Environmental Engineering Reference
In-Depth Information
residues” and 49.2 Mt of “wood charcoal” (FAO 2011e). Charcoal production
(using the FAO's recommended conversion factor of 6.0) required 295 Mm
3
wood,
resulting in a grand total of 2.195 Gm
3
of wood. Assuming averages of about
0.65 t/m
3
and 15 GJ/t of air-dried wood, this converts to approximately 21.5 EJ or
1.43 Gt. As already noted (in chapter 2), in many countries most woody phytomass
does not come from forests, and that is why the FAO's total greatly underestimates
the actual use.
My estimate for the year 2000 is about 2.5 Gt of air-dried wood (about 2 Gt of
absolutely dry matter containing about 35 EJ, including wood needed to make
charcoal), and the burning of crop residues in i elds and their recycling and feeding
to animals left about 20% of the entire residual phytomass (about 10 EJ in the year
2000) to be used as household fuel. My most likely estimate of phytomass combus-
tion is thus 45 EJ in the year 2000, an equivalent of 3 Gt of air-dried wood (Smil
2010b). This compares to 2.06 Gt (nearly 31 EJ) of all fuel phytomass (of which
1.32 Gt was wood) estimated only for the low-income countries in the year 1985
by Yevich and Logan (2003) and to a range of 45
10 EJ estimated by Turkenburg
et al. (2000). The most recent (and the most detailed) estimate, based on the com-
pilation of many national totals, ended up with 2.457 Gt of solid phytomass fuels
in the year 2000, with about 75% as wood (1.85 Gt), 20% (0.5 Gt) as crop residues
and the rest as dung and charcoal, and with 80% of the total consumed by house-
holds and 20% by workshops and industries (Fernandes et al. 2007).
Their aggregate solid biofuel consumption translates to at least 37 EJ. Published
estimates of solid phytomass combustion thus converge to around 40 EJ, or around
2.5-2.7 Gt of all solid phytomass fuels in the year 2000. Given the many inherent
uncertainties in estimating this supply, plausible totals for the year 2000 range
between 35 and 45 EJ, or between 2.3 and 3.0 Gt of wood equivalent, of which
wood accounts for 30-35 EJ, or between 2 and 2.35 Gt. This range implies an
increase of nearly 70% between 1950 and 2000 and a doubling of solid biofuel
harvests during the twentieth century. But this absolute growth has been accompa-
nied by substantial declines in per capita uses in all high-income countries, with a
rising or stable per capita supply found only in some countries of sub-Saharan Africa
and in rural areas of low-income economies in Asia.
The global harvest of 40 EJ of solid biofuels in the year 2000 was just over 10%
of all primary energy supply when we compare the initial energy contents of fuels
and primary electricity: that year's total was about 382 EJ, with fossil fuels account-
ing for about 305 EJ and hydroelectricity and nuclear electricity generation contrib-
uting respectively about 10 EJ and 25 EJ. But because of much higher conversion
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