BELT MANURE REMOVAL AND GASIFICATION SYSTEM
TO CONVERT MANURE TO FUEL: THE “RE-CYCLE” SYSTEM
J.B. Koger1, G.A. Wossink2, B.A. Kaspers1, and T.A. van Kempen1
1Department of Animal Science, NCSU, Raleigh, NC
2Department of Agricultural and Resource Economics, NCSU, Raleigh, NC
Introduction
Animal production provides significant agricultural receipts for many state economies, but the environmental impact of animal industries is coming under increasing scrutiny. In North Carolina, hog production accounted for $1.6 billion, or 22.2%, of the 2000 agricultural receipts (North Carolina Agricultural Statistics, 2001). However, the nearly 10 million hogs in this state annually produce 70,000 tons of nitrogen, 22,000 of phosphorus, and 29,000 of potassium. The current waste management strategy of flushing waste from houses, storing it in lagoons, and ultimately applying it to dedicated spray fields has led to public outcry. In response, the “Smithfield Agreement” (2000) was devised to support research into alternative methods of waste management that could address the public’s concerns. The Agreement identified five objectives that must be met by new systems: 1) eliminate discharge of wastes to surface and ground waters through discharge, seepage, or runoff, 2) substantially eliminate emissions of ammonia, 3) substantially eliminate off-farm odor emissions, 4) substantially eliminate release of disease vectors and airborne pathogens, and 5) substantially eliminate nutrient and heavy metal contamination of soil and ground water.
The “RE-Cycle” system was developed in response to these criteria and the system effectively addresses all the identified concerns. The first component of this system is a belt-based waste harvesting system that separates the liquid from the solid waste and partially dries the fecal portion with normal ventilation air. The liquid waste is then directed to an enclosed vessel for treatment in the Ammonia Recovery Process. This process captures the valuable nitrogen from urine in the form of an ammonium sulfate fertilizer. Feces are trucked to a centralized steam reforming gasifier facility for ultra-high temperature processing. The only two products of this process are sterile gases and ash. The product gas mixture, or “syngas” as it is called, has many potential end-uses giving the entire system additional flexibility for adjusting to changing power markets without having to revamp the entire hog production system. Syngas can be converted to liquid fuels, such as ethanol or diesel fuel, by a process known as catalytic liquefaction, or it can be used to generate electricity, produce steam, or synthesize chemicals and plastics. The ash, a sterile by-product, may be used to produce fertilizer pellets or processed directly into animal feed. Either way, it reenters the production cycle without open storage of waste or its application to land. It has been so reduced in mass that transportation to areas of need is no longer prohibitive. Thus, the “RE-Cycle” system eliminates lagoons, provides “green” or renewable energy, recycles mineral nutrients, avoids environmental eutrophication, and reduces odor and ammonia emissions from swine operations. In short, it achieves all the objectives identified by the Smithfield Agreement.
Belt-based Manure Management
The belt-based manure separation system is an important first step for the “RE-Cycle” program since the downstream gasification process requires a feedstock of 60-80% dry matter (DM). Shown schematically in Fig. 1, the belt system has been designed for retrofitting into the current flush system of hog housing. The belt runs beneath a slatted portion of the pen, the usual flush area, and takes advantage of the fact that the animals naturally dung away from the feeding and sleeping quarters in the solid-floored portion of the pen. The slanted belt allows urine to drain into a long, sloped gutter that empties directly into a covered storage container. Feces are dried to approximately 50% DM during their residence time on the belt. A demonstration, belt-based housing unit for 100 animals has been installed at NCSU and its performance has been evaluated.
The belt-based collection system addresses the Smithfield Agreement objectives and offers many advantages over the flush method. There is no potential for waste discharge to surface and ground waters, or for environmental eutrophication, since there is no in-ground storage or land application of raw waste material. Ammonia emissions are reduced since the separation of urine from feces, and fecal microbes, reduces the conversion of urea to ammonia and carbon dioxide. Ammonia emissions are further reduced by the fact that the urine is immediately directed to a closed container thereby restricting its atmospheric contact. Roughly 60% of excreted nitrogen is in the urine, so curtailing ammonia emissions from the liquid waste stream has a dramatic effect. Furthermore, fecal nitrogen compounds are slow to release ammonia; restricting urinary ammonia emissions addresses the principal source of the problem. Studies with the demonstration-housing unit have shown a 60% reduction, relative to literature values, for ammonia emissions from housing. Given the absence of lagoons and spray fields in the “RE-Cycle” system, the belt method of waste harvesting is expected to reduce overall ammonia emissions by at least 80%. Odor emissions are also reduced since microbial metabolism, the source of much of the odor, is inhibited by the drier condition of the waste. Consequently, an 80% reduction in odor emissions is also expected (Aarnink, 2000).
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Figure 1. Schematic of the belt-based hog pen design
Ammonia Recovery Process
An ammonia recovery process has been developed by ThermoEnergy to capture nitrogen from human waste. The same process will be applied to the liquid hog waste so that the nitrogen can be captured as ammonium sulfate for resale as a fertilizer. Potential eutrophication by excess nitrogen is avoided since the fertilizer can be shipped to nitrogen-deficient areas. Studies are currently underway with this system.
Steam Reforming Gasification
Steam reforming gasificationis the technology of choice for recovering the energy and mineral nutrients of fecal waste. Gasification is a clean technology yielding only energy and mineral ash while completely disposing of the waste. The process temperature is 1470°F (800°C), a temperature too low for dioxin formation but far higher than required for destroying pathogens. In addition, there is little or no odor produced. A gasifier schematic, Figure 2, helps to explain the process. The feedstock is metered into the heated gasifier chamber, and, in the case of a fluidized bed gasifier, is mixed with a sand-like bed material that assists in heat
 |
Figure 2. Schematic of a steam reforming gasifier (Reed and Gaur, 1999)
transfer. Super-heated steam helps to keep the bed in motion, or fluidized, and to thermally “crack”, or decompose, the organic compounds into low molecular weight gases as shown in equation 1 (Reed and Gaur, 1999):
Eqn. 1: CH1.4O0.6 + 0.35 O2 à 0.4 CO + 0.6 H2 + 0.4 CO2 + 0.1 H2O + 0.2 C
Fluidization with steam, as opposed to air, yields a product gas of high hydrogen content undiluted by nitrogen. Thus, the gas has an energy value of 350-500 Btu per standard cubic foot (Btu/scf) and is considered a medium energy gas. Gases and fine particulates percolate through the fluidized bed and are partially separated from one another in the disengagement zone and the subsequent cyclone (not shown). Particulates may be returned to the gasifier for further cracking. The gases are collected, cooled, and scrubbed of any entrained particulates, hydrogen sulfide, and ammonia.
Hog waste offers many advantages as a gasifier feedstock and a source of renewable energy. In the first place, it is continuously available rather than being seasonally limited, as is the case with crop sources. Being a waste material, it is a low to zero cost feedstock. The proximate and ultimate analyses of the fecal waste are shown in Table 1. The energy content of 14.3 Btu/g (6,500 Btu/lb) and the 45% carbon content suggest an excellent feedstock for thermal cracking processes and liquid fuel synthesis. Moreover, hog fecal waste has a low amount of nitrogen and sulfur; consequently, concerns about NOx and SOx emissions are minimal. The low chlorine value is also important for avoiding agglomeration within the gasifier. Indeed, analysis of the bed material at the end of each of four gasification trials showed no formation of aggregates that could foul the gasifier and prevent its optimal performance.
Table 1. Proximate and ultimate analyses of hog feces
|
|
Weight Per Cent |
|
As Received |
Dry Basis |
|
Proximate Analysis |
||
|
Moisture |
23.2 |
------- |
|
Ash |
9.4 |
12.2 |
|
Volatile Matter |
57.4 |
74.8 |
|
Fixed Carbon |
10.0 |
13.0 |
|
HHV (Btu/g) |
14.3 |
18.6 |
Ultimate Analysis |
||
|
Moisture |
23.2 |
------- |
|
Carbon |
34.6 |
45.0 |
|
Hydrogen |
5.3 |
6.9 |
|
Nitrogen |
3.1 |
4.0 |
|
Sulfur |
0.3 |
0.4 |
|
Ash |
9.4 |
12.2 |
|
Chlorine |
0.2 |
0.3 |
|
Oxygen (by difference) |
24.0 |
31.2 |
The steam reformer used in these studies was a 0.5 lb/h unit owned and operated by ThermoChem (Manufacturing and Technology Conversion International, Inc., Baltimore, MD). Four trials were conducted to determine the fluidization medium and velocity best suited for conversion of swine waste to syngas with a 1.05:1 ratio of H2:CO since this ratio is optimal for synthesis of ethanol, the chosen end-product. Bed temperature was maintained at 800oC +/- 15o based on earlier work with poultry litter (Manufacturing and Technology Conversion International, Inc., 2000). Fluidization with a mixture of steam and carbon dioxide gave the desired gas composition, but the 40% decrease in the energy content of the gas was unacceptable. When steam was used as the sole fluidization medium and oxygen source, a H2:CO ratio of 2.1:1 was obtained and the energy value of the product gas was 382 Btu/scf. Composition of the resulting syngas, shown in Figure 3, showed that hydrogen constituted over 40% of the product with carbon monoxide and carbon dioxide each constituting approximately 20%. Ethylene and methane are the only other hydrocarbons present at greater than 1% of the total. Nitrogen from the feedstock is emitted as ammonia and is removed in the gas clean-up train.
Figure 3. Syngas composition from hog waste; stem fluidized bed
Mineral analysis of swine feces by inductively coupled plasma spectroscopy suggests that the ash by-product of gasification could be useful as a feed additive. Results of the analysis are shown in Table 2. The calcium to phosphorus ratio of 1.5 is adequate when the phosphorus is highly bio-available, as is expected to be the case in an ash material. This ratio promotes more efficient utilization of phosphorus (National Research Council, 1998). Ash from the trials conducted to date fails to demonstrate such a favorable composition. Contamination of the ash, presumably by the magnesium oxide bed material and the non-refractory lined test unit, results in an ash containing more magnesium than was supplied in the feedstock. The trials were run, however, in a very small test unit (0.5 lb/h) with no cyclone to remove particulates from the ash. It is expected that full-scale gasification will yield an ash product that more closely reflects the feedstock composition.
Table 2. Mineral Analysis of Hog Fecal Ash
|
|
ppm |
|
|
Elementa |
|
|
|
Calcium |
16,880 |
21,968 |
|
Copper |
172 |
224 |
|
Iron |
1,418 |
1,845 |
|
Phosphorus |
12,820 |
16,684 |
|
Potassium |
15,868 |
20,651 |
|
Sodium |
2,901 |
3,776 |
|
Zinc |
653 |
850 |
|
a Magnesium values are not reported as they far exceeded the feedstock input value. This is thought to have resulted from contamination of the ash by the magnesium oxide bed material. |
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Steam reforming gasification offers many benefits as a waste disposal method. First, the only products are energy and a sterile mineral ash. Both of these are value-added products that generate additional jobs and revenues. Second, the process completely eliminates the manure with little or no odor or noxious emissions and no residual sludge or waste stream. Third, pathogens and bioactive compounds, such as antibiotics, are destroyed. Finally, the technology is not limited by season or process retention time, since materials are thermally decomposed in a matter of minutes, not days or weeks.
Catalytic Liquefaction
Catalytic liquefaction, or the synthesis of liquid compounds such as ethanol or diesel fuel from gaseous starting materials, can generate valuable end products from the synthesis gas. For ethanol synthesis, inorganic catalysts are preferred to the biological, fermentative route for product-purification ease. While the catalytic method suffers from some lack of specificity, the conversion efficiency is excellent, mass transfer problems are greatly reduced, and there is no residual waste requiring disposal. Metal complexes, such as molybdenum sulfide, cause the carbon monoxide and hydrogen in syngas to form a mixture of alcohols. “Ecalene”ä produced in this way by Power Energy Fuels (Laramie, WY) is 75% ethanol and 25% higher alcohols (propanol to hexanol). Various catalysts are available and estimates of ethanol yield from one ton of hog waste range from 95 to 190 gallons.
System Implementation
Implementation of the RE-Cycle system involves on-farm and off-farm components. The general layout of components is shown in Figure 4. The solid waste collected in the barns (Panel A) is deposited on a transverse belt that carries it to another belt for loading into in a truck bed. The material is then trucked to a centralized, off-farm gasification facility where the waste is gasified and the syngas is converted to a liquid fuel or electric power. The liquid waste (Panel B) is collected in a treatment vessel that captures the ammonia on a resin. The resin is also delivered to the centralized facility where it undergoes further processing to recover ammonium sulfate and to regenerate the resin for return to the farm. In Duplin county, for example, there are roughly two million hogs in a 900 square mile area. Locating four gasifier installations in
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Figure 4. Layout of the “RE-Cycle” system. Panel A: collection of the fecal waste and its delivery to the off-site gasification facility. Panel B: management of the liquid waste
that county would provide each with 250 tons of fecal dry matter per day. Each gasifier would be within 10 miles of the farms it served, hence, within a reasonable transportation distance.
Economic Analysis
Economic Analysisof the “RE-Cycle” system has been undertaken using fuel ethanol as the model end-product of the liquefaction process. The revenues, costs, and potential profit for the “RE-Cycle” system are summarized in Table 3 and result in a net profit of $4.63 per pig place. (The data do not include any costs or profits from the ammonia recovery process.) The potential ethanol yields and system profit, shown in Table 4, can be up to $37 million on the state level. National figures are roughly 6-fold greater as can be seen from the animal numbers. Optimizing the gasification process to yield more CO, at the expense of CO2, would further improve the profit potential.
Table 3. Annual profit potential per pig place
Revenues |
|
|
Ethanol and Ash |
$24.82 |
|
|
|
|
Costs |
|
|
Housing |
$ 7.35 |
|
Technologies |
$11.27 |
|
Transportation |
$ 1.47 |
|
|
|
Profit |
$ 4.63 |
Table 4. Annual state and national ethanol production and profit potential
|
|
North Carolina (millions) |
USA (millions) |
|
Number of pig places |
8 |
50 |
|
Fecal Dry Matter (tons) |
1.5 |
9.2 |
|
Potential ethanol yield (gallons) |
140 |
870 |
|
Potential profit (US Dollars) |
$37 |
$222 |
Summary
The “RE-Cycle” system promises to reduce the threat of agricultural pollution while supplying a clean source of renewable energy and returning minerals to the production cycle. It provides a mechanism whereby all the Smithfield Agreement objectives can be achieved with a system that does not require rebuilding hog facilities or managing a secondary waste stream created by the treatment process. The system’s ability to direct syngas to a variety of value-added fuels and end-products gives it the flexibility to adjust to changing energy markets without changing the entire waste collection system. “RE-Cycle” can generate additional jobs and revenues in a North Carolina region that needs such benefits. As a source of renewable energy, hog waste provides a constantly replenishing feedstock, so no off-season storage is required. The treatment retention time is negligible, so waste processing can keep pace with waste production. Using waste to provide renewable energy addresses two problems at once: environmentally sound disposal of a waste material, and a sustainable, renewable domestic energy supply.
References
Aarnink, A. J. A., 2000. Institute for Environmental and Agritechnology, Wageningen, The
Netherlands, personal communication.
Manufacturing and Technology Conversion International, Inc., 2000. Cost-Effective, Clean and
Modular Biomass Power System for the Utilization of Farm Animal Waste. Phase I Final Report. Prepared for the US Department of Energy, Award #DE-FG02-99-ER82822, Small Business Innovation Research Program, pp 1-1 to 2-43.
National Research Council. 1998. Nutrient Requirements of Swine, 10th Revised Edition; p. 51.
National Academy Press, Washington, DC.
North Carolina Agricultural Statistics, 2001. B.C. Meadows, ed., published by North Carolina
Agricultural Statistics.
Reed, T. B., and S. Gaur, 1999. “Current Status of Biomass Gasification” A Survey of Biomass
Gasification 2000. The National Renewable Energy Laboratory and The Biomass Energy Foundation (Golden, CO), pp 1-1 to 1-32.
Smithfield Agreement, 2000, by and between Attorney General of North Carolina; Smithfield
Foods, Inc.; Brown’s of Carolina, Inc.; Carroll’s Foods, Inc.; Murphy Farms, Inc.;
Carroll’s Foods of Virginia, Inc.; and Quarter M Farms, Inc.
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