Biogas1

Introduction

Biogas is solar energy concentrated and processed by plants and animals.

Whatcom County is Washington State's largest producer of dairy products west of the Cascades. There are some 60,000 dairy cows on 200 dairy farms in Whatcom County. At a human equivalent of 2.5 people per cow, this equates to an un-sewered population of 125,000 human equivalents. Untreated dairy waste has a significant impact on water quality and constitutes a significant source of biomass for methane production.

Growth and concentration of the livestock industry creates opportunity for the proper disposal of large quantities of manure generated at dairy farms. The potential pollutants from decomposing livestock manure include biochemical oxygen demand (BOD), pathogens, nutrients, methane, and ammonia emissions. The major pollution problems associated with these wastes are surface and groundwater contamination and surface air pollution caused by odors, dust, and ammonia. There is also concern about the contribution of methane emissions to global climate change. Consequently, manure management systems that enable pollution prevention and produce energy are becoming increasingly attractive.

Anaerobic digestion of manure not only provides pollution prevention but also can convert a manure problem into a new profit center. Economic evaluations and case studies of operating systems indicate that the anaerobic digestion (AD) of livestock manures is a commercially available bioconversion technology with considerable potential for providing profitable coproducts, including a cost-effective renewable fuel for livestock production operations.

This biogas section examines some of the current opportunities for the recovery of methane from AD animal manures. U.S. livestock operations currently employ four types of anaerobic digester technology: slurry, plug-flow, complete-mix, and covered lagoon. An introduction to the engineering economies of these technologies is provided, and possible end-use applications for the methane gas generated by the digestion process is discussed. The economic evaluations are based on engineering studies of digesters that generate electricity from the recovered methane.

Case studies of operating digesters, with project and maintenance histories and the operators' "lessons learned" are contained in the National Renewable Energy lab publication, Methane Recovery from Animal Manures. The Current Opportunities Casebook.2 Factors necessary for successful projects, as well as a list of reasons explaining why some projects fail, are also provided in the above reference. The role of farm management is key; not only must digesters be well engineered and built with high-quality components, they must also be sited at farms willing to incorporate the uncertainties of a new technology.

More than two decades of research has provided much information about how manure can be converted to an energy source; however, farmers have not been motivated to adopt new practices. More cost-effective and easily managed manure management techniques are still needed to encourage farmers to use animal manure for conversion into energy and nutrients, especially for smaller farms.

Anaerobic digestion benefits farmers monetarily and mitigates possible manure pollution problems, thereby sustaining development while maintaining environmental quality. Moreover, rural economic development will benefit from the implicit multiplier effect resulting from jobs created by implementing digester systems. Promising future waste-to-profit activities may enhance the economic performance of the overall farm manure management system. New end-use applications that can provide added value to coproducts and maximize nutrient utilization include advanced technologies to generate electricity and process heat; greenhouses; and, algae, plant and fish aquaculture. The use of attached greenhouses can also provide enhanced plant growth rates if the available carbon dioxide is captured. Wastewater effluent can also be discharged into ponds and used as a growth culture for high-protein content algae or other aquatic plants. A combination of these activities could be incorporated on farms with multi-function production systems.

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Introduction to Anaerobic Digestion (AD)

Biogas is formed solely through the activity of bacteria, unlike composting in which fungi, insects, and worms are also involved in the degradation process. Microbial growth and biogas production is very slow at ambient temperatures. Bacteria tend to occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of animals, and in the anaerobic interiors of landfill sites.

The overall process of AD occurs through the symbiotic action of a complex bacteria consortium. Hydrolytic microorganisms, including common food spoilage bacteria, break down complex organic wastes. These subunits are then fermented into short-chain fatty acids, carbon dioxide, and hydrogen gases.

Synthrophic microorganisms then convert the complex mixture of short-chain fatty acid to acetic acid with the release of more carbon dioxide and hydrogen gases. Finally, methanogenesis produces biogas from the acetic acid, hydrogen, and carbon dioxide. Biogas is a mixture of methane, carbon dioxide, and numerous trace elements. According to some, the two key biological issues are determining the most favorable conditions for each process stage and how non-optimal circumstances affect each process stage as a whole, and the governing role of hydrogen generation and consumption.

Sulfate-reducing bacteria, which reduce sulfates and other sulfur compounds to hydrogen sulfide, are also present during the process. Most of the hydrogen sulfide reacts with iron and other heavy metal salts to form insoluble sulfides, but there will always be some hydrogen sulfide in the biogas.

The widespread natural occurrence of methane bacteria demonstrates that AD can take place over a wide temperature range from 400° F to more than 2120° F and at a variety of moisture contents from around 60 percent to more than 99 percent. This distinguishes the methane bacteria favorably from most aerobic microorganisms involved in the composting process.

AD occurs in the psychrophilic temperature range (less than 680° F), and is routinely observed in marsh gas and in the ambient temperature lagoons used for livestock. Conventional anaerobic digesters are commonly designed to operate in either the mesophilic temperature range (950-1050° F) or thermophilic temperature range (1200-1350° F).

There are usually two reasons why the mesophilic and thermophilic temperatures are preferred. First, a higher loading rate of organic materials can be processed and, because a shorter hydraulic retention time (HRT) is associated with higher temperatures, increased outputs for a given digester capacity result. Second, higher temperatures increase the destruction of pathogens present in raw manure.

During the energy crises of the mid- and late 1970s, the search for alternative energy resources led to investigation of small- and medium-scale anaerobic digesters developed in India and China to determine whether these technologies were directly transferable to farms in the United States. Unfortunately, although these technologies are useful in providing fuel for cooking and lighting in developing economies, most are much too small to be useful to most American farmers. For example, the typical small-scale digester daily produces about the same amount of energy as contained in one gallon of propane.

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Complete-mix Digester

The greater energy requirements of the larger American livestock operations led to the design and installation of several demonstration projects that transferred state-of-the-art sewage treatment plant technology to the farm. Although complete-mix digesters can operate in the thermophilic temperature range, the demonstration projects at facilities such as the Washington State Dairy Farm in Monroe operated only in the mesophilic temperature range. At the Monroe project the digester was sized for the manure volume produced by a milking herd of 180-200 Holstein cows.

Although these first-generation complete-mix digesters generally produced biogas at the target design rate, they suffered from high capital costs and significant operation and maintenance requirements. In practical application on the farm, solids settling, scum formation, and grit removal often presented major problems.

Today's complete-mix digesters can handle manures with Total Solids (TS) concentrations of 3-10 percent, and generally can handle substantial manure volumes. The reactor is a large, vertical, poured concrete or steel circular container. The manure is collected in a mixing pit by either a gravity-flow or pump system. If needed, the TS concentration can be diluted, and the manure can be preheated before it is introduced to the digester reactor. The manure is deliberately mixed within the digester reactor. The mixing process creates a homogeneous substrate that prevents the formation of a surface crust and keeps solid in suspension. Mixing and heating improve digester efficiency. Compete-mix digesters operate at either the mesophilic or thermophilic temperatures range, with a HRT as brief as 10-20 days.

A fixed cover is placed over the complete-mix digester to maintain anaerobic conditions and to trap the methane-rich biogas that is produced. The methane is removed from the digester, processed, and transported to the site of end-use application. The most common application for methane produced by the digestion process is electricity generation using a modified internal combustion engine. Both the digester and the mixing pit are heated with waste heat from the engine cooling system. Complete-mix digester volumes range considerably from about 3,500-70,000 cubic feet. This represents daily capacities of about 35,000 gallons of manure per 500,000 gallons of digester. Multiple digesters usually handle larger volumes.

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Plug-flow Digesters

By the late 1970s researchers at Cornell University were able to reduce the capital costs and the operational complexities associated with the early complete-mix digesters by using a simple extension of Asian AD technology.

These "plug-flow" digesters were adopted with some success in the cooler climate of the Northeast, where dairy farms primarily use scraping systems for manure removal. The 1979 project at the Mason Dixon Dairy Farms in Gettysburg, Pennsylvania was the first plug-flow digester operated on a commercial farm. At the Mason Dixon project, the plug-flow digester was originally sized for a manure volume produced by a milking herd of 600 Holstein cows.

The basic plug-flow digester design is a long linear trough, often built below ground level, with an airtight expandable cover. Manure is collected daily and added to one end of the trough. Each day a new "plug" of manure is added, slowly pushing the other manure down the trough. The size of the plug-flow system is determined by the size of the daily "plug." As the manure progresses through the trough, it decomposes and produces methane that is trapped in the expandable cover. To protect the flexible cover and maintain optimal temperatures, some plug-flow digesters are enclosed in simple greenhouses or insulated with a fiberglass blanket. Plug-flow digesters usually operate at the mesophilic temperature range, with a HRT from 20-30 days. An often vital component of a plug-flow digester is the mixing pit, which allows the TS concentration of the manure to be adjusted to a range of 11-13 percent by dilution with water. Many systems use a mixing pit with a capacity roughly equal to one day's manure output to store manure before adding it to the digester.

The complete-mix and plug-flow digestion technologies are not suited for use on farms that use hydraulic flushing systems to remove manure and anaerobic lagoons to treat waste. Hydraulic flushing substantially dilutes the manure, with TS concentrations often far less than three percent. An anaerobic lagoon is a popular method used to treat and store manure. A properly designed and operated anaerobic lagoon system, in which the HRT exceeds 60 days, may produce significant quantities of methane.

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Covered Anaerobic Lagoon

In the early 1980s, the concept of using a floating cover to collect biogas as it escapes from the surface of an anaerobic lagoon was transferred from industry to the farm. The first successful floating cover that recovered biogas from an anaerobic lagoon operating in the psychrophilic range was sponsored by the California Energy Commission at the Royal Farm operation in Tulare, California. The Royal Farm's digester used the manure from a 1,600 sow farrow-to-finish farm

The North Carolina Energy Division and North Carolina State University constructed the first full-scale covered anaerobic lagoon digester on the East Coast at the Randleigh Dairy in 1988. The digester processed the manure from 150 dairy cows. The project objective was to educate dairy producers through practical demonstration and outreach about the merits of a low-cost and easily maintained digester suitable for use on farms using hydraulic flush manure management systems. The project provided information about the amount of biogas that can be recovered, along with cost information from which the economic merit of the technology can be evaluated.

Placing a floating, impermeable cover over the lagoon captures the methane produced in an anaerobic lagoon. The cover is constructed of an industrial fabric that rests on solid floats laid on the surface of the lagoon. The cover can be placed over the entire lagoon or over the part that produces the most methane.

Once the cover is installed, the methane produced under the covered area of the lagoon is trapped. The biogas is harvested using a collection manifold, such as a long perforated pipe, that is placed under the cover along the sealed edge of the lagoon. Methane is removed by the pull of a slight vacuum on the collection manifold (by connecting a suction blower to the end of the pipe) that draws the collected biogas out from under the cover and on to the end-use application.

The cover is held in position with ropes and anchored by a concrete footing along the edge of the lagoon. Where the cover attaches to the edge of the lagoon, an air-tight seal is constructed by placing a sheet of the cover material over the lagoon bank and down several feet into the lagoon, and clamping the cover (with the footing) onto the sealed bank. Seals are formed on the remaining edges with a weighted curtain of material that hangs vertically from the edge of the floating cover into the lagoon.

The covered lagoon digester has several merits. First, it has good potential for widespread adoption in the United States, especially in the southeast and southwest regions, because many dairy and swine facilities use hydraulic flushing to collect manure and anaerobic lagoons to treat waste. Second, covered lagoon digester operation and maintenance is simple and straightforward compared to complete-mix and plug-flow digesters. Third, the capital costs for this type of digester can be less than those required for the complete-mix and plug-flow types of conventional digesters.

Covering an anaerobic lagoon and harvesting the biogas can be a simplified technology; however, the approach raises at least three significant concerns. A key issue is that digestion rate is dependent on temperature; therefore, biogas production varies seasonally if the lagoon is not externally heated. This means that methane production is greatest in the warm, summer months and lowest during the cooler, winter months. At the Randleigh Dairy, daily biogas production during the summer averaged 35 percent more than during the winter. This may make end-use applications more problematic than they are with conventional digesters, which have less significant seasonal variations in methane production. A second concern is that it can take an anaerobic lagoon as long as one to two years to achieve its "steady-state" biogas production potential. It is best to start a project of this type in late spring or early summer to take advantage of warm weather. Digesters that are started during cool months are subject to upset from overfeeding. Moreover, any anaerobic lagoon (covered or not) is impractical in areas with a high water table because of the potential for groundwater contamination. Lagoons built into highly permeable soils must be adequately lined to prevent groundwater contamination.

The complete-mix and plug-flow digesters for dairy waste, and the covered anaerobic lagoon are the only anaerobic digestion methods now recognized by the USDA's Natural Resource Conservation Service (NRCS) in the form of "National Guidance Provided to States." Other types of AD systems may have the technical and economic potential to process animal manures. Odor control is probably the main reason livestock farmers have installed anaerobic digesters in the United States.

According to the On-Farm Biogas Production Manual,3 Holstein dairy cows can yield 42 cubic feet of 60 percent methane biogas per 1000 pounds of Live Animal Weight (LAW) per day. The sixty thousand dairy cows in Whatcom County therefore have a biogas generation potential of some 3.15 million cubic feet (1250 pounds per cow times 60000 cows divided by 1,000 pounds times 42 cubic feet) per day. At 550 Btu/ft3 for 60 percent methane biogas, this is 1,732 Mbtu/day. The Farm Ware program listed in the Current Opportunities Casebook (4) uses a slightly higher Live Animal Weight (1,400 pounds) and consequent higher daily biogas value (51 cubic feet per 1000 pounds LAW) per day.

1,732 Mbtu/day divided by 3413 Btu/kWh equals 507,471 kWh/day at 100 percent electrical conversion efficiency. A typical engine generator converts only 20 percent to 25 percent of the biogas energy to electrical energy. The bulk of the energy is converted to heat. Heat exchangers typically recover 40 percent to 50 percent of the energy in the biogas as hot water. Thus the production of both heat and electricity from an engine generator, called cogeneration, is 60 percent to 70 percent efficient. As much as half the available heat may be required to heat the digester in the winter. Therefore, existing electrical generation capacity that could be displaced by biogas would likely not exceed 50 percent of 507,471 kWh/day or 253,735 kWh/day. At a residential rate of $.07/kWh, the value of potential electricity from biogas would be $17,761/day or approximately $6.5 million/year and could displace some 10.5 MW of generation capacity.

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Hypothetical Digester

For purposes of estimating capital cost and energy production rates, a hypothetical plug-flow digester is assumed to be located in Whatcom County, Washington. The herd size was assumed to be 500 Holstein dairy cows with a total Live Animal Weight (LAW) of 700,000 pounds.

The farm's manure management was assumed to be a scrape system. The total amount of manure collected was 56,000 pounds per day. With an average Volatile Solids (VS) production of 8.5 pounds/1,000 pounds LAW, total daily VS production was 5,950 pounds. It was assumed that all the manure generated was collected and that the manure had a 12.5 percent TS concentration. Any flushing that may occur in the milking parlor was not included. Inclusion of manure deposited in the milking parlor would increase estimations by 12.5 percent.

The next step was to estimate digester size. The key values for determining digester size are HRT and loading rate. The HRT was assumed to be 20 days, fairly standard for preliminary estimates of digester size. The total digester volume was estimated to be 22,227 cubic feet. Assuming a depth of 10 feet, the linear surface dimensions were estimated to be 106 feet by 21 feet, presenting a total surface area of 2,223 square feet to be covered with an industrial fabric like HDPE or XR-5.4

The analysis estimated that the digester would, on average, produce 35,700 cubic feet of biogas per day. With a heat rate of 14,000 Btu/kWh, daily electricity production was estimated to be 1,607 kWh. The 14,000 Btu/kWh would fuel a 67-kW engine/generator at 24 percent electrical efficiency (there are actually 3413 Btu's in a thousand Watt-hours). Assuming an annual capacity factor of 80 percent and subtracting for the parasitic losses required for the digester's electrical equipment like mixing motors and solid separators, it was estimated that a net of 452,980 kWh could be produced annually.

Thermal energy produced by combusting biogas in an engine/generator is also recoverable. On a dairy farm, hot water is commonly used for sanitary washing throughout the entire year. Thermal energy can also be used for space heating offices and nearby buildings, as well as for other uses such as preheating livestock drinking water. Assuming that 3,750 Btu can be recovered per kWh generated and that 60 percent of this available energy was used during the year, 10.48 million Btu could be recovered.

The complete digester system consists of three components: an energy plant, a digester plant, and a solids processing plant. As already discussed, the normal efficiency of a small internal combustion engine that converts biogas into electricity is about 24.4 percent, a heat rate of 14,000 Btu/kWh. The basic cost of this type of engine/generator including heat recovery and controls was estimated to be $1,050/kW. The engine/generator is assumed to be housed in a 30 by 30 foot building. A stand-by flare should also be nearby to burn off biogas in the event something prevents it from being combusted in the engine.

The digester is assumed to be a plug-flow reactor with a receiving pit that has storage capacity equivalent to two days' loading rate. Although not factored into this analysis, the digester system should also have a liquid storage tank with enough capacity to hold six-months' worth of liquid filtrate.

The last component of the digestion system is the solids recovery plant. Each day, a fraction of the digested slurry is discharged and collected in a discharge tank, and fresh feedstock is added to the digester. The collected digestate will be sent to an on-site solids recovery building where mechanical separators divert the fiber and liquid fractions. It was assumed that the project would use a screw-press mechanical separator that is housed in a 20 by 20 foot building.

Although the three components of the digestion system are assumed to be "turn key" installed, digester engineering fees and a contingency rider are added to the overall capital cost assumptions. Engineering fees are assumed to be 10 percent of the entire plant cost. Contingency was also assumed to be 10 percent of the entire plant cost.

The engine and induction generator was estimated to cost $70,284. Including the cost of the flare and other necessary engine plant subsystems, such as the building to house the engine/generator, piping, biogas meters, switchgear, and intertie components required by most electric utilities, the total capital cost for the energy plant was $109,684. The cost of constructing the digester was estimated to be $97,825, including the cost of the cover, as well as all gas handling and transmission costs. The cost of the screw-press separator and building was estimated to be $31,000. Engineering costs and the contingency rider were each estimated to be $23,851. Thus, the total capital expense was estimated to be $286,211.

Engine operation and maintenance costs were estimated to be $0.0125/kWh of electricity generated, which came to a total annual expense of $5,662 during the first year of operation. The first year expenses associated with the digester system were $9,783 and solids recovery operation and maintenance was $1,550. Total expenses during the first year of the project were projected to be $16,995.

In determining the offset cost of purchased power, the electricity rate used was $0.113/kWh. Given the kWh generated during the course of a year, this system could offset enough purchased power sufficient to save $51,187 during the first year of operation. The value of the thermal energy recovery system was based on offsetting the delivered cost of purchased propane that was assumed to cost $0.58/gal. This translates into a cost of about $12.41/million Btu. Given the total amount of thermal energy produced during the course of a year, this system could provide a saving of $10,476 in propane costs during the first year of operation. Fiber product sales were estimated to be $16,671. Total revenue for the first year of the project summed to $78,333. The simple payback period (SPP) for this project was 5.3 years.

The Langerwerf Dairy is a plug-flow digester in California that has been rebuilt after 16 years of profitable service.5 Original cost was $200,000. Electricity sales alone over 16 years was $350,000. Total earnings for 16 years was $540,000. Annual maintenance costs were 1 percent of capital cost. Total system upgrade good for 10 more years cost $50,000.

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References Cited

1. Northwest Indian College National Indian Center for Marine and Environmental Research and Education. Feasibility Studies for Potential Application of Renewable Energy Technologies at Tribal Colleges and Universities Supplemental Announcement 07. Volume III - Draft Phase 1 Final Report. Submitted: June 30,2001. <http://nwic-research.org/DOE%20PhaseII%20Vol3.htm>

2. Methane Recovery from Animal Manures. The Current Opportunities Casebook. September 1998. National Renewable Energy lab publication NREL/SR-580-25145. (DOE contract No. DE-AC36-83CH10093).

3. Parsons, Robert. 1984. On-Farm Biogas Production Manual. Northeast Regional Agricultural Engineering Service (NRAES –20).

4. Agricultural Waste Management Field Handbook. 1992. Natural Resource Conservation Service. 210-AWMFH. Department of Agriculture, Washington, DC.

5. Langerwerf Dairy. A 16 year success story… making bucks and electricity from dairy cow waste. <http://www.westbioenergy.org/june1999/wbqjune9902.htm>

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