Pellet Ash Composition and i

Potential Effects on Stove and B

Performance and the Environm

R. W. Rice

University of Maine
Increasing Complaints about Sinter, Sla

Clinkers in Biomass Burner/boiler Sys

Increasing concerns about particula

criteria gas pollutants
Combustion of Wood
Closeup of Coal
Volatiles and Char: Where is the Fuel V

Coal Wood

Should a volatiles test be required as part of the pellet fuels 
Pollution, Particulates and Ash from Bio

Burning Stoves, Furnaces and Applian

Gas

Particles

Combustion

AerosolsGasses fr

combustiincluding 

Fly Fin

Bottom Ash
Pollution, Agglomeration and Ash from Biomass 

Function of:• Chemical Composition of fuel

• Temperature• Burner design characteristics (airflow)

• Moisture content• Combustion Time

• Other...
Particulate Sizes Released in Combu

Small Burner Systems

Adapted from :http://www.dustconf.com/CLIENT/DUSTCONF/UPLOAD/S4/EHRLICH_.PDF

Stove burning: Mean Particulates PM10 PM2.5 

mg/cubic meter % % 

Hardwood logs 83 98.6 93 Softwood logs 82.5 99.2 97.6 Pellets (clean) 21 99 95.3 
Slagging Potential of Bottom Ash: Burner Characte

Source: Ohman et al, 2004

Pure Wood

Forest Residues
If the wood/pellet/coal etc. was 

dry when fired, would there

still be fly ash?
Bottom Ash Temperature

The agglomeration of ash particles as a result

of increased temperature exposure. A – Ash

sample at 525°C. B – Ash sample at 1025°C.
Soot: Burner Design and Compo

The lignin content of biomass tends to

increase the amount of soot.

• Hardwood lignin content, on

average: 28% by weight

• Softwood lignin content, on averag

20% by weight
Composition of the Fuel and

Combustion
The Formation of Pollutants fr

Biomass and Pellet Fuels
Creosote

HEARTH.COM ARTICLES
The Dynamics of Pollution from w

biomass and fuel pellets

Fuel species

characteristics

Air flow dynamics of

stove or furnace

Products formed by

reactions: NO, NO2,

N2O, HCl etc

Vapors (aerosols):

potassium, sodium, etcsalts. (inorganic)

Unburned fuel due to p

mixing and air flowdynamics: smoke, PAH, VOC’s

Adapted from Williams, et al, 2012 and Bryers, 1996
Particle Size and Pollution: MC 

Temperature
The Components of Pure Wood As

European to Eastern US (Percent of a

Oxide European Eastern US

Al2O3 6.76 3.63CaO 28.9 33.1Fe2O3 5.12 2.68K2O 13.2 16.34MgO 5.4 6.78MnO 2.77 2.34Na2O 1.42 2.05P2O5 2.64 3.52SO3 2.67 2.65SiO2 31.6 18.67

Where is the Cl and N?
Levels of Cl and N in Biofue

Obernberger et al 2006
Importance

• Ca and Mg normally increase the meltin

temperature of ashes.

• K and Na decrease the melting point of 

leading to “stickiness”

• Si and Ca form large portions of bottom

ash/clinkers and fly ash particulates
Slag and Clinker Formation

Large particles containing Ca, Si,

Mg, Mn, K, Al,

Vapors

Aerosols an

particulates

CaO

Na, K, S, Cl, SOx and SO2,CO, CO2 heavy metals

KCl, K2SO4,Na

compounds

Melting andCondensation,“Stickiness”

Nucleation and

Condensation,“Stickiness”

Agglomeration/sinter/slag Agglomeration/sinter

SOx and SO2, CO, CO2 etc
Where is the Potassium?

Comparison of composition before as

Werkelin et al Fuel 89 (2010) 481–493

Species %

Wood Spruce 0.0215

Pine 0.0407Birch 0.0315Aspen 0.137

Bark Spruce 0.203

Pine 0.318Birch 0.171Aspen 0.473

Twigs Spruce 0.356

Pine 0.304Birch 0.302Aspen 0.587

Needles Spruce 0.427

Pine 0.477

0

0.10.20.30.40.5

Wood Bark 

PercentK in Category

Spruce
One Specific Case: Bottom an

Agglomerated Ash

Element Pellet Ash Fly Ash

Normalized %

Ca 54% 31%

K 30% 53%

Mg 5% 3%

Na 0% 1%

S 2% 7%

Si 3% ----
Bottom Ash Depletion: Oxide Losses On H

0.0000.1000.2000.3000.4000.5000.600

525°C 825°C 1025°C

Oxide/CaO

Mixed Hardwood Pellet Ash
Bottom Ash Depletion: Oxide Losses On H

0.0000.1000.2000.3000.4000.5000.6000.7000.800

525°C 825°C 1025°C

Oxide/CaO

Pine Botton Ash
Oxide Losses On Heating

0.0000.1000.2000.3000.4000.5000.6000.700

525°C 825°C 1025°C

Oxide/CaO

Pine Botton Ash

CK2O

0.0000.1000.2000.3000.4000.5000.600

525°C 825°C 1

Oxide/CaO

Mixed Hardwood Pelle
Fly Ash Composition:

8.3μm Ash Fraction Aerosol Ash Fraction

Source: Chenevert, 1998
Thank You