Re: What causes different woods to burn hotter?

In general, in forestry practice, wood burns best for heating purposes 
when it is at about 20% moisture content.  For this purpose we will use this 
for our basis in our discussion.  Some woods burn hotter than others for a 
number of reasons, but principle among them would be the density of the 
wood or in general terms, the hardness of the wood.  Hickories typically 
burn hottest at approximately 30.8 - 32.1 million BTU's per cord. (a 
cord of wood is 128 cubic feet or otherwise occupying a space 4ft x 4ft x 
8 ft.  For the most part, the harder the wood, the hotter it will burn.  
One exception is yellow pine and the cedars and junipers, which burn in the 
middle of the heat range (22 - 21 BTU's per cord) burn hotter than come 
of their soft wood cousins due to their high resin contents.  They possess 
oils which help them burn hotter than some of their counterparts. 

After hickory, the oak family burns next hottest, from 29.6 - 30.8 BTU's 
per cord.  The least hot burning wood is white pine, what many 
superstructures of wood frame homes are built from.  Keep in mind that 
these woods burn much quicker due to the fact of their density. A good web 
site with all this information is from the 
Tennessee Department of 
Agriculture's Forestry Division.  
Following is the page pasted for your information.  

Thanks,
Donald E Duggan 


Forest owners can convert leftovers from thinnings,
improvement cuttings, or commercial harvests into firewood.
They have the option of selling on a "you-cut" basis, cutting
and selling on-site, selling to (or at) a firewood lot, or delivering
to the customer.

Buyers can get the best price by shopping around. "Want
ads", yellow pages, bulletin boards, word of mouth, or roadside
signs are means of finding and advertising firewood. Some
landowners, utilities, landfills, wood products industries and
state forests will let "do-it-yourselfers" cut wood for a fee.
Dense or heavy woods such as hickory and oak burn long at a
sustained rate and contain the greatest amount of energy per
cord. Light woods provide a quick bright fire. They contain
only ½ to 2/3 the energy of heavy woods and should be priced
somewhat lower.

Yellow pine contains pitch that burns hot but is sooty. Yellow
pine and especially cedar pop a lot, which can be enjoyable and
safe behind glass fireplace doors.

Oak, beech, ash and hackberry are good, easy-to-split firewoods.
Sycamore, blackgum and elm are almost impossible to split by
hand. Hickory is difficult to split, but it and sugar (hard) maple
make good beds of coals.

Firewood is sold by the cord, rick and pickup load. A cord is a
stack of wood 4'wide, 4' high and 8' long. A rick is a fraction of
a cord. An 8'x4'x24" rick contains ½ cord. A rick of 16" pieces
is 1/3 cord. A load in a full-sized pickup can vary from 1/3 to ½
cord, which is 1 to 1½ ricks of 16" wood. A cord of heavy wood
weighs about 2 tons cured and 3 tons green.

A rick might be stacked loosely or tightly. Small round pieces
should fill the voids between large round pieces, and split pieces
should fit closely. Unless the price is right, avoid firewood
made up mainly of small limbs, which make for a loose rick and
a lot of bark. Avoid knotty pieces that are too big for your
fireplace or stove. Cover stacks to keep wood from
deteriorating. Two to three cords will usually last a winter.

Wood should be air-dried for at least 6 months, and preferably
9. Well-cured firewood is grayish on the end, with radial cracks.
"Green" wood is difficult to light, burns cool, smokes a lot, and
can leave deposits of tar in the chimney that can cause
dangerous flue fires. Those who burn uncured wood might need
their chimney swept often. Woodcutters can speed up curing by
felling and leaving trees in summer. The leaves will draw much
of the water out of the wood.

Heat content (million BTU/cord), 20% moisture:
Hickories 30.8-32.1
Oak: willow, swamp white 29.6-30.8
post, scarlet, swamp chestnut 28.7
chestnut, southern red, white 28.3
northern red, overcup, water 27.0
black 26.1
Locust, black 28.3
Beech 27.4
Maple, sugar 27.0
Elm, rock 27.0
Ash, white 25.7
Walnut, black 23.6
Maple, red 23.2
Sweetgum 22.3
Hackberry 22.1
Pine, yellow 21.8
Cherry, black 21.4
Elm, American 21.4
Sycamore 21.0
Yellow-poplar 18.0
Sassafras 17.5
Cottonwood 17.1
Hemlock 17.1
Willow 16.7
Pine, white 15.0

Firewood can be difficult to identify. With some practice, a
simple "heft test" can distinguish dense from light firewood.

Information is from www.state.tn.us/agriculture/forestry/lit/06.pdf.  This is a PDF 
download, so you will need  Adobe Acrobat 
Reader 
to be able to extract the information.