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Manure to Energy - The Utah project

by 5m Editor
16 August 2003, at 12:00am

By Theo van Kempen, Swine Nutrition Specialist, North Carolina State University - At the 2003 Banff Pork Seminar, 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.


Dr Theo van Kempen
Swine Nutrition Specialist

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 --> COOH-(CH2) n-CH3 --> 4CH4 + 2CO2
Manure + water --> mixture of volatile fatty acids --> methane + carbon dioxide

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 --> 3H2 + CO or CH4 + 2H2O --> 4H2 + CO2

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 <-- --> CO + H2O

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 --> CH3OH or CO2 + 3H2 --> CH3OH + H2O

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: www.bestbiofuels.com/index.html.

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.

Reproduced Courtesy

Source: North Carolina State University Swine Extension - July 2003