Environmental Engineering Reference
In-Depth Information
Charcoal and Pellets: Energy-Dense Solid Biomass
The value of woody biomass as a fuel depends on its density and moisture content. Wood
with a moisture content above 67 per cent will not burn. Freshly felled timber, which has a
moisture content of 50-60 per cent, burns poorly, while wood that has been left to dry for a
year, with a typical moisture content below 30 per cent, burns well. The most combustible
of all wood products is charcoal; its moisture content is negligible, its energy density is
similar to coal, and when burnt it produces very little smoke and soot. This explains why
charcoalwastheindustrialfuelofchoiceuntilthenineteenthcentury,usedforfiringbricks,
making lime and smelting metal. Charcoal is produced by burning wood in a low-oxygen
environment. This is traditionally done by covering a wood pyre with clay, leaving small
air holes at the bottom. Wood, like all biomass, is composed mainly of carbon, hydrogen
and oxygen. The heat expels most of the hydrogen and oxygen in the form of water vapour,
leaving almost pure carbon with an energy density almost twice that of the original wood.
Charcoal is therefore an energy-dense, clean and convenient wood-derived product.
Today,its most common use is in barbeques, rather than industry.Amore modern densified
form of bioenergy is wood pellets, which are produced by compressing dried sawdust.
Because of their low moisture content and higher density, pellets have a much higher
calorific value than wood. Pellets are used instead of logs in modern domestic stoves and
boilers (see
Figure 4.40
)
, and are already competitive with fossil fuels in terms of price.
