Daniel L. A. Fernandes, Susana R. Pereira, Luisa S. Serafim, Dmitry V. Evtuguin and Ana M. R. B. Xavier CICECO, Department of Chemistry, University of Aveiro

Portugal

1. Introduction

The world is facing a reduction of global fossil fuels resources, like petroleum, natural gas, or charcoal, while energy requirements are progressively growing up. Fossil fuels should be replaced, at least partially, by biofuels once the current fuel supply is suspected to be unsustainable in the foreseen future. In fact, the search for sustainable alternatives to produce fuel and chemicals from non-fossil feedstocks has attracted considerable interest around the world, to face the needs of energy supply and to response to climate change issues. Alternative resources of energy are being explored in order to reduce oil dependence and increase energy production by exploring of solar, wind, hydraulic and other natural phenomena. Besides these sources of energy, also biomass possesses a potential target for fuel and power production as well as for chemicals or materials feedstocks. Thus biomass can efficiently replace petroleum-based fuels for a long term. (Sanchez et al. 2008; Alvarado- Morales et al. 2009; Brehmer et al. 2009; Gonzalez-Garcia et al. 2009; Singhania et al. 2009; Mussatto et al. 2010; Sannigrahi et al. 2010).

Many countries in Europe, North and South America and Asia are replacing fossil fuels by biomass-based fuels according to international regulations. One of the directives of European Union (2009/28/CE) imposes a quota of 10% for biofuels on all traffic fuel until 2020 (Rutz et al. 2008; Xavier et al. 2010). Also economic incentives for research on biofuels are being implemented all over the world. Bioethanol can be produced from different raw materials containing simple sugars, starch or more complex substrates as lignocellulosics. New methodologies for biofuels (e. g. ethanol and biodiesel) production have been developed in the last years, to achieve new and non cost-intensive technologies for bioconversion of lignocellulosic renewable resources. The most common renewable fuel is ethanol, which is produced from direct fermentation of sugars (e. g. from sucrose of sugarcane or sugar beet) or polysaccharides (e. g. starch from corn and wheat grains) (Gonzalez-Garcia et al. 2009; Mussatto et al. 2010). The selection of the best raw material is strongly dependent on the local conditions where feedstock is obtained. Evidently, ethanol in Brazil is produced from sugarcane, whereas, in North America or Europe the ethanol industry is based on starchy materials. Besides, energy considerations should be attained: not only the energy input required for ethanol production and the content in fermentable

sugars of the feedstock must be considered, but also the annual ethanol yield per cultivated hectare. As suggested, for beet molasses, the yield of ethanol per ton of feedstock is lower than that for corn, but on the other hand, when compared to starchy materials the beet productivity per cultivated hectare, expressed in L/(ha year), is considerably higher, (Sanchez et al. 2008).

The growth of the biofuels industry raised questions regarding the sustainability of these "first generation" biofuels. The feedstocks described play an essential role in human and animal food chains, therefore the rise of prices of food all over the world resulted in social disturbance (Gonzalez-Garcia et al. 2009; Mussatto et al. 2010; Xavier et al. 2010). These raw materials were also expected to be limited due to the reassign of arable lands from food to fuel production leading to competition for feedstocks (Gray et al. 2006; Bacovsky et al. 2010). Moreover, first generation biofuels were accused of not contributing to reduce gas emissions, therefore the use of this technology was highly criticized. For all these reasons additional research in this area is mandatory, in order to search for non-food crops, like wastes from agriculture and/or industry as sources of raw-material. European Union strongly incentives research focusing biotechnological solutions for energy and chemical demands from renewable resources, such as, forestry wastes, agricultural biomass residues and food industrial wastes for "second generation" biofuels production.

The great advantage for the choice of lignocellulosic biomass as feedstock is the non­interference with food chain, which allows the production of bioethanol without using arable lands (Sanchez et al. 2008; Zhang 2008). Lignocellulosic biomass is a complex raw material which can be processed in different ways to obtain other value-added compounds contributing to the possibility of establishing a biorefinery. Different value-added products such as lactic acid, acetic acid, furfural, methanol, hydrogen and many other products can be obtained from its sugars. Lignin, the non-carbohydrate component, can be used for the production of advanced materials, polymers and aromatic aldehydes (Sanchez et al. 2008; Zhang 2008; Sannigrahi et al. 2010; Santos et al. 2001). In this way, lignocellulosic biomass can be used as substrate for the production of second generation biofuels, contributing to the diversification of energy supply and gas mitigation, offering less competition for the food and feed industry (Rutz et al. 2008; Bacovsky et al. 2010). The use of these raw materials to produce fuel, power and value-added chemicals, fits well into the biorefinery concept invoked to decrease the dependence from fossil resources and to improve the economic sustainability (Alvarado-Morales et al. 2009; Xavier et al. 2010). However, for a world massive utilization of fuel ethanol, a cost-effective technology for ethanol production is also required. In other words, ethanol production costs should be lowered (Sanchez et al. 2008). In a biorefinery, different technologies, including fermentation, biocatalytic, thermal and chemical technologies, must be used simultaneously for biomass conversion for the production and the purification of different value-added products (Alvarado-Morales et al. 2009).

Bioethanol is one of the products that can be obtained via biorefinery using bio-based resources. It is one of the most attractive biofuels, since it can be easily produced in large amounts and blended with gasoline or used pure as a "green" fuel. Furthermore, due to the higher oxygen content, ethanol allows a better oxidation of the gasoline and reduces CO and particulate emissions. Other advantages of ethanol versus gasoline are the higher octane number, broader flammability limits, higher flame speeds, heat of vaporization and compression ratio and a shorter burn time (Balat et al. 2008; Mussatto et al. 2010). The use of bioethanol can also contribute for the reduction of CO2 build-up, while the CO2 content of fossil fuels will remain in storage (Sanchez et al. 2008; Gonzalez-Garcia et al. 2009; Chen et al. 2010a; Balat 2011). Moreover, combustion of ethanol results also in lower NOx emissions, being free of sulphur dioxide. However, as disadvantages, ethanol has an energy density lower than gasoline, it is fully miscible in water and its lower vapour pressure makes motor cold start more difficult (Balat et al. 2008; Gonzalez-Garcia et al. 2009; Chen et al. 2010a; Mussatto et al. 2010; Balat 2011). Simultaneously, bioethanol is a building block for the production of several other chemicals, usually petrochemical-based, like acetaldehyde, ethane, ethylene, propylene, butadiene, carbon monoxide or hydrogen (Idriss et al. 2000; Wang et al. 2008; Yu et al. 2009; Lippits et al. 2010; Oakley et al. 2010; Song et al. 2010). Today nearly 95% of hydrogen is produced from fossil-based materials such as methane and naphtha. Bioethanol as chemical reagent for hydrogen production could be a way to support hydrogen economy from a renewable and clean energy source (Yu et al. 2009; Lippits et al. 2010). Besides, the production of olefins from ethanol has attracted much attention since it valorises bioethanol production under a biorefinery context (Thygesen et al. 2010).

In this context Hardwood Sulphite Spent Liquor (HSSL) is a subproduct of pulp and paper industry that results from the acidic sulphite pulping process in high amounts per day. The main objective of acidic sulphite pulping process is to remove lignin and hemicelluloses from wood and to maintain cellulose integrity as much as possible. In this process, lignin and hemicelluloses are hydrolysed and released in the aqueous phase. HSSL can be a suitable substrate for 2nd generation bioethanol production as well as other biobased products since it is rich in monosaccharides obtained during the acidic sulphite pulping process.