5.2 Temperate and tropical crop co-products
Worldwide, residues from the two main temperate cereal crops – wheat and maize (corn) – amount to more than a billion tonnes per year, with an estimated energy content of 15–20 EJ, For use as a bioenergy source, straw must be baled, removed from the fields, stored in a dry atmosphere and transported to its point of use. Although it has a reasonable energy density of up to 15 GJ t−1, it has a relatively low mass density and occupies a substantial volume, making transport and storage expensive.
Several European countries already have wide experience of straw burning for electricity generation, as you will see in the large power station at Avedore near Copenhagen shown in this short video clip.
There are also energy co-products from tropical food crops.
The energy content of the annual residues of two major tropical food crops, sugar cane and rice, is estimated at about 18 EJ, and significant quantities of tropical crop residues are already being used as fuels.
Bagasse, the fibrous residue of sugar cane, is used in sugar factories as a fuel for raising steam and to produce electricity for use in the plant, in much the same way as wood residues are used in pulp and paper mills.
Rice husks are among the most common agricultural residues in the world, making up about one-fifth of the dry weight of un-milled rice. Their uniform texture makes them suitable for technologies such as gasification.
In the next section you move on to types of waste used as secondary energy sources.