Perennial grasses have in the past been used as fodder crops but now they are consid­ered suitable as energy crops because of their high content of cellulose and lignin. This gives the plant biomass a high heating value. Some examples of the heating values of biomass types are compared with fossil fuels in Fig. 4.2. The figure shows that the energy density of all the biomass types is lower than coal and especially gas and oil. As a consequence more biomass will be required to produce an equivalent amount of energy and thus more biomass will need to be transported.

Trials with a large number of perennial grasses have been carried out for energy in both the USA and Europe (Table 4.3). The criteria that were used in the selection as an energy crop were as follows:

1. Suitable for the climate in the region.

2. Easily propagated.

3. A consistent and high yield of biomass per hectare, probably the most important.

4. Positive balance of energy input versus output.

5. The crop can be cultivated in a sustainable manner.

6. Resistance to pests and diseases.

Fig. 4.2. Energy content of fossil fuels, a: SRC and perennial grasses.

7. Broad genetic diversity to enable species to be adapted to prevailing conditions.

8. Harvesting possible with existing technology.

9. Perennial.

10. Competitive on cost with food crops.

The four that have been chosen for further study are Miscanthus, switchgrass, reed canary grass and giant reed. Some of the properties of these grasses are compared with short rotation coppice of willow and poplar in Table 4.4.

Table 4.4. Properties of biomass crops. (From Powlson et al., 2005; Lewandowski etal., 2003.) Crop Poplar (SRC) Willow (SRC) Miscanthus sp. Switchgrass Reed canary grass Giant reed Yield t/ha/year 7 7 (15-30) 12 (5-44) (1 0-25) 10 (5-23) (15-35) 8 (7-13) 5-23 Establishment time 3 years+ 3 years+ 3 years+ 2-3 years+ 1 -2 years 1 -2 Photosynthetic pathway C3 C3 C4 C4 C3 C3 Fertilizer Low/medium Low/medium Low Very low Medium Moderate Water supply Wet Wet Not tolerant to stagnant water Drought tolerant Drought tolerant Drought tolerant Pesticide Low Low Low Very low Low Low Establishment costs High High Very high Very low Very low High Pest/disease — Beetle rust None None Some insect problems Few Day/length Long Long Long Short Long Long Plantation longevity 20 years 20 years 20 years 20 years 10 years n /a Energy content GJ /t 15 15 17.6-17.7 1 7.4 16.5-17.4 17.3-18.8 Output GJ/ha/year 105 105 260-530 (262-525) 174-435 (1 75-437) 240-600 (262-613) 88-403