Environmental Engineering Reference
In-Depth Information
However, there are major limitations in their successful implementation, being
the economic feasibility of the technology the most important. Firstly, the recovery
of such bio-oil from algae is very challenging task. The algal broth produced in the
biomass production generally needs to be further processed to recover the biomass
[207] and then the concentrated biomass paste is extracted with an organic solvent
(e.g. hexane) to recover the algal oil that can be transesterified into biodiesel.
Secondly, microalgal oil is rich in long-chain polyunsaturated acids including
eicosapentaenoic (20:5 n-3, EPA) and docosahexaenoic acids (22:6
ω
-3, DHA)
which are generally undesirable in conventional biodiesel due to the negative impact
of the polyunsaturations on oxidation stability. The presence of EPA and DHA is not
contemplated in the EU (EN 14214 and EN 14213, biodiesel for transport and heat-
ing) and US (ASTM D6751) quality biodiesel standards that specify a limit of 130 g
(EN 14213) and 120 g (EN 14214) iodiene/100 g biodiesel (iodine value). The stor-
age issues arising from the oxidation instability may either be overcome through
chemical transformations (e.g hydrogenations) of the polyunsaturated compounds
[208]. It is yet unclear how the presence much more saturated FAME will affect
cold performance (CFPP) of the biodiesel.
These main drawbacks will undoubtedly put up the costs of an already costly pro-
cess in which problems related to capital infrastructure costs, contamination through
open pond systems and costs associated with harvesting, drying and valorisation of
the rest of the algae may have also a major contribution. A full and precise esti-
mation of the economics of the process, that have been argued to be far too good
from what Chisti [74, 75] originally reported, is needed in order to demonstrate its
feasibility [76, 207].
8.4.1.2 Second Generation Bioalcohols
There are two critical issues that need to be addressed for the succesful develop-
ment of the second generation bioalcohols from biomass via biological conversion.
Firstly, the development of an efficient pre-treatment process in order to break up
the fibre structure of the biomass is needed because the methodologies investigated
(mechanical, thermal, chemical, enzymatic-cellulase- and combinations of them)
have been proven to be unsuitable due their high costs, low yields, produced waste
or undesired by-products. Secondly, an efficient microorganism for the fermenta-
tion of pentoses, present in hemicellulose, needs to be developed. These strategies
may also open up interesting possibilities to employ more user-friendly microorgan-
isms (e.g. Saccharomyces cerevisiae ) for biofuels production. Therefore, there is a
need for a joint effort from chemists, microbiologists and chemical and biochemical
engineers in order to demonstrate the potential of second generation bioethanol via
biological conversion.
Bioalcohols obtained from the gasification of biomass does not have signifi-
cant differences in properties compared to that obtained by biological conversion.
However, the processes are remarkably dissimilar. The conventional gasification
step is a costly process compared to the relatively inexpensive biological conver-
sion. Another important issue that needs to be addressed is the lack of standards for
Search WWH ::




Custom Search