Page 99 - Advances in bioenergy (2016)

emissions. Fossil fuel inputs linked to building the infrastructure for growing algae, such as
ponds, are not always included (e.g., Ref 19). And, as emerged in the previous section, future
fossil fuel inputs in generating autotrophic microalgal biodiesel are highly uncertain. Life cycle
N O emissions have been neglected or estimated using assumptions, which were not based on
2
empirical data about the cultivation of autotrophic microalgae and the downstream fate of N-
compounds in waste water of such cultivation. 123 Reference land use, which allows for
estimates of changes in ecosystem carbon stocks, is excluded from currently available LCAs.
This exclusion may impact estimated greenhouse gas balances. This impact would be small in
settings that sequester little C, such as deserts, but may matter much in settings where forests
27
would be natural ecosystems. Indirect effects of land use change on ecosystem carbon stocks
are also not included in available LCAs.

As it stands, available LCAs of greenhouse gas emissions linked to autotrophic microalgal
biofuels do not seem to allow for firm estimates of future greenhouse gas emissions linked to
such biofuels.

LIFE CYCLE POLLUTION LINKED TO AUTOTROPHIC

MICROALGAL LIPID-BASED BIOFUELS

Life cycle pollution linked to autotrophic microalgal lipid-based biofuels has been addressed
−1
−1
by Hou et al., 100 assuming a yield of 109 mg dry weight biomass ha year and a lipid content
of 45%. When waste water treatment is not included, emissions of substances characterized by
human toxicity and ecotoxicity and of eutrophying substances were estimated to be larger for
90
autotrophic microalgal lipid-based biofuels than for fossil fuels. Lardon et al., who included
waste water treatment in their assessment, found microalgal biodiesel and fossil diesel
performing roughly similar as to eutrophication, but also found microalgal biodiesel to do
worse than fossil diesel regarding aspects such as acid deposition, human and marine toxicity,
68
and photochemical oxidation. Clarens et al., who also included waste water treatment, found
autotrophic microalgal biofuels in terms of pollution to be a larger environmental burden than
biofuels derived from switchgrass or canola. Thus, apart from greenhouse gases, life cycle
pollutant emissions of autotrophic microalgal biodiesel may well be more of an environmental
burden than the life cycle emissions linked to fossil diesel or biofuels based on canola or
switchgrass.

CONCLUSIONS

Near-term liquid autotrophic microalgal lipid-based biofuel production with an EROI of more
than 5 and costs which are not much higher than the prices for fossil fuels is highly unlikely.
Whether the prospects for microalgal biofuel production in the more distant future will much

improve is uncertain. As discussed above, in several cases, assumptions about the future
impact of proposals for improving energy return on investment seem overly optimistic. In other
cases, the impact of proposals for improvement of EROI is highly uncertain because research