The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 146

Table 6.3 Specific exergy on a dry basis of re-
presentative biomass samples (Hermann, 2006)

Biomass type Exergy (dry), MJ/kg

Eucalyptus 19.9

Poplar 19.2

Corn stover 18.2

Bagasse 17.8

Water hyacinth 15.2

Brown kelp 10.9

exergy content of dry biomass is around 810 Gtoe 10 . The theoretical biomass po-
tential was estimated by Johansson et al. (2004) at around 92 TW, which implies
an available exergy capacity of 70 Gtoe per annum (Table 6.8).
Mankind uses about 16 TW of the land productivity. From this, about 5 TW
contributes to the consumption of 1.5 TW in the form of wood fuel and around
0.2 TW goes into the production of 20 GW of ethanol (Hermann, 2006). The
world biomass production energy potential varies greatly depending on the assump-
tions taken into account. IPCC (2001) estimates a raw biomass energy potential of
10.4 Gtoe/yr (14 TW) and a liquid biofuel energy potential of 3.6 Gtoe/yr (4.8 TW),
while Moreira (2002) 31.2 Gtoe/yr (41.5 TW) and 10.8 Gtoe/yr (14.4 TW), res-
pectively (Table 6.8). According to IPCC (2007b), current biomass consumption is
around 1.5 TW.

6.6.6 Fossil fuels

Fossil fuels constitute reservoirs of chemical exergy. They represent the remains of
plants or animals that gathered their energy from the sun millions of years ago.
Around 40 GW of biological matter are buried under sediments and will eventually
form fossil fuels (Berner, 2003). As in the case of other minerals, they are not
renewable, since they cannot be replenished, at least not in Man's lifetime. The main
commercial types of fossil fuels are coal, oil and natural gas. Other unconventional
fossil fuels include tight gas sands, coal bed methane (or coal seam gas), clathrate
hydrates, tar sands, shale and heavy oil. These technologies are still in their early
stages.
Fossil fuels are by far the most important sources of energy in the 20th and
early 21st century, accounting for 87% of the world energy consumption. Indeed
production of fossil fuels at the end of 2010 reached over 10,442 Mtoe (BP, 2011).
The remaining 13% came from nuclear, hydroelectric power and renewables. See
Fig. 6.4 for the world distribution of energy consumption.

10 This number represents the total exergy one could extract from biomass, if it were not renewable.