Environmental Engineering Reference
In-Depth Information
Another complication in converting wood to a common mass denominator is its
highly variable water content. The water content of freshly felled roundwood is
almost never less than 40% and can be well above 100% (when measured on an
oven-dry basis). Poor people who collect their own fuelwood do so because they
obviously do not have any tools (saws or axes) to cut down trees, but there is a
major advantage to this limitation: fallen or dead branches and twigs are much drier
than green stemwood and can be used immediately for cooking or heating, while
wood with a water content above 67% will not even ignite. Green stemwood must
be split into smaller pieces and air-dried, often for long periods of time, before it
reaches its minimum equilibrium moisture of no less than 20%-25%. Moreover,
twigs and branches are easily carried bundled as head loads.
A simple formula is used to convert volumes of freshly cut roundwood to oven-
dry weight: oven-dry weight = specii c density
×
1,000 (specii c density of m
3
of
(1 + wood moisture/100), and tables list these values according to wood's
water content and specii c gravity (Glass and Zelinka 2010). Green heartwood has
a water content as low as 35% in some pine and i r species and as high as 80%-90%
in some hardwoods, but most values will be between 40% and 60% by weight.
With a moisture content of about 40% and a specii c gravity of 0.4, a freshly felled
cubic meter of coniferous wood will have an oven-dry weight of about 560 kg, and
1 m
3
of hardwood with a specii c gravity of 0.6 and a moisture content of 60% will
have a dry weight of 960 kg. The mean of these two common combinations is 760
kg/m
3
, while wood with a specii c gravity of 0.5 and a moisture content of 50%
has an almost identical mass of 750 kg/m
3
. I will use this value later in the topic
when converting roundwood volume to mass.
Inevitable errors in converting fresh volumes of commercial roundwood to the
common denominator of absolutely dry phytomass are further compounded by
uncertainties regarding the actually harvested volume, as illegal logging has been
common in several major wood-producing countries, including Russia, Brazil,
China, Indonesia, Malaysia, Congo, Gabon, and Ghana (Global Timber 2011). The
challenge is even greater when trying to account for wood used by low-income
households. No statistics are kept regarding the volumes or weights of woody phy-
tomass collected by families in rural areas. Short-term studies of actual collecting
activities have to reckon with different sizes of typical head loads or animal loads,
and if fuelwood is stacked near a house for future use, then (depending on the
species, size of wood pieces, and the tightness of stacking) 1 m
3
can have a mass of
310-460 kg.
water)
×
