As introduced above, a strong argument in favour of biofuels is that they emit less GHGs than fossil fuels when combusted (i. e. without taking into account emis­sions created during fuel extraction, growing, production and/or refining) and therefore mitigate a number of environmental issues associated with conventional fuels. Yet, it should be noted that all biofuels may not be equally environmentally friendly since the nature of the gases emitted depends on the specific composition of the biofuel in question, together with engine specifications. This section will provide a brief overview of bioethanol and biodiesel contents, the gases that they release and their respective health impacts.

Bioethanol contains oxygen, which helps create a more complete combustion of the fuel itself. An E10 blend of bioethanol (10 % bioethanol), for example, reduces the level of carbon monoxide produced by 30 % and particulate materials by 50 % in comparison with conventional gasoline (Whitten 2004). Benzene, which accounts for 70 % of toxic emissions from conventional gasoline, is also reduced by 25 % when E10 is combusted (EPA 2002). Furthermore, bioethanol contains no sulphur. As a result, there is no potential threat of sulphur emissions, which can contribute to the formation of acid rain. However, if the blended fuel contains a low percentage of bioethanol (e. g. less than 10 %), some low-level ozone could be emitted, though not to the extent of 100 % conventional gasoline (Natural Resources Defence Council 2006). In contrast to high-level ozone, which protects people from ultraviolet rays, low-level ozone can adversely affect the human respiratory system, together with plant life. By way of contrast, a higher percentage of conventional fuel in bioetha­nol blends produces carbon monoxide, unburned hydrocarbons, benzene and nitrous oxides (Demirbas 2009). When these combine with moisture and suspended air particulates, smog is formed. High-bioethanol-content fuels, such as E85, may also have negative effects on human health. They release aldehydes, such as acetalde­hyde, which causes nasal and eye irritation, and even breathing problems if the con­centration is high (McCarthy and Galvin 2006). Table 3 below presents a synopsis of the percentage variation of emissions from two blends of bioethanol in comparison with conventional gasoline.

Like bioethanol, the oxygen content in biodiesel is higher (usually 10-12 %) than for petroleum diesel. This reduces the emission of smog-forming particulate materi­als such as carbon monoxide by 11 % and unburned hydrocarbons by 21 % (EPA 2002). Though biodiesel may contain traces of sulphur, the risk of sulphur oxides and sulphate emissions is minimal. Some blends of biodiesel such as B20, however, could emit 2 % more nitrous oxide than conventional diesel (EPA 2002). This affects

the quality of air since nitrous oxide undergoes a chemical reaction in the presence of sunlight and causes smog formation. Table 4 above summarizes the findings of the US Environmental Protection Agency on the exhaust emissions from two vari­ants of biodiesel, viz. B100 and B20, compared to conventional diesel.

As mentioned earlier, emissions also vary by engine type. Vehicles with conven­tional catalytic converters are capable of minimizing the emission of aldehydes from bioethanol blends of up to 23 % ethanol. These engines can be easily adapted for using high-bioethanol-content fuels such as E85 (Greene 2004). More advanced engines were found to reduce formaldehyde emission by 85 % and acet­aldehyde by approximately 58 % (MECA 1999). With regard to biodiesel, Kousoulidou et al. (2008) concluded, from studies conducted in the USA, that pre — 1998 diesel engines emit less nitrous oxide than 2004 diesel engines equipped with exhaust gas recirculation (EGR) and that the percentage of emissions increases with the share of biodiesel in the fuel blend. 2Of particular concern is the high per­centage of nitrogen dioxide (NO2), the most harmful of all nitrous oxides, released when such blends are used in modern (e. g. Euro 4) engines (Kousoulidou et al. 2008). The emission of particulate matter is usually low for all types of engines, except for those which emit a high soluble fraction and consume more lube oil.3

From the above discussion, it appears that the combustion of biofuels, in gen­eral, affects the environment to a lesser extent than fossil fuels. However, tail­pipe emissions are only the end result and therefore do not really explain the

2 These assertions are based on the findings of EPA (2002) and Sze et al. (2007).

3 Refer to Dwivedi and Sharma (2013) for further details on emissions from the various varieties of biofuels, together with engine specifications.

net emission or absorption of GHGs throughout the life cycle of biofuels, which includes cultivation of feedstock, the processing of the biomass and, finally, its combustion for end use.