The glucose concentration obtained after hydrolysis from wheat straw pretreated with different levels/degrees of severity (conditions ranging from 160°C, 10 minutes to 200°C, 20 minutes) is demonstrated in Fig. 7. The pretreatment at 200°C over 20 minutes (severity factor 18000; log(R0)=4.26) achieved the highest sugar concentration, converting about 100% cellulose during the hydrolysis. Recirculation strategies with wheat straw were developed, where the sugar solution of a first hydrolysis reaction was recycled twice to fresh straw and the subsequent hydrolysis reaction. The glucose concentration was further increased by a recirculation process to fresh washed solids and subsequent hydrolysis from 30 g/l to 143 g/l

(20% solids, third hydrolysis). After fermentation with Saccharomyces cerevisiae, an ethanol concentration of 7.5%(vol) was obtained. In Fig. 8, the final glucose concentrations after recirculation processes with inhibitor-controlled wheat straw as well as bioethanol yields after fermentation are shown.

Corn stover was pretreated at 190°C for 10 minutes. Initially, 10% of the dry substance corn stover was hydrolyzed and fermented. Here, the sugar concentration was 32 g/l glucose and 10 g/l xylose yielding 1.9% bioethanol (Table 3). The dry substance was increased to 15% and 20%, yielding considerably higher sugar and bioethanol concentrations (Table 3).

Wheat straw was moistened with water before steam explosion pretreatment. Ethanol was added during pretreatment (10 minutes at 200°C) to test for a possible effect on the hydrolysis and fermentation step. The wet straw was hydrolyzed with the enzymes and fermented with yeast. Additional ethanol during the pretreatment process did not influence the sugar and bioethanol content (Table 4).

Alternatively, pellets from mixed straw were used to increase the dry substance already during the pretreatment step. It was possible to increase the glucose concentration from wet straw pellets to 60 g/l resulting in 2.5%(vol) bioethanol (from 10 % dry substance).