part 2, biomass fuels and conversion technologies 5eures ... · pdf filechp generation from...

CHP generation from biomass fuels

Part 2, Biomass fuels and conversion technologies

5Eures International training, Joensuu

Seppo Hulkkonen

14.6.2006

2 S. Hulkkonen 6/06

Contents of part 2

• 2) Biomass fuels

• Properties

• Problem areas

Slagging&Fouling

Corrosion

• 3) Conversion technologies

• Boilers

Grate firing

Fluidized beds

• Gasification

3 S. Hulkkonen 6/06

Fuel properties

Source: Tekniikan käsikirja

Volatile content, %

Peat

WoodBro

wn

coal

Sand

coa

ls

Gas

san

dcoa

ls

Gas

coa

ls

Anr

asite

Anr

asiti

c co

als

4 S. Hulkkonen 6/06

• Interesting fuels in Finland

• Wood chips

• Wood pellets

• Bark

• Forest residues

• Reed canary grass

• Rape seed

• Cereal crops, barley

• Sugar beet

• Peat

• Woody biomass

• Forest and plantation wood

• Wood processing industry, by-products and residues

• Used wood (not demolition wood)

• Herbaceous biomass

• Agriculture and horticulture herb

• Herb processing industry, by-products and residues

• Fruit biomass

• Orchard and horticulture fruit

• Fruit processing industry, by-products and residues

• Blends and mixtures

Biomass fuel classification CEN/TS 14961technical specification

5 S. Hulkkonen 6/06

Biomass fuel propertiesFuels C H2 S O2 N2 Ash Cl Na K

%-daf %-daf %-daf %-daf %-daf %-dry %-daf mg/kg-d mg/kg-dPeat 55 5,5 0,2 33 1,7 5Wood, coniferous 51 6,3 0,02 42 0,1 0,3 0,01 20 400Wood, deciduous 49 6,2 0,02 44 0,1 0,3 0,01 50 800Bark, coniferous 54 6,1 0,1 40 0,5 4 0,02 300 2 000Bark, deciduous 55 6,1 0,1 40 0,3 5 0,02 100 2 000Willow 49 6,2 0,05 44 0,5 2 0,03 200 3 000Poplar 49 6,3 0,03 44 0,4 2 0,01 3 000Straw, wheat, rye, barley 49 6,3 0,1 43 0,5 5 0,4 500 10 000Straw, rape 50 6,3 0,3 43 0,8 5 0,5 500 10 000Reed canary grass, summer harv. 49 6,1 0,2 43 1,4 6,4 0,6 200 12 000Reed canary grass, delayed harv. 49 5,8 0,1 44 0,9 5,6 0,1 200 2 700

6 S. Hulkkonen 6/06

• ALKALI METALS (Sodium, potassium)

•Slagging/fouling

•Hot corrosion

•Fluid bed sintering

• CHLORINE

•Hot corrosion

•Fouling

•HCl emissions

•Dioxines

• SULPHUR

•SO2 emission

•Low-temperature corrosion

Fuel related problems in boilers

• NITROGEN

•NOx emissions

• HEAVY METALS

•Emissions

•Corrosion

•Ash treatment

• OTHERS

•Fuel moisture

•Particle size

7 S. Hulkkonen 6/06

Fuels sorted by harmful components

Fuel High alkaliNa+K

High chlorine High sulfur High moisture High ash

Plywood waste XForest residue XOlive pits XAspen bark XRubber tree XStraws X XReed canary grass (X) (X)REF XRDF X (X) (X)Bark XPeat (X) XBio-sludge X (X) X XPrimary-sludge X X

8 S. Hulkkonen 6/06

Slagging

Source: Miles

• Slagging – furnace

9 S. Hulkkonen 6/06

Fouling

Source: Miles

• Ash deposits on the heat transfer surfaces• Inertial impaction

• Thermophoresis

• Condesation

• Chemical reaction

10 S. Hulkkonen 6/06

Superheater foulingMECHANISM

• At low temperature melting components present in the flue gases impact and deposit on heat transfer surfaces

• Typically when amount of melt is 15-70%, ash is sticky. Corresponding temperatures T15 and T70.

• Typical sticky alkali compounds are (NaCl, KCl, Na2CO3, K2CO3)

• Aluminium is also highly problematic – melting temperature of metallic aluminiun 660 °C.

AFFECTING FACTORS

• Flue gas temperature

• Fuel bound Na, K, Cl, and Al

• High chlorine content decreases ash melting temperature

• S/Cl ratio determines the amount of free Cl.

• Alkalies (Na, K) increase the amount of melt

• High moisture assists HCl formation


Melting temperatures

Source: Miles


Ash melting (Chemkin-calculation)

0

5

10

15

20

25

500

525

550

575

600

625

650

675

700

725

750

775

800

825

850

875

900

925

950

975

1000

1025

1050

1075

1100

1125

1150

1175

1200

Temperature [C]

Am

ount

of m

elt [

wt%

]

Wood 50% - Olive pumace 50%Cl: 0,15 wt-% S/Cl: 0,68 (mole ratio)Alkali index: 0,36 kg alk.ox./GJNa2O: 1,71 wt-% of the ashK2O: 7,7 wt-% of the ashAsh: 7,88 wt-% of dsT0: 685 CT15: 960 C

Source: Hulkkonen


• Different indices are used to estimate the fouling behavior

• Alkali-index shows sodium and potassium content as related to fuel heating value

• (K2O+Na2O)/GCV (kg/GJ)

• Threshold value 0,17 kg/GJ, when fouling is probable and 0,34 kg/GJ when it is unavoidable

• Another index shows alkalies as related to silica

• (K2O+Na2O)/SiO2

• When larger than 1, fouling probable

• Slagging/fouling depends on many factors: indices are only tools for asessing the possible behaviour of ash

Source: Miles

Alkali-index


Furnace average temperatureBFB, Arvika

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

11,0

12,0

1100 1200 1300 1400 1500 1600

T/K, Average

Distance from bed surface (m)

FEGT


Boiler design criteria

• Flue gas temperature before the superheater less than T15. T15 means the temperature where 15% of the ash is in melted phase.

• Typically the temperatures 650 - 750 °C with difficult fuels and 900-950 °C with good fuels.

• Wide tube spacing• 120 mm when temperature > 600 °C

• 60 mm when temperature < 600 °C

• 40 mm in economiser area

• Efficient sootblowing - Tube bank thickness less than 1.5m

• Downwards gas flow to help soot blowing

• Additives for binding the alkalies


Corrosion• Furnace

• Reducing conditions

• Molten ash – components that stay molten in low

temperatures

• Superheaters – hot corrosion

• High temperature

• Chlorides – hot corrosion

• Fouling

• Last heat transfer surfaces – low temperature

corrosion

• Fuel sulphur and chlorine content

• Mainly dewpoint corrosion


Superheater hot corrosion• In combustion gaseous Cl, S, Na, K release to flue gases

• Alkali chlorides condense on the heat transfer surfaces – deposits

• In the deposit the chlorides react with sulphur dioxide in the flue gas releasing gaseous chlorine

• The alkali chlorides on the tube surface break the oxide layer protecting the tube

• Chlorine reacts with iron forming FeCl2

• Chlorine transfers iron (Fe) from the metal surface -> corrosion

• FeCl2 oxidices releasing Cl back to the deposit

• Threshold temperature for FeCl formation is 460 °C

• Strong corrosion may be experienced with biomass fuels when temperature higher than 480 °C and alkalimetals and chlorine in the fuel

• With steam temperature of 400-420 °C no chlorine based corrosion


Corrosion mechanism

Source: Riedl


Superheater materials

Lähde: Salmenoja


Corrosion protection in boilers

•SUPERHEATER

•Low flue gas temperature before superheater to avoid formation of tenacious deposits - cooling chamber and screens

•Wide tube spacing to reduce the gas velocity and fouling

•Low steam temperature to minimize the rate of corrosion (< 420 C)

•Co-current flow in the hottest superheater

•Corrosion resistant materials in the hottest superheaters (AC 66)

•High alloy shielding of the tubes in the sootblower area

•Avoid radiant superheaters

•According to studies

•Strong corrosion when S/Cl molar ratio below 2, moderate when S/Cl 2-4 and unlikely when S/Cl higher than 4

•Memory effect– Chlorine remains in the ”memory” of the metal


Boilers for biomass

• Grate fired boilers

• Fluidized bed boilers


Grate types• Fixed and mechanical grates

• Fixed grates in small units and mechanical in bigger plants

• Grate types

• Fixed plane grate

• Fixed inclined or step grate

• Underfeed grate

• Mechanical chain grate

• Mechanical inclined grate

• Grates are either air or watercooled. Water cooled typically

large and for high heating value fuels.

• Grates in Finland typically in small applications

• In Finland Wärtsilä supplies rotating grates – BioGrate


Grate firing principle

• Combustion

• Drying and heating

• Volatiles combustion

• Fixed carbon combustion

• Output limited by ignition- or combustion trate

• Grate surface area

• Coals 1-1.6 MW/m2

• Wet biomass, 60% 0.2-0.4 MW/m2

• Dry biomass, 30% 0.6-0.8 MW/m2


KABLITZ GRATES Kablitz grate

RECIPROCATING GRATE

The Detroit Reciprograte Stoker has met with wide approval for burning unprepared municipal and industrial solid waste as fuel.


Travelling grate

TRAVELLING GRATE

The Detroit RotoGrate stoker is a continuous ash discharge, traveling grate, spreader stoker that is perfect for a broad range of applications. It is recognized worldwide for its efficiency in generating steam and power from coal and refuse


VIBRATING GRATE

The spreader firing principle is the most widely accepted, proven and user friendly means of burning biomass fuels. Sized fuel is metered to a series of distribution devices which spread it uniformly over the stoker grate surface.

Fine particles of fuel are rapidly burned in suspension assisted by carefully designed overfire air turbulence systems. Coarser, heavier fuel particles are spread evenly on the grate forming a thin, fast-burning fuel bed. The combination of suspension and the fast-burning bed makes this method of firing extremely responsive to load demand.

Vibrating grate


+ Robust technology+ Fuel flexible+ Plenty of references- sensitive to moisture and

particle variation+/- scarce outfit- maintenance of grate- small tube spacing- headers in gas flow- water circulation not the

safest- lower boiler efficiency- emissions may be

problem- unburnt carbon in ash- throat required for

mixing, possible slagging problem

Grate boiler, Standard Kessel


Biograte - Wärtsilä

- Wärtsiläsupplies grate boilers in size class of 3-20 MWth

• Conical, rotating grate

• Fuel feeding with stoker screw to the center of the grate

• 3-5 rotating grate rings

• 2-4 fixed grate rings

• Ash grate

• Wide grate area for complete combustion

• Flexible, controlled primary air distribution

• Wet bottom ash system

• Flue gas recirculation used for grate cooling


Wärtsilä Biopower- fire tube boiler

Source: Wärtsilä Biopower


Wärtsilä water tube boiler

Etupesä

Lieriö

I-tulistin

Lippuhöyrystin

EkoTulipesä

II-tulistin

Verho

SV-säiliö

PA-syöttö

Etupesä

Lieriö

I-tulistin

Lippuhöyrystin

EkoTulipesä

II-tulistin

Verho

SV-säiliö

PA-syöttö


Fluidized bed combustion•Fluidized bed combustion was introduced to energy

production in 1970’s and widened to biomass fuels in 1980’s

•In fluidized bed combustion inert bed material is fluidized by blowing air through the material layer

•Fuel is combusted inside or above the fluidizing bed

•The heat capacity of the fluidized bed maintains and evens out the combustion

•Benefits with biomass fuels•Fuel flexibility

•Efficient combustion

•Low emissions

•Bubbling fluidized bed best for biomass fuels

•Circulating fluidized bed when burning coal or high heating value biomass fuels

•Suppliers in Finland•Kvaerner (Metso), Foster Wheeler, Putkimaa, Noviter


Fluidized bed types

• BFB, Bubbling fluidized bed

• CFB, Circulating fluidized bed

Lähde: Huhtinen

FIXED BED BUBBLING BED CIRCULATING BED

Fluidizing velocity

log dp


Differences between BFB and CFB

BFB• bed material diameter 0.5-1.2mm• primary air flow 40%• fluidizing velocity 1-3 m/s• furnace loading 0.7-3 MW/m2• bed height 0.5 m• bed temperature 850 °C• furnace temperature 900-1000°C• best for high reactive fuels• sulphur removal 50%• suitable for boiler conversions• lower price than CFB• lower auxiliary power consumption• largest boilers abt. 300 MWth

CFB• bed material diameter 0.1-0.5 mm• primary air flow <40%• fluidizing velocity 4-8 m/s• furnace loading 0.7-5 MW/m2• bed temperature 850 °C• applicable to coal, 100 %• sulphur removal 90-95%• largest bioboilers abt. 500 MWth


Bed sinteringMECHANISM

• The alkali metal (Na, K) present in the fuel reacts on the surface of sand particles forming alkali silicates

• The surface of particles becomes gluish thus sticking particles together

• Results in disturbances in fluidisation, local hot spots and accelerating agglomeration

AFFECTING FACTORS

• Bed temperature

• Ash composition (Sodium and Potassium)

• Ratio between fuel and bed mass

POSSIBLE WAYS TO AVOID SINTERING

• Bed temperature decrease

• Continuous change of bed material

• Alternate bed materials

• Diabas, volcanic stone

• Additives

• Caolin


BFB by Kvaerner

Source: Salmenoja


Large BFB boiler by Kvaerner

Source: Salmenoja


Small ACZ- plant for waste fuels (by Kvaerner)

Source: Salmenoja


Large CFB - Kvaerner

Lähde: Salmenoja


CFB for waste- Foster Wheeler

+ Intrex- high steam T+ Fluidized bed benefits+ Applicable for coal+ Plenty of references+ Bed sulphur removal

- Expensive- High aux.power- Part load operation ??- Higher N2O emissions


Gasification• In gasification process wood and other biomass

materials are gasified to produce so called

´producer gas´ for heat or electricity generation.

Gasification system consists of a gasifier unit,

purification system and energy converters - burner

or engine.

• Gasifier types

• Fixed bed gasifiers

Down draft

Up draft

• Fluidized bed gasifiers

• Entrained flow gasifiers


Gasifier types

Ud

Dd


Fluidized bed gasifiers


Gasifier applications

Source: Rensfelt/Synbios 2005


Summary

• Biomass fuels

• High volatile content – good combustion

• Slagging and fouling may be a problem with high alkali

fuels

• Hot corrosion with high chlorine fuels

• In Finland fluidized bed combustion is

dominating in peat and wood fired boilers

• Grate firing in smaller size scale applications

• Gasifiers under development

• Fixed bed gasifiers in small size scale

• Higher power-to heat ratio as main benefit

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