Page 41 - Advances in bioenergy (2016)

coming from Europe. 8

However, production of these biofuels from plants like corn or rapeseed also competes for
arable cropland needed for food. This adds undesirable price sensitivities between biofuels
and food and has already shown adverse effects on food prices. Transforming forests or
existing cropland can also sometimes have the effect of increasing greenhouse gas emissions,
counteracting the carbon emissions benefit of biofuels. 9

The primary cost for producing biofuels is the cost of the feedstock: 60% in the case of corn
ethanol and 80% for soybean biodiesel. 10,11 Even with gains in process yield, current crop-
based feedstocks will still limit the overall profitability of biofuels. Currently, upward of half
of the production cost of these biofuels needs to be supported by government subsidies. 6

The next generation of feedstocks will need to have lower land requirements and lower
production cost, yet maintain high production capacity to bring biofuels closer to economic
viability. Metabolic engineering allows us to bridge the feedstock gap by enabling the
utilization of cheaper and more sustainable substrates by introducing catabolic pathways and
optimizing metabolic networks for the conversion of feedstock to fuel. Indeed, yield
optimization has been a critical aspect of virtually all biochemical engineering processes in
recent history. Metabolic engineering of organisms toward this end only serves to continue this

tradition, pushing yields beyond what is naturally observed. Furthermore, microbe-based
biofuel production also reduces the cropland requirements compared to crop-based methods,
decreasing competition with food production.

Engineering for Improved Fuels

Although new feedstocks are explored, a simultaneous search continues for the next generation
of fuel types. Current biofuels have some persistent disadvantages that limit their incorporation
into existing infrastructure.

Ethanol, although widely produced, has relatively poor fuel characteristics. Ethanol is
hygroscopic, capable of absorbing water, which can lead to corrosion. The energy content is
also low, containing only 70% of the energy per volume of gasoline. Also, as ethanol is
produced by fermentation, the resulting beer is dilute, containing roughly 10% ethanol.
Subsequent distillation to separate the ethanol is very energy intensive. 10

Biodiesel is a better fuel, but also has some disadvantages. It is not well suited for use at low
temperatures because of a high cloud point, and still often requires large quantities of

petroleum-derived methanol as part of its production. It also has only 89% of the energy
content of its analog, petrodiesel. 11

Current biofuel characteristics limit their integration into existing infrastructure. Because of
this, there is a high transition barrier to adoption of biofuels, and both ethanol and biodiesel
are often blended only at low concentrations into conventional fuels.

Development of better fuels that have high energy density and can be integrated into existing
pipelines and engines will be needed if biofuels are to be more widely adopted and have a