Energy crops belong to biomass. In general, the term "biomass" is applied to renewable energy sources, referring to organic materials that are viable as sources of energy or may be converted to biofuels, which may be used as energy sources. The majority of biomass resources fit into the broad categories of energy crops, residues (by-products of existing crops) and waste products produced directly or indirectly from the solar conversion process.
According to Bassam (1998) who provided a well-established definition, "energy crops" are "those annual and perennial plant species that can be cultivated to produce solid, liquid or gaseous energy feedstock. The organic residues and wastes from the most widely diverse types of plant production, also used for producing energy, do not fall under this term but nevertheless represent a large potential." Silvicultural plants are mostly excluded too. Worldwide, approximately 300 plant species have been domesticated as crops for agriculture to supply food, feed and fibres. Of these, more than 60 species play an important role as feedstock for energy conversion processes (Fig. 5.1).
Energy crops can be divided into three principal groups: oil, cellulose and sugar/ starch plant species. The first group is used for the production of liquid biofuels. Oil pressed or extracted from seeds (e.g. sunflower and rape) is mainly supplied as straight vegetable oil (SVO), or as ester (biodiesel, RME and fatty acid methyl ester [FAME]) for power generation, or as a fuel in the transport sector. The second group is represented by ligno-cellulosic plant species, mostly used as whole plants for biomass conversion processes (combustion, gasification and synthesis). Ligno-cellulosic feedstock (annual and perennial crops) is processed into solid, gaseous and liquid energy carriers for heat and/or power generation. The third group presently cultivated on a large scale for ethanol production comprises sugar and starch crops (e.g. cereals, maize, potato and sugar beet). The most frequent use of ethanol in Europe is, however, for conversion into derivates such as Ethyl tert-butyl ether (ETBE). Many crop species are multipurpose, i.e. they can be used to produce more than one type of energy carrier, for example, cereals (ethanol and solid biofuel). Hence, biomass (wet) rich in oil, sugar and starch is also suitable for
Fig. 5.2 The main crop-to-energy chains. BtL: Biomass-to-Liquid, GtL: Gas-to-Liquid, ETBE: Ethyl ieri-butyl ether, MTBE: Methyl ieri-butyl ether, MeOH: Methanol, DME: Dimethyl ether. Pyrolysis oil, HTU-Diesel (Hydro Thermal Upgrading), ethanol and hydrogen from ligno-cellulosic species are not considered here because of their minor practical relevance in the near future
Fig. 5.2 The main crop-to-energy chains. BtL: Biomass-to-Liquid, GtL: Gas-to-Liquid, ETBE: Ethyl ieri-butyl ether, MTBE: Methyl ieri-butyl ether, MeOH: Methanol, DME: Dimethyl ether. Pyrolysis oil, HTU-Diesel (Hydro Thermal Upgrading), ethanol and hydrogen from ligno-cellulosic species are not considered here because of their minor practical relevance in the near future biogas production by anaerobic digestion. In Germany, biogas produced is most frequently used for heat and power generation. However, upgraded compressed biogas can be used as an engine fuel (Gas-to-Liquid [GtL]), which currently represents a niche market (Fig. 5.2).
Aquatic species (e.g. algae) are not considered in this study, which relates strictly to terrestrial energy crops that are classified as woody or herbaceous. Woody crops are predominantly plantation trees, frequently grown in short rotation intervals of 1-20 years (short rotation coppice [SRC]). Cultivating practices in energy plantations (e.g. willow and poplar) have only been established in a few countries (Scholz 2004). In most cases production practices for herbaceous crops (annual and perennial) resemble those of agricultural crops, although in both woody and herbaceous crop production the end use of the biomass determines the management and cultivation inputs, as well as practices employed to optimise the production system (Heiermann et al. 2007). Actual and/or potential energy crops and forms of use are listed in Table 5.1 according to their current distribution and temperature requirements.
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