Plant Biomass as Biofuels

Abstract

Sustainable and renewable fuel sources are imperative to maintain future global commerce, mobility and economic prosperity, to alleviate current dependency on fossil fuels and to mitigate greenhouse gas emissions from combustion in vehicles. Plant biomass and its derivatives represent an abundant carbon source for the production of biobased alternatives to petroleum‐based transport fuel. The production of first‐generation biofuels is currently the most advanced, with bioethanol being manufactured commercially in many countries. However, the major disadvantage of first‐generation biofuels is that they introduce competition for land on which human food crops are grown. Second‐generation biofuels from lignocellulose address this issue, but their production is not yet fully commercial. Advanced biofuels from algae are an alternative biofuel technology currently in the early stages of development. Minimising renewable plant biomass feedstock cost, improving biomass feedstock yields, establishing efficient, coordinated processes for carbon neutral commercial production and changing policy in favour of developing existing and future biofuel technologies are some of the challenges that must be overcome before biofuels can become competitive with fossil fuels.

Key Concepts

  • Global prosperity is closely linked to transport of goods and people; however, for practical, political and environmental reasons, the combustion of fossil fuels is not sustainable. Biofuels offer an alternative that is minimally disruptive to the existing infrastructure at this early stage in the transition towards a biobased economy.
  • Biofuels are mainly derived from plants and are characterised according to the source of the plant material used in their production; ‘first‐generation’ (1G) biofuels (ethanol and biodiesel) are derived from the fermentation of sugar‐ or transesterification of oil‐rich food crops; ‘second‐generation’ (2G) biofuels come from plant biomass that is not suitable for human or animal consumption, such as straw, wood, energy crops or inedible plant oils; ‘advanced biofuels’ are generated by microbes, notably, oleaginous yeasts or microalgae, where the microbe does not convert the substrate to fuel but is the immediate source of the fuel.
  • Advanced biofuels can further be classified as ‘third generation’ if they are made by naturally occurring microbes or ‘fourth generation’ if the biofuels are produced by synthetic biology in engineered microbes.
  • First‐generation ethanol and biodiesel are relatively simple to produce and used in fuel blends throughout the world.
  • Lignocellulose (LC) forms the structure of plant cell walls and is the most abundant natural polymer on the Earth but is very resistant to degradations; to produce 2G ethanol, lignocellulosic biomass must be pretreated to release the sugars, which are then fermented to ethanol.
  • Advanced biofuels are largely at the experimental phase of development.
  • Whatever the source, biofuels face a number of challenges including the cost of the feedstock, the cost of conversion to biofuel and slow progress from laboratory to pilot‐scale production.

Keywords: biofuel; biomass; bioethanol; biodiesel; bioenergy; lignocellulose; first‐generation biofuel; second‐generation biofuel; advanced biofuel

Figure 1. Summary of first‐, second‐ and third‐generation biofuels and the feedstocks from which they are made.
Figure 2. First‐generation ethanol production from glucose and starch. (a) Diagram of the anaerobic conversion of glucose to ethanol by yeast (Saccharomyces cerevisiae) via the action of fermentation. (b) Schematic representation of the principle steps involved in starch hydrolysis (saccharification), before fermentation.
Figure 3. Schematic representation of the transesterification of storage lipid (triacylglycerides) with alcohol to produce biodiesel (fatty acid methyl ester; FAME) and glycerol.
Figure 4. Simplified process flowchart of the production of (a) biogas and (b) syngas.
Figure 5. Schematic representation of lignocellulose. Two molecular groups are shown, representing the structure of cellulose and a potential structure of xylan, one of the components of hemicellulose. Cellulose is composed of repeating units of glucose bound by B‐1,4,glucosidic bonds. Conversely, Xylan contains not only bonds between the xylose monomers but also additional components branching from the main chain.
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Wojcik, Emilia Z, Singleton, Chloe, Chapman, Liam NM, Parker, David A, and Love, John(Jun 2017) Plant Biomass as Biofuels. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023716]