Comparison and Evolution of Particle Size 

Distributions from a High Efficiency Wood Boiler Using 

Two Types of Fuel: Wood Chips and Wood Pellets

Suresh Raja, James Laing, Thomas Holsen, Philip K. Hopke

Center for Air Resources Engineering and Science, Clarkson 

University, Potsdam, NY 13699
High Efficiency Wood Boiler

 Hamont CATfire (Hamont

Consulting and Engineering, 

Austria) 

 150kW (514,000 Btu.h-1) 

Output High Efficiency 

Wood Boiler 

 High efficiency due to large 

heat exchanger surface area 

and good insulation

 Automated fuel feed system

 Duel air staging process

 Oxygen sensor to control 

Lambda air to fuel ratio

 Can burn wood as pellets, 

shavings or chips

PKH1
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Philip Hopke, 10/20/2009
Wood Pellets and Wood Chips

Wood Pellets: 

• made from compacted sawdust 

• debarked clean wood used

• low moisture content due to 

pelletizing process 

Wood Chips: 

• urban wood waste comprised 

primarily of chipped wood pallets

• higher level of contamination, 

therefore, higher contaminants 

expected from stack emissions
Wood Pellets and Wood Chips

Wood Pellets Wood Chips

heat content (MJ/lbs) 8.49 6.72

moisture (%) 4.80 26.6

ash (d.w. %) 0.60 1.79

carbon (d.w. %) 51.5 45.0

nitrogen (d.w. %) 0.13 0.37

sulfur (d.w. ppm) 70.1 175

 Wood Pellets have higher heat content, therefore, higher combustion 

efficiency

 Higher fuel bound nitrogen and sulfur content of the wood chips expected 

to lead to higher NOxand SO2emissions

 Higher ash content expected to lead to higher PM2.5 emissions
Source Sampling System Schematics

Dilution Chamber

Dilution air

Sampling 

ports

142 mm quartz 

filter

PUF Exhaust

HEPA 

filter

PM2.5 cyclone 

& nozzle

Probe Hot box

Stack

FDMS & FMPS

TEOM

CO

NOx

SO2

CO2

in-line filter
Concentration of criteria pollutants (CO, NOx

, SO2 and 

PM2.5) during steady state operation of boiler

Time

 11:58 12:08 12:18 12:28

C

O

C

o

n

c

e

ntr

atio

n (m

g.m-3)

100

1000

C

o

n

c

e

ntr

atio

n (m

g.m-3)

0.1

1

10

100

1000

CO

NOx

SO2

 PM2.5

Note: Dashed line indicate data for Wood Pellets

Solid line indicate data for Wood Chips
Concentration of criteria pollutants 

(CO, NOx, SO2 and PM2.5) during shutdown of boiler

Time

 16:54 17:04 17:14 17:24 17:34 17:44 17:54

C

O

C

o

n

c

e

n

t

r

a

tio

n (m

g.m-3)

10

100

1000

10000

C

o

n

c

e

n

t

r

a

tio

n (m

g.m-3) 0.001

0.01

0.1

1

10

100

1000

CO

NOx

SO2

 PM2.5

Note: Vertical line indicates time of boiler shutdown process initiation. Wood Chips used as fuel.

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Philip Hopke, 10/20/2009
Average Ultra-Fine PM Emission due to Combustion of 

Wood Chips and Pellets in a HE Boiler

Time

 11:00 13:00 15:00 17:00 19:00

A

v

e

r

a

g

e

P

a

rticle

Siz

e (n

m)

0

20

40

60

80

100

120

140

Wood Chips (Steady State)

Wood Pellets (Steady State)

Wood Chips (Shutdown)
Average particle size distribution at full load during 

steady state operation and during boiler shutdown

Electrical Mobility Diameter [nm]

1 10 100 1000

dN/dlo

g

D

p [#/c

m

3] Ste

a

d

y

State

0

20x106

40x106

60x106

80x106

100x106

dN/dlo

g

D

p [#/c

m

3] B

oile

r

S

h

utd

o

w

n

0.0

5.0e+8

1.0e+9

1.5e+9

2.0e+9

2.5e+9

3.0e+9

3.5e+9

Pellets - Steady State

Chips - Steady State

Chips - Shutdown
Criteria emissions from various types of 

wood-fired burners and boilers

Note: 

* – measurements include particles with diameters less than 2.5μm, cyclone used 

for PM control

♦ – multiclones used in series for PM control

● – no controls for PM

Emission Species Residential:

Emission 

factors (EPA, 

1980)

This study:

Average 

emission factors 

Commercial:

Emission factors 

(EPA, 2008)

This study: 

Average emission 

factors

FirePlace 

(g/kg)

Wood 

Stove 

(g/kg)

Wood 

Chips 

(g/kg)

Wood 

Pellets 

(g/kg)

Wet 

Wood

(μg/BTU

)

Dry 

Wood

(μg/BTU)

Wood 

Chips 

(μg/BTU)

Wood 

Pellets 

(μg/BTU)

PM 13●9.1●0.54* 0.47* 158.8♦136.1♦50.6* 26.5*

SO2 0.2 0.2 0.02 0.005 11.3 11.3 1.8 0.28

NOx 2 0.49 1.84 0.34 99.8 222.3 137 19.4

CO 67 180 2.13 2.16 272.2 272.2 158 122
Conclusions

 Particle size distributions similar for steady state combustion 

of pellets and chips

 During shutdown, GMD decreased and particle number 

increased

 Gaseous and particle emissions are lower than normal wood 

boilers due to quasi-complete combustion.

 During shutdown, CO increased, while NOx, SO2and PM2.5 

decreased

 Higher fuel nitrogen and sulfur content of the wood chips may 

have resulted in higher NOxand SO2emissions

 Higher ash content of the wood chips may have resulted in 

higher PM2.5 emissions
Acknowledgments 

.

This study was funded by the New York State 

Energy Research and Development Authority 

(NYSERDA) under contract 10672.

We want to thank David Dungate of ACT

Bioenergy for his assistance in these studies