state of the art of small scale biomass combustion

BIOS BIOENERGIESYSTEME GmbH

Inffeldgasse 21b, A-8010 Graz, Austria

TEL.: +43 (316) 481300; FAX: +43 (316) 4813004

E-MAIL: [email protected]

HOMEPAGE: http://www.bios-bioenergy.at

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EN E R G Y B I O M A S

S

A S H

Prof. Ingwald Obernberger

State-of-the-art of small-scale

biomass combustion in boilers

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Content

Classification of different small-scale

biomass combustion systems

Technologies for small-scale biomass boilers

Manually fed boilers for log wood

Automatically fed boilers for wood chips

Automatically fed boilers for wood pellets

Dual-fuel boilers

Automatically fed boilers for non-wood fuels

Emissions from small-scale biomass boilers

CFD based combustion design

Summary and conclusions

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Classification of different

small-scale

biomass combustion system

Biomass fuels

Wood fuels Non-wood

fuels

Manually fed

systems

Automatically

fed systems

Tiled stoves •logwood

•briquettes

Stoves •logwood

•briquettes

Tiled stoves •pellets

Stoves •pellets

Boilers •pellets

•wood chips

Automatically

fed systems

Boilers •straw pellets

•corn

•miscanthus

Boilers •logwood

•briquettes

Dual fuel

boilers

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Manually fed boilers

for log wood (1)

State-of-the-art:

Downdraft boilers

(show a more stable combustion than in over-fire boilers (old technology))

Capacity range:

Usually 15 to 70 kW

Fuels used:

Wood logs with 33 or 50 cm length

Use of wood briquettes possible (not usual)

Features:

Manual fuel loading, automatic operation during combustion batches

Air staging (primary and secondary combustion zone

with separate air feed)

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for log wood (2)

Downdraft boiler Source: [Fröling GmbH (A)]

Automatic flue gas

Suction of smoke

suction fan

Storage room

Microprocessor

Secondary combustion

(cyclone combustion chamber)

when opening door

controller

Types:

Draught regulation by

combustion air fan

Very low emissions

(improved mixing)

Automatic control –

typically with lambda (O2)

sensor

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for log wood (3)

Performance data: modern log wood boilers achieve efficiencies of

more than 90% (compared to approximately 55% in the early 1980´s)

Year

Eff

icie

nc

y [

%]

Source: [Wörgetter et al., 2005]

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Automatically fed boilers for

wood chips (1)

State-of-the-art:

Fully automatic operation (only ash box emptying needed)

Micro-processor controlled

(load and combustion control similar to pellet boilers)

Proven automatic fuel feeding (screw conveyors, agitators) with burn-

back protection

Application in residential heating and micro grids

Air staging (primary and secondary combustion zone with separate air

feed)

Typically vertical fire tube boilers with automatic or semi-automatic boiler

cleaning (with a clear tendency towards automatic boiler cleaning

systems)

Flue gas condensation partly applied

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wood chips (2)

Automatically underfed wood chip stoker burner (50-100 kWth)

1

2 1

3

4

5

Explanations:

1… storage container

2… feeding screw

3… combustion

chamber with

radiation plate

4… heat exchanger

with turbolators

and cleaning

system

5… ash container

Source: [KWB (A)]

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wood chips (3)

Capacity range: >15 up to several 100 kW

Fuels used: wood chips according to EN 14961 – Part 4

Types: underfed burners, horizontally fed burners,

moving grate systems

Performance data [Wörgetter et al., 2005]:

Efficiencies of wood chips boilers in the early 1980´s: around 70%

Average efficiency of modern wood chips boilers: 91%

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wood pellets (1)

State-of-the-art:

Fully automatic operation (only ash box emptying once or twice a year)

Micro-processor controlled (load and combustion control)

- Load control by regulation of the fuel and primary air feed guided by the feed

water temperature

- Combustion control by regulation of the secondary air feed usually guided

by the O2 concentration in the flue gas (lambda sensors)

Proven automatic fuel feeding (flexible or inflexible screw conveyors,

pneumatic systems, agitators or combinations) with burn-back protection

Air staging (primary and secondary combustion zone with separate air

feed)

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wood pellets (2)

State-of-the-art (continued):

Typically vertical fire tube boilers with automatic or semi-automatic boiler

cleaning (with a clear tendency towards automatic boiler cleaning

systems)

Pellet boilers with flue gas condensation units already available

Capacity range: 6 to 300 kW (usual capacities in Austria)

Fuels used: wood pellets according to EN14961-2

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wood pellets (3)

Types of pellet burners depending on the fuel feeding system:

1) underfed burners

2) horizontally fed

burners

3) overfed burners

Underfed burners

Horizontally fed burners

Overfed burners

Source: [Van Loo et al., 2002]

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Automatically fed boilers for wood

pellets (4) - basic designs

Retort furnaces

(only in combination with

underfed burners)

Grate furnaces

(only in combination with

horizontally or overfed

burners)

Fixed grate

Source: [KWB Kraft und Wärme aus Biomasse GmbH,

Austria]

Source: [Windhager Zentralheizung GmbH, Austria]

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Automatically fed boilers for wood

pellets (5) - basic designs

Hinged grate

Step grate

Source: [GUNTAMATIC Heiztechnik GmbH, Austria]

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Automatically fed boilers

for wood pellets (6) –

overfed pellet boiler

1

4

3

2

6

8 7

5

Explanations:

1 …. fuel container

2 …. stoker screw

3 …. primary combustion chamber

with primary air addition

4 .... secondary air addition

5 .… secondary combustion

chamber

6 .… heat exchanger with cleaning

device

7 .… bottom ash container

8 …. fly ash container

Source: [Fischer Guntamatic (A)]

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for wood pellets with

integrated flue gas condensation

Source: [ÖkoFEN, Austria, 2004]

First pellet boiler with

integrated flue gas

condensation

Market introduction: 2004

Efficiency: up to 103%

(according to type test by BLT

Wieselburg)

depends on return

temperature and excess O2

Nominal capacities:

8, 10, 15 and 20 kW

feed water

return

flue gas path

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Boiler efficiency as a function of flue

gas temperature and O2 concentration

in the flue gas

Source: [BIOS BIOENERGIESYSTEME GmbH, Austria, 2002]

80

85

90

95

100

105

20 40 60 80 100 120 140 160 180 200

Flue gas temperature [°C]

Bo

ile

r e

ffic

ien

cy

[%

]

O2 content = 6%

O2 content = 8%

O2 content = 10%

O2 content = 12%

Calculated for wood pellets as fuel

(moisture content: 8 wt% w.b.,

H content: 5.7 wt.% d.b.)

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Fine particulate emissions and

efficiencies of pellet boilers

with and without flue gas condensation

Fine particulate emissions and efficiencies of pellet boilers

with and without flue gas condensation

0

2

4

6

8

10

12

14

16

With flue gas

condensation

Without flue gas

condensation

Fin

e p

art

icu

late

s

[mg

/MJ]

90

92

94

96

98

100

102

104

106

With flue gas

condensation

Without flue gas

condensation

Eff

icie

nc

y [

%]

Source: [Obernberger et al., 2006]

Explanations: operating conditions for furnace with flue gas condensation: load 20 kW; flue gas temperature 33.7°C; return

temperature 22.4°C; O2 concentration in the flue gas 8.3 vol.% dry flue gas; box plots based on 9 measurements;

operating conditions for furnace without flue gas condensation: load 18.2 kW, flue gas temperature 144 °C, return

59.8 °C, O2 concentration in the flue gas 8.6 vol.% dry flue gas, box plots based on 5 measurements

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Dual-fuel boilers (1)

Proven concepts are already available on the market

Wood pellets / wood chips combinations

(automatic operation for both fuels)

Wood pellets / firewood combinations

(different levels of automation depending on the type):

- Simple boiler concepts have to be adjusted and operated manually

in case of operation with firewood

- Sophisticated boilers identify the fuel automatically

- For both boiler types several applications are available

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Dual-fuel boilers (2) - Boiler for

wood pellets / wood chips operation

Source: [KWB – Kraft und Wärme aus Biomasse GmbH, Austria]

Nominal thermal capacity:

15 - 300 kW

Innovative rotary grate furnace

(robust, calm bed of embers,

optimised gasification conditions)

Innovative cyclone combustion

chamber (optimised by CFD

simulations, efficient fly ash

separation, flue gas recirculation)

Fuels: wood chips (up to w50)

and wood pellets

Automatic operation for all fuels

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Dual-fuel boilers (3) - Boiler for

wood pellets / firewood operation

Nominal thermal capacity:

15 – 40 kW

Fuels: wood pellets and

firewood

Automatic fuel

identification

No manual adaptations

needed

Automatic operation for

all fuels

(ignition of firewood with

the pellets burner)

Explanations:

1 process control system

2 fan

3 chamotte

4 combustion chamber

for firewood

5 pellet burner

6 isolation

7 secondary combustion

chamber Source: [Fröling Heizkessel- und Behälterbau Ges.m.b.H, Austria]

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for non-wood fuels (1)

State-of-the-art:

Guntamatic Powercorn: for corn and pellets,

nominal thermal capacity: 30, 50 and 75 kW [Guntamatic, Austria, 2011]

Hargassner Agrofire: for corn, straw pellets, miscanthus, wood chips

and pellets, nominal thermal capacity: 25 and 40 kW [Hargassner,

Austria, 2011]

Ökotherm Compactanlage: for corn, straw pellets, miscanthus,

rapeseed cake and wood chips, nominal thermal capacity:

30 – 800 kW [Ökotherm, Germany, 2011]

Heizomat RHK-AK: for miscanthus, corn, rape straw, maize cobs,

pellets and wood chips, nominal thermal capacity: 30 – 1,000 kW

[Heizomat, Germany, 2011]

The potential for further development is rather high due to the

novelty of these technologies

The gaseous as well as particulate emissions are considerably

higher in comparison to wood fuels

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Guntamatic POWERCORN

1. Furnace doors

2. Step grate – primary air

3. Combustion chamber

4. Fill level indicator

5. Spiral jet – secondary air

6. Reaction pipe

7. Cleaning cover

8. Vibrating mechanism

9. Tube bundle heat exchanger

10. Induced draught fan

11. Cleaning of heat exchanger

12. Smoke pipe

13. Lambda probe

14. Flue gas sensor

15. Cleaning/ grate drive

16. Ash auger

17. Removable ash container

18. Menu-guided control

Source: [Guntamatic Heiztechnik GmbH (A)]

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Hargassner AgroFire:

Source: [Hargassner GesmbH (A)]

moving grate

(step grate)

heat exchanger

with automatic

cleaning system

secondary combustion

chamber

fan

automatic ash

removal system

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1

163

22

87

29

9

73

24

0

20

40

60

80

100

120

140

160

180

200

CO TOC NOx (NO2) dust

Em

issio

n f

acto

r [m

g/M

J (

NC

V)]

test stand measurements field measurements limiting value

(500)

Emissions - overview

Source: [Spitzer et al, 1998;

BLT Wieselburg, 2006]

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0

50

100

150

200

250

300

1996 - 1998 1999 - 2001 2002 - 2004 2005

CO

[m

g/M

J]

CO emissions – comparison between

old and new pellet furnaces

Source: [Jungmeier et al, 1999;

BLT Wieselburg, 2006;

results from test stand

measurements]

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0

10

20

30

40

50

60

1996 - 1998 1999 - 2001 2002 - 2004 2005

Du

st

[mg

/MJ]Dust emissions – comparison between

old and new pellet furnaces

Source: [Jungmeier et al, 1999;

BLT Wieselburg, 2006;

results from test stand

measurements]

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NOx emissions and N conversion

versus N content in the fuel

Fuel selection relevant

Primary measures especially relevant

Combination of primary and secondary measures if needed

0

100

200

300

400

500

600

700

0,01 0,10 1,00 10,00

Fuel N [wt% (d.b.)]

NO

X e

mis

sio

n

[mg

/Nm

³, d

ry f

lue g

as, 11 v

ol%

O2]

0%

10%

20%

30%

40%

50%

60%

70%

80%

0,01 0,10 1,00 10,00

Fuel N [wt% (d.b.)]w

t% o

f fu

el-

N c

on

vert

ed

to

N in

NO

X e

mis

sio

n

wood

SRC

waste wood

kernels

straw

whole crops

energy grasses

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NOx emissions versus air ratio in the

primary combustion chamber

Fuel: chipboard N content: 3.7 wt% (d.b.)

Fuel: wood chips (spruce) N content: 0.09 wt% (d.b.)

Explanations: Source: [Hesch et.al., 2011, Hesch et.al., 2011 (submitted)], measurements performed

in a moving grate furnace (nominal boiler capacity: 150 kWth), air staging and flue gas recirculation

above grate applied, temperature in the primary combustion chamber for all tests: 1,000 °C

0

100

200

300

400

500

600

700

800

0.3 0.5 0.7 0.9 1.1 1.3 1.5

air ratio in the primary combustion chamber

NO

x e

mis

sio

ns

[mg

/Nm

3 (

dry

flu

e g

as, 13%

O2)]

0

20

40

60

80

100

120

140

160

180

0.3 0.5 0.7 0.9 1.1 1.3 1.5

air ratio in the primary combustion chamber

NO

x e

mis

sio

ns

[mg

/Nm

3 (

dry

flu

e g

as

, 1

3%

O2)]

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Influence of the fuel used

on the mass of aerosols formed

• Emissions at boiler outlet

• Particle emissions related to dry flue gas and 13 vol% O2

10

100

1.000

100 1.000 10.000 100.000

concentration of aerosol forming elements in the fuel (K+Na+Zn+Pb)

[mg/kg (d.b.)]

pa

rtic

les <

1 µ

m (

ae

.d.)

[m

g/N

m³]

softwoodhardwoodbarkwaste woodstraw

appropriate fuel selection relevant

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Average chemical compositions of

aerosols from small-scale biomass

boilers and stoves

Primary measures of great relevance

(complete combustion, optimisation of partload operation,

reduction of the release of ash vapours)

0%

20%

40%

60%

80%

100%

full load 50%

load

full load 50%

load

full load 50%

load

full load full load

modern pellet boilers modern wood chip boilers modern logwood

boilers

old

stoves

modern

stoves

wt%

(d

.b.)

alkalimetal sulfate alkalimetal chloride alkalimetal carbonate oxides non-carbonate carbon

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Factors influencing aerosol formation –

burnout quality

statistic significance: p<0.05

R2 = 0.85

1

10

100

1,000

10 100 1,000 10,000 100,000

CO [mg/MJ]

PM

1 [m

g/M

J]

Explanations: mean values from test runs performed with different old and modern

residential heating systems (modern pellet boiler, modern wood chip boiler, modern

logwood boiler, old logwood boiler, modern stove, old stove, modern tiled stove)

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CFD based combustion design (1) -

modelling of a biomass fired underfeed

stoker grate furnace (20 kW)

Scheme of the grate used for simulations

Fuel inlet

Primary air

inlets

Picture of the grate

Grate

Fuel bed

chimney duct

Heat exchanger

tubes

Secondary air nozzles

Fuel feed

Primary combustion

zone

Secondary

combustion zone

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particle and flue gas temperatures [°C]

Explanations: Biomass fuel: softwood pellets;

Moisture content= 8.12 wt.% (wet base); λtotal= 1.58; λprim= 0. 64, no flue gas recirculation

Flue gas temperatures T [°C] Particle temperatures

Visual observation

T [°C]

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particle tracks coloured

by release rates [mg/s]

H2O

CO CO2

Volatiles

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H2O

CO CO2

volatiles


contours of release rates [mg/s]

Contours of release rates at a vertical cross section through the grate axis; x= 5cm

x

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Summary and conclusions (1)

The following market-proven small-scale boiler technologies are

available for wood fuels

Manually fed boilers for log wood

Automatically fed boilers for wood chips

Automatically fed boilers for wood pellets

Dual-fuel boilers (wood pellets / wood chips and wood pellets / firewood)

For non-wood fuels only a few manufacturers offer solutions but they

are available on the market. Emissions and ash related problems are

generally higher for non-wood fuels

Flue gas condensation units offer a potential to increase efficiency.

Over 100% (related to the NCV of the fuel) compared to approx. 90% for

conventional technology. Flue gas condensation is especially

interesting for wet fuels and / or requires low return temperature

(30 – 40 °C) to work efficiently

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Summary and conclusions (2)

Small-scale boiler technologies have considerably improved in the last

two decades regarding CO, TOC and dust emission reduction

Further improvements are possible regarding the reduction of NOx and

PM1 emissions

Regarding primary measures an efficient air staging strategy

is very important

The influence of the fuel itself (N, ash and alkali metal contents vary) is

of great relevance the technologies have to be adapted to the fuels

used

Regarding partial load operation, load changes and the process

control system a considerable potential for improvements still exists

CFD simulations show increasing importance as an efficient design

tool for small-scale combustion systems

BIOS BIOENERGIESYSTEME GmbH

Inffeldgasse 21b, A-8010 Graz, Austria

TEL.: +43 (316) 481300; FAX: +43 (316) 4813004

E-MAIL: [email protected]

HOMEPAGE: http://www.bios-bioenergy.at

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CON O

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P Ca

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EN E R G Y B I O M A S

S

A S H

Thank you for your attention

state of the art of small scale biomass combustion

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