Page 90 - Advances in bioenergy (2016)

comparative life cycle water consumption and greenhouse gas emissions, while comparing
autotrophic microalgal lipid-based biofuels with terrestrial biofuels. 27,29

It may be noted that there are also other ways to produce liquid biofuels from autotrophic
microalgae than from lipids. The microalga Botrycoccus braunii generates terpenoids, which
30
may be upgraded to liquid fuels. Alternatively, microalgal biomass may be thermochemically
converted to liquid biofuels. 31-35 In this context, there has been research into hydrothermal
liquefaction, (preferably) catalytic pyrolysis, and gasification, followed by methanol or
Fischer–Tropsch synthesis. 31-35 Research into these alternatives for autotrophic microalgal
lipid-based liquid biofuels is still at an early stage, and here the focus will be on the better
researched autotrophic microalgal lipid-based liquid biofuels, especially biodiesel.

In the following, first, the major ways to produce biomass from which autotrophic microalgal
lipid-based biofuels can be derived and their yields will be surveyed. An energetic criterion
for suitable energy sources will be discussed. Then, life cycle assessment (LCA) that can
address the energetic and environmental performance of biofuels will be briefly outlined.
Thereafter, life cycle energetic inputs and outputs and the life cycle emissions of greenhouse
gases and pollutants will be considered.

TECHNOLOGIES FOR THE PRODUCTION OF

AUTOTROPHIC MICROALGAE AND THEIR BIOMASS

AND LIPID YIELDS

There are essentially two ways to grow autotrophic microalgae: in closed bioreactors and in
open systems such as ponds. 11-18, 36-46 Open ponds are often raceway ponds with a depth of 10–
35 cm, with water mixed by paddlewheels. Three types of closed bioreactors have been found
suitable for large-scale algal cultivation. These are the flat plate or flat panel type, the
horizontal tubular type, and the vertical column type, whether or not with circulating loop (e.g.,

Refs 9, 34, 42, and 45). If compared with ponds, bioreactors have the advantage of higher
−2
yields (m ) of area used for algal cultivation and lower evaporative loss of water and the
disadvantage of higher inputs of energy and materials and higher costs. There is no agreement
on the best type of photobioreactor in view of the efficiency in converting solar irradiation to
algal biomass. 9,41,43 Some have argued in favor of vertical column reactors, 41,43 whereas others
9
have come out preferring flat plate reactors with short light path length and little shading. In
the latter case, east–west orientation and between-panel distances ranging from 0.2 to 0.4 m
44
are favorable to relatively high yields. There are also approaches to algal biomass
production in which autotrophic microalgae are firstly cultivated in closed bioreactors and
subsequently in open ponds. 40,47

When microalgal biofuels are to make a large contribution to future energy supply, the use of
open systems would seem to be preferable from the perspective of cost and scale. 11,14,17,37,38

Following harvesting of algal biomass, several options are open. One option is to concentrate