Page 82 - Materials Chemistry, Second Edition

68 N. E. Korres

If the LCA aims to compare biofuels with their fossil substitutes (i.e., gasoline),
the utilization stage is crucial (Singh et al. 2010); the final energy produced from
tank for a given end use (transport/heat/electricity) depends on the combustion
performances of that engine using that fuel (Gnansounou et al. 2009; Murphy and
Power 2009). Many researchers use the ‘‘well-to-tank’’ system boundary to
compare environmental impact of biofuels with fossil fuels while many others use
‘‘well-to-wheel’’ or ‘‘cradle-to-grave’’ system (Singh et al. 2010).

5.2.3 Processing and Conversion

Current technologies for ethanol production from lignocellulosic material are
based on chemical or enzymatic conversion of the substrate to fermentable sugars
followed by fermentation process using a microorganism (Xiros and Christako-
poulos 2009). However, enzymatic hydrolysis of lignocellulosic biomass without
pretreatment is usually not so efficient due to the high resistance of the materials to
microbial degradation (Taherzadeh and Karimi 2008).
In addition to biochemical hydrolysis (i.e., enzymatic or chemical/acidic
hydrolysis) (Fig. 5 and Table 2), there is also the thermochemical approach to
second-generation bioethanol production. Both of these approaches can be used to
produce a wide variety of fuels. In the biochemical approach, enzymes (biological
catalysts, usually obtained from microorganisms) or acid is used to break down
cellulosic materials to sugars that are then fermented into alcohols (such as eth-
anol) by microorganisms. These are separated through distillation. In the ther-
mochemical approach, heat, pressure, chemical catalysts, and water are used to
break down biomass in much the same way that petroleum is refined. Thermo-
chemical technologies include gasification, fast pyrolysis, and hydrothermic pro-
cessing. These technologies can be used to convert almost any kind of biomass
into fuel, from grass to turkey feathers, giving them a potential advantage over
biochemical technologies that rely on developing specific enzymes to break down
specific plant matter (Bransby 2007; Lange 2007).

5.2.4 GHG Emissions

GHG emissions and savings are the center of attention in most LCA studies in
comparison to a reference system (Gnansounou et al. 2009; Liska et al. 2009)
along with other midpoint impacts such as eutrophication, acidification, ozone
depletion. Nevertheless, very few studies have considered these impacts since they
are site specific, thus limiting generalization of the results and pollution shifting
phenomena (Cherubini et al. 2009).