swine oil
Industrial-scale farming has a serious waste problem, but new technology can convert it into oil.
By Yuanhui Zhang, Kim Ocfemia, and Malia Appleford
American
industrial agriculture is executed on an enormous scale. To look at just
one slice: More than 100 million head of hogs and pigs are slaughtered
in the United States every year. That's one hog for every household
in America.
But to say that American agriculture is breathtaking is to reveal
an unpleasant secret. As the pork industry provides food for the table,
pigs themselves produce a considerable amount of waste. The large-confinement
swine farms have become intensive point sources of pollution—not
just of wastewater and sludge, but of odor emission as well. The impact
of swine farming on the environment has raised concerns from government
agencies, the general public, and the pork industry itself. Swine manure,
once considered a valuable natural fertilizer, has now become an expensive
burden on the pork industry.
This doesn't have to be the case. Manure can be converted to energy
through biological and chemical processes. The tremendous amount of swine
manure produced each year can be an alternative, renewable energy source
that can supplement the ever-dwindling reserve of fossil fuels.
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One exciting new technology for turning waste to fuel is thermochemical
conversion. Thermochemical conversion, or TCC, is a chemical process that
reforms organic matter in a heated enclosure, usually in an environment
with little or no oxygen.
Thermochemical processes were studied using primarily coal, peat, and
wood sludge as feedstock during the 1970s. The technology was found to
be technically sound. Recently, a few pilot or pre-commercial TCC processes
have begun operation, but the technology has not been developed into commercial
processes for energy production, mainly because of its economic inefficiency.
Thermochemical conversion technology can be applied to the treatment of
swine manure, which at the moment is a burden for farmers, not a resource.
The treatment of swine manure through thermochemical processes can greatly
reduce wastewater intensity and odor emission. Meanwhile, the thermochemical
process produces heat, which can be used as energy for the process itself,
making the process potentially self-sustaining.
A Stinky
Mess
With nearly 60 million hogs in feedlots around the nation (Iowa, famously,
has five times as many hogs as citizens), the issue of just how to deal
with manure from these operations—as well as from chicken farms,
and dairy and cattle lots—is becoming too large to ignore. A 150-pound
hog produces more than 10 pounds of manure a day. Over the course of a
year, a single mature dairy cow can produce as much as 20 tons of wet
manure.
As one can well imagine, this stinks. Confining animals to small areas,
such as feed lots, ratchets up the level of odor pollution. Nuisance odor
from animal waste is largely due to the release of volatile organic compounds
from the fermentative degradation of fecal residues. That is, as bacteria
break down the organic material in the manure, they produce foul-smelling
chemicals.
Historically, manure had been spread onto cropland as a cheap and natural
fertilizer. At present, however, managing livestock manure is far more
involved than simply spreading it over a field, because of the density
of animals found in so-called intense confinement operations. The increasing
concerns over the pollution from swine farms have put the industry on
notice that farmers must be more circumspect in how they dispose of animal
waste.
Runoff from the operations is a source of high concentrations of bacteria,
suspended and dissolved solids, and chemical and biological depletion
of dissolved oxygen. High nitrogen concentrations leaking into ground
and surface waters contribute to the aging of streams, and nitrates in
drinking water can harm infants. Manure in water drives transmissions
of infectious disease organisms to people, livestock, and wildlife.
With more and more head of livestock being raised in industrial-style
confinement facilities, proper management of animal waste has become an
economic and environmental imperative.
Swine manure management includes the collection, transport, storage, handling,
treatment, disposal, and utilization. The most widely used methods of
swine manure treatment are biological, including aerated lagoons, anaerobic
digestion, and composting.
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| Farmers often collect livestock manure in
vast lagoons (top and above), relying on naturally occurring bacteria
to digest it. Mechanical aerators (below) may speed up the process
and reduce the smell, which can be overpowering. All photos courtesy of the U.S. Dept. of Agriculture |
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Most swine waste lagoons are aerobic—large pounds of waste exposed
to the air so that naturally occurring microbes digest the material. Naturally
aerated lagoons require extremely large surface areas to treat often highly
diluted liquid wastes. Due to problems with odor emanating from these
lagoons, many farmers have adopted mechanical aeration systems designed
to cut back on the smell.
Anaerobic digesters are also used in swine waste treatment. The process
is more expensive than open-air lagoons, but it has the advantage of enabling
the capture of methane.
Composting renders the organic waste biologically stable after a period
of time under high temperature with sufficient oxygen and the proper amount
of moisture. The compost product is suitable for land disposal. However,
it may still emit nuisance odor from its operation.
Chemical treatment methods include chlorination, flocculation, hydrolysis,
and pyrolysis. However, none of them has been put into commercial practice
so far, primarily for economic reasons.
Until recently, small-scale operations and relatively remote locations
have enabled manure management to escape public scrutiny. That's
beginning to change as suburban housing increasingly encroaches on farming
areas. And disasters, such as a breach in a waste lagoon retaining wall,
can send millions of gallons of partially digested manure coursing into
waterways.
Environmental concerns and public reactions over the intensive livestock
production facilities have led some state legislatures to pass new regulations
on manure management. As the regulations become more stringent, livestock
farming becomes more costly. Traditional treatment processes no longer
satisfy environmental concerns. New technically and environmentally sound
technologies are highly desirable, not only for the sustainability of
the livestock industry, but also for environmental protection.
Better still would be a means to convert the manure—which is now
a liability—to a product of some value. One obvious product that
can be made from agricultural waste is energy.
Tapping
Biomass
Organic material, or biomass, can be converted to various forms of energy
by many technical processes, depending upon the raw material and the type
of product desired. Biomass encompasses a wide variety of biological materials
with distinctive physical and chemical characteristics, such as woody
or ligno-cellulosic materials, various types of herbage, especially grasses
and legumes, and crop residues. As a result, a wide variety of conversion
schemes has been developed to take best advantage of the properties of
the biomass to be processed.
Historically, biomass has been burned. But that is a fairly inefficient
means of extracting the latent energy value locked in a mass of dung or
wood, and it doesn't lend itself to large-scale market distribution.
A more complex way of extracting energy from biomass is processing the
material to make a liquid or gas fuel. Ethanol made from corn is a common
example of biochemical conversion of biomass.
Perhaps a more promising means of tapping this resource is through thermochemical
conversion. This is the application of heat to break down the long chains
of polymers in the organic material into smaller molecules.
One method of doing this is pyrolysis, which is thermal decomposition
of organic matter in the absence of air or oxygen. Thermal decomposition
in an oxygen-deficient environment (that is, one in which less oxygen
is present than required for complete combustion) can also be considered
to be true pyrolysis as long as the primary products of the reaction are
solids or liquids.
Conventional pyrolysis is characterized by a slow feedstock heating rate
(less than 10°C/s), relative low temperatures (less than 500°C)
and long gas and solids residence times. The primary products are tar
and char, such as coke. A related method, flash pyrolysis, involves rapidly
heating dry matter to moderate temperatures (400°C to 600°C)
to produce liquids. The focus on creating liquid fuel means that less
char and gas—the traditional products of biomass conversion—are
generated.
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| Pigs generally spend less than a year on a farm (above) before going to market. Modern farming practices place large numbers of animals in tight confines (below and bottom). Although efficient, this type of operation concentrates waste. |
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In another variation, pyrolysis combined with combustion to provide heat
for the endothermic pyrolysis reactions in a process known as gasification.
The oxygen-deficient thermal decomposition of organic matter in gasification
yields non-condensable fuel or synthesis gases as the principal reaction
products.
Another thermochemical conversion method, liquefaction, was historically
linked to hydrogenation and other high-pressure decomposition processes
that employed reactive hydrogen or carbon monoxide carrier gases to produce
a liquid fuel from organic matter at moderate temperatures.
Liquefaction was initially developed for turning coal into liquid fuels,
but recently, the technique has been applied to a number of feedstocks.
In liquefaction, carbonaceous materials are converted to liquefied products
through a complex sequence of physical structure and chemical changes.
The main purpose of liquefaction is increasing the hydrogen-to-carbon
ratio—and decreasing the prevalence of oxygen—of the product
oil relative to that present in the feedstock.
Researchers have been investigating the thermochemical conversion of animal
waste for more than 30 years, but their conclusions have been mixed at
best. Perhaps the most celebrated project has been created by Changing
World Technologies of West Hempstead, N.Y. According to the company, any
type of organic waste—offal, tires, plastics, old computers, municipal
sewage sludge, paper-pulp effluent, or oil-refinery residues—can
be processed into oil, clean fuel-gas, and solid products.
CWT's method begins by pulping the feedstock into a slurry and
heating it under pressure to 200°C to 300°C. Then, the pressure
on the slurry is quickly reduced, and the oil created by this first stage
is separated from water. Finally, the oil is heated to a higher temperature
to crack it into light hydrocarbon and leave behind a solid product—a
mix of carbon and metals.
Little of the chemistry of the patented process has been released by CWT.
The company claims that fatty acids such as palmitic acid, which is common
in plant and animal tissue, is converted into a bio-derived fuel ester
through this process. The esterification is advantageous because ester
has a lower melting point than the fatty acid, making handling and storage
easier. The conversion, however, requires more heat for combustion because
the ester has a higher boiling point than palmitic acid.
After conducting tests in a small batch reactor, CWT built a pilot plant
at the Philadelphia Naval Yard. Recently, a full-scale commercial plant
was built in Carthage, Mo., in a joint venture with ConAgra Foods Inc.
to process 200 tons of turkey offal a day. The Carthage plant is slated
to produce some 500 barrels a day of fuel-grade oil, plus additional methane
and carbon solids. This plant, however, is still in the start-up phase
of operations.
Feed
Into Fuel
Our team at the University of Illinois has examined a similar system for
digesting hog manure. We fed small batches of a raw animal waste slurry
into an experimental liquefaction processor and subjected them to moderate
heat and high pressure for about 20 minutes. The conversion of swine manure
to oil, it turned out, was easier than converting other biomass to oil
because the manure contained less lignin, which is difficult to decompose.
In a sense, the hogs turn biomass—animal feed—into a form
that is more suitable for energy conversion.
Still, the manure had less energy content than other potential biomass
feedstocks, and it had less hydrogen, and more oxygen, per carbon atom
than was optimal. To run the liquefaction process, some sort of reductive
chemical reagent, such as hydrogen or CO, is needed to increase the oil
production rate.
The final oil yield for the conversion process ran as high as 63 percent
of the solids in the manure feedstock. And just how good was that oil?
Based on the chemistry principle that "like dissolves like,"
benzene solubility is one parameter to characterize oil quality: The more
the oil product dissolves in benzene, the more oil-like components it
contains, thus the better quality of the oil.
The oil samples produced at temperatures of 285°C to 305°C
had a benzene solubility of 82 to 90 percent. The average heating value
of the oil was estimated at 30,500 kJ per kilogram, about the same as
common furnace oil.
These results point to the potential for thermochemical conversion to
be an economically feasible alternative for handling hog manure. Not only
does conversion reduce the overall mass of the waste stream, but the fuel
produced can provide an additional revenue source for hog farmers. We've
estimated that the oil produced through this means could yield about half
a barrel per hog. If just half of the nation's hog producers adopted
this technology, it would reduce imports of foreign oil by some $1.5 billion
a year.
Granted, that's just a drop in the bucket—about one-tenth
of the oil imported from a major producer such as Mexico, Saudi Arabia,
or Venezuela. Even the conversion of manure from cattle and chicken operations
could not replace fossil fuel imports. But it could displace some of those
imports, and do it in a way that is more beneficial to the environment
than the current situation—both in terms of petroleum extraction
and animal waste disposal.
Yuanhui Zhang is a professor of agricultural and
biological engineering at the University of Illinois at Urbana-Champaign.
Kim Ocfemia and Malia Appleford are graduate students there.
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