Environmental Engineering Reference
In-Depth Information
other production costs, increased competition from unconventional liquid fossil fuels
(e.g., oil sands, coal, heavy oil, shale), the emergence of cellulosic ethanol as a low-
cost competitor, and new policies to reduce greenhouse gas (GHG) emissions that
could favor advanced biofuels over corn ethanol.
Biodiesel
is another type of biofuel experiencing expansion. Although its production
costs are higher than ethanol, biodiesel has some environmental advantages, includ-
ing biodegradability and lower sulfur and carbon dioxide emissions when burned.
Biodiesel production in the United States increased rapidly from less than 2 million
gallons in 2000 to about 500 million gallons in 2007. Policy incentives in the Energy
Independence and Security Act of 2007 are expected to sustain demand for 1 billion
gallons per year of this fuel after 2011. A variety of oil-based feedstocks are con-
verted to biodiesel using a process known as
transesterification.
. This is the process of
exchanging the organic group R′′ of an ester with the organic group R′ of an alcohol.
These reactions are often catalyzed by the addition of an acid or base as shown below:
Transesterification: Alcohol + Ester → Different alcohol + Different ester
The oil-based feedstocks include vegetable oils (mostly soy oil), recycled oils and
yellow grease, and animal fats such as beef tallow. It takes 3.4 kg of oil/fat to produce
1 gallon of biodiesel (Baize, 2006). Biodiesel production costs are high compared to
ethanol, with feedstocks accounting for 80% or more of total costs.
The biodiesel industry consists of many small plants that are highly dispersed
geographically. Decisions about plant location are primarily determined by local
availability and access to the feedstock. Recent expansion in biodiesel production is
affecting the soybean market. Achieving a nationwide target of 2% biodiesel blend in
diesel transportation fuel, for example, would require 2.8 million metric tons (MT)
of vegetable oil, or about 30% of current U.S. soybean oil production (USDA, 2012).
One of the key biodiesel byproducts is glycerin. At the present time there is con-
cern about producing glycerin mainly because there are no existing markets for the
product; however, recent technological developments include an alternative chemical
process that would produce biodiesel without glycerin. In addition, new processes
are being tested that further transform glycerin into propylene glycol, which is used
in the manufacture of antifreeze.
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eedstocks
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Agricultural residues
, a second-generation biomass feedstock, offer a potentially
large and readily available biomass resource, but sustainability and conservation
constraints could place much of it out of reach. Given current U.S. cropland use, corn
and wheat offer the most potentially recoverable residues; however, these residues
play an important role in recycling nutrients in the soil and maintaining long-term
fertility and land productivity. Removing too much residue could aggravate soil ero-
sion and deplete the soil of essential nutrients and organic matter.
Safe removal rate methodologies, based on soil erosion, have been developed.
Methodologies to determine removal rates while safeguarding soil fertility and
meeting conservation objectives still need to be developed. Studies have shown that
