Environmental Engineering Reference
In-Depth Information
Box 9.2
Charcoal Requirements for Iron Smelting
In 1810, the earliest year for which the nationwide data are available, the United States
smelted about 49,000 t of pig iron. With an average rate of 5 kg of charcoal (at least
20 kg of wood) per kilogram of hot metal, this production required approximately 1
Mt of wood. Even if all that wood were harvested in old-growth eastern hardwood
forests, which contained around 250 t of aboveground phytomass per hectare (Brown,
Schroder, and Birdsey 1997), and even if all stem, bark, and branch phytomass could
be used in charcoaling (both being excessively generous assumptions), an area of about
4,000 km
2
(a square with a side of about 63 km) had to be cleared in that year. By
1840 all U.S. iron was still smelted with charcoal, but by 1880 nearly 90% of it was
produced with coke.
Without the switch from charcoal to coal-based coke, even the forest-rich United
States could not carry on for another century: in 1910 the country smelted 25 Mt of
iron, and even with just 1.2 kg charcoal (or about 5 kg of wood) per 1 kg of hot metal
it would have needed 125 Mt of wood. Harvesting this wood from a mixture of i rst-
and second-growth forests averaging 125 t of aboveground phytomass per hectare
would have claimed 10,000 km
2
/year; in just ten years an unchanging rate of iron
smelting would have cleared 100,000 km
2
of forest, an area only slightly smaller than
Pennsylvania—but by 1920 the U.S. output was more than 35 Mt of iron, and annual
wood harvests would have to be (even with efi ciency improvements) about a third
higher.
These realities should be kept in mind by all advocates of green energy futures. Blast
furnaces produced about 50 Mt of iron in 1900, 580 Mt in 2000, and nearly 1,000
Mt (1 Gt) in 2010. By 1910 the average worldwide ratio of dry coke to hot metal had
declined to just 0.45:1, which means that the global need for pig iron smelting was
about 450 Mt of coke. Replacing all of this with charcoal would require—even when
assuming just 0.7 t of charcoal per 1 t of hot metal and a 3.5:1 wood: charcoal ratio—
about 2.5 Gt of wood, the total roughly equal (assuming average 650 kg/m
3
) to the
world's harvest of about 3.9 Gm
3
of roundwood. And even if that wood were to come
from high-yielding tropical eucalyptus (producing annually 10 t/ha), its growth would
occupy some 250 Mha, an equivalent of more than half of Brazil's Amazon forest.
