690 o C 6 4 2 0 600 700 800 900 1000 600 700 800 900 1000 600 700 800 900 1000 Temperature, o C

Adhesion of ash particles to a surface Amount of melt dominating reason for - large impacting particles (> 10 mm) - front side of tubes (wind side) Physical & physico-chemical forces important for - small particles (< 1 mm) - electrostatic forces, van der Waal s forces - around the tube (both wind & lee) Chemical reactions in the deposit sometimes important - Ca particles reactive with SO 2 & CO 2

Formation of a troublesome deposit: Fuel Formation of ash particles Transportation of ash particles to a surface Adhesion of ash particles to a surface Densification of ash particles on a surface

Densification of ash particles Sintering - small amount of freezing melt, partial melting - slow flowing of amorphous glas -phase viscous flow sintering - chemical reactions between particles and gas - solid particles growing together solid state sintering

Ash related problems - Principles - Summary Large effect from used fuel (what is fed into the boiler) ash composition (how the feed behaves thermally) flow field (where the particles go) temperature (amount of melt)

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Opportunity fuels Annual biomasses Forrest residues & prunings Agricultural rests, -shells Olive residues Sorted wastes Sludges Coal slurry, pet coke Others MBM (meat and bone meal) Solid animal excrement

ÅA fuel database Analyzed per 2006 18 wood bark fuels 46 wood based fuels (trunk, forest residues, construction residues) 11 annual biomass fuels 18 peats 15 coals 37 other (sorted wastes, sludges, biomassbased wastes, chicken litter) TOT 145 fuels

Analyzing ash forming elements in a fuel Conventional - done on the ash of the fuel - ashing + element analysis from ash - all other elements except S & Cl - ashing affects the analysis Advanced - done directly on fuel - dissolving of fuel + element analysis of solution - all elements - no ashing For ex. selective leaching (=chemical fractionation) various microscopic methods (SEM/EDS, CCSEM) others

Stepwise leaching /Benson & Holm 1985, Baxter 1994, Zevenhoven 2001/ Total mineral matter - all major ash-forming elements H 2 O Water leachible - alkali sulfates/carbonates/chlorides NH 4 Ac Buffer solution leachible - organically associated HCl Acid leachible - carbonates/sulfates Rest - silicates, unsoluble rest

Stepwise leaching /Benson & Holm 1985, Baxter 1994, Zevenhoven 2001/ Total mineral matter All major ash-forming elements Easily soluble Water leachible - alkali sulfates/carbonates/chlorides Buffer solution leachible - organically associated Mineral part Acid leachible - carbonates/sulfates Rest - silicates, unsoluble rest

Ash forming elements, g/kg dry fuel 140 120 100 Stepwise leaching /ÅA fuel database, 2006/ Leached in H 2 O Leached in Acetate Leached in HCl Rest fraction Untreated fuel 267 g/kg 312 g/kg 80 60 40 20 0 Coal Peat Bark Wood AB Other

Ash-forming elements, weight-% d.b. Ash forming elements, g/kg d.b. 30 Ash-forming elements in fuels /ÅA fuel database, 2006/ 300 25 250 20 200 15 150 10 100 5 50 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others 0

Ash-forming elements, weight-% d.b. Ash forming elements, g/kg d.b. 30 Ash-forming elements in fuels /ÅA fuel database, 2006/ 300 25 Ash-forming elements, easily soluble Ash-forming elements, mineral part 250 20 200 15 150 10 100 5 50 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others 0

Sulphur in fuel, mg/kg d.b. 18000 Sulphur in fuels /ÅA fuel database, 2006/ 16000 14000 12000 10000 8000 6000 4000 2000 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Chlorine in fuel, mg/kg d.b. 18000 Chlorine in fuels /ÅA fuel database, 2006/ 16000 14000 12000 10000 8000 6000 4000 2000 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Potassium in fuel, mg/kg d.b. 18000 Potassium in fuels /ÅA fuel database, 2006/ 16000 14000 12000 10000 8000 6000 4000 2000 Potassium, easily soluble Potassium, mineral part 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Sodium in fuel, mg/kg d.b. 18000 16000 14000 Sodium, easily soluble Sodium, mineral part Sodium in fuels /ÅA fuel database, 2006/ 12000 10000 8000 6000 4000 2000 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Calcium in fuel, mg/kg d.b. 100000 90000 80000 Calcium, easily soluble Calcium, mineral part Calcium in fuels /ÅA fuel database, 2006/ 70000 60000 50000 40000 30000 20000 10000 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Silicon in fuel, mg/kg d.b. 100000 90000 80000 70000 60000 50000 40000 30000 20000 10000 Silicon, easily soluble Silicon, mineral part Silicon in fuels /ÅA fuel database, 2006/ 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

EU heavy metals in fuel, mg/kg d.b. 18000 EU heavy metals in fuels /ÅA fuel database, 2006/ 16000 14000 12000 10000 8000 6000 4000 2000 0 Wood Forest res. Bark Const res. An. biom. Peat Coal Others

Summary Opportunity fuels Opportunity fuels challenging, from an ash point-of-view don not necessarily increase the ash amount increase clearly ash aggressiveness High Chlorine, Alkali (Na, K), Calcium sulphur, silicon as in conventional fuels ( heavy metals higher than ) in conventional fuels

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Contractual fuels in the large FBC deliveries 2001-2002 Wood based 16/23 Peat 10/23 Coal 8/23 Sludges 8/23 Pet Coke 4/23 Co-firing vs single-fuel-firing 20 vs 3 /Hupa 2003; FBC plenary session/

Slagging & Fouling Co-firing, effect on slagging & fouling Fuel A 50% Fuel B /Hupa 2003; FBC plenary session/

Deposition, g/m 2 h Co-firing, effect on slagging & fouling Deposit probe measurements Full-scale BFB 60 50 3h 10h 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 Bark share in rice husk, weight-% /Skrifvars et al 2004; Energy & Fuels/

Deposition, g/m 2 h 100 90 80 70 60 50 40 30 Full-scale deposit probe measurements - Deposition vs fuel mix - /ÅA deposit probe measurements, 2006/ 20 10 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Biomass Thermal share Peat or Coal

Deposition, g/m 2 h) Peat/straw co-firing lab-scale drop-tube tests 160 140 120 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 90 100 Peat Share of straw in peat, weight-% Straw /Theis 2006; Dr thesis /

Deposition, g/m 2 h Cl & S in fuel vs deposition - lab-tests - 160 140 120 100 80 60 40 20 Peat-straw Peat-bark 2 KCl + SO 2 + ½ O 2 + H 2 O K 2 SO 4 + 2 HCl 0 0.0 0.1 0.2 0.3 0.4 0.5 Molar ratio of Cl/S in fuel 72 /Theis 2006; Dr thesis /

Cl in deposit, weight-% 40 35 30 25 20 15 10 5 Full-scale deposit probe measurements - Cl i deposit vs fuel mix - /ÅA deposit probe measurements, 2006/ Wind Lee 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Biomass Thermal share Peat or coal

Co-firing Summary seldom a linear ash behavior co-firing worse than single-fuel-firing if fuel ashes cause a melt when mixed together silicate-based ashes may function as cleaning agents, through an erosive effect sulphur may stop chlorine from getting into deposits

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Traditional corrosion protection of steel Compact steel-oxide layer - protects from further oxidation - requires the presence of oxygen Steel

Traditional corrosion protection of steel Does not work if - the steel-oxide layer breaks - oxygen is abscent - the steel-oxide layer is porpous Steel

Traditional corrosion protection of steel Usually handleled by materials people only Novel thinking needed - challenging interface, steel-deposit-gas - chemistry-physics-engineering

Other low melting compounds (Ni-P, V 2 O 5, MoO 3, etc.) Sulfidation High-temperature corrosion mechanisms Sulfur Low melting compounds OXYGEN O 2 CO/CO 2 H 2 /H 2 O Carburization Carbon Molten salts corrosion Molten salts /Salmenoja 2001, Dr Thesis/

Corrosion caused by Chlorine Fe Cl S /Salmenoja 2001, Dr Thesis/

/Westen-Karlsson 2008, Lic. Thesis/ ÅA, laboratory-scale corrosion tests Before Heat treatment After Heat treatment For SEManalysis

Corrosion layer thickness, (µm) /Westen-Karlsson 2008, Lic. Thesis/ ÅA, laboratory-scale corrosion tests - SEM analysis - 200 100 0 Mean, median, most frequent, max 0 5 10 15 Cross-section, mm

Corrosion layer thickness, mm /Skrifvars et al, Corr. Sci. 2008/ ÅA, laboratory-scale corrosion tests (Na, K) 2 SO 4 + 0.0 p-% Cl 140 120 100 80 60 40 20 0 Synthetic ash T 0 = 834 o C 0% Cl 10% K

Corrosion layer thickness, mm /Skrifvars et al, Corr. Sci. 2008/ ÅA, laboratory-scale corrosion tests (Na, K) 2 SO 4 + 0.2 p-% Cl 140 Synthetic ash 120 T 0 = 526 o C 100 0.2% Cl 10% K 80 60 40 20 0

Corrosion layer thickness, mm /Skrifvars et al, Corr. Sci. 2008/ ÅA, laboratory-scale corrosion tests (Na, K) 2 SO 4 + 1.2 p-% Cl 140 Synthetic ash 120 T 0 = 522 o C 100 1.2% Cl 10% K 80 60 40 20 0

Cl in deposit, weight-% Deposit probe measurements, full-scale boilers - Cl in deposit vs probe surface temperature- 40 35 30 25 20 15 10 5 /ÅA deposit meaurements database, 2006/ 0 100 200 300 400 500 600 700 Probe surface temperature, o C? Wind Lee

Corrosion Summary alkali chlorides enhance corrosion strongly already a small amount of Cl increases corrosion sulphur may stop chlorine to get into the deposit increase of steam temperature very challenging

Ash and ash deposition for solid fuels Content 1. Ash related problems Principles Facts 2. Co-firing 3. Corrosion 4. Summary

Summary 1(4) From ash-forming elements to deposits Strong influence of fuel fired (what is fed into the boiler) ash composition (how the element react to ash in the boiler) flow fields (where the ash particles flow/impact) temperature (amount of melt in the ash/deposit)

Summary 2(4) New fuels, Opportunity fuels challenging, from an ash point-of-view don not necessarily increase the ash amount increase clearly ash agressiveness High Chlorine, Alkali (Na, K), Calcium sulphur, silicon as in conventional fuels ( heavy metals higher than ) in conventional fuels

Summary 3(4) Co-firing seldom a linear ash behavior co-firing worse than single-fuel-firing if fuel ashes cause a melt when mixed together silicate-based ashes may function as cleaning agents, through an erosive effect sulphur may stop chlorine from getting into deposits

Summary 4(4) Corrosion alkali chlorides enhence corrosion strongly already a small amount of Cl increases corrosion sulphur may stop chlorine to get into the deposit increase of steam temperature very challenging

Acknowledgments Prof. Rainer Backman @ University of Umeå Assoc. Prof. Flemming Frandsen, @Technical University of Denmark Dr. Mischa Theis @Bayer, Germany Prof. Mikko Hupa Mr. Tor Laurén Mr. Linus Silvander Dr. Johan Werkelin Dr. Patrik Yrjas Dr. Maria Zevenhoven @Åbo Akademi University Ms. Micaela Westén-Karlsson @Finnsementti