Can renewable energy power the world?
Can renewable energy power the world?

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Can renewable energy power the world?

4.3 Biomass as a fuel

Fuels are materials from which useful energy can be extracted. In using biomass as fuel, this release of energy usually involves burning – known as combustion.

Some essential features of combustion are:

  • it needs air – or to be more precise, oxygen
  • the fuel undergoes a major change of chemical composition
  • heat is produced, i.e. energy is released.

Consider for example, methane – the principal component of the fossil fuel natural gas and also one form of gaseous biofuel. As shown in Figure 4, each methane molecule consists of one carbon and four hydrogen atoms: CH4. Atmospheric oxygen has molecules consisting of two atoms (O2). In combustion, each methane molecule reacts with two oxygen molecules to produce carbon dioxide and water. The heat energy released is the difference between the chemical energy of the original fuel plus oxygen, and the chemical energy of the resulting carbon dioxide plus water. It is the energy content (or heat content) of the methane.

Figure 4 Chemical equation representing the combustion of methane yielding carbon dioxide, water and heat

Table 1, shows the energy produced in burning one tonne, or one cubic metre, of various biological materials.

Table 1 Typical heat energy content per unit of dry mass, and per unit volume, of various forms of biomass

Category Major energy-rich components Structural strength/resistance to natural decay Examples Typical yields of dry matter /4 ha-1 y-1
Woody biomass

Lignin/lignocellulose

(complex carbohydrates)

High Trees (deciduous or hardwoods) 10 (temperate) to 20 (tropics)
Cellulosic biomass

Lignin/lignocellulose

(complex carbohydrates)

Medium Grasses (e.g miscanthus). Water hyacinth, seaweeds 10 (temperate) to 60 (tropical aquatics)
Starch/sugar crops Simpler carbohydrates Low Cereaks (maize, sugar cane, wheat) 10 (temperate cereals) to 35 (sugar-cane)
Oily crops Lipids (i.e. oils/fats) Low Oilseeds (rape, sunflower, oil palm. Jatropha) 8 to 15
Micro organisms Oils Low Microalgae Unknown – still speculative

Let’s now move on to discuss the energy that can be derived from plant biomass.

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