Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 175

CHAPTER 11

Alternative fuels and green aviation

Emily Nelson

11.1 INTRODUCTION

For engineers and scientists who are new to the field of biofuel production for aviation, it can be
bewildering to come to termswith the conflicting conclusions drawn in the literature or evaluate the
claims found in a casual web search. It is a multidisciplinary field that spans chemical, petroleum,
environmental, mechanical, aerospace, materials and industrial engineering. The field is also
overlaid by a web of geographically varying public policies that profoundly impact economic
feasibility and the pace and directions in technology development. It is a field that has grown
rapidly in all of these areas, so that there is a vast, rich literature base to peruse. Finally, there are
passionate and articulate advocates for every conceivable side. This can make the leap across the
interdisciplinary aspects rather challenging. Some of the most embedded sources of confusion
arise from the following:

• Terminologymay be conflicting or otherwise difficult to understand. For example, the naming of
the same set of hydrocarbon compounds in petroleum engineering differs from the conventions
in organic chemistry.
• The standard units inwhich values are reported varywidely. Often this is because the convention
had already been established in various disciplines and geographical regions.
• There may be no clear standards or protocols for measurements, which leads to difficulty in
comparing values, e.g., the manner in which to quantify biomass yield from algae.
• Some studies do not clearly state their assumptions and identify their uncertainties, which are
critical to evaluating its conclusions.

•

In order to create assessments for global applications, the numbers used to describe the physics
may be scaled up from laboratory results which grossly magnify the uncertainties present in
the small-scale data (e.g., processing conditions to yield 500 milliliters (mL) of crude algal oil
over a few days in a laboratory) to huge numbers (e.g., annual production of millions of barrels
per hectare per year (MMbbl/ha/y) of biodiesel).

With regard to scaleup, a number of factors come into play: To be useful, the lab-scale experi-
ment must meticulously keep track of all nutrients and additives, gas exchange and energy input.
Sometimes the processes or equipment that are useful for lab experiments for mixing, separation
or combustion do not translate up to industrial scale facilities. For example, centrifuging to remove
excess water from algal broth is both simple and effective for small quantities of fuel, but it is too
expensive for commercial production. Finally, the lab has the luxury of a controlled environment
and may run its tests over only a few days, whereas commercial producers and refiners will be
subject to the vagaries of seasonal and diurnal temperature swings, cloudy days, too much or too
little rainfall, and the intermittent interest of predators or competitors.
The next generation of engineers, scientists and policymakers must find a way to bridge these
incompatibilities in order to provide the most useful, comprehensive, evidence-based data on
current technologies and new strategies to make those technologies commercially viable. The
purpose of this monograph is to present basic concepts in the key areas and provide a foundation
for critical thinking. Where available, uncertainties in the data are given. Such uncertainties

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