There are a variety of ways to use biofuels for transport. The first category focuses on electric traction, which currently accounts for about 1% of energy use in the transportation sector worldwide (de la Rue du Can and Price 2008). Electric trac­tion is common in train transport, but there are also ships powered by electricity, and a battery-powered small airplane has been demonstrated (Sanderson 2008). All­electric cars currently have limited application, but more recently there has been a rapid increase in the use of hybrid cars that use both internal combustion engines and electromotors (Mom and Kirsch 2001; Wurster and Zittel 2007; H0yer 2008).

Electricity can, for instance, be generated in power plants fired by biomass and stored in batteries. Also, electricity can be generated by onboard fuel cells fed with, for example, H2 derived from biomass or H2-producing organisms. Hydrogen used in fuel cells is, from a life cycle perspective, more energy efficient than the applica­tion of H2 in Otto or diesel motors (EUCAR et al. 2007; Hussain et al. 2007; Kleiner

2007) . Fuel cells may also be used for the propulsion of ships and airplanes (Little­field and Nickens 2005; Lapena-Rey et al. 2008; Sanderson 2008). Introduction of hydrogen as a major transport fuel requires concerted action of many stakehold­ers (Wurster and Zittel 2007) and includes large changes in fuelling infrastructure and a major effort to reduce fire and explosion risks (MacLean and Lave 2003; Ag — nolucci 2007; Astbury and Hawksworth 2007; Markert et al. 2007; Melaina 2007; Ng and Lee 2008). Also, major advances in several key components of motorcars are necessary for a successful large-scale introduction of all-electric or H2-powered cars (Chalk and Miller 2006; Matheys et al. 2007; H0yer 2008; Lache et al. 2008; Samaras and Meisterling 2008).

In practice, wood, animal wastes, harvest residues, municipal and industrial or­ganic wastes, landfill gas, ‘energy’ grasses (such as reed canary grass) and veg­etable oils have been used in power generation (e. g. Reijnders and Huijbregts 2005; Berggren et al. 2008; Heinimo 2008; Junginger et al. 2008; Reijnders and Huijbregts

2008) . Sewage sludges and wastewater treatment sludges are also applied, though these tend to be net users instead of net producers of energy due to their high water content (Wang et al. 2008c).

There is, furthermore, scope for the co-production of electricity and ethanol from sugar cane (Macedo et al. 2008). In producing electricity, both direct burning of biomass and burning after gasification or fermentation are practiced (Wheals et al. 1999). Problems in generating electricity from biomass have arisen due to slagging, corrosion and fouling mainly linked to the presence of inorganic elements such as Cl and K; in the case of gasification, fouling has also been linked to tar formation (Monti et al. 2008). Ways to decrease such problems, such as lowering Cl and K concentrations by judicious choice of feedstocks, have been researched (Monti et al. 2008), though there are types of biomass, such as macroalgae, that still appear un­suitable for direct combustion or gasification (Ros et al. 2009).

The second possibility is to produce liquid or gaseous biofuels that can be burnt in transport engines that currently burn fossil fuels. In 2006, such biofuels accounted for about 1% of energy use in the transportation sector worldwide (de la Rue du Can and Price 2008). Various engines operate under a variety of conditions, and not all liquid and gaseous biofuels are suitable to all applications. Quantitatively speaking, two engine types dominate road transport, and also transport in general: the diesel engine and the Otto motor. A variety of gaseous and liquid biofuels produced have been proposed for these engine types. As to the way these biofuels are produced, most of them can be allocated to three categories (Ahman and Nilsson 2008). The first category relies on the biochemical conversion of biomass into transport bio­fuels. Biochemical conversion is now used for the production of ethanol, butanol and methane. The second category is based on lipids (oils and fats) derived from organisms. Such oils may be applied directly or after processing (e. g. transesterifi­cation or catalytic cracking). The third category uses thermochemical conversion of biomass via pyrolysis or gasification into a variety of fuels.

A part of the transport biofuels which have been proposed are currently produced on an industrial scale and widely applied in means of transport. Ethanol obtained from starch or sugar by fermentation and biodiesel based on lipids from terrestrial plants are currently the main transport biofuels. Other substances that have potential as transport biofuels are produced on an industrial scale but hardly or not applied in Otto and diesel motors. A third category of transport biofuels include those in the laboratory and pilot plant stage. All these are shown in Table 1.1.

The energy contents of the liquid and gaseous transport biofuels mentioned in Table 1.1 may be different from the fossil petrol and diesel that they replace. Table 1.2 gives a survey of the energy contents (lower and higher heating values) in megajoules (MJ) of the liquid fossil and biofuels per kilogram (kg) and per litre (l). The lower heating value (LHV) represents net energy content, and the higher heating value (HHV) represents gross energy content (including the heat of condensation of water vapour produced by combustion (Piringer and Steinberg 2006)).

The differences in heating values indicate that when the amount of transport kilo­metres for a full tank is to be maintained, a substantial adaptation of tank size may be necessary when transport fuels contain high percentages of biofuels with relatively low heating values, such as dimethylether and ethanol (Semelsberger et al. 2006). This is not the only adaptation that may be necessary when switching to biofuels. Table 1.3 gives a brief summary of other adaptations for a number of biofuels.

Table 1.3 Problems and adaptations necessary for the use of biofuels Biofuel Problems and adaptations

Ethanol — Ethanol is relatively corrosive, and ethanol-gasoline blends may sep­

arate in pipelines; this limits the scope for pipeline transport. Also, ethanol is hygroscopic, and high water concentrations may lead to phase separation. So, in storage and distribution, exposure to water should be severely limited (Antoni et al. 2007; Atsumi et al. 2008).

— Limited admixture of ethanol (whether or not as ETBE: the tertiary butylether of ethanol) up to 5% is possible without adaptation of cars. If ethanol-fossil hydrocarbon blends with percentages of ethanol over 5% are used, however, changes in cars are needed (Antoni et al. 2007). Such changes regard the fuel-sending unit, the fuel injector, the fuel filter, fuel management and flame arrestors. When the percentage of bioethanol be­comes 85 or 100%, changes necessary for the engine become substantial (Antoni et al. 2007; Hammond et al. 2008). This has led to the develop­ment of flex vehicles that are able to run on blends with high percentages of ethanol, and also on conventional petrol.