Agricultural residues are of a wide variety of types, and the most appropriate energy conversion technologies and handling protocols vary from type to type. The most significant division is between those residues that are predominantly dry (such as straw) and those that are wet (such as animal slurry).
Sources of Agricultural Residues
Many agricultural crops and processes yield residues that can potentially be used for energy applications, in a number of ways. Sources can include:
These include those parts of arable crops not to be used for the primary purpose of producing food, feed or fibre, used animal bedding and feathers:
These are residues and wastes that have a high water content as collected. This makes them energetically inefficient to use for combustion or gasification, and financially and energetically costly to transport. It is therefore preferable to process them close to production, and to use processes that can make use of biomass in an aqueous environment. Typical wet residues include:
Any moisture content must be driven off before combustion can take place, either in advance before storage or as part of the combustion process (which then uses part of the energy of the fuel); in either case reducing overall energetic efficiency. Equally, gasification also requires relatively low moisture content (<10-15%). Other processes, however make use of biomass in an aqueous slurry, and these therefore are particularly suitable for 'wet' materials with a very high moisture content.
Considering Alternative Uses of Agricultural Rresidues
Many of the above agricultural residues may have alternative uses or markets, and any decision to use them for energy must be made in the context of these alternatives.
In particular, many such residues are presently widely used for soil nutrient recycling and improvement purposes and may therefore be displacing significant quantities of synthetic fertilizers or other products. As the manufacture of many of these products entails significant CO2 emissions and energy inputs, their substitution for agricultural residues should not be undertaken lightly.