We have combined biological production of ethanol, hydrogen, and methane in the Maxifuel concept (Fig. 2). The concept is designed to address the major barriers for bioethanol production from lignocellulosic materials. The overall process outline has been defined to yield the maximum amount of biofuels per unit of raw material and to increase the process benefit by utilization of the residues for further energy conversion and by-product refining. The main product is bioethanol for use as transportation fuel and emphasis has been on optimizing ethanol production. The supply and efficient conversion of the raw material is the major economic burden of bioethanol production and full and optimized use of the raw material is a key to success. Production of other bio­fuels such as methane and hydrogen, and other valuable by-products such as
a solid fuel from the parts of the biomass not suitable for ethanol production, adds full value to the overall process. The concept exploits an environmen­tally friendly way of producing bioethanol where recirculation and reuse of all streams produced in the process are integrated into the process. This is in contrast to most other bioethanol process schemes where water has to be added continuously and toxic waste water is left after the process. The basic ideas of producing biogas along with bioethanol and then to recycle the pro­cess water, or part of the process water, within the process are patented [14]. A combination of these innovative ideas along with the best available tech­nologies has ensured an economic feasibility with a competitive advantage over other concepts. The development of the optimized process of bioethanol production from lignocellulosic biomass can be further integrated into a con­ventional bioethanol production where corn or grain fibers will be a residue of low value. Conversion of this fraction into ethanol can increase the produc­tivity by up to 20% along with an improvement of the protein feed produced in the process (Fig. 3).

The Maxifuel concept is patented and consists of the following process steps (Fig. 2):

• Pretreatment

• Hydrolysis

• Fermentation of C6 sugars

• Separation

• Fermentation of C5 sugars

• Anaerobic digestion of process water and recirculation

All fermentable carbohydrates in the raw material are converted into ethanol and hydrogen, while much of the unused fraction such as residues from

tion can be recirculated to the pretreatment unit and used together with fresh raw material.

4.1