Page 98 - Advances in bioenergy (2016)

0.6 g/L, have to be concentrated to about 10–15% solids in water, which is resource
intensive. 18,115 Also, algal cell walls are sturdy, which requires relatively large life cycle
energy inputs to get the lipids out. 113 Supercritical technologies are furthermore characterized

by relatively large life cycle energy inputs if compared with conventional catalytic
transesterification. 116 Also, supercritical transesterification of polyunsaturated fatty acids, as
3-6
commonly present in autotrophic microalgae, may lead to substantial reductions in biodiesel
yield. 117 In the case of in situ transesterification of algal biomass, furthermore, very high inputs
of methanol and methanol recovery are needed, 114 which are not conducive to life cycle energy

efficiency. 116 Thus, the assumptions about the beneficial effects of wet technologies on EROI
may be overly optimistic.

It has furthermore been argued that energy inputs may be reduced when benthic cyanobacteria
are used for biofuel production, as these may form dense mats at the surface of the culture
medium. 118 Another option which has been suggested to facilitate harvesting is culturing
18
attached microalgal biofilms. Liquid biofuel yields achievable by these approaches are as
yet highly uncertain as research into these options is at an early stage.

It has also been suggested that the increase in photon demand linked to high lipid
concentrations might be countered by the excretion of lipids from algal cells, which may be

achieved by genetic engineering of algae or cyanobacteria. 56,119,120 This would also circumvent
the need for breaching sturdy cell walls to get the lipids out. 113 What liquid biofuel yields
might be achieved by using algae or cyanobacteria which excrete lipids is, however, again
highly uncertain as research into this option is at an early stage.

The many uncertainties about the future development of biofuel production on the basis of
autotrophic microalgal lipids do not allow for firm predictions about future EROIs. However,
it seems likely that an EROI of more than 5 will remain elusive unless major breakthroughs
emerge, which may, or may not, occur. This also seems to hold for alternative options to

generate liquid biofuels from autotrophic algae. 121,122

LCAs OF GREENHOUSE GAS EMISSIONS LINKED TO

AUTOTROPHIC MICROALGAL LIPID-BASED BIOFUELS

Nonenergetic environmental aspects have been addressed in LCAs in a relatively limited way.
Quite a number of LCAs have however addressed life cycle greenhouse gas emissions of
lipids or methanol from autotrophic microalgae (e.g., Refs 19, 27, 38, 68, 86, 87, 90, 91, 93,
94, 100, 101, 121, and 122). Several types of emissions can be expected to contribute
27
significantly to such emissions. These are the CO emissions linked to fossil fuel inputs, life
2
cycle N O emissions largely linked to nutrient N inputs, and CO emissions or sequestration
2
2
27
linked to changes in ecosystem carbon stocks. CO emissions linked to fossil fuel inputs have
2
been addressed in LCAs, but the questions previously raised regarding the system boundaries
chosen, such as the exclusion of waste water treatment, also apply to estimates of these