Lignin is separated out after glucose fermentation in the Maxifuel concept. Using a filter-type separator, it is possible to obtain the high dry weight lignin necessary to avoid simultaneous removal of xylose and ethanol still present in the liquid phase after initial hydrolysis and fermentation.

4.4

Fermentation

Biomass or agricultural residues consist of the polymers cellulose, hemicel — lulose, pectin, protein, and lignin. Of the carbohydrate monomers, xylose is second-most abundant after glucose in most plant cell walls [21]. Because the raw material cost is > 50% of the overall cost of the ethanol process, fermen­tation of xylose is needed to improve the yield and lower the production cost of ethanol since many biomasses and agricultural wastes contain xylose, in the order of 10-40% of the total carbohydrate mass. Fermentation of both xy­lose and glucose is therefore crucial to reduce the costs of ethanol production from lignocellulosic raw materials.

The baker’s yeast Saccharomyces cerevisiae is a desired process organism for fuel ethanol production due to its extensive use in current large-scale industrial ethanol production processes. Also, the excellent ethanol produc­tivity and tolerance towards ethanol and the inhibitors found in biomass hydrolysates are important reasons for using this organism, even though its natural xylose utilization capability is poor [22].

In the Maxifuel concept, a pentose and hexose fermenting thermophilic microorganism Thermoanaerobacter BG1 is used to ferment the residual sugars in the hydrolysate left after yeast fermentation [23]. Similar to the in­dustrial yeast strains, the thermophilic microorganism is able to grow under the harsh conditions provided by the hydrolysate whilst fermenting sug­ars efficiently. This genetically modified strain has been shown to produce

38.7 g/L or 5.4% v/v of ethanol in a continuous system running directly with non-detoxified lignocellulosic hydrolysate material. The yield from the process is 0.40 g/g total influent sugar or 78% of the theoretical possible value, and productivity is 0.85 g/L/h. The strain is tolerant to 7% of ethanol and higher dry weight in the pretreatment could be used for reaching this concentration.

Furthermore, it grows in temperatures of up to 75 °C, which eases the dis­tillation of ethanol from the reactor. Operation of the fermentation process at thermophilic conditions counteracts contamination by other bacteria, which is generally a problem for mesophilic yeast fermentation. During the residual sugar fermentation, between 0.5 and 1.1 mol of hydrogen/mol of substrate is produced. This is in the same magnitude as hydrogen yields from ded­icated dark fermentation of complex substrates such as sugar beet extract (1.0-1.7 mol hydrogen/mol substrate) [24] and molasses (0.52-1.58 mol hy­drogen/mol substrate) [8]. BG1 and all its mutants are covered by different patent applications.

To optimize the feasibility of the bioethanol production process the ther­mophilic fermentation is conducted in an immobilized reactor system. The immobilization of the fermenting organism in an up-flow reactor brings an array of important traits to the fermentation process like increased ethanol tolerance, high substrate conversions, and decreased sensitivity towards pro­cess imbalances.

4.5