1.2 Acidic sulphite wood pulping process and (H)SSL composition

In pulp-and-paper industry the removal of lignin (fibre consolidating material) from wood is carried out during the pulping process to obtain a fibre material (cellulose pulp) suitable for papermaking or as a chemical feedstock. About 10% of chemical pulps are produced worldwide employing sulphite methods. The acidic sulphite chemical pulping is carried out under acidic conditions (pH 1-2) at 135-145 °C for 6-12h in batch digesters using SO2/MeHSO3 (Me — pulping base) aqueous solution (Sjostrom 1993). During sulphite pulping process, lignin and part of hemicelluloses (about 50% based on wood) are dissolved in sulphite spent liquors (SSLs) composed by monomeric sugars already in the fermentable form. Roughly 1 ton of solid waste is dissolved in the spent liquor (SSL 11-14% solids) per ton of pulp produced. SSLs are produced in large amounts, about 90 billion litres annually worldwide (Lawford 1993). SSL is usually burned, for chemical and energy recovery after its concentration by evaporation (Fig. 7). The utilization of SSL is considered for a long time to produce value-added products fitting well to the biorefinery concept (Lawford et al. 1993; Marques et al. 2009).

Fig. 7. Representation of acidic sulphite wood pulping process with Spent Sulphite Liquor release

For this reason, the use of raw materials like SSL is advantageous over other agro-forestry wastes, since the more complex lignocellulosic components were previously hydrolysed, releasing most of the sugars as monosaccharides. Consequently this process is already cost — effective for pulp production, improving the 2nd generation bioethanol process economy from SSL (Lawford et al. 1993; Helle et al. 2008; Marques et al. 2009; Xavier et al. 2010). However, besides monosacharides, SSL contains several fermentation inhibitors that require a preliminary detoxification step (Lawford et al. 1993; Xavier et al. 2010).

The major organic components of SSLs are, lignosulphonates, and sugars, and their composition varies notably among softwoods and hardwoods (Table 2). Softwood sulphite spent liquor (SSSL) from coniferous, yields a high proportion of hexose sugars content (about 76%), mainly mannose and glucose, while HSSL, from hardwood Eucalyptus globulus, produces a liquor with high content of pentose sugars (xylose about 70%). Hexoses bioprocessing is well studied and already implemented in different processes, while pentoses are difficult to use as feedstock for industrial bioprocesses, because pentoses are not fermented by the yeasts currently used on ethanol production, namely Saccharomyces cerevisiae. Therefore, while the use of SSSL has been studied since 1907, when SSSL was used in Sweden for bioethanol production and also during the World War II, for yeast production as a source of protein and vitamins, the HSSL bioprocessing only recently become investigated (Lawford et al. 1993; Helle et al. 2008; Marques et al. 2009; Xavier et al. 2010). Pichia stipitis, recently reclassified as Scheffersomyces stipitis (Kurtzman and Suzuki, 2010), is the most studied yeast capable to convert pentoses to ethanol. However, this yeast is highly sensitive to HSSL inhibitors, namely formic and acetic acids, furfural, levulinic acid and phenolics. For this reason, HSSL needs a special pretreatment for inhibitors removal, which is another technical issue to consider (Helle et al. 2008; Xavier et al. 2010).