Lignocellulosics and algae have been recently considered the most promising alternatives for the production of later-generation biofuels. A full account of the production of a wide range of second-generation biofuels from lignocellulosic biomass (e. g. wood, grasses, agricultural and forestry waste) is given in Parts III and IV of this monograph. The process is identical to that described in the production of first-generation bioethanol: decomposition of the material into fermentable sugars (hydrolysis) and transformation of the sugars into bioethanol (fermentation). The main changes are in the processing technologies and the feedstocks that usually account for the majority of the plant cost. Lignocellulosic biomass comprises three main components: cellulose and hemicellulose (complex carbohydrate polymers), accounting roughly for about a 70-75 wt% of the lignocellulose, and lignin (Fig. 1.2).

A mixture of enzymes (cellulases and hemicellases) different from those used in the first-generation bioethanol production are employed in the hydrolysis step. Lignin is obtained as a by-product of the process that can be burned to produce heat and power for the processing plant and potentially for surrounding homes and businesses. It has also a great potential as it is hoped to become a future source of aromatic chemicals and materials. Alternative organisms also need to be employed due to the impossibility of traditional yeast and bacteria to process the pentose (C5) sugars derived from hemicellulose.20 We refer the readers to

Chapters 8 and 14-18 for more detailed information on advanced technologies for the processing of lignocellulosic feedstocks.

Algae is the second relevant feedstock with a great potential for future development. It has not been included as such in the monograph, but we believe that Chapters 4 and 8 will give some details about these microorganisms for the production of biofuels.

Microalgae are sunlight-driven cell organisms that convert atmospheric CO2 (via photosynthesis) into a plethora of chemicals, including methane, hydrogen, polysaccharides and oil.21-23 Interestingly, the production of algal oil is remarkably more efficient compared with conventional oil crops, providing higher oil yields (up to a 75% dry weight) and lower land area utilisation (Tables 1.1 and 1.2).

The process involves the extraction of the oil from microalgae and subsequent transesterification with alcohols using homogeneous or heterogeneous catalysts (in a similar way to that of biodiesel obtained from (non)edible feedstocks) to give biodiesel.

Despite significant advances in the field, which have been recently reported in the area of biofuels produced from algal oil, there are several drawbacks that

currently limit its widespread utilisation, primarily the economic feasibility of the technology.24

The recovery of such bio-oil from algae is a very challenging task. The algal broth produced in the biomass production generally needs to be further processed to recover the biomass24,25 and then the concentrated biomass paste is extracted with an organic solvent (e. g. hexane) to recover the algal oil that can be transesterified into biodiesel. Furthermore, the valorisation of the dry residue of the algae is not normally taken into account in current processes, and this largely implies a significant increase in costs as these algal residues need to be disposed of/removed upon extraction.

On the other hand, algal oil is rich in long-chain polyunsaturated acids, including eicosapentaenoic (EPA; 20:5 n-3) and docosahexaenoic acids (DHA; 22:6 ro-3), which are generally undesirable in conventional biodiesel due to the negative impact of the polyunsaturation on the oxidation stability. The presence of EPA and DHA is not contemplated in the EU (EN 14214 and EN 14213, biodiesel for transport and heating) and US (ASTM D6751) quality biodiesel standards that specify a limit of 130 g (EN 14213) and 120 g (EN 14214) iodine/100 g of biodiesel (iodine value). Storage issues arising from the oxidation instability may be overcome through either chemical transformations (e. g partial catalytic hydrogenations of the polyunsaturated compounds in the oil)26 or genetic modification of certain species.7,24 It is yet unclear as to how the presence of much more saturated FAM/ EE will affect the cold performance (CFPP) of the biodiesel.

These main drawbacks remarkably influence the economics of the process, in which problems related to capital infrastructure costs, contamination through open — pond systems and costs associated with harvesting and drying of the algae may also have a major contribution. A full and precise estimation of the economics of the process is therefore needed in order to demonstrate its feasibility,23-25 in which the valorisation of the algal residue (potentially via gasification to syngas and/or other biofuels) is believed to be critical to improve the economics of the process.

The potential for biofuels has been recognised throughout the twentieth century, but the new century has brought with it a widespread realisation that the petroleum age is coming to an end. The use of petrol fuel replacements has generated a lot of controversy; ideally, they should contribute to global sustainability, ensuring the energy supply and meeting the GHG targets (as well as being profitable and cost — competitive as much as possible) without compromising the economies, culture, societies and the environment of our future.

There are in our views exaggerated expectations from second-generation technologies which probably will take a long time to materialise, with topics such as fuel ‘versus’ food and the consequence of land use changes on GHG emissions being ‘politicised’.

These important issues should however not let us get distracted from the potential benefits of biofuels and, more widely, of biomass exploitation. It should rather encourage us to redouble our efforts to research low-carbon technologies

for the production of later-generation biofuels (and biochemicals) from low-value waste biomass, with properly measured and reported environmental impacts. A combined effort from politicians, economists, environmentalists and scientists is needed now, more than ever, to address the issues of the progressive incorporation of biofuels in our society and to come up with alternatives, policies and choices to advance the key technologies for a more sustainable future.

1.7 Acknowledgements

R. L. gratefully acknowledges Ministerio de Ciencia e Innovation, Gobierno de Espana, for the provision of a Ramon y Cajal contract (RYC-2009-04199).

1.8 Sources of further information

www. rsc. org; Royal Society of Chemistry, keyword: biofuels http://www. refuel. eu/ and links therein. http://www. biofuelstp. eu/