Wood Combustion Basics
Topics
Wood Combustion Basics
Wood Composition
Combustion ProcessCombustion Process
Moisture Content and Seasoning
Q ti ?Questions ?
Presented By: Rick Curkeet, PE
Chief Engineer
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Chief Engineer
EPA Workshop March 2, 2011
Wood CompositionWood Composition
All wood is made up of primarily Cellulose, Hemi-Cellulose and Lignin.
Cellulose – (C6H10O5)x
Hemi-Cellulose –xylose, mannose, galactose, rhamnose, and arabinose
(sugars)(sugars).
Lignin - C9H10O2, C10H12O3, C11H14O4
These are all essentially complex hydrocarbons which form chains and
fib ith li i ti dh i bi di it ll t thfibers with lignin acting as an adhesive binding it all together.
Common
Name
Cellulose Hemi-
Cellulous
Lignin Other
Softwoods 42.2 28 24-35 Ca 0-1
Hardwoods 42.2 38 15-20 Ca 0-1
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Wood CompositionWood Composition
All common wood is made up of roughly 50 ±3% Carbonp g y
6 ±1% Hydrogen and 44±3% Oxygen with the rest
inorganic ash. Softwoods tend toward higher Carbon
and lower Oxygen content than hardwoodsand lower Oxygen content than hardwoods.
When burned completely about ½ the wood mass is
converted to Carbon Dioxide and about ½ to waterconverted to Carbon Dioxide and about ½ to water.
This process liberates about 8600 Btu’s per pound (20
MJ/kg) of heat energy for hardwoods and 9000 Btu/lb (21g) gy (
MJ/kg) for softwoods.
This is essentially the same amount of energy the tree
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soaked up from the sun when it created a pound wood.
Wood Combustion BasicsWood Combustion Basics
Wood is a complex fuel thatWood is a complex fuel that
undergoes dramatic changes as it
burns.
When heated to 500 600 °F wood
Wood Higher Heating Value
per CSA B415
40 000
When heated to 500-600 °F, wood
undergoes pyrolysis which
liberates organic gases and
leaves behind carbon rich
20,000
30,000
40,000
KJ/KG
10,000
15,000
Btu/lb
Average HHV
leaves behind carbon rich
charcoal.
Pyrolysis is an exothermic reaction
d t d t b lf t i i
-
10,000
0 10 20 30 40 50 60 70 80 90 100
Percent of Fuel Consumed
-
5,000
and tends to be self sustaining
once started.
Percent of Fuel Consumed
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Wood Combustion BasicsWood Combustion Basics
Up to 85% of the mass and 60% of the heating valueUp to 85% of the mass and 60% of the heating value
from wood is contained in gases produced by
pyrolysis.
Smoke and creosote represent unburned fuel.
Creosote is the condensed portion of smoke.p
Smoke contains over 100 chemical compounds
including CO, CH4, and many hydrocarbons. All can
be burned.
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Primary and Secondary CombustionPrimary and Secondary Combustion
“Primary Combustion” is the burning of solid material
directly. In wood combustion this is the burning of
the charcoal – “hot coals” “embers”the charcoal hot coals , embers .
“Secondary Combustion” is the burning of gas fuels
which produces the flames of a fire.c p oduces e a es o a e
Do not confuse with Primary and Secondary air as
these terms are used with gas appliances.
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Complete CombustionComplete Combustion
Complete combustion of wood produces only Carbon
Dioxide (CO2) and Water (H2O).
No visible smoke, no creosote, no harmful emissions.
Releases the full heating potential of the fuel.
Natural wood has no significant sulfur or metals
content.
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Incomplete CombustionIncomplete Combustion
Incomplete combustion results in production of
significant levels of CO, and many hydrocarbons.
These unburned components represent lost heating
value, pollutant emissions and potential creosote
formationformation.
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Wood Combustion BasicsWood Combustion Basics
Complete combustion requires adequate oxygenComplete combustion requires adequate oxygen
and “the 3 T’s”.
• TIMETIME
• TEMPERATURE
• TURBULENCE• TURBULENCE
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The 3 T’sThe 3 T s
TIME Sufficient time at the right• TIME - Sufficient time at the right
conditions for combustion reactions to go
to completion.p
•TEMPERATURE - High enough
temperature for fuel gas mixture to ignitetemperature for fuel gas mixture to ignite.
•TURBULENCE - To mix combustible
gases with oxygen.
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Conditions for Good Wood
CombustionCombustion
Temperatures in the 1100 to 1500°F range.Temperatures in the 1100 to 1500 F range.
Air/Fuel ratio of 10 to 12 pounds of air per
pound of fuel.p
Residence time of 2-4 seconds in high
temperature zone.
No flame contact with “cold” surfaces that
quench the flame and stop combustion.
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Combustion ChemistryCombustion Chemistry
H
H H O
O
C
H
HH
OC
O
O
Photosynthesis
H
H H
O
+ HEATCH4 + 2O2 ⇒ CO2 + 2H2O + ε
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Wood Combustion FormulaWood Combustion Formula
OjHhNhNuO 2222α ++⇒++ 2COOHC dCBA j 2222 2CBA
“Stoichiometric” (ideal) Combustion:
1 lb of dry wood + 6.4 lb air 1.83 lb CO2 + 0.52 lb H2O + 5.05 lb N2
Of course, we never achieve ideal combustion. We always need
significant “excess air” to approach 100% combustion. Usually 50significant excess air to approach 100% combustion. Usually 50
to 100% excess air is required. And we never actually get 100%
Combustion. So the real chemistry looks more like this.
YX HkCOjHhNhNuO +++++⇒++ 2222α 22 OCOCOOHC gedCBA
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MOISTURE CONTENTMOISTURE CONTENT
Moisture Content refers to water that is contained
within a piece of wood, but in not part of the wood
molecules. We often refer to it as “free water”.
Moisture content is expressed on either a wet basisMoisture content is expressed on either a wet basis
or dry basis. This refers to whether the dry or wet
weight of the wood is used as the denominator.
MC i d t i d b i hi th d d i it
+
MC is determined by weighing the wood, drying it
completely at ~220 ºF and weighing again.
MC Dry = 100 x (Wt wet – Wt dry)/Wt dryy ( wet dry) dry
MC Wet = 100 x (Wt wet – Wt dry)/Wt wet
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MOISTURE CONTENTMOISTURE CONTENT
Example:p
Wet Weight = 1.25 lbs
Dry Weight = 1.00 lbs
MC Dry = 100 x (1.25-1.00)/1.00 = 25%
MC Wet = 100 x (1.25 – 1.00)/1.25 = 20%
Note that “100% MC Dry” means 1 lb of water per 1 lb of dry wood SameNote that 100% MC Dry means 1 lb of water per 1 lb of dry wood. Same
as 50% MC Wet.
A 5 pound piece of wood with a 25% MC(dry) contains a pint of “free” water.
Electronic Moisture Meters are calibrated to determine MC on a dry basis.
We will use “dry basis” Moisture Content in the remainder of this
presentation.
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Electronic Moisture MetersElectronic Moisture Meters
Dry wood is a good insulatorDry wood is a good insulator.
Water in wood is a good conductor.
Moisture meters measure the electrical
resistance through a fixed distance which
is inversely proportional to the moisturey p p
content.
There are two types of probes or pins – short
uninsulated pins and longer insulated pins
that can be driven deep into the wood.
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Electronic Moisture MetersElectronic Moisture Meters
Wood dries from the outside in.
During drying there will normally be a large
difference between MC between outer layers and
the center.
However, as MC approaches equilibrium, it becomes
more uniform.
So for wetter wood the insulated type probe isSo for wetter wood the insulated type probe is
needed to get an accurate measurement of MC.
But for well seasoned wood the short probes are
d tadequate.
Note: The short uninsulated probes read MC of the
“wettest” area they contact – in normal conditions
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this will be at the tips.
Electronic Moisture MetersElectronic Moisture Meters
In general readings taken at ¼In general readings taken at ¼
to 1/5 the wood thickness
will be close to the average
fif the wood has had a month
or two of drying time since
cutting.g
Take readings parallel to grain.
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Electronic Moisture MetersElectronic Moisture Meters
Moisture meters are usually calibrated for DouglasMoisture meters are usually calibrated for Douglas
Fir at 70 ºF.
Corrections are used for other species and
temperatures.
Current popular models have built in species and
temperature correction functionstemperature correction functions.
Moisture Contents above about 30 to 40% are in the
fiber saturation range where the resistance is notg
significantly affected by increasing MC and thus
are not accurately determined with a Moisture
Meter.
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MOISTURE CONTENT AFFECTSMOISTURE CONTENT AFFECTS
Wood burning appliances are tested for emissions and efficiency
f ith d th t i i i t t t f b t 18 tperformance with wood that is in a moisture content range of about 18 to
28% (15 to 22%wet basis).
This represents the normal range most people use most of the time – or at
l t h t th h ldleast what they should use.
Both higher or lower moisture content can have significant negative
consequences.
High moisture reduces efficiency and makes it harder to start and sustain
good secondary combustion. This is due to its cooling effect that slows
down combustion and cools down the gases produced by pyrolysis.
Very dry wood tends to burn faster and can evolve gases at a rate that
outstrips the appliance’s ability to supply adequate air resulting in oxygen
starvation. This can result in higher emissions, pulsating combustion and
overheating
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overheating.
Fuel Wood MoistureFuel Wood Moisture
When wood is freshly cut it can have very high moisture content.When wood is freshly cut it can have very high moisture content.
Even trees that have been dead for years can have MC’s as high
as 50% or more. Living trees cut during the growing season can
have MC’s of over 150%
High moisture is not good for fuel wood.
• It soaks up heat to boil it off – Over 1030 Btu’s per pound of
water.
• It has a cooling effect on the combustible gases making it more
difficult to burn them.
• It allows micro-organisms (mold and fungus) and insects to
consume the wood.
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Effective Heating Value v MCEffective Heating Value v. MC
Effect of MC on Heating Value
8100
8600
Dry
90%
95%
100%
g
7600
8100
aueBtu/lbD
Wood
75%
80%
85%
90%
ofDryHeating
Value
6600
7100
HeatingVl
W
55%
60%
65%
70%
Percento
V
6100
0 20 40 60 80 100 120 140 160 180
Moisture Content (dry basis %)
50%
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Heating Value % of Dry HHV
Drying Fuel WoodDrying Fuel Wood
To maximize heating efficiency and minimize poor
combustion and emissions, it is essential to properly
dry or “season” fuel wooddry or season fuel wood.
Fortunately this is easily handled for us by time and
mother nature.o e a u e
The process requires little more than properly
managing fuel supplies through cutting, splitting and
storing to allow the wood to dry naturally.
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Drying Fuel WoodDrying Fuel Wood
Wood will naturally dry to an “equilibrium moisture
content” if it remains in a stable temperature and
humidity environment long enoughhumidity environment long enough.
But “long enough” can be years and the environment
that fuel wood is exposed to is far from stable.a ue ood s e posed o s a o s ab e
Still, wood seasoning can be accomplished in less than
a year if certain practices are followed.
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Equilibrium MCEquilibrium MC
Relative
EMC Table
Relative
Humidity
Percent
0 30 40 50 60 70 80 90 100 110 120 130
5 1.4 1.4 1.4 1.3 1.3 1.3 1.2 1.2 1.1 1.1 1
10 2.6 2.6 2.6 2.5 2.5 2.4 2.3 2.3 2.2 2.1 2
Ambient Air Temperature - Degrees Fahrenheit
15 3.7 3.7 3.6 3.6 3.5 3.5 3.4 3.3 3.2 3 2.9
20 4.6 4.6 4.6 4.6 4.5 4.4 4.3 4.2 3 3.9 3.7
25 5.5 5.5 5.5 5.4 5.4 5.3 5.1 5 4.9 4.7 4.5
30 6.3 6.3 6.3 6.2 6.2 6.1 5.9 5.8 5.6 5.4 5.2
35 7.1 7.1 7.1 7 6.9 6.8 6.7 6.5 6.3 6.1 5.9
40 7 9 7 9 7 9 7 8 7 7 7 6 7 4 7 2 7 6 8 6 640 7.9 7.9 7.9 7.8 7.7 7.6 7.4 7.2 7 6.8 6.6
45 8.7 8.7 8.7 8.6 8.5 8.3 8.1 7.9 7.7 7.5 7.2
50 9.5 9.5 9.5 9.4 9.2 9.1 8.9 8.7 8.4 8.2 7.9
55 10.4 10.4 10.3 10.2 10.1 9.9 9.7 9.5 9.2 8.9 8.7
60 11.3 11.3 11.2 11.1 11 10.8 10.5 10.3 10 9.7 9.4
65 12.4 12.3 12.3 12.1 12 11.7 11.5 11.2 11 10.6 10.3
70 13.5 13.5 13.4 13.3 13.1 12.9 12.6 12.3 12 11.7 11.3
75 14.9 14.9 14.8 14.6 14.4 14.2 13.9 13.6 13.2 12.9 12.5
80 16.5 16.5 16.4 16.2 16 15.7 15.4 15.1 14.7 14.4 14
85 18.5 18.5 18.4 18.2 17.9 17.7 17.3 17 16.6 16.2 15.8
90 21 21 20.9 20.7 20.5 20.2 19.8 19.5 19.1 18.6 18.2
95 24 3 24 3 24 3 24 1 23 9 23 6 23 3 22 9 22 4 22 21 5
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95 24.3 24.3 24.3 24.1 23.9 23.6 23.3 22.9 22.4 22 21.5
98 26.9 26.9 26.9 26.8 26.6 26.3 26 25.6 25.2 24.7 24.2
Drying Fuel WoodDrying Fuel Wood
The goal of “seasoning” fuel wood should be to get it to a relatively
uniform moisture content of 20 to 25%.
It helps to understand how wood dries when discussing whatIt helps to understand how wood dries when discussing what
should be done to season fire wood.
• Once a tree is cut, free water trapped in the structure will begin
to work its way to the much lower humidity ambientto work its way to the much lower humidity ambient
environment.
• But the wood structure tends to make this moisture migration
very slow.
• The easiest path for water to work its way out is through the
ends of the log – i.e. moving parallel to the wood grain.
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ends of the log i.e. moving parallel to the wood grain.
Drying Fuel WoodDrying Fuel Wood
• Drying out the ends essentially utilizes the samey g y
paths for water movement that the tree used to draw
water from its roots when it was alive.
• But if the logs are long, the path is long and the
process can be very slow. So cutting wood to short
useable lengths right away helps speed theg g y p p
seasoning process.
• Still, it may take a long time for larger logs to dry
down if left in full rounds.
• Splitting fuel wood can substantially reduce drying
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times.
Why Split Wood for Drying?Why Split Wood for Drying?
Spring
Wood
Moisture
Path
Summer
Wood
Heart
Wood
Moisture
Summer wood is
relatively dense and
slows moisture migration
Gradient
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slows moisture migration.
Why Split Wood for Drying?Why Split Wood for Drying?
Spring
Wood
Moisture
Path
Summer
Wood
Heart
Wood
Moisture moves more
easily through less dense
spring wood and has a
shorter path to the
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shorter path to the
surface in splits.
Recommendation – Split Almost EverythingRecommendation – Split Almost Everything
Split rounds from about 3” to 8” diameter into halves.
Split rounds 8” and larger into at least quarters.
Chunk large diameter logs into pieces with a maximum
dimension of 8 to 12”.
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Moisture Distribution
Fresh Cut 1-2 Months 3-6 Months Eqilibrium
Moisture DistributionreContentMoistu
Outer Surface ' ' Center
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Location
Factors That Speed DryingFactors That Speed Drying
Temperature –each 20 ºF increase in
temperature doubles the drying rate.
Stack in a sunny location if possible.y p
Wind – Air movement speeds drying.
Ventilation – Stack wood loosely ande a o S ac ood oose y a d
perpendicular to prevailing wind.
Covering – cover the top of the pile only
ith l ti t t k ff iwith plastic or tarp to keep off rain.
Surface wetting will dry off quickly.
Elevate off ground
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Elevate off ground.
How to tell wood is seasonedHow to tell wood is seasoned.
• A moisture meter is, of course, the best way to
determine MC. But there are other simple
indicators that wood is reasonably well
seasoned.
• Bark loose or falling off.
• Checking and splitting• Checking and splitting.
• Ringing sound vs. dull thump when struck.
• Relative WeightRelative Weight.
• Easier to split.
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Drying Times – Depend on Wood and
Weather US F t P d t L b E ti t f Ai D i Ti fWeather (US Forest Products Lab- Estimates of Air Drying Times for
Several Hardwoods and Softwoods)
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Drying Times – Depend on Wood and
WeatherWeather
In general – Cut, split and stack your wood by
April and it will be ready to use by October.
If you can cut two years ahead, and you stack
and store cordwood reasonably, the MC
should be in a 15 to 25% range.
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Questions?Questions?
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