Environmental Engineering Reference
In-Depth Information
biofuels are more complicated than for starch ethanol and biodiesel, and less is known about the
costs and environmental impacts of these biofuels. This is because very few demonstration- or
commercial-scale facilities exist for these biofuels at this time, yet this situation may soon change
because of recent industry and government initiatives in research and commercial development.
For example, as of 2010 there are 26 cellulosic ethanol facilities in various stages of construction in
the United States with capacities ranging from a low of 20,000 gallons/year to 100 million gallons/
year (RFA 2010). Additionally, in the United States, demonstration-scale facilities of similar scale
are in the planning and construction phase for production of some other types of advanced biofuels
(Figure 8.1).
The remaining sections of this chapter describe some of the main features of conventional
and advanced biofuel technologies. Technological barriers are discussed and recent approaches
to overcome them are presented. The chapter ends with a discussion of the relative advantages
and disadvantages of each conversion technology from cost and environmental impact
perspectives.
8.2
Plant oIls: conversIon to BIodIesel and hydrocarBons
8.2.1 i introduction to thE p rocESSing c oncEptS
This section focuses on the conversion of plant-derived oils to distillate transportation fuels.
Two distinctly different processing pathways are commercially available: (1) base-catalyzed
transesterification of triacylglycerol (TAG)-rich feedstocks with methanol to produce fatty acid
methyl esters (FAME), referred to as biodiesel; and (2) catalytic reaction of TAG and free fatty
acid (FFA)-rich feedstocks with hydrogen to produce hydrocarbon-based fuels such as hydrotreated
renewable diesel (HRD) and hydrotreated renewable jet (HRJ) fuel. Biodiesel can also be produced
from FFA-rich feedstocks using a more severe acid-catalyzed esterification process.
To date, most of the existing capacity for producing diesel fuel from plant oils has centered
on biodiesel. Although its use in blends with petroleum diesel is already widespread, there are
some limitations to biodiesel that may ultimately limit its future use. These limitations include
fuel storage stability, cold flow properties in winter climates, and high solvency that can lead to
engine compatibility issues. The implementation of an ASTM specification for biodiesel (ASTM
D6751-09) has helped to overcome issues related to fuel contamination with methanol, glycerol,
and partially converted glycerides and fatty acids. To minimize the chance of engine problems,
some car manufacturers have recommended capping the amount of biodiesel in petroleum blends at
approximately 5% (McCormick 2009).
In the hydroprocessing pathway to biofuels, plant oils can be processed in pure form or
co-processed with petroleum feedstock. HRD and HRJ are hydrocarbon liquids produced from
plant oil feedstock using advanced hydroprocessing technology. Their physical properties (e.g.,
viscosity, flash point, distillation, freeze point) are similar to petroleum-derived fuels and their
chemical composition is similar to paraffin-rich Fischer-Tropsch (FT)-derived fuels (Koers et al.
2009). Because these fuels are fully deoxygenated, they have excellent storage stability and can be
used in any proportion with existing petroleum fuels.
Although commercial production of hydrocarbon-based biofuels is not yet widespread, sufficient
production capacity has been brought on stream to demonstrate fuel quality and compatibility with
existing engine technology. A significant increase in the HRD and HRJ production rate is expected
over the next 3-5 years.
A more detailed discussion of the chemistry and processing steps for each processing
pathway is provided in the paragraphs that follow. Process yields and conversion efficiencies
are compared as well as fossil fuel and total energy requirements. A discussion of the current
state of commercialization is also provided, including a brief discussion of sustainable feedstock
selection.
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