|
The burning of wood, wood waste, and other biofuels results
in a great deal of ash that is similar in properties to activated carbon. This
similarity, due to the incomplete combustion of wood at temperatures greater
than 700°C, is what makes wood ash such a beneficial material.
The carbonization process that occurs during the combustion
of wood fuels causes the random bonding of carbon in ash. This results in
defects at the molecular level, forming voids within the carbon that give wood
ash a high surface area. This property of wood ash is what helps it absorb
odors, which makes it a useful additive to compost and biosolid fertilizers.
(Rosenfeld, 2000)
Rosenfeld (2000)
analyzed specific physical properties
of wood boiler ash. In his study, he used high carbon wood ash formed from
burning 90% log yard waste and 10% paper sludge at a Kimberly Clark mill in
Everett, Washington. The sample wood ash was found to have a surface area of
about 85 m2/g and a bulk density of 0.57 g/cm3. The ash was 33% carbon, had a
calcium carbonate equivalency of 11%, and contained 32,000 mg/kg of calcium and
9,400 mg/kg of magnesium.
These values can only be used as examples for wood ash
because generally the properties of the ash will vary. Everything from the type
of tree and the soil it was grown in to the temperature of combustion and the
amount of co-fired fuels affects the properties of wood ash.
One physical property that is common to nearly all ash is its
dusty quality. The ash must either be stored where it won't be exposed to wind
or it has to be wetted down to prevent the material from blowing around. The ash
is hard on equipment as well because it is so fine and because it contains such
a large amount of carbon. (Goldstein,
1993) Although the fineness of the
ash can be a problem, there are ways to minimize dust. Generally it is
more beneficial to correct the problem and use the ash rather than just
disposing of the material.
There hasn't been a great deal of information published on
the use of wood ash as a replacement for cement. A Nigerian study (Udoeyo and
Dashibil, 2002) described the use of sawdust ash as a cement replacement.
According to the study's test results, the use of sawdust ash decreased slump
and increased the expansion of the ash/cement mortar as it hardened.
The ash also caused an increase in the initial and final
setting times for the concrete mixes. This is due to slower hydration and a
slower evolution of heat. This makes ash-containing concrete beneficial for jobs
where a large amount of early heat could cause problems.
The addition of ash also causes a decrease in strength. From
experimental compressive strength data, Udoeyo and Dashibil
(2002)(282)
determined that the optimum replacement of cement by sawdust ash is
approximately 10-20%. This results in a decrease in 28-day strength by 4-25%.
More research has to be done on how to successfully incorporate wood ash into
concrete before it can become common practice.
|
