Page 51 - Advances in Eco-Fuels for a Sustainable Environment

28 Advances in Eco-Fuels for a Sustainable Environment

future option for blending with diesel because it contains more oxygen than biodiesel,
potentially leading to emissions reduction from these engines. Butanol also has a
higher heat of evaporation than ethanol, which is beneficial for reducing combustion
temperature and possibly reducing NO x formation in the process. Properties of the
several butanol isomers are different, but applications are similar. All butanol isomers
can be produced from both fossil fuels and biomass. Table 2.1 compared some of the
main physical and chemical properties of gasoline, diesel, ethanol, and n-butanol.
The main disadvantage at the moment, if compared with other biofuels, is its low
production; however, this could be changed if current testing on microalgae, and espe-
cially on Nannocholoropsis gaditana, turns out to be industrially successful.
According to values reported in Table 2.1, n-butanol could overcome some draw-
backs of other lower carbon alcohols used as fuel or additives. First of all, n-butanol
presents a higher heating value than ethanol with almost double energy density by vol-
ume, meaning that an engine running on n-butanol should have a lower fuel consump-
tion and better mileage compared to ethanol. Also, n-butanol will have less of a
tendency to vaporization; a higher flash point indicates that n-butanol is potentially
safer during transportation and use at high temperatures. Higher carbon-number alco-
hols are easier to blend with gasoline (intersolubility) due to their nonpolar long
hydrocarbon chains and lower affinity to water (less hygroscopic). n-Butanol also
has higher viscosity and lubricity, is noncorrosive, and may protect some engine com-
ponents against wear problems [3].
Biobutanol production using conventional biomass such as agricultural biomass
from crops is controversial for the balance with their nutritional needs for population
while microalgae, the third-generation feedstock, are considered a very promising
feedstock for biobutanol production due to their high growth rate and carbohydrate
content.

2.4 Production technologies

Some of the main biofuel production technologies are discussed in greater detail in
this section, including various technological alternatives for obtaining the same bio-
fuel from various feedstocks.

2.4.1 AD process
AD is the main process for biogas generation. In terms of both environmental and eco-
nomic properties, it is superior to incineration or combination of digestion and com-
posting because of an improved energy balance (Table 2.8) and reduced emission of
volatile compounds. Biosynthesis of methane follows a series of metabolic steps in
which organic matter is converted to biogas in the presence of microorganisms, as
illustrated in Fig. 2.2. The first process step is hydrolysis in which organic substrates
such as carbohydrates, proteins, and fats are depolymerized to the respective mono-
mers or oligomers (sugars, glycerol; amino acids; long chain fatty acids). These
undergo another degradation, called acidogenesis, operated by fermentative