The pretreatment of the straw was carried out in a steam explosion pilot unit using a reactor with a reaction volume of 0,015 m3. Explosion was carried out into a cyclonic separator to separate the treated straw from the vapour phase. The vapour was then condensed in a regenerative cooler. The maximum steam temperature of the steam generator was 200°C

(equivalent to a steam vapour pressure of 1.55 MPa). The operation temperature in the reactor is reached via a temperature ramp. In most experiments the mass of added water was 1.0 kg per kg of straw. The operation temperature was generally 200°C and the duration of the treatment was usually 10 minutes. This results in a severity factor of 9500 (log (R0) = 3.98).

The bulk density of the straw in the reactor was 60 kg m-3 for chopped straw. When straw pellets (mixture of Triticale (Triticosecale Wittmack) and wheat straw) were pretreated, the bulk density increased to 520 kg m-3. However, in these cases the volumetric use of reactor volume had to be reduced. Also the ratio of added water was lower.

The steam consumption in the pilot tests was more than two times the calculated value due to only partial thermal insulation of the reactor. In the case of a cold start of the system, steam consumption was even higher.

Figs. 5a and 5b show an example of wheat straw before and after pretreatment. The scanning electron microscope (SEM) images show wheat straw with intact bundles of fibres before preatrement (Fig. 5a) and the same material after pretreatment (Fig. 5b), where the morphological structure has been broken down. This material is now accessible to the cellulytic enzyme complex.

Fig. 5a. Wheat straw untreated (SEM)

Fig. 5b. Wheat straw treated (SEM)