NCSU Extension Swine Husbandry
A more printable version of Swine News in Adobe Acrobat.
August, 2003 . Volume 26, Number 07
THE UTAH MANURE-TO-ENERGY PROJECT
This February, Smithfield Foods announced it would invest $20 million in a new environmental initiative at its Utah swine farms. The objective of the project is to recover energy in animal manure in a usable form: bio-diesel. Although this project has received considerable notice on television and in newspapers, most of the coverage did not provide substantial details on the actual processes involved. This article will explain what is going to happen in Utah. It is based on a paper being prepared by Prince Dugba, the Smithfield Foods engineer responsible for this project.
Animal manure holds a tremendous energy potential. Within U.S. animal agriculture, approximately 250 million tons of dry fecal material are produced yearly, with an energy value comparable to wood (on a dry-matter basis). If this energy could be harvested in gasoline, it would be equivalent to 21 billion gallons. In animal manure treatment systems using lagoons, this energy is not recovered. Instead, bacteria break down most of the organics contained in the manure, producing gases such as methane and carbon dioxide, the former of which may have a negative impact on the environment, as it is a potent greenhouse gas.
In many parts of the world, energy contained in animal manure is already harvested. For example, feces are used for cooking in Africa and Asia. In the U.S., though, both farmers and researchers have searched for many years for processes to harvest this energy. Given that in the U.S., swine manure is typically harvested in a very diluted form, this recovery has been more difficult. One technology that is effective with wet manure is to use a digester (or bio-gasification). This technology actually acts very much like an improved lagoon. Bacteria break down the organics in the manure to produce methane and carbon dioxide. Instead of letting these gases escape to the atmosphere, though, they are captured and used as an energy source.
A challenge with methane is that it is a gas that is difficult to store. In the past, it was typically used as an energy source by immediately combusting it in a generator for the production of electricity. Although a valuable energy source, difficulties marketing electricity have resulted in many of these projects being economically unfeasible. Thus, alternative uses of this methane-energy were needed.
The Utah project is a combination of technologies that recover the energy contained in swine manure and turn it into a usable and marketable fuel. This project will tie into Smithfield Foods' Skyline complex and include 23 farms with a total of 257,000 finisher pigs, all within a 5-mile radius. A diagram of the process is shown on page 1.
Three steps are involved in this project: Step 1: Manure (approximately 40,000 tons of dry matter per year) is collected from the swine houses by conventional technology. Thus, it is flushed from the buildings, the flush-water being recycled water. Rather than ending up in a lagoon for treatment and storage, the manure goes to a storage basin where it is held for one to two days. From this basin, the manure, with a total solids content of 1.5 to 2 percent, is pumped to a central location and into one of four gravity thickeners. These thickeners use gravity to produce a more concentrated solids fraction (70 to 80 percent of the total solids are concentrated in a volume of 30 percent of the original, resulting in a solution with roughly 4.5 percent total solids), and a liquid fraction. This liquid fraction is returned to the farms, treated in a lagoon, and used for flushing.
The thickened fraction is pumped into one of
four digesters. These digesters are in-ground, heated
(mesophilic) digesters kept at 95ºF. Within these
digesters, bacteria break down about two thirds of the solids that
were in the thickened manure. These solids include
proteins, carbohydrates, and fats; all these are broken down
into volatile fatty acids by bacteria known as acidifiers.
Subsequently, methanogens break down the volatile fatty acids
to produce methane and carbon dioxide.
C6H13O5 + xH2O
Methane is not soluble in water and will bubble to
the surface of the digester. Carbon dioxide, although
soluble, also will escape at the surface, and both gases are
captured by means of an impermeable membrane covering
these digesters. The digester gas produced contains
approximately two thirds methane, one third carbon dioxide, and
1 percent trace gases. This biogas, produced at a rate
of approximately 1.2 million cubic feet per day, is pumped to
a processing plant (see step 2).
The digested manure now is low in organics, as
these have been converted to biogas. As a result, it is
nearly odorless. In the Utah project, this fraction will be sent
back to the lagoons at the individual farms and used as
flush water.
The reason for using a mesophilic or heated digester
is that bacteria work much more expediently at higher
temperatures. Thus, the digestion occurs at a much faster rate
than when the digester was left at ambient temperature.
This means that a smaller but heated digester can handle
the same flow of manure as a larger but unheated digester.
This also is the reason the manure is pre-concentrated;
smaller digesters are needed, thus improving efficiency.
Step 2: The biogas that is recovered first will be
cleaned up, as it contains some impurities that can affect
further processing. Of highest concern is hydrogen sulfide, as
it can damage processing equipment. Removal is achieved
by washing the biogas with a sodium-hydroxide
solution because hydrogen sulfide is captured in this.
The cleaned-up biogas then is converted to
methanol. This is a two_stage process. In the first stage, the biogas
is heated to approximately 700ºC together with
super-heated steam, causing the methane and water to react with
each other. Products formed are predominantly hydrogen
gas, carbon monoxide, and carbon dioxide.
CH4 + H2O
This process is called steam reformation: steam is
split into hydrogen gas and oxygen that typically ends up
in carbon monoxide or carbon dioxide after reacting
with carbon (in, for example, methane). Another reaction that
can occur under these conditions is the water-gas shift
reaction, in which carbon dioxide reacts with hydrogen, resulting
in the production of carbon monoxide and water. This
reaction is fully reversible, mainly dependent on the reaction
temperature.
CO2 + H2
Using this reversible reaction, the balance
between carbon dioxide and hydrogen can be controlled.
Technically, the ideal ratio for methanol
(CH3OH) production is 2 H2 and
1 CO, or 3 H2 and 1 CO2. Practically, this depends upon
the catalyst and the reaction conditions used for
methanol production (not disclosed). In contrast to traditional
steam reforming of natural gas for the production of
methanol, biogas already contains substantial portions of
carbon dioxide, alleviating the need to source this material,
for example, from burning methane.
In the second stage, the hydrogen and carbon
monoxide/carbon dioxide are catalytically recombined in a
Fischer-Tropsch reactor to form methanol. This process uses a
metal catalyst based on nickel, typically in a slurry-based
reactor. This reaction occurs at much lower temperatures,
typically around 200ºC, and at elevated pressures. In essence,
the product gas from the first stage is simply pumped into
the reactor under the desired pressure and temperature,
and methanol is formed. This reaction is also exothermic; thus,
it produces heat that needs to be removed.
CO + 2H2
In the Utah project, the steam reformation of biogas
and the Fischer-Tropsch production of methanol will
occur adjacent to the digesters, and is expected to yield
7,000 gallons of methanol per day. These processes should
be considered industrial-scale processes, thus requiring
a highly trained staff and high-tech equipment. By
contrast, in-ground digesters are relatively simple technology.
Step 3: The resulting methanol is subsequently taken
off-site to a bio-diesel plant, the location of which has not
been decided at this point. Bio-diesel production is a
well-established and relatively simple process. Fats and
methanol are the two key components in the reaction. The
fats, consisting of fatty acids and glycerol, are combined
with methanol (in a 6:1 ratio). Under moderate heating
(around 200ºF) and using a catalyst such as sodium hydroxide,
the fatty acids dissociate from the glycerol backbone, and
they react with methanol to form a methanol ester of the fatty
acid and free glycerol. These methanol esters are what
constitute bio-diesel.
After separating the bio-diesel from the glycerol
and removing other impurities (through distillation), the
product is ready for marketing. The Utah project is expected to
yield 40,000 gallons of bio-diesel and 9,000 gallons of glycerin
per day.
Virtually any type of fat can be used for
bio-diesel production. These include animal fats from
rendering, vegetable oils such as soybean or corn oil, or
restaurant grease. The choice of fat has not been made for the
Utah project, and likely will depend on market prices and
the availability of the different sources.
Bio-diesel is a clean-burning alternative to
fossil-fuel-derived diesel. Diesel engines were actually invented to
run on bio-diesel, and they run cleanest on such fuel as
bio-diesel, is essentially free of sulfur and aromatics
(organic compounds with a ring structure). Bio-diesel can be used
as a fuel by itself, in which case minor engine
modifications may be required. Alternatively, bio-diesel can be
blended with fossil-fuel-based diesel in a 20:80 ratio to yield
B20, which can be used in conventional diesel engines. For
the Utah project, the objective is to produce B20. For
more information on biodiesel, go to: www.biodiesel.org.
Smithfield Foods has formed a new subsidiary to run
this project, named Best Biofuel, LLC. This company works
with several partners to realize the Utah project. More
information about this company can be found at:
The main benefit of the Utah project is energy
recovery, odor abatement, and reduction of the biological and
chemical oxygen demand of the manure. The project in its current
form does not address minerals such as nitrogen and
phosphorus, which are the minerals based on which
land-application of manure is or will be regulated. The potential to
harvest energy from a waste stream, though, is a tremendous
step toward not only more sustainable animal production but
also a more sustainable U.S. economy. It reduces dependence
on foreign energy sources while reducing emissions that
may harm the environment.
—Theo van Kempen
—Frank Hollowell and David Lee
Manure + water
Breeder
Address
Breeds
Bob Ivey*
314 N.C. 111 S, Goldsboro 27530
L, D, H, Y, CW, X
Wesley Looper*
4695 Petra Mill Rd., Granite Falls 28630
L, D, H, Y, X
Thad Sharp, Jr., & Sons
5171 N.C. 581 Hwy., Sims 2788030
D, Y, X
Tommy Spruill
Rt. 1, Box 149, Columbia 27925
L, X
Thomas Farms
8251 Oxford Rd., Timberlake 27583
X
*Real-Time Ultrasound
Last modified July 28, 2003.